Effects Of Chestnut Shell Carbon On Electrochemical Properties Of Li₂FeSiO₄ Materials

With the development of energy crisis and environmental pollution,more and more attention has been paid to the application of biomass.Biomass is not only rich in raw materials but also reproducible and low cost.Biomass as carbon source not only can save the production cost,but also can reduce the environmental pollution caused by biomass incineration.Li₂FeSiO₄ theoretically can reversibly insert two Li⁺,its theoretical capacity is high,relatively strong Si-O bond makes the material more stable in thermal stability.Its raw material is environmentally friendly,and it has a promising future,but both low electrical conductivity and low diffusivity of lithium ions are its defects.In this paper,aiming at the existing defects of lithium iron silicate,Li₂FeSiO₄/C composite was synthesized by hydrothermal method using biomass chestnut shell as carbon source.Activator KOH was used to activate chestnut shell carbon,and melamine was used as nitrogen source to modify the chestnut shell carbon,Fe²⁺in lithium iron silicate has been substituted by rare earth Nd³⁺.In addition,X ray diffraction,scanning electron microscopy,transmission electron microscopy,Raman spectroscopy,Xinwei tester and electrochemical workstation were used to systematically characterize the structure and electrochemical performance of the synthesized samples,some meaningful results are obtained.The nano silica as silicon source,lithium hydroxide as the source,ferrous sulfate as iron source,chestnut shell as carbon source,carbon coated Li₂FeSiO₄/C composite was synthesized by hydrothermal method.The optimum addition amount of chestnut shell carbon was 20%.The chestnut shell carbon was activated by KOH.Li₂FeSiO₄/AC composites have orthorhombic structure with the space group of Pmn2₁.The particle size of the composite is uniform,and the individual spindle is cross-linked into petal,and the optimum activation mass ratio between activator KOH and chestnut shell carbon is 3:1.The activated chestnut shell carbon was modified with melamine as the nitrogen source,and then the Li₂FeSiO₄/AC-N composite was obtained by coating lithium iron with modified chestnut shell carbon,which has a monoclinic and microporous structure with the space group of P2₁/n and the uniform particle size.The electrochemical properties of Li₂FeSiO₄/AC-N composite are obviously superior to those of Li₂FeSiO₄/AC composites,and the charge transfer impedance is effectively reduced.Li₂Fe_1-xNd_xSiO₄/AC-N?x=0,0.01,0.03,0.05?composite was prepared by substitution of Fe²⁺with rare earth Nd³⁺in Li₂FeSiO₄/AC-N composite,which has a monoclinic structure with the space group of P2₁/n.Substitution of the rare earth Nd³⁺does not change the crystal structure,whereas the lattice of the material expands slightly with the increase of rare earth Nd³⁺amount.The composite Li₂Fe_0.97Nd_0.03SiO₄/AC-N has the best electrochemical performance during the synthesized samples.The discharge specific capacity of the composite is up to 196.4 mAh·g^-1 at 0.1 C,this is equivalent to the removal of 1.18 lithium ions,which is much higher than 133 mAh·g^-1 of the un-substituted sample.The composites also have good cyclic stability and minimum charge transfer impedance,and all the electrochemical properties are superior to those of the other synthesized samples.