Switching Optimally Balanced Fe–N Interaction Enables Extremely Stable Energy Storage

The interaction between electrode materials and charge carriers is one of the central issues dominating underlying energy storage mechanisms. To address the notoriously significant volume changes accompanying intercalation or formation of alloy/compounds, we aim to introduce and utilize a weak, reversible Fe–N interaction during the (de)intercalation of ammonium ions (NH₄⁺) within iron(Ⅲ) hexacyanoferrate (FeHCF), inspired by manipulating the electrostatic adsorption between N and Fe in the early stages of ammonia synthesis (Bosch-Harber Process, Chemical Engineering) and steel nitriding processes (Metal Industry). Such strategy of switching well-balanced Fe–N interaction is confirmed in between the nitrogen of ammonium ions and high-spin Fe in FeHCF, as observed by using X-ray absorption spectroscopy. The resulting material provided an extremely stable energy storage (58 mAh g⁻¹ after 10000 cycles at current density of 1 A g⁻¹) as well as high-rate performance (23.6 mAh g⁻¹ at current density of 10 A g⁻¹).