The electrical power grid faces a growing need for large-scale energy storage over a wide range of time scales due to costly short-term transients, frequency regulation, and load balancing. The durability, high power, energy efficiency, and low cost needed for grid-scale storage pose substantial challenges for conventional battery technology. (1, 2) Here, we demonstrate insertion/extraction of sodium and potassium ions in a low-strain nickel hexacyanoferrate electrode material for at least five thousand deep cycles at high current densities in inexpensive aqueous electrolytes. Its open-framework structure allows retention of 66% of the initial capacity even at a very high (41.7C) rate. At low current densities, its round trip energy efficiency reaches 99%. This low-cost material is readily synthesized in bulk quantities. The long cycle life, high power, good energy efficiency, safety, and inexpensive production method make nickel hexacyanoferrate an attractive candidate for use in large-scale batteries to support the electrical grid.

Recent battery research has focused on the high power and energy density needed for portable electronics and vehicles, but the requirements for grid-scale energy storage are different, with emphasis on low cost, long cycle life, and safety. 1, 2 Open framework materials with the Prussian Blue crystal structure offer the high power capability, ultra-long cycle life, and scalable, low cost synthesis and operation that are necessary for storage systems to integrate transient energy sources, such as wind and solar, with the electrical grid. We have demonstrated that two open framework materials, copper hexacyanoferrate and nickel hexacyanoferrate, can reversibly intercalate lithium, sodium, potassium, and ammonium ions at high rates. These materials can achieve capacities of up to 60 mAh/g. The porous, nanoparticulate morphology of these materials, synthesized by the use of simple and inexpensive methods, results in remarkable rate capabilities: e.g. copper hexacyanoferrate retains 84% of its maximum capacity during potassium cycling at a very high (41.7C) rate, while nickel hexacyanoferrate retains 66% of its maximum capacity while cycling either sodium or potassium at this same rate. These materials show excellent stability during the cycling of sodium and potassium, with minimal capacity loss after 500 cycles.

The electrical energy grid has a growing need for energy storage to address short-term transients, frequency regulation, and load leveling. Though electrochemical energy storage devices such as batteries offer an attractive solution, current commercial battery technology cannot provide adequate power, and cycle life, and energy efficiency at a sufficiently low cost. Copper hexacyanoferrate and nickel hexacyanoferrate, two open framework materials with the Prussian Blue structure, were recently shown to offer ultralong cycle life and high-rate performance when operated as battery electrodes in safe, inexpensive aqueous sodium ion and potassium ion electrolytes. In this report, we demonstrate that the reaction potential of copper-nickel alloy hexacyanoferrate nanoparticles may be tuned by controlling the ratio of copper to nickel in these materials. X-ray diffraction, TEM energy dispersive X-ray spectroscopy, and galvanostatic electrochemical cycling of copper-nickel hexacyanoferrate reveal that copper and nickel form a fully miscible solution at particular sites in the framework without perturbing the structure. This allows copper-nickel hexacyanoferrate to reversibly intercalate sodium and potassium ions for over 2000 cycles with capacity retentions of 100% and 91%, respectively. The ability to precisely tune the reaction potential of copper-nickel hexacyanoferrate without sacrificing cycle life will allow the development of full cells that utilize the entire electrochemical stability window of aqueous sodium and potassium ion electrolytes.