|Habrioux, A.; Surble, S.; Berger, P.; Khodja, H.; D'Affroux, A.; Mailley, S.; Gutel, T.; Patoux, S. Nucl. Instrum. Methods Phys. Res., Sect. B , 290, 13-18 (2012).|
During the past few years much progress has been made in development of electrodes and electrolytes for lithium ion batteries, however capacity fading during cycling remains a major problem limiting battery lifetime. This fading phenomenon could be due to the formation of SEI by electrolyte degradation or by a migration of lithium ion at the interface. Several methods have been applied to understand the origins of this loss of capacity. The well-known LiFePO4 cathode material has been the subject of original studies employing a combination of X-ray photoelectron spectroscopy, X-ray diffraction, and X-ray fluorescence. Discriminating FePO4 and LiFePO4 phases indirectly allows local state-of-charge (SOC) evaluation, but none of these mentioned techniques can display directly the local stoichiometry of lithium. (1-4)
Conversely, nuclear microprobe is a powerful tool to simultaneously investigate concentrations and distributions of the different constituent elements of a positive LiFePO4 electrode as a function of its state of charge (SOC). (5)
The developed method combines the use of two beam conditions (alpha at 2900 keV and protons at 2600 keV) and several analytical techniques (RBS, PIGE and PIXE).
It allows determining the concentration of light elements such as Lithium, and simultaneously the whole composition of the electrode (including majors as well as traces such Mn). We confirmed theoretical compositions at different states of charge (SOC) but a partial immobilization of lithium is observed (Fig. 1), which could be responsible for the loss capacity.
Lithium maps show some strong heterogeneities which can be account for the immobilization of lithium. Thanks to dedicated software, the lithium concentration can be estimated: on a 100% SOC electrode we found areas with Li content 28% higher in the region of interest than in the full map. Likewise, for example manganese clusters have been found also (Fig. 2).
These observed heterogeneities could be responsible of the capacity fading. As the used method is non-invasive, it opens a potential in-situ characterization technique with a specially designed lithium-ion battery cell.
Contact CEA : Suzy Surblé.
Maj : 07/11/2013 (2217)