A Reversible Dendrite-Free High-Areal-Capacity Metallic Lithium Electrode
Authors: Wang Hui , Matsui Masaki , Kuwata Hiroko , Matsuda Yasuaki , Shang Xuefu , Takeda Yasuo , Yamamoto Osamu , Imanishi Nobuyuki 
Published: February 2020

A reversible dendrite-free high-areal-capacity metallic lithium electrode (MLE) is pivotal to develop beyond lithium ion batteries-LIBs (aqueous Li-air or soluble cathodes, Li-S, Li-O2, etc.), whose practical specific energy strongly depends on the realization of the high-areal-capacity of reversible MLEs, and to develop post-LIBs without safety concerns as well. Recent advances of reviving MLE studies further shed new light on the key role of electrolytes.
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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 co-solvent of 1,3-dioxolane (DOL).
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] 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.[
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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+.[
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] Herein, we first unveil the significant impact of the introduction of cyclic voltammetry (CV) premodulation, prior to the lithium plating onto Cu or Li working electrode, on lithium electrodeposition process and surface film chemistry. With CV premodulation, a Li/Li cell can be cycled at 5.0 mA cm-2 for 100 cycles, and a Cu/Li cell can be cycled at 5.0 mA cm-2 for 450 cycles with a perfect Coulombic efficiency (CE) of 100% unless taking into account of the tiny loss of lithium source during CV premodulation, where both cells exhibit the exceptional high-areal-capacity of 12 mAh cm-2 at 25 oC. In-situ optical observation clearly demonstrated that two-dimensional layer-by-layer dendrite-free growth mode is the prevailing growth mode of lithium electrodeposits in the SIL of LiFSI-2G4 using DOL as the co-solvent with the assistance of CV premodulation. Post-mortem analyses (XPS, SEM, EDX) indicated that the outer G4-derived SEI films along the planar lithium particles, that can retard the surge of lithium ions at the preferential spots due to the strong binding energy of G4 and Li+ (ca. -2.8 eV for nLi
+:nG4=1:2) at high current densities, should be subjected to a redistribution process from the top-centered region to the step-edge region along initial planar lithium particles upon CV premodulation, which render lithium adatoms to deposit in the two-dimensional layer-by-layer growth mode because of the decline in the Ehrlich-Schwoebel barrier. The combination of a novel SIL and CV premodulation paves a new avenue to developing practical MLEs for beyond or Post LIBs. 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 premodulation technique. Details of this study will be presented in a paper under the peer-review.

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