Abstract
High capacity Li2MnSiO4/C nanocomposite with good rate performance was prepared via a facile sol-gel method using ascorbic acid as carbon source. It had a uniform distribution on particle size of approximately 20 nm and a thin outlayer of carbon. The galvanostatic charge-discharge measurement showed that the Li2MnSiO4/C electrode could deliver an initial discharge capacity of 257.1 mA h g−1 (corresponding to 1.56 Li+) at a current density of 10 mA g−1 at 30°C, while the Li2MnSiO4 electrode possessed a low capacity of 25.6 mA h g−1. Structural amorphization resulting from excessive extraction of Li+ during the first charge was the main reason for the drastic capacity fading. Controlling extraction of Li+ could inhibit the amorphization of Li2MnSiO4/C during the delithiation, contributing to a reversible structural change and good cycling performance.
1. Introduction
Environmental pollution and energy crisis promote people to search for renewable resources and energy storage device, such as batteries. Rechargeable Li-ion secondary batteries are attracting more and more attention due to their high energy density and environmental friendly features [1]. Recently, orthosilicate Li2MSiO4 (M = Fe, Co, Mn) materials were regarded as the candidate for cathodes [2–4] due to their high theoretical capacity (e.g., Li2MnSiO4: 330 mA h g−1). Among this family, Li2MnSiO4 is much easier to achieve the transformation of Mn2+/Mn3+ and Mn3+/Mn4+ to carry out extraction of two Li+ in comparison to other orthosilicates [5, 6]. Li et al. [7] prepared Li2MnSiO4/C through a solution route. The composite electrode exhibited a discharge capacity of 209 mA h g−1 at the initial cycle and 140 mA h g−1 after ten cycles. Bhaskar et al. [8] fabricated Li2MnSiO4/C by a facile nanocomposite gel precursor route. This composite electrode showed an initial discharge capacity of 330 mA h g−1 and a stable discharge capacity of 115 mA h g−1 for 30 cycles at room temperature. Devaraju et al. [9] adopted supercritical fluid process to synthesize Li2MnSiO4/C with a mean particle diameter of 4-5 nm. It showed a high capacity of about 320 mA h g−1 at the first cycle and 190–220 mA h g−1 after 50 cycles. Carbon coating plays a critical role in the improvement of the electrical conductivity of the Li2MnSiO4 and consequently enhancement of discharge capacity. However, this material still has a large irreversible capacity loss and low capacity retention rate upon cycling. Structure change resulting from J-T effects and the dissolution of Mn caused by the disproportionate reaction (Mn3+ → Mn4+ + Mn2+) were reported to be among other reasons for the drastic capacity fading of the Li2MnSiO4 material [6–8, 10, 11]. Recently, a new possible explanation regarding the amorphization transformation of the Li2MnSiO4 material during lithium ions extraction/insertion was also reported [7, 12, 13]. Although the theory has been put forward by Dominko et al. [13] in 2005, this explanation was not reinforced by substantial experimental results.
Herein, we prepared the Li2MnSiO4/C nanocomposite with a homogenous carbon distribution by a facile sol-gel method. The structure, morphology, and electrochemical performance of the Li2MnSiO4 materials were characterized by XRD, SEM, TEM, galvanostatic charge-discharge test, and CV. Possible capacity fading mechanism was presented.
2. Experimental
2.1. Synthesis
Li2MnSiO4/C nanocomposite was prepared by a facile sol-gel method using LiCH3COO2H2O, Mn(CH3COO)24H2O, and Si(OC2H5)4 as raw materials. Ascorbic acid served as chelating agent and carbon source. A solution containing Li and Mn in the molar ratio of 2 : 1 was stirred for half an hour, followed by the addition of ascorbic acid in aqueous solution, and thereafter, Si(OC2H5)4 dissolved in ethanol was added. The mixed solution was continuously stirred at 80°C for 24 h and then heated in oven at 80°C overnight. Thereafter, the obtained xerogel was calcined at 650°C for 10 h in a furnace with a constant flow of argon and then left to cool down to room temperature. In order to make comparison, pure Li2MnSiO4 was prepared by similar process without the addition of ascorbic acid.
2.2. Measurements
X-ray powder diffraction (XRD) was collected by diffractometer (SmartLab, Japan) with Cu K radiation. The scanning angle 2 was ranging from 10 to 80° at a step of 0.02°. The morphology and particle distribution were observed by field emission scanning electron microscopy (FE-SEM, HITACHIS-4800, Japan) and transmission scanning electron microscopy (TEM, JEM-2100F, Japan). Carbon content in the composite was measured by elemental analyzer (Vario EL III, Germany).
