Preparation And Modification Of High-Vibration Lithium Iron Phosphate

Lithium iron phosphate positive electrode material has good thermal stability,high safety performance,long cycle life,and it has attracting attention because of rich reserves of raw materials and no environmental pollution.Therefore,it has become the key research object of lithium ion battery' s production and development in the field of power and energy storage currently.However,LiFePO4 has some disadvantages due to its structural characteristics,such as low electrical conductivity and lithium ion diffusion rate,low tap density,and poor discharge performance under low temperature conditions.These problems have restricted the application of LiFePO4 cathode materials.In this thesis,lithium iron phosphate was prepared by co-precipitation and solid-phase method.On the one hand,optimizing the synthesis process to reduce the product size and uniform the particle size distribution.Firstly,use co-precipitation method to prepare dense iron oxide precursors,adjusting the dispersant,precipitation temperature and other processes to improve the material density,particle size,and appearance from the source.Then synthesize the high vibration lithium iron phosphate by optimized roasting process on this basis.On the other hand,by doping a metal heteroatom,the electrical conductivity and the lithium ion diffusion rate of the material are structurally improved.The material was tested and characterized by SEM/EDS,XRD,laser particle size measurement,EIS,CV and other means.The effect of sodium naphthenate as a dispersant is better than polyethylene glycol and Tween 80.With the increase of added dispersant' s amount,the crystallinity of the iron oxide powder was first enhanced and had no significant change after.Consistent with this,the SEM image and the particle size distribution diagram show that the iron oxide particles are smaller and the distribution is more uniform when the dispersant reaches 10%.With increasing water bath temperature,the crystallinity of the iron oxide powder gradually increased,while the SEM image and particle size distribution diagram showe that the of the iron oxide particles became smaller and more uniform first and then agglomeration increased.When using lithium nitrate as a lithium source,the powder particle size is better than using lithium hydroxide and lithium carbonate.When prepare the lithium iron phosphate cathode material,the crystallinity of the positive electrode material is increased as the sintering temperature is increased,but it is not changed when the temperature continuously increased.The particle size distribution diagram shows that the particle size of the material slightly decreased slightly first and then the particles reunification increased.The electrochemical performance analysis shows that the 700? sample has the highest specific discharge capacity,the best cycle performance,good reversibility,and low impedance.With the prolongation of sintering time,the crystallinity of the positive electrode material had no obvious change after the first increase,but the particle size first slightly decreased and then increased,that is,agglomeration occurs.Among them,6h sample has the best discharge chemical property.The specific volume of the sample prepared at 700? and 6 h was 140.94 mAh/g.The optimal proportion of doped metal elements was explored.LiFe0.95Co0.05 PO4 and LiFe0.9Ni0.1PO4 have the best discharge performance,with discharge capacities of 145.11 mAh/g and 147.96 mAh/g at 0.2 C rate,2.9%and 5.0%higher than undoped material,respectively.After 50 cycles,the specific discharge capacities are 137.70 mAh/g and 143.85 mAh/g,which are 2.8%and 7.4%higher than that of the undoped material,respectively;the transfer resistance are 335.6 ? and 317.0 ?,and are both smaller than pure lithium iron phosphate of 382.1 ?,indicating that doping metal elements can increase the conductivity of the material.