Synthesis Strategies for Highly Stable Sodium and Potassium Manganese Hexacyanoferrates
Authors: Boutelle Ethan , Bai Peng 
Published: August 2024

Sodium- and potassium-ion batteries (NIBs and KIBs) have been recognized as a promising alternative to lithium-ion batteries (LIBs), especially for stationary grid-scale applications that favor inexpensive storage over high energy density.1 NASICON-structured phosphates and alluaudite sulfates offer cheap synthesis routes but demonstrate low energy density2,3. Layered oxides can provide a higher energy density but require complex synthesis routes and degrade quickly4. Prussian Blue Analogues (PBAs) benefit from facile precipitation synthesis, flat voltage plateaus, and specific capacities greater than 150 mAh g-1.5 However, capacity fading due to Jahn-Teller distortion, manganese dissolution, and residual interstitial water limit their cycle life6,7. In this study, synthesis parameters and post-synthesis treatment of Na2Mn[Fe(CN)6] and K2Mn[Fe(CN)6] are discussed. Furthermore, the addition of solubilized conductive carbon additives to synthesis solutions, both to enhance electronic conductivity and provide a flexible framework to reduce strain during cycling, is explored. Na2Mn[Fe(CN)6] electrodes demonstrate a specific capacity of 149.6 mAh g-1 and surpass 250 cycles at 1C before dipping below 80% capacity retention. K2Mn[Fe(CN)6] electrodes demonstrate a specific capacity of 155.1 mAh g-1 with negligible capacity fading over the first 50 cycles at 0.1C. Our results indicate that lowering the temperature during synthesis improves morphology and cycling performance even when a chelating agent is used to control the reaction speed.

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