Synthesis Of Acrylate Water-based Binder And Its Application In LiFePO₄ Cathode

Lithium-ion batteries occupy a vital position in the current secondary battery field,and the LiFePO4 cathodes is widely used in lithium-ion batteries due to its high safety,excellent long-cycle stability and low cost.As one of the most important polymer materials in lithium batteries,although the electrode binder occupies a relatively small proportion,it plays a really important role in connecting active materials,conductive agents,and current collectors to form a complete conductive network,which has a significant impact on battery performance.At present,the commercial binder for LiFePO4 is mainly polyvinylidene fluoride(PVDF).However,PVDF needs a sufficiently dry environment and toxic N-methylpyrrolidone(NMP)solution in the application,which make it cost-ineffective and environmentally unfriendly.Therefore,exploring low-cost,environmentally friendly water-based binders has become a top priority.This article aims to design a environmentally friendly acrylic water-based binder with a simple synthesis method for the LiFePO4 cathode,which can achieve excellent electrochemical performance with industrial ratio.The specific work is as follows:(1)Triblock copolymer PSt-b-PEHA-b-PSt(SEHAS)was designed and controllably synthesized via reversible addition-fragmentation chain transfer radical polymerization(RAFT)emulsion polymerization and applied in LiFePO4 cathode.This triblock copolymer provides mechanical strength and electrolyte swelling ability with different blocks.And change the ratio of its first and last blocks.Compared with the traditional PVDF binder,the LiFePO4 electrode with SEHAS shows similar cycle performance and better rate performance.(2)The reactive emulsifier NRS1230 was selected to synthesize P(St-co-EHA)random copolymers through simpler traditional emulsion polymerization.After optimizing binders with different monomer compositions,the performance of the preferred random copolymer binder-PSEHA-1:11 is comparable to block copolymers.Compared with the commercial PVDF and water-based SBR,the PSEHA-1:11 adhesive has higher peel strength,better cycle and rate performance.After 100 cycles at a high rate of 1 C,its discharge capacity is still 126 mAh/g,and the capacity retention rate is 96%.In the follow-up industrial application test,the pounch full cell prepared by industrial process using PSEHA-1:11 binder gained an excellent cycle and rate performance,which initial discharge capacity at 1 C is 370.8 mAh,and after 1100 cycles,the specific capacity can still be maintained at 341.8 mAh,the capacity retention rate is up to 92.2%.Its average coulombic efficiency is 99.83%,which is always stable at a high level.Meanwhile,it also shows good rate performance(0.2 C-2 C,stable discharge capacity;5 C,about 340 mAh;10 C,about 280 mAh).In addition,the slurry obtained with PSEHA-1:11 binder has good stability and the electrode has excellent processability.(3)Design and synthesis of styrene-isooctyl acrylate-hexafluorobutyl acrylate terpolymer and applied in LiFePO4 cathode.By comparing the electrochemical performance of LiFePO4 cathode prepared with F-x%(x=1,3,5,x represents the mass fraction of fluorinated monomer)and explored the influence of the addition amount on the swelling ratio and the electrochemical performance.The result shows that the addition of C7H6F6O2 can effectively reduce the electrolyte swelling,and improve cycle stability.For the best-performing F-3%binder,the initial discharge capacity at 1 C is 126.6 mAh/g,and the capacity retention rate after 185 cycles is up to 100.6%,while the F-0%is 92.8%for 185 cycles.Compared with F-0%,the capacity retention rate has been significantly improved,which can prove that the addition of C 7H6F6O2 in a proper proportion can effectively improve the cycle stability without affecting the specific discharge capacity.