Li-ion batteries are the electrochemical energy storage technology of choice of today’s electrical vehicles and grid applications with a growing interest for Na-ion and K-ion systems based on either aqueous or non-aqueous electrolyte for power, cost, and sustainable reasons. The rate capability of alkali-metal-ion batteries is influenced by ion transport properties in the bulk of the electrolyte, as well as by diverse effects occurring at the vicinity of the electrode and electrolyte interface. Therefore, identification of the predominant factor affecting the rate capability of electrodes still remains a challenge and requires suitable experimental and computational methods. Herein, we investigate the mechanistic of the K+ insertion process in the Prussian blue phase, in both aqueous and non-aqueous electrolytes, which reveals drastic differences. Through combined electrochemical characterizations, electrochemical-quartz-crystal-microbalance and ac-electrogravimetric analyses, we provide evidences that what matters the most for fast ion transport is the positioning of the partially solvated cations adsorbed at the material surface in aqueous as opposed to non-aqueous electrolytes. We rationalized such findings by molecular dynamics simulations that establish the K+ repartition profile within the electrochemical double layer. A similar trend was earlier reported by our group for the aqueous versus non-aqueous insertion of Li+ into LiFePO4. Such a study unveils the critical but overlooked role of the electrode–electrolyte interface in ruling ion transport and insertion processes. Tailoring this interface structuring via the proper salt–solvent interaction is the key to enabling the best power performances in alkali-metal-ion batteries.