Synthesisi And Research On High Capacity Sioc Based Anode Materials For Lithium Ion Batteries

With the rapid development of electronic vehicles?EVs?,the conventional lithium ion batteries?LIBs?are far from meeting the ever-growing demands of EVs for LIBs equipped with high energy density and long life cycliability.Among the various attempts,improving the capacity of anode material is a direct way to improve the energy density of LIBs.Owing to the low voltage plateau and highly stable cycliability,graphite is the most widely used commercialized anode material.However,its low capacity is far from meeting the demands for next generation LIBs.Silicon oxycarbide?SiOC?has gained considerable attention due to its high capacity,low expansion coefficient and good cycling performance.Nevertheless,most traditional SiOC-based anode materials show bulk structure and irregular particle size distribution,which usually result in a poor rate performance and long activation process.These drawbacks largely hinder the practical application of SiOC anode materials.Therefore,in this paper,we start from regulating the microstructure of SiOC materials and successfully synthesized morphology controllable samples,such as low carbon SiOC microspheres,nano-sized Si/SiOC composite,2D-SiOC/rGO composite,2D-SiOC/graphite nanosheets composite,3D-macroporous SiOC and3D-hierarchical porous SiOC.Furthermore,the synthetic method,structural property and electrochemical performance of each material is systemic studied.Bulk sized SiOC is synthesized by pyrolyzing organosilicon resin in an inert atmosphere,and the influence of pyrolysis temperature,elemental composition and morphology change on the electrochemical performance of SiOC anode materials are systematically studied.Our results show that the pyrolysis temperature mainly affects the graphitization degree of free carbon phase in SiOC.When the pyrolysis temperature is relatively low??600??,the grafted organic groups only partial carbonized and can not induce the rearrangement reaction of Si-O and Si-C bonds,resulting in a relatively low capacity of only²4 mAh g^-1.At the elevated temperature??800??,phenyl groups are fully carbonized and the rearrangement reaction is complete,generating the SiOC anode material characterized with the nanodomain model.After 50th cycles at 0.05 A g^-1,as-received anode material still shows high reversible capacity of⁴65 mAh g^-1 with capacity retention of 86.9%.In addition,both the reversible capacity and cyclic stability of SiOC anodes increase largely with the increase in the content of free carbon phase.To improve the electrochemical performance of SiOC anode materials,polyphenylsilsesquioxane?PPSQ?and polymethylsilsesquioxane?PMSQ?spheres were synthesized by emulsion method.Owing to the thermoplastic property of PPSQ sub-micron spheres,as-synthesized SiOC still reveal a similar electrochemical performance with that of bulk materials.In contrast,the thermosetting property of PMSQ spheres avoid the glass transition process,leading to the formation of microsized SiOC spheres.Spherical SiOC exhibits a high reversible capacity of⁵91 mAh g^-1 after 200 cycles at 0.5 A g^-1.However,owing to its low electrical conductivity caused by the low content of free carbon phase,a poor rate performance was observed with capacity of⁸5 mAh g^-1.To improve the carbon content and suppress the condensation of PPSQ particles,nano-Si is introduced and achieved a positive result owing to the dispersion effect.At an optimized condition,Si/SiOC-0.6 exhibits a high capacity of⁵50 mAh g^-1after 500 cycles at 0.5 A g^-1,with a capacity retention of 80%.When the current density is enhanced to 5.0 A g^-1,the composite could still retain a capacity of³03mAh g^-1.To study the influence of two-dimensional structure on the electrochemical performance of SiOC anodes,SiOC/rGO composites with the thickness of SiOC layer around⁴0 nm are synthesized using GO as the substrate.Owing to the oxidation of SiOC by the oxygen-containing functional groups in GO,SiOC is partial converted into SiO2,which largely restrict the columbic efficiency at first cycle?56.4%?.The composite shows an improved rate performance by 10 times higher than that of bulk SiOC.By replacing the GO with flake graphite?20-30layers?,the oxidation of SiOC during the pyrolysis is successfully avoided,and as-derived SiOC/Graphite?SG?composite shows a high initial columbic efficiency?71.5%?as well as a excellent rate performance.After 1000 cycles at 2.0 A g^-1,the composite still retain a capacity of¹84 mAh g^-1.Cyclic voluntary curves reveal that the peak current shows a liner relation with the scan rate^1/2(?^1/2),indicating that the electrode reaction is controlled by the diffusion process.GITT test reveals that2D SG shows higher Li⁺diffusion coefficient han that of pure SiOC during the whole charging state and stables at².9×10^-12 cm² s^-1 and¹.1×10^-12 cm² s^-1,respectively.Through the combination of sol-gel method and precursor transformation method,SiOC materials with macroporous and hierarchical porous structures are synthesized with propyl?Pr SA?and phenyl?Ph SA?incorporated SiO2 aerogel,respectivly.The macroporous SiOC-7 sample shows good rate performance,and the capacity retention is still as high as¹7.9%at 5.0 A g^-1.CV test shows that the high oxygen content in the sample leads to the formation of irreversible Li2O in the lithiation process,which reduces the initial coulombic efficiency by only 32.7%.The hierarchical porous structure of SiOC-1000 exposes more active sites for lithium insertion,increases the utilization rate of the active material and thus increases the reversible capacity(⁶09 mAh g^-1).In addition,the porous structure also helps shorten the Li⁺transmission pathways and enhances the electrolyte penetration,resulting in an improvement in the rate performance.SiOC-1000 shows a reversible capacity of²27 mAh g^-11 even at 2.5 A g^-1.dQ/dV curve shows that,in the first lithiation cycle,Li⁺reacts with SiO2 at⁰.24 V and⁰.27 V,respectively,which generates irreversible Li4SiO4 and partially reversible Li2Si2O5 compounds,resulting in a reduction in the initial coulomb efficiency?30.9%?.