In Situ Electrochemical Polymerization Enabling High‐Performance Quasi‐Solid‐State Batteries

ABSTRACT In situ polymerization of liquid electrolytes is promising for achieving solid batteries with enhanced safety and low interfacial resistance. However, most current in situ polymerization methods, such as in situ thermal polymerization (THPO), are impacted frequently by issues such as uncontrollability, uneven polymerization, and low Li + conductivity. In this context, we propose an innovative in situ electrochemical polymerization (ELPO) technique that enables a controllable and uniform polymerization reaction on the cathode side by potentiostatically charging the battery to a high voltage (4.2 V). The degree of polymerization could be monitored via in situ scanning Raman spectroscopy. The reaction pathways are revealed via both experiments and theoretical calculations. It is interesting to find that, without adding any extra conventional initiators, the dehydrogenation reaction of fluoroethylene carbonate (FEC) molecules at high voltages could initiate the polymerization of 1,3‐dioxolane (DOL). The LiFePO 4 ||Li cells utilizing ELPO demonstrate an extraordinary capacity retention of 88.86% after 740 cycles and largely enhance the safety compared to their regular counterparts. These findings provide a simple, practically feasible, and controllable in situ polymerization method to achieve high‐performance quasi‐solid‐state batteries (QSSBs) that are compatible with current battery production.