Magnesium Fluoride Interlayers Enabled by Wet‐Chemical Process for High‐Performance Solid‐State Batteries

Abstract Garnet‐type solid‐state electrolytes (SSEs) exemplified by Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT) are chemically unstable when exposed to air, leading to the formation of impurities and poor wettability with Li metal. Herein, a protocol to address this Li/LLZT interface challenge is demonstrated by constructing a lithiophilic MgF₂ nanofilm on the LLZT pellet. Specifically, a solution‐based process is developed for the surface engineering of LLZT, utilizing magnesium trifluoroacetate (MTF) as the molecular precursor while poly(acrylic acid) (PAA) as the coordinating agent in a sol‐gel process. It is demonstrated that a facile spin‐coating treatment followed by high‐temperature annealing reliably forms crack‐free MgF₂ nanofilms with precise thickness control. Introduction an MgF₂ interlayer transforms the LLZT pellet into a highly lithophilic, facilitating close contact with the lithium anode, thereby leading to a significantly reduced interfacial resistance from 1190 Ω cm² to 6 Ω cm². Such an interfacial engineering enables stable cycling of full batteries with high reversibility and rate capability using commercial LiFePO₄ and LiNi_0.83Co_0.07Mn_0.1O₂ as cathodes. This study unfolds the possibility of a solution‐based method as a facile and scalable process for the construction of fluoride nanofilms, which is promising to address the critical interfacial challenges of solid‐state batteries (SSBs) to facilitate its practical applications.