Synthesis And Application Of Polyimide Binder For Silicon/Graphite Anode In Lithium-ion Batteries

Lithium-ion secondary batteries are listed as one of the most competitive energy storage technologies due to its environmental friendliness,long cycle life,high output voltage,and low self-discharge performance.In recent years,with the boom of electric vehicles and large power grids,people’s demand for lithium-ion batteries has been more reflected in high energy density.Although the traditional graphite anode has good cycle stability,its theoretical specific capacity is low(375 mA h g-1),which does not meet the development requirements of high energy density lithium-ion batteries.The silicon-based anode is one of the most potential lithium-ion battery anode materials due to its environmental friendliness,high theoretical specific capacity(4200 mA h g-1),low operating voltage,etc.However,as an anode material,silicon suffers from some problems such as poor conductivity,large volume change,and instability of the solid electrolyte interface(SEI)during the cycle,which hinders its wide application.As one of the important components of the electrode,the binder plays a role in maintaining the contact integrity between the active material,the conductive additive and the collector,thereby reducing the crushing and shedding of silicon particles during charge and discharge process.Due to its excellent mechanical properties and adhesion properties,polyimide is considered to be a highly potential silicon-based anode binder.In this paper,the polyimide was modified and crosslinked,and three binders were prepared to improve the cyclic defects of the Si/Graphite anode.The specific work mainly includes the following three parts:(1)First,the chain modified polyimide was synthesized through a tri-monomer condensation,and then was cross-linked by Trimesic acid to obtain a modified cross-linking polyimide(C-PI-OH)as a binder for Si/Graphite anode.The elastic network of C-PI-OH binder can well adapt to the volume expansion and contraction of silicon particles during the process of lithium deintercalation,and ensure the stability of the SEI film of the active material surface,effectively improving the cycle stability of the Si/Graphite anode.The Si/Graphite/C-PI-OH electrode present 77.03%at first coulombic efficiency,1112 mA h g-1 after 200 cycles at a current density of 0.1A g-1,and the capacity retention rate is 65.87%.(2)The three-dimensional network tannie-polyimide(PI-T)binder was prepared through the crosslinking process of tannic acid and polyimide polymer,applying to Si/Graphite anode.The PI-T binder has excellent mechanical properties and strain resistance,and its three-dimensional network structure can effectively inhibit the rapid deformation of silicon particles during high-current cycling.The Si/Graphite/PI-T2%electrode present 90.59%at first coulombic efficiency,801 mA h g-1 after 200 cycles at a current density of 1A g-1,and the capacity retention rate is 66.75%.(3)A dual cross-linked network structure polyimide binder(D-PI)was designed for Si/Graphite anode,in which the addition of ethylene glycol and trimesic acid can crosslink side and end groups respectively.The ethylene glycol serves as a segment to regulate the mechainical properties of the polymer,and the trimesic acid acts as a cross-linking agent to form a dual network with PI,which displays outstanding mechanical properties and strain resistanc,maintaining the stability of Si/Graphite electrode during high-current long cycling.The Si/Graphite/D-PI-0.7 electrode present 92.93%at first coulombic efficiency,943 mA h g-1 after 300 cycles at a current density of 1A g-1,and the capacity retention rate is 74.25%.