Direct Synthesis of Nanotubular Si-TiO2for Lithium-Ion Batteries Anodes By Anodizing
Authors: Valente Renato Castro , Dick Luís Frederico Pinheiro 
Published: February 2020

Industry and the scientific community have been concentrating efforts on the production and development of energy storage systems ESS [1].By the energy density improvements of lithium ion battery electrodes for electronic devices (EDs) and energy vehicles (EVs), and cost of manufacturing must be taken into account, so that new developed materials can with the currently used batteries[2-4].Silicon is an important candidate as anode material for the next generation of lithium ion batteries(LIBs) because of its extremely high specific capacity (4200 mAh/g). However, Si particles degrade during the volumetric change related to lithiation/delithiation processes.Efforts have been made in the last decade in an attempt to minimize the instabilities caused by the large volumetric variation [5,6]. The most effective approaches in this regard include: (a) reduction of particle size to the nanometer size range to lower mechanical stress, (b) formation of a porous structure promoting a stable solid electrolyte interface SEI (c) dispersing Si nanoparticles in a conductive matrix to accommodate the volumetric variations and preserve the mechanical integrity of the electrode, (d) formation of amorphous SiO2 structure in small dimensions, and the(e) formation of intermetallic composite with structure layers to accommodate Si particles[7]. In the other hand, TiO2also incorporates Li(330 mAh/g)and shows a fast lithium intercalation reaction, without major volumetric effects during Li intercalation/deintercalation.
In this work the aim is to produce Si nanoparticles embedded in a nanotubular TiO2 matrix, avoiding mechanical stress problems, due to volumetric changes and the formation of SEI directly between Si nanoparticles(SiNP) and the electrolyte. For this, Ti-Si alloys with 2 and 4 at.%Si were prepared by arc melting in an Ar atmosphere. A Si-Ti solid solution phase was obtained by furnace heating to 1200 ºC and rapidly cooling to a hard martensitic phase.Thin sheets of these alloys were cut and anodized in different electrolytes aiming to avoid the dissolution of the Si Particles. The structure and morphology of the Si rich TiO2 nanotubes were investigated by TEM, XPS, SEMand GID/DRX. Cell voltage transients of the anodizing of Ti2Si and Ti4Si indicate different levels of SiNP accumulation. Different electrolyte compositions, organic and inorganic, with and without fluoride were used for the growth of the porous structures. The effect of lithium addition on electrolytes is observed in favor of porous structure. SiNPs embedded in TiO2 were observed by SEM, TEM and characterized by GID/DRX.1% at. Si was measured by EDS onregular tubular TiO2 detached from Ti-2 at.%Si anodized in a fluoride based electrolyte, indicating that around 50% of the Si was not lost to the electrolyte during anodization. In the other hand, metallic Si phase was detected by grazing angle diffraction indicating that Si NP are formed.
The developed process shows to be an efficient way for the direct fabrication of self-organized titania nanotubes containing silicon nanoparticles.
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