Control And Research Of Nano-spherical Lithium Iron Phosphate Crystal Morphology

LiFePO₄ is an important cathode material in lithium batteries,which has the advantages of good cycle performance,high safety,and environmental friendliness.However,its low conductivity,low ion diffusion coefficient,and low tap density have restricted its development.The core factors that affect its performance are morphology,nanometerization,and electrical conductivity.This thesis focuses on the research of controlling nanometerization and electrical conductivity.In order to achieve LiFePO₄ nanometerization,we first start with FePO₄ precursors.During the growth of iron phosphate crystals,surfactants are added to control the crystal morphology and nanometerization.The influence of the synthesis method on lithium iron phosphate is also studied,especially focusing on the coating method and coating materials.The specific work and research results of this article are as follows:Firstly,Fe（NO₃）₃·9H₂O and NH₄H₂PO₄ were selected as raw materials to synthesize FePO₄ precursors by hydrothermal method.The phase analysis of XRD,SEM,TG,FTIR and other phases were used to explore the different p H,reaction time and reaction temperature on precursor influences.Experiments show that the ferric phosphate prepared by hydrothermal method is FePO₄ H2 O,p H is 1.45,reaction temperature is 120 ℃,reaction time is 6h,and products with good crystal and morphology can be obtained.The precursor morphology and structure were modified by adding four different surfactants CTAB,ascorbic acid,PVP and Tween-20.The results show that when 3% CTAB is added,the resulting product has the most regular morphology,small size and particle size,and a spherical controllable micronano FePO₄ precursor can be obtained.Subsequently,LiFePO₄ anode materials were prepared by coprecipitation method,one-step hydrothermal method and two-step hydrothermal method.The first discharge capacity of LiFePO₄ anode material prepared by two-step hydrothermal method was 120.3 m Ah/g,and the capacity retention rate was 91.8% after 100 charge and discharge cycles.The LiFePO₄ cathode material prepared by the coprecipitation method has a specific discharge capacity of 103.2 m Ah/g for the first time.The LiFePO₄ cathode material synthesized by one-step hydrothermal method has a first discharge specific capacity of 113.4 m Ah/g.By comprehensively comparing three different preparation methods to synthesize LiFePO₄ cathode material,by analyzing and comparing its morphology structure and electrochemical performance,LiFePO₄ cathode material prepared by two-step method has the best performance,The materials prepared by the two-step hydrothermal method were determined for subsequent coating modification.Finally,the effects of different carbon sources and different amounts of carbon on LiFePO₄ cathode materials were discussed.The effects of organic carbon source glucose and ascorbic acid and inorganic carbon source acetylene black on the coating effect were discussed.The experimental results show that when glucose is used as the carbon source for carbon coating,the specific discharge capacity is 128.1m Ah/g at 0.2C for the first time,when ascorbic acid is the carbon source,the specific capacity is 132.5m Ah/g.After 100 charge and discharge cycles,the specific discharge capacity is still about 126.2m Ah/g,and the capacity retention rate is 95.9%,and When acetylene black is used as the carbon source for coating,the specific capacity is 120.1 m Ah/g,.The results show that the carbon source coated with ascorbic acid has the best electrochemical performance.When ascorbic acid was used as the carbon source,the effects of different contents on the product performance showed that when the carbon content was 5%,7%and 10%,the specific discharge capacities were 132.5 m Ah/g,156.9 m Ah/g,and 130.8 m Ah/g.Therefore,when the carbon content is 7%,it can effectively improve its electrochemical performance,maintain a good cycle,and maintain a stable discharge specific capacity at different rates.The electrochemical performance is the best,reaching more than 90% of the theoretical capacity of LiFePO₄,and the capacity retention rate after 100 charge and discharge cycles is 96.4%.