| The overlithiation of xLi2MnO3·(1-x)LiNi0.2Co0.2Mn0.6O2, a ternary cathode material with high-manganese content, was systematically studied, mainly investigating the effects of lithium amount, synthesized method, synthesized conditions, pe-cycled treatment, surface coating and doping on the properties of materials. The crystal structure, surface morphology and electrochemical properties were investigated using X-ray diffraction(XRD), scanning electron microscope(SEM), galvanostatic charge-discharge test, cyclic voltammetry(CV) and electrochemical impedance spectrum(EIS).The materials with different amount of lithium-rich(x = 0.7, 0.6, 0.5, 0.4) were synthesized. The results showed that the material with x = 0.5, namely the optimal ratio of Li2MnO3- like phase and ternary phase was 1:1 exhibited the optimum layered structure, the lower cation mixed degree, the highest specific capacity and the best cycling stability.The 0.5Li2MnO3·0.5LiNi0.2Co0.2Mn0.6O2 cathode material was synthesized via high temperature solid-state method, using transition metal acetate and Li2CO3 as the raw materials. The effects of calcining temperature(750, 800, 850, 900℃) and calcining time(9, 12, 15, 18 h) on the materials’ properties were investigated. The cathode material calcined at 800 ℃ for 12 h exhibited the good layer structure and the charge- discharge specific capacities at 0.1 C- rate were 218, 162.1 mAh/g, respectively, with a coulomb efficiency of 74.36 %. The capacity retention rate was 106.9 % when cycled at 0.1 C- rate for 5 times and was 93.93 % when cycled at 0.2 C- rate for 10 times.Pre-treatment could prevent the damage to the material structure caused by charging to higher voltage directly, improving the rate capability. Thin MgO layer restrained the side reaction between cathode material and the HF coming from the decomposed of LiPF6, reduced the dissolution of metal ions and stabilized the surface structure of the active material during cycling. The optimal amount of MgO-coated was 2 wt. %.The precursor of 0.5Li2MnO3·0.5LiNi0.2Co0.2Mn0.6O2 cathode material was synthesized by carbonate co-precipitation reaction, using transition metal sulfate solution, Na2CO3 solution and ammonium hydroxide as the raw materials. The effects of transition metal ions concentration(0.5, 1.0, 1.5, 2.0 mol/L) and the value of precipitation solution(8.0, 8.5, 9.0) on the materials’ properties were investigated. The cathode material synthesized at the optimal conditions with a transition metal ions concentration of 1.0 mol/L and a pH value of 8.5 exhibited the good layer structure and desired cation mixed degree, and the charge- discharge specific capacities at 0.1 C- rate were 350.9, 280.7 m Ah/g, respectively, with a coulomb efficiency of 79.99 %. The capacity retention rate was 106.9 % when cycled at 0.1 Crate for 5 times and was 93.93 % when cycled at 0.2 C- rate for 10 times.Pre-treatment could enhance the initial charge-discharge capacities and the rate capability of the cathode material to some extent. The additive sodium caused the formation of the secondary phase with the structure of Na0.7MnO2.05, demonstrating an improved rate capability and cycle performance, which could be attributed to the enhanced electron conductivity and lithium ion diffusion. The optimal material with a nominal formula of Li1.11Na0.06Ni0.10Co0.10Mn0.63O2 exhibited the charge- discharge specific capacities at 0.2 C- rate were 359.2, 277.9 mAh/g, respectively, with a coulomb efficiency of 77.37 %. The capacity retention rate was 94.90 % when cycled at 0.5 C- rate for 50 times. |
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