(306f) Synthesis of Layer-By-Layer Thick Mesoporous Titania Films with Vertically Oriented 2D-HCP Nanopores and Their Use in Lithium Ion Batteries As Negative Electrodes
Mesoporous titania films with 2D Hexagonal Close Packed (HCP) cylindrical nanopores have been synthesized by the evaporation-induced self-assembly (EISA) technique with Pluronic surfactant F127 (with average structure (EO)106(PO)70(EO)106 where EO is an ethylene oxide unit and PO is a propylene oxide unit) as the structure-directing agent. To provide vertical alignment of the pores, surface modification of substrates with crosslinked surfactant F127 has been used to provide a chemically neutral surface. According to Koganti et. al (Nano Lett. 2006, 6, 2567), thick films do not have perfect vertical alignment of the pores and better vertical HCP orientation is best achieved by sandwiching the films between two modified surfaces. Here, we report the synthesis of relatively thick films with vertically oriented 2D HCP cylindrical nanopores by layer-by-layer synthesis. Such films do not require sandwiching for vertical orientation of the pores. The hypothesis being tested is whether surfactant micelles from an existing layer will interact with micelles from a freshly deposited layer to provide epitaxial self-assembly of vertically oriented cylindrical micelles in registry from layer to layer. Using this technique, titania films with vertically oriented cylindrical nanopores as much as 1 µm thick were obtained. The thickness and vertical orientation were characterized using XRD, SEM and profilometry. These films were then used as negative electrodes in Li-ion batteries because of advantages of titania including good cycling stability, small volume expansion (~3%) during intercalation/extraction and high discharge voltage plateau. The high surface area and small wall thickness of these titania films provide better lithium ion insertion and reduced Li ion diffusion length compared to nonporous titania films or titania nanotube arrays. However, maintaining accessibilty of the full pore space in the thick films is a challenge to be discussed. Furthermore, the mass transport across these films is studied using electrochemical impedence spectroscopy (EIS). Using equivalent circuit modeling, the fraction of the accessible area of the nanoporous film coated electrode was calculated and analyzed as a function of the number of layers deposited.