| The exploration of new battery technologies beyond lithium-ion batteries(LIBs)is important for the next generation of high-energy storage devices as the energy density of current LIBs approaches their limitation.Lithium-sulfur(Li-S)batteries are expected to be one of next generation energy storage systems because of the abundant reserves and high theoretical specific capacity of sulfur.In the past decade,Li-S batteries have achieved significant progress in academy and commerce but are poor in practical application.They still face many problems such as shuttle effect,fast capacity decay,poor electrical conductivity,and lithium dendrites.Sulfurized polyacrylonitrile(SPAN),as a sulfur-active material,has good cycling stability in ester electrolytes due to its special polymer structure.Its charging/discharging mechanism is different from conventional Li-S batteries-the "solid-solid" reaction mechanism.This reaction mechanism does not have this process of dissolving lithium polysulfide in the ester electrolyte,but there is still dissolution of lithium polysulfide and loss of active sulfur in the ether electrolyte.Although the shuttle effect can be avoided when the SPAN cathode are cycled in the ester electrolyte,the growth of lithium dendrites is unavoidable.The lithium dendrites can pierce the separator and lead to the short circuit of the battery.Therefore,some reasonable experimental investigations are conducted to address the above mentioned problems in both electrolyte systems.The details of this thesis are as follows.The inhibition effect of 4-aminobenzoic acid(4-ABA)as an ether electrolyte additive on the shuttle effect of polysulfide is investigated in the SPAN cathode.The capacity retention of the cell containing 1% 4-ABA electrolyte is 88.81% for 100 cycles,which is better than 65.56% of the cell containing blank electrolyte.The cells cycled in the electrolyte containing the additive have more stable discharge voltage and lower interfacial charge transfer impedance.The reconstituted cells also further demonstrate that 4-ABA has an positive effect on the sulfur cathode but not the lithium anode.The cathode electrolyte interphase(CEI)film formed in 1% 4-ABA electrolyte protects the SPAN cathode,inhibiting the dissolution of lithium polysulfide and reducing the loss of active material.Based on the short cycle life of the cell in ether electrolyte,the problems faced by the battery cycling in ester electrolyte will be the focus.Phenyl Methanesulfonate(ph MS)is applied to improve the electrochemical performance of Li/SPAN,inhibiting the formation of lithium dendrites and extending the cycle life of the cells.After a series of electrochemical performance and Li-Li symmetric cell tests,it is found that the electrolyte containing 1% ph MS enhances the cycling performance of Li/SPAN cells.The capacity retention of the cell is only 71.12% for 100 cycles in blank electrolyte compared to 90.17% in 1% ph MS electrolyte.However,the overpotential of Li-Li symmetric cell is still high and its cycle life is not extended at high current densities.Subsequent characterization tests show that ph MS is involved in the formation of SEI film which could protect the lithium anode to some extent.Tris(2,2,2-trifluoroethyl)borate(TTFEB)is selected as an ester electrolyte additive to participate in the formation of SEI film on the lithium anode side.The stronger SEI film can induce the uniform deposition of lithium ions and prevent side reactions between lithium metal and electrolyte.The experimental results show that the addition of TTFEB can indeed enhance the cycling performance of Li/SPAN cells.The Li-Li symmetric cell is further assembled and tested.The results show that this additive effectively reduces the overpotential of lithium-ion deposition and extends the cycle life of the Li-Li symmetric cell.The reasons for the improved electrochemical performance of Li/SPAN cells are analyzed via the relevant characterizations.The capacity retention of the cell is 58.09% in blank electrolyte and 74.81% in 1% TTFEB electrolyte after 400 cycles. |
