Na2-xMn[Fe(CN)6] Prussian blue analog cathodes for Na-ion batteries - From fundamentals to practical demonstration

Prussian blue analogues (PBAs) hold significant promise as potential cathode materials for sodium-ion batteries (SIBs) due to their various merits, such as large interstitial voids enabling efficient diffusive pathways, high theoretical capacity, ease of synthesis and lower cost. However, the structural water unavoidably generated during the synthesis significantly impacts the practical applications of PBAs. While it provides structural support, it can also undergo side reactions with sodium, compromising the stability and overall performance. To address this, we here in focus on the specific role of interstitial structural water in Na2-xMn[Fe(CN)6]1-y center dot square y center dot nH2O analogue, leading to the formation of hydrated H-NaMnHCF and dehydrated D-NaMnHCF. This allows us to elucidate the impact of interstitial structural water on the charge storage mechanisms in a comparative manner, using a combination of ex-situ and in-situ tools, including solid-state NMR, electrochemical quartz crystal microbalance (EQCM), and IR fiber-optic evanescent-wave-spectroscopy (IR-FOEWS). From this gained knowledge, we elaborated a processing protocol enabling the straightforward assembly of NaMnHCF 18650 cells using hard carbon (HC) anodes, demonstrating capacities of 548 mAh and high-rate capabilities (72 % of initial capacity at 10C). We believe that this contribution is of special interest to accelerate the commercial development of NaMnHCF PBA-based SIBs.