Improving the cycling performance, rate capacity, and thermal stability of LiCoO2 by doping high-valence ions into the Li+ site

The poor rate performance and low discharge capacity of LiCoO2 limit its applications. Therefore, in this work, we synthesized Li1-xMxCoO2 (M = Na, Zr, Nb) via solid-phase synthesis to improve its properties. X-ray diffraction (XRD) results suggest that the doping elements were successfully doped into LiCoO2. The electrochemical properties showed that the samples doped with the high-valence elements Zr and Nb had a higher capacity, better cycle stability, and better rate performance than those doped with the low-valence element Na. In particular, the capacity retention of LiCoO2, Li0.97Na0.03CoO2, Li0.99Zr0.01CoO2, and Li0.99Nb0.01CoO2 was 68, 42, 85, and 87%, respectively, after 80cycles at a rate of 10 C at 55 degrees C. However, doping of Zr and Nb into the Li+ site of LiCoO2 will reduce the content of Li+. And, less Li+ extracted in the cathode material resulting in low discharge capacity under low current density. The larger radius of Na+ is incorporated into the Li slab and enlarged the interlayer spacing of the (003) plane. The larger (003) interplanar spacing can significantly facilitate the lithium diffusion and is also favorable to the rate capability. The differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis results demonstrated that the Zr-doped and Nb-doped LiCoO2 had a higher thermal stability in the charged state than the Na-doped LiCoO2. Additionally, the resistances of the Zr-doped and Nb-doped electrodes were much lower than that of the undoped electrode. Our research results indicate that doping with high-valence elements is a very effective strategy for optimizing the electrochemical performance of LiCoO2 and that this method can also be extended to other cathode materials.