Effect of Solid Electrolyte Distribution in Composite Cathode on Electrochemical Properties of Solid-State Li-Air Batteries

Li-air batteries have increasing attention as energy sources for automotive application due to high theoretical energy density over 3000 Wh/kg which is about 10 times greater than that of Li-ion battery [1-2]. Typical nonaqueous Li-air batteries used organic liquid electrolyte and oxygen from the air in atmosphere as active cathode material. Oxygen can be reduced and oxidized at three phase boundary in electrolyte filled-porous electrode.  Excess organic liquid electrolyte may be needed to fill porous electrode and occupy nearly 70% of total weight, which makes the specific energy density of practical Li-air batteries reduced [3].

Recently, the studies for solid-state Li-air batteries with solid electrolyte have been reported [4]. Using solid electrolytes such as polymer and inorganic ionic conductors makes it possible to control proper amount in composite cathode and keeps the electrolyte from being squeezed out compared with organic liquid electrolyte. However, lithium ion conductivity of solid electrolyte is lower than that of liquid one and it is difficult to make a good interface between solid electrolyte and carbon in composite cathode. The design and internal structure in composite cathode is important to maintain the performance of solid-state Li-air batteries.

In this study, the electrochemical properties and pore structure in various composite cathodes were measured from solid-state Li-air batteries. PEO (polyethylene oxide)-based electrolyte containing LiTFSI (lithium bis(trifluoromethanesulfonyl) imide) was used as a solid electrolyte in composite cathode. The mixture of carbon and PEO was directly coated on LATP (Ohara Inc.) by screen printing method.  Solid-state Li-air cell was assembled by stacking Li metal, interlayer, LATP with composite cathode and GDL (gas diffusion layer), respectively.

The AC impedance was conducted to get internal resistance in fresh composite cathode at non faradic condition. The activation energy for each resistance components was calculated by fitting the impedance spectra. The activation energy of R_inf (interface resistance between LATP and cathode) is higher than those of R_el (resistance of LATP) and R_ion(resistance of Li ion transfer in cathode), which indicates kinetic barrier for Li ion transfer between LATP and solid electrolyte in cathode.

We also have studied the change of internal resistance in composite cathode with various carbons and electrolyte composition and will present our investigation.

References

[1] K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 143 (1996) 1.

[2] A. Ogasawara, A. Debart, M. Holtzapfel, P. Novak, P. G. Bruce, J. Am. Chem. Soc.129 (2006) 1390.

[3] J.-G. Zhang, D. Wang, W. Xu, J. Xiao, R.E. Williford,  J. Power Sources 195 (2010) 4332.

[4] H. Kitaura, H. Zhou, Energy Environ. Sci. 5 (2012) 9077.