The researchers note that their current MLD alucone-coated C−S electrodes in carbonate-based electrolyte still present a number of challenges, including unsatisfactory cycle performance at room temperature. These issues are related to the limited conductivity of the MLD coating; the nanostructure of the carbon host; and the components of carbonate based solvents. Future work is aimed at addressing these issues.

Lithium-sulfur batteries are considered as highly promising candidate applied for EVs due to their high specific energy. However, a long-standing—and ignored—challenge is the safety hazard that arises when Li-S batteries operate at elevated temperature—critical in EV applications.

Scheme of MLD alucone-coated C-S electrode (left) and the cycle performance of stabilized high temperature Li-S batteries (right). Credit: ACS, Li et al. Click to enlarge.

State-of-art ether based electrolytes for Li-S batteries suffer from low boiling and flash points, and therefore pose significant safety risks for operation at elevated temperatures. In addition, the commonly used LiNO₃ additive is an oxidizing agent and provides further safety concerns. Moreover, high temperatures also promote lithium polysulfide dissolution into the electrolyte, resulting in poor cycle life. These safety concerns have considerably restricted the potential application of Li-S batteries in EVs with the use of ether based electrolyte and may involve the re-designation of sulfur cathodes in practical applications.

One possible solution in addressing these temperature issues for Li−S batteries is revisiting the use of traditional carbonate based Li-ion electrolytes, which have been developed and adopted for lithium-ion batteries (LIBs) over three deca- des. Unfortunately, attempts in employing carbonate based electrolytes for Li−S batteries were rarely [successful] due to side reactions between carbonate solvents and electrochemical intermediates such as lithium polysulfide species, resulting in the irreversible electrochemical behavior of batteries. Very recently, a possible approach has been developed to solve this issue by capturing sulfur within fine holes tailored into carbon hosts. The confined sulfur molecules undergo solid-state electrochemical conversion to avoid solution based side reactions. However, it still needs specially manufactured microporous carbon−sulfur composites, which requires delicate control in laboratory-scale fabrication. Thereby, a critical question now arises: Is there a simple and versatile approach to be used in sulfur cathodes with carbonate based electrolytes to realize the safe operation of Li−S batteries at high temperature?

—Li et al.

Addressing these challenges, the research team led by Prof. Andy Xueliang Sun demonstrated that MLD alucone coating can take advantage of carbonate-based electrolyte normally used in Li ion batteries, and offers a safe and versatile approach toward Li-S batteries at elevated temperature.

Prior work has shown that atomic layer deposition (ALD) aluminum oxide coating for sulfur cathodes can improve cycle performance as well as stabilize and prolong cycle life for Li−S batteries.

The MLD alucone film in the new study functions as an effective protecting layer and enables a reversible charge/discharge behavior for carbon−sulfur electrodes in carbonate-based electrolytes by preventing side reactions from occurring between sulfide/polysulfide intermediates and carbonate solvents.

As a result, rather than having to elaborately design microporous carbon for use as an electrode material, the researchers could use commercially available mesoporous carbon is employed as a sulfur host to demonstrate the universal application of MLD.

X-ray studies at Advanced Light Source (LBNL) and Canadian Light Source (CLS) revealed the mechanism and interaction between sulfur and alucone MLD coating. By using synchrotron-based near edge X-ray absorption fine structure (NEXAFS) and high energy X-ray photoelectron spectroscopy (HEXPS), it demonstrated the alucone coating ends up passivating the surface of the electrode and hindering the unwanted side reactions.

Resources

• Xia Li, Andrew Lushington, Qian Sun, Wei Xiao, Jian Liu, Biqiong Wang, Yifan Ye, Kaiqi Nie, Yongfeng Hu, Qunfeng Xiao, Ruying Li, Jinghua Guo, Tsun-Kong Sham, and Xueliang Sun (2016) “Safe and Durable High-Temperature Lithium–Sulfur Batteries via Molecular Layer Deposited Coating” Nano Letters doi: 10.1021/acs.nanolett.6b00577