The Catalytic Behavior of Lithium Nitrate in Li-O2 Batteries

Abstract

Over the last decade extra efforts were invested in the development of aprotic Li-O2 cells. Early research presented optimistic results that showed the great potential of this system. However in recent years more research works observed that it is hard to find suitable cell components that will enable prolong cycling of Li-O2 cells. Many challenges need to be addressed. Two dominant subjects were given special attention: the carbon cathodes and the electrolyte solutions. These factors governed Li-O2 cells’ operation during both the oxygen reduction reaction (ORR) and the oxygen evaluation reaction (OER). Despite many attempts to find solvents that are stable toward active oxygen species formed by oxygen reduction (super-oxide, peroxide moieties) , no solvent was found to be fully stable during ORR & OER. Several sovents were explored and although none of them was found to be stable, they presented difference features that can affect positively the ORR. One parameter is the Guttmann number of the solvent. High donor number solvents like dimethyl sulfoxide (DMSO), dimethylacetamide (DMA) and dimethylformamide (DMF) are able to stabilize the soluble superoxide intermediate during ORR. Stabilization of superoxide moieties in the course of oxygen reaction can help the growth mechanism of the main product of ORR in aprotic media, namely, Li2O2. Similar effect was accomplished by adding water contaminations to the electrolyte solution consequently enhancing formation of soluble species. However, solvents and solution additives that stabilize superoxide species can undergo parasitic, side reactions. Soluble charged species were investigated as red-ox mediators that can reduce the over-potential of OER. Redox mediators used as additives in solution can easily oxidize the Li2O2 by accepting charge at relatively low potentials and transfer it to the carbon cathode. In this work we show that by using lithium nitrate (LiNO3) as an electrolyte, we can positively affect ORR mechanism and reduce OER over-potential. LiNO3 is a well-known passivating agent for lithium metal anodes. This phenomenon can be used in Li metal based batteries such as Li-S systems. In Li-O2 cells LiNO3 was first introduced as a passivating agent for Li metal in cells based on amide solvents like DMA1. We show that LiNO3 by-product NO2 -, from the reaction of NO3 - with lithium metal, can behave as a redox mediator that reduces OER over-potential below 4V. We suggest that the oxidation of NO2 - to NO2 forms a NO2/NO2 - couple which serves as a good red-ox mediator. . NO2 can oxidize the Li2O2 and, as a result, the NO2 is reduced back to NO2 -. As with other redox mediators, only a small amount is needed to complete the process. The ability of the soluble NO2 - anions to be oxidized on the carbon surface and then oxidize thick Li2O2 surface films, enables to reduce pronouncedly the OER over-potential. Recent work2 suggested that anions that can complex with the lithium cations reduce their Lewis acidity. As a result, soluble species like superoxide can take part as intermediates in the Li2O2 formation. Lithium nitrate moieties are associated in poly-ethers more than any other lithium salt used in lithium batteries. This means that nitrate anions can be coordinate strongly to solvated Li cations thus enabling formation of mobile superoxide moieties. We show that even using cathodes with flat area, we can achieve high capacity of ORR, including formation of submicron Li2O2 particles due to the influence of the NO3 - ionson the growth mechanism as presented in Figure. 1. Figure 1. HR-SEM images of gold electrodes discharged to 2 V in 1M LiNO3 diglyme electrolyte solution. References (1) Walker, W.; Giordani, V.; Uddin, J.; Bryantsev, V. S.; Chase, G. V; Addison, D. J. Am. Chem. Soc. 2013, 135 (6), 2076. (2) Gunasekara, I.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. a.; Abraham, K. M. J. Electrochem. Soc. 2015, 162 (6), A1055. Figure 1