Performance Study Of Tin Anode Operated In "Water-in-salt" Electrolytes

In the research field of secondary batteries,metallic tin?Sn?is regarded as a kind of promising anode material for lithium ion batteries.However,organic electrolytes were adopted in most of literatures enabling tin as anode material while there was hardly any literatures related to the applications of tin anode in aqueous electrolytes,which means that it's very innovative to investigate the electrochemical behaviors of tin anode in aqueous electrolytes.The non-flammability,low toxicity,high operability of aqueous electrolytes compared to those of organic electrolytes even make this investigation have practical significance.What's more,it's worthy to explore the possibility of tin anode applied to aqueous electrolytes.Firstly,the redox reaction of tin anode in aqueous solutions exhibits relatively high theoretical capacity?451.54mAh/g,two-electron transfer?.In addition,tin has a relatively mild standard electrode potential(-0.1375 V vs.SHE,Sn²⁺+2e^-=Sn),with high overpotential of hydrogen evolution and environmental friendliness.In recent years,a new type of aqueous electrolytes has appealed to the research field of secondary batteries---named“water-in-salt”.This kind of highly concentrated electrolytes has some advantages compared to conventional aqueous electrolytes.A research group had enabled the high reversibility of zinc anode by virtue of a“water-in-salt”electrolyte and achieved great success,which inspired us to attempt to use“water-in-salt”electrolytes to enhance the electrochemical performances of tin anode,similarly.In this thesis,two original“water-in-salt”electrolytes were utilized to improve the electrochemical performances of tin anode in aqueous solutions.A series of electrochemical measurements and characterizations were adopted to evaluate the enhancements.?1?Utilizing the“water-in-salt”electrolyte of“1 m Sn?OTf?₂+20 m LiTFSI”to improve the performances of tin anode.In this work,we had successfully prepared“1 m Sn?OTf?₂+20 m LiTFSI”electrolyte and use it to enhance the performances of tin anode in aqueous electrolytes while an electrolyte with 0.25 M Sn?OTf?₂ was also prepared for comparision.In the Sn/Sn symmetrical cell tests,the“water-in-salt”electrolyte showed extremely strong suppression to tin dendrite formation compared to that of 0.25 M Sn?OTf?₂ electrolyte.In the experiments of Sn²⁺plating/stripping on Pt disk electrode,the coulombic efficiencies?CEs?of Sn²⁺plating/stripping reactions in“water-in-salt”electrolyte were obviously higher than those in 0.25 M Sn?OTf?₂ electrolyte,implying that the side reaction of hydrogen evolution was significantly inhibited in“water-in-salt”electrolyte.Finally,we selected LiFePO₄ as cathode material and assembled the Sn-LiFePO₄ full cells.In the galvanostatic tests,the discharge voltage of the full cells was 0.6 V and.In cyclability test,the full cells could cycle steadily for about 40cycles at 0.2 C rate with a CE over 95%and a discharge capacity over 107 mAh/g,which was a high utilization of the theoretical capacity of tin anode?over 23%?.Even at a high rate of 2.0 C,the cells could cycle for 30 cycles,demonstrating the feasibility of tin anode for aqueous secondary batteries.?2?Utilizing the“water-in-salt”electrolyte of“Sn?OTf?₂+NaClO₄”to improve the performances of tin anode.In last work,though we had successfully utilize“1 m Sn?OTf?₂+20 m LiTFSI”electrolyte to enhance the performances of tin anode,the high cost of this electrolyte still prevented it from practical applications.To reduce the cost of“water-in-salt”electrolytes,in this work,we had chosen the cheap NaClO₄ salt to substitute the expensive LiTFSI salt and prepared a new“water-in-salt”electrolyte with Sn?OTf?₂and NaClO₄.After multiple times of optimization of the composition of the electrolyte,it was determined to be“0.5 m Sn?OTf?₂+8 m NaClO₄”.Finally,Sn-Na₃V₂?PO₄?₃full cells were attained and tested.The discharge voltage of the full cells were measured to be 0.86 V.In the cyclability tests,they can cycle for 100 cycles with CEs over 85%,revealing their potential to some extent.However,the suppression to tin dendrite formation,the reversibility of tin anode and the battery performances at low rates using“0.5 m Sn?OTf?₂+8 m NaClO₄”electrolyte were far inferior to those using“1 m Sn?OTf?₂+20 m LiTFSI”electrolyte.The only advantage of“0.5 m Sn?OTf?₂+8 m NaClO₄”electrolyte was its battery performances at high rates compared to those of“1 m Sn?OTf?₂+20 m LiTFSI”electrolyte.Overall,better performances were expected for this electrolyte,which needed further optimization of composition to improve the pH value of the electrolyte.