Synthesis of MnO2 Phases from LiMn2 O 4 in Aqueous Acidic Media: Mechanisms of Phase Transformations, Reactivity, and Effect of Bi Species

Abstract

The nature of the phases obtained by acid digestion of LiMn2O4 phases prepared at 800 degrees C from a mixture of MnO2 (EMD) and Li2CO3 was investigated. We found that the complete transformation toward alpha-MnO2 and then gamma-MnO2 observed for LiMn2O4 treated in 2.5 M H2SO4 for 24 h at 95 degrees C is highly dependent on the amount of water in the reaction medium. The lambda --> alpha/gamma transformation was found to be the result of a dissolution-crystallization mechanism that can be completely avoided by adding a soluble Bi, Pb, or Tl salt to the reaction medium. By coupling energy dispersive spectroscopy analysis, infrared spectroscopy, and potentiometric titration, we demonstrated the presence of Bi species adsorbed at the surface of the lambda-MnO2 oxide thus modifying its reactivity. In addition, the kinetics of the lambda --> alpha/gamma phase transformation was found to depend on the amount of added Bi salt, suggesting the complexing role of Bi toward Mn (Bi-Mn complexes), thereby affecting the crystallization step of the reaction. The same treatment was applied to LiMn2O4 in the presence of a Bi salt in anhydrous electrolyte (LiPF6/ethylene carbonate/dimethyl carbonate). In this case, the spinel oxide dissolution slows down and BiF3 precipitates. With respect to recent findings about the mechanisms involved in the electrochemical capacity failure at elevated temperature in Li-ion LiMn2O4 cells, these results open new alternatives to solve this recurrent problem.