A team at Dalhousie University led by Professor Jeff Dahn, with colleagues from Tesla Canada R&D and the University of Waterloo, has demonstrated stable cycling of an anode-less Li metal cell by using a dual-salt electrolyte (1 M lithium difluoro(oxalate)borate (LiDFOB) and 0.2 M lithium tetrafluoroborate (LiBF₄) in a fluoroethylene carbonate (FEC):diethyl carbonate (DEC) solvent). The researchers suggest that their results could shift research focus away from solid-state batteries (SSBs) as a rechargeable, high-energy-density solution.

Dahn’s team’s anode-free lithium-metal pouch cells retained 80% capacity after 90 charge–discharge cycles—the longest life demonstrated to date for cells with zero excess lithium. In their paper in Nature Energy, the researchers explained that the liquid electrolyte enables smooth dendrite-free lithium morphology comprising densely packed columns even after 50 charge–discharge cycles. NMR measurements show that the electrolyte salts responsible for the excellent lithium morphology are slowly consumed during cycling.

Replacing the conventional graphite anode with lithium metal is one of the most popular approaches [to improve energy density], as this can increase the cell energy density by 40–50%. However, this substantial increase in cell energy is achieved only if the excess thickness of the lithium anode is limited. Unfortunately, lithium-metal cells reported in the literature often use extremely thick anodes containing over 10 times the amount of lithium actually being cycled. This huge excess could never be used in a practical cell and makes interpretation of results more difficult, as cycling stability becomes artificially enhanced. As a result, researchers have called for limiting the lithium excess to less than 50 μm.

Limiting lithium excess is a challenge, as lithium metal is prone to form dendrites with high surface area, which reduce cycling efficiency by increasing the reactivity of the anode with the electrolyte and forming isolated metallic lithium. The low cycling stability of lithium metal is especially apparent in the anode-free or zero-excess configuration, where cells are built with a bare copper anode and the lithium is plated directly from the cathode on the first charge cycle. Since there is no excess lithium built into the cell, volume is minimized and energy density is maximized, but performance may be very poor since there is no reservoir of fresh lithium to replenish the cell during cycling.

… Many different approaches have been pursued to improve cycling stability in liquid electrolytes, including high salt concentration, ether solvents, fluorinated compounds, electrolyte additives, anode surface coatings and external pressure. … Another potential path to enable the lithium-metal anode is the use of solid-state electrolytes, which is regarded by many as the most viable way forward. However, solid-state electrolytes have not been successful in completely eliminating dendrites, and it is unclear how compatible these technologies will be with existing lithium-ion manufacturing infrastructure, in which billions of dollars have been invested. If liquid electrolytes can be used to create safe, long-life lithium-metal cells, then existing manufacturing equipment can be used to rapidly commercialize high-energy-density cells.

In this work, we demonstrate a practical concentration (∼1.2 M) dual-salt lithium difluoro(oxalato)borate (LiDFOB)/LiBF₄ liquid electrolyte that enables the longest cycle life for anode-free cells seen thus far: 80% capacity retention after 90 cycles. The lithium-metal anode is dendrite free and composed of tightly packed lithium columns 50μm in diameter even after 50 cycles. Compared with single-salt electrolyte compositions, the dual-salt blend performed better at varied cell voltages and was less dependent on external pressure to achieve good cycle performance. The electrolyte salt was observed to be continuously consumed during cycling, which is a key finding to lead further development of liquid electrolytes. This report demonstrates that stable cycling of lithium-metal cells may be possible with practical liquid electrolytes, which we believe could shift the research focus in this field away from solid-state batteries.

—Weber et al.

With the new cell, Li ions are extracted from the cathode and electrodeposited as metallic Li onto the current collector (Cu) during the initial charging process. During discharge, Li ions are stripped from the Cu current collector and intercalated back into the cathode.

This research was financially supported by Tesla Canada and NSERC under the Industrial Research Chairs Program.

In 2016, NSERC, Dalhousie University and Tesla Motors established the NSERC/Tesla Canada Industrial Research Chair that Jeff Dahn will hold to 7 June 2021. It is possible that the Chair will be renewed in 2021. The goals of the Chair are to develop lithium-ion batteries with longer lifetime, higher energy density and lower cost.

Our goal is to do something useful, not publish papers in Nature and similar journals.

—Jeff Dahn

Resources

• Rochelle Weber, Matthew Genovese, A. J. Louli, Samuel Hames, Cameron Martin, Ian G. Hill & J. R. Dahn (2019) “Long cycle life and dendrite-free lithium morphology in anode-free lithium pouch cells enabled by a dual-salt liquid electrolyte” Nature Energy doi: 10.1038/s41560-019-0428-9