Preparation And Improved Properties Of Li_1.2Mn_0.6Ni_0.2O₂ Cathode Materials For Lithium-ion Batteries

Li-rich manganese cathode material of Li1.2Mn0.6Ni0.2O2(LMNO)has high specific capacity(>250 mAh·g-1),which makes it have great potential application in energy storage,high-power battery,etc.However,the material still has some problems to be resolved,such as low first coulombic efficiency,serious capacity and voltage attenuation,and poor rate performance.Based on this,in this paper,Li-rich manganese cathode material,Li1.2Mno 6Nio.2O2,was prepared by combustion synthesis,and the properties were improved by Fe3+and Mg2+ ions doping and the surface coating of MgF2.The corresponding mechanisms were discussed in detail.LMNO cathode material powders were prepared by combustion synthesis,using LiNO3,Ni(CH3COO)2·4H2O and Mn(CH3COO)2·4H2O as raw materials and PEG400 as combustion promoter,which were characterized by XRD and SEM,and the electrochemical properties were tested.The effects of calcination time,preheating temperature and lithium amount on the phase structure,microstructure and electrochemical properties of LMNO were studied systematically.The optimum synthesis procedure was proposed for combustion method.Namely,,the precursor was preheated at 500? for 2h,and then calcined at 900? for 21 h,with 5wt%more lithium source.The initial discharge specific capacity of the as-prepared LMNO was 277.2 mAh·g-1 and the coulomb efficiency was 73.5%at C/20 rate.The specific capacity of the material reached 174.3 mAh g-1 at 1C rate,and the capacity retention rate was 80.2%after 200 cycles.The performance of LMNO cathode material was modified by Fe3+and Mg2+ions doping.It was found that Fe3+ doping increased the lattice spacing of the crystal,which was available for the quick intercalation and de-intercalation of Li+.Meanwhile,Fe3+ doping effectively inhibited the lattice transformation.The electrochemical performance tests showed that the material with 1%Fe3+ doping had an initial discharge capacity of 299.0 mAh·g-1 at C/20 rate,and a specific capacity of 190.3 mAh·g-1 at 1C rate and 98.1 mAh·g-1 at 10C rate.At the same time,its capacity retention rate increased to 85.4%after 200 cycles at 1C rate.The LMNO cathode material doped by Mg2+ ions were studied.Results showed that the prepared sample could maintain better structural stability and electrochemical performance under the condition of deep lithium removal when the Mg2+ content was 2%.The specific capacity of the sample was 171.8 mAh·g-1 at 1C rate,and the capacity retention rate reached to 90.3%after 200 cycles.Also,it showed good high rate performance,with a specific capacity of 111.8 mAh·g-1 at 5C.The MgF2 coated LMNO cathode material was prepared by liquid phase method.The results showed that MgF2 coating with an appropriate amount did not affect the crystal structure and morphology of LMNO.When the content was 2wt%,the discharge specific capacity of the sample was 146.4 mAh·g-1 after 300 cycles at 1C rate,with a capacity retention rate of 94.0%,indicating that MgF2 coating could improve significantly the structural stability of LMNO.