Sandwich-Structured Lithiophilic Layer with Mixed Ionic–Electronic Conductivity for Lithium Metal Batteries

Precise regulation of Li nucleation and dense deposition are needed for the practical application of lithium metal batteries (LMBs). Herein, MgF2 nanoparticles were sandwiched between two thin N-doped graphitized carbon layers (NG@MgF2@NG) via atomic layer deposition and chemical vapor deposition. Upon full lithiation, a sandwich-structured lithiophilic mixed ionic–electronic conductive (LMIEC) layer is formed in situ, which reduces the nucleation overpotential and induces lateral Li growth with reduced volume fluctuation, leading to dense and uniform deposition. Furthermore, Li deposition behavior and its underlying mechanisms on the LMIEC layer were investigated by using in situ optical microscopy, in situ TEM observation, and theoretical calculations. A Coulombic efficiency of 99.0% over 1000 cycles in asymmetric cells and a lifespan exceeding 4200 h in symmetric cells were achieved. The full cell with a LiFePO4 cathode maintained 76.2% capacity retention after 1000 cycles at 0.5 C (N/P = 2).