Structure and electrochemical properties of LiMnO_4 thin films

摘要

The structural and electrochemical properties of LiMnO_4 films depend upon the deposition and annealing conditions. Films which were deposited by rf magnetron sputtering of LiMn_2O_4 in Ar+N_2 gas mixtures and annealed in O_2 at temperatures between 400 to 1000 deg C had the cubic spinel structure with an a-axis length that increased linearly from 8.13 to 8.25 A with increasing anneal temperature. Thin-film lithium cells with cathodes of different a-axis lengths exhibited marked differences in their voltage profiles. In particular, the ratio of the capacities at 4 V and 3 V increased with the a-axis length. A defect model of LiMn_2O_4 which is consistent with the structural and electrochemical data is represented by (Li_1-y+zMn_y-z~2+)_8a(Mn_z~2+)_16c(LI_xMn_2-x)_16dO_4. Based on this model and the results of in-situ XRD measurements, it is proposed that Mn ions migrate from 8(a) tetrahedral sites to 16(c) octahedral sites on charging the cells in the 5V plateau.