A review of recent developments in polymeric materials for battery energy storage

Abstract

This review explores the innovative role of polymer electrolytes in energy storage systems, emphasizing their characteristics, fabrication methods, and practical applications in advanced electrochemistry. It begins by outlining various types of polymeric material, including conductive polymers, redox-active polymers, and polymer electrolytes, that improve the lifetime, efficiency, and capacity of energy storage devices. A comprehensive review of polymeric electrochemistry is provided, focusing on how polymers maintain electrical insulation and enable ion transport, two essential functions for devices such as supercapacitors and batteries. Advanced fabrication techniques, including solution casting, melt processing, electrospinning, and in-situ polymerization are emphasized for tailoring polymer electrolytes to achieve optimal microstructural and electrochemical properties. The study further explores the electrochemical characteristics of polymer electrolytes, focusing on the electrochemical impedance spectroscopy technique. By offering a thorough understanding of both the theoretical and practical aspects of polymer electrolytes, this study contributes to the development of more effective and long-lasting energy storage systems. This discussion aligns with the global trend toward innovative and eco-friendly energy technologies while advancing the field of materials research.

Keywords

Polymer electrolytes electrochemistry fabrication techniques energy storage battery

Introduction

There are some types of energy storage systems, such as batteries, thermal energy storage, flywheels, pumped-hydro storage, compressed-air storage, and hydrogen storage. The battery energy storage systems are easy to operate and require a relatively small space for large amounts of energy, which make them perfect for residential and commercial energy storage solutions. In the quest for effective and sustainable battery energy storage solutions, polymeric materials have become essential components. Their unique properties and adaptability make them crucial in the development of contemporary energy systems. As global energy demands continue to rise, the need for advanced materials with improved energy storage and management capabilities has become urgent.

Traditionally, battery energy storage systems have relied on a wide range of materials, each with its unique advantages and disadvantages. Research has consistently aimed to discover the ideal material, ranging from heavy metals to organic compounds and ceramics. The development of polymeric materials marked a significant breakthrough, providing enhanced functionality that earlier materials were unable to achieve, as illustrated in Figure 1. Due to their remarkable mechanical flexibility, chemical stability, and electrical properties, polymeric materials have addressed many of the challenges faced by conventional energy storage systems.^1–4

Figure 1.

Research progress on energy storage systems.

Heavy metals including lead, cadmium, and mercury have been commonly used in traditional battery technologies; however, they pose serious threats to human health and the environment.⁵ For example, lead-acid batteries while reliable and affordable, are widely utilized in backup power systems and automotive applications.^6,7 However, the well-documented environmental damage from lead mining and the health risks associated with lead exposure have spurred a growing demand for safer alternatives. Likewise, nickel-cadmium batteries are known for their long cycle life and high energy density, but their toxicity and the “memory effect” of cadmium present significant drawbacks.⁸ Mercury, once widely utilized in button cell batteries, has been phased out due to its extreme toxicity. The challenges posed by heavy metals highlight the urgent need for the development of new materials, as demonstrated in Figure 2.⁹ Polymeric materials offer a promising alternative, presenting reduced environmental impact and the potential for improved performance characteristics such as flexibility, reduced weight, and higher conductivity. Their ability to be modified through chemical synthesis makes them ideal candidates for next-generation energy storage solutions, aligning with the worldwide trend towards sustainability and reduced environmental footprints.^10,11

Figure 2.

Mercury (Hg) coin cell and its anatomy.

One of the most critical aspects of these polymeric materials is their application in polymeric electrochemistry, a field focused on the electrochemical behavior of polymers. This area is particularly relevant to energy storage technologies, as the electrochemical properties of materials determine the efficiency, capacity, and life span of storage devices. Understanding the electrochemical mechanisms of polymers not only enhances the fundamental knowledge but also facilitates improvements in their practical applications.

This review delves into the multifaceted roles of polymeric materials in energy storage systems, emphasizing the latest advancements in polymeric electrochemistry and their real-world applications. It aims to provide a comprehensive exploration of the synthesis and properties of key polymeric materials, their underlying electrochemical mechanisms, and practical applications, particularly in enhancing the performance and efficiency of energy storage devices.

This study is organized to guide readers through logical progression of topics, beginning with an introduction to the various types of polymeric materials used in energy storage systems. It then focuses on different categories of polymeric materials, with particular attention to polymer electrolytes and their classifications. Following this, it delves into the fundamentals of polymer electrolytes’ electrochemistry, followed by a discussion of the design and operation of these materials in batteries.

As illustrated in Figure 3, research interest in polymeric materials for energy storage has grown significantly. The number of publications in this area increased from 3540 in 2015 to 20,778 in 2024, underscoring both the rapid progress and the importance of these materials in energy storage systems.¹²

Figure 3.

Publication trend on polymeric materials for energy storage systems.¹²

In summary, as the global energy sector moves toward more sustainable and high-efficiency solutions, the insights presented in this work are timely and essential for advancing materials, science, and energy technology. By the end of this review, readers will gain a thorough understanding of the potential of polymeric materials in transforming energy storage systems, equipped with the knowledge to drive further innovations in this dynamic field.

Polymeric materials for energy storage

Polymeric materials stand out in the rapidly evolving field of energy storage technologies due to their adaptability and versatility. The advancement and integration of these materials into energy storage systems consistently improve their efficiency, capacity, and longevity, as shown in Figure 4. The sorts of polymers utilized in these systems are divided into three categories in this thorough discussion: conductive polymers, redox-active polymers, and polymer electrolytes. Each category plays a distinct contribution to the development of energy storage solutions.^13–15

Figure 4.

Variety types of polymer materials are used in energy storage systems.

Conductive polymers

Conductive polymers are renowned for their intrinsic ability to conduct electricity, a property stemming from the conjugation in their molecular structures and further enhanced through doping processes. Polyaniline (PANI), polypyrrole (Ppy), and polythiophene are prominent examples of these materials. These polymers are extensively used in supercapacitors and as electrode materials in batteries. Recent advancements in polyaniline, such as nano-structuring have increased its surface area and improved interface interactions, significantly boosting its capacitance and overall performance in supercapacitors (Figure 5).¹⁶ Additionally, progress in doping techniques has enhanced the conductivity and stability of polythiophene, making it increasingly suitable for flexible electronic applications. Notably, the development of PEDOT:PSS composites with graphene oxide (Figure 6), has led to improved thermal stability and mechanical strength, making them ideal for high-performance energy storage devices.¹⁷ This observation is consistent with recent reviews, emphasizing the role of carbon nanostructured materials in enhancing the performance of supercapacitors.¹⁸

Figure 5.

Nano-engineering of PANI structures.

Figure 6.

Surface modification of PEDOT: PSS with graphene oxide (GO).¹⁷

Table 1 summarizes the key characteristics and applications of these conductive polymers in energy storage, based on various studies.

Table 1.

Summary of key properties of conductive polymers in energy storage.

Material	Method	Electrolyte solution	Specific capacitance (F/g)	Cyclic performance/retention (%)	Energy density (wh/kg)	Power density (W/kg)	Reference
GNP/PANI nanocomposite electrode	Conventional in-situ chemical oxidative polymerization	H₂SO₄	1088	2000,93	30.68	272.71	¹⁹
Fe-PAni	Electrochemical polymerization	Oxalic acid	83	—	2.47	774	²⁰
Fe-PAni/MnO₂	Electrochemical polymerization	Oxalic acid	278	—	3.75	2250	²⁰
PANi@CNF	Electrodeposition method	H₂SO₄	285.3	9000, 70.3	106.5	769	²¹
SPANi/MXene	In-situ polymerization	HCl	512.45	10,000,95.2	15.18	200	²²
PANI nanofibers	Chemical oxidative synthesis	H₂SO₄	832.5	1000, 95.3	—	—	²³
CDP-PANI/PC	In-situ polymerization	HCl	739.2	5000,81	—	—	²⁴
PPy@PANI nanotubes electrode	Chemical synthesis, dilute solution method	H₂SO₄	765	1000,86.3	—	—	²⁵
Ni@PPy nanocomposite	Growth of Ni NPs followed by pyrrole polymerization on the paper substrate.	PVA and H₂SO₄	544	3000,68.3	48	400	²⁶
PPy@NCS-5	Solvothermal	KOH	2011.6	5000,93.3	44.6	15,005.6	²⁷
g-C3N4/ppy electrode	In-situ oxidation polymerization	H₂SO₄	555	500,84	86	300	²⁸
Ppy/rGO	In-situ polymerization and hydrothermal technique	KOH	675	1000,85	72.23	400.85	²⁹
SDBSDPPy-HC	Polymerization	KOH	1086	2500,90	195.05	1574	³⁰
PPy nanosheets	Polymerization	H₂SO₄	586	5000,91	—	—	³¹
CNT/PTH composite	Polymerization		0.057	—	3.5		³²
PTh/PbO/PbS		KOH	992	10,000,96	972	6086	³³
poly(indole/thiophene)(P7-P)	Polymerization		1170	—	56.5	2614.4	³⁴
PTh/BTB	Electro polymerization	H₂SO₄	443.5	10,000,79.2	9.7	5000	³⁵
(PDBrTh)	Electrochemical polymerization		229.6	1000,>80	—	—	³⁶
PEDOT:PSS/rGO	—	sulfuric acid	215.6	5000,93.2	11.6	500	³⁷
(P(TT-co-EDOT))	Electrochemical copolymerization	ACN-Bu₄NPF₆	193	3000	—	—	³⁸
polythiophene/50%Al₂O₃ (PTCA5)	Chemical polymerization	PVA-KOH	780.40	5000,90.54	58.04	533.02	³⁹

Redox-active polymer

Redox-active polymers are distinguished by their ability to undergo reversible redox reactions, making them ideal candidates for redox flow battery applications. Polymers such as polyviologens, polyanthraquinones, and polyimides provide significant benefits in terms of energy density and cyclic stability.^13,40 Functional group modifications to polyviologens have notably enhanced their solubility and redox activity, which are crucial for boosting the energy density of redox flow batteries. A breakthrough in synthesizing poly(anthraquinonyl-sulfide), patented by Gomez et al., has expanded its application in aqueous flow batteries, providing a cost-effective and efficient solution for large-scale energy storage.⁴¹ The schematic representation of the redox reaction of polyanthraquinone sulfide (PAQS) is depicted in Figure 7.⁴² Moreover, the development of hyperbranched polyimides with superior thermal stability and electrochemical activity positions them as suitable materials for high-temperature applications. Figure 8 shows the schematic of the integration of calcium fluoride (CaF₂) nanoparticles into the polyimide (PI) matrix.⁴³ Table 2 provides an overview of the most prevalent redox-active polymers and their composites, highlighting their key properties and performance metrics.

Figure 7.

Two-step redox reaction of PAQS. Reprinted from Phadke et al.⁴² with permission from Wiley.

Figure 8.

Pi/CaF₂ nanocomposites. Reprinted from Li et al.⁴³ with permission from Elsevier.

Table 2.

Summary of key properties of redox-active polymers in energy storage.

