Lithium (​Li) metal is an ideal anode material for rechargeable Li batteries due to its extremely high theoretical specific capacity (3,860 mAh g⁻¹), low density (0.534 g cm⁻³) and the lowest negative electrochemical potential (−3.040 vs standard hydrogen electrode). Extensive attempts have been made to use Li as an anode in rechargeable Li batteries since the 1970s, but several seemingly insurmountable barriers, including dendritic​Li growth and limited Columbic efficiency (CE) during repeated​Li deposition/stripping processes, have prevented their large-scale applications.

… Electrolyte is one of the most critical elements that affects the cycling stability of Li metal anodes. Aurbach et al. indicated that Li is thermodynamically unstable with any kinds of organic solvents. The interactions between electrolyte components and Li metal results in significant side reactions that not only lead to a low CE but also consume Li metal and the electrolyte. This produces a solid-electrolyte interphase (SEI) film that may eventually grow into a thick layer, leading to high-impedance-failure of the battery instead of a short circuiting failure due to dendritic​Li growth. This phenomena becomes serious especially at high current densities.

… Here we demonstrate that the use of highly concentrated electrolytes composed of ether solvents and the salt lithium bis(fluorosulfonyl)imide (LiFSI or​LiN(SO₂F)₂) results in the dendrite-free plating of Li metal at high rates and with high CE. This exceptional performance cannot be achieved when lower concentration electrolytes are used (with or without LiFSI) and when LiFSI is substituted with other salts.

—Qian et al.

The researchers explored the morphology of Li deposition in different electrolytes using coin-type Cu|Li cells. After the initial deposition, the coin cells were disassembled to collect the Li films deposited on the Cu substrates for microscopic analysis by scanning electron microscopy (SEM) without exposing to air. In cells with the ether-based LiFSI electrolytes, the anode developed a thin, relatively smooth layer of lithium nodules that didn’t short-circuit the battery instead of growing dendrites.

(a) and (b) show the cross-section and surface morphologies of the Li films deposited in a typical carbonate-based electrolyte (1 M LiPF6 in PC), showing extensive dendritic Li metal deposition. In contrast, a nodule-like Li deposition without dendrite formation was obtained when Li film was deposited from the ether-based LiFSI-DME electrolytes with different salt concentrations. The typical dimension of the nodule-like Li particles is on the order of ~10 μm which restricts their ability to penetrate the porous separators. (c) and (d) show the cross-section and surface morphologies of the Li film obtained in a concentrated 4M LiFSI-DME electrolyte. Credit: Qian et al. Click to enlarge.

After 1,000 repeated charge and discharge cycles, cells with the new electrolyte retained 98.4% of the initial energy while carrying 4 mA cm^-2. The researchers found that greater current densities resulted in slightly lower efficiencies. For example, a current density as high as 10 mA cm^-2, the test cell maintained an efficiency of more than 97%. And a test cell carrying just 0.2 mA cm^-2 achieved 99.1% efficiency. Most batteries with lithium anodes operate at a current density of 1 mA cm^-2 or less and fail after less than 300 cycles.

Results indicated that 4 M LiFSI in​DME is an optimized salt concentration to obtain the stable cycling of Li metal in this electrolyte system.

The reactivity of these electrolytes is low (resulting in very limited side reactions and thus a high CE) and the large amount of Li⁺ cations available enables high current densities to be used for​Li metal deposition. For more dilute electrolytes, the solvent is found to react with the plated Li metal to a much greater extent, which lowers the CE of the Li plating/stripping. Although the thickness of the SEI layer still grows with increasing cycle numbers as a result of the non-perfect CE (~99%) for Li plating/stripping in the 4M LiFSI-DME electrolyte, the highly conductive nature of the SEI layer leads to a highly stable voltage profiles during the cycling of Li electrode. In addition, the highly compact feature of the SEI layer also prevents further corrosion of the Li metal electrode and results in excellent stability of the electrode in the highly concentrated LiFSI-DME electrolytes.

—Qian et al.

The electrolyte needs to be refined before it’s ready for mainstream use. Zhang and his colleagues are evaluating various additives to further enhance their electrolyte so a lithium battery using it could achieve more than 99.9% efficiency, a level that would be needed for commercial adoption. They are also examining which cathode materials would work best in combination with their new electrolyte.

The new electrolyte’s remarkably high efficiency could also open the door for an anode-free battery, Zhang noted. The negative electrodes in today’s batteries consist of metal current collectors (e.g., copper) coated in active materials such as graphite or lithium. An electrolyte with more than 99% efficiency means there’s potential to create a battery that only has a negative current collector, without an active material coating, on the anode side.

Resources

• Jiangfeng Qian, Wesley A. Henderson, Wu Xu, Priyanka Bhattacharya, Mark Engelhard, Oleg Borodin & Ji-Guang Zhang (2015) “High Rate and Stable Cycling of Lithium Metal Anode,” Nature Communications doi: 10.1038/ncomms7362