1. Introduction
The portability of electrical energy is a fundamental part of our daily life, [1] due to the applications of this energy in devices such as cell phones, tablets, laptops, and hybrid, and electric cars. Nowadays, these devices require greater efficiency from batteries [2] . There are numerous reports from studies that have been done on the development of new materials that could improve volumetric and gravimetric properties of batteries [3] .
Since their commercialization in the 1990s, LiCoO2 cathodes have been a conventional component of lithium-ion batteries, [4] [5] [6] due to their high electrical capacity and their electrochemical stability [7] [8] . They have been widely used to enhance electrochemical properties and to provide electrical solutions in response to a growing demand for electrical portability [8] [9] [10] .
Researchers have proposed two methods for improving the performance of LiCoO2 cathodes [1] [11] . The first is to introduce different doping metals into the matrix (Fe, Cr, and M) [12] ; the second is to coat the cathode with different metal oxides (such as Al2O3, B2O3, and TiO2) [13] . Although researchers have focused on using certain groups of chemical elements, they have neglected rare earths, which have been shown to have properties that could be key to overcoming technological challenges [4] [11] .
In 2007, Yanhuai et al. reported on the doping effects of Re = lanthanum, cerium, and praseodymium in Li [Ni1/3Co1/3Mn1/3]1−XReXO2 cathodes, showing that the cathode matrix can be doped with rare earths, provoking a distortion of the crystalline structure at a maximum doping level of lanthanum at 0.03 mol%. The optimization of the cathode’s electrochemical properties was related to crystal deformation of LiNi1/3Co1/3Mn1/3O2 which increased the cell parameters and enabled the lithium ions to diffuse [14] .
The structural stability of a Sm 0.01 mol%-doped LiNi0.5Mn1.5O4 cathode was reported in 2014 by Mingyue Mo et al. [15] ; this low content of Sm in the cathode favored the conductivity of the lithium ion. In 2016, Xudong Meng et al. [16] increased the charge retention capacity of the LiFePO4/C cathode by 96%, following doping with Sm at 6 mol%; this study revealed that samarium ions increase the conductivity and diffusion of lithium ions, improving their electrochemical performances and, as a result, their charge retention capacity.
The inclusion of rare earth elements in Li cathodes is an alternative to satisfy the real high energy source requirements. Therefore, to the best of our knowledge, in order to determine the effects of Sm on the structural and morphological properties of LiCoO2, the system LiCo1−XSmXOy reported in this study was synthesized by the sol-gel method, for the first time. The characteristics of the Sm-doped powders obtained were structurally and morphologically compared with a non-doped system.
2. Experimental Procedure
2.1. Synthesis of Powders
LiCoO2 and LiCo1−XSmXOy powders (X = 0.002, 0.004, 0.006, 0.008 and 0.01) were prepared by the sol-gel method. The synthesis procedure was as follows: 22 mmol of lithium nitrate (LiNO3; Sigma Aldrich) was mixed with 20 ml of distilled water. After 20 min of constant stirring, 20 mmol of cobalt nitrate (Co (NO3)3 8H2O; ≥98% Sigma Aldrich) were added to the solution and magnetically stirred for 40 min to synthesize non-doped LiCoO2 powders. Then an appropriate quantity of samarium nitrate (Sm (NO3)3 8H2O; 99.9% Aldrich) (X = 0.002, 0.004, 0.006, 0.008, and 0.01) was incorporated into the non-doped precursor solution to produce a LiCo1−XSmXOy-doped precursor solution. A second solution was prepared as follows: 0.219 mmol of citric acid (HOC(CO2H)(CH2CO2H)2 H2O; ≥99.0% Sigma Aldrich) was dissolved in 5 ml of distilled water for 20 minutes, and this second solution was then added to the LiCo1−XSmXOy-doped precursor solution. After a 140˚C drying treatment for 2 hours, non-doped and Sm-doped gels were obtained. The resulting powders were heat-treated at 700˚C for 24 hours, to yield LiCoO2 and LiCo1−XSmXOy crystalline powders.
