Elucidating the Mechanisms and Optimizing Electrode Chemistry for Advanced Sodium Ion Batteries (SIBs)

Abstract

The global transition to renewable energy and the electrification of key sectors require energy storage technologies that are efficient, sustainable, and economically viable. While lithiumion batteries dominate the present market, their long-term scalability is hampered by finite lithium deposits, growing prices, and unequal regional distribution. Sodium-ion batteries (SIBs) have emerged as a possible alternative, benefiting from sodium's natural availability, affordability, and chemical similarity with lithium. However, their practical application is hindered by persisting problems in electrode materials, such as structural instability, capacity fading with extended cycling and low initial coulombic efficiency (ICE). This dissertation tackles these difficulties through four interrelated studies: cathode synthesis, development, anode optimization, and full-cell integration. We produced sodium-deficient layered oxides (NaxMnO2, x = 0.6, 0.7 and 0.8) with a custom-built ultrasonic spray pyrolysis (USP) technique. Meticulously changing precursor stoichiometry, carrier gas flow, and temperature gradients, phase-pure cathodes with controllable morphology and crystallinity were synthesized, providing stable cycling, good capacity retention, and competitive rate performance. Second, a robust Ni-substituted layered oxide, Na0.67Mn0.67Ni0.33O2, was produced using improved solid-state methods. A comparison of dry and wet-milled precursors highlighted the intricate interaction of phase purity, microstructure, and electrochemical reversibility, with the drymilled sample displaying improved long-term stability and capacity retention. Third, bio-wastederived hard carbon was produced from walnut shells using a designed two-step thermal process that included pre-oxidation and high-temperature carbonization. This method resulted in a disordered carbon with enlarged interlayer spacing and hierarchical porosity, attaining a reversible capacity of more than 300 mAh g-1 and an ICE over 80%. This represents a substantial improvement for sustainable anodes. Finally, full-cell configurations that combine the optimized hard carbon anode with Ni doped cathodes were successfully demonstrated. The designed electrodes demonstrated high compatibility and practical potential. This study increases the knowledge of structure–property–performance correlations in sodium-ion battery electrodes and presents scalable synthesis methodologies for both cathodes and anodes. The main contributions of this work are: (i) fabrication of an inexpensive USP framework for NaxMnO2 cathodes; (ii) optimization of Nisubstituted Na0.67Mn0.67Ni0.33O2 with improved reversibility; (iii) customized pre-treatment method for hard carbon derived from bio-waste with ICE > 80%; and (iv) Full-cell configurations combining the optimized hard carbon anode with Ni-doped cathodes were successfully demonstrated, delivering an initial reversible capacity of ~135 mAh g-1 and retaining ~44% capacity after 50 cycles within a 1.5–4.7 V window.