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Document Type : Original Article

Authors

Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq

Abstract

The spinel compound of LiCo0.5Mn1.5O4 (LCMO) was synthesized as cathode active material for lithium–ion batteries by using sol–gel technique in this study. The substance was subjected to a thermo-gravimetric (TGA) study. The sample was calcined in the air for 12 hours at 700, 850, and 1000 ᵒC to examine differences in structural and morphological features. The thermogravimetry (DTG) curve of sample was fabricated at 850 °C. A loss region took place before 400 °C related to the evaporation regarding the adsorbed water molecules and the elimination of oxygen-containing functional groups. The powder crystallized in the phase structure space group Fdm and has a random orientation, according to the XRD measurement. The LCMO powder's surface morphology comprises nano-grains crystallite with a homogeneous coverage of the sample surface and randomly oriented grains. LiCo0.5Mn1.5O4 synthesized at 850°C was taken after one charge/discharge cycle. The interfacial layer resistance, or Rf, is attributed to the impedance of Li+ ion migration through SEI film. From the EIS measurements, the diameter of the semicircle equal to charge transfer resistance, which is approximately 700 Ω. The capacity of the charging and discharging were of 121 mAhg-1 and 122 mAhg-1 delivered by the LiCo0.5Mn1.5O4 cathode. Likewise, a columbic efficiency of 94% was obtained for the cathode.

Graphical Abstract

Study the Effect of Different Temperatures on Structural and Morphological Features of Co-Doped LiMnO4

Keywords

Main Subjects

Introduction

As the specific capacity of lithium-ion batteries based on intercalated cathodes approaches the theoretical value, increasing the energy density becomes significantly difficult. By increasing the battery output voltage, the energy density of the battery may be greatly improved [1, 2]. For a high energy density LIBs, Li [Ni1/2Mn3/2] O4 and LiCoMnO4 (LCMO) show the processes of the lithium insertion at 5V, which has been considered as a highly advantageous characteristic [3, 4]. In spite of several attempts, the substantial liquid electrolyte decompositions at high operating voltage values (5V) make it extremely difficult to increase the cyclability of 5 V materials [5, 6]. 5-V materials have reemerged in recent years as a positive electrode for all-solid-state LIB [7, 8] due to the advantages of solid electrolytes over liquid-based electrolytes in overcoming their constraints. These all-solid states LIBs exceed their conventional liquid-based versions in terms of security, cyclability, high power density, and a wide range of the operating temperatures [9, 10]. Higher energy densities and enhanced dependability are the potential outcomes of using all-solid-state LIBs, which may offer the higher operating voltages. Due to LCMO's excellent dimensional stability, which results in little changes in the lattice's dimensions throughout reactions, it is hoped that it will exhibit a greater cyclability by lowering the risk of electrolyte breakdown. The development of stable contact between active substance and solid electrolyte is significantly aided by this technique [10]. Therefore, LCMO has been considered as one of the best cathode materials for all-solid-state LIB. However, the electrochemical performance achieved so far is below average, with a reversible capacity that peaked at 100 mAh/g (about 60% of the theoretical capacity) and insufficient columbic efficiency (~80%) throughout the charge and discharge [11, 12]. The LCMO charge-discharge behavior is further subpar at high operational voltage values like 5V. Reversible capacities greater than 120 mAh/g require charge-end voltages of at least 5.4 V [13]. In addition to the aforementioned characteristics, the oxygen deficiencies develop inside the material because oxygen is liberated from the crystal lattice of LCMO at high temperatures (>500 °C). Therefore, the conditions under which LCMO was produced have a significant impact on its electrochemical characteristics [14]. This incident compromises the LCMO's stoichiometric, which reduces the substance's electrochemical activity [15].

Materials and Methods

Sol-gel synthesis method was used to prepare LiCoMnO4 cathode material. A 60 mL of distilled water containing dissolved amounts of LiCH3COO·2H2O (99%, fluka, India), Co(CH3COO)2·4H2O (98%, fluka, India), and Mn(CH3COO)2·4H2O (99%, fluka, India), a solution with a Li:Co:Mn stoichiometric ratio of 1:0.5:1.5 was heated to 120 °C while being agitated until a gel formed. In a muffle furnace, gel was heated to 300 °C for six hours before being calcined in the air for 12 hours at 700, 800, and 1000 °C.

