Degradation Mechanism of LiCoPO4 As a Cathode for Li-Ion Batteries
LiCoPO4 was prepared by electrostatic spray deposition (ESD) method. ESD method is a thin film preparation method. Fine spray of a precursor solution is generated and deposited onto the heated substrates. High voltage around 15 kV are applied between spray nozzle and the substrate during deposition. We prepared precursor solution to dissolve lithium ethoxide, cobalt nitrate and phospholic acid into the mixed solution of ethanol and n-butyl carbitol. The deposition samples on Pt foil were annealed at 600 ºC in air to increase the crystallinity of LiCoPO4.
LiCoPO4 thin film consisted of 300 nm particles was fabricated by ESD method. Thin film obtained was measured by XRD. From the results of XRD, the profiles were in good agreement with the orthorhombic, olivine-like structure. Fig. 1 shows the charge-discharge profiles of the LiCoPO4 thin films. The charge profiles give 2 plateaus at around 4.77 and 4.86 V respectively. 2 plateaus also appear the 4.8 and 4.7 V respectively during discharge. This sample shows low coulomb efficiency for initial few cycles because of electrolyte decomposition in the charge. Discharge capacity decreased during the charge-discharge cycles. TEM observation was performed in order to consider the cause of this capacity degradation. Fig. 2 shows HAADF-STEM (High-angle Annular Dark Field – Scanning Transmission Electron Microscopy) images for the bulk of the LiCoPO4 electrode after charge-discharge cycle. It was observed that the cobalt and phosphorus ordered regularly and anti-site defects were not observed in the pristine sample. On the other hand, the anti-site defects are observed in the sample after charge-discharge test for 5 cycles. In addition, amount of anti-site defects at the surface were more than at the bulk. From these results, we think the discharge capacity might decrease with the charge-discharge cycle as the anti-site defects might prevent Li-ion diffusion in the LiCoPO4during the discharge. Therefore the anti-site defects should be suppressed to improve the cycle performance.
: K. Phadhi, K.S.Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188