Titanium and fluorine co-modification strengthens high-voltage electrochemical performance of LiCoO2

Abstract

The cathode material for lithium-ion batteries, LiCoO 2 (LCO), is suffered from the structural damage and rapid performance degradation at high voltage. In this article, a hydrolysis-calcination method was explored to modify the single-crystal LCO particles with Ti&F co-modification, aiming at strengthening the structural stability through decreasing the energy barrier of lithium ion diffusion. Evaluated at high voltage (3.0–4.55 V), the optimal sample LCO-TF2 has the initial discharge capacity of 199.3 mAh g −1 and capacity retention rate of 75% after 100 cycles at 1 C and 45 °C, significantly higher than that of the pristine sample (192.8 mAh g −1 and 35%). Characterized by XRD, TEM, XPS, etc., it is illustrated that the Ti&F co-modified sample LCO-TF2 has the rock-salt phase on its surface besides the layered structure inside. GITT results indicate that Li + diffusion coefficients of LCO-TF2 are more than twice times of those of LCO under the voltage range of 4.0–4.55 V. The in-situ XRD tests reflect that during the charge-discharge cycle the crystal structure variation of LCO-TF2 is suppressed during Li + extraction and insertion. Further, DFT calculations disclose that with the co-modification of Ti and F the surface film of LiCoO 2 possesses lower energy barrier of Li-ion diffusion. These results would provide useful guidance on improving electrochemical performance of LiCoO 2 at high cut-off voltage. Ti&F co-modification enhanced the structure stability of LiCoO 2 , reduced the energy barrier of Li-ion diffusion, thus significantly improved the high voltage performance. • A hydrolysis-calcination method using (NH4)2TiF6 and carbamide was developed to modify LiCoO 2 . • Ti&F co-modified sample (LCO-TF2) showed higher capacity and retention rate at 3.0–4.55 V. • The surface film of LCO-TF2 has the rock-salt phase. • DFT calculations disclosed that Ti&F co-modification can significantly decrease the energy barrier of Li-ion diffusion.