Investigation of the Interactions between Electrodes in Li4Ti5O12 – Based Batteries with Complementary Surface Analysis Techniques (XPS, SAM, ToF-SIMS)

Abstract

Li4Ti5O12(LTO)-based negative electrode for lithium-ion batteries is of interest for electrical vehicles due to its safety, low cost and cycling stability [1]. In this study, the effect of the positive electrode on the electrochemical performances of LTO electrodes, in relation with the Solid Electrolyte Interphase (SEI) properties, has been investigated [2]. Full cells LTO/LiNi3/5Co1/5Mn1/5O2 (NMC) and LTO/LiMn2O4 (LMO) were cycled at 40°C over 100 cycles and the electrodes were analyzed by XPS, Scanning Auger Microscopy (SAM) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) after one and 100 cycles (figure 1). Moreover, LTO/LTO symmetrical cells were also analyzed in order to be free of the positive electrode impact. For each system, LTO electrodes are homogeneously covered by surface layers since the first cycle which induces an irreversible capacity loss. This latter is more important for LTO/LMO compared to LTO/NMC and LTO/LTO. Both SEI layers are composed of organic (polyethylene oxides, oxalates) and inorganic species (LiF, phosphates and fluorophosphates) but in different proportions and with different 2D and 3D spatial distributions: fluorine species are detected deeper in the electrode than organic species and in higher quantities for LTO/LMO for instance [3]. Moreover, the SEI is thicker on the LTO electrode when cycled versus LMO compared to NMC and contains small amounts of manganese, homogeneously spread over the surface and deeply inserted in the SEI, which entails an increase of the system impedance. In conclusion, a thick SEI associated with the presence of metallic species could alter the passivating role of the SEI and explain the less efficient electrochemical performance of LTO/LMO cells. [1] Tarascon et al. Nature 2001, 414, 359–367. [2] El Ouatani et al. Journal of The Electrochemical Society, 156(6): A468, 2009. [3] Nicolas Gauthier et al. Journal of The Electrochemical Society, 165(13): A2925-A2934, 2018. Figure 1