NASICON‐Based Solid State Li‐Fluoride Conversion Batteries Enabled by Constructing a Fluorine‐Rich Trap for Ti⁴⁺

Abstract The application of solid‐state Li metal batteries based on NASICON‐type Li_(1+x)Al_xTi_(2‑x)(PO₄)₃ (LATP) electrolytes has been hindered by the interface instability, originating from the spontaneous Ti⁴⁺ reduction reaction between LATP and Li. Here, a mesoporous fluorination interlayer based on high‐surface‐area AlF₃ is proposed to improve the stability and compatibility of the LATP/Li interface and enable the reversible operation of NASICON‐type Li||FeF₃ conversion batteries. The high Lewis acidity and porosity in the interlayer can buffer the volume evolution, increase the Li‐ion transference number and promote the formation of LiF domains. Time‐of‐flight secondary‐ion mass spectroscopy (ToF‐SIMS) proves the in situ construction of an F‐rich trap zone for Ti⁴⁺, which can pin the easily‐reduced Ti⁴⁺ in LATP and retard its conversion to Ti³⁺ during the electrochemical process. An electro‐fusion process at the LATP/Li interface occurs and successfully welds the solid electrolyte, AlF₃ interlayer, and anode into a whole to mitigate the anode resistance, guaranteeing the fast interface migration of Li⁺. The corresponding Li||Li symmetric cells exhibit a stable cycling performance with small voltage hysteresis for at least 500 h. The ceramic‐type solid‐state Li‐Fe‐F cells modulated by the F‐rich transition layer can deliver a high capacity of 696.7 mAh g⁻¹ and show a good cyclability of the conversion reaction.