Lithium-ion batteries (LIBs) have experienced substantial growth and have become dominant in various applications, such as electric vehicles and portable devices, ever since their commercialization by Sony Corporation in 1991 ^[1][2][3][1,2,3]. Despite the advantages of LIBs, such as their high energy density and long lifespan, concerns regarding safety and their extremely high energy density have become limitations hindering further progress. The pursuit of even higher energy density has led to safety concerns, as packing more energy into a confined space can increase the risk of thermal runaway and safety incidents. Balancing energy density with safety is now a critical challenge in the battery industry. Moreover, extremely high energy density can result in more rapid capacity degradation and reduced cycle life, which affects the longevity and reliability of LIBs. In recent years, LIBs, also known as Li-ion batteries, have made rapid advancements due to their usage in grid energy storage, electric vehicles (EVs), and portable electronic devices [4]. Enhancing cycling performance and ensuring safety in LIBs heavily relies on the development of high-performance electrolyte materials. Conventional non-aqueous liquid electrolytes containing combustible organic solvents such as carbonates and ethers pose safety issues such as fire, explosion, and electrolyte leakage, presenting a challenging situation for LIB development [5]. These safety concerns become even more critical in post-LIBs, such as Li-metal batteries, lithium-sulfur (Li-S) batteries, and lithium-oxygen (Li-O₂) batteries, which offer higher energy density [6]. Commercial LIBs currently employ liquid organic electrolytes, which possess high conductivity and excellent wetting ability on electrode surfaces. However, liquid electrolytes have inherent drawbacks, including electrochemical instabilities, potential hazards, and limited ion selectivity.

In contrast, solid electrolytes offer superior safety and thermal stability compared to liquid electrolytes since they can physically separate positive and negative electrodes, preventing thermal runaway under high temperatures or impact. Additionally, solid electrolytes effectively suppress the growth of lithium dendrites, enabling the use of a lithium metal anode. However, challenges such as inadequate ionic conductivity and insufficient interface contact must be addressed for developing solid lithium batteries ^{[7][8][9][10]}[7,8,9,10]. Key requirements for solid electrolytes to meet commercial demands include excellent ionic conductivity, favorable mechanical properties, and exceptional interfacial stability with the electrodes [11].

Polymer electrolytes, a type of electrolyte used in LIBs, combine polymers and ionic salts. Their integration into LIBs has resulted in significant advancements in battery technology, including improved safety, increased capacity, and longer cycle life. The history of polymer electrolytes in LIBs dates back to the late 1990s. The first commercial application of polymer electrolytes in LIBs was in the early 2000s. Sony Corporation introduced these batteries in their consumer electronics products, marking a significant milestone in the battery industry. Sony’s introduction of the first polymer electrolyte-based LIBs represented a departure from traditional liquid electrolyte-based batteries [12].

