Gas Evolution Studies on Emerging Cathode Materials for Lithium-Ion Batteries

Lithium-ion batteries have had a huge societal impact in the last fifty years, playing animportant role in the widespread availability of mobile electronics and with them unprecedented global access to information. Now as the effects of anthropogenic climate change have become evident in weather patterns across the globe, and therefore the impetus to address them even more dire, lithium-ion batteries again have the potential to change the world./nOnce considered an antiquated technology, electric vehicles have had a massive resurgencein the past few decades, supported in a large part by developments in lithium-ion battery technology. First demonstrated in the Tesla Roadster (2008), lithium-ion batteries for electric vehicles are the first type of battery with the ability to rival fossil fuels in performance while drastically reducing emissions from the transportation sector./nDespite their adoption as the industry standard for electric vehicles, lithium-ion batteriesstill have inadequacies that need to be addressed before they can replace fossil fuels as the optimal vehicle power source. Principal among these issues is the inability to match the energy density of gasoline. This significant drawback is the origin of the phenomenon of range anxiety, in which consumers fear that the battery will run out of charge before reaching a refueling station, resulting in their vehicle becoming stranded. While the issue can be partially mitigated by adding more batteries to the vehicle, cost and weight considerations limit the effectiveness of this solution. Modifications to the chemistry of the battery itself will be required to address this problem while maintaining a low cost and weight./nAs the limiting component in terms of energy density, the cathode has been the subject ofintense research in recent years. In this dissertation, two major research branches in emerging cathode materials are addressed: lithium-excess and high-nickel. Both of these categories of cathode material use the same components as commercial materials (lithium, a combination of transition metals, and oxygen) with slight variations in their ratio, so they are expected to be ready for production at industrial scale within a shorter time frame of a few years as opposed to a few decades for those technologies that require a complete cathode redesign./nThis work is unique in the field of battery research due to its use of gas evolution techniques,which measure gases evolved from a battery during or after operation. Conclusions can be drawn about the degradation mechanisms responsible for the observed outgassing based on the identity, amount, and state-of-charge at which gases are evolved. The two major sources of cathode instability addressed in this work are (i) cathode reactivity with the carbonate electrolyte, which often produces CO2 gas and deposits degradation products onto the cathode surface; and (ii) inherent cathode instabilities at high voltage, which are often characterized by permanent alteration of the cathode structure via the oxidation of the oxide component of the cathode into O2 gas./nAfter an introduction to energy storage research and the goals of this dissertation in Chapter1, chapters 2 and 3 focus on lithium-excess cathode materials. These materials are designed to contain more lithium than can be compensated for by the transition metals, resulting in increased capacity and oxygen redox activity (a phenomenon previously seen in lithiumair batteries). Novel isotope tagging techniques and titration methods are also introduced. This work shows that stable oxygen redox can occur in lithium-excess materials but that oxidation of residual carbonate on the cathode surface triggers irreversible oxygen activity. Only irreversible oxygen activity is detected for Li2MnO3, a material that represents the maximum lithium to transition metal ratio while maintaining a layered structure. Further work is needed to determine whether stable manganese redox occurs in Li2MnO3, a process that could make Li2MnO3 a viable cathode material./nChapters 4 and 5 focus on high-nickel cathode materials. The advantage of these materialsis based on the superior charge compensation ability of nickel compared to other transition metals, resulting in enhanced capacity and attainable energy density. However, as a result of nickel’s reactivity, they are structurally unstable, undergoing phase transitions that have consequences in terms of outgassing and cathode structure. This work shows that outgassing is closely linked with particle cracking, with the onset of gas evolution occurring at the same voltage at which cracks which originated inside the particle extend to the cathode/electrolyte interface. In addition, the unique capability of an ex-situ acid titration developed to study surface contaminants to separately detect contaminants located inside particles is demonstrated. Lastly, single crystal morphologies are addressed as a possible solution to structural instability in high-nickel materials. While the change in morphology partially mitigates cracking and outgassing, particle agglomerates show the same degradation issues as polycrystalline materials, with cracks propagating from stacking faults where two single crystal particles have merged. Additional studies are necessary to produce single crystal materials that are well-dispersed to take full advantage of their enhanced stability compared to polycrystalline materials./nWhile this work is rooted in scientific discovery more so than development of industriallyrelevant materials, it demonstrates that the commercialization of both of these types of emerging cathode materials is within reach. Gas evolution studies on these materials, as well as those that involve more drastic changes to the cathode structure, are integral to the progression of these technologies from ideas to practice.

Advisor: McCloskey, Bryan D.