| With increasing emphasis on energy and environmental issues and the rapid development of modern technologies,the requirements for batteries performance are increasing.Lithium ion batteries are attractive for using in the field of mobile power source with the favorable properties of high energy density,long cycle life,low self-discharge rate,no memory effect and green environmental protection.Lithium-ion batteries are mainly composed of cathode materials,anode materials,diaphragms and electrolytes.The cathode materials occupy more than 40%of the total cost of lithium-ion batteries.The performance of lithium-ion batteries are depended on the cathode materials.Cathode materials occupy a core position in lithium-ion batteries.The olivine-type lithium iron phosphate(LiFePO4)has been considered to be a promising cathode material because of its low cost,non-toxicity,abundant raw materials,high,theoretical capacity(170 mAh·g-1)and good thermal stability.However,the practical application is limited by the low electrionic conductivity,low lithium-ion transport kinetic and the low density.To improve the electrochemical properties,researches have been focused on improving the electrical conductivity of LiFePO4 materials.Surface modification and metal-ion doping are simpler and more effective approachs to improve the electrochemical performance of LiFePO4 materials.In this study,LiFePO4 materials were prepared via high-temperature ball milling route.The influence of the mixing process,ultrasonic dispersion time,high temperature ball milling temperature and high temperature ball milling time on the morphology,structure and electrochemical properties of LiFePO4 materials were investigated.We explored the effect of the co-doping of V and F and graphene coating on the properties of LiFePO4,and obtained a superior process for the preparation of LiFePO4/C by high temperature ball milling route.(1)LiH2PO4,Li2CO3,Fe2O3,C6H12O6·H2O and PEG12000 were used as raw materials.The solid phase reaction temperature was determined by analyzing the TGA-DSC curve of LiFePO4/C precursor.LiFePO4/C was prepared by solid phase synthesis and high temperature ball milling,respectively.The initial discharge capacity of LiFePO4/C prepared by high-temperature ball milling method was 143.6 mAh·g-1 at 0.1C-rate with the discharge efficiency was 95.0%.Compared with the high temperature solid-state reaction method,the performance of LiFePO4/C synthesized by high-temperature ball milling method was much better.(2)LiFePO4/C materials were synthesized by high temperature ball milling route with different mixing processes.The effects of different mixing processes on the structure and electrochemical properties of LiFePO4/C materials were investigated.The results indicate that the ultrasonic dispersion mixture was capable of bringing the reactant molecules into contact sufficiently to achieve a sufficient mixing of the materials.The obtained LiFePO4/C material had the smallest particle size and uniform distribution,and exhibited the better electrochemical performance.Its initial discharge capacity was 151.4 mAh·g-1.The discharge capacity was 146.9 mAhg-1 at 0.1C-rate over 50 cycles with capacity retention of 96.8%.(3)LiFeP04/C materials were prepared via high-temperature ball milling route with ultrasonic dispersion as mixing process.The initial discharge capacities of LiFePO4/C samples were used as the evaluation index by the Box-Behnken experimental design.The experimental factors of ultrasonic time,ball milling temperature and high temperature ball milling time are optimized.Analysis of variance showed that the model proved to be high significance and accuracy.The response surface method optimization results show that the ultrasonic time was 83 min,the high temperature ball milling temperature was 628?,and the high-temperature ball milling time was 8.8 h.Under this condition,the maximum initial discharge specific capacity(156.9 mAh·g-1)of LiFePO4/C sample could be obtained.According to the response surface methodology,we prepared LiFeP04/C sample when the values of ultrasonic time,ball milling temperature and ball milling time were 85 min,630? and 9 h.Its initial discharge capacity was 158.1 mAh·g-1 at a 0.1C-rate which was in consistence with the predicted discharge capacity of 156.9 mAh·g-1,the results indicated that the experimental values were consistent with the predicted values and the fitness was high.Moreover,its specific capacities were 150.3(0.5C),142.1(1.0C),133.8(2.0C),120.5(5.0C)and 102.6 mAh·g-1(10C),respectively.(4)V and F co-doped LiFePO4/C cathode material was synthesized by high-temperature ball milling route with ultrasonic dispersion as mixing process using V2O5 and LiF as dopants.The experimental results showed that V and F doping made the cell volume of LiFePO4 become larger which can broaden the lithium ion diffusion path,shorten the lithium ion diffusion path,enhance the lithium ion diffusion.LiFe0.9gV0.02(PO4)0.98F0.06/C delivered initial discharge capacities of 166.5 mAh·g-1,which was close to the theoretical value of 170 mAh·g-1.Moreover,its discharge capacities were 155.7(0.5C),151.1(1C),143.1(2C),135.6(5C)and 120.3 mAh·g-1(10C)after 100 cycles,respectively.Significant capacity decay was not observed.(5)LiFePO4/graphene(LiFePO4/G)cathode material was prepared by a facile high-temperature ball milling route with ultrasonic dispersion as mixing process.The results exhibited that graphene combined with the LiFePO4 clusters giving rise to a conductive network,which can broaden the lithium ion diffusion path and shorten the path of lithium ion between the cathode material and the electrolyte.Graphene improved the electronic conductivity of the material,making electron transfer between the surface of LiFePO4 particles and graphene much easier,which can enhance the.diffusion of lithium ions,and reduce the resistance of lithium ion deintercalation.The initial discharge capacity of LiFePO4/G was 164.1 mAhg-1 at 0.1 C-rate,and over 50 cycles the capacity retention was 99.2%.Moreover,at various rates,its capacity retention rates were 98.1%(0.5C),97.3%(1C),96.4%(2C),96.1%(5C)and 95.9%(10C)over 100 cycles,no significant capacity decay was observed.LiFePO4/G exhibited excellent electrochemical rate performance and cycle stability. |
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