(Invited) In-Situ Analysis of Lithium Ion Batteries

Abstract

Twenty-five years have passed since the first lithium ion battery (LIB) was commercialized, which work on the basis of ion shuttle mechanisms. Reactions in LIBs are very simple at first glance; insertion and extraction of Li+at positive and negative electrodes, and electron transfer occurs through a schottky contact between current collectors and active masses. When we look insight of the reaction, the situation is not so simple as the first glance. Even during insertion and extraction of positive ions, i.e. Li+, charge neutrality through the system must be retained, which means that the valences of the elements, mainly transition metals, composing the host lattice must change during the reaction. The valence change causes structural changes in the lattice, namely, phase transition should occur.Carbon materials, generally graphite, are used as the negative active mass in LIB. The fully charged state of graphite shows the potential as low as that of lithium metal, 90 – 200 mV vs Li/Li+. Organic species composing the electrolyte and binder of LIB are inherently unstable against such reductive circumstances. Nevertheless LIB shows stable and long life compared with other rechargeable batteries and is now produced in an enormous scale and widely used. This is due to the surface stabilization of the graphite negative electrode by the formation of a passivating film called SEI.Its energy density has increased over twice since the commercialization. There remain some issues to enhance the property and extend the application of LIBs.Innovation in materials for LIB is essential for advancing LIB properties but it is no less important to clarify reactions occurring in LIBs. The latter would supply information and cues to develop innovative materials for future LIBs.Reactions proceeding inside LIB range widely in scale of time and space. A variety of analytical technologies including FT-IR, Raman, SPM, NMR, SR-X-ray diffraction and absorption, neutron diffraction, SEM, TEM, EC-MS have been developed and used to elucidate reactions inside LIBs in situ and ex situ.The author’s group has developed some of these technologies, some of that are summarized and explained here.There are plenty of analytical technologies further-more that have high potential for clarifying reactions of batteries. Main battery reactions should be focused and in addition to the main reactions side reactions and slow reactions that give major influences on life and safety of battery required to be solved. Each analytical technology has some merits and demerits in focusing on reactions to be clarified. Adequate technologies should be selected depending on the phenomena to be solved. Figure 1