Electrochemical Research Of Functional Organic Molecules In Lithium Selenium Batteries

Lithium-ion?Li-ion?batteries have been widely used in cell phones and laptops,however,the cathodes have limited capacities which limit the energy densities of Li-ion batteries.In order to develop high-energy-density batteries,high-specific-capacity cathode materials are needed.Elemental selenium is a potential high-capacity cathode material.However,lithium polyselenides are formed upon cycling in batteries,which are soluble in electrolyte resulting in shuttle effect and instability of cycling.Organosulfide molecules as cathode materials can provide high capacities and energy densities,and the functional groups can change the redox reaction pathways.Based on the advantages of elemental selenium and organosulfide,they are combined to alter the redox reaction pathways of selenium in order to overcome the intrinsic issues of elemental selenium.First,we design elemental selenium-diphenyl trisulfide as an inorganic-organic hybrid cathode for rechargeable lithium batteries.Highly conductive selenium nanowires and carbon nanotubes interwined as the electrode substrate can physically trap and adsorb cycled products,therefore the battery performance can be improved.The phenyl groups in diphenyl trisulfide are electron-withdrawing,and the S-S bond breakage/reformation upon cycling can cause structural changes.Diphenyl trisulfide is dissolved in electrolyte,and then added into the Se nanowire/carbon nanotube composite electrode.Upon cycling in the battery,some selenium atoms are transferred into the molecular structures of diphenyl trisulfide and disulfide,realizing atomic fixation of selenium.Therefore,the redox reaction pathway is changed and the battery cycling stability is improved significantly.Next,we use diphenyl diselenide as an electrolyte additive for Li-Se batteries.Each diphenyl diselenide molecule can store two lithium ions upon the breakage of Se-Se bond,possessing the thoeretical capacity of 172 m Ah/g.In the oxidation process,Se-Se bonds could be directly formed by the phenyl selenium radical?PhSe.?and selenium radical?Se.?derived from the delithiation of lithium phenyl selenide and lithium selenide,respectively,avoiding the oxidation of lithium selenide?Li₂Se?to form high-order lithium polyselenides?Li₂Se_x,x>3?.The formed diphenyl triselenide can adhere to the walls of carbon nanotubes.By changing the electrochemical pathway and forming insoluble charged product,the reversibility of battery cycling is improved.Li-Se batteries still show long stable cycle performance under high current rate.Therefore,the application of this kind of organic small molecules can improve the capacity retention rate of the battery,providing an effective method for the next generation high-energy-density batteries.