Solid electrolyte interphase surface-related process

Figure 18 Various models proposed for the surface films that cover Li electrodes in nonaqueous solutions. The relevant equivalent circuit analog and the expected (theoretical) impedance spectrum (presented as a Nyquist plot) are also shown [77]. (a) A simple, single layer, solid electrolyte interphase (SEI) (b) solid polymer interphase (SPI). Different types of insoluble Li salt products of solution reduction processes are embedded in a polymeric matrix (c) polymeric electrolyte interphase (PEI). The polymer matrix is porous and also contains solution. Note that the PEI and the SPI may be described by a similar equivalent analog. However, the time constants related to SPI film are expected to be poorly separated (compared with a film that behaves like a PEI) [77]. (With copyrights from The Electrochemical Society Inc., 1998.)...

The irreversible part of the potential curve in Fig. 5.11 is the charge loss, which corresponds to the formation of a passivation layer at the graphite surface. This so-called solid electrolyte interphase (SEI) layer is formed by electrolyte decomposition products. The SEI layer is essential for the reversible electrochemical process because it passivates the graphite surface against further electrolyte decomposition. " However, the charge loss related to the SEI layer formation needs to be minimized, since it detreases the energy density of the ceU. 

The surface films discussed in this section reach a steady state when they are thick enough to stop electron transport. Hence, as the surface films become electrically insulating, the active electrodes reach passivation. In the case of monovalent ions such as lithium, the surface films formed in Li salt solutions (or on Li metal) can conduct Li-ions, and hence, behave in general as a solid electrolyte interphase (the SEI model ). See the basic equations 1-7 related to ion transport through surface films in section la above. The potentiodynamics of SEI electrodes such as Li or Li-C may be characterized by a Tafel-like behavior at a high electrical field and by an Ohmic behavior at the low electrical field. The non-uniform structure of the surface films leads to a non-uniform current distribution, and thereby, Li dissolution from Li electrodes may be characterized by cracks, and Li deposition may be dendritic. The morphology of these processes, directed by the surface films, is dealt with later in this chapter. When bivalent active metals are involved, their surface films cannot conduct the bivalent ions. Thereby, Mg or Ca deposition is impossible in most of the commonly used polar aprotic electrolyte solutions. Mg or Ca dissolution occurs at very high over potentials in which the surface films are broken. Hence, dissolution of multivalent active metals occurs via a breakdown and repair of the surface films. 

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See also in sourсe #XX -- [ ]

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