Comprehensive analysis of TEM methods for LiFePO4/FePO4 phase mapping: spectroscopic techniques (EFTEM, STEM-EELS) and STEM diffraction techniques (ACOM-TEM)

Abstract

Transmission electron microscopy (TEM) has been used intensively in investigating battery materials, e.g. to obtain phase maps of partially (dis)charged (lithium) iron phosphate (LFP/FP), which is one of the most promising cathode material for next generation lithium ion (Li-ion) batteries. Due to the weak interaction between Li atoms and fast electrons, mapping of the Li distribution is not straightforward. In this work, we revisited the issue of TEM measurements of Li distribution maps for LFP/FP. Different TEM techniques, including spectroscopic techniques (energy filtered (EF)TEM in the energy range from low-loss to core-loss) and a STEM diffraction technique (automated crystal orientation mapping (ACOM)), were applied to map the lithiation of the same location in the same sample. This enabled a direct comparison of the results. The maps obtained by all methods showed excellent agreement with each other. Because of the strong difference in the imaging mechanisms, it proves the reliability of both the spectroscopic and STEM diffraction phase mapping. A comprehensive comparison of all methods is given in terms of information content, dose level, acquisition time and signal quality. The latter three are crucial for the design of in-situ experiments with beam sensitive Li-ion battery materials. Furthermore, we demonstrated the power of STEM diffraction (ACOM-STEM) providing additional crystallographic information, which can be analyzed to gain a deeper understanding of the LFP/FP interface properties such as statistical information on phase boundary orientation and misorientation between domains.

Advanced Transmission Electron Microscopy for Electrode and Solid‐Electrolyte Materials in Lithium‐Ion Batteries

“…Energy‐filtered TEM (EFTEM) using a specific energy range can provide phase and elemental distribution maps from substantially larger specimen areas. Furthermore, a new STEM diffraction technique, named automated crystal orientation mapping (ACOM), can even provide crystallographic information at each sampling point of the phase maps …”

Scanning transmission electron diffraction methods

“…Through this approach the domino-cascade model of lithium insertion, where individual particles of FePO 4 transform serially rather than through a continuous parallel absorption, was confirmed (Brunetti et al, 2011). Further to this, the spatial resolution in TEM allowed those particles where partial lithium insertion had occurred to be studied in greater detail to understand the interface properties between the pristine and converted material and so understand the potential limitations of crystallography on the capacity for charge storage (Mu et al, 2016).…”

Structure solution and refinement of metal-ion battery cathode materials using electron diffraction tomography

“…One could argue that, as for the colloidal nanocrystals mentioned above, it is possible to find the correct zones among randomly oriented crystallites if there are many crystallites available. However, the state of charge might slightly differ depending on the proximity of the particle to the electrode of the cell and many metal-ion battery cathode materials even inherently cycle with a two-phase process (Ariyoshi et al, 2004;Mu et al, 2016;Drozhzhin et al, 2016). Combining direct space imaging data from different crystals will give incorrect results in such case.…”

“…For the electrode mounted on a current collector (typically Al), the diffraction pattern is dominated by strong reflections from the metallic foil, which is often rolled and highly textured causing strong non-uniaxial preferred orientation effects not taken into account by common preferred orientation functions, thus hampering a multi-phase Rietveld refinement. The active electrode material might also consist of two or more phases, as is the typical case for compounds demonstrating a two-phase (de)intercalation mechanism, such as LiFePO 4 (Mu et al, 2016). Last but not least, only a very small amount of material is often available for the diffraction studies.…”