Interlayer Confined Water Enabled Pseudocapacitive Sodium-Ion Storage in Nonaqueous Electrolyte.

Electrochemical capacitors have faced the limitations of low energy density for decades, owing to the low capacity of electric double-layer capacitance (EDLC)-type positive electrodes. In this work, we reveal the functions of interlayer confined water in iron vanadate (FeVO·HO) for sodium-ion storage in nonaqueous electrolyte. Using an electrochemical quartz crystal microbalance, Raman, and X-ray diffraction and X-ray photoelectron spectroscopy, we demonstrate that both nonfaradaic (surficial EDLC) and faradaic (pseudocapacitance-dominated Na intercalation) processes are involved in the charge storages. The interlayer confined water is able to accelerate the fast Na intercalations and is highly stable (without the removal of water or co-intercalation of [Na-diglyme]) in the nonaqueous environment. Furthermore, coupling the pseudocapacitive FeVO·HO with EDLC-type activated carbon (FeVO-AC) as the positive electrode brings comprehensive enhancements, displaying the enlarged compaction density of ∼2 times, specific capacity of ∼1.5 times, and volumetric capacity of ∼3 times compared to the AC electrode. Furthermore, the as-assembled hybrid sodium-ion capacitor, consisting of an FeVO-AC positive electrode and a mesocarbon microbeads negative electrode, shows a high energy density of 108 Wh kg at 108 W kg and 15.3 Wh kg at 8.3 kW kg. Our results offer an emerging route for improving both specific and volumetric energy densities of electrochemical capacitors.