Gao, Xuan; (2025) Break the Trilemma: design of high-loading, high-capacity, high-stability zinc-ion battery cathode towards industrialization. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Aqueous zinc-ion batteries (AZIBs) are increasingly seen as promising options for stationary energy storage, given their high volumetric capacity, intrinsic safety, and affordability. To enable their widespread adoption for large-scale applications, improvements in AZIB performance and stability, particularly in cathode materials like MnO2, are essential. Enhancing cathode properties—specifically achieving high-loading, high-capacity, and high-stability—is critical for AZIBs industrialization. High-loading cathodes accommodate more active material per unit area, increasing energy density and reducing the battery's footprint. High-capacity cathodes store more energy per unit mass or volume, crucial for applications requiring long-duration energy storage, such as grid-level energy balancing. Moreover, high-stability cathodes ensure long-term performance and reliability, crucial for industrial environments where downtime is costly. Nevertheless, with the escalation of unit mass loading, the dynamics of electron and ion transport within the electrode diminish, significantly impacting both capacity and cycle stability. This phenomenon is commonly referred to as the "Trilemma" in battery technology. This thesis proposes a decoupling enhancement strategy for surface and bulk materials to achieve these cathode properties simultaneously, advancing AZIBs industrialization. By focusing on electron and ion transport dynamics in Mn-based cathodes of AZIBs, the research develops a high-loading cathode based on designs of electrode structure (free-standing three-dimensional network), material property (double-ion pre-intercalation), fabrication process design (slurry treatment), facilitating high-loading cathode applications in Ah-level full cells. The study employs physicochemical and electrochemical characterizations, along with ex-situ material characterization and computational simulations, to elucidate cathode engineering details. Overall, this research aims to address critical factors in AZIBs development, laying the groundwork for their widespread industrial use in energy storage applications. The PhD project encompasses three primary works, outlined below: (1) MnO2-based cathodes in AZIBs offer stability and safety yet face challenges in slow kinetics due to low electrical conductivity, crucial for rapid charging devices. Addressing these hurdles, a sodium-intercalated manganese oxide (NMO) with 3D varying thinness carbon nanotubes (VTCNTs) network is proposed as a binder-free cathode (NMO/VTCNTs) without heat treatment. This novel network, utilizing low-thinness CNTs (LTCNTs) and high-thinness CNTs (HTCNTs), enhances specific capacity and mass loading. The interconnected CNTs withstand deformation, providing extra Zn2+ storage and efficient ion/electron migration routes. The NMO is evenly distributed within CNTs, improving structural stability and transport rates. The cathodes achieve loading of 5 mg cm−2, retaining high specific capacities of 329 mAh g−1 after 120 cycles at 0.2 A g−1, 225 mAh g−1 after 200 cycles at 1 A g−1, and 158 mAh g−1 after 1000 cycles at 2 A g−1. This construction strategy offers insights into achieving high mass loading and capacity, presenting significant potential for industrial application. (2) This study introduces a dual-ion co-intercalation strategy to enhance Mn-based cathodes in AZIBs. By incorporating both sodium and copper ions into δ-MnO2 (NCMO), stable cycling performance and high specific capacity are simultaneously achieved, even under high mass loading. Pre-intercalated Na+ boosts the Cu2+-driven activation of the Mn2+/Mn4+ redox process, while the smaller ionic radius of Cu2+ accelerates diffusion for improved charge/discharge kinetics. At lower mass loadings, the synergistic action of Na+ and Cu2+ sustains a prolonged capacity enhancement, whereas at higher loadings it enables a reversible Mn deposition/dissolution process. Ex-situ XRD and XPS analyzes clarify how Cu2+ promotes these redox dynamics. Remarkably, the NCMO cathode displays a continuous activation process under low mass loading condition, delivering 576 mA h g-1 capacity after 100 cycles at 0.5 A g-1. Under higher mass loading (∼10.9 mg cm-2), it exhibits a high areal capacity of 2.10 mA h cm-2 at 1.09 mA cm-2. These findings underscore co-intercalated Mn-based cathodes as promising candidates for practical, high-performance energy storage applications. (3) In this study, acetic acid (HAc) was introduced during the mixing process to enhance the chemical activity of the cathode surface. This addition led to notable improvements, particularly under low loading conditions, where the surface ratio increased, resulting in a very high specific capacity and prolonged activation process of the cathode. As the loading gradually increased, the activation process diminished due to a decrease in the surface ratio. Remarkably, the traditional coating process achieved a high load capacity of 21 mg cm-2. Moreover, the 8 Ah pouch cell with the HAc-treated cathodes exhibited an impressive 89% capacity recovery in the 100th cycle at a current density of 0.2 A g-1. These results underscore the effectiveness of HAc treatment in enhancing cathode performance, highlighting its potential for practical application in battery technology. Investigations into the underlying mechanisms and optimization based on in-situ Raman ex-situ XRD, ex-situ XPS, and X-ray Computed Tomography (XCT) of the HAc treatment process has been discussed for more significant advancements in AZIBs electrochemical performance and lifespan.

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