Electrochemical Study on Additive Effect of Porous V2O5 to Sulfur/Carbon Composite Cathode

Abstract

The increase in the demand for clean and sustainable energy has prompted intense effort towards the realization of high-performance energy storage technologies. Among the many available energy storage technologies, lithium(Li)-ion batteries with Li-intercalation compounds as positive electrodes have achieved great success for use in portable electronic devices [1, 2]. A typical rechargeable Li-ion battery requires heavy host compounds (e.g., CoO2) to accommodate guest atoms (e.g., Li), which greatly limits its energy density to less than 210 Wh kg-1. On the other hand, a Li-sulfur (S) battery as a typical integration reaction battery system [4] may open a new horizon for electric vehicles. As the sulfur cathode has a theoretical specific capacity of 1672 mAh g-1, a Li-S battery can achieve an energy density of 2654 Wh kg-1 when combined with a lithium metal anode, which is 3-5-fold higher than those of current Li-ion batteries [5]. In addition, sulfur is a highly abundant element source on earth and is not geographically isolated [6], which may reduce the cost of cathode materials. However, Li-S batteries have serious problems [7-9], including low utilization of the active material and a poor cycle life because of the insulating property of sulfur and the high solubility of lithium polysulfides in organic electrolytes. To overcome the above-mentioned problems, much effort has been devoted to designing S-based cathodes, thus far. Various elegantly designed carbons have been studied for use as sulfur hosts to enhance the electronic conductivity of sulfur and trap polysulfides within a carbon framework simultaneously. However, carbon is an electrochemically inactive material in the integration reaction of Li-S batteries, and therefore its quantity in the electrode should be reduced as much as possible. Vanadium pentoxide (V2O5) is one of the most common cathode materials in the field of Li batteries and has a few advantages when used as an additive in sulfur cathodes. It is an electrochemically active material in the cut-off voltage range from 1.5 to 4 V, which closely matches that for sulfur cathodes (from 1.5 to 3 V), and it also positively contributes to the capacity of sulfur cathodes. In this study, S/Carbon/porousV2O5 nanocomposites were prepared by a novel synthesis route and their physical and electrochemical properties were further investigated. Porous V2O5 particles were firstly prepared by spray pyrolysis (SP) with NH4NO3 additive. The pore structure of obtained samples were analyzed by N2 adsorption-desorption isotherm measurements. The V2O5 particles with pore size less than 100 nm could be prepared at 500 ºC by SP with a 0.272 mol L-1 NH4NO3 additive, and the porous V2O5 electrode exhibited a first discharge capacity of 400 mAh g-1 at 20 mA g-1.The as-prepared porous V2O5 were packed with acetylene black and S at a targeted ration in a closed reactor. The reactor was heated at different temperatures ranging from 120 to 200 ºC. The effect of S content, heating temperature and heating time were investigated systematically. Figure 1 shows the cycle performance of the S/C/porous V2O5 composite electrode at a charge-discharge rate of 100 mA g-1. The cycle performance of the S/C composite electrode is also shown in the figure as a comparison. The S/C/porous V2O5 composite electrode exhibits a discharge capacity of 666 mAh g-1 at 50th cycles, while the S/C composite electrode shows a discharge capacity of 379 mAh g-1 at 50th cycles . As seen from this fact, the S/C/porous V2O5 composite electrode shows larger discharge capacities and better cycle performance than the S/C composite electrode. This fact indicate that the V2O5 additive to the S/C composite electrode leads to enhance its electrochemical performance.