Design And Preparation Of Polyimides And The Application As High Nickel Ternary Cathode Functional Binder

As the core of new energy technology,lithium-ion batteries have been widely used in various small electronic devices and gradually expanded to emerging fields such as power vehicles and energy storage technologies.The rapid development of power energy storage technology puts forward larger demands for lithium-ion batteries with high energy density,long endurance,and high safety.Cathode materials play a vital role in the energy density,cycle life,safety,and cost of lithium-ion batteries.Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）,has been spotlighted as the most promising candidate for the new-generation high-energy cathode material,due to its high theoretical specific capacity(270 m Ah g^-1)and low cost.Although high nickel content satisfies high capacity,it leads to poor structural stability,serious electrode/electrolyte interface side reactions,and poor thermal stability,eventually influencing the cycle,rate,and safety performances of NCM811 materials.Many studies have shown that binders with facile-preparation and low-cost can improve the structure and performance of cathode material.The commercial polyvinylidene fluoride（PVDF）binder lacks the ability to adapt well with NCM811,because of its low adhesion,poor swelling property,and weak high temperature/voltage endurance.Therefore,exploring high-performance binders to replace traditional PVDF binder is crucial and urgent.Among many materials,polyimide（PI）,which possesses the thermal stability,chemical stability,outstanding mechanical property,flammability,and designable molecular structures,has been considered as an ideal material for a new generation binder.Therefore,PI has attracted wide attention.Previous studies show that PI binders can improve the cycle performance and safety performance of cathode materials.However,the molecular structure of PI is diverse and the factors influencing the performance of lithium-ion batteries are complex.It is considerable to select an adaptive PI molecular structure as a cathode binder for lithium-ion batteries.The current study lacks the exploration of influences on the PI molecular structure,the PI preparation process for cathode morphology,and the explanation of the mechanism between PI binder and cathode.Herein,without changing the thermal and mechanical properties of PI itself,PI binders with multifunctional polar groups,rigid-soft segments,and three-dimensional cross-linked networks were designed.In this paper,the molecular structure,the preparation process,and the performance of PI binders are completely explored.The influence of PI binders on the morphology and electrochemical performance of cathode as well as the mechanism of PI binder on the cathode are deeply analysed.The research contents and the achievements of this paper are divided into three parts as followed:（1）The PI binders with multifunctional polar groups were designed and prepared.By optimizing the monomer compositions,the polar functional groups like-CF₃,-O-,-OH,and-COOH were introduced into the PI bone by copolymerization.Two PI binders（PI-OH and PI-COOH）with different molecular structures were synthesized.The polar functional groups on the PI binders can improve the redox resistance,enhance the adhesion ability,and regulate the molecular chain flexibility.The precursor polyamic-acid（PAA）was used as the glue to prepare the cathode slurry and then heated the cathode slurry to prepare the cathode with PI binder.During this process,a large density of-COOH and-CONH groups in the PAA chain can form bonding reactions with the active particles,which is contribute to a uniform nano-scale surface coating layer.Heating process makes PI coating layer with high thermal stability and high mechanical strength.The experimental results show that both PI-OH and PI-COOH binders have good thermal stability,low electrolyte swelling,and high adhesive strength.The PI-OH and PI-COOH surface coating layers prevent the cathode/electrolyte contact,improve the cathode/electrolyte interface stability,suppress the surface phase transition,and inhibit the transition metals dissolution.The NCM811 half-cells with PI-OH and PI-COOH binders display a high-rate performance,a high-voltage cycle life,and a stable thermal property.Compared with PI-OH binder,PI-COOH binder with high bonding energy exhibits better properties.The half-cells with PI-COOH binder show a high reversible capacity of 220 m Ah g^-1 in the voltage range of 2.5-4.7 V and a high rate performance of 151 m Ah g^-1 at a current of 1000 m A g^-1,as well as a high capacity retention of 72%after 100 cycles at a current of 40 m A g^-1.The cathode using PI-COOH binder shows an increased peak temperature to 235°C and a decreased heat production to 34 J g^-1 at a 4.5 V fully-charged station.（2）Poly（imide-siloxane）（PIS）binder with an adjustable rigid-soft chain structure was designed and prepared.By copolymerizing flexible aliphatic polydimethylsiloxane（DMS）into the rigid aromatic PI,the liner PIS binder was obtained.The flexible aliphatic DMS segment increases the free volume of PIS binder,which benefits the electrolyte infiltration.The negatively charged Si-O bond in the DMS structure can form a complexing site with Li⁺,thereby promoting the migration of Li⁺.The precursor polyamic acid（PAA）of PIS was used as the glue to prepare the slurry,and then heating to 300 to get the cathode with PIS binder.Similarly,PIS not only acts as a binder to guarantee the cathode stability but also acts as a surface coating layer to protect the layered NCM811 structure.The experimental results show that introducing the flexible DMS segments will increase the electrolyte affinity and improve the lithium-ion migration rate in PIS binder.The lithium-ion diffusion rate,rate performance,and high-voltage cycle performance of the half-cells with PIS binder are highly improved.The half-cells with PIS binder show a high reversible capacity of 215 m Ah g^-1 in the voltage range of 2.5-4.7 V and a high rate performance of 163 m Ah g^-1 at a current of 1000 m A g^-1,as well as a high capacity retention of 61%after 300 cycles at a current of 40 m A g^-1.（3）Self-adaptive gel poly（imide-siloxane）（c PIS）binder with 3D cross-linked structure was designed and prepared.The flexible linear DMS acts as a bridge to graft rigid PI to form crosslinked structures through the formation of acid-base salts between carboxyl groups in PI and amino groups in DMS under the room temperature.The rigid PI segments with imide rings and benzene rings can bear the swelling and maintain the high heat resistance,while the flexible DMS segments with abundant lone-pair electrons on Si-O bonds act as transfer sites to promote the Li⁺diffusion and guarantee high free volume.The gel c PIS terminated with abundant amide groups can offer adhesive sites on NCM811particles by forming hydrogen bonds.The three-dimensional cross-linked network structure of c PIS binder provides cohesion and high elasticity.The experimental results show that c PIS binder has good electrolyte affnity,high adhesive strength,excellent scalability,and high elasticity.This gel c PIS binder with a self-adaptive polymer network and high adhesion on NCM811 particles can offer elasticity to ensure cathode-electrolyte interface stability during lithiation/delithiation.The half-cells with c PIS binder exhibit fast lithium-ion diffusion rate,excellent rate performance,high voltage cycle stability and low impedance.The half-cells with c PIS binder exhibit a high reversible capacity of202 m Ah g^-1 in the voltage range of 2.5-4.3 V and a high rate performance of150 m Ah g^-1 at a current of 1000 m A g^-1,as well as high capacity retentions of100%,87%and 65%after 100 cycles at a current of 40 m A g^-1in the voltage range of 2.5-4.3 V,2.5-4.5 V and 2.5-4.7 V,respectively.The c PIS binder can be adapted to the practicle cathode preparation process without high temperature heat treatment,which has a broad industrial application prospect.