Alginate Hydrogel-assisted Synthesis And Performances Study Of Silicon-based Composite Anode Materials For Lithium-Ion Batteries

Owing to the rapid development of electric vehicles and electronic devices,lithium-ion batteries(LIBs)were required to have a high energy density,high power and light weight.However,limited by the theoretical specific capacity of graphite anode(372mAh/g),the energy density of LIBs is difficult to improve.Therefore,it is imperative to develop a higher capacity anode material.Silicon(Si)has been considered as the promising next generation of LIBs anode material,because it has the high theoretical capacity(4200 mAh/g),low working potential(<0.5V vs Li/Li+)and rich reserves in the earth.However,the Si anode has a huge volume change during the charge/discharge process,which causing pulverization of Si material structure.In addition,as Si is a semiconductor material,its electrical conductivity is poor.To solve these problems of Si anode,the Si/M/C composite(M-electrical conductive additive),is one of the effective means to improve the electrochemical proformance of Si anode.However,most of these reports used the synthetic methods required to be carried out under strict conditions and time consuming,the raw materials used are expensive and toxic to humans.In this paper,we use the sodium alginate,which is a cheap material,and commercial Si nanoparticles as the raw materials.The Si/M/C composites have been synthesized by a simple alginate technology(cross-linking of alginate with different cross-linking agent).The effects of different electrical conductive additives and different cross-linking agents on the electrochemical properties of the the Si/M/C composites were studied.Including the following research:1.In this chapter,the Si/rGO/C composite is synthesized by the cross-linking of alginate with Ca2+,in which,graphene as the electrical conductive additive.Material properties are shown that most of Si nanoparticles are wrapped between the rGO sheets and the carbon layer;the rGO sheets and carbon layer provide a double protection against the Si nanoparticles.On the one hand,as a buffer for the volume change for Si nanoparticles and improve the electrical conductivity of composite;On the other hand,prevent the direct contact of the Si nanoparticles with electrolyte,help to form a stable SEI film.The Si/rGO/C composite delivers the reversible capacity of 1000mAh/g and 77.6%capacity retention afer 100 cycles,when the current density is 1.0A/g.Furthermore,the discharge capacity is still 643.7mA/g at the current density of 6.0A/g,showing the excellent electrochemical cycle and rate performance.2.The Si nanoparticles were embedded into the hydrogel containing Cu2+ by cross-linking of alginate with Cu2+,and after high temperature carburization treatment,the Si/Cu/C composite was prepared.In the composite preparation process,the Cu2+as a cross-linking agent,also as a precursor of Cu element and conversion into the electronic conductive additive,after high temperature reduction,resulting in improve the conductivity of the composite.The results show that part of Cu reacts with Si to form Cu3Si,and the initial coulombic efficiency of Si/Cu/C composite has a greater decrease,compare to Si nanoparticles.Furthermore,with the prolongation of high temperature carbonization time,the decrease of the initial coulombic efficiency(Si/Cu/C composite)is greater.However,the cycle and rate performance of the Si/Cu/C composite are improved.The Si/Cu/C-3h composite delivers a capacity of 900mAh/g and 76.9%capacity retention at the current density of 0.2A/g after 100 cycles.3.The Si/rGO/C composite was fabricated through the electrostatically crosslinking of alginate with chitosan(form an insoluble polyelectrolyte complex)and carbonization process.In the material preparation process,chitosan as the cross-linking agent,at the same time,avoid the reaction of with Si.The results show that rGO(electronic conductive additive)and carbon layer work together to effectively improve the conductivity of the composite and suppress the huge volume change of the Si nanoparticles.The Si/rGO/C composite delivers the reversible capacity of 1163.5mAh/g and 72.7%capacity retention afer 100 cycles,when the current density is 0.2A/g.Furthermore,the discharge capacity is still 615.7mA/g at the current density of 6.0A/g,showing the excellent electrochemical cycle and rate performance.