Effect Of Precursors Morphological Characteristics On The Electrochemical Performance Of LiFePO₄/C As Cathode Material For Lithium Ion Batteries

This paper focused on the preparation and morphological modification of FePO₄and Fe₃?PO₄?₂ as precursors of LiFePO₄/C cathode materials for lithium ion batteries.Lamellar nanostructured FePO₄ and porous hollow structure Fe₃?PO₄?₂/C composite were prepared by liquid crystallization and hydrothermal methods respectively and its were used as precursors to further synthesize LiFePO₄/C cathode materials.The effects of precursors morphological characteristics on the electricalchemical performance of lithium iron phosphate cathode materials were investigated and analyzed.The FePO₄ precursor was synthesized by liquid phase crystallization using ferric nitrate,phosphoric acid and nitric acid as raw materials.The effects of nitric acid concentration,reaction time and raw material ratio on the quality?n?Fe?/n?P??and conversion rate of samples were discussed.Secondly,the spindle-like ferric phosphate precursor with lamellar nanostructure was prepared by making use of the effect of the Al?OH?₃ colloid formed by adding NaAlO₂ in the system on the growth direction of crystal surface during the crystallization process and immersing in subsequent acetic acid solution.And the effects of the addition amout of NaAlO₂ and the immersing concentration of acetic acid solution on the quality and morphology of iron phosphate were discussed.The iron phosphate precursor was further prepared into lithium iron phosphate by high temperature solid state reduction.The samples were characterized by XRD,FT-IR,SEM,TEM,analysis of BET surface area and porosity,laser particle size analysis and electrochemical performance test.The results showed that when the reaction temperature is 95 C,the concentration of nitric acid is 0.6 mol/L,the reaction time is 150 minutes and the ratio of raw materials is n?H₃PO₄?/n??FeNO₃?₃?=3:1,the obtained ferric phosphate sample?FP?has highest quality?n?Fe?/n?P?=0.98?and large conversion rate?60%?.When the NaAlO₂ content is m?NaAlO₂?/m??FeNO₃?₃?=8%and the immersion concentration of acetic acid solution is 60wt%,the obtained ferric phosphate precursor?L-FP2?has the best microscopic morphology of lamellar nanostructure and quality.The first discharge specific capacity of the LiFePO₄/C prepared by the lameller nanostructure FePO₄ precursor with 8%NaAlO₂ addition at0.1C increased by 20%,reaching 151.48 mAh/g,and the electrode charge transfer resistance was reduced by 75%,just 27.23?,compared with the LiFePO₄/C that prepared by non lameller nanostructure FePO₄.And the retention rate of capacity reached 95%after 50 cycles at 0.5C rate.Meanwhile,the mechanism of Al?OH?₃colloid affecting the formation of lammellar nanostructure FePO₄ was analyzed.At the same time,the carbon-coated Fe₃?PO₄?₂/C composite precursor with porous hollow sphere structure were prepared by hydrothermal method using ferric nitrate,dihydrogen phosphate and glucose as raw materials.The Fe₃?PO₄?₂/C precursor is crystal form of graftonite.Under the catalysis of Fe³⁺,glucose was dehydrated and polycondensed into a polymer to slowly reduce Fe³⁺to Fe²⁺and then Fe²⁺combined with PO₄^3-to react in hydrothermal reaction.The effects of the glucose concentration,reaction time and the ratio of water/ethylene glycol on the preparation of Fe₃?PO₄?₂/C composite precursor were discussed.The Fe₃?PO₄?₂/C precursor was homogeneously mixed with nanospherical Li₃PO₄ prepared by coprecipitation with lithium hydroxide and phosphoric acid as raw materials,and lithium iron phosphate was further prepared by high temperature solid state method.The samples were characterized by XRD,FT-IR,TG-DSC,SEM,analysis of BET surface area and porosity,laser particle size analysis and electrochemical performance test.The results show that when the glucose concentration is 0.5M,the reaction time is 48h and the ratio of water/ethylene glycol is all water,1:3,1:1 and 3:1,the Fe₃?PO₄?₂/C composites can be prepared.The morphology and in-situ carbon content of the Fe₃?PO₄?₂/C precursor prepared at different water/glycol ratios were different.When the water/ethylene glycol ratios is 1:1,the carbon coated content of the Fe₃?PO₄?₂/C precursor is about 13.78%and it has the best porous hollow morphology.LiFePO4/C material further prepared by this Fe₃?PO₄?₂/C precursor exhibited better electrochemical performance.The initial discharge specific capacities of the LiFePO₄/C material at 0.1C is up to 165mAh/g,and the discharge specific capacities at 0.2C,0.5C,1C,5C and 10C are 158,139,139,120,95 and 66mAh/g respectively.It has better stability for 100 cycles at 0.5C and the retention rate of capacity reached96%.At the same time,the formation mechanism of sphere Fe₃?PO₄?₂/C precursor with porous hollow structure and the reaction mechanism of using Fe₃?PO₄?₂/C precursor and Li₃PO₄ to synthetise LiFePO₄/C by solid state reaction are analyzed theoretically.