Title: Improved cyclic stability of Li-alloying type anode materials through nanostructuration for Li-ion batteries.
Authors: Abirdu Woreka Nemaga, Jeremy Mallet and Mathieu Morcrettea.
a. Laboratoire de Réactivité et Chimie des Solides, LRCS, CNRS UMR 7314, Université de Picardie
Jules Verne, 33 Rue Saint-Leu, 80039 Amiens Cedex, France.
b. Laboratoire de Recherche en Nanosciences, LRN EA4682, Université de Reims ChampagneArdenne, Campus Moulin de la Housse, BP 1039, 51687 Reims Cedex, France.
As demand of high energy density rechargeable batteries is growing, the Li-alloying type anode materials (e.g. Si, Ge) have gained great attention due to their high theoretical capacities (3-10 times higher than that of commercial graphite anode, 372 m Ah/g) and high energy densities. However, the large lithium storage capacities of Si and Ge lead to huge volume change during lithiation/delithiation process. This huge change in volume causes the breaking of the active materials, loss of electrical contact and continuous breaking/reformation of solid electrolyte interface (SEI), and finally to rapid capacity fading.
To address these concerns, nanostructures such as nanotubes, nanowires and nanorods have been used to enhance the cyclability because nanomaterials are able to accommodate stress, provide fast ionic/electronic transport and large surface area. Despite these interests, the nanostructures synthesis has been an issue since then because of many factors like high cost, complex processes (high temperature and low vacuum techniques) and “non-scalable” technics. Moreover, most fabrication processes are using inactive templates which need to be removed to realize battery electrodes. This 2nd step contributes to the destruction of the nanostructures.
In this work, we addressed those challenges using designed, high surface area and electro-active template as a host matrix for Si and Ge nanostructured anode materials. Self-organized TiO2 nanotubes have been used as a host matrix of Si or Ge to fabricate amorphous Si@TiO2 or Ge@TiO2 nanotube composites. The host template does not need to be removed since the TiO2 nanotubes themselves are active versus lithium storage. To form the nanocomposite electrodes, first, TiO2 nanotubes are grown by the electrochemical anodization of a titanium foil and second a Si or Ge conformal thin film is electrodeposited using ionic liquid electrolyte containing the appropriate Si or Ge precursor. Electrochemical methods are not only known to be most accessible, cost effective and scalable, but also known to grow nanostructure with well controlled structure, morphology and compositions due to the easily controllable parameters such as voltage, current, time and electrolyte composition. In this strategy, the TiO2 nanotube arrays plays key roles through providing strong mechanical support to buffer the volume expansion, a high surface area to increase active mass of Si or Ge before reaching the critical thicknesses. Besides, the direct contact of the nanotubes with the Ti current collector facilitates 1D electron transport and avoids the need of adding inactive binders or conductive additives. Taking an example of our amorphous Si@TiO2 nanotube composite electrode with optimized loading, a specific capacity of 900 mA h/g with an efficiency of 85% have been obtained after 90 cycles.