Theoretical insight into charging process in a Li sub(3)PO sub(4) (100)/LiFePO sub(4) (010) coherent interface system

• Published in: Solid state ionics 2016-02, Vol.285, p.59-65
• Main Authors: Sumitaa, Masato, Tanakab, Yoshinori, Ikedaa, Minoru, Ohnoa, Takahisa
• Format: Article
• Language: English
• Subjects: Charging; Coherence; Density functional theory; Energy of formation; Formations; Gain; Lithium; Vacancies
• ISSN: 0167-2738
• DOI: 10.1016/j.ssi.2015.09.029

We have investigated the introduction of Li vacancy (V sub(Li) super() in the Kroger-Vink notation) into a Li sub(3)PO sub(4) (100)/LiFePO sub(4) (010) coherent interface with the density functional theory in order to gain an insight into the initial stage of the charging process. The V sub(Li) super() introduction results in the formation of a Li ion vacancy (V sub(Li)') and hole (h). When one V sub(Li) super() is introduced in each bulk LiFePO sub(4) and Li sub(3)PO sub(4) materials (both V sub(Li)' and h are donated to the position in the proximity of the initial V sub(Li) super() position), the formation energies of the Li vacancy (E sub(V)) are quite different between them. On the other hand, when introduced into the Li sub(3)PO sub(4)/LiFePO sub(4) interface system, the values of E sub(V) in the LiFePO sub(4) and Li sub(3)PO sub(4) bulk regions of the interface become almost equal. This is because irrespective of the introduced V sub(Li) super() position, the associated h is always donated to the LiFePO sub(4) region while the Li sub(3)PO sub(4) region keeps its insulating properties. Although the Li atoms near the interface have smaller E sub(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 Li sub(3)PO sub(4)/LiFePO sub(4) interface.