Nanocarbon Materials Modified Lithium Iron Phosphate And Its Lithium Ion Battery Performance

Olivine-type structure of lithium iron phosphate is considered to be used in portable electronic products and electric vehicles on one of the ideal cathode material.Lithium iron phosphate has the advantages of a low price,high charge-discharge capacity,green non-toxic,good thermal stability and so on.However,lithium iron phosphate has the shortcomings of a complex preparation process,slow diddusion rate of lithium-ion and poor conductivity,which leads to the low capacity,poor stability and other issues of lithium iron phosphate itself.In this paper,the particles of lithium iron phosphate particles was enhanced by coating a nanocarbon layer onto LiFePO4 to prepare C@LFP.Then the(nitrogen doped)carbon nanotubes with one-dimensional structure and good electrical conductivity was used as a conductive agent to be dispersed in the C@LFP for enhancing the conductivity between the C@LFP particles,thereby improving the specific capacity,rate capability and cycle life of lithium iron phosphate.The composition and structure of the composites were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),specific surface and pore structure analysis and thermogravimetric analysis(TGA).Cyclic voltammetry and AC impedance were used to test the electrochemical properties of the materials.First,the C@LFP were prepared by the pyrolysis method using the glucose as the precursor.The influence of heating temperature and using amount of glucose on the texture,structure and electrochemical properties of C@LFP were investigated.The optimal condition for preparing C@LFP were determined at the temperature of600?,the ratio of lithium iron phosphate and glucose of 10: 1.Glucose pyrolysis results in a carbon coating onto the surface of lithium iron phosphate,which enhances the particle conductivity.The electrochemical impedance spectroscopy(EIS)showed that the charge transfer impedance of C@LFP was significantly decreased.The discharge capacity of C@LFP at 0.1C rate was 144.8 mAh·g-1,which was higher than that of pure lithium iron phosphate(133.7mAh·g-1)at constant current charge-discharge test.The specific capacity of C@LFP was reduced from 120.7mAh·g-1 to 113.6 mAh·g-1 at a rate of 1 C after 200 cycles,and the retention rate was94.1%.The specific capacity of LFP decreased from 106 m Ah·g-1to 93.5 mAh·g-1at1 C rate,and the retention rate was 88.2%.At 0.1C rate,compared to pure LiFePO4,the capacity of C@LFP by 8.3%.Moreover,in order to improve the performance of LFP,carbon nanotubes(CNTs)and nitrogen-doped carbon nanotubes(NCNTs)were used as conductive agents to enhance the electric conductivity between particles.The results showed that CNTs-C@LFP prepared by adding the CNTs of 3 wt% has a specific discharge capacity of 158.6 mAh·g-1 at 0.1C rate.NCNTs-C@LFP were prepared as followedprocedure.Firstly,the carbon nanotubes coated with polypyrrole(PPy@CNTs)were prepared,which then were then mixed with C@LFP and heat-treated to convert PPy@CNTs into NCNTs,thereby obtaining NCNTs-C@LFP.The results show that the NCNTs are uniformly dispersed in the C@LFP particles,therefore,the conductive network consisting of the NCNTs between the LFP and the carbon layer coating LFP.When the doping amount of PPy@CNTs is only 1wt%,the specific capacity of LFP is162.3mAh·g-1 at 0.1C rate.That is,the specific capacity of the NCNTs-C@LFP obtained through the carbon-coaing and nitrogen-doped carbon nanotube addition increases by 21% compared with pure LFP.Meanwhile,NCNTs-C@LFP also shows the improved the performance of rate capability and charge-discharge cycle.