Selenium-Carbon Nanotubes Composite Cathode with Long Cycle Life for Rechargeable Li-Se Batteries
Authors: Cui Yi , Zhou Xinwei , Guo Wei , Liu Yuzi , Li Tianyi , Fu Yongzhu , Zhu Likun 
Published: February 2020

In the past two decades, lithium-ion batteries (LIBs) have been extensively utilized in portable electronic devices as well as electric vehicles (EVs). However, conventional LIBs depending on the lithium intercalation reactions of anode and transitional metal oxide cathode materials have limited capacities and low energy densities, which prevent their broader deployment as power sources for prolonged usage. High energy density and durable electrical energy storage devices are the key factors to meet the crucial demands for high-performance portable electronic devices and EVs. Selenium (Se), as a congener of sulfur in the periodic table, has been proposed as an alternative cathode material for rechargeable lithium batteries. Although Se has lower theoretical capacity (678 mAh g-1) than sulfur, it can provide a high volumetric capacity of 3253 mAh cm-3 because of its high mass density (4.8 g cm-3). In addition, selenium has a much higher electronic conductivity (1×10-3 S m-1), which implies that Se could provide better electrochemical activity. However, Se cathode is still facing similar challenges as sulfur such as poor cycling life and low Coulombic efficiency, which are attributed to the dissolution and shuttle effect of the formed high-order polyselenides as redox intermediates in the ether-based electrolyte. In addition, the significant volume change increases the difficulty for Li-Se batteries to be used in practical applications. During the last decade, lots of effort have been put into developing approaches to address these problems, thereby increasing energy density and cycle life of Li-Se batteries.
In this contribution, we synthesized Se nanowires in the presence of carbon nanotubes (CNTs) by a facile method. The experimental process for preparing the Se/CNT composite consists of dissolving selenium oxide (SeO2) and beta-cyclodextrin (β-cyclodextrin) and dispersing the CNT bundles in an ethanolic mixture with de-ionized water. The vigorously stirring and ultrasonication procedure in the initial mixture cause fine dispersion of CNTs. In addition, the ethanol extenuates the hydrophobic activity of CNTs, and hence accelerates the dispersion-weave process by ultrasonication. The addition of ascorbic acid solution ensures the product of selenium nanowires. Upon another ultrasonication for selenium nanowires and CNTs to be perfectly woven, and then filtered and dried, the Se/CNT composite electrode was obtained. The filter paper has a diameter of 7 cm, which decides the overall size of the composite electrode.
Se nanowires are weaved with CNTs to form a uniform binder-free composite electrode, which has superior electrical conductivity. This synthesis method provides a path for fabricating the Se cathodes with controllable mass loadings and thicknesses. At initial state, Se is in the form of branched nanowires with a diameter of <150 nm and length of 1-2 µm, interweaving with CNTs. By studying the composites electrodes with different mass loadings of active materials and different thicknesses, it is found that the electrode thickness has influence on the accumulation of redox products upon repeated cycling. The structural and morphological changes are analyzed by scanning electron microscope (SEM). Finally, the electrochemical phenomena are revealed experimentally, and enhanced cycling performance is achieved. The composite electrode with 23 µm thickness and 60% Se loading shows a high initial capacity of 537 mAh g-1, followed by stable cycling performance with a capacity of 401 mAh g-1 after 500 cycles at 1C rate. This study reports a synthesis strategy to obtain Se/CNT composite cathode with long cycle life for rechargeable Li-Se batteries.

Figure 1