Grain Welding Strategy Suppressing Chemomechanical Failure in Polycrystalline Cathode for High‐Voltage Fast‐Charging Sodium‐Ion Batteries

Abstract Chemomechanical failure is critical bottleneck for Na‐based layered oxides (especially polycrystalline material) to achieve the stable operating at high‐voltage with fast‐charging ability. Although attempts like coating and doping are presented to alleviate this issue, the inherent intergranular crevices are inevitable during the synthetic process, which facilitates the chain reaction of chemomechanical failure. Herein, a concept of grain welding strategy is presented to achieve densification of powder particle by introducing liquid phase sintering process, strengthening intergranular adhesion force. The designed polycrystalline NaLi 0.05 [Ni 1/3 Fe 1/3 Mn 1/3 ] 0.95 O 2 shows dense structure attributed to the formation of eutectics which lowers the melting point with fast mass transfer. The Li doping also well manipulates the interlayer properties in unit cell, suppressing the high‐voltage phase transition. Multiple characterizations like cross‐sectional images and synchrotron‐based full‐field transmission X‐ray microscopy confirm that the modified material suppresses the generation of microcracks during electrochemical cycles, impeding the chemomechanical breakdown. Consequently, it shows advanced capacity of 152.8 mAh g −1 , excellent fast charging capability (121.6 mAh g −1 at 5 C, 12 min), as well as long‐life of 1000 cycles.