Chemical and Electrochemical Differences in Nonaqueous Li–O₂ and Na–O₂ Batteries

We present a comparative study of nonaqueous Li–O₂ and Na–O₂ batteries employing an ether-based electrolyte. The most intriguing difference between the two batteries is their respective galvanostatic charging overpotentials: a Na–O₂ battery exhibits a low overpotential throughout most of its charge, whereas a Li–O₂ battery has a low initial overpotential that continuously increases to very high voltages by the end of charge. However, we find that the inherent kinetic Li and Na–O₂ overpotentials, as measured on a flat glassy carbon electrode in a bulk electrolysis cell, are similar. Measurement of each batteries’ desired product yield, <i>Y</i>_NaO2 and <i>Y</i>_Li2O2, during discharge and rechargeability by differential electrochemical mass spectrometry (DEMS) indicates that less chemical and electrochemical decomposition occurs in a Na–O₂ battery during the first Galvanostatic discharge–charge cycle. We therefore postulate that reactivity differences (Li₂O₂ being more reactive than NaO₂) between the major discharge products lead to the observed charge overpotential difference between each battery

Theoretical Limits to the Anode Potential in Aqueous Mg–Air Batteries

The aqueous Mg–air battery is an attractive candidate for some electric vehicle applications due to its high theoretical specific energy, environmentally and physiologically benign properties, and implied low cost from using earth-abundant materials. However, the experimentally observed potentials (1.6–1.2 V) are far from the thermodynamically predicted value of 3.09 V, based on the free energy of formation for the reaction Mg (s) + 1/2 O₂ (g) + H₂O (l) ⇌ Mg­(OH)₂ (s). It is generally believed that this large difference is principally due to the presence of Mg corrosion giving rise to a net corrosion potential, and that it would be possible to nearly obtain the full potential of 3.09 V if corrosion were completely suppressed. In this contribution, we present a density functional theory study of the hydroxide-assisted Mg anodic dissolution mechanism in the aqueous Mg–air battery. We show that the Mg surface is expected to be highly OH*-covered in the anodic dissolution process, and that the calculated intrinsic limiting potentials are in fact in reasonable agreement with experimentally observed potentials. These limiting potentials are dictated by sequential electrochemical adsorption of hydroxide to the Mg surface, and therefore, the bulk free energy of Mg­(OH)₂ (s) formation cannot be used to predict the intrinsic anode potential in the aqueous Mg–air battery. These intrinsic limits imply that completely suppressing Mg corrosion will not significantly increase the potential available for the Mg–air battery

Al–Air Batteries: Fundamental Thermodynamic Limitations from First-Principles Theory

The Al–air battery possesses high theoretical specific energy (4140 W h/kg) and is therefore an attractive candidate for vehicle propulsion. However, the experimentally observed open-circuit potential is much lower than what bulk thermodynamics predicts, and this potential loss is typically attributed to corrosion. Similarly, large Tafel slopes associated with the battery are assumed to be due to film formation. We present a detailed thermodynamic study of the Al–air battery using density functional theory. The results suggest that the maximum open-circuit potential of the Al anode is only −1.87 V versus the standard hydrogen electrode at pH 14.6 instead of the traditionally assumed −2.34 V and that large Tafel slopes are inherent in the electrochemistry. These deviations from the bulk thermodynamics are intrinsic to the electrochemical surface processes that define Al anodic dissolution. This has contributions from both asymmetry in multielectron transfers and, more importantly, a large chemical stabilization inherent to the formation of bulk Al­(OH)₃ from surface intermediates. These are fundamental limitations that cannot be improved even if corrosion and film effects are completely suppressed

Multi-ion Conduction in Li₃OCl Glass Electrolytes

Antiperovskite glasses such as Li3OCl and doped analogues have been proposed as excellent electrolytes for all-solid-state Li ion batteries (ASSB). Incorporating these electrolytes in ASSBs results in puzzling properties. This Letter describes a theoretical Li3OCl glass created by conventional melt–quench procedures. The ion conductivities are calculated using molecular dynamics based on a polarizable force field that is fitted to an extensive set of density functional theory-based energies, forces, and stresses for a wide range of nonequilibrium structures encompassing crystal, glass, and melt. We find high Li+ ion conductivity in good agreement with experiments. However, we also find that the Cl– ion is mobile as well so that the Li3OCl glass is not a single-ion conductor, with a transference number t+ ≈ 0.84. This has important implications for its use as an electrolyte for all-solid-state batteries because the Cl could react irreversibly with the electrodes and/or produce glass decomposition during discharge–charge

