The electrochemistry of
TiS2
,
ZrS2
,
RuO2
,
Co3O4
, and
V2O5
has been studied in several organic solvents containing
normalMgfalse(ClO4)2
in view of their application as positive electrodes in rechargeable magnesium batteries. Only
V2O5
showed promising coulombic capacity and reversibility. Mg2+ insertion into this oxide depends on the ratio between the amounts of
H2O
and Mg2+ as well as on the absolute amount of
H2O
in the electrolyte. Water molecules preferentially solvating Mg2+ions appear to facilitate the insertion process. The highest coulombic capacities of up to 170 Ah/kg were reached in acetonitrile solutions containing
1M normalMgfalse(ClO4)2+1M H2O
.
Thin films of nickel hydroxide deposited on gold electrodes have been characterized in detail by in situ surface Raman spectroscopy in conjunction with electrochemical techniques. Raman spectra were obtained for film thicknesses varying from less than one equivalent monolayer to several hundred monolayers, as determined from the faradaic charge for the cyclic voltammetric oxidation of
normalNifalse(OH)2
. For the thinnest films, Raman bands at 455 cm−1 and at 480 and 560 cm−1were obtained for the reduced and oxidized films, respectively, using 647.1 nm excitation at roughened gold. These signals, identified with Ni‐OH and Ni‐O vibrations from deuterium isotope data, were diagnosed as arising from surface‐enhanced Raman scattering (SERS) in view of their absence for the reduced film when using smooth gold surfaces and/or green/blue laser excitation. Raman spectra were obtained using the latter conditions for thicker oxidized films, which were consistent with resonance Raman scattering (RRS). Analysis of the dependence of the
480/560 cm−1
band intensities as a function of film thickness under conditions where SERS or RRS predominates enables the relative contributions of these mechanisms as well as the influence of film absorbance to be assessed quantitatively. Raman spectra were also obtained in air and after “film aging” by potential cycling or heating. The spectral changes following the latter treatments support the evolution of a less hydrogen‐bound
normalNifalse(OH)2
film structure, i.e., the transformation of
α‐normalto β‐normalNifalse(OH)2
. By combining cyclic voltammetry with analysis of the dissolved film using atomic absorption spectroscopy, the effective oxidation state of nickel in the oxidized films is determined to be
+3.7 false(±0.1false)
. On the basis of the spectroscopic and electrochemical measurements, the most likely structure for the oxidized film is a hydrated form of
Mfalse(NiO2)3
, where M is the supporting electrolyte cation.
Due to their rather low molecular weight and their favorable electrochemical and solid-state properties, first row transition metal oxides seem to be specially attractive as cathode materials in electrochemical energy storage systems. Therefore, we undertook a detailed overview, covering electrochemical, conductivity, ion diffusivity, spectroscopic, and other physico-chemical data on metal oxides in relation to their behavior in batteries. Metal oxide-based primary batteries have achieved a high technological level and yield energy densities of up to 300 Wh kg-' or 880 Wh 1 1. Oxide-based secondary batteries, on the other hand, typically yield less than 100 Wh kg-L Based on the present review, V, Cr, Mn, and Co oxides seem to be the most promising solid-state cathode materials for future high performance secondary batteries.
gElectrochemical energy storage will become increasingly important with increasing complexity of our power distribution systems, increasing environmental pollution, and decreasing resources of fossil fuels. Rechargeable batteries seem to be prime candidates for storing renewable energies, e.g., solar or wind, over periods of hours to days, for load leveling, and for delivering rather unpolluting power to electric cars and to remote areas. Unfortunately, energy densities of commercial secondary batteries (typically <50 Wh kg-') are well below energy densities of nonrenewable and polluting fuels such as gas or oil (ca. 12,000 Wh kg-'). In addition, energy densities of electrochemical power sources are much lower than those calculated from the weight and the thermodynamic properties of battery active material (up to > 1000 Wh kg-1). Hopefully, the large discrepancy between theoretical and practical energy densities of batteries will be lowered considerably by future research.Any chemically founded selection of suitable active material for batteries should start with the Periodic Table.
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