Pillared layered transition metal oxides for high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) are emerging as cost-effective and sustainable alternatives to lithium-ion batteries (LIBs) for large-scale energy storage, owing to the abundance and low cost of sodium. Among the candidate cathodes, layered transition-metal oxides (NaₓTMO₂, 0.5 ≤ x ≤ 1.0, TM = transition metals) are especially attractive due to their high theoretical capacity, structural flexibility, wide voltage window, and favourable Na+ kinetics. However, their electrochemical performance is often limited by structural degradation in the Na-depleted state. To overcome this, recent studies highlight the use of Ca²⁺ or K⁺ substitution in the Na layer as structural pillars to improve cycling stability. This thesis systematically investigates the pillaring effect of Ca²⁺ and K⁺ on NaₓTMO₂. Chapter 3 examines the role of Ni/Mn ratio in Ca-pillared O3-type MnFeNi-based oxides. For Ni-rich compositions - Na₁₋₂ₓCaₓNiₒ.₂₅Mnₒ.₂₅Feₒ.₅O₂ (x = 0.00, 0.03), Ca enhances cycling stability and reduces polarization but is less effective at suppressing Fe migration, while excessive Ca doping reduces rate capability. In Mn-rich compositions - Na₁₋₂ₓCaₓNiₒ.₁₇Mnₒ.₃₃Feₒ.₅O₂ (x = 0.00, 0.04), Ca pillaring provides stronger benefits, improving structural integrity, redox reversibility, and suppression of Fe migration. Chapter 4 demonstrates a high-performance O3-Na₁₋₂ₓCaₓNiₒ.₃Feₒ.₄Mnₒ.₃O₂ (x = 0.03) material, delivering 128 mAh g⁻¹ initial capacity with 92% retention after 100 cycles. By using a combination of operando X-ray diffraction, and Mössbauer Spectroscopy, it is revealed that Ca incorporation broadens the solid-solution reaction, suppresses Fe migration, and mitigates structural distortion during cycling. Chapter 5 compares Ca- and K-pillared materials within a unified host (P2-Naₒ.₇₆Mnₒ.₆Feₒ.₂₅Cuₒ.₁₅O₂), confirming enhanced structural reversibility relative to the unpillared sample. K-pillared materials show greater improvements in cycling stability due to expanded Na layer spacing and stronger TM–O interactions. Chapter 6 explores 6% transition-metal vacancies in a P2-type MnCuFe-based oxide (Naₒ.₆₇Mnₒ.₅₈Feₒ.₁₉Cuₒ.₁₇[]ₒ.ₒ₆O₂), achieving 96.3 mAh g⁻¹ capacity with 103% retention after 200 cycles. Improved stability arises from progressive Mn redox activation, enhanced O redox reversibility, and suppression of the detrimental Z phase by vacancy introduction.