Enhanced ion intercalation in Ni _x K_1-2x TiNbO₅ enabled by redox active Ni exchange for potassium-ion batteries

The ultimate goal of potassium-ion batteries (KIBs) is to become a serious competitor to lithium-ion batteries (LIBs). Achieving this requires the development of high energy density negative electrode materials, with transition metal oxides emerging as the most promising candidates. However, despite their high theoretical capacities, most transition metal oxides still struggle to achieve high performance, often necessitating substantial nanostructuring. Ion-exchange presents a facile and effective process for enhancing material properties, yet the demonstration of the exchanged ions undergoing redox activity has not been previously reported for KIBs. Herein, this work reports Ni_0.25K_0.5TiNbO₅, synthesized through the ion-exchange between K⁺ and Ni²⁺, as a novel negative electrode material for KIBs. The ion-exchanged material achieves a specific capacity of 304 mAh g^-1 in the first cycle and 162 mAh g^-1 after 10 cycles, corresponding to a 240% and 156% increase compared to the pristine, unexchanged KTiNbO₅ at the same cycle numbers. The structure-performance relationship was investigated in detail, shedding light on the previously unknown relationships between the level of hydration, degree of exchange and the performance of ion-exchanged materials. Furthermore, the exchanged Ni was demonstrated to be reversibly redox active, contributing to the observed capacity and representing a first for ion-exchanged materials in the KIB literature.