Surface Chemistry Consequences of Mg-Based Coatings on LiNi0.5Mn1.5O4 Electrode Materials upon Operation at High Voltage

LiNi0.5Mn1.5O4 epitomizes the challenges imposed by high electrochemical potential reactivity on the durability of high energy density Li-ion batteries. Postsynthesis coatings have been explored as a solution to these challenges, but the fundamentals of their function have not been ascertained. To contribute to this understanding, the surface of LiNi0.5Mn1.5O4 microparticles was modified with Mg2+, a coating component of literature relevance, using two different heat treatment temperatures, 500 and 800 °C. A combination of characterization tools revealed that Mg2+ was introduced mainly as an inhomogeneous MgO coating in the sample treated at 500 °C, and into the spinel lattice at the subsurface of the particles at 800 °C. Comparing the properties of these two different materials with an unmodified baseline afforded the opportunity to evaluate the effect of varying surface chemistries. Coulometry in Li metal half cells was used as a macroscopic measure of side reactions at the electrode–electrolyte interfaces. This magnitude was comparable in all the materials at room temperature. In contrast, a significant drop in efficiency was observed in the untreated material when the cycling temperature was raised to 50 °C, but not in the modified materials. The origin of the reduced reactivity of the materials after introducing Mg-based modifications was evaluated by probing the chemical changes at the Ni–O bonds using soft XAS. Taken together, the results of this study revealed that incorporation of Mg stabilizes highly oxidized Ni–O species, which can be related to the better stability toward the electrolyte. They point to a pathway toward the guided design of efficient surface modifications to yield battery electrode materials with increased stability against the electrolyte.