Electrochemically Formed Na_xMn[Mn(CN)₆] Thin Film Anodes Demonstrate Sodium Intercalation and Deintercalation at Extremely Negative Electrode Potentials in Aqueous Media

The development of electrode materials for Na-ion batteries has been substantially accelerated recently with respect to application in grid energy storage systems. Specifically, development of Na-ion batteries operating in aqueous media is considered more promising for this application due to safety issues. Many different types of cathode materials for aqueous Na-ion batteries have been proposed; however, the number and performance of contemporary anode materials are still insufficient for practical deployment. In this work, we demonstrate that electrochemically deposited Na_xMn[Mn(CN)₆] thin films are very promising anode materials for aqueous Na-ion batteries. Na_xMn[Mn(CN)₆] films exhibit (i) very low half-charge potential ca. -0.73 V vs SHE (ca. -0.93 V vs SSC) being one of the lowest among those reported in the literature for the electrode materials, which also inhibit hydrogen evolution reaction; (ii) a specific capacity of ca. 85 mA h g^-1 and (iii) only ∼3% loss of capacity and high round-trip efficiency (99.6%) after 3,000 cycles. Surprisingly, the choice of the electrolyte composition has a very strong influence not only on the intercalation process but also on the long-term performance of battery anodes and their electrode surface morphology.