Revealing the Origin of Transition‐Metal Migration in Layered Sodium‐Ion Battery Cathodes: Random Na Extraction and Na‐Free Layer Formation

Abstract Cation migration often occurs in layered oxide cathodes of lithium‐ion batteries due to the similar ion radius of Li and transition metals (TMs). Although Na and TM show a big difference of ion radius, TMs in layered cathodes of sodium‐ion batteries (SIBs) can still migrate to Na layer, leading to serious electrochemical degeneration. To elucidate the origin of TM migration in layered SIB cathodes, we choose NaCrO 2 , a typical layered cathode suffering from serious TM migration, as a model material and find that the TM migration is derived from the random desodiation and subsequent formation of Na‐free layer at high charge potential. A Ru/Ti co‐doping strategy is developed to address the issue, where the doped active Ru is first oxidized to create a selective desodiation and the doped inactive Ti can function as a pillar to avoid complete desodiation in Ru‐contained TM layers, leading to the suppression of the Na‐free layer formation and subsequent enhanced electrochemical performance.