Results and Discussion

The XRD patterns of LiCo0.5Mn1.5O4 synthesized at various temperatures, including 700, 850, and 1000 °C are displayed in Figure 1, which shows their spinel structure. The main peaks of the diffraction are designated to (111), (220), (311), (222), (400), (331), (422), (511), (440), and (531) crystal planes of spinel structure [16]. As factors of atomic scattering of the ions of cobalt and manganese are considerably larger than that of the lithium ions, cation distribution of the transition metal ions in spinel structure highly influences the intensity ratio of the diffraction lines in the XRD patterns. To be more specific, the relative intensity of 220 lines is increased in proportion to transition metals’ occupancy at tetrahedral 8a site. The very weak (220) and strong (111) lines of diffraction have shown that cation distribution in spinel structure was Li[CoxMn2-x]O4 in which ions of Mn situated at 16d site in a cubic FCC structure (space group Fdm) are substituted with Co ions [17]. The peak located at roughly 2θ = 45° is ascribed to the 400 diffraction of LiCo0.5Mn1.5O4 [18].

Particle size is one of the most important parameters for characterization of nanostructured materials determined by means of Scherrer formula. The data obtained from the calculation of particle size is approximate and they are based on the peak broadening. The Scherrer formula is as follows:

Where, K represents particle shape factor which equals 0.9, λ represents the X-ray wave-length, βhklrepresents (hkl) reflection’shalf-width, and θ=2θ/2 represents Bragg angle that corresponds to (hkl) reflection [19]. The LiCo0.5Mn1.5O4 nanoparticle crystal sizes that have been estimated at 700, 850, and 1000 °C, respectively, are 53.6, 65.3, and 91.8 nm, implying that the calcination temperature had an apparent impact on the crystallinities of the LiCo0.5Mn1.5O4 nanoparticles.

A few of major diffraction peaks, such as (111), (220), (311), (222), (400), (331), (422), (511), (440), and (531), are either completely absent or have extremely weak signals in XRD sample synthesized at a temperature of 700 °C. However, peaks of (220) and (111) attributed to sample synthesized at 850 °C are visible in the XRD pattern, showing that the spinel structure is Li [CoxMn2-x]O4, where the Mn ions at the 16d site have been replaced by Co ions. This indicates that there is only a tiny (3%) transition metal occupancy at tetra-hedral 8a sites since the intensity of (220) line is < 1% of main peak’s intensity.

Among the samples, the one synthesized at 850 °C manifests the sharpest and strongest peak at 2θ = 45°. It is worth mentioning that increasing the temperature to 1000 °C led to disappearing all the diffraction peaks observed in the sample fabricated at 850 °C. Besides, the temperature decrease from 850 to 700 °C had resulted in a considerable reduction in the intensity of the diffraction peaks. With its good crystallinity, it is expected that the powder synthesized at 850 °C demonstrates better electrochemical performance compared with other samples.

To investigate morphology of synthesized powders, SEM images were taken, as displayed in Figure 2. Clearly, the temperature increase caused an increment in the crystal size of the particles. The particles synthesized at 700 °C demonstrate a size of approximately 100 nm. On the other hand, the sample obtained at 850 °C exhibits a size of roughly 400 nm. The particles subjected to 1000 °C showed the largest size which was more than 1 µm. According to the SEM image of the powder synthesized at 700 °C, the particles indicate near-spherical structure and the surface has not grown efficiently, as depicted in Figure 2a and b. By increasing the temperature to 850°C, the sufficient growth along with an appropriate crystallization occurred, giving rise to the formation of polyhedral structure, as demonstrated in Figure 2c and d, which is consistent with the XRD data. The further increase in the temperature resulted in a rapid growth in the size of the particles, and the sample fabricated at 1000 °C manifests truncated octahedral structure, as illustrated in Figure 2e and f [20]. Based on the XRD data and SEM images, the sample subject to the 850 °C shows the best morphology and crystallinity.

The EDX patterns regarding the synthesized cathodic powders are depicted in Figure 3, and their elemental analysis is listed in Table 1. Accordingly, the presence of the peaks related to Co, Mn, and O elements testifies the LiCo0.5Mn1.5O4synthesis. The atomic percentages of above-mentioned elements are the same.

