| Author: | Liu, Tiancheng |
| Title: | Synthesis, modification, and degradation mechanism of single-crystal cobalt-free Ni-rich cathode materials |
| Advisors: | Huang, Haitao (AP) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Cathodes Electrodes -- Materials Lithium ion batteries Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xxvi, 179 pages : color illustrations |
| Language: | English |
| Abstract: | Cobalt has been regarded as a crucial component in Ni-rich layered oxide cathodes due to its ability to alleviate Li/Ni mixing, improve electronic conductivity, and ensure structural stability. However, cobalt is a scarce and expensive metal with significant price volatility and an unstable supply chain. Therefore, the removal of Co from the Ni-rich layered cathode is a significant step toward low-cost cathode materials. Single-crystal Co-free Ni-rich cathodes Li(NixMn1-x)O2 have been proposed as a promising candidate due to their low cost, excellent structural stability, and thermal stability. However, low Li-ion diffusion kinetics, highly reactive Ni4+, and stress-induced intragranular micro-cracking restrict its further application. The objective of this thesis is to synthesize, modify, and investigate the degradation mechanism of single-crystal Co-free Ni-rich cathode Li(NixMn1-x)O2. Firstly, we report the dual-functional boron modification (doping and coating) on a cobalt-free single-crystal layered cathode (B-LiNi0.75Mn0.25O2) via a simple solid-state method. The boron atoms prefer to occupy the tetrahedral interstices in the Li layer, which enlarges the c-axis for fast Li-ion diffusion kinetics and can also serve as a pillar to achieve an ultra-low (1.62%) c-axis lattice contraction at 80% state of charge (SOC). Boron doping passivates the lattice oxygen, inhibits the irreversible phase transition and endows excellent thermal stability at high cut-off voltage of 4.5 V. On the other hand, the byproduct LiBO2 can improve Li-ion diffusion and alleviate side reactions at the electrode/electrolyte interface by serving as a protective and Li-ion conductive coating layer. As a result, the B-LiNi0.75Mn0.25O2 is almost free of inter-and intragranular micro-cracking with faster Li-ion diffusion kinetics. This routine not only provides a low-cost and scalable method to remove Co from conventional Ni-rich layered cathode, but also reveals the commercial feasibility of highly safe single-crystal layered cathode materials. Secondly, to further investigate the capacity fading mechanism of Li(NixMn1-x)O2 at high voltage, large-sized single-crystal cathodes LiNi0.6Mn0.4O2 (NM64) and LiNi0.8Mn0.2O2 (NM82) are selected as model materials. By modulating the SOC, we found that NM82 undergoes faster capacity decline at 4.5 V cut-off voltage than NM64 at 4.6 V. Using a combination of experiments and theoretical calculations, we found that the Li/Ni mixing plays a significant role in mitigating the anisotropic lattice contraction in NM64. On the other hand, with more Ni4+ at the high SOC, NM82 presents serious irreversible phase transition, accompanied with loss of lattice O and undesirable reactions. Therefore, the damage of “high” voltage should be assessed objectively compared with other factors, such as anisotropic lattice contraction and highly reactive Ni4+. Developing single-crystal Li(NixMn1-x)O2 with rational Ni/Mn contents can promote the commercialization of highly safe single-crystal Co-free cathodes at low cost. Thirdly, the role of cobalt in terms of capacity contribution has been disclosed. We found that the Co-free single-crystal cathode, LiNi0.88Mn0.12O2, exhibits superior cycling performance compared to the Co-containing cathode, LiNi0.83Co0.05Mn0.12O2. Our results revealed that 5% Co facilitates the early onset of the phase transition to release more capacity. Nevertheless, the Co-containing cathode suffers from more irreversible surface reconstruction and rapid capacity decay. What’s more, Ni dominates the change in oxidation state compared with Co, revealing the essence of capacity improvement in Co-containing cathode. As a result, Co itself is stable due to less reactivity at 4.3 V, while Ni in the Co-containing cathode is over-oxidated, leading to severe Ni4+ reduction, dissolution, and deposition at the anode. Finally, high-entropy doping approaches are developed to further enhance the stability of the Co-free single-crystal cathode NM88. Our findings indicate that the high-entropy doped NM88 exhibits an impressive capacity retention rate of over 95% after 200 cycles. In situ XRD analysis reveals that the anisotropic lattice contraction is mitigated in the high entropy doped NM88. Additionally, the high entropy doped NM88 exhibits reversible phase transition for exceptional structural stability, indicating that the high-entropy doping strategy is feasible to develop highly stable Li-ion batteries. |
| Rights: | All rights reserved |
| Access: | open access |
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