Oxygen Dimerization-Driven Cation Migration Induces Voltage Hysteresis in Disordered Rocksalt Cathodes

Despite the potential to increase the energy limit of Li-rich cathodes by using oxygen redox, its practicality has been limited by the accompanying structural changes and voltage hysteresis. While voltage hysteresis is commonly associated with transition metal (TM) migration and oxygen dimerization, the specific contribution of each is unclear. We provide a mechanistic insight into how each of these changes induces hysteresis in a representative Li-rich disordered rocksalt cathode, Li_1.2Mn_0.4Ti_0.4O₂. We reveal that the formation and cleavage of oxygen dimers can occur exceptionally rapidly during the electrochemical process, suggesting that the dimerization process is not directly the cause of voltage hysteresis, contrary to prevailing arguments. Instead, oxygen dimers are found to indirectly exacerbate hysteresis by instigating TM migration, which leads to the evolution of dimer-rich and TM-rich regions within the structure. We demonstrate that TM migration is relatively slower than dimerization and as such contributes to hysteresis by dissipating internal energy during the relaxation of charged electrodes and by inducing cation rearrangement with each cycle.