Synthesis Strategies for Highly Stable Sodium and Potassium Manganese Hexacyanoferrates

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. References (1) Liu, J. Addressing the Grand Challenges in Energy Storage. Advanced Functional Materials. February 25, 2013, pp 924–928. https://doi.org/10.1002/adfm.201203058. (2) Zhang, X.; Rui, X.; Chen, D.; Tan, H.; Yang, D.; Huang, S.; Yu, Y. Na 3 V 2 (PO 4 ) 3 : An Advanced Cathode for Sodium-Ion Batteries. Nanoscale. Royal Society of Chemistry February 14, 2019, pp 2556–2576. https://doi.org/10.1039/c8nr09391a. (3) Niu, Y.; Zhao, Y.; Xu, M. Manganese‐based Polyanionic Cathodes for Sodium‐ion Batteries. Carbon Neutralization 2023, 2 (2), 150–168. https://doi.org/10.1002/cnl2.48. (4) Xiao, J.; Li, X.; Tang, K.; Wang, D.; Long, M.; Gao, H.; Chen, W.; Liu, C.; Liu, H.; Wang, G. Recent Progress of Emerging Cathode Materials for Sodium Ion Batteries. Materials Chemistry Frontiers. Royal Society of Chemistry May 21, 2021, pp 3735–3764. https://doi.org/10.1039/d1qm00179e. (5) Wang, Q.; Li, J.; Jin, H.; Xin, S.; Gao, H. Prussian-Blue Materials: Revealing New Opportunities for Rechargeable Batteries. InfoMat. John Wiley and Sons Inc June 1, 2022. https://doi.org/10.1002/inf2.12311. (6) Shang, Y.; Li, X.; Song, J.; Huang, S.; Yang, Z.; Xu, Z. J.; Yang, H. Y. Unconventional Mn Vacancies in Mn–Fe Prussian Blue Analogs: Suppressing Jahn-Teller Distortion for Ultrastable Sodium Storage. Chem 2020, 6 (7), 1804–1818. https://doi.org/10.1016/j.chempr.2020.05.004. (7) Ge, J.; Fan, L.; Rao, A. M.; Zhou, J.; Lu, B. Surface-Substituted Prussian Blue Analogue Cathode for Sustainable Potassium-Ion Batteries. Nat Sustain 2022, 5 (3), 225–234. https://doi.org/10.1038/s41893-021-00810-7.