The Redox Processes in Lithium Batteries : Beyond the Oxidation State Formalism
Authors: Ouvrard Guy , Pradon Alexandre , Caldes Maria Theresa , Boucher Florent 
Published: February 2020

In general, electrochemistry is based on electron exchange between different species (ions, atoms, molecules,grains…), in a liquid or solid environment, and this electron exchange is usually written in using the concept of oxidation state of the different elements constituting the species. Redox processes are thus introduced, where oxidation and reduction for a specie correspond to the loss or a gain of electrons, respectively. This concept is generally well adapted to electrochemistry because the oxidation state of an element corresponds to the number of electrons which is lost or gained in a chemical bond, supposed to be purely ionic, to form a cation or an anion, respectively. The electronic structure of an ion or a molecule is described by discrete energy levels which are occupied by a small and integer number of electrons. So, during an electrochemical process, electrons are added or removed and this electronic transfer can be explained by using quite simple concept. The problem is different in a solid, The problem is different in a solid for which the only way to describe the electronic structure is to use the band structure model: atomic orbitals of all elements are more or less mixed together to form the bands. Consequently, the number of valence electrons is very large and the ionic model is very often unsuitable. In this presentation we will show through different examples that use of the oxidation state can be misleading in order to explain the true behavior of a solid electrode material in lithium batteries. We will propose a more general description.
First, we will focus on X-ray Absorption Spectroscopy (XAS), one of the most powerful tools to analyze precisely the state of charge of an element. It has been showed many times, on chalcogenides (1) and oxides (2), that this technique combines very nicely with the calculated electronic structure of a solid. It will be evidenced that in a redox process all elements of a material are modified by the electron transfer.
Secondly, we will show how the oxidation state concept could lead the researchers to exclude a priori some reactions because they would put some elements in unusual low or high oxidation states (3).
Finally, we will show recent results obtained on Li2MnO3 and HE-NMC materials (4-6). From them, it appears clearly that the oxidation and reduction processes in solid are complex phenomenon. They generally impact all elements which have to be considered as possible redox centers.
The general model we propose is based on solid electronic band structure for solid, considered as an electron reservoir which can be emptied (oxidation) or filled (reduction). The reservoir is built a priori by the orbitals of all elements. The fact that an electrochemical reaction occurs, becomes reversible, or leads to degradation of the material, has to be discussed in terms of energetic stability of a given atomic structure. This can be related to the shape and the filling of the electronic reservoir.
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