Role of Disproportionation in the Dissolution of Mn from Lithium Manganate Spinel

Dissolution of Mn from lithium manganate spinel has hindered its commercialization as a cathode material in Li-ion batteries. This dissolution has been widely attributed to disproportionation of near-surface Mn(III), in the presence of acid, with the resultant divalent Mn being vulnerable to dissolution. To what extent moisture in the cell (as opposed to the organic electrolyte) acts as the solvent for Mn ions has not been established. Simulations by Leung show that a small displacement of trivalent Mn from its equilibrium site at an LiMn2O4(001)/ethylene carbonate interface leads to its reduction to Mn(II). In the present work, thermodynamic integration is performed based on first-principles molecular dynamics simulations within the Blue-Moon ensemble to investigate the detachment of Mn ions at the LiMn2O4(001)/water interface. The results show that reduction of Mn(III) to Mn(II) occurs also in the case of an aqueous interface. The simulations were performed for both neutral and acidic water (in the presence of HF) with the coordination number of the dissolving Mn ion (to substrate oxygen ions) taken as the reaction coordinate. The simulations indicate that an F- ion strongly binds to a surface Mn(III) ion and weakens its adhesion to the substrate. Owing to this weakening, a surface MnF complex traverses regions of phase space at room temperature where disproportionation becomes energetically favorable. Although this disproportionation occurs close to the substrate where the Mn coordination number is only slightly lowered from its equilibrium value, we argue that the likelihood of reattachment after disproportionation is small (Leung arrived at a similar interpretation in the case of the LiMn2O4(001)/EC interface). We suggest that the "critical role of F- in promoting dissolution is to weaken the Mn binding to the substrate to enable disproportionation. The partially detached MnF complex may then undergo additional interaction with the solvent to form, e.g., MnF2, which would enable transport away from the substrate. The electron paramagnetic resonance measurements of Shilina et al. that appear to show Mn(III) as the predominant solvated species are discussed.