The solvent-assisted microstructure increased the energy density of the cathode to 300Wh/kg, from just under 180Wh/kg for the dry-mixed microstructure, by significantly improving the utilization rate of active material. Image: University of Houston.To date, electric vehicles (EVs) account for only around 2% of vehicles on the road, but that is projected to rise to 30% by 2030. A key step toward improving the commercialization of EVs is to heighten the gravimetric energy density of their batteries – measured in watt hours per kilogram (Wh/kg) – using safer, easily recyclable materials that are abundant. Lithium-metal anodes, rather than the conventional graphite anodes, are considered the 'holy grail' for improving the energy density of EV batteries, in the race to reach more competitive energy density at 500Wh/kg.
Yan Yao, professor of electrical and computer engineering in the Cullen College of Engineering at the University of Houston (UH), and UH post-doctorate Jibo Zhang are taking on this challenge with colleagues at Rice University. In a paper in Joule, Zhang, Yao and their team report a two-fold improvement in energy density for organic-based, solid-state lithium batteries by using a solvent-assisted process to alter the cathode microstructure.
"We are developing low-cost, earth-abundant, cobalt-free organic-based cathode materials for a solid-state battery that will no longer require scarce transition metals found in mines," said Yao. "This research is a step forward in increasing EV battery energy density using this more sustainable alternative." Yao is also principal investigator with the Texas Center for Superconductivity at UH (TcSUH).
All batteries contain an anode, also known as the negative electrode, and a cathode, also known as the positive electrode, separated by a porous membrane. Lithium ions flow through an ionic conductor – the electrolyte – which allows for the charging and discharging of electrons that generate electricity for, say, a vehicle.
Electrolytes are usually liquid, but they can also be solid, a relatively new concept. This novelty, combined with a lithium-metal anode, can prevent short-circuiting, improve energy density and lead to faster charging.
Cathodes typically determine the capacity and voltage of a battery, and are often the most expensive part, due to the use of scarce materials like cobalt. But such cobalt-based cathodes display excellent performance, leading to their widespread use in solid-state batteries. Only recently have organic compound-based lithium batteries (OBEM-Li) emerged as a more abundant, cleaner alternative that is more easily recycled.
"There is major concern surrounding the supply chain of lithium-ion batteries in the United States," said Yao. "In this work, we show the possibility of building high energy-density lithium batteries by replacing transition metal-based cathodes with organic materials obtained from either an oil refinery or biorefinery, both of which the US has the largest capacity in the world."
Cobalt-based cathodes can generate 800Wh/kg of material-level specific energy, or voltage multiplied by capacity, as do OBEM-Li batteries, which were first demonstrated by the team in an earlier study. But previous OBEM-Li batteries were limited to a low-mass fraction of active materials due to the non-ideal microstructure of the cathodes, capping their total energy density.
Yao and Zhang uncovered how to improve the energy density in OBEM-Li batteries by optimizing the cathode microstructure for improved ion transport. To do this, they altered the microstructure of the organic cathode pyrene-4,5,9,10-tetraone (PTO) with a familiar solvent – ethanol.
"Cobalt-based cathodes are often favored because the microstructure is naturally ideal, but forming the ideal microstructure in an organic-based solid-state battery is more challenging," said Zhang.
On an electrode level, the solvent-assisted microstructure increased the energy density to 300Wh/kg, from just under 180Wh/kg for the dry-mixed microstructure, by significantly improving the utilization rate of active material. Previously, the amount of active materials could be increased but the utilization percentage was still low, near 50%. With Zhang's contribution, the utilization rate improved to 98%, which resulted in a higher energy density.
"Initially, I was examining the chemical properties of PTO, which I knew would oxidize the sulfide electrolyte," Zhang said. "This led to a discussion on how we might be able to take advantage of this reaction. Together with colleagues at Rice University, we investigated the chemical composition, spatial distribution and electrochemical reversibility of the cathode-solid electrolyte interphase, which can provide us with hints as to why the battery could cycle so well without capacity decay."
Over the past 10 years, the cost of EV batteries has declined to nearly 10% of their original cost, making them commercially viable. So, a lot can happen in a decade. This research is a pivotable step in the process toward more sustainable EVs and a springboard for the next decade of research.
This story is adapted from material from the University of Houston, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.