Luận án Tiến sĩ: Tổng hợp và nghiên cứu tính chất điện hóa của vật liệu oxit (Na, Li)MnO₂ pha tạp và đồng pha tạp Co, F ứng dụng làm điện cực dương trong pin ion natri

The escalating demand for sustainable energy storage has positioned sodium-ion batteries (SIBs) as a compelling alternative to conventional lithium-ion batteries (LIBs), which face limitations due to lithium's finite resources and increasing cost. Despite their promise, SIBs encounter challenges, particularly in developing high-performance cathode materials that can ensure high energy density, cycle stability, and fast charge/discharge rates. Addressing this critical bottleneck, this study focuses on the synthesis and comprehensive electrochemical characterization of novel (Na,Li)MnO2-based oxide materials. Specifically, it investigates the impact of Co and F doping, both individually and in combination, on the structural and electrochemical properties of these materials, aiming to optimize their performance as advanced cathode candidates for SIB applications.

Researchers and academics in materials science, electrochemistry, and battery technology, particularly those focused on next-generation energy storage solutions and sodium-ion batteries.

This doctoral thesis comprehensively explores the synthesis and electrochemical properties of (Na,Li)MnO2-based oxide materials, strategically doped with Co and F, for their application as high-performance cathodes in sodium-ion batteries (SIBs). The research aims to overcome the inherent limitations of SIBs, such as lower energy density and structural instability, often associated with the larger Na+ ion. Two primary synthesis methods, solid-state reaction and sol-gel, were employed to prepare the doped materials. Structural and morphological analyses, utilizing techniques like XRD, SEM, TEM, EDX, ICP-MS, and XPS, revealed that doping significantly influences the crystal lattice parameters. For instance, Co doping via solid-state reaction simultaneously reduced lattice parameters 'a' and 'c', while sol-gel synthesis with Co and F co-doping led to a reduced 'a' and an increased 'c'. These structural modulations are crucial; a smaller 'a' enhances structural stability and cycle efficiency, whereas a larger 'c' facilitates Na+ ion diffusion, improving specific capacity and reaction kinetics. Electrochemical characterizations using CV, GCD, and EIS confirmed these improvements. Co doping directly contributes to capacity through additional redox couples (Co2+/Co3+) and enhances electrical conductivity. F doping, by inducing charge compensation through Mn4+/Mn3+ and Co3+/Co2+ reductions, increases active sites and, consequently, the material's capacity. The study successfully developed hexagonal P63mmc structured materials with particle sizes below 5µm, demonstrating high discharge capacities (e.g., CoF-NLM achieving 135 mAh/g at 8 mA/g) and excellent cycle stabilities (up to 94% retention over 100 cycles). This research provides a valuable scientific foundation for understanding doping mechanisms in SIB cathodes and offers practical, cost-effective synthesis routes for scalable production. The findings are instrumental for designing advanced sodium-ion battery technologies and serve as a significant reference for material scientists and battery researchers.