Fabrication and Electrochemical Performance of TiO2-TiN/Sn-SnO2 Composite Films for High Capacity and High Conductivity Lib Anodes

Abstract

Recently, anodic nanoporous TiO2 films have gained widespread attention as promising anode candidates for Li ion batteries to replace traditional C-based anodes. TiO2 delivers very small volume expansion (<4%) and works at a relatively high working voltage (1.7 V vs. Li/Li+) during Li+ insertion/extraction. This eliminates the influence from the solid electrolyte interface (SEI) layer and avoid the risk of lithium electrodeposition during charge/discharge process, thus further ensuring the cycling stability and safety of batteries. However, the development of TiO2 materials as LIB anodes is limited by its poor electric conductivity and low theoretical capacity. Therefore, we propose a new approach to fill Sn-based materials, which possess around 3 times theoretic capacity higher that the C-based anode, into the nanoporous anodic TiO2 films. Since the TiO2 nanostructure can alleviate the volume expansion of Sn-based materials, the synergistic effect of Sn with high capacity and TiO2 with large surface area is expected.In this study, we first formed nanoporous TiO2-TiN composite films on Ti foils by a smart anodization in NO3 –-based solutions. The inclusion of conductive TiN component in TiO2 matrix film improves the electronic conductivity effectively, which enables the successive electrodeposition on anodic TiO2 film. Then, the nanoporous TiO2-TiN composite films were used as matrix films to deposit Sn and/or SnO2 nanoparticles by a cathodic electrodeposition in acidic baths containing different Sn ions. The electric conductivity and nanostructures were tailored through anodizing process to perform successive electrodeposition. The microstructures, chemical composition, and crystalline structure of the anodized specimens before and after annealed were investigated FE-SEM (EDS), TEM (FIB), and XPS. Moreover, the cyclic voltammetric behaviors, impedance spectra, and charge-discharge performances of various composite films on Ti foils were investigated as binder-free anode materials for Li ion secondary batteries.Fig.1a-b shows the FE-SEM images of surface and cross section of TiO2-TiN composite films. An ordered nanoporous structure is obtained, with parallel pores of 50-100 nm in diameter and around 3-μm thickness after anodization for 90 min. Fig.1c shows the surface morphology of a TiO2-TiN/Sn composite film by electrodeposition in a Sn2+-based solution for 25 min. A granular Sn-based film with particle size of 100-300 nm in diameter formed on the anodic TiO2-TiN film. This indicates that the prolonged electrodeposition led to the filling of Sn not only inside but also on the anodic TiO2-TiN film. XPS analysis revealed that Sn deposits existed in metallic Sn and Sn oxides as SnO and/or SnO2. Moreover, it is confirmed by TEM observation that the nanocrystalline Sn-SnO2 deposits filled in the porous TiO2-TiN film. It is normally that the particles emerging on the film surface are larger than the pores of TiO2-TiN film.Fig.1d demonstrates the cycling performance and coulombic efficiency of TiO2-TiN/Sn-SnO2 and TiO2-TiN composite films on Ti sheets at 30 mA cm-2 for the first cycle and then 80 mA cm-2. The initial capacity of TiO2-TiN/Sn-SnO2 is 2100 μA h cm-2, which is around 5 times larger than the TiO2-TiN matrix film of 396 μA h cm-2. After 40 cycles, the capacity of the composite film remains 1153 μA h cm-2 and 4.6 times higher than the film without Sn electrodeposition. Since the nanoporous structure of TiO2-TiN can alleviate the volume expansion of Sn-based particles inside, the capacity deterioration can be mainly ascribed to the Sn-based particles overflowed on the surface, which may fall off or crush during charge/discharge process by the large volume expansion. Moreover, it was found from the EIS measurement that the internal electrical resistance of the TiO2-TiN composite film decreased considerably after the electrodeposition of the Sn-based materials, in proportional to the deposition time. Therefore, electrodeposition of Sn-based material in porous TiO2-TiN films is an effective approach to enhance the capacity and to improve the conductivity of the electrode material. Nevertheless, optimizing the conditions of anodization Ti base materials and Sn-based electrodeposition are necessary and will be an issue in the future study. Figure 1