Material	Electrolyte	Gravimetric energy density (wh/kg)	Volumetric energy density (J/cm³)	Specific capacity (mAh/g)	Cyclic performance	Application	Reference
PHV-Cl	Magnesium-based electrolytes.	227	—	171	50	Dual-ion mode in MIBs	⁴⁴
Li–PBV-Cl2 DIB	LiCl/DMSO	470	—	183	300, 88%	Dual-ion batteries	⁴⁵
PXVCl₂/G	—	—	—	140	60	Rechargeable batteries	⁴⁶
PBVCl2 cathode	TBMACl-based electrolyte	391	—	179.1	120, 83.8%	Chloride ion batteries	⁴⁷
PAQS derived activated carbon electrode(C-KOH-H)	(SBP-BF4)	46	—	—	20,000,98%	Supercapacitors	⁴⁸
CGO/PDAAQ@MXene	H₂SO₄	41	—	—	5000, 83%	Supercapacitor	⁴⁹
PAT polymer	LiPF₆ in EC/DMC			1770	400	Lithium-ion battery,	⁵⁰
TNAQ	LiClO₄ in EC/ DEC	—	—	231	300,81%	Li-organic batteries (LOBs)	⁵¹
IEP-11\|\|Ni(OH)₂	KOH	—	—	150	22,730,75%	Alkaline rechargeable batteries	⁵²
PDHA	H₂SO₄	—	—	140	500, 97%	Battery	⁵³
0.87PI-0.13PAA	—	—	8.9	—	20,000		⁵⁴
NH2-POSS functionalized PI films	—	0.7611	3.45	—	>50,000, >90%	High-temperature energy storage	⁵⁵
PI1/CNT cathode	Non-nucleophile Mg(HMDS)2-4MgCl2/2THFPP14TFSI	—	—	183	Over 8000	Rechargeable magnesium batteries	⁵⁶
PI/MOFs hybrid electrode	Ionic liquid electrolyte.	—	—	83	1800, 88%	Al-organic batteries.	⁵⁷

Polymer electrolytes

Polymer electrolytes can be categorized into three types according to their compositions, as follows: Dry solid polymer electrolytes, gel polymer electrolytes, and composite polymer electrolytes.⁵⁸

The first type, dry solid polymer electrolytes, is formed through the interaction between metal ions and polar polymers after dissolving inorganic salts into the polymer matrix, resulting in ion-conducting solid polymers. The second type, gel polymer electrolytes, also known as plasticized polymers, are produced by adding a plasticizer to a polymer, causing it to swell. These electrolytes are typically prepared by heating and cooling the mixture to achieve a gel-like state. A schematic illustration of the gel polymer electrolytes is shown in Figure 9.⁵⁹ The third type, composite polymer electrolytes, has been emerged to improve the performance of polymer electrolytes, particularly in terms of ionic conductivity. These electrolytes are created by the incorporation of additional materials into solid polymer electrolytes. Several fabrication techniques have been employed, including polymer blending, nano particles doping, and reinforcement with fillers. Figure 10 depicts the different types of composite polymer electrolytes.⁵⁹ In general, polymer electrolytes are widely used in solid-state batteries and supercapacitors.^58,60

Figure 9.

Schematic illustration of gel–polymer electrolytes.⁵⁹

Figure 10.

Different types of composite polymer electrolytes.⁵⁹

Solid-state batteries rely significantly on polymer electrolytes such as poly (methyl methacrylate) (PMMA) and polyethylene oxide (PEO). These materials enhance the mechanical stability, safety, and longevity of batteries, particularly in applications requiring flexibility, such as wearable devices. They facilitate ionic movement between electrodes while maintaining electrical insulation.^61,62 Recent studies, including work by Feng et al., have demonstrated that introducing nanostructured ceramics into polyethylene oxide has improved its mechanical strength and ionic conductivity, making PEO-based electrolytes more effective even at lower temperatures.⁶³ Feng's comparative analysis of different ceramic types hybridized with a PEO matrix, shown in Figure 11, highlights these improvements.

Figure 11.

Ionic conductivity comparison with different types of ceramic hybridization to PEO matrix polymer.⁶³

In a separate study conducted by Alharbi and Rajeh, the effect of doping cobalt oxide nanoparticles (Co₃O₄ NPs) into poly (vinylidene fluoride) (PVDF)/poly(ethylene oxide) (PEO) composites was evaluated, revealing an impressive ionic conductivity of 6.5 × 10^–4 Scm⁻¹. Figure 12 illustrates the proposed interaction mechanism between PEO, PVDF, and Co₃O₄ nanocomposites.⁶⁴ PMMA-based electrolytes have also been enhanced through blending with ionic liquids, which improves ionic transport mechanisms and boosts solid-state battery performance.^65–69 Furthermore, polysiloxanes are under investigation for their unique properties, including high flexibility and thermal stability, positioning them as promising candidates for cutting-edge battery technology.⁷⁰ Table 3 summarizes the most widely studied polymer electrolytes and their composites, showcasing their key characteristics and performance data.

Figure 12.

Possible mechanism for interaction between PEO, PVDF, and Co₃O₄ nanocomposites.⁶⁴

Table 3.

Summary of key properties of polymer electrolytes in energy storage.

Material	Conductivity (S/cm)	Breakdown strength (MV/m)	Energy density (J/cm³)	ESW) (V)	Cyclic performance	Specific capacity (mAh/g)	Application	Reference
BT@PDA/PVDF-PMMA nanocomposite	—	378	11.15	—	—			⁷¹
PVDF/SiO2-g-P(MMA-co-HEMA) GPE	2.63 × 10⁻³	—	—	4.8	50	102	Rechargeable Li batteries	⁷²
Sandwiched polymer nanocomposites with poly(methyl methacrylate) layer	—	559	17.1	—	600	—	Capacitive energy storage applications	⁷³
PMMA/PVDF-based organic composite	—	685.67	21.89	—	—	—	Energy storage and conversion for clean energy	⁷⁴
PVDF/PMMA/TiO₂ blending nanodielectric material	8.3 × 10⁻¹⁴	387	5.8	—	—	—	High-performance energy-storage dielectric materials	⁷⁵
P(VDF-TrFE)-g-PMMA	—	220	9.5	—	—	—	Organic electrical energy storage.	⁷⁶
PMMA/SiO₂/PP@GPE	4.1 × 10⁻⁴	—	—	4.9	116	10 880	Solid state Li−O₂ battery	⁷⁷
B_0.25PEG_0.25IL_0.5	1.9 × 10^{− 3}	—	—	—	Over 100	144	Lithium-ion batteries	⁷⁸
P(VDF-TrFE-CFE) and PMMA polymer blend composites	—	520	9.3	—	—	—	Electrostatic capacitor application	⁷⁹
GO-modified PEO electrolyte	1.54 × 10⁻⁵	—	—	5	100	142	Solid-state lithium metal battery	⁸⁰

Principle of polymeric electrochemistry

Electrochemistry is the study of chemical reactions involving electron movements that generate electrical currents. It deals with the interconversion between chemical changes and electrical energy, predominantly observed in redox reactions characterized by electron transfer during oxidation and reduction processes. These reactions are fundamental to energy generation and storage in devices such as fuel cells and batteries, making electrochemistry essential for understanding and developing energy storage systems. It provides both the mechanisms for energy conversion and reliable techniques for energy storage and release.

The controlled movement of electrons in a defined system forms electrochemistry's role in energy production and storage. In battery applications, electrochemical processes enable the flow of electrons between different materials through an external circuit, generating electrical power. During charging, these processes are reversed by applying electrical energy, converting it back into chemical potential energy. The reversibility of these electrochemical reactions is a critical advantage, facilitating the extensive use of batteries and similar systems for energy storage. This feature provides a dependable method to store energy and release it as needed, supporting stable and efficient energy management.

The presence of conjugated systems along their molecular backbones allows polymeric materials to participate in electrochemical interaction despite their non-metallic nature.⁸¹ Furthermore, the synthetic versatility of polymeric materials allows for structural tuning to improve properties such as electrical conductivity and ion exchange capacity. This tunability makes polymers highly desirable for a variety of energy storage applications, ranging from large-scale storage systems to flexible wearable devices.

The movement of protons or electrons within polymer matrices is essential to their function in electrochemical applications. In conductive polymers, electron transport is facilitated by π-conjugated systems that enable electron delocalization and mobility along the polymer chain.⁸² This mobility is crucial for the electrical conductivity and electrochemical reactivity of polymers. Proton transport, on the other hand, happens through basic or acidic groups integrated into the polymer structure. In hydrophilic polymers, these groups allow protons to move along the polymer backbone or through associated water molecules (Figure 13).⁸³

Figure 13.

Ionic conductive channel visualized by TEM images.⁸³

Polyaniline (PANI) is a dual-conductivity polymer, capable of facilitating both electron and proton transport. Its amine groups aid in proton conduction, while electrons are transported along its conjugated backbone.^7,84,85 This property makes PANI highly suitable for use in battery electrodes and electrochromic devices. Nafion^®, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, is another noteworthy example, known for its proton conductivity through sulfonic acid groups. This property is crucial for its role as proton exchange membranes (PEMs) in fuel cells (Figure 14).⁸⁶ Hydration enhances proton conduction in Nafion^®, facilitating efficient proton hopping. When combined with polystyrene sulfonate (PSS), poly(3,4-ethylenedioxythiophene) (PEDOT) exhibits improved stability and processability, conducting electrons through its π-conjugated structure and proving effective for energy storage applications.⁸⁷

Figure 14.

Nafion^® used in PEM water electrolyzer. Reprinted from Rao et al.⁸⁶ with permission from Elsevier.

Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) is an electrolyte frequently employed in energy storage systems, particularly in lithium batteries and super-capacitors. While PVDF-HFP does not inherently conduct electrons, it acts as a host matrix for conductive fillers such as carbon black or graphene, which promote electron transport. With its porous structure capable of absorbing liquid electrolytes that enable ion transport, PVDF-HFP functions as a gel polymer electrolyte supporting proton movement. This feature is essential in applications where high ionic conductivity must be complemented by the mechanical and chemical stability of the polymer matrix.

