Chemical Composition and Morphology of the SEI

Chemical Composition and Morphology of the SEI 16.2.2.1 Ether-Based Liquid Electrolytes... 

It is now generally accepted that the surface chemistry and morphology of the edge planes of graphite play a major role in the chemical and electrochemical reactivity of this material in contact with electrolyte. In order to determine whether there is a correlation between the composition and morphology of the SEI formed on the HOPG and on the real anode in lithium-ion batteries, we... 

Andersson AM, Edstrom K (2001) Chemical composition and morphology of the elevated temperature SEI on graphite. J Electrochem Soc 148 A1100-A1109. doi 10.1149/1.1397771... 

The chemical composition of the SEI formed on carbonaceous anodes is, in general, similar to that formed on metallic lithium or inert electrodes. However some differences are expected as a result of the variety of chemical compositions and morphologies of carbon surfaces, each of which can affect the i() value for the various reduction reactions differently. Another factor, when dealing with graphite, is solvent co-intercalation. Assuming Li2C03 to be a major SEI building material, the thickness of the SEI was estimated to be about 45 A [711. 

It should be noted here, that not only the (chemical and morphological) composition of the protective layers at the basal plane surfaces and prismatic surfaces is different, but that these layers also have completely different functions. At the prismatic surfaces, lithium ion transport into/ffom the graphite structure takes place by intercalation/de-intercalation. Here the formed protective layers of electrolyte decomposition products have to act as SEI, i.e., as transport medium for lithium cations. Those protective layers, which have been formed on/at the basal plane surfaces, where no lithium ion transport into/from the graphite structure takes place, have no SEI function. However, these non-SEI layers still protect these anode sites from further reduction reactions with the electrolyte. 

According to Peled s model, the existence of an SEI constitutes the foundation on which lithium ion chemistry could operate reversibly. Therefore, an ideal SEI should meet the following requirements (1) electron transference number 4 = 0 (otherwise, electron tunneling would occur and enable continuous electrolyte decomposition), (2) high ion conductivity so that lithium ions can readily migrate to intercalate into or deintercalate from graphene layers, (3) uniform morphology and chemical composition for ho-... 

Qjr depends on the electrolyte type (solvent and salts), the impurity level of the carbon and the electrolyte, the real surface area of the carbon including inner pores which the electrolyte can enter, the surface morphology, and the chemical composition of the carbon. It typically decreases in the order powders > microbeads > fibers. Impurities such as acids and alcohols, water, or heavy metals may contaminate the SEI, causing side-reactions [1, 2] such as hydrogen evolution and electrolyte reduction this results in larger Qjr values [99, 100]. 

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