Batteries with metallic anodes, such as lithium metal, promise significantly higher storage capacity. However, a significant hurdle barring their successful implementation has been the uneven deposition of the metal during the charging process, which leads to formation of dendrites. After longer uses of the battery, these dendrites can grow so extensive that they short-circuit the battery.

In addition, there are undesirable side-reactions between the reactive metal electrodes and the electrolyte, which significantly reduces the lifetime of the batteries. The formation of a stable, passivating layer that prevents further contact would be an ideal solution; however, the constant expansion and contraction of the electrode upon charging and discharging destroys the layer and exposes the metal to the electrolyte for more reactions. Other approaches include artificial films or physical barriers.

The success of these methods in simultaneously suppressing dendritic deposition and parasitic side reactions appear to hinge upon formation of a SEI enriched with species such as LiF that facilitate fast Li-ion diffusion at the electrolyte–metal interface. It also underscores the importance of fundamentally based strategies able to control the dendrite nucleation processes at reactive metal/liquid electrolyte interfaces.

… Here, we report a new approach to diffusion barrier minimization that exploits the effects of the solvent (and electrolyte) at the interface. The key idea is to use strong interactions of the solvent with the electrodeposited atom to weaken its binding to the electrode surface and flatten the energy landscape for atom motion in the plane. Aprotic solvents used in battery electrolytes will interact most strongly with charged species. Stable charging of the surface atom should be possible under these conditions by employing a difference in electropositivity between the deposited atom and the electrode. We illustrate these ideas using indium metal coatings on lithium metal anodes formed by an in situ electroless plating technique. The high electropositivity of lithium relative to indium is expected to result in (partially) positively charged lithium atoms on the In surface.

—Choudhury et al.

Using straightforward electroless ion-exchange chemistry, the team produced indium coatings on lithium. Some of the indium is deposited on the surface of the lithium electrode as metal and the lithium ion concentration in the electrolyte simultaneously increases.

The indium layer is uniform and self-healing when the electrode is in use, if small amounts of the indium salt are added to the electrolyte. It remains intact during charge/discharge cycles, its chemical composition remains unchanged, and side-reactions are prevented. Dendrites are also eliminated, leaving the surface smooth and compact.

Using computer modeling, the researchers showed that lithium ions loosely bind to the indium coating, thus enabling faster surface transport to form more uniform Li electrodeposits. They form an alloy with the indium, which allows them to move very rapidly over the surface before they cross it and are deposited on the underlying lithium electrode.

Using a suite of characterization tools, including electron microscopy and X-ray spectroscopy, the researchers showed that the In layer is uniform and stable even after battery cycling.

In complete cells with commercial cathodes (LTO and NCM), these new indium–lithium hybrid electrodes were stable over more than 250 cycles, retaining about 90% of their capacity.

Top: Half-cell cycling with Lithium Titanate cathode using the electrolyte of 1 M EC/DMC LiPF6 with the additives 12 mM In(TFSI)3 and 10 vol% FEC. The areal capacity is 3 mAh cm-2 and the C-rate is 1 C. Bottom: Cycling with Lithium Nickel Cobalt Manganese Oxide cathode using the same electrolyte. The areal capacity is 2 mAh cm-2. The charging cycling rate is C/2 and the discharging rate is 1 C. Choudhury et al. Click to enlarge.

Resources

• Choudhury, S., Tu, Z., Stalin, S., Vu, D., Fawole, K., Gunceler, D., Sundararaman, R. and Archer, L. A. (2017), “Electroless Formation of Hybrid Lithium Anodes for Fast Interfacial Ion Transport.” Angew. Chem. Int. Ed. doi: https://doi.org/10.1002/anie.201707754doi: