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Open Access Publications from the University of California

Scholarly Works (13 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2020)

With an ever-growing number of applications, from portable electronics to electric vehicles, that rely on Li-ion battery technologies, advancements in this field are now shaping the convenience, efficiency, and sustainability of modern society. Further performance

improvements are still highly anticipated, pushing the scientific community to develop a novel strategy to design a battery material with enhanced properties. An in-depth understanding of the synthesis process is critical to achieve this goal, since it can provide

insights towards controlling the material’s chemical and physical properties, such as crystal structure, morphology, and defects. In this study, a polyol method has been developed as a versatile technique for the synthesis of high-performance, dispersive cathode nanoparticles. Although the polyol method is a promising synthetic process that offers many advantages such as low cost, ease of use, and proven scalability for industrial applications, the scope of previous studies has been mainly limited to simple metals and metal oxides. As a novel method to synthesize battery materials, a detailed reaction mechanism study was conducted using a combination of in situ and ex situ characterization. Nanoscale dynamics that occur during the synthesis process were reported with a focus on the material’s structural and chemical transformation. The detailed knowledge of the reaction mechanism helped provide an insight towards finding the optimum synthesis conditions for a variety of cathode materials - including layered LiNi0.4Mn0.4Co0.2O2, spinel LiNi0.5Mn1.5O4, and olivine LiCoPO4. Using scanning transmission electron microscopy, polyol-synthesized LiNi0.4Mn0.4Co0.2O2 was further revealed to be enriched with highly coherent twin boundary defects. The positive role of twin boundary defects was thus proposed, as mitigated anisotropy and volume expansion were observed in the polyol sample during the charge and

discharge process. Lastly, in order to increase the capacity, the scope of the studied material was expanded beyond the conventional transitional metal redox and towards high capacity anionic redox materials, referred to as cation-disordered rock salt cathode materials. The strategies to develop the synthesis method that can control the morphology of this material have been proposed, along with the precautions that need to be taken when studying this type of cathode nanoparticles.

