Stabilizing solid electrolyte/Li interface via polymer-in-salt artificial protection layer for high-rate and stable lithium metal batteries

Solid electrolytes are fundamental for next-generation solid-state Li-metal batteries that promise high energy density and safety. However, various solid electrolytes suffer from chemical/electrochemical instability against Li metal and poor interfacial contact with electrodes. Herein, a polymer-in-salt (PiS) artificial protection layer composed of fluoropolymer in highly-concentrated lithium salt (labeled as PiSPL) is introduced to address these problems using NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) as a typical solid electrolyte. The resulted PiSPL layer exhibits small thickness (10 μm), good chemical/electrochemical stability, and excellent ionic conductivity (0.5 and 1.8 mS cm−1 at 25 and 60 °C, respectively). These properties endow the LATP/Li interface with improved contact, favorable Li+ diffusion, and, most importantly, inhibited interfacial reactions, which is examined using ex-situ X-ray photoelectron spectroscopy and electrochemical analyses and morphological observations. Consequently, Li||[email protected]||Li symmetric cells can deliver small overpotentials at various current densities and long lifetime at 0.16 mA cm−2 over 400 h. On the contrary, the same configuration cells with LATP die after only 50 h. Moreover, the LiFePO4||[email protected]||Li solid-state full cells exhibit excellent rate capability (124.0 mA h g−1 at 2.0C) and cyclability (148.9 and 140.6 mA h g−1 after 200 cycles at 0.2C and 300 cycles at 1.0C, respectively). The PiS strategy represents a general and effective approach to stabilize various solid electrolyte/Li interfaces to push solid-state Li-metal batteries forward.