The Synthesis And Electrochemical Performance Of Si-based Anode Materials For LIBs

Secondary Li-ion batteries are promising energy storage sources for many aspects of applications,such as portable electronic devices,electric vehicles?EVs?,and stationary grid storage.To meet the increasing energy density requirements of the EVs,many materials with high energy density,high power density and long cycle life are extensively studied.Among those materials,Silicon-based materials are promising candidates to substitute currently commercialized graphite anodes due to their safety,natural abundance,relatively low discharge potential?<0.5 V?,and high theoretical specific capacity of 4200 mAh g^-1,which is 10 times higher than that of graphite(372 mAh g^-1).However,Si suffers from huge volume expansion/contraction?>300%?during lithiation and delithiation processes,leading to the pulverization and electrical disconnection of Si particles,unstable solid electrolyte interface?SEI?film formation and thus poor cycle life of the electrode.Furthermore,poor electronic conductivity of Si results in inferior rate performance and severe capacity fade.Therefore,it is highly desirable to prepare Si-based anode materials with excellent electrochemical performance for industrial applications.The major targets of this dissertation are to prepare Si-based anode materials with excellent electrochemical performance using different methods.The results are listed as follows:?1?A sandwich-structured graphite-metallic silicon@C?MS-G@C?composite shows good electrochemical performance compared to metallic silicon or graphite-metallic silicon.The improved electrochemical performance may be ascribed to the sandwich structure of the composite with the carbon coating on metallic Si and to the metallic Si particles with high electronic conductivity according to the results of four probe tester measurement and a density functional theory study.When evaluated as an anode material for LIBs,the low cost MS-G@C anodes exhibit 830 mAh g^-1 at 0.5 C,650 mAh g^-1 at 1 C,and 251.2 mAh g^-1 at 5 C,respectively,and after 100 cycles with an average capacity loss of only 0.02% per cycle at 0.5 C.The synthetic method is provided a facile and low-cost strategy for the large-scale production of silicon-based material as an anode in LIBs.?2?A composite with ultrafine SiOx?x = 1.57,around 2 nm?nanoparticles confined in carbon framework is synthesized by a simple thermopolymerization process and subsequent heat treatment.In the composite,the carbon framework can provide conductive network to enhance the electrical conductivity of the composite and buffer the volume change to prevent the composite peeling from the current collector.The ultrafine SiOx nanoparticles can alleviate mechanical stress during lithiation/delithiation process and decrease the diffusion/transport distance of lithium ions and electrons.In consequence,when used as an anode material for lithium ion battery,the as-prepared composite exhibits a high reversible capacity of 540 mAh g^-1 at a current density of 500 mA g^-1 after 200 cycles.The composite delivers good electrochemical performance,making it a promising candidate for the next-generation high-energy lithium ion battery.?3?Hollow silica-copper-carbon?H-SCC?nanocomposites are first synthesized using copper metal-organic frameworks as skeletons to form Cu-MOF@SiO₂ and then subsequent heat treatment.In the composites,the hollow structure and the void space from the collapse of the MOF skeleton can accommodate the huge volume change,buffer the mechanical stress caused by lithium ions insertion/extraction and maintain the structural integrity of the electrode and a long cycling stability.The ultrafine copper with uniform size of around 5 nm and carbon with homogeneous distribution from the decomposition of the MOF skeleton can not only enhance the electrical conductivity of the composite and preserve the structural and interfacial stabilization,but also suppress the aggregation of silica nanoparticles and cushion the volume change.In consequence,the resulting material as an anode for LIBs delivers a reversible capacity of 495 mAh g^-1 after 400 cycles at a current density of 500 mA g^-1.A high-coulombic-efficiency silicon-based anode material with Cu and C substances dispersed in Si host and a RGO-CNT framework is designed and synthesized by a facile method.The obtained composite could deliver a high initial coulombic efficiency of 82.3% and a reversible capacity of 1268.9 mAh g^-1 at 2 Ag^-1 after 100 cycles,which is much better than that of the Si-Cu-C composite.This excellent electrochemical performance can be ascribed to the following factors.Cu and C substances in Si host can enhance the electronic conductivity of Si and facilitate the transfer of electrons.The RGO-CNT framework can prevent the direct contact of Si nanoparticles with the electrolyte to avoid the continuous formation of SEI film.?4?Silicon-carbon composite is a promising anode material for high-energy Li-ion batteries.However,it still remains a great challenge to fabricate high-performance Si-based materials at large-scale and low cost.Here,we for the first time report the preparation of an interconnected three-dimensional?3-D?porous Si-hybrid architecture by using a spray drying method.In this unique structure,the highly robust C-CNT-RGO cages not only can enhance the electrical conductivity of the electrode,but also accommodate the volume change and suppress the Si nanoparticles aggregation.As a result,the 3D Si@po-C/CNT/RGO electrode achieves long-life cycling stability at high rates(a reversible capacity of 854.9 mAh g^-1 at 2 A g^-1 after 500 cycles and capacity loss less than 0.013% per cycle)and good rate capability(1454.7,1198.8,949.2,597.8 and 150 mAh g^-1 at current densities of 1,2,4,10 and 20 A g^-1,respectively).Moreover,this novel electrode could deliver high reversible capacities and long-life cycling stabilities even with high mass loading density(764.9 mAh g^-1 at 1.0 mg cm^-2 after 500 cycles,and 472.2 mAh g^-1 at 1.5 mg cm^-2 after 400 cycles,respectively).This facile,low-cost and scalable method can be extended to fabricate other high capacity anode materials with the large volume expansion?Sn,Ge,transition-metal oxides?and 3D porous carbon for other potential applications.