| Developing renewable energy is an important strategy to improve energy structure and solve environmental problems.High-energy-density and stable energy storage systems are crucial for extensive deployment of renewable energy due to their intermittent and unstable natures,such as solar and wind systems.Lithium-sulfur flow battery is a new type of flow battery system combining lithium-sulfur battery and flow battery,its anode is lithium,and its cathode is sulfur or sulfur composite suspension(catholyte).Lithium-sulfur flow battery has the advantages of high energy density,low cost,wide working range and non-toxic etc.,and is a very promising energy storage technology.However,the main problems in lithium-sulfur battery,such as poor conductivity of sulfur electrode and shuttle effect of lithium polysulfides(LPS),still exist in lithium-sulfur flow battery,especially shuttle effect is more serious in the flow system as more electrolyte must be used to form flow configuration.Meanwhile,the sulfur suspension catholyte also has the problems of poor stability and high viscosity.In this dissertation,the stability of the suspension catholyte was improved by the structure optimization design of catholyte materials;the shuttle effect was modulated by ionic liquid polysulfieds bridgebuilder,and the related mechanism was analyzed;high-energy-density and low-temperature suspension catholyte was achieved by the functional processing of the catholyte materials;the effects of flow mode and flow velocity on the charge/discharge performance of the suspension electrode were studied.Major innovative results are summarized as follows:(1)High-energy-density sulfur suspension catholyte was designed and prepared.The Sulfur-Ketjenblack(S-KB)composite was prepared in situ by reduction deposition to improve the conductivity of sulfur.Compared with the sulfur suspension catholyte prepared by mechanical mixing sulfur and KB,the S-KB catholyte showed higher discharge capacity and better rate discharge performance.The viscosity was reduced by introducing Triton X-100 into the S-KB suspension catholyte,meanwhile,Triton X-100 inhibited the loss of active substances and improved the cycle stability of suspension catholyte.(2)A self-stabilized suspension catholyte was designed and prepared by regulating the microstructure of sulfur composite.The special sandwich-structure Sulfur-Ketjenblack@reduced Graphene Oxide(S-KB-rGO)composite was prepared in situ by controlling Zeta potential.When S-KB-rGO was dispersed in the electrolyte,the hyperbranched KB and the interconnected rGO sheets formed a three-dimensional conductive and load network,the in-situ deposition of sulfur in the network had good suspensibility,flowability and electrochemical stability.No deposition was found in the S-KB@rGO suspension after 30-days rest,the specific capacity of the catholyte was 1532 mAh g-1,upon 1000 cycle was achieved,and the self-discharge rate was 1.1%per day.The S-KB@rGO catholyte was operated in the flow battery equipmennt and showed high electrochemical activity and stability at flow mode.Meanwhile,the viscosity of S-KB@rGO suspension catholyte reduced with increasing the flow velocity,which facilitated ion transfer,therefore,the discharge voltage rose with the increase of flow rate.The design concept of self-stabilized catholyte provides references for other suspension electrodes.(3)The shuttle effect was modulated and cycle performance of the suspension catholyte was effectively improved by ionic liquid polysulfides bridgebuilder.The dissolution of LPS is a double-edged sword,on one hand,it is the root cause of shuttle effect,which leads to the failure of flow cell;on the other hand,it improves electrochemical reactivity of the catholyte.Therefore,the kernel is controlling LPS migration in cathode zone to balance cycle performance and kinetic performance.An ionic liquid nanoparticle,SiO2 tethered 1-methyl-1-propylpiperidinium chloride(SiO2-PPCl),was adopted to control shuttle effect,it formed chemical bonding with carbon carrier materials and polysulfides by its methoxyl and amino groups,which made Si02-PPCl act as a bridge between carbon carrier materials and polysulfides,the polysulfides dissolution and consequent shuttle effect were modulated,and the kinetic reaction of catholyte was improved.The SiO2-PPCl functionalized catholyte showed over 1000 cycels with a capacity retention ratio of 95.4%,and the coulombic efficiency was about 99%.The approach of exploiting polysulfides bridgebuilder to control LPS shuttle also offers a new direction to develop lithium-sulfur batteries.(4)A high-energy-density,low-temperature sulfur suspension catholyte was designed and prepared.The S-KB-G@P suspension catholyte was prepared by surface functionalization treatment of PVP on sulfur composite.The energy density and peak power density of the S-KB-G@P suspension catholyte was 445 Wh L-1,22.5 mW cm-2 respectively at-30?,and the cycle was stable(200 cycles without obviously dacay).The excellent low-temperature performance of the S-KB-G@P catholyte stems from the fact that anchored PVP on the surface weakens agglomeration between materials,and thus reduces the viscosity of the suspension,which is beneficial for ion transfer;meanwhile,PVP enhances the dispersion of carbon carrier materials in the suspension,thus a continuous conductive network is formed by rGO and KB,therefore,S-KB-G@P catholyte has high ion and electron conductivity at low temperature.Amphipathic PVP formed chemical bonding with carbon carrier and polysulfides,which inhibited the migration of polysulfides and improved the cyclic stability. |
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