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Condens. Matter 2021, 6, 28 6 of 11 Presumably, other short components discussed before can affect the measured value of τ1 (as the Al contribution). In any case, the measured values follow the trend indicated by the theoretical calculations, which leads us to think that the description is going in the right direction. The criteria of the PALS analysis previously studied by Parz et al. [37] is different. These authors used a different criterion for the Al contribution subtraction. However, the results of the cathode oxide follow the same tendency of the average positron lifetime result found in the present work (Table 2). 2.3. Doppler Broadening Figure 3 shows the evolution of the S parameter measured for the LiXCoO2 cathode samples as a function of the positron implantation energy. These measurements were performed by means of a positron beam. Figure 2b shows the set-up scheme. Positrons are implanted at different energies with a characteristic implantation profile that depends on the mass density of the studied material. The implantation depth of the positrons was lower than 1 μm for the higher energy used. The S parameter is mainly correlated with the annihilation of the valence electrons of the material, and its values are associated with the chemical environment surrounding annihilation sites. The reference sintered graphite measurement results almost constant and the average value (SGraphite = 0.509 (1)) is indicated with a dashed black line. For implantation energies higher than 3 keV, the S parameter of the discharged cathodic material (LiCoO2, green symbols) tends to be higher than the partially charged material (Li0.5CoO2, brown symbols), although the differences are not significant within the experimental error. A higher value of the central part of the peak as a function of the Li contains was observed by Barbiellini et al. [43] using X-ray Compton scattering measurements. These correspond to an increase of S parameter in the present work. Instead, Parz et al. [37] observe that the behavior of the S parameter tends to be opposite with the Li concentration. In fact, this is an argument to be studied in more detail because, in general, the S parameter follows the lifetime tendency. Presumably, the other extra-cathode materials measured in this work and the influence of the chemistry outside the oxide grain (mainly graphite) can affect the positronic parameters. This effect could be rationalized by a strong distortion of the positron wave-function induced by graphite reported by Cartier et al. [44]. In fact, if the positron wave-function spills over the grains it becomes less sensitive to the Li ion vacancies. Therefore, in this case, X-ray Compton and the Doppler broadening experiments can give similar results. Figure 3 shows a best-fit procedure for the experimental data (green and brown dashed lines) called VEPFIT [45] based on the solution of the diffusion equation for positrons in layers, considering the energy-dependent positron implantation profiles (Makhov profiles). To fit the experimental data, a one-layer model was used, comprising the sample surface and the bulk. It was possible to estimate a set of parameters as the positron diffusion length and S parameter of the surface and bulk knowing the film mass density (Table 3). The positron diffusion length L+ is about 60 nm, and it tends to be shorter in the case of Li0.5CoO2, but it does not differ much within experimental error. Table 3. Positron diffusion length and S parameter of the surface and bulk. Cathode LiCoO2 Li0.5CoO2 L+ (nm) 60 (3) 55 (3) Ssurface 0.506 (1) 0.505 (1) Sbulk 0.4760 (7) 0.4742 (7)PDF Image | Positron Annihilation Spectroscopy LiCoO2 Cathode of Lithium-Ion Batteries
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