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Rechargeable lithium-ion (Li-ion) batteries continue to be the most prevalent form of electrical energy storage technology with applications in portable electronics and electric vehicles. One of the limiting factors in their performance is chemical and morphological changes undergone by the electrode materials, such as LiFePO4, during operation. In order to understand phase transformations in solids, tools that characterize the onset and propagation of these transitions with nansocale spatial and chemical resolution are critical. Transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS) can provide extremely high spatial resolution but is limited in sample thickness and induces radiation damage, which could affect the battery material. Synchrotron-based soft X-ray microscopy coupled with X-ray absorption spectroscopy (XAS) has been able to provide chemical mapping correlated with morphology with nanoscale spatial resolution while inducing lower rate of radiation damage.
PiFM, with its capability to correlate chemical makeup with morphology with spatial resolution of ~5 nm, can be an excellent tool of choice for studying such phase transformation of solid inorganic materials. The figure below shows PiFM and associated AFM topography of partially delithiated LixFePO4 plates with submicron dimensions. Based on the FTIR spectra obtained from a literature, one can see that FePO4 and LiFePO4 should be distinctively identifiable by using the wavenumbers 941 cm−1 and 1054 cm−1 respectively. The PiFM images at those two wavenumbers do highlight different regions of the sample, and the combined PiFM image shows that the delithiation process seems to have occurred more effectively at the rim of the larger plates and more thoroughly in an isolated and smaller plate.
We thank Prof. Jordi Cabana of Univ. of Illinois, Chicago for the samples.
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