Near‐Micron Single‐Crystalline Mn‐Rich Cathodes With Exceptional Cycling Stability

ABSTRACT Achieving high‐energy‐density and long‐term cycling stability in Mn‐redox‐based cathodes is a critical step toward advancing Ni‐ and Co‐free Li‐ion chemistries to diversify supply chains and lower battery costs. Here, we demonstrate an unusually high performance benchmark for Mn‐redox cathodes using near‐micron single‐crystalline δ‐phase cation‐disordered rocksalt (δ‐DRX), derived from Mn‐rich DRX, delivering ≈640 Wh kg −1 with ≈97% capacity retention over 400 cycles. The very high energy density and cycle life were achieved by using a high Mn content, to suppress oxygen redox activity, and careful control of the synthesis parameters to tune single crystal particle size. Using high‐throughput automated electron microscopy, we identify interparticle compositional heterogeneity present in the early stage of phase‐pure DRX formation as the origin of orthorhombic LiMnO 2 impurity formation during cooling, a finding that rationalizes the long high‐temperature dwell times required and the associated particle coarsening in conventional synthesis. We demonstrate that an accelerated cooling strategy kinetically suppresses localized impurity formation, reducing long high‐temperature exposure, and enables downsizing of single‐crystalline Mn‐rich DRX particles to the near‐micron regime (D50 = 1.5 µm). This work provides a mechanistic basis for particle‐size control in Mn‐rich DRX synthesis, and presents δ‐DRX as a stable, sustainable, high‐performance, Ni‐ and Co‐free cathode material.