The Research On Synthesis With Solid-State Method And Electrochemical Properties Of LiFePO₄

A new kind of lithium ion batteries cathode material, lithium iron phosphate, was prepared by solid-state method. The charging-discharging performance and the microstructure of different samples prepared by this method were taken as the investigation targets of optimization, and the technological conditions were optimized. The compatibility of the sample with nine different kinds of electrolytes was investigated. The influences of some other factors: the granularity of sample, the content of conductive additive—acetylene black and the content of binder—polytetrafluoroethylene ( PTFE ) in the electrode membrane and the charging-discharging current density etc, on the charging-discharging performances of the sample were investigated by the orthogonal method. The sample was coated by different carbon sources, and the influence of carbon coating on the structure and electrochemical properties of the sample was investigated. The experimental results are as follows:1. The influence degree order of the decomposition conditions on D₃ ( the discharging capacity in the third cycle ) is: decomposition temperature > flow rate of protection gas > decomposition time > milling time. The best combination of the technological conditions is: the ball milling time is 2h, the decomposition temperature is 350℃, the decomposition time is 5h, and the flow rate of protection gas is 1.5L/min, while D₃ is 121.78mAh/g andη3 ( the Coulombic efficiency of the third cycle ) is 98.56%.2. The influence degree order of the synthesis conditions on D₃ is synthesis temperature > Li/Fe/P molar ratio > synthesis time > briquetting pressure. In the best combination of the technological conditions, the Li/Fe/P molar ratio is 1.05:1:1, the synthesis temperature is 650℃, the synthesis time is 18h, and the briquetting pressure is 60MPa, while D₃ is 127.49mAh/g andη3 is 99.46%. The influence degree order of the synthesis conditions on crystalline cell volume ( V ) and average size of crystallites ( D₁₃₁ ) is: synthesis temperature > Li/Fe/P molar ratio > synthesis time > briquetting pressure. The best combination of the technological conditions is: the Li/Fe/P molar ratio is 1.05:1:1, the synthesis temperature is 650℃, the synthesis time is 18h, and the briquetting pressure is 60MPa, while V is 0.29172nm3, which is most close to the crystalline cell volume ( V ) of the theoretical value, and the D₁₃₁ is 34.383nm, which is suitable for the lithium ions to diffuse into the central part of the crystallites. It is obvious that the best combination of the technological conditions determined by microstructure is the same with those determined by charging-discharging performances. They must be optimized simultaneously.3. The sample shows good charging-discharging potential platforms during charging-discharging in nine different electrolytes and good electrochemical performance in the electrolytes using LiClO₄ as solute, EC+DEC (1:1,v:v), EC+DMC (1:1,v:v) and PC+DME (1:1,v:v) as solvent. The best electrochemical performance of the sample can be obtained in 1mol/L LiClO₄/EC+DMC (1:1), the cyclic performances are worse and the fading capacities are larger in the electrolytes using LiPF₆ and LiBF4 as solute.4. The factors such as the granularity of sample, the content of acetylene black and the content of PTFE in the electrode membrane and the charging-discharging current density also make influences on D₃. The influence degree order of above four factors on D₃ is: content of acetylene black in the electrode membrane > charging-discharging current density > granularity of sample > content of PTFE in the electrode membrane. In the best combination of the four factors, the granularity of sample is -400 mesh, the content of PTFE in the electrode membrane is 5%, the content of acetylene black in the electrode membrane is 20%, and the charging-discharging current density is 10mA/g, while D₃ is 131.25mAh/g andη₃ is 99.37%.5. The crystal structure is not perfect and the electrochemical properties are not significantly enhanced when the samples are coated by acetylene black, but both of them can be great improved after coated by using glucose as the carbon source, and then the samples demonstrate good charging-discharging potential platform properties and rate capabilities. After coating with phenol formaldehyde resin as the carbon source, samples of high purity and small average size of crystallites are prepared, the pyrolytic carbon forms good coating structure, the particles'size of samples is very small and uniform, even nano-meter particles are prepared, the electrochemical properties of the samples are greatly improved and the samples show excellent charging-discharging potential platforms and rate capabilities. While the coating carbon content is 10%, the sample possesses the best electrochemical properties, then D₃ is 164.89mAh/g andη₃ is 99.20%, and D₃ could achieve 149.12mAh/g at 1C rate.