Electrochemical lithiation of silicon electrodes neutron reflectometry and secondary ion mass spectrometry investigations

In situ neutron reflectometry and ex situ secondary ion mass spectrometry in combination with electrochemical methods were used to study the lithiation of amorphous silicon electrodes. For that purpose specially designed closed three electrode electrochemical cells with thin silicon films as the working electrode and lithium as counter and reference electrodes were used. The neutron reflectometry results obtained in situ during galvanostatic cycling show that the incorporation, redistribution and removal of Li in amorphous silicon during a lithiation cycle can be monitored. It was possible to measure the volume modification during lithiation, which is found to be rather independent of cycle number, current density and film thickness and in good agreement with first principles calculations as given in literature. Indications for an inhomogeneous lithiation mechanism were found by secondary ion mass spectrometry measurements. Lithium tracer diffusion experiments indicate that the diffusivities inside the lithiated region D gt; 10 amp; 8722;15 m2 s amp; 8722;1 are considerably higher than in pure amorphous silicon as known from literature. This suggests a kinetics based explanation for the occurrence of an inhomogeneous lithiation mechanis

Lithium insertion into silicon electrodes studied by cyclic voltammetry and operando neutron reflectometry

Operando neutron reflectometry measurements were carried out to study the insertion of lithium into amorphous silicon film electrodes during cyclic voltammetry CV experiments at a scan rate of 0.01 mV s amp; 8722;1. The experiments allow mapping of regions where significant amounts of Li are incorporated released from the electrode and correlation of the results to modifications of characteristic peaks in the CV curve. High volume changes up to 390 accompanied by corresponding modifications of the neutron scattering length density which is a measure of the average Li fraction present in the electrode are observed during electrochemical cycling for potentials below 0.3 V lithiation and above 0.2 V delithiation , leading to a hysteretic behaviour. This is attributed to result from mechanical stress as suggested in the literature. Formation and modification of a surface layer associated with the solid electrolyte interphase SEI were observed during cycling. Within the first lithiation cycle the SEI grows to 120 for potentials below 0.5 V. Afterwards a reversible and stable modification of the SEI between 70 delithiated state and 120 lithiated state takes plac

Host Atom Diffusion in Ternary Fe-Cr-Al Alloys

In the Fe-rich corner of the Fe-Cr-Al ternary phase diagram, both interdiffusion experiments [1048 K to 1573 K (775 °C to 1300 °C)] and 58Fe tracer diffusion experiments [873 K to 1123 K (600 °C to 850 °C)] were performed along the Fe50Cr50-Fe50Al50 section. For the evaluation of the interdiffusion data, a theoretical model was used which directly yields the individual self-diffusion coefficients of the three constituents and the shift of the original interface of the diffusion couple through inverse modeling. The driving chemical potential gradients were derived using a phenomenological Gibbs energy function which was based on thoroughly assessed thermodynamic data. From the comparison of the individual self-diffusivities of Fe as obtained from interdiffusion profiles and independent 58Fe tracer diffusivities, the influence of the B2-A2 order–disorder transition becomes obvious, resulting in a slightly higher activation enthalpy for the bcc-B2 phase and a significantly lower activation entropy for this phase