Abstract
A lithium-rich manganese-based cathode material, Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2, was prepared using a polyvinyl alcohol (PVA)-auxiliary sol-gel process using MnO2 as a template. The effect of the PVA content (0.0–15.0 wt%) on the electrochemical properties and morphology of Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 was investigated. Analysis of Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 X-ray diffraction patterns by RIETAN-FP program confirmed the layered α-NaFeO2 structure. The discharge capacity and coulombic efficiency of Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 in the first cycle were improved with increasing PVA content. In particular, the best material reached a first discharge capacity of 206.0 mAhg−1 and best rate capability (74.8 mAhg−1 at 5 C). Meanwhile, the highest capacity retention was 87.7% for 50 cycles. Finally, electrochemical impedance spectroscopy shows that as the PVA content increases, the charge-transfer resistance decreases.
1. Introduction
Rapid development of lithium-ion battery technology and the environmental impact of traditional non-renewable energy sources are driving considerable interest in efficient “green” energy storage devices [1]. Currently, the energy storage devices of new energy vehicles and various electronic devices (laptops, cell phones, power tools, bluetooth devices, etc.) usually use lithium ion secondary batteries (LIBs). LiCoO2 was the first cathode material used for commercial LIBs because of its high operating voltage and ease of preparation [2, 3]. But, the high cost, and toxicity of layered LiCoO2 limit its use for large-scale high-power applications. Replaceable cathode materials, Li1.2Ni0.2Mn0.6O2, LiMn2O4, LiFePO4, LiNi1/3Co1/3Mn1/3O2, and LiNi0.8Co0.15Mn0.05O2, have previously been synthesized [4–8]. Despite its unsatisfactory capacity (120–140 mAhg−1), the layered-structure LiNixCoyMn1−x−yO2 is considered a promising cathode material because of its relatively low cost and reduced toxicity [9, 10]. To address the capacity issue, researchers have launched a series of explorations to identify high-capacity lithium storage cathode materials.
The specific capacity and stability of LIBs can be effectively improved by doping or changing the surface morphology or through surface modification. Mohan et al. synthesized LiFexNi1−xO2 (0.00 ≤ ≤ 0.20) nanoparticles with a single-layer structure using a sol-gel approach. The particle size was reduced with the substitution of iron, thereby improving the electrochemical properties of the cathode material [11]. The surface-modified layer of the cathode improves the cycle efficiency and thermal stability for high-rate discharge and improves the conductivity of the material surface [12–14].
In this work, we used the well-known sol-gel process to synthesize the target material Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 by controlling the PVA content (0.0–15.0 wt%). Scanning electron microscopy (SEM), electrochemical tests, and X-ray diffraction (XRD) were used to investigate the relationship between the PVA doping ratio and specific capacity of the synthesized cathode materials. Pretreatment can significantly improve the rate performance and initial coulomb efficiency and also help to improve cycle stability and suppress voltage attenuation. These properties are expected to improve the capacity utilization and rate capability of this material [15].
2. Experimental Section
2.1. Material Synthesis
Using a method previously reported in the literature, MnCO3 microspheres were prepared. First, 1.183 g MnSO4 was dissolved in a mixture of ultrapure water (conductivity of 0.05 μScm−1) and alcohol labeled “A.” Then, 5.53 g NH4HCO3 was dissolved in 490 ml of ultrapure water without alcohol labeled “B.” When both A and B are fully dissolved, the two solutions A and B are mixed together to form a suspension. The suspension was stirred constantly at room temperature for 5 h. Then, the white precipitate in the suspension was separated by centrifugation; the precipitate was identified as MnCO3. Next, the MnCO3 was washed with a mixed solution of alcohol and ultrapure water. After cleaning, place MnCO3 on a drying table and dry it in a 60°C air atmosphere for 6 h.
Second, MnO2 hollow microspheres were prepared by mixing the precursor MnCO3 and an aqueous 0.033 M KMnO4 solution. After removing the MnCO3 core with 0.03 M HCl, MnO2 hollow microspheres were obtained after centrifugation. The microspheres were cleaned 3 times with ultrapure water. Next, MnO2 was dried using a drying oven under an air atmosphere.
