Preparation Of Triazolium-based Ionic Liquids And Their Applications In Zinc Ion Batteries

The electrolyte is an important component of batteries,which affects the safety,electrochemical stability window（ESW）,cycling performance,and charge/discharge performance of the batteries.At present,most of the research on electrolytes for zinc ion batteries（ZIBs）focuses on aqueous electrolytes,but there are still some problems,such as narrow ESW,zinc dendrite growth,and hydrogen/oxygen evolution reaction.Ionic liquids（ILs）usually possess wide ESWs and liquid temperature ranges,and the absence of water inhibits the dendrite growth fundamentally.Triazolium-based ILs display high thermal stability and wide ESWs,and could be good electrolyte materials for ZIBs.Therefore,in this paper,the research based on triazolium-based IL was carried out and mainly included the following aspects:Triazolium-based ILs（T1,T2 and T3）with or without terminates hydroxyl groups were prepared via Cu（Ⅰ）catalysed azide-alkyne click chemistry and their properties were investigated using various technologies.The hydroxyl groups affected their physicochemical properties obviously,where with a decrease in the number of hydroxyl groups,their conductivity,stability and ESW were enhanced.Among them,T1displayed the best performance with a thermal decomposition temperature of 301 ^oC and a conductivity of 8.80×10^-4 S cm^-1（30 ^oC）and an ESW of 4.76 V.T1S-20 was obtained via the addition of zinc trifluoromethanesulfonic acid（Zn（CF₃SO₃）₂）and lithium bis（trifluoromethanesulfonyl）imide（Li TFSI）to T1,and displayed conductivity and ESW values of 1.55×10^-3 S cm^-1 and 6.36 V at 30 ^oC,respectively.Subsequently,a Zn/Li₃V₂（PO₄）₃ battery was assembled using T1S-20 as electrolyte and its performances at 30 ^oC and 80 ^oC were investigated.The battery showed a capacity at 0.2 A g^-1 current density was 81 m A h g^-1 at 30 ^oC,and its capacity retention rate was 89%after 50 cycles.At 80 ^oC,the initial capacity of ZIB with T1S-20 system was 111 m A h g^-1,and the capacity retention after 100 cycles（93%）was much higher than that of the WS-20system（71%）.Simultaneously,the T1S-20 electrolyte-based battery exhibited a good charge/discharge efficiency,and its Coulomb efficiency was 99%.Consequently,the T1S-20 electrolyte displayed a better performance in the Zn/Li₃V₂（PO₄）₃ battery than that with the WS-20 system electrolyte.To explore the influence of substituents of ILs on their electrochemical properties,triazolium-based ILs T4 with methyl substituent and T5 with methoxypropyl substituent were synthesized,the influence of IL’s chain length on electrochemical performance was explored.The results showed that the conductivities,ESWs and thermal decomposition temperatures were all decreased gradually with the increase of chain length.T4 with the shortest chain displayed the best performance.As a comparison,imidazolium-based IL（IM）with the same substituent as T4 was prepared,and IMS-20 and T4S-20 electrolytes were applied to Zn/Li₃V₂（PO₄）₃ batteries.The capacity of the battery with T4S-20 electrolyte system at 0.2 A g^-1 current density was94 m A h g^-1 at 30 ^oC,and the capacity retention was 97%after 50 cycles,which was higher than that of IMS-20 under the same conditions(capacity of 80 m A h g^-1 and capacity retention of 92%).At 80 ^oC,the capacity of the T4S-20 system(125 m A h g^-1)was still higher than that of the IMS-20 system(118 m A h g^-1).At the same time,a good charge/discharge efficiency of 99%was achieved.In comparison with the battery capacity of T1S-20 and T5S-20 electrolytes,the capacity of ZIBs decreased as the chain length increase,and the T4S-20 electrolyte with the shortest chain exhibited the best electrochemical performance in Zn/Li₃V₂（PO₄）₃ battery.Also,the electrolyte was applied to Zn/Mn O₂ and Zn/V₂O₅ batteries,and they displayed high discharge specific capacity and charge/discharge efficiency of 97%,indicated the electrolyte was suitable for different types of ZIBs.The above investigations showed that the T4 possessed the best electrochemical performance,but its conductivity(2.38×10^-3 S cm^-1)could be further improved,so the electrolyte was prepared by mixing the T4 with an aqueous electrolyte formulated from Zn（CF₃SO₃）₂ and Li TFSI at the salt concentration ratio of c _Zn:c _Li=1:1 in different ratios,The conductivity of T4W-8 reached 2.70×10^-3 S cm^-1,and T4W-8 was applied as electrolyte in Zn/Li₃V₂（PO₄）₃ battery,which displayed high charge/discharge capacities of 98 and 131 m A h g^-1 at 30 ^oC and 80 ^oC,respectively,as well as good capacity retention（96%and 97%）and charge/discharge efficiency（both 99%）.It can be found that the combination of small amount of water in ILs could enhance the electrochemical performance of ZIBs.To verify the universality of electrolytes in ZIBs,Zn/Mn O₂ and Zn/V₂O₅ full batteries were assembled.The initial discharge capacities of Zn/Mn O₂ and Zn/V₂O₅ batteries were 145 and 273 m A h g^-1 respectively at 0.1 A g^-1 current density,demonstrating the universality of the T4W-8 electrolyte in ZIBs.In summary,based on the advantages of triazolium-based ILs,a series of ILs with different structures were synthesized and applied in ZIBs.The influences of structures on the electrochemical performances were investigated,which provided a reference for preparing electrolyte for ZIBs.