Research On Optimized Synthesis And Modification Of LiNi_0.5Mn_0.5O₂Cathode Material For Lithium-ion Battery

LiNi0.5Mn0.5O2is considered as one of the most promising candidate materials to replacethe commercialized LiCoO2due to its advantages of higher specific capacity, excellentthermal stability, low cost and environment friendly. However, this material still has someproblems to be overcome in order to applied in the future, such as the serious Li+/Ni2+cationmixing, low actual capacity and poor rate capacity. So it is a great significance to furtherimprove the electrochemical properties of LiNi0.5Mn0.5O2for accelerating itscommercialization. This paper is focusing on improving the electrochemical performance ofLiNi0.5Mn0.5O2. The effects of synthesis conditions, cation/anion doping and surface coatingon the electrochemical performance were extensively studied by TG、XRD、SEM、EDS、CV、EIS、DSC and galvanostatic charge-discharge test.The layered structure LiNi0.5Mn0.5O2cathode material with ball-like shape wassuccessfully prepared by carbonate co-precipitation method. The effects of differentcomplexants, the amount of complexing agent and the calcination temperature on themorphology, crystal structure and electrochemical performance were thoroughly investigated.The optimal synthesis condition was as follows: using Na2CO3-NH4HCO3asprecipitant-complexant system to prepare the precursor, the mole ratio of NH4HCO3to themetal ions was1.5, and calcining first at850oC for6h then at950oC for6h. For the bestLiNi0.5Mn0.5O2, an initial discharge capacity of191.2mAhg-1is obtained between2.5-4.6V at0.1C; cycled at0.2C for50cycles, the retention rate is90.9%.The LiNi0.5Mn0.5O2material was first modified by doping with various Li contents, andthe electrochemical performances test showed that doped with appropriate Li+resulted inhigher specific capacity and better cycling performance, Li1.05(Ni0.5Mn0.5)0.95O2exhibits thebest electrochemical properties. On this basis, we investigated the Li+-F-co-dopedLi1.05(Ni0.5Mn0.5)0.95O2-yFy(0≤y≤0.1) materials. The results revealed that, although the fluorinesubstituted materials show somewhat lower initial capacity, the cycling stability issignificantly improved compared with Li1.05(Ni0.5Mn0.5)0.95O2because of the stabilization ofthe host structure. Li1.05(Ni0.5Mn0.5)0.95O1.95F0.05shows the best electrochemical property witha capacity retention of96.2%and93.4%after50cycles at room and elevated temperatures,respectively, and the tap density is also enhanced.In addition, the Li+-F-co-doped Li1.05(Ni0.5Mn0.5)0.95O1.95F0.05material was coated withdifferent amount of FePO4. It is demonstrated that with proper amount of FePO4coating layer(1wt.%-3wt.%), significant improvements in cycle performance, rate capability and thermalstability are achieved. Specifically, the sample coated with3wt.%FePO4shows the optimumcyclability (with a capacity retention of97.3%after50cycles) and rate capability(deliverscapacity of62.1%at3C based on the value of0.2C). EIS results indicated that the FePO4 coating layer significantly suppressed the increase of charge transfer resistance with cycling.Overall, the combination of Li+-F-co-doping and FePO4coating was effective inimproving both cycling performance and rate capability of LiNi0.5Mn0.5O2material.