Synthesis And Application Of Novel Conductive Binders For High-Capacity Anodes In Li-ion Batteries

With lots of merits including high theoretical specific capacity,low platform and widespread availability,tin and silicon anodes have been considered as the most promising anode candidates for the next-generation LIB.However,these two anodes will suffer from huge volume change during the repeated charge/discharge process,which can lead to pulverization of the electrode and unstable solid-electrolyte interphase(SEI),eventually causing fast capacity fading and limitation of their commercial application.There are some main strategies implemented in the academic fields to improve the cycling performance of Sn and Si anodes,such as preparing nanosized mateials,formimg alloy with other metals and combining active materials with conductive carbon materials.But these mentioned methods will either cause high cost,or sacrifice specific capacities of electrodes to reduce volume change,which is exactly a compromise.It has been found that novel binders can enhance the electrochemical performance of high-capacity anodes.For instance,conventional watersoluble binders including caboxymethyl cellulose(CMC)?polyacrylate acid(PAA)and Polysaccharides have been investigated as binder for high-capaicty anode and show good performance.And the only thing that these binders have in common is lots of polar functional groups in the polymer,which is crucial for the cycling stability of anode materials.Furthermore,conductive binders can act as a conductive bridge,endowing the whole electrode with electrical integrity despite enormous volume change,thus achieving more stable cycling.In this pater,new types of water-soluble conductive polymers possessing the advantages of both traditional water-soluble binders and liposoluble conductive binders are developed and investigated as binders for high-capacity anode mateirals.The main research contents are presented as follows:(1)A novel water-soluble conductive binder PF-COONa consisting of an n-type polyfluorene backbones and massive side chains with polar carboxyl groups was designed and investigated as the silicon anode binder.Abundant polar groups in the polymer can enhance the adhesion ability of PF-COONa,while the conjugated backbone endows PF-COONa,with higher electronic conductivity under low potential conditions.In addition,carboxyl groups can form covalent bonds with hydroxyl groups in the surface of Si particles,keeping active materials in close contact with conductive matrix.The Si/PF-COONa electrode showed excellent electrochemical performance.(2)The effect of PF-COONa on the electrochemical performance of Sn anode was investigated.Although Sn anode will suffer from pulverization of active materials during cycling,the conductive binder forms an excellent conductive network,preventing Sn nanoparticles losing electronic contact.At the same time,the existence of polymer PF-COONa contributes to the formation of stable solid-electrolyte interface(SEI).Thus,Sn anodes using the conductive binder PF-COONa achieved good electrochemical performance.(3)10 mol% phenanthraquinone(PQ)groups were introduced into the backbone of PFCOONa and a new water-soluble conductive binder PFPQ-COONa was reported.Similar to PF-COONa,PFPQ-COONa is able to form covalent bonds with Si,which guarantees the tight connect between the conductive polymer network and Si particles,thus promoting the electron transfer between the interface.Meanwhile,PQ groups in the backbone can be electrochemically reduced at a strong reducible condition,bringing about significantly improved electric conductivity.With aforementioned merits as well as good mechanical properties,Si anodes based on PFPQ-COONa show better rate performance and cycling performance than that of Si/PF-COONa.