Design And Properties Of Composite Water-soluble Binder For Silicon Anode Of Lithium-ion Battery

At present,lithium-ion batteries are playing an increasingly important role in electric vehicles such as mobile and portable electronic devices.Silicon（Si）has a high theoretical specific capacity of 4200 mAh g^-1,and has gained wide attention as a negative electrode material for lithium-ion battery.However,in the process of charge and discharge of the Si anode,the stress caused by the insertion and detachment of lithium ions from the Si anode will cause the drastic volume change and rupture of the electrode,thus leading to the decline of lithium-ion battery cycle performance.As an important component of an electrode,binders can alleviate the drastic changes of Si volume effectively.Therefore,in order to further improve the structural stability of Si electrode,the selection of binder is particularly important.The traditional polyvinylidene fluoride（PVDF）binder cannot form a strong interaction with the surface of Si particles,thus failure to effectively alleviate the expansion of Si,and ultimately destroys the overall electrode structure.In addition,when PVDF is used as binder,toxic N-methylpyrrolidone（NMP）is needed as solvent.To sum up,it is an important research direction to design the non-toxic water binder for the Si anode of lithium-ion battery.In this paper,two kinds of binders are developed in the Si anode of lithium-ion battery,and significantly improve the cycle stability of the electrode.（1）We have developed potassium tripolyphosphate（PTP）as an inorganic polymer binder for the Si anode.PTP has abundant P-O^-functional groups,so it can form a strong ion-dipole interactions with hydroxyl groups on the surface of Si（Si-OH）,which can reduce the fragmentation of conductive network caused by the expansion of Si particles in the electrode.In addition,PTP,as an inorganic oligomer,is better dispersed in aqueous solution,thus facilitating the uniform coating of Si nanoparticles,which makes the stress distribution received by the Si electrodes more uniform.Therefore,the specific capacity of nano Si electrode based on PTP binder is 1739.6mAh g^-1after 80 cycles,and the corresponding capacity retention rate is as high as98.8%.（2）we prepared a composite network binder（c-PTP-Alg binder）based on the PTP binder used in the first system by combining the PTP binder with the Alg binder through a crosslinking reaction.In the c-PTP-Alg binder,Alg as the binder skeleton,which makes up for the lack of long-range mechanical properties of short-chain structures of PTP.Besides,P-O^-in PTP can interact with polar groups like-OH/-COOH in Alg by ion-dipole forces,thus constructing a robust cross-linking network to provide rich active site-forming interactions with Si-OH.Most importantly,PTP with high dispersion in water allows polar groups in c-PTP-Alg binder to utilize fully,so the c-PTP-Alg binder with uniform texture can better disperse the stress generated by Si particles.In the selection of active material of electrode,we choose micron Si.Although micron Si is cheap,it can produce greater stress than nano Si to cause its own fragmentation during the charging and discharging.When the PTP and Alg binders with mass ratio of 3:2 to prepare c-PTP-Alg（3:2）binder for micron Si anode,the capacity retention of the corresponding electrode is 84.13%after 80 cycles.Finally,based on c-PTP-Alg（3:2）binder,we also constructed Si|S-PAN with a high energy density,which fully demonstrates the practical potential of this binder.