Electrochemical comparison and deposition of lithium and potassium from phosphonium- and ammonium-tfsi ionic liquids
Vega, Jose A.
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In this work, ionic liquids (ILs) were investigated for use as battery electrolytes. The ILs were synthesized from quaternary ammonium and phosphonium salts and TFSI-. A dendrite free lithium metal anode was demonstrated by deposition of a lithium-potassium alloy. Several phosphonium ILs were synthesized using the TFSI- and PF6- anions until a room temperature IL was obtained. The smaller size, highly symmetric PF6- anion yielded high melting point salts, while TFSI- yielded much lower melting point ILs. When a room temperature IL, Bu3HexP+TFSI-, was obtained the analogous ammonium IL, Bu3HexP+TFSI-, was synthesized and compared. The phosphonium-based ionic liquid showed improved stability and physical properties compared to the analogous ammonium-based IL. The phosphonium-based IL had higher conductivity, 0.43 mS/cm, than the ammonium-based IL, 0.28 mS/cm. The addition of LiTFSI to both ILs led to a decrease in conductivity and increase in viscosity. The lower viscosity and higher stability of the phosphonium-based IL led to higher current density and stability for electrodeposited lithium metal. IL reduction interfered with lithium deposition reflecting lower coulombic efficiencies and giving the appearance of an unstable lithium couple. An optimum deposition potential was found which was bounded by the electrochemical stability of each IL. The stability of lithium in the ILs increased at lower temperature due to slower reactivity with the IL. Addition of higher quantities of lithium ions caused a higher fraction of the cathodic current going to lithium deposition that was reoxidized. The stability of lithium in the ILs increased at lower temperature due to slower reactivity with the IL. The electrodeposition and reoxidation of potassium was also demonstrated. Deposition of a lithium-potassium alloy caused slight increases in the cathodic and anodic currents along with higher coulombic efficiencies. Also, it was found that a lithium-potassium alloy could be deposited at high current for long times without the occurrence of dendrites.