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

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Electrochemical capacitors (ECs) are limited by low energy density, primarily due to the low capacity of electric double-layer capacitance (EDLC)-type positive electrodes.
• Iron vanadate (FeV₃O_8.7·nH₂O) is explored as a novel material for sodium-ion storage.

Purpose of the Study:

• To investigate the role of interlayer confined water in iron vanadate for sodium-ion storage in nonaqueous electrolytes.
• To enhance the energy density and capacity of electrochemical capacitors using a hybrid electrode design.

Main Methods:

• Electrochemical quartz crystal microbalance (EQCM)
• In situ Raman spectroscopy
• Ex situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)
• Fabrication of hybrid electrodes (FeVO-AC) and sodium-ion capacitors

Main Results:

• Demonstrated both nonfaradaic (EDLC) and faradaic (Na⁺ intercalation pseudocapacitance) charge storage mechanisms in FeV₃O_8.7·nH₂O.
• Confirmed that interlayer confined water accelerates Na⁺ intercalation and remains stable in nonaqueous electrolytes.
• Hybrid FeVO-AC positive electrodes showed significant improvements: ~2x compaction density, ~1.5x specific capacity, and ~3x volumetric capacity compared to AC electrodes.
• Assembled sodium-ion capacitor achieved high energy densities (108 Wh kg^-1 at 108 W kg^-1 and 15.3 Wh kg^-1 at 8.3 kW kg^-1).

Conclusions:

• Interlayer confined water plays a crucial role in enhancing sodium-ion intercalation kinetics and stability in iron vanadate.
• The hybrid FeVO-AC electrode design offers a promising strategy to significantly boost both specific and volumetric energy densities of electrochemical capacitors.
• This study presents a novel pathway for developing high-performance sodium-ion capacitors for advanced energy storage applications.