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Fundamental electrochemistry of batteries 'beyond Li-ion', with a specific focus on metal-air chemistries
The lithium-ion battery is a truly remarkable technology; one which has found commercial application in numerous devices requiring portable energy storage. However, through decades of development, Li-ion batteries are now reaching physical limits in terms of the amount of energy they can store per unit volume. Our research will broadly focus on new battery chemistries that have promise to improve upon the energy density of Li-ion batteries. Among the many alternatives being explored, metal-air batteries possess very high theoretical energy densities, but suffer from poor rechargeability and Coulombic efficiency. Our laboratory’s specific focus will be to assess, and then address, the shortcomings of metal-air batteries through a systematic approach involving state-of-the-art electrochemical and spectroscopic characterization. Current projects include novel electrolyte development for Li-air batteries, fundamental characterization of Na-air batteries, corrosion suppression of Mg metal anodes in aqueous electrolytes, and outgassing of high voltage Li-excess Li-ion battery electrodes.
High Li ion transference number polymer electrolytes to enable high energy Li-ion batteries
In typical liquid electrolytes, most of the ionic current is initially carried by the anion, with a lithium ion transference number (t) ranging from 0.1-0.4 (i.e., only 10-40% of the ionic current is carried by Li ions). As a result of this low t, and in combination with the consumption/formation of the electroactive Li ions at the battery cathode/anode, salt concentration gradients are established within the battery in order to maintain electroneutrality, deleteriously affecting the battery performance. Specifically, as a battery discharge proceeds, the ionic concentration within the porous battery cathode, where Li ions are consumed, continuously decreases until it reaches zero at some shallow depth within the electrode, limiting the ultimate electrode thickness. High t electrolytes could significantly reduce, or perhaps eliminate, concentration polarization at high discharge rates, enabling thicker electrodes to be used, which would increase the ratio of electrochemically active battery components to passive cell components (such as the electrolytes), thereby increasing the energy density of the cell, particularly at high current rates desirable for electric vehicles. Our group works to understand the effect of various polymer properties, both chemical and physical, on the electrode/electrolyte interfaces and the polymer conductivity, thereby allowing us to optimize electrolyte composition to impart low impedance and enhanced Li-ion battery performance.
The stability and performance of Li-ion cathode materials
The traditional cathode in a lithium-ion battery is a layered lithium transition metal oxide. This stoichiometric oxide delivers reversible capacity by utilizing the redox capability of the transition metal. The achievable reversible capacity of these oxides, however, is much lower than the theoretical capacity, and despite the breadth of research in this field, the nature of this limitation is still highly debated. Our focus is to understand this limitation by decoupling the linked roles of electrolyte and oxide degradation by using a combination of spectroscopic and in-situ techniques. Our lab is also interested in several new classes of cathode materials that depart from the traditional layered stoichiometric oxides. These new classes of materials promise much higher theoretical capacities than the traditional oxides, but suffer from poorly understood limitations in stability and cyclability. Our lab wishes to understand how the identity of the transition metal, the extent of lithium content, and the crystal structure, as well as the electrolyte, all play a role in the stability and reversible capacities of these new cathode materials.
Electrocatalysis, with a focus on photocatalytic CO2 reduction
Using inputs of only sunlight, electricity, carbon dioxide, and water, photocatalytic CO2 reduction imitates nature by producing fuels directly from the sun. These solar fuels lead to greater independence from fossil fuels and mitigate the effects of climate change by consuming CO2 that would otherwise be released into the atmosphere. Our research specifically targets a strategy to improve efficiency and selectivity of CO2 electrochemical reduction, both of which currently hinder the practicality of CO2-to-hydrocarbon conversion. Plasmon-assisted photocatalytic CO2 reduction leads to greater selectivity and lower overpotentials by unlocking unique mechanistic pathways. The electron dynamics in an irradiated plasmonic nanoparticle can alter the electronic coupling with surface adsorbed CO2 and reaction intermediates, thereby changing the binding energy of these species and the catalytic properties of the plasmonic metals. The wide tunability of the plasmon resonance frequency with shape, size, and material confers fine control over these catalytic mechanisms, allowing for optimization of the photocatalytic performance. Our laboratory is exploring novel composite electrodes of nanostructured materials that exhibit strong plasmonic and electrocatalytic behavior, leading to enhanced CO2 reduction.