Design And Application Of Single-ion Quasi-solid-state Electrolytes Based On Covalent Organic Frameworks

The invention of Li batteries has changed our lives in drastic ways.As a new generation of batteries,Li batteries have been widely utilized in various electronics devices and electric vehicles,due to the advantages such as light weight,high open-circuit voltage,no memory effect and environmentally friendly.With the progress of society,the requirements of batteries become increasingly high,for instance,higher energy density,better safety,longer life and more economical.Nowadays,most Li batteries contain dual-ion liquid electrolytes.This kind of electrolyte is not only of high mass,but also has potential safty hazards,due to its toxic,easy to leak,flammable and explosive disadvantages.Moreover,dual-ion liquid electrolytes inevitably cause anion concentration gradient during the charing and discharging process,leading to the generation of lithium dendrite and dead lithium,which reduce safety and battery life.In terms of current issues,single-ion all-solid-state/quasi-solid-state electrolytes is becoming an area that gains increasing concern.These novel electrolytes eliminate the safety issues caused by electrolyte leakage,and prevent the generation of lithium dendrite and dead lithium,leading to higher safety and longer battery life.Single-ion all-solid-state/quasi-solid-state electrolytes require lithium-ion transport channels which are long-range and highly ordered.To fulfill this requirement,we chose covalent organic frameworks(COFs)as the backbone of the novel single-ion quasi-solid-state electrolyte,due to its porosity and high regularity,which are beneficial to building the lithium-ion transport channels.Meanwhile,single-ion all-solid-state/quasi-solid-state electrolytes also require good lithium dissociation ability.In response to this issue,we synthesized a series of lithium storage molecules with strong electron-withdrawing groups or large-scale delocalized?bonds.On this basis,we fixed the lithium storage molecules to the COFs with post-synthetic modification,and prepared a series of single-ion quasi-solid-state electrolytes with good electrochemical performance.The contents of this paper are as follows:1.Inspired by the structures of bistrifluorosulfonimide(TFSI)and bisphenylsulfonimide(SPSI),we synthesized four kinds of lithium storage molecules with excellent lithium-ion dissociation ability and suitable groups for post-synthetic modification on COFs.They were lithium((trifluoromethyl)sulfonyl)((4-vinylphenyl)sulfonyl)amide,lithium(phenylsulfonyl)((4-vinylphenyl)sulfonyl)amide,lithium((4-(2-((2-aminoethyl)thio)ethyl)phenyl)sulfonyl)((trifluoromethyl)sulfonyl)-amide(N-S-TFSILi)and lithium((4-(2-((2-aminoethyl)thio)ethyl)phenyl)sulfonyl)-(phenylsulfonyl)amide(N-S-SPSILi).All four molecules were characterized by ¹H NMR,¹³C NMR and FT-IR,which provided a basis for the subsequent synthesis work.2.We synthesized COF-V,which was an imine linkage COFs with vinyl side chain.Considering the large distance of interlayer is not suitable for lithium-ion transportation,while the flexible side chain can compensate for this short coming.We chose to fix 4-bz-TFSILi and 4-bz-SPSILi to COF-V with 1,2-ethanedithiol as the linker.With this method,we prepared two novel single-ion quasi-solid-state electrolytes,COF-S-S-TFSILi and COF-S-S-SPSILi.The ionic conductivity of COF-S-S-TFSILi reached4.3×10^-4 S cm^-1 at room temperature with a lithium transference number of 0.87,while that of COF-S-S-SPSILi reached 2.6×10^-4 S cm^-1 at room temperature with a lithium transference number of 0.86.3.We synthesized COF-5,which was a boronate-ester linkage COFs.Unlike the electrolytes based on COF-V,we inserted the lithium storage molecules into the interlayer of COF-5 with coordination between N atom and B atom to shorten the distance of lithium-ion binding sites.N-S-TFSILi and N-S-SPSILi can be fixed to COF-5 backbone with the coordination between N atom and B atom.With this method,we prepared two novel single-ion quasi-solid-state electrolytes,COF-5-N-S-TFSILi and COF-5-N-S-SPSILi.The ionic conductivity of COF-5-N-S-TFSILi reached 6.3×10^-4 S cm^-1 at room temperature with a lithium transference number of 0.85,while that of COF-5-N-S-SPSILi reached 5.3×10^-4 S cm^-1 at room temperature with a lithium transference number of 0.78.