Werner Scheifele

A new type of nano-sized silicon/carbon composite electrode for reversible lithium insertion

A new type of nano-sized silicon/carbon composite was developed. It shows superior electrochemical cycling properties as negative electrode material for possible use in lithium-ion batteries with respect to high reversible and low irreversible capacity, and low fading.

Morphology of electrochemically prepared polyaniline. Influence of polymerization parameters

The kinetics of electrochemical aniline polymerization in aqueous acidic solutions have been investigated by chronocoulometry as a function of the electrolyte anion (i.e. HSO4–, Cl–, NO3–, ClO4–, CF3CO2–), concentration of aniline and para-phenylenediamine (PPDA) as an additive and also in terms of potential or current parameters during film preparation. The morphology of the resulting films was examined by scanning electron microscopy and by porosity measurements. Porosity values were determined based on film thicknesses (40–2000 µm) of polyaniline (PANI) layers saturated with electrolyte solution. While the standard recipes for electrochemical aniline polymerization yield films of porosities P>90%, much denser films (P≈60%) were prepared from aqueous solutions of 0.5 mol dm–3 aniline in 1.0 mol dm–3 HClO4 in the presence of 25 mmol dm–3 PPDA. The results are discussed in terms of a qualitative model for film growth and in view of potential applications in batteries.

Magnesium insertion batteries — an alternative to lithium?

Abstract Li + , Na + and Mg 2+ insertion in five hydrated, layered vanadium bronzes, LiV 3 O 8 , NaV 3 O 8 , KV 3 O 8 , Mg(V 3 O 8 ) 2 and Ca(V 3 O 8 ) 2 , was studied with regard to their use as electroactive materials in ion-transfer batteries. The behaviour of all bronzes is similar: variation in the content of bound lattice water in the bronzes is responsible for a difference in the electrochemical properties of the same material dried at different temperatures. The presence of this water is essential; the amount of H 2 O in the NaV 3 O 8 ·(H 2 O) γ and Mg(V 3 O 8 ) 2 ·(H 2 O) γ bronzes has been optimized to get the best electrochemical performance. The specific charges of both optimized bronzes calculated from the first discharge are about 330 and 150 mAh/g for Li + and Mg 2+ insertion, respectively.

In Situ Determination of Gravimetric and Volumetric Charge Densities of Battery Electrodes Polyaniline in Aqueous and Nonaqueous Electrolytes

Polyaniline (PANI) electrodes with redox capacities >1 C/cm2 were investigated by in situ electrogravimetry in aqueous electrolytes of pH values varying from <0 to 3 and in nonaqueous, propylene carbonate‐based media. Film mass, volume, and bulk density were determined in situ as a function of electrode potential. For aqueous electrolytes, the gravimetric analysis supports a charge‐transfer mechanism which depends on pH and involves the exchange of protons, electrolyte anions, and . Protonation equilibria of reduced PANI were monitored gravimetrically in aqueous , , and. In nonaqueous electrolytes, however, the main charge‐compensating process appears to be reversible insertion of anions. For carbonate solutions, it was shown that up to 0.9 electrons can be transferred per aniline unit, resulting in experimental charge densities of 270 Ah/kg, based on dried reduced PANI. Upon immersion and cycling in nonaqueous electrolytes, the practically usable charge density was lowered to 160 Ah/kg due to sorption of solvent and incomplete oxidation. Based on in situ gravimetry, a detailed analysis of mass and volume requirements for electroactive material and electrolyte in rechargeable PANI batteries (e.g., Li/PANI) was derived.

Colorimetric Determination of Lithium Content in Electrodes of Lithium-Ion Batteries

Graphites, as well as other intercalation materials used in lithium-ion batteries, change their color upon electrochemical insertion of lithium ions. In this study, in situ colorimetry was developed as a straightforward technical method to measure the local state of charge of lithium-ion battery electrodes. A laboratory cell with a glass window was built for in situ characterization of intercalation materials. Calibration curves of red, green, and blue color values vs state of charge were acquired and used for mapping of lithium distribution in battery electrodes. The lithium distribution in anodes of aged lithium-ion batteries was found to be highly heterogeneous.

CO2 Gas Evolution on Cathode Materials for Lithium-Ion Batteries

The gas evolution related to the film formation on cathode materials for lithium-ion batteries was studied using differential electrochemical mass spectrometry and subtractively normalized interfacial Fourier transform infrared spectroscopy. With both methods the oxidative formation of CO 2 was observed in standard battery electrolytes. We show the strong influence of the type of the electrolyte and especially of the additive, vinylene carbonate (VC), as well as the effect of the temperature on the CO 2 gas formation rate. The VC additive significantly reduces the gas formation rate in the commonly used voltage window between 3.0 and 4.3 V. Long cycling experiments show that test cells containing VC have a higher cycling stability compared to cells cycled without this additive. Cycling at elevated temperatures (60°C) results in a high, enduring CO 2 gas evolution, already starting at a lower cell voltage of about 3.5 V.

Graphite electrodes with tailored porosity for rechargeable ion-transfer batteries

Abstract A correlation between porosity and electrochemical behavior of thin graphite electrodes has been found. To enlarge the electrode/electrolyte interface area and thus to enhance the maximum current density. LiCl and NH4HCO3 were used as pore-forming additives during the preparation of graphite electrodes. By adjusting the porosity, the electrochemical performance of graphite electrodes was improved. The porosity optimization led to ∼500 μm thick graphite electrodes. Up to 30 cycles at 1.5 mA/cm2 (about 50 μA/mg C) have been performed on a number of these electrodes, confirming that a stable specific charge of > 300 Ah/kg (with respect to the graphite mass) can be achieved. Moreover, the irreversible charge loss in the first cycle was moderate, typically only ∼20% of the charge for lithium de-intercalation.

A novel electrochemical cell for in situ neutron diffraction studies of electrode materials for lithium-ion batteries

Lithium-ion batteries are based on the principle of intercalation of lithium ions in host materials, both at the anode and at the cathode. These materials are in general crystalline and, during the operation of the battery, they undergo numerous phase transitions and structural rearrangements, often amplified by the presence of an applied potential difference. While in situ X-ray diffraction is an established technique in this field, in situ neutron diffraction is still in its pioneering stages and only a few attempts have been made to design an electrochemical cell suitable for these experiments. The technical development of such a device, along with a discussion of its serviceability to combine electrochemical measurements with neutron diffraction experiments, is hereby presented.