In a paper published in the ACS journal Nano Letters, the team suggests that the new molecular-level insights gained about transient species and the associated anode failure mechanism are crucial to developing effective strategies to accelerate the development of Li–S battery technologies.

Compared with conventional Li-ion batteries (LIBs), Li−S chemistry has the attractive potential to afford transformational improvements in specific energy and cost reductions for next-generation energy storage systems. Theoretically, the energy conversion of sulfur is based on the electrochemical reactions with lithium resulting in phase transitions between S₈ and Li₂S (corresponding to a specific energy of 2500 Wh kg⁻¹). The formation of soluble lithium polysulfides as intermediates in the Li−S chemistry is a critical factor in harnessing the high energy stored in the system, although the dissolved species are also an initiator of cell failure.

… To date, there have been no reports of in situ techniques that can simultaneously identify the different reaction species and provide quantitative information about how these species change with time during discharge/charge processes in a functioning Li−S battery. … Although some of these soluble species redeposit on the sulfur cathode at 2.1 V, a significant portion of the dissolved polysulfides diffuse gradually to the Li metal anode, constituting the well documented “shuttle process,” which contributes to an internal leakage current and lowers the cell efficiency. It is also possible for the polysulfides to irreversibly accumulate on other cell components such as the separator in the form of various S-containing products. The collective effects lead to a gradual loss of active sulfur from the cathode and increased cell impedance, both of which are detrimental to the performance and cycle life of Li−S batteries.

… Here, we take advantage of the fact that Li⁺ cations ubiquitously exist in the cathode, electrolyte, and anode to quantitatively monitor the electrochemical and chemical reactions incurring in the whole Li−S system by using an in situ NMR technique with an in-house cylindrical micro-battery. The transient chemical environments around the Li⁺ cations in the discharge/charge process of the polysulfides are captured, which provides previously unattainable information and complements ex situ NMR observations.

—Xiao et al.

The in situ ⁷Li single pulse (SP) NMR experiments were performed on a Varian-Agilent 300 MHz NMR spectrometer, corresponding to ⁷Li Larmor frequencies of 116.58 MHz. A specially designed in situ NMR battery capsule case made of PCTFE (polychlorotrifluoroethylene) held the cylindrical Li−S battery. All of the cells were tested at a C/5 rate (1C = 1680 mA g⁻¹) between 1.0 and 3.0 V at room temperature.

The in situ experiments not only provided detailed information about the reaction sequences but also showed reaction pathways composed of mixed species rather than discrete step-by-step reactions. The technique revealed that the polysulfide redox reactions involve charged free radicals as intermediate species that are difficult to detect in ex situ NMR studies.

Resources

• Jie Xiao, Jian Zhi Hu, Honghao Chen, M. Vijayakumar, Jianming Zheng, Huilin Pan, Eric D. Walter, Mary Hu, Xuchu Deng, Ju Feng, Bor Yann Liaw, Meng Gu, Zhiqun Daniel Deng, Dongping Lu, Suochang Xu, Chongmin Wang, and Jun Liu (2015) “Following the Transient Reactions in Lithium–Sulfur Batteries Using an In Situ Nuclear Magnetic Resonance Technique” Nano Letters doi: 10.1021/acs.nanolett.5b00521