Structural and electrochemical investigation of NA⁺ insertion into high-voltage spinel electrodes

The emerging interest in Na⁺-ion batteries should increase the relative importance of higher voltage positive electrodes as a response to the lower voltage imparted by the Na/Na⁺ redox reaction relative to that of Li/Li⁺. The 4.7 V LiMn_1.5Ni_0.5O₄ spinel is already an attractive candidate for high-voltage lithium-ion positive electrodes, and its host structure, -Mn_0.75Ni_0.25O₂, could be of even greater importance for Na batteries. The electrochemical and structural characteristics of higher-voltage Na_xMn_1.5Ni_0.5O₄ relative to Na_xMn₂O₄ during ion insertion are investigated for the first time. High-resolution electrochemistry, coupled with in situ and ex situ X-ray diffraction, is utilized to provide initial insights into the mechanism of Na⁺ insertion. Distinct electrochemical challenges brought forth by Na⁺ ion insertion into the spinel structure are also discussed in detail.