Encasing Si particles within a versatile TiO2-xFx layer as an extremely reversible anode for high energy-density lithium-ion battery

The chemical phenomena occurring at the electrode-electrolyte interfaces profoundly determine the cycle behavior of a lithium ion battery. In this work, we report that silicon-based anodes can attain enhanced levels of capacity retention, rate performance and lifespan when a versatile protective layer of, F-doped anatase is applied towards taming the interfacial chemistry of the silicon particles. With careful choice of titanium fluoride as a precursor, internal voids can be generated upon in-situ fluoride etching of the native oxide layer and are used to alleviate the mechanical stress caused by volume expansion of silicon during cycling. In the course of F-doping, part of the Ti4+(d(0)) ions in anatase are reduced to Ti3+(d(1)), thereby increasing charge carriers in the crystal structure. Hence, the multifunctional F-doped TiO2-x, coating, not only minimizes the direct exposure of the Si surface to the electrolyte, but also improves the electronic conductivity via inter-valence electron hopping. The best performing composite electrode, Si@TiO2-xFx-3, delivered a satisfactory performance in both half-cell and full-cell configurations. Furthermore, we present a study of 1) the Si valence change at the buried interface using synchrotron based hard X-ray photoelectron spectroscopy, and 2) the phase transformation of the electrode monitored in operando using X-ray diffraction. Based on these characterizations, we observe that the Li+ conducting intermediate phase (LixTiO2-xFx-3) formed inside the surface coating enables deep lithiation and delithiation of the silicon during battery operation, and thus increase the capacity that can be accessed from the electrodes. (C) 2016 The Authors. Published by Elsevier Ltd.