Charging lithium-ion batteries too fast can permanently reduce the battery capacity, as portions of the energy storage structure will be destroyed and deactivated by doing so. These structural changes have been visualised for the first time by DESY researcher Dr. Ulrike Bösenberg along with her team at DESY’s X-ray source PETRA III. Their fluorescence studies show that even after only a few charging cycles, damage to the inner structure of the battery material is clearly evident. The results of the studies will be published in the latest edition of the academic journal Chemistry of Materials.
Damaged After 1,000 Times Of Charging
Lithium-ion batteries are very common because they possess a high charge density. Typically, the storage capacity is significantly diminished after one thousand charges and discharges. A promising candidate for a new generation of such energy storage systems, particularly due to their high voltage of 4.7 Volts, are what are known as lithium-nickel-manganese-oxide spinel materials or LNMO spinels. The electrodes consist of miniature crystals, also referred to as crystallites, which are connected with binder material and conductive carbon to form the thin layer.
The team around Bösenberg, which also includes researchers from the University of Giessen, University of Hamburg and from Australia’s national science agency CSIRO, studied the negative electrodes of this LiNi0.5Mn1.5O4 compound at PETRA III’s X-ray microfocus beamline P06.
The Faster The Charging, The Higher The Destruction
In their present study, the researchers exposed different battery electrodes to twenty-five charging and discharging cycles each, at three different rates and measured the elementary distribution of the electrode components. The scientists could show that during fast charging, manganese and nickel atoms are leached from the crystal structure. In their investigation, the researchers spotted defects such as holes in the electrode with up to 100 microns (0.1 millimetre) diameter. The destroyed areas can no longer be utilised for lithium storage.
Utilising the X-ray fluorescence method in their studies, the researchers took advantage of the fact that X-rays can excite chemical elements into fluorescence, a short-term radiation emission. The wavelength or energy of the fluorescent radiation is a characteristic fingerprint for each chemical element. This way, the distribution of the individual materials in the electrode can be precisely determined.
Damages Revealed By Hamburg’s High-Intensity PETRA III X-Ray Beam
The narrow and high-intensity PETRA III X-ray beam could precisely scan the sample surface, which measured approximately 2×2 square millimetres, with a resolution of half a micrometre. Investigating each point took merely a thousandth of a second. “It is the first time that we could localize these inhomogeneities with such a high spatial resolution over so large an area,” says Bösenberg. “We hope to better understand the effects and to create the foundation for improved energy storage devices.”
What is still puzzling is where the dissolved nickel and manganese atoms end up – this is a question the researchers would like to resolve in further studies.
source and further details:
Correlation between Chemical and Morphological Heterogeneities in LiNi0.5Mn1.5O4 Spinel Composite Electrodes for Lithium-Ion Batteries Determined by Micro-X-ray Fluorescence Analysis; Ulrike Bösenberg, Mareike Falk, Christopher G. Ryan, Robin Kirkham, Magnus Menzel, Jürgen Janek, Michael Fröba, Gerald Falkenberg and Ursula E. A. Fittschen; Chemistry of Materials, 27 (7), 2015, DOI: 10.1021/acs.chemmater.5b00119