The electrochemical performance of the material was characterized in a CR2032 coin cell. The working electrode was prepared by mixing 80 wt.% active materials and 10 wt.% carbon black with 10 wt.% polyvinylidene fluoride (PVDF) via using N-methyl-2-pyrrolidone as a solvent and thenceforward pasted the slurry on Al foil and subsequently dried in vacuum at 100°C overnight. The coin cell was assembled in a glove box filled with high pure argon using Li metal as the counter and reference electrode, Celgard 2400 film as the separator, and 1 M LiPF6 dissolved in EC and DMC (1 : 1 w/w) as the electrolyte. Galvanostatic charge-discharge measurements (Land, China) were performed in the voltage range of 1.5–4.8 V or 1.5–4.15 V at the current density of 10 mA g−1 at 30°C. Cyclic voltammogram (CV) was carried out by CHI660D between 1.5 and 4.8 V at a scanning rate of 0.1 mV s−1.
In order to investigate the structural change during charge-discharge process, the tested batteries were disassembled, and then the cathodes were washed by dimethyl carbonate (DMC) and alcohol and then dried in vacuum at 50°C. Ex situ X-ray diffraction analysis was performed on electrodes which were charged or discharged to different states using Cu K radiation with 2 from 10 to 60° at a step of 0.02°.
3. Results and Discussion
3.1. Structure and Morphology
Figure 1 shows the XRD patterns of as-prepared Li2MnSiO4 and Li2MnSiO4/C composite. The observed diffraction peaks of both samples are in good well agreement with orthorhombic structure with a space group of Pmn21, which are the same as the previous reports [14–16]. Compared to the XRD pattern of Li2MnSiO4, the diffraction peaks of Li2MnSiO4/C seem to be somewhat weaker, indicating that the presence of carbon could prevent growth of Li2MnSiO4 grains, thus decreasing the particle size. There are no obvious peaks of carbon observed maybe due to the low proportion or amorphization.
SEM image in Figure 2(a) presents the typical morphology of the Li2MnSiO4/C composite, which is composed of nanoparticles with an aggregation. TEM picture (Figure 2(b)) shows that it possesses the irregular shape and a uniform size distribution with a mean size of approximately 20 nm. Carbon wraps the Li2MnSiO4 particle to form a uniform coating with a thickness of 5 nm, as shown in Figure 2(c). This carbon film could suppress the particle growth during heat treatment and therefore shorten the pathway of lithium ion migration and effectively improve the electrical conductivity [17, 18]. The carbon content of the Li2MnSiO4/C composite is determined to 17.08 wt.% by the elemental analyzer.
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3.2. Electrochemical Performance
Figure 3 shows the CV curves of the Li2MnSiO4/C electrode. Two cathodic peaks located at 2.6 V and 4.4 V during first charge may be attributed to the side reaction and Mn oxidation, respectively. But during first discharge, it shows two broad reduction peaks located at 2.9 and 3.7 V maybe due to Mn3+/Mn2+ and Mn4+/Mn3+, respectively. In subsequent cycles, the CV profile displays no obvious discrepancy, demonstrating that the Li2MnSiO4/C electrode has a good electrochemical reversibility.
Figure 4 shows the charge and discharge curves of Li2MnSiO4 and Li2MnSiO4/C electrodes for the initial cycles. The discharge capacity at the first, second, and fifth cycles of Li2MnSiO4/C composite is 257.1, 250.2, and 225.4 mAh g−1, respectively, while the corresponding values are only 25.6, 23.8, and 25.9 mAh g−1 for Li2MnSiO4. The rate performance of the electrodes after the first cycle is presented in Figure 5. The discharge capacities of the Li2MnSiO4/C electrode are 250.1, 180.4, 168.5, and 32.9 mAh g−1 at 0.03 C, 0.1 C, 0.5 C, and 1 C (1 C = 330 mA g−1), respectively. While the Li2MnSiO4 electrode almost could not discharge when increasing the current density, the superior discharge capacity and rate performance of the Li2MnSiO4/C electrode are attributed to the enhancement of the electrical conductivity and the short pathway for Li-ion migration.
