| Silicate cathode materials have attracted much attention due to their unique characteristics, including high theoretical capacity, high safety, low exothermicity, low cost, nonpoisonous, environmental friendliness and easy synthesis. However, as a kind of polyanion-type compounds, silicate material suffers from the low electronic conductivity and slow lithium-ion diffusion, which prevent it from further applications. It is reported that carbon coating, decreasing particle size to nano-scale and doping are three effective approaches to overcome these obstacles. Up to now, sol-gel method and in-situ carbon coating process are pursued to prepare silicate cathode materials, which exhibit pure phase, small particle size, and good electrochemical performance. Usually, the soluble compounds are used as raw materials in sol-gel method, but the morphologies of as-prepared materials are not controlled by researchers, although the products have small particle size. Transition-metal oxides are considered as the impurities which are easily generated in preparation of silicate materials. Hitherto, it has not been reported that silicate cathode materials are synthesized by sol-gel method with transition-metal oxides. Is it possible to control the particle size of active material via a simple sol-gel route? In this work, the assumption has been proved to be feasible.Li2Fe Si O4/C composite was prepared by sol-gel method with 500 nm Fe2O3 microsphere as iron source. Li2 Fe Si O4 crystal was attributed to monoclinic structure with space group of P21/n. The product had pure phase, and carbon was amorphous state in the product. Li2 Fe Si O4/C exhibited sphere-like morphology, and their particle size was close to Fe2O3 microsphere. Li2 Fe Si O4/C delivered a discharge capacity of 160 m Ah g-1 at 0.1 C rate, which was about 96% of the theoretical capacity reversibly exchanging 1 mol Li+ per formula unite. This cathode exhibited stable cycle performance at different rates from 0.1 C to 2 C.Pure-phased Li2 Fe Si O4/C was synthesized with 50 nm Fe2O3 particle. Particle size of Li2 Fe Si O4 was about 50 nm, and it was close to Fe2O3 particle. This Li2 Fe Si O4/C cathode delivered a discharge capacity of 166 m Ah g-1, corresponding to reversibly exchanging 1 mol Li+ per formula unite. This cathode exhibited stable cycle performance at different rates from 0.1 C to 5 C. After 100 cycles, it delivered discharge capacity of 140 m Ah g-1, 125 m Ah g-1, 120 m Ah g-1, 110 m Ah g-1 and 90 m Ah g-1 for 0.1 C, 0.5 C, 1 C, 2 C and 5 C rate, respectively.Li2Fe0.98Mg0.02 Si O4/C was prepared with Fe2O3 nanoparticle. Li2Fe0.98Mg0.02 Si O4 crystal was attributed to monoclinic structure with space group of P21/n. Compared with Li2 Fe Si O4, the cell volume of Li2Fe0.98Mg0.02 Si O4 slightly shrank. Mole ratio of Li, Fe, and Mg in the product was close to the feed ratio, and it indicated that Mg was successfully doped in the lattice of Li2 Fe Si O4 and Mg occupied the corresponding position of Fe. The size of Li2Fe0.98Mg0.02 Si O4 was about 50 nm, and it meant that doping Mg had not effect on the morphology, particle size, and carbon coating of Li2 Fe Si O4/C composite. Li2Fe0.98Mg0.02 Si O4/C delivered a discharge capacity of 193.3 m Ah g-1 at 0.1 C rate, corresponding to reversibly exchanging 1.16 mol Li+ per formula unite, and the capacity retention was 92% after 100 cycles. By investigating the effect of doping Mg on low-temperature performance of Li2 Fe Si O4/C cathode, it was found that doping Mg was able to increase the diffusion coefficient of lithium ion and improve the electrochemical performance of Li2 Fe Si O4/C cathode under low temperature.Pure-phased Li2 Mn Si O4/C was synthesized with Mn3O4 nanoparticle. Li2 Mn Si O4 crystal was attributed to monoclinic structure with space group of P21/n. Li2 Mn Si O4/C composite was formed by a tight accumulation of nanoparticles, and particle size ranged from 20 to 30 nm. Li2 Mn Si O4/C cathode delivered a discharge capacity of 240 m Ah g-1 at current density of 8 m A g-1, which was corresponding to reversibly exchanging 1.44 mol Li+ per formula unite. The cycle performance of Li2 Mn Si O4/C cathode at different current densities from 8 to 320 m A g-1 was studied, and it was found that capacity retention was improved with the increasing of current density. The concentrations of Mn in electrolyte after the initial and 30 th cycles at different current densities were measured, and it was observed that Mn dissolution gradually decreased with the increasing current density. By investigating the impedance responses of Li2 Mn Si O4/C cathode after the initial and 30 th cycles at different current densities, it was found that the surface film generated under low current density could be more beneficial to protect the cathode from the corrosive reaction of electrolyte. Basing on the results of ex-situ X-ray diffraction measurement, it was inferred that low degree of irreversible distortion for Li2 Mn Si O4 may result in the improved capacity retention at high current density. |
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