|dc.description.abstract||The surging demand for electrified transportation (both ground and air), robots, and other battery-powered devices has driven the development of electrochemical energy storage systems that can provide high gravimetric/volumetric energy density and cost-effectiveness.
Li-ion batteries (LIBs) have been highlighted as the most promising electrochemical energy storage system among the various options. They are the key component for electric vehicles. However, although LIBs have been intensively developed and commercialized, their energy density still needs to be improved to achieve goals in cost and energy efficiency. To improve the energy density of lithium layered transition metal oxide (LixTMO2) positive electrodes for LIBs, oxygen redox in the high-valent state of the lithium-rich layered oxide (LRLO, Li1+xTM1-xO2) materials has been extensively explored because it enables the high reversible capacity that exceeds the material’s theoretical capacity (if one only accounts for transition metal redox). However, the local structural distortion and reactive oxygen release accompanied by anion redox have hindered the harnessing of oxygen redox for commercialized LIBs.
In addition to LIBs, sodium ion batteries (NIBs) have emerged as an alternative to LIBs owing to earth-abundant, low cost and environmentally-safe sodium. Although sodium is heavier and exhibits ~ 0.3 V higher redox potential than lithium, the energy penalty of NIBs is small enough to compete with LIBs. To improve the structural stability and energy density of sodium layered transition metal oxides (NaxTMO2, 0 ≤ x ≤ 1), substituting transition metals with different types of metals (e.g. Li, Mg, and Sn) has been suggested as one of the effective strategies. Meanwhile, oxygen redox in sodium layered oxides has also been actively explored, especially for earth abundant Mn and Fe containing sodium layered metal oxides. However, the local structural environment for triggering oxygen redox in NaxTMO2 still requires further investigation.
Along with increasing capacity and raising the redox potential of positive electrode materials to improve the energy density of LIBs and NIBs, developing safe non-flammable electrolytes has become a critical issue due to thermal runaway of batteries caused by high operation voltage. In this regard, all-solid-sate Li-ion batteries (ASSBs) employing non-flammable solid electrolytes are emerged as a safe and sustainable energy storage system. While high Li-ion conductive sulfide solid electrolytes have been widely explored as a solid electrolyte for ASSBs, their poor electrochemical stability (S2- oxidation at ~ 2.5 V vs. Li/Li+) requires a protective coating layer on 4 V-class positive electrode materials. Therefore, developing solid electrolyte which can provide high ionic conductivity together with wide electrochemical stability window and chemical stability to the positive electrodes is necessary to fully utilize cation redox, and possibly anion redox together, of the high voltage positive electrode materials in ASSBs.
This thesis presents various approaches for improving energy density of LIBs, NIBs and ASSBs with respect to harnessing the cation and anion redox. In Chapter 3 a strategy to enhance the cycle performance of a LRLO cathode is demonstrated by scavenging the evolved reactive oxygen species with a polydopamine (PDA) surface coating. PDA, a well-known oxygen radical scavenger, provides a chemically protective layer that diminishes not only the growth of the undesirable cathode electrolyte interphase (CEI), but also results in less oxygen gas release compared to an unprotected surface, and significantly suppressed phase transformation at the surface. These factors lead to improved rate capability and diminished capacity fading on cycling; namely a capacity fade of 82% over 200 cycles at a C rate for the PDA-coated LRLO, compared to 70% for the bare LRLO material.
Chapter 4 reports the control of high-valent oxygen redox in the Na-ion positive electrode P2-Na0.67-x[Fe0.5Mn0.5]O2 (NMF), where Fe is partially substituted with Cu (P2-Na0.67-x[Mn0.66Fe0.20Cu0.14]O2, NMFC) or Ni (P2-Na0.67-x[Mn0.65Fe0.20Ni0.15]O2, NMFN). The combined study of electrochemistry, local structural evolution, and synchrotron radiation spectroscopies demonstrates the correlation between the degree of structural disorder and the emergence of oxygen redox in charged NMF, NMFC, and NMFN electrodes. Importantly, we show that the presence of significant anion redox from charged NMF and NMFC, but with different extent, without the widely accepted requirement of an A-O-A’ (where A= Na and A’= Li, Mg, vacancy) local configuration in the pristine materials. The density of states calculations demonstrate that the extent of oxygen redox is more favored when oxygen 2p band dominates the band near the Fermi level in the TM-O bond. This study further suggests that the amount of Jahn-Teller active Fe4+ ion and the oxygen redox mechanism are additional related factors to be considered to establish the correlation between the degree of structural disorder and the extent of anion redox in the materials.
In Chapter 5, layered Na2-xMn3O7 is synthesized in its hexagonal phase, and its local structural evolution that stabilizes oxygen redox is investigated. In Na2Mn3O7, ordered Mn site vacancies in the MnO2 layer generate a Na-O-Vacancy configuration which provides an environment for oxygen redox. By combining experimental (electrochemistry and synchrotron radiation spectroscopies) and theoretical (first principle calculation) studies for the partially desodiated Na2-xMn3O7 electrode, we demonstrate that coulombic interactions between oxidized oxide and the negatively charged Na vacancy can stabilize hole polarons on lattice oxygen and disfavor O-O dimerization at 4.2 V vs. Na/Na+ which are commonly known as an evidence of oxygen redox. This study highlights the role of coulombic interactions for stabilizing highly oxidized oxygen for developing high energy density layered oxide materials.
Chapter 6 introduces a new class of polyanion-type Na+ insertion materials for Na-ion batteries. By virtue of its moderately inductive polyanionic framework, the air and moisture stable sodium cobalt selenite, Na2Co2(SeO3)3, shows a redox potential of ~ 4 V vs. Na/Na+ that utilizes the Co2+/3+ redox couple, rendering it compatible with conventional liquid organic electrolytes. A microwave hydrothermal synthesis route is developed for the rapid synthesis of nanostructured Na2Co2(SeO3)3 and its conductive graphene oxide composite. These studies reveal good structural and electrochemical reversibility of the Na2Co2(SeO3)3 positive electrode.
Chapter 7 reports a new metastable trigonal phase of Li3YbCl6 with an ionic conductivity of 1.0×10−4 Scm−1, and mixed-metal halide solid electrolytes, Li3−xYb1−xZrxCl6 with conductivities up to 1.1 mS·cm−1 at room temperature. Combined neutron, single-crystal, and powder X-ray diffraction methods reveal that Zr-substitution for Yb in Li3YbCl6 triggers a trigonal-to-orthorhombic phase transition and forms new, lower energy pathways for Li-ion migration. All-solid-state cell cycling with uncoated >4 V-class cathodes is enabled by the high electrochemical oxidation stability of the mixed-metal halide solid electrolyte.||en