SEI formation

This chapter addresses several issues dealing with the mechanism of SEI formation on inert substrates, lithium, carbonaceous materials and tin-based alloys. Attention is currently focused on the correlation between the composition and morphology of the solid-electrolyte interphase forming on the different planes of highly ordered pyrolytic graphite (HOPG) and different types of disordered carbon electrodes in lithium-ion cells. 

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The excess charge consumed in the first cycle is generally ascribed to SEI formation and corrosion-like reactions of Li C6[19, 66, 118-120]. Like metallic lithium and Li-rich Li alloys, lithiated graphites, and more generally lithiated carbons are thermodynamically unstable in all known electrolytes, and therefore the surfaces which are exposed to the electrolyte have to be kinetically protected by SEI films (see Chapter III, Sec.6). Neverthe-... 

Film-forming chemical reactions and the chemical composition of the film formed on lithium in nonaqueous aprotic liquid electrolytes are reviewed by Dominey [7], SEI formation on carbon and graphite anodes in liquid electrolytes has been reviewed by Dahn et al. [8], In addition to the evolution of new systems, new techniques have recently been adapted to the study of the electrode surface and the chemical and physical properties of the SEI. The most important of these are X-ray photoelectron spectroscopy (XPS), SEM, X-ray diffraction (XRD), Raman spectroscopy, scanning tunneling microscopy (STM), energy-dispersive X-ray spectroscopy (EDS), FTIR, NMR, EPR, calorimetry, DSC, TGA, use of quartz-crystal microbalance (QCMB) and atomic force microscopy (AFM). 

Sloop and Lerner [132] showed that SEI formation can be affected by treatment of the cross-linked polymer, poly-[oxymeth-ylene oligo(oxyethylene)] (PEM) with an alkylating agent. Cross-linked films of PEM do not form a stable interface with lithium however, upon treatment with methyl iodide, / Ej stabilizes at 2000 Hem"1. Such an SEI is characterized by low conductivity, from 10 to 10 Q-Icm2, which is linear over the temperature range of 25-85 °C. 

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. 

SEI formation control is the key to good performance and the safety of the whole lithium ion battery, as not only anode operation but also cathode properties are strongly affected by the SEI formation process (the cathode is the lithium cation source of lithium ion cells). Apart from control of the graphite (surface) properties, an appropriate composition of the electrolyte is usually helpful for creation of an effective SEI. 

SEI FORMATION AND SEI STABILITY ON LITHIUM STORAGE METALS AND ALLOYS... 

Electrolyte effects on the cycling stability of lithium storage metals and alloys indicate the importance of SEI formation in this case, too. Very early measurements suggest that additives such as CO2 do not only improve the cycling stability of metallic lithium [41] and graphitic carbons (see above), but also that of lithium storage metals (Fig. 18), which may be related with the electrical properties of the SEI (Fig. 19) [13]. 

Figure 20. SEI formation on different anodes for rechargeable Li batteries (A) lithium metal, (B) graphitic carbon, and (C) metals and intermetallics. Different colors of the SEI indicate SEI products formed at different stages of charge and discharge (and do not indicate different composition) [42],...

H. J. Santner, M. R. Wagner, G. Fauler, P. Raimann, C. Veit, K. C. Moller, J. O. Besenhard, M. Winter (2003). An Overview on SEI Formation Processes of Lithium Battery Anodes in Organic Solvent Based Electrolytes, Taipei Power Forum and Exhibition (TPF2003), December 1-3, 2003, Taipei (Taiwan) Invited lecture. 

The characteristic reduction semi plateau associated with the SEI formation appears in the 800-600mV region in EC-DMC/LiPF6 electrolyte under C/10 discharge rate [23],... 

Figure 11. (a) Initial IV2 cycles of a Li/petroleum coke cell. The cell was cycled at a rate of 12.5 h for Ax = 0.5 in Li sG6. (b) Initial IV2 cycles of a Li/graphite cell. The cell was cycled at a rate of 40 h for Ax= 0.5 in Li sG6. F denotes the irreversible capacity associated with SEI formation, E the irreversible capacity due to exfoliation, and I the reversible capacity due to lithium intercalation into carbon. 1.0 M LiAsEe in EC/PC was used as electrolyte. (Reproduced with permission from ref 36 (Eigure 2). Copyright 1990 The Electrochemical Society.)... 

There has been considerable controversy concerning the mechanism of SEI formation on a carbonaceous anode, but it is generally agreed that the initial electrolyte decomposition is responsible and that a competition among a variety of reactions involving the solvent as well as the salt components is also present. 

Peled s Model Anode/Electrolyte Interface Film. In their proposal of SEI formation on a carbonaceous electrode in nonaqueous electrolytes, Dahn actually adopted Peled s model for lithium s surface and extended it to carbonaceous electrodes. By this model, a two-dimensional passivation film is established via a surface reaction. 

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