Development of Materials for Aqueous Lithium Batteries
Authors: Imanishi Nobuyuki , Wang Hui 
Published: February 2020

A reversible high-areal-capacity metallic lithium electrode (MLE) is pivotal to develop protected lithium electrodes (PLE) for aqueous lithium-air batteries, whose practical specific energy strongly depends on the realization of the high-areal-capacity of reversible MLEs as well as the sustainable unclosed air-breathing cathode/catholyte couple. Recent advances of reviving MLE studies further shed new light on the key role of electrolytes (i.e., the anolyte in the PLE part).
[1]

In this study, a novel electrolyte has been prepared by the complexation of lithium cations supplied by lithium bis(fluorosulfonyl)imide (LiFSI) and tetraethylene glycol dimethyl ether (G4) with the additive of 1,3-dioxolane (DOL).
[2] The matrix has been disclosed to enable MLE to deliver a high-areal-capacity at elevated temperature in our latest publication. The composition of LiFSI-2G4 (the molar ratio of G4 to Li+ is 2) is found to be a solvate ionic liquid (SIL) with appealing attributes of suppressing lithium dendrite formation at 60 oC, possibly due to the fact that the high fraction of surfactant-role G4 in the solid-electrolyte interphase (SEI) film and elevated temperature could lower the Ehrlich-Schwoebel barrier to facilitate adatoms migration down the descending step that favor two-dimensional layer-by-layer lithium deposition mode.[3]
To make this SIL work at ambient temperature, decreases in the viscosity and interfacial resistance are of great importance. The addition of DOL into LiFSI-2G4 has been demonstrated to lead to the decrease in the viscosity but the absence of alteration of the solvation manner of Li+.[4] Herein, we first unveil the significant impact of the introduction of cyclic voltammetry (CV) modulation, prior to the lithium plating onto Cu or Li working electrode, on lithium plating process and surface film chemistry. With CV modulations, a lithium/lithium cell can be cycled at 5.0 mA cm-2 for 100 cycles, and a copper/lithium cell can be cycled at 5.0 mA cm-2 for 80 cycles with a coulombic efficiency (CE) of 100%, where both cells exhibit the exceptional high-areal-capacity of 12 mAh cm-2 at 25 oC. The combination of a novel SIL and CV modulations paves a new avenue to developing practical MLEs and thus PLEs. 

Figure (a) shows the CV measurements of Li/LiFSI-2G4-50 vol% DOL/Cu exhibited a sudden change in the 6th scan, where the onset potential for lithium deposition (Eld) fell from the initial -0.153 V to -0.083 V and substantial increases in the peak current density for lithium deposition and stripping. The decrease of Eld continued with the sweeping but got into a steady value of -0.042 V until the 23rd scan as shown in Figure (b). Current density of 5 mA cm-2 occurred at -0.283 V, -0.139 V, -0.070 V in the 5th, 6th, 30th cathodic sweep, respectively, which suggests that the lithium deposition onto Cu substrate is supposed to evolve faster with the cycle number. Other electrolytes (e.g. LiFSI-2G4, LiFSI-G4, LiTFSI-2G4, LiFSI-2G4-50 vol% G4, 1M LiPF6-EC-DEC, LiFSI-5G4) were also investigated in the same condition. Only the electrolyte of LiFSI-2G4 was found to exhibit the comparable descending trend of Eld but much lower degree of the increase in the Ip. Under the CV pre-modulation, the CE measurement of Li/LiFSI-2G4-50 vol%/Cu cell was performed at 5.0 mA cm-2 and 25 oC with a constant lithium deposition capacity of 12 mAh cm-2 as shown in Figure (c). Surprisingly, the CE of 100% can be sustained up to 80 cycles. In the same condition, the cell of Li/Li can be cycled for 100 cycles without short-circuit by the CV pre-modulation and modulation during the intervals after each half-cycle. To the best of our knowledge, MLE is first reported to achieve a high specific areal-capacity of 12 mAh cm-2 at 5 mA cm-2 and ambient temperature by the combination of a novel SIL and the CV modulation technique. Details of this study will be presented in a paper under the submission.

[1] J. Qian, J.-G. Zhang et al., Nat Commun
2015, 6; b) Y. Lu, L. A. Archer et al., Nat Mater.,
2014, 13, 961-969; c) W. Li, Y. Cui et al., Nat Commun.,
2015, 6.

[2] D. Aurbach, J. Power Sources.,
2000, 89, 206-218.

[3] a) H. Wang, N. Imanishi et al., ChemElectroChem
2015, 2, 1144-1151; b) Q. Chen, K. Geng, K. Sieradzki, J. Electrochem. Soc.,
2015, 162, A2004-A2007.

[4] H. Wang, N. Imanishi et al., in The 56th Battery Symposium in Japan, Nagoya, 2015, 3A03.

Figure 1