Single‐ or Poly‐Crystalline Ni‐Rich Layered Cathode, Sulfide or Halide Solid Electrolyte: Which Will be the Winners for All‐Solid‐State Batteries?

Abstract Two newly emerging materials for application in all‐solid‐state batteries, namely, single‐crystalline Ni‐rich layered oxide cathode and halide solid electrolyte (SE), are of utmost interest because of their superior properties (good microstructural integrity and excellent electrochemical oxidation stability, respectively) to conventional polycrystalline layered oxides and sulfide SEs. In this work, four electrodes employing single‐ or polycrystalline LiNi 0.88 Co 0.11 Al 0.01 O 2 (NCA) and Li 3 YCl 6 or Li 6 PS 5 Cl 0.5 Br 0.5 are rigorously characterized by complementary analyses. It is shown that the synergy of employing cracking‐free single‐crystalline NCA and oxidation‐tolerable Li 3 YCl 6 can be achieved by considering intercoupled engineering factors that are prone to overlook, such as size, lightness, and mixing of particles. Accordingly, the highest level of performances in terms of discharge capacity (199 mA h g −1 at 0.1C), initial Coulombic efficiency (89.6%), cycling performance (96.8% of capacity retention at the 200th cycle), and rate capability (130 mA h g −1 at 4C) are demonstrated at 30 °C. Severe side reactions occurring at the Li 6 PS 5 Cl 0.5 Br 0.5 /NCA interfaces are also quantified and probed. Importantly, an overlooked but significant contribution of the side reaction of Li 6 PS 5 Cl 0.5 Br 0.5 to the detrimental electrochemo‐mechanical degradation of polycrystalline NCA is revealed for the first time by postmortem scanning electron microscopy and operando electrochemical pressiometry measurements.