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Condens. Matter 2021, 6, 28 4 of 11 2.2.2. Cathode Samples The PALS measurements were performed in real LiXCoO2 cathodes used in batteries and produced with a thickness devoted to improving the battery performance. The mea- surement represents a challenge from the PALS point of view, as the dimensions are not optimized to avoid annihilation in extra-cathodic zones. This cathode material was studied previously by means of PALS [37] using more idealized materials. Figure 2a shows the schematic setup used for the PALS measurements. These were performed using a sandwich setup, in which two pairs of identical cathode samples were placed with the 22Na source of positrons in between. Two external annealed Al samples (1.5 mm thick) close the sandwich. The mean mass density of the LiCoO2 cathode samples were obtained from gas pycnome- ter measurements (4.52 ± 0.01 g/cm3). Table 2 shows that the thickness of the cathode electrodes is about 60 μm and the Al foil substrate 15 μm (d1 and d2 in Figure 2a). After the subtraction of the Kapton and spurious components from the spectra (see Materials and Methods section), following the formulation of Brandt [42] with an approximative version of a multilayer formulation used for the β+ emission 22Na profile, it was possible to estimate the fraction of positrons implanted into the LiXCoO2 grains embedded with a low percentage of graphite and polyvinyl difluoride (PVDF) (see Materials and Methods section). This fraction, after the subtraction of the Kapton contribution, results about 91 (3)%; the other positron fraction is implanted inside the Al supporting foil and into the external Al polycrystalline sample. Considering the thicknesses of the materials and the typical diffusion length inside the crystal oxide (~60 nm, see Table 3 in Section 2.3) and Condens. Matter 2021, 6, x FOR PEER REVIEW 5 of 12 in the Al support (~100 nm), we can assume that the implantation fraction practically corresponds to the annihilation fraction. Figure 2. Schematic set-up for the LiXCoO2 measurements. (a) PALS measurements using a sand- Figure 2. Schematic set-up for the LiXCoO2 measurements. (a) PALS measurements using a sandwich wich configuration; here, the positrons are emitted fro2m2 the 22Na source located between two Kap- configuration; here, the positrons are emitted from the Na source located between two Kapton foils ton foils (7.5 μm each). In this case, positrons are emitted with a continuous distribution of kinetic (7.5 μm each). In this case, positrons are emitted with a continuous distribution of kinetic energies energies up to 546 keV. (b) Doppler measurements using a positron beam with variable implanta- up to 546 keV. (b) Doppler measurements using a positron beam with variable implantation energy. tion energy. Each Doppler measurement is performed with a fixed energy with positrons implanted Each Doppler measurement is performed with a fixed energy with positrons implanted with a range with a range of kinetic energies from 1 to 17 keV. In this case, the maximum mean implantation of kinetic energies from 1 to 17 keV. In this case, the maximum mean implantation depth is about depth is about 1 μm. 1 μm. Table 2. Thickness, positron lifetime components, relative intensities and average positron lifetime obtained in the cathode oxides. The initial thickness of the LiCoO2 and graphite/PVDF mixture is the same 59 (2) nm, after the charge process the macroscopic thickness is increased about 14% in the case of the mixture with Li0.5CoO2 grains. Cathode Thickness (μm) LiCoO2 59 (2) Li0.5CoO2 67 (2) τ1 (ps) 163 (2) 181 (2) τ2 (ps) 315 (3) 327 (3) I1 (%) 66 (2) 73 (2) I2 (%) 34 (2) 27 (2) τav (ps) 215 (4) 220 (4) The PALS spectra measured in the composite oxide LiXCoO2 samples have a complex unresolved distribution that can be analyzed with two lifetime components; the longer ofPDF Image | Positron Annihilation Spectroscopy LiCoO2 Cathode of Lithium-Ion Batteries
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