Study On Synthesis And Electrochemical Properties Of Lithium-rich Material Based On Nickel–manganese–chromium System

In this research, a series of Li[Li1/3Mn2/3]O2–LiCrO2–LiNi1/2Mn1/2O2compounds weresynthesized, and the influence of synthesis method on their structures, element valencesand electrochemical behavior were investigated. Furthermore, lithium-rich cathodematerial of manganese–nickel–chromium ternery system, Li–Mn–Ni–Cr–O, wassynthesized by spray-drying process and hydrothermal method, respectively. Their crystalstructures, element valences and the electrochemical performances were investigated byXRD, XPS, ICP, SEM, TEM and so on.As to their crystal structures, Li1+2xMn0.3+xNi0.3–3xCr0.4O2（x=1/60,1/30,1/20,1/15,1/12, called SD1–SD5, hereinafter） and Li1.2Mn0.4+xNixCr0.4–2xO2（x=0,0.05,0.10,0.15,0.20, called SD6–SD10, hereinafter） materials prepared by spray-drying method have alayered-NaFeO2structure （R3m space group）, while hydrothermal-obtainedLi_2+4xMn_0.6+2xNi0.6–6xCr0.8O4materials （x=1/60,1/30,1/20,1/15,1/12, called H1–H5,hereinafter） have a cubic spinal structure, whose space group is Fd3m. All SD compoundsexhibited strong hygroscopicity. The cells were rapidly broken down during thecharge-discharge cycle if using the non-treated SD samples as the cathode.ICP and XPS analysis revealed that Li+and Cr6+ions were mainly dissolved from thecompounds into water by washing them with water. The elemental analyses indicate thatthe co-existence of Li2CrO4in the Li–Mn–Ni–Cr–O resulted in the high moistureabsorption. This is mainly attributed to the amorphous Li2CrO4, which can be removed bywater processing. There was no Cr6+ion in H1–H5samples, therefore no hygroscopicitywas observed.There is remarkable difference in terms of the electrochemical behavior between bothmaterial systems: the SD series of compounds exhibit a solid state solution behavior,whereas the H series of compounds are similar to that of LiNi0.5Mn1.5O4, whose working voltage is about4.7V. The initial discharge capacity for a series of SD compounds increasewith the increase in the Li and Mn contents. As to the SD5sample, the discharge capacityis202.8mA h g-1when cell was operated at a current density of20mA g-1. While thedischarge capacities of SD6and SD9are246.9and243.9mA h g-1, respectively. In thecase of H series of the compound, H5sample is the best whose discharge capacity is107.3mA h g-1.The capacity retentions of SD5, SD6and SD9after20cycles are87.92%,73.11%and69.82%, respectively. The capacity retention of H5is84.90%.