SEI Composition

Silicon Nanowire Electrodes for Lithium-Ion Battery Negative Electrodes 

Further charging to 10 mV (Fig. 1.25b] revealed a very different type of film that consisted of a composite of different kinds of 

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Figure 11. Depth profile of estimated SEI composition on the basal (B) and cross section (CS) plane of HOPG, 1.2 mol L"1 LiAsF6 / EC-DEC (1 2) (Chapter 111 Sec, 2),...

The SEI is formed by parallel and competing reduction reactions and its composition thus depends on i0, t], and the concentrations of each of the electroactive materials. For carbon anodes, (0 also depends on the surface properties of the electrode (ash content, surface chemistry, and surface morphology). Thus, SEI composition on the basal plane is different from that on the cross—section planes. 

Figure 17. The basal plane and prismatic surfaces of graphite have different functions with respect to lithium intercalation and de-intercalation (= charge, discharge, self-discharge, etc.). As a consequence, only the electrolyte decomposition product layers at the prismatic surfaces have SEIfunction. Any processes related with electrolyte decomposition product layers at the basal plane surfaces (= non-SEI layers) therefore can not be directly related to electrochemical data such as charge, discharge, self-discharge, etc. The situation is even more complex as the SEI composition and morphology at the basal and prismatic surface...

Quantum dots are detectable by SEM by topographical contrast with secondary electron imaging (SEI), compositional contrast with backscattered electron imaging (BEI), and scanning transmission electron imaging. By SEI, the quantum dots appear as particles on exposed surfaces that can resemble normal biological structures. However, the CdSe core and ZnS coating impart... 

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. 

The processes on the metallic lithium electrode are simple anodic dissolution with the formation of Li" " ions under discharge and cathodic deposition under charging. In a freshly assembled battery, the lithium surface is covered by SEI (see Chapter 11) that includes the products of interaction between lithium and components of electrolyte. Under cycling, the SEI composition changes in the course of the first several... 

Peled E., Golodnitsky D., Ulus A., Yufit V. Effect of carbon substrate on SEI composition and morphology, Electrochim. Acta 2004, 50, 391-395. 

Figure 11. Depth profile of estimated SEI composition on the basal (B) and cross section (CS) plane of...

In many cases there is a good correlation between the SEI composition and the reactivity of electrolyte components toward LiF and As-F-O species are found in the SEI formed in electrolytes containing LiAsF. " BF and CIO/ are much less reactive toward e/ (k < 10 ) and LiCl and B" are rarely found in the SEI in y-BL solutions." y-BL is expected to have a high k, similar to that of acetone. Ether is kinetically stable vs e/ (k <10 ), thus in ether-based solutions, the anion may be reduced. Indeed, in ether-based solutions containing LiBF., B° was found in the SEI." When COj, which has a high k, is added to the electrolyte, more Li COj is found in the SEI. EC is so far the best SEI-forming... 

In order to obtain information on the chemical composition of the SEI and the depth distribution of SEI-forming materials, high-resolution XPS spectra were recorded for different sputtering times. The intensity, the shape and the position of the main peaks in the Cls and 01s spectra of the cross-section and basal SEI were found to change on sputtering, indicating different SEI composition on the solution-side surface, in the bulk and at the bottom. 

Figure 7 depicts the proposed SEI composition as a function of sputtering time. The cross-section SEI (Eigure 7a) consists mainly of LiF (about 90 atomic % in the bulk of the SEI) with some polymers at the solution side of the SEI and some LiOC groups. Li O was found only at the bottom of the SEI. It should be mentioned that no lithium carbonate was found either on the solution-side surface of the cross-sectional SEI, or in the bulk. The basal SEI (Eigure 7b) contains much less LiE and much more polymeric material (about 50% at the surface of the SEI). From such compositional difference it was deduced that the solvents are preferentially reduced on the basal plane and the salt anion is preferentially reduced on the cross section. 

Figure 7 Estimated SEI composition on HOPG in LiPF,j EC DMC electrolyte. Reproduced from [74] by permission of the Journal of New Materials for Electrochemical Systems.

Figure 8 Estimated SEI composition on the cross-section and basal planes in LiAsF electrolyte.

Figure 23 Effect of the type of carbonaceous material on the SEI composition in LiAsE electrolyte. Reproduced from [84] by permission of Elsevier Science Ltd.

Overview of SEI Composition and Properties in Different Carbon/ Non-aqueous Electrolyte Systems... 

THEORETICAL INSIGHTS INTO THE SEI COMPOSITION AND FORMATION MECHANISM DENSITY FUNCTIONAL THEORY STUDIES... 

Theoretical Insights into the SEI Composition and Formation Mechanism 229... 

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