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The carbon coating effectively improved the charge and discharge performance of the Li2MnSiO4 material. However, the Li2MnSiO4/C electrode exhibited poor cycling stability as shown in Figure 6. The capacity retention rate after 50 cycles was only 30.6% (78.6 mAh g−1), while the Li2MnSiO4 electrode has good cycling performance without capacity loss, although its capacity is low. The reason for serious capacity fading of the Li2MnSiO4/C electrode is probably related to excessive Li+ extraction from host lattice, leading to structural collapse and/or irreversible structural change during the charge process. This speculation will be discussed later in this work.
We employ the ex situ XRD analysis to confirm whether the drastic capacity fading was caused by structural change during cycling. Figure 7 shows the XRD patterns of the Li2MnSiO4/C electrode with different charge and discharge states at the first cycle ( is the amount of extraction/insertion of Li+). The more Li+ gets out, the weaker diffraction peak intensity becomes. When more than 1.66 Li+ was delithiated, the diffraction peaks corresponding to the Li2MnSiO4 phase vanished. Note that the crystalline structure was not recovered after the subsequent discharge. This demonstrates that the crystalline Li2MnSiO4 transforms to amorphous state by the excessive delithiation. This result is consistent with that of Dominko’s [13] and may be the main reason which causes the capacity fading. In order to confirm whether there is reversible structure change when the extraction of Li+ is less than 1.66 Li+, we control charge cut-off voltage at 4.15 V. Figure 8 shows the XRD pattern of Li2MnSiO4/C composite charged and discharged to different states at the first cycle (the voltage range of 1.5 to 4.15 V). The crystal structure of Li2MnSiO4 can be recovered after the first discharge, indicating that the Li2MnSiO4 structure did not change under this charge and discharge term. From the partial enlarged figure in Figure 8, the diffraction peak shifts to the right with Li+ extraction during charge and moves back by lithiation during discharge. This means that the structure change is reversible during charge and discharge if the exchanged amount of Li+ is low. Therefore, controlling exchanged amount of Li+ can restrain Li2MnSiO4 transforming into amorphous state. This is also confirmed by the XRD pattern of Li2MnSiO4/C electrode after 21 cycles, as shown in Figure 9.
Figure 10 shows the charge and discharge curves of the Li2MnSiO4/C electrode at the first, second, and fifth cycles, where the electrode shows similar discharge capacity, indicating that the Li2MnSiO4/C electrode exhibits good reversible reactions between the potential of 1.5 and 4.15 V. The cycling performance of the Li2MnSiO4/C electrode between 1.5 and 4.15 V is shown in Figure 11. Note that the capacity retention rate of the Li2MnSiO4/C electrode after 21 cycles is significantly enhanced to 81.8%, which is much higher than 55.3% when the electrode is charged to 4.8 V. This good cycling performance is ascribed to the high structural stability achieved by controlling the Li content in the electrode. Based on ex situ XRD experiments and electrochemical measurements, the structural amorphization caused by excessive Li+ extraction from host lattice is the main reason for drastic capacity fading. Controlling extraction of Li+ can inhibit this amorphization and achieve reversible structural change and good cycling performance.
4. Conclusions
An in situ carbon coated Li2MnSiO4/C has been prepared by a facile sol-gel method using ascorbic acid as carbon source. XRD, SEM, and TEM studies confirm the formation of the Li2MnSiO4 with orthorhombic crystal and its particle size is approximately 20 nm, surrounded by a uniform carbon coating. High electrical conductivity and small particle size obtained by in situ carbon formation contribute to the enhancement of the rate performance. The Li2MnSiO4/C nanocomposite exhibits good electrochemical performance with a discharge capacity 257.1 mA h g−1 at the first cycle, while the pure Li2MnSiO4 only shows a low discharge capacity of 25.6 mA h g−1. However, capacity retention of Li2MnSiO4/C is only 30.6% after 50 cycles. The ex situ XRD analysis reveals that the structural amorphization caused by excessive Li+ extraction from host lattice is the main reason for this drastic capacity fading. Controlling extraction of Li+ can inhibit this amorphization and contributes to the reversible structural change and good cycling performance. Hence, it is suggested that the effective way to achieve a high capacity of the Li2MnSiO4 material is to find useful methods, such as doping or substitution, to stabilize its structure, rather than only modifying the surface state.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by the China Postdoctoral Science Foundation (2012M521064). The support from the National Natural Science Foundation of China (51201089) and the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD) was also acknowledged.