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High performance Li2MnSiO4 prepared in molten KCl–NaCl for rechargeable lithium ion batteries
Electrochimica Acta 119 (2014) 131–137
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
High performance Li2 MnSiO4 prepared in molten KCl–NaCl for rechargeable lithium ion batteries Fei Wang a,∗ , Yanming Wang a , Dengming Sun a , Lei Wang a , Jun Yang b , Haiping Jia c a b c
School of Chemistry and Materials Science, Huaibei Normal University, Huaibei, Anhui 235000, China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Meet Battery Research Center, Institute of Physical Chemistry, University of Muenster, Corrensstrasse 46, 48149 Muenster, Germany
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Article history: Received 11 October 2013 Received in revised form 5 December 2013 Accepted 9 December 2013 Available online 24 December 2013 Keywords: Lithium ion battery Molten salt method Lithium manganese silicate Cyclability
a b s t r a c t Li2 MnSiO4 /C composites have been prepared by a facile molten salt method followed by a carbon coating process. Submicron Li2 MnSiO4 particles are obtained in KCl–NaCl molten phase with a short reaction time of 3 h. The orthorhombic structure and sphere-like morphology are confirmed by X-ray diffraction and scanning electron microscope. Ex-situ XRD study confirms amorphous transition of Li2 MnSiO4 during the first charge process. Galvanostatic charge-discharge tests display high initial charge and discharge capacities of 265 and 194 mAh g−1 , respectively, at 0.05 C rate for the Li2 MnSiO4 /C composite prepared at 700 ◦ C. At 0.1 C rate, it maintains a discharge capacity of 165 mAh g−1 and its capacity retention at the 50th cycle is up to 78%, showing superior cycling stability. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Great attention has been paid to next-generation safe lithium batteries with high energy and power densities for vehicle applications and energy storage. The low practical capacity and safety issue of currently commercialized lithium transition metal oxides cathode materials, such as LiCoO2 , LiMn2 O4 , etc., can not meet the demand of high performance battery systems [1,2]. Lithium orthosilicates, Li2 MSiO4 (M = Fe, Mn and Co), have recently attracted tremendous interests on account of their high theoretical capacities (>330 mAh g−1 ) corresponding to a two electron (2 Li+ ) reaction and good thermal stability from strong Si-O bonding [3–6]. Due to the possible oxidation of the Mn3+ /Mn4+ couple rather than Fe3+ /Fe4+ and Co3+ /Co4+ couples within the potential range of present electrolyte systems, the insertion/extraction of two Li+ ions is much more feasible in Li2 MnSiO4 than that in Li2 FeSiO4 and Li2 CoSiO4 [7–9]. However, Li2 MnSiO4 has two main shortcomings, low electronic conductivity (∼10−16 S cm−1 ) and slow kinetics, which would result in a poor electrochemical activity [10,11]. Similar to LiFePO4 material, several approaches including surface carbon coating, metal ion doping and nanoparticles preparation have been pursued to achieve the high capacity of Li2 MnSiO4 [12–15]. Recently, the nano-sized Li2 MnSiO4 /C composites with the
∗ Corresponding author. Tel.: +86 561 3802235; fax: +86 561 3806281. E-mail address: [email protected] (F. Wang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.12.057
particle size less than 50 nm were successfully synthesized by solution route [16], solvothermal method [17], sol-gel route [18–20] and microwave-solvothermal process [21,22]. The initial discharge capacities of the composites were beyond 200 mAh g−1 , while the capacity dropped dramatically after several cycles and its capacity retention was less than 50% after 20 cycles. The fast capacity fade may be ascribed to the conversion from crystal structure to amorphous phase during the electrochemical cycling [16,19,22]. More recently, Liu et al. [23] reported a solid-state reaction for carbon coated Li2 MnSiO4 nanoparticles using citric acid as the carbon source. At the rate of 0.05 C, the Li2 MnSiO4 /C composites delivered an initial discharge capacity of 268 mAh g−1 and showed a reversible capacity of 136 mAh g−1 up to 140 cycles. Zhao et al. [24] synthesized a carbon coated Li2 MnSiO4 composite, which was uniformly distributed on reduced graphene oxide (RGO) networks. The RGO@Li2 MnSiO4 @C composite presented excellent cyclability with a stable capacity of 150 mAh g−1 for 700 cycles at 1 C. The above results indicate that the cycling performance of Li2 MnSiO4 /C is strongly affected by the preparation process and the microstructural perfection of the material. The molten salt method has been widely employed to prepare multi-component oxides powders with controllable particle morphology and excellent electrochemical performance [25,26]. Molten salts as a reaction media could provide a liquid reaction environment for reactants, thereby accelerating the reaction rate and providing a homogeneous structure of the final product [27]. However, to the best of our knowledge, the molten salt synthesis of Li2 MnSiO4 has rarely been studied so far.
