Synthesis And Characterization Of High Capacity Li-rich Manganese-based Cathode Materials

Due to the ultra-high specific capacity (>250mAh·g-1), special charge-dischargemechanism, good thermal stability, safe, environmental friendliness and low-cost, etc.,the lithium-rich manganese-based cathode material has been paid more and moreattention and has considerable potential for market application. However, the researchand development of this material is still in its infancy and far away from the marketapplication. There are still some problems to be solved, e.g., the low initial coulombicefficiency caused by the high voltage platform in the first cycle, the low ratecapability attributed to poor electronic conductivity and the defects of the synthesismethod (namely, complex process, much variables to control and poor consistencefrom batch to batch), which greatly limit the industrial production and marketapplications for lithium-rich manganese-based materials.In this paper, in order to improve the electrochemical performances of thelithium-rich manganese-based materials, the xLi2MnO3·(1-x)Li(Ni1/3Co1/3Mn1/3)O2(0.4≤x≤0.6) and Li(1+y)Ni0.166Co0.166Mn0.667O(2+δ)(0.3≤y≤0.6) materials weresynthesized by simple spray drying method, which can be easily applied in large scalepreparation. The efforts of two-phase proportions and the lithium content on theproperties of the material were studied in succession. When x=0.5, y=0.4, theLi1.4Ni0.166Co0.166Mn0.667O2+δsample shows an initial discharge capacity up to277.5mAh·g-1with the initial coulombic efficiency of81.2%at0.1C between2.0and4.8V.The capacity retention after20cycles is95.3%, the rate capacity retention at the ratesof0.2C、0.5C、1C and2C exhibits to be92.3%、85.1%、76.4%and69.8%,respectively..According to the preliminary result, Li1.4Ni0.166Co0.166Mn0.667O2+δwasoptimized for the research object. The effect of sintering process on the physical andelectrochemical performance was discussed. The results show that the sinteringtemperature and sintering time have direct impacts on the crystal structure andprimary particle sizes of the lithium-rich manganese-based solid solution, therebyaffecting the electrochemical properties. The sample pre-calcined at600oC for10hand then sintered at920oC for16h shows the best electrochemical performance.When chargerd-discharged at0.2C rate between2.5and4.65V, the sample discharges an initial capacity of230.0mAh·g-1. After cycling for40times, its capacity can stillreach to219.7mAh·g-1with the capacity retention of95.5%.The Li1.4Ni0.166Co0.166Mn0.667O2+δsample was modified with carbon nanotubes(CNTs) by spray drying method. It is found that the initial coulombic efficiency andrate capability of the material was effectively improved after CNTs wiring. Whenchargerd-discharged at0.2C rate between2.5and4.65V, the0.5%CNTs-wiredsample exhibites an initial discharge capacity of250.5mAh·g-1with the initialcoulombic efficiency of77.4%. While the initial capacity and coulombic efficiencyfor the unwired sample is only222.6mAh·g-1and71.1%respectively. Whenchargerd-discharged at2C, the0.5%CNTs-wired sample delivers a capacity of199.5mAh·g-1at2C, in contrast to the value of135.6mAh·g-1for the unwired sample. Themechanism of CNTs wiring on the enhancement of electrochemical performance forthe lithium-rich manganese-based materials has been analyzed by cyclicvoltammogram (CV), four-point probe method and electrochemical impedancespectroscopy (EIS).