Electric states of spinel LixMn2O4 as a cathode of the rechargeable battery

We calculated the electric states of spinel LixMn2O4 using the discrete variational Xa cluster method. The magnitude of the distortion of the LixMn2O4 lattice due to the Jahn-Teller effect is in good agreement with the experimental result. Moreover, our calculated results suggest that the critical valency of the manganese ion in the distortion of the LixMn2O4 is 3.5, which is consistent with the experimental result that the tetragonal Li2Mn2O4 phase appears at x = 1 of LixMn2O4. The dependence of the open-circuit voltage on x was also calculated. The change in the open-circuit voltage in the region of 0 less than or equal to x less than or equal to 1 is 0.52 V and the drastic change in the open-circuit voltage at x = 1 is 0.84 V. These calculated results are in good agreement with the observed open-circuit voltage. We conclude that the drastic change in the open-circuit voltage at x = 1 is due to the sum of the contribution of the valency-change of manganese ions (Mn-3.5 + to Mn3+) during the phase separation, 0.52 V, and the contribution of the occupancy of lithium ions in 16c sites, 0.32 V. The repulsive electrostatic force between a lithium ion in a 16c site and two lithium ions in 8a sites has no influence on the energy level of Mn 3d orbitals.