Optimal Ti-Substitution in Layered Oxide Cathodes for Na-Ion Batteries

Abstract

Sodium layered oxides Na_xMO₂ (x ≤ 1 and M = transition metal ions) gain interest as sodium-ion battery (NIB) cathodes due to their high energy density and cost-effectiveness. The nature of transition metal ions (M) defines the material properties, and the substitution of M with redox inactive Ti⁴⁺ is often seen as beneficial in reducing phase transitions during cycling and thus improving the cycle life. In this respect, our present study focuses on understanding the origin of this improvement by studying the highly substituted P2 Na_0.67Ni_0.30Zn_0.03Mn_0.67–yTi_yO₂ (0 ≤ y ≤ 0.67) phases based on their electrochemical performance combined with structural analyses and DFT calculations. The results indicate that Ti⁴⁺, by increasing the M–O bond ionicity, disrupts the Na⁺-vacancy ordering at lower voltages (<4 V, until ∼60% SOC) and reduces the participation of O 2p in the redox process, thereby suppressing Na-removal and the extent of P2–O2 phase transition at high voltages. We show that this effect becomes maximum for y = 0.52 (P2 Na_0.67Ni_0.30Zn_0.03Mn_0.15Ti_0.52O₂) and beyond, for which we observe a nearly solid-solution-like behavior of the P2-type structure. However, the d⁰ Ti⁴⁺ is prone to cation migration leading to poor structural reversibility as observed from operando XRD analyses, making the highly Ti⁴⁺-substituted material less suitable for practical applications. An optimum ratio of y = 0.3 (Na_0.67Ni_0.3Zn_0.03Mn_0.37Ti_0.3O₂) is beneficial for the cycle life as well as rate capability, and the study points to the importance of carefully selecting transition metal combinations in the finest ratio to achieve the best performing sodium layered oxide electrode materials.