2.2. Characterization of Powders
Both powders, the non-doped LiCoO2 and the doped LiCo1−XSmXOy (X = 0.002, 0.004, 0.006, 0.008, and 0.01) were labeled as shown in Table 1. The crystalline structure of the powders was studied using a Bruker eco D8 Advance diffractometer with Kα1 radiation, with a Cu-Kα radiation source (λ = 1.5406), and 2θ values ranging from 10˚ to 80˚, with a step size of 0.02˚ s−1. The morphological observations were carried out using a scanning electron microscope (JEOL JSM- 6390LV) operated at 15 kV.
3. Results and Discussion
3.1. Structural Properties
The x-ray diffraction patterns obtained from the LiCo1−XSmXOy powders (X = 0.002, 0.004, 0.006, 0.008, and 0.01) (see Table 1), heat-treated at 700˚C, are shown in Figure 1(a).
These powders exhibit a rhombohedral crystalline structure (ICSD 98-004-8103) characteristic of LiCoO2 [17] [18] [19] with no secondary phases. The space group associated with this structure is R-3m (166), known as a high-temperature structure, or HT-LiCoO2 [20] ; it is formed of alternating cobalt and lithium layers with oxygen anions, as reported by Bruno et al. [21] ; a schema of this structure is shown in Figure 1(b).
The rhombohedral structure obtained corresponds to the α-NaFeO2 type [18] [19] . The LT-LiCoO2 to HT-LiCoO2 transformation was caused by the distortion of the face-centered cubic structure (FCC) along the cell parameter c, manifesting itself in a clear separation of the crystalline planes (006) and (012), which are found between 35˚ and 40˚, and planes (018) and (110), located between 65˚ and 70˚ [17] [19] [22] (see Figure 1(a)). Another indicator of hexagonal crystallization is the high intensity of planes (003) and (004), which demonstrates that the desired HT-LiCoO2 structure was obtained [20] [23] .
The shift in the Bragg peaks was analyzed focusing on plane (003) (see Figure 2(a)). The 2-θ degree shifts in the samples were ascertained and compared. The
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Table 1. Terms used and different Sm concentrations of the LiCo1−XSmXOY powders.
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Figure 1. (a) XRD pattern and (b) scheme of the rhombohedral structure.
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Figure 2. (a) XRD patterns of Sm doped LiCo1−XSmXOY powders heat-treated at 700˚C; (b) 2θ values shift of (0 0 3) hkl Bragg reflection.
2θ shift in values between the non-doped powders (LiCoO2) and the Sm-co-doped ones (Sm = 0.002, 0.004, 0.006, 0.008, and 0.1) was 0.11˚. The data obtained are summarized in Table 2. Figure 2(b) shows the increases in degree as the samarium concentration increases to 1 mol%.
To demonstrate the distortion of cell parameters, Rietveld refinement was performed on all the synthesized samples, yielding acceptable confidence values (GOF). The results are shown in Table 3.
Figure 3 illustrates the dimensional changes in cell parameters a, b, and c of the rhombohedral structure. The black line indicates a size increase of 0.0134 Å in parameters a and b; shown in red is an increase in cell parameter c, equal to 0.0183 Å.
By increasing the concentration of Sm in the LiCoO2 crystalline structure, the cell parameters increase (Figure 3). This is due to Sm3+, which has an ionic radius of 1.09 Å [24] [25] , greater than that of Co, at 0.545 Å [26] , and which provokes a distorted crystalline structure.
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Table 2. 2θ values of (0 0 3) hkl Bragg reflection of LiCo1−XSmXOY powders.
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Figure 3. Lattice constants a = b and c for samples heat-treated at 700˚C.
This distortion of the unit cell enables lithium ions to migrate and thereby improves the electrochemical reactions [24] . The integrated intensity ratio between the (003) and (004) Bragg reflections (see Figure 4) indicates the degree of cation order in the LiCoO2 [27] [28] structure. A value of I(003)/(104) higher than 1.2 promises to improve electrochemical performances of the cathode materials [27] [29] . Table 3 shows the values obtained from the integrated intensities corresponding to the LiCoO2 Sm 0.2%, Sm 0.4%, Sm 0.6%, Sm 0.8% y Sm 1% powders. The values for each sample are higher than 1.2, with the highest value corresponding to the sample doped with Sm at 0.1, equal to 1.54. This suggests that the alternating layers of Co and O allowed lithium ions to diffuse during the charge/discharge process.
