Porosity and stoichiometry as efficient means to optimize Li storage in anatase

Anatase TiO2 is a stable, cheap, non-toxic and environmentally benign candidate for use as an anode active material for lithium ion batteries (LIB). Due to the comparably high lithium insertion potential of around 1.8 V vs. Li/Li, typical battery electrolytes can be used in their thermodynamic stability window. Anatase offers high capacity and high cycling stability, and due to the oxidic framework, the anode cannot contribute to combustion reactions upon LIB failure, leading to improved cell safety. Despite all advantages, anatase electrodes suffers from poor chemical diffusivity of lithium; both electron and lithium ion transport are extremely slow and therefore lithium storage kinetics and rate performance are quite limited in this material. Here, several techniques come into play, that allow one to optimize ionic transport and electronic transport towards and inside the particles. Note that the storage of Li (= Li + e) in an electrode implies conductivity of lithium ions and electrons. A simple way of improving lithium ion transport in battery materials (“ionic wiring”) is the introduction of open porosity. Fast Li transport is available in the liquid electrolyte. If the electrolyte penetrates the pores, it brings Li ions to the interior of electrode particles, and therefore reduces the length for the slow solid state diffusion transport. Liquid transport systems with hierarchical pores of different diameters have proven to be extremely efficient distribution systems. (This is a similar principle as in many biological systems, where liquids in transport channels [e.g. blood in blood vessels, or sap in trees] are employed for fast ion transport.)