3.1 CRYSTALLOGRAPHY
Figure 2a shows the diffractogram of the NMC333 powder used to produce the target. Narrow and intense peaks indicate a compound with a high order of crystallinity, indexed to a α-NaFeO2 hexagonal layered structure, with space group R-3m [7, 9, 26]. Based on the diffractogram and applying the Scherrer equation, while using silicon as an instrumental-standard sample for determining the FWHM value, the crystallite size of the target was estimated in 24.5 nm [17].
The diffraction patterns of as-deposited TCM films under different conditions are shown in Fig. 2b. All diffractograms exhibited three peaks: the first, around 35º (2θ), belonging to the Ti phase; the second, at 38.0° (2θ), corresponding to the cubic phase of a conductor aluminum film with preferred orientation towards the direction of [111]. The highest intensity peak of aluminum can be attributed to the structure factor (F(hkl)) of an FCC-type lattice, which implies an increase in the intensity of the scattered X-rays. The third, at 44.0° (2θ), is related to the TCM film whose Miler index is (104). The set of planes appears to be spatially oriented in parallel to the substrate, in the [104] direction, and his preferential orientation may be due to the minimization of the surface energy, since these films have a lesser thickness [20, 27, 28].
The crystallite size of the TCM films are shown in Table 2. Table 3, by its turn, presents the correlations between the deposition parameters (power and pressure). It can be seen that the power has a subtle, but statistically significant, effect on the size of the crystallite, while the pressure does not seem to influence this response. The positive value of the main effect related to power indicates that an increase in this factor causes an increase in the crystallite size. Such occurrence is expected because, with the rise of the sputtering power, the atoms ejected from the target reach the substrate with greater kinetic energy, favoring the crystallization of the film. The pressure main effect and the incidence of interaction values close to zero indicate that these effects exert no influence on the crystallite size.
Table 2
Crystallite size calculated by the Scherrer equation. The value of the k factor chosen was 0.9 and the error was estimated from the FWHM
Sample
|
Crystallite size (nm)
|
Po + Pr+
|
4.93 ± 0.53
|
Po-Pr-
|
3.51 ± 0.57
|
Po + Pr-
|
4.85 ± 0.32
|
Po-Pr+
|
3.21 ± 0.15
|
Table 3
Table of effects for the 22 factorial design of crystallite sizes
|
Effects
|
Crystallite size (nm)
|
Main Effects
|
Power
|
1.53 ± 0.1
|
Pressure
|
-0.11 ± 0.1
|
Interaction
|
Power-Pressure
|
0.19 ± 0.1
|
The intrinsic crystallinity of the film obtained via RFMS deposition is an important factor, which demonstrates the feasibility of this process in obtaining crystalline films. Furthermore, such factor is crucial for eliminating post-thermal treatments, allowing for the manufacture of electrodes especially designed for flexible micro-devices, since they do not support high temperatures.
3.2 ElemeNtal quantification
Table 4 presents elemental quantitative results obtained from the X-ray fluorescence of the ternary films. Through the analysis of the data, it is possible to notice that the films presented a stoichiometry different from that of the target. All TCM films showed a high concentration (in mol %) of manganese (45%), an intermediate concentration of cobalt (30%), and a low concentration of nickel (25%). The discrepancy between stoichiometry values (1/3:1/3:1/3) can be due to the different sputtering yields for each atomic species. According to the factorial planning, and considering that no specific order was followed during the depositions, XRF results revealed that the deposition parameters (power and pressure) do not have a great impact on the stoichiometry of the films.
Table 4
Results of energy-dispersive X-ray fluorescence for planned TCM films
Element
|
Po + Pr+
(mol %)
|
Po + Pr-
(mol %)
|
Po-Pr-
(mol %)
|
Po-Pr+
(mol %)
|
Mn
|
44.45
|
44.35
|
43.28
|
42.70
|
Co
|
31.64
|
29.95
|
31.40
|
31.67
|
Ni
|
26.00
|
25.68
|
25.31
|
25.62
|
3.3 ATOMIC FORCE MICROSCOPY
Figure 3 exhibits the atomic force microscopy (AFM) images of the surface of (a) clean glass - g; (b) titanium-coated glass - g/Ti; (c) aluminum-coated glass - g/Al; and (d) glass initially covered with titanium, then by aluminum - g/Ti/Al. Titanium was used as an anchoring layer not only due to its strong bond with the oxygen present in the glass substrate, but also due to its small atomic size, close to those of aluminum and silicon, which strengthen the adherence of the aluminum layer, contributing to lower surface tensions. Roughness values are shown in Table 5. The titanium film seems to make surfaces smoother, whether it is deposited on top of glass (b) or under aluminum (d). This effect causes a decrease in the number of dispersion centers for conduction electrons, located in grain boundaries, causing a decrease in the resistivity [29].
