Improving cycling performance of the NaNiO₂ cathode in sodium-ion batteries by titanium substitution

AbstractO3-type layered oxide cathodes, such as NaNi_0.5Mn_0.5O₂, have garnered significant attention due to their high theoretical specific capacity while using abundant and low-cost sodium as intercalation species. Unlike the lithium analog (LiNiO₂), NaNiO₂ (NNO) exhibits poor electrochemical performance resulting from structural instability and inferior Coulomb efficiency. To enhance its cyclability for practical application, NNO was modified by titanium substitution to yield the O3-type NaNi_0.9Ti_0.1O₂ (NNTO), which was successfully synthesized for the first time via a solid-state reaction. The mechanism behind its superior performance in comparison to that of similar materials is examined in detail using a variety of characterization techniques. NNTO delivers a specific discharge capacity of ∼190 mAh g^-1 and exhibits good reversibility, even in the presence of multiple phase transitions during cycling in a potential window of 2.0‒4.2 V vs. Na⁺/Na. This behavior can be attributed to the substituent, which helps maintain a larger interslab distance in the Na-deficient phases and to mitigate Jahn-Teller activity by reducing the average oxidation state of nickel. However, volume collapse at high potentials and irreversible lattice oxygen loss are still detrimental to the NNTO. Nevertheless, the performance can be further enhanced through coating and doping strategies. This not only positions NNTO as a promising next-generation cathode material, but also serves as inspiration for future research directions in the field of high-energy-density Na-ion batteries.