| Lithium-rich cathodes x Li2MnO3·(1-x)LiMO2have become one of the ideal power battery cathode materials due to theirhigh specific capacitance, long cycle life and excellent safety performance. In the present study, layered lithium-rich cathodes Li1.2Mn0.54Ni0.13Co0.13O2 was synthsised via coprecipitation and sol-gel method. The obtained material was characterized using X-ray diffraction(XRD), scanning electron microscopy(SEM), cyclic voltammetry(CV), electrochemical impedance spectroscopy(EIS) and galvanostatic chargeedischarge, which investgated the crystal structure, physical morphology and electrochemical performance.Firstly, the effect of pH value, choice of the source of lithium, calcination time and temperature was investigated in the coprecipitation process. Experimental found that the the optimal conditions were conducted as the following : using acetate as raw materials, NaOH and NH4·H2O as precipitant, gettinghydroxide precursor under the condition of pH=11.0, adding LiOH·H2O inhydrate precursor as source of lithium, presintering 500℃ for 5h, sintering at 900℃ in air atmosphere for 12 h to get the target materials. The material revealed a specific capacitance of 283.1mAh/g at the current density of 0.5C for the first discharge, with coulomb efficiency ashigh as 80.7%. The capacitance maintained 179.5mAh/g after 50 cycles at current density of 0.2C, retaining 76.7%, maintaining 118.9mAh/g after 100 cycles at current density of 1C, retaining 73.3%. The material still possessed a specific capacitance of 108.2mAh/g at current density of 2C.In order to improve the cycle stability and rate capability of the material, the material was coated and modified using MoO3 and Al2O3 respectively. The material coating Mo O3 at the best proportion of 1% showed a specific capacitance of of 256.7mAh/g at the current density of 0.5C, with coulomb efficiency ashigh as 83.4%. The capacitance maintained 107.3mAh/g after 100 cycles at current density of 1C, retaining 78.4%. The material coating Al2O3 at the best proportion of 5% retained 87.8% capacitance after 100 cycles at 1C. Al2O3 or MoO3 existed in the coating layer for the pristine material, decreasingthe electrolyte/electrode surface area and further inhibiting undesirable reactions,thus achieving an excellent cycling performance.The charging and discharging mechanism of the material prepared by coprecipitation method was explored, and revealed that the best charge cut-off voltage of 4.6V, which could reduce the electrolyte decomposition and inhibit corrosion on active material. The best charging cut-off current attenuated constant current constant voltage charging current to 1/20 of the original.Secondly, Li1.2Mn0.54Ni0.13Co0.13O2 was synthesized using sol-gel method. The optimum process was stated as follows: using acetate as raw materials and glycolic acid as complexing agent(glycolic acid: metal ion as 1.0), using ammonia to adjust pH between 7.0 to 8.0, presintering 450℃ for 5h, sintering at 900℃ in air atmosphere for 12 h and getting the target materials. The material revealed a specific capacitance of 275.9mAh/g at the current density of 0.5C for the first discharge, with coulomb efficiency ashigh as 74%. The capacitance maintained 81.3mAh/g after 100 cycles at current density of 0.2C, retaining 39.4%, The material synthesise by Sol-gelhave a layered structure and a better dispersion, the active substance can be fully utilized, and the material well dispersed, large surface area, Li+ for fast deintercalation, the loop stable material and co-precipitation rate performance is better than synthetic materials.In order to further improve the electrochemical performance of the material, the material was doped and modified using Mg2+ and PO43- in the sol-gel method. The material Mg2+-doped at the best proportion of x=0.01 retained 64.9% capacitance after 100 cycles at 0.2C. retaining 97.0%, maintaining 139.4mAh/g after 100 cycles at current density of 1C. The material PO43--doped at the best proportion of x=0.02 retained retained 52.1% capacitance after 100 cycles at 0.2C. This is because the ions doped with increased space sterichindrance, reduced the ions are mixed and circulation in the process of phase change, maintain the stability of the crystal structure, doped material after the reunion, poor dispersibility, Li + embedded distance is longer, the ratio performance degradation Furthermore, the material coating rGO at the best proportion of 3% retained 80.8% capacitance after 100 cycles at 0.2C and 91.8% capacitance after 100 cycles at 1.0C. The cycle stability and rate capability of the materialhave greet improvement. Graphene not only can reduce the corrosion of active material, electrolyte raise the cycle stability of the material, and itshigh ion and electron conductivity, make the material times permeance is also improved.Finally, nano-materials Li1.2Mn0.54Ni0.13Co0.13O2 was synthesized using sol-gel method with polyvinylpyrrolidone(PVP) as the complexing agent. The size of materials is about 100 nm. the effect of complexing agent PVP was investigated in the sol-gel process. The material PVP at the best proportion of PVP:M=1.0(M is metal ions). The material revealed a specific capacitance of 256.8mAh/g at the current density of 0.5C for the first discharge, with coulomb efficiency ashigh as 74.4%. The capacitance maintained 137.1mAh/g after 100 cycles at current density of 0.2C, retaining 68.6%, maintaining 126.7mAh/g after 100 cycles at current density of 1C, retaining82.8%. The cycle stability and rate capability of the material are excellent. This is because the nanometer material of embedded path is shorter, carrier to take off the embedded speeding up, the large current discharge performance is improved, and nano materialhas larger specific surface, ion embedded loci increased, more efficient use of active substances andhigh discharge capacity. |
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