There is an increasing demand for high power, long cycle life, inexpensive batteries for behind-the-meter (BTM) applications including uninterruptible power supplies (UPS), demand charge management, and electric vehicle (EV) fast support. For the past decade most battery manufacturers have focused on increasing energy density while decreasing the cost of cells optimized for EVs, which are not designed for BTM duty cycles. Innovations in materials and design to enable higher power and longer cycle life at a low cost have received little attention.

This presentation introduces a new battery technology based on cells containing Prussian blue electrodes and an organic sodium-ion electrolyte. Prussian blue (ferric hexacyanoferrate) is a pigment found in a variety of consumer products. Prussian blue and its transition-metal substituted analogues have a unique open framework crystal structure containing channels and interstices larger than de-solvated sodium ions. This allows sodium ions to be inserted and removed from the Prussian blue structure rapidly and reversibly with near-zero strain to the structure.¹

For device performance to benefit from these unusual characteristics, both of the two electrodes in the cell must have comparable rate capability and cycle life. For this reason, Natron Energy has developed a cell in which the cathode and anode active materials are both Prussian blue analogues. The cathode undergoes a single electron redox of Fe^III/II(CN)₆ at +0.9 V vs. S.H.E., while the anode undergoes a unique Mn^II/I(CN)₆ redox at -0.7 V vs. S.H.E. in which a unique low-spin Mn(I) valence state forms during charging.² The resulting cell has a nominal voltage of 1.6 V, can be fully discharged at rates of up to 50C, and fully recharged at rates of up to 16C. Because the cathode and anode each undergo low-strain single phase reactions, neither electrode limits the life of the cell. After over one year of 10C-10C cycling (60,000 cycles), just 2% capacity loss is observed.

The high power and long cycle life of the Prussian blue cell lead to new opportunities for pack and system design optimization. This presentation will highlight two real-world examples of BTM applications that make use of these device properties: data center UPS, and EV fast charge support. The high rate capability of the Prussian blue cell allows rack-mounted UPS battery trays based on this chemistry to deliver nearly double the power density of conventional lead acid trays. In addition, a simulation of energy storage photovoltaic (PV) solar system sizing was performed for electricity cost minimization for a four-charger direct current fast charge (DCFC) station using one year of real-world operational load data. The dynamic rate capability and deep discharge cycle life of the Prussian blue battery allows a smaller, lower cost system to deliver a competitive return on investment. These two examples indicate the potential value of high rate capability and long cycle life for BTM applications.

References:

1. Wessells, C. D., et al., Nickel Hexacyanoferrate Electrodes For Aqueous Sodium And Potassium Ion Batteries, Nano Letters, 11, 5421 (2011).
2. Firouzi, A., et al., Monovalent manganese based anodes and co-solvent electrolyte for stable low-cost high-rate sodium-ion batteries, Nature Communications, 9, 861 (2018).