ABSTRACT
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.
ABSTRACT
The self-diffusion of lithium in congruent LiNbO3 single crystals was investigated at low temperatures between 379 and 523 K by neutron reflectometry. From measurements on (6)LiNbO3 (amorphous film)/(nat)LiNbO3 (single crystal) samples, Li self-diffusivities were determined in single crystals down to extremely low values of 1 × 10(-25) m(2) s(-1) on small length scales of 1-10 nm. The measured diffusivities are in excellent agreement with (extrapolated) literature data obtained by experiments based on Secondary Ion Mass Spectrometry and Impedance Spectroscopy. The tracer diffusivities can be described by a single Arrhenius line over ten orders of magnitude with an activation enthalpy of 1.33 eV, which corresponds to the migration energy of a single Li vacancy. A deviation from the Arrhenius behaviour at low temperatures, e.g., due to defect cluster formation is not observed.
ABSTRACT
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.
Subject(s)
Electric Power Supplies , Lithium/chemistry , Silicon/chemistry , Electrochemical Techniques , Electrodes , Ions/chemistry , Neutron DiffractionABSTRACT
Li self-diffusion in Li(2)O-deficient LiNbO(3) single crystals is investigated in the temperature range between 423 and 773 K (150-500 °C) by secondary ion mass spectrometry. A thin layer of ion-beam sputtered isotope enriched (6)LiNbO(3) was used as a tracer source, which allows one to study pure isotope interdiffusion. The diffusivities can be described by the Arrhenius law with an activation enthalpy of (1.33 ± 0.03) eV, which is in acceptable agreement with the migration energy of a single Li vacancy as determined by ab initio calculations given in the literature. Charge diffusivities as derived from impedance spectroscopy measurements on the same type of samples are identical to the tracer diffusivities within error limits. No indication of the formation of defect-complexes at low temperatures could be found in the diffusion behaviour.
