Synthesis Of Aqueous Composite Binder And Its Interfacial Regulation Mechanism In Lithium Storage Process Of Silicon Anode

Lithium-ion battery has received widespread attention from industry and academia because of its high energy density,long cycle life and environmental friendliness.Silicon is considered to be the most promising anode material for high performance lithium-ion batteries because of its highest theoretical specific capacity and low working potential.However,silicon has a severe volume expansion problem,which seriously hinders its commercial development.Binder can firmly adhere electroactive material and conductive agent to the collector and effectively alleviate the silicon volume effect,thus maintaining the integrity of the electrode structure and realizing the lithium-ion battery with high electrochemical performance.Two aqueous composite binders that can regulate the silicon anode interface have been designed in this research.The main contents are as follows:(1)Small molecule CA with abundant carboxyl groups was selected as the modifier to construct covalent cross-linking CA@CMC binder with rigid polysaccharide CMC by esterification reaction.The Si@CA@CMC electrode materials with different crosslinking degrees were prepared by adjusting the amount of CA and CMC.When the amount of CA and CMC is 1:1,Si@CA@CMC-1 showed a high initial discharge specific capacity of 3390.57 mAh·g-1 and a high initial coulomb efficiency of 82.03%at the current density of 0.1 A·g-1.The excellent electrochemical performance of Si@CA@CMC-1 was attributed to the bonding of-COOH on the CA@CMC-1 with-OH on the silicon surface,which successfully constructed a double-crosslinked network structure with a high degree of esterification and ensured the structural stability of the silicon anode during frequent lithiation/delithiation process.At the same time,the results of in situ EIS decoupling and XPS with different cycles showed that CA@CMC binder is conducive to the formation of a stable interface on the silicon surface,and effectively regulates the SEI composition and proportion,thus contributing to the formation of thin and stable solid electrolyte interface.Therefore,Si@CA@CMC-1 exhibited a high initial coulomb efficiency.(2)To further improve the electrochemical performance of the silicon anode,PA containing rich phosphate groups was selected as dopant to form ester bond with PAA to generate the PA@PAA binder by in situ thermal crosslinking,and dynamic cross-linking network was constructed with rich hydroxyl groups in silicon.The Si@PA@PAA materials with different crosslinking degrees were obtained by adjusting the heat treatment temperature.When the temperature reached 220℃,electrode material Si@PA@PAA-220 has excellent electrochemical performance.The electrode still had the discharge specific capacity of 1322.1 mAh·g-1 after 510 cycles at the current density of 0.5 A·g-1.Through characterizations and DFT,it is found that the ester bond between PA and PAA,together with the interactions between PA@PAA binder and silicon,can effectively alleviate the volume expansion of silicon in the alloying/dealloying process.Based on the analysis of DRT and XPS after cycling,it is found that PA@PAA binder can regulate the interfacial chemical reaction,build a relatively stable interface,accelerate the conduction of lithium ions,and also adjust the ratio of organic layer-inorganic layer in SEI,so as to enhance the mechanical stability of SEI layer during cycling process.In conclusion,two aqueous composite binders were designed in this research,and the composition ratio/heat treatment temperature were adjusted to obtain electrode materials with different degrees of cross-linking.Si@CA@CMC-1 electrode with high initial coulomb efficiency and Si@PA@PAA-220 with excellent cycling stability were obtained by electrochemical tests.By combining DRT theory with XPS analysis,the binders were confirmed to be involved in the electrochemical reaction process of interface,and the composition and relative content of the organic layer-inorganic layer in SEI were adjusted,which provides a new idea for in-situ regulation of the interfacial reaction at the electrode.Moreover,the synthesis method was simple with a wide range of raw materials and a controlled preparation process,which is an effective method to optimize the performance of silicon anode.