Simultaneously Modulating Li+ Transport Kinetics and Structural Stability in Li‐Rich Mn‐Based All‐Solid‐State Electrodes by Dual‐Scale Particle Engineering

Li‐rich Mn‐based oxides (LRMOs) are promising cathodes for all‐solid‐state lithium batteries (ASSLBs) due to their high theoretical capacity. However, their practical application is hindered by sluggish Li + transport and interfacial instability. Herein, it is demonstrated that primary and secondary particle sizes of LRMOs play crucial roles in influencing Li + transport kinetics and interfacial stability. With a fixed primary particle size (0.1 μm), large secondary particles (≈10 μm) impede Li + transport by creating tortuous transport paths and inducing stress‐induced cracks. Reducing the secondary particle size enhances Li + transport kinetics; however, excessively small secondary particles (≈1 μm) lead to poor point‐contact geometry at the LRMO/solid‐state electrolyte (SSE) interface and increased oxygen release, triggering phase transformation and SSE oxidation, which further obstructs Li + transport. An optimal secondary particle size of ≈5 μm provides a balance between Li + transport efficiency and interfacial structural integrity. Furthermore, increasing the primary particle size to ≈0.46 μm reduces grain boundary resistance, enhancing Li + transport and minimizing side reactions. This dual‐scale optimization results in a high capacity of 200.2 mAh g −1 at 0.05 C and excellent cycling stability with 67.4% capacity retention after 500 cycles at 0.3 C, highlighting the importance of dual‐scale particle engineering for LRMO‐based ASSLBs.