Johanna Rahn

Lithium Transport through Nanosized Amorphous Silicon Layers

Lithium migration in nanostructured electrode materials is important for an understanding and improvement of high energy density lithium batteries. An approach to measure lithium transport through nanometer thin layers of relevant electrochemical materials is presented using amorphous silicon as a model system. A multilayer consisting of a repetition of five [(6)LiNbO3(15 nm)/Si (10 nm)/(nat)LiNbO3 (15 nm)/Si (10 nm)] units is used for analysis, where LiNbO3 is a Li tracer reservoir. It is shown that the change of the relative (6)Li/(7)Li isotope fraction in the LiNbO3 layers by lithium diffusion through the nanosized silicon layers can be monitored nondestructively by neutron reflectometry. The results can be used to calculate transport parameters.

Li diffusion and the effect of local structure on Li mobility in Li2O-SiO2 glasses.

Aimed to improve the understanding of lithium migration mechanisms in ion conductors, this study focuses on Li dynamics in binary Li silicate glasses. Isotope exchange experiments and conductivity measurements were carried out to determine self-diffusion coefficients and activation energies for Li migration in Li2Si3O7 and Li2Si6O13 glasses. Samples of identical composition but different isotope content were combined for diffusion experiments in couples or triples. Diffusion profiles developed between 511 and 664 K were analyzed by femtosecond laser ablation combined with multiple collector inductively coupled plasma mass spectrometry (fs LA-MC-ICP-MS) and secondary ion mass spectrometry (SIMS). Analyses of diffusion profiles and comparison of diffusion data reveal that the isotope effect of lithium diffusion in silicate glasses is rather small, consistent with classical diffusion behavior. Ionic conductivity of glasses was measured between 312 and 675 K. The experimentally obtained self-diffusion coefficient, D(IE), and ionic diffusion coefficient, D(σ), derived from specific DC conductivity provided information about correlation effects during Li diffusion. The D(IE)/D(σ) is higher for the trisilicate (0.27 ± 0.05) than that for the hexasilicate (0.17 ± 0.02), implying that increasing silica content reduces the efficiency of Li jumps in terms of long-range movement. This trend can be rationalized by structural concepts based on nuclear magnetic resonance (NMR) and Raman spectroscopy as well as molecular dynamic simulations, that is, lithium is percolating in low-dimensional, alkali-rich regions separated by a silica-rich matrix.

Self-Diffusion of Lithium in Amorphous Lithium Niobate Layers

Abstract We investigated lithium self-diffusion in amorphous lithium niobate layers between 298 and 423 K. For the experiments, amorphous 6LiNbO3/ 7LiNbO3 isotope hetero-structures were deposited by ion beam sputtering and analysed after diffusion annealing by secondary ion mass spectrometry (SIMS). This arrangement allows one to study pure isotope interdiffusion. The results show that the diffusivities obey the Arrhenius law with an activation enthalpy of 0.7 eV, which is considerably lower than the activation enthalpy found for LiNbO3 single crystals in literature. Consequently, the Li diffusivities are higher by at least eight orders of magnitude in the amorphous samples in the temperature range studied.

Low-Temperature DC Conductivity of LiNbO3Single Crystals

Abstract We report on conductivity measurements of LiNbO3 single crystals with congruent composition in ambient atmosphere between 449 and 727 K which include the lowest temperatures covered so far. The ionic DC conductivity observed along the c-axis at, e. g., 650 K is 6.7 × 10-9 S/cm and shows an activation energy of 1.33 eV. The corresponding Li+ diffusion coefficients range between 2.5 × 10-22 and 1.4 × 10-16 m2/s. These results are perfectly consistent with our conductivity results in the high-temperature regime as well as SIMS measurements published recently (Rahn et al., Phys. Chem. Chem. Phys. 14 (2012) 2427). From the Li+ diffusion coefficients obtained here a Haven ratio of 0.7(2) can be deduced.

Li Diffusion in (110) Oriented LiNbO3 Single Crystals

Li diffusion is investigated in Li2O-deficient, (110) oriented LiNbO3 single crystals in the temperature range between 523 and 673 K by secondary ion mass spectrometry. A thin layer of ion-beam sputtered isotope enriched 6LiNbO3 was used as a tracer source, which allows one to study pure isotope interdiffusion. The diffusivities coincide with those of (001) oriented single crystals and follow the Arrhenius law with an activation enthalpy of 1.33 eV. The results prove the existence of a three-dimensional diffusion mechanism.

Lithium Diffusion in Ion-Beam Sputtered Amorphous LiAlO2

Abstract We investigated lithium self-diffusion in amorphous lithium aluminate (LiAlO2) layers between room temperature and 473 K. For the experiments, amorphous 6LiAlO2 (30 nm)/7LiAlO2 (1200 nm) isotope hetero-structures were deposited by ion-beam sputtering on sapphire substrates. Diffusion profiles were analysed by secondary ion mass spectrometry (SIMS). The results show that the diffusivities obey the Arrhenius law with an activation enthalpy of (0.94 ± 0.02) eV. This is not much different to the activation enthalpy of 1.14 eV found for LiAlO2 single crystals by impedance spectroscopy. It rationalizes the only modest enhancement of diffusivities in amorphous lithium aluminate compared to single crystals of three to five orders of magnitude in the temperature range studied, when compared with, e.g., lithium niobate.

A SIMS Study on Li Diffusion in Single Crystalline and Amorphous LiNbO3

We investigated lithium self-diffusion in amorphous and single crystalline lithium niobate at low temperatures of 323, 423 and 623 K. The diffusivity was studied by secondary ion mass spectrometry (SIMS), using ion beam sputtered 6LiNbO3 as a tracer source. Our intention was to get information how structural disorder influences ionic diffusivity, while chemical composition remains unchanged. The results indicate an increase of the Li diffusivity by about eight orders of magnitude in the amorphous state.

Lithium Diffusion in Li-Rich and Li-Poor Amorphous Lithium Niobate

The diffusion of lithium in amorphous lithium niobate layers is studied as a function of temperature between 293 and 423 K. About 800 nm thick amorphous 7LiNbO3 layers were deposited on sapphire substrates by ion-beam sputtering. As a tracer source about 20 nm thin 6LiNbO3 layers were sputtered on top. Isotope depth profile analysis is done by secondary ion mass spectrometry. Compared are amorphous samples which show a ratio of Li : Nb < 1 (Li-poor) and of Li : Nb > 1 (Li-rich) close to the stoichiometric composition of Li : Nb = 1 for crystalline LiNbO3. The results reveal that the diffusivities of both types of samples obey the Arrhenius law with an activation enthalpy of 0.70 eV and 0.83 eV, respectively. The diffusivities of the sample containing a higher amount of Li are lower by a factor of about two to ten. This demonstrates that variation of the Li content in amorphous samples over the stability range of the crystalline LiNbO3 phase has only a modest influence on diffusivities and activation enthalpies.