Aprotic metal-oxygen batteries: recent findings and insights

During the last two decades, we have observed a dramatic increase in the electrification of many technologies. What has enabled this transition to take place was the commercialization of Li-ion batteries in the early nineties. Mobile technologies such as cellular phones, laptops, and medical devices make these batteries crucial for our contemporary lifestyle. Like any other electrochemical cell, the Li-ion batteries are restricted to the thermodynamic limitations of the materials. It might be that the energy density of the most advance Li-ion battery is still too low for demanding technologies such as a full electric vehicle. To really convince future customers to switch from the internal combustion engine, new batteries and chemistry need to be developed. Non-aqueous metal-oxygen batteries—such as lithium–oxygen, sodium–oxygen, magnesium–oxygen, and potassium–oxygen—offer high capacity and high operation voltages. Also, by using suitable polar aprotic solvents, the oxygen reduction process that occurs during discharge can be reversed by applying an external potential during the charge process. Thus, in theory, these batteries could be electrically recharged a number of times. However, there are many scientific and technical challenges that need to be addressed. The current review highlights recent scientific insights related to these promising batteries. Nevertheless, the reader will note that many conclusions are applicable in other kinds of batteries as well.     

The dependence of the electronic conductivity of carbon molecular sieve electrodes on their charging states

The dependence of the electronic conductivity of activated carbon electrodes on their potential in electrolyte solutions was examined. Kapton polymer films underwent carbonization (1000 degrees C), followed by a mild oxidation process (CO(2) at 900 degrees C) for various periods of time, to obtain carbons of different pore structures. A specially designed cell was assembled in order to measure the conductivity of carbon electrodes at different potentials in solutions. When the carbon electrodes possessed molecular sieving properties, a remarkable dependence of their conductivity on their charging state was observed. Aqueous electrolyte solutions containing ions of different sizes were used in order to demonstrate this phenomenon. As the average pore size of the activated carbons was larger, their molecular sieving ability was lower, and the dependence of their conductivity on their charging state regained its classical form. This behavior is discussed herein.     

A brief review: Past,present and future of lithium ion batteries

The review summarizes the development of lithium ion batteries beginning with the research of the 1970–1980s which lead to modern intercalation type batteries. Following the history of lithium ion batteries, material developments are outlined with a look at cathode materials, electrolyte solutions and anode materials. Finally, with lithium sulfur and lithium oxygen batteries two post intercalation type lithium batteries are discussed. The focus of the material discussions lies on basic understanding, problems and opportunities related to the materials.     

New cathode materials for rechargeable Mg batteries: fast Mg ion transport and reversible copper extrusion in CuyMo6S8 compounds

We report on a discovery of fast cathode materials, ternary Chevrel phases (CPs), CuyMo6S8, for rechargeable magnesium batteries; the related electrochemical process displays a unique coupling between reversible Mg insertion, and Cu extrusion/ reinsertion; this coupling results in an entirely new intercalation mechanism which combines the total chemical reversibility of the electrochemical reaction of MgxCuyMo6S8 with irreversibility of its separate stages once Cu extrusion stage is reached (in MgxCuyMo6S8: 0.5x + y > 4).     

On the mechanism of triclinic distortion in Chevrel Phase as probed by in-situ neutron diffraction

This work presents, for the first time, a general mechanism of a rhombohedral (R)-triclinic (T) phase transition in Chevrel Phases (CPs) with small cations (radius<1 A), which was unclear in spite of intensive studies of these important materials in the past. In contrast to previous interpretation of the R<-->T transition in some CPs as cation ordering, T-distortion is regarded here as a particular case of general adaptation of the framework to cation insertion, which includes the deformations of the coordination polyhedra and their tilting. The research is based on a combination of experimental studies (in-situ neutron diffraction at different temperatures) for one model compound, MgMo6Se8, and structural analysis for a variety of known CPs. This analysis shows that the structure flexibility is fundamentally different for the R and T forms. As a result of the lower flexibility, in the R form, a strict correlation exists between the compression of the framework along the -3 symmetry axis and the cation position in the structure (the so-called 'delocalization'). The decreasing delocalization in the R-CPs, which occurs on cooling, leads to excessive repulsion within the cations pairs (R-Cu1.8Mo6S8 case) or undesirable asymmetry in the cation polyhedra (R-MgMo6Se8 case). The higher flexibility of the T framework allows for relaxation of these structural strains by increasing the cation-cation distances and forming a more symmetric cation environment, sometimes with higher coordination number (CN), like CN=5 in the T-Fe2Mo6S8 type. Thus, this work also proposes possible driving forces for T-distortion in CPs.     

Effect of sonochemistry: Li- and Mn-rich layered high specific capacity cathode materials for Li-ion batteries

Li- and Mn-rich layered Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode material was synthesized using sonochemical method followed by annealing at 700, 800, and 900 °C for 10 h. The material was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and electrochemical techniques. Its performance as a cathode material for Li-ion batteries was examined. With the sample annealed at 900 °C, an initial specific capacity of 240 mAh g^?1 was obtained, which decreased to 215 mAh g^?1 after 80 cycles, thus retaining about 90 % of its initial capacity. In contrast, samples annealed at lower temperatures exhibited lower capacity retention upon cycling. Thus, the final annealing temperature was found to have a significant effect on the electrochemical stability of this material in terms of capacity, average voltage, and rate capability. The advantage of this synthesis, which includes a sonochemical stage, compared with a conventional co-precipitation synthesis, was also confirmed.     

Rechargeable lithiated silicon-sulfur (SLS) battery prototypes

Amorphous columnar structured silicon film electrodes were prepared and electrochemically tested in dioxolane based electrolyte solution, containing LiNO₃. The electrochemical performance of prelithiated amorphous silicon anodes coupled with sulfur composite cathodes was evaluated in full Si-Li-Sulfur (SLS) cells. The reversible capacity at the first 10 cycles was 600 mAh/g sulfur with gradual fading to ~ 380 mAh/g sulfur after 60 cycles which is the highest obtained capacity reported for SLS full cells. Possible reasons for this capacity fading are discussed.     

Formation and stability of alkoxyimidates in the cyclobutane bicyclobutane system

The reactions of CN^- with methyl-3-chlorobicyclobutanecarboxylate and 3-chloro and 3-bromobicyclobutanecarbonitrile in MeOH were investigated. The MeO^- present in the reaction mixture adds reversibly to the carbonitrile function of the various products. The equilibrium constants for the imidate formation were determined. The general notion that the imidate form is favored by an electron withdrawing group was reconfirmed. The order found was CN>Cl ≈ Br>C(NH)OMe>CO₂Me. It was also found that the central bond of bicyclobutane is a good mediator for transmittance of electronic effects. The unique stability of the alkoxyimidates compared to the low stability of the corresponding adducts with CN^- is discussed.     

The effect of milling on the performance of a Mo6S8 Chevrel phase as a cathode material for rechargeable Mg batteries

In the present study, we explored how milling Mo₆S₈ Chevrel phase in inert or air atmosphere affects their electrochemical behavior as a Mg-ion insertion material for rechargeable Mg batteries. Electrochemical tools such as slow scan rate cyclic voltammograms and potentiostatic intermittent titration technique have been used in conjunction with X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy. In contrast to the deterioration observed for milling Mo₆S₈ in air, its milling under Ar results in specific capacity increase due to improved Mg-ion diffusion kinetics. It was shown that in spite of the conservation of the bulk crystallographic structure, both for air and the Ar-milled materials, they differ significantly in the average particle sizes and the degree of surface oxidation state.Dedicated to Prof. G. Horanyi on the occasion of his 70th birthday