Electrochemical reactions in polymer electrolytes

In this section, we investigate the electrochemical reactions of polymer electrolytes in batteries and fuel cells. Figure 15(a) shows the schematic use of polymer electrolytes in a rechargeable battery, where a polymer electrolyte acts as a membrane to separate the cathode and the lithium metal anode, which serve as the positive and negative electrodes, respectively. During battery operation, chemical reactions occur during both charging and discharging processes. In the discharge process, represented by the forward reaction in Figure 15(a), energy is released as high-energy materials are converted into low-energy compounds. In contrast, during charging, the reverse reaction takes place, and energy stores with the aid of an external power source. As illustrated in Figure 15(a), the cathode material (FePO₄ or LiFePO₄) is combined with conductive carbon (black spheres) to increase the electronic conductivity, followed by embedding in the polymer electrolyte matrix to enhance conductivity and facilitate Li⁺ transport to the active sites. On the other hand, the lithium metal anode exhibits high conductivity for both Li⁺ and electrons, enabling it to function effectively as a conductive sheet. Overall, the polymer electrolyte must transfer Li⁺ ions while blocking electron transfer electrolytes to maintain proper battery function. Additionally, it must have sufficient mechanical strength to divide the cathode and anode. Figure 15(b) illustrates a hydrogen fuel cell, another application where polymer electrolytes are used, though with some differences compared to lithium batteries. As shown in the figure, the system operates based on forward electrochemical reactions, at both anode and cathode. Although these reactions are theoretically irreversible, only the forward direction is utilized, leading to hydrogen consumption and water production, which is typically released into the atmosphere. Similar to lithium batteries, polymer electrolytes in hydrogen fuel cells serve as membranes for ion transport. However, instead of Li⁺ ions, H⁺ are charge carriers in this system. The fundamental role of polymer electrolytes remains the same: enabling selective ion transport. This selectivity is achieved due to the covalently bonded anionic groups to the polymer backbone, preventing their movement and minimizing energy losses. Additionally, controlling the hydration of polymer membranes is essential in fuel cells. Efficient proton transport in fuel cells relies on the proper hydration of polymer electrolytes. Excess water can cause polymer swelling, resulting in mechanical stress failure. On the other hand, insufficient hydration can impair ion transport and cause the membrane to detach from the electrodes. Overall, Figure 15(a) and (b) highlights a key insight that polymer electrolytes are versatile and can be effectively used in both open systems like fuel cells and closed systems like lithium batteries.⁸⁸

Figure 15.

(a) Polymer electrolyte in a rechargeable battery, (b) polymer electrolyte in a hydrogen fuel cell. Reprinted from Hallinan and Balsara⁸⁸ with permission from Annual Reviews.

Application and fabrication methods in energy storage systems

Polymeric materials, particularly polymer electrolytes, have significantly contributed to the development of energy storage systems by offering properties that surpass those of conventional materials. Their applications range from large-scale industrial systems where stability, safety, and energy density are critical, to small-scale electronic devices. The inherent versatility and impressive electrochemical performance of polymers are key to enabling next-generation energy storage solutions. This section emphasizes the critical role of polymers in bridging theoretical electrochemistry with practical applications while introducing readers to the advanced fabrication and analytical techniques driving progress in this field.

Applications of polymer electrolytes in lithium batteries

Flexible lithium-ion batteries

Nowadays, with the rapid increase in the demand for electronic devices, the need for flexible lithium-ion batteries is also growing. To enhance the performance of these batteries, replacing liquid electrolytes with polymer electrolytes has become common due to their advantages including improved flexibility and safety. Several studies have investigated the use of polymer electrolytes in flexible lithium-ion batteries.⁸⁹ Xu et al. developed a pouch cell prototype using a flexible PVAc/PVDF-HFP/SiO₂-based gel polymer electrolyte, fabricated through hot calendar “Bellcore” technology. As shown in Figure 16, the schematic illustrates the application of this novel GPE in the flexible lithium-ion battery. The battery exhibited excellent flexibility, withstanding bending from 0° to 180°, and maintained its electrochemical performance by powering a 2.5 W electric fan while bending. It also demonstrated high mechanical tolerance without any sign of thermal runaway.⁹⁰

Figure 16.

Schematic illustration of flexible PVAc/PVDF-HFP/SiO₂-based gel polymer electrolyte preparation method in a pouch cell prototype. Reprinted from Xu et al.⁹⁰ with permission from Elsevier.

Recently, Zhu et al. fabricated a P(BA-co-EGDMA)/PVDF-HFP/LiTFSI/HNT-based gel polymer electrolyte to enhance the electrochemical performance of a semi-solid lithium-ion battery. To improve the flexibility of the battery, they employed UV curing technology, as illustrated in Figure 17. This novel battery exhibited a high ionic conductivity of 1.22 × 10⁻³ S/cm and retained 89% of its capacity after 120 cycles.⁹¹ Weng et al. reported a polymer electrolyte composed of PEO and LiTFSI, sandwiched between a LiMn₂O₄ (LMO)/carbon nanotubes (CNTs) cathode and a Li₄Ti₅O₁₂ (LTO)/CNTs anode. This highly flexible battery design demonstrated excellent stretchability, up to 400%, with minimal change in electrochemical performance, which is consistent with recent reviews on the role of carbon nanomaterials in advanced energy storage systems. The schematic configuration of this flexible battery is shown in Figure 18(a)–(d).^89,92 Overall, these studies confirm that polymer electrolytes enhance both flexibility and electrochemical stability in lithium-ion batteries.

Figure 17.

Schematic illustration of preparation methodology of flexible polymer PHHNT electrolyte. Reprinted from Zhu et al.⁹¹ with permission from Elsevier.

Figure 18.

Schematic configuration of flexible lithium-ion battery and the stretchable battery fixed on a moving stage lighted up an LED before and under stretching strain.⁸⁹

High-voltage lithium-ion batteries

The application of certain polymer electrolytes in high-voltage lithium-ion batteries is attributed to their exceptional chemical stability, which prevents them from reacting or degrading easily under high-voltage conditions, a crucial property derived from their molecular structure.⁸⁹ In a recent study, Huang et al. developed a GPE composed of a polyacrylonitrile (PAN) porous membrane and 2,2,3,44,4-hexafluorobutyl acrylate grafted pentaerythritol tetraacrylate (PETEA), synthesized via an in-situ polymerization method. This GPE exhibited a high ionic conductivity of 4.06 × 10⁻³ S/cm and an electrochemical stability window up to 5.35 V, making it a promising candidate for high-voltage lithium-ion batteries.⁹³ Similarly, Poiana et al. developed a non-flammable gel polymer electrolyte by incorporating 1-butyl-1-methylpyrrolidinium hexafluorophosphate ([Py₁₄]PF₆) into a PVDF-HFP-based membrane. As illustrated in Figure 19, the addition of ionic liquid significantly reduced flammability, thereby enhancing the safety of the battery. Additionally, the GPE demonstrated a high specific capacity of 110 mAh/g and excellent stability over 150 cycles, confirming its potential for high-voltage applications.⁹⁴ Dong et al. fabricated a GPE incorporating bacterial cellulose and poly (methyl vinyl ether-alt-maleic anhydride), showcasing oxidation resistance up to 5.2 V vs. Li/Li⁺ and maintained 85% capacity retention after 700 cycles. These characteristics highlight its suitability for high-voltage lithium metal/LiCoO₂ batteries.⁸⁹

Figure 19.

Images during flammability tests of the samples. Reprinted from Poiana et al.⁹⁴ with permission from Elsevier.

Smart lithium-ion batteries

An advanced type of lithium-ion battery is classified as a smart battery, which can adapt its characteristics and behavior to external stimuli such as humidity and temperature. These batteries can be divided into different groups based on their responsiveness. Transient Li-ion batteries, for example, respond to humidity and are excellent candidates for applications ranging from environmental sensors to biomedical applications due to their ability to disintegrate after a specific operational time. In this regard, as demonstrated in Figure 20(a) and (b), Fu et al., reported a PEO-based GPE in a transient lithium-ion battery that fully dissolved after a defined operational period, leaving no harmful residues, thereby minimizing environmental impact. To prevent thermal runaway caused by high temperatures, another category, thermal-responsive lithium-ion batteries, has been developed. These batteries can self-protect by reacting to high temperatures. Kelly et al., developed a thermally sensitive battery composed of PEO, lithium salt, and ionic liquid, which undergoes phase separation above its critical temperature, thereby reducing ionic conductivity and offering a safe mechanism to prevent overheating. This process is shown in Figure 21(a) and (b).⁸⁹ In another study, Jabbary et al. introduced a smart lithium-ion battery with a shape memory function, capable of recovering its original shape after deformation. This battery demonstrated a discharge capacity of approximately 140 mAh/g, with 92% capacity retention after 100 cycles, making it ideal for wearable electronics.⁹⁵ In summary, smart lithium-ion batteries with polymer electrolytes show remarkable adaptability to external factors, making them ideal for energy storage applications. However, further research is needed to improve their long-term stability, and real-world application.

Figure 20.

(a) Schematic illustration of the transient Li-ion batteries. (b) Transient test of the battery. Reprinted from Zhou et al.⁸⁹ with permission from Elsevier.

Figure 21.

(a) Photographs for the PEO/IL phase separation. (b) The conductivities of materials. Reprinted with permission from Kelly et al.¹⁰³ Copyright 2015 Royal Society of Chemistry. Reprinted from Zhou et al.⁸⁹ with permission from Elsevier.

Lithium-metal batteries

To enhance battery performance, the use of lithium metal as the anode has become common due to its high energy storage capacity and high voltage. However, the use of lithium metals has also drawbacks, including the formation of dendrites during charging and discharging processes. To address this challenge, polymer electrolytes have shown promise in several ways. These materials can strengthen the mechanical stability of the batteries, which helps decrease dendrite growth and extend the battery's lifetime. Additionally, polymer electrolytes can regulate the movement of lithium ions, which should be the only ions moving between electrodes. Thanks to their unique mesh-like structure, polymer electrolytes help prevent the movement of other ions and ensure a more uniform dispersion of lithium ions, both of which are critical for reducing dendrite formation.⁸⁹ Bao et al. reported a polymer electrolyte consisting of PEO, LiTFSI, and ox-PIL@GO in lithium-metal electrolyte, which improved the Li+ transference number from 0.21 to 0.61, resulting in high ionic conductivity of 1.01 × 10⁻⁴ S/cm and discharge specific capacity of 116 mAh/g for 300 cycles.⁹⁶ Additionally, Zhu et al. developed a non-flammable liquid polymer electrolyte, without using any molecular solvent, for the lithium-metal batteries. This electrolyte demonstrated a high ionic conductivity of 1.09 × 10⁻⁴ S/cm and enabled operation at 60–90 °C for more than 1000 cycles.⁹⁷ In conclusion, polymer electrolytes showcase potential to stabilize lithium-metal batteries by limiting dendrite growth, although long-term reliability is still under investigation.

