Polymer electrode interphase

Later Thevenin and Muller suggested several modifications to the SEI model (1) the polymer-electrolyte interphase (PEI) model in which the lithium in PC electrolyte is covered with a PEI composed of a mixture of LijCOj, P(PO),and LiClO, P(PO), is polypropylene oxide, formed by reduction-induced polymerization of PC (2) the solid-polymer-layer (SPL) model, where the surface layer is assumed to consist of solid compounds dispersed in the polymer electrolyte (3) the compact-stratified layer (CSL) — in this model the surface layer is assumed to be made of two sublayers. The first layer on the electrode surface is the SEI, while the second layer is either SEI or PEI. The first two... 

Crasto, A.S., Own, S.H. and Subramanian, R.V. (1988). The influence of the interphase on composite properties Poly(cthylene-co-acrylic acid) and poly(methyl vinyl ether-co-maleic anhydride) electrode-posited on graphite fibers. Polym. Composites 9, 78-92. 

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.)...

Although the electronic conductivity of an interphase that is present on an electrode can be related to various optoelectronic properties that are also measurable with spectroscopic techniques, the direct measurement of surface conductivities is not a spectroelectrochemical method. It is nevertheless a surface sensitive method that provides results closely related to those of other methods discussed in this book. Data on the electrosorption of alcohols on gold electrodes [45] or the electrode potential dependent conductivity of intrinsically conducting polymers [46] have been obtained with in situ surface conductivity measurements. Figure 4.4 shows the electrical resistance of a poly(2-propylaniline) film measured in situ under experimental conditions suppressing any influence of solution phase conduction. The influence of... 

The rate of this process in aprotic electrolytes is rather high the exchange current density is fractions to several mA/cm. As pointed out already, the first contact of metallic lithium with electrolyte results in practically the instantaneous formation of a passive film on its surface conventionally denoted as solid electrolyte interphase (SEI). The SEI concept was formulated yet in 1979 and this film still forms the subject of intensive research. The SEI composition and structure depend on the composition of electrolyte, prehistory of the lithium electrode (presence of a passive film formed on it even before contact with electrode), time of contact between lithium and electrolyte. On the whole, SEI consists of the products of reduction of the components of electrolyte. In lithium thionyl chloride cells, the major part of SEI consists of lithium chloride. In cells with organic electrolyte, SEI represents a heterogeneous (mosaic) composition of polymer and salt components lithium carbonates and alkyl carbonates. It is essential that SEI features conductivity by lithium ions, that is, it is solid electrolyte. The SEI thickness is several to tens of nanometers and its composition is often nonuniform a relatively thin compact primary film consisting of mineral material is directly adjacent to the lithium surface and a thicker loose secondary film containing organic components is turned to electrolyte. It is the ohmic resistance of SEI that often determines polarization of the lithium electrode. 

Peled, E. Golodnitsky, D. Ardel G. Advanced model for solid electrolyte interphase electrodes in liquid and polymer electrolytes, J. Electrochem Soc. 1997,144, L208-L210. 

The examples of composite electrodes and composite films above represent an extension of the electrode itself into the chemically modified interphase with the solution. Another means of extending the dimensions of the electrode is by using films of conductive polymers (117), such as electropolymerized poly(pyrrole) (118), in electrode modification, or by simply replacing the electrode with a conducting polymer or semiconductor. This allows expansion of the intrinsic surface chemistry that is available from electrode materials. 

As part of an overall study of the electrode/electrolyte interphase of the electronically conducting polymer, polypyrrole, the surface structure and electronic properties have been investigated. 

Polymer Electrolyte Fuel Cells (PEFCs), Introduction, Fig. 4 Electrode layer (interphase) with three phase boundary (schematic) of a polymer electrolyte fuel cell (cathode side). Blue, polymer electrolyte black, carbon particles grey, platinum nanoparticles (Adopted from L. Gubler)... 

It is well known that graphite is unstable in some aprotic electrolytes. For instance, when propylene carbonate (PC) is used as a solvent, the cointercalation of solvent molecules and the Li ions will lead to the exfoliation of graphite layers Only in some selected electrolyte systems such as LiPF in EC/DEC (EC for ethylene carbonate and DEC for diethyl carbonate), can graphite show better cycling behavior. Solvent decomposition on the surface of conductive carbon or lithium electrodes will lead to the formation of a passivating layer. Peled named this layer as solid electrolyte interphase (SEI). ° It is an ionic conductor but electron insulator, mainly composed of LijCOj and various lithium alkylcarbonates (ROCO Li) as well as small amounts of LiE, LijO, and nonconductive polymers. These compounds have been detected on carbon and Li electrodes in various electrolyte systems. Therefore, it would be an interesting question whether semiconductive nano-SnO anode is also sensitive to electrolyte and electrolyte decomposition takes place on it. This section will characterize the structures and compositions of the... 

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