UC Berkeley

The rapid growth in demand for portable energy storage technologies, driven by the proliferation of electric vehicles and portable electronics, has created a tremendous need for low-cost, resource-friendly energy storage technologies with high energy densities. To date, lithium-ion (Li-ion) batteries have been the primary choice for these burgeoning applications, largely due to their favorable energy and power densities. To meet this ever-growing demand, low-cost, high energy density Li-ion batteries constructed from materials with resource-friendly compositions must be developed and mass-produced.

The cost, energy density, and resource footprint of modern commercial Li-ion batteries is largely determined by the electrode materials. Specifically, the layered transition metal oxide cathode that is commonly used in commercial Li-ion batteries comprises a large fraction of the cost, possesses a limiting capacity that significantly constrains the achievable energy density, and often contains scarce and expensive transition metals such as Co and Ni. Furthermore, from a humanitarian perspective, a large fraction of the Co that goes into the global Li-ion battery supply chain is sourced via ‘artisinal’ mining in the Democratic Republic of Congo, which commonly features horrible working conditions, low wages, and insufficient protection from occupational hazards. As such, there is an urgent need to develop low-cost, high-capacity, resource-friendly cathode materials.

One type of cathode material that has received significant attention in recent years is the lithium-excess, cation-disordered rocksalt (DRX) material. DRX materials are transition metal oxides/oxyfluorides that occupy the same crystal structure as traditional layered transition metal oxide cathode materials. In contrast to the layered materials, in which lithium and transition metals are arranged in alternating layers within the oxide lattice, the cations in DRX materials possess no long-range ordering and instead are distributed throughout the material in a disordered manner. Furthermore, as designated by the term ‘Li-excess,’ DRX materials contain more than one Li per formula unit. A few key consequences of the cation-disordered nature and Li-excess composition emerge as features which make DRX materials attractive candidates for next-generation cathode materials. First, due to the lack of the requirement to form any sort of long-range ordering, a wide array of transition metals may be accommodated as electrochemically-active redox centers. This compositional flexibility enables the employment resource-friendly redox couples like Mn3+/4+. Second, due to a reshuffling of the electronic structure caused by the broad distribution of local coordination environments, a secondary high-voltage bulk charge compensation process known as oxygen redox is activated. The additional charge compensation provided by oxygen redox allows for delithiation beyond what can be achieved relying on transition metal redox alone, thereby allowing DRX materials to achieve capacities as high as 300 mAh g−1. These two features, compositional flexibility and high capacity, position DRX materials as a unique and promising candidate for the development of low-cost, resource-friendly, high-capacity cathode materials.

Despite these promising advantages, DRX materials suffer from a few key drawbacks that prevent their application in commercial Li-ion batteries. Perhaps the most significant of these drawbacks is high interfacial reactivity. The wide voltage window needed to achieve high capacities from DRX materials requires the electrode to depart the electrolyte stability window, driving extensive electrolyte degradation. This electrolyte degradation leads to depletion of the electrolyte and formation of various degradation products. The formation of these degradation products can cause numerous undesirable processes, including deposition of insulating species on electrode surfaces resulting in a rise in cell impedance, downstream reaction of degradation products with cell components causing additional degradation, and accumulation of gaseous degradation products leading to a rise in pressure within the cell. These deleterious processes lead to significant performance decay and limit the long-term cyclability of cells containing DRX cathodes.

This dissertation explores the bulk electrochemistry and interfacial reactivity that are characteristic of DRX materials during electrochemical cycling. A broad suite of characterization techniques, some traditional and others novel, are employed to obtain insights into the fundamental chemistry that underlies both the initial performance and the long-term stability of cells containing DRX cathodes. Principal among these techniques, gas evolution measurements conducted by differential electrochemical mass spectrometry provide crucial insights into the electrochemical conditions under which and the extent to which interfacial degradation processes occur.

In Chapter 1, the fundamentals of Li-ion batteries are presented. This introduction describes the basic principles of Li-ion batteries as well as the important factors that determine their promise as energy storage devices for use in applications such as electric vehicles and portable electronics. Chapter 2 investigates the effect of fluorine substitution on the bulk redox and interfacial degradation reactions that occur during electrochemical cycling of Mn/Nb-based DRX materials. The results of this investigation demonstrate that adding fluorine to the DRX material shifts the balance between Mn3+/4+ and oxygen redox, suppresses oxygen loss, and causes dissolution of fluorine from the DRX surface. Chapter 3 examines the reactivity of a set of highly fluorinated Mn/Nb-based DRX materials synthesized by high-energy ball milling. It is shown that the degree of fluorination can be used to tune the balance between Mn2+/4+ and oxygen redox in these materials. This study also reveals that these materials possess extremely high interfacial reactivity that drives a variety of interfacial degradation processes, including fluorine dissolution and sustained electrolyte degradation that generates CO2 and electrolyte-soluble acidic species. Chapter 4 explores the effect of voltage window on interfacial degradation and performance decay during cycling of Mn/Ti-based DRX cathodes. This work demonstrates that heightened interfacial degradation brought about by cycling to extreme ends of the voltage window leads to drastic performance decay during extended cycling. Finally, Chapter 5 summarizes the work presented throughout this dissertation and highlights its implications.