The crystal structure of the inorganic surface films formed on Mg and Li intercalation compounds and the electrode performance

A crystallographic approach was applied to elucidate the influence of the nature of the surface films on the electrochemical behavior of Li and Mg intercalation compounds. This paper presents two examples: (1) protection of graphite electrodes by Li₂CO₃ surface films, and (2) the unique electrochemical behavior of Mg-containing Chevrel phases (MgCP) obtained by different synthetic routes. In the former case, the elucidation of the protection mechanism and the explanation of the high performance of such protected electrodes are based on the analysis of possible Li-ion motion in the carbonate crystal structure. In the latter case, a combination of synthesis, electrochemistry and XRD analysis was used to explain an unusual phenomenon: the difference between the excellent electrochemical behavior of the Chevrel phase (CP) based on Cu-leached Cu₂Mo₆S₈ (CuCP), and the poor electrochemical activity of the high-temperature synthesized MgCP, with the same phase composition. It is shown that this phenomenon is caused by MgO formation on the surface of the latter material. The different surface chemistry of the MgCPs obtained by the two different synthetic routes was substantiated by revealing the correlation between the electrochemical activity and the chemical stability of these materials under ambient atmosphere conditions. Dedicated to Prof. Mikhail A. Vorotyntsev on the occasion of his 60th birthday.     

Effect of the Structure of Nonuniform Conducting Polymer Films on Their Electrochemical Impedance Response

Electrochemical impedance spectroscopy is used to characterize thin p-doped polypyrrole (PPy) films in propylene carbonate (PC) solutions and poly(trifluorophenyl)thiophene (PTFPT), in solutions based on sulfolane (SF). It appears that the latter film is much less swelled compared to the former one. One consequence of this difference is that the PTFPT film shows a much higher bulk resistance compared to that for the PPy film. Another important consequence is that the swelling of the PTFPT film is essentially physically non-homogeneous. Two parallel, uncoupled paths, with different chemical diffusion coefficients, model the experimental results adequately. In order to quantify the impedance spectra for both polymer films, we use a model proposed by Rubinstein et al. explaining the difference in the diffusion coefficients of Ru(bpy)^3+/2+ ₃ within a thin Nafion film. The model can also predict the impedance spectra for composite powdery electrodes containing different particle sizes, such as composite cathodes and graphite anodes used in lithium batteries.     

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     

Structural analysis of electrolyte solutions for rechargeable Mg batteries by stereoscopic means and DFT calculations

We present a rigorous analysis of unique, wide electrochemical window solutions for rechargeable magnesium batteries, based on aromatic ligands containing organometallic complexes. These solutions are comprised of the transmetalation reaction products of Ph(x)MgCl(2-x) and Ph(y)AlCl(3-y) in different proportions, in THF. In principle, these reactions involve the exchange of ligands between the magnesium and the aluminum based compounds, forming ionic species and neutral molecules, such as Mg(2)Cl(3)(+)·6THF, MgCl(2)·4THF, and Ph(y)AlCl(4-y)(-) (y = 0-4). The identification of the equilibrium species in the solutions is carried out by a combination of Raman spectroscopy, multinuclear NMR, and single-crystal XRD analyses. The association of the spectroscopic results with explicit identifiable species is supported by spectral analyses of specially synthesized reference compounds and DFT quantum-mechanical calculations. The correlation between the identified solution equilibrium species and the electrochemical anodic stability window is investigated. This study advances both development of new nonaqueous solution chemistry and possible development of high-energy density rechargeable Mg batteries.     

