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Development of Advanced Li-S Batteries

Lithium–sulfur (Li-S) batteries have long been regarded as one of the most promising battery technologies that are attractive for various applications from portable electronics to electric drives (EV/HEV/PHEVs). As one of the most abundant elements on earth, sulfur is an attractive cathode which offers the highest theoretical capacity of ~1675 mAh/g, among available solid cathode candidates. This capacity is almost an order of magnitude higher than that of conventional insertion compound cathodes, such as LiCoO2 (~135 mAh/g) and LiFePO4 (~170 mAh/g). As a result, Li-S batteries have the potential to provide a high energy density up to ~ 400 - 600 Wh/kg, which is two or three times higher than that of state-of-the-art Li-ion batteries. In addition, several other benefits can be obtained based on S cathodes, such as (1) low-cost S replacing expensive transition metals like Co, (2) low operating voltages (~2.15V vs. Li/Li+) which provides enhanced safety, and (3) environmentally benign capability of S, when compared to certain toxic transition-metal compounds. Therefore, Li-S batteries with low-costs and high energy density are appealing not only to EVs but also for grid energy storage for renewable energy applications (such as solar and wind), if long cycle life and high system efficiency can be achieved.

Currently available Li-S batteries based on liquid electrolyte suffer from columbic inefficiencies and rapid electrode degradation, which can be attributed to the dissolution and migration of polysulfides in the electrolyte, or the so-called polysulfide shuttle phenomenon. Additionally, safety issues attributed to dentritic growth resulting from the cycling of Li metal anodes is also a concern. Solid-state electrolytes offer a promising solution to these issues because they can provide a typically impermeable electrolyte layer that prevents polysulfide migration and lithium dendrite penetration. In addition, improved electrochemical, mechanical and thermal stability can also be achieved using these electrolytes.

Bioenno Power has conducted extensive leading research on a new-generation of Li-S battery technologies based on advanced nanostructured cathodes and solid-state electrolyte materials. This R&D effort will eventually lead to the production of novel Li-S cells and the resultant battery modules/packs with high energy/power density, long service/cycle life, excellent safety, high reliability, and low costs. For cathode development, nanostructured S/graphene cathode composites are being developed, with an aim of combining several attractive advantages of the two components (e.g., high capacity of S and high electric conductivity and high specific surface area of graphene), thereby achieving high capacity > 800 mAh/g, along with enhanced cyclability (>200 cycles). In addition, another type of composite cathode based on Li2S is also being investigated. For solid-state electrolytes, a ceramic electrolyte based on cubic garnet structured Li7La3Zr2O12 (LLZO) with carefully selected substitution-doped elements is being developed. Using these novel dopants, Li-ion conductivity of LLZO is expected to be enhanced up to ~1×10-3 S/cm, attractive for practical solid-state Li-ion/Li-S battery applications. In the meantime, a nanocomposite solid-state electrolyte material based on LLZO and a novel sulfide compound electrolyte is also being developed, based on a complementary effect between these two promising electrolyte components.