We present an operando neutron reflectometry study on the electrochemical incorporation of lithium into crystalline silicon for battery applications. Neutron reflectivity is measured from the ⟨100⟩ surface of a silicon single crystal which is used as a negative electrode in an electrochemical cell. The strong scattering contrast between Si and Li due to the negative scattering length of Li leads to a precise depth profile of Li within the Si anode as a function of time. The operando cell can be used to study the uptake and the release of Li over several cycles. Lithiation starts with the formation of a lithium enrichment zone during the first charge step. The uptake of Li can be divided into a highly lithiated zone at the surface (skin region) (x ∼ 2.5 in LixSi) and a much less lithiated zone deep into the crystal (growth region) (x ∼ 0.1 in LixSi). The total depth of penetration was less than 100 nm in all experiments. The thickness of the highly lithiated zone is the same for the first and second cycle, whereas the thickness of the less lithiated zone is larger for the second lithiation. A surface layer of lithium (x ∼ 1.1) remains in the silicon electrode after delithiation. Moreover, a solid electrolyte interface is formed and dissolved during the entire cycling. The operando analysis presented here demonstrates that neutron reflectivity allows the tracking of the kinetics of lithiation and delithiation of silicon with high spatial and temporal resolution.

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-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 Å for potentials below 0.5 V. Afterwards a reversible and stable modification of the SEI between 70 Å (delithiated state) and 120 Å (lithiated state) takes place.

The solid electrolyte interphase (SEI) is a complex and fragile passivation layer with crucial importance for the functionality of lithium-ion batteries. Due to its fragility and reactivity, the use of in situ techniques is preferable for the determination of the SEI's true structure and morphology during its formation. In this study, we use in situ neutron reflectometry (NR) and in situ atomic force microscopy (AFM) to investigate the SEI formation on a carbon surface. It was found that a lithium-rich adsorption layer is already present at the open circuit voltage on the carbon sample surface and that the first decomposition products start to deposit close to this potential. During the negative potential sweep, the growth of the SEI can be observed in detail by AFM and NR. This allows precise monitoring of the morphology evolution and the resulting heterogeneities of individual SEI features. NR measurements show a maximum SEI thickness of 192 Å at the lower cutoff potential (0.02 V vs Li/Li), which slightly decreases during the positive potential scan. The scattering length density (SLD) obtained by NR provides additional information on the SEI's chemical nature and structural evolution.

Most commercial lithium-ion batteries and other types of batteries rely on liquid electrolytes, which are preferred because of their high ionic conductivity, and facilitate fast charge-transfer kinetics at the electrodes. On the other hand, hybrid battery concepts that combine solid and liquid electrolytes might be needed to suppress unwanted shuttle effects in liquid electrolyte-only systems, in particular if mobile redox systems are involved in the cell chemistry. However, at the then newly introduced interface between liquid and solid electrolytes, a solid−liquid electrolyte interphase forms. In this study, we analyze the formation of such an interphase between the solid electrolyte lithium phosphorous oxide nitride (Li x PO y N z , "LiPON") and various liquid electrolytes using in situ neutron reflectometry, quartz crystal microbalance, and atomic force microscopy measurements. Our results show that the interphase consists of two layers: a nonconducting layer directly in contact with "LiPON" and a lithiumrich outer layer. Initially, a fast growth of the solid−liquid electrolyte interphase is observed, which slows down significantly afterward, resulting in a thickness of about 20 nm eventually. Here, a formation mechanism is proposed, which describes the solid−liquid electrolyte interphase growth as the fast deposition of a film, which mostly covers the "LiPON", with only a little degree of remaining porosity. The residual void space is then slowly filled, thus blocking the remaining channels for ionic conduction, which leads to increasing resistance of the interphase. The results obtained imply that hybrid battery concepts with liquid electrolyte and solid electrolyte can be hampered by highly resistive interphases, whose formation cannot be simply slowed down or suppressed. Further research is required regarding possible countermeasures.

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.