Molecular Design And Properties Research Of Multifunctional Network Binder For High-loading Lithium-sulfur Batteries

The current energy density of lithium-ion batteries is approaching its limit,so it is important to develop new battery technology for high energy storage.Lithium-sulfur(Li-S)batteries,consisting of lithium negative electrode,sulfur cathode,electrolyte and separator,have a theoretical energy density of 2600 Wh kg-1 and are considered to be the next generation of high specific energy storage systems that can replace conventional lithium-ion batteries.However,the dissolution and shuttling of polysulfides during the cycling process of Li-S batteries has led to the loss of active material and the large volume change of S particles during the cycling process,which has seriously affected the electrochemical performance of Li-S batteries.As an important component of Li-S batteries,the binder plays a crucial role in the sulfur cathode to stabilize the integrity of the electrode,enhance the mechanical properties of the electrode,and effectively prevent the diffusion of polysulfides.Compared with the conventional oil-based PVDF binder,we designed two aqueous 3D network binders to effectively enhance the electrochemical performance of high-capacity Li-S batteries.(1)We designed a strong affinity elastic network binder by coupling tannic acid(TA)and polyurethane(PU)for use in high-performance Li-S batteries.First,TA possesses abundant hydrogen bonds that can be closely linked with PU to form a binder network with excellent mechanical properties,so that the sulfur electrode will not break during cycling,thus ensuring the integrity of the electrode.Secondly,because TA has abundant phenolic hydroxyl groups,thus showing strong adsorption ability to polysulfides,which can effectively suppress shuttle effect.Therefore,the P1T1(Polyurethane: Tannic acid = 1:1)network binder formed by cross-linking also has strong adsorption ability to polysulfides,which can rapidly adsorb polysulfides in a short time.The cycling stability of Li-S battery with P1T1 binder was significantly improved,and the capacity was still maintained at 602.5 m Ah g-1 after 500 cycles at the current density of 0.5 C.When the S loading was increased to 3.63 mg cm-2,the LiS battery with P1T1 binder still showed good cycling performance.The discharge capacity can be maintained at 708.1 m Ah g-1 after 50 cycles at the current density of0.1 C.(2)In this study,potassium tripolyphosphate(PTP)inorganic oligomers was used to induce a stretchable network structure binder for Li-S batteries.The tamarind polysaccharide gum(TSG)is rich in hydroxyl groups,and PTP is an inorganic oligomer and contains a large number of strongly electronegative P-O-groups.TSG films also have excellent tensile properties,which can effectively prevent the volume expansion of sulfur active material during the charging and discharging processes of Li-S batteries.Meanwhile,the large amount of-OH in TSG and P-O-bonds in PTP can effectively adsorb polysulfide and thus suppress the shuttle effect,which enables the Li-S battery with this binder to exhibit good cycling performance.When the S loading reaches 4.29 mg cm-2,the area specific capacity reaches 3.37 m Ah cm-2 after 70 cycles.In summary,two bifunctional 3D network binders were designed in this paper,which not only significantly stabilized the mechanical structure of the high loading sulfur cathode,but also effectively anchored the lithium polysulfide.This paper provides a new idea for the design of binders for high loading Li-S batteries.