Enabling Moisture and Interfacial Stability in Sulfide Solid Electrolytes via a Processable Organic Coating Strategy for High‐Voltage All‐Solid‐State Batteries

ABSTRACT Sulfide solid electrolytes (SEs) are excellent candidates for solid‐state batteries (SSBs), but their extreme sensitivity to moisture and lack of oxidative stability with uncoated high‐voltage cathodes incur processing complexity and cost. Here, we present a simple and cost‐effective decanoate fatty‐acid (DA) coating strategy for argyrodite (Li 6 PS 5 Cl, LPSCl) that stabilizes it to exposure at 39% relative humidity for up to 2 h, while preserving its structure, ionic conductivity, and increasing its anodic stability. Cells employing 2 wt%‐coated LPSCl (DA–LPSCl) as the catholyte, with a bare NCM85 cathode and Li‐In anode, deliver a capacity of 175 mAh.g −1 , and 96% capacity retention over 150 cycles at 0.2 C, while bare LPSCl retains only 61% capacity. Symmetric Li|DA–LPSCl|Li cells cycle for 1000 h, in contrast to bare LPSCl cells, which short‐circuit after ∼230 h. Moreover, full cells using a lithium metal anode with the DA–LPSCl SE showed remarkable performance compared to state‐of‐the‐art SSBs, retaining 81% of their capacity after 300 cycles at 0.2 C. High‐loading cells with areal capacities up to 3.2 mAh cm −2 are also demonstrated. This work showcases the potential of a low‐cost, processable, and flexible coating to address key limitations of sulfide SEs, advancing the commercial viability of SSBs.