Study on passivation film stability on LiNi0.5Co0.2Mn0.3O2 electrode using linear sweep thermammetry(LSTA)

Abstract

Nowadays, application range of lithium ion batteries (LIB) is begin to change from small power devices (e.g. IT devices) to large scale energy storage systems (e.g. electrical vehicle (EV), large scale energy storage systems (ESSs)). From this paradigm change, cycleability of fabricated battery system is more and more important than previous one. And especially, commercialized electrode materials are stable at long-term operation. In this case, passivation film stability is a point to be considered. Therefore, stabilization of this interphase is crucial. In this sense, various additives are currently being studied to stabilize the surface film for enhancing cycleability of battery system. Previously, evaluation sequences of suggested additives are time and cost consuming process. This process included long-term galvanostatic charge/discharge step, AC impedance for resistance measurement and XPS technique for seeing surface characteristics. As we can see the name of analysis technique, these methods are not economically favorable and also time involved cases. However, in industrial aspect, fast with accuracy and economy is needed for testing suggested additives. In this study, we suggest LSTA(Linear Sweep Thermmametry) technique for testing surface film characteristics with various additives and diverse temperature. LSTA shows cathodic or anodic currents on working electrode at fixed voltage with temperature sweep. In this case, LSTA is preceded after formation of surface film, therefore, in this experiment, anodic current shown by LSTA with temperature sweep means stability of surface film. From this basis, we predict surface film stability with additive variation at certain temperature. And by using stable electrode material with layered structure, LiNi0.5Co0.2Mn0.3O2 (NCM 523), we can apply this measured relative surface film stability to predict long-term cycleability of various temperatures at certain voltage cut-off cycling with used additives. In this research, we use non-additive included electrolyte, vinylene carbonate (VC) and propane sultone (PS) 2 wt. % added electrolyte solution. In previous reported studies, VC shows unstable feature and PS demonstrates stable characteristic at high temperature and voltage cycling. From selection of these known additives, we can define validation of suggested tool. Also, by choose two temperature cases with room temperature (25 0C) and high temperature (60 0C), temperature dependency of additives are also predicted and confirmed. In room temperature (25 0C) case, three electrolytes show no meaningful difference in anodic current measured by LSTA technique. And this tendency is also found at cycle performance of three electrolyte solution with NCM 523 active material. Furthermore, increment of resistance measured by AC impedance shows same trend with LSTA and cycle data. At high temperature (60 0C) condition, three electrolytes show different stability of surface film with PS, non-additive, VC, in sequence. Therefore, we can predict cycleability as that sequence. And in real evaluation of cycle performances of three electrolyte solutions with same active material, NCM 523, demonstrate same tendency as same with we predicted in LSTA data. In nyquist plots of three samples shows resistance difference of three samples with same trends, and also that sequence is also found at film thickness after cycling by XPS spectra. From these results, we can state that LSTA technique shows stability of surface film generated by additive variation. By using this technique for testing additive validation, time and cost saving at industrial field is expected.