Changes in Mn3+/Mn4+ ratio, resistance values in electrochemical impedance spectra, and rate capability with increased lithium content in spinel LixMn2O4

Spinel-structured LixMn2O4 (x = 1.0, 1.02, 1.04, and 1.06) cathode materials have been synthesized by a solid-state method using Mn3O4 and Li2CO3 as raw materials. The Mn4+ content gradually decreases from 84.80% to 82.62% and then increases from 82.62% to 88.98%, when the Li content increases from 1.00 to 1.06. The rate capability of the LiMn2O4 samples evidently increases as the lithium content increases from 1.00 to 1.06. Electrochemical impedance spectroscopy results indicate that the Rct values gradually increase from 58.33 Ω to 133.1 Ω and then decrease from 133.1 Ω to 68.79 Ω when the Li concentration increases from 1.00 to 1.06. The Rsf values also increase from 25.67 Ω to 37.84 Ω and then decrease from 37.84 Ω to 16.69 Ω, when the Li concentration increases from 1.00 to 1.06. These changes in Mn4+ content, Rsf, and Rct with the increase in Li content are related to changes in the second phase in LiMn2O4.