Activation energy of diffusion determined from a single in-situ neutron reflectometry experiment

We present a new method for the determination of self-diffusivities in solids and the corresponding activation energy of diffusion using in-situ Neutron Reflectometry. In contrast to the classical ex-situ approach based on a sequence of isothermal measurements at different temperatures, the in-situ method allows one to work with a single experiment based on ramping the temperature with a constant rate. Our experiment demonstrates the success of the method for the model system of amorphous germanium. The activation energy of 2.2 eV and the absolute values of diffusivities achieved by the new method are in good agreement with the results of the classical approach, while a significantly lower amount of experimental time and samples are necessary. The presented method allows for an all-in-one type of experiment which can provide clearer and quicker results than similar methods using isothermal annealing procedures

Volume expansion of amorphous silicon electrodes during potentiostatic lithiation of Li-ion batteries

Large volume modifications during electrochemical cycling of electrodes in Li-ion batteries often limit successful applications due to stress formation, electrode fracture and delamination from the current collector. In this study, we carried out investigations on the volume changes taking place during potentiostatic lithiation of the high capacity electrode material amorphous silicon. Thin film electrodes were investigated at potentials of 0.45, 0.28, 0.19 und 0.06 V vs Li/Li+ during lithiation using in-operando neutron reflectometry. We found a strongly non-linear correlation between volume and state-of-charge for each potential applied in strong contrast to the results of galvanostatic lithiation. A possible explanation might be that for high current densities occurring at the beginning of each potentiostatic lithiation step free volumes are created in the electrode material leading to disproportionate volume expansion

Lithium-ion diffusion in near-stoichiometric polycrystalline and monocrystalline LiCoO2

Lithium-metal-oxide-based cathode materials like LiCoO2 are an essential part of lithium-ion batteries, which are intensively researched and continuously improved. For a basic understanding of kinetic processes that control lithium incorporation and removal into/from electrodes, the lithium-ion transport in the cathode material is of high relevance. This concerns lithium diffusivities, as well as defect structures, transport mechanisms, and confined diffusion paths, such as grain boundaries. In the present study, lithium tracer self-diffusion is investigated by means of isotope exchange and secondary ion mass spectrometry in polycrystalline sintered bulk samples of stoichiometric LiCoO2 with an average grain size of about 70 nm and in single crystalline LiCoO2 in the ab-plane and c-axis in the temperature range between 200 and 700 °C. For the polycrystals, we found an activation enthalpy of ΔH = 0.75 eV. In the single crystal, the lithium-ion diffusivities along the ab-plane are identical to the diffusivities in polycrystalline LiCoO2. This indicates that diffusion along grain boundaries is similar to bulk diffusion and does not play a dominating role for the overall lithium-ion migration. Along the c-axis, diffusivities are some orders of magnitude lower, but only a slightly higher activation energy of 0.94 eV is found. This provides the experimental evidence of the often-claimed sluggish lithium diffusion along the c-axis. We suggest that lithium diffusion along the c-axis is most likely determined by fast diffusion in the ab-plane and a slow transfer of lithium ions across the CoO2 layers

Self-diffusion of lithium in amorphous lithium niobate layers

We investigated lithium self-diffusion in amorphous lithium niobate layers between 298 and 423 K. For the experiments, amorphous 6LiNbO 3/ 7LiNbO 3 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 LiNbO 3 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. © by Oldenbourg Wissenschaftsverlag, München

Slow Lithium Transport in Metal Oxides on the Nanoscale

This article reports on Li self-diffusion in lithium containing metal oxide compounds. Case studies on LiNbO3, Li3NbO4, LiTaO3, LiAlO2, and LiGaO2 are presented. The focus is on slow diffusion processes on the nanometer scale investigated by macroscopic tracer methods (secondary ion mass spectrometry, neutron reflectometry) and microscopic methods (nuclear magnetic resonance spectroscopy, conductivity spectroscopy) in comparison. Special focus is on the influence of structural disorder on diffusion. © 2017 Walter de Gruyter GmbH, Berlin/Boston