Georg Lieser

LixFeF6 (x = 2, 3, 4) battery materials: structural, electronic and lithium diffusion properties

Lithium iron fluoride materials have attracted recent interest as cathode materials for lithium ion batteries. The electrochemical properties of the high energy density Li(x)FeF6 (x = 2, 3, 4) materials have been evaluated using a combination of potential-based and DFT computational methods. Voltages of 6.1 V and 3.0 V are found for lithium intercalation from Li2FeF6 to α-Li3FeF6 and α-Li3FeF6 to Li4FeF6 respectively. The calculated density of states indicate that Li2FeF6 possesses metallic states that become strongly insulating after lithium intercalation to form α-Li3FeF6. The large energy gain associated with this metal-insulator transition is likely to contribute to the associated large voltage of 6.1 V. Molecular dynamics simulations of lithium diffusion in α-Li3FeF6 at typical battery operating temperatures indicate high lithium-ion mobility with low activation barriers. These results suggest the potential for good rate performance of lithium iron fluoride cathode materials.

Unravelling the mechanism of lithium insertion into and extraction from trirutile-type LiNiFeF6 cathode material for Li-ion batteries

LiNiFeF6 was used as cathode material in lithium-ion cells and studied by in situ X-ray diffraction (XRD), in operando X-ray absorption spectroscopy (XAS) and 7Li MAS NMR spectroscopy. An optimised electrochemical in situ cell was employed for the structural and electrochemical characterisation of LiNiFeF6 upon galvanostatic cycling. The results for the first time reveal the lithium insertion process into a quaternary lithium transition metal fluoride with a trirutil-type host structure (space group P42/mnm). The in situ diffraction experiments indicate a preservation of the structure type after repeated lithium insertion and extraction. The lithium insertion reaction can be attributed to a phase separation mechanism between Li-poor Li1+x1NiFeF6 and Li-rich Li1+x2NiFeF6 (x1 ≲ 0.16 ≲ x2), where not only the weight fractions, but also the lattice parameters of the reacting phases change. The insertion of Li ions into [001]-channels of the trirutile structure causes an anisotropic lattice expansion along the tetragonal a-axes. An overall increase in the unit cell volume of ~6% and a reduction in the c/a ratio of ~4% are detected during discharge. Changes of atomic coordinates and distances suggest the accommodation of intercalated lithium in the empty six-fold coordinated 4c site. This is confirmed by 7Li MAS NMR spectroscopy showing two Li environments with similar intensities after discharging to 2.0 V. Furthermore, in operando XAS investigations revealed that only Fe3+ cations participate in the electrochemical process via an Fe3+/Fe2+ redox reaction, while Ni2+ cations remain electrochemically inactive.

Luminescence study on the solvation of Cm(III) in binary aqueous solvent mixtures

Abstract The preferential solvation of Cm(III) in binary aqueous mixtures of MeOH, tBuOH, DMSO, MeCN and acetone is studied at varied solvent composition. Solvation is investigated by time-resolved laser fluorescence spectroscopy and emission spectra and fluorescence lifetime data are obtained. At high mole fractions of organic solvent, preferential solvation of Cm(III) increases in the order: Me2CO < tBuOH ≈ MeOH < MeCN < H2O < DMSO. Thermodynamic data are derived from the spectroscopic results showing small positive standard Gibbs energies for the transfer of Cm(III) into the various solvent mixtures except for DMSO mixtures where negative values are found. The spectroscopically obtained enthalpies of transfer are fitted to a solvation model using the model parameters Δ Δ Hº12 and (αn+βN)º, rendering valuable information on the interaction of Cm(III) with the solvent molecules. Cm(III)-solvent interaction increases in the order: tBuOH < MeCN < MeOH < DMSO.