Theoretical insight into charging process in a Li3PO4 (100)/LiFePO4 (010) coherent interface system

We have investigated the introduction of Li vacancy (VLi× in the Kröger-Vink notation) into a Li3PO4 (100)/LiFePO4 (010) coherent interface with the density functional theory in order to gain an insight into the initial stage of the charging process. The VLi× introduction results in the formation of a Li ion vacancy (VLi′) and hole (h). When one VLi× is introduced in each bulk LiFePO4 and Li3PO4 materials (both VLi′ and h are donated to the position in the proximity of the initial VLi× position), the formation energies of the Li vacancy (EV) are quite different between them. On the other hand, when introduced into the Li3PO4/LiFePO4 interface system, the values of EV in the LiFePO4 and Li3PO4 bulk regions of the interface become almost equal. This is because irrespective of the introduced VLi× position, the associated h is always donated to the LiFePO4 region while the Li3PO4 region keeps its insulating properties. Although the Li atoms near the interface have smaller EV than in the bulk regions, it is suggested that only a fraction of them may be extracted at the initial stage of charging which is not enough to lead to the Li depletion at the interface and as a result the effect of the space charge layer may be negligible in the Li3PO4/LiFePO4 interface.