Modification On LiFePO₄and Li₂FeSiO₄Cathode Materials For Lithium Ion Batteries

In order to overcome the defects of low electronic conductivity and low lithium iondiffiusion coefficient of LiFePO₄and Li₂FeSiO₄cathode materials, different approacheshave been employed, such as carbon coating, metal oxide coating, and so on. The resultsare as follows:LiFePO₄/C composites have been successfully prepared by high-temperature solid-statereaction route using citric acid as carbon source. The experimental results indicate thatcarbon content of LiFePO₄/C composite decreased with increasing temperature, anddetrimental effects on electrochemical performance of LiFePO₄/C sample have caused bylarger particle size under higher sintering temperature. The sample synthesized at700°Cshows the best electrochemical performance with moderate carbon content and uniformparticle distribution. The LiFePO₄/C electrode could deliver discharge capacity of110.9mAh g^-1and108.8mAh g^-1at10C of the first and50thtime, respectively, with theretention ratios of98.1%; Under20C, The discharge capacity is up to78.4mAh g^-1, andthe capacity retention ratio is92.7%after50cycles. LiFePO₄-C, LiFePO₄-G andLiFePO₄-（C+G） composites have been successfully synthesised by carbonthermalreduction method by using glucose, activated graphite and glucose-activated graphite ascarbon source, respectively. Among the three samples, LiFePO₄-（C+G） has the best highrate performance, which results from the increased degree of graphitization of residualcarbon, the decreased charge transfer resistance and the reduced particles size. A novelCdO and carbon co-coated LiFePO₄cathode material has been successfully synthesized.Combined with the results of XRD, XPS, XAS and TEM, it reveals that CdO-incorporatedLFP particles are coated with a mixed amorphous carbon and CdO layer with a thicknessof2³nm. Compared to the LiFePO₄/C electrode, the capacity retention ratio and highC-rate performance for LiFePO₄/（C+CdO） have been effectively improved, which is due tothe enhanced structural stability, the reduced charge-transfer resistance, and the increasedelectronic conductivity. Therefore, the CdO and carbon co-coating is an effective surfacemodification technique.Compared with the electrochemical performance of Li₂FeSiO₄/C composites preparedby solid stated reaction, the Li₂FeSiO₄/C composite synthesized by sol-gel method hasdisplayed a better electrochemical performance. The results indicate that the samplesprepared by the two methods contain Li2SiO3and Fe3O4impurties and the sample prepared by sol-gel sample exhibit a better performance because of the smaller particle size, itexhibits a discharge capacity as high as227.8mAh g^-1,219mAh g^-1at the first and secondcycle at0.1C, respectively, and achieves more than1mol Li+intercalation/deintercalation.At1.0C and3.0C the sample delivers a reversible discharge capacity as high as151.7mAh g^-1,130.4mAh g^-1after50cycles. But it has lower degree of crystallinity andstronger polarization effect in the process of the charge-discharge.Nano-Li₂FeSiO₄/C/MWCNTs composite with enhanced rate performance and cyclestability has been successfully synthesized via sol-gel method. SEM images show thedecreased particle size, and EIS results confirm the decreased charge-transfer resistanceand improved Li+diffusion coefficient after MWCNTs-incorporation. Therefore, theLi₂FeSiO₄/C/MWCNTs electrode exhibits a discharge capacity as high as206.8mAh g^-1at2nd cycle, corresponding to1.24mol Li+intercalation and deintercalation during thecharge/discharge process. Furthermore, the Li₂FeSiO₄/C/MWCNTs displays excellenthigh-rate capacity and cycling stability, i.e., it delivers a reversible discharge capacity ashigh as82mAh g^-1at20C after500cycles, which is attributed to the high conductivenetwork between the MWCNTs and the Li₂FeSiO₄/C particles, and the reducing particlesize. Our results suggest that the nano-Li₂FeSiO₄/C/MWCNTs composite is a promisingcathode material for lithium ion batteries.