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transferred to a tube furnace and heated at 350 ◦ C for 2 h, followed by treatment at different temperatures (660, 700 and 750 ◦ C) for 3 h in H2 /Ar (5 vol%) atmosphere. Finally, the product was washed with deionized water to dissolve any remaining salt, filtered and dried in vacuum. The obtained Li2 MnSiO4 powder was mixed with a certain amount of sucrose and acetylene black, and then calcined at 600 ◦ C in Ar atmosphere for 2 h to obtain Li2 MnSiO4 /C composites. 2.2. Sample analysis
Fig. 1. XRD patterns of Li2 MnSiO4 and Li2 MnSiO4 /C prepared at various temperatures.
In this work, well-dispersed Li2 MnSiO4 particles have been synthesized by a facile molten salt method using the mixture of KCl and NaCl as the reaction medium. To enhance the surface electronic conductivity, sucrose and acetylene black are adopted as carbon sources to coat the Li2 MnSiO4 particles with a thin carbon layer. The effects of the material structure and morphology on the electrochemical performances of Li2 MnSiO4 /C are discussed and compared. 2. Experimental 2.1. Sample Synthesis Li2 MnSiO4 particles were prepared by a molten salt method. An equal molar ratio of KCl and NaCl with a melting point of 658 ◦ C acts as flux after drying at 100 ◦ C for 2 h under vacuum. Li2 CO3 , MnCO3 , SiO2 (10–20 nm) and KCl–NaCl in a molar ratio of 1:1:1:4 were well mixed with a mortar and pestle. Then, the mixed powders were
Powder X-ray diffraction (XRD) patterns of the prepared Li2 MnSiO4 were recorded on a Rigaku D/Max-2200 diffractometer using Cu K␣ radiation (40 kV, 30 mA). The morphologies of the Li2 MnSiO4 and Li2 MnSiO4 /C samples were observed by a field emission scanning electron microscope (FESEM, JEOL JSM-7401F) with an accelerating voltage of 5.0 kV and a high resolution transmission electron microscope (HRTEM, JEOL JEM-2100) operating at 200 kV. The carbon content of the Li2 MnSiO4 /C composites was measured with PE 2400IIelemental analyzer. 2.3. Electrochemical Measurements Electrochemical measurements were performed using CR2016 coin-type cells with Celgard 2400 membrane as separator and metallic lithium foil as anode. The cathode was composed of 85 wt% Li2 MnSiO4 /C, 5 wt% Super P conductive carbon and 10 wt% polyvinylidene difluoride (PVDF). 1 M LiPF6 in 50:50 (v/v) ethylene carbonate (EC)/dimethyl carbonate (DMC) was used as the electrolyte. Galvanostatic charge/discharge cycling was carried out on a Land battery testing system (CT2001, Wuhan, China) in a potential range of 1.5–5.0 V at room temperature. Cyclic voltammetry (CV) was performed on an electrochemical workstation (CHI660D, Shanghai, China). Electrochemical impedance spectroscopic measurement (EIS) was conducted on a Solartron SI1287 electrochemical interface in the frequency rang of 100 kHz to 10 mHz. The specific capacity was calculated based on the mass of pure orthosilicate active material.
Fig. 2. SEM images of Li2 MnSiO4 and Li2 MnSiO4 /C (inset) prepared at 660 ◦ C (a), 700 ◦ C (b) and 750 ◦ C (c); TEM image of the Li2 MnSiO4 /C composite (d).
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Fig. 4. CV curves of the Li2 MnSiO4 /C for the first 3 cycles (a) and the 30th cycle (b) at 0.1 mV s−1 , dQ/dV vs. potential plot for the 30th charge curve (c). Fig. 3. Galvanostatic charge-discharge curves of Li2 MnSiO4 /C composites prepared at 660 ◦ C (a), 700 ◦ C (b) and 750 ◦ C (c) at 0.05 C rate.