Finally, Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 was prepared using the following PVA-auxiliary sol-gel process. Weigh a certain amount of LiOH·H2O, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, the MnO2 microspheres were dissolved in 60 ml of ultrapure water, and the lithium/metal (Ni, Mn, Fe) molar ratio is 1.25. Since lithium is depleted under high temperature conditions, it is necessary to add an excessive amount of lithium. In order to ensure sufficient reaction of raw materials, PVA and citric acid were added successively in different time. The mass fractions of PVA were = 0.0, 2.5, 5.0, 7.5, 9.0, 10.0, 11.0, 12.5, and 15.0 wt%. The molar ratio of citric acid to total metal ions was 1:1 [16]. The mixture was then stirred at 80°C until a viscous gel was obtained [17]. The treatment of the precursor was divided into two steps, first drying at 120°C for 12 h and then heating to 450°C for 6 h in air.
The final products were obtained by finely grinding the gel followed by heat treatment at 950°C under a non-rare gas atmosphere for 12 h.
3. Results and Discussion
3.1. Sample Characterization
We first investigated the structural features of the MnO2 hollow microsphere samples and PVA-auxiliary lithium-rich manganese-based cathode samples using SEM. The spherical shell structure and broken hollow microspheres of the MnO2 are shown in Figures 1(a) and 1(b), respectively. Both of their diameters were approximately 1 – 2 μm. All of the samples exhibited a shell structure that was gradually destroyed with increasing while the layered structure was aggregated together, which greatly increased the contact surface area in the scanned area. The tendency toward aggregation decreased with increasing in Figures 1(c)–1(i). For x = 10.0 wt%, the sample has the smallest particle size and the most uniform distribution. It is well known that electrochemical lithium insertion/extraction is much easier in small particles, because of the reduction of diffusion pathways for Li+ ions, which also enables faster electronic transport through the size effect [19].
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4. Characterization
The X-ray diffraction (Rigaku SmArtlab-3KW) patterns were collected on a D8 Advance instrument with Cu Kα radiation operated at 40 KV, 40 mA and scanning step 2 degrees per minute at room temperature from 10° to 80°. The XRD results were analyzed using the RIETAN-FP program. SEM (JEOL JSM-6390) was usually applied to analyze the morphology of the resulting materials.
Electrochemical Measurements. The electrochemical measurements of terrestrial CR2025 batteries were performed at room temperature using CR2025 coin cells. The active substances, acetylene black and poly (vinylidene fluoride) were dissolved in N-methyl-2-pyrrolidone (NMP) at a ratio of 8:1:1, and then the cathode compounds were evenly coated on the aluminium foil by a smearing machine. Firstly, the smear was dried in 50°C air for 6 hours, and then dried in 120°C vacuum for 12 hours. The coin cells were assembled in a glove box (H2O < 1ppm, O2 < 1ppm) using Li metal foil as the counter electrodes and 1 M LiPF6 in a 1:1 v/v mixture of ethylene carbonate and diethyl carbonate as the electrolyte. The assembled battery was subjected to a charge and discharge cycle test at different magnifications, and the voltage range was controlled within 2.5 to 4.5 V. The electrochemical impedance spectroscopy spectra were obtained in the frequency range of 10−1 Hz to 105 Hz using a PARSTAT 2273 electrochemical work station [18].
Figure 2 presents a prototypical example of the XRD powder pattern of Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 (x = 10.0 wt%). The pattern was obtained using the RIETAN-FP program. The powder pattern reproduces well the typical features of the α-NaFeO2 structure and can be indexed as the R-3m space group, as indicated by the solid curve. The splitting of the (006), (012) and (018), (110) reflections suggests the formation of a highly ordered hexagonal layered structure [20]. We further performed a similar Rietveld analysis for other compounds, and software analysis results are shown in Table 1. The final refinements were satisfactory, with and (reliable factor based on the integrated intensity) being sufficiently small. The lattice parameters of all the samples differed slightly. Figure 3 shows the regularity of (a) lattice parameters a and c and (b) lattice volume V with the change of PVA mass fraction. The lattice parameters had minimum values at x = 10.0 wt%, indicating the stability of the lattice of these samples. Compared with = 0.0 wt%, the lattice volume of Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 (x = 10.0 wt%) was reduced by 0.94%.
