Effects of carbon on the structure and electrochemical performance of Li₂FeSiO₄ cathode materials for lithium-ion batteries

Abstract

A Li₂FeSiO₄/C composite cathode material was prepared by a solid-state method with sucrose as a carbon source. The effect of carbon on the structure and electrochemical performance of Li₂FeSiO₄/C cathode materials for lithium-ion batteries was investigated. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), galvanostatic charge–discharge tests and electrochemical impedance spectroscopy (EIS). SEM images show that the obtained Li₂FeSiO₄/C materials consist of partially agglomerated nanoparticles with an average particle size of 100 nm. TEM images confirm that the carbon layer formed on the surface of Li₂FeSiO₄/C particles enhances the electronic conductivity and inhibits the agglomeration of the active particles during the annealing process. The electrochemical measurement results reveal that the Li₂FeSiO₄/C composite with 7.5 wt% carbon shows a good electrochemical performance with an initial discharge capacity of 141 mA h g⁻¹ at 0.1 C. After 50 cycles, the discharge capacity of the Li₂FeSiO₄/C composite remains 94.2% of the initial capacity at a discharge rate of 0.5 C.

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Introduction

In recent years, cathode materials containing polyanions, such as XO₄³⁻ (X = P, Si) for lithium ion batteries have attracted much attention due to their safety and excellent electrochemical performance in comparison with the other established cathode materials, like LiCoO₂ and LiNiO₂.^1,2 It has been found that the polyanions can beneficially lower the redox energies of chemically more stable couples, such as Fe²⁺/Fe³⁺, through an inductive effect and, thereby, increase the cell voltage.³ The polyanion-type compound of Li₂FeSiO₄ can be considered as the potential cathode material because of its high theoretical specific capacity of 166 mA h g⁻¹ for one Li⁺ exchange and 332 mA h g⁻¹ if the second Li⁺ exchange can be achieved.^4,5 Moreover, Li₂FeSiO₄ is inexpensive, nontoxic, environmentally friendly and very stable both chemically and thermally in comparison with the high cost, toxic and relatively unstable traditional cobalt-based compounds.^6,7 However, Li₂FeSiO₄ also suffers from the drawback of poor electronic conductivity and the slow diffusion rate of lithium ions, which leads to a low initial charge capacity and poor cycle stability.⁸ So far, many efforts have been conducted to optimize the performance of Li₂FeSiO₄ materials by reducing the particle size to the nano-scale,^9,10 coating the surface with conductive materials^11–13 and assuring porosity with a composite.¹⁴ Consequently, the capacity and rate capability have been improved significantly.^15–17 It has been reported that Li₂FeSiO₄ can be prepared through diffident kinds of methods, such as solid-state reactions,^18–20 hydrothermal processes^21–23 and sol–gel methods.^24,25 It has been indicated that carbon-coated Li₂FeSiO₄ can exhibit superior electrochemical performance.²⁶ In general, carbon coating on the surface of active materials can control the particle growth and make the particles more homogeneous, both of which are favorable for the transfer of electrons and ions. Therefore, carbon has an important effect on the structure and electrochemical performance of Li₂FeSiO₄/C cathode materials for lithium-ion batteries.

In this paper, Li₂FeSiO₄/C composite cathode materials were prepared by a solid-state method with sucrose as a carbon source and the effect of the carbon content on the crystalline structure, morphology and electrochemical performance of the composite were investigated in detail.

Experimental

Synthesis of samples

The Li₂FeSiO₄ cathode material was prepared by the solid-state reaction of stoichiometric amounts of Li₂CO₃ (99.5%, Kermel Chem), FeC₂O₄·2H₂O (99.7%, Kermel Chem), nano-SiO₂ (99.5%, Wacker Chemical GmbH, N20). The addition of different amounts of sucrose into the above mixture was conducted to prepare the Li₂FeSiO₄/C materials. The starting materials were first mixed using a planetary high energy ball milling machine (Fritsch pulverisette 7) in acetone. The rotating speed and ball milling time were 600 rpm and 6 h, respectively. After the ball milling procedure, the samples were annealed in a horizontal quartz tube oven at 700 °C for 10 h in an argon atmosphere. The mass percentage of carbon in the obtained Li₂FeSiO₄/C cathode material was fixed at 2.5 wt%, 5 wt% and 7.5 wt% according to the different content of sucrose in the precursor mixtures.

Characterization and measurements

The phase structure of the samples was studied by an X-ray powder diffraction (Rigaku D/max-γB) analyzer equipped with Cu–Kα radiation. The morphology and microstructure of the samples were investigated by scanning electron microscopy (FEI Quanta 200FEG) and transmission electron microscopy (JEOL, JEM-2010F).