3. Results and discussion 3.1. Sample characterization Fig. 1 shows the XRD patterns of Li2 MnSiO4 and Li2 MnSiO4 /C prepared at various temperatures. The well distinguished sharp peaks of all samples suggest the high crystallinity of Li2 MnSiO4 synthesized via the molten salt method. The diffraction patterns of Li2 MnSiO4 samples can be indexed to the orthorhombic crystal structure with Pmn21 space group, which is the same as the previous reports [23,28,29]. Due to the high ion diffusion rate and strong
dissolving ability of the molten salt [30,31], the well-crystallized Li2 MnSiO4 can rapidly be formed at a moderate temperature. The peaks at 35.0◦ and 40.6◦ identified as MnO impurity are observed in all samples. Additionally, the Mn2 SiO4 impurity is also detected in Li2 MnSiO4 prepared at a higher temperature of 750 ◦ C. It is extremely difficult to obtain phase-pure Li2 MnSiO4 , and the presence of those impurity phases during the synthesis of Li2 MnSiO4 is also reported in many literatures [5,19,32,33]. The diffraction peaks for Li2 MnSiO4 /C composites indicate that all Li2 MnSiO4 samples maintain their original crystal structures after carbon coating process. SEM images of Li2 MnSiO4 particles prepared at various temperatures are displayed in Fig. 2a–c. The sample synthesized at 660 ◦ C
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Fig. 5. Rate capability of Li2 MnSiO4 /C composites obtained at various temperatures.
mostly consists of crystalline well-shaped spherical particles with the size of 250–600 nm. The particle size enlarges with the increase of the temperature and the sample obtained at 750 ◦ C presents a particle distribution ranging from 600 nm to 1 m. The morphology of Li2 MnSiO4 particles prepared by molten salt method is round and smoother than those synthesized by conventional solid-state reaction and sol-gel route [19,23]. In addition, no visible aggregation is observed for as-synthesized samples, which can be attributed to the existence of molten KCl–NaCl medium during the reaction process [26]. Surface carbon coating is considered as an effective strategy to improve the cyclability and lithium extraction/insertion kinetics of the poor-conducting active material. Hence, sucrose and acetylene black are used as carbon sources to prepare carbon coated Li2 MnSiO4 . Carbon dispersed among Li2 MnSiO4 particles can be clearly observed from SEM images of Li2 MnSiO4 /C composites in the inset in Fig. 2a–c. As shown in Fig. 2d, a carbon layer with a thickness of 10–25 nm is covered on the surface of Li2 MnSiO4 nanoparticle. Based on elemental analysis, the amount of carbon was approximately 15 wt% in the Li2 MnSiO4 /C composite. 3.2. Electrochemical properties Figs. 3a–c compare the typical galvanostatic charge-discharge curves of the Li2 MnSiO4 /C composites prepared at different temperatures between 1.5 and 5.0 V with a rate of 0.05 C (16.65 mA g−1 ). It is interesting to find that the first charge curve is different from the subsequent ones, while the first and the following discharge
Fig. 6. Cycling performance of Li2 MnSiO4 /C composites obtained at various temperatures.
Fig. 7. Ex-situ XRD patterns of the Li2 MnSiO4 /C electrode: uncharged (a), charged to 4.5 V (b), charged to 4.8 V (c), discharged to 1.5 V (d) and discharged to 1.5 V at the 3rd cycle (e).
curves are similar. This phenomenon is related to the structural rearrangements of Li2 MnSiO4 in the first charge process [18,22]. As seen in Figs. 3a-c, there are no apparent potential plateaus in the first and second charge-discharge curves. However, the 30th charge-discharge curves present two sloping plateaus at two different broad potential ranges at around 3.0 and 4.2 V, corresponding to the Mn2+ /Mn3+ and Mn3+ /Mn4+ redox couples. The Li2 MnSiO4 /C synthesized at 700 ◦ C delivers the initial charge capacity of 265 mAh g−1 against discharge capacity of 194 mAh g−1 , corresponding to 1.6 Li+ ions extraction and ∼1.2 Li+ ions insertion per formula unit. The initial discharge capacities of Li2 MnSiO4 /C synthesized at 660 and 750 ◦ C are 187 and 177 mAh g−1 , respectively, indicating more than one Li+ ion transfer per formula unit. Among all three samples, the Li2 MnSiO4 /C obtained at 700 ◦ C delivers a discharge capacity of 157 mAh g−1 (charge capacity 165 mAh g−1 ) with the columbic efficiency of 95% at the 30th cycle, exhibiting the highest capacity retention of 81%. The apparent charge-discharge plateaus and high columbic efficiency indicate that the electrochemical activity and reversibility of Li2 MnSiO4 are obviously improved though a KCl–NaCl assisted molten salt process. The inferior electrochemical performance of the sample prepared at 750 ◦ C may be attributed to its large particle size and the presence of Mn2 SiO4 impurity phase. Cyclic voltammetry curves of the Li2 MnSiO4 /C are investigated at a potential scan rate of 0.1 mV s−1 between 1.5 and 5.0 V, and the first three CV curves are shown in Fig. 4a. Great difference between the 1st and the following curves is discerned. For the 1st CV curve,
Fig. 8. Nyqvist plots of the Li2 MnSiO4 /C electrode at the fully discharged state after various cycles.
