LiNixCoyMnzO2 cathode materials for lithium (-ion) batteries

• Main Author: Chen, Zhen
• Other Authors: Shen Zexiang
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Science::Chemistry::Physical chemistry::Electrochemistry
• Online Access: https://hdl.handle.net/10356/90001; http://hdl.handle.net/10220/46482
• Institution: Nanyang Technological University

The current state-of-the-art lithium ion batteries (LIBs) have dominated the small format battery market for portable electronic devices， i.e., 3C products, namely, computer, communication and consumer electronics. The implementation of such technologies into automotive like hybrid (HEVs), plug-in (PHEVs), or fully battery electric vehicles (BEVs) is also quite successful. However, the realization of pure electrical propulsion requires battery materials that enable high energies to meet demands of a ∼500 km driving range, and a 10-year lifetime with driving range of 250,000 km. To achieve such a goal, the key challenge for both chemists and electrochemical engineers lies in increasing the battery energy density, while retains a long cycle life. At the moment, cathode materials remain the bottleneck due to their rather low capacity, compared with anode materials (generally graphite). Among all the common cathode materials, layered LiNixCoyMnzO2 (NCM) has overwhelming advantages for its high reversible capacity, rather low cost, good environmental benignity and high structural stability. However, the current NCM still suffers from two notable shortcomings: 1) poor rate capability especially at high C-rates due to rather sluggish Li+ diffusion kinetics; 2) fatal capacity degradation upon prolonged cycles mainly resulting from side reactions between electrode and electrolyte. Therefore, further improvements are highly desired to address these issues and satisfy the requirements for practical applications. Bearing these targets in mind, the main aims of my Ph.D. project are: 1) synthesis of NCM cathode materials with high C-rate capability and stable long-term cyclability; 2) demonstration of their practical feasibility in full cell applications, both by fabricating full lithium-ion cells with a combination of our modified cathode electrode, and via directly employing metallic lithium as anode electrode to assemble lithium metal cells (in ionic liquid electrolyte system). To address the issue of poor C-rate capability, we employed a strategy of preparation of 1D bar-like LiNi0.4Co0.2Mn0.4O2 with preferentially exposed {010} electrochemically active facets, which provide more open structure for unimpeded Li+ migration. In addition, the diffusion pathways of lithium ions are greatly reduced because of the bar shape. All these factors together contribute to the achievement of significantly enhanced lithium ion diffusivity, and thus superior high C-rate capability. Aiming at addressing the strong capacity fading issue, we adopted a surface coating strategy to stabilize the electrode/electrolyte interface and prevent detrimental side reactions. The amorphous MnPO4 coating layer acts as an ideal protective layer via physically isolating the contact between NCM active material and electrolyte. Additionally, the MnPO4 coating enhances the lithium de-/intercalation kinetics in terms of the apparent lithium ion diffusion coefficient. As a result, MP-NCM-based electrodes achieve greatly enhanced C rate capability and superior cycling stability – even under exertive conditions like extended operational potential windows, elevated temperature, and higher active material mass loadings. Through a combination of MP-NCM and commercial graphite, we further fabricated graphite/MP-NCM full lithium-ion batteries, which show remarkable long-term cycling stability (capacity retention ratio up to 74.1% over 2000 cycles at 1C), and deliver a high gravimetric energy density of 376.2 Wh kg-1, demonstrating great practical feasibility. Employing an intrinsically safer electrolyte system, ca., ionic liquid as alternative, the as-fabricated lithium metal batteries allow for outstanding cycling stabilities with a capacity retention of well above 85% after 2000 cycles. Currently, the commercial electrode fabrication generally employs a large portion of toxic components and processing solvents (i.e., NMP), particularly for cathode electrodes. To design greener LIBs, in this thesis, we also dedicated to replacing the conventional organic PVDF-based binder with aqueous CMC-based binder and investigating its effects in electrochemical performance.