    Cover page: Tuning Nanoparticle Synthesis to Enable Anionic and Cationic Redox in Cathode Materials
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2021)
    Lithium-rich nickel manganese cobalt oxide (LRNMC) is being explored as an alternative to stoichiometric nickel manganese cobalt oxide (NMC) cathode materials due to its higher, initially accessible, energy-storage capacity. This higher capacity has been associated with reversible O oxidation; however, the mechanism through which the change in O chemistry is accommodated by the surrounding cathode structure remains incomplete, making it challenging to design strategies to mitigate poor electrode performance resulting from extended cycling. Focusing on LRNMC cathodes, we identify nanoscale domains of lower electron density within the cathode as a structural consequence of O oxidation using small-angle X-ray scattering (SAXS) and operando X-ray diffraction (XRD). A feature observed in the small angle scattering region suggests the formation of nanopores, which first appears during O oxidation, and is partially reversible. This feature is not present in traditional cathode materials, including stoichiometric NMC and lithium nickel cobalt aluminum oxide (NCA) but appears to be common to other Li-rich systems tested here, Li2RuO3 and Li1.3Nb0.3Mn0.4O2.
      Cover page: Nanostructure Transformation as a Signature of Oxygen Redox in Li-Rich 3d and 4d Cathodes
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2018)
        Cover page: Intercalation and Conversion Reactions of Nanosized β‑MnO2 Cathode in the Secondary Zn/MnO2 Alkaline Battery
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2020)
        Li-rich oxides have generated widespread interest for their use as next generation high energy density Li-ion battery cathodes. Among Li-rich candidates, cation disordered phases are novel lithium intercalation compounds that warrant further fundamental characterization. Here, we compare Li1.3Nb0.3Mn0.4O2 and Li1.3Nb0.3Fe0.4O2 system to explore differences in charge compensation and electrochemical performance. Using Operando XANES/EXAFS measurements examine how the Mn/Fe/Nb local environments and oxidation states change and their effect on electrochemistry. Li1.3Nb0.3Mn0.4O2 displays capacities beyond the traditional transition metal redox (Mn3+/4+), which is considered accessible through oxygen redox. Li1.3Nb0.3Fe0.4O2 exhibits charge capacities at similar potentials to that of anionic redox but our investigations suggest the charge compensation mechanism displays more Fe3+/4+ redox in origin. By using a suite of characterization techniques, we explore the capacity contributions of cation and anionic components in these systems.
          Cover page: Comparing Charge Compensation Mechanisms of Li1.3Nb0.3Mn0.4O2 and Li1.3Nb0.3Fe0.4O2 As Novel Cathode Material
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2020)
          Li-rich disordered rock salt (DRS) oxides are a promising class of cathode materials with the wide chemical space to be explored. The high capacity (>300mAh/g) of this class of material can be explained by the reversible redox chemistry of the oxide anions, which sets it apart from the conventional layered cathode materials that rely only on the transition metal redox. However, these materials suffer from poor ionic and electronic transport properties: Most previous studies report electrochemical performance at low current rate and elevated temperature. Even then, the particle size needs to be reduced to sub-micrometer size, often by high-energy ball milling, to get reasonable capacities. To mitigate this issue, we performed a detailed study of the synthesis of three different Nb-based Li-rich DRS materials - Li3NbO4, Li1.3Fe0.4Nb0.3O2, and Li1.3Mn0.4Nb0.3O2. Systematic evaluation shows that both the synthesis conditions and the reagents used have a large effect on the phase and morphology of the material synthetized, and therefore on its electrochemical performance. Without varying the synthesis method, the extent of cation ordering, the particle morphology, and the degree of elemental segregation can be controlled by a careful choice of the metal oxide precursors. This study helps the community to distinguish important synthesis criteria in order to design Li-rich DRS cathode materials with improved electrochemical performance.
            Cover page: Morphology Control to Enable High Capacity Li-Rich Disordered Rock Salt Cathodes
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2017)
            Electrochemical capacitors and lithium-ion batteries have seen little change in their electrolyte chemistry since their commercialization, which has limited improvements in device performance. Combining superior physical and chemical properties and a high dielectric-fluidity factor, the use of electrolytes based on solvent systems that exclusively use components that are typically gaseous under standard conditions show a wide potential window of stability and excellent performance over an extended temperature range. Electrochemical capacitors using difluoromethane show outstanding performance from -78° to +65°C, with an increased operation voltage. The use of fluoromethane shows a high coulombic efficiency of ~97% for cycling lithium metal anodes, together with good cyclability of a 4-volt lithium cobalt oxide cathode and operation as low as -60°C, with excellent capacity retention.
              Cover page: Liquefied gas electrolytes for electrochemical energy storage devices
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2020)
                Cover page: Nano-Ceramic Cathodes via Co-sputtering of Gd–Ce Alloy and Lanthanum Strontium Cobaltite for Low-Temperature Thin-Film Solid Oxide Fuel Cells
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2019)
                A modified co-precipitation method is introduced to synthesize P2-type Na0.67Fe1/4Mn3/4O2 cathode for low-cost Na-ion batteries. The meso-structure of the obtained material is well controlled through regulated cooling after a high temperature calcination process. The material via slow-cooling consists of sphere-like secondary particles with a uniform dispersion while the quenched material exhibits a hexagonal plate-like primary particle without meso-structure. Meso-structure is found to have a distinct effect upon the electrochemical performance of P2-type layered cathode. The slow-cooled sample exhibits a larger capacity and improved cyclability compared with the quenched sample, which is attributed to the larger surface area, reduced surface contamination, and surprisingly higher transition metal redox activity. This work demonstrates that cooling rate plays the key role in controlling the formation of spherical meso-structure for sodium iron-manganese oxides with enhanced electrochemical performance.
                  Cover page: Meso-Structure Controlled Synthesis of Sodium Iron-Manganese Oxides Cathode for Low-Cost Na-Ion Batteries
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2017)

                  A functional current collector was developed to mitigate the internal short circuit in high-voltage lithium-ion batteries.

                    Cover page: Internal short circuit mitigation of high-voltage lithium-ion batteries with functional current collectors
                    • Article
                    • Peer Reviewed
                    UC San Diego Previously Published Works (2021)

                    Synthesis parameters, storage conditions, and electrolyte chemistry are all critical parameters limiting the cycling performances of disordered rock salt cathodes.

                      Cover page: Experimental considerations to study Li-excess disordered rock salt cathode materials
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