Lithium-ion batteries (LIBs) are crucial for achieving sustainable energy goals due to their high energy density and long cycle life. They dominate markets like consumer electronics, electric vehicles, and stationary energy storage systems. However, current LIBs use liquid electrolytes, which are toxic, flammable, and their liquid state does not resist dendrite growth, causing battery capacity decline and failure. Additionally, the limited availability of lithium and other metals makes liquid-based LIBs less sustainable. On the other hand, solid polymer electrolytes (SPEs) offer a safer alternative as they are non-volatile and can resist dendrite growth. However, ion transport in solids is much more restricted than in liquids, while imperfect solid-solid interfaces contribute to interfacial resistance leading to lower ionic conductivity and increasing Ohmic losses or requiring battery operation at elevated temperatures. Chemical and mechanical degradation of these interfaces can also result in battery capacity fade, and poorer cyclic performance compared to liquid electrolytes. Understanding the ionic transport mechanisms in SPEs is critical for designing and optimizing the nanostructure of polymers and polymer/electrode interfaces to overcome these limitations. In this review, the fundamental mechanisms of ion transport in SPEs will first be explored. Various state-of-the-art approaches for addressing the key challenges in SPEs and their solutions are then discussed. Furthermore, the current status of SPEs is analyzed to determine their potential for replacing liquid electrolytes in the future.

In this work, a series of crosslinked poly(poly(ethylene glycol) methyl ether methacrylate-co-acrylonitrile) [P (PEGMA-co-ACN)] copolymers has been designed as gel polymer electrolytes (GPEs) for high-safety and high voltage lithium ion batteries. Two approaches including conventional free radical polymerization (FRP) and control radical polymerization (CRP) through one-step reversible addition-fragmentation chain transfer (RAFT) polymerization in the presence of 2-cyano-2-propyl dodecyl trithiocarbonate (CPDT) were used to synthesize GPEs with different molar ratios of ACN and PEGMA. Although there was not observed remarkable advantage of polymerization method in different monomer compositions, P(PEGMA-co-ACN) by 50 mol.% ACN and PEGMA prepared via FRP exhibited the maximum ionic conductivity of 5.1 * 10(-3) S cm(-1), transference number (t+) of 0.55, and exceptional swelling ratio value of 150% in 30 min. Interestingly, it affords a supreme electrochemical stability window up to 5.13 V vs. Li/Li+, the charge capacity of 205 mAh g(-1) at 0.1 ?, and capacity retention of 89% after 100 cycles.

Starch-based electrolytes are used here to achieve safe, efficient, inexpensive, and eco-friendly lithium ion batteries (LIBs). Carboxymethyl starch (CMS) and starch acetate (SA) are synthesized as starch amorphous derivatives from corn starch, and then crosslinked by poly(vinyl alcohol) (PVA) to form a polymer network. In the following, the electrochemical properties of the obtained electrolytes in both solid and gel states are investigated. At room temperature, the ionic conductivity for solid CMS and gel SA electrolytes are 9.2*10(-3) S cm(-1 )and 1.13*10(-2) S cm(-1), respectively. Other remarkable results of these electrolytes are the wide electrochemical stability window, stable cyclic performance, charge capacity higher than 210 mAh/g, CE = 100 % before 10 cycles charge-discharge for both CMS and SA, and good electrode/electrolyte compatibility. The unparalleled electrochemical performance of CMS and SA, along with their unique properties, make them a unique alternative to liquid electrolytes in LIB.

A structural design and appropriate morphology of polymer-functionalized graphene oxide are occupied to optimize nanocomposite polymer electrolytes. The nanocomposite gel polymer electrolytes (GPEs) based on poly (poly (ethylene glycol) methyl ether methacrylate and RAFT agent-functionalized graphene oxide [P(PEGMA/ GO-S-(thiobenzoyl)thioglycolic acid (STTA))] have been successfully prepared by in-situ polymerization method in different ratio of GO-STTA and crosslinking by poly(ethylene glycol) diallyl (PEGDA). The P(PEGMA/ GO-STTA) GPE with 0.5 wt% of GO-STTA exhibited a high ionic conductivity of 5.5 mS cm(-1) at room tem-perature, a superior lithium transfer number (t+) value of 0.61, and electrochemical window up 4.7 V. The GPE based P(PEGMA/GO-STTA0.5%) indicated 92% coulombic efficiency, the charge capacity value of 191.7 mAh g(-1) potential for lithium ion battery with high safety and long cycle life. , and capacity retention was about 92% after 100 cycles at 0.1C. P(PEGMA/GO-STTA) GPEs showed great potential for lithium ion battery with high safety and long cycle life.

