Ultrathin Superhydrophobic Coatings for Air‐Stable Inorganic Solid Electrolytes: Toward Dry Room Application for All‐Solid‐State Batteries

Abstract Inorganic solid electrolytes (SEs), such as sulfides and halides, are crucial for developing practical all‐solid‐state batteries (ASSBs) owing to their high ionic conductivities and mechanical sinterabilities. However, their sensitivity to humid air necessitates stringent dry‐room conditions during processing, which increases production costs. This study demonstrates that ultrathin (≈5 nm) superhydrophobic polydimethylsiloxane (PDMS) or fluorinated PDMS (F‐PDMS) protective layers can enhance the stability of air‐sensitive sulfide (Li₆PS₅Cl (LPSCl)) and halide (Li_2.5Zr_0.5In_0.5Cl₆) SEs in ASSBs. The (F)‐PDMS coatings are applied using a scalable, straightforward vapor‐phase deposition process, achieving high Li⁺ conductivity retention (92%, from 2.5 to 2.3 mS cm⁻¹ at 30 °C). The protective layers effectively inhibit LPSCl degradation under practically relevant dry room conditions (dew point of −50 °C or −10 °C): e.g., from 2.3 to 0.97 mS cm⁻¹ for PDMS‐coated LPSCl versus from 2.5 to 0.57 mS cm⁻¹ for bare LPSCl. Surprisingly, the superhydrophobic coatings facilitate the recovery of Li⁺ conductivity via vacuum heat treatment. This new phenomenon, known as regeneration, is achieved by the facile elimination of adsorbed water. Furthermore, the regenerated (F)‐PDMS‐coated LPSCl demonstrates significant performance in NCM||Li‐In ASSB cells. These findings suggest that superhydrophobic (F)‐PDMS coatings are a promising solution for practical all‐solid‐state technologies.