Improvement Of Electrochemical Performance And Synthesis Of Phosphate-Based Composite Cathode Materials For Lithium-ion Battery

Phosphate-based cathode materials have attracted much attention in recent years.Among them,LiFePO4 has been commercialized because of its advantages of environmental friendliness,low cost and good thermal stability,but its energy density is low.In this dissertation,LiFe0.4Mn0.4Co.2PO4/C was prepared by using Mn and Co elements instead of Fe in LiFePO4 to improve the energy density of the material.Then the optimized preparation method was used to improve electrochemical properties.At the same time,the electrochemical performance of LiFe0.15Mn1.85PO4/C was further improved by using a novel aqueous binder.LiFe0.4Mn0.4Co0.2PO4/C composites were successfully prepared by coprecipitation assisted wet-milling by using stearic acid as a carbon source.The as-prepared LiFe0.4Mno.4Co0.2PO4/C can deliver a high initial discharge capacity of 163.3 mAh g-1 at 0.1 C,together with a capacity retention rate of 86.6%after 50 cycles.Even at a high rate of 1 C,the nanocomposite can exhibit also a high discharge capacity of 104.7 mAh g-1 after 100 cycles.And it has confirmed that the LiFe0.4Mn0.4Co0.2PO4/C is solid solution through XRD analysis.The above results are mainly attributed to the uniform distribution of elements and carbon coating in LiFe0.4Mn0.4Co0.2PO4/C.Sa-son seed gum(SG)was used as the binder for LiFe0.15Mn0.85PO4/C composites.Compared with PVDF binder,it was found that electrodes with SG binder showed excellent performance.At 0.2 C,the discharge capacity of the low-loading electrode(?4 mg cm-2)was 139.7 mAh g-1 after 100,cycles with capacity retention of 98.5%.And at the same rate,the discharge capacity of the high-loading electrode(7.5 mg cm-2)was 138.6 mAh g-1 after 140 cycles with capacity retention of 98.6%.At 0.5 C,the discharge capacity of the low-loading electrode maintained at 131.4 mAh g-1 after 350 cycles with capacity retention of 98.3%,and the discharge capacity of the high-loading electrode maintained at 123.7 mAh g-1 after 280 cycles with capacity retention of 92.7%.At 3 C,the discharge capacity of the electrode(-3.5 mg cm-2)with SG binder was 94 mAh g-1,while the discharge capacity of the electrode(-2.5 mg cm-2)with PVDF binder was almost 0 mAh g-1.The cyclic voltammetry showed that the electrode with SG binder exhibited lower polarization.Comparing the SEM images after the electrode cycle shows that the integrity of the electrode can be well maintained with the SG adhesive,and good conductive contact between the electrode material and the current collector can be achieved.