Ambient Air Operation in Non-Aqueous Li-Air Batteries: Influence of Water on Electrochemical Li Cycling

Non-aqueous Li-air batteries have created considerable interest in recent years for their potential application to energy storage devices of electric vehicles as well as smart grids.  Theoretically, Li-air batteries have much higher energy density than currently-developed Li-ion batteries, based on inexhaustible O₂gas from outside of the battery and high capacity of Li metal.  But in reality, the energy density of the Li-air batteries is strongly governed by 1) precipitates as discharged product inside/outside carbon pores, 2) electrochemical stabilities of electrolyte, carbon-based cathode and Li anode and 3) electrode/cell structures, which do not fully satisfy the aforementioned theoretical prediction.  Additionally, there have many hurdles to be overcome in the non-aqueous Li-air batteries, for example, power density, energy efficiency, durability and so on.  Since 2006, intensive researches on electrolyte and cathode (i.e. carbon/catalyst/binder) materials have accelerated all over the world, to solve the fundamental problems and to maximize the advantages of the Li-air battery system.  

Ambient air operation is one of the key points to realize the Li-air battery system.  So far, to simplify the mechanistic understanding of problems and to highlight the material aspects obviously, pure O₂ gas has been preferably used for this study.  However, the utilization of ambient air into the system is undoubtedly essential to make it viable.  A few preliminary studies have just considered the effect of water on Li-air battery performances [1-3], but much effort need to be devoted to this research.  We have already demonstrated ionic liquid based Li-air batteries [4-7].  Some ionic liquids have great chemical and electrochemical properties against O₂radical species as well as Li metal.  Furthermore, since ionic liquids have a promising feature of non-volatility, they possess some advantages as the battery electrolyte considering ambient air operation.

In this presentation, we will focus on a Li anode side in the ionic liquid electrolyte under presence of water [8].  Some ionic liquids were compared with typical organic solvents as described in Figure 1.  Figure 1 shows the voltage profiles of electrochemical Li cycling in the (a) PC-LiTFSI organic electrolyte and (b) PP13TFSI-LiTFSI ionic liquid electrolyte.  Black and red lines represent the electrolyte without and with 1% of water, respectively.  These cells were operated at room temperature at a constant current density of 0.1 mA/cm².  In the PC based electrolyte without water, the potentials observed during Li cycling were continuously much lower than that in the PP13TFSI based electrolyte under the same condition.  However, in the PC based system, 1 % of water drastically changed the electrochemical behavior during Li cycling, while it did not have such big impact in the PP13TFSI based system.  This difference should be originated from not only nature of solid electrolyte interface (SEI) but also state of water in the media.  This discussion will become some clues of future material design as well as entire cell design, leading to the enhancement of Li-air battery performances.

References

[1] T. Kuboki et al., J. Power Sources, 146 (2005) 766.

[2] S. Meini et al., Electrochem. Solid-State Lett., 15 (2012) A45.

[3] X-Z. Yuan et al., J. Electrochem. Soc., 161 (2014) A451.

[4] F. Mizuno and K. Takechi et al., Electrochemistry, 79 (2011) 876.

[5] K. Takechi and F. Mizuno et al., ECS Electrochem. Lett., 1 (2012) A27.

[6] S. Higashi, K. Takechi and F. Mizuno et al., J. Power Sources, 240 (2013)14.

[7] N. Imanishi, A. C. Luntz and P. G. Bruce (Editors), Springer, The Lithium Air Battery: Fundamentals, Chapter 2 (2014).

[8] F. Mizuno and K. Takechi, submitted for publication (2015).