SEI Formation Processes

Alkali and alkaline-earth metals have the most negative standard reduction potentials these potentials are (at least in ammonia, amines, and ethers) more negative than that of the solvated-electron electrode. As a result, alkali metals (M) dissolve in these highly purified solvents [9, 12] following reactions (1) and (2) to give the well-known blue solutions of solvated electrons. 

These reactions proceed to equilibrium when the potential of the solvated-electron electrode equals that of the alkali metal 113]  

This dissolution process takes place in many solvents to an extent governed by Eq. (3). Solvated electrons can be formed in all solvents by many means. Their kinetics is best studied with the use of pul.se radiolysis. 

In this paper we shall discuss four types 

When an inert metal (or any electronic conductor) is negatively polarized in the electrolyte (typically from 3 V versus the lithium reference electrode (LiRE) to 0 V versus LiRE), the following reactions take place at different potentials and at varying rates, depending on, on concentration, and on (q for ch of the following elec-trochemically active materials (1) solvents, (2) anions of the salts, and (3) impurities such as H2O, O2, HF, CO2 etc. (Fig. 1). Some of the products, especially at more positive potentials, may be soluble and diffuse away from the electrode, while others will precipitate on the surface of the electrode to form the SEI. At potentials lower than a few hundred millivolts, solvated electrons will be formed. These will also react with impurities, solvents and salts (e o scavengers) to produce similar products. The lifetime r, (r = 0.69A gfS] = rate constant  

The charge needed to complete the formation of the SEI (about 10-3 mAh cm-2 [8, 14]) increases with the real surface area of the electrode and decreases with increase in the current density and with decrease in the electrode potential (below the SEI potential). Tn practice, it may take from less than a second to some hours to build an 

Practical primary or secondary alkali-metal or alkaline-earth batteries can be made only if the dissolution of the anode by reactions (16.1) and (16.2) (and by other corrosion reactions) can be stopped. Since attacks both the electrolyte and the cathode, the electrolyte must be designed to contain at least one material that reacts rapidly with lithium (or with the alkali-metal anode) to form an insoluble SEI. On inert electrodes, the SEI is formed by reduction of the electrolyte. This type of electrode (completely covered by SEI), was named [1, 2] the SEI electrode.  

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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. 

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. 

Based on the understanding of the formation mechanisms of SEI on graphite anode, it is possible to improve performance of lithium-ion battery by tailoring the structure and chemistry of an SEI. The application of additives is the most successful tailoring SEI. Numerous additives which assist in SEI formation process have been extensively explored, and a few of them have been widely utilized in commercial lithium-ion battery. The successful utilization of SEI formation additives boosts the applications of lithium-ion battery technology in our daily life and, the ultimate goal in future, electric vehicles. 

The voltage of SEI formation (V gj) correlates with the reactivity of the electrolyte components towards e as well this reactivity, in turn, is directly related to i . In the case of reactive components like AsF ", CO and EC, Vsg, is more positive. However, for more kinetically stable (lower kj substances, like CIO., (and probably PE " and imide), Vsg, approaches the LifLi potential, i.e. the overpotential of the SEI formation process is higher. In order to estimate the relative contribution of the EME and i to the value of V g, let us consider the following example. The OCV of the Li/SO/C cell and of the Li/EC, PC, DEC, DMC/C cells is about the same, 2.8-3 V. However, the SEI formation voltage on a carbonaceous (Li C ) electrode in the SO -containing electrolyte is 2.4 V, almost 1 V more positive than that in EC, PC, DEC, DMC solutions (where varies from 0.6 to 1.5 V). This indicates that the kinetics has a more profound effect on the SEI formation voltage than does the thermodynamic parameter (OCV). 

The SEI formation processes and the properties of the resulting SEI greatly depend on the kind of solvent. The most important and interesting fact is that propylene carbonate (PC), which has been used as a solveait in primary lithium cells, has a poor compatibility with gr hite anodes [65-68]. Wheai gr hite is polarized in PC-based electrolyte solution, ceaseless solvent decomposition and intensive exfoliation of graphene sheets take place, and does not give an effective surface film. It seems that co-intercalation of PC is much more vigorous than that of EC. This is the reason why EC-based solutions are exclusively used in commercially available LIBs. Nevertheless, PC-based solutions are attractive as electrolj e solutions for LIBs because of... 

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