Polymer electrolytes are one of the most effective alternatives to liquid electrolytes to extend lithium ion batteries (LIBs) safety. However, they suffer from low conductivity and charge capacity. In this work, to overcome these tremendous disadvantages, cellulose acetate (CA) and oxidized carboxymethyl cellulose (OCMC) polymer electrolytes obtained from sugarcane and cotton, respectively, are used as polymer electrolytes in LIBs. Then, the effect of short (citric acid) and long (polyvinyl alcohol (PVA)) chain crosslinker to create polymer networks and solid and gel state of electrolytes on the electrochemical properties are investigated. Gel CA-PVA and solid OCMC-PVA electrolytes indicated the best results. Additionally, each CA and OCMC, in both gel and solid state exhibited ionic conductivity in order of 10(-2) S cm(-1), charge capacity over 230 mAh g(-1), capacity retention of 90% after 100 cycles, excellent electrochemical stability upper than 4.5 V, supreme lithium transfer number (similar to 0.8), and good thermal stability. The electrolytes prepared here bring safe and highly efficient LIBs and are a very serious competitor to liquid electrolytes.

Poly(ethylene oxide) (PEO) is one of the most famous polymer electrolytes; however, its low conductivity and capacity have prevented its commercial applications. This study utilizes carboxymethyl starch (CMS) and oxidized carboxymethyl cellulose (OCMC) natural polymers with a high potential to dissolve lithium ions (Li+) in to help PEO ionic conductivity. The semi-interpenetrating polymer networks (semi-IPNs) consist of crosslinked poly(ethylene glycol) methyl ether methacrylate (PEGMA) with poly(ethylene glycol) diallyl (PEGDA) and free CMS/OCMC chains. Effect of increasing the amount of natural polymer on the electrochemical properties of semi-IPNs is investigated. Semi-IPN CMS50% and semi-IPN OCMC50% deliver excellent results such as high conductivity (in order 10–2 Scm−1) at room temperature, electrochemical stability window higher than 4.5 V, high Li+ transfer number, high discharge capacities (191 and 203 mAh g–1 with capacity retention of 85 and 88.5% after 100 cycles at 0.2 C, respectively), and stable cyclic behavior.

Nanocomposite gel polymer electrolyte (GPE) films based on poly(poly[ethylene glycol] methyl ether methacrylate)/graphene oxide (GO) (P[PEGMA-GO]) have been prepared by in situ conventional free radical polymerization (FRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization as controlled radical polymerization (CRP) using 2-cyano-2-propyl dodecyl trithiocarbonate (CPDT) as RAFT agent. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) showed that prepared nanocomposite films had amorphous structure and also, thermogravimetric analysis (TGA) showed appropriate thermal stability of samples. GPEs prepared via FRP showed slight higher ionic conductivity that GPEs prepared via RAFT polymerization. In both synthetic approaches, an optimum GO amount of 0.3 wt. % was obtained considering ionic conductivity. At ambient temperature, P(PEGMA-GO) with 0.3 wt. % GO prepared via FRP indicated the highest ionic conductivity of 4.05 x 10(-3) S cm(-1), a satisfactory lithium ion transference number (t(+)) of 0.6, and the excellent interface compatibility with electrodes. The lithium ion battery with P(PEGMA-GO0.3%) as GPE also exhibited electrochemical stability window up to 4.6 V vs. Li/Li+, a high charge-discharge capacity of 179 mAh g(-1) at 0.1 C, and capacity retention of 91% after 100 cycles.