Carboxylate-derived conductive, sodium-ion storable surface of Prussian Blue with a stable cathode-electrolyte interface

Considering the high binding affinity between carboxylate ligands (−O–CO) and alkaline Na ions, Prussian blue (PB) with a carboxylate-derived conductive, Na-ion storable surface is developed as a cathode material for Na-ion rechargeable batteries. Here, citric acid (CA) (C6H8O7) is introduced as a carboxylate source because, in a neutral aqueous solution, it loses two H ions of each molecule to reach an acid-base equilibrium and is stabilized in a chemical form with two −O–CO ligands that are accompanied by two delocalized electrons. Consequently, these functional groups newly formed from CA remain on the surfaces of PB particles even when PB is co-precipitated using CA and sodium citrate as dual chelating agents. Specifically, they strongly bind to high-spin Fe (FeHS) ions on the PB surface, providing additional active sites for Na-ion storage via a quasi-reversible, surface redox process (i.e., FeHS–R–C–O– ↔ FeHS–R–C–O–Na) in the low-voltage region close to 3.0 V (vs. Na+/Na). During charge/discharge cycling, inserted Na ions prefer to be stored in the activated surface sites instead of being used to produce unstable by-products by electrolyte decomposition, which resultantly inhibits the thick growth of cathode-electrolyte interface (CEI) layers on the PB particles. As a result, 2 wt% CA-assisted PB exhibits high first discharge capacity (∼110 mA h g−1) and low average capacity decay rate (−0.39 mA h g−1/cycle) at a current density of 0.2 C.