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. 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. We have recently 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 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. In a battery half-cell using metallic lithium as counter electrode, the orientation-engineered CaTi₅O₁₁ film discharged to 155 mA h g^-1 at a rate of 60C, corresponding to a time of 60 s to fully discharge the capacity, at the 100th cycle, delivering specific power of ~20 kW kg^-1. 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. The film microstructure and interfacial atomic structure were characterized by atomic resolution transmission electron microscopy. In addition, we have employed novel in situ TEM to study the Li-intercalation of the films. Revealed by TEM of electrochemical lithiation in TiO₂-B, many defects were induced by strain relaxation upon Li-induced TiO₂-B lattice expansion. Depending on Li intercalation direction in the crystal structure, either high-symmetry structural transformation or plain shears was generated. These results provide the basic knowledge needed to realize and utilize TiO₂-B single crystals, while also supporting theoretical studies with determinate experimental data.