Design,Farbication And Properties Of Silicon Oxycarbide-Based Nanocomposites As High Performance Lithium/Potassiumion Battery Anodes

Lithium-ion batteries（LIBs）and potassium-ion batteries（PIBs）with high energy and power densities are increasingly required for the rapid development of modern society.Thus,the design and preparation of the anodes with high performance are the key factors to improve the performance of LIBs/PIBs.In this paper,the design,preparation and characterization of high-performance anode materials for LIBs/PIBs were carried out from SiOC polymer-derived ceramics,which possesses a high lithium storage capacity and long-cycle stability.Two kinds of（quasi）three-dimensional graphene modified SiOC nanocomposites（3D-GNS/SiOC and 3D-GNS/SiOC_f）were prepared by compositing 3D-interconnected conductive network and nano-sized（porous）SiOC matrix,which effectively tailored the electron conductivity and accelerated the ion diffusion rate in SiOC-based materials.Meanwhile,the bi-continuous porous carbon materials（BC-NCS）derived from SiOC and SiOC stabilized bismuth nanoparticles composite（SiOC/Bi）were designed and prepared for obtaining a continuous pore structure and stable coating respectively,which relieved the large volume expansion and improved the long-cycle stability for carbon-/bismuth-based anodes for PIBs.We further made clear the promotion mechanism of three-dimensional interconnected graphene and nano-sized（porous）structure to the electron conductivity and ion diffusion rate of SiOC electrode materials,as well as the bi-continuous pore structure and SiOC stable layer for improving the long-cycle performance by maintaining the stability of structure of carbon-/bismuth-based anodes.The details are as following:（1）SiOC nanolayer coated 3D-graphene nanocomposites（3D-GNS/SiOC）were prepared,using three-dimensional graphene aerogel（3D-GNS）as preform,through the impregnation and pyrolysis of polysiloxane precursor.The effect of graphene network and SiOC nanolayers on the electrochemical performance was studied.The 3D-GNS framework in the nanocomposite provides enough channels for electron transport,while the uniform SiOC nanolayer coated on the surface of GNS shortens the diffusion distance of Li⁺ions,as well improves the utilization of SiOC as active sites,thus improving the electron/ion conductivity for the composite.Therefore,3D-GNS/SiOC,as an anode for LIBs possesses a high reversible capacity and excellent rate performance(586 m Ah g^-1/311μAh cm^-2 at 0.5 A g^-1 after 200 cycles).Significantly,in this work,it is proposed that incorporating a three-dimensional connected network into a nano-sized matrix is helpful to improve both the electron and ion conductivity,which provides an efficient method to prepare SiOC-based anodes with improved electrochemical performance.（2）For preparing high-performance SiOC-based flexible electrodes,a quasi 3D-graphene modified SiOC fiber cloth（3D-GNS/SiOC_f）with high SiOC loading（～90wt.%）was obtained by immersing a flexible porous SiOC fiber cloth,which was prepared by electrospinning polysiloxane precursor,into GO solution followed by freeze-drying and thermal reduction of GO.In the 3D-GNS/SiOC_f,the porous structure of SiOC fibers provides sufficient active sites for the insertion/extraction of Li⁺ions,while the 3D-GNS framework improves the electron conductivity in the whole composite.Thus,3D-GNS/SiOC_f,as a flexible anode for LIBs,possesses excellent rate and long-cycle performance,which can achieve 686 m Ah g^-1 after 500 cycles at 0.5 A g^-1.Significantly,a“reverse”strategy for preparing flexible electrodes was proposed by directly constructing the high capacity active material into a flexible form.Different with loading active materials on flexible carbon matrix,this method is helpful to achieve a high loading of high capacity active material,and then realize the high-energy and high-power densities for flexible electrodes.（3）Bi-continuous porous carbon spheres（BC-NCS）with interconnected nanopores and continuous carbon network were prepared by etching carbon-rich SiOC spheres with molten KOH,in which the SiOC spheres were prepared by a sol-gel method using phenyl triethoxy silane and four ethoxy silane as precursors.The“scaffold-like”structure of SiOC consisting of bi-continuous carbon framework and SiO_x phase guarantees the formation of bi-continuous structure in BC-NCS after etching the SiO_x phase.The as-prepared BC-NCS not only provides enough space to alleviate the volume expansion and supplies fast ion diffusion channels owing to the unique interconnected nano-pore structure,but also maintains an excellent electron conductivity taking advantage of the remaining interconnected carbon framework.When tested as anodes in half-cells for PIBs,the BC-NCS possesses a high reversible capacity,high rate and long cycle stability(the capacity maintained at 191 m Ah g^-1 after2000 cycles of 0.5 A g^-1).Significantly,the designed bi-continuous structure solves the inherent contradiction between the reduction of conductivity and the improvement of porosity in the porous materials reported in the literature,which provides a new idea/structure for preparing high-performance electrodes with long-cycle stability and rate performance for PIBs.（4）SiOC stabilized Bi nanoparticles（SiOC/Bi）were prepared by direct grinding and pyrolysis the mixture of bismuth acetate and polysiloxane.The stability effect and mechanism of SiOC coatings on Bi nanoparticles were studied.In the composite,SiOC matrix can effectively avoid the direct contact between Bi nanoparticles and the electrolyte,inhibit the serious expansion and pulverization of Bi during the charging and discharging process,and prevent the formation of solid electrolyte interface layer（SEI）repeatedly,contributing to a long-cycle stability for SiOC/Bi composite.When tested as anodes in half-cells for PIBs,SiOC/Bi possess a high reversible capacity and excellent long-cycle stability,which can deliver a reversible capacity of 223 m Ah g^-1after 230 cycles at 1.0 A g^-1,while under the same conditions,the capacity of pure Bi_acnanoparticles is about 0 m Ah g^-1.This work demonstrates the excellent stability mechanism of SiOC as a stable layer for Bi electrodes,and proves that SiOC as a new stable agent can improve the long-cycle performance for other electrode materials with high volume expansion.