Orientation-dependent arrangement of antisite defects in lithium iron(II) phosphate crystals.

The distribution and local concentration of point defects in crystal lattices such as dopants and atomic vacancies have been recognized as significant factors that govern the overall electrical and optical properties of inorganic crystals. The intentional use of impurities in semiconductors and the formation of ionic vacancies in ion-conducting metal oxides are well-known examples of displaying the correlation between atomic-scale chemical variations and resulting physical properties. Furthermore, as the chemically different environment induced by point defects leads to breaking of the ordered arrangement of atoms in crystals, mass and charge transport behaviors are also considerably affected by the presence of the defects. In many lithium intercalation compounds, an ordered array of lithium is usually maintained. Therefore, the control of point defects, including cation disorder, is of major significance for application to electrodes in rechargeable batteries. A variety of investigations on lithium vacancies and cation intermixing have been reported for layered oxides. In contrast, few experimental details showing the atomic-scale point defects in olivine-type lithium metal phosphates LiMPO4 (where M = Fe, Mn, Ni, Co), are yet available, although these phosphates have attracted a great deal of attention as alternative cathode materials in lithium-ion batteries over the past decade. As illustrated in Figure 1a, the lithium and the metal (M) ion in LiMPO4 having an ordered olivine structure occupy different octahedral inter-