Unravelling the Mechanism of Al2O3 Atomic Layer Deposition on Li6PS5Cl for All-Solid-State Batteries

Sulfide-based all-solid-state batteries (ASSBs) are considered a promising alternative to state-of-the-art Li-ion batteries due to their high gravimetric and volumetric energy density, as well as improved safety. Unfortunately, sulfide solid-state electrolyte, such as Li 6 PS 5 Cl (LPSCl), can undergo chemical and electrochemical reactions with cathode materials during cycling, leading to significant chemo-mechanical challenges. Moreover, LPSCl is highly sensitive to moisture and air, resulting in the evolution of H 2 S gas and the formation of electrochemically inactive and resistive interfacial layers. Atomic layer deposition (ALD) can play a critical role in suppressing the decomposition of LPSCl by forming an ultra-thin, conformal, and chemically/electrochemically stable buffer layer that protects the particle surface from degradation. With proper design, this layer can also enhance ionic conductivity and mechanical properties while reducing electronic conductivity. However, the reaction mechanism of ALD on LPSCl has not yet been systematically studied. Here, we elucidate the mechanism for Al 2 O 3 ALD using trimethyl aluminum (TMA) and H 2 O on LPSCl for ASSBs through a combination of in situ and ex situ experiments supported by density functional theory (DFT) calculations. In situ Fourier transform infrared (FTIR) spectroscopy measurements identified the functional groups on the LPSCl surface that participate in the TMA chemisorption and the subsequent H 2 O reaction during the first Al 2 O 3 ALD cycle. The FTIR measurements also revealed the steady Al 2 O 3 growth on the LPSCl with repeated ALD cycles. Ex situ X-ray photoelectron spectroscopy (XPS) measurements unveiled the chemical bonding following the TMA and H 2 O reactions and ex situ Raman spectroscopy measurements showed that there are no bulk changes in the LPSCl structure as a result of the Al 2 O 3 ALD. DFT calculations helped to discriminate between candidate reactions of the ALD precursors on the LPSCl surface. This work not only provides insights into optimizing ALD process parameters for LPSCl but also informs broader efforts in designing interfacial modifications for a wide range of sulfide-based solid electrolytes.