Study of Antisite Defects in LiCoPO4 Cathode Material and Their Change during Initial Charge/Discharge Cycle

Antisite cation exchange is the typical point defects in crystal lattices which have been the topic of extraordinary research interest in solid state physics and chemistry. The optical properties, electrical conductivity, ionic diffusion and resulting chemical properties, mass and charge transport behavior of the materials are heavily affected by the concentration and distribution of these defects in the crystal lattices, which has triggered considerable efforts in the control the exchange disorder to provide optimal performance in devices.

In lithium intercalation compounds, the lithium diffusion along the channel is highly anisotropic which is strongly depend on the cation ordering within the crystal lattices. The cation exchange disorder, namely the occupation of Li sites by transition metals, inevitably blocks the lithium ion diffusion pathway, thus directly affects the cathode performance in lithium-ion batteries. It has been clearly confirmed the presence of antisite defect in lithium-intercalated transition-metal oxides and phosphates by theoretical calculations, neutron diffraction and direct observation by advanced electron microscopy. It now turns to research on the local variations, transition metal relocation, structural change and their correlation to the voltage/capacity fading during the charge/discharge cycling. Many intensive investigation on the diffusion of transition metal ions on the layered metal oxides during the cycling process. However, the relocation of metal ions in ordered lithium metal phosphates during the intercalation-deintercalation reactions, remains unknown, although this relocation undoubtedly influence mass and charge transport behavior.

In this presentation, by a combination of high-angle annular dark-field (HAADF) and annular bright-field (ABF) STEM, we observed the local variation of cobalt ions from M2 sites to vacancy M1 sites in olivine lithium cobalt phosphates upon the delithiation process (Ref. 1). The finding provides the intrinsic evidence for the capacity fading of  LiCoPO₄ cathode materials.

1. Truong, Q. D., Devaraju, M. K., Tomai, T. & Honma, I. Direct observation of antisite defects in LiCoPO₄ cathode materials by annular dark- and bright-field electron microscopy. ACS Appl. Mater. Interfaces 5, 9926−9932 (2013).