| Author: | Wei, Zhenyao |
| Title: | Fabrication of flexible polymer-based high-voltage solid-state lithium metal batteries |
| Advisors: | Zheng, Zijian (SFT) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Department: | School of Fashion and Textiles |
| Pages: | xxi, 123 pages : color illustrations |
| Language: | English |
| Abstract: | Commercial lithium-ion batteries have been widely applied in electric vehicles, portable electronics, and large-scale energy systems. The emergency of solid-state electrolytes could increase energy density and improve the safety concerns of batteries. However, many challenges must be addressed to meet the requirements of industrial applications. Firstly, the interfacial contact between solid-state electrolytes needs to be improved due to the solid-solid contact. Secondly, the electrochemical stability of polymer electrolytes restricts the further enhancement of energy density because of the poor compatibility with high-voltage cathodes. Impressively, the solid-state feature of solid-state electrolytes enables the realization of internal bipolar configuration once the compatibility of the current collector for lithium metal anode and cathodes are optimized. Firstly, UV-cured polymer electrolyte is synthesized via photo-polymerization of poly (ethylene glycol) methyl ether methacrylate and poly (ethylene glycol) diacrylate. The obtained polymer electrolyte exhibits a high ionic conductivity of 2.95 × 10⁻⁵ S cm⁻¹ at 30°C, a wide electrochemical stable window of up to 4.69 V (vs. Li/Li⁺), and excellent compatibility against lithium metal electrodes over 800 h. Besides, an integrated cathode/electrolyte interface is constructed by pouring the polymer electrolyte precursor onto the cathode layer. This integrated cell exhibits faster Li-ion diffusion in cathodic electrochemical reactions than conventional cells. Moreover, LiMn0.8Fe0.2PO₄||Li cells with integrated cathode/electrolyte interface deliver a reversible capacity of 164.7 mAh g⁻¹ at 0.1 C and retain a capacity of 134.4 mAh g⁻¹ after 240 cycles at 0.2 C. Furthermore, the integrated cells perform satisfactorily under disastrous conditions, presenting high safety. The UV cross-linked polymer electrolyte is a promising candidate for high energy density all-solid-state lithium metal batteries. Secondly, we designed and synthesized a fluorine-based polymer electrolyte with poly(ethylene glycol) diglycidyl ether copolymerizing with glycidyl 2,2,3,3-tetrafluoropropyl ether for safe and stable all-solid-state lithium batteries. Introducing TFE enhances the ionic conductivity, widens the electrochemical stable window, and improves the stability with the lithium metal anode. Moreover, the constructed LiF-rich solid electrolyte interface ensures the symmetric lithium metal cell cycling for 2000 h under 0.2 mA cm⁻². The assembled all-solid-state lithium batteries paired with LiNi0.6Co0.2Mn0.2O₂ demonstrate outstanding capacity retention after 300 cycles with coulombic efficiency of 99.95% and long-term charge/discharge stability with LiFePO₄ for 700 cycles. Notably, the assembled pouch cell exhibits excellent flexibility with a capacity retention of 93.8% after 3000 bending cycles under a radius of 4 mm. Therefore, introducing fluorine-based components into the polymer structure effectively stabilizes high-voltage cathode during long-term operation. Thirdly, nickel-coated PET (Ni-PET) fabric was fabricated through a polymer-assisted metal deposition method to support cathodic and anode materials. Ni-PET fabric shows excellent flexibility after bending 10000 cycles under a bending radius of 5.0 mm. The electrochemical performance of Ni-PET batteries is similar to those with Al current collectors, delivering capacity retention of 85.4% after 500 cycles at 0.5 C. In addition, Ni-PET can also act as a bipolar substrate due to the compatible stability of nickel metal and lithium metal. The polarization voltage of the internal bipolar battery is lower than the external bipolar configuration. The capacity retention of assembled bipolar batteries with Ni-PET is 95.4% after 250 cycles. Furthermore, the bending process decreases resistance, increasing capacity after 200 bending cycles. The coulombic efficiency of the bipolar battery is up to 99.8% during the dynamic bending test, showing great flexibility with PET-based current collectors. In conclusion, this thesis offered effective strategies to improve the energy density of all-solid-state lithium metal batteries. The interfacial contact between the electrolyte and cathode was addressed by constructing the integrated structure. Fluorine-based polymer electrolytes enhanced the compatible stability with high-voltage cathodes. Moreover, the bipolar configuration of solid-state batteries was optimized by introducing a Ni-PET current collector. The proposed strategies provide a solid foundation for industrial applications in the future. |
| Rights: | All rights reserved |
| Access: | open access |
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