Design Principles for Electrolytes in Li-O2 Batteries

Abstract

Among the “beyond Li-ion” battery chemistries, non-aqueous Li-O2 batteries have the highest theoretical specific energy[1] and, as a result, have attracted significant research attention over the past decade. A critical challenge facing non-aqueous Li-O2 batteries is the electronically insulating nature of the primary discharge product, lithium peroxide, which passivates the battery cathode as it is formed, ultimately leading to low ultimate cell capacities. Recently, it has been shown that, to resolve the passivation of cathode, we need to enhance the solubility of the intermediate LiO2 *.[2],[3] This can be done by either solvating the Li+ cation using high donor number (DN) solvents such as (DMSO) or by solvating superoxide anion O2 - using small amount of high acceptor number (AN) solvent additives like water, alcohols. However both these approaches lead to electrolyte formulations that become unstable. High donor number solvents have been shown to be susceptible to nucleophilic attack from superoxide ion (O2 -) formed from the dissolution of LiO2 * intermediate leading to solvent degradation.[4] Water is also known to react with superoxide ion and lithium, increasing the parasitic reactions. This leads to a further decrease in the overall electrochemical stability of the system, thereby deleteriously affecting the rechargeability of the battery. In this study, we report that a significant enhancement (greater than fourfold) in Li-O2 cell capacity is possible by appropriately selecting the salt-solvent combination in the electrolyte solution. A route to decouple this trade-off is to enhance the solvation through appropriate electrolyte salt selection. To quantify the increase in battery capacity for different salt-solvent combinations, we have to develop a theory for determining the Gibbs free energy of Li+ in the electrolyte solution. To a first approximation, the Gibbs free energy of ionic species in solution is primarily dependent on its first solvation shell. In this study, we develop a modified Ising model to derive the solvation shell composition and in turn the Gibbs free energy of Li+. In the Ising model, we describe the possible energetic interactions between all species as functions of the DN and AN of each species that can occupy the solvation shell. Within the mean-field approximation, we derive average occupation for each species in the solvation shell. The average occupation is then used to derive the Li+ Gibbs free energy. We compare the results from the developed model to the 7Li NMR chemical shift and this agrees well for both low DN, DME and high DN solvents, DMSO. From the model, we find that high DN salt anions mixed with low DN solvents give enhanced dissolution of LiO2 * and in turn the cell capacity as compared to high DN solvents. However, high DN salts do not offer additional enhancement of the dissolution rate and capacity when mixed in high DN solvent. However, as pointed out earlier, high DN solvents are unstable and thus, we suggest using low DN solvents with high DN salt anions to give the same increase in capacity without compromising stability. Using this strategy, the challenging task of identifying an electrolyte solvent that possesses anti-correlated properties of high intermediate solubility and solvent stability is alleviated, potentially providing a pathway to develop an electrolyte that affords both high capacity and rechargeability. We will discuss extensions of the model to include interface effects, which become important for cells with thin electrolyte layers. We believe the model and strategy presented here will be generally useful to enhance Coulombic efficiency in many electrochemical systems where improving intermediate stability in solution could induce desired mechanisms of product formation. One limitation of the solubility based increase of cell capacity is the transport of Li+ and O2 - ions in solution will limit the current and thus the power density delivered by the cells. We will present extensions that will explore the trade-off between ion mobility or equivalently current density (or power density) and cell capacity of the Li-O2battery chemistry. [1] Christensen J, Albertus P, Sanchez-Carrera R S, Lohmann T, Kozinsky B, Liedtke R, Ahmed J & Kojic A (2011) A critical review of li/air batteries J. Electrochem. Soc. 159 R1–30 [2] N. B. Aetukuri, et. al. (2015) Solvating additives drive solution-mediated electrochemistry and enhance toroid growth in non-aqueous Li–O2 batteries. Nat. Chem. 7(1), 50-56. [3] Johnson L, et al. (2014) The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li–O2 batteries. Nature Chem. 6(12):1091-1099. [4] Khetan A, Luntz A, & Viswanathan V (2015) Trade-offs in capacity and rechargeability in nonaqueous Li–O2 batteries: solution-driven growth versus nucleophilic stability. J. Phys. Chem. Lett.:1254-1259 Figure 1