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.
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.
Introduction
Faced with the reality of limited fossil fuel resourcesand their environmental consequences, use of lithium-ionbatteries as the onboard energy storage for vehicles isdrawing much attention. However, the high cost, safetyconcerns, and limited energy density associated with thecurrently available insertion-compound electrodes remainto be an impediment to adopt them. Development ofalternative low-cost cathode materials with high energyand power densities is critical for successfully utilizingthe lithium-ion technology for vehicle applications.Rechargeable lithium-sulfur batteries are attractive inthis regard as sulfur is abundant and exhibits an order ofmagnitude higher theoretical capacity (1675 mAh g-1)compared to the currently used cathodes. However, pooractive material utilization, rate capability, and cyclabilityresulting from the high electrical resistivity of sulfur anddissolution of intermediate polysulfides (Li2Sx with 2 < x≤ 8) into the electrolyte during cycling remain to be majorobstacles to employ the Li-S batteries. With thisperspective, we present here a self-weaving sulfur/multiwallcarbon nanotube (MWCNT) composite cathodeswith unique structural/morphological features andoutstanding performance advantages for high-raterechargeable lithium-sulfur batteries.
