Interface‐Driven Electrochemical K ⁺ Ion Storage in Bi ₅ Nb ₃ O ₁₅

Understanding the synergy of different fundamental components of a rechargeable aqueous battery is quite pivotal for making technological advances. Herein, the aqueous electrolyte‐enabled electrochemical K ⁺ ion storage in Bi ₅ Nb ₃ O ₁₅ is illustrated for the first time. While achieving a satisfactory electrochemical cycling stability remains a bottleneck, it is shown that a favorable electrochemical interface between the electrode material and current collector can alleviate this issue. The crucial role of this active‐inactive interface is unveiled using electrochemical impedance spectroscopy, electron microscopy, corrosion, and contact angle measurements. It reveals that the interface resistance plays a significant role in the achievement of long‐term cycling stability. Graphitic substrate shows least corrosion rate among the others, whereas it has the highest polarization resistance. A stable capacity above 100 mAh g ⁻¹ is achieved with the graphite current collector, whereas the other substrates, such as Ti, Ni, and stainless steel, are not beneficial. The wettability of the interface is also found to be crucial in the ion insertion phenomena. This work definitely serves as a pertinent guide to wring out the best of an electrode material in attaining high energy K ⁺ ‐ion aqueous batteries.