Materials with a disordered rocksalt (DRX) structure have been the focus of much recent work in the field of lithium-ion battery (LIB) cathode materials as they can make use of a variety of earth-abundant elements and have relatively high theoretical capacities based on transition metal (TM) and oxygen redox. These lithium transition metal oxides with general formula Li_1+xTM_1-xO₂ (0<x<0.3) are named for the random or disordered arrangement of lithium and transition metal cations across the cation sites of the rocksalt structure.¹

However, the disordered phase of these materials can be challenging to obtain as often a more ordered phase is thermodynamically favoured. One approach to favour disorder is to include transition metal ions with no d-electrons (d⁰ ions) within the structure which helps by accommodating octahedral distortions. As well as the inclusion of d⁰ ions, high temperature or high-energy ball milling are often required to favour the pure disordered rocksalt phase.^2,3 Computational studies have used phase diagrams to investigate the energy required to favour certain DRX phases⁴ but in practice it can take several optimisations of the procedure to obtain a pure rocksalt phase.

This work will focus on the effect of synthetic parameters such as precursor type and particle size, temperature, reaction times and atmosphere in the obtaining of Li-excess disordered rocksalt materials. These DRX materials contain Ni²⁺ ions as redox-active element and Ti⁴⁺ or Nb⁵⁺ as d⁰ ions. Ni²⁺-based DRXs are of interest for LIB cathodes as they are relatively stable in air and have the possibility of accessing 2-electron Ni^2+/4+ redox upon cycling. Techniques including high-temperature X-ray diffraction (HT-XRD) (Figure 1) are used to study the phase changes taking place during calcination while techniques such as scanning electron microscopy (SEM) are used to compare particle size and morphology.

Figure 1: High-temperature X-ray diffraction measurement monitoring calcination in the Li_1.2 Ni_0.2Ti_0.6O₂ disordered rocksalt synthesis.