(521b) 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.