The electrochemical performance of the samples was investigated using coin-type cells (CR2025). The cell was composed of a lithium metal negative electrode and a positive electrode, which were separated by a microporous polypropylene film. 1 mol L⁻¹ LiPF₆, in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a 1 : 1 volume ratio, was used as the electrolyte. The cathode electrodes were fabricated from a mixture of the active materials in the presence of polyvinylidene fluoride (PVDF) and acetylene black in a weight ratio of 8 : 1 : 1 in 1-a methyl-2-pyrrolidinone (NMP) dispersion. The resultant slurry was then coated on aluminum foil and dried at 120 °C for 10 h in a vacuum oven. The cathode was punched into circular discs. The cell was assembled in a glove box filled with high purity argon gas. The electrochemical performance of the cells was examined by a battery testing system (Neware, BST-5 V1 mA) at different charge/discharge rates between 1.5 V and 4.75 V.

Electrochemical impedance spectroscopy (EIS) measurements were carried out using an electrochemical workstation (Princeton Applied Research, M2273). The amplitude of the AC signal was 10 mV in the frequency range of 0.01 Hz to 100 kHz. All electrochemical measurements were performed at room temperature.

Results and discussion

Fig. 1 shows the XRD patterns of Li₂FeSiO₄ and Li₂FeSiO₄/C composites with various contents of carbon prepared by a solid-state method. To obtain Li₂FeSiO₄ with very high purity, it was necessary to control the synthesis conditions precisely, such as the reaction temperature and the ratio of the starting materials. From Fig. 1, it can be seen that no impurities were detected in the pristine Li₂FeSiO₄ and Li₂FeSiO₄/C materials prepared by the solid-state process. All the diffraction peaks of the samples were in good agreement with the orthorhombic structure (space group Pmn2₁) of Li₂FeSiO₄.⁵

Fig. 1 XRD patterns of the Li₂FeSiO₄ and Li₂FeSiO₄/C samples

The particle morphology of the Li₂FeSiO₄/C composite with different contents of carbon were observed by SEM and the images are shown in Fig. 2. All the four samples show a similar morphology and the powders consist of agglomerates of the primary particles. It is obvious that the primary particles have an average size of 100 nm, which can be attributed to the extensive ball milling process. As the content of carbon is increased, the size of the Li₂FeSiO₄/C particles tends to decrease gradually, which means that carbon plays an important role in hindering the growth of Li₂FeSiO₄ particles during the sintering process. The small particle size is key to enhance the electrochemical performance because it shortens the distance of Li⁺ diffusion in the solid phase. From the SEM images, it can be concluded that carbon acts as a dispersing agent and agglomeration inhibitor. Therefore, the excessive growth and agglomeration of Li₂FeSiO₄ particles is hampered effectively.

Fig. 2 SEM images of Li₂FeSiO₄ (a) and Li₂FeSiO₄/C with different carbon contents (b) 2.5 wt%C, (c) 5 wt % and (d) 7.5 wt %

In order to understand the dependence of the electrochemical performance of the Li₂FeSiO₄/C composites on the carbon content, charge/discharge tests of the Li₂FeSiO₄/C materials with various carbon contents were conducted and the charge/discharge curves after the first cycle are shown in Fig. 3. The cells were tested between 1.5 V and 4.75 V using a current rate of 16 mA g⁻¹ at room temperature. With an increasing carbon content, it can be found that the discharge capacity of pristine Li₂FeSiO₄ and Li₂FeSiO₄/7.5 wt%C increases from 39 mA h g⁻¹ to 150 mA h g⁻¹, respectively. From Fig. 3, it can be seen that pristine Li₂FeSiO₄ has a poor capacity performance, which is mainly due to its intrinsic poor electronic conductivity as a polyanion-type cathode material. Moreover, in comparison with pristine Li₂FeSiO₄, it can be seen that the difference between the charge and discharge potentials for the Li₂FeSiO₄/C composites become smaller, which can be explained by the Li₂FeSiO₄/C materials showing better reversibility and less polarization during the charge/discharge process. The enhanced performance of the Li₂FeSiO₄/C materials is associated with the carbon coating, which provides a conductive network in the electrode and enhances the conductivity of this kind of cathode material.

Fig. 3 Charge/discharge curves for Li₂FeSiO₄ and Li₂FeSiO₄/C with different carbon contents

Fig. 4 shows the electrochemical impedance spectra of the Li₂FeSiO₄/C electrodes with various contents of carbon after 10 cycles. Each profile is a combination of a depressed semicircle in the high-to-intermediate frequency region, which represents the total charge transfer resistance in the complex interfacial processes, and an inclined line in the lower frequency region, which is related to the lithium-ion diffusion in Li₂FeSiO₄/C particles. The charge transfer resistance of the Li₂FeSiO₄/C composites decreases gradually as the content of carbon increases, which means that carbon improves the electronic conductivity of the Li₂FeSiO₄/C materials effectively.

