Encasing Si particles within a versatile TiO_2−xF_x 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 (TiO_2−xF_x), 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⁺(d₀) ions in anatase are reduced to Ti³⁺(d₁), thereby increasing charge carriers in the crystal structure. Hence, the multifunctional F-doped TiO_2−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@TiO_2−xF_x-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 (Li_xTiO_2−xF_x) 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.