2. Potential Applications of Polymer Electrolytes in Li-Based Batteries

Polymer electrolytes have garnered substantial attention in the realm of lithium-based batteries, offering a unique set of advantages and disadvantages that can significantly impact their potential applications. On the positive side, polymer electrolytes have the potential to revolutionize battery safety. Unlike liquid electrolytes, which are highly flammable and can pose serious safety risks, polymer electrolytes are non-flammable, reducing the likelihood of battery-related fires and explosions. This enhanced safety is especially critical in applications where batteries are subjected to physical stress or high-impact situations, such as in electric vehicles or portable electronics. Furthermore, the flexibility and versatility of polymer electrolytes open up a world of possibilities for novel battery designs. These materials can be tailored to possess various mechanical properties, including flexibility and stretchability, making them suitable for use in flexible and wearable electronics. This flexibility allows for the creation of batteries that conform to irregular shapes and can be integrated seamlessly into clothing and other non-traditional form factors. Advancements in polymer chemistry have also led to polymer electrolytes with improved ionic conductivity. Although they still lag behind liquid electrolytes in this regard, these materials have made significant strides, enabling their use in a wide range of portable electronic devices. Additionally, some polymer electrolytes can operate at higher voltages than traditional liquid electrolytes, potentially increasing the energy density of lithium-based batteries. Another critical advantage of polymer electrolytes is their ability to reduce the growth of lithium dendrites. Dendrites are undesirable needle-like structures that can form on the surface of the lithium anode, leading to short circuits and potentially catastrophic failures. Polymer electrolytes can help inhibit dendrite growth, improving the cycle life and overall reliability of lithium-based batteries. From an environmental standpoint, many polymer electrolytes can be produced using sustainable materials and processes, aligning with the growing demand for eco-friendly battery technologies. This aspect is particularly important as society becomes increasingly conscious of the environmental impact of energy storage solutions. However, it is essential to acknowledge the challenges and disadvantages associated with polymer electrolytes. One of the primary drawbacks is their lower ionic conductivity compared to liquid electrolytes. This limitation can result in reduced power output and may necessitate larger battery sizes to achieve comparable performance, which can be problematic in applications with stringent space constraints. Additionally, polymer electrolytes often exhibit a narrower operating temperature range than liquid electrolytes. Extreme temperatures can adversely affect their performance, limiting their suitability for applications in harsh environments or regions with extreme climates. The complex manufacturing processes required to produce high-performance polymer electrolytes can be a significant barrier to widespread adoption. Precise control over the composition and structure of these materials is necessary, increasing production costs and complexity. Some polymer electrolytes may also exhibit thermal instability at high temperatures, which can degrade their performance and reliability. Moreover, not all electrode materials are compatible with polymer electrolytes, limiting their applicability in certain types of lithium-based batteries. Lastly, the materials used in polymer electrolytes can be more expensive than traditional liquid electrolytes, contributing to higher overall battery production costs. Overall, polymer electrolytes offer substantial advantages in terms of safety, flexibility, and potential environmental benefits, making them an attractive option for lithium-based battery applications. However, their lower ionic conductivity, limited temperature range, complex manufacturing processes, potential thermal instability, material compatibility issues, and higher production costs represent significant challenges that researchers are actively working to address. As these challenges are overcome, polymer electrolytes have the potential to play a pivotal role in shaping the future of energy storage technology. In LIBs, polymer electrolytes enable the development of solid-state cells, eliminating the need for flammable liquid electrolytes and enabling the use of higher energy density electrode materials. Additionally, polymer electrolytes find potential applications in Li-air, Li-sulfur, and solid-state Li batteries. Their ability to accommodate different lithium salts, plasticizers, and fillers opens doors for customization and optimization, driving innovation in advanced energy storage systems. These different types of LIBs with polymer electrolytes demonstrate the versatility and