Aqueous-processable Polymer Binders For Cathode Of Lithium Sulfur Batteries

Lithium-sulfur batteries?LSBs?,based on sulfur active materials,have been treated as an ideal candidate for next-generation advanced energy storage system because of its exceptional theoretical capacity,and the wild availability and cheapness of sulfur.However,the shuttle effect of lithium polysulfides?LiPS?during charge-discharge process results in the loss of active materials and the reduction of capacity,even the failure of battery.Meanwhile,the low conductivity of active materials and drastic volume change during charge-discharge process also lead to the poor cycling performance of LSBs.The sulfur cathode is mainly composed of active materials?usually sulfur powder?,conductive addictive and binder.As the important composition of sulfur cathode,binder plays a significant role in alleviating the shuttle of LiPS and maintaining the integrity of cathode.Nevertheless,the traditional binder for LSBs,polyvinylidene fluoride?PVDF?,is insufficient in the confinement of LiPS and the preservation of structure in the sulfur cathode.Besides,PVDF also suffers from the use of hazard solvent?N-methyl-2-pyrrolidone?during the preparation of slurry.For the further development of LSBs,novel aqueous-processable polymer binders are urgently needed to improve the energy density and long-term cycling performance of LSB,and replace PVDF to be next generation high-performance binders of LSB.To address this issue,this dissertation introduced a series of aqueous-processble polymer binders with different chemical structuress to improve the electrochemical stability of sulfur cathode.The binders demonstrate the strong affinity to LiPS and could effectively alleviate the shuttle effect in the cathode as convinced by ultraviolet-visible spectroscopy and density functional theory calculations.1.To prepare catechol conjugated chitosan sulfate?CCS?,the water-insoluble chitosan was firstly sulfated to form chitosan sulfate and then conjugated with hydrocaffeic acid.This novel aquoues-processable polymer binder can enhance the cycling stability and rate performance of sulfur cathode.The CCS based sulfur cathodes demonstrate high capacity retention of 80.14%after 400 cycles at 0.5 C?0.0496%fading per cycle?,and retention of79.52%and 77.30%after 300 cycles at 1 C and 2 C,respectively.The specific capacity of the CCS-based cathode is 402 mAh g^-1 at a high rate of 4 C,which is much higher than the chitosan,PVDF based sulfur cathodes.2.The holey reduced graphene oxide and chitosan sulfate are applied to form a novel composite conductive binder,CS-HrGO.The CS-HrGO can enhance the conductivity of the binder and improve the rate performance of sulfur cathode.The CS-HrGO based sulfur cathode demonstrates high initial capacity of 849 mAh g^-1 at a rate of 0.5 C.After 200 cycles,the cathode still maintains about 100 mAh g^-1 higher than the CS based cathode.At a high rate of 4 C,the specific capacity of the CS-HrGO based sulfur cathode is 542 mAh g^-1,which is much high than that of the PVDF based sulfur cathode.3.A robust 3D crosslinked polymer network was prepared by aldimine condensation and coordination reaction,which demonstrates excellent mechanical property and strong affinity to LiPS.The crosslinked chitosan sulfate network?CCSN?highly enhances the cycling stability and rate performance of sulfur cathode.The CCSN based cathode exhibits high initial capacity of 824 mAh g^-1 at a rate of 1 C,and the capacity remains at 485 mAh g^-1 after500 cycles.At a high rate of 4 C,the cathode exhibits high capacity retention of 84.8%after300 cycles.When the sulfur loading was increased to 2.5 mg cm^-2,the CCSN based cathode performs high initial capacity of 806 mAh at 0.5 C,and 473 mAh g^-1 with high Coulombic efficiency of 96.8%after 200 cycles,indicating the significance of CCSN in preserving the high capacity and stable cycling performance of the sulfur cathode.4.A novel aqueous-processable polymer,chitosan sulfate ethylamide glycinamide?CSEG?,is synthesized by the conjugation of acryloyl glycinamide and chitosan sulfate.The dual-amide structure of the CSEG highly enhances its affinity to polysulfide species and empowers the CSEG based electrodes to realize stable cycling performances at 1 C for 700cycles and 6 C for 450 cycles with only 0.049%,0.0895%capacity fading per cycle,respectively.The CSEG-based cathode demonstrates superior rate performance with a specific capacity of 194.4 mAh g^-1 at an ultrahigh rate of 20 C.When the sulfur loading is increased to6.8 mg cm^-2,the CSEG-based cathode still shows a high capacity of 548 mAh g^-1(areal capacity of 3.72 mAh cm^-2)at a rate of 0.2 C.The superior electrochemical performance of the sulfur cathode with the CSEG binder has demonstrated the great advantages of this novel aqueous-processable binder.This work provided new insights into the development of eco-friendly high-performance LSBs.5.A novel hydrophilic dual-crosslinked polymer network,polyphosphate acid crosslinked chitosan ethylamide carbamide?PACEC?,is synthesized by the in situ crosslink reaction of polyphosphate acid and chitosan ethylamide carbamide.The sulfur cathodes based on this novel hydrophilic binder exhibit excellent flexibility and electrochemical performance.When the sulfur loading is 3.5 mg cm^-2,the sulfur cathode demonstrates 0.055%capacity fading per cycle within 800 cycles at a rate of 0.5 C.When the sulfur loading is increased to 6mg cm^-2,the cathode demonstrates high areal capacity of 4.8 mAh cm^-22 and capacity retention of 83.4%after 250 cycles at 2 mA cm^-2.The energy density of this cathode outperforms the commercially available Lithium ion battery(⁴ mg cm^-2).When the sulfur loading is increased to 13.1 mg cm^-2,the cathode performs high areal capacity of 15.6 mAh cm^-2.These results demonstrate the significance of PACEC in the development of high energy density LSB,and pave new ways to realize the practical application of the flexible LSBs.