Researchers have discovered why the crucial boundary layer in lithium-ion batteries is much thicker than it should be. The behavior of the precursor molecules is responsible for the ‘impossible’ growth of the solid-electrolyte interface (SEI): contrary to what was thought, they do not crystallize directly on the electrode, but grow first in the solvent.
They are found in mobile phones, laptops, cameras, and also in electric cars and airplanes: Lithium ion batteries They are almost indispensable as mobile electricity providers. Typically, their anode is made of graphite and the cathode of metal oxides such as lithium cobalt oxide.
The third in the group is a liquid electrolyte containing lithium ions. When a battery is first charged, a critical component to the performance and service life of the battery forms between the anode and the electrolyte: the solid electrolyte interface (SEI). It acts as a passivating layer and prevents further decomposition of the electrolyte.
The very thick boundary phase puzzle
But the strange thing is that this boundary layer should be only 2 to 3 nanometers thick. This is because the electrons required for the passivation layer cannot penetrate from the anode to the organic solvent environment. So the growth of the limiting stage must quickly stop again. But this is not the case: the thickness of the SEI in lithium-ion batteries is usually from 50 to 100 nanometers. Thus the boundary phase is several times thicker than it should be.
but why? So far there have been some hypotheses about how this apparent paradox might be resolved. However, none of the proposed mechanisms can be proven—also because the exact formation mechanisms of the SEI layer have only been partially elucidated so far. “Even if the underlying interfacial processes and the chemistry that govern them are well known, the mechanisms of formation and degradation of the SEI on the medium scale remain open,” explain Meisam Ismailpour of the Karlsruhe Institute of Technology (KIT) and colleagues.
SEI formation in 50,000 simulations
Researchers have now solved the puzzle of the “ultra-thick” passivation layer. To do this, they developed a 2D model that reconstructs the processes based on the chemical components of the electrolyte and the anode. In more than 50,000 simulations, they tracked how the SEI layer and its precursor formed under different reaction conditions.
As expected, lithium compounds, known as the precursors of the boundary layer, formed a few tens of microseconds after a voltage was applied. “All three intermediates originate near the electrode surface,” the team says. This fits with the assumption that the anode electrons are essential for these reactions.
Nucleation away from the anode
But then something surprising happened: the precursor molecules didn’t stay close to the anode, nor were they directly deposited on it. Instead, they spread farther from the pole, Ismail Pour and his colleagues note. Some of these particles have moved so far that they are no longer visible in the section recorded in the simulation. This drift of the precursor molecules had crucial consequences for the formation of the solid-electrolyte interface:
As the simulations further revealed, the first crystallization nuclei of the passivation layer do not form directly on the anode, but at a distance of about 20 nm from its surface. There, these spores grow into larger and larger clumps. Only then do they come into contact with the anode surface again and grow together to form an immobile porous layer – a solid electrolyte interface.
One of the greatest mysteries that solves the most important interface in liquid electrolyte batteries.
“This is the first evidence of a solution-mediated growth process for SEI,” the scientists wrote. “Mass transfer of SEI precursors away from the surface is a critical step for the nucleation and growth of the passivation layer.” In their simulations, the SEI layer became thicker the farther away from the anode the first crystallization nuclei from which the particles formed.
“With this we have solved one of the biggest puzzles regarding the most important interface in liquid electrolyte batteries – also in lithium-ion batteries, where we all use them every day,” says senior author Wolfgang Wenzel from KIT. In their study, the team was also able to identify other reaction parameters that determine the thickness of the passivation layer.
“In the future, this will enable the development of suitable electrolytes and additives to control the properties of the SEI and thus improve the performance and service life of the batteries,” says Saibal Gana, Ismailpour Fellow. (Advanced Energy Materials, 2023; doi: 10.1002/year 202203966)
Source: Karlsruhe Institute of Technology (KIT)
This article was written by Nadja Podbregar
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