Comparison between Na-Ion and Li-Ion Cells: Understanding the Critical Role of the Cathodes Stability and the Anodes Pretreatment on the Cells Behavior.

The electrochemical behavior of Na-ion and Li-ion full cells was investigated, using hard carbon as the anode material, and NaNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2 as the cathodes. A detailed description of the structure, phase transition, electrochemical behavior and kinetics of the NaNi0.5Mn0.5O2 cathodes is presented, including interesting comparison with their lithium analogue. The critical effect of the hard carbon anodes pretreatment in the total capacity and stability of full cells is clearly demonstrated. Using impedance spectroscopy in three electrodes cells, we show that the full cell impedance is dominated by the contribution of the cathode side. We discuss possible reasons for capacity fading of these systems, its connection to the cathode structure and relevant surface phenomena.

Catalytic Behavior of Lithium Nitrate in Li-O2 Cells.

The development of a successful Li-O2 battery depends to a large extent on the discovery of electrolyte solutions that remain chemically stable through the reduction and oxidation reactions that occur during cell operations. The influence of the electrolyte anions on the behavior of Li-O2 cells was thought to be negligible. However, it has recently been suggested that specific anions can have a dramatic effect on the chemistry of a Li-O2 cell. In the present paper, we describe how LiNO3 in polyether solvents can improve both oxygen reduction (ORR) and oxygen evolution (OER) reactions. In particular, the nitrate anion can enhance the ORR by enabling a mechanism that involves solubilized species like superoxide radicals, which allows for the formation of submicronic Li2O2 particles. Such phenomena were also observed in Li-O2 cells with high donor number solvents, such as dimethyl sulfoxide dimethylformamide (DMF) and dimethylacetamide (DMA). Nevertheless, their instability toward oxygen reduction, lithium metals, and high oxidation potentials renders them less suitable than polyether solvents. In turn, using catalysts like LiI to reduce the OER overpotential might enhance parasitic reactions. We show herein that LiNO3 can serve as an electrolyte and useful redox mediator. NO2(-) ions are formed by the reduction of nitrate ions on the anode. Their oxidation forms NO2, which readily oxidizes to Li2O2. The latter process moves the OER overpotentials down into a potential window suitable for polyether solvent-based cells. Advanced analytical tools, including in situ electrochemical quartz microbalance (EQCM) and ESR plus XPS, HR-SEM, and impedance spectroscopy, were used for the studies reported herein.

Self-assembled monolayers of perfluoroterphenyl-substituted alkanethiols: specific characteristics and odd-even effects.

Self-assembled monolayers (SAMs) formed by perfluoroterphenyl-substituted alkanethiols (C(6)F(5)-C(6)F(4)-C(6)F(4)-(CH(2))(n)-SH, FTPn) with variable length of the aliphatic linker (n = 2 and 3) were prepared on (111) Au and Ag and characterized by a combination of several complementary spectroscopic and microscopic techniques. A specific feature of these systems is the helical conformation of the FTP moieties, which, along with the high electronegativity of fluorine, distinguishes them from the analogous non-fluorinated systems and makes them attractive for different applications. The SAMs were found to be well-defined, highly ordered, and densely packed, which suggests a perfect correlation between the orientations and, in particular, twists of the FTP helices in the adjacent molecules. Significantly, the SAM exhibited pronounced odd-even effects, i.e. a dependence of the molecular orientation and packing density on the length of the aliphatic linker in the target molecules, with parity of n being the decisive parameter and the direction of the effects on Au opposite to that on Ag. The presence of the odd-even effects in the FTPn system brings new aspects into the discussion about the origin and mechanism of these phenomena. Specifically, the helical conformation of the FTP moieties in the dense phase excludes a variation of the intramolecular torsion and molecular twist as the mechanism behind the odd-even effects.