Applications for polymer electrolytes in supercapacitors

Another important application of polymer electrolytes is in supercapacitors, which are classified into three groups: electrochemical double-layer capacitors (EDLCs), pseudocapacitors, and hybrid systems. Supercapacitors are known for their rapid charge–discharge ability and long cycle life, and recent studies have demonstrated major progress in electrode design, enhancing their performance. Similar to their roles in batteries, the incorporation of polymer electrolytes in supercapacitors can increase the performance by improving ionic conductivity and mechanical flexibility.^98–101 Several studies have demonstrated these advantages using different types of polymer electrolytes in supercapacitors. For example, Je and Choi fabricated triple-network hydrogel polymer electrolytes (TNPEs) for solid-state supercapacitors, achieving a high ionic conductivity of 5.6 × 10⁻³ S/cm at −30 °C, a specific capacitance of 24 F/g, and an energy density of 17 Wh/kg.¹⁰² In another study, Zaidi et al. developed a chitosan-reinforced gelatin hydrogel polymer electrolyte for flexible solid-state supercapacitors, reporting high ionic conductivities of 72 mS/cm at 26 °C and 39 mS/cm at −30 °C.¹⁰³ Lim et al. prepared an environmentally friendly polymer electrolyte consisting of sulfonated methyl cellulose (LiSMC) and LiClO₄ salt, which delivered a specific capacitance of 8.93 mF/cm² at 0.1 mA/cm² and retained 95% of its stability after 30,000 cycles.¹⁰⁴ Similarly, Shin et al. developed a PAN-based gel polymer electrolyte for photothermal supercapacitors. When combined with a carbon black layer, the supercapacitors operated at −60 °C, and achieved an energy density of 94 μWh/cm² at 0.4 mW/cm².¹⁰⁵ More recently, Badi et al. fabricated solid flexible polymer blend electrolytes (PVA–PVP doped with NaSCN) and integrated them into supercapacitor devices with graphene oxide (GO)-based electrodes. Their optimized composition achieved an ionic conductivity of 8.1 × 10⁻⁵ S/cm, an electrochemical stability of 1.5 V, and an approximate specific capacitance of 13–14 F/g.¹⁰⁶ This finding is consistent with recent studies emphasizing graphene aerogels as highly porous materials for charge storage in supercapacitors.¹⁰⁷ These examples demonstrate that polymer electrolytes play a crucial role in enhancing ionic conductivity, flexibility, and cycling stability of supercapacitors.

Fabrication methods

The fabrication of polymer-based energy storage devices is a complex process that requires precise control over material properties to achieve optimal performance. This subsection delves into key manufacturing methods, including solvent casting, melt-processing, electrospinning, and In-situ polymerization, which are essential for producing high-efficiency polymer electrolytes. It also examines the influence of these techniques on the microstructural and electrochemical characteristics of the polymer electrolytes, highlighting their impact on the overall performance of energy storage devices.

Solution casting

The solution casting technique is a widely used and highly regarded method for fabricating polymer electrolytes due to its simplicity and efficiency in producing uniform, defect-free films. In this technique, a polymer is dissolved in an appropriate solvent to form a homogenous solution, which is subsequently cast and dried to create a thin solid polymer film. This technique allows for precise control over the film's thickness and morphology, both of which are critical for the performance of energy storage devices. Solution casting has been a fundamental technique in polymer research since the early 2000s, as evidenced by the numerous studies employing this method. The significant body of literature underscores its essential role in developing novel polymeric materials, with numerous publications focusing on process parameter optimization, including solvent type, polymer-solvent ratios, drying conditions, and the mechanical characteristics of the final films.¹⁰⁸

The solution casting process typically follows a systematic approach to ensure high-quality polymer films. It begins with the preparation of the polymer solution, where the choice of solvent is vital for efficient dissolution and compatibility with the polymer's chemical structure. Commonly used solvents include acetone, 1-methyl-2-pyrollidone, and N,N-dimethylformamide (DMF) due to their solvating properties and relatively low boiling points.^109–111 Once prepared, the solution is cast onto a mold or a flat surface. The film's final thickness can be controlled by adjusting the volume of the solution and its concentration. The drying phase is crucial, with temperature and time meticulously regulated to promote slow, uniform evaporation without compromising the polymer's structure. For example, when using DMF, drying is typically conducted at a temperature slightly above the solvent's boiling point to guarantee solvent removal without subjecting the polymer to heat damage.¹¹²

An application of solution casting is seen in the development of PVDF-HFP-based polymer electrolytes, which are widely used in lithium batteries and supercapacitors due to their outstanding ionic conductivity and mechanical resilience. As shown in Figure 22, PVDF-HFP is dissolved in a solvent such as NMP with continuous stirring to form a clear, homogenous solution. The solution is then uniformly spread onto a glass plate, and the drying process is carried out at around 80 °C, slightly above acetone's boiling point, to facilitate effective solvent removal.^113,114 The outcome is a smooth, flexible electrolyte membrane ready for use in energy storage devices. The success of solution casting hinges on the specific interactions within the polymeric matrix, which influence the solubility, viscosity, and the film's final properties. For PVDF-HFP, the fluorine groups in the copolymer enhance solubility by interacting well with solvent molecules, while the semi-crystalline nature of PVDF-HFP allows for efficient chain packing during solvent evaporation. These intrinsic polymeric interactions are critical to the successful fabrication of high-quality polymer electrolytes through solution casting methods.

Figure 22.

Example flowchart for solution casting method to produce PVDF-HFP sample.

Melt processing

Melt processing is a significant fabrication method employed in the production of polymer electrolytes. Unlike solution casting, which relies on solvents, melt processing involves heating the polymer above its melting point to create a viscous molten polymer that can then be directly processed into films or other required shapes, bypassing the challenges associated with solvent handling and removal.¹¹⁵ This method is particularly advantageous for creating scalable and environmentally sustainable polymer electrolyte membranes. By eliminating the use of volatile organic compounds (VOCs) as solvents, melt processing provides a safer and more eco-friendly approach suitable for both industrial and laboratory settings. Its capability for continuous processing is especially crucial for large-scale production, offering enhanced control over the polymer's morphology, factors essential for optimizing electrolyte performance in energy storage applications.

The suitability of a polymer for melt processing relies on two primary factors: thermal stability and melt viscosity.¹¹⁶ Polymers suitable for melt processing must withstand high processing temperatures without degrading and maintain an appropriate melt viscosity to ensure consistent film formation.¹¹⁷ Polymers that meet these criteria typically demonstrate excellent ionic conductivity and strong mechanical properties when used as electrolytes. Polyethylene oxide (PEO) is particularly well-suited for polymer electrolyte fabrication through melt processing, especially in the context of solid-state batteries. PEO's inherent properties, including its ability to dissolve lithium salts and facilitate ionic conduction, make it an optimal choice for energy storage applications. PEO's molecular structure facilitates effective interaction with lithium ions, enabling their movement at elevated temperatures, which is crucial for high-performance battery operations.^118,119 This feature is particularly advantageous in solid-state batteries, where the stability and efficiency of the electrolyte can significantly influence the overall battery performance and safety.

The process of creating PEO-based polymer electrolytes begins with the mechanical blending of PEO and lithium salt, such as lithium trifluoro-sulfonate (LiTFSI). The initial mixing at room temperature ensures an even distribution of salt within the polymer matrix, which is crucial for uniform ion distribution and effective ionic conduction throughout the electrolyte. After achieving a homogenous mixture, the PEO-salt blend is heated to a temperature slightly above PEO's melting point, typically between 65 °C and 75 °C. This heating phase transitions the blend into a viscous, molten state, facilitating subsequent processing and shaping. Temperature control is essential at this stage to prevent polymer degradation while ensuring sufficient melt fluidity for extrusion or casting. During processing, the molten PEO–salt mixture is extruded or cast into thin films or desired shapes, with the method chosen based on specific battery design requirements and the desired thickness and uniformity of the electrolyte. The final step involves controlled cooling, which solidifies the polymer while maintaining the desired crystalline structure and phase distribution. Rapid cooling helps preserve the amorphous regions within the polymer matrix, which are essential for maintaining high ionic conductivity at room temperature. Proper solidification is key to achieving the mechanical strength and ionic pathways crucial for optimal performance in solid-state batteries.

The success of melt-processing highly depends on the polymeric interactions during the melting process. For PEO, the ether oxygen in the polymer backbone coordinates with lithium ions, facilitating ion transport. The rapid cooling phase locks these interactions in place, ensuring the formation of amorphous regions necessary for effective ionic conduction.

Electrospinning

Electrospinning is a versatile and highly efficient method for producing ultrafine fibers from polymer melts or solutions. This method is valuable for polymer electrolytes due to its ability to produce fibrous membranes with high surface area-to-volume ratios, porosity, and interconnected structure.^120–122 These features improve the mechanical strength and ionic conductivity, making the technique ideal for high-performance energy storage systems such as supercapacitors and batteries. Electrospinning offers nanoscale control over fiber morphology and structure, as shown in Figure 23, significantly impacting the electrochemical characteristics of the electrolyte. This technique can produce nonwoven polymer fiber mats with diameters ranging from a few nanometers to micrometers. These mats possess high porosity and interconnected pore structures, which accelerate ion transit and offer substantial ion migration channels, both of which are essential for energy storage device performance. Furthermore, electrospinning allows for the straightforward incorporation of functional additives, such as ceramic materials or nanoparticles, into the polymer matrix to enhance performance, making it essential for the development of composite materials.

Figure 23.

SEM micrographs show fibrous morphology for polymer electrolyte samples fabricated using the electrospinning method. [Reprinted with permission from Springer Nature publisher].

The electrospinning process begins with the preparation of a polymer solution. For instance, poly (vinylidene fluoride) (PVDF), known for its exceptional electrochemical characteristics, is typically dissolved in a solvent mixture such as acetone and dimethylformamide (DMF). This combination of solvents is selected for its ability to optimize the solution viscosity and effectively dissolve PVDF, both essential for a successful electrospinning process. The concentration of PVDF must be carefully controlled to achieve a viscosity that enables the formation of continuous, fine fibers without breaking during electrospinning. Achieving a homogenous polymer solution is vital to ensuring consistent fiber characteristics and quality in subsequent steps. Once the polymer solution is prepared, it is placed in a syringe equipped with a metallic needle and incorporated into an electrospinning setup. The syringe is connected to a pump that precisely regulates the solution's flow rate. The flow rate is a critical parameter, influencing the fiber diameter and stability of the ejected polymer jet. During the electrospinning process, a steady, continuous jet of polymer solution is maintained by adjusting the flow rate properly. A high voltage is applied between the needle tip and a collecting screen positioned opposite the needle, creating an electric field that charges the surface of the polymer solution. When the electrostatic repulsion of the charged solution exceeds its surface tension, a fine polymer jet is ejected from the needle tip. This stage requires careful control of the applied voltage to initiate and maintain the jet while avoiding spray-like patterns or instability. As the polymer jet travels toward the collector, the solvent evaporates rapidly due to the surrounding air and the electric field, resulting in the formation of solid polymer fibers. The distance between the needle and the collector, as well as the voltage and flow rate, are precisely adjusted to control the fiber diameter, morphology, porosity, and thickness of the nonwoven fabric produced. The controlled environment ensures uniform fiber deposition, resulting in a mat with desired mechanical and electrochemical properties. Finally, the fibrous mat is carefully removed from the collector and subjected to further drying to eliminate any residual solvent completely. This step is crucial to ensure the structural integrity and performance of the electrospun polymer electrolyte. The drying process must be thorough yet gentle to prevent any deformation or loss of structural features of the fibers. Once fully dried, the nonwoven fabric is ready for use as a high-performance polymer electrolyte in energy storage devices.^10,123,124

The suitability of a polymer for electrospinning depends on its solution properties, such as viscosity, conductivity, and molecular weight. The solution must enable sufficient chain entanglements to maintain a continuous jet during the electrospinning process.¹²⁵

In-situ polymerization

For the preparation of solid-state lithium batteries and gel polymer electrolytes, the in-situ polymerization method is highly suitable. This process begins with the injection of a liquid precursor, typically containing a monomer, lithium salt, initiator, and solvent, into the battery cell, followed by polymerization through UV irradiation or the heating process. Due to its simplicity and effectiveness in forming gel-like or solid-state electrolytes, this method is widely used, resulting in the creation of a strong bond between the electrode and electrolytes. The most common monomers in the precursor solution include trimethylolpropane ethoxylate triacrylate (ETPTA), poly(ethylene glycol) diacrylate (PEGDA), trimethylolpropane propoxylate triacrylate (TPPTA), and di(trimethylolpropane) tetraacrylate (DTPTA). For polymer electrolytes used specifically in solid-state batteries, there are two mains in situ polymerization strategies, shown in Figure 24. Figure 24(a), illustrates the separator-assisted method, where a separator is placed between the anode and cathode in a sandwich-like structure. Then the precursor solution is injected to fill the separator, followed by polymerization. This method is straightforward and easily separates electrolytes. However, the use of a separator may decrease ionic conductivity. Additionally, some types of monomers may react with lithium metal, leading to side reactions that must be carefully managed. Figure 24(b) represents the direct deposition method, where the precursor solution is directly applied onto the surface of the electrodes, followed by polymerization. In some cases, polymerization occurs on the cathode alone, before integrating the anode, in others, the solution precursor is applied on both electrodes, before assembling the batteries. By eliminating the separator, this method avoids issues related to the extra weight of the separator and low ionic conductivity. However, it requires solid polymer electrolytes with strong mechanical stability to ensure the performance.^126,127 A table listed the comparative analysis of the manufacturing processes (Table 4).

