Abstract:
The study of surfaces and interfaces is one of the main fields of material science. This domain requires specific techniques of surface analysis such as X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Secondary Ion Mass spectrometry (TOF-SIMS) or Scanning Probe Microscopies (AFM – STM). In this field, redox processes and surface and interface phenomena (usually linked) occurring in Li(Na, Mg, K….) batteries during cycling (including liquid or solid electrolyte) play a key role for their performances as well. Solid Electrolyte Interphase (SEI) formed upon cycling leads to a double-edged problematic: its formation lowers the coulombic efficiency and causes irreversible capacity loss, but it also passivates the electrode from the electrolyte and prevents further aging processes.
Several systems were considered in this keynote to illustrate the relevance of such surface analyses in the understanding of redox phenomena in batteries:
First is the study of Full cells as Li4Ti5O12(LTO)/LiNi3/5Co1/5Mn1/5O2 (NMC) and LTO/LiMn2O4 (LMO). The interactions between the two electrodes during cycling are investigated, especially the deposition and insertion of metallic compounds within the LTO electrode, which can directly influence on the stability of the cells and their electrochemical performances. More specifically, we focus this presentation on the results obtained by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) which could give in-depth elemental and molecular information about the interfacial layers through sputter-depth-profiling experiments. Thanks to a high sensitivity and 2 D and 3D imaging capability, it will be particularly useful to follow the deposition of low amounts of metallic species and especially manganese within the SEI layer. Moreover, the evolution of the SEI chemical composition and spatial distribution upon cycling is also reported to better understand the protective role of the SEI.
Secondly, innovative K-ion batteries were considered; potassium is a promising alternative to lithium as K i) is much more abundant than lithium in the earth's crust and concomitantly much cheaper ; ii) has a low redox potential in non-aqueous solvent so that high voltage is expected and iii) has the lowest Lewis acidity and desolvation energy (compared to Na+ and Li+), which should lead to higher ionic conductivity and faster electrode/electrolyte interface diffusion kinetics so that high power potassium-ion batteries (KIBs) are expected.
However, for the practical use of KIBs, high energy density cathode materials are required. In that direction, polyanionic compounds offer various structural frameworks working at high voltage. Among them, KVPO4F showed reversible capacity up to 105 mAh.g-1 with an average discharge potential of 4.3 V vs. K+/K with excellent rate performance. However, the exact electrochemical redox processes of KxVPO4F remains to be better understood, especially above 4.5 V (i.e. from x=0.5 to x=0), to further improve its electrochemical performance.To fill this gap, the carbon-coating impact of the KVPO4F material on the electrolyte reactivity and the polarization will first be presented. Then, the vanadium average oxidation state of KxVPO4F-C was followed upon charge/discharge in half cell using X-ray photoelectron spectroscopy. Importantly, it will be shown that the obtained results not only validate the occurrence of a redox process from x=0.5 to x=0 but also provide the extent of this process, which was never reported before. Also, a severe electrolyte degradation issue above 4.5 V was observed.
IPREM-UPPA (FRANCE) Contributors : N.Gauthier, C. Courrèges, L. Madec, L.Caracciolo
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