Preview

PDF, English - main document Download (54MB) | Terms of use

Abstract

In this thesis, conversion type anode materials including transition metal oxides (MoO3, MoO2,WOx), disulfides (WS2) and the insertion reaction-based carbides with MXene-structure (Ti3C2,Nb2C, V2C), as well as their composites, were investigated as potential anode materials for next generation lithium ion batteries (LIBs). MXenes were prepared by an selective etching-based process. When used as anode materials forLIBs, the synthesized MXenes electrodes exhibit excellent cycling stability due to their high electronic conductivity, layered structure as well as good mechanical properties. In order toimprove the specific capacity (<300 mAh g-1) of the MXenes, composites based on Nb2C- andV2C-MXenes and conversion-based high-capacity anode materials (MoO2 and MoO3) were produced. The here presented MoO3/Nb2C was synthesized by a ball-milling method and MoO2/C/V2C by an electrostatically assisted hydrothermal method. Crucial experimental parameters for the ball-milled MoO3/Nb2C (ball-milling time, ball-milling speed, and mass ratio of components) were varied to optimize the morphology and thus the battery performance. The best properties are obtained for MoO3/Nb2C composite synthesized with a mass ratio of 1:1 where a capacity of 261 mAh g-1 is found after 300 cycles at a current density of 100 mA g-1. The uniquely structured hydrothermally synthesized MoO2/C/V2C composites consist of uniformly distributed MoO2 in the hierarchical V2C/C structure. When used as anode materials for LIBs, the composites show outstanding cycling stability and superior rate capability with, e.g., 96% capacity retention (605 mAh g-1) at a high current density of 1000 mA g-1 after 400 cycles. Lastly, carbon-coated tungsten oxides based on low-cost carbon sources (CTAB or PVP) were synthesized by a hydrothermal, carbonization process. An additional sulfurization process yielded carbon-coated disulfides. When used as anode materials for LIBs, the CTAB-assisted tungsten oxide carbon composite (c-WOx/C), tungsten disulfide carbon composite (c-WS2/C), and mixedphase (c-WOx/C-WS2/C) electrodes show outstanding cycling stability and rate performance compared to pristine ones. Particularly, the c-WS2/C electrode shows superior long-term cycle stability of 97% retention after 500 cycles at a high current density of 500 mA g-1. Similarly, the PVP-assisted WS2/C (p-WS2/C) electrode displays a capacity retention of 80% after 500 cycles. This work, therefore, presents a scalable and low-cost route to prepare carbon-coated tungsten oxide and disulfide for high performance LIBs, which can be extended for the preparation of other carbon-coated metal-based materials.