No abstract

Electrical energy storage is one of the most critical needs of 21st century society. Applications that depend on electrical energy storage include portable electronics, electric vehicles, and devices for renewable energy storage from solar and wind. Lithium-ion (Li-ion) batteries have the highest energy density among the rechargeable battery chemistries. As a result, Li-ion batteries have proven successful in the portable electronics market and will play a significant role in large-scale energy storage. Over the past two decades, Li-ion batteries based on insertion cathodes have reached a cathode capacity of ∼250 mA h g(-1) and an energy density of ∼800 W h kg(-1), which do not meet the requirement of ∼500 km between charges for all-electric vehicles. With a goal of increasing energy density, researchers are pursuing alternative cathode materials such as sulfur and O2 that can offer capacities that exceed those of conventional insertion cathodes, such as LiCoO2 and LiMn2O4, by an order of magnitude (>1500 mA h g(-1)). Sulfur, one of the most abundant elements on earth, is an electrochemically active material that can accept up to two electrons per atom at ∼2.1 V vs Li/Li(+). As a result, sulfur cathode materials have a high theoretical capacity of 1675 mA h g(-1), and lithium-sulfur (Li-S) batteries have a theoretical energy density of ∼2600 W h kg(-1). Unlike conventional insertion cathode materials, sulfur undergoes a series of compositional and structural changes during cycling, which involve soluble polysulfides and insoluble sulfides. As a result, researchers have struggled with the maintenance of a stable electrode structure, full utilization of the active material, and sufficient cycle life with good system efficiency. Although researchers have made significant progress on rechargeable Li-S batteries in the last decade, these cycle life and efficiency problems prevent their use in commercial cells. To overcome these persistent problems, researchers will need new sulfur composite cathodes with favorable properties and performance and new Li-S cell configurations. In this Account, we first focus on the development of novel composite cathode materials including sulfur-carbon and sulfur-polymer composites, describing the design principles, structure and properties, and electrochemical performances of these new materials. We then cover new cell configurations with carbon interlayers and Li/dissolved polysulfide cells, emphasizing the potential of these approaches to advance capacity retention and system efficiency. Finally, we provide a brief survey of efficient electrolytes. The Account summarizes improvements that could bring Li-S technology closer to mass commercialization.

The limitations in the cathode capacity compared with that of the anode have been an impediment to advance the lithium-ion battery technology. The lithium-sulphur system is appealing in this regard, as sulphur exhibits an order of magnitude higher capacity than the currently used cathodes. However, low active material utilization and poor cycle life hinder the practicality of lithium-sulphur batteries. Here we report a simple adjustment to the traditional lithium-sulphur battery configuration to achieve high capacity with a long cycle life and rapid charge rate. With a bifunctional microporous carbon paper between the cathode and separator, we observe a significant improvement not only in the active material utilization but also in capacity retention, without involving complex synthesis or surface modification. The insertion of a microporous carbon interlayer decreases the internal charge transfer resistance and localizes the soluble polysulphide species, facilitating a commercially feasible means of fabricating the lithium-sulphur batteries.

A conductive multiwalled carbon nanotube (MWCNT) interlayer acting as a pseudo-upper current collector not only reduces the charge transfer resistance of sulfur cathodes significantly, but also localizes and retains the dissolved active material during cycling.

The success of rechargeable lithium-ion batteries has brought indisputable convenience to human society for the past two decades. However, unlike commercialized intercalation cathodes, high-energy-density sulphur cathodes are still in the stage of research because of the unsatisfactory capacity retention and long-term cyclability. The capacity degradation over extended cycles originates from the soluble polysulphides gradually diffusing out of the cathode region. Here we report an applicable way to recharge lithium-sulphur cells by a simple charge operation control that offers tremendous improvement with various lithiumsulphur battery systems. Adjusting the charging condition leads to long cycle life (over 500 cycles) with excellent capacity retention (499%) by inhibiting electrochemical reactions along with severe polysulphide dissolution. This charging strategy and understanding of the reactions in different discharge steps will advance progress in the development of lithiumsulphur batteries.