Mechanisms of manganese spinels dissolution and capacity fade at high temperature

Tests on the stability of spinels stored at different states of charge were performed. It was found that Mn dissolution took place irreversibly from the charged state with formation of MnF2, ramsdellite–Li0.5MnO2. In the discharged state, the Mn at the surface of the LiMn2O4 was in equilibrium with a soluble species, which leads to an increase in the cathode resistance. It was found that the amount of the dissolved Mn increased with increasing LiPF6 concentration and that 2 equiv. of PO2F2− were generated. Concurrent formation of carbonate decomposition products, such as CO2, C2H4, EtOH, Et2O, and AcOEt, suggests that the mechanism of Mn dissolution from the charged state was composed of the following steps: the initial decarboxylation of electrolyte catalyzed by λMnO2 to give EtOH, followed by reduction of λMnO2 by EtOH to give acetate and MnO, and then Mn dissolution via Mn---O bond activation assisted by LiPF6.