Enhanced Electrochemical performance of Li2FeSiO4/C as cathode for lithium-ion batteries via metal doping at Fe-site

In this paper, Li2Fe0.98 × 0.02SiO4/C (X = Ag, Zn, Ti, Cu, Pb, Al, W, Mo, Ga, Cs, V, Mn) was synthesized for lithium ion battery as cathode material by solid-state method and modified by doping metal ion on the Fe-site. The constant current charge-discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were chosen to investigate the electrochemical performance of Li2Fe0.98 × 0.02SiO4/C. The results shows that the materials doped with Ag, Zn, Ti and Cu on the Fe-site have good electrochemical properties. Then Li2Fe1−yXySiO4/C (X = Ag, Zn, Ti, Cu; y = 0.01, 0.02, 0.03, 0.05) were synthesized by solid-state method. After their electrochemical properties were compared, we found Li2Fe0.98Ag0.02SiO4/C, Li2Fe0.98Zn0.02SiO4/C, Li2Fe0.97Ti0.03SiO4/C and Li2Fe0.98Cu0.02SiO4/C have excellent electrochemical performance. The first discharge capacity of Li2Fe0.98Zn0.02SiO4/C and Li2Fe0.98Cu0.02SiO4/C were 201 mAh/g, which is equivalent to 1.2 Li+ deintercalation. The initial discharge capacity of Li2Fe0.97Ti0.03SiO4/C is 104 mAh/g, which is higher than that of other materials doped with Ti. Li2Fe0.98Ag0.02SiO4/C has good cyclic stability, the first discharge capacity is 143.4 mAh/g, the diffusion coefficient of lithium ion is two orders of magnitude higher than that of pure phase, and the electrochemical properties are stable. XRD and SEM tests were conducted on Li2Fe0.98Ag0.02SiO4/C, Li2Fe0.98Zn0.02SiO4/C, Li2Fe0.97Ti0.03SiO4/C and Li2Fe0.98Cu0.02SiO4/C. It can be seen from the XRD patterns that there are no characteristic peaks of Fe or Li2SiO3 impurities in the materials mentioned above. The impurity peak of Li2Fe3O4 was not found in the Li2Fe0.98Ag0.02SiO4/C material, which indicated that the purity of the material was high. It can be seen from the SEM diagram that the particle size of Li2Fe0.98Ag0.02SiO4/C is more uniform and there is no obvious agglomeration. Then, Li2Fe0.98Ag0.02SiO4/C with better performance was analyzed by EDS, XPS and FT-IR spectra. From the EDS analysis, it can be seen that the theoretical value of each element's scale were close to each other. By the data got from XPS spectrum the existences of the characteristic peaks of Li, Fe, Si and O in the sample were confirmed, which proves Ag+ has succeeded in replacing Fe2+ in the crystal structure of Li2FeSiO4. The positions of the absorption peaks of Li2Fe0.98Ag0.02SiO4/C in FT-IR spectra are consistent with those of Li2FeSiO4, indicating that stable Li2Fe0.98Ag0.02SiO4/C materials have been synthesized.