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The electrochemical performance of the original and PVA-auxiliary lithium-rich manganese-based cathode samples was investigated at 2.5 and 4.5 V at different rates using lithium metal as the anode. The charge curves for various cycles at a rate of 0.1 C are presented in Figure 4(a). The smooth curves indicate that the electrode material had a stable structure within the tested voltage range [21]. For the pristine sample, the first discharge capacity was 141.5 mAhg−1 and after 50 cycles, it faded to 122 mAhg−1, and the capacity loss was 16%.
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For the PVA-auxiliary lithium-rich manganese-based sample (x = 10.0 wt%), the first discharge capacity was 206.0 mAhg−1 and after 50 cycles, it faded to 180.7 mAhg−1, and the capacity loss was only 14%. It can thus be concluded that the use of a moderate PVA-auxiliary sol-gel process can enhance the specific capacity of the sample. This result occurs because the surface area is affected by the PVA content, and the increased surface area provides favorable conditions for the lithium ion insertion/extraction process. As the number of cycles increases, the discharge capacity of all samples decreases slowly, most likely due to structural deformation and/or side reactions during charge and discharge.
Figure 4(b) shows the cycle performances of all the materials under different rate. Classically, at high current rates, the specific volume drops, which may be due to an increase in electrode polarization during the cycle observed by previous researchers.
Under the various rates, specific capacities of PVA-auxiliary sample (x = 10.0 wt%) are always higher than other samples. Especially in 2 C rate, compared with PVA-auxiliary sample (x = 0.0 wt%), PVA-auxiliary sample (x = 10.0 wt%) has 2.5-fold superior rate capabilities. More importantly, when the magnification returns to 0.1 C, the capacity can be closest to the initial value. This suggests that the cathode material (x = 10.0 wt%) has an excellent reversibility. The results in Figures 4(a) and 4(b) indicate that the optimal electrochemical performance of coin cells was achieved for the PVA-auxiliary sample (x = 10.0 wt%).
To determine the rate capability of the sample, the battery was cycled between 2.5 and 4.5 V at different current rates of 0.1 C. The results are shown in Figure 5. The rate capability of the PVA-auxiliary sample is significantly higher than the original sample. Compared with other samples, the PVA-auxiliary sample (x = 10.0 wt%) has an excellent rate performance, because the PVA-auxiliary method reduces the charge-transfer resistance and enhances the reaction kinetics [22].
To study the reaction kinetics of the electrode materials, as shown in Figure 6, the EIS is tested with three electrode units in the frequency range of 10−1 to 105 Hz. In this region, a slanted line and semicircle were observed. The Z’-intercept between the composition of the battery and the electrolytic resistance in the high frequency region corresponds to the ohmic resistance ().
The middle-frequency semicircle is associated with the charge-transfer resistance () at the interface of the electrode and electrolyte [23]. The lithium ion scattering relationship in the cathode material is reflected on the oblique line of the low velocity region of the Warburg impedance (). The charge-transfer resistance of the x = 10.0 wt% sample was 300 Ω, which is smaller than that of the pristine sample (400 Ω). One possible explanation for this result is that the ion transport of the PVA-auxiliary sample is relatively faster. The reduction in charge transfer resistance clearly demonstrates the importance of electron conductivity and Li+ diffusion when cycling at high current rates, which is closely related to rate capability and charge and discharge cycle data.
5. Conclusions
In this study, a lithium-rich manganese-based cathode material, Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2, was synthesized using a PVA-auxiliary sol-gel process, and the effects of the PVA content on the crystal structure and morphology were investigated using XRD and SEM analyses. Compared with the pristine material, the PVA-auxiliary material (x = 10.0 wt%) after the 50 cycles of 0.1 C, the capacity holding rate is 27.7%, and the capacity at 5 C is 74.8 mAhg−1. The improved performance of the PVA-auxiliary sample resulted from the effective reduction of the charge-transfer resistance and enhancement of the reaction kinetics. Because of the role of PVA, the rate capability and circulation of Li1.25Ni0.2Co0.333Fe0.133Mn0.333O2 are improved.
Data Availability
The parameters and data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (NSFC, 11144007) and National Natural Science Foundation of Shandong Province (ZR2016AM27).