Beyond Lithium Ion Batteries
Continuous developments in technology lead to the quick growth of both the number of applications requiring autonomous power and size of the energy storage devices. Because of their light weight and highly electropositive nature, lithium ions are likely to be used as charge carriers for at least the next few years. However, future affordable energy storage solutions could utilize less expensive electrode materials and so called “beyond lithium ions” (BLIs: Na+, K+, Mg2+, Zn2+, Al3+, etc.) in electrolytes. Our focus on Beyond Lithium Ion Batteries is motivated by the global need to identify alternate intercalation chemistries that can alleviate the supply and geographic constraints associated with sourcing the lithium for Li-ion batteries, particularly for large-format storage. Although heavier than lithium, in addition to their lower price, these ions have other advantages. For example, larger monovalent ions (Na+ and K+) require lower desolvation energy than the smaller Li+ ion, thus improving the kinetics of the ion insertion process at the electrode/electrolyte interface, which is important for high power. In addition, unlike lithium, sodium does not form an alloy with aluminum. Therefore, heavy and expensive copper current collectors can be replaced with aluminum for anodes in Na-ion batteries. At the same time, for applications where volumetric performance plays a key role, such as wearables and small electronics, high energy density in a small volume can be achieved by combining the best 2D electrode materials with multivalent ions (Mg2+ and Al3+).
However, beyond lithium ions are either larger in size than lithium ion, or they carry a higher charge, which limits their diffusion in the crystal lattice of electrode materials. In addition, Na-ion batteries (SIBs) and Mg-ion batteries (MIBs) operate at lower voltages than Li-ion batteries, and thus higher capacity cathode materials are necessary to increase the energy storage of SIBs and MIBs. In the MEG laboratory, we design, synthesize, characterize and test new electrode materials for BLI batteries with the aim to achieve record high electrochemical performance in these emerging energy storage systems.
Capacitive Water Deionization
Clean water is essential for life, but attaining affordable, fresh, and uncontaminated water still remains a grand challenge. Only ~0.3% of the water present on Earth is freshwater in the liquid form on the surface, while remaining freshwater resources exist in the form of glaciers/ice and groundwater. As a result, it is of great interest to develop cost-effective and high efficiency approaches to desalinate the large supply of brackish or salty water available on our planet. Currently, the most common methods used to desalinate water are reverse osmosis, electrodialysis, and thermal separation, but these processes require high operating cost and/or significant energy inputs. Capacitive deionization (CDI) represents a less expensive and more energy efficient technology that does not produce secondary pollution and allows for easy regeneration and maintenance. CDI functions via charge storage mechanism analogous to supercapacitors. In this process, saline water flows by or through two electrode materials, while a potential is applied across these electrodes. In this first step, ion electrosorption, ions are extracted out of the water and absorbed into the structure of the electrodes, resulting in desalinated water that exits from the system. On the subsequent step, ion desorption, the potential is brought to zero or a negative potential is applied, forcing ions out of the structure of the electrodes to be washed from the system and thus regenerating the electrodes for the next ion electrosorption step.
The performance of a CDI system to the large extend depends on two parameters: (1) the choice of electrode material and (2) the electrode architecture. In the MEG laboratory, we focus on designing, synthesizing and exploring water desalination properties of the faradaic electrode materials in hybrid capacitive deionization system. This type of system takes advantage of the ability of the faradaic electrode to undergo redox reactions with cations (analogous to the reactions occurring at the cathode in a battery), thus removing them from solution. This type of electrode has the potential to achieve much higher desalination capacities compared to the surface-based ion adsorption exhibited by carbon electrodes. In addition, we assemble high performing materials to construct electrodes with highly porous architecture and good transport of both electrons and ions to maximize performance characteristics.