Li ion batteries have transformed portable electronics and will play a key role in the electrification of transport. However, the highest energy storage possible for Li-ion batteries is insufficient for the long-term needs of society, for example, extended-range electric vehicles. To go beyond the horizon of Li-ion batteries is a formidable challenge; there are few options and one of them is Li-S batteries.
Under intense examination for well over two decades, the cell in its simplest configuration consists of sulphur as the positive electrode and lithium as the negative electrode. Li-S cells operates on a principle very different from the Li ion battery described above. The redox couple, described by the reversible reaction lies near 2.2 V with respect to Li+/Li0, a potential about 2/3 of that exhibited by conventional positive electrodes.
S8 + 16Li = 8Li2S
However, this is compensated by the very high theoretical capacity afforded by the non-topotactic `assimilation’ process, of 1675mAh g-1. Thus, compared with intercalation batteries, Li-S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Values can approach 2500Wh kg-1 or 2800Wh l-1 on a weight or volume basis respectively, assuming complete reaction to Li2S. Despite its considerable advantages, the Li-S cell is plagued with problems that have slowed down its widespread practical realization.
One of the problems in Li-S batteries is sulfur´s low electrical conductivity. Another problem is the fast that the polysulfides which are generated at the cathode during chargin are soluble into most of the utilized electrolytes and thus they migrate to the anode where they react with lithium electrode to generate to form lower-order polysulfides which then are transported back to the sulfur cathode and regenerate the higher form of polysulfides. Such a
polysulfide “shuttle” process decrease the uitilization of the overall active material mass during discharge, triggers current leakage, poor cyleability and reduced columbic efficiency of the battery.
Some significant progress in overcoming these two very important challenges associated with Li-S batteries commence after the pioneer work of Nazar at al. They have demonstrated for the first time that those cathodes based on nanostructured sulphur/mesoporous carbon materials can overcome these challenges to a large degree, and the Li-S cell can exhibit stable, high, reversible capacities (up to 1320mAh g-1) with good rate properties and cycling efficiency. The proof-of-concept studies are based on CMK-3, the most well-known member of the mesoporous carbon family obtained from replicating SBA 15 silica.
The carbon framework not only acts as an electronic conduit to the active mass encapsulated within, but also serves as a minielectrochemical reaction chamber. The entrapment ensures that a more complete redox process takes place, and results in enhanced utilization of the active sulphur material. This is vital to the success of all conversion reactions to ensure full reversibility of the back-reaction. Following this report, the topic related to the development of novel cathodes for Li-S cells based on nanostructured carbon/sulfur composites flourished and more and more interest is dedicated to improving the performance of Li-S cells.
In our group we investigate the capability of various sustainable nanostructured carbon materials with various functionalities to act as efficient encapsulating materials for the polysulfides resulting in Li-S batteries with the final aim of improving the cycle performance of Li-S batteries and maintaining a high energy density.
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