The huge growth of the EV market has spurred significant development of the EV charging industry and a host of fast-charging station competitors. However, public EV charging station owners have had a hard time turning a profit because of the highly volatile demand charges that electric utilities charge their commercial customers due to the unpredictable power profile. These fees can represent as much as 90 percent of a public charging station’s monthly utility bill, according to the Rocky Mountain Institute (RMI). In order to lower those fees, charging station owners and developers are looking at the incorporation of stationary energy storage capability. However, batteries based on the incumbent lithium-ion technology tend to degrade quickly when charged and discharged 2-3 times per day.
Chevron Technology Ventures is placing a bet on a new robust battery for fast-charging stations that uses Prussian blue electrode chemistry. The venture arm of the petrochemical company has invested in Natron Energy, a developer of batteries built around Prussian blue dye. The dye, commonly used in blueprints, stores and releases energy in the form of sodium ions. Unlike the electrode materials found in most lithium-ion batteries, Prussian blue enjoys a widespread availability and low cost that make batteries based on Prussian blue electrodes an economically attractive, environmentally friendly technology.
The unique chemistry enables these batteries to store energy much more rapidly and reversibly than other types of battery electrodes. This means that such a battery is the first where the chemistry does not limit the lifetime of the system. Prussian blue chemistry permits batteries to be fully charged and discharged tens of thousands of times at very high power. Additionally, the batteries offer improved safety compared to lithium ion, as they are non-flammable during overcharge, hard short-circuit, overheating from external sources or when penetrated by foreign objects such as nails.
Using Prussian blue analogue electrodes and a sodium-ion electrolyte, Natron’s electrodes charge and discharge by single-phase reactions within the stability window of that electrolyte. This eliminates the conversion reactions and electrolyte decomposition that limit the lifetime of lead acid and lithium-ion cells. CEO Colin Wessells describes the crystal lattice of Prussian blue as a big cage like a Rubik’s cube, except that the cubes are empty with lots of pass-through space. “You can remove ions and the structure doesn’t care,” he said. “The lattice does not have to expand or contract, and the particles don’t change size.” In contrast, lithium-ion tends to expand or contract, causing the battery to degrade over time.
Natron’s batteries are designed for stationary applications including industrial and grid-scale storage markets. The batteries are already in demonstration with 5 data storage customers, and Paris-based EDF is conducting evaluations of the technology, including lifetime testing.
While demand at certain kinds of charging sites may be more predictable and thus less expensive, such as corporate campus EV parking lots, Wessells says the batteries are ideal for such sites as on-the-road fast charging infrastructure. “A truck stop on the I-80 corridor could be problematic, with 5-15 minute charging happening irregularly any time of day, incurring large demand charges. This is where Natron batteries could be very helpful.”