The Computational Aim

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. 

From a practical point of view there are two main approaches to avoid, mitigate, or quench unwanted electrolyte redox reactions either increasing the electrochani-cal stability window of the electrolytes or deliberately introducing an additional well-defined redox reaction, i.e., by using additives in what then is coined functional or role-assigned electrolyte [1]. Both approaches rely on careful design of the electrolyte from the level of individual species and up, with special attention taken to the specific electrode chanistries. 

To this end, various computational approaches have proven to be important tools for making a priori predictions of the electrochemical stability of solvents and salts, as well as additives. More precisely the aim of the modelling is to access the electronic energy levels of the molecules/materials, which for many of the methods used are easily accessible, and then directly or indirectly correlate these with the observed experimental data or electrode potentials on an absolute or relative energy scale - to truly test the predictive power of electrochemical stability. 

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