Solid-State Siloxane Polymer Electrolyte for Lithium-Air (O2) Batteries

The need to reduce our dependence on fossil fuels and curb our contribution to global warming has increased interest in the development of electric vehicles. Current electric vehicles are mostly powered by lithium-ion batteries; the most energy dense batteries commercially available. However, lithium-ion batteries do not have the energy density needed for electric vehicles to compete effectively with gasoline-powered cars.^1-2 Therefore, newer battery chemistries are needed. One battery chemistry that can provide energy densities an order of magnitude greater than current lithium-ion batteries is lithium-air (O₂).³

Lithium-air (O₂) batteries utilize lithium metal as the anode, and a high surface area cathode (e.g., carbon) with oxygen serving as the active material. Oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) are believed to occur during discharge and charge of a Li-O₂ cell respectively.³ Despite the promises of high energy densities, several challenges such as electrolyte and electrode (e.g., carbon) instability,^4-6 low rate capability,⁷ and poor round-trip efficiency,^{6, 8} must be resolved before possible commercialization. In addition, the use of volatile, flammable liquid organic electrolytes pose severe safety concerns. To address some of these challenges, our group has focused on the development of solid polymer electrolytes for lithium-air.

We have utilized a solid polymer electrolyte with a siloxane (Si—O) backbone and alkoxy-like side chain that has a low glass transition temperature (T_g) and no melting transition. Although the synthesized polymer is viscous, it becomes a flexible solid when complexed with a lithium salt—a sign of physical crosslinking between the adjacent alkoxy chains and lithium ions. Ionic conductivity in polymer electrolytes is believed to occur through the segmental motion of the polymeric chain, and ion transport primarily in the amorphous region of the polymer.⁹ Therefore, the lack of a melting transition, which corresponds to a lack of crystallinity, and the low T_g which is due to the highly flexible siloxane chain, supports ionic transport in this polymer electrolyte.

This polymer-salt complex was then incorporated in a Li-O₂ cell as a solid electrolyte and the discharge and charge performance of the Li-O₂ cell examined. In addition, the polymer electrolyte appears stable in the highly reactive lithium-air cell environment. Our work has shown that solid polymer electrolytes are a great alternative to flammable liquid electrolytes and can serve as the physical separator in the cell, can support the Li-O₂ electrochemistry, and with further optimization can move these novel batteries closer to commercialization. 

1.            Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M., Li-O2 and Li-S batteries with high energy storage. Nature materials 2011, 11 (1), 19-29.

2.            Girishkumar, G.; McCloskey, B.; Luntz, A.; Swanson, S.; Wilcke, W., Lithium- air battery: promise and challenges. The Journal of Physical Chemistry Letters 2010, 1 (14), 2193-2203.

3.            Lu, Y.-C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y., Lithium–oxygen batteries: bridging mechanistic understanding and battery performance. Energy & Environmental Science 2013, 6 (3), 750-768.

4.            McCloskey, B.; Speidel, A.; Scheffler, R.; Miller, D.; Viswanathan, V.; Hummelshøj, J.; Nørskov, J.; Luntz, A., Twin problems of interfacial carbonate formation in nonaqueous Li–O2 batteries. The Journal of Physical Chemistry Letters 2012, 3 (8), 997-1001.

5.            Freunberger, S. A.; Chen, Y.; Drewett, N. E.; Hardwick, L. J.; Bardé, F.; Bruce, P. G., The Lithium–Oxygen Battery with Ether‐Based Electrolytes. Angewandte Chemie International Edition 2011, 50 (37), 8609-8613.

6.            Freunberger, S. A.; Chen, Y.; Peng, Z.; Griffin, J. M.; Hardwick, L. J.; Bardé, F.; Novák, P.; Bruce, P. G., Reactions in the rechargeable lithium–O2 battery with alkyl carbonate electrolytes. Journal of the American Chemical Society 2011, 133 (20), 8040-8047.

7.            Lu, Y.-C.; Kwabi, D. G.; Yao, K. P.; Harding, J. R.; Zhou, J.; Zuin, L.; Shao-Horn, Y., The discharge rate capability of rechargeable Li–O2 batteries. Energy & Environmental Science 2011, 4 (8), 2999-3007.

8.            Gallant, B. M.; Mitchell, R. R.; Kwabi, D. G.; Zhou, J.; Zuin, L.; Thompson, C. V.; Shao-Horn, Y., Chemical and morphological changes of Li–O2 battery electrodes upon cycling. The Journal of Physical Chemistry C 2012, 116 (39), 20800-20805.

9.            MacCallum, J. R.; Vincent, C. A., Polymer Electrolyte Reviews. Elsevier Applied Science: 1987.