Solid electrolyte interface, SEI

Table 3, which presents the various surface compounds formed on Li electrodes in the various solutions, together with reaction schemes 1-10, describes well the basic surface chemistry developed on the carbons. Similar results concerning the surface chemistry developed on carbons in alkyl carbonate mixtures have also been obtained by others [363-365], Hence, carbon electrodes are also solid electrolyte interface (SEI) electrodes, similar to lithium i.e., the overall insertion process of Li into the carbons requires the necessary step of Li ion... 

As already mentioned, salt-containing liquid solvents are typically used as electrolytes. The most prominent example is LiPF6 as a conductive salt, dissolved in a 1 1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as 1 molar solution. It should be mentioned that this electrolyte is not thermodynamically stable in contact with lithium or, for example, LiC6. Its success comes from the fact that it forms an extremely stable passivation layer on top of the electrode, the so-called solid-electrolyte interface (SEI) [35], Key properties of such SEI layers are high Li+ and very low e conductivity - that is, they act as additional electrolyte films, where the electrode potential drops to a level the liquid electrolyte can withstand [36],... 

Fig. 12. Solid electrolyte interface (SEI) model for surface formed on lithium metal and graphite electrodes in nonaqueous electrolytes.

Another important feature for lithium graphite intercalation compounds in Li -containing electrolytes is the formation of solid electrolyte interface (SEI) film. During the first-cycle discharge of a lithium/carbon cell, a part of lithium atoms transferred to the carbon electrode electrochemically will react with the nonaque-ous solvent, which contributes to the initial irreversible capacity. The reaction products form a Lb-conducting and electronically insulating layer on the carbon surface. Peled named this film as SEI. Once SEI formed, reversible Lb intercalation into carbon, through SEI film, may take place even if the carbon electrode potential is always lower than the electrolyte decomposition potential, whereas further electrolyte decomposition on the carbon electrode will be prevented. 

During the 1970s, propylene carbonate (PC) was found to be a suitable solvent for lithium batteries, but this does not mean that PC is stable on lithium metal. PC is decomposed by a reduction process, after which a passivation layer [so-called solid electrolyte interface (SEI)] is formed on the surface of the lithium metal. In fact, most organic solvents are not stable at the potential of lithium metal. In addition, during the 1980s, no one believed that any organic solvent could be stable at more than 4 V. Thus, the electrochemical environment in LIB produces severe demands... 

Wang F.-M., Shieh D.-T, Cheng J.-H., Yang C.-R. An investigation of the salt dissociation effects on solid electrolyte interface (SEI) formation using linear carbonate-based electrolytes in hthium ion batteries, Solid State Ionics 2010, 180,1660-1666. 

To be able to understand how computational approaches can and should be used for electrochemical prediction we first of all need to have a correct description of the precise aims. We start from the very basic lithium-ion cell operation that ideally involves two well-defined and reversible reduction and oxidation redox) reactions - one at each electrode/electrolyte interface - coordinated with the outer transport of electrons and internal transport of lithium ions between the positive and negative electrodes. However, in practice many other chemical and physical phenomena take place simultaneously, such as anion diffusion in the electrolyte and additional redox processes at the interfaces due to reduction and/or oxidation of electrolyte components (Fig. 9.1). Control of these additional phenomena is crucial to ensure safe and stable ceU operation and to optimize the overall cell performance. In general, computations can thus be used (1) to predict wanted redox reactions, for example the reduction potential E ) of a film-forming additive intended for a protective solid electrolyte interface (SEI) and (2) to predict unwanted redox reactions, for example the oxidation potential (Eox) limit of electrolyte solvents or anions. As outlined above, the additional redox reactions involve components of the electrolyte, which thus is a prime aim of the modelling. The working agenda of different electrolyte materials in the cell -and often the unwanted reactions - are addressed to be able to mitigate the limitations posed in a rational way. 

Once the cell assembly process is complete, the final step in the overall production process shifts to the formation and aging of the cells. This applies to cylindrical, prismatic, flat plate, and polymer cell constructions. Li-ion cells are assembled in the discharged condition and must be activated by charging. The first charge is called formation, which activates the active materials in the cells and establishes their ability to function. The first charge typically starts at a lower current to properly form the protective solid electrolyte interface (SEI) layer on the graphite/carbon anode and then increases to the normal current at about 30% into the charge period. 

In addition to one-dimensional and two-dimensional silicon anodes, several forms of three-dimensional nanostructured silicon have been explored. For example, silicon nanotubes (Fig. 15.9) were investigated by Cho et al. [21] as an anode material for lithium-ion batteries. Both interior and exterior surfaces of the nanotubes are accessible to the electrolyte and lithium ions. Through carbon coating, a stable solid electrolyte interface (SEI) was generated on the inner and outer surfaces of the silicon nanotubes. These silicon/carbon assemblies showed a reversible capacity as high as 3,247 mAh/g (based on the weight of silicon) and good capacity retention. 

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