Preparation Of Soybean Protein Based Aqueous-processable Binders And Their Applications In Lithium Sulfur Batteries

Rechargeable Li-S batteries have drawn much attention because of their high theoretical energy density(2567 Wh kg-1),low cost and environmental friendliness.However,the"shuttle effect" originating from the dissolution and migration of lithium polysulfide in electrolytes limits the cycling stability of Li-S batteries.In addition,the large volume changes(-80%)during the charge-discharge process will destroy the structural stability of electrode,which greatly shorts the lifetime of Li-S batteries.Binder is an essential component in the typical Li-S batteries.It plays a critical role in maintaining the structural integrity of electrodes and ensuring the effective connection between active materials and conductive agents.As the commonly used commercial binder for cathode,poly(vinylidene fluoride)(PVDF)exists several problems for directly applied in sulfur cathode.First,PVDF needs to be dissolved in the toxic organic solvent N-methyl pyrrolidone(NMP).Second,PVDF will swell in the organic electrolyte.In addition,PVDF has a weak affinity for active sulfur and its discharge products.To address above problems,we use soybean protein as the main raw materials to develop a series of water-based multifunctional polymer binders for sulfur cathode.The developed binders in this work play an important role in inhibiting the dissolution and shuttle of polysulfide,maintaining the structure stability of sulfur electrode and accelerating electron/ion conduction.The main contents are as follows:1.The water-based phytic acid-crosslinked supramolecular binder is developed to prolong charge-discharge cycling of Li-S batteries.The supramolecular binder is fabricated with a mixture of phosphorylated soybean protein isolate(P-SPI),poly(ethylene oxide)(PEO)and phytic acid(PA),which is denoted by SPP.The strong adsorption of polysulfides by the SPP binder is verified by using UV-vis spectroscopy with an in situ battery.The high discharge capacity of 932.8 mAh g-1 is achieved by SPP-based Li-S batteries at a high sulfur loading of 8.9 mg cm-2 at 0.1C.The discharge capacity at 1C is 629.7 mAh g-1,with stable cycling over 800 charge-discharge cycles and the capacity attenuation is only 0.0298%per cycle.The coulombic efficiency remains at 99.7%.This novel water-based supramolecular polymer binder shows great potential in high-energy-density Li-S batteries.2.We fabricate a self-healing,water-based,and double cross-linked soy protein isolate-polyacrylamide(SPI-PAM)binder for the sulfur cathode,which is facilely synthesized by copolymerization of methacrylated SPI and acrylamide.It is demonstrated that methacrylated SPI can act as a macro-cross-linker,combining with dynamic hydrogen bonding cross-linking from PAM,endowing the SPI-PAM polymer binder satisfactory bonding strength and excellent self-healing ability.Moreover,the SPI-PAM exhibits superior lithium polysulfide anchoring capability to impede the dissolution and diffusion of lithium polysulfides in the electrolyte.Li-S batteries with such a robust SPI-PAM binder can stabilize the charge and discharge for 400 cycles at a high rate of 6C;the average specific capacity loss per cycle is only 0.0545%,and even at an ultrahigh current density of 20C,the specific capacity still remains at 148.2 mAh g-1.With a sulfur loading of 2.3 mg cm-2,the SPI-PAM-based sulfur cathodes exhibits remarkable cycling performances at 0.5C for 200 cycles,and the capacity remains at 707.7 mAh g-1.This green binder derived from bioresources modified with PAM shows great potential for application in high-energy density Li-S batteries.3.We fabricate a biopolymer binder named PPG with a three-dimensional(3D)cross-linked structure for Lithium-sulfur batteries(LSBs),composed of guar gum(GG),phytic acid(PA)and soy protein isolate(SPI).The strong lithium polysulfide adsorption ability of the PPG binder has been demonstrated through the ultraviolet-visible(UV-vis)measurement and further been certified by calculation with the density functional theory(DFT).Its outstanding mechanical strength and adhesion performance is beneficial to sustain the integrality of sulfur electrodes during cycling.PPG binder-based LSBs exhibit significant promotion in prolonged cycling tests with a 79.7%capacity retention after 700 cycles at 1C.The capacity of PPG binder-based LSBs can still maintain 933.6 mAh g-1 at 0.1C after 50 cycles even with a high sulfur mass loading of 4.9 mg cm-2.This result demonstrates the significance of a natural polymer in the design of a binder for a sulfur cathode to achieve LSBs with splendid cycling performance.4.We fabricate a novel dual-network conductive(DNC)binder.This as-developed binder consists of a conductive network(composed of poly(3,4-ethylenedioxythiophene)(PEDOT)and poly(styrenesulfonate)(PPs)),and a crosslinking absorbing network composed of phosphorylated soy protein isolate(P-SPI),polyethylene ox-ide(PEO),and phytic acid(PA),demonstrating an excellent conductivity and polysulfide adsorption capability.The role of different binder components in Li-S battery are also investigated via detailed properties analysis and molecular dynamics simulations,and the optimum ratio of the two networks is obtained(SPP:PPs=9:1,named S9P1 binder).Remarkably,the S9P1 binder-based Li-S batteries with a sulfur loading of～1.7 mg cm-2 possess an excellent long-term stability,which has a high retention capacity of 751.9 mAh g-1 after 400 cycles at 0.5C with a coulombic efficiency of 99.57%.A good rate performance of 692.4 mAh g-1 at 4C under a sulfur loading of～1.2 mg cm-2 is also achieved.5.We have developed a three-dimensional cross-linked polyacrylic acid-vinylphosphonic acid(AA-VPA)/phosphorylated soy protein(P-SPI)multifunctional binder(named:APL binder).The three-dimensional network structure endows the binder a strong bonding force and good mechanical properties,which can maintain the structure integrity of the electrode during long-term cycling.The abundant polar groups in the APL polymer endow the sulfur cathode with a good polysulfide anchoring capacity and retard the shuttle effect of the lithium-sulfur battery.The ultraviolet-visible absorption spectroscopy,molecular dynamics simulation and density functional theory calculations(DFT)have been employed to verify the strong polysulfide anchoring ability of APL binders.Therefore,the APL binder based sulfur cathode has a high specific capacity of 940 mAh g-1 when the sulfur load is 4.8 mg cm-2.After 250 cycles at a current density of 0.5C,its specific capacity is still as high as 805 mAh g-1.In addition,we also verify that the APL-based water-based sulfur-carbon composite slurry has a good affinity with the carbon cloth through finite element simulation,DFT simulation and the SEM images of the electrode.The slurry can penetrate into the carbon cloth and firmly adhere to the surface of carbon fiber.The flexible electrode remains intact after bending.This work offers a viable approach for the application of multifunctional binders in flexible lithium-sulfur batteries.