Figure 1: The XRD patterns of LiCo0.5Mn1.5O4 synthesized at 700 °C, 850 °C, and 1000 °C

Figure 2: The SEM images of LiCo0.5Mn1.5O4 synthesized at a,b) 700 °C, c,d) 850 °C, and e,f) 1000 °C with different magnifications

Figure 3: The EDX patterns of LiCo0.5Mn1.5O4 synthesized at (a) 700 oC, (b) 850 oC, and (c) 1000 oC

Table 1: Elemental analysis of LiCo0.5Mn1.5O4 synthesized at (a) 700 oC, (b) 850 oC, and (c) 1000 oC

Spectrum 4        

Element

Line Type

Weight %

Weight % Sigma

Atomic %

O

K series

51.46

0.32

78.77

Mn

K series

35.15

0.30

15.67

Co

K series

13.39

0.29

5.56

Total

 

100.00

 

100.00

Spectrum 5

 

 

 

 

Element

Line Type

Weight %

Weight % Sigma

Atomic %

O

K series

50.22

0.33

77.93

Mn

K series

36.00

0.31

16.27

Co

K series

13.78

0.30

5.80

Total

 

100.00

 

100.00

Spectrum 1

 

 

 

 

Element

Line Type

Weight %

Weight % Sigma

Atomic %

O

K series

44.43

0.31

73.64

Mn

K series

41.53

0.31

20.04

Co

K series

14.04

0.30

6.32

Total

 

100.00

 

100.00

Raman spectroscopy can be defined as a significant mean to characterize the structure regarding LiCo0.5Mn1.5O4, which provides valuable information about its structure. Figure 4 displays the Raman spectra of LiCo0.5Mn1.5O4 synthesized at 700, 850, and 1000 °C. In Raman spectrum of sample fabricated at 700 °C, a well-defined high intensity band is observed at 639 cm-1, which has been ascribed to symmetric Mn-O stretching mode of MnO6 octahedral (A1g) [21]. Li2MnO3 typically indicates two characteristic bands at about 493 and 612 cm-1 [22]. Accordingly, the band located at 479 cm-1 can be a result of the Li2MnO3presence in sample subjected to the 700 °C. These results are evidences for the LiCo0.5Mn1.5O4synthesis. In Raman spectrum of sample that has been synthesized at 850 °C, two distinguished bands are seen at 500 and 661 cm-1. As mentioned previously, Li2MnO3 typically shows two characteristic bands at approximately 493 and 612 cm-1. In addition, the peaks that are seen at 500 and 661cm-1 could be ascribed to Li2MnO3. Furthermore, the band found at 661 cm-1 is associated with symmetric Mn-O stretching mode regarding MnO6 octahedral (A1g). In comparison with the sample fabricated at 700 °C, the band related to the Li2MnO3 in the sample synthesized at 850 °C show shifts toward a higher intensity. It is worth mentioning that the Li2MnO3 band is not found in the sample fabricated at 1000 °C, which is in agreement with the XRD analysis.

The AFM images of topographical structure of LiCo0.5Mn1.5O4 active material acquired for powders synthesized at 700, 850, and 1000 ᵒC are demonstrated in 3D in Figure 5. The scanning area of this measurement was 5 µm x 5 µm. According to Table 2 and the RMS data regarding the surface roughness, the increase in annealing temperature led to the formation of bigger particles. Surface roughness went up from 0.16 nm in the sample made at 700 ᵒC to 0.2 nm in the powder made at 1000 ᵒC. The particle size also rose, going from 93 nm for the sample made at 700 ᵒC to 1150 nm for the sample made at 1000 ᵒC. The direct impact of the annealing temperature on structure of the LiCo0.5Mn1.5O4 active materials is responsible for the increase roughness in and average particle size. As a result of an increase in ion diffusion, agglomeration, and grain formation, when the temperature rises, the material receives more energy and is more likely to produce large particles [23].

Figure 4: Raman spectra of LiCo0.5Mn1.5O4 synthesized at 700 °C, 850 °C, and 1000 °C

Figure 5. AFM images of LiCo0.5Mn1.5O4

Table 2: The topographical features of LiCo0.5Mn1.5O4 active synthesized at various temperatures

Parameter

700 ᵒC

850 ᵒC

1000 ᵒC

Average grain diameter (nm)

93

350

1150

Average roughness

0.11

0.12

0.14

Root mean square (RMS) (nm)

0.16

0.18

0.2

 

Figure 6 displays the derivative thermogravimetry (DTG) curve of sample fabricated at 850 °C. A loss region takes place before 400 ᵒC, related to evaporation regarding adsorbed water molecules and the elimination of oxygen-containing functional groups. The loss region occurs at 900 ᵒC is relevant to the oxygen loss from the LiCo0.5Mn1.5O4 decomposition [24, 25].

