Magnetic measurements as a viable tool to assess the relative degrees of cation ordering and Mn3+ content in doped LiMn1.5Ni0.5O4 spinel cathodes

Abstract

The magnetic properties of the doped high-voltage spinels LiMn_1.5Ni_0.42M_0.08O₄ (M = Cr, Fe, Co, Cu, Al, and Ga) as well as the Mn-rich spinels LiMn_1.5+xNi_0.5−xO₄ (x = 0, 0.05, 0.1) synthesized by a tank-reactor coprecipitation method have been investigated. It is found that the Curie temperatures correlate well with the degree of cation ordering determined by measuring the capacity value at ∼2.7 V, which corresponds to the insertion of additional lithium into the empty 16c octahedral sites of the spinel lattice. Additionally, it is found that the saturation magnetization near 0 K provides a reliable means of calculating the Mn³⁺ content in each sample based on the predicted magnetic exchange interactions of the spinel structure. The magnetic results match well with the capacity values obtained in the 4 V region of the samples when assembled as cathodes in lithium-ion cells. The Mn-rich spinels offer insight into the relationship between Mn³⁺ content and degree of cation ordering and how they affect the magnitude of the capacity in the ∼2.7 V region. The effects of Mn³⁺ content and cation ordering were isolated by examining the relationship among lattice parameter, Mn³⁺ content, capacity contribution at ∼2.7 V, and Curie temperature. Thus, it is posited that the Curie temperature is a reliable qualitative means of measuring the degree of cation order in a variety of doped or Mn-rich spinel oxides.