Figure 24.

In situ polymerization approaches of solid-state lithium batteries. (a) Separator-assisted approach. (b) Direct deposition approach. Reprinted from Liu et al.¹²⁶ with permission from John Wiley and Sons.

Table 4.

Comparative analysis of the manufacturing processes.

Fabrication methods	Reaction condition	Advantages	Disadvantages	Applications
In-situ polymerization	Monomers and initiators are introduced, and polymerization occurs directly within the structure.	Good interfacial contact with electrodes; uniform electrolyte distribution; reduced interfacial resistance.	Requires precise control of polymerization; residual monomers may affect performance; may need UV/thermal initiation.	Solid polymer electrolytes, gel electrolytes, integrated directly into battery cells.
Electrospinning	High-voltage field is used to produce ultrafine polymer fibers, forming a porous fibrous structure.	High surface area; tunable porosity; good ionic conductivity; mechanically robust.	Complex setup; scalability challenges; fiber uniformity depends on parameters; may need post-treatment.	Separator-supported polymer electrolytes, composite polymer electrolytes.
Melt-casting	Polymer and additives are melted and then cast into films by cooling.	Simple process; solvent-free and environmentally friendly; good mechanical stability.	Limited to thermally stable polymers; poor control over microstructure; risk of salt aggregation.	Thermoplastic-based polymer electrolytes.
Solution casting	Polymer and salt are dissolved in a solvent, cast into a film, then solvent is evaporated.	Simple and versatile; uniform salt dispersion; easy to prepare thin films.	Solvent residue may remain; film porosity and mechanical strength can be low; drying required.	Widely used for preparing polymer-salt complexes and gel polymer electrolytes.

In this subsection, we examined four primary fabrication techniques: Solution casting, melt processing, electrospinning, and in-situ polymerization. Solution casting is useful for fabricating thin films through solvent evaporation, ideal for laboratory prototypes. Melt processing, which involves heating polymers above their melting point to form shapes without solvents, offers an environmentally friendly option suitable for large-scale production. Electrospinning, employing an electric field to create fibrous mats, excels in producing materials with high surface areas and porosity, beneficial for enhancing ionic conductivity. In-situ polymerization, which is a highly effective fabrication method, begins with the injection of solution into the battery structure, followed by a polymerization process. These methods, each tailored to specific polymer properties and application requirements, set the stage for the next section on advanced analytical technique, EIS, which assesses the electrochemical properties of polymer electrolytes.

Performance analysis

Polymeric materials play a crucial role in the field of energy storage systems due to their diverse applications and functions. Comprehensive data gathering and analysis are essential for understanding and optimizing these materials.¹²⁸ This section covers ionic conductivity and electrochemical performance evaluation through electrochemical impedance spectroscopy (EIS), with Nafion-based polymer electrolytes as a representative example.

Electrochemical impedance spectroscopy analysis

Electrochemical impedance spectroscopy (EIS) is an indispensable technique in the analysis of advanced materials for energy storage systems, particularly for evaluating polymer electrolytes. EIS helps to probe the electrochemical properties of these materials under conditions that closely simulate practical use. This section outlines the foundational principles of EIS, its importance, and how it aids in deriving the ionic conductivity of polymer electrolytes. EIS measures the response of a material system (electrolyte, electrode, and interfaces) to a small alternating current (AC) over a range of frequencies. This method produces complex impedance data, which is graphically represented on a Nyquist plot. A Nyquist plot displays the imaginary part of the impedance against the real part and is pivotal in identifying various electrochemical and physical properties of the system. EIS provides detailed insights into several key aspects of material behavior, including the resistance to ion flow (bulk resistance), interfacial charge transfer characteristics, and the capacitive behavior of the material. Understanding these factors is vital for the design and improvement of energy storage devices such as batteries and supercapacitors, where efficient ion transport and minimal internal resistance are desired for high performance.

Nyquist plot and bode plots and other analysis

The Nyquist plot is a fundamental tool in EIS analysis, particularly valuable in the research and development of advanced materials for energy storage systems. This graphical representation is key to understanding the complex electrochemical behaviors of materials such as polymer electrolytes used in batteries and supercapacitors. A Nyquist plot visualizes the complex impedance of a material system by plotting the imaginary component of the impedance (Z′′) against the real component (Z′). By displaying how impedance changes across a range of frequencies, it provides a signature of the material's electrochemical properties, making it a fundamental resource for analyzing the dynamic responses of polymer electrolytes under operational conditions.

Nyquist plots are critical in advanced material research for energy systems as they allow for the precise characterization of electrochemical properties that directly affect device performance. The plot's ability to distinguish between different types of resistance and capacitive behaviors helps optimize materials for specific applications, ensuring efficiency and durability. For instance, understanding impedance behaviors aids in improving charge storage capacity and power output by minimizing energy losses from internal resistances. One of the fundamental parameters evident from the Nyquist plot is the bulk resistance (R_b). This is typically observed as the interception of the plot on the real axis at high frequencies, representing the inherent resistance to ion flow within the polymer electrolyte. Bulk resistance is a direct indicator of the electrolyte's ability to conduct ions; lower values signify better ionic conductivity, facilitating faster and more efficient charge transport across the electrolyte, which is vital for high-performance energy storage applications. Another critical aspect discernible from the Nyquist plot is the interfacial resistance, often manifested as a semicircle. The appearance and size of this semicircle provide insights into the resistance at the electrode–electrolyte interface, a key area affecting the rates of charge transfer. The interfacial resistance is influenced by the compatibility of the electrode material with the electrolyte and the surface characteristics of the electrode. Effective management of this resistance through material selection and surface engineering can lead to enhanced charge transfer efficiencies, improving the overall performance of energy storage devices. Capacitive behavior is another crucial electrochemical element visible in the Nyquist plot, depicted by the diameter of the semicircles. This behavior is related to the capacitive properties of the material and interfaces within the system. The capacitance extracted from the plot informs how well the electrolyte material can store and release charge.

Lastly, the plot often features Warburg impedance, observed as a sloped line in the low-frequency region of the plot, indicative of diffusion-controlled processes. This impedance element reflects the material's response to ionic diffusion, an essential factor in the rate at which ions can move through the electrolyte under gradient-driven forces. Understanding and optimizing Warburg impedance through material composition and structural design can significantly enhance the kinetic properties of the electrolyte, improving the dynamic charge and discharge capabilities of the energy storage system.

Considering the Nafion-CMC solid-state electrolyte (SSE) samples, the Nyquist plots provide critical insights into their electrochemical performance. Samples like C005Naf and C015Naf, displaying multiple semicircles across the frequency range, indicate complex charge transfer processes due to the interaction between Nafion and CMC. Such behaviors suggest that varying Nafion content influences the internal charge transfer dynamics, as evidenced by the changes in the width of the semicircles across different samples. For instance, sample C010Naf, which shows the smallest semicircle in its Nyquist plot, corresponds to lower bulk resistance and higher DC ionic conductivity. This sample demonstrates optimal electrochemical performance, likely due to an ideal balance of Nafion within the CMC matrix that facilitates smoother ion transport. Conversely, the sample C015Naf exhibits the largest semicircle, indicating higher resistance and lower ionic conductivity, which suggests that excessive Nafion may hinder effective charge transfer.

These observations underscore the importance of carefully optimizing the content of fillers like Nafion in polymer electrolytes to enhance their ionic conductivity and overall electrochemical performance. The Nyquist plot not only elucidates the impact of material composition on electrochemical behaviors but also guides the design and optimization of energy storage materials to achieve desired properties and performance.^129–131

Electrochemical stability window

The electrochemical stability window of polymeric materials used in battery energy storage defines the safe operating voltage range before oxidative or reductive decomposition, and it influences compatibility with cathodes and anodes.^132,133 Conventional PEO-based solid polymer electrolytes are typically stable within the range of 4.0–4.2 V vs. Li/Li⁺, limiting their use with high-voltage cathodes, while carbonate or fluorinated polymers, such as PVDF-HFP and polycarbonates, extend stability from 4.5 to 5.0 V.¹³⁴ Gel polymer electrolytes largely inherit the stability of their liquid solvents, but careful selection of salts, solvents, polymer and inorganic salt interactions is needed to suppress side reactions and widen the stability window.^135–137 Incorporating ceramic fillers or designing single-ion conducting polymers can further stabilize the electrochemical interface and suppress parasitic reactions.¹³⁸ Polymeric binders and separators, although not primary charge carriers, must also resist electrochemical degradation, especially in different voltage environments, where functionalized binders or fluorinated separators enhance resistance to oxidative attack. Overall, tuning the polymer backbone chain, inorganic salt and polymer interactions, and composite architectures is essential to extend the electrochemical stability window.

Thermal properties

Thermal properties of polymeric materials in battery energy storage are critical for safety, stability, and performance under operating and abuse conditions. Conventional polyolefin separators (PE, PP) have relatively low melting points (120–165 °C) and can shrink or collapse at elevated temperatures, posing potential risks.^139–142 To overcome this problem, multilayer designs and ceramic coating are widely employed to enhance thermal resistance and introduce shutdown functions. Gel polymer electrolytes, though offering high conductivity, can suffer from solvent evaporation and thermal instability, whereas crosslinked networks and plasticizers with a high boiling point improve retention at elevated temperatures. Solid polymer electrolytes such as PEO exhibit improved thermal stability compared to liquid systems but soften near their glass transition or melting point, and high crystallinity can influence both mechanical and thermal behavior. Polymer composite electrolytes filled with ceramic, aramid and PVDF separators display superior thermal resistance, maintaining integrity above 200 °C and offering enhanced flame retardancy.¹⁴³ Polymeric binders, while not primarily thermal management components, can impact electrode stability under cycling heating, with functional binders and thermally stable backbones improving interfacial adhesion at high temperature. Overall, enhancing polymeric thermal stability through composite architectures, ceramic integration, and chemical modification is central to advancing safer and more resilient battery systems.

