Cycling of a [Li | [Li(G4)][TFSA]/CO2(5 MPa) | Cu] cell (1 hr of 400 µA current application corresponding to 0.2 mA.cm-2, 0.72 C.cm-2 with 10 min rest periods, at 30 ºC) allows quantitative evaluation of the efficiency of the reversible electrochemical deposition and dissolution of Li from this medium, as shown in Figure 1(a). For comparison, analogous data recorded in a high pressure Ar rather than CO2 medium are also shown. It is noteworthy that the data recorded in the presence of CO2 provides a considerably improved performance in terms of cycle life, and thus indicates that the CO2 plays an important role in the deposition process. This is assumed to be in part due to the improved conductivity of the binary medium, which may be varied as a function of pressure, as shown in Figure 1(b). Conductivity observed upon dilution of the SIL with conventional molecular solvents is shown for comparison.4
Microscopic (FE-SEM) investigation of the deposited lithium surface (deposition of lithium onto the clean Cu surface for 10 hours at 400 µA), see Figure 1(c), indicates the deposited lithium has a particle-like structure, and there are only minor structural differences apparent in the presence of CO2 compared to Ar. Further analysis of the surface chemistry of these deposits has revealed that the Li deposited under CO2 conditions is of higher purity, as will be elaborated further in the presentation.
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(a) Coulombic efficiency for deposition (1 hour) and dissolution of Li, under pressure, with cell configuration [Li | [Li(G4)][TFSA]/gas(5 MPa) | Cu], 30 °C, 0.4 mA (0.2 mA.cm-2).
(b) Ionic conductivity as a function of Li+ concentration for [Li(G4)][TFSA]/CO2 (mole fraction of CO2 is varied with pressure). *Molecular solvents shown for comparison, from Ref.4.