Venkatasubramanian Viswanathan

Solvating additives drive solution-mediated electrochemistry and enhance toroid growth in non-aqueous Li–O2 batteries

1 IBM Almaden Research Center, San Jose, CA, 95120 2 Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA, 94720 3 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 4 Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213 5 SUNCAT, SLAC National Accelerator Laboratory, Menlo Park, CA 94025Given their high theoretical specific energy, lithium-oxygen batteries have received enormous attention as possible alternatives to current state-of-the-art rechargeable Li-ion batteries. However, the maximum discharge capacity in non-aqueous lithium-oxygen batteries is limited to a small fraction of its theoretical value due to the build-up of insulating lithium peroxide (Li₂O₂), the batterys primary discharge product. The discharge capacity can be increased if Li₂O₂ forms as large toroidal particles rather than as a thin conformal layer. Here, we show that trace amounts of electrolyte additives, such as H₂O, enhance the formation of Li₂O₂ toroids and result in significant improvements in capacity. Our experimental observations and a growth model show that the solvating properties of the additives prompt a solution-based mechanism that is responsible for the growth of Li₂O₂ toroids. We present a general formalism describing an additives tendency to trigger the solution process, providing a rational design route for electrolytes that afford larger lithium-oxygen battery capacities.

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.

Li-O2 Kinetic Overpotentials: Tafel Plots from Experiment and First-Principles Theory.

We report the current dependence of the fundamental kinetic overpotentials for Li-O2 discharge and charge (Tafel plots) that define the optimal cycle efficiency in a Li-air battery. Comparison of the unusual experimental Tafel plots obtained in a bulk electrolysis cell with those obtained by first-principles theory is semiquantitative. The kinetic overpotentials for any practical current density are very small, considerably less than polarization losses due to iR drops from the cell impedance in Li-O2 batteries. If only the kinetic overpotentials were present, then a discharge-charge voltaic cycle efficiency of ∼85% should be possible at ∼10 mA/cm(2) superficial current density in a battery of ∼0.1 m(2) total cathode area. We therefore suggest that minimizing the cell impedance is a more important problem than minimizing the kinetic overpotentials to develop higher current Li-air batteries.

Unifying the 2e– and 4e– Reduction of Oxygen on Metal Surfaces

Understanding trends in selectivity is of paramount importance for multi-electron electrochemical reactions. The goal of this work is to address the issue of 2e(-) versus 4e(-) reduction of oxygen on metal surfaces. Using a detailed thermodynamic analysis based on density functional theory calculations, we show that to a first approximation an activity descriptor, ΔGOH*, the free energy of adsorbed OH*, can be used to describe trends for the 2e(-) and 4e(-) reduction of oxygen. While the weak binding of OOH* on Au(111) makes it an unsuitable catalyst for the 4e(-) reduction, this weak binding is optimal for the 2e(-) reduction to H2O2. We find quite a remarkable agreement between the predictions of the model and experimental results spanning nearly 30 years.

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

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 batterys 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.

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

Lithium and oxygen vacancies and their role in Li2O2 charge transport in Li–O2 batteries

It is now well accepted that charge transport through Li2O2 is a serious limitation in Li–O2 batteries. We report formation energies for the different charge states of Li, O and O2 vacancies in Li2O2 that could have important implications for charge transport through Li2O2. Charge transition levels are given as a function of the location of the Fermi level in Li2O2 relative to the valence band maximum (VBM). We argue that in Li–O2 discharge/charge electrochemistry, the Fermi level is pinned by LiO2 interface states at ∼0.35 eV above the VBM and this causes the vacancies to be in positively charged states (weakly violating local charge neutrality). We show by non-equilibrium Greens function calculations of charge transport through a Au|Li2O2 + VLiq |Au metal–insulator–metal structure (with VLiq a Li vacancy of charge state q), that the +1 and 0 charge state induces considerable scattering for tunneling holes. This tunneling is the previously proposed dominant mechanism of charge transport in Li–O2 batteries at practical current densities at room temperature, although we also proposed a contribution from hole polaron migration at very low currents and higher temperatures. We suggest that scattering of the tunneling holes by the positively charged vacancies (and possibly hole polarons) is the origin of the resistive loss observed in Li–O2 discharges in bulk electrolysis cells (where other forms of resistance are negligible). Thus, we argue that charged vacancies hinder charge transport through Li2O2 in Li–O2 electrochemical discharges.

Nanoscale Limitations in Metal Oxide Electrocatalysts for Oxygen Evolution

Metal oxides are attractive candidates for low cost, earth-abundant electrocatalysts. However, owing to their insulating nature, their widespread application has been limited. Nanostructuring allows the use of insulating materials by enabling tunneling as a possible charge transport mechanism. We demonstrate this using TiO2 as a model system identifying a critical thickness, based on theoretical analysis, of about ∼4 nm for tunneling at a current density of ∼1 mA/cm(2). This is corroborated by electrochemical measurements on conformal thin films synthesized using atomic layer deposition (ALD) identifying a similar critical thickness. We generalize the theoretical analysis deriving a relation between the critical thickness and the location of valence band maximum relative to the limiting potential of the electrochemical surface process. The critical thickness sets the optimum size of the nanoparticle oxide electrocatalyst and this provides an important nanostructuring requirement for metal oxide electrocatalyst design.

Solvent Degradation in Nonaqueous Li-O2 Batteries: Oxidative Stability versus H-Abstraction

Developing rechargeable Li-O2 batteries hinges on identifying stable solvents resistant to decomposition. Here, we focus on solvent stability against adsorption-induced H-abstraction during discharge. Using a detailed thermodynamic analysis, we show that a solvents propensity to resist H-abstraction is determined by its acid dissociation constant, pKa, in its own environment. Upon surveying hundreds of solvents for their pKa values in different media, we find linear correlations between the pKa values across various classes of solvents in any two given media. Utilizing these correlations, we choose DMSO as the common standard to compare the relative stability trends. We construct a stability plot based on the solvents HOMO level and its pKa in DMSO, which reveals that most solvents obey a correlation where solvents with lower HOMO levels tend to have lower pKa values in DMSO. However, this is at odds with the stability requirement that demands deep HOMO levels and high pKa values. Thus, stable solvents need to be outliers to this observed correlation.

Selective electrochemical generation of hydrogen peroxide from water oxidation

Water is a life-giving source, fundamental to human existence, yet over a billion people lack access to clean drinking water. The present techniques for water treatment such as piped, treated water rely on time and resource intensive centralized solutions. In this work, we propose a decentralized device concept that can utilize sunlight to split water into hydrogen and hydrogen peroxide. The hydrogen peroxide can oxidize organics while the hydrogen bubbles out. In enabling this device, we require an electrocatalyst that can oxidize water while suppressing the thermodynamically favored oxygen evolution and form hydrogen peroxide. Using density functional theory calculations, we show that the free energy of adsorbed OH* can be used to determine selectivity trends between the 2e(-) water oxidation to H2O2 and the 4e(-) oxidation to O2. We show that materials which bind oxygen intermediates sufficiently weakly, such as SnO2, can activate hydrogen peroxide evolution. We present a rational design principle for the selectivity in electrochemical water oxidation and identify new material candidates that could perform H2O2 evolution selectively.