Self-supported anodes based on carbon and transition metal sulfides for sodium-ion batteries

• Main Author: Wang, Haisheng
• Other Authors: Shen Zexiang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/147838
• Institution: Nanyang Technological University

This thesis aims to develop self-supported anode materials based on hard carbon and cobalt sulfides for sodium-ion batteries (SIBs), which are regarded as promising low-cost alternatives to the prevailing lithium-ion batteries. As hard carbons are considered as the most promising first-generation Na-storage anodes, chapter 4 aims to design Na-storage hard carbon with both long cycling stability and a large reversible capacity. In order to achieve higher energy density and ensure the possibility for assembling full cells, chapter 5 aims to develop Na-storage cobalt sulfides with a high initial Coulombic efficiency (ICE). To move further toward practical applications, chapter 6 aims to design Na-storage cobalt sulfides with enhanced cycling stability, a high reversible capacity, and a high ICE. In chapter 4, the hollow interconnected carbon foam (HICF) is developed by one-step pyrolysis of a commercial and low-cost melamine sponge. The integration of interconnected network structure and hollow feature can not only enable strong mechanical stability and extra inner space to effectively accommodate the structural deformation from Na+ insertion/extraction, but also achieve fast sodium-ion and electron transport. As a self-supported anode for SIBs, HICF-1 delivers a large reversible capacity (305.7 mAh g-1 at 100 mA g-1) with a high initial Coulombic efficiency of 80.2%, an ultralong cycle life (86.4% capacity retention after 1000 cycles at 1000 mA g-1), as well as superior rate performance (170.1 mAh g-1 at 1000 mA g-1). In addition, the excellent Na-storage performance is also contributed by the maximum content (63.24%) of pseudo-graphitic phase (d002-spacing between 0.36 and 0.40 nm) in HICF-1 realized by tuning pyrolysis holding time, because the pseudo-graphitic phase could store more sodium ions and maintain more stable microstructure owing to its appropriate d-spacing than highly disordered phase (d002-spacing above 0.40 nm). Furthermore, kinetic analysis based on cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) verifies the adsorption-intercalation mechanism, in which highly disordered carbon phase absorbs Na ions fast in the sloping region and then Na ions intercalate into the pseudo-graphitic phase in the plateau region. This work provides a very promising anode candidate for the future commercialization of low-cost SIBs. In chapter 5, the CoSx NF@GF composite has been developed by growing CoSx (Co9S8/CoS) nanoflakes on the highly conductive and flexible substrate of graphite foam (GF) through a one-pot solvothermal route. As a freestanding anode for SIBs, to the best of our knowledge, CoSx NF@GF in diglyme-based electrolyte achieves the highest ICE of 99.4% among the previously reported cobalt sulfide-based Na-ion anodes. Through a systematic investigation of several factors that can potentially influence the ICE, such a high ICE could be ascribed to the following three aspects, including i) the negligible side reactions between diglyme-based electrolyte and Co9S8/CoS, owing to much higher Fermi level of diglyme reduction than anode potential μA of Co9S8/CoS, blocking transfer of electrons from anode to electrolyte, ii) the highly reversible conversion reaction of Co9S8/CoS, and iii) the rather low initial capacity loss of substrate GF. Furthermore, CoSx NF@GF in diglyme-based electrolyte achieves excellent cycling and rate performance, primarily resulting from alleviated volume expansion and facilitated ionic and electronic kinetics ensured by ultrathin nanoflakes vertically aligned with GF, as well as negligible side reactions at the interface of electrode/electrolyte. The revealed underlying rules serve as general guidelines in the development of sodium-ion anodes to achieve superb ICE. In chapter 6, by taking the structural advantages of both cobalt-based metal organic frameworks (Co-MOFs) and self-supported GF, Co-MOFs nanosheets are first grown on the GF substrate. After the processes of carbonization and sulfurization, carbon nanosheet arrays embedding Co9S8 nanoparticles are anchored on GF substrate (Co9S8-C NS@GF). The confinement of Co9S8 nanoparticles within carbon nanosheet combined with the excellent conductivity and flexibility of GF network could not only effectively alleviate the mechanical stress from conversion reactions to boost cycling stability, but also ensure fast transport of electrons and Na ions to achieve a high reversible capacity. As a self-supported anode for SIBs, the Co9S8-C NS@GF exhibits a high reversible capacity of 401 mAh g-1 at 0.5 A g-1 with a high ICE of 93.2% and a large capacity retention of 85.4% over 300 cycles. Interestingly, the superior Na-storage performance also benefits from the optimization of both the thickness and degree of graphitization of carbon nanosheets tuned by the pyrolysis temperature.