Solid electrolyte interface layer

In silicone electrodes, a factor that affects the cell performance is the formation of a solid electrolyte interface layer on the sUicon surface (47). Initially, the surface of the silicon material has a thin native oxide layer on it which has a low conductivity. During the first charge, this layer is replaced by a solid electrolyte interface layer of higher ionic conductivity formed from reactions with the electrolyte and reduction of the solvents. 

A stable solid electrolyte interface layer with good ionic conductivity that can withstand the volume changes is essential to the proper working of the cells, and in this regard certain solid electrolyte interface products are much better than others. 

We shall use the familiar Gouy-Chapman model (3 ) to describe the behaviour of the diffuse double lpyer. According to this model the application of a potential iji at a planar solid/electrolyte interface will cause an accumulation of counter-ions and a depletion of co-ions in the electrolyte near the interface. The disposition of diffuse double layer implies that if the surface potential of the planar interface at a 1 1 electrolyte is t ) then its surface charge density will be given by ( 3)... 

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],... 

B.E. Conway, The Solid/Electrolyte Interface, NATO Conf. Ser. 6, Vol. 5 on Atomistics of Fracture, R.M. Latanision, Ed., Plenum (1983) 497. (Review, emphasis on metals double layers and water structure near charged surfaces.)... 

Thus, in the metal/YSZ systems of solid-state electrochemistry, AC-impedance spectroscopy provides concrete evidence for the formation of an effective electrochemical double layer over the entire gas-exposed electrode surface. The capacitance of this metal/gas double layer is of the order of 100-500 pF cm-2 of superficial electrode surface area and of the order 2-10 pF cm-2 when the electrode roughness is taken into account and, thus, the true metal/gas interface surface area is used, comparable to that corresponding to the metal/solid electrolyte double layer. Furthermore AC-impedance spectroscopy... 

Fig. 49 Schematic representation of a metal catalyst electrode deposited on a O2 - and a Na+-conducting solid electrolyte, showing the location of the metal-solid electrolyte double layer and that of the effective double layer created at the metal/gas interface due to potential-controlled ion migration (backspillover). (Reprinted with permission from Ref. 23, Copyright 2001 by Kluwer/Plenum Publishers).

The backspillover ions (O, Na, etc.) are each accompanied by their compensating (screening) charge in the metal, thus forming surface dipoles. Consequently, these surface dipoles form an "effective electrochemical double layer" on the gas-exposed, i.e., catalytically active, catalyst surface, in addition to the classical double layer which exists at the metal-solid electrolyte interface (Figure 13). 

Porous photoelectrochemical systems consist of an insulating or semiconducting solid network permeated with a conducting electrolyte solution the dimensions of the solid structures and pores are in the 1-500-nm range. A typical semiconduct or/electrolyte interface has a width of between 0.5 nm (the Helmholtz layer in a concentrated electrolyte solution) and 100 nm (typical depletion layer in a semiconductor). Thus, the width of the solid/electrolyte interfacial layer can be... 

S. Sevastyanov, and A. Popov, J. Electroanal. Chem. Interfacial Electrochem. 145(2), 225 (1983) B. E. Conway, The Solid-Electrolyte Interface, Nato Conf. Ser., Ser. 6(5), 497 (1983) G. A. Martynov and R. R. Salem, Electronic Capacitor at a Metal/Electrolyte Interface, Elektrokhimiya 19, 1060-1070 (1983) and G. A. Martynov and R. R. Salem, Lecture Notes in Chemistry, Vol. 33 Electrical Double Layer at a Metal-Dilute Electrolyte Solution Interface, Springer-Verlag, Berlin (1983) also B. W. Ninham, Surface Forces— The Last 30 Angstrom, Pure Appl. Chem. 53, 2135-2147 (1981). 

Another important feature for lithium graphite intercalation compounds in Li -containing electrolytes is the formation of solid electrolyte interface (SEI) film. During the first-cycle discharge of a lithium/carbon cell, a part of lithium atoms transferred to the carbon electrode electrochemically will react with the nonaque-ous solvent, which contributes to the initial irreversible capacity. The reaction products form a Lb-conducting and electronically insulating layer on the carbon surface. Peled named this film as SEI. Once SEI formed, reversible Lb intercalation into carbon, through SEI film, may take place even if the carbon electrode potential is always lower than the electrolyte decomposition potential, whereas further electrolyte decomposition on the carbon electrode will be prevented. 

During the 1970s, propylene carbonate (PC) was found to be a suitable solvent for lithium batteries, but this does not mean that PC is stable on lithium metal. PC is decomposed by a reduction process, after which a passivation layer [so-called solid electrolyte interface (SEI)] is formed on the surface of the lithium metal. In fact, most organic solvents are not stable at the potential of lithium metal. In addition, during the 1980s, no one believed that any organic solvent could be stable at more than 4 V. Thus, the electrochemical environment in LIB produces severe demands... 

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