Mechanical properties

Polymeric materials used in battery energy storage devices must balance ionic transport with robust mechanical properties to ensure safety and long-term stability. Solid polymer electrolytes are often mechanically weak in their crystalline state but can be strengthened through crosslinking, though this may reduce ionic conductivity. Gel polymer electrolytes provide flexibility and dimensional stability but generally have lower tensile strength, requiring reinforcement or crosslinking to resist dendrite penetration. Polymer composite electrolytes filled with ceramic achieve higher modulus and puncture resistance by incorporating rigid ceramic fillers, improving dendrite suppression without fully sacrificing processability. Polyolefin separators made from PE and PP exhibit sufficient tensile strength and elasticity but can shrink or melt at elevated temperatures. Polymeric binders primarily serve to provide adhesion and elasticity within electrodes, where functional binders with hydrogen bonding and ionic crosslinks improve electrode cohesion during large volume changes. Overall, mechanical reinforcement using crosslinking, filler, and suitable functional groups may improve polymer stability against dendrites, thermal stress, and electrode expansion.

Challenges and future directions

Despite the many benefits of polymer electrolytes, several areas still require further exploration. First, sustainability and recyclability must become central to future research. With the rapid expansion of battery production, environmentally friendly solutions, such as bio-based polymers, green solvents, or solvent-free fabrication methodologies, will be crucial for reducing ecological impacts. Exploring alternative salts to lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) represents another promising direction for future research. Salts with broader electrochemical windows should be investigated for their potential to enhance their performance. Environmentally friendly additives such as ionic liquids also present promising opportunities for improved ionic conductivity, especially in gel polymer electrolytes. Another pressing challenge lies in scaling up lab-based innovation for commercial applications. Some advanced materials, while effective at the lab scale, may be expensive for industrial use. Therefore, a balance between performance and economic viability must be maintained. Finally, the application of prediction models, particularly machine learning, can accelerate the prediction of new polymer electrolytes and organic batteries. These tools can help identify candidates with high porosity, ionic conductivity, and mechanical stability. More research is needed in this area to harness the potential of new designs and the development of high-performance and sustainable polymer electrolytes for future solutions.^144,145 A schematic of prospects for polymer electrolytes in energy storage devices is illustrated in Figure 25.

Figure 25.

Future research directions for polymer electrolytes in energy storage applications.

Conclusion

This review delves into the integration and significance of polymeric materials in the development of advanced energy storage systems. By examining key categories, conductive polymers, redox-active polymers, and polymer electrolytes, it outlines the performance and sustainability of these systems. Particular attention is given to polymer electrolytes, discussing their role in facilitating ion transport while ensuring electrical insulation, both of which are essential for the operation of devices such as solid-state batteries and supercapacitors. The review examines synthesis processes and electrochemical mechanisms, emphasizing the importance of tailoring properties of polymer electrolytes to achieve high energy density, enhanced stability, and improved safety. Further, advanced fabrication techniques such as solution casting, melt processing, electrospinning, and in-situ polymerizations are also presented, each contributing unique microstructural and electrochemical characteristics. These techniques are essential for engineering polymer electrolytes that meet specific performance goals in energy storage systems. Additionally, various characterizations are discussed for a deeper understanding of the electrochemical properties of polymer electrolytes. The insights presented here aim to guide the development of next-generation energy storage solutions, highlighting the transformative potential of polymer electrolytes in meeting the global demand for reliable and efficient energy storage. The review provides a thorough understanding of the multifaceted roles of polymer electrolytes in energy storage systems, bridging fundamental electrochemical research with practical applications. It concludes by reinforcing the significance of continuous innovation and material optimization in enhancing the performance and sustainability of energy storage technologies.

Footnotes

ORCID iD

Muhammad Remanul Islam

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

References

Mupit

Islam

Azam

, et al. Magnetic particle-filled polyaniline-doped graphene oxide nanocomposite-based electrode in application of supercapacitor. Energy Environ 2024; 35: 1987–2007.

Mitali

Dhinakaran

Mohamad

. Energy storage systems: a review. Energy Storage Sav 2022; 1: 166–216.

Nasib

Islam

Yahya

ANA

, et al. Effects of lithium perchlorate (LiClO₄) on the electrochemical performance of Pvdf-Hfp/Ca composite-based solid-state electrolytes. DOI: 10.2139/ssrn.4743334.

Nasib

Islam

Firouzi

, et al. Effects of cellulose acetate on electrochemical performance in poly vinylidene fluoride-co-hexafluoropropylene solid-state electrolytes. Ionics 2025. DOI: 10.1007/s11581-025-06539-z.

Kajal

Singh

Gupta

, et al. Metal organic frameworks for electrochemical sensor applications: a review. Environ Res 2022; 204: 112320.

West

Pearce

Bentham

, et al. Issue profile: environmental issues and the geological storage of CO2. Eur Environ 2005; 15: 250–259.

Smdani

Islam

Ahmad Yahaya

, et al. Performance evaluation of advanced energy storage systems: a review. Energy Environ 2023; 34: 1094–1141.

Koehler

. Chapter 2—general overview of non-lithium battery systems and their safety issues. In: Garche

Brandt

(eds) Electrochemical power sources: fundamentals, systems, and applications, Li-battery safety. Amsterdam, Netherlands: Elsevier, 2019, pp.21–46.

Pavithra

SundarRajan

Kumar

, et al. Mercury sources, contaminations, mercury cycle, detection and treatment techniques: a review. Chemosphere 2023; 312: 137314.

10.

Yang

Liu

Pei

, et al. The critical role of fillers in composite polymer electrolytes for lithium battery. Nano-Micro Lett 2023; 15: 74.

11.

Polu

Rhee

Kim

. New solid polymer electrolytes (PEO20–LiTDI–SN) for lithium batteries: structural, thermal and ionic conductivity studies. J Mater Sci Mater Electron 2015; 26: 8548–8554.

12.

“Scopus Database”. https://www-scopus-com-s.web.bisu.edu.cn/term/analyzer.uri?sort=plf-f&src=s&sid=0618c36a0b41dd0de3079d80f7693406&sot=a&sdt=a&sl=43&s=polymeric+materials%2C+energy+storage+systems&origin=resultslist&count=10&analyzeResults=Analyze+results.

13.

Casado

Mecerreyes

. Introduction to redox polymers: classification, characterization methods and main applications. In: Redox polymers for energy and nanomedicine. The Royal Society of Chemistry, 2020. DOI: 10.1039/9781788019743-00001.

14.

Kondratiev

Holze

. Intrinsically conducting polymers and their combinations with redox-active molecules for rechargeable battery electrodes: an update. Chem Pap 2021; 75: 4981–5007.

15.

Tajik

Beitollahi

Nejad

, et al. Recent developments in conducting polymers: applications for electrochemistry. RSC Adv 2020; 10: 37834–37856.

16.

Kaitsuka

Hayashi

Shimokawa

, et al. Synthesis of polyaniline (PANI) in nano-reaction field of cellulose nanofiber (CNF), and carbonization. Polymers (Basel) 2016; 8: 40.

17.

Il Park

Lee

, et al. Preparation and characterization of PEDOT/PSS hybrid with graphene derivative wrapped by water-soluble polymer. Appl Chem Eng 2014; 25: 581–585.

18.

Anil Kumar

Koyyada

Ramachandran

, et al. Carbon materials as a conductive skeleton for supercapacitor electrode applications: a review. Nanomaterials 2023; 13: 1049.

19.

Alam

Husain

. Engineering 3D-interconnected graphene nanoplatelets and polyaniline nanocomposite for high energy density energy storage. J Energy Storage 2024; 80: 110304.

20.

Rabbani

Mollah

MYA

Susan

MABH

, et al. In situ electrodeposition of conducting polymer/metal oxide composites on iron electrode for energy storage applications. Mater Today Proc 2020; 29: 1192–1198.

21.

Han

Tong

Tang

, et al. Boost anion storage capacity using conductive polymer as a pseudocapacitive cathode for high-energy and flexible lithium ion capacitors. ACS Appl Mater Interfaces 2020; 12: 10479–10489.

22.

Bai

Zhou

, et al. Sulfonated polyaniline/MXene composite electrode with high cycling stability for anti-freezing flexible supercapacitor. Chem Eng J 2024; 496: 153730.

23.

Rahman

Röse

Shah

AUHA

, et al. An amazingly simple, fast and green synthesis route to polyaniline nanofibers for efficient energy storage. Polymers (Basel) 2020; 12: 1–19.

24.

Shan

Lin

Huo

, et al. Preparation of cyclodextrin polymer-functionalized polyaniline/porous carbon composites for use in high-performance supercapacitors. Mater Lett 2022; 324: 132771.

25.

Tian

Cheng

, et al. The ordered polyaniline nanowires wrapped on the polypyrrole nanotubes as electrode materials for electrochemical energy storage. Electrochim Acta 2021; 398: 139328.

26.

Devi

Tapadia

Kant

, et al. A low-cost paper-based flexible energy storage device using a conducting polymer nanocomposite. New J Chem 2020; 44: 13446–13457.

27.

Zhang

, et al. Design and construction of conductive polymer PPy anchored NiCo bi-metal sulfide composite electrode materials for high-performance hybrid supercapacitor and electrochemical hydroquinone sensor. J Energy Storage 2024; 87: 111427.

28.

Arora

Pal

Suman

. In-situ composited g - C 3 N 4 / polypyrrole nanomaterial applied as energy-storing electrode with ameliorated super-capacitive performance. 2023; 30: 98589–98600.

29.

Ramya

Raja

, et al. Effect of conducting polymer-decorated Ppy@rGO hybrid electrodes with high specific capacitance and long-term stability for energy storage devices. Chem Pap 2023; 77: 6907–6918.

30.

Bhardwaj

Singh

Kharangarh

, et al. Surfactant decorated polypyrrole-carbon materials composites electrodes for supercapacitor. Diam Relat Mater 2020; 108: 107989.

31.

Dubal

Lee

Kim

, et al. Porous polypyrrole clusters prepared by electropolymerization for a high performance supercapacitor. J Mater Chem 2012; 22: 3044–3052.

32.

Qureshi

Nimauddin

Mazari

, et al. Ultrasonic-assisted synthesis of polythiophene-carbon nanotubes composites as supercapacitors. J Mater Sci Mater Electron 2021; 32: 16203–16214.

33.

Vinodhini

Xavier

. Electrochemical evaluation and structural characterization of polythiophene surfaces modified with PbO/PbS for energy storage applications. Mater Chem Phys 2024; 318: 129233.

34.

Chen

, et al. Facile preparation of poly(indole/thiophene) for energy storage and sensor applications. Electrochim Acta 2020; 358: 136919.

35.

