Surface Modification of Nanostructured Materials for Lithium-Ion Batteries

Abstract

Lithium-ion batteries (LIBs) continue to attract a tremendous amount of interest because they are the most promising candidate to power hybrid electric vehicles (HEV), electric vehicles (EV) and stationary energy storage systems. However, to meet the needs of the soaring markets, new generations of LIBs are required with increased energy and power density, longer cycle life, improved safety, and lower cost. To achieve this goal, electrode materials discovery and development is necessary. In particular, anode materials remain a key factor in improving the performance of LIBs due to their notably higher capacities than cathode materials. However, for a large fraction of anode materials, major critical challenges are the poor cyclic stability and safety issue during the repeated charging and discharging operations. Therefore, it is still greatly important to develop new materials for overcoming these problems. In this thesis, a two-step strategy comprising synthesis and surface modification was adopted to develop various novel anodes materials for LIBs. First, we synthesized morphology-controlled TiO2, Li4Ti5O12, and MoS2 nanomaterials, and then used surface modification methods including doping, calcination, composites and coating to improve the electrochemical performance of prepared materials. It was found that the undoped TiO2 showed an initial capacity of 201 mAh g-1 but only had 44.1% of the initial capacity retained after 50 cycles at mixed current densities of 30, 150, and 500 mA g-1 at 55 °C, while the Mn-doped one exhibited an initial capacity of 190 mAh g−1 and 91.4% capacity retention with superior reversible capacity under the same test conditions. A similar result was also observed in the nitrogen-treated porous TiO2, which exhibited a higher capacity of 293 mAh g-1 than that of air-treated sample (187 mAh g-1) after 50 cycles. In addition, the nitrogen-treated Li4Ti5O12-TiO2 sample showed significantly improved capacity, good rate capability, and cycling stability compared with pure Li4Ti5O12. It delivered capacities of 220 mAh g-1, 213 mAh g-1 at a current density of 30 mA g-1 when tested at room temperature and 55 °C, respectively; it still had a capacity of 184 mAh g-1, 197 mAh g-1 after 50 cycles, which was noticeably higher than the known theoretical capacity of pure Li4Ti5O12 (175 mAh g-1). Furthermore, compared with the previous results, the hierarchical nanocomposites of MoS2@Li4Ti5O12 showed much improved capacity (433 mAh g-1 after 70 cycles at various current densities), good rate capability (320 mAh g-1 and 210 mAh g-1 at 500 mA g-1 and 2000 mA g-1, respectively), outstanding cycling stability (174 mAh g-1 after 1000 cycles at 5000 mA g-1) and wide operating temperature range extending from -15 to 55 ºC when used as anode materials for lithium ion batteries. Finally, the mechanism of surface modification on the improvement of nanomaterials’ performance has been studied, and simple methods to fabricate promising candidates for the next generation anode materials are given.