DOE JCESR team significantly improves Li-S performance under lean electrolyte with soft swellable gel
Researchers from Pacific Northwest National Laboratory (PNNL) and Sandia National Laboratories, all members of the DOE’s Joint Center for Energy Storage Research (JCESR), have significantly improved the performance of Li-sulfur batteries under lean electrolyte conditions by using a soft PEO10LiTFSI polymer swellable gel as a nanoscale reservoir to trap the polysulfides. A paper on their work is published in the ACS journal Nano Letters.
Li-sulfur batteries are looked to as a likely next-generation higher energy density energy storage system due to the high theoretical capacity, low cost and high earth abundance of sulfur. The system faces barriers to commercialization, however, including degradation of the Li-metal anode, polysulfide dissolution and electrolyte decomposition.
|Credit: ACS, Chen et al.|
Great progress has been made using rigid nanostructured porous carbon or metal oxide hosts to encapsulate sulfur on the cathode, together with advances in electrolyte additives and other electrode protection strategies. The rigid encapsulation approaches relies on the interfacial binding between the substrate and the polysulfides and charge transport across the interfaces.
The majority of investigation reported in the literature was conducted with a high electrolyte amount. The careful analysis suggested that the amount of electrolyte needs to be significantly reduced in order for Li-S battery technology to achieve the desired high energy density. Currently, most studies use a high electrolyte amount with an electrolyte to sulfur (E/S) ratio in the range of 10 -50 mLE/gS (flooded electrolyte condition). There have been very few reports on rechargeable Li-S batteries using a low electrolyte amount with an an E/S ratio less than 5 mLE/gS (lean electrolyte condition). It is increasingly recognized now that good electrochemical performance obtained with flooded electrolytes cannot be reproduced under lean electrolyte condition.
… The change to lean electrolyte condition is critical for high pack-level energy density, but places severe restrictions on wetting, charge transport between the interface, and electrochemical reaction kinetics. In addition, when a Li-S cell operates under a lean electrolyte condition, its charge/discharge behavior, cell failure model, etc. may be substantially different from those with flood electrolyte.— Chen et al.
In the approach described in the paper, the DOE team confine the electrolyte and the polysulfides in the swellable, flexible polymer which itself is ion-conducting, thus providing more efficient encapsulation and better ion transport.
The soft gel provides strong binding ability within the sulfur cathode scaffold, absorbs electrolyte and forms an efficient Li+ conducting gel network within the cathode. The high polysulfide solubility in the gel also enables good polysulfide confinement, improving cycling stability under a lean electrolyte condition.
Using the soft gel encapsulation approach, a Li-s cell with a E/S of 4gE/gS could deliver a capacity of 1200 mAh g-1, 4.6 Ah/cm2 and good cycle life.
Junzheng Chen, Wesley A. Henderson, Huilin Pan, Brian R. Perdue, Ruiguo Cao, Jian Zhi Hu, Chuan Wan, Kee Sung Han, Karl T. Mueller, Ji-Guang Zhang, Yuyan Shao, and Jun Liu (2017) “Improving Lithium–Sulfur Battery Performance under Lean Electrolyte through Nanoscale Confinement in Soft Swellable Gels” Nano Letters doi: 10.1021/acs.nanolett.7b00417