Lithium Doesn't Behave As We Thought: The Discovery Could Change Future Batteries
Solid-state batteries are considered one of the most promising technologies for the generation of energy storage systems due to their potential to offer greater safety, higher energy density, and better performance compared to traditional liquid electrolyte lithium-ion batteries. Despite these expectations, many unknowns remain regarding the behavior of lithium within solid materials during charge and discharge cycles.
A group of researchers from the Institut Laue-Langevin (ILL) and partner institutes has succeeded, for the first time, in directly observing the movement of lithium ions within a solid-state battery while it operates, using neutron diffraction during the normal functioning of the cell. The study, published in Advanced Energy Materials, allowed for real-time tracking of structural evolution in materials and the identification of phenomena that had previously remained hidden.
Unlike X-rays, which primarily interact with electrons and are not very effective in analyzing light elements like lithium, neutrons interact directly with atomic nuclei. This characteristic makes them particularly sensitive to lithium atoms and also allows them to penetrate thick samples without damaging them, providing a comprehensive view of the inside of the battery rather than just the surface.
To make the experiment possible, a solid-state battery was created with a cathode based on NMC622, a solid electrolyte of mixed halide argyrodite type, and a lithium-indium alloy anode. The need to obtain a sufficiently intense neutron signal required the use of about 140 mg of active material and the construction of a cell approximately 2.5 millimeters thick, a significantly higher value than that of cells normally used in the laboratory. Such a configuration noticeably increases internal resistance, complicating ion transportation.
To compensate for this limitation, the researchers employed a new electrolyte characterized by an ionic conductivity about six times greater than traditional Li₆PS₅Cl (sulfide-based with an argyrodite crystal structure) materials. Thanks to this solution, it was possible to extract about 55% of the lithium contained in the active material, a result comparable to that obtained in conventional experimental cells despite the increased thickness of the battery.
However, observations made during the first charge and discharge cycle revealed unexpected behavior. Even using an extremely slow charging rate (C/60), lithium did not distribute uniformly within the positive electrode. The researchers found coexisting two different structural phases of the material, called H1 and H2, indicating that different regions of the electrode were achieving different charge levels simultaneously.
According to the team, this phenomenon can be attributed to an uneven distribution of electric current within the thick electrode, causing a sort of “bottleneck” in the transport of lithium ions. In other words, some areas of the battery advance in the charging process more quickly than others, reducing the homogeneity of the electrochemical reaction.
A second experiment showed how an increase in temperature could significantly modify this behavior. By repeating the test at 100 °C, the coexistence of the two phases completely disappeared, and the reaction occurred through a single structural phase. The increase in ionic conductivity favored by the temperature indeed uniformed lithium transport and current distribution within the electrode. Moreover, the solid electrolyte maintained its structural stability throughout the entire operating cycle, with no significant observable changes in neutron diffraction data. This result is considered encouraging for the development of solid-state batteries based on sulfur electrolytes, as it suggests good robustness of the material even during operation.
According to the authors, the ability to monitor in real-time the distribution of lithium, variations in lattice parameters, and the evolution of different crystalline phases offers new insights for understanding the limitations of solid-state batteries. The evidence gathered indicates that the design of electrodes, the conductivity of the electrolyte, and thermal management are critical factors for achieving a more uniform ion distribution and improving the performance of future batteries intended, among other uses, for electric vehicles.