Referring to XRD data and SEM images, the sample subject to the 850 °C demonstrates the best morphology and crystallinity. Therefore, this temperature was selected as the optimum temperature and the electrochemical measurements were carried out for the sample fabricated at this temperature. The Nyquist plot of LiCo0.5Mn1.5O4 synthesized at 850 ᵒC was taken after one charge/discharge cycle, as depicted in Figure 7. Two semi-circles in the high-to-medium range of frequency, along with the linear section in low-frequency range, make up a Nyquist plot in most cases. The interfacial layer resistance, or Rf, or high-frequency semicircle, is attributed to the impedance of Li+ ion migration through SEI film. Along with a line (Warburg impedance, W0), relevant to the solid-phase diffusion of the Li+ ion in most of intercalation compound, the semi-circle at medium frequency (charge transfer resistance, Rct) has been relevant to charge transfer impedance and kinetic of the electro-chemical reactions in the electrode [26, 27]. According to Figure 7, the Rs attributed to the electrolyte resistance is roughly 50 Ω. Furthermore, the semi-circle diameter is equal to charge transfer resistance, which is approximately 700 Ω.

Figure 8 demonstratesthe cyclic voltammetry curves of LiCo0.5Mn1.5O4 synthesized at 850ᵒC. The peak located at about 3.5 V is associated with redox couple Mn3+/Mn4+. In addition, the redox peaks observed at 5/5.5 are ascribed to redox contribution from Co3+/Co4+. Since all manganese in the material stoichiometric is Mn4+, the Mn3+ presence is an evidence for an off- stoichiometric of the material, which is a typical chemical composition of the Fdm [16].

Figure 6: DTG curve of LiCo0.5Mn1.5O4 synthesized at 850 °C

Figure 7: Nyquist plot of LiCo0.5Mn1.5O4 synthesized at 850 ᵒC

Figure 8: Cyclic voltammetry curve of LiCo0.5Mn1.5O4 synthesized at 850 ᵒC

The first charge/discharge cycle of LiCo0.5Mn1.5O4 cathode, which was produced at 850 ᵒC, as depicted in Figure 9. It was captured by using a C/10 rate in the 3.0 to 5.5 V voltage range. The minor oxygen non-stoichiometry degree associated with the addition of more Mn3+ component was coupled with a little plateau located about 4.0. Two plateaus at about 5.5v have been a result of the structural reorganization occurring throughout lithium de(intercalation). The LiCo0.5Mn1.5O4 cathode had an approximate charge capacity of 121 mAh/g and an approximate discharge capacity of 122 mAh/g, indicating that the capacity loss in the first cycle was minimal. Likewise, a columbic efficiency of roughly 100% was obtained for the cathode, meaning that the capacity fade between the discharge and charge cycles was negligible.

Figure 9: The first charge/discharge cycle of LiCo0.5Mn1.5O4 synthesized at 850ᵒC recorded at C/10 rate in the range of voltage between 3 and 5.5 V

Conclusion

The sol-gel process was used to successfully synthesize LiCo0.5Mn1.5O4 for lithium–ion battery cathode. Likewise, the structural and compositional studies have shown that these materials can be synthesized in spinel structures with a high crystallinity and well-defined compositions. Moreover, Raman results accord with the XRD results in terms of the order of phase changes and temperatures at which they happen. The stoichiometric synthesis of LCMOs was achieved by synthesizing LCMOs under stable conditions (crystallized at 850 ᵒC for 12 hours) and tuning their particle size (controlled by crystallization temperature) to achieve the greater reversible capacity. The LiCo0.5Mn1.5O4 cathode indicated around 121 mAhg-1 charge capacity and approximately 122 mAhg-1 discharge capacity. A columbic efficiency of 94% was obtained for the cathode. This indicates that there was little any capacity fading between the discharge and charge cycles.

Acknowledgments

The authors are grateful for the University of Baghdad, College of Science, Department of Physics, for providing the paper all the supporting tools and materials

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contributions

All authors contributed to data analysis, drafting, and revising of the paper and agreed to be responsible for all the aspects of this work.

Conflict of Interest

There are no conflicts of interest in this study.

HOW TO CITE THIS ARTICLE

Waleed K. Mahmooda, Asama N. Najea. Study the effect of different temperatures on structural and morphological features of Co-Doped LiMnO4. Chem. Methodol., 2022, 6(12) 985-996

https://doi.org/10.22034/CHEMM.2022.354750.1588

URL: http://www.chemmethod.com/article_156791.html

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