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

Garnet-type solid-state electrolytes (SSEs) exemplified by Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (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 MgF 2 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 MgF 2 nanofilms with precise thickness control. Introduction an MgF 2 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 cm 2 to 6 Ω cm 2 . Such an interfacial engineering enables stable cycling of full batteries with high reversibility and rate capability using commercial LiFePO 4 and LiNi 0.83 Co 0.07 Mn 0.1 O 2 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.