LiFePO 4 Cathode Material Prepared By Sol-gel Method And Doping Modification

With the energy and environmental problems highlighted increasingly,the lithium-ion battery has become a hot topic in current research.Li FePO₄ has attracted broad attention because of its wide range sources,low price,high safety,long cycling life,and environmental friendly,but compared with other types of cathode materials of lithium-ion battery,LiFePO₄ has a low electronic conductivity and lithium ion diffusion rat,which restrict its application.In this paper,we synthesized the LiFePO₄/C composites by sol-gel method,using low-cost raw materials as sources,through carbon doping,metal cation doping and cation-anion co-doping to study the effect on its performance,and discussed the influence of Mn²⁺ doping on Fe-site and the Mn²⁺ and F-co-doping,the main conclusions were concluded as follows:1.The LiFePO₄/C composites were synthesized by sol-gel method by using LiOH·H₂O,FeCl₃·6H₂O,H₃PO₄ and citric acid as raw materials.The results show that the samples have olivine type crystal structure and small particle size,which indicates that the carbon didn't affect its structure,and doping a certain amount of carbon can effectively limit the growth of crystal.2.Through optimize the synthesis parameters of the materials,we studied the influence of citric acid adding quantity,sintering temperature,and sintering time on its performance.The results showed that the samples prepared at 650?,8 hours and the citric acid ration n?FeCl₃?:n?citric acid?=1:0.6 can display the best electrochemical performance.The electrochemical test results showed that the first discharge capacity measured at 0.1C,0.5C,1C,and 5C was 134.5 mAh·g^-1,109.2 mAh·g^-1,101.8 mAh·g^-1,73.9 mAh·g^-1,respectively,and have a different degrees of degradation after 20 cycles.3.A series of Fe-site Mn²⁺ doped of Li Fe_1-xMn_xPO₄/C composites were prepared by sol-gel method.The results showed that the materials can remain the olivine-type crystal structure,even at a large doping proportion of 20%.The appropriate doping proportion of Mn²⁺ can improve its charge and discharge capacity and cycle performance,and the material with doping 5% of Mn²⁺ has the best electrochemical performance.The first discharge capacity measured at 0.1C can reach 145.0 mAh·g^-1.In addition,under 0.5C,1C,and 5C,its capacity was 127.3 mAh·g^-1,123.0 mAh·g^-1,90.1 mAh·g^-1,respectively,and the basic capacity after 20 cycles without degradation.4.In the experiment of cation-anion co-doping of LiFePO₄/C composites,we synthesized a series of Mn²⁺ and F-co-doped LiFe_0.95Mn_0.05?PO₄?1-yF3y/C?0.01?y?0.03?materials by using Mn?CH₃COO?₂ and LiF as dopant.The results showed that compared with the cationic doping,the Mn²⁺ and F-co-doping has increased particle size and decreased the electrochemical performance.The LiFe_0.95Mn_0.05?PO₄?_0.99F_0.03/C material measured at 0.1C,its first discharge capacity was 138.3 mAh·g^-1,after recycle 20 times,the capacity retention rate was just 98.2%.And at 0.5C,1C,5C discharge rate,the capacity was 116.1 mAh·g^-1,109.6 mAh·g^-1,85.5 mAh·g^-1,respectively.After 20 cycles,can remain 97.7%,96.1%,94.8%,respectively.