Designs And Application Studies Of Binders For Silicon Anodes In Lithium-Ion Batteries

Lithium-ion battery stood out among many energy storage devices due to its high energy efficiency,long life,portability,and light weight design.It has achieved great success in the past few decades.However,new technologies represented by electric vehicles,artificial intelligence,and virtual reality urgently require lithium-ion batteries with ever-increasing energy densities while ensuring safety.Silicon(Si)holds the advantages of high theoretical specific capacity(4200 mAh g-1),low operation voltage and abundant resources,making it very promising as the anode material for next-generation high-energy-density lithium-ion batteries.However,the huge volume change of Si(up to 300%)will destroy the integrity of the Si electrode,resulting in a rapid decline in capacity.The scientific community has recently realized that binders can greatly improve the cycle stability of the Si anodes.The traditional polyvinylidene fluoride(PVDF)binder is not suitable for Si anodes due to its weak Van der Waals interactions with Si and copper current collectors.Researchers have made tremendous efforts in designing and preparing of binders for Si anodes,and developed many binders that effectively enhanced the cycling performances of Si anodes.However,how to solve the issues of low initial Coulombic efficiency,low active material content,and low areal capacity of Si anodes via reasonable design of binders are still big challenges.In view of the abovementined shortcomings in the research of binders for Si anodes,in this dissertation,a variety of new binders for Si anodes were designed and prepared in order to promote the research of Si anodes from the persperctive of binders The main research contents and results are summerized as follows:(1)Preparation and research of a trifunctional binder for silicon anodes with high initial Coulombic efficiency and high areal capacity.This work first prepared partially lithiated polyacrylic acid and Nafion,named as P-LiPAA and P-LiNF.Then hard P-LiPAA and soft P-LiNF were mixed to construct a soft/hard modulated trifunctional binder(N-P-LiPN).Due to the three functions of the N-P-LiPN,i.e.,excellent mechanical properties,strong adhesion strength,and transport facilitation of Li-ions,the Si electrodes using N-P-LiPN as the binder(Si@N-P-LiPN)achieve high initial Coulombic efficiencies and stable long-term cycling performances.Specially,the Si@N-P-LiPN electrode demonstrates an ultrahigh areal capacity of 49.59 mAh cm-2 In addition,the soft pack full cell with the Si@N-P-LiPN anode also exhibits a good cycling performance,demonstrating the great practical potential of the N-P-LiPN binder.(2)Peach gum and epichlorohydrin(ECH)crosslinked peach gum(PG)were used as binders for high-areal-capacity Si anodes.In this work,ECH was used as the crosslinking agent to chemically crosslink PG to prepare a robust network binder(PG-c-ECH).The abundant bonding sites between the binder and the Si particles endow the binder strong interfacial adhesion.Chemical crosslinking improves the mechanical strength of the binder,which could maintain the integrity of electrode during huge volume change of Si.The Si anode using PG-c-ECH as the binder(Si@PG-c-ECH)demonstrates a stable long-term cycle performances at 2 A g-1.Moreover,the PG-c-ECH binder enables an ultrahigh-areal-capacity of 60.00 mAh cm-2(3)Biopolymers modified by heat treatment in air atmosphere were used as the binders for Si anodes with high silicon content.In this work,a series of biopolymers(sodium carboxymethyl cellulose,sodium alginate and carboxylated chitosan)were used as raw materials,followed by heat treatment in air atmosphere.After heat treatment,the polar groups were destroyed,resulting in gradually reduced hydrogen bonding sites.However,the dispersion force increases due to the decrease of the surface tension.Meanwhile,the electronic conductivity of the polymer increases after heat treatment.The product obtained after the heat treatment acts as both the binder and conductive additive in the Si anode,which can make the Si anode cycle stably with a Si content of 90 wt%without any other conductive additives.(4)The compound glue of ?-carrageenan and konjac gum was used as a binder for high-performance Si anodes in lithium-ion batteries.Inspired by the compounding useage of food gums in food processing,high-safety ?-carrageenan(KCG)and konjac gum(KG),which have been widely used as food additives,were chosen for this study.A new three-dimensional network binder(N-KCG-KG)was prepared via the interactions between KCG and KG.Due to the synergistic effect of KCG and KG,the Si anodes using the N-KCG-KG binder exhibit stable cycling performances at a low binder content(10 wt%),high Si content(80 wt%)and high Si loading(1 mg cm-2).(5)Guar gum grafted polyacrylic acid was used as the binder for high-performance Si nanoparticles and Si microparticles anodes.In this work,the ion-conductive biopolymer guar gum(GG)and acrylic acid(AA)were used as the raw materials for graft polymerization to prepare the GG-g-PAA.The abundant ether groups of GG can effectively promote the transportation of lithium ions,and the abundant oxygen-containing functional groups in GG and PAA can interact strongly with the Si particles,so that GG-g-PAA owns ion-conductivity and strong adhesion.More importantly,GG-g-PAA shows excellent mechanical properties and can effectively restrain the pulverized Si particles during cycling.Electrochemical tests show that the Si microparticles anodes using GG-g-PAA as the binder have good cycling stabilities,while the Si nanoparticles anodes using GG-g-PAA as the binder also exhibits excellent cycle performances.