Sol-gel Synthesis And Modification Of Li₂ FeSiO₄ /C Composites As Cathode Material For Lithium Ion Battery

Orthosilicate Li₂FeSiO₄ is considered as a promising cathode material for lithium ion batteries, due to its advantages such as inexpensive, high theoretical capacity, excellent safety performance, cycling stability and environmental benignity. However, it suffers from low electronic conductivity and poor lithium ion diffusion, which prevents it from commercial use. In the aims of improving the electrochemical properties of Li₂FeSiO₄ as a cathode material, Li₂FeSiO₄/C composites with in situ carbon coating were synthesized via sol-gel method based on hydrolysis/condensation of tetraethoxysilane (TEOS), whereby the electronic conductivity was enhanced. Based on the in situ carbon coating technique, nanostructured Li₂FeSiO₄/C were synthesized by sol-gel method through which surfactant PEG was introduced to control the particle size and thus the improvement of the lithium ion diffusion. The structure of Li₂FeSiO₄/C composites was investigated by XRD, RM, FE-SEM and HR-TEM, respectively. The electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge measurements were conducted to analyze the electrochemical performances of Li₂FeSiO₄/C. The effects of different carbon additives and introduction of PEG on the structure and electrochemical performances were studied. In addition, the effects of calcination atmosphere and temperature on the structure and electrochemical performances were also studied.Li₂FeSiO₄/C composites were successfully synthesized with sucrose and L-ascorbic acid as carbon additives. The results indicate that structure of the residual carbon is graphene-rich, whereby the electronic conduction is enhanced. The specific surface area of the samples prepared with sucrose and L-ascorbic acid are 14.1 m² g^-1 and 395.7 m² g^-1, respectively. The EIS results show lithium ion diffusion of the samples prepared with sucrose and L-ascorbic acid are 3.62E^-13 cm² s^-1 and 1.35E^-12 cm² s^-1, respectively. At C/16 rate, the Li₂FeSiO₄/C composites synthesized with sucrose exhibit an initial discharge capacity of 125.0 mAh g?1. However, the Li₂FeSiO₄/C composites synthesized with L-ascorbic acid deliver an initial discharge capacity of 135.3 mAh g?1, and show cycling stability at high rate (C/1), which outperforms the composites synthesized with sucrose.Based on the in situ carbon coating technique, as-obtained Li₂FeSiO₄/C nanocomposites synthesized with PEG as surfactant and L-ascorbic acid as carbon additive possess dispersed spherical particles (⁵0 nm) with narrow particle size distribution, embedded in a continuous carbon matrix. As a result, nanostructured Li₂FeSiO₄/C composites exhibit less additional barrier and higher electronic conductivity during the charge/discharge process. Advantageously, the uniform carbon coated on the particles restrains the side reactions occurring at the electrode-electrolyte interface. At C/16 rate, the nanostructured Li₂FeSiO₄/C composites undergo obviously small polarization between the charge and discharge plateaus. The nanocomposites deliver an initial specific capacity of 138.2 mAh g?1 and remain good capacity retention at various rates.