The lithiation onset of amorphous silicon (a-silicon) films up to 10% state of charge (SOC) is characterized by a Li⁺-uptake region around 0.5 V vs a Li reference electrode. In the literature, this is commonly attributed to surface processes such as the formation of a solid electrolyte interphase layer and/or the reduction of the surface native oxide, and more seldom to bulk processes such as reduction of oxygen contaminations inside the silicon film and to silicon lithiation. This work presents evidence that this process is associated with the lithiation of elemental silicon using electrochemical and non-electrochemical measurements and including a discussion of literature data. Cyclic voltammetry performed on pre-lithiated a-silicon and a-silicon films with different oxygen concentrations and with different film thicknesses reveals a lithiation process that is not associated with the reduction of oxygen. Elemental depth-profiling with secondary ion mass spectrometry (SIMS) reveals a two-phase lithiation mechanism at the lithiation onset, which sharply delimits non-lithiated silicon from a Li-poor phase of constant Li content (Li_0.3Si). The published operando neutron reflectometry data suggest that this is also the case for oxygen-free Si single crystal wafers. SIMS measurements further show that the whole a-silicon thin film is converted into the Li-poor phase. This phase remains stable in the vicinity of the current collector even at 100% SOC, indicating that mechanical stress may play an important role.

Redox studies on dense zirconia-doped ceria pellets were carried out by thermogravimetric investigations and dilatometry. Up to 1600 K reduction parameters determined by both methods correspond to each other. At higher temperatures, however, thermogravimetry overestimates the degree of reduction since mass loss is not only due to oxygen exsolution but also to selective evaporation of CeO₂ whose vapour pressure is considerably higher than that of ZrO₂. As a consequence surface segregation of zirconia occurs in (Ce,Zr)O_2−δ pellets leading to a porous surface zone of Ce₂Zr₂O₇ pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO₂ or H₂O since re-oxidation temperatures of (Ce,Zr)O_2−δ are known to be shifted towards lower temperatures with increasing ZrO₂ content. Thus, very harsh reduction conditions should be avoided for the (Ce,Zr)O_2−δ redox system. The kinetics investigations comprised the high temperature reduction step (T ≅ 1600 K) and the “low” temperature oxidation reaction with a carbon dioxide atmosphere (T ≅ 1000 K). The reduction kinetics (at around 1600 K and an oxygen activity of 7 × 10⁻⁴ in the gas phase) directly yield the (reduction) equilibrium exchange rate of oxygen in the order of 10⁻⁷ mol·O/(cm³·s) as the kinetics are surface controlled. The oxidation step at around 1000 K, however, occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling oxygen exchange coefficients, K, and oxygen diffusivities, D, were determined for so-called equilibrium experiments as well as for non-equilibrium measurements. From the obtained values for K and D the (oxidation) equilibrium exchange rates for differently doped ceria samples were determined. Their dependency on the oxygen activity and the nature and the concentrations of a tetravalent dopant (Zr) and trivalent dopants (La, Y, Sm) could be semi-quantitatively rationalised on the basis of a master equation for the equilibrium surface exchange rate.

The element silicon is currently considered as one of the most promising alternative electrode materials for lithium-ion batteries. During lithiation, silicon experiences a large volume increase, which often leads to material failure and significant irreversible loss of capacity. The production of amorphous thin layers of lithium-silicon compounds as pre-lithiated electrode material is a promising approach to reduce significant capacity losses and to “homogenize” volume expansion. We demonstrate that it is possible to produce stable amorphous lithium-silicon thin films with variable stoichiometry using ion beam co-sputtering. The electrochemical experiments show that the different starting compositions of the electrode material (different x for Li _x Si) compared to pure amorphous silicon cause significant differences in the electrochemical behavior and in the electrode kinetics. With an increasing amount of lithium in the lithium-silicon electrode, the specific charge and discharge capacity decreases, but irreversible capacity losses are reduced in the first cycle and the Coulomb efficiency stabilizes at a value of almost 100%. Thin films with a low Li concentration, such as Li _0.06 Si, are likely not stable to the lithiation process during long term cycling due to material failure, whereas Li _0.4 Si shows a much better charge/discharge stability up to 100 cycles.

Silicon is highly desired as high-energy density active Li storage material in Li-ion batteries, but usually does not withstand extended cycling. This work examines the C-rate capability up to Li plating and the long term cycling for ion-beam sputter-deposited amorphous (Si/C) × 10 multilayers (MLs) (with individual layer thicknesses between 5 and 27 nm), as well as for amorphous silicon and carbon single layers (with film thicknesses between 14 and 230 nm). Differential capacity plots were analyzed to examine the lithiation and delithiation mechanism. The silicon single-layers are stable for the first five cycles only, with a behavior of thinner films similar to supercapacitors. The carbon single layers show good cycling stability but also low capacities similar to graphite. The combination of silicon and carbon within Si/C MLs improved capacity and cycling behavior. The Li ⁺ insertion and extraction process from the Si/C MLs is reversible and dominated by silicon. It coincides even at high currents (10C) and after hundreds of cycles with that of the thicker silicon film at its initial cycles. The MLs combine the positive property of carbon (reversible cycling) and of silicon (high capacity). Thinner carbon layers in the ML increase the silicon capacity for all cycles. The topic of irreversible Li-losses is discussed.