|Fig. 1: Sulfur recovered from hydrocarbons and stockpiled for shipment in North Vancouver, British Columbia. (Source: Wikimedia Commons)|
High-performance energy storage devices are crucial for meeting the ever-growing demand for high energy density applications such as portable electronics and electric-vehicles. Secondary lithium-ion batteries have raised intense attention due to the high energy density, high power density and excellent cycle behavior.  The key parts of a rechargeable battery are anodes and cathodes which could store energy inside and deliver it out. Research into anode materials in lithium-ion battery field has seen tremendous progress including the demonstration of nano-structured silicon, nano-structured phosphorus and lithium metal. [2-4] Although specific capacities of all these advanced anode materials can reach up to 3000-4000 mAh/g, the practical specific capacity for most commercial cathodes (transition metal oxides) is only limited to around 200 mAh/g. The huge capacity mismatch in the cathode/anode pair as well as relatively slow research progress on cathode materials become a big hurdle in fully exploiting the potential of lithium-ion batteries. Among emerging advanced cathode materials for lithium-ion system, sulfur cathodes could deliver a gravimetric specific capacity of as high as 1675 mAh/g when pairing with lithium anode which is dramatically higher than conventional cathode materials.  In addition, the natural abundance, low cost, and environmental friendliness further contribute to the potential of sulfur as ideal lithium-ion battery cathodes (Fig. 1).
Nevertheless, the practical application of lithium-ion batteries with sulfur cathodes has been hindered by a series of technical obstacles, including poor battery life, limited rate capability and low utilization of battery materials. These issues could be attributed to the insulating nature of sulfur (conductivity of both sulfur and its discharge product, lithium sulfide, can be as low as 5x10-30 S/cm at room temperature) as well as the notorious phenomenon called polysulfides dissolution and shuttle effect. According to the battery chemistry described in the literature, during discharge process, elemental sulfur transforms through a series of intermediate products known as lithium polysulfides, into solid final products (lithium sulfide).  In the subsequent charge process, these discharge products are converted back reversibly into elemental sulfur. Polysulfides possess a high solubility in ether-based electrolyte and tend to diffuse into the electrolyte. The dissolved lithium polysulfides migrate back and forth between battery electrodes continuously, with structures switching between higher-order long chain polysulfides and lower-order polysulfides.  This effect significantly reduces the cycling stability, active material utilization, electrode capacity and Coulombic efficiency of batteries.
Various strategies have been developed to address the above issues of lithium-sulfur batteries. Building a composite electrode (composed of sulfur and the framework) with sulfur as the main active material could be an effective solution. An ideal framework for composite sulfur electrode should be a conductive (ionically and electrically) matrix with the ability to effectively trap polysulfides. Carbon-based hosts, such as carbon nanotubes, mesoporous carbon, carbon fiber, and graphene, have been extensively studied for sulfur cathodes.  These sulfur/carbon composites have been shown to have good physical confinement of sulfur as well as polysulfides. Another rational design consists of hindering polysulfides dissolution by chemical adsorption through the employment of conductive polymer or metal oxide additives.  These advanced composite electrodes with sulfur as the main active material exhibit improved electrochemical performances and open up a new avenue for lithium-ion batteries.
© Zheng Liang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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