Materials and mechanisms of sodium and zinc-based energy storage

• Main Author: Jia, Guichong
• Other Authors: Fan Hongjin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Nanostructured materials; Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/136977
• Institution: Nanyang Technological University

The tremendous development of mobile electronics and renewable energy technology facilitate the research on energy storage devices. Electrochemical energy storage system is an effective technique to store and release electricity reversibly. The rechargeable lithium ion battery as the conventional electrochemical energy storage system has a great development in recent two decades. However, it has hampered by the increasing cost of lithium resource and severe safety issues. Thus, the exploration of alternative rechargeable batteries is significant and emergent. Sodium and zinc-based batteries show the promising prospect because of the low cost and high safety, especially for the grid-scale energy storage system. In this thesis, the materials and mechanisms of sodium and zinc-ion batteries have been investigated. Two-dimensional layered transition-metal dichalcogenides (TMDs) are widely studied as anode materials in sodium-ion batteries (SIBs) in recent years. However, their storage mechanisms of sodium ions are vague. There are two problems, the first one is the reversibility of conversion reaction during cycles, and the second one is the specified potential ranges for conversion reaction and intercalation process. In chapter 3, the reversibility of the layered TMDs in SIB has been investigated. The thin nanosheets MoSe2 uniformly embedded within an N-doped carbon matrix was synthesized. Its electrochemical properties were measured in detail, which exhibited better rate and cycle performance than those of pure MoSe2 in SIB. And the storage mechanism of sodium ions was studied with ex-situ X-ray diffraction characterization, which revealed the irreversible conversion reaction of MoSe2 during the first cycle within the potential range from 0.01 to 3.0 V vs. Na+/Na. Meanwhile, layered TMDs, as the anode materials in SIBs, always suffer from the poor capacitance retention due to the damage of the layered structure, but the research on its protecting conditions is few. Moreover, some ex-situ measurements could be influenced by the ambient environment, so the direct and continuous observation with in-situ characterization techniques in working condition is highly significant. Herein, in chapter 4, the specified potential ranges of conversion reaction and intercalation process have been studied in detail. The MoS2-xSex/graphene foam was synthesized via a Se substitution reaction of S in MoS2/graphene foam. The controlled potential ranges were applied on MoS2-xSex/graphene foam in SIBs to investigate the triggering condition of damaging the layered structure. The composition changes and crystal structure evolution are characterized by the combined in-situ Raman spectroscopy and ex-situ XRD measurements, which also revealed the specified potential ranges for conversion reaction and intercalation process. MoS2/graphene foam with only intercalation process shows better capacitance retention than that with intercalation process and conversion reaction simultaneously. And MoS2-xSex/graphene foam exhibited better rate performance than that of MoS2/graphene foam within the chosen potential range of intercalation process, which indicated the Se substitution of S in MoS2 is an effective method to enhance the electrochemical performance, owing to the expanded interlayer spacing. This work provides direct observation on mechanism study and effective strategy to improve the electrochemical performance. For the alternative battery of zinc-ion battery (ZIB), it has the advantages of low cost and high safety because of the cheap cost and aqueous electrolyte. Vanadium-based materials are one main type of cathode materials for ZIBs. However, the main studies are focused on the vanadium-based materials with high valence states of vanadium, such as the V4+ and V5+. The vanadium-based materials with low valence states have not been reported yet. Herein, the synchrotron-based in-situ X-ray diffraction and density functional theory (DFT) calculation have been applied to explore the materials. In chapter 5, the VOOH with a low valence state (V3+) has been synthesized and investigated, as cathode material, in aqueous ZIB, which exhibited a high specific capacity and good rate performance. And the electrochemical reaction kinetics was studied with galvanostatic intermittent titration technique (GITT) and redox current contribution analysis. Its composition changes and crystal structure evolution were also characterized by synchrotron-based in-situ X-ray diffraction. Combined with density functional theory (DFT) calculation, a new storage mechanism of zinc ions in vanadium-based materials have been proposed. This work can provide an insight to explore new electrode materials for electrochemical energy storage. All in all, the elaborate control and design of experiments and advanced characterization techniques are effective strategies for material exploration and mechanism study.