Miran Gaberšček

Impact of the Carbon Coating Thickness on the Electrochemical Performance of LiFePO4 / C Composites

Porous, well crystalline LiFePO 4 /C composites with different amounts of carbon have been prepared by a sol-gel technique. The thickness of carbon coatings (paintings) has been determined by high-resolution transmission electron microscopy. It is shown that carbon coating thickness can be controlled by the amount of carbon and it has an impact on the obtained reversible capacity. Furthermore, it is shown that atomic ratio of nonactive Fe(III) phase (presumably Fe 3 P) in as-synthesized LiFePO 4 /C composites depends on the amount of carbon in the composite. Using Mossbauer spectroscopy, we have confirmed that the nonactive Fe(III) remains nearly unchanged in the composite during cycling. The lowest amount of carbon in LiFePO 4 /C composites obtained from citrate anion as a gelling agent was 3.2 wt % and this particular amount corresponds to the carbon coating thickness of about 1-2 nm. The reversible capacity obtained from the above-mentioned composite delivers close to 80% of the theoretical capacity at room temperature with a current density of 170 mA/g (C/1 rate).

The thermodynamic origin of hysteresis in insertion batteries.

Lithium batteries are considered the key storage devices for most emerging green technologies such as wind and solar technologies or hybrid and plug-in electric vehicles. Despite the tremendous recent advances in battery research, surprisingly, several fundamental issues of increasing practical importance have not been adequately tackled. One such issue concerns the energy efficiency. Generally, charging of 10(10)-10(17) electrode particles constituting a modern battery electrode proceeds at (much) higher voltages than discharging. Most importantly, the hysteresis between the charge and discharge voltage seems not to disappear as the charging/discharging current vanishes. Herein we present, for the first time, a general explanation of the occurrence of inherent hysteretic behaviour in insertion storage systems containing multiple particles. In a broader sense, the model also predicts the existence of apparent equilibria in battery electrodes, the sequential particle-by-particle charging/discharging mechanism and the disappearance of two-phase behaviour at special experimental conditions.

The role of carbon black distribution in cathodes for Li ion batteries

Abstract The influence of carbon black distribution/arrangement in cathode composite on cathode performance is studied using three types of active materials: LiMn2O2-spinel, LiCoO2, and LiFePO4. To the active materials, carbon black is added in two different ways: (a) using a conventional mixing procedure and (b) using a novel coating technology (NCT) invented in our laboratory. Different technologies yield different arrangement (distribution) of carbon black around active particles. It is shown that the uniformity of carbon black distribution affects significantly the cathode kinetics, regardless of the type of active particles used. A simple model explaining the influence of carbon black distribution on cathode kinetics is presented.

The Importance of Interphase Contacts in Li Ion Electrodes : The Meaning of the High-Frequency Impedance Arc

Li insertion electrodes are made by pressing a mixture of active material and additives on a metallic substrate. Here we estimate how various interphase contacts affect the electrode kinetics. We apply variable external mechanical pressure onto different cathodes and measure their impedance response. Similar experiments are performed on dry composites in contact with: Al or Cu foil, or Ag paste. Most surprisingly, we find that the high-frequency impedance arc is due to the contact impedance between the metal and the electrode material. This is in fundamental contradiction with previous interpretations. We propose an equivalent circuit explaining the observed phenomena.

On the Interpretation of Measured Impedance Spectra of Insertion Cathodes for Lithium-Ion Batteries

In the literature, the interpretation of electrochemical impedance spectra measured on insertion cathode materials is far from being unique. In most cases, various arbitrarily selected equivalent circuits have been used for analysis of spectra whereby the criterion of merit has mainly been the quality of fit. Herein, we propose a different approach. We try to explain the main features such as the high and medium frequency arcs and the low frequency diffusional tail using convenient (simplified) equivalent circuits derived from a quite general description of impedance due to a particulate (porous) system. The proposed models have a clear physical background. The meaning of selected circuit parameters is experimentally verified using carefully modeled experiments on LiFeP0 4 and LiCoO 2 materials. In particular, we discuss the effects of state of charge, external pressure, electrode mass (thickness), and electrolyte concentration on the measured and simulated equivalent circuits. In the last part, we discuss in certain depth the complications arising from poor electronic or ionic contacting (wiring) between different phases constituting electrodes.

