Study Of Silicon-Based Anode Materials For High Performance Lithium-ion Batteries

Lithium-ion batteries(LIBs)are a common secondary battery that rely on lithium ions to shuttle between positive and negative electrodes to achieve charge and discharge.The voltage and specific energy of lithium-ion batteries are several times higher than those of Ni-Cd and Ni-MH batteries.The cycle life is high,and they have excellent safety performance.Because of these advantages,lithium-ion batteries have been widely used in electronic equipment such as 3C products.In recent years,emerging technologies such as new energy vehicles,large-scale energy storage equipments,5G technology,and AI have gradually developed,which have set higher requirements for the energy density of lithium-ion batteries.The anode material,as an important component of lithium-ion batteries,plays an important role in the overall performance of the battery and will be an important breakthrough to improve energy density.At present,graphite materials are mainly negative electrode materials that are commercially used.Graphite has an abundant sources and low cost,but the specific capacity is low(375 mAh g-1).Therefore,silicon(4200 mAh g-1,Li4.4Si)with ten times the theoretical specific capacity of graphite has attracted researchers'attention.Silicon has the potential for large-scale industrial applications due to its abundant sources and mature preparation process.At the same time,among many negative electrode materials,the working voltage of silicon is only slightly higher than that of graphite,which can exert a higher energy density and better safety performance.However,silicon is also facing serious constraints.Firstly,silicon undergoes a large volume change(approximately 300%)during the charge and discharge process,which leads to structural damage to the silicon material and the electrode,and the active material is detached from the current collector,causing a decrease in reversible capacity and cycle stability.Secondly,the silicon material reacts with the electrolyte,and forms a solid electrolyte layer(SEI)on the surface of the material,which irreversibly consumes the electrolyte and lithium ions.At the same time,the poor ion conductivity of SEI will cause the increase of material's polarization.In addition,the volume expansion of silicon will cause the SEI to rupture,leading to the contact between the internal silicon and electrolyte,then generate SEI again.As the cycle progresses,the SEI continues to thicken,the negative polarization becomes larger,and the reversible capacity continues to decrease.The solutions to the shortcomings of silicon mainly include using nanosized Si and their composites,and SiO is also a promising alternative.Nanostructures improve the ability of silicon to withstand the stress from the volume change and help maintain the integrity of the structure.Si composite mainly modify the surface properties of materials and inhibit the excessive growth of SEI.SiO is one of the important forms of silicon-based anode materials.The internal structure is mainly short-range ordered Si and SiO2 micro-domains.Amorphous SiO2 will react with lithium ions to form lithium silicate and lithium oxide,which can alleviate the volume change of Si to a certain extent to keep the overall structure intact.However,this advantage also creates the main shortcoming of SiO:low cycling Coulombic efficiency.One of the main solutions to this shortcoming is to convert SiO2 into a phase that does not react with lithium ions or has less reactivity,such as a silicate or a SiO2 phase with higher crystallinity.Taking the above solution as a guide,we successfully prepared a high-performance core-shell material of silicon oxide-coated nano-silicon particles,and proposed the method that using potassium ions to induce the conversion of amorphous SiO2 into crystalline cristobalite phase in SiO,which successfully improved their Coulombic efficiency and cycling stability.The main research contents include as follows:(1)A SiOH-groups modified SiOx coating was successfully coated on the surface of the nano-sized silicon particles by utilizing the hydration/dehydration reaction of silicon.The coating effectively relieves the expansion of the nano-silicon particles,stabilizes the overall structure of the electrode,and significantly improves the capacity retention rate and reversible capacity of the silicon anode material.The SiOH-groups can undergo an esterification reaction with the binder to form a stronger bond.At the same time,the activity of the silicon particles is improved,the polarization is reduced,and the reversible capacity is increased.(2)Using the principle of reducing the SiO2 bond energy via potassium ions,the amorphous SiO2 in SiO is converted into a cristobalite phase with higher crystallinity,which reduces its reactivity with lithium,thereby significantly improves the initial coulombic efficiency(ICE)and subsequent cycle efficiency.The reduced reactivity of SiO2 also helps to generate less lithium silicate,and the electrode structure become more stable with increased lithium ion conductivity inside SiO,reduced polarization,and improved reversible capacity.In addition,our study found that prolonging the treatment time is helpful to improve the cyclic stability of SiO.However,the reversible capacity is reduced due to more generated crystalline SiO2.