Design Grain Boundary Strengthening Phase for Enhanced Stability and Ionic Conductivity in Nickel-Rich Cobalt-Free High-Entropy Layered Oxide Cathodes

High-entropy nickel-rich cobalt-free layered cathode materials have garnered attention for their high energy density in lithium–ion batteries, thanks to their impressive reversible capacity and enhanced structural stability. However, their thermodynamic stability is challenged by kinetic inhomogeneity during cycling, leading to strain mismatches and failure at grain boundaries. To address this, we propose a grain boundary engineering strategy that introduces a strengthening phase with both high mechanical strength and ionic conductivity. This phase reinforces grain boundaries by creating strong chemical bonds, buffering stresses, and facilitating rapid lithium–ion transport. As a result, our modified cathode exhibits a remarkable 1.75-fold increase in capacity retention over 400 cycles, and a pouch cell with 14.17 Ah capacity and 361.40 Wh kg–1 energy density shows stable cycling for more than 200 cycles. These findings highlight the potential of grain boundary engineering in high-entropy materials for developing next-generation cathodes for high-energy-density batteries.