A Solid‐State Battery Cathode with a Polymer Composite Electrolyte and Low Tortuosity Microstructure by Directional Freezing and Polymerization

Abstract Solid‐state Li metal batteries (SSLMBs) combine improved safety and high specific energy that can surpass current Li ion batteries. However, the Li⁺ ion diffusivity in a composite cathode—a combination of active material and solid‐state electrolyte (SSE)—is at least an order of magnitude lower than that of the SSE alone because of the highly tortuous ion transport pathways in the cathode. This lowers the realizable capacity and mandates relatively thin (30–300 μm) cathodes, and hence low overall energy storage. Here, a thick (600 μm) hybrid cathode comprising vertically aligned LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811)‐rich channels filled with a [LiTFSI+PEGMA+MePrPyl TFSI] polymer composite electrolyte is fabricated by an innovative directional freezing and polymerization method. X‐ray micro‐computed tomography, ion mobility simulations, and DC depolarization show that the cathode structure improves Li⁺ ion diffusivity in the cathode from 4.4 × 10⁻⁹ to 1.4 × 10⁻⁷ cm² s⁻¹. In a SSLMB full cell at 25 ^oC, the cathode provides gravimetric capacities of 199 and 120 mAh g⁻¹, and ultra‐high areal capacities of 16.7 and 10.1 mAh cm⁻² at 0.05 and 1 C, respectively. The work demonstrates a scalable approach to realizing composite cathode structures with kinetically favorable ion transport characteristics in SSLMBs.