Polyethylene Oxide-Based Composite Solid Electrolytes for Lithium Batteries: Current Progress, Low-Temperature and High-Voltage Limitations, and Prospects

Lithium-ion batteries (LIBs) are considered to be one of the most promising power sources for mobile electronic products, portable power devices and vehicles due to their superior environmental friendliness, excellent energy density, negligible memory effect, good charge/discharge rates, stable cycling life, and efficient electrochemical energy conversion, which distinguish it from other power devices. However, the flammable and volatile organic solvents in carbonate-containing liquid electrolytes can leach, resulting in thermal runaway and interface reactions, thus significantly limiting its application. The use of polymer solid electrolytes is an effective way to solve this safety issues, among which poly (ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) have attracted much attention because of their stable mechanical properties, ease of fabrication, excellent electrochemical and thermal stability. Unfortunately, PEO-SPEs with their low room-temperature ionic conductivity, narrow electrochemical windows, poor interface stability, and uncontrollable growth of lithium dendrites cannot meet the demand for high energy density in future LIBs. Therefore, this review firstly describes the ion transport mechanisms and challenges that are crucial for PEO-SPEs, and then provides a comprehensive review of current approaches to address the challenges, including novel and efficient lithium salts, additives, composite electrolytes, stable solid electrolyte interfaces, 3-D lithium metals and alloys, cathode protection layers and multi-layer electrolytes. Finally, future research directions are proposed for the stable operation of PEO-SPEs at room temperature and high voltage, which is imperative for the commercialization of safe and high energy density LIBs.