Impact of LiFePO4 ∕ C Composites Porosity on Their Electrochemical Performance

Fe(III) citrate was used as a source for synthesis of microsized porous LiFePO 4 /C particles. All samples, prepared either by solid-state or by sol-gel techniques, are phase-pure triphylite phases, which, however, have different morphology highly influenced by the type of synthesis and synthesis parameters. Their common feature is porosity due to thermal decomposition of citrate anion. The impact of particle porosity on the electrochemical behavior is discussed in terms of qualitative results obtained from scanning electron microscopy (SEM) micrographs and in terms of quantitative results obtained from N 2 adsorption isotherms. The best electrochemical behavior (above 140 mAh/g at C/2 rate during continuous cycling) was obtained with composites prepared at a relatively high heating rate (above 5 K/min). This suggests that interlaced pores were formed inside particles. A strong correlation between the electrochemical results and the heating rate was observed, which could easily be explained based on SEM micrographs and on some trends found in porosity measurements. The latter reveal the main difference between samples prepared by solid-state and by sol-gel techniques.

Influence of carbon black distribution on performance of oxide cathodes for Li ion batteries

Abstract The influence of carbon black content and carbon black distribution on performance of oxide-based cathodes, such as LiCoO 2 and LiMn 2 O 4 , is investigated. The electronic conductivity of oxide material/carbon black composites is compared with electrochemical characteristics of the same composites. Uniformity of carbon black distribution in cathode composites is achieved using novel coating technology in cathode preparation. In this technology, the active particles are first pretreated in a gelatin solution. The adsorbed gelatin then controls the deposition of carbon black so that carbon black particles are uniformly distributed in the final composite. The influence of various parameters, such as pH of gelatin, amount of gelatin and concentration of carbon black on the uniformity of carbon black distribution is investigated. It is shown that the conventional technology of cathode preparation yields quite non-uniform distribution of carbon black in cathode material. At the end, we demonstrate that uniformity of carbon black distribution has a crucial impact on reversible capacity, especially at high current densities.

Cellulose as a binding material in graphitic anodes for Li ion batteries: a performance and degradation study

Four types of cellulose, in particular carboxy methyl cellulose (CMC), are tested as potential binding materials in graphitic anodes for lithium ion batteries. It is shown that a minimum content of a cellulose which gives acceptable anode properties (reversible capacity>300 mA h g−1 during the first 10 cycles, irreversible loss<20%) is about 2 wt.%, which is less than in the case of conventional polymeric binders (5–10 wt.%). Kinetics of insertion–deinsertion and passivation processes seem not to be affected by the presence of cellulose. Explanation for the electrode failure at cellulose contents lower than 1 wt.% is given based on X-ray diffraction and microscopy investigations. Finally, the structure (distribution) of cellulose in the composite anode material is discussed and (indirectly) checked with a series of experiments. Most results are compared with the corresponding results obtained either with gelatin or conventional polymeric binders or both.

Understanding controlled drug release from mesoporous silicates: theory and experiment.

Based on the results of carefully designed experiments upgraded with appropriate theoretical modeling, we present clear evidence that the release curves from mesoporous materials are significantly affected by drug-matrix interactions. In experimental curves, these interactions are manifested as a non-convergence at long times and an inverse dependence of release kinetics on pore size. Neither of these phenomena is expected in non-interacting systems. Although both phenomena have, rather sporadically, been observed in previous research, they have not been explained in terms of a general and consistent theoretical model. The concept is demonstrated on a model drug indomethacin embedded into SBA-15 and MCM-41 porous silicates. The experimental release curves agree exceptionally well with theoretical predictions in the case of significant drug-wall attractions. The latter are described using a 2D Fokker-Planck equation. One could say that the interactions affect the relative cross-section of pores where the local flux has a non-vanishing axial component and in turn control the effective transfer of drug into bulk solution. Finally, we identify the critical parameters determining the pore size dependence of release kinetics and construct a dynamic phase diagram of the various resulting transport regimes.

New Insight into Platinum Dissolution from Nanoparticulate Platinum‐Based Electrocatalysts Using Highly Sensitive In Situ Concentration Measurements

Time‐ and potential‐resolved electrochemical Pt dissolution from commercial Pt and prepared PtCu alloy nanoparticulate catalysts have been studied under potentiodynamic conditions in 0.1 M HClO4 by using on‐line inductively coupled plasma mass spectrometry (ICP‐MS). For the first time the exact amount of dissolved Pt per cycle has been measured on real electrocatalysts. Results show clearly that Pt dissolution depends on the particle size: approximately seven times as much Pt is released into the solution from commercial 3 nm Pt particles as from a commercial 30 nm Pt sample. The stability of our prepared PtCu electrocatalyst is higher than that of a commercial 3 nm electrocatalyst, which is, however, still slightly lower than that of a commercial 30 nm Pt electrocatalyst.