Epitaxial CaTi5O11 and TiO2-B Thin Films for High Rate Lithium-Ion Batteries.

Abstract

The bronze polymorph of titanium dioxide (TiO2-B) is interesting for many applications including high rate energy storage, solar cells, photocatalysis, thermoelectrics and sensing, owing to its uniquely layered structure and highly asymmetric unit cell. However, such a metastable phase is extremely hard to obtain with high purity and crystallinity, significantly impeding its development in these fields. This dissertation is devoted to the waterless synthesis, structural characterization and property testing of both TiO2-B and a related novel material, CaTi5O11, in the form of highly crystalline thin films, with a specific emphasis on their application as anode materials in lithium-ion batteries.

Although known to have advantages over anatase or rutile, high quality bronze phase TiO2-B specimens that demonstrate good electrochemical properties thus far have exclusively been nano-structured powders prepared by hydrothermal methods, as first synthesized in 1980. Aided by first-principles calculation and atomic resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), it has been discovered that Ca can stabilize the bronze structure, forming a variant phase CaTi5O11, which has then been successfully synthesized in epitaxial single-crystalline thin films by pulsed laser deposition (PLD), a completely waterless process. Due to the near-perfect lattice match, the CaTi5O11 film can be further used as a template layer to grow high quality, water-free TiO2-B films on top, which facilitates the synthesis and application of both materials on a wide variety of substrates, including SrTiO3, Nb:SrTiO3, LaAlO3, LSAT and SrTiO3 buffered Si.

Lithium ion transport in the bronze structure is highly anisotropic. By utilizing substrates with a different orientation to align the more open channels with out-of-plane directions, extremely high rates of lithium ion transport, up to 600C (1C=335 mA g-1), with extraordinary structural stability has been achieved. Post-mortem examinations by x-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed that both the TiO2-B and CaTi5O11 structures were essentially unchanged after aggressively cycling for more than 60 days.

As the methods and equipment required are readily accessible to the extended research community, further studies on and applications of these materials, which are attractive in realms that extend beyond electrochemistry, may emerge.