Encapsulating Sulfur into Magnéli Phase Ti4O7 Nanotube Array for Lithium Sulfur Battery Cathode
Authors: WU Hsiwen , HU Xijun , Chen Guohua 
Published: February 2020

Lithium sulfur (Li-S) battery is considered the next generation lithium ion batteries for energy storage. However, several issues have to be addressed before the full potential of Li-S battery can be realized. The intrinsically low electronic conductivity of sulfur renders incomplete material utilization; the dissolution of charge/discharge intermediates, lithium polysulfides, results in the shuttle phenomenon that leads to low a coulombic efficiency and fast capacity fading upon cycling; the 80% volume expansion of sulfur upon full lithiation changes the electrode structure and shortens battery cycle life. Magnéli phase Ti4O7 is known for its high electronic conductivity, which is as high as that of metal, and the polar nature of the compound allows strong chemisorption of lithium polysulfides onto Ti4O7 surface. In this study, Magnéli phase Ti4O7 nanotube array (Ti4O7 NTA) grown on a titanium nitride mesh is synthesized via anodization of titanium mesh followed by high-temperature reduction under a hydrogen atmosphere. The effect of the duration of the reduction reaction is investigated, and from X-ray diffraction patterns, a 45-minute reaction time generates a product that most closely resembles pure Magnéli phase Ti4O7. Sulfur is introduced inside the Ti4O7 NTA via electrodeposition followed by melt-infusion under vacuum. The electrodeposition of sulfur onto the free-standing Ti4O7 NTA electrode allows a precise control of the areal loading of sulfur, simply via adjusting the duration of electrodeposition under a constant current density of 20mA/cm2. A conductive carbon coating is applied onto the sulfur-incorporated Ti4O7 NTA mesh to further confine sulfur inside the Ti4O7 nanotubes. This produces a composite material of C-S-Ti4O7 The Ti4O7 NTA provides sulfur with a large electronically conductive and strong polysulfide-absorbing surface that facilitates the redox chemistry of sulfur. The NTA structure promises ample access of electrolyte to sulfur, and allows a high areal sulfur loading in the cathode. Excellent battery performance is obtained using this composite as the cathode material.
Under a moderate sulfur loading in the range of 1.3~2.8mg/cm2, a high initial specific capacity (1604mAh/g at 0.05C), ultra-low capacity decay rates (0.0322% per cycle, Figure (a)) for 1800 cycles, and good rate capability (660mAh/g at 2C and 500mAh/g and 4C, Figure (b)) are achieved. Under a higher sulfur loading in the range of 3.5~4.8mg/cm2, stable cycling (capacity decay rates below 0.10% per cycle, Figure (c)) with very high values areal capacity (4.97mAh/cm2 at 0.05C, 3.82mAh/cm2 0.1C, and 2.74 mAh/cm2 at 0.2C) are obtained, making a high-volumetric-energy-density Li-S battery more practical. The low capacity decay rate and the high areal capacity in this study are among the best reported polar sulfur hosts such as graphitic carbon nitride C3N4, α-MnO2, MXene phase Ti2C, Co9S8, and Magnéli phase Ti4O7 nanoparticles. The interaction between Magnéli phase Ti4O7 and lithium polysulfides is investigated via X-ray photoelectron spectroscopy. It is discovered that the interaction is of a redox nature, where titanium atoms on Ti4O7 NTA are partially reduced while sulfur atoms in lithium polysulfides are partially oxidized with peaks corresponding to sulfate and sulfite becoming more dominant when lithium polysulfides are in contact with Ti4O7 NTA. The positive effects of Ti4O7 NTA and those of carbon coating are studied via cyclic voltammetry and electrochemical impedance spectroscopy. It is found that the Ti4O7 NTA decreases the charge transfer resistance of the Li-S cell, and that the carbon coating produces a stable cycling performance through gradually decreasing polarization as reflected by a lowering of overpotential during charge and discharge.

Figure 1