Research On Synthesis And Modification Of Lithium-Rich Cathode Material Li[Li_0.2Ni_0.15Mn_0.55Co_0.1]O₂ For High-Performance Lithium-Ion Battery

Lithium ion batteries（LIBs） are widely used for energy storage in number of power supplies owing to high energy, high capacity, and environmental compatibility. The developments of electric vehicles, smart grid and large-scale energy storage have put forward high demands on LIBs with high energy density and power density. Because of the desirable features, the novel solid solution system has become appealing as promising cathode material for LIBs. However, during the application there are still some key challenges to be addressed, including large initial irreversible capacity and poor rate performance. In the present study, Li[Li_0.2Ni_0.15Mn_0.55Co_0.1]O₂ was successfully prepared, and the content of Li was optimized, then the materials were modified by single doping and co-doping with cations and anions as well as surface coating. The acquired materials were characterized by SEM, XRD, EDS, cyclic voltammetry, charge and discharge tests etc.The Li[Li_0.2Ni_0.15Mn_0.55Co_0.1]O₂ materials with different ration of Li were prepared by sol-gel methode. It is indicated that the optimal Li content is 105% through the investigation of the structure, morphology and the electrochemical properties. Furthermore, the maierials were doped by Cr or Cr and Al, which enhanced the stability of the materials structure, because of the decrease of cation anti-site in the crystals. The electrochemical performance exhibited that the comprehensive property was the best with the 1% doping. After 50 cycles at 0.2C, the capacity retention ratio of Li[Li0.2Ni0.15Mn0.55Co0.1]0.99Cr0.01O2 and Li[Li0.2Ni0.15Mn0.55Co0.1]0.98（CrAl）0.01O2 both maintained 93%. At 1C current density the discharge specific capacity was 180.4 mAh/g at room temperature and 198.6 mAh/g at 55℃. Overrall, the rate performance and the recyclable ability of Li[Li_0.2Ni_0.15Mn_0.55Co_0.1]O₂ would be improved by the Cr and Al co-doping.Li[Li0.2Ni0.15Mn0.55Co0.1]0.98（CrAl）0.01O2 materials were doped by F- on the basis of the cationic doping. The XRD indicated that all samples possess typical α-NaFeO2 layered characteristics without any impure phase. And the diameter of particles was decreased from the SEM images. The results of charge and discharge tests showed that the cyclic property of materials was significantly improved by F- doping and the optimal doping amount was 6%. At room temperature, for the sample of Li[Li0.2Ni0.15Mn0.55Co0.1]0.98（CrAl）0.01O1.94F0.06, the first discharge capacity was 240.5 mAh/g at the condition of 0.1C and the discharge capacity and capacity retention ratio was 221.3 mAh/g and 95%, respectively, after 50 cycles at 0.2C. In addition, the discharge capacity was 189.5 mAh/g（1C）.Lithium-rich cathode material Li[Li_0.2Ni_0.15Mn_0.55Co_0.1]O₂ was modified by surface coating of TiO₂, MnO₂ and A12O3. The characterization of SEM、EDS and XRD indicated that TiO₂, MnO₂ and A12O3 were attached to the surface of the materials successfully, and the laminated structure was still well maintained. The materials that had been coated showed better cycle performance and rate performance and the loss of initial irreversible capacity became smaller. The first discharge capacity of the sample modified by 3wt% TiO₂ was 247.2 mAh/g, and the capacity and capacity retention ratio was 219.8 mAh/g and 95%, respectively, after 50 cycles at 0.2C. Furthermore, the electrochemical performance of the materials was improved by the coating of MnO₂. When the MnO₂ coating amount was 3wt%, the loss of initial irreversible capacity decreased to 57.1 mAh/g from the original 107.1 mAh/g, and the coulomb efficiency was 85%. After 50 cycles at 0.2C, the discharge capacity was 213.8 mAh/g. The result of Cyclic Voltametric test showed that the peak became sharp and the potential difference decreased which means that the reversible property of the materials has improved after MnO₂ coating. The enhanced cyclic performance was mainly attributed to the separation between active materials and electrolyte by the coating layer. That allowed the electrode/electrolyte interphase to be stable and the SEI film became steadier. A12O3 coating layer could restrain the erosion of the cathode material by HF and improve the cycle performance. That was mainly due to the reaction between A12O3 and HF will generate AlF3 which could remain stable in the electrolyte.