Fig. 4 EIS of the Li₂FeSiO₄/C electrodes with various contents of carbon after 10 cycles

From the electrochemical measurement and EIS results, Li₂FeSiO₄/7.5 wt%C possesses the best electrochemical performance in comparison to pristine Li₂FeSiO₄ and the other Li₂FeSiO₄/C samples. Therefore, a comprehensive study of the structure and electrochemical performance of this kind of Li₂FeSiO₄/C material was conducted.

Fig. 5 shows the TEM images of the Li₂FeSiO₄/C materials with 7.5 wt% carbon prepared by the solid-state method. It can be clearly seen from the TEM images that nanosized crystalline Li₂FeSiO₄ was dispersed in the carbon matrix homogeneously, which is beneficial to enhance the electronic conductivity and reduce the contact resistance between the Li₂FeSiO₄ particles. The HR-TEM images show that the nanocrystalline Li₂FeSiO₄ particles have an average size distribution of 15∼20 nm, which is helpful to reduce the distance of lithium-ion intercalation and de-intercalation during cycling.

Fig. 5 TEM (a, b) and HR-TEM (c, d) images of Li₂FeSiO₄/C materials with 7.5 wt % carbon

Fig. 6 shows the charge–discharge curves of the Li₂FeSiO₄/C composites with 7.5 wt% carbon in the potential range of 1.5–4.75 V (vs. Li/Li⁺). The electrodes were first galvanostatically charged and subsequently discharged at a current density of 16 mA g⁻¹. The Li₂FeSiO₄/C cathode shows charge and discharge potentials of about 3.1 V and 2.6 V, respectively, which is in good agreement with the oxidation and reduction peaks of the CV curve shown in Fig. S1 (ESI)†. Moreover, a higher charge potential of about 4.6 V was observed in the first cycle, which was also reported by another research group.⁵ This phenomenon can be attributed to a structure improvement process.^5,19 The charge potentials of the Li₂FeSiO₄ sample declined and stabilized within the subsequent cycles.

Fig. 6 Charge and discharge curves of Li₂FeSiO₄/C with 7.5 wt % carbon

The rate discharge performance of Li₂FeSiO₄/C with 7.5 wt% carbon at different charge–discharge rates is shown in Fig. 7. It can be found that the initial discharge capacity is about 141 mA h g⁻¹ at 0.1 C and the discharge capacity increases to 150 mA h g⁻¹ after the first two cycles. The increase in the discharge capacity is mainly caused by the reactivation of the positive materials. In addition, the Li₂FeSiO₄/C composite delivers a lower discharge capacity of 92 mA h g⁻¹ and 80 mA h g⁻¹ at 0.5 C and 1 C, respectively. At the higher rates of 0.5 C and 1 C, charge-transfer resistance plays an important role in the potential drop due to kinetic limitations, which result in the deterioration of the electrochemical performance at high charge/discharge rates. From Fig. 7 it can also be found that the reversible capacity of the Li₂FeSiO₄/C material returns to about 140 mA h g⁻¹ at 0.1 C after 30 cycles, which reveals the excellent structural stability of this kind of cathode material.

Fig. 7 Discharge capacity of the Li₂FeSiO₄/C with 7.5 wt % carbon at different rates

To investigate the cycling stability of the Li₂FeSiO₄/C composite with 7.5 wt % carbon, it was tested at the rate of 1 C and 0.5 C for 50 cycles and the results are shown in Fig. 8. From Fig. 8 it can be seen that the initial discharge capacity of the Li₂FeSiO₄/C composite at 0.5 C is 103.6 mA h g⁻¹, and remains at 97.6 mA h g⁻¹ after 50 cycles, which means that the discharge capacity retains 94.2% of the initial capacity in this kind of material. At the discharge rate of 1 C, the initial discharge capacity is 88.6 mA h g⁻¹ and 88.0% of the initial discharge capacity after 50 cycles remains.

Fig. 8 Cycle performance of Li₂FeSiO₄/C with 7.5% carbon

Conclusions

Li₂FeSiO₄/C composite cathode materials were prepared by a solid-state method with sucrose as a carbon source. The effect of the carbon content on the structure and electrochemical performance of the Li₂FeSiO₄/C cathode materials for lithium-ion batteries were investigated and the optimal carbon content was determined to be 7.5 wt%. The Li₂FeSiO₄/C composite with 7.5 wt % carbon has a uniform particle distribution with an average size of 100 nm and shows excellent electrochemical characteristics with a large initial reversible capacity of 141 mA h g⁻¹ and good cycling performance. Therefore, these results indicate that carbon can act as a bridge, interconnecting the isolated Li₂FeSiO₄ particles and providing a valid conductive network in the electrode, which results in an improved cycle stability. The Li₂FeSiO₄/C material with 7.5 wt% carbon has an initial reversible capacity of 103.6 mA h g⁻¹ and retains 97.6 mA h g⁻¹ after 50 cycles at 0.5 C.

Acknowledgements

This work was partially supported by the Natural Science Foundation of China (No. 50902038), the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT.NSRIF. 2008.25) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (200802131064).

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra20552a/

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