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Condens. Matter 2021, 6, 28 8 of 11 filled glovebox and the cathode electrode is thoroughly washed with anhydrous dimethyl carbonate to remove the excess electrolytes. The positron annihilation lifetime experiments were performed using a fast–fast coin- cidence lifetime setup (see [46]) with a time resolution of ~235 ps (FWHM). Time calibration used for a multi-channel analyzer (MCA) card was 25.35 ps per channel. The positron source 22Na (2–3 μCi), deposited between two 7.5 μm thick Kapton foils, was sandwiched between two samples (see Figure 2a the scheme of the cathode oxides measurements). All lifetime measurements were performed at room temperature (21 ± 1 ◦C) at atmospheric pressure. Positron lifetime spectra of ~4–6 × 106 annihilation events each were recorded. Each positron spectrum required 36–48 h of acquisition time. The intensity of the source component was about 12% for Al samples and 14% for the oxide (LiXCoO2) samples with a value of 382 ps and it was considered throughout the positron data analysis. PALS spectra were fitted using the LT software version 10 [47]. In general, after the Kapton subtraction, three discrete lifetime components τ1 < τ2 < τ3 appear together with their respective in- tensities, I1, I2 and I3. The goodness parameter value of the fit (χ2/dof) lay between 0.99 and 1.15. The third component τ3, normally called “spurious”, has, in general, a very low intensity I3 (<0.05%) when a metal with a smooth surface is measured, if the source is done well as in this case. The value of τ3 is around 2 ns and is attributed to positronium formed at the interface between the Kapton foil and the studied material. In the case of the cathode oxides (LiXCoO2) and the sintered graphite the intensity of this component was between 0.2 and 0.4% due to the surfaces are porous or not well smoothed. In this work an analysis with two lifetime components after subtraction the source and the spurious contributions is presented. Doppler broadening (DB) of the annihilation radiation was used to monitor the microstructure and defects associated with the cathode oxides. In order to obtain depth- resolved annihilation data, positrons were implanted in the sample at various depths using a variable energy positron beam (from 1 to 17 keV, see Supporting Information of [48] to know the positron beam characteristics). Two high pure HPGe gamma detectors (Ortec, relative efficiency ~50% at 1.33 MeV) were used to measure the spectrum of the annihilation radiation. The PAS measurements were performed at room temperature in a vacuum environment of ~10−7 mbar. The annihilation peak (511 keV) is broadened by the Doppler effect due to the motion of the electrons annihilating with positrons. For characterization purposes, it is convenient to distinguish the area around the maximum of the annihilation peak and define a parameter called Shape or S parameter. The S parameter is associated with the fraction of annihilating positron-electron pairs with momenta |pL| ≤ 0.456 atomic units, corresponding to the energy range within 511 ± 0.85 keV. The total area of the annihilation peak is taken in the energy range 511 ± 4.25 keV. The S parameter corresponds to annihilation of positrons with valence electrons in the sample (and, occasionally, para- positronium). 4. Conclusions In this work, it is demonstrated that positron annihilation spectroscopy is a crucial tool in order to study the effect of the state of charge on the structural features and morphology of cathodic materials. In detail, for a cathode corresponding to a battery state of charge of 50% (x = 0.5), the measured first component of the positron lifetime of 180 ps is in excellent agreement with the calculated value. This finding implies that the bulklike state in the grain [49] becomes very similar to a true positron bulk state. However, in the case of the discharged battery (x = 1.0), the positron spillover increases and the lifetime for the bulklike state is about 30 ps higher than the calculated bulk value. Since the intensity of the positron surface states is high both for x = 1 and x = 0.5, this state could in principle be used to monitor in operando non-homogeneous lithiation states at the grain boundaries of the cathode [50]. Therefore, one could detect possible lithium bottlenecks at the grain-graphite interface.PDF Image | Positron Annihilation Spectroscopy LiCoO2 Cathode of Lithium-Ion Batteries
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