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Designing and Diagnosing Novel Electrode Materials for Na- ion Batteries : : Potential Alternatives to Current Li-ion Batteries

Abstract

Owing to outstanding energy density, Li-ion batteries have dominated the portable electronic industry for the past 20 years and they are now moving forward powering electric vehicles. In light of concerns over limited lithium reserve and rising lithium costs in the future, Na-ion batteries have re-emerged as potential alternatives for large scale energy storage. On the other hand, though both sodium and lithium are alkali metals sharing many chemical similarities, research on Na-ion batteries is still facing many challenges due to the larger size and unique bonding characteristics of Na ions. In this thesis, a series of sodium transition metal oxides are investigated as cathode materials for Na-ion batteries. P2 - Na₂/₃[Ni₁/₃Mn₂/₃]O₂ is firstly studied with a combination of first principles calculation and experiment, and battery performance is improved by excluding the phase transformation region. Li substituted compound, P2-Na₀.₈[Li₀.₁₂Ni₀.₂₂Mn₀.₆₆]O₂, is then explored. Its crystal / electronic structure evolution upon cycling is tracked by combing in situ synchrotron X-ray diffraction, ex situ X-ray absorption spectroscopy and solid state NMR. It is revealed that the presence of Li-ions in the transition metal layer allows increased amount of Na-ions to maintain the P2 structure during cycling. The design principles for the P2 type Na cathodes are devised based on this in-depth understanding and an optimized composition is proposed. The idea of Li substitution is then transferred to O3 type cathode. The new material, O3 - Na₀.₇₈Li₀.₁₈Ni₀.₂₅Mn₀.₅₈₃O₂, shows discharge capacity of 240 mAh/g, which is the highest capacity and highest energy density so far among cathode materials in Na-ion batteries. With significant progress on cathode materials, a comprehensive understanding of Na₂Ti₃O₇ as anode for Na-ion batteries is discussed. The electrochemical performance is enhanced, due to increased electronic conductivity and reduced SEI formation with carbon coating. Na full cell with high operating voltage is demonstrated by taking advantage of the ultra-low voltage of Na₂Ti₃O₇ anode. The self-relaxation for fully intercalated phase, Na₄Ti₃O₇, is shown for the first time, which results from structural instability as suggested by first principles calculation. Ti⁴⁺ / Ti³⁺ is the active redox couple upon cycling based on XANES characterization. These findings unravel the underlying relation between unique properties and battery performance of Na₂Ti₃O₇ anode, which should ultimately shed light on possible strategies for future improvement

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