Improvement Effect Of Different Graphenes On The Ultrahigh Current Charge-discharge Performance Of The Doped Lithium Iron Phosphates

The olivine LiFePO4 cathode material has many advantages,such as good cycle performance,stable structure,low cost,and high safe.However,it has the disadvantages of low density,low rate performance,bad discharge performance at low temperature.This dissertation investigated the key factor which determined the charge-discharged performance of the doped lithium iron phosphate discharged at high current.The structure,surface morphology and electrochemical properties of the samples were studied by using charge discharge test,cyclic voltammetry,electrochemical impedance,x-ray diffaction analysis and scanning electron microscopy.The main research conclusions are as follows:1.The electrochemical properties of the cathode of the mixture of the titanium doped LiFePO4,acetylene black,polyvinylidene fluoride,graphene treated were studied.The experimental results show that the optimal sample with a olivine structure prepared by mixing the titanium doped LiFePO4,polyvinylidene fluoride,graphene treated and acetylene black in the proportion of 0.425:0.15:0.0065:0.01 in a weight ratio.The optimal sample exhibits the 1St cycle capacity of 129.4mAh/g and exhibit the 40th cycle capacity of 132.9mAh/g while discharged at the 50C rate.The capacity retention rate is 102.7%in 40 cycles.At 55?,the sample exhibits the 1St cycle capacity of 152.4mAh/g(0.2C rate)and 142.8mAh/g(100C rate).Therefore,the optimal sample exhibits the excellent charge and discharge performance while discharged with high current at room temperature or at 55?.2.The electrochemical properties of the cathode of the mixture of the boron doped LiFePO4,acetylene black,polyvinylidene fluoride,graphene treated were studied.The experimental results show that the optimal sample with an olivine structure prepared by mixing the boron doped LiFePO4,polyvinylidene fluoride,graphene treated and acetylene black in the proportion of 0.425:0.15:0.0085:0.01 in a weight ratio.The optimal sample exhibits the 1st cycle capacity of 145.0mAh/g(0.2C rate)and 120.8 mAh/g(100C rate).At 55?,the sample exhibits the 1st cycle capacity of 146.5mAh/g(0.2C rate)and 139.2mAh/g(100C rate).Therefore,the optimal sample exhibits the excellent charge performance with high current while discharged either at room temperature or at 55?.3.The electrochemical properties of the cathode of the mixture of the boron doped LiFePO4,acetylene black,polyvinylidene fluoride,organic/inorganic graphene composites were studied.The experimental results show that the optimal sample prepared by sintering the mixture at 400?,which was obtained by mixing the boron doped LiFePO4 and organic/inorganic graphene composites in the proportion of 3:5 in a weight ratio.The surface and particales gap of the optimal sample observed to be covered with graphene particles.The partial graphene seems to become the conductor with the morphology similar to nanotube by crimp and winding.Even discharged at the 200C rate,the opmital sample exhibits the 1st cycle capacity of 122.3 and columbic efficiency of 84.3%,and the 40th cycle capacity of 128.4mAh/g,respectively.The capacity retention rate is 105%in 40 cycles.At 55?,the sample exhibits the 1st cycle capacity of 150.3mAh/g(0.2C rate)and 116.5mAh/g(100C rate).At-22?,the sample exhibits the 1st cycle capacity of 91.5 mAh/g(2C rate)and columbic efficiency of 81.6%,and the 40th cycle capacity of 71.4mAh/g,respectively.And the capacity retention rate is 78%in 40 cycles.Therefore,the optimal sample exhibits the excellent charge performance with ultrahigh current while discharged either at room temperature,or-22? or at55?.