Synthesis And Electrochemical Investigation Of Cathode Material LiNi_1/3Co_1/3Mn_1/3O₂

Layered Li[Ni-Co-Mn]O₂ has been considered as one of the most promising cathode materials for power lithium ion batteries because of its lower cost, better thermal stability and higher capacity. It combines the merits of LiCoO₂, LiNiO₂ and LiMnO₂ cathode materials, and its performance is better than any of them. However, the cycle performances of the Li[Ni-Co-Mn]O₂ material as a cathode of lithium ion batteries need to be improved, especially when charged to higher voltage or discharged at higher rates. In this paper we studied the preparation, structure, doping modification, electrochemical performance and electrode kinetics of the LiNi_1/3Co_1/3Mn_1/3O₂ material in detail.The effects of Co content on the performance of the LiNi_0.5-2xCo_2xMn_0.5-2xO₂ were investigated in detail. Co can stabilize the layered structure, inhibit the cation mixing of Ni²⁺ and Li⁺, facilitate Li⁺ insertion and exsertion, increase the conductivity and improve the cycling performance. The results showed that the material with 1/3 Co content had better layered structure, and the electrochemical performance was excellent no matter that the cell was charged to high or low voltages. However, extra Co led to the decrease of lattice parameters of c and a, thus reducing the volume of the lattice, and decreased the reversible capacity of the materials.We investigated the effect of three types of precipitators (NaOH, Na₂CO₃ and (NH4)₂CO₃) on the layered LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials synthesized via a co-precipitation method. The precipitation conditions were optimized for these three precipitators. The XRD analysis showed that the sample synthesized by Na₂CO₃ had lower cation mixing and better layered structure. After 100 cycles between 2.8 and 4.5V vs. Li/Li⁺ at 2C discharge rate, the LiNi_1/3Co_1/3Mn_1/3O₂ system using Na₂CO₃ as the precipitator retained 97% of its initial discharge capacity. The materials prepared by different calcination conditions were investigated in detail. The results showed the sample prepared at 500°C for 5h, then calcined at 900°C for 24h had better layered structure, lower cation mixing and higher electrochemical performance.The effects of four different electrolytes,LiPF₆/EC+DEC(l:l), LiPF₆/EC+DMC(l:l), LiPF₆/EC+EMC(1:1) and LiPF₆/EC+PC+DMC(l:l:l), on the performance of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ were investigated. The results showed that the material used 1mol·L-1LiPF₆/EC+PC+DMC(l:l:l) as the electrolyte exhibited better electrochemical performance. After 50 cycles between 2.8 and 4.5V vs. Li/Li+, the LiNi_1/3Co_1/3Mn_1/3O₂ system with 1mol·L-1LiPF₆/EC+PC+DMC(l:l:l) electrolyte retained 88.58% of its initial discharge capacity.The obtained compounds were characterized by X-ray diffraction (XRD), ICP and X-ray photoelectron spectra (XPS). The XRD analysis showed that the LiNi_1/3Co_1/3Mn_1/3O₂ materials presented layered hexagonal structure and the cation mixing was low. The ICP results showed the Li:Ni:Co:Mn ratio in the material was 1.01:0.338:0.330:0.343. The XPS results showed that the valence states of Ni, Co and Mn in the Li[Ni_1/3Co_1/3Mn_1/3]O₂ were mainly +2, +3 and +4, respectively. However, Ni and Mn also partially existed in +3 states. Electrochemical Impedance Spectroscopy (EIS) was used to discuss electrochemical reaction mechanism. The results showed that the charge transfer resistance Rct was decreased when the voltage increase and increased when the cycling number increased. The exchange current density of the layered Li(Ni_1/3Co_1/3Mn_1/3)O₂ was increased when the voltage increased, and the value was between 0.16¹.05mA·cm^-2.In order to improve the electrochemical properties of the LiNi_1/3Co_1/3Mn_1/3O₂ at high charge end voltage (4.6V), a series of the doped compounds, LiNi1/3Co1/3-xMn1/3MxO₂-yFy (M= Mg,Cr,Al,x=0.05, y=0.1), were synthesized. The effects of doping element on the structure and electrochemical performances of the LiNi_1/3Co_1/3Mn_1/3O₂ were investigated. The results showed that the tap-density was obviously increased with doping different elements, and the tap-density of the Cr-doped material could reach 2.51 g·cm-3. The XRD results showed that all doped materials kept the same layered structure with R3m space group as the LiNi_1/3Co_1/3Mn_1/3O₂. In the voltage range of 2.8⁴.6V, the Cr and F doped material showed highest capacity retention, it was 87.69% and 108.67% after 50 cycles respectively. The capacity retention was 55.1% and 56.4% for the Cr and F doped materials when the discharge rate was 5C. Furthermore, we investigated the effect of the doped quantity of Cr on the structure and electrochemical performance of the LiNi_1/3Co_1/3-xMn_1/3Cr_xO₂. The results showed that when x=0.04, the material had better layered structure, lower cation mixing, higher initial discharge specific capacity, better cycling performance and rate performance.