The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes

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.