Realization of Stable Cathode-Electrolyte Interfaces in DMSO Based Li-Air Batteries: Experimental and Theoretical Perspectives

Abstract

Electrochemical power sources based on metal anodes have specific energy density much higher than conventional Li ion batteries, due to the high energy density of the metal anode (3842mAh/g1 for Li). Rechargeable aprotic Li-O2batteries consume oxygen from the surrounding environment during discharge to form Li oxides on the cathode scaffold, using reactions (1) [anode] Li(s) ↔ Li+ + e− (2) [cathode] Li+ + ½ O2 (g)+ e− ↔ ½ Li2O2(s), (3) [cathode] Li+ + e− + ¼ O2 (g) ↔ ½ Li2O (s), The cathode reaction requires large over-potentials for charging due to the mass transfer resistance of reagents to the active sites on its surface, decreasing the round trip efficiency, making recharge of the Li-O2cell difficult. To overcome these problems, the cathode needs good electrical conductivity and a porous structure that enables facile diffusion of oxygen and can accommodate the reduced oxygen species in the pores. Two significant challenges exist in the use of the traditional activated carbon material as the cathode of the Li-O2 system. First, in the presence of Li2O2 the carbon electrode becomes relatively unstable even at low voltages (<3.5V). (Itkis et al., 2013) Second, cathode structures must be porous to accommodate a substantial amount of Li–peroxide (Li2O2) without blocking ion transport channels in the cathode. An additional major obstacle for the realization of the non-aqueous Li-O2 battery is finding a solvent that is chemically and electrochemically stable under cell operating conditions. Dimethyl sulfoxide (DMSO) is an attractive candidate for rechargeable Li-O2 battery studies(Peng, Freunberger, Chen, & Bruce, 2012); however, there is still significant controversy regarding its stability on the Li-O2cathode surface(Kwabi et al., 2014). We report here results from a model cathode systems which enable determination of the effects of various catalysts on the OER/ORR reactions in the DMSO based Li-O2 cell. Mesoporous CNT sponge is used as the model cathode material, decorated with catalyst nanoparticles by nucleation-controlled atomic layer deposition (ALD) of Ru, RuO2, MnOx, and Pt catalyst components whose loading and composition are controlled by manipulating the ALD conditions. We performed multiple experiments (in-situ XPS, FTIR, Raman, and XRD) which assess the stability of the DMSO-Li2O2 interface and report perspectives on previously published studies. Our electrochemical experiments show long term stable cycling of a DMSO-based operating Li-O2 cell with a Catlyst@carbon nanotube core-shell cathode fabricated via atomic layer deposition. This work is complemented by density functional theory calculations of DMSO degradation pathways on Li2O2. Both experimental and theoretical evidence strongly suggests that DMSO is chemically and electrochemically stable on the surface of Li2O2 under the reported operating conditions.