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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 > 10−15 m2 s−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 mechanism.

Amorphous silicon is a promising high-capacity anode material for application in lithium-ion batteries. However, a huge drawback of the material is that the large capacity losses taking place during cycling lead to an unstable performance. In this study we investigate the capacity losses occurring during galvanostatic lithiation of amorphous silicon thin film electrodes by in-situ neutron reflectometry experiments for the first ten cycles. As determined from the analysis of the neutron scattering length density and of the film thickness, the capacity losses are due to irreversible storage of lithium in the electrode. The amount of stored lithium increases during cycling to 20% of the maximum theoretical capacity after the 10th cycle. Possible explanations are discussed.

Electrodes made of amorphous silicon are a promising alternative to graphite as anode material for the next generation of high-capacity Li-ion batteries. In order to optimize such batteries the mechanism of lithium incorporation into the electrode during charging has to be elucidated. In the present study we measured the modification of lithium distribution taking place during galvanostatic lithiation of about 600 nm thick amorphous silicon film electrodes at low current densities of about 30 μA/cm2 (∼ C/14) by Secondary Ion Mass Spectrometry. The results were confirmed by X-ray Photoelectron Spectroscopy. The results indicate a two-step lithiation procedure where the electrode is transformed first into a homogeneously lithiated Li x Si phase with a low Li content of x ≈ 0.3. During later stages of the lithiation process, the results are in agreement with the penetration of a highly lithiated phase via a moving phase boundary, as also observed for crystalline silicon.

Amorphous silicon is a promising high-capacity anode material for the next generation of lithium-ion batteries. However, the enormous volume expansion of the active material during lithiation up to...

Neutron reflectometry is used to study in situ the intercalation of lithium into amorphous silicon electrodes. The experiments are done using a closed three-electrode electrochemical cell setup. As a working electrode, an about 40 nm thick amorphous silicon layer is used that is deposited on a 1 cm thick quartz substrate coated with palladium as a current collector. The counter electrode and the reference electrode are made of lithium metal. Propylene carbonate with 1 M LiClO4 is used as an electrolyte. The utility of the cell is demonstrated during neutron reflectometry measurements where Li is intercalated at a constant current of 100 μA (7.8 μA cm(-2)) for different time steps. The results show (a) that the change in Li content in amorphous silicon and the corresponding volume expansion can be monitored, (b) that the formation of the solid electrolyte interphase becomes visible and (c) that an irreversible capacity loss is present.

Experiments on self-diffusion in amorphous silicon between 400 and 500 °C are presented, which were carried out by neutron reflectometry in combination with 29Si/natSi isotope multilayers. Short range diffusion is detected on a length scale of about 2 nm, while long range diffusion is absent. Diffusivities are in the order of 10−19–10−20 m2/s and decrease with increasing annealing time, reaching an undetectable low value for long annealing times. This behavior is strongly correlated to structural relaxation and can be explained as a result of point defect annihilation. Diffusivities for short annealing times of 60 s follow the Arrhenius law with an activation enthalpy of (0.74 ± 0.21) eV, which is interpreted as the activation enthalpy of Si migration.