Table 5
Substrate roughness calculated with basis on AFM images, using the Gwyddion 2.53 software, in an area of 5x5 µm2
Sample
|
Roughness
(nm)
|
Glass
|
1.7 ± 0.6
|
Glass-Ti
|
2.0 ± 0.3
|
Glass-Al
|
4.2 ± 0.6
|
Glass-Ti-Al
|
2.1 ± 0.5
|
AFM images of the glass/Ti/Al/TCM films are shown in Fig. 4, in a way to facilitate the analysis of the influence of deposition parameters on the roughness and on the shape of surface grains. When horizontally comparing the images, we can observe the influence of the power on the morphology of the film, while a vertical comparison allows for an analysis of the influence exerted by the deposition pressure. Roughness values are presented in Table 6 and geometrically interpreted in Fig. 4. Results indicate that the highest roughness value was obtained under (Po-, Pr+) conditions. Additionally, the main effect of power occurs due to the kinetic energy of the ions colliding with the target. An increase in power (Po+) causes the atoms to be ejected from the target with greater kinetic energy, which leads to a faster coalescence and favors the formation of a flatter surface.
Table 7 shows the analysis concerning the main effect of Po and Pr, along with the interactions between the two parameters. Power and pressure influenced the roughness response, but the negative value (-7.2) assumed for their interaction not only suggests a synergic effect between them, but also an inversely proportional dependence encompassed by both parameters and the roughness coefficients. Therefore, considering their interdependency, it is not recommended to individually analyze each parameter, i.e., only their main effects. However, we can not neglect the significant main effect of power (-15.3), which indicates a major influence on roughness, but in an inverse mode (should the power increase, the roughness decreases).
Table 6
Roughness of TCM thin films calculated with basis on AFM images, using the Gwyddion 2.53 software, in an area of 5x5 µm2
Sample
|
Roughness
(nm)
|
Po + Pr+
|
6.9 ± 1.2
|
Po-Pr-
|
14.6 ± 2.8
|
Po + Pr-
|
6.5 ± 0.9
|
Po-Pr+
|
29.5 ± 4.7
|
Table 7
Table of effects for the 22 factorial design of the roughness (nm) of TCM thin films
|
Effects
|
Roughness (nm)
|
Main Effects
|
Power
|
-15.3 ± 2.8
|
Pressure
|
7.6 ± 2.8
|
Interaction
|
Power-Pressure
|
-7.2 ± 2.8
|
A scanning electron microscopy (SEM), detailed in Fig. 5, revealed that the surface of the films was free from deep cracks, which indicates a good adhesion of the Ti/Al films to the glass substrate. The spherical structures observed in all images differ in diameter, suggesting that pressure was the main acting factor, once Pr + films presented smaller grains than those of Pr- (vertical analyses). Additionally, if we compare the images concerning the deposition power, it is possible to observe that Po + films tend to be smoother and more compact, with feasible alterations in their density. However, no appreciable difference was observed between Po- and Po+ (horizontal analyses). After confronting Po-Pr- and Po + Pr-, it was possible to observe the presence of spheres with a larger diameter when compared to those of the samples of the top row. Furthermore, Po + Pr- showed grains linked to each other. Po + Pr+, by its turn, also presented joint grains, if compared to Po-Pr+. Such findings are in agreement with the roughness obtained via AFM, showing that these samples are smoother than Po-Pr + and Po-Pr-.
