Hybrid conductive-lithophilic-fluoride triple protection interface engineering: Dendrite-free reverse lithium deposition for high-performance lithium metal batteries

Lithium metal batteries (LMBs) with high energy density are impeded by the instability of solid electrolyte interface (SEI) and the uncontrolled growth of lithium (Li) dendrite. To mitigate these challenges, optimizing the SEI structure and Li deposition behavior is the key to stable LMBs. This study novelty proposes a facile synthesis of MgF₂/carbon (C) nanocomposite through the mechanochemical reaction between metallic Mg and polytetrafluoroethylene (PTFE) powders, and its modified polypropylene (PP) separator enhances LMB performance. The in-situ formed highly conductive fluorine-doped C species play a crucial role in facilitating ion/electron transport, thereby accelerating electrochemical kinetics and altering Li deposition direction. During cycling, the in-situ reaction between MgF₂ and Li leads to the formation of LiMg alloy, along with a LiF-rich SEI layer, which reduces the nucleation overpotential and reinforces the interphase strength, leading to homogeneous Li deposition with dendrite-free feature. Benefiting from these merits, the Li metal is densely and uniformly deposited on the MgF₂/C@PP separator side rather than on the current collector side. Furthermore, the symmetric cell with MgF₂/C@PP exhibits superb Li plating/stripping performance over 2800 h at 1 mA cm⁻² and 2 mA h cm⁻². More importantly, the assembled Li@MgF₂/C@PP|LiFePO₄ full cell with a low negative/positive ratio of 3.6 delivers an impressive cyclability with 82.7% capacity retention over 1400 cycles at 1 C.