Yazar

Arvas

Gürkan

. Synthesis of polythiophene with zigzag morphology on pencil graphite electrode and investigation of energy storage properties in supercapacitors. J Mater Sci 2024; 59: 10936–10952.

36.

Guo

Deng

, et al. High performance polythiophene derivative with good electrochromic and energy storage properties electrochemical synthesized in boron trifluoride diethyl etherate. Dye Pigment 2023; 220: 111709.

37.

Wang

Liu

, et al. Heterostructural conductive polymer with multi-dimensional carbon materials for capacitive energy storage. Appl Surf Sci 2021; 558: 149910.

38.

Wang

Gao

, et al. One-step electropolymerized thieno[3,2-b]thiophene-based bifunctional electrode with controlled color conversion for electrochromic energy storage application. Chem Eng J 2022; 445: 136731.

39.

Vijeth

Ashokkumar

Yesappa

, et al. Camphor sulfonic acid assisted synthesis of polythiophene composite for high energy density all-solid-state symmetric supercapacitor. J Mater Sci Mater Electron 2019; 30: 7471–7484.

40.

Sandford

Edwards

Klunder

, et al. A synthetic chemist’s guide to electroanalytical tools for studying reaction mechanisms. Chem Sci 2019; 10: 6404–6422.

41.

Gomez

Leonet

Alberto Blazquez

, et al. Poly(anthraquinonyl sulfides): high capacity redox polymers for energy storage. ACS Macro Lett 2018; 7: 419–424.

42.

Phadke

Cao

Anouti

. Approaches to electrolyte solvent selection for poly-anthraquinone sulfide organic electrode material. ChemSusChem 2018; 11: 965–974.

43.

Dong

, et al. Wide-bandgap fluorides/polyimide composites with enhanced energy storage properties at high temperatures. Chem Eng J 2022; 435: 135059.

44.

Ikhe

Naveen

Sohn

, et al. Polyviologen as a high energy density cathode in magnesium-ion batteries. Electrochim Acta 2018; 283: 393–400.

45.

Wang

, et al. A high-energy dual-ion battery based on chloride-inserted polyviologen cathode and LiCl/DMSO electrolyte. Energy Storage Mater 2022; 50: 658–667.

46.

Xia

Zhu

Miao

, et al. Polyxylylviologen chloride as an organic electrode material for efficient reversible chloride-ion storage. ACS Appl Energy Mater 2022; 5: 6980–6985.

47.

Xue

Chen

, et al. Crown ether electrolyte additive enables high-rate and stable polyviologen cathode material for chloride ion batteries. Small 2024; 20: 1–11.

48.

Ahmad

Wang

Z-F

Khan

, et al. Self-sulfur doped mesoporous novel carbon electrodes derived from poly-anthraquinone sulfide for high-performance supercapacitors! A comparative study. J Energy Storage 2024; 100: 113763.

49.

Shao

, et al. Graphene oxide coated polyaminoanthraquinone@MXene based flexible film electrode for high-performance supercapacitor. J Energy Storage 2023; 57: 106180.

50.

Kang

Liu

, et al. Polyanthraquinone-triazine—a promising anode material for high-energy lithium-ion batteries. ACS Appl Mater Interfaces 2018; 10: 37023–37030.

51.

Huang

Sun

, et al. Designing anthraquinone-based conjugated microporous polymers with dual-ion storage behavior towards high-performance lithium-organic batteries. J Power Sources 2022; 550: 232149.

52.

Grieco

Molina

Sanchez

, et al. A significantly improved polymer||Ni(OH)₂ alkaline rechargeable battery using anthraquinone-based conjugated microporous polymer anode. Mater Today Energy 2022; 27: 101014.

53.

Oka

Murao

Kataoka

, et al. Hydrophilic anthraquinone-substituted polymer: its environmentally friendly preparation and efficient charge/proton-storage capability for polymer-air secondary batteries. Macromolecules 2021; 54: 4854–4859.

54.

Dai

Bao

Ding

, et al. Scalable polyimide-poly(amic acid) copolymer based nanocomposites for high-temperature capacitive energy storage. Adv Mater 2022; 34: 1–9.

55.

Dong

Qiu

, et al. Scalable polyimide-organosilicate hybrid films for high-temperature capacitive energy storage. Adv Mater 2023; 35: 1–8.

56.

Wang

Liu

Wang

, et al. π-Conjugated polyimide-based organic cathodes with extremely-long cycling life for rechargeable magnesium batteries. Energy Storage Mater 2020; 26: 494–502.

57.

Zhou

, et al. Polyimide/metal-organic framework hybrid for high performance Al-organic battery. Energy Storage Mater 2020; 31: 58–63.

58.

Ngai

Ramesh

, et al. A review of polymer electrolytes: fundamental, approaches and applications. Ionics 2016; 22: 1259–1279.

59.

Sashmitha

Rani

. A comprehensive review of polymer electrolyte for lithium-ion battery. 2023; 80: 89–135.

60.

Aziz

Woo

Kadir

MFZ

, et al. A conceptual review on polymer electrolytes and ion transport models. J Sci Adv Mater Devices 2018; 3: 1–17.

61.

Chu

Reddy

Kao

. Novel composite polymer electrolyte comprising mesoporous structured SiO₂ and PEO/Li. Solid State Ionics 2003; 156: 141–153.

62.

Huang

Wang

, et al. Determination of the interaction using FTIR within the composite gel polymer electrolyte. J Mol Struct 2013; 1031: 30–37.

63.

Feng

Wang

Chen

, et al. PEO Based polymer-ceramic hybrid solid electrolytes: a review. Nano Converg 2021; 8: 2.

64.

Alharbi

Rajeh

. Processing polymer film nanocomposites of poly(vinylidene fluoride)/poly(ethylene oxide) and cobalt oxide nanoparticles for flexible energy storage systems. Opt Quantum Electron 2024; 56: 1–25.

65.

Manthiram

Wang

. Lithium battery chemistries enabled by solid-state electrolytes. Nat Rev Mater 2017; 2: 1–16.

66.

Goodenough

Hong

Kafalas

JARG

. Graves and JB Smathers. Mater Res Bull 1976; 11: 77843.

67.

Hong

HYP

. Crystal structure and ionic conductivity of Li14Zn(GeO4)4 and other new Li+ superionic conductors. Mater Res Bull 1978; 13: 117–124.

68.

Inaguma

Chen

Itoh

, et al. High lithium ion conductivity in the perovskite-type compounds. Solid State Commun 1993; 86: 689–693.

69.

Geiger

Alekseev

Lazic

, et al. Crystal chemistry and stability of ‘Li7La3Zr2O12’ garnet: a fast lithium-ion conductor. Inorg Chem 2011; 50: 1089–1097.

70.

González Calderón

Contreras López

Pérez

, et al. Polysiloxanes as polymer matrices in biomedical engineering: their interesting properties as the reason for the use in medical sciences. 2020; 77: 2749–2817.

71.

Wang

Yang

Hessien

, et al. Flexible barium titanate@polydopamine/polyvinylidene fluoride/polymethyl methacrylate nanocomposite films with high performance energy storage. Adv Compos Hybrid Mater 2022; 5: 2106–2115.

72.

Jamalpour

Ghahramani

Ghaffarian

, et al. The effect of poly(hydroxyl ethyl methacrylate) on the performance of PVDF/P(MMA-co-HEMA) hybrid gel polymer electrolytes for lithium ion battery application. Polymer 2020; 195: 122427.

73.

Shen

Yang

, et al. Ultrahigh charge-discharge efficiency and enhanced energy density of the sandwiched polymer nanocomposites with poly(methyl methacrylate) layer. Compos Sci Technol 2021; 202: 108591.

74.

Zhang

Feng

, et al. Significantly improved energy storage performance of PVDF ferroelectric films by blending PMMA and filling PCBM. ACS Sustain Chem Eng 2021; 9: 16291–16303.

75.

Liu

Yang

, et al. Enhanced energy storage performance in a PVDF/PMMA/TiO₂ blending nanodielectric material. Mater Chem Phys 2020; 250: 1–6.

76.

Chu

, et al. Enhancing released electrical energy density of poly(vinylidene fluoride-co-trifluoroethylene)-graft-poly(methyl methacrylate) via the pre-irradiation method. Appl Surf Sci 2019; 465: 643–655.

77.

Liu

Xin

Shen

, et al. Poly(methyl methacrylate)-based gel polymer electrolyte for high-performance solid state Li-O₂ battery with enhanced cycling stability. ACS Appl Energy Mater 2021; 4: 3975–3982.

78.

Chien

Safanama

Goh

, et al. A thermostable ionic liquid-methacrylate-based polymer electrolyte for energy storage application. J Energy Storage 2024; 89: 111741.

79.

Zhu

Jiang

Huang

. Poly(vinylidene fluoride)terpolymer and poly(methyl methacrylate)composite films with superior energy storage performance for electrostatic capacitor application. Compos Sci Technol 2019; 179: 115–124.

80.

Wen

Zhao

Jiang

, et al. Graphene oxide enabled flexible PEO-based solid polymer electrolyte for all-solid-state lithium metal battery. ACS Appl Energy Mater 2021; 4: 3660–3669.

81.

Yang

Sun

Wang

, et al. A novel flexible supercapacitor based on cross-linked PVDF-HFP porous organogel electrolyte and carbon nanotube paper-conjugated polymer film electrodes. ACS Sustain Chem Eng 2015; 3: 2067–2076.

82.

Mauger

Julien

Paolella

, et al. A comprehensive review of lithium salts and beyond for rechargeable batteries: progress and perspectives. Mater Sci Eng R Rep 2018; 134: 1–21.

83.

Liu

Xie

Xue

, et al. Magnetic-field-oriented mixed-valence-stabilized ferrocenium anion-exchange membranes for fuel cells. Nat Energy 2022; 7: 329–339.

84.

Sumdani

Islam

Yahaya

ANA

, et al. Recent advancements in synthesis, properties, and applications of conductive polymers for electrochemical energy storage devices: a review. Polym Eng Sci 2022; 62: 269–303.

85.

Deng

Zhang

Lai

, et al. Electrochemical impedance spectroscopy study of a lithium/sulfur battery: modeling and analysis of capacity fading. J Electrochem Soc 2013; 160: A553–A558.

86.

Rao

Manjunatha

Jayarama

, et al. Power enhancement of passive micro-direct methanol fuel cells with self-sulfonation of P(VDF-TrFE) copolymer during lamination on Nafion membrane. Int J Hydrogen Energy 2019; 44: 30375–30387.

87.

Zhao

, et al. Design of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate-polyacrylamide dual network hydrogel for long-term stable, highly efficient solar steam generation. Sep Purif Technol 2022; 300: 121889.

88.

Hallinan

Balsara

. Polymer electrolytes. Annu Rev Mater Res 2013; 43: 503–525.

89.

Zhou

Shanmukaraj

Tkacheva

, et al. Polymer electrolytes for lithium-based batteries: advances and prospects. Chem 2019; 5: 2326–2352.

90.

