Structural Analysis of the LiCoPO<sub>4</sub> Electrode/NASICON-Type Li<sub>1.3</sub>Al<sub>0.3</sub>Ti<sub>1.7</sub>(PO<sub>4</sub>)<sub>3</sub> Solid Electrolyte Interface
Fumihiko Ichihara, Kodai Niitsu, Yoshinori Tanaka, Y. Niwa, Kazutaka Mitsuishi, Shogo Miyoshi, Takahisa Ohno, Takuya Masuda
Abstract
The mixed composites of a high-voltage cathode material in lithium-ion batteries, LiCoPO 4 (LCP), and an oxide-based solid electrolyte, Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP), were sintered at a relatively high temperature, and the structure of the bulk and interface was comprehensively analyzed by various techniques, including X-ray diffraction, X-ray absorption fine structure (XAFS), and transmission electron microscopy, together with selected-area electron diffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy (EELS). The crystal structure and electronic state of LCP and LATP remained unchanged throughout the temperature range, but amorphous reaction layers were formed at their boundaries at temperatures over 700 °C. The reaction layers were composed of Li, O, P, and Co, and P and Co were richer and poorer than LCP, respectively. Linear combination fitting of XAFS data and microanalysis of EELS identified the major species of reaction layers to be Li 3 PO 4 and divalent Co compounds such as CoO. Li 3 PO 4 presumably promoted the crystal growth of LATP and Li-ion transfer.