Solid electrolyte interphase film

S. Li, X. Xu, X. Shi, B. Li, Y. Zhao, H. Zhang, Y. Li, W. Zhao, X. Cui, L. Mao, J. Power Sources 2012, 217, 503-508. Composition analysis of the solid electrolyte interphase film on carbon electrode of lithium-ion battery based on lithium difluoro(oxalate)borate and sulfolane. 

Xing, L.D. Li, W.S. Xu, M.Q. Li, T.T. Zhou, L., The reductive mechanism of ethylene sulfite as solid electrolyte interphase film-forming additive for lithium ion battery, J. Power Sources, 2011,196,7044-7047. 

AUiata D., Kotz R., Novak P., Siegenthaler H. Flectrochemical SPM investigation of the solid electrolyte interphase film formed on HOPG electrodes, Electrochem. Commun. 2000, 2, 436-440. 

Chen X, Li X, Mei D, Feng J, Hu MY, Hu J, Engelhard M, Zheng J, Xu W, Xiao J, Liu J, Zhang J-G (2014) Reduction mechanism of fluoroethylene carbonate for stable solid-electrolyte interphase film on silicon anode. Chemsuschem 7 549-554. doi 10.1002/ CSSC.201300770... 

An overview about more than 10 years of R D activities on solid electrolyte interphase (SEI) film forming electrolyte additives and solvents at Graz University of Technology is presented. The different requirements on the electrolyte and on the SEI formation process in the presence of various anode materials (metallic lithium, graphitic carbons, and lithium storage metals/alloys are particularly highlighted. 

In other words, these films behave as a solid electrolyte interphase (SEI)... 

When the anode is first charged, it slowly approaches the lithium potential and begins to react with the electrolyte to form a film on the surface of the electrode. This film is composed of products resulting from the reduction reactions of the anode with the electrolyte. This film is called the solid electrolyte interphase (SEI) layer [30], Proper formation of the SEI layer is essential to good performance [31-34], A low surface area is desirable for all anode materials to minimize the first charge related to the formation of SEI layer. Since the lithium in the cell comes from the lithium in the active cathode materials, any loss by formation of the SEI layer lowers the cell capacity. As a result, preferred anode materials are those with a low Brunauer, Emmett, and Teller (BET) surface area... 

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.)...

These surface films passivate the electrode, and thus further reduction of most of the solution species is inhibited. Therefore, in contrast to TAA-based solutions, there is no bulk solution reduction. At this stage, the cathodic potential limit becomes Li deposition at 0. V lAllA. This process is not inhibited by the surface films because most of these surface films are good Li ion conductors (as described by the solid electrolyte interphase model for Li electrodes in these solutions [27]). 

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. 

We have seen that the idea of an electrode film system is useful for electrochemistry of molten salts including low-temperature ionic liquids. It is not restricted, however, to this field only. As an example, the protective layer on lithium metal in aprotic organic electrolytes could be mentioned. This layer, so-called solid electrolyte interphase (SEl), exhibits properties of a polyfunctional conductor with high ionic conductivity (Li ions are the carriers) and low electronic conductivity of semiconductive nature. Some peculiarities of film systems with semiconductive character of electronic conductivity are considered below. 

Not long after Dahn s work, Shenoy et al. employed molecular dynamics simulations to study the formation and growth of solid electrolyte interphase for the case of EC, DMC, and mixtures of these two solvent on lithium metalhc electrode [61]. In their work, they investigated the constitutes and structures of SEI on lithium metal electrode with the dependence of electrolyte composition and temperature change. The results show that the SEI films grow faster in the case of EC compared to DMC, with EC+DMC mixtures falling in between, as shown in Fig. 5.22. 

Fig. 7.14 ExsituHRTEM near the surface of an epitaxial graphene film on a SiC [2 12 0] substrate after cathodic polarization in a Li metal cell, showing the structure of the SiC, the graphene layers, and the solid electrolyte interphase (SEl) with LiF crystals.

From the results summarized by chart (Fig. 6.9) it is apparent that carbon-based anodes can contribute as much as two-thirds of the internal cell resistance. This is mostly due to the resistive solid electrolyte interphase (SEI) film formation on the... 

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. 

Electrolyte Formulation, Irreversible Capacity and the SEI. Various electroljttes have been used in Li-ion batteries. The solvents used must be stable at both the anodie and cathodic potentials found in Li-ion cells, 0 V to 4.2 V vs. lithium. No practical solvents are thermodynamically stable with lithium or Li Cg near 0 V vs. Li, but many solvents undergo a limited reaction to form a passivation film on the electrode surface. This film spatially separates the solvent from the electrode, yet is ionicaiiy conductive, and thus allows passage of lithium ions. The passivation film, termed the solid electrolyte interphase (SET), therefore imparts extrinsic stability to the system allowing the fabrication of cells that are stable for years without significant degradation. " ... 

Wang et al. C80 Electrolyte A C80 calorimeter was used to study the thermal behaviors of electrolyte. C80 results show that alone shows one exothermic peak, which is attributed to the solid electrolyte interphase (SEI) decomposition, Li-electrolyte reaction as well as new SEI film formation, new SEI film decomposition, and Li with PVDF/other products reactions [34]... 

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