Trade-Offs in Capacity and Rechargeability in Nonaqueous Li–O₂ Batteries: Solution-Driven Growth versus Nucleophilic Stability

Abstract

The development of high-capacity rechargeable Li-O2 batteries requires the identification of stable solvents that can promote a solution-based discharge mechanism, which has been shown to result in higher discharge capacities. Solution-driven discharge product growth requires dissolution of the adsorbed intermediate LiO2*, thus generating solvated Li+ and O2(-) ions. Such a mechanism is possible in solvents with high Gutmann donor or acceptor numbers. However, O2(-) is a strong nucleophile and is known to attack solvents via proton/hydrogen abstraction or substitution. This kind of a parasitic process is extremely detrimental to the battery's rechargeability. In this work, we develop a thermodynamic model to describe these two effects and demonstrate an anticorrelation between solvents’ stability and their ability to enhance capacity via solution-mediated discharge product growth. We analyze the commonly used solvents in the same framework and describe why solvents that can promote higher discharge capacity are also prone to degradation. Solvating additives for practical Li-O2 batteries will have to be outliers to this observed anticorrelation.

Enhancing electrochemical intermediate solvation through electrolyte anion selection to increase nonaqueous Li–O 2 battery capacity

“…However, there is tremendous scope in tuning the electrolyte anion in low-DN solvents to obtain high discharge capacities. Given that it should be simpler to identify anions stable to the Li-O 2 cathode electrochemistry than high-DN solvents (36,37), anion selection in combination with low-DN solvents potentially provides a route to avoid the unfavorable capacity/stability trade-off observed in high-DN solvents, such as DMSO (14,15,38,39).…”

“…Furthermore, using quantitative measures of battery rechargeability, high-DN solvents, such as DMSO and N-methyl pyrrolidone, have been observed to be less stable than low-DN solvents, such as acetonitrile and DME (14). Recently, Khetan et al used a thermodynamic analysis to show that an organic solvent's ability to induce the solution mechanism is anticorrelated with its stability toward nucleophilic attack (15 (16,17). We found that electrolytes containing a high concentration of NO 3 − exhibited higher donicity, as verified using 7 Li NMR, and provided an increase in battery capacity greater than fourfold compared with a battery using exclusively TFSI − as the electrolyte anion, while not decreasing battery rechargeability, as measured using quantitative oxygen consumption and evolution.…”

Experimental and Computational Analysis of the Solvent‐Dependent O2/Li+‐O2− Redox Couple: Standard Potentials, Coupling Strength, and Implications for Lithium–Oxygen Batteries

“…[50] As Li + solvation structures can vary greatly among similar solvents (for example, glymes), [51,52] and across different classes of solvents such as ionic liquids, [53] À potentials. These results highlight the importance of the interplay between ion-solvent and ion-ion interactions in understanding and controlling the intermediate species energetics, reaction product morphology, discharge capacity, and solvent stability in aprotic metal-O 2 batteries.…”

Examining the Role of Electrolyte and Binders in Determining Discharge Product Morphology and Cycling Performance of Carbon Cathodes in Li-O2Batteries

“…27 Ideally, future electrolyte solvents should possess high AN or DN to facilitate toroid formation and maximize discharge capacity without exacerbating by-product formation. 28,29 The search for more stable electrolyte and cathode pairings for Li-O 2 batteries has become a critical area of research and will likely determine the long term prospects of the Li-O 2 system. 30,31 To date, the pursuit of sufficiently stable electrolytes (i.e.…”