Improved electrochemical performance of Li2FeSiO4/C as cathode for lithium-ion battery via metal doping

Abstract

Li2FeSi0.98M0.02O4/C (M = Ti, Ag, Cu, V, Pb) was synthesized as cathode material for lithium-ion battery by the solid-state method. The electrochemical performance of Li2FeSi0.98M0.02O4/C was investigated by constant current charge–discharge test, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results show that the materials doped with Ti or Ag at the Si site deliver good initial discharge capacity. Li2FeSi1-xMxO4/C (M = Ti, Ag; x = 0.01, 0.02, 0.03, 0.05) was synthesized via the solid-state method. By comparing the electrochemical properties, it can be observed that Li2FeSi0.98Ti0.02O4/C and Li2FeSi0.98Ag0.02O4/C have good initial discharge capacity. The initial discharge capacity of Li2FeSi0.98Ti0.02O4/C is 164.1 mAh/g, which is equivalent to 0.98 Li+ deintercalation. The capacity of Li2Fe0.98Ti0.02SiO4/C is 155.8 mAh/g after 10 cycles under 0.1 C, and the capacity retention rate is 94.9%. The initial discharge capacity of Li2FeSi0.98Ag0.02O4/C is 166.6 mAh/g, which is better than other materials. The capacity of Li2Fe0.98Ag0.02SiO4/C is 132.8 mAh/g after 10 cycles under 0.1 C, and the capacity retention rate is 79.7%. The charge–discharge cycle performance of Li2FeSi0.98Ti0.02O4/C is more stable than Li2FeSi0.98Ag0.02O4/C. The Li+ diffusion coefficient of Li2FeSi0.98Ti0.02O4/C is higher than that of pure phase material by two orders of magnitude. The Li2FeSi0.98Ti0.02O4/C and Li2FeSi0.98Ag0.02O4/C were tested by XRD and SEM. The XRD patterns show that there are no characteristic peaks of Fe or Li2SiO3 impurities in the materials, which indicates that the crystal structure of Li2FeSiO4 has not been changed after doping metal ion at the Si site. The SEM images indicate that the particle size of materials is quite uniform and no obvious agglomeration is detected in the materials. Li2FeSi0.98Ti0.02O4/C was analyzed by EDS, ICP, XPS, and FT-IR spectra since it delivers better performance when compared with other materials. EDS and ICP show that the values which were measured according to the ratio of each element are found to be similar to the theoretical values. The XPS spectrum confirms the existence of the characteristic peaks of Li, Fe, Si, and O in samples, which could also prove that Si4+ is successfully replaced by Ti4+ in the crystal structure of Li2FeSiO4. The position of each absorption peak in the infrared spectrogram coincides with that reported in the literatures, which indicates that the stable materials are formed.