Introduction. Recently, nanoporous or nano-tubular TiO₂ films on Ti foils formed by anodization have attracted great attention as promising candidate anode materials for Li-ion batteries (LIBs) to substitute the commercially used graphite. TiO₂ materials have a theoretical specific capacity of 330 mAh g^-1, which is slightly lower than graphite of 372 mAh g^-1. Importantly, TiO₂ the operating potential of Li insertion is at ~1.8 V vs Li⁺/Li, which is much higher than graphite (0.2 V vs. Li⁺/Li), thus assuring an enhanced battery safety. Especially, nanostructured anodic TiO₂ films can provide a high surface area that facilitates the intercalation of Li ions, with limited volume change less than 4% upon lithium insertion/deinsertion, thus improving the safety and lifetime of LIBs. Usually, anodic TiO₂ films were formed by anodizing Ti foils in ethylene glycol containing corrosive F^- ions and a small amount of water, which may cause environmental concerns. It is highly desirable to explore an aqueous electrolyte with less corrosion and low cost, considering on the practical applications. Therefore, the present study proposed a facile anodizing process to fabricate nanoporous TiO₂-TiN composite films on Ti foils by using a nitric-based aqueous electrolyte system without containing corrosive F^-ions. The microstructures, chemical composition, and the electrochemical charge-discharge performance as binder-free electrode materials for LIBs were investigated.

Experimental. Titanium foils (99.5%, 20 × 40 × 0.1 mm) were used as staring specimens without mechanical or electro-polishing. The specimens, after ultrasonically cleaned for 10 min in acetone and then dried under flowing Ar gas were anodized in various acidic electrolytes containing NH⁺ and/or NO₃^- ions in a constant current density or a voltage mode for 1 – 3 h.   Moreover, in order to investigate the effects of crystalline structure of the nanoporous anodic TiO₂-TiN films on the electrochemical properties, the anodic specimens were annealed at 423 – 573 K for 1 – 4 h. The morphology, chemical composition, chemical state, and crystalline structure of the anodized specimens before and after annealed were investigated FE-SEM, EDS, TEM (FIB), XRD, and XPS. Moreover, various TiO₂-TiN composite films on Ti foils after heating at different temperatures were used as binder-free anodes for LIBs. The charge-discharge performances were investigated using a home-made pouch cell assembled in an Ar-filled glove box. The cathode was utilized with a lithium foil, and the electrolyte solution was made of 1M LiPF₆ in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volumetric ratio of 1:1:1. The charge-discharge performances were investigated by using a multi-channel battery test unit in a galvanostatic mode over 1.0 – 3.0 V (vs. Li/Li⁺) at 303 K at 5 – 50 mA cm⁻²for 1 – 50 cycles.

Results and Discussion. Figure 1a demonstrated a cross-sectional TEM image of as-anodized titania film on Ti foil. The anodic film possessed a nanoporous structure with cylindrical pores of ~Φ50 nm and nano-laminated layers with layer thickness of 250 – 600 nm, indicating a 3D nanostructure. Figure 1b shows the XRD patterns of the anodic titania film before and after annealed at 773 K for 4 h. The as-anodized specimen exhibited a crystalline structure with sharp peaks at 37.1º and 43.3º, close to the (111) and (200) facets of cubic TiO, as well as to TiN. Whereas the specimen after annealed delivered several new peaks that indexed to anatase TiO₂. Figure 1c gives the XPS spectra of N1s for the as-anodized specimen. The spectrum for the film surface showed two peaks corresponding to NO₃^- and NH⁺, whereas the spectra after Ar⁺ sputtering revealed a strong peak close to the binding energy of TiN, thus confirming the formation of TiO₂-TiO-TiN composite film.

Moreover, it was found that the performance or the specific area capacity of the nanoporous TiO₂-TiO-TiN composite films was greatly dependent on the anodizing conditions and the crystalline structure. Especially, the anodized specimen after annealed at 523 K for 1 h exhibited the highest initial discharge capacity of 440 μAh cm^-2 upon ~1 μm film, in a charge-discharge test as anode material for LIBs at a constant current density of 5 μA cm^-2.