Preparation And Performance Study Of LiFe_1-x-yMn_xMg_yPO₄/C Composites

Lithium ion batteries are favored by people because of the advantages of high energy density,green and environmental friendliness.As one of the positive materials,the LiFePO₄ has the advantages of stable structure,long cycle life,safety and so on.With the wide use of lithium ion batteries,people have put forward higher requirements for it.It is hoped that there is a higher energy density and longer cycle life.This article takes LiFe_1-x-yMn_xMg_yPO₄/C as the research object.Because the4.1V voltage platform of Mn³⁺/Mn²⁺is higher than the 3.5V voltage platform of Fe³⁺/Fe²⁺,a higher energy density can be obtained by doping Mn.Nanoscale porous material LiFe_1-xMn_xPO₄/C（x=0.25,0.50,0.75）was prepared by sol-gel method.In situ carbon coating is carried out at the same time as Mn doping.The effects of sintering temperature,carbon content and Mn doping amount on the structure,morphology and electrochemical performance of the materials were investigated.The LiFe_0.75Mn_0.25PO₄/C material was optimized by the use of surfactant Cetyltrimethyl ammonium bromide（CTAB）and Mg doping.The basic research on the thermal stability of Li FePO₄/C-LiMn₂O₄ blended cathode materials is carried out from the point of view of safety.The main conclusions are as follows:（1）LiFe_0.75Mn_0.25PO₄/C is double mesoporous materials,the ratio of mesopore in diameter 9.5 nm increases with the increase of sintering temperature.The optimum sintering temperature is 750℃.When the carbon content is 9.04%（wt%）,the initial discharge specific capacity is 120.28 mAh/g at 10C.And discharge specific capacity keeps 80%after 400 cycles.When the carbon content is 10.97%,the primary particle size is tens of nanometers,the specific surface area is 90.06m²/g,the diameter of the mesopore is 12.10 nm.The initial discharge specific capacity is115.69 mAh/g at 10C,and discharge specific capacity keeps 80%after 490 cycles.Nanoscale prime particle,larger mesopore size,in-situ carbon coating and high crystallinity are beneficial to the diffusion of lithium ions and the transfer of electrons,therefore,the material has good electrochemical performance.LiFe_0.75Mn_0.25PO₄/C was optimized by using CTAB,the carbon content is 14.66%（wt%）,the initial discharge specific capacity is 77.88 mAh/g at 10C.As Mg stabilizes the crystal structure,the Li Fe_0.75Mn_0.24Mg_0.01PO₄/C has good capacity retention.（2）The optimum sintering temperature of nanoscale porous material LiFe_0.50Mn_0.50PO₄/C is 800℃.The carbon content is 14.36%,and the prime particle size is tens of nanometers.The thickness of the coated conductive carbon layer is about 3nm,and the excess carbon is coated around the second particles.The initial discharge specific capacity is 90.32 mAh/g at 10C,and discharge specific capacity keeps 80%after 300 cycles.The optimum sintering temperature of LiFe_0.25Mn_0.75PO₄/C is 725℃,the carbon content is 20.31%,and the thickness of coated conductive carbon layer is about 5nm.The initial discharge specific capacity is 103.40mAh/g at 10C,and discharge specific capacity is over 80%after 600 cycles.The good electrochemical performance of the materials is attributed to the existence of pores,nano size particle,the presence of conductive carbon,and the better dispersion.（3）The thermal stability and chemical stability of the LiFePO₄/C-LiMn₂O₄blended cathode materials is worse than that of two single components.In Ar 500℃,the oxygen is released and the impurity phases appear.The appearance of the impurity phases is the result of the chemical reaction of two components under the heating condition.The discharge capacity of the material after heat treatment is obviously lower than that before heat treatment.This is the result of the appearance of the impurity phases.Therefore,the LiFePO₄/C-LiMn₂O₄ blended cathode materials needs strict thermal management and control in use.