The Study of Various Anode Materials for Sodium (or Lithium)-Ion Batteries

room-temperature sodium-ion batteries (NIBs or SIBs) have raised a great deal of attention for grid-level applications considering the sustainability advantages of NIBs. Significant progress has been made for NIB cathodes by adapting the knowledge learned on lithium-ion batteries (LIBs). Simultaneously, numerous attempts have been made to find suitable anodes for NIBs, however, the research to improve NIB technologies rema ns a challenge. This thesis presents fundamental studies of various anode materials for NIBs from different aspects. Surface and interface engineering of nanostructured anatase TiO2 anode through Al2O3 surface modification and wise electrolyte selection is conducted. The results show that Al2O3 coating provides beneficial effects to the TiO2-based anodes and the modified TiO2 exhibits significant improvements in cycling performance using electrolyte with binary ethylene carbonate (EC) and propylene carbonate (PC) solvent mixture without the need of the commonly used fluoroethylene carbonate (FEC) additive. The achieved excellent electrochemical performance (a high reversible capacity of 188.1 mAh g−1 at 0.1C after 50 cycles, good rate capability up to 5C, and long-term cycling performance for 650 cycles at a high rate of 1C) can be ascribed to the synergistic effects of surface and interface engineering enabling the formation of a stable and highly ionic conductive interface layer in EC:PC based electrolyte which combines the native SEI film and an ‘artificial’ SEI layer of irreversibly formed Na−Al−O. A dual-modification approach of Mo doping combined with AlF3 coating is also introduced to enhance the sodium storage activity of anatase TiO2. The Mo-doped anatase TiO2 synthesized by a simple co-precipitation method delivers an enhanced reversible capacity compare to pristine TiO2 (139.8 vs. 100.7 mAh g−1 at 0.1C after 50 cycles) due to enhanced electronic/ionic conductivity. Via further coating AlF3 using a modified solid-state method, a much higher reversible capacity of 178.9 mAh g−1 with good cycle stability and excellent rate capabilities up to 10C can be finally obtained. The experimental results indicate that AlF3 surface coating could effectively reduce solid electrolyte interfacial resistance, enhance electrochemical reactivity at the surface/interface region, and lower polarization during cycling. As for alloy-type anode of Sn with high theoretical capacity of 847 mAh g−1 but experiences a high volume expansion of 420% upon sodiation, we carry out a fundamental study of the degradation mechanisms that occur in Sn during sodiation-desodiation by employing a Sn thick film as the anode. Electron microscopy reveals new deformation mechanisms, as multiple Sn whiskers nucleate on the surface of the Sn, while pores form within the Sn (over the Na-ion penetration distance) after electrochemical cycling. These mechanisms are in addition to the dry lake-bed fracture that is also observed. Such whiskers and pores may be more-subtle at the nanoscale, and therefore have not been reported for sub-micron Sn particles in porous electrodes. The simplified planar geometry of the Sn sheet allows to dispense with the influence of the binder and carbon additives that are required in porous electrodes and the implementation of the Randles-Sevick equation provides a first experimental estimate for the diffusion coefficient of Na+ in Sn as 6.45×10−12 cm2 s−1. Finally, we explore facile synthesis of carbon materials from low cost carbon source of CaC2 using a novel sulfur-based thermo-chemical etching technique. Comprehensive analysis using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and N2 adsorption−desorption isotherms, reveals a highly graphitized mesoporous structure for the CaC2-derived carbon with a specific surface area of 159.5 m2 g−1. Microscopic analysis displays micron-scale mesoporous frameworks (4–20 μm) with a distinct layered structure along with agglomerates of highly graphitized nanosheets (about 10 nm in thickness and 1–10 μm of lateral size). The application of the as-prepared carbon materials as anode for NIBs and LIBs is also preliminarily studied.