Design and Electrochemical Performance of Cathode Additives for Lithium Sulfur Batteries
鋰硫電池正極添加劑的設計以及電化學性能研究
Student thesis: Doctoral Thesis
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Award date | 9 Aug 2023 |
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Permanent Link | https://scholars.cityu.edu.hk/en/theses/theses(b73ed398-f6b8-4e91-8290-8de55ef56cce).html |
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Abstract
Lithium-sulfur batteries (LSBs) are regarded as next-generation energy storage solution owing to their remarkable theoretical energy density (2600 Wh kg-1) beyond other battery systems. However, the commercialization of LSBs is beset with some tenacious issues, including the insulation nature of the S or Li2S (the discharged product), the unavoidable dissolution of the reaction intermediate products (e.g., lithium polysulfides (LiPSs)), and the shuttling effect of subsequent LiPSs across the separator, which result in the continuous loss of active material, anode passivation, and low coulombic efficiency.
Containment methods of introducing the high-electrical conductivity host are commonly used in improving the electrochemical performances of LSBs. However, such prevalent technologies will reduce energy density since they require additional host materials. Trace amount of catalysts that dominates the redox reaction between S/Li2S and Li2Sn (3 < n ≤ 8) shows an ingenious design, which not only accelerates the conversion reaction between the solid and dissolved S species, alleviating the shuttling effect, but also expedites the electron transport and reduces the polarization of the electrode.
Li6.4La3Zr1.4Ta0.6O12 (LLZTO), commonly used in solid-state batteries, was first used as a catalyst and lithium-ion transport mediator in the cathode. We find that LLZTO can act as a fast Li-ion transport pathway for the cathode, improving Li2S nucleation and dissolution kinetics, and the battery with LLZTO had good redox-conversion reversibility. Attributed to these improvements, the cell with LLZTO shows a slow capacity decay of 0.046% per cycle over 1000 cycles. Moreover, LLZTO can regulate the conversion of Li2S2 to Li2S, eliminating the production of “dead” sulfur and increasing the use of elemental sulfur.
Secondly, we introduced an easy-to-synthesize, defect-rich, and costless Co3O4/Co4N heterojunction as a cathode additive by plasma-induced transformation approach. The plasma-engineered MOF-derived Co3O4/Co4N has a well-defined interface to adsorb LPSs to eliminate the shuttling effect and exhibits remarkable electrocatalytic performance for the complex conversion of LPSs. As a result, the sulfur cathode coupled with Co3O4/Co4N performs long-term cycling stability with a low-capacity decay of 0.041% per cycle after 1100 cycles at 1C. In the scenario of high sulfur loading of 8 mg, the cell still shows a high specific capacity of 675 mAh g-1 at 0.5 C with a low E/S ratio (~8 μL mg-1). This study reveals the excellent electrochemical performance of heterojunction material on LPSs conversion and adsorption of lithium-sulfur battery.
Thirdly, we introduced graphene quantum dots (GQDs) as the structure regulator to synthesize CoS/GQDs composites as the cathode catalyst. The 2D sheet-like morphology of CoS was regulated as a nano-flower type. The novel composite material greatly improved the rate performance of the battery, which can reach a relatively high capacity of around 725 mAh g-1 at 8 C. Besides, it can also strongly adsorb the lithium polysulfides to prevent the shuttling effect. Due to these advantages, the cell has a competitive rate and cycling performances. Moreover, the structure regulators of GQDs can provide new ideas for synthesizing novel catalysts for lithium-sulfur batteries.
In summary, this thesis studied three types of cathode additives for lithium-sulfur batteries, which focused on different functions. All of them can greatly improve the performance of the cells in various aspects, which could provide new ideas for the design of cathode additive in lithium-sulfur batteries.
Containment methods of introducing the high-electrical conductivity host are commonly used in improving the electrochemical performances of LSBs. However, such prevalent technologies will reduce energy density since they require additional host materials. Trace amount of catalysts that dominates the redox reaction between S/Li2S and Li2Sn (3 < n ≤ 8) shows an ingenious design, which not only accelerates the conversion reaction between the solid and dissolved S species, alleviating the shuttling effect, but also expedites the electron transport and reduces the polarization of the electrode.
Li6.4La3Zr1.4Ta0.6O12 (LLZTO), commonly used in solid-state batteries, was first used as a catalyst and lithium-ion transport mediator in the cathode. We find that LLZTO can act as a fast Li-ion transport pathway for the cathode, improving Li2S nucleation and dissolution kinetics, and the battery with LLZTO had good redox-conversion reversibility. Attributed to these improvements, the cell with LLZTO shows a slow capacity decay of 0.046% per cycle over 1000 cycles. Moreover, LLZTO can regulate the conversion of Li2S2 to Li2S, eliminating the production of “dead” sulfur and increasing the use of elemental sulfur.
Secondly, we introduced an easy-to-synthesize, defect-rich, and costless Co3O4/Co4N heterojunction as a cathode additive by plasma-induced transformation approach. The plasma-engineered MOF-derived Co3O4/Co4N has a well-defined interface to adsorb LPSs to eliminate the shuttling effect and exhibits remarkable electrocatalytic performance for the complex conversion of LPSs. As a result, the sulfur cathode coupled with Co3O4/Co4N performs long-term cycling stability with a low-capacity decay of 0.041% per cycle after 1100 cycles at 1C. In the scenario of high sulfur loading of 8 mg, the cell still shows a high specific capacity of 675 mAh g-1 at 0.5 C with a low E/S ratio (~8 μL mg-1). This study reveals the excellent electrochemical performance of heterojunction material on LPSs conversion and adsorption of lithium-sulfur battery.
Thirdly, we introduced graphene quantum dots (GQDs) as the structure regulator to synthesize CoS/GQDs composites as the cathode catalyst. The 2D sheet-like morphology of CoS was regulated as a nano-flower type. The novel composite material greatly improved the rate performance of the battery, which can reach a relatively high capacity of around 725 mAh g-1 at 8 C. Besides, it can also strongly adsorb the lithium polysulfides to prevent the shuttling effect. Due to these advantages, the cell has a competitive rate and cycling performances. Moreover, the structure regulators of GQDs can provide new ideas for synthesizing novel catalysts for lithium-sulfur batteries.
In summary, this thesis studied three types of cathode additives for lithium-sulfur batteries, which focused on different functions. All of them can greatly improve the performance of the cells in various aspects, which could provide new ideas for the design of cathode additive in lithium-sulfur batteries.