Darko Hanzel

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).

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.

Crystal Structure of a New Polymorph of Li2FeSiO4

We report on the crystal structure of a new polymorph of Li(2)FeSiO(4) (prepared by annealing under argon at 900 degrees C and quenching to 25 degrees C) characterized by electron microscopy and powder X-ray and neutron diffraction. The crystal structure of Li(2)FeSiO(4) quenched from 900 degrees C is isostructural with Li(2)CdSiO(4), described in the space group Pmnb with lattice parameters a = 6.2836(1) A, b = 10.6572(1) A, and c = 5.0386(1) A. A close comparison is made with the structure of Li(2)FeSiO(4) quenched from 700 degrees C, published recently by Nishimura et al. (J. Am. Chem. Soc. 2008, 130, 13212). The two polymorphs differ mainly on the respective orientations and alternate sequences of corner-sharing FeO(4) and SiO(4) tetrahedra.

On the Origin of the Electrochemical Capacity of Li2Fe0.8Mn0.2SiO4

On the Origin of the Electrochemical Capacity of Li2Fe0.8Mn0.2SiO4 Robert Dominko, Chutchamon Sirisopanaporn,* Christian Masquelier, Darko Hanzel, Iztok Arcon, and Miran Gaberscek** National Institute of Chemistry, SI-1001 Ljubljana, Slovenia ALISTORE-European Research Institute, 80039 Amiens Cedex, France Universite de Picardie Jules Verne, 80039 Amiens, France Jozef Stefan Institute, SI-1000 Ljubljana, Slovenia University of Nova Gorica, SI-5000 Nova Gorica, Slovenia Faculty for Chemistry and Chemical Technology, University of Ljubljana, 1000 Ljubljana, Slovenia

Electrochemical Behavior of Li2FeSiO4 with Ionic Liquids at Elevated Temperature

Ionic liquids, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide with 0.7 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or 0.5 M LiPF 6 , were successfully tested as electrolytes for Li 2 FeSiO 4 cathodes operating at elevated temperatures of 60°C. The electrolytes based on ionic liquids show good ionic conductivity (from 3.3 × 10 ―3 to 4.5 × 10 ―3 S cm ―1 ) together with good electrochemical stability up to 5 V vs Li/Li + . The electrochemical stability of electrolytes based on ionic liquids against an aluminum current collector was comparable to a diethyl carbonate:ethylene carbonate (DEC:EC) 1 M LiPF 6 electrolyte and even better when the electrolytes were tested with blank electrodes (only aluminum current collector, carbon black, and binder). In the first cycle the electrochemical testing of Li 2 FeSiO 4 showed a slightly lower reversibility in ionic liquids when compared to a DEC:EC 1 M LiPF 6 electrolyte; at higher cycles the reversibility and the obtained capacity were comparable to the one obtained in a DEC:EC 1 M LiPF 6 electrolyte. All electrochemical results show that LiTFSI can be used as a salt in ionic liquid-based electrolytes. These properties allow their potential application in large-scale lithium-ion batteries with improved safety.

Detailed In Situ Investigation of the Electrochemical Processes in Li2FeTiO4 Cathodes

The possible mechanism of lithium exchange from Li 2 FeTiO 4 is discussed on the basis of three different in situ structural characterization methods. The in situ X-ray diffraction (XRD) showed that the mechanism of lithium exchange was through a solid solution formation whereby the change in volume during the oxidation was 1.4%. The reversibility in terms of charge recuperation was complete; however, after the first reduction, the change in volume was still 0.4%. In situ Mossbauer spectroscopy experiment showed complete Fe(II) oxidization to Fe(III), while the reduction was not reversible. After the complete first cycle, the number of local environments in the Mossbauer spectra needed for a successful fit increased by at least two. Several local environments can be explained on the basis of extended X-ray absorption fine structure results. In the as-prepared sample, Fe cations are coordinated to three oxygen atoms at 2.03 A and another three at 2.13 A. With the oxidation of Fe, the splitting of two oxygen atoms gradually increases to 0.18 A, and the octahedra deformation gradually changes from 3 + 3 to 4 + 2. This deformation is not reversible and it can explain changes in the long-range order as observed from in situ XRD.