Design,Synthesis And Electrochemical Properties Of High Specific Capacity Silicon Oxide-Based Anode Materials

Lithium-ion batteries（LIBs）have shown great brilliance in many fields due to their high energy density,long cycle life and safety.However,the current mainstream LIBs with graphite as the anode still cannot meet the ever-increasing demand for energy density.Therefore,developing anode materials with high specific capacity to enhance the energy density of LIBs has become an important direction for the future development of energy storage devices.Recently,silicon oxide（SiO_x）-based anodes have received more and more attention due to its high theoretical specific capacity,smaller volume expansion compared with Si-based anode,abundant reserves and low price,but the relatively low Li⁺/e^-conductivity of SiO_x-based anode and the huge volume expansion during lithium intercalation/deintercalation still seriously limit its practical applications.In addition,most of the non-stoichiometric SiO_x-based anodes reported in the literature are prepared from the hydrolysis of tetraethyl orthosilicate（TEOS）,and these SiO_x-based materials usually feature low specific capacity;therefore,finding suitable silica sources to prepare SiO_x with high capacity is also challenging.To solve these problems,this paper,started from the selection of precursors,optimization of structures and regulation of composite components,fabricates a series of SiO_x-based composites by nanostructure design and compounding with conductive materials.The physical and electrochemical properties of these composites are systematically characterized,and the relationship between structure,composition and electrochemical properties is explored.The main results obtained are summarized as follows:（1）Synthesis and electrochemical properties of interconnected SiO_xC_y amorphous nanospheres.Interconnected SiO_xC_y amorphous nanospheres were prepared by a two-step process of trimolecular sol-gel strategy and high-temperature carbonization using3-aminopropyltrimethoxysilane（APTMS）,formaldehyde and resorcinol as silica and carbon sources,respectively.Both carbon and SiO_x in amorphous nanospheres are chemically intertwined via Si-C and Si-O-C bonds,showing a homogeneous distribution throughout amorphous nanospheres at the atomic scale.Meanwhile,these amorphous spheres are cemented through interfacial junctions,thereby forming a continuous electron transport passage,and the electrochemical stability of the interconnected SiO_xC_y nanospheres is not affected even if the carbon content is reduced.More importantly,the specific surface area of these interconnected amorphous nanospheres is only 16 m² g^-1,which effectively reduces the contact area between the electrolyte and the active material and thus reduces side reactions.SiO_xC_y nanospheres exhibit excellent electrochemical performance when used as the LIBs anode,with a specific capacity of 731 m Ah g^-1 after 200 cycles at a current density of 0.2 A g^-1.Even at a high current density of 0.5 A g^-1,it shows ultra-long cycle stability,and the reversible specific capacity is as high as 697 m Ah g^-1 after 500 cycles.（2）Construction and electrochemical properties of high-capacity hollow-structured SiOx nanocube anode.We proposed a novel sequential coating strategy to construct hollow-structured high-capacity SiO_x nanocube anode.Zeolitic imidazolate framework（ZIF）-8 was chosen as self-sacrficial template,two silica sources,TEOS and 3-aminopropyltrimethoxysilane（APTMS）,were hydrolyzed and sequentially coated on the surface of ZIF-8 to fabricate high-capacity SiO_x.Finally,the hollow SiO_xnanocubes with sandwich structure were obtained by phenolic resin coating and one-step pyrolysis.These nanocubes are composed of an innermost nitrogen-doped porous carbon network（NC）,a middle mesoporous SiO_x layer,and the outermost mesoporous carbon layer（m-C）.The incorporation of high-capacity SiO_x components in the sequential coating process greatly improves the specific capacity of SiO_x-based anode.Both of NC in the inner layer and the m-C in the outermost layer synergistically enhance the overall electronic conductivity of the anode material,and the m-C layer effectively prevents the direct contact between electrolyte and SiO_x to reduce the side reactions.Meanwhile,the hollow structure is very efficient in alleviating the volume expansion of SiO_x during lithium intercalation/deintercalation.This unique nanostructure design endows the SiO_x-based anode with excellent electrochemical performance,including high specific capacity,high pseudocapacitive behavior and lithium ion diffusion coefficient,outstanding rate performance and long-cycle stability,especially with a high specific capacity of 583 m Ah g^-1 after 500 cycles at a high current density of 1 A g^-1.（3）Construction and electrochemical properties of yolk-shell gradient-structured SiO_x-based anode.We prepared SiO_x-based anodes with yolk-shell gradient structure and porous structure derived from periodic mesoporous organosilica spheres（PMOs）by selective etching method.Two kinds of organic-inorganic hybrid silica nanospheres were synthesized using TEOS,1,2-bis（triethoxysilyl）ethane（BTSE）and TEOS,1,4-bis（triethoxysilyl）benzene（BTSB）as double silica sources,respectively,and then the inorganic silica component in the hybrid silica nanospheres was removed by sodium carbonate etching to prepare yolk-shell structured PMOs and porous PMOs.Finally,the yolk-shell structured SiO_x-based anode（YSG-SiO_x/C@C）and porous SiO_x-based anode（P-SiO_x/C@C）were obtained by carbon coating.Both the yolk-shell structure and the porous structure reserve a certain void space for the volume expansion of SiO_xand effectively relieve the stress caused by the volumetric effect of SiO_x.Therefore,both YSG-SiO_x/C@C and P-SiO_x/C@C show excellent stability during cycling.It was found that YSG-SiO_x/C@C retains more polysilsesquioxane and thus exhibits higher reversible specific capacity during cycling;YSG-SiO_x/C@C delivers a specific capacity of 680 m Ah g^-1 after 150 cycles at a current density of 0.2 A g^-1.Even at a high current density of 2 A g^-1,YSG-SiO_x/C@C still demonstrates a high specific capacity of 519 m Ah g^-1 after 550 cycles.（4）Preparation and electrochemical properties of core-shell structured SiO_x-TiO₂@C ultrafine particles.Based on Chapter 2,SiO_x-TiO₂@C ultrafine particles with core-shell structure were synthesized by using TEOS and APTMS as dual silica sources through a simple sol-gel process.Compared with using TEOS as the sole silica source,using TEOS and APTMS as dual silica sources reduced the particle size of SiO_x;electrochemical tests also showed that the introduction of APTMS improved the specific capacity of SiO_x.Differential capacity curves confirmed that the introduction of TiO₂ reduced the electrode polarization and improved the electrochemical kinetics of SiO_x.IR indicated that the Si-O-Ti bond formed between TiO₂ and SiO_x enhanced the structural stability of the SiO_x-based anode.SiO_x-TiO₂@C showed excellent cycling performance at 0.2 A g^-1,maintaining a specific capacity of 556 m Ah g^-1 after 250cycles.