Preparation And Electrochemical Properties Of Silicon Suboxide-based Anode Materials For Lithium Ion Batteries

Graphite is the conventional anode material for commercial lithium ion batteries(LIBs).It has a theoretical specific capacity of 372 mAh g-1,which cannot meet the ever-increasing demands for energy density of secondary battery by energy storage system.Therefore,the exploration of high-performance anode material becomes the critical point for the development of high energy density lithium ion bateries.This thesis aimed at developing high performance silicon suboxide(SiOx)novel anode materials.To address the issue of poor structural stability induced by the large volume change during charge/discharging process and the low intrinsic electrical conductivity of SiOx material,several strategies were attempted,including nanotechnology,hollow structure design and highly conductive phase incorporation,with the aim of improving the structural stability of electrode and enhance the electrode reaction kinetics.The effects of processing parameters on the particle morphology and phase structure were investigated,and the relationship between material chemistry,microstructurc and electrochemical properties were studied.Based on deep optimization,the prepared SiOx-based anode materials show high specific capacity,long cycle life and excellent rate capability.The main results are listed below.(1)Investigation of SiOX/C@rGO composite:During the preparation process,polyvinylpyrrolidone(PVP)is introduced as a surfactant to help achieving SiOx/C@rGO composite with graphene sheets uniformly coating on SiOx particles.The PVP surfactant links with SiOx particle and graphene sheet through hydrogen bonds and meanwhile carbonizes to amorphous carbon during heat treatment,which uniformly coats on SiOx particles;The uniform graphene and carbon coating layer not only facilitate the fast electron transport and thus enhance the electrode reaction kinetics,but also alleviate the structural stress induced by the uneven volume expasion during lithiation process,thereby maintaining the structural integrity of SiOx/C@rGO composite.In addition,the graphene and carbon coating layer can prevent the active SiOx particle from peeling off from electrode.The effect of PVP amount on the microstructures and electrochemical performance are investigated.The SiOx/C@rGO composite prepared with the optimized SiOx/PVP weight ratio of 9:40 presents the best electrochemical performance.The electrode showed a reversible specific capacity of 1184 mAh g-1 after 100 cycles with a current density of 0.1 C(1C=1000 mA g-1),corresponding to a capacity retention of 90.7%.(2)Self-template fabrication of porous Si/SiOx/C anode material for lithium ion battery:the porous structure Si/SiOx/C composite is designed and prepared via a novel self-template method with subsequent carbon coating process.The porous structure can accommodate the volume variation during lithiation/delithiation process,mainiting the structural integrity of Si/SiOx/C composite.The carbon coating layer can effectively homogenize the current density and electrode reactions on the surface of Si/SiOX particles,which can alleviate the stress-induced pulverization,improving the structural durability.The effects of carbon content on microstructure and electrochemical performance are studied.The electrochemical test results showed that the Si/SiOx/C anode material with carbon content of 37.8 wt.%presented a reversible specific capacity of 1072 mAh g-1 after 30 cycles at 0.1C,corresponding to a capacity retention of 82.3%.(3)Investigation on C/SiOx@Graphene composite with sandwich-like structure constructed by SiOx nanoparticles uniformly distributing between graphene sheets and amorphous carbon layer:the sandwich-like structured C/SiOX@Graphene composite was prepared via a room-teimperatured alcoholysis reaction with subsequent heat treatment using silicon tetrachloride(SiCl4)as silicon source.Ethylene glycol acts as a bridge to connect graphene sheets and SiO2 nanoparticles derived from the alcohilysis reaction of SiCl4,forming the SiO2@Graphene composite with SiO2 nanoparticles uniformly dispersing on graphene sheets.Subsequently,the SiO2@Grapliene precursor successively underwent magnesiothermic reduction and carbon coating process to form C/SiOx@Graphene composite.The sandwich-like structure not only effectively facilitate the fast electron transport among SiOx nanparticles,but also accommodate the volume expansiona and thus improve the structural stability of C/SiOx@Graphene electrode during charge/discharge process.Owing to the well-engineered structure,the C/SiOx@Graphene electrode exhibited excellent cyclic staility and rate capability.A reversible specific capacity of 655 mAh g-1 can be achieved after 100 cycles at 0.1C,relating to a capacity retention of 74%.Even at a current density as high as 5C,a specific capacity of about 400 mAh g-1 can still be maintained.(4)A high ionic and electronic conductive phase is introduced into SiOx matrix,forming a dual-oxides/carbon anode material with high electrochemical performance.SiO2-TiO2 precursor was prepared by a sol-gel method,which ismixed with carbon and then underwent a carbothermal reduction reaction to form SiOx-TiO2/C anode material.The uniformly dispersed TiO2 nanacrystals in SiOx nanoparticle matrix form LiXTiO2 nanocrystals with high lithium ionic and electronic conductivity during the lithiation process,which enhance the electrode reaction kinetics,thus improving the cyclic stability and rate capability.The effects of TiO2 amounts on microstructure and electrochemical performance of SiOx-Tio2/C composite are investigated.The test results indicated that the SiOx-TiO2/C composite with TiO2 con.tent of 17.6 wt.%showed the optimal electrochemical performance.A reversible specific capacity of 910 mAh g-1 can be achieved after 200 cycles at 0.1C.Even at 6.4C,a specific capacity of 375 mAh g-1 can still be maintained,demonstrating an excellent rate capability.Moreover,the optimal SiOx-TiO2/C electrode delivers 700 mAh g-1 after 600 cycles at 1C without obvious capacity decline,showing the outstanding long cyclic stability.(5)A porous nano-SiOx@C anode material was prepared by micro-emulsion method.The dandelion-like structured SiOx@Cnanoparticle is constructed with a porous nano-SiOx skeleton coated with carbon layer on the surface.The large specific surface area of porous SiOx skeleton facilitates the full contact with carbon,resulting in full reduction of SiO2 at relative low temperature(700?)and thus SiOx component with high lithium storage capability.In addition,the abundant inner pores can effectively accommodate the volume variation during charge/discharge process,guaranteeing the structural stability of SiOx@C electrode Moreover,the 3D conformal carbon layer can homogenize current density on particle surface,which can prevent the inner structural stress induced by uneven electrode reaction,thus ensuring the structural integrity during lithiation/delithiation process.Owing to the synergetic effects,the prepared porous nano-SiOx@C material delivers outstanding electrochemical performance.A reversible specific capacity of 1116 mAh g-1 can be achieved after 200 cycles at 0.1C,relating to a capacity retention of 88.5%.Even at 2C,the electrode maintained at 650 mAh g-1 after 1000 cycles without obvious capacity degradation.