Nanostructured titanium-based oxide anode materials for lithium-ion and sodium-ion batteries

Abstract

Commercial lithium-ion batteries (LIBs) with graphite anodes pose safety concerns because of the proximity of the lithiation potential of graphite to the reduction potential of lithium. In this regard, titanium-based anode materials, due to their high lithiation potential (> 0.8 V vs. Li+/Li), and excellent thermal and cycling stability, are promising alternatives to graphite. Nevertheless, the limited rate-capability performance of titanium-based oxide (TBO) anodes, particularly Li4Ti5O12, due to their low theoretical capacity and low electrical conductivity (< 10-11 S m-1), cannot fulfill the ever-growing energy and power demand of LIBs. On the other hand, some TBO anodes, like TiO2(B), though having higher capacity, suffer from severe irreversible capacity loss, thereby hindering their commercialization. This dissertation study aims to circumvent these intrinsic problems of TBO anodes, by rationally designing and synthesizing the nanostructures, with detailed materials and electrochemical characterizations. Li4Ti5O12 distributed within and around 3D hollow core mesoporous shell carbon matrix (LTO/HCMS-C) are synthesized to address the low electrical conductivity of Li4Ti5O12. Benefiting from the unique combination between Li4Ti5O12 nanoparticles and high-surface area HCMS carbon with abundant, regular, interpenetrating mesopores and macropores, LTO/HCMS-C manifests a superior room temperature and sub-zero temperature anode performance for LIBs due to facile lithium ion and electron transport to Li4Ti5O12. TiO2(B) with a capacity of 335 mAhg-1, almost twice the theoretical capacity of Li4Ti5O12, is explored. On the other hand, there is severe irreversible capacity loss (ICL) of TiO2(B). Li4Ti5O12/TiO2(B) composite (LTO-TB), consisting of Li4Ti5O12 nanocrystals preferentially grown at both ends of TiO2(B) nanofibers, are synthesized. LTO-TB not only demonstrates a mere 7-9 % initial irreversible capacity loss, but also enhanced rate-capability. Although the introduction of Li4Ti5O12 can mitigate some of the ICL of TiO2(B), there is insufficient Li4Ti5O12 coverage on TiO2(B). Therefore, another synthesis route is designed to coat a conformal Li4Ti5O12 layer to fully cover the TiO2(B). The resulting core/shell Li4Ti5O12/TiO2(B) nanofibers (LTO-TB-II) even exhibits smaller irreversible capacity loss than LTO-TB, due to the full protection of TiO2(B) surface from electrolyte by the Li4Ti5O12 layer avoiding capacity and voltage loss from a solid electrolyte interphase (SEI). The Li4Ti5O12/TiO2(B) composite (LTO-TB and LTO-TB-II) can be a new class of high-performance alternatives to Li4Ti5O12. Besides LIBs, sodium-ion batteries (NIBs), the most promising alternatives to LIBs, have also been studied in this dissertation to address the ever-increasing concern about the high cost and limited abundance of lithium. To promote the realization of NIBs into the society, a facile and scalable carbon-assisted solid-state synthesis is developed to synthesize TBO anodes. A series of Na2Ti3O7/Na2Ti6O13 nanorods with tunable composition is fabricated and applied as anodes for NIBs. The carbon added provides an alternative route for the formation of Na2Ti3O7 and Na2Ti6O13 due to the extra local heat generation and CO2/CO release from carbon oxidation. By controlling the carbon content in the synthesis, the composition of Na2Ti3O7/Na2Ti6O13 can be tuned to achieve the optimized performance based on the high theoretical capacity of Na2Ti3O7 and extended cyclability of Na2Ti6O13. The synthesized Na2Ti3O7/Na2Ti6O13 nanorods exhibit the best rate-capability and cyclability among the reported Na2Ti3O7 or Na2Ti6O13 prepared by the conventional solid-state method.