3.4 FOURIER-TRANSFORM INFRARED SPECTROSCOPY
Figure 6 shows the FTIR spectra of Li(Ni1/3Co1/3Mn1/3)O2 powder and of the films deposited by RFMS, as planned by the factorial design of experiments. Results show that the ternary lithium oxide was successfully formed, as indicated by the characteristic peaks for layered oxides with α-NaFeO2 structure, in a wavenumber range of 800 − 400 cm− 1. Absorption bands localized at 440 − 420 cm− 1 can be attributed to the Li-O bond in LiO6 octahedrons [30]. Bands at 642–648 cm− 1, by their turn, are result of the asymmetric stretching of M-O (M = Ni, Co, Mn), and the band at 525 cm− 1 is due to the O-M-O bending [30, 31]. The M-O stretching occurs because the metal occupies an (MO6) octahedral site [32–34]. The band at 950 cm− 1 can be attributed to the Al-O bound [35], and the band at 870 cm− 1 can be related to the CO32− out-of-plane bending [36]. Bands at 1613 and 1443 cm− 1 can be respectively associated attributed to COO− antisymmetric and symmetric stretches [34, 36], probably due to the presence of lithium carbonate. The band at 1613 cm− 1 can be attributed to the O-H bending [31], and the bands at 2850, 2920, 3165 and 3563 cm− 1 can be associated to the O-H stretching [31, 34, 37].
3.5 ELECTROCHEMICAL CHARACTERIZATION OF NMC333 films
The obtained open-circuit voltages of the lithium half-cells containing NMC films were: Po + Pr + = 3.5 ± 0.2 V, Po-Pr- = 3.5 ± 0.1 V, Po + Pr- = 3.3 ± 0.1 V, and Po-Pr + = 3.4 ± 0.1 V. These values are compatible with the ones expected for lithium half-cells containing lithium transition metal oxides.
Figure 7 presents the chronopotentiometric cycles of the films, in a potential range between 2.20 and 4.20 V, at a current density of 15.0 mAg− 1. Po + Pr + and Po + Pr- films exhibited lower discharge capacities when compared to those reported for conventional bulk NMC333 electrodes, while Po-Pr- and Po-Pr + showed a significant improvement in capacity. In general, the obtained discharge capacities were higher than those of conventional cathode composites, which require the addition of binders and electronic conductors [6, 26]. Furthermore, the attained values were close to the theoretical capacity, showing that all lithium ions could be reversibly de-intercalated from the host material structure. The Po-Pr- sample, on the other hand, showed good reversibility up to the ninth cycle and presented a high discharge capacity (250 mAhg− 1), even with a cut-off potential of 4.20 V. The increase in the discharge capacity, observed from the first to the ninth cycle, can be attributed to the lattice accommodation and to the activation of more ionic intercalation sites. The discharges capacities are summarized in Table 8.
Table 8
Discharge capacity for TCM thin films at 9th galvanostatic discharge, tested at a current density of 15.0 mAg− 1.
Sample
|
Discharge Capacity
(mAhg− 1)
|
Po + Pr+
|
100 ± 2
|
Po-Pr-
|
250 ± 2
|
Po + Pr-
|
45 ± 2
|
Po-Pr+
|
180 ± 2
|
Table 9 describes the effects exerted by deposition parameters on the discharge capacity. The high negative value of the power main effect (-167.1) indicates that an increase in this parameter would cause a decrease in the discharge capacity, whereas the opposite is valid for the pressure (since it exhibited a positive value). Due to its high value, the power effect exerted more influence on the response of the discharge capacity. On the other hand, as far as roughness is concerned, the interaction effect was positive, showing a synergistic interaction between Po and Pr. In this case, though, the positive value demonstrates that both parameters directly influenced the response.
Table 9
Table of effects for the 22 factorial design of the discharge capacity (mAhg− 1), at a current density of 15.0 mAg− 1
|
Effects
|
Discharge Capacity
(mAhg− 1)
|
Main Effects
|
Power
|
-167.1 ± 3.0
|
Pressure
|
53.4 ± 3.0
|
Interaction
|
Power-Pressure
|
25.0 ± 3.0
|
The highest capacity was observed in films deposited with lower power (Po-), demonstrating its dependency on physical properties, like crystallite size and roughness. A small crystallite size improves the kinetic performance, whereas the elevated roughness can generate a greater number of lithium diffuser channels and expand the contact area between the film and the electrolyte, improving the diffusion of lithium ions in the material and increasing its charge capacity [38, 39].