| Author: | Raza, Hassan |
| Title: | High entropy materials (HEMs) for high-performance lithium-sulfur batteries |
| Advisors: | Chen, Guohua (ME) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Lithium ion batteries Storage batteries -- Design and construction Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxiii, 161 pages : color illustrations |
| Language: | English |
| Abstract: | This thesis reports the preparation and utilization of high entropy materials (with Sconfig>1.5R) in lithium-sulfur (Li-S) batteries to suppress the shuttle effect and accelerate the redox conversion kinetics of lithium polysulfides (LPSs). First, a single-phase high-entropy oxide (HEO) was fabricated by calcination of metal-organic frameworks (MOFs). This novel method is simple and requires a temperature of 850 °C. It effectively circumvented the typical complex synthesis procedures and high temperature calcination (> 1000 °C). The material generated, denoted as HEO850, displayed a single-phase rocksalt crystalline structure, an entropy dominant phase stability for a temperature range between 750 and 850 ℃, and a uniform elemental distribution of Ni, Co, Cu, Mg, Zn. Furthermore, HEO850 offers affluent active sites for LPSs anchoring and facilitates the sulfur redox kinetics. It was integrated with sulfur to make the cathode of Li-S batteries. As a result, the sulfur (S; 70 wt.%), ketjen black (KB, 20 wt.%) and HEO850 (10 wt.%) composite cathode (HEO850/S/KB) realized a high initial discharge capacity of ~1244 mAh g-1 at 0.5 C compared to medium entropy oxide (with 1R<Sconfig<1.5R) (MEO/S/KB; ~979 mAh g-1), low entropy oxide (with Sconfig<1R) (LEO/S/KB; ~908 mAh g-1), and sulfur ketjen black composite cathodes (S/KB; ~966 mAh g-1). It also exhibited an excellent cycling stability with a low-capacity fade rate of 0.043% per cycle over 800 cycles at 0.5 C with a sulfur loading of ~1.2 mg cm-2. The ex-situ XPS and XRD analyses revealed that the electrochemical performance of Li-S batteries was enhanced because of the cumulative effect of the metal elements and the structural stability of HEO850 during redox cycling. Next, titanium-based high entropy oxides (Ti-HEOs) were produced with the developed technique but with Cu being replaced by Ti from HEO850 (Ni, Co, Cu, Mg, Zn) in order to increase the electrical conductivity. Because of its efficient catalytic properties and increased conductivity, the Ti-HEO/S/KB cathode showed further improved electrochemical performance than that of HEO850/S/KB at high sulfur loadings. At a sulfur loading of ~3.4 mg cm-2, it displayed an initial discharge capacity of ~1246 mAh g-1 at 0.5 C and retained at ~542 mAh g-1 with a small capacity decay rate of 0.056% per cycle over 1000 cycles. In comparison, the HEO850/S/KB cathode had a lesser initial discharge capacity (~1178 mAh g-1) and degraded to ~302 mAh g-1 after 800 cycles at a moderate degradation rate of 0.093% per cycle under equivalent sulfur loading. Also, the voltage hysteresis between the discharge and charge curves of the Ti-HEO/S/KB cathode (~184 mV) is lower than that of the HEO850/S/KB (~200 mV), resulting in superior rate performance. It delivered a high initial discharge capacity of ~985 mAh g-1 at 1 C, which outperformed HEO850/S/KB (~505 mAh g-1) at high sulfur loading (~3.4 mg cm-2). Furthermore, the Ti-HEO/S/KB cathode exhibited a better calendar life with less self-discharging capacity loss. Ti-HEO/S/KB attained higher initial discharge capacity of ~1256 mAh g-1 at 0.1 C and a capacity retention of 84% after 20 days of resting during the 3rd and 5th discharge cycles, respectively. In comparison, HEO850/S/KB cathode showed an inferior capacity retention of 72%, and S/KB cathode experienced a sudden failure because of sluggish conversion of LPSs and probable shuttle effect. In addition, high-entropy sulfides (HES) generated from glycerate template were investigated as a sulfur host to accelerate the kinetics of LPSs conversion. The HES spheres were fabricated utilizing a two-fold hydrothermal procedure at reaction temperatures of 150 °C and 160 °C, respectively. Three types of HES samples denoted as GS-1 (Ni, Co, Fe, Mg, and Zn), GS-2 (Ni, Co, Cu, Mg, and Zn) and GS-3 (Ni, Co, Fe, Mg, and Ti) were prepared successfully. They were employed as both the working and counter electrodes in symmetric batteries containing Li2S4, Li2S6 and Li2S8 mixture electrolyte. The GS-3/KB electrode demonstrated a higher current response than that of GS-1/KB, GS-2/KB, Ti-HEO/KB, HEO850/KB, and bare KB, indicating accelerated conversion kinetics of LPSs. Thus, the typical Li-S battery with GS-3/S/KB cathode (sulfur loading ~2.3 mg cm-2) displayed a stable cycling for 1500 cycles at 0.5 C. It attained an initial discharge capacity of ~1061 mAh g-1 and retained at ~550 mAh g-1 with a lower capacity fade rate of 0.032% per cycle. In comparison, GS-1/S/KB demonstrated a lower discharge capacity (~945 mAh g-1) in the first cycle at the same current rate with a slightly higher capacity fade of 0.034% per cycle over 1500 cycles. GS-2/S/KB displayed a slow conversion kinetics with a rapid fade rate of 0.086% per cycle over 500 cycles. Overall, the high entropy materials (HEOs/HES) with proper composition can have positive impact on the electrochemical performance of lithium-sulfur batteries. It could be attributed to their catalytic properties and availability of multiple active sites for LPSs anchoring because of the high configurational entropy and uniform elemental distribution in the crystal structure of HEMs. |
| Rights: | All rights reserved |
| Access: | open access |
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