Synthesis And Modification Of LiNi_1/3Co_1/3Mn_1/3O₂ As Cathode Material Of Lithium-ion Batteries At High Voltage

The cathode material LiNi_1/3Co_1/3Mn_1/3O₂ was prepared via solid state method with commercialized Li2CO3 and Ni1/3Co1/3Mn1/3（OH）2 which was mixed through a high speed mixture as the starting material. The effects of calcination temperature, calcination time, mole ratio of n（Li）:n（Ni1/3Co1/3Mn1/3（OH）2） and other factors on electrochemical properties of LiNi_1/3Co_1/3Mn_1/3O₂ were discussed. Then, elements substitution and surface coating of LiNi_1/3Co_1/3Mn_1/3O₂ were studied. The microscopic structural features and electrochemical properties were investigated using XRD、SEM、ICP analysis and electrochemical properties were also investigated. The cycling stability was obviously improved by determining the optimal amount of elements substitution and surface coating at high cutoff voltage.Measurement results demonstrated that the optimum technologic parameters were as followed: the material should be calcined at 550℃ for 5h for a pretreatment and then calcined at 900℃ for 10 h. n（Ni1/3Co1/3Mn1/3（OH）2）:n（ Li） was 1:1.04. LiNi_1/3Co_1/3Mn_1/3O₂ synthesized at the optimal conditions delivered a discharge capacity of 173.2 mAh/g at 0.1C and coulomb’s efficiency of first time is 89.1%. The capacities of the synthesized material at 0.2C、0.5C and 2C rate are 167.9、160.9、149.1 mAh/g, respectively. The capacity retention of LiNi_1/3Co_1/3Mn_1/3O₂ was 96.7% for 50 cycles, while when the voltage was enhanced to 4.6V, the capacity retention has been dropped to only 91.9%.In order to improve the electrochemical properties of the LiNi_1/3Co_1/3Mn_1/3O₂ at high cutoff voltage（4.6V）, a series of the doped compounds, Li[Ni1/3Co1/3Mn1/3]1-xMxO2（M=Mg、Ti、Zr, x=0.01⁰.03）, were synthesized. The effects of doping element on the structure and electrochemical performances of the LiNi_1/3Co_1/3Mn_1/3O₂ were investigated. Of all the doped samples, 1% Mg-substituted, 3% Ti-substituted, 1% Zr-substituted cathode materials showed better cyclic performance. Furthermore, the Zr-substituted sample has the best electrochemical properties of a discharge capacity of 196.1mAh g-1 and the capacity retention of 95.2% after 50 cycles. The improved electrochemical properties of Zr-doped sample result from the more stable structure and lower resistance during the electrochemical cycling.Al₂O₃ and MgF2 coated cathode materials were synthesized successfully. The LiNi_1/3Co_1/3Mn_1/3O₂ was coated with Al₂O₃ using nano Al₂O₃, Al（OH）3 and Aluminium isopropoxide as the starting materials, respectively. The effects of different Al₂O₃ sources coating on the surface of LiNi_1/3Co_1/3Mn_1/3O₂ material on the electrochemical properties were investigated. Of all the samples, the sample that using Aluminium isopropoxide as the starting material has the best electrochemical performance. The improved cycling behavior of the Al₂O₃-coated sample is attributed to the Al₂O₃ coating layer acts as a stabilizer and hence protects the oxide structure from damage during long-term cycling. 2% Al₂O₃-coated, and 2% MgF2-coated cathode materials showed better cyclic performance than other samples. The initial discharge capacities of 2% Al₂O₃-coated and 2% MgF2-coated cathode materials were 196.1mAh/g, 195.8 mAh/g, respectively. After 50 cycles at 1 C rate, the capacity retention ratio of the coated samples were 95.6% and 94.8%, respectively.