We have investigated the introduction of Li vacancy (V-Li(x) in the Kroger-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 V-Li(x) introduction results in the formation of a Li ion vacancy (V-Li') and hole (h). When one V-Li(x), is introduced in each bulk LiFePO4 and Li3PO4 materials (both V-Li' and h are donated to the position in the proximity of the initial V-Li(x) position), the formation energies of the Li vacancy (E-v) 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 V-Li(x) 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 E-v 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. (C) 2015 Elsevier B.V. All rights reserved.