Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin , Austin, Texas 78712, United States.
National Synchrotron Radiation Laboratory, University of Science and Technology of China , Hefei, Anhui 230026, China.
Nano Lett. 2017 Oct 11;17(10):6273-6279. doi: 10.1021/acs.nanolett.7b02958. Epub 2017 Sep 7.
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 (VOPO) 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 VOPO 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 VOPO host layers, density functional theory calculations are also performed to understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO nanosheets. Because of the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, intercalated VOPO 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 VOPO nanosheets. Our results may also shed light on possible multivalent-ion based energy storage such as Mg and Al.
二维（2D）能量材料在锂离子存储方面表现出了有前途的电化学特性。然而，在电极处理过程中，2D 材料的不可逆堆叠导致其比锂离子存储更具潜力的钠离子或钾离子存储的活性表面减少，电荷/质量传输缓慢，这仍然是一个重大挑战。在这里，我们开发了一种通用的层间工程策略，通过使用二维超薄钒磷酸氧(VOPO)纳米片作为具有挑战性的钠离子存储的模型材料，来解决上述挑战。通过控制有机分子（如三乙二醇和四氢呋喃）的嵌入，VOPO 纳米片中的钠离子传输得到了显著改善。除了包括 X 射线衍射、高分辨率透射电子显微镜和 X 射线吸收精细结构在内的先进表征来表征层间和有机嵌入剂与 VOPO 主体层之间的化学键合/结构外，还进行了密度泛函理论计算，以了解钠离子在纯 VOPO 纳米片和 TEG 嵌入 VOPO 纳米片中的扩散行为。由于层间间距的扩大以及钠离子扩散的能量势垒降低，嵌入 VOPO 纳米片显示出明显改善的钠离子传输动力学性能，大大提高了钠离子存储的倍率性能和循环稳定性。我们的结果提供了对层间工程策略的深入理解，以提高 VOPO 纳米片的钠离子存储性能。我们的结果还可能为基于多价离子的储能（如镁和铝）提供启示。