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Fig. 9. SEM images of the Li2 MnSiO4 /C electrode: before cycling (a), after 10 cycles (b) and after 20 cycles (c); TEM image of the Li2 MnSiO4 /C composite after 30 cycles (d).
a broad peak at 4.4 V corresponds to the oxidation of Mn2+ to Mn3+ [20,21,34]. Another shoulder peak around 4.8 V might be attributed to the conversion from Mn3+ to Mn4+ and irreversible side reactions due to the formation of solid electrolyte interface film [35]. The inconspicuous reduction peaks in the range of 4.0 and 2.5 V are associated with the lithium ions insertion. The oxidation peaks in the 2nd and 3rd CV curves obviously shift toward the low potential at 3.4 and 4.2 V, suggesting a possibility of structural rearrangements during the first cycle. The positions of oxidation/reduction peaks at 3.1/2.8 and 4.2/4.1 V in the 30th CV curve (Fig. 4b) are ascribed to the Mn2+ /Mn3+ and Mn3+ /Mn4+ redox processes. The CV data are in good agreement with the charge-discharge behavior in Fig. 3. To further clarify the charge-discharge characteristics of Li2 MnSiO4 , the dQ/dV vs. potential plot for the 30th charge curve is presented in Fig. 4c, where the peaks correspond to the potential plateaus in the charge-discharge curve. Two oxidation peaks located at 3.1 V and 4.3 V in the charge process can be ascribed to the oxidation of Mn2+ to Mn3+ and Mn3+ to Mn4+ , respectively, which corresponds well with the CV and charge-discharge curves. Fig. 5 exhibits the rate capability of the Li2 MnSiO4 /C composites prepared at various temperatures. The charge rate remains constant at 0.05 C and the discharge rates are varied from 0.05 C to 1 C. The discharge capacities of all samples remarkably decrease with increasing current. The rapid decrease in capacity is possibly due to the poor electronic and ionic conductivity of Li2 MnSiO4. The capacity retentions of the samples prepared at 660 and 700 ◦ C at 0.5 C is 63% and 61% against the capacities at 0.05 C, respectively. In contrast, this value is only 56% for the sample prepared at 750 ◦ C, which can be attributable to long lithium ion diffusion distance within the large particles. Fig. 6 gives a comparison of the cycling performance of three Li2 MnSiO4 /C composites. The discharge capacity of the Li2 MnSiO4 /C composite prepared at 700 ◦ C decreases from 165 to 129 mAh g−1 up to 50 cycles and 78% of the initial capacity can be retained at 0.1 C rate. For the Li2 MnSiO4 /C composites prepared at
660 and 750 ◦ C, the capacity retention are also up to 74% and 70%, respectively, in spite of a relatively lower cycle capacity. The cycling stability of the submicron Li2 MnSiO4 particles prepared by molten salt method is much higher than that of most previously reported Li2 MnSiO4 nanocomposites [18–22,28]. Fig. 7 shows the ex-situ XRD patterns of the Li2 MnSiO4 /C electrode at different charge-discharge stages. XRD peaks associated with the crystal Li2 MnSiO4 phase are observed from the fresh cathode sheet. However, the diffraction peaks intensity of Li2 MnSiO4 decreases significantly when the electrode is charged to 4.5 V. When charged to 4.8 V, the peaks can hardly be distinguished from the background, indicating a transition from crystal Li2 MnSiO4 to an amorphous state after lithium ion extraction in the first charge. Furthermore, no obvious diffraction peaks corresponding to Li2 MnSiO4 are observed when the electrode is discharged to 1.5 V. Most importantly, after the subsequent two cycles, the initial crystal structure is not recovered, which means that the amorphous transition of Li2 MnSiO4 during the first charge process is irreversible and the electrochemical behaviors after the first charge reflect the characteristic of the amorphous Li2 MnSiO4 , in accordance with other reports concerning the crystal structure studies of the Li2 MnSiO4 [12,32,36,37]. The interfacial property between electrode and electrolyte is further examined by EIS measurement. Fig. 8 shows the Nyqvist plots of Li2 MnSiO4 /C at the fully discharged state of the 1st , 10th , 20th and 30th cycles. All Nyquist plots contain a semicircle in the high-to-medium frequency region and a slope line in the low frequency region. The semicircle can be attributed to the charge transfer impedance between the electrolyte and electrode material, and the slope line is related to the Li+ diffusion in the electrode material. In the inset of Fig. 8, an equivalent circuit is proposed to model the Nyquist plot. The symbols Re, Rct, CPE, and Zw represent the ohmic resistance of the cell, charge transfer resistance, double layer capacitance, and Warburg diffusion impedance, respectively. It is seen that the Li2 MnSiO4 /C electrode exhibits a relatively huge
