Polymer-electrolyte interphase

The availability of tunable infrared diodes limits the accessible spectral range in the study by Deng et al. [330], data for the range 600-1000 cm could be obtained. LiOH was identified as the dominant species in the lithium/solid polymer electrolyte 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... 

One of the major problems of lithium polymer electrolyte systems is the development of high interfacial resistance at the lithium/polymer electrolyte interphase. This resistance grows with time and could be as high as 10 kO cm. This resistance layer is due to the reactions of lithium with water, other impurities, and the salt anions. Similar to nonaqueous electrolytes, the solid electrolyte interphase (SET) also exists in the lithium/polymer electrolyte systems. In this case, the solid electrolyte interface (SEI) consists of the inorganic reduction products of the polymer electrolyte and its impurities. 

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 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. 

In summary, two processes should be considered as indicated above when the polymer is reduced and can be represented by the equivalent circuit of Figure 3a. In this diagram, represents the solution resistance, Q refers to the double layer capacitance, Ri is the intrinsic resistance due to charge transfer of the redox process within the polymer film, W is the equivalent to the ionic diffiision at the film/electrolyte interphase, Q is the film capacitance, and Rf is the film resistance. In all cases, an electronic resistance component of the polymer film is considered to be connected in series with the solution resistance. Similar equivalent circuits were described in the literature for poly(2,5-di-(-2-thi yl)-thiophene) films earlier in the literature (25). The value of e electrical r istance of the polymer film varies considerably according to the applied potential to the polymer film, the film thickness and the electrol3rtic medium in which the measurement is taking place (26). Polymer film parameters are summarized in Table 1 for both oxidized and reduced states, respectively. 

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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