Orientation‐Dependent Arrangement of Antisite Defects in Lithium Iron(II) Phosphate Crystals

Abstract

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. [1][2][3] The intentional use of impurities in semiconductors [1] and the formation of ionic vacancies in ion-conducting metal oxides [2] 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. [4] 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.[5] In contrast, few experimental details showing the atomic-scale point defects in olivine-type lithium metal phosphates LiMPO 4 (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.[6] As illustrated in Figure 1a, the lithium and the metal (M) ion in LiMPO 4 having an ordered olivine structure occupy different octahedral inter-

Advanced analytical electron microscopy for lithium-ion batteries

“…Although the amount of the anti-site defects can be easily determined using neutron diffraction, the distribution can be studied only using atomic-resolution annular BF-STEM imaging, which has an ultrahigh spatial resolution and is also sensitive to light elements such as lithium. 9,10 With the aid of this technique, Chung et al 33 discovered the localized aggregation behavior of Fe Li . 34 On the basis of their observations, a vacancy-driven mechanism was proposed for the formation of the localized defect segregation (Figure 3a).…”

Cation exchange mediated elimination of the Fe-antisites in the hydrothermal synthesis of LiFePO4

“…After Graetz's in-situ X-ray diffraction studies of LiFePO 4 antisite defects, [18,19] Iversen and co-workers demonstrated using neutron scattering [20] that the antisite defects are mainly represented by Fe-ions occupying M1 sites, without any presence of Li-ions in M2 sites. Other studies sought to clarify and control the role of antisites, for example by using vanadium [21,22] or niobium dopants [23], thermal annealing [24] or first principle calculations [25,26]. Fe 2 + ions occupying the M1 sites [23,27] represent an obstacle to the diffusion of lithium ions, and therefore cause degradation of the electrochemical performance.…”

“…Other studies sought to clarify and control the role of antisites, for example by using vanadium [21,22] or niobium dopants [23], thermal annealing [24] or first principle calculations [25,26]. Fe 2 + ions occupying the M1 sites [23,27] represent an obstacle to the diffusion of lithium ions, and therefore cause degradation of the electrochemical performance.…”

Vacancy-Driven Anisotropic Defect Distribution in the Battery-Cathode MaterialLiFePO4

“…Recently, Chung et al reported that iron antisite defects (Fe Li ) form preferentially in only a few channels along the b axis instead of being homogeneously distributed in the lattice [5]. This kind of segregation is important, because only a few diffusion channels are blocked, leaving most of them available for Li diffusion.…”