potential of polymer electrolytes in enabling advancements in energy storage technologies across various industries. Lithium-Ion Polymer Batteries (LiPo): These batteries utilize polymer electrolytes as solid or gel-like materials that offer flexibility in battery design. Due to their thin and lightweight form factor, LiPo batteries find applications in portable electronic devices such as smartphones, tablets, and wearable devices. In a study by Lee et al. ^[13][93], the effect of unreacted monomer on the functionality of LiPo batteries was investigated. The polymer electrolytes used in the batteries are made of cross-linked polyethylene glycol diacrylate and are created by free radical polymerization. As the proportion of unreacted monomer in the electrolytes increases, particularly at low temperatures and high discharge rates, the discharge capacities of test cells containing polymer electrolytes made with various initiator concentrations drop. This behavior indicates a correlation between the increase in unreacted monomer content and the rise in interfacial resistance within the test cells. The cause of this high interfacial resistance is believed to be the reaction of the unreacted monomer at the electrode surface during the charging process, resulting in the formation of a resistive film. In another work, polymer electrolytes for Li-ion polymer batteries were investigated using various combinations of lithium difluoro(oxalato)borate (LiODFB) and lithium bis(oxalate)borate (LiBOB) salts, along with an ionic liquid (N-methyl-N-propyl pyrrolidinium bis(trifluoro methane sulphonyl)imide or 1-ethyl-3-methyl imidazolium bis(tri fluoro methane sulphonyl)imide, sulfolane (TMS), and poly(vinylidene fluoride). The preparation of polymer electrolytes involved combining quaternary lithium salt, ionic liquid, PVdF, and TMS using the casting technique ^[14][94]. Lithium-Ion Solid-State Batteries: Solid-state batteries employ polymer electrolytes as a solid material, eliminating the need for flammable liquid electrolytes. They offer improved safety, higher energy density, and longer cycle life. Solid-state batteries are considered for electric vehicles (EVs) and grid energy storage due to their enhanced stability and potential for increased energy storage capacity. A novel design for SPEs was proposed by Wang et al., utilizing electrospinning to create an ultrathin and rigid blend ^[15][95]. The blend consisted of a bio-polyamide with an N-substituted pyrrolidone ring (referred to as IBD) and PEO/Li bis(tri fluoro methane sulfonyl)imide. The experimental results confirmed that IBD exhibited a strong affinity for Li⁺ ions, making it the primary component responsible for ion transport in the SPE. The combination of IBD with the flexible chain segments of PEO resulted in a synergistic effect, facilitating the transport of ions. This mechanism involved IBD promoting the dissociation of ion pairs, leading to dynamic interactions between the mobile cations and the long-chain molecules that constituted the SPE. Additionally, this combination contributed to the widening of the ion transport pathway. The resulting SPE exhibited an impressive ionic conductivity of up to 4.26 × 10⁻⁴ S cm⁻¹ at 50 °C. In another work, Zhu et al. proposed a novel design of inorganic-polymer gel electrolyte/anode interphase in quasi-solid-state LIBs ^[16][96]. This study aimed to develop a gel electrolyte and anode electrode employing helical mesoporous silica nanofibers (HMSFs) as the underlying structure to improve the electrochemical performance of LIBs. This innovative approach allows for the seamless integration of the electrolyte and anode interface, resulting in remarkable electrochemical properties, as observed through measurements. Several noteworthy characteristics were achieved by incorporating HMSFs into a P(VDF-HFP) matrix to form electrolyte membranes referred to as NPCGE. These included exceptional thermal stability, withstanding temperatures of up to 372 °C, a wide electrochemical window of 5.30 V, and a high room temperature ionic conductivity of 1.2 × 10⁻³ S cm⁻¹. Lithium-Air Batteries: Li-air batteries, also known as lithium-oxygen batteries, are designed to utilize oxygen from the surrounding air as a cathode material. Polymer electrolytes play a crucial role in these batteries by providing a stable and conductive medium for lithium-ion transport. Lithium-air batteries have the potential to offer high energy density and could be used in electric vehicles and aerospace applications. Yoon et al. recently proposed a new GPE for Li-air batteries with excellent ionic conductivity, lithium stability, and redox activity ^[17][97]. The introduction of IL-PTZ in Li-air cells leads to significantly improved electrochemical performance. The cell exhibits a four-times-longer cyclability than the cell without the addition and a remarkable initial discharge capacity of 4685 mAh g⁻¹. Previously, Song et al. revealed that the performance of solid-state lithium-air batteries is greatly improved by including a continuous 3D