Plot of permittivity (ε) and dielectric breakdown (Ebd) for a range of materials including the AEB as originally targeted by the Stanford engineering team in 2010. Corresponding gravimetric densities at top. Advanced experimental Li-sulfur batteries have shown volumetric densities in the range of 400 Wh/L, while Li-air batteries theoretically could have volumetric capacities of ~1500 Wh/L—i.e., roughly the same as the target. Source: Stanford. Click to enlarge.

The “All Electron Battery” the Stanford engineering team set out to develop represented “a completely new class of electrical energy storage devices for electric vehicles that has the potential to provide ultra-high energy and power densities, while enabling extremely high cycle life.”

Electrons are lighter and faster than the ion charge carriers in conventional Li-ion batteries. Because the technology relies on electrical energy stored as electrons rather than ions, small and light devices with high storage capacities are possible. Furthermore, electron transport allows for fast charge and discharge.

Further, the technology uses a novel architecture that has potential for very high energy density because it decouples the two functions of capacitors: charge separation and breakdown strength. This increases both the life of the battery and the amount of energy it can store. The battery could be charged 1000s of times without showing a significant drop in performance.

In 2010 (also the year QuantumScape was founded), ARPA-E awarded Stanford, with Honda and Applied Materials as project partners, $1,498,681 for a two-year project to further the AEB.

In patents awarded to Stanford, the Stanford researchers explained that the improved energy storage is provided by exploiting two physical effects in combination.

• The All-Electron Battery (AEB) effect relates to the use of inclusions embedded in a dielectric structure between two electrodes of a capacitor. Electrons can tunnel through the dielectric between the electrodes and the inclusions, thereby increasing the charge storage density relative to a conventional capacitor.

• The area enhancement effect relates to the use of micro-structuring or nano-structuring on one or both of the electrodes to provide an enhanced interface area relative to the electrode geometrical area. Area enhancement is advantageous for reducing the self-discharge rate of the device.

In the patent document, the inventors note that important design parameters for AEBs include:

• electrode area enhancement factors;
• the spacing between electrodes and inclusions;
• the spacing between inclusions;
• the size, shape, and number density of inclusions;
• the tunneling energy barrier between electrodes and inclusions;
• the tunneling energy barrier between inclusions;
• dielectric constants; and
• work functions.

The charge and discharge rates and storage capacities of the devices can be selected by appropriate geometrical design and material choice. Charge and discharge rates depend on the gaps between inclusions and the dielectric constant of the dielectric material, therefore the rates can be altered by changing the distances between inclusions, and/or the dielectric constant. Charge and discharge rates further depend upon the electron affinity of the dielectric material and of the inclusions.

If Volkswagen has invested in QuantumScape (and if QuantumScape is basing its technology on the Stanford AEB), then we may see relatively soon what types of design and optimization decisions the startup has made on top of the basic technology with an eye toward vehicle applications.

Resources

• All-electron battery having area-enhanced electrodes US 8524398 B2