Mechanochemically Disordered Li 2 Ta 2 OCl 10 Oxyhalide Unlocks Superionic Conductivity (>10 mS.cm −1 )

All‐solid‐state lithium‐metal batteries (ASSBs) offer a promising route to surpass the energy‐density limits of conventional lithium‐ion technology. However, their commercialization is hindered by interfacial instability, chemo‐mechanical degradation, and the limited ionic conductivity of current solid electrolytes, driving the search for new fast‐ion conductors, particularly among lithium oxyhalides. Here, we report a new lithium tantalum oxychloride, Li 2 Ta 2 OCl 10 , synthesized via a simple low‐temperature route. The material crystallizes in a triclinic structure composed of isolated [Ta 2 OCl 10 ] 2− dimers and exhibits a room‐temperature ionic conductivity of 0.002 mS.cm − 1 . Ball milling dramatically enhances Li + transport, increasing the conductivity by a factor of 5000 to 11 mS.cm −1 while reducing the activation energy from 0.46 to 0.27 eV. Combined structural, spectroscopic, and computational analyses reveal that this enhancement originates from the formation of a highly disordered, liquid‐like Li + sublattice. Li 2 Ta 2 OCl 10 further demonstrates a stability window of 1.89–3.70 V vs. Li x In and enables ASSBs with low polarization and excellent capacity retention (700 cycles at 1C with decay less than 20%). These results establish Li 2 Ta 2 OCl 10 as a promising superionic conductor and highlight the Li–M–O–Cl family as a fertile design space for next‐generation solid electrolytes.