Lars Dörrer

Neutron reflectometry studies on the lithiation of amorphous silicon electrodes in lithium-ion batteries

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.

Lithium Transport through Nanosized Amorphous Silicon Layers

Lithium migration in nanostructured electrode materials is important for an understanding and improvement of high energy density lithium batteries. An approach to measure lithium transport through nanometer thin layers of relevant electrochemical materials is presented using amorphous silicon as a model system. A multilayer consisting of a repetition of five [(6)LiNbO3(15 nm)/Si (10 nm)/(nat)LiNbO3 (15 nm)/Si (10 nm)] units is used for analysis, where LiNbO3 is a Li tracer reservoir. It is shown that the change of the relative (6)Li/(7)Li isotope fraction in the LiNbO3 layers by lithium diffusion through the nanosized silicon layers can be monitored nondestructively by neutron reflectometry. The results can be used to calculate transport parameters.

Self-Diffusion in Amorphous Silicon.

The present Letter reports on self-diffusion in amorphous silicon. Experiments were done on ^{29}Si/^{nat}Si heterostructures using neutron reflectometry and secondary ion mass spectrometry. The diffusivities follow the Arrhenius law in the temperature range between 550 and 700 °C with an activation energy of (4.4±0.3)  eV. In comparison with single crystalline silicon the diffusivities are tremendously higher by 5 orders of magnitude at about 700 °C, which can be interpreted as the consequence of a high diffusion entropy.

Self-Diffusion of Lithium in Amorphous Lithium Niobate Layers

Abstract We investigated lithium self-diffusion in amorphous lithium niobate layers between 298 and 423 K. For the experiments, amorphous 6LiNbO3/ 7LiNbO3 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 LiNbO3 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.

Kinetics of Lithium Intercalation in Titanium Disulfide Single Crystals

Abstract Single crystals of titanium disulfide TiS2 were synthesized by chemical vapor synthesis and subsequently intercalated with n-butyl lithium (BuLi) in n-hexane. Experiments were carried out using a butyl lithium concentration between 0.8 and 10 mol L-1 and the temperature range was from 248 K to 328 K. The duration of the intercalation was varied from 2 h to 30 d. After the intercalation experiments concentration profiles of lithium, titanium and sulfur were measured parallel to the ab-plane of the crystal by LA-ICP-OES (LASER Ablation — Inductively Coupled Plasma - Optical Emission Spectroscopy). Chemical diffusion coefficients (D) were determined by fitting of the profiles to the specific solution of Ficks 2nd law for the given boundary conditions. The measured diffusivity in the ab-plane (D|| a/b) varies between 10-13 and 10-15 m2 s-1 at room temperature. These variations of D cannot be correlated to the reaction time. On the other hand a systematic increase of D|| a/b with the concentration of butyl lithium (cBuLi) was observed, with a corresponding decrease in activation energy from 59.6 ± 7.6 kJ mol-1 (cBuLi=1.6 mol L-1) to 42.6 ± 11.7 kJ mol-1 (cBuLi=10 mol L-1). Furthermore, profiles measured on the same crystals reveal D|| a/b values differing by up to a factor of 3. ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectromety) images gave evidence of inhomogenous insertion of lithium along crystal edges. These findings indicate that stress induced by widening of the crystal layers plays a crucial role in the intercalation kinetics. SIMS profiling perpendicular to the ab-plane gives evidence that D⊥ a/b is at least four orders of magnitude lower than D|| a/b.

Li Diffusion in (110) Oriented LiNbO3 Single Crystals

Li diffusion is investigated in Li2O-deficient, (110) oriented LiNbO3 single crystals in the temperature range between 523 and 673 K by secondary ion mass spectrometry. A thin layer of ion-beam sputtered isotope enriched 6LiNbO3 was used as a tracer source, which allows one to study pure isotope interdiffusion. The diffusivities coincide with those of (001) oriented single crystals and follow the Arrhenius law with an activation enthalpy of 1.33 eV. The results prove the existence of a three-dimensional diffusion mechanism.

Lithium Transport through Thin Silicon Layers: On the Origin of Bragg Peaks in Neutron Reflectometry Experiments

Lithium transport through ultrathin silicon layers can be measured non-destructively by neutron reflectometry (NR) using a multilayer composed of silicon layers embedded between solid state Li reservoirs. An established model system is a multilayer with a repetition of five [Si / natLiNbO3 / Si / 6LiNbO3] units. Two types of Bragg peaks are detectable in reflectivity patterns. These Bragg peaks result from the interference of neutrons reflected at periodic interfaces. One type of Bragg peak originates from the periodicity of the LiNbO3/Si chemical contrast (first order peak), while the other Bragg peak results from a superstructure with double periodicity. This superstructure may arise from 6Li/7Li isotope contrast or alternatively from periodic thickness variations, as shown by simulations based on the Parratt algorithm. The intention of the present paper was to elucidate the origin of the second Bragg peak. Experiments done by Secondary Ion Mass Spectrometry (SIMS) isotope sensitive depth profiling showed in a direct way that annealing at 360 °C destroys indeed the 6Li/7Li contrast, whereas the LiNbO3/Si chemical contrast remains unchanged. This evidences that the experimentally observed decrease of the second Bragg peak in the reflectivity pattern during annealing is a measure for Li transport through the Si layer.

Interdiffusion in ternary Fe–Cr –Al alloys with variable molar volume

Abstract For interdiffusion profiles obtained at 1000ºC in the Fe-rich corner of the ternary system Fe –Cr – Al the evaluation of these profiles with the method proposed by Dayananda and Sohn in 1999 has been performed. Further, an alternative mathematical model is presented which directly yields element mobilities and Kirkendall velocities from experiments if the Gibbs free energy of the system is given as a function of composition, temperature and pressure. Computer simulations show that, interestingly enough, already fairly weak deviations from (thermodynamic) ideality will lead to pronounced up-hill diffusion effects for the majority component, i.e. Fe.

Slow Lithium Transport in Metal Oxides on the Nanoscale

Abstract 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.

Erratum: Self-Diffusion in Amorphous Silicon [Phys. Rev. Lett. 116, 025901 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.025901.