The formation of oxygen vacancies and Mn 3+ is a dominant feature when LMNO is synthesized at a more O-poor (n-type) condition. The electrochemical performance of LMNO synthesized under those conditions can be improved due to the increase in cation disorder and the proportion of Mn 3+ . 12 While Mn 3+ species is redox active which improves the electronic conductivity, it is subject to JT distortion and promotes Mn dissolution. Therefore, [Mn 3+ ] should be controlled to achieve optimum performance. Ni-rich rock salt impurity phases are also likely to form under n-type conditions. This is not desirable as it leads to a decrease in capacity.