Researchers demonstrated that a long-standing explanation for low energy efficiency in lithium-ion batteries does not hold.
The lithium-ion batteries used today are dominant due to their energy efficiency and charge storage capacity. However, the charge storage capacity of lithium-ion batteries is not half the capacity that lithium-enriched oxide cathodes could deliver. The problem with the lithium-enriched oxide cathodes is that it has a lower efficiency. In other words, you will need to spend significantly more power to charge up the battery than it will ultimately provide.
To combine the advantages of that technology with existing lithium-ion batteries, researchers have to understand the mechanism behind their inefficiency and exactly where the lost energy goes. Researchers from Skolkovo Institute of Science and Technology provide experimental evidence refuting the previously held explanation of the phenomenon known as voltage hysteresis.
As a lithium-ion battery gets charged, the lithium ions travel between the anode and cathode electrodes. The ions that migrate towards the anode leave behind vacancies in the cathode. The other half of the cycle involves lithium ions going back as the energy gets expended.
“In the meantime, however, some of the transition metal atoms making up the cathode might have temporarily invaded the vacancies and then pulled back again, spending valuable energy on this jumping around. Or so the old theory of voltage hysteresis went,” study co-author and Skoltech Ph.D. student Anatoly Morozov said.
The researchers used a transmission electron microscope at Skoltech’s Advanced Imaging Core Facility to monitor the atomic structure of a lithium-enriched battery cathode.
“Our findings inspired the team to seek the origin of voltage hysteresis elsewhere. What gives rise to the phenomenon is not reversible cation migration but rather the reversible transfer of electrons between the atoms of oxygen and transition metals. As the battery gets charged, some of the electrons from iron are hijacked by the oxygen atoms. Later on, they go back. This reversible transfer consumes some of the energy,” explained Professor Artem Abakumov, who heads the Center of Energy Science and Technology at Skoltech.
“Understanding voltage hysteresis in terms of electron transfer might have immediate implications for mitigating this unwelcome effect to enable next-generation lithium-ion batteries with record-high energy density for powering electric cars and portable electronics,” he added. “To enable that next step, chemists could manipulate the electron transfer barriers by varying the covalency of the cation-anion bonding, guided by the periodic table and such concepts as ‘chemical softness.'”
“This demonstrates the power of advanced transmission electron microscopy for deciphering local structures of extreme complexity. It is really great that young researchers at Skoltech have direct and easy access to such sophisticated equipment as aberration-corrected electron microscopes, and opportunities for further training. This enables us to contribute to top-level battery research in collaboration with our international peers in both academia and the industry,” Morozov says.
The research appeared in the journal Nature Chemistry.