Frank Berkemeier

Thickness dependent ion conductivity of lithium borate network glasses

Lithium borate network glasses are possible candidates for separator membranes in all-solid-state batteries. Thin films of a Li2O-borate glass were produced by argon beam sputtering and their specific ionic conductivities were measured by impedance spectroscopy. The conductivity of as-sputtered films is about two orders of magnitude higher compared to the conductivity of bulk glasses produced from the melt. Furthermore, thin films with a thickness of 7–125nm reveal a remarkable finite size effect after annealing: with decreasing thickness the specific dc conductivity increases about three orders of magnitude.

Mixed-alkali effect of tracer diffusion and ionic conduction in Na-Rb borate glasses as a function of total alkali content

Abstract The mixed-alkali effect is studied in Y [X Na2O · (1−X) Rb2O] · (1−Y) B2O3 glasses as a function of the total alkali content Y. To this aim the ionic transport in glasses with Y = 0.3 was investigated by impedance spectroscopy and by the radiotracer diffusion technique. The results are compared with data obtained earlier by our group on a glass system with Y = 0.2. In both systems the relative compositions X = Na/(Na + Rb) are varied from 0.0 to 1.0 in steps of 0.2. The dc electrical conductivity is higher in Y = 0.3 glasses than in Y = 0.2 glasses and exhibits a minimum in each system near X = 0.4. Also near X = 0.4 a pertaining maximum in activation enthalpy occurs being more pronounced for Y = 0.3 as compared to Y = 0.2. The glass-transition temperature Tg is almost identical for glasses with Y = 0.2 and 0.3 with a minimum near X = 0.2. The diffusivities of 22Na and 86Rb measured at a constant temperature (380 °C) show in both glass systems (Y = 0.3 and Y = 0.2) a similar dependence on the composition parameter X. The 86Rb diffusivity decreases exponentially with decreasing Rb content. The 22Na diffusivity decreases with decreasing Na content but reaches an almost constant plateau on the Rb-rich side. For each relative composition X the diffusivities of the two isotopes increase by nearly the same constant factor, when Y is increased from 0.2 to 0.3. In both systems 22Na- and 86Rb-diffusivities as functions of the composition parameter X intersect near X = 0.2 explaining the minimum in dc-conductivity. Composition dependent common Haven ratios were obtained by comparing the conductivity with the diffusion data.

Nanoanalysis and Ion Conductivity of Thin Film Battery Materials

Abstract Thin film ion-conductive materials may represent essential components of future all-solid-state batteries. Besides, they are also of interest from a fundamental point of view. In this article, the deposition of thin films of complex oxides and amorphous glasses is discussed. Methods of local chemical analysis by analytical electron microscopy and atom probe tomography are described and studies of atomic transport in sputtered network glasses are presented. As experimental examples, thin films of Li borate and silicate glasses, LiCoO2, V2O5, and Li4Ti5O12 are addressed.

Controlling the optical properties of sputtered-deposited LixV2O5 films

This study examines the influence of lithium intercalation on the optical properties of vanadium pentoxide films. The films with a thickness between 400 and 1000 nm were prepared by DC magnetron sputter deposition. Cyclic voltammetry and chronopotentiometry were used to set a well defined lithiation state of the LixV2O5 films between x = 0 and x = 1. The optical properties of these films were measured by optical reflectometry in the wavelength range between 500 and 1700 nm. From the reflectance data, the refractive index and the extinction coefficient of the films were finally calculated as a function of the wavelength using Cauchys dispersion model. The results confirm that the optical behavior of LixV2O5 films varies significantly upon lithium insertion. It is demonstrated that the changes produced in the optical properties are completely reversible within the limits of permanent structure changes.

Ion transport and phase transformation in thin film intercalation electrodes

Abstract Thin film battery electrodes of the olivine structure LiFePO4 and the spinel phase LiMn2O4 are deposited through ion-beam sputtering. The intercalation kinetics is studied by cyclo-voltammetry using variation of the cycling rate over 4 to 5 orders of magnitude. The well-defined layer geometry allows a detailed quantitative analysis. It is shown that LiFePO4 clearly undergoes phase separation during intercalation, although the material is nano-confined and very high charging rates are applied. We present a modified Randles–Sevcik evaluation adapted to phase-separating systems. Both the charging current and the overpotential depend on the film thickness in a systematic way. The analysis yields evidence that the grain boundaries are important short circuit paths for fast transport. They increase the electrochemical active area with increasing layer thickness. Evidence is obtained that the grain boundaries in LiFePO4 have the character of an ion-conductor of vanishing electronic conductivity.