Exploration of Novel Cathode Active Materials for Sodium-Ion Batteries

Abstract

Lithium-ion batteries (LIBs) have led the energy storage market for decades due to their excellent performance. However, the increasing scarcity and cost of lithium resources have shifted research interest toward alternative technologies. Sodium-ion batteries (SIBs), with their abundant sodium resources and competitive electrochemical properties, have emerged as promising candidates. In particular, O3-type layered oxide cathodes are attractive for practical applications due to their high sodium content and potential for high reversible capacity. This dissertation focuses on optimizing Ni- and Mn-rich O3-type layered oxide cathodes for SIBs. Using NaNiO2 (NNO) as a prototype material, the work first addresses the challenges associated with irreversible phase transitions caused by Na+-de/intercalation-induced interlayer gliding and Jahn-Teller (JT) distortion from Ni3+. To improve the structural reversibility and electrochemical performance, Ti4+ was introduced to produce NaNi0.9Ti0.1O2 (NNTO), which was successfully synthesized via a solid-state reaction for the first time. NNTO exhibited a high specific capacity of 190 mAh/g, but suffered from chemo-mechanical degradation and irreversible lattice oxygen loss. To further address these limitations, Ca2+ was introduced as a pillaring ion. Among the various substitution levels explored, Na0.95Ca0.025Ni0.9Ti0.1O2 (CaNNTO) exhibited the largest interlayer spacing, along with best capacity retention and rate performance. In situ X-ray diffraction (XRD), scanning electron microscopy (SEM), acoustic emission (AE), and differential electrochemical mass spectrometry (DEMS) analyses confirmed that Ca2+ effectively mitigates interlayer gliding and volume collapse, thereby enhancing the reversibility of both nickel and oxygen redox reactions and increasing the structural and interfacial stability during electrochemical cycling. The study was further extended to Mn-rich O3-type layered oxides. NaMnO2 (NMO), a cost-effective and environmentally friendly cathode candidate, is also known to undergo complex and irreversible phase transitions, along with significant manganese migration at high potentials. In this work, precipitated Mn3O4 was employed as a precursor for NMO synthesis. To enhance its structural and electrochemical properties, Ti4+ was introduced to form NaMn0.9Ti0.1O2 (NMTO). Titanium substitution effectively stabilized the O3 phase, suppressed the JT distortion associated with Mn3+, and facilitated the transformation from polycrystalline to quasi-single-crystalline morphology. The optimized NMTO also exhibited an enlarged interlayer spacing, enhanced capacity retention ([42-70] % after 50 cycles), and suppressed oxygen evolution, while effectively preventing the formation of the O1 phase, as confirmed by in situ XRD and DEMS measurements. In conclusion, this dissertation demonstrates effective strategies for improving the structural and electrochemical stability of both Ni-rich and Mn-rich O3-type layered oxides. The mechanistic insights and material design principles presented herein provide valuable guidance for the development of high-performance sodium-ion cathode materials.