Nanostructured Titanium Oxide Anodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have been deemed a promising low-cost battery technology for grid-scale energy storage in view of the abundance of sodium resources. The large ionic size (1.02 Å) and different electronegativity of Na+ nevertheless, make it difficult to reversibly insert/extract into/out of most host materials, resulting in inferior battery performance. Searching for viable anode materials with high efficiency that can accommodate the Na+ while achieving long-term stable electrochemical performance is a tough issue for SIBs. With the characteristics of high theoretical capacity (335 mAh g-1), reasonable insertion voltage (~0.75 V vs. Na/Na+), chemical stability, and abundant reserves, titanium oxides have shown promise as anodes for SIBs. The slow progress on titania-based SIBs, however, has been due mainly to their inferior conductivity and sluggish Na+ reaction kinetics. In this doctoral work, several strategies, including atomic-level P-doping, surface structural engineering, constructing two dimensional (2D) superlattices, and optimizing Na+ intercalation kinetics by tailoring-designed nanostructures, have been investigated to enhance the electrochemical reactivity of titania anodes. Structural modulation and surface engineering have remarkable advantages for fast and efficient charge storage. In Chapter 4, a phosphorus modulation strategy is presented that can induce surface structural-disorder while realizing interior phosphorus-doping to enhance the Na+ storage capability of TiO2. The P-modulated TiO2 nanocrystals present an improved structural stability and Na+ diffusion kinetics, achieving excellent high rate performance and long cycling life without dramatically capacity fading, even after 5000 cycles at a current density of 30 C. Meanwhile, the Na+ insertion mechanism was symmetrically studied using in situ synchrotron X-ray diffraction (XRD). The results show that the phosphorus modulation strategy can enable negligible volume expansion of only 0.1% during cycling.