Highly Stable Suspension Electrodes For Lithium Ion Slurry Flow Batteries

Flow batteries have been studied for grid-scale energy storage applications because their capacity and power output can be controlled independently.Lithium ion slurry flow cells(LSFCs)are expected to break through the restriction of active material solubility in electrolyte,and provide higher energy density to flow systems.Herein,two typical electrode materials,Li4Ti5O12(LTO)anode and LiFePO4(LFP)cathode are selected to prepare slurry electrodes.The LTO flow batteries are particularly attractive because of its ease of availability,dimensional stability,as well as high security,ultra-long cycle life and environmental friendship.LFP is considered as the best favorable cathode compound for the batteries used in electric vehicles(EVs)and hybrid electric vehicles(HEVs),due to its adequate flat voltage plateau(3.4 V versus Li/Li+),inherent thermal security,environmentally friendship,and availability of numerous iron resources in natural surroundings.This dissertation focuses on the research and development of highly stable suspension,which contain continuous conductive network to deliver high efficiency and long cyclic performance.The feasibility of the suspension making design verified using LTO(anolyte)and LFP(catholyte)by assembling and testing coin-cells work in static mode.Typical progresses are presented as the following:(1)The fabrication of high performance of lithium ion suspension electrodes,which are usually comprised of electrolyte,active material and other additives,is an effective way to enhance the energy density of flow batteries for its relatively high active material loading per unit of volume.However,stable suspension electrodes are difficult to be realized mainly for two reasons,the high density of common electrode materials and the poor conductive networks in the suspension.Firstly,a stable Li4Ti5O12(LTO)suspension anolyte was successfully prepared with the aid of polyethylene oxide(PEO)and carbon nanotubes(CNTs),in which PEO stables the anolyte by intramolecular repulsion force,and CNT forms an integrated conductive network.This anolyte delivered a high reversible capacity of over 140 mAh/g under 0.5C rate,and it maintained more than 80%of its initial capacity after 200 cycles.This strategy is hopefully suitable for the design of other suspension electrodes,such as graphite and LiFePO4,which will shine a light on the development of high energy density flow battery.(2)A stable LiFePO4(LFP)suspension catholyte was successfully prepared with the presence of polyethylene oxide(PEO)and ketjen black(KB),in which PEO would stable the anolyte by intramolecular repulsion force,and KB supplied an integrated conductive network.This highly stable LFP-catholyte,containing a robust conductive network,could deliver an electrochemical capacity over 155mAh/g at 0.5C and 80%of its initial capacity was kept after 200 cycles.(3)Furthermore,lithium ion slurry flow cells employing triethylene glycol dimethyl ether(Triglyme/G3)and tetraethylene glycol dimethyl ether(Tetraglyme/G4)electrolyte solutions with LiFePO4 catholyte were studied.G3 and G4 could form solvated ionic liquid(SIL)with equal molar Lithium bis(trifluoromethanesulfonyl)imide(LiTFSI).Considering SIL is a very promising novel electrolyte system,therefore,we also prepared suspension electrode with it.A highly stable LiFePO4(LFP)suspension catholyte was successfully prepared with the aid of CNTs and KB;their combined effect favored the formation of an integrated conductive network.This catholyte delivered a reversible capacity of over 155 mAh/g under 0.5 C rate,and 74.19%of its initial capacity was kept after 200 cycles.In summary,the physical and electrochemical stability of suspension electrodes could be modulated by introducing conductive agents,dispersant and new electrolyte.Our findings will be helpful for the fabrication of high performance lithium ion slurry flow batteries.