Two room-temperature ionic liquids, Bu(3)HexP(+)TFSI(-) [TFSI=bis(trifluoromethanesulfonyl)imide] and Bu(3)HexN(+)TFSI(-), were synthesized and their electrochemical behavior was investigated using CV. The phosphonium-based ionic liquid (IL) 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 lower viscosity and higher stability of the phosphonium-based IL led to higher current density and stability for electrodeposited lithium metal. The addition of LiTFSI to both ILs led to a decrease in conductivity and increase in viscosity. An optimum deposition potential was found that 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. The electrodeposition and reoxidation of potassium was also demonstrated. It was found that a lithium-potassium alloy could be deposited at high current for long times without the occurrence of dendrites.