Study Of Structure Design And Electrochemical Performance Of Silicon-based Anode Materials For Lithium Ion Batteries

Lithium ion batteries have the advantages of high specific energy,high voltage,long cycle life,low self-discharge rate,no memory effect,and green environmental protection,which have shown broad application prospects in the fields of portable digital electronic products,electric vehicles and electric energy storage.In order to further meet the market's requirements for the use of high specific energy and safety batteries,vigorously developing electrode materials has become the focus of current research,and the level of lithium-storage capacity of electrode materials as a key part of the battery is crucial to the improvement of the specific capacity of lithium ion batteries.Compared with the current commercial graphite anode material(372 mAh/g),Si material has the highest theoretical specific capacity(4200 mAh/g,Li₂₂Si₅ alloy phase),low voltage platform,abundant earth resources,which is the most potential choice for the next generation of high specific energy lithium ion anode materials.There are two main bottlenecks in the use of Si materials:one is that huge volume change effect will occur during the repeated extraction of lithium,resulting in poor cycle stability of the Si anode material,the other is that the Si material has poor conductivity,which is not conducive to electron conduction of electrode materials.Aiming at the above two bottlenecks of Si,this paper further proceed to in-depth design of micron-level dispersed Si based composites,coated porous Si based composites,coral-like porous Si based composites and Al-MOF structures to improve the cycle stability.The primary nano-sized pores particles in the micron-sized porous Si material can not only shorten the diffusion distance of Li⁺,but also effectively improve the volume expansion problem.Meanwhile,the micron-sized secondary particles can guarantee high-pressure tap density and increase the specific capacity of Si electrode materials.The detailed research content is summarized as follows:(1)In response to the high impurity content of Si particles cut from diamond wire in the photovoltaic industry,the SiO₂ layer,attached trace metal impurities and the adhesive PEG layer on the surface of Si particles were removed by 3-step surface modification treatments,and the corresponding removal rates of total metal and organic impurities reached 67.4 and 43.7%,respectively.Ultimately,the micron-sized dispersed PSi/PA-C(1:6)composite was prepared by liquid phase mixing and high temperature pyrolysis,and the discharge and charge specific capacities reached 463.1 and 458.5 mAh/g after 100 cycles at 0.1 A/g,showing good cycling stability.This is due to the proper amount of amorphous PA-C covering the Si particles uniformly,which can alleviate the macroscopic Si volume change during lithiation and delithiation processes,and the PA-C in the electrode material forms an interconnected conductive network.(2)On the basis of the above research,a new process of HF corrosion coupled high temperature cracking PEG is further proposed to modify and optimize the surface of Si particles,which reveals the surface modification model,the mechanism of action and the influence of different pretreatments on the electrochemical characteristics of the Si particles.The removal rates of total metal and organic impurities of Si particles after surface modification treatments reached 66.76 and 70.42%,respectively,and the purity of finally obtained Si particles is about 4.5 N.After 100 cycles,the discharge and charge specific capacities of different surface modification samples SiO,Sil and Si2 at 0.1 A/g reached 0.9/0.7,33.4/33.2 and 230.0/229.2,respectively.It can be seen that the effective removal of the SiO₂ and PEG fouling layer on the surface of the Si particles can reduce the hindrance of the migration of Li⁺to the Si interface,reduce the consumption of Li⁺caused by the irreversible side reactions on the surface of the Si particles,thereby improving the cycle stability of the Si electrode material.Meanwhile,a new method of using NaNO₂ as a catalyst to perforate surface-modified micron-sized Si particles was proposed.Porous Si@SiO₂ structure with nano-clusters pores was prepared under the conditions of HF/HNO₃/NaNO₂/DI solution system with 16:4:1:80 for 120 min through thermodynamic analysis and process optimization.The nano-pores can provide convenient channels for the rapid insertion and extraction of Li⁺,and also provide buffer space for the volume expansion of Si,and the coated SiO₂ layer can avoid direct contact between Si and electrolyte,which can reduce the occurrence of irreversible side reactions,while acting as a transition layer to enhance the interface bonding force between Si and C in the form of Si-O-C functional groups.The micron-sized Porous Si@SiO₂@C composites were obtained by the treatments of surface PVP adsorption and high temperature pyrolysis.The discharge and charge specific capacities reached 1051.4 and 1038.2 mAh/g after 100 cycles at 0.1 A/g,showing good cycling performance.The coated C layer can improve the conductivity of this composite structure and also has the function of consolidating the stability of the material structure.(3)In order to obtain a micron-sized Si based composite material with a more stable porous structure and simpler operation,low-cost Al-Si alloy particle was selected as the raw material,and a simple process flow of dealloying,pre-oxidation and high-temperature pyrolysis was proposed to prepare a novel double-shell constrained coral-like Porous Si@SiO₂@C composite,in which the double shell thickness of SiO₂@C is about 44?72 nm.The discharge and charge specific capacities of Porous Si@SiO₂@C reached 933.2 and 929.2 mAh/g after 100 cycles,respectively.The composition and mechanisms of the primary Si and eutectic Si in coral-like porous were studied.The coral-like porous structure provides a more stable transport channel for the rapid insertion and extraction of Li⁺,in which the micron primary Si plays a supporting skeleton stabilization role,and the eutectic Si holes obtained by HCl corrosion in eutectic structure provide buffer space for volume expansion.(4)On the basis of the above research,the most superior coral-like Porous Si@SiOx composites were obtained by orthogonal design with Al-Si alloy powder(-6 ?m).The influence of porosity,conductive sites and SiOx layers of coral-like porous structure on electrochemical characteristics was revealed.The concentrations of Si,Al and O reached 83.39,5.73 and 10.64 wt.%,respectively,and the corresponding porosity was about 82.04%,between 75.0?85.0%,which can theoretically meet the 300%volume expansion of Si.3D coral-like eutectic Si rods will collapse when the concentration of Al in the eutectic is over leached.Meanwhile,the influence of the composition and proportion of Si valence state(Si⁰,Si⁴⁺/Si⁰,Si³⁺ and Si⁴⁺)in the 10 nm SiOx layer on the electrochemical characteristics is elucidated.The first discharge and charge specific capacities of the composite were 3058.7 and 2364.4 mAh/g,and 1367.9 and 1340.8 after 100 cycles,respectively.The composite still reached 903.2 and 899.7 mAh/g at 1.0 A/g after 300 cycles,showing excellent electrochemical performance.(5)This paper presents a self-assembly hydrothermal method using PVP auxiliary modification for the regeneration and utilization of AlCl₃ solution formed after Al-Si dealloying.A novel brick Al-MOF structure with particle size of 878.6 nm,pore size of 2.5 nm was prepared by hydrothermal method modified by PVP under the condition of 150?and 9 h.Under the acidic condition of pH=4?5,the Al ion and the O in the organic ligand will form an octahedron A106 structure.The octahedron A106 clusters are further connected by the PTA ligands to form a 3D structure with rhombus,which is beneficial to the rapid transmission and deintercalation of Li⁺.The first discharge and charge specific capacities of the brick Al-MOF at 0.1 A/g reached 392.4 and 379.2 mAh/g,respectively.The corresponding specific capacities were 372.3 and 370.8 mAh/g after 100 cycles,respectively,showing superior capability of capacity recovery,and finally realizing the regeneration and utilization of AlCl₃ solution.