Crystallite size was estimated using the Scherrer equation (Equation (1)), where k = 0.9, λ = wavelength (1.5418 Å), β = FWHM, and Θ = the Bragg angle [30] [31] . The average size of the LiCoO2, Sm 0.2%, Sm 0.4%, Sm 0.6%, Sm 0.8% y Sm 1% was 30 nm (see Figure 5). The smallest crystallite (24 nm) was observed for the Sm 0.8 mol%-doped powder, and in the samples, the size increased up to 36 nm for the Sm 1 mol% sample.
(1)
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Figure 4. I(003)/I(004) ratios of the LiCo1−XSmXOY powders.
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Table 3. Lattice constants and I(003)/(004) ratios for different LiCo1−XSmXOY powders.
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Figure 5. Crystallite size for Sm doped LiCo1−XSmXOY powders.
3.2. Morphological Results
Figure 6 shows the micrographs obtained by scanning electron microscopy (SEM) for the powders prepared by the sol-gel method, doped at concentrations of Sm 0.2%, Sm 0.4%, Sm 0.6%, Sm 0.8% y Sm 1%, and for the non-doped LiCoO2 system.
Figure 6(a) shows the morphology of the LiCoO2; it is a well-rounded morphology with a particle size of approximately 136 nm. Adding citric acid during the synthesis process generates smaller particles, depending on the citric acid/metal ratio, as reported by Wein-duo et al. [17] .
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Figure 6. SEM micrographs of the (a) LiCoO2, (b) Sm 0.2%, (c) Sm 0.4%, (d) Sm 0.6%, (e) Sm 0.8%, (f) Sm 1% of the LiCo1−XSmXOY powders heat-treated at 700˚C.
Figures 6(b)-(f) show the morphologies of the samples containing Sm 0.002, 0.004, 0.006, 0.008, and 0.1 mol%. When these results are compared with those of the non-doped LiCoO2 powders, it can be seen that the particle size decreases as the concentration of samarium increases. The particle sizes of Sm 0.2%, Sm 0.4%, Sm 0.6%, Sm 0.8% y Sm 1% powders were 126 nm, 123 nm, 122 nm, 121 nm, and 118 nm, respectively.
The TEM images of the Sm 0.4 and Sm 0.8 mol% powders, heat-treated at 700˚C, are shown in Figure 7. Nanometric particles with an interplanar distance between of 0.22 nm and 0.25 nm can be seen, associated with plane (003). This increase in interplanar distance is the result of the insertion of Sm into the crystalline structure of LiCoO2; as mentioned above, when the proportion of Sm increases, the interplanar distance increases as well.
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Figure 7. TEM micrographs of the (a) Sm 0.4% and (b) Sm 0.8% powders, heat-treated at 700˚C.
4. Conclusions
LiCo1−XSmXOY-doped powders (X = 0, 0.002, 0.004, 0.6, 0.008 and 0.01) were synthesized by the sol-gel method and heat-treated at 700˚C. The XRD results confirmed the successful insertion of Sm into the characteristic rhombohedral crystal structure of HT-LiCoO2, with no secondary phases. The displacement of the crystallographic planes, specifically in the (003) plane, and the deformation of the unit cell, reflected in an increase in the network parameters, confirm that the doping was successful. The morphology revealed by SEM shows rounded particles that decrease in size from 136 nm to 118 nm as the proportion of Sm decreases.
The integrated intensities (I(003)/I(004)) indicate an improvement in the electrochemical properties of the powders (Sm 0.002, 0.004, 0.006, 0.008, and 0.1 mol%) when a value higher than 1.2 is reached. In sum, the powders obtained can be considered candidates for use in cathodes in lithium-ion batteries.
Acknowledgements
The authors gratefully acknowledge the financial support of this work by SIP- IPN projects 20211265 and 20211548 and by CNMN and Lab. CREA C.I.I.TEC. IPN for experimental support. Conacyt project A1-S-28234. Víctor H. Colín Calderón acknowledges the Conacyt Ms S scholarship. This work was supported by Instituto Politécnico Nacional (Grant N˚ 31903). The authors also would like to thank Henry Jankiewicz for the editing work that he did for this paper and M. García Murillo for her assistance.