The Catalytic Behavior of Lithium Nitrate in Li-O2 Batteries

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.