Multi-Wall Carbon Nanotubes As Cathodes for Lithium-Air Batteries: Effect of Catalysts and Additives

Batteries with high specific energy densities have attracted a lot of attention due to their demand in electric vehicles (EVs). Due to the increase in demand for energy storage devices for applications such as load-levelling of electrical energy supply and demand [1], propulsion for electric vehicles, and microelectro-mechanical devices, the research for new classes of electrode materials that provide high energy, high power and cycle life longer than lithium-ion batteries is being motivated [2,3]. Lithium air batteries have high specific energy density and they can achieve four times higher energy density than the current lithium ion batteries [2, 3]. Lithium air batteries also have higher theoretical energy density than lithium ion batteries [4].

Catalysts have been shown to improve both the battery capacity and the recyclability of these batteries when used in cathodes. Several catalysts have been discovered for lithium air batteries. Debart et al. used several catalysts for lithium oxygen batteries with non-aqueous electrolyte. The electro-catalysts used by them included Pt, La_0.8Sr_0.2MnO₃, Fe₂O₃, NiO, CuO, CoFe₂O₄. Co₃O₄ exhibited highest discharge capacity and best cycling performance. Later Debart et al. examined manganese oxides as catalysts for lithium oxygen batteries. Mn₃O₄, bulk Mn₂O₃, bulk α, β, λ, γ-MnO₂, α-MnO₂ nanowires and β-MnO₂ nanowires were used as catalysts [6]. The air cathode with α-MnO₂ as the catalyst showed the highest capacity of about 3000 mAh/g (carbon) Lu et al. reported ORR studies on polycrystalline metal catalysts of palladium, platinum, ruthenium, gold and glassy carbon surfaces in non-aqueous electrolyte [5].

In this study, we investigate various modes of loading palladium-based catalysts and their effect on the capacity and round-trip efficiency of lithium-air batteries. The aim of our study is to increase the first discharge capacity, improve the recyclability, increase capacity retention and improve the recyclability of the battery. The loading and morphology the catalyst-CNT cathodes were studied using transmission electron microscopy. Electrochemical testing using electrochemical impedance spectroscopy and charge-discharge cycling were also investigated.

The cathode consisted of conductive carbon cloth coated with MWCNT or palladium inside MWCNT. Celgard 2040 was used as a separator. LiTFSi (Bis Trifluoromethane sulfonamide Lithium salt) in TEGDME (Tetraethyl glycol di-methyl ether) was used as electrolyte. The battery was assembled in the Swagelok cell. Figure 1 shows the assembly of the battery. Figure 2 shows the TEM images of palladium nanoparticles inside MWCNT. Figure 3 shows Pristine CNT with first cycle at a current rate of 200 mA/g showing a discharge capacity of  9300 mAh/g and a charge capacity of 8250 mAh/g. Figure 4 shows catalyst-loaded CNT with first cycle at a current rate of 200 mA/g showing a discharge capacity of  13800 mAh/g and a charge capacity of 3000 mAh/g.

References:

1.             J R Miller, A.F.B., ECS Interface, 2008: p. 17,53.

2.             Nobuyuki Imanishi, O.Y., Rechargeable Lithium air batteries:Characteristics and prospects. Materials Today 2014. 17.

3.             Girishkumar, M., Luntz, Swanson, Lithium air battery: Promise and Challenges. Journal of physical chemistry letters, 2010. 14(1): p. 2193-2203.

4.             Abraham, K.M., Jiang, A polymer electrolyte-based rechargeable lithium/oxygen battery. Journal of Electrochemical Society, 1996. 143(1): p. 1-5.

5.             Lu, Y.C., H.A. Gasteiger, and Y. Shao-Horn, Catalytic activity trends of oxygen reduction reaction for nonaqueous Li-air batteries. J Am Chem Soc, 2011. 133(47): p. 19048-51.

6.             Debart, A., et al., Alpha-MnO₂ nanowires: a catalyst for the O2 electrode in rechargeable lithium batteries. Angew Chem Int Ed Engl, 2008. 47(24): p. 4521-4.