Maximilian Fichtner

Publications (108) View all

• Source

  Available from: Anji Reddy Munnangi

  Article: CFx Derived Carbon–FeF 2 Nanocomposites for Reversible Lithium Storage

  M Anji Reddy, Ben Breitung, Venkata Sai, Kiran Chakravadhanula, Clemens Wall, Michael Engel, Christian Kübel, Annie K Powell, Horst Hahn, Maximilian Fichtner, [......], M Wall, C Engel, Prof A K Kübel, Prof H Powell, M Hahn, Fichtner, M Fichtner, V S K Chakravadhanula, C Kübel, Prof A K Powell
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  ABSTRACT: Lithium ion batteries are key energy storage devices that power today's consumer electronics. [ 1–4 ] However, their energy den-sity still falls short for transportation and large scale power storage applications. One way to increase the energy density of a battery is to use high energy density electrode materials. Current commercial Li-ion batteries mostly use insertion-based LiCoO 2 or LiFePO 4 as positive electrode materials. While LiCoO 2 is a layered compound with a specifi c capacity of 150 mAh g − 1 , [ 5 ] LiFePO 4 is a framework compound whose capacity is 170 mAh g − 1 . [ 6 ] Although both compounds show excellent reversibility with lithium, the specifi c capacity is lim-ited to single electron redox reaction per molecule or even less. A valid approach to increase the energy density of electrode material is to utilize all possible redox states of a metal ion. [ 7–9 ] Good candidates for this purpose are metal fl uorides as they reversibly react with lithium at relatively high voltage. [ 10–16 ] A disadvantage, however, is that metal fl uorides are electrical insulators. Further, when micrometer-sized metal fl uoride par-ticles are used the capacity fades rapidly with cycling. [ 17 , 18 ] To address these problems carbon-metal fl uoride nanocomposites (CMFNCs) were prepared using mechanical milling, which show superior electrochemical performance. [ 17–19 ] In addi-tion, considerable efforts have been made to understand and improve the electrochemical performance of metal fl uoride sys-tems. [ 20–33 ] Recently, a ferrocene/cobaltocene based method was developed for the synthesis of C-Fe-LiF/C-Co-LiF nanocompos-ites which showed excellent cycling stability. [ 34–36 ] It was shown that the microstructure and the iron-carbon contact are crucial for both reversibility and capacity. [ 34 , 35 ] Among the various metal fl uorides, iron fl uorides are an important class due to their low cost and low toxicity. In this context, FeF 2 is an interesting cathode material which has a thermodynamic reduction potential of 2.66 V versus lithium and a theoretical specifi c capacity of 571 mAh g − 1 , which leads to a theoretical gravimetric energy density of 1519 Wh kg − 1 . [ 12 , 23 , 32 ] FeF 2 is an electrical insulator and needs to be in intimate con-tact with electronic conductors at the nanoscale in order to become electrochemically active. A good choice for this purpose is graphitic carbon. However, the synthesis of graphitic carbon-metal fl uoride nanocomposites by ball milling leads to less stable interfaces and carbon may detach from the active mate-rial during its phase transformation, which is accompanied with expansion and shrinkage, so that more and more volume elements of the composite become inactive. Moreover, carbon coating by pyrolysis of nanometer-sized metal fl uorides with organic carbon precursor may not be effective as it is in the case of metal oxides. While the carbon becomes graphitic only above 600 ° C, metal fl uorides can decompose or agglomerate to micron-sized particles at this temperature. Instead, a fabri-cation method would be desirable which leads to a nanoscale dispersion and a stable anchoring of FeF 2 in a graphitic matrix. Herein, we report a new and facile one-step method for the chemical synthesis of novel carbon-metal fl uoride nanocom-posites where the active material is formed by "reactive inter-calation" inside a matrix which changes its properties into the desired direction at the same time, during the reaction. The FeF 2 product nanoparticles are clamped between graphitic carbon layers inside the matrix thus providing a very good and stable electrical contact between the active material and the electrically conducting carbon additive which both form from precursor compounds. The electrochemical properties of carbon–FeF 2 nanocomposites are demonstrated to show revers-ible lithium storage with high capacities. The composite is produced by reaction of a volatile Fe com-pound with chemically modifi ed graphite, graphite fl uoride (CFx) in that case. We found that iron pentacarbonyl Fe(CO) 5 , a liquid source for iron metal, reacts well inside the CFx matrix. CFx has a high thermodynamic reduction potential vs. lithium (4.2 V) so that it can react with iron metal to form FeF 2 , which has a thermodynamic reduction potential of 2.66 V vs. lithium. During the synthesis CFx acts both as source of fl uoride for the iron and as source of graphitic carbon for the matrix. The overall reaction proceeds according to the following equation.
  Advanced Energy Materials. 12/2012; 2013(3):308.
• Article: Effect of a Ti-based additive on the desorption in isotoped labeled LiB(H,D)4 – Mg(H,D)2 nanocomposites

