A group of scientists has identified the causes behind the degradation observed in some latest generation lithium ion batteries, known as single crystal cathode batteries, even though they were adopted to improve reliability and durability.
The research was conducted by experts from the Argonne National Laboratory and the Pritzker School of Molecular Engineering at the University of Chicago, with the aim of understanding why this technology, despite having been conceived to solve durability problems, does not always meet expectations, and sometimes shows signs of premature deterioration.
The results of this investigation were published in the journal Nature Nanotechnology, providing data and insights into the behavior of cathode materials at the nanoscopic level and the relationships between chemical composition and degradation mechanisms observed.
Lithium-ion batteries used in electric vehicles are based on nickel-rich cathode materials. In traditional designs, these cathodes consist of polycrystalline structures, made up of numerous small crystals. These structures, despite having been the industrial standard for years, have limitations linked to the fractures that develop along the edges of the crystals during the charging and discharging cycles. To overcome this problem, in recent years research has pursued the adoption of single crystal cathodes, without “boundaries” between crystals.
The underlying logic predicts that by eliminating the grain boundaries, the crack initiation points could be reduced, increasing the mechanical stability of the material. However, single crystal cathodes, despite the absence of grain boundaries, have nevertheless proven susceptible to degradation phenomena, with performances that do not always exceed those of polycrystalline cathodes.
To investigate this phenomenon, the researchers employed advanced analysis techniques including X-ray diffraction via synchrotron-type sources and very high resolution electron microscopy. These tools allowed us to examine cathode structures at the nanometric level, revealing that, within each individual particle of material, chemical reactions do not occur uniformly during use cycles. Different areas of the single particle react at different speeds, generating internal tensions that lead to the formation of fractures from within the material itself.
This heterogeneous reaction dynamics differs from the mechanisms observed in polycrystalline materials and constitutes the main driver of degradation in single crystal cathodes.
Another result that emerged from the study concerns the role of chemical components within the composition of cathode materials. In batteries with polycrystalline cathodes, cobalt is traditionally associated with increased fracturing but is also considered useful for preventing structural instability. Tests conducted by the research team, however, indicated that in single-crystal cathodes the behavior of these elements may be different. In experiments in which experimental cells were constructed based on distinct combinations of nickel with cobalt and nickel with manganese, it emerged that manganese causes greater mechanical damage, while the presence of cobalt seems to improve the overall durability of the material.
In parallel with the analyzes on degradation mechanisms, the scientists also considered the impact of material costs. Cobalt, although showing a stabilizing effect in single crystal cathodes, is an expensive element and for this reason the group of researchers indicated that future challenge is the search for lower cost materials that can replicate its contribution to structural stability without significantly impacting the overall costs of the batteries.
