Journal of Nanomaterials

Volume 2014 (2014), Article ID 368071, 6 pages

http://dx.doi.org/10.1155/2014/368071

Research Article

Structural Evolution and Electrochemical Performance of Li₂MnSiO₄/C Nanocomposite as Cathode Material for Li-Ion Batteries

Min Wang, Meng Yang, Liqun Ma, Xiaodong Shen, and Xu Zhang

College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China

Received 11 November 2013; Accepted 4 December 2013; Published 19 January 2014

Academic Editor: Xiangyu Zhao

Copyright © 2014 Min Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

Figure 1: XRD patterns of as-prepared Li₂MnSiO₄ and Li₂MnSiO₄/C powders.

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.

(a)

(b)

(c)

Figure 2: Images of the Li₂MnSiO₄/C nanocomposite (a) FE-SEM; (b) and (c) TEM.

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 3: Cyclic voltammetry (CV) curve of Li₂MnSiO₄/C electrode at a scanning rate of 0.1 mV s⁻¹.

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.

(a)

(b)

Figure 4: Charge and discharge curves of Li₂MnSiO₄ and Li₂MnSiO₄/C electrodes during the first, second, and fifth cycles.

(a)

(b)

Figure 5: Charge-discharge plots of Li₂MnSiO₄ and Li₂MnSiO₄/C electrodes at various rates.

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.

Figure 6: Cycling performance of Li₂MnSiO₄ and Li₂MnSiO₄/C electrodes between 1.5 and 4.8 V.

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 7: XRD patterns of Li₂MnSiO₄/C at each stage during first charge and discharge cycle ( is the amount of extraction/insertion of Li⁺).

Figure 8: XRD patterns of Li₂MnSiO₄/C electrodes charged and discharged to different states in the first cycle (the voltage range of 1.5 to 4.15 V).

Figure 9: XRD patterns of Li₂MnSiO₄/C charged and discharged between 1.5 and 4.15 V after 21 cycles.

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.

Figure 10: Charge-discharge curves of Li₂MnSiO₄/C electrode during the first, second, and fifth cycles between 1.5 and 4.15 V.

Figure 11: Cycling performance of Li₂MnSiO₄/C electrodes at a voltage range of  V.

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.

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