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Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2017)

The Li-ion battery is one of the best rechargeable energy storage techniques due to its exceptional high energy density and long cycle life. It has dominated the portable electronic industry for the past 20 years and is going to be applied for large scale energy storage. However, concerns over limited lithium reserve and rising lithium costs have arisen, therefore Na-ion batteries are being considered as the alternative for grid storages.

In this thesis, Na2Ti3O7 is first investigated as anode materials for Na-ion batteries. By carbon coating, the cyclability and coulombic efficiency are significantly enhanced for Na2Ti3O7. The self-relaxation behaviour for fully intercalated phase, Na4Ti3O7, is shown for the first time, which results from structural instability as suggested by first principles calculation. Another SnS2-reduced Graphene Oxide (rGO) composite material is investigated as an advanced anode material for Na-ion batteries, which can deliver a reversible capacity of 630 mAh g-1 with negligible capacity loss and exhibits superb rate performance. The energy storage mechanism of it and the critical mechanistic role of rGO are revealed in detail.

On the cathode side, we introduce a novel layered oxide cathode material, Na0.78Ni0.23Mn0.69O2. This new compound provides remarkable rate and cycling performances owning to the elimination of the P2-O2 phase transition upon Na deintercalation. The first charge process yields an abnormally excess capacity which has yet to be observed in any other P2 layered oxides. It is proposed that part of the charge compensation mechanism during the first cycle takes place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient. Atomic layer deposition (ALD) is known to improve the cycling performance, coulombic efficiency of batteries, and maintain electrode integrity for LIBs. Therefore we examine carefully the effect of Al2O3 ALD coating to P2 electrode materials. X-ray photoelectron spectroscopy (XPS) is used to elucidate the cathode electrolyte interphase (CEI) on ALD coated electrodes, which clearly reveal the effectiveness of the ALD coating. We believe that by optimizing and controlling the materials surface, Na layered oxide material with higher capacities can be designed.

    Cover page: Beyond Lithium: Novel Electrode Materials for Room Temperature Na-ion Batteries
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2017)
    Atomic layer deposition (ALD) is a commonly used coating technique for lithium ion battery electrodes. Recently, it has been applied to sodium ion battery anode materials. ALD is known to improve the cycling performance, Coulombic efficiency of batteries, and maintain electrode integrity. Here, the electrochemical performance of uncoated P2-Na2/3Ni1/3Mn2/3O2 electrodes is compared to that of ALD-coated Al2O3 P2-Na2/3Ni1/3Mn2/3O2 electrodes. Given that ALD coatings are in the early stage of development for NIB cathode materials, little is known about how ALD coatings, in particular aluminum oxide (Al2O3), affect the electrode-electrolyte interface. Therefore, full characterizations of its effects are presented in this work. For the first time, X-ray photoelectron spectroscopy (XPS) is used to elucidate the cathode electrolyte interphase (CEI) on ALD-coated electrodes. It contains less carbonate species and more inorganic species, which allows for fast Na kinetics, resulting in significant increase in Coulombic efficiency and decrease in cathode impedance. The effectiveness of Al2O3 ALD coating is also surprisingly reflected in the enhanced mechanical stability of the particle which prevents particle exfoliation.
      Cover page: Improvement of the Cathode Electrolyte Interphase on P2-Na2/3Ni1/3Mn2/3O2 by Atomic Layer Deposition
      • Article
      • Peer Reviewed
      UC Santa Barbara Previously Published Works (2017)
      Large-scale electric energy storage is fundamental to the use of renewable energy. Recently, research and development efforts on room-temperature sodium-ion batteries (NIBs) have been revitalized, as NIBs are considered promising, low-cost alternatives to the current Li-ion battery technology for large-scale applications. Herein, we introduce a novel layered oxide cathode material, Na0.78Ni0.23Mn0.69O2. This new compound provides a high reversible capacity of 138 mAh g-1 and an average potential of 3.25 V vs Na+/Na with a single smooth voltage profile. Its remarkable rate and cycling performances are attributed to the elimination of the P2-O2 phase transition upon cycling to 4.5 V. The first charge process yields an abnormally excess capacity, which has yet to be observed in other P2 layered oxides. Metal K-edge XANES results show that the major charge compensation at the metal site during Na-ion deintercalation is achieved via the oxidation of nickel (Ni2+) ions, whereas, to a large extent, manganese (Mn) ions remain in their Mn4+ state. Interestingly, electron energy loss spectroscopy (EELS) and soft X-ray absorption spectroscopy (sXAS) results reveal differences in electronic structures in the bulk and at the surface of electrochemically cycled particles. At the surface, transition metal ions (TM ions) are in a lower valence state than in the bulk, and the O K-edge prepeak disappears. On the basis of previous reports on related Li-excess LIB cathodes, it is proposed that part of the charge compensation mechanism during the first cycle takes place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient. We believe that by optimizing and controlling oxygen activity, Na layered oxide materials with higher capacities can be designed.
        Cover page: Exploring Oxygen Activity in the High Energy P2-Type Na0.78Ni0.23Mn0.69O2 Cathode Material for Na-Ion Batteries
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2014)
        A layered SnS2-reduced graphene oxide (SnS2-RGO) composite is prepared by a facile hydrothermal route and evaluated as an anode material for sodium-ion batteries (NIBs). The measured electrochemical properties are a high charge specific capacity (630 mAh g-1 at 0.2 A g-1) coupled to a good rate performance (544 mAh g-1 at 2 A g-1) and long cycle-life (500 mAh g-1 at 1 A g -1 for 400 cycles). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
          Cover page: Layered SnS2‐Reduced Graphene Oxide Composite – A High‐Capacity, High‐Rate, and Long‐Cycle Life Sodium‐Ion Battery Anode Material
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2018)
          Metal-ion doping can improve the electrochemical performance of Na3V2(PO4)3. However, the reason for the enhanced electrochemical performance and the effects of cation doping on the structure of Na3V2(PO4)3 have yet been probed. Herein, Mg2+ is doped into Na3V2(PO4)3/C according to the first-principles calculation. The results indicate that Mg2+ prefers to reside in the V site and an extra electrochemical active Na+ is introduced to the Na3V2(PO4)3/C crystal to maintain the charge balance. The distribution of Mg2+ in the particle of Na3V2(PO4)3/C is further studied by electrochemical impedance spectroscopy. We find that the highest distribution of Mg2+ on the surface of the particles leads to facile surface electrochemical reactions for Mg2+-doped samples, especially at high rates.
            Cover page: Understanding the Electrochemical Mechanisms Induced by Gradient Mg2+ Distribution of Na-Rich Na3+x V2–x Mg x (PO4)3/C for Sodium Ion Batteries
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