Post lithium-ion batteries: lithium-sulfur batteries

Probing the discharge and charge mechanism of lithium-sulfur batteries

The ever-rising demand for portable power sources has prompted a renewed interest in next-generation energy storage systems, including batteries, fuel cells and electrochemical capacitors. The applications of these energy storage devices range from consumer electronics to nextgeneration electric vehicles. Among the various energy storage technologies, batteries are by far the most successful products in the consumer market. Although Li-ion batteries (theoretical specific energy of C/LiCoO2 is ~387 Wh kg-1) still dominate the portable electronic device market, their capacity cannot reach the requirements of batteries for electric vehicles. Next-generation Li batteries, such as Li-S batteries, have attracted attention recently, mainly due to their low cost and high theoretical specific energy of ~2567 Wh kg-1. However, many problems associated with the battery performance, such as dendrite formation at the Li anode and dissolution of intermediates from the S/C cathode, still limit the practical application of Li-S batteries.

   To overcome these technical challenges regarding Li-S batteries, a detailed understanding of the discharge/charge mechanism is needed, as all the electrochemical reactions occurring on the cathode and anode are based on the fundamentals of electrochemical thermodynamics and kinetics. Hence, we developed operando Raman/FT-IR/UV-Vis spectroscopy to examine the electrochemical reactions in the electrolyte and at the electrolyte-electrode interface under operating conditions together with real-time analysis of the reaction products. During the discharge process, in situ Raman spectroscopy obtained from the sulfur-carbon cathode showed that long-chain polysulfides (S82–) were formed via S8 ring opening in the first reduction process and that short-chain polysulfides, such as S42–, S4, S3•–, and S2O42–, were observed with continued discharge in the second reduction process. All the formations of these intermediate species are reversible during the charge process.

Figure. 1 In situ Raman spectra of the sulfur-carbon cathode shown at representative potentials during discharge from 3.2 to 1.5 V and after recharging to 3.2 V

These operando techniques enable us to study the formation of intermediate species, the formation/decomposition rate (kinetics) and the interaction between intermediate species and additives during cycling. The decomposition of the electrolyte on anode and cathode surfaces was observed using operando FT-IR spectroscopy. Performing operando Raman/FT-IR/UV-Vis spectroscopy with different spectral regions provides complementary information for the mechanism on the molecular level. Based on these findings, novel materials and electrolytes can be designed to achieve better performance.

Figure 2. Illustration of the operando Raman/FT-IR/UV-Vis measurements

Heng-Liang Wu, Minjeong Shin, Yao-Min Liu, Kimberly A. See, and Andrew A. Gewirth. (2017). Thiol-based electrolyte additives for high-performance lithium-sulfur batteries. Nano Energy, 32, 50-58.

Dr. Heng-Liang Wu
Assistant Research Fellow
Center for Condensed Matter Sciences