Lithium-ion batteries have become an integral part of our everyday life. But one crucial component of these batteries has been a mystery up until now: the solid-electrolyte interface (SEI). This layer forms on the anode when the battery is first charged and is essential for the life of the battery. Only now has a research team clarified exactly how this passivation layer forms. Contrary to what was thought, it does not form directly at the electrode, but initially grows in the solvent. The porous boundary layer then forms only from these “seeds”. This new finding also clarifies how this layer can reach its typical thickness.
From smartphones to notebooks to electric cars – lithium-ion batteries are used almost everywhere that mobile power supply is required. The solid electrolyte interphase (SEI) is one of the decisive factors for the reliable operation of these batteries. This passivation layer forms on the anode when voltage is first applied and has a significant impact on the electrochemical performance and service life of a lithium-ion battery. If the SEI breaks open, the electrolyte is further decomposed and the capacity of the battery steadily decreases. Despite the great importance of the boundary layer for battery performance, its formation has only been partially clarified. "Even if the fundamental interface processes and the chemistry that governs them are well known, the mechanisms of formation and degradation of the SEI on the mesoscale remain unclear," write Meysam Esmaeilpour from the Karlsruhe Institute of Technology (KIT) and his colleagues.
SEI formation in 50,000 simulations
One thing in particular has puzzled researchers so far: Electrons from the anode are needed to form the boundary layer from the electrolyte. According to studies, however, these can only penetrate around two to three nanometers into the organic solvent environment. The growth of the passivation layer should actually come to a standstill after just a few nanometers, as Esmaeilpour and his team explain. Instead, however, the SEI in lithium-ion batteries is typically 50 to 100 nanometers thick. So far there have been some hypotheses as to how this apparent paradox can be resolved. However, none of the proposed mechanisms could be proven - also because neither experimental nor computer-aided approaches have succeeded in fully decoding the complex growth processes of the SEI, which take place on very different sizes and length scales.
In order to solve the mystery of the passivation layer, the scientists have now reconstructed the processes in a two-dimensional model. To do this, they created a set of more than 50,000 simulations representing different reaction conditions based on the chemistry of the electrolyte and the anode. In the simulations, as expected, the first lithium compounds, which are known as precursors for the interface, formed just a few tens of microseconds after applying a voltage. "All three intermediate compounds originate near the electrode surface," reports the team. This fits with the assumption that the electrons of the anode are necessary for these reactions. However, what happened next was surprising: the precursor molecules did not stay close to the anode, nor did they deposit directly on it. Instead, they diffused away from the electrode, as observed by Esmaeilpour and his colleagues. Some of these molecules moved so far away that they were no longer visible in the section recorded in the simulation.
SEI germs do not develop directly on the electrode
This drift of the precursor molecules has a decisive effect on the formation of the solid-electrolyte interface, as the further course of the simulations revealed: The first crystallization nuclei for the formation of this passivation layer do not form directly on the anode, but around 20 nanometers from its surface , as the team reports. There, these germs grow into larger and larger clumps. Only then do they come into contact with the anode surface again and grow together to form a porous, immobile layer – the solid-electrolyte interface. "This is the first evidence for a solution-mediated growth process of the SEI," the scientists write. "Mass transport of the organic SEI precursors away from the surface is a critical step for nucleation and growth of the passivation layer." In their simulations, the SEI layer became thicker the further away from the anode the first crystallization nuclei formed from the precursor molecules.
"We have thus solved one of the great mysteries of the most important interface in liquid-electrolyte batteries - also in lithium-ion batteries, as we all use them every day," says senior author Wolfgang Wenzel from KIT. In their study, the team also managed to identify other reaction parameters that determine the thickness of the passivation layer. "In the future, this will make it possible to develop electrolytes and suitable additives to control the properties of the SEI and thus improve the performance and lifespan of the batteries," says Esmaeilpour's colleague Saibal Jana.
Source: Meysam Esmaeilpour (Karlsruher Institute for Technology) et al., Advanced Energy Materials, doi: 10.1002/aenm.202203966