Two-dimensional layered cobalt sulfide derived from metal organic framework (MOF) as anode for lithium-ion batteries

• Main Author: Guo, Ziqi
• Other Authors: Huang Yizhong
• Format: Thesis-Master by Research
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/145174
• Institution: Nanyang Technological University

Lithium-ion batteries (LIBs) as clean electrochemical energy devices have gained much attention because conventional fossil fuels and renewable energies are having limitations. LIBs can be applied in areas such as energy storage power stations, electronic vehicles, and portable electronic devices (smartphones, laptops, etc.). However, current LIBs in the market could not meet the ever-growing demand due to the capacity of the commercial graphite anode material has already reached its theoretical limits (372 mAh g-1). Thus, developing alternative anode materials with a higher specific capacity, longer cycle life, better cycling stability, and more outstanding rate capability holds great promise. Nowadays, transition metal sulfides have attracted great interests as LIBs anode materials, due to their higher specific capacity (500-900 mAh g-1), moderate rate capability and cycling stability. Among these, cobalt sulfides (CoS) have advantages including relatively high theoretical capacity (589 mAh g-1), abundant resources, and low cost. However, CoS as anode materials also face challenges including low electronic conductivity, capacity attenuation due to large volume expansion during cycling. The motivation of this research is to improve CoS’s capacity fading issue by reducing the large volume expansion during cycling, as well as to increase their electric conductivity. In this research, two-dimensional layered cobalt sulfides (CoS) were prepared using a facile two-step in-situ formation method: Synthesizing cobalt metal-organic framework (Co-MOF) as precursor first, followed by sulfidation/carbonization at high temperatures. This novel MOF-converted CoS could fulfill nano-sizing and composition with carbon in one shot. Four calcination temperatures were investigated, samples synthesized at 700˚C appear to have the best cycling stability and rate capability. The discharge capacity of the 2nd cycle is 828.1 mAh g-1, while capacity still has a 50.9% retention rate after 100 cycles under the current density of 100 mA g-1, which is 421.5 mAh g-1. The reversible capacities of samples sulfurized at 700˚C are 700.45, 586.45, 455.60, 364.20, and 273.29 mAh g-1 at current densities of 100, 200, 500, 1000, 2000 mA g-1, respectively. When reverting back to 100 mA g-1, the capacity of CoS increased back to 618.68 mAh g-1，implying good reversibility. During the second cycle of the galvanostatic charge-discharge cycling test, the coulombic efficiency is as high as 93.6%. The discharge capacity is higher than the theoretical capacity of graphite (372 mAh g-1), indicating that samples in this research are a promising candidate for the anode of LIBs. Compared with bulk materials, nano-structures are more stable upon reactions, resulting in smaller volume change. Moreover, the nano-sizing structure provides a high surface to volume ratio and more active sites for Li+ storage, leading to the capacity increase. The short Li+ diffusion path across nano-particles allows easy ion diffusion that enhances the power density and high electron transfer rate. With the incorporation of carbon, a carbon layer is wrapped around CoS nano-particles. Not only could this carbon layer provide mechanical support to prevent large volume expansion, but also it increases the electric conductivity of the anode materials, resulting in a better electrochemical performance of LIBs.