Controllable Fabrication of Multi-tiered Nanoporous Anodic TiO₂–TiN Composite Films as High-Performance Anode Materials for Lithium-Ion Batteries

The present study proposed a novel approach to fabricate multi-tiered nanoporous TiO₂–TiN composite (NTTC) films on Ti foils by facile anodization in an aqueous electrolyte containing NH₄NO₃. The NTTC films possess a unique laminated nanostructure with a tier thickness of 250–600 nm, where each tier comprises cylindrical pores of ϕ30–60-nm diameters. The NTTC films contained 2.2–6.0 at.% nitrogen in the forms of TiN and adsorbed NH₄⁺ ions, which is attributed to the electrochemical/chemical reactions during anodization in NH₄NO₃ electrolytes. Moreover, novel dual-hierarchical NTTC films with large (ϕ250–300 nm) and small (ϕ80 nm) pores on the top and bottom layers, respectively, are fabricated by a successive anodization in sulfate- and nitrate-based electrolyte systems. Specifically, the dual-hierarchical NTTC film formed in (NH₄)₂SO₄ ⇒ NH₄NO₃ solutions delivers a high-specific capacity of around 2504 mAh cm⁻³, and exhibit excellent cycling stability with nearly 100% capacity retention during lithium-ion battery tests. The high capacities of the NTTC films can be attributed primarily to the inclusion of highly conductive TiN in the TiO₂ matrix (improving the anodic TiO₂ film conductivity) and their highly nanoporous structure (facilitating Li⁺ insertion/de-insertion because of the high surface area and presence of fast ion diffusion paths.)