Exceptional electrochemical performance of Si-nanowires in 1,3-dioxolane solutions: a surface chemical investigation

The effect of 1,3-dioxolane (DOL) based electrolyte solutions (DOL/LiTFSI and DOL/LiTFSI-LiNO(3)) on the electrochemical performance and surface chemistry of silicon nanowire (SiNW) anodes was systematically investigated. SiNWs exhibited an exceptional electrochemical performance in DOL solutions in contrast to standard alkyl carbonate solutions (EC-DMC/LiPF(6)). Reduced irreversible capacity losses, enhanced and stable reversible capacities over prolonged cycling, and lower impedance were identified with DOL solutions. After 1000 charge-discharge cycles (at 60 °C and a 6 C rate), SiNWs in DOL/LiTFSI-LiNO(3) solution exhibited a reversible capacity of 1275 mAh/g, whereas only 575 and 20 mAh/g were identified in DOL/LiTFSI and EC-DMC solutions, respectively. Transmission electron microscopy (TEM) studies demonstrated the complete and uniform lithiation of SiNWs in DOL-based electrolyte solutions and incomplete, nonuniform lithiation in EC-DMC solutions. In addition, the formation of compact and uniform surface films on SiNWs cycled in DOL-based electrolyte solutions was identified by scanning electron microscopic (SEM) imaging, while the surface films formed in EC-DMC based solutions were thick and nonuniform. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy were employed to analyze the surface chemistry of SiNWs cycled in EC-DMC and DOL based electrolyte solutions. The distinctive surface chemistry of SiNWs cycled in DOL based electrolyte solutions was found to be responsible for their enhanced electrochemical performances.     

The electrochemical properties of a series of phenylene-thienyl polymers in tetramethylene sulfone based electrolytes

Electronically conducting polymers based on polythiophene and polyparaphenylene exhibit generally, both n-doping and p-doping. In most cases, however, the n-doping process is associated usually with poor reversibility and stability. One approach to improve the doping behavior is to modify the structure of the monomer repeat unit at the molecular level. In this paper we report on the electrochemical properties of the polymer series: pTHP, pTFP (1,4-bis(2-thienyl)-2-fluorobenzene), pTF₂P (1,4-bis(2-thienyl)-2,5-difluorobenzene), and pTF₄P (1,4-bis(2-thienyl)-2,3,5,6-tetrafluorobenzene) in tetramethylene sulfone (sulfolane) TBABF₄ based electrolytes.     

Unusually high stability of a poly(alkylquaterthiophene-alt-oxadiazole) conjugated copolymer in its n and p-doped states

Incorporation of electron accepting units (oxadiazole) into the 2,5-thienylene conjugated chain leads to a significant improvement in the n-doping-undoping redox stability of the resulting polymer.     

Nonaqueous magnesium electrochemistry and its application in secondary batteries

A revolution in modern electronics has led to the miniaturization and evolution of many portable devices, such as cellular telephones and laptop computers, since the 1980s. This has led to an increasing demand for new and compatible energy storage technologies. Furthermore, a growing awareness of pollution issues has provided a strong impetus for the science and technology community to develop alternatives with ever-higher energy densities, with the ultimate goal of being able to propel electric vehicles. Magnesium's thermodynamic properties make this metal a natural candidate for utilization as an anode in high-energy-density, rechargeable battery systems. We report herein on the results of extensive studies on magnesium anodes and magnesium insertion electrodes in nonaqueous electrolyte solutions. Novel, rechargeable nonaqueous magnesium battery systems were developed based on the research. This work had two major challenges: one was to develop electrolyte solutions with especially high anodic stability in which magnesium anodes can function at a high level of cycling efficiency; the other was to develop a cathode that can reversibly intercalate Mg ions in these electrolyte systems. The new magnesium batteries consist of Mg metal anodes, an electrolyte with a general structure of Mg(AlX(3-n)R(n)R')(2) (R',R = alkyl groups, X = halide) in ethereal solutions (e.g., tetrahydrofuran, polyethers of the "glyme" family), and Chevrel phases of MgMo(3)S(4) stoichiometry as highly reversible cathodes. With their practical energy density expected to be >60 Wh/Kg, the battery systems can be cycled thousands of times with almost no capacity fading. The batteries are an environmentally friendly alternative to lead-acid and nickel-cadmium batteries and are composed of abundant, inexpensive, and nonpoisonous materials. The batteries are expected to provide superior results in large devices that require high-energy density, high cycle life, a high degree of safety, and low-cost components. Further developments in this field are in active progress.