Synthesis And Surface Modification Of Li-rich Cathode Material For Lithium Ion Batteries

With the development of mobile communication,portable devices,electric vehicles and energy storage,Li-ion battery as a kind of high energy density and long life energy storage system gets more concentration.Further improvement of the energy density of Li-ion batteries has been widely studied by researchers.Li-rich cathode material performs higher capacity and energy density than other traditional cathode materials to become a strong contender for the next generation Li-ion battery cathode material.However,the problem of cycling stability of Li-rich material,especially the voltage fading among the cycle,becomes a bottleneck that affects its application.We synthesized the Li-rich cathode material and used different methods to improve its electrochemical performance.1.We synthesised aluminum base metal organic frameworks(MOFs)MIL-100(Al)as Li-rich material coating layer precursor.The experiment result showed,the electrochemical performance of Li-rich cathode material obviously improved,especially the cycle stability obtained great improvement.There was still 202.0 mAh/g of discharge capacity after 200 cycles at 1C(2.0-4.8V).The capacity retention was 94.3%,a figure which was much better than that of the bulk material.We figured out the reasons for the improvement of electrochemical performance through comparing the effect of different coating source.The coating layer avoids the direct contact and side reactions between electrolyte and active material,so as to avoid the growth of surface SEI film and impedance.The coating layer is homogeneous and not completely dense,so will not prevent the transmission of lithium ion while it avoids the direct contact between electrolyte and active material.In addition,the coating layer is also beneficial to stability of crystal structure.2.We opened a new path to solve the voltage fading of Li-rich cathode material.A hollow micro sphere structure was built by MnO2 template.Capacity retention of HLRMS was 94.7%after 200 cycles at the rate of 1C.Then the average voltage was 97.0%remained after 100 cycles and 93.5%remained after 200 cycles at 1C.The cycle stability of hollow micro sphere Li-rich material(HLRMS)was obviously superior to that of infarctate micro sphere Li-rich material ILRMS.The improvement of electrochemical performance is attributed to the hollow secondary particles,which can stabilize the secondary structure of material and prevent the transformation from layered phase to spinel phase.3.We used surface Mg doping to build a stable support in surface of material,which is able to inhibit the spinel transformation from the outside to inside.Compared with bulk material,the electrochemical performance was improved obviously.The capacity retention increased from 73.2%to 93.4%,and the average voltage retention increased from 91.1%to 98.6%.It can be seen that the surface doping strategy has greatly improved the stability of the material cycle,especially the retention of voltage.