Ultrathin polymer electrolyte film prepared by in situ polymerization for lithium metal batteries

Polymer materials demonstrate tremendous potential in constructing high-performance solid-state lithium metal batteries (LMBs). However, the polymer-based solid-state batteries assembled via the ex situ process generally exhibit poor electrolyte/electrode interfacial contact. Herein, we report an in situ solidification method based on ring-opening polymerization of ε-caprolactone (ε-CL), which enables an ultrathin film (11 μm) as solid polymer electrolytes (SPEs). Thanks to the mild reaction conditions and simple process, such SPEs are obtained efficiently and exhibit intimate interfacial contact with electrodes. It is worth noting that the initiator—stannous octoate—can react with the lithium metal anode to form a LiSn alloy layer, which possesses excellent lithium dendrite resistance. The addition of 15 wt% propylene carbonate leads to a semisolid-state battery with further improved ionic conductivity, which manifests outstanding cell performance at both 60 °C and room temperature. As a result, the semisolid-state LiFePO4/Li cell delivers a high discharge capacity of 150 mAh/g even after 100 cycles at room temperature, further highlighting the enhanced interfacial stability after the in situ solidification of ε-CL on the electrode surface. It is believed that such an in situ polymerization process to prepare polymer electrolytes has great potential for building the better solid-state LMBs.