garnet network composite polymer electrolyte ^[18][19][98,99]. Lithium-Sulfur Batteries: Polymer electrolytes can also be used in lithium-sulfur batteries, which utilize a sulfur-based cathode. These batteries offer high energy density and are being explored for applications in electric vehicles, renewable energy storage, and portable electronics. Hui et al. reported on a cathode material and additive called poly(sulfur-1,3-diisopropenylbenzene) (PSD), based on an organosulfide, was synthesized, and incorporated into a P(VDF-HFP) polymer electrolyte. The resulting composite, named P(VDF-HFP)-10%PSD, exhibited superior ionic conductivities compared to the liquid electrolyte, reaching up to 2.27 × 10⁻³ S cm⁻¹ at room temperature ^[20][100]. In another work, a novel flame-retardant polymer electrolyte was developed to improve the Li-ion conductivity, aiming to enhance the safety of lithium-sulfur batteries ^[21][101]. The same group of researchers introduced a pioneering approach of in situ electrochemical polymerization to fabricate a nonflammable polyether electrolyte in Li-S@pPAN batteries ^[22][102]. This innovative technique enhances lithium compatibility (exceeding 3000 h) and modifies the cathode interphase, improving reaction kinetics. The resulting polyether-rich interphase ensures remarkable battery performance, including a higher capacity of 1645.3 mAh g⁻¹ based on S, faster charging capability (up to 10 °C), and exceptional cycling stability (maintaining 99.5% retention over 400 cycles). Moreover, it achieved an ultrahigh average Coulombic efficiency (CE) of over 99.9995%, indicating minimal irreversible reactions on the cathode. In a pouch cell configuration, the battery exhibited a high sulfur utilization rate of 91.2% and maintained stable performance with 84.6% capacity retention over 60 cycles. A novel SPE was developed in recent work through the insitu polymerization of 1,3-dioxolane (DOL) in the presence of a multifunctional indium phthalocyanine (PDOL@InPc) ^[22][102]. Including InPc in the SPE enhanced ionic conductivity and effectively mitigated the shuttle effect. Furthermore, InPc served as an effective additive that facilitated uniform lithium deposition by restructuring the electric field at the interface. A new family of block copolymer polyelectrolytes with single-ion conducting properties was successfully synthesized using the reversible addition–fragmentation chain transfer polymerization technique. These copolymers consist of blocks of poly(lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethylsulfonyl)imide) and poly(ethylene glycol) methyl ether methacrylate. The resulting polyelectrolytes exhibit a range of low T_g values from −61 to 0.6 °C, demonstrating high ionic conductivity at different temperatures (up to 2.3 × 10^–6 and 1.2 × 10^–5 S cm^–1 at 25 and 55 °C, respectively). Additionally, they demonstrate wide electrochemical stability, withstanding potentials up to 4.5 V vs. Li⁺/Li. Furthermore, they possess a lithium-ion transference number close to unity, specifically 0.83. Due to the combination of these exceptional properties, the prepared polymer materials have been employed as solid polyelectrolytes and binders to develop lithium-metal battery prototypes. These prototypes display remarkable charge/discharge efficiency and outstanding specific capacity (up to 130 mAh g^–1) at a C/15 rate ^[24][104]. In another work, Porcarelli et al. successfully developed an innovative polymer electrolyte system by controlling the mobility of traditional EO-based backbones. This newly designed polymer electrolyte enables the creation of all-solid lithium-based polymer cells, exhibiting remarkable cycling performance in both rate capability and stability across a wide range of operating temperatures. The polymer electrolytes were synthesized through UV-induced (co)polymerization, facilitating effective interlinking between the PEO chains, which are plasticized by tetraglyme at various lithium salt concentrations. The resulting polymer networks demonstrate exceptional mechanical robustness, high flexibility, and exhibit a homogeneous and highly amorphous structure. These polymer electrolytes exhibit impressive ionic conductivity values exceeding 0.1 mS cm⁻¹ at ambient temperatures. They also display a wide electrochemical stability window, capable of withstanding potentials greater than 5 V vs. Li⁺/Li. Furthermore, these electrolytes possess an excellent lithium-ion transference number, exceeding 0.6, and display favorable interfacial stability. An important advantage of these polymer electrolytes is their effective resistance to the nucleation and growth of lithium dendrites. This property makes them highly promising for utilization in the next generation of all-solid Li-metal batteries operating under ambient conditions ^[25][105]. Overall, Porcarelli et al. demonstrated the successful architecture of a novel polymer electrolyte system with impressive properties, paving the way for enhanced performance and safety in advanced all-solid lithium-based batteries.