The solid glass electrolyte also has a surface that is wet by metallic lithium or sodium, which allows the reversible plating/stripping of an alkali-metal anode without dendrites, and an energy-gap window > 9 eV that makes it stable on contact with both an alkali-metal anode and a high-voltage cathode without the formation of an SEI.

Traditional rechargeable batteries use a liquid electrolyte and an oxide as a cathode host into which the working cation of the electrolyte is inserted reversibly over a finite solid-solution range. The energy-gap “window” E_g = 1.23 eV of an aqueous electrolyte restricts rechargeable batteries with a long shelf life to a voltage V ≲ 1.5 V. The organic-liquid electrolyte of the lithium-ion battery has an energy-gap window E_g ≈ 3 eV, but its LUMO is below the Fermi level of an alkali-metal anode, it is flammable, and it is not wet by an alkali-metal anode; therefore, the anode of a rechargeable battery having an organic-liquid electrolyte does not contain an alkali-metal component lest anode dendrites form and grow across the electrolyte to the cathode during charge to short-circuit the battery cell with incendiary consequences.

Moreover, if the battery cells have an anode with a chemical potential (Fermi level) E_F < 1.3 eV below the E_F of lithium metal, a passivating solid/electrolyte interphase (SEI) is formed on the anode to prevent reduction of the electrolyte on contact with the anode. If the Li-ion battery is fabricated in a discharged state, as may be required for high-voltage cathodes, the Li⁺ of the SEI layer comes from the cathode, further restricting the operational capacity of the cathode. The carbon anodes of the Li-ion batteries that today power hand-held and portable devices have a low volumetric capacity and restrict the rate of charge since metallic lithium is plated on the carbon at higher charging voltages; also, oxide cathodes providing a cell voltage V 4 4.3 V versus lithium tend to be unstable if overcharged.

Therefore, the multiple cells of a large-scale battery stack require an expensive management system. Attempts to develop Li-alloy anodes have generally failed to provide the volumetric energy density required for portable batteries. Finally, sodium is cheaper than lithium and widely available from the oceans, which makes a sodium battery preferable to a lithium battery, but insertion hosts for Na⁺ have lower capacities than insertion hosts for Li⁺.

In this paper, we report a new strategy for a safe, low-cost, all-solid-state rechargeable sodium or lithium battery cell that has the required energy density and cycle life for a battery that powers an all-electric road vehicle.

—Braga et al.

The all-solid-state metal-plating batteries with the cathode strategy are simpler to fabricate at lower cost and offer much higher energy densities, longer cycle life, and acceptable charge/discharge rates, the researchers said.

For the study reported in EES, the UT team fabricated Li⁺ and Na⁺ glass electrolytes. Cathodes consisted of a redox center (an S₈ or ferrocene molecule or an MnO₂ particle) embedded in a mix of electrolyte and carbon contacting a copper current collector. The cathode composite was pressed against the electrolyte membrane in a coin-cell configuration.

Schematic representation of a plating cathode: the plating process on discharge with a redox center, the electrode energies, and the EDL capacitances at electrode/electrolyte interfaces are shown. Braga et al. Click to enlarge.

The researchers demonstrated that their new battery cells have at least three times as much energy density as today’s lithium-ion batteries. The UT Austin battery formulation also allows for a greater number of charging and discharging cycles—more than 1,200 cycles with low cell resistance.

Additionally, because the solid-glass electrolytes can operate, or have high conductivity, at -20 degrees Celsius, this type of battery in a car could perform well in subzero degree weather. This is the first all-solid-state battery cell that can operate under 60 degree Celsius.

Braga began developing solid-glass electrolytes with colleagues while she was at the University of Porto in Portugal. About two years ago, she began collaborating with Goodenough and researcher Andrew J. Murchison at UT Austin. Braga said that Goodenough brought an understanding of the composition and properties of the solid-glass electrolytes that resulted in a new version of the electrolytes that is now patented through the UT Austin Office of Technology Commercialization.

Goodenough and Braga are continuing to advance their battery-related research and are working on several patents. In the short term, they hope to work with battery makers to develop and test their new materials in electric vehicles and energy storage devices.

This research is supported by UT Austin, but there are no grants associated with this work. The UT Austin Office of Technology Commercialization is actively negotiating license agreements with multiple companies engaged in a variety of battery-related industry segments.

Resources

• M. H. Braga, N. S. Grundish, A. J. Murchison and J. B. Goodenough (2017) “Alternative strategy for a safe rechargeable battery” Energy Environ. Sci., 10, 331-336 doi: 10.1039/C6EE02888H