Lithium-chalcogen batteries—e.g., lithium-sulfur (Li-S) and lithium selenium (Li-Se) systems— are promising candidates for high energy electrical storage solution.

Earlier this year, a team of researchers at the General Motors Research and Development Center in Warren, MI, with colleagues at Optimal CAE and Pacific Northwest National Laboratory (PNNL), reported demonstrating a rationally designed hierarchical porous carbon (SPC) electrode architectures with maximum micro-, meso- and macro-level porosities as the conductive framework for the lithium-chalcogen batteries.

Cell level calculations suggests that the hierarchical electrode architectures have the potential to increase the specific energy to more than 350 Wh kg⁻¹—much higher than what can be achieved using the materials and parameters reported in the literature. A paper on their work is published in the journal Nano Energy.

Scheme of SPC synthesis route. Dai et al.

Lithium-chalcogen batteries, such as lithium-sulfur (Li-S) and lithium- selenium (Li-Se), have been recognized as promising systems beyond conventional Li-ion systems due to their higher specific capacity. Elemental sulfur (S) has a theoretical specific capacity of 1670 mAh/g, while Selenium (Se) has a theoretical specific capacity of 675 mAh/g. However, some major issues prohibit their practical applications.

For example, low electronic conductivity of S (5 × 10^-30 S/cm) and shuttling effect highly affect the electrochemical performance. Although the conductivity of Se (10^-5 S/cm) is several orders higher than S, a conductive backbone is still necessary to enable the redox process. Therefore multi-functional frameworks were adopted to provide reactive sites, electronic conducting channels, and polysulfide constraining reservoirs. A corresponding chalcogen cathode structure, which is composed of a chalcogen element (S or Se), a conductive framework, and a polymer binder, is well-accepted for most studies.

On the cell level, a fundamental challenge is the conflict exists between the overall electrochemical performance and the active material loading. It has now well recognized that excellent performance could be achieved with a low sulfur to carbon ratio and a low total S mass loading in the electrode. However, in order to achieve competitive energy density compared to current Li-ion batteries (i.e. > 3 mAh/cm²), a high total S loading and a high area capacity are required and critical for a Li-S system. Therefore certain parameters to meet the minimum requirements, such as 65 wt.% S content and 2 mg/cm² S mass loading, were recommended for future evaluations. Higher loadings (i.e. 7 mg S/cm²) was also suggested for specific practical applications, such as electrical vehicle applications. Unfortunately, achieving excellent electrochemical performance under such conditions is challenging when using conventional sulfur/carbon (S/C) and selenium/carbon (Se/C) composites.

… In this work, a hierarchical carbon (SPC) electrode architecture was developed for high loading Li-S and Li-Se batteries and control the electrochemical reactions on various levels.

—Dai et al.

The maximum total pore volume of the SPC reaches 4.67 cm³/g—among the highest reported value for porous carbon materials. The hierarchical pore distribution also helps to improve the utilization of S and Se, as well as providing long cycling stability.

Without any conducting additive in composite/electrode fabrication, or performance booster in electrolyte, or additional interlayer structure/membrane modification in the cell configuration, the Li-S and Li-Se cells show higher specific capacity and better cycling stability compared with results published in the literature involving high loading studies of either S or Se.

Resources

• Fang Dai, Jingmei Shen, Anne Dailly, Michael P. Balogh, Peng Lu, Li Yang, Jie Xiao, Jun Liu, Mei Cai (2018) “Hierarchical electrode architectures for high energy lithium-chalcogen rechargeable batteries,” Nano Energy, Volume 51, Pages 668-679 doi: 10.1016/j.nanoen.2018.07.015