Preparation And Performance Of Aqueous Soy Protein-based Binder And Current Collector For Cathode Of Lithium-Sulfur Batteries

The advantages of high specific capacity,high energy density,abundant source of raw materials and low manufacturing cost make lithium-sulfur battery become one of the research highlights of the next generation of high energy density energy storage equipment.However,there are three major problems in lithium-sulfur battery:low conductivity of sulfur;shuttle effect of polysulfide dissolved in electrolyte,which increases the viscosity of electrolyte,decreases the transport rate of lithium ion;and huge volume change of active material in the process of battery operation,which makes the sulfur cathode structure vulnerable to damage,and eventually lead to a sharp decline in specific capacity and even an early termination of battery life.These problems will be further magnified in the batteries with high load of active materials,and ultimately limit the rapid development of high energy density lithium-sulfur batteries.In order to prepare high-sulfur-loading lithium-sulfur battery with excellent performance,this paper proposed useful solutions from two aspects of binder and collector in cathode,and explored the principle of improving the electrochemical performance of the battery.In the first work,catechol group and phosphate group were grafted onto the molecular chain of SPI through two steps of amidation and phosphorylation.The corresponding molecules of products were crosslinked under the hydrogen bond andπ-πinteraction between hydroxyl group,amino group,carboxyl group and catechol group on the molecular chain to form a soybean protein isolate based three-dimensional network multifunctional binder named as SHP.It has been used in the traditional sulfur/carbon composite cathode.The introduction of catechol group can significantly improve the adhesive strength of SHP in the polar environment of electrolyte,and improve the internal crosslinking degree of SHP.The introduction of phosphate group significantly improved the lithium polysulfide adsorption capacity of SPI,and appropriately reduced the degree of cross-linking in the binder,which made the binder have better reversible deformation ability and help to maintain the integrity of the electrode structure.Moreover,the stronger adsorption capacity of lithium polysulfide can also inhibit the increase of viscosity of the electrolyte,ensure the unobstructed lithium ion transmission channel,and finally improve the discharge capacity,rate performance and cycle stability of the battery.With a sulfur-loading of 1.3 mg cm^-2,the specific capacity of lithium-sulfur battery with SHP binder can reach 899.6 m Ah g^-1in the first cycle at the rate of1 C.After cycling for 400 times,the specific capacity can still maintained at 561.5 m Ah g^-1,and the capacity decay rate of each cycle is as low as 0.094%.When the sulfur-loading increased to 6.74 mg cm^-2,the specific capacity of lithium-sulfur battery with SHP binder can reach 672.9 m Ah g^-1in the first cycle at the rate of 0.1 C.After cycling for 200 times,the specific capacity can still maintain at 405.8 m Ah g^-1,and the coulombic efficiency is up to95.3%.These results indicate that SHP is a multifunctional bio-based three-dimensional network binder with excellent comprehensive performance,so it has a very broad application prospect in high-loading lithium-sulfur battery.In the second work,the method of heterogeneous growth and post doping carbonization is used to introduce the graphene oxide（GO）into the carbonized melamine foam.There are tungsten carbide（WC）particles with similar structure of platinum and cobalt（Co）monatomic particles loaded on the flake-like GO.Finally,a tungsten carbide/cobalt@graphene oxide@nitrogen doped carbon foam named as WCoGN is obtained.It is also used as a self-supporting current collector in the cathode of lithium-sulfur battery without binder and conductive agent.WC and Coparticles have excellent catalysis and electrical conductivity,which can promote the solid-to-solid transition of Li₂S₂to Li₂S and improve the discharge capacity of the battery.Moreover,the introduction of these two kinds of particles can accelerate the mutual transformation between chalcogenide species,improve the reversibility of electrochemical reaction,and improve the rate performance of the battery.The introduction of catalyst particles also increased the content of carbon atoms with a structure of sp²in WCoGN,increased its conductivity,and accelerated the electron transfer rate during the charge-discharge process.With a sulfur-loading of 3.5 mg cm^-2,the discharge capacity of battery with WCoGN in the first cycle is 472.6 m Ah g^-1at 1 C.After cycling for 500 times,the specific capacity can still maintained at 421.8 m Ah g^-1,and the capacity decay rate per cycle is as low as 0.021%.When the sulfur-loading increased to 7 mg cm^-2,the discharge capacity of battery with WCoGN is 303.3 m Ah g^-1in the first cycle at 0.1 C.After cycling for80 times,the specific capacity is 316.1 m Ah g^-1and the coulombic efficiency is 91.5%.The results show that the light carbon foam current collector can significantly increase the energy density of the battery under the same active material loading.Therefore,the WCoGN carbon foam self-supporting current collector has great potential to play a greater role in the electrolyte deficient and high-loading lithium-sulfur battery.