Polymer additives in liquid electrolytes for advanced lithium batteries

Compared to traditional energy storage devices, lithium-ion batteries (LIBs) have the advantages of high energy density, good cycling performance, and low self-discharge rate. Therefore, LIBs have been widely used as the main energy storage devices in various industries. As the blood of the battery, the electrolyte plays a key role in ion transport, formation of the interface layer, protection of electrode materials, etc. The commonly investigated electrolytes include liquid electrolytes, gel electrolytes, and solid or quasi-solid electrolytes. Liquid electrolytes have higher ionic conductivity, which is more conducive to the transport of lithium ions. Therefore, batteries based on liquid electrolytes often exhibit better electrochemical performance. In a liquid electrolyte, the additive is also an indispensable component to ensure the high efficiency of the electrolyte, which plays an important role in regulating the solvation structure of lithium ions, the formation of the solid-electrolyte interface layer, improving the safety performance of batteries, and maintaining operability under extreme conditions (such as low temperature). Unlike previous reviews that focused on small molecule additives, this review herein mainly reviews the application of polymer additives in liquid lithium batteries. Firstly, the functional mechanisms of different types of additives in liquid electrolytesfor lithium batteries are outlined and the advantages and disadvantages of different types of additives are summarized. Then, the research progress of polymers as additives in liquid lithium batteries in recent years is discussed in detail. According to the role of additives, the involved polymer additives are divided into five categories: molecular crowding agents, film-forming agents, HF scavengers, antifreeze agents, and flame retardants. A detailed explanation of the mechanisms related to the efficacy of polymers as additives is also provided. Finally, we present some perspectives on the limitations and future development trends of polymers as additives in liquid lithium batteries and other devices.