Positivitat in Kultur und Religion (1) : Ein Gesichtspunkt aus dem wir die europaische Kultur anzusehen haben

In the field of energy storage devices the pursuit for cheap, high energy density, reliable secondary batteries is at the top of the agenda. The Li–O₂ battery is one of the possible technologies that, in theory, should be able to close the gap, which exists between the present state-of-the-art Li-ion technologies and the demand placed on batteries by technologies such as electrical vehicles. Here we present a redox probing study of the charge transfer across the main deposition product lithium peroxide, Li₂O₂, in the Li–O₂ battery using outer-sphere redox shuttles. The change in heterogeneous electron transfer exchange rate as a function of the potential and the Li₂O₂ layer thickness (∼depth-of-discharge) was determined using electrochemical impedance spectroscopy. The attenuation of the electron transfer exchange rate with film thickness is dependent on the probing potential, providing evidence that hole transport is the dominant process for charge transfer through Li₂O₂ and showing that the origin of the sudden death observed upon discharge is due to charge transport limitations

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/cm². 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

An Electrochemical Impedance Study of the Capacity Limitations in Na–O₂ Cells

Electrochemical impedance spectroscopy, pressure change measurements, and scanning electron microscopy were used to investigate the nonaqueous Na–O₂ cell potential decrease and rise (sudden deaths) on discharge and charge, respectively. To fit the impedance spectra from operating cells, an equivalent circuit model was used that takes into account the porous nature of the positive electrode and is able to distinguish between the electrolyte resistance in the pores and the charge-transfer resistance of the pore walls. The results obtained indicate that sudden death on discharge is caused by, depending on the current density, either accumulation of large NaO₂ crystals that eventually block the electrode surface and/or a thin film of NaO₂ forming on the cathode surface at the end of discharge, which limits charge-transfer. The commonly observed sudden rise in potential toward the end of charge may be caused by a concentration depletion of NaO₂ dissolved in the electrolyte near the cathode surface and/or an accumulation of degradation products on the cathode surface

Factors Affecting the Electron Conductivity in Single Crystal Li₇La₃Zr₂O₁₂ and Li₇P₃S₁₁

One of the serious challenges in all solid-state Li ion batteries is neutral Li intrusion into the solid-state electrolyte that can ultimately cause catastrophic failure. One possibility for this is due to n-type electron conductivity that induces the reaction Li+ + e– → Li0 at sites where the potential is less than the Li+/Li potential. This paper reports hybrid density functional theory calculations of the electronic conductivity in two prototype single crystalline solid-state electrolytes, cubic Li7La3Zr2O12 (c-LLZO) and Li7P3S11 (LPS). The formation energies of important point defects that can affect electron conductivity are determined, and we find that the mechanism of n-type electron conductivity for both solid-state electrolytes is via “small” electron polaron hopping, where the quotes signify that substantial Li ion rearrangement is associated with the polaron formation and its migration. In both electrolytes, the formation energies for the small polarons at the Fermi energy are too high to generate measurable electron conductivity at room temperature. For c-LLZO, the concentration of electron polarons necessary to ensure charge neutrality from positively charged oxygen vacancies formed in synthesis can be significantly higher. Hence, the electron conductivity could be significant when measured with ion-blocking metal electrodes, and we discuss how the synthesis conditions could affect this magnitude. However, in the solid-state battery, these polarons are replaced by negatively charged Li vacancies so that the electron conductivity should remain minimal. For LPS single crystals, the inherent minimal electron conductivity is independent of synthesis conditions. We also show that the cost of forming Li0 in bulk c-LLZO is enormous due to strain effects so that it could only potentially form at voids, grain boundaries, or around vacancy defects which relax the lattice strain

Razvoj eksplozivne moči v košarki, pri kadetih v predtekmovalnem obdobju

Lithium–O₂ (Li–O₂) batteries are currently limited by a large charge overpotential at practically relevant current densities, and the origin of this overpotential has been heavily debated in the literature. This paper presents a series of electrochemical impedance measurements suggesting that the increase in charge potential is not caused by an increase in the internal resistance. It is proposed that the potential shift is instead dictated by a mixed potential of parasitic reactions and Li₂O₂ oxidation. The measurements also confirm that the rapid potential loss near the end of discharge (“sudden death”) is explained by an increase in the charge transport resistance. The findings confirm that our theory and conclusions in ref , based on experiments on smooth small-area glassy carbon cathodes, are equally valid in real Li–O₂ batteries with porous cathodes. The parameter variations performed in this paper are used to develop the understanding of the electrochemical impedance, which will be important for further improvement of the Li–air battery