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charge transfer resistance of 315 after the 1st cycle, which could due to the formation of sold electrolyte interface film on the surface of Li2 MnSiO4 /C composite [32]. The values of Rct decrease with the increase of the cycle number, up to 116 after the 30th cycle. Moreover, the value of Rct after the 30th cycle is close to that after the 10th cycle, suggesting that the passivating film becomes very stable after ten charge-discharge cycles and offers high ion conductivity. Fig. 9 shows the SEM and TEM images of the Li2 MnSiO4 /C electrode before cycling and after different charge-discharge cycles. Well-shaped Li2 MnSiO4 particles dispersed among Super P conductive carbon can be clearly observed from the SEM image of the fresh electrode (Fig. 9a). As shown in Fig. 9b-d, Li2 MnSiO4 particles basically retain the original spherical morphology and particle size after 10, 20 and 30 cycles, respectively. This result confirms that the crystal structure transition during cycling does not make submicron Li2 MnSiO4 break down into smaller particles. Based on the literature, the poor capacity retention of Li2 MnSiO4 /C is probably ascribed to the structural instability of the delithiated phase and manganese dissolution in the electrolyte [20,22]. According to the charge-discharge results (Fig. 3 and Fig. 4b), the amorphous Li2 MnSiO4 displays fairly good electrochemical activity and reversibility during cycling. Benefiting from the liquid reaction environment, smooth-surfaced Li2 MnSiO4 products with a larger particle size can be successfully prepared in the KCl–NaCl molten phase. The smooth surface and relatively low specific surface are beneficial to decreasing the side reactions at the electrode/electrolyte interfaces and forming stable interfacial film, which contributes to the suppression of manganese dissolution into the electrolyte. In summary, the improved electrochemical performance of Li2 MnSiO4 /C composite can be mainly ascribed to the perfect surface morphology and stable electrode/electrolyte interfacial property during charge-discharge process. 4. Conclusions In conclusion, the well-dispersed Li2 MnSiO4 particles have been successfully prepared by the KCl–NaCl molten salt method with short reaction time of 3 h. The Li2 MnSiO4 particles are coated with a relatively uniform carbon layer using sucrose and acetylene black as carbon source. The ex-situ XRD analysis indicates the amorphization of Li2 MnSiO4 during the first charge process. Additionally, the existence of two redox couples determined by CV together with the dQ/dV analysis further confirms two lithium insertion/extraction processes. At a low rate of 0.05 C, the Li2 MnSiO4 /C composite prepared at 700 ◦ C delivers an initial discharge capacity of 194 mAh g−1 . Furthermore, about 78% of the initial discharge capacity is retained after 50 cycles at 0.1 C rate. The smooth surface morphology and stable electrode/electrolyte interfacial property are favorable for improving the reversible capacity and cyclability of Li2 MnSiO4 /C composites. Acknowledgement This work was supported by Anhui Provincial Natural Science Foundation, China (No. 1308085QB41) and Special Foundation for Outstanding Young Scientists of Anhui Province, China (No. 2012SQRL226ZD). References [1] Y.-K. Sun, C.S. Yoon, S.-T. Myung, I. Belharouak, K. Amine, Role of AlF3 coating on LiCoO2 particles during cycling to cutoff voltage above 4.5 V, J. Electrochem. Soc. 156 (2009) A1005. [2] Y. Chen, K. Xie, Y. Pan, C. Zheng, Nano-sized LiMn2 O4 spinel cathode materials exhibiting high rate discharge capability for lithium-ion batteries, J. Power Sources 196 (2011) 6493.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
High performance Li2 MnSiO4 prepared in molten KCl–NaCl for rechargeable lithium ion batteries Fei Wang a,∗ , Yanming Wang a , Dengming Sun a , Lei Wang a , Jun Yang b , Haiping Jia c a b c
School of Chemistry and Materials Science, Huaibei Normal University, Huaibei, Anhui 235000, China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Meet Battery Research Center, Institute of Physical Chemistry, University of Muenster, Corrensstrasse 46, 48149 Muenster, Germany
