Construction Of Polyimide Cathode Binders With High Temperature And High Voltage Tolerance For Li-Ion Battery By Controlling Chain Flexibility

Lithium-ion batteries（LIBs）have become the focus of widespread attention in recent years due to the rapid development of renewable energy industry.At present,LIBs need to improve in terms of high safety and high energy density.Among them,increasing the cutoff voltage is one of the effective measures to achieve high energy density.Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）is promising cathode material candidates for high-energy LIBs.As an important part of the electrode,the binder is responsible for firmly joining the electrode components and has a significant impact on battery performances.Polyvinylidene fluoride（PVDF）is an extensively used cathode binder,but it has poor thermal stability,easy oxidation and low adhesion,which cannot maintain the integrity of the electrode structure after long term cycling.Therefore,it is necessary to develop novel binders to adapt to the rapid development of LIBs.As one of the organic polymers with excellent comprehensive performance,polyimide（PI）is an advanced candidate material for LIB binders because of its excellent high temperature tolerance,chemical stability,mechanical strength and strong designability.In this paper,the micro molecular structure design method was used to construct and synthesize the PI binders containing ether bond structure and biphenyl structure,of which the ether bond structure enhances the flexibility and improves the diffusion of lithium-ion,while the rigidity of biphenyl structure endows the material with excellent high temperature tolerance so as to improve the safety of the battery.Thus,we constructed the PI（ODPA-ODA）/PI（BPDA-PDA）blend binders,and the PI（ODPA-BPADA-ODA）and PI（BPDA-BPADA-PDA）copolymer binders.（1）The PI（ODPA-ODA）flexible precursor and the PI（BPDA-PDA）rigid precursor were synthesized and blended in different mass ratios to construct the PI（ODPA-ODA）/PI（BPDA-PDA）blend binders,abbreviated as PI（OO/BP=x:y）（x and y represent the mass ratios of flexible precursor and rigid precursor in the system,x=3,5,7;y=7,5,3,respectively）.And we investigated the effect of PI（OO/BP=x:y）binders on the performance of LIBs.（2）We designed and synthesized PI（ODPA-BPADA-ODA）copolymer binders with larger flexibility and PI（BPDA-BPADA-PDA）copolymer binders with less flexibility by molecular structure optimization,in which abbreviated as PI（OBO）and PI（BBP）,respectively.The properties of binders and batteries were tested and analyzed.In addition,we used molecular simulation（MS）to study and analyze the mechanism of PI（OBO）and PI（BBP）binders deeply.The batteries have better electrochemical performance（i.e.rate performance and high voltage cycling stability）by using the PI（OO/BP=x:y）binders.In particular,PI（OO/BP=3:7）cell possesses the discharge capacity of126.9 m Ah·g^-1 at 5 C in 4.3 V and the capacity retention rate of 52%after 100cycles in 2.5-4.5 V,which are higher than PVDF cell(115.1 m Ah·g^-1 and 25%,respectively).The cell with PI（BBP）copolymer binder possesses the capacity retention rates of 88%and 62%after 100 cycles in the voltage range of 2.5-4.3 V and 2.5-4.5 V,respectively,and the discharge capacity of 127.7 m Ah·g^-1at 5 C in 4.3 V,showing excellent electrochemical performance.In addition,compared with PVDF cathode(108.01 J·g^-1),the heat release of the 4.5 V fully-charged PI（BBP）cathode was simply 66.19 J·g^-1 in the DSC test.This study supplies a novel perspective for the construct of cathode binder with high temperature and high voltage tolerance for lithium-ion batteries.