Mukherjee: Mesoscale Interactions in Li-Air Batteries
Partha P. Mukherjee
As we look to reduce our carbon footprint and fossil fuel dependence – particularly in transportation fuels – scientists and engineers are working to identify technologies and solutions to overcome the current barriers to vehicle electrification. The primary challenge to vehicle electrification centers on developing rechargeable batteries that will provide enough energy capacity and power to meet or exceed the standards of current combustion engines. A vehicle’s energy capacity refers to its driving range, and consumers generally expect vehicles to travel approximately 300 miles on a fill-up or single charge. The power of a vehicle indicates its ability to accelerate at a rate consistent with current vehicles on the road.
Other challenges to vehicle electrification include safety, weight, durability, environmental impact, and a host of additional considerations.
The majority of rechargeable batteries in transportation currently employ lithium-ion (Li-ion) batteries. While providing sufficient specific power to meet consumer expectations, these vehicles generally have a short driving range.
To understand how Li-ion batteries can provide both suitable power and energy, researchers are looking deeply into many areas to exploit this technology. However, batteries are complex, dynamical systems that include a multitude of processes and interactions that are not currently fully understood.
Dr. Partha Mukherjee, an assistant professor in the Department of Mechanical Engineering, has been investigating the multiscale electrode physics of energy storage within Li-ion batteries – analyzing electrochemical-thermal-mechanical interactions and physicochemical processes in battery electrodes, which he calls “electrodics.” He has been investigating the mesoscale physics resulting from microstructure-transport interactions and the dynamic interplay across spatio-temporal, or time and space scales.
These investigations have led him to look at other technologies and implementations that may have higher potential for meeting the demand for specific power and energy capacity.
Lithium-air (Li-Air) batteries are an emerging alternative battery technology that shows promise for a revolution to the status quo in electric drive vehicles, potentially providing ten times the specific energy of Li-ion batteries.
Because of this potential, Mukherjee and his research group are expanding their understanding from Li-ion batteries, coupled with expertise in lithium-sulfur batteries and polymer electrolyte membrane fuel cells, to Li-Air battery technology.
A typical Li-Air battery consists of a lithium metal anode, a porous air cathode, and a nonaqueous or aqueous electrolyte. As the battery is discharged, positive lithium ions are formed by the oxidation of lithium at the negative electrode (anode). The positive ions travel through the electrolyte to the positive electrode (cathode) where they react with oxygen to form Lithium Peroxide (Li2O2) deposits (for nonaqueous Li-air battery) that eventually become a layer. When the battery is charged, the process is reversed. This process only works well, however, when the Li2O2 layer does not become so pervasive in the battery that it inhibits reactions.
Therefore, it is critical to understand the how and why the Li2O2 forms a layer, so that the layer can be precisely controlled or managed. The complex microstructure of the cathode plays a key role in determining how the ions and electrons flow, how the Li2O2 layer is formed, and how the Li2O2 layer behaves.
Mukherjee and his collaborators have hypothesized that the design of the cathode must factor in the interaction of the physico-chemical processes in the microstructure of the cathode that affect flow, impact total surface area and volume, and contribute to the ability for a Li2O2 layer to form, but not inhibit reactions.
To learn how they might prevent the inhibition of reactions and other undesirable outcomes from the Li2O2 deposits and eventual layering, research will be conducted to fully understand why and how the Li2O2 is deposited, so that Mukherjee and his team can then control and direct the formation of the Li2O2 deposits and layer. This will be accomplished through mesoscale modeling of the behavior coupled with considerations for microstructural stochastics – the unpredictable and random formation of deposits.
Following that work, electrochemical experiments and nanoscale characterization will be conducted to make associations between the battery’s composition and dynamics and the performance of the electrode.
Through this research, Mukherjee and his team hope to outline the basic mechanisms that lead to the formation and accumulation of Li2O2 and how it affects performance within the battery. This understanding will allow engineers to design microstructures that can limit the blockage of pores and other undesirable behaviors during the discharging and charging process. Eventually, this could lead to the development of highly efficient Li-Air batteries with ample power and energy for the future of transportation.
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