High Voltage Reactivity in Lithium Batteries
A major limit of electric vehicle performance is the energy density of available mobile energy storage systems. Lithium ion (Li-ion) batteries are now well-known and have enabled many portable electronics as well as electronic vehicles. However, for greater market penetration and range competitiveness, a battery with greater practical energy density must be designed. This dissertation focuses on three methods of improving energy density in lithium based batteries: lithium oxygen (Li-O2) batteries, Li-rich cathodes for Li-ion batteries, and high voltage operation of simple transition metal oxide cathodes for Li-ion batteries. To evaluate each of these technologies, the reactivity of each system is monitored.
Li-O2 batteries, a “beyond Li-ion" battery technology, have a very high theoretical energy density that is difficult to realize. After initial excitement surrounding the novel chemistry, many challenges associated with Li-O2 batteries have been highlighted in the past decade. Among these challenges, the reactivity of oxygen in the system is one of the most pressing. In this dissertation, the possible stability of LiO2, an advantageous discharge product to the typical Li2O2, is examined alongside binder degradation in the system.
As the workings of a Li-ion battery require the removal and intercalation of Li, the theoretical capacity is determined by the amount of Li available for extraction from the cathode. Consequently, a method of increasing energy density in Li-ion batteries is to increase the stoichiometric ratio of Li in the cathode material. These "Li-rich" cathode materials demonstrate large capacities, but must compensate the additional Li with cation double redox or anion redox. The electrochemical cells must be operated to > 4.5 V vs Li/Li+ to reach these additional capacities, and this operation results in greater reactivity and instabilities. This dissertation examines the trends of high voltage instability, especially as it relates to oxygen redox, in these Li-rich cathode materials.
Typical Li-ion batteries utilize only a fraction of the theoretical capacity available, only extracting around half of the available lithium from the layered transition metal oxide cathode material during charge. This is due to enhanced degradation mechanisms and reduced cyclability when additional Li is extracted at the necessitated higher voltages. Enabling operation of layered transition metal oxides at high voltages would result in increased capacity without the need for “beyond Li-ion" technologies. But first, the instabilities associated with Li extraction at voltages beyond the typical cut-off must be well-studied. Currently, the stability and degradation mechanisms of cathode materials even as common as LiCoO2 remain unclear. In studies presented in this dissertation, high voltage reactivity for layered transition metal oxide cathode materials is investigated.
The main conclusions of the studies presented here are drawn from measurements monitoring the reactivity of each of the aforementioned technologies. Among the measurements presented in these studies, outgassing and gas evolution measurements by differential electrochemical mass spectrometry have proved paramount in utility. As cell reactivity and instability at high voltages is often accompanied by outgassing, these measurements have assisted in the elucidation of instability origins. Practical application of Li-O2 batteries remains elusive as additional instabilities are discovered, Li-rich cathode materials show promise as various methods of mitigating high voltage instabilities are discussed, and the major sources of high voltage reactivity of layered transition metal oxide cathode materials are evaluated here.