Origin of Site Disorder and Oxygen Nonstoichiometry in LiMn1.5Ni0.5-xMxO4 (M = Cu and Zn) Cathodes with Divalent Dopant Ions

The high-voltage LiMn1.5Ni0.5O4 spinel cathodes offer high power capability but are plagued by capacity fade, particularly at elevated temperatures. With an aim to develop a better understanding of the factors influencing the electrochemical properties, a systematic investigation of LiMn1.5Ni0.5-xMxO4 (M = Cu and Zn and x = 0.08 and 0.12), in which Ni2+ ions are substituted by divalent Cu2+ and Zn2+ ions, are presented. The effects of dopant ion content, their site occupancy, degree of cation ordering, and oxygen nonstoichiometry have been investigated with respect to the electrochemical properties by cycling at room temperature and elevated temperatures. It is found that although both Zn and Cu are divalent with ionic radii similar to those of Ni2+, they behave quite differently with respect to cation ordering and site occupancy, and higher levels of doping lead to distinct differences in cycling and rate performances. The understanding of the role of site preferences of the cations and the degree of cation ordering could help to develop high-performance high-voltage spinel cathode compositions.