Cell reactions lithium solid electrolyte

When the anode is first charged, it slowly approaches the lithium potential and begins to react with the electrolyte to form a film on the surface of the electrode. This film is composed of products resulting from the reduction reactions of the anode with the electrolyte. This film is called the solid electrolyte interphase (SEI) layer [30], Proper formation of the SEI layer is essential to good performance [31-34], A low surface area is desirable for all anode materials to minimize the first charge related to the formation of SEI layer. Since the lithium in the cell comes from the lithium in the active cathode materials, any loss by formation of the SEI layer lowers the cell capacity. As a result, preferred anode materials are those with a low Brunauer, Emmett, and Teller (BET) surface area... 

What makes the sodium-sulfur cell possible is a remarkable property of a compound called beta-alumina, which has the composition NaAlnOiy. Beta-alumina allows sodium ions to migrate through its structure very easily, but it blocks the passage of polysulfide ions. Therefore, it can function as a semipermeable medium like the membranes used in osmosis (see Section 11.5). Such an ion-conducting solid electrolyte is essential to prevent direct chemical reaction between sulfur and sodium. The lithium-sulfur battery operates on similar principles, and other solid electrolytes such as calcium fluoride, which permits ionic transport of fluoride ion, may find use in cells based on those elements. 

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. 

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. 

In most cases a mixture of propylene carbonate and dimethoxy methane with lithium perchlorate or lithium trifluoromethane sulfonate as electrolyte salt is applied. Mixtures of tetrahydrofurane, butyrolactone, and dioxolane are used also. As an example of a passivation layer on the lithium metal anode in a cell with a solid cathode and a fluid organic electrolytic solvent we see here the dense and stable layer of lithium carbonate as the reaction product of lithium with propylene carbonate. 

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