  N. Boucharat, D. Wang, E. G. Bardají, M. Fichtner, W. Lohstroh
  05/2012;
• Source

  Available from: Elisa Gil Bardaji

  Article: Hindered Rotational Energy Barriers of BH4– Tetrahedra in β-Mg(BH4)2 from Quasielastic Neutron Scattering and DFT Calculations

  D. Blanchard, J. B. Maronsson, M. D. Riktor, J. Kheres, D. Sveinbjörnsson, E. Gil Bardají, A. Léon, F. Juranyi, J. Wuttke, K. Lefmann, B. C. Hauback, M. Fichtner, T. Vegge
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  ABSTRACT: In this work, hindered rotations of the BH4– tetrahedra in Mg(BH4)2 were studied by quasielastic neutron scattering, using two instruments with different energy resolution, in combination with density functional theory (DFT) calculations. Two thermally activated reorientations of the BH4– units, around the 2-fold (C2) and 3-fold (C3) axes were observed at temperatures from 120 to 440 K. The experimentally obtained activation energies (EaC2 = 39 and 76 meV and EaC3 = 214 meV) and mean residence times between reorientational jumps are comparable with the energy barriers obtained from DFT calculations. A linear dependency of the energy barriers for rotations around the C2 axis parallel to the Mg–Mg axis with the distance between these two axes was revealed by the DFT calculations. At the lowest temperature (120 K) only 15% of the BH4– units undergo rotational motion and from comparison with DFT results it is expectedly the BH4– units with the boron atom closest to the Mg–Mg axis, although dynamics related to local disorder existing at the boundary of the antiphase domains or to the presence of solvent in the sample cannot be strictly excluded. No long-range diffusion events were observed.
  01/2012;
• Article: Effect of several metal chlorides on the thermal decomposition behaviour of -Mg(BH4)2

  E. G. Bardají, N. Hanada, O. Zabara, M. Fichtner
  07/2011;
• Source

  Available from: Maximilian Fichtner

  Article: Raman and inelastic neutron scattering study on a melt-infiltrated composite of NaAlH4 and nanoporous carbon.

  D Colognesi, A Giannasi, L Ulivi, M Zoppi, A J Ramirez-Cuesta, A Roth, M Fichtner
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  ABSTRACT: Inelastic neutron scattering and Raman scattering spectra of a melt-infiltrated composite of NaAlH(4) and active carbon fibers have been measured at low temperature for two sample conditions: as prepared and subjected to hydrogen desorption-absorption cycling. After a careful data analysis, the present experimental results have been compared to the corresponding spectroscopic data taken from bulk NaAlH(4) and Na(3)AlH(6). Evident signatures induced by infiltration process onto the NaAlH(4) phonon bands have been detected, showing up as a strong peak broadening and smoothing together with, in some cases, an energy shift. Traces of Na(3)AlH(6), appearing as an extra intensity between 130 and 200 meV, seem also confirmed. A substantial agreement between neutron and Raman measurements has been found for the as-prepared melt-infiltrated sample, while for the cycled sample the two techniques produced rather dissimilar results. However, this apparent discrepancy can be explained by considering the different penetration depths of the two spectroscopic probes. Further work, both experimental and based on ab initio simulations, is surely needed in order to rationalize the finding of the present measurements.
  The Journal of Physical Chemistry A 06/2011; 115(26):7503-10. · 2.95 Impact Factor