A Full Prussian Blue Cell Containing a Hexacyanomanganate(II/I) Anode and a Cosolvent Electrolyte

Abstract

The continued deployment of variable generation assets including solar and wind has resulted in widespread concerns regarding electric grid stability.1-2 Energy storage in the form of batteries can smooth the volatile power outputs of solar and wind, alleviating the impact of that volatility on the rest of the grid.2 Unfortunately, existing battery technologies do not offer a long enough cycle life or high enough power to be economically used for this application. We previously showed that copper hexacyanoferrate, a Prussian Blue analogue (PBA) cathode material, can survive for over 40,000 deep discharge, high rate cycles.3 This cathode cycle life was an order of magnitude longer than incumbent batteries under consideration for renewables support. To achieve a significant lifetime advantage in a practical battery however, an anode having comparable cycle life was needed. In this presentation, we describe a new Na+cell containing PBA as both the cathode and the anode and a nonflammable aqueous-organic cosolvent electrolyte. By using a PBA as the active material for both electrodes, neither electrode limits the lifetime or kinetics of the cell. In this cell, a transition metal hexacyanoferrate cathode is paired with a new manganese(II) hexacyanomanganate(II/I) (MnHCMn) anode. Soft X-ray absorbance spectroscopy (sXAS) shows that the MnHCMn anode operates by a novel Mn(II/I) redox in which low-spin 3d6 MnI(CN)6 complexes are formed during charging (reduction). Pairing this anode with a hexacyanoferrate-based cathode results in a 1.6 V average cell voltage, which is nearly twice the 950 mV cell voltage of previously reported full PBA cells.4 Lab-scale full pouch cells containing have a hexacyanomanganate(II/I)-based anode and a hexacyanoferrate-based cathode have an average projected lifetime of over 30,000 cycles at 100% depth of discharge based on >2,000 cycles and six months of testing. This long cycle life is achieved because both of the electrodes cycle by near-zero strain single-phase insertion reactions. The addition of organic cosolvents to the aqueous electrolyte prevents dissolution of the PBAs while retaining the high Na+ conductivity and nonflammability of aqueous electrolytes. This results in an extension of the projected cell calendar life from just months to years. Finally, the rapid kinetics of the PBA electrodes and cosolvent electrolyte allow access to 80% of total cell energy above 1.15 V during 12C discharge. The long cycle life and high power capability of these cells make them particularly attractive for short-duration renewables support applications such as solar intermittency smoothing.5