Activation volumes in lead halides and other solids

Chemomechanical Interactions Dictate Lithium Surface Diffusion Kinetics in the Solid Electrolyte Interphase

The solid electrolyte interphase (SEI) plays a pivotal role in enabling fast ionic transport and preserving the battery electrodes from parasitic reactions with solvents. However, due to large volume changes of lithium (Li) electrodes, the SEI layer can potentially undergo mechanical failure, resulting in electrolyte degradation. The mechanical stability of the SEI is a critical aspect that needs to be modulated for designing rechargeable metal batteries with optimal performance. In this work, we perform density functional theory calculations to investigate the mechanical properties of lithium fluoride (LiF) and lithium oxide (Li₂O) nanofilms and quantify the Li surface diffusion kinetics over these two SEI materials. Based on our analysis, it is identified that Young's modulus and the ideal strength of the SEI are strong functions of the nanofilm thickness and crystallographic direction. Interestingly, we find that mechanical strain substantially alters the Li surface diffusion behavior on the SEI. For a strain of 4%, while the Li surface diffusion rate decreases by two orders of magnitude on the stretched Li₂O film, it increases two times on the stretched LiF film, indicating critical implications on the morphological stability of the metal anode. A fundamental correlation between inherent SEI properties and Li plating behavior is revealed, suggesting a potential pathway to achieve dendrite-free electrodeposition via SEI modulation.

Defects and lithium migration in Li2CuO2

Li₂CuO₂ is an important candidate material as a cathode in lithium ion batteries. Atomistic simulation methods are used to investigate the defect processes, electronic structure and lithium migration mechanisms in Li₂CuO₂. Here we show that the lithium energy of migration via the vacancy mechanism is very low, at 0.11 eV. The high lithium Frenkel energy (1.88 eV/defect) prompted the consideration of defect engineering strategies in order to increase the concentration of lithium vacancies that act as vehicles for the vacancy mediated lithium self-diffusion in Li₂CuO₂. It is shown that aluminium doping will significantly reduce the energy required to form a lithium vacancy from 1.88 eV to 0.97 eV for every aluminium introduced, however, it will also increase the migration energy barrier of lithium in the vicinity of the aluminium dopant to 0.22 eV. Still, the introduction of aluminium is favourable compared to the lithium Frenkel process. Other trivalent dopants considered herein require significantly higher solution energies, whereas their impact on the migration energy barrier was more pronounced. When considering the electronic structure of defective Li₂CuO₂, the presence of aluminium dopants results in the introduction of electronic states into the energy band gap. Therefore, doping with aluminium is an effective doping strategy to increase the concentration of lithium vacancies, with a minimal impact on the kinetics.

Lithium diffusion in Li5FeO4

The anti-fluorite type Li₅FeO₄ has attracted significant interest as a potential cathode material for Li ion batteries due to its high Li content and electrochemical performance. Atomic scale simulation techniques have been employed to study the defects and Li ion migration in Li₅FeO₄. The calculations suggest that the most favorable intrinsic defect type is calculated to be the cation anti-site defect, in which Li⁺ and Fe³⁺ ions exchange positions. Li Frenkel is also found to be lower in this material (0.85 eV/defect). Long range lithium diffusion paths were constructed in Li₅FeO₄ and it is confirmed that the lower migration paths are three dimensional with the lowest activation energy of migration at 0.45 eV. Here we show that doping by Si on the Fe site is energetically favourable and an efficient way to introduce a high concentration of lithium vacancies. The introduction of Si increases the migration energy barrier of Li in the vicinity of the dopant to 0.59 eV. Nevertheless, the introduction of Si is positive for the diffusivity as the migration energy barrier increase is lower less than that of the lithium Frenkel process, therefore the activation energy of Li diffusion.

Composition and temperature dependence of self-diffusion in Si1-x Ge x alloys

The knowledge of diffusion processes in semiconducting alloys is very important both technologically and from a theoretical point of view. Here we show that, self-diffusion in Si_1-x Ge _x alloys as a function of temperature and Ge concentration can be described by the cBΩ thermodynamic model. This model connects the activation Gibbs free energy of point defects formation and migration with the elastic and expansion properties of the bulk material. The approach allows the systematic investigation of point defect thermodynamic parameters such as activation enthalpy, activation entropy and activation volume, based on the thermo-elastic properties (bulk modulus and its derivatives, mean atomic volume and thermal expansion coefficient) of the two end-members of the Si_1-x Ge _x alloy. Considerable deviations from Vegard's law are observed, due to the diversification of the bulk properties of Si and Ge, in complete agreement with the available experimental data.

Charge carrier accumulation in lithium fluoride thin films due to Li-ion absorption by titania (100) subsurface

The thermodynamically required redistribution of ions at given interfaces is being paid increased attention. The present investigation of the contact LiF/TiO(2) offers a highly worthwhile example, as the redistribution processes can be predicted and verified. It consists in Li ion transfer from LiF into the space charge zones of TiO(2). We not only can measure the resulting increase of lithium vacancy conductivity in LiF, we also observe a transition from n- to p-type conductivity in TiO(2) in consistency with the generalized space charge model.

Solid-state 207Pb NMR studies of mixed lead halides, PbFX (X=Cl, Br, or I)

Solid-state 207Pb NMR studies have been conducted on mixed lead(II) halides of the type PbFX, where X=Cl, Br, or I. NMR data for the mixed halides are compared to the solid-state NMR data for the divalent, binary lead halides, PbX2 (X=F, Cl, Br, I). The NMR data are evaluated in the context of the structures of the compounds and the effects of the mixed halides on the electronic structure of the divalent lead. Data sets for the mixed halides are discussed and compared to those for the regular lead(II) halides.