Vikram Pande

Enhancing electrochemical intermediate solvation through electrolyte anion selection to increase nonaqueous Li–O2 battery capacity

Significance The Li–air battery has attracted significant interest as a potential high-energy alternative to Li-ion batteries. However, the battery discharge product, lithium peroxide, is both electronically insulative and insoluble in nonaqueous electrolytes. It therefore passivates the battery cathode as it is uniformly deposited and disallows the battery to achieve even a modest fraction of its potential electrochemical capacity. Our objective is to circumvent this challenge by enhancing the solubility of electrochemically formed intermediate species. We present a rational basis for electrolyte (i.e., solvent and salt) selection for nonaqueous Li–air batteries and demonstrate a selection criterion for an electrolyte salt that increases the stability of Li+ in solution, thereby triggering a solution-based process that allows significantly improved battery capacities. Among the “beyond Li-ion” battery chemistries, nonaqueous Li–O2 batteries have the highest theoretical specific energy and, as a result, have attracted significant research attention over the past decade. A critical scientific challenge facing nonaqueous Li–O2 batteries is the electronically insulating nature of the primary discharge product, lithium peroxide, which passivates the battery cathode as it is formed, leading to low ultimate cell capacities. Recently, strategies to enhance solubility to circumvent this issue have been reported, but rely upon electrolyte formulations that further decrease the overall electrochemical stability of the system, thereby deleteriously affecting battery rechargeability. In this study, we report that a significant enhancement (greater than fourfold) in Li–O2 cell capacity is possible by appropriately selecting the salt anion in the electrolyte solution. Using 7Li NMR and modeling, we confirm that this improvement is a result of enhanced Li+ stability in solution, which, in turn, induces solubility of the intermediate to Li2O2 formation. Using this strategy, the challenging task of identifying an electrolyte solvent that possesses the anticorrelated 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 believe the model and strategy presented here will be generally useful to enhance Coulombic efficiency in many electrochemical systems (e.g., Li–S batteries) where improving intermediate stability in solution could induce desired mechanisms of product formation.

Comment on “Cycling Li-O2 batteries via LiOH formation and decomposition”

Based on a simple thermodynamic analysis, we show that iodide-mediated electrochemical decomposition of lithium hydroxide (LiOH) likely occurs through a different mechanism than that proposed by Liu et al. (Research Article, 30 October 2015, p. 530). The mismatch in thermodynamic potentials for iodide/triiodide (I−/I3−) redox and O2 evolution from LiOH implies a different active iodine/oxygen electrochemistry on battery charge. It is therefore possible that the system described in Liu et al. may not form the basis for a rechargeable lithium-oxygen (Li-O2) battery.

Quantifying Confidence in DFT-Predicted Surface Pourbaix Diagrams of Transition-Metal Electrode–Electrolyte Interfaces

Density functional theory (DFT) calculations have been widely used to predict the activity of catalysts based on the free energies of reaction intermediates. The incorporation of the state of the catalyst surface under the electrochemical operating conditions while constructing the free-energy diagram is crucial, without which even trends in activity predictions could be imprecisely captured. Surface Pourbaix diagrams indicate the surface state as a function of the pH and the potential. In this work, we utilize error-estimation capabilities within the Bayesian ensemble error functional with van der Waals correlations exchange correlation functional as an ensemble approach to propagate the uncertainty associated with the adsorption energetics in the construction of Pourbaix diagrams. Within this approach, surface-transition phase boundaries are no longer sharp and are therefore associated with a finite width. We determine the surface phase diagram for several transition metals under reaction conditions and electrode potentials relevant for the oxygen reduction reaction. We observe that our surface phase predictions for most predominant species are in good agreement with cyclic voltammetry experiments and prior DFT studies. We use the OH* intermediate for comparing adsorption characteristics on Pt(111), Pt(100), Pd(111), Ir(111), Rh(111), and Ru(0001) since it has been shown to have a higher prediction efficiency relative to O*, and find the trend Ru > Rh > Ir > Pt > Pd for (111) metal facets, where Ru binds OH* the strongest. We robustly predict the likely surface phase as a function of reaction conditions by associating confidence values for quantifying the confidence in predictions within the Pourbaix diagram. We define a confidence quantifying metric, using which certain experimentally observed surface phases and peak assignments can be better rationalized. The probabilistic approach enables a more accurate determination of the surface structure and can readily be incorporated in computational studies for better understanding the catalyst surface under operating conditions.

Role of Disorder in NaO2 and Its Implications for Na–O2 Batteries

There is a need for more energy dense batteries, and Na–O2 batteries have emerged as an attractive option. In this work, we present a density functional theory (DFT) study utilizing the Hubbard U correction to probe structural and magnetic disorder in NaO2, the primary discharge product of Na–O2 batteries. We show that NaO2 exhibits a large degree of rotational and magnetic disorder. A three-body Ising model is necessary to capture the subtle interplay of this disorder. Our Monte Carlo (MC) simulations demonstrate for the first time that energetically favorable, ferromagnetic (FM) phases near room temperature consist of alternating bands of O2 dimers oriented along two of four cubic cell body diagonals. Using hybrid density functional theory calculations, we find that bulk structures are insulating, with a subset of FM structures showing a moderate gap (<2 eV) in one spin channel. The insulating nature of NaO2 implies that growth of the discharge product is most likely occurring due to the solution mechan...

Understanding Ion Pairing in High-Salt Concentration Electrolytes Using Classical Molecular Dynamics Simulations and Its Implications for Nonaqueous Li–O2 Batteries

A precise understanding of solvation is essential for rational search and design of electrolytes that can meet the performance demands in Li-ion and beyond Li-ion batteries. In the context of Li–O2 batteries, ion pairing is decisive in determining battery capacity via the solution-mediated discharge mechanism without compromising heavily on electrolyte stability. We argue that models based on coordination numbers of the counterion only in the first solvation shell are inadequate to describe the extent of ion pairing, especially at higher salt concentrations, and are often not consistent with experimental observations. In this study, we use classical molecular dynamics simulations for several Li salt anion (NO3–, BF4–, CF3SO3–, and (CF3SO2)2N–) and nonaqueous solvent (dimethyl sulfoxide, dimethoxyethane, acetonitrile, tetrahydrofuran, and dimethylacetamide) combinations to improve the understanding of ion paring with the help of a new metric of ion pairing. We proposed a metric that defines the degree of c...