Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Mechanical Engineering | en_US |
| dc.contributor.advisor | Yu, Xiaoliang (ME) | en_US |
| dc.contributor.advisor | Chen, Guohua (ME) | en_US |
| dc.creator | Hu, Liang | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13604 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Tailoring electrolyte solvation and electrode-electrolyte interphases for high-performance sodium metal batteries | en_US |
| dcterms.abstract | Lithium-ion batteries (LIBs) have dominated the global market of consumer electronics, electric vehicles, and grid-scale energy storage because of their high energy/power density, long cycle life, and safety. However, limited reserves and geographical distribution of lithium have intensified the search for alternatives. Sodium-ion batteries (SIBs) have emerged as a promising one, because of their low cost, abundant existence, and compatible technologies with existing LIB production. Nevertheless, gaps still exist between state-of-the-art SIBs and commercial LIBs, particularly in terms of energy density and stability. Employing sodium metal anode presents a promising pathway to bridge this gap, because of its high theoretical capacity of 1166 mAh g⁻¹ and low electrochemical potential (-2.71 V vs. reversible hydrogen electrode). The reversible and durable operation of sodium metal batteries (SMBs) at low temperatures is essential for cold-climate applications but is plagued by dendritic Na plating and unstable solid-electrolyte interphase (SEI) formation. Current Coulombic efficiencies (CE) of sodium plating/stripping at low temperatures fall significantly below 99.9%, representing a substantial barrier to practical implementation. | en_US |
| dcterms.abstract | In this research, the challenges stated above were addressed strategically first by modifying the solvation structure of a conventional 1M NaPF₆ in diglyme (G2) electrolyte through facile cyclic ether (1,3-dioxolane, DOL) dilution. This DOL diluent helps achieve an impressive Na⁺ ion conductivity, 5.46 mS cm⁻¹ at 25 °C, and facilitates the desolvation of Na⁺ through weakening the chelation by G2, leading to a decrease in the Na⁺ ion desolvation energy from 282.7 to 245.1 kJ mol⁻¹. More importantly, this modification promotes concentrated electron cloud distribution around PF₆⁻ in the solvates, favoring their preferential decomposition, and an inorganic-rich SEI with compositional uniformity, high ionic conductivity, and high Young's modulus of 1.1 Gpa. As a result, a record-high CE exceeding 99.9% is achieved at an ultralow temperature of -55 °C, and a 1-Ah capacity pouch cell employing an initial anode-free sodium metal battery (AFSMB) configuration retains 95% of the initial discharge capacity over 100 cycles at -25 °C. | en_US |
| dcterms.abstract | High-voltage cathodes of SMBs are promising for increasing energy density. However, the elevated charging cut-off voltage necessitates better oxidation stability of the electrolyte. Although ether-based electrolytes are highly compatible with Na metal anodes, their applications in high-voltage SMBs remain limited because of their relatively low value of highest occupied molecular orbital (HOMO) energy levels. High-concentration electrolytes have proven effective in decreasing solvent activity and enhancing the electrolyte oxidative stability of electrolytes. However, concentrating ether electrolytes based on fluorine-rich sodium salts is challenging because of their high lattice energies and limited solubility in ether solvents. The next part of the work reveals a unique solubilization effect of cyclic ethers for enhanced solubility of NaPF₆ in G2. The introduction of solubilizing DOL co-solvent into the NaPF₆-G2 electrolyte promotes the coordination of G2 solvent with Na⁺ and decreases the free G2 solvent ratio from 35 to 24%. The ratio of contact ion pair (CIP) and aggregate (AGG) solvates increases from 23 to 32%, promoting the formation of stable NaF-rich cathode electrolyte interphase (CEI). Both factors significantly eliminate the decomposition reaction at high-voltage battery operation, enabling an average charge/discharge CE of 99.7% for the Na₃V₂O₂(PO₄)₂F cathode. More intriguingly, the as-obtained electrolytes retain a relatively low viscosity of 10.83 mPa·s at 25 °C, ensuring fast Na⁺ ion mobility with a diffusion coefficient of 1.17 ×10⁻¹⁰ m² s⁻¹ and thus smooth charge transfer kinetics during charge/discharge. Consequently, the as-fabricated Na//Na₃V₂O₂(PO₄)₂F cell retains 89.9% capacity over 2000 cycles at a high operation voltage of 4.4 V. | en_US |
| dcterms.abstract | Solvent engineering involves molecular structure regulation of solvents, such as optimizing the structure of alkyl groups in solvent molecules, decreasing the ether oxygen atom content in solvent molecules, and substituting atoms of the solvent molecules with halogen atoms or halogen groups. While improving oxidative stability, these modifications weaken the salt dissociation ability of solvents and decrease electrolyte ionic conductivity. Furthermore, fluorinated solvents have high costs and pose potential environmental hazards. In the last part of the research, the stabilization of the ether electrolyte at high operation voltages by forming a crown-like solvation structure is demonstrated. It enables the coordination of active oxygen atoms in the ether solvent with Na⁺ ions. Unlike the above-mentioned solvent engineering strategies, this new approach promotes an oxidatively stable electrolyte, achieving a 55% ratio of solvent-separated ion pair solvates, favorable salt dissociation up to 2.5 M, and ensuring a Na⁺ ion conductivity of 1.13 mS cm⁻¹ at 25 °C. The high-voltage reversibility of the cathode can be enhanced by using NaBF₄ to replace NaPF₆, which produces more favorable B-containing CEIs. Moreover, further concentrating the electrolyte results in more crown-like solvates and boosts the charge/discharge CE to a record-high 99.9%. In consequence, the high-voltage Na₃V₂O₂(PO₄)₂F cathode delivers 95.5% capacity over 1000 cycles. Fabricated AFSMB shows a superior cycling performance of 94% capacity retention over 200 cycles at 1 C. | en_US |
| dcterms.extent | xxiv, 177 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2025 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.accessRights | open access | en_US |
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