Synthesis And Electrochemical Performances Of Layered Rich-lithium Cathode Materials For Lithium-ion Battery

With the development of economics, rechargeable lithium-ion batteries（LIBs） have been considered as the most promising power sources in the field of hybrid electric vehicles（HEVs） and electric vehicles（EVs）. At present, many researchers have been devoted to improving the electrochemical performances of layered rich-lithium cathode materials, to meet the public demand for the energy density and power density. However, several major problems still exist for these lithium-rich layered oxides. For example, a large irreversible capacity loss in first charge/discharge cycle, severe capacity fading, poor rate capability and voltage decay during subsequent charge-discharge cycles, all of which limits its commercialization. Therefore, it is a challenge work to improve the layered rich-lithium electrochemical performances.In our research, Zr⁴⁺ is selected as a dopant to substitute for Mn⁴⁺ in Li₂MnO₃, aiming to improve the cycle performances and voltage decay. 0.6Li[Li_1/3Mn_2/3]O₂·0.4LiMn_5/12Ni_5/12Co_1/6O₂ is coated by ZrO₂, aiming to improve the cycle performances. Our work is concluded as following:（1） a series of layered lithium-rich oxides 0.6Li[Li_1/3Mn_2/3(_1-x)Zr_2/3x]O₂·0.4LiMn_5/12Ni_5/12Co_1/6O₂（0 ≤ x ≤ 10%） have been prepared by spray-dry method. The crystal structural and morphological properties of all samples have been studied by XRD, XPS, SEM, HRTEM and SAED. XRD results reveal Zr⁴⁺ ions are doped into the lattice without any change. HRTEM results suggest Zr⁴⁺ ions can stabilize the layered structural feature during cycles. The electrochemical properties are remarkably upgraded by Zr⁴⁺ ions doping. The discharge capacity of Zr4% doped samples remains 218.9 mA h g^-1 after 100 cycles with a capacity retention of 84% at 20 mA g^-1 between 2.0 and 4.8 V, while the undoped samples drop to 168.6 mA h g^-1 with a capacity retention of 72%. Moreover, Zr4% doped samples show the lowest voltage decay, about 0.16 V lower than the undoped samples after 100 cycles.（2） The 0.6Li[Li_1/3Mn_2/3]O₂·0.4LiMn_5/12Ni_5/12Co_1/6O₂（named as LNMCO）sample is firstly prepared by spray-dry method, then a series of ZrO₂-coated LNMCO samples are synthesized by sol-gel method. XRD, XPS and TEM results suggest ZrO₂ only distribute on the surface of LNMCO sample and do not change the crystal structure. Among these samples, 1.5% ZrO₂ coated LNMCO sample show the highest initial discharge capacity 279.3 mA h g^-1 with the highest initial coulombic efficiency 87.2% and the irreversible loss only 55.3 mA h g^-1. After 70 cycles, the discharge capacity 1.5% ZrO₂ coated LNMCO sample is 244.3 mA h g^-1, whose electrochemical properties is much better than LNMCO sample. However, the rate properties of ZrO₂ coated LNMCO samples are worse than LNMCO sample, suggesting the research method should be further revised.