Polymer electrolytes have attracted great interest in advanced lithium batteries. Recently, it has been found that there are many functional applications of polymer electrolytes in lithium batteries, which are very important for the development of high-energy-density lithium batteries. In this review, the functional applications of polymer electrolytes are reviewed in terms of protecting lithium metal anode, coating for high-voltage cathodes, and improving the safety of batteries. Finally, the remaining challenges and future perspectives of polymer electrolytes in these functional applications are presented. This review aims to provide a comprehensive understanding of polymer electrolytes and promotes their functional application in high-energy-density lithium batteries.

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The interface compatibility and ionic conductivity of polymer electrolytes play crucial roles in electrochemical behaviors of polymer-based solid-state batteries. Advanced characterization techniques are essential to explore interface compatibility and ion transport mechanisms. In this review, the recent application of multi-scale characterization techniques in polymer-based solid-state lithium batteries (PSSLBs) is summarized, in terms of microscopy techniques, X-ray techniques, spectroscopy techniques, and neutron techniques. These characterization technologies span the atomic scale, micro scale, mesoscale, and macroscopic scale. In particular, the in situ characterization with high time resolution is highlighted in each of the techniques, in order to promote the exploration of dynamic interface evolution in operating batteries. It is hoped that this review will contribute to the development of multi-scale in situ characterization techniques, thus promoting the improvement of energy density for PSSLBs.