Effective Interlayer Engineering of Two-Dimensional VOPO₄ Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage

Abstract

Two-dimensional (2D) energy materials have shown the promising electrochemical characteristics for lithium ion storage. However, the decreased active surfaces and the sluggish charge/mass transport for beyond-lithium ion storage that has potential for large-scale energy storage systems, such as sodium or potassium ion storage, caused by the irreversible restacking of 2D materials during electrode processing remain a major challenge. Here we develop a general interlayer engineering strategy to address the above-mentioned challenges by using 2D ultrathin vanadyl phosphate (VOPO₄) nanosheets as a model material for challenging sodium ion storage. Via controlled intercalation of organic molecules, such as triethylene glycol and tetrahydrofuran, the sodium ion transport in VOPO₄ nanosheets has been significantly improved. In addition to advanced characterization including X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption fine structure to characterize the interlayer and the chemical bonding/configuration between the organic intercalants and the VOPO₄ host layers, density functional theory calculations are also performed to understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO₄ nanosheets. Because of the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, intercalated VOPO₄ nanosheets show much improved sodium ion transport kinetics and greatly enhanced rate capability and cycling stability for sodium ion storage. Our results afford deeper understanding of the interlayer-engineering strategy to improve the sodium ion storage performance of the VOPO₄ nanosheets. Our results may also shed light on possible multivalent-ion based energy storage such as Mg²⁺ and Al³⁺.