Table of Contents:
The scope of this work is the investigation of ion dynamics in solid state electrolytes, such as ion conducting alkali containing calcium polyphosphate glasses, mixed alkali borosilicate glasses or praseodymium-manganese oxides. The ion transport properties are studied by means of the technique of bombardment induced ion transport (BIIT), which was recently developed and time-of-flight secondary ion mass spectrometry (ToF-SIMS).
Alkali containing calcium-polyphosphate glasses with the common composition x CaO - (55-x)M2O - 45 P2O5 (M = Na, K, Rb, Cs; x = 20, 30, 40) are synthesized and subsequently investigated by means of BIIT and ToF-SIMS concerning their structure-function-relation. For this purpose the ionic conductivity and activation energy for dc ion transport are measured as a function of composition. It is observed that the ionic conductivity increases with increasing molar alkali ion content, whereas the activation energy as well as the glass transition temperature decrease related to the cross-linking of Ca2+-ions. However, the ionic conductivity does not follow a certain trend. In Ca40Y glasses the conductivity decreases in the sequence of σNa > σK > σRb > σCs. This trend changes by increasing the molar content of the alkali ion, which results in a sequence of ionic conductivities in the order σCs > σNa > σRb > σK in Ca20Y glasses. Additionally, a special situation characterized by similar sizes of the alkali ion and the calcium ion is observed, leading to an increased conductivity. To clarify how and to which extend the measured properties are influenced by the dynamics of the calcium ions, the following experiment is performed: Glasses of the above-mentioned glass system are bombarded for a longer period of time with the same alkali species present in the glass system prior to the bombardment. Afterwards, the glasses are analyzed by means of ToF-SIMS generating concentration profiles of calcium ions. It is observed that a part of the calcium ions is replaced by the incorporated alkali ions. Consequently, also calcium ions contribute to the overall conductivity. Nevertheless calcium ions are not as mobile as alkali ions, since alkali ions are more likely being depleted than calcium ions. The analysis of the platinized back side of the glass subsequent to the bombardment reveals that the alkali ions define the most mobile ion species through the glass resulting in their electrodeposition and neutralization at the electrode-glass interface. Additionally, electrochemical degradation of the glass is observed.
In another investigation a mixed sodium and potassium containing borosilicate glass is investigated, concerning the question of how far the two alkali ions are contributing to the overall conductivity. Hence, the glass is bombarded with cesium ions for a long period of time and subsequently concentration profiles of each alkali ion are generated ex-situ via ToF-SIMS. These electrodiffusion profiles reveal that native sodium and potassium ions are replaced by cesium ions down to a depth of 200 nm below the bombarded surface. In the further course of the concentration profiles (in direction of ion transport) potassium is accumulated to a value above its former bulk concentration, while sodium is further depleted and replaced by potassium ions. A simulation based on coupled NERNST-PLANCK and POISSON equations, matching these concentration profiles yields the concentration dependence of the diffusion coefficients for sodium and potassium ions. At the back side electrode ToF-SIMS analysis shows that only sodium is accumulated at the glass-electrode-interface, since the potential at the back side electrode does not support potassium electrodeposition. Thus, a sodium containing interphase is formed. By means of transmission electron microscopy
(TEM) the incorporation of cesium ions at the surface of the glass as well as the formation of the interphase at the electrode-glass interface is verified at electron transparent thin lamellas cutted out by a focused ion beam (FIB).
Finally, praseodymium-manganese oxide (PMO) known as good electronic conductor is investigated by means of low energy K+-bombardment with respect to the PMO’s ionic conductivity. In contrast to former BIIT experiments the sample surface is not charged due to the higher conductivity of PMO.
Instead, the adsorbed potassium ions are neutralized at the sample surface. The analysis of the bombarded PMO by means of TEM and energy-dispersive X-ray spectroscopy (EDX) at FIB-prepared thin lamellas shows that potassium ions are deposited at the surface and diffuse into the PMO. Grain boundaries are observed in columnar-like structures due to the interface of two crystallites in different orientations. By ToF-SIMS analysis two different diffusion coefficients of potassium in PMO can be determined.