Research Progress on Polymer-Based Solid-State Electrolyte Materials

Compared with inorganic solid-state electrolyte, solid polymer electrolyte (SPEs) is an ideal electrolyte material for solid-state lithium metal batteries due to its high flexibility, excellent processability and good interfacial contact. The ionic conductivity, electrochemical window and interface stability of SPEs with electrodes play a crucial role in the overall performance of solid-state lithium batteries. According to the different electrochemical stability windows, this review represents research progress on two types of typical SPEs systems, i.e., low-voltage stable polyethers and high-voltage stable polyesters. Polyether-based electrolytes, which mainly outlines the problems and challenges faced by four types of polyether, such as poly(ethylene oxide) (PEO), poly(ethylene glycol monomethyl ether acrylate) (pPEGMEA), poly(1,3-oxolane) (PDOL), and polyoxymethylene (POM)-based SPEs. These electrolytes have a better compatibility with lithium metal, but a poor oxidation resistance and a low ionic conductivity at room temperature. The molecular structure design of polymers and the introduction of functional groups (—F, —CN, —C=O) can reduce the HOMO value of SPEs and improve the antioxidant ability of electrolytes. The crystallinity of PEO-SPEs can be reduced via compounding, blending and cross-linking with inorganic fillers to improve the ionic conductivity. In addition, the balanced interactions between Li⁺, lithium salts, ether groups, anions and polymer matrix structure also need to be considered to achieve high ionic mobility and ionic conductivity. Polyester electrolytes mainly outline the challenges and solution strategies of four types of polyester, i.e., poly(ε-caprolactone) (PCL), poly(trimethylene carbonate) (PTMC), poly(propylene carbonate) (PPC) and polyoxalate (POE). This type of electrolyte is a good match for high-voltage cathode materials, but is easily reduced by lithium metal. The construction of a stable solid electrolyte interface membrane (SEI) layer can improve the stability with lithium metal via introducing multifunctional groups. In addition, the development of in-situ polymerisation processes is also reviewed, considering that solid-state electrolytes suffer from a poor electrode–electrolyte interfacial contact, a low ionic conductivity and complex manufacturing processes. Therefore, this review outlines the advantages and disadvantages of the two types of polyether-based and polyester-based SPEs, and analyzes their key scientific issues as well as recent research progress of the modification methods to provide a reference for future research and practical applications.

Despite important advances in solid polymer electrolytes (SPEs), polyether and polyester electrolytes and in-situ polymerisation technologies still face several challenges before they can be applied commercially. Improving ionic conductivity and electrochemical stability while maintaining good mechanical properties and interfacial compatibility are key issues in the development of SPEs. In addition, the economics and scale-up of preparation are important considerations for commercial applications, and greener and more efficient preparation processes also need to be explored. The development of SPEs in solid-state lithium-ion batteries focuses on the following aspects, i.e., 1) designing new molecular structures, exploring the constitutive relationship between molecular structure and properties, and improving the ionic conductivity and antioxidant properties of electrolytes, 2) since all electrochemical reactions occur at the interface, constructing a stable cathode-electrolyte (CEI) and SEI membrane between SPEs and cathode and anode to improve the interfacial stability and inhibit the interfacial reactions, 3) improving the ionic conductivity of SPEs through strategies such as grafting, copolymerisation and organic-inorganic composites, and 4) exploring the recycling methods of lithium salts and polymers to further reduce the manufacturing cost of SPEs.

For quasi-solid state electrolytes (QSSEs) with a high ionic conductivity, it is necessary to solve the following problems, i.e., 1) to improve the molecular stability of polymers for high-voltage cathode materials and low-voltage anode materials, 2) to enhance the liquid-locking ability of polymer molecules, inhibit the interfacial side reactions and improve the safety of the battery, 3) to regulate the interactions among the components of polymer, solvent and lithium salt to optimize the solvation structure of Li⁺ and achieve the rapid migration of Li⁺, and 4) the electrolyte film thickness and electrode active substance loading must be as close as possible to or exceed the targets of commercial liquid batteries. In addition, the production of large capacity energy storage batteries (i.e., >200 A·h) , results in local inhomogeneities within the cell due to the inhomogeneous heating within the cell. The influence of various factors (i.e., initiator activity, initiator dosage, and polymerisation temperature) on the polymerisation process should be investigated. The "step-by-step in-situ curing" route should be adopted, so that the polymer is cured in the battery separator, cathode and anode, and the polymer is cured in a step-by-step process. The polymer is in-situ cured at the battery diaphragm, positive electrode and negative electrode, and then laminated to assemble the solid-state battery. In summary, the synergistic development of polyether electrolyte, polyester electrolyte and in-situ polymerisation technology has an important foundation for the commercial application of lithium solid-state batteries. In the future, high-performance, low-cost SPEs, especially the development of in-situ curing, are expected to be applied in a large scale, becoming the first practical solid-state battery system.