Passivity layer-electrolyte interface

The simple model of a homogenous passive layer of Figure 5.6 becomes more complicated if a second alloy component is present as shown in Figure 5.30. The composition of the passive layer is then determined by the oxidation rates of the components A and B at the metal surface, fheir transfer rates through the film, and their transfer across the passive layer-electrolyte interface, i.e., their individual corrosion rates in the passive state. The reaction rates at both interfaces may be decisive for the layer composition. One example is the preferential dissolution of Fe " ions due to the extremely slow cation transfer of Cr " ions at the surface of the film, which leads to an accumulation of Cr(lll) wifhin the film for FeCr alloys. Another example is the preferential oxidation of A1 of an A1 alloy containing 1% Cu. Cu does not enter the film and is accumulated at the metal surface while an AI2O3 film is formed. These examples are discussed in defail in the following. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

The electrode/solution interface, in the simpler case (no passive layer on the electrode surface) can be modeled using the following equivalent circuit (Figure 1.18), where Rel stands for the electrolyte resistance, CDL for the double layer capacitance, and ZF the faradic impedance. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

For alloys the corrosion properties, as well as the composition of the passive layers, depend strongly on the chemical properties of the alloy components. For an alloy of chemically very different components, the noble metal tends to stay within the metal matrix, whereas the non-noble partner enters preferentially the oxide matrix or is dissolved more readily. The more-noble component enters the passive layer or is dissolved only if the potential is sufficiently positive. The more-noble component will be oxidized also later on a time scale if the potential is sufficiently positive. Besides thermodynamics also the kinetic properties of the system under study have a decisive influence on the various reactions. This involves the rate of transfer reactions at the metal/oxide and oxide/electrolyte interface, as well as the transfer of the cations and anions across the oxide matrix. 

As already mentioned, salt-containing liquid solvents are typically used as electrolytes. The most prominent example is LiPF6 as a conductive salt, dissolved in a 1 1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as 1 molar solution. It should be mentioned that this electrolyte is not thermodynamically stable in contact with lithium or, for example, LiC6. Its success comes from the fact that it forms an extremely stable passivation layer on top of the electrode, the so-called solid-electrolyte interface (SEI) [35], Key properties of such SEI layers are high Li+ and very low e conductivity - that is, they act as additional electrolyte films, where the electrode potential drops to a level the liquid electrolyte can withstand [36],... 

Oxide formation and passivation occurs in region III. This region describes the nature and the composition of the oxides formed as corrosion products. By using the Pourbaix diagrams, it is possible to identify the combinations of potential and pH that wiU stop the corrosion of metals and form passive oxide layers. At any given pH, it is possible to describe conditions at the metal-electrolyte interface that help to evaluate the equilibrium potential as a function of pH. 

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