Research Of High Specific Capacity And High Tap Density LiFePO₄ And LiMnFePO₄ Battery Materials

The current carbon coating technology has a low tap density(0.8 g cm^-3).But in this dissertation,with the improvement of carbon coating process of lithium iron phosphate and lithium iron manganese phosphate materials.It has been efficaciously improved the compactness between the primary particles of activity material when the super-conductive material Ketjen Black was used as a addition of carbon source.As the discharge specific capacity of the material was improved,the tap density was also increased.At the same time,providing a direction of simultaneously improve the discharge specific capacity and tap density of the material.In addition,we also use the pre-doping method of the precursor to achieve the in-situ doping of titanium ions in the lithium iron phosphate crystal.It also greatly improved the specific discharge capacity of the material.We have studied that the effect of different amounts of Ketjen Black on lithium iron phosphate and lithium iron manganese phosphate materials when carbon coating on the primary particles of lithium iron phosphate and lithium manganese phosphate materials with Ketjen Black and glucose as composite carbon sources.The influence of the electrical conductivity and the degree of primary particle aggregation,and the impact on the specific discharge capacity and tap density of the material.The experimental results show the performance of lithium iron phosphate with the different amount of Ketjen Black.It was determined that when the addition of Ketjen Black is 5 wt%of glucose,the synthesized material not only has a significant improvement in electrochemical performance(161.6 m Ah g^-1at 0.2 C),and the tap density of the material was also improved(1.6 g cm^-3).For lithium iron manganese phosphate materials,when the mass of Ketjen Black added was 5 wt%of glucose,the composite material exhibits the best electrochemical performance.After 500 cycles,the discharge capacity at a rate of 0.2 C is 159.3m Ah g^-1,the capacity retention rate is 96.8%.In addition,this material also shows the excellent performance in 14,500 cylindrical full cell,and after 100 cycles,it has an excellent capacity retention rate of approximately 96.9%at 1 C rate.The effect of titanium ion doping on the crystal structure of the synthesized lithium iron phosphate was studied.After tried the various doping methods,it was found that the titanium ions were Pre-doped（the titanium ions in the iron phosphate were completed during the preparation of iron phosphate）.The incorporation of lithium in the lattice can effectively achieve the in-situ doping of titanium ions in the lithium iron phosphate crystal.In this way,the titanium ions can replace apart amount of the iron ions in the lithium iron phosphate crystal.It can be seen from the transmission diagram,the spacing of lattices is slightly shortened.The lithium iron phosphate material synthesized by in-situ doping of titanium ions has a smaller primary particle diameter,which greatly shorted the transmission path of lithium ions and greatly improved the diffusion rate of lithium ions.And the lithium ion exchange rate was greatly improved the low-temperature performance of lithium iron phosphate,and the low-temperature environment of-20℃and 1 C discharge specific capacity is 122.3 m Ah g^-1.