Preparation And Modification Of LiFePO₄ Material For The Batteries

LiFePO₄ is considered as a potential candidate for power lithium ion batteries because of high theoretical capacity,friendly environment,long cycle life and good thermal stability.However,LiFePO₄ has bad consistency from batch to batch,low conductivity and low lithium ion diffusion coefficient,which hinder the further development of LiFePO₄ as power battery.In this work,the performance of LiFePO₄ was improved systematically from the preparation method to the modification with electrochemically active polymer.With the purpose to improve the consistency of LiFePO₄ with no impurities,LiFePO₄ was prepared with Li₃PO₄ as main lithium source and other lithium salts as ancillary lithium source.Effects of Li₂CO₃,LiOH,CH₃COOLi,C₆H₅Li₃O₇ and Li₃PO₄ ancillary lithium source on the performances of LiFePO₄ had been investigated respectively.The results showed that LiFePO₄ with Li₂CO₃ as ancillary lithium source had no impurities and high crystallinity.LiFePO₄ with Li₂CO₃ as ancillary lithium source had better electrochemical property with initial capacity of 140.1mAh/g at 0.1C and 109mAh/g at 1.0C and retention of 95.2%after 50 cycles.Carbon coating is a common way to improve the conductivity of LiFePO₄,but carbon decreases the specific capacity of active materials.In this work LiFePO₄ was coated with electrochemically active polymer poly-aniline（PANi）.H₂O₂ oxidized aniline（An）to prepare LiFePO₄/PANi in the HCl solution with high ionic strength.PANi is like a thin gauze coating the surface of LiFePO₄ particles.LiFePO₄ with weight percent of 4%poly-aniline showed good performance with capacity of 142.6mAh/g at 0.1C and retention of 91.3%.At last,we designed the polymer-metal conductive matrix composite to solve the problem of low conductivity through the introduce of metal.LiFePO₄ coated with PANi-Ag matrix composite had been investigated.AgNO₃ and aniline directly reacted to form PANi-Ag composites.We respectively investigated effects of concentration of formic acid and aniline/AgNO₃ mol ratio on the morphology,conductivity and electrochemical performance of PANi-Ag composite.The results showed that PANi-Ag composite possessed more poly-aniline and moderate conductivity at formic concentration of 0.4M and An/AgNO₃ mol ratio of 0.6.LiFePO₄/PANi-Ag composite was synthesized at the best condition.Compared with LiFePO₄/C composite,LiFePO₄/PANi-Ag composite showed more excellent properties.LiFePO₄/PANi-Ag composite had capacity loss of 1%at 0.1C after 50 cycles and capacity of 103.2mAh/g with retention of 67.8%at 5.0C high rate.The LiFePO₄/PANi-Ag cathode had low polarization and the cathode surface was smooth after discharging at 5.0C.