Suppression of Mn-Ion-Dissolution of LiNi0.5 Mn1.5 O4 Electrodes in a Highly Concentrated Electrolyte Solution at Elevated Temperatures

Mn-based active materials, such as LiMn2O4, are widely used for positive electrodes in lithium ion batteries, and spinel LiNi0.5Mn1.5O4 is drawing much attention to realize 5-V class batteries. However, the oxidative decomposition of electrolyte solution at high voltages and Mn-dissolution of LiNi0.5Mn1.5O4 are serious problems to be solved. These two drawbacks are more marked at elevated temperatures, and should be caused by free solvent molecules in electrolyte solution. In this study, highly concentrated electrolyte solution, which contains few free solvent molecules, was investigated to solve the problems. LiNi0.5Mn1.5O4 electrodes worked at 50oC in nearly saturated 7.25 mol kg−1 LiBF4/ propylene carbonate (PC) electrolyte solution, whereas not in the nearly saturated 4.30 mol kg−1 LiPF6/PC. In addition, Mn-ion dissolution from LiNi0.5Mn1.5O4 was significantly suppressed in highly concentrated electrolyte solutions, and correlated to the fraction of free PC molecules in them.