The Carbon-Free Ag Electrode for Non-Aqueous Li-O2 Batteries

In the past decade, worldwide attention has been focused on the development of lithium-ion batteries with high energy density for automotive applications, e.g. plug-in hybrid vehicles (PHEVs) and electric vehicles (EVs). However, the current lithium-ion battery systems are unable so far to fulfil the energy demand for those applications. Among the next generation batteries, Li-air batteries have attracted great interests due to their very high energy density  that is about 10 times higher than that of commercialized Li-ion batteries.¹ Present Li-air batteries, however, have still many problems to be satisfactorily applied to electrical vehicles.² One of them is the low round-trip efficiency.³ The cause of low round-trip efficiency is largely due to the formation of Li₂CO₃ during cycling of Li-air batteries because Li₂CO₃ decomposes at a potential higher than 4V.⁴ Li₂CO₃ is typically formed by decomposition of the carbon materials in the electrodes and the organic solvent. Li₂CO₃ formed by decomposition of carbon materials occupies half of whole Li₂CO₃ formed by decomposition of carbon in electrode and solvent in electrolyte. Therefore, to reduce the polarization in the oxygen evolution reaction (OER), it is required to develop a carbon-free electrode.

There have been many attempts to prevent decomposition of the carbon materials in the air electrode during cycling.⁵ It has shown by density functional calculation that the structural defects in the carbon materials of the air electrode easily reacted with the electrolyte. It also demonstrated that Al₂O₃ coating on the carbon electrode by atomic layer deposition (ALD) prevented the decomposition of the tetraethylene glycol dimethyl ether (TEGDME) by blocking the reaction of the TEGDME solvent molecules with the defect sites on the carbon surface. D. M. Itkis’s group also confirmed that activated double bonds or aromatics of the carbon materials react with the oxygen radical on the epoxy groups and carbonates during discharge. Meanwhile, carbon-free materials of air electrodes: Ru/indium tin oxide (Ru/ITO) and TiC were investigated. The carbon-free materials indeed suppressed the formation of Li₂CO₃ and enhanced the rechargeability of Li-air batteries.

In this study, we employ Ag-coated air electrode prepared by electrodeposition. The electrodeposition is one of the simplest metal deposition methods and easily adaptable to a large scale production. Sun’s group have, in fact, applied the electrodeposition method to deposit Co₃O₄ on the air electrode for the first time. The Pt/Co₃O₄-deposited carbon-free electrode showed very low polarization during the charge process but high polarization during the discharge process. Since Ag has very high electronic conductivity and good catalytic property for the oxygen reduction reaction, it is surmised that the Ag-deposited electrode can potentially outperform the previously studied carbon-free materials of air electrodes.

References

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[3] Y. Shao, S. Park, J. Xiao, J.-G. Zhang, Y. Wang, and J. Liu, ACS Catal., 2012, 2, 844−857.

[4] B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov, and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997−1001.

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