Zhao

Xie

, et al. A rigid-flexible coupling gel polymer electrolyte towards high safety flexible Li-Ion battery. J Power Sources 2021; 499: 229944.

91.

Zhu

Hao

Cao

, et al. Ultraviolet-cured heat-resistant and stretchable gel polymer electrolytes for flexible and safe semi-solid lithium-ion batteries. J Power Sources 2024; 613: 234944.

92.

Kumar

Alagarasan

Ramachandran

, et al. The landscape of energy storage: insights into carbon electrode materials and future directions. J Energy Storage 2024; 86: 111119.

93.

Huang

Shen

Robertson

, et al. Fluorine grafted gel polymer electrolyte by in situ construction for high-voltage lithium metal batteries. Chem Eng J 2023; 475: 145802.

94.

Poiana

Lufrano

Tsurumaki

, et al. Safe gel polymer electrolytes for high voltage Li-batteries. Electrochim Acta 2022; 401: 139470.

95.

Jabbari

Yurkiv

Rasul

, et al. A smart lithium battery with shape memory function. Small 2022; 18: 1–11.

96.

Bao

Wang

, et al. Poly(ionic liquid)-functionalized graphene oxide towards ambient temperature operation of all-solid-state PEO-based polymer electrolyte lithium metal batteries. Chem Eng J 2022; 437: 135420.

97.

Zhu

Zhang

Liu

Q-S

, et al. Non-flammable solvent-free liquid polymer electrolyte for lithium metal batteries. Nat Commun 2023; 14. DOI: 10.1038/s41467-023-40394-8.

98.

Moniruzzaman

Reddy

Ramachandran

, et al. Sodium symphony: crafting the future of energy storage with sodium-ion capacitors. J Energy Storage 2024; 95: 112566.

99.

Kumar

Roy

Ramachandran

, et al. Shaping the future of energy: the rise of supercapacitors progress in the last five years. J Energy Storage 2024; 98: 113040.

100.

Noor IJ, Raheek II, and Manal K.O, et al. A review on Supercapacitors: types and components. J Physics: Conference Series, 2021, 1973: 012015.

101.

Suriyakumar

Bhardwaj

Grace

, et al. Role of polymers in enhancing the performance of electrochemical supercapacitors: a review. Batter Supercaps 2021; 4: 571–584.

102.

Choi

. Triple-network hydrogel polymer electrolytes: enabling flexible and robust supercapacitors for extreme conditions. Chem Eng J 2024; 483: 149386.

103.

Zaidi

SFA

Saeed

V-C

, et al. Chitosan-reinforced gelatin composite hydrogel as a tough, anti-freezing, and flame-retardant gel polymer electrolyte for flexible supercapacitors. Int J Biol Macromol 2023; 234: 123725.

104.

Lim

Kim

Nguyen

V-T

, et al. Functionalized methyl cellulose/LiClO₄ composite as an environmentally friendly quasi-solid polymer electrolyte for solid-state electrochromic devices and cellulose-based supercapacitors. Mater Today Energy 2023; 33: 101263.

105.

Shin

Yao

Lin

, et al. Photothermal supercapacitors with gel polymer electrolytes for wide temperature range operation. ACS Energy Lett 2023; 8: 1911–1918.

106.

Badi

Theodore

Alghamdi

, et al. Fabrication and characterization of flexible solid polymers electrolytes for supercapacitor application. Polymers (Basel) 2022; 14: 3837.

107.

Ramachandran

Roy

Hegazy

, et al. From graphene aerogels to efficient energy storage: current developments and future prospects. J Alloys Compd 2025; 1010: 177248.

108.

Kufian

Osman

. Prospects and challenges in the selection of polymer electrolytes in advanced lithium-air batteries. Energy Mater Fundam Appl 2021: 313–333. DOI: 10.1016/B978-0-12-823710-6.00009-1.

109.

Kim

J-C

Ryu

. Characteristics of PVdF-HFP/TiO₂ composite electrolytes prepared by a phase inversion technique using dimethyl acetamide solvent and water non-solvent. Macromol Mater Eng 2006; 291: 1495–1502.

110.

Shenbagavalli

Muthuvinayagam

Revathy

. Electrical properties of Mg²⁺ ion-conducting PEO: p(VdF-HFP) based solid blend polymer electrolytes. Polymer 2022; 256: 125242.

111.

Khan

Ryan

Blau

, et al. The effect of solvent choice on the mechanical properties of carbon nanotube–polymer composites. Compos Sci Technol 2007; 67: 3158–3167.

112.

Filip

Zelenkova

Peer

. Electrospinning of a copolymer PVDF-co-HFP solved in DMF/acetone: explicit relations among viscosity, polymer concentration, DMF/acetone ratio and mean nanofiber diameter. Polymers (Basel) 2021; 13: 3418.

113.

Song

Cheng

Wang

, et al. Microstructure of poly(vinylidene fluoride)-based polymer electrolyte and its effect on transport properties. J Electrochem Soc 2002; 149: A1230.

114.

Gohel

Kanchan

MacHhi

, et al. Gel polymer electrolyte based on PVDF-HFP:PMMA incorporated with propylene carbonate (PC) and diethyl carbonate (DEC) plasticizers: electrical, morphology, structural and electrochemical properties. Mater Res Express 2020; 7: 025301.

115.

Jiang

. Introduction to fuel cells: electrochemistry and materials. 2021.

116.

Gilbert

. Chapter 4—relation of structure to thermal and mechanical properties. In: Gilbert

(ed.) Brydson’s plastics materials. 8th ed. Amsterdam, Netherlands: Butterworth-Heinemann, Elsevier, 2017, pp.59–73.

117.

Spreij

. Creep behaviour of magnesia carbon refractory aluminium content on creep development creep behaviour of magnesia carbon refractory.

118.

Sim

Gan

Chan

, et al. ATR-FTIR studies on ion interaction of lithium perchlorate in polyacrylate/poly(ethylene oxide) blends. Spectrochim Acta - Part A Mol Biomol Spectrosc 2010; 76: 287–292.

119.

Ahn

J-H

Wang

Liu

, et al. Nanoparticle-dispersed PEO polymer electrolytes for Li batteries. J Power Sources 2003; 119-121: 422–426.

120.

Mupit

Azam

Islam

. Facile preparation of polyaniline/graphene oxide composite towards electrode materials. Energy Environ 2024; 35: 2476–2496.

121.

Zulkifli

Rani

NAM

Shahitha

. Carboxymethyl cellulose nanofibres impregnated with silver nanoparticles for tissue engineering applications. Mater Today Proc 2019; 16: 1715–1721.

122.

Yusuf

SNF

Yusof

Kufian

, et al. Preparation and electrical characterization of polymer electrolytes: a review. Mater Today Proc 2019; 17: 446–458.

123.

Wan

Xie

Kong

, et al. Ultrathin, flexible, solid polymer composite electrolyte enabled with aligned nanoporous host for lithium batteries. Nat Nanotechnol 2019; 14: 705–711.

124.

Chen

, et al. PEO/Garnet composite electrolytes for solid-state lithium batteries: from ‘ceramic-in-polymer’ to ‘polymer-in-ceramic. Nano Energy 2018; 46: 176–184.

125.

Ahmad

Jawad

Low

, et al. A cellulose acetate/multi-walled carbon nanotube mixed matrix membrane for CO₂/N₂ separation. J Memb Sci 2014; 451: 55–66.

126.

Liu

Wang

. Toward practical solid-state polymer lithium batteries by in situ polymerization. Adv Energy Mater 2023, 13: 2300798.

127.

Chae

Kim

Ryoo

, et al. A brief review of gel polymer electrolytes using in situ polymerization for lithium-ion polymer batteries. Polymers (Basel) 2023; 15: 803.

128.

Song

M-K

Kim

Y-T

Kim

, et al. Thermally stable gel polymer electrolytes. J Electrochem Soc 2003; 150: A439.

129.

Abubakar Abdulkadir

Ojur Dennis

Abdullahi Adam

, et al. Preparation and characterization of solid biopolymer electrolytes based on polyvinyl alcohol/cellulose acetate blend doped with potassium carbonate (K₂CO₃) salt. J Electroanal Chem 2022; 919: 116539.

130.

Yang

Peng

Wang

, et al. Crystallization behavior of poly(ε-caprolactone)/layered double hydroxide nanocomposites. J Appl Polym Sci 2010; 116: 2658–2667.

131.

Scrosati

Croce

Persi

. Impedance spectroscopy study of PEO-based nanocomposite polymer electrolytes. J Electrochem Soc 1718; 147: 2000.

132.

Zhao

Liang

, et al. Advances of solid polymer electrolytes with high-voltage stability. Next Materials 2025; 7: 100364.

133.

Méry

Rousselot

Lepage

, et al. A critical review for an accurate electrochemical stability window measurement of solid polymer and composite electrolytes. Materials (Basel) 2021; 14: 3840.

134.

Zhang

Xie

Zhang

, et al. High-temperature and high-voltage gel polymer electrolytes for lithium-metal batteries: integrating hybrid molecular engineering and in situ polymerization strategy. Adv Funct Mater 2025. DOI: 10.1002/adfm.202510652.

135.

Aruchamy

Ramasundaram

Divya

, et al. Gel polymer electrolytes: advancing solid-state batteries for high-performance applications. Gels 2023; 9: 585.

136.

Zheng

Liu

, et al. Progress in gel polymer electrolytes for sodium-ion batteries. Energy Environ Mater 2022; 6.

137.

Kojitsu

Fujimoto

, et al. Sulfolane-based gel polymer electrolytes enhanced by 2D boron nitride nanosheets for optimized ion dynamics and stability in lithium metal batteries. ACS Appl Polym Mater 2025; 7: 3296–3308.

138.

Zheng

Zhou

Zhu

, et al. A near-single-ion conducting polymer-in-ceramic electrolyte for solid-state lithium metal batteries with superior cycle stability and rate capability. Chem Eng J 2024; 500: 157572.

139.

Sun

, et al. Enhancing safety in lithium batteries: a review on functional separators controlling substance and heat during thermal runaway. Adv Funct Mater 2025; 35. DOI: 10.1002/adfm.202425698.

140.

Liu

Jiang

, et al. Safer lithium-ion batteries from the separator aspect: development and future perspectives. Energy Environ Mater 2021; 4: 336–362.

141.

Zhao

Zhou

, et al. Towards establishing uniform metrics for evaluating the safety of lithium metal batteries. Adv Powder Mater 2023; 2: 100139.

142.

Roth

Doughty

Pile

. Effects of separator breakdown on abuse response of 18650 Li-ion cells. J Power Sources 2007; 174: 579–583.

143.

Yang

Liu

Chai

, et al. High performance polyimide-based separator for 4.5 V high voltage LiCoO₂ battery with superior safety. Mater Chem Phys 2022; 282: 125975.

144.

Mecerreyes

Casado

Villaluenga

, et al. Current trends and perspectives of polymers in batteries. Macromolecules 2024; 57: 3013–3025.

145.

Ram

Prasanth S

Prasannavenkadesan

Katiyar

, et al. Future directions for lithium-ion batteries. R Soc Chem 2025; 3: 499–531.