Although rechargeable lithium (Li) ion batteries (LIBs) are widely studied for application in portable electronic devices and vehicle electrification, they cannot store sufficient energy for the extended driving range required by electric vehicles. Alternative energy-storage systems with much higher theoretical specific energy are needed. Among these technologies, Li–sulphur (Li–S) batteries hold great promise. Theoretically, a Li–S battery has a specific capacity and energy of 1,675 mAh g^-1 and 2,600 Wh kg^-1, respectively, much higher than those of traditional LIBs.

Sulphur is also of low cost, and is abundant and non-toxic, making this system attractive for large-scale applications. However, many obstacles in this system still need to be overcome for practical applications. The major issue associated with Li–S cells is the formation of soluble long-chain polysulphides during discharge/charge. The gradual loss of active mass from the cathode into the electrolyte and onto the Li metal anode leads to ‘shuttle reactions’, severe self-discharge, low efficiency and fast capacity decay on cycling, which are commonly observed in Li–S batteries.

—Huang et al.

Most lithium-sulfur battery research to date has centered on stopping sulfur leakage from the cathode. But PNNL researchers determined stopping that leakage can be particularly challenging. Further, recent research has shown a battery with a dissolved cathode can still work. The PNNL team, led by Dr. Jun Liu, thus focused on the battery’s other electrode by adding a protective shield to the anode.

PNNL’s graphite shield moves the sulfur-side reactions away from the anode’s lithium surface, preventing it from growing the debilitating interference layer. Combining graphite from lithium-ion batteries with lithium from conventional lithium-sulfur batteries, the researchers dubbed their new anode a hybrid of the two.

A graphite film is placed between the Li foil and separator, and then electrically connected with the Li metal to form a parallel anode. Once immersed in the electrolyte, Li ions immediately intercalate into the graphite, which persists in the lithiated state because this hybrid anode is, in essence, a shorted cell.

Using this anode concept, the hybrid Li–S cell delivers a reversible capacity of 4900 mAh ^g-1 at 1,370 mA g^-1 (~0.8 C). At a rate of 13,790 mA g^-1 (~8 C), cell capacity exceeds 450 mAh g^-1, demonstrating exceptional rate performance.

The PNNL team also observed marked improvement in capacity retention after repeated cycling; significantly improved cycling stability was observed at all rates. At 612 mA g^-1, the discharge capacity became stable after 50 cycles, maintaining at ~850 mAh g^-1 for more than 200 cycles with a small initial capacity loss.

Columbic efficiency is always more than 99.5%, reflecting that the shuttle mechanism is significantly mitigated in this fundamentally different design of Li–S cells, the authors found. Similar performance enhancements were observed at increased rates and over longer cycling times strongly suggesting that this innovative anode concept plays a key role in extending the cycling life of this system.

Electrochemical behaviors of Li–S cells using the proposed hybrid anode. (a) Charge–discharge curves of hybrid Li–S cells at different rates. (b) Cycling ability of the hybrid Li–S cell at a rate of 1,737 mA g-1. (c,d) Long-term cycling behavior and corresponding Coulombic efficiencies of hybrid Li–S cells at different rates. Huang et al. Click to enlarge.

In summary, a radically different hybrid design has been introduced for Li–S cells to mitigate the loss of active material and harmful parasitic reactions on the anode by using an integrated structure composed of electrically connected graphite and Li metal. Although further studies are required, preliminary findings, combined with available literature results, support the idea that lithiated graphite functions as an electrochemical artificial SEI that decouples the Li⁺ extraction/reinsertion from side reactions caused by soluble polysulphides. The anode reaction front is shifted from the Li metal to the graphite surface and the interaction with dissolved polysulphide species is primarily confined to graphite. The reduced irreversible sulphur loss on the Li metal anode, as well as the physical barrier benefit provided by the lithiated graphite, collectively leads to the exceptional electrochemical performances observed in Li–S batteries. This simple hybrid concept provides important clues in the future development of Li–S batteries and may be broadly adapted in many energy-storage technologies utilizing metal anodes.

—Huang et al.

This and most other lithium-sulfur battery research is conducted with small, thin-film versions of the battery that are ideal for lab tests. Liu noted tests with a larger battery system would better evaluate the performance of PNNL’s new hybrid anode for real-world applications.

This study was primarily supported by the Department of Energy’s Office of Science (BES), with additional support from DOE’s Advanced Research Projects Agency-Energy, and DOE’s Office of Energy Efficiency and Renewable Energy. Some of this research was performed at EMSL, DOE’s Environmental and Molecular Sciences Laboratory at PNNL.

Resources

• Cheng Huang, Jie Xiao, Yuyan Shao, Jianming Zheng, Wendy D. Bennett, Dongping Lu, Saraf V. Laxmikant, Mark Engelhard, Liwen Ji, Jiguang Zhang, Xiaolin Li, Gordon L. Graff & Jun Liu (2014) “Manipulating surface reactions in lithium-sulfur batteries using hybrid anode structures,” Nature Communications doi: 10.1038/ncomms/4015