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 (FeV₃O_8.7·nH₂O) for sodium-ion storage in nonaqueous electrolyte. Using an electrochemical quartz crystal microbalance, in situ Raman, and ex situ 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 FeV₃O_8.7·nH₂O 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^-1 at 108 W kg^-1 and 15.3 Wh kg^-1 at 8.3 kW kg^-1. Our results offer an emerging route for improving both specific and volumetric energy densities of electrochemical capacitors.