With the demand for high-energy-storage devices, the rechargeable metal-oxygen battery has attracted attention recently. Sodium-oxygen batteries have been regarded as the most promising candidates because of their lower-charge overpotential compared with that of lithium-oxygen system. However, conflicting observations with different discharge products have inhibited the understanding of precise reactions in the battery. Here we demonstrate that the competition between the electrochemical and chemical reactions in sodium-oxygen batteries leads to the dissolution and ionization of sodium superoxide, liberating superoxide anion and triggering the formation of sodium peroxide dihydrate (Na2O2·2H2O). On the formation of Na2O2·2H2O, the charge overpotential of sodium-oxygen cells significantly increases. This verification addresses the origin of conflicting discharge products and overpotentials observed in sodium-oxygen systems. Our proposed model provides guidelines to help direct the reactions in sodium-oxygen batteries to achieve high efficiency and rechargeability.

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Dissolution and ionization of sodium superoxide in sodium-oxygen batteries.

Understanding and controlling the kinetics of O2 reduction in the presence of Li(+)-containing aprotic solvents, to either Li(+)-O2(-) by one-electron reduction or Li2 O2 by two-electron reduction, is instrumental to enhance the discharge voltage and capacity of aprotic Li-O2 batteries. Standard potentials of O2 /Li(+)-O2(-) and O2/O2(-) were experimentally measured and computed using a mixed cluster-continuum model of ion solvation. Increasing combined solvation of Li(+) and O2(-) was found to lower the coupling of Li(+)-O2(-) and the difference between O2/Li(+)-O2(-) and O2/O2(-) potentials. The solvation energy of Li(+) trended with donor number (DN), and varied greater than that of O2 (-) ions, which correlated with acceptor number (AN), explaining a previously reported correlation between Li(+)-O2(-) solubility and DN. These results highlight the importance of the interplay between ion-solvent and ion-ion interactions for manipulating the energetics of intermediate species produced in aprotic metal-oxygen batteries.

Experimental and Computational Analysis of the Solvent-Dependent O2/Li(+)-O2(-) Redox Couple: Standard Potentials, Coupling Strength, and Implications for Lithium-Oxygen Batteries.

Solvation effects play a major role in determining the cycling characteristics of the non-aqueous rechargeable Li-air battery. We use a mixed cluster/continuum solvent model with varying number of explicit solvent molecules (n = 4–10) to calculate the solvation free energies (
 $$ \Updelta G_{\text{solv}}^{*} $$
 ) of Li+ and O2
 − ions and neutral LiO2, Li2O2, LiO, and Li2O species in acetonitrile solvent. Calculations for complexes with the full first solvation shell around Li+ (n = 4) and O2
 − (n = 8) show excellent agreement with the solvation free energies obtained using the cluster pair approximation (the error is below 2.0 kcal/mol). The use of the pure continuum model fitted to reproduce the experimental values of $$ \Updelta G_{\text{solv}}^{*} $$
 (Li+) and $$ \Updelta G_{\text{solv}}^{*} $$
 (O2
 −) gives the solvation free energies of various lithium–oxygen species (Li
 x
 O
 y
 ; x, y = 1, 2) that are in excellent agreement with the results obtained using mixed cluster/continuum models (n ≥ 8). This provides a theoretical framework for including solvent effects in the theoretical models of oxygen reduction and evolution reactions in the aprotic Li-air battery.

Calculation of solvation free energies of Li+ and O2− ions and neutral lithium–oxygen compounds in acetonitrile using mixed cluster/continuum models

The solution structures, stabilities, physical properties, and reactivities of sodium diisopropylamide (NaDA) in a variety of coordinating solvents are described. NaDA is stable for months as a solid or as a 1.0 M solution in N,N-dimethylethylamine (DMEA) at −20 °C. A combination of NMR spectroscopic and computational studies show that NaDA is a disolvated symmetric dimer in DMEA, N,N-dimethyl-n-butylamine, and N-methylpyrrolidine. Tetrahydrofuran (THF) readily displaces DMEA, affording a tetrasolvated cyclic dimer at all THF concentrations. Dimethoxyethane (DME) and N,N,N′,N′-tetramethylethylenediamine quantitatively displace DMEA, affording doubly chelated symmetric dimers. The trifunctional ligands N,N,N′,N″,N″-pentamethyldiethylenetriamine and diglyme bind the dimer as bidentate rather than tridentate ligands. Relative rates of solvent decompositions are reported, and rate studies for the decomposition of THF and DME are consistent with monomer-based mechanisms.

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Sodium Diisopropylamide: Aggregation, Solvation, and Stability

A quantum-mechanical investigation on Li poly(ethylene oxide)-based ionomers was performed in the cluster-continuum solvation model (CCM) that includes specific solvation in the first shell surrounding the cation, all surrounded by a polarizable continuum. A four-state model, including a free Li cation, Li+-anion pair, triple ion, and quadrupole was used to represent the states of Li+ within the ionomer in the CCM. The relative energy of each state was calculated for Li+ with various anions, with dimethyl ether representing the ether oxygen solvation. The population distribution of Li+ ions among states was estimated by applying Boltzmann statistics to the CCM energies. Entropy difference estimates are needed for populations to better match the true ionomer system. The total entropy change is considered to consist of four contributions: translational, rotational, electrostatic, and solvent immobilization entropies. The population of ion states is reported as a function of Bjerrum length divided by ion-pair separation with/without entropy considered to investigate the transition between states. Predicted concentrations of Li+-conducting states (free Li+ and positive triple ions) are compared among a series of anions to indicate favorable features for design of an optimal Li+-conducting ionomer; the perfluorotetraphenylborate anion maximizes the conducting positive triple ion population among the series of anions considered.

Cluster-continuum quantum mechanical models to guide the choice of anions for Li(+)-conducting ionomers.

The role that alkali cations in non-aqueous solvents play in organic reactions continues to be a topic of interest. In particular it has been observed that these cations can alter the stereoselectivity of organic reactions. Our interest is to first understand the nature of cation–ether complexes, then to investigate the role that the cation plays in the reaction. We have used the electronic structure techniques Hartree-Fock (HF), Second-order Moller-Plesset perturbation theory (MP2), and the Becke three-parameter exchange functional coupled with the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP) to study the structure and properties of tetrahydrofuran (THF) and dimethyl ether (DME) solvation complexes with Li+, Na+, K+, Cu+, and MgCl+. The values calculated for DME complexes were compared with existing experimentally determined data. The B3LYP/6-31+G∗ model chemistry was found to be the most accurate and efficient method of modeling the cation–DME molecular system. The energetic trends observed in the DME results were also observed in the THF data. Based on the accuracy of the calculations and the computational cost of the calculations, B3LYP was found to be the most desirable method of modeling these types of systems.

Michael J. Ziegler

Papers

Solvation of Metal Cations in Non-aqueous Liquids