Li-ion migration

From the R and C values of the time constants a-c in the model, it was possible to estimate the thickness and resistivity of layers comprising the compact part of the surface films. The temperature dependence of these three time constants (e.g., linear Arrhenius plots for the different resistivities calculated that reflect different activation energy for Li+ ion migration in each layer), as well as their dependence on the solution composition and the experimental conditions, revealed that the model has a solid physicochemical ground [48,49,186],... 

Figure 6.15. Time dependent pair correlation function Gj(r,t) in composition 35iO.5Li2O-O.5K2O] 65Si02 for (a) Lf ion migration to Lf vacancy, (b) ion migration to Li vacancy, (c) Li ion migration to vacancy, and (d) ion...

FIGURE 19.10 A schematic diagram af a solid-state lithium battery. Lithium metal is the anade, and TiS2 is the cathode. During operation, Li ions migrate through the solid polymer electrolyte from the anode to the cathode while electrons flow externally from the anode to the cathode to complete the circuit. 

L1XV2O5 is a non-stoichiometric compound (x 1) and acts as a Li atom store. When a small potential is applied across the cell, Li ions migrate from the lithium polymer electrolyte into the WO3 layer forming a tungsten bronze (see equation 22.42 and discussion). Its formation results in a colour change from colourless to blue. 

On discharge, Li ions migrate through the separator from the anode to the cathode. Charging reverses the migration. 

Electrochemical impedance spectroscopy (EIS) provides indirect information about the surface phenomena of all kinds of electrodes [32]. The high-frequency part of impedance spectra of electrodes is usually attributed to surface phenomena such as Li-ion migration through surface films, surface film capacitance, and interfacial charge transfer [33]. However, it should be noted that EES provides very ambiguous information. A special skill, as well as experience, is needed for a reliable assignment of spectral features to the time constants of a complicated electrochentical system such as that of composite electrodes [34]. 

The subject of surface films on electrodes in non-aqueous solutions is mostly important for the field of batteries. The performance of both Li and Li-ion batteries depends strongly on passivation phenomena that relate to surface film formation on both the anodes and the cathodes. Lithium and lithiated carbon anodes reduce all the solvents and salt anions in electrolyte solutions relevant to Li batteries. The products of these surface reactions always contain insoluble Li salts that precipitate on the electrodes as surface films. All charge transfer processes of Li, Li-C, and Li alloy anodes in Li batteries involve the critical step of Li-ion migration through the surface films. Thereby, the composition, structure, morphology, and electrical properties of surface films on Li, Li-C, and Li alloy electrodes were smdied very intensively over the years. In contrast, reversible magnesium electrodes can function only in surface film-free conditions. ... 

As discussed in the first sections, the surface films formed in these systems, which comprise insoluble Li salts, allow Li-ion migration through them, but block electron transfer from the Li-C electrode to the solution species. In general, the irreversible capacity of carbon electrodes depends on many factors, including the type of carbon, its morphology, its surface area, and, of course, the solution composition. This highly important issue is dealt with in depth in the next section related to failure and stabilization... 

The surface films formed on toe carbon electrodes may be similar to those formed on Li and noble metal electrodes (polarized to low potentials) in the same solutions, as discussed in sections 1 and 2 above. Hence, it is possible to describe Li-ion migration through toe surface films formed on carbon electrodes by electrical models (i.e., equivalent circuit analogs) similar to those used to describe toe behavior of Li and noble metal electrodes (covered by surface films) in Li salt solutions [25,31]. 

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