Simulation of Aqueous Dissolution of Lithium Manganate Spinel from First Principles

Constrained density functional theory at the GGA+U level, within the Blue Moon ensemble, as implemented in the VASP code, is applied to simulate aqueous dissolution of lithium manganate spinel, a candidate cathode material for lithium ion batteries. Ions are dissolved from stoichiometric slabs of composition LiMn₂O₄, with orientations (001) and (110), embedded in a cell with 20 Å water channels between periodically repeated slabs. Analysis of the Blue Moon ensemble forces for dissolution of Li, Mn, and O ions from lithium manganate indicate that bond breaking occurs sequentially, ordered from weak to strong bonds, where bond breaking occurs when a bond length is stretched about 50% relative to its equilibrium value. Substrate ions are displaced to maintain bond lengths close to equilibrium for bonds other than that the one being broken. The predicted free energies required to break the chemical bonds with the LiMn₂O₄ substrate are Mn³⁺, 1.4; O^2–, 1.0; Mn²⁺, 0.8; and Li⁺, 0.35, in eV; an existing experimental measurement (Lu, C. H.; Lin, S. W. J. Mater. Res.2002, 17, 1476) had yielded an effective dissolution activation energy of 0.7 eV. A mechanism for the role of acid in promoting lithium manganate dissolution is discussed.