Preparation And Mechanism Of Transition Metal Silicate Nanomaterials For Lithium Storage

In order to meet the requirements of LIBs materials with high power/energy density for the emerging energy storage field,designing and developing new anode materials with high electrochemical performance for LIBs becomes more and more crucial.Researchers devote extensive studies into transition metal silicate?MSiO?anode materials because of their merit of abundant source,low price and non-toxicity.So far,the researches about MSiO centralize in materials modification part,and their electrochemical performances improve by turning into nanocomposites.This work focuses on the transformation of tetravalent silicon(Si⁴⁺)in MSiO,different with the study mentioned above.The transformation is boosted by preparing SiO₂ nano particles as the precursor and introducing transition metal elements,utilizing the catalytic ability to make valence of Si⁴⁺decrease,thus the lithium storage capability of MSiO gets enhanced and reversible specific capacity is improved significantly.?1?Preparing SiO₂/G nanocomposites as the precursor by the simple alcoholysis reaction of SiCl₄,and introducing transition metal elements to synthesize Fe₂SiO₄/G.X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FT-IR?,thermogravimetric analysis?TGA?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and electrochemical tests were resorted to investigate the phase,structure and electrochemical performance of samples.The results show that Fe₂SiO₄/G possesses a multilevel structure,Fe₂SiO₄ nanocrystalline is well dispersed on graphene nanosheets and has an ultrasmall size.As a kind of anode material for lithium-ion battery,Fe₂SiO₄/G delivers the reversible specific capacity reaches 814 mAh g^-1 after 400 cycles at the current density of 200 mA g^-1,and the initial Coulombic efficiency is 68%.This section of work has reference significance for studying transition metal silicate nanocomposites with ultrasmall sizes.?2?Aiming at the shortcoming of tedious preparation in?1?,proposing a novel synthetic method:using SiCl₄ as the Si source to prepare Co₂SiO₄/SiO₂/G composites through a step solvothermal method.Different from the hollow nanosphere structure of MSiO as reported in the past article,the amorphous gel-like film structure of Co₂SiO₄/SiO₂/G benefits the transmission rates of electrons and ions.The Co₂SiO₄/SiO₂/G shows the highest reversible capacity(1382 mAh g^-11 at 500 mA g^-1)and longest life span?500 cycles?in all known silicates-based materials.Besides,this section of work also prepared SiO₂/G,CoO/G and CoO/SiO₂/G three materials to contrast their electrochemical behaviors with Co₂SiO₄/SiO₂/G.Ex-situ X-ray photoelectron spectra?XPS?and other tests were conducted to illustrate the reason for the significantly improved performances and put forward the reasonable hypothesis,offering a new idea for interpreting the lithium storage mechanism of MSiO.In conclusion,Fe₂SiO₄/G and Co₂SiO₄/SiO₂/G were obtained by preparing SiO₂/G nanocomposites as the precursor and introducing transition metal elements Fe and Co.The composites with graphene and nanostructures made the electrochemical performances rise,having guiding significance for designing and developing new anode materials for LIBs.Besides,the hypothesis for hyper-capacity phenomenon was proposed and demonstrated,providing new evidence for the electrochemical catalytic mechanism in transition metal silicates for lithium storage.