Degradation Mechanism of LiCoPO4 As a Cathode for Li-Ion Batteries

Rechargeable lithium ion batteries are recently installed in hybrid electric vehicle (HEV) and electric vehicle (EV). Polyanionic cathodes such as olivine type LiCoPO₄, LiNiPO₄ and Li₂CoPO₄F have attracted much attention as high voltage cathodes for high energy batteries and high power batteries. Especially LiCoPO₄ shows high redox potential 4.8 V vs. Li/Li⁺ and a high theoretical capacity 170 mAh/g^[1]. However this material has poor cycle performance. It was reported that the capacity decreases to 50% of initial discharge capacity during an initial few dozen cycles^[2]. Therefore it is necessary to improve the cycle performance of LiCoPO₄ for commercial applications. We analyzed the bulk structure of LiCoPO₄ before and after the charge-discharge process to consider the mechanism of capacity degradation.

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.

References;

[1]: K. Phadhi, K.S.Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188

[2]: H.H. Li, J.Jin, J.P. Wei, Z. Zhou, J. Yan, Electrochem. Commun. 11 (2009) 95-98