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Article history: Received 11 October 2013 Received in revised form 5 December 2013 Accepted 9 December 2013 Available online 24 December 2013 Keywords: Lithium ion battery Molten salt method Lithium manganese silicate Cyclability
a b s t r a c t Li2 MnSiO4 /C composites have been prepared by a facile molten salt method followed by a carbon coating process. Submicron Li2 MnSiO4 particles are obtained in KCl–NaCl molten phase with a short reaction time of 3 h. The orthorhombic structure and sphere-like morphology are confirmed by X-ray diffraction and scanning electron microscope. Ex-situ XRD study confirms amorphous transition of Li2 MnSiO4 during the first charge process. Galvanostatic charge-discharge tests display high initial charge and discharge capacities of 265 and 194 mAh g−1 , respectively, at 0.05 C rate for the Li2 MnSiO4 /C composite prepared at 700 ◦ C. At 0.1 C rate, it maintains a discharge capacity of 165 mAh g−1 and its capacity retention at the 50th cycle is up to 78%, showing superior cycling stability. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Great attention has been paid to next-generation safe lithium batteries with high energy and power densities for vehicle applications and energy storage. The low practical capacity and safety issue of currently commercialized lithium transition metal oxides cathode materials, such as LiCoO2 , LiMn2 O4 , etc., can not meet the demand of high performance battery systems [1,2]. Lithium orthosilicates, Li2 MSiO4 (M = Fe, Mn and Co), have recently attracted tremendous interests on account of their high theoretical capacities (>330 mAh g−1 ) corresponding to a two electron (2 Li+ ) reaction and good thermal stability from strong Si-O bonding [3–6]. Due to the possible oxidation of the Mn3+ /Mn4+ couple rather than Fe3+ /Fe4+ and Co3+ /Co4+ couples within the potential range of present electrolyte systems, the insertion/extraction of two Li+ ions is much more feasible in Li2 MnSiO4 than that in Li2 FeSiO4 and Li2 CoSiO4 [7–9]. However, Li2 MnSiO4 has two main shortcomings, low electronic conductivity (∼10−16 S cm−1 ) and slow kinetics, which would result in a poor electrochemical activity [10,11]. Similar to LiFePO4 material, several approaches including surface carbon coating, metal ion doping and nanoparticles preparation have been pursued to achieve the high capacity of Li2 MnSiO4 [12–15]. Recently, the nano-sized Li2 MnSiO4 /C composites with the
∗ Corresponding author. Tel.: +86 561 3802235; fax: +86 561 3806281. E-mail address: [email protected] (F. Wang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.12.057
particle size less than 50 nm were successfully synthesized by solution route [16], solvothermal method [17], sol-gel route [18–20] and microwave-solvothermal process [21,22]. The initial discharge capacities of the composites were beyond 200 mAh g−1 , while the capacity dropped dramatically after several cycles and its capacity retention was less than 50% after 20 cycles. The fast capacity fade may be ascribed to the conversion from crystal structure to amorphous phase during the electrochemical cycling [16,19,22]. More recently, Liu et al. [23] reported a solid-state reaction for carbon coated Li2 MnSiO4 nanoparticles using citric acid as the carbon source. At the rate of 0.05 C, the Li2 MnSiO4 /C composites delivered an initial discharge capacity of 268 mAh g−1 and showed a reversible capacity of 136 mAh g−1 up to 140 cycles. Zhao et al. [24] synthesized a carbon coated Li2 MnSiO4 composite, which was uniformly distributed on reduced graphene oxide (RGO) networks. The RGO@Li2 MnSiO4 @C composite presented excellent cyclability with a stable capacity of 150 mAh g−1 for 700 cycles at 1 C. The above results indicate that the cycling performance of Li2 MnSiO4 /C is strongly affected by the preparation process and the microstructural perfection of the material. The molten salt method has been widely employed to prepare multi-component oxides powders with controllable particle morphology and excellent electrochemical performance [25,26]. Molten salts as a reaction media could provide a liquid reaction environment for reactants, thereby accelerating the reaction rate and providing a homogeneous structure of the final product [27]. However, to the best of our knowledge, the molten salt synthesis of Li2 MnSiO4 has rarely been studied so far.
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transferred to a tube furnace and heated at 350 ◦ C for 2 h, followed by treatment at different temperatures (660, 700 and 750 ◦ C) for 3 h in H2 /Ar (5 vol%) atmosphere. Finally, the product was washed with deionized water to dissolve any remaining salt, filtered and dried in vacuum. The obtained Li2 MnSiO4 powder was mixed with a certain amount of sucrose and acetylene black, and then calcined at 600 ◦ C in Ar atmosphere for 2 h to obtain Li2 MnSiO4 /C composites. 2.2. Sample analysis
Fig. 1. XRD patterns of Li2 MnSiO4 and Li2 MnSiO4 /C prepared at various temperatures.
In this work, well-dispersed Li2 MnSiO4 particles have been synthesized by a facile molten salt method using the mixture of KCl and NaCl as the reaction medium. To enhance the surface electronic conductivity, sucrose and acetylene black are adopted as carbon sources to coat the Li2 MnSiO4 particles with a thin carbon layer. The effects of the material structure and morphology on the electrochemical performances of Li2 MnSiO4 /C are discussed and compared. 2. Experimental 2.1. Sample Synthesis Li2 MnSiO4 particles were prepared by a molten salt method. An equal molar ratio of KCl and NaCl with a melting point of 658 ◦ C acts as flux after drying at 100 ◦ C for 2 h under vacuum. Li2 CO3 , MnCO3 , SiO2 (10–20 nm) and KCl–NaCl in a molar ratio of 1:1:1:4 were well mixed with a mortar and pestle. Then, the mixed powders were
Powder X-ray diffraction (XRD) patterns of the prepared Li2 MnSiO4 were recorded on a Rigaku D/Max-2200 diffractometer using Cu K␣ radiation (40 kV, 30 mA). The morphologies of the Li2 MnSiO4 and Li2 MnSiO4 /C samples were observed by a field emission scanning electron microscope (FESEM, JEOL JSM-7401F) with an accelerating voltage of 5.0 kV and a high resolution transmission electron microscope (HRTEM, JEOL JEM-2100) operating at 200 kV. The carbon content of the Li2 MnSiO4 /C composites was measured with PE 2400IIelemental analyzer. 2.3. Electrochemical Measurements Electrochemical measurements were performed using CR2016 coin-type cells with Celgard 2400 membrane as separator and metallic lithium foil as anode. The cathode was composed of 85 wt% Li2 MnSiO4 /C, 5 wt% Super P conductive carbon and 10 wt% polyvinylidene difluoride (PVDF). 1 M LiPF6 in 50:50 (v/v) ethylene carbonate (EC)/dimethyl carbonate (DMC) was used as the electrolyte. Galvanostatic charge/discharge cycling was carried out on a Land battery testing system (CT2001, Wuhan, China) in a potential range of 1.5–5.0 V at room temperature. Cyclic voltammetry (CV) was performed on an electrochemical workstation (CHI660D, Shanghai, China). Electrochemical impedance spectroscopic measurement (EIS) was conducted on a Solartron SI1287 electrochemical interface in the frequency rang of 100 kHz to 10 mHz. The specific capacity was calculated based on the mass of pure orthosilicate active material.
Fig. 2. SEM images of Li2 MnSiO4 and Li2 MnSiO4 /C (inset) prepared at 660 ◦ C (a), 700 ◦ C (b) and 750 ◦ C (c); TEM image of the Li2 MnSiO4 /C composite (d).
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Fig. 4. CV curves of the Li2 MnSiO4 /C for the first 3 cycles (a) and the 30th cycle (b) at 0.1 mV s−1 , dQ/dV vs. potential plot for the 30th charge curve (c). Fig. 3. Galvanostatic charge-discharge curves of Li2 MnSiO4 /C composites prepared at 660 ◦ C (a), 700 ◦ C (b) and 750 ◦ C (c) at 0.05 C rate.
3. Results and discussion 3.1. Sample characterization Fig. 1 shows the XRD patterns of Li2 MnSiO4 and Li2 MnSiO4 /C prepared at various temperatures. The well distinguished sharp peaks of all samples suggest the high crystallinity of Li2 MnSiO4 synthesized via the molten salt method. The diffraction patterns of Li2 MnSiO4 samples can be indexed to the orthorhombic crystal structure with Pmn21 space group, which is the same as the previous reports [23,28,29]. Due to the high ion diffusion rate and strong
dissolving ability of the molten salt [30,31], the well-crystallized Li2 MnSiO4 can rapidly be formed at a moderate temperature. The peaks at 35.0◦ and 40.6◦ identified as MnO impurity are observed in all samples. Additionally, the Mn2 SiO4 impurity is also detected in Li2 MnSiO4 prepared at a higher temperature of 750 ◦ C. It is extremely difficult to obtain phase-pure Li2 MnSiO4 , and the presence of those impurity phases during the synthesis of Li2 MnSiO4 is also reported in many literatures [5,19,32,33]. The diffraction peaks for Li2 MnSiO4 /C composites indicate that all Li2 MnSiO4 samples maintain their original crystal structures after carbon coating process. SEM images of Li2 MnSiO4 particles prepared at various temperatures are displayed in Fig. 2a–c. The sample synthesized at 660 ◦ C
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Fig. 5. Rate capability of Li2 MnSiO4 /C composites obtained at various temperatures.
mostly consists of crystalline well-shaped spherical particles with the size of 250–600 nm. The particle size enlarges with the increase of the temperature and the sample obtained at 750 ◦ C presents a particle distribution ranging from 600 nm to 1 m. The morphology of Li2 MnSiO4 particles prepared by molten salt method is round and smoother than those synthesized by conventional solid-state reaction and sol-gel route [19,23]. In addition, no visible aggregation is observed for as-synthesized samples, which can be attributed to the existence of molten KCl–NaCl medium during the reaction process [26]. Surface carbon coating is considered as an effective strategy to improve the cyclability and lithium extraction/insertion kinetics of the poor-conducting active material. Hence, sucrose and acetylene black are used as carbon sources to prepare carbon coated Li2 MnSiO4 . Carbon dispersed among Li2 MnSiO4 particles can be clearly observed from SEM images of Li2 MnSiO4 /C composites in the inset in Fig. 2a–c. As shown in Fig. 2d, a carbon layer with a thickness of 10–25 nm is covered on the surface of Li2 MnSiO4 nanoparticle. Based on elemental analysis, the amount of carbon was approximately 15 wt% in the Li2 MnSiO4 /C composite. 3.2. Electrochemical properties Figs. 3a–c compare the typical galvanostatic charge-discharge curves of the Li2 MnSiO4 /C composites prepared at different temperatures between 1.5 and 5.0 V with a rate of 0.05 C (16.65 mA g−1 ). It is interesting to find that the first charge curve is different from the subsequent ones, while the first and the following discharge
Fig. 6. Cycling performance of Li2 MnSiO4 /C composites obtained at various temperatures.
Fig. 7. Ex-situ XRD patterns of the Li2 MnSiO4 /C electrode: uncharged (a), charged to 4.5 V (b), charged to 4.8 V (c), discharged to 1.5 V (d) and discharged to 1.5 V at the 3rd cycle (e).
curves are similar. This phenomenon is related to the structural rearrangements of Li2 MnSiO4 in the first charge process [18,22]. As seen in Figs. 3a-c, there are no apparent potential plateaus in the first and second charge-discharge curves. However, the 30th charge-discharge curves present two sloping plateaus at two different broad potential ranges at around 3.0 and 4.2 V, corresponding to the Mn2+ /Mn3+ and Mn3+ /Mn4+ redox couples. The Li2 MnSiO4 /C synthesized at 700 ◦ C delivers the initial charge capacity of 265 mAh g−1 against discharge capacity of 194 mAh g−1 , corresponding to 1.6 Li+ ions extraction and ∼1.2 Li+ ions insertion per formula unit. The initial discharge capacities of Li2 MnSiO4 /C synthesized at 660 and 750 ◦ C are 187 and 177 mAh g−1 , respectively, indicating more than one Li+ ion transfer per formula unit. Among all three samples, the Li2 MnSiO4 /C obtained at 700 ◦ C delivers a discharge capacity of 157 mAh g−1 (charge capacity 165 mAh g−1 ) with the columbic efficiency of 95% at the 30th cycle, exhibiting the highest capacity retention of 81%. The apparent charge-discharge plateaus and high columbic efficiency indicate that the electrochemical activity and reversibility of Li2 MnