UT Austin team devises new strategy for safe, low-cost, all-solid-state rechargeable Na or Li batteries suited for EVs
Researchers at the University of Texas at Austin, including Prof. John Goodenough, known around the world for his pioneering work that led to the invention of the rechargeable lithium-ion battery, have devised 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.
As reported in their paper in the RSC journal Energy & Environmental Science, the cells use a solid glass electrolyte having a Li+ or Na+ conductivity σi > 10-2 S cm-1 at 25°C with a motional enthalpy ΔHm ≈ 0.06 eV, which promises to offer acceptable operation at lower temperatures. The glass also has a surface that is wet by metallic lithium or sodium, which allows reversible plating/stripping of an alkali-metal anode without dendrites, and an energy-gap window Eg > 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.
The glass also contains electric dipoles that endow it with a large dielectric constant. Optimal properties are only obtained after aging, which requires more than 10 days at 25°C, but only a few minutes at 100°C.
With this glass, a rechargeable battery with a metallic lithium or sodium anode and an insertion-compound as cathode may require a polymer or liquid catholyte in contact with the cathode. However, the team notes, the all-solid-state metal-plating batteries are simpler to fabricate at lower cost and offer much higher energy densities, longer cycle life, and acceptable charge/discharge rates.
To fabricate test cells for their study, the researchers took Na+ and Li+ glass electrolytes and introduced them into either a fiberglass sheet or a thin sheet of recycled paper from a slurry of the glass particle sin ethanol. They heated the membranes to outgas the ethanol and reform the solid glass electrolyte without grain boundaries, then pressed these against an anode of Li or Na foil contacting a stainless-steel cell container.
The electrolyte membrane was 0.06 mm thick. The cathode consisted of a redox center (an S8, or ferrocene Fe(C5H5) molecule or MnO2 particle) embedded in a mix of electrolyte and carbon contacting a Cu current collector.
This cathode composite was pressed against the electrolyte membrane in a coin-cell configuration. The sealed cell was then aged. None of the components were optimized before the electrochemical performance measurements.
|Schematic of an all-solid-state Li-S cell with the glass electrolyte; during discharge the metallic-lithium anode is plated on the cathode carbon-copper composite current collector. Click to enlarge.|
The ability to plate/strip reversibly an alkali-metal anode from a solid electrolyte invites a complete rethink of rechargeable-battery strategies. With the Li-glass and Na-glass electrolytes, we have demonstrated in this paper one possible new strategy in which the cathode consists of plating the anode alkali-metal on a copper-carbon cathode current collector at a voltage V > 3.0 V. Replacement of a host insertion compound as cathode by a redox center for plating an alkali-metal cathode provides a safe, low-cost, all-solid-state cell with a huge capacity giving a large energy density and a long cycle life suitable for powering an all-electric road vehicle or for storing electric power from wind or solar energy.—Braga et al.
Maria Helena Braga, Nicholas S. Grundish, Andrew J. Murchison and John B Goodenough (2016) “Alternative Strategy for a Safe Rechargeable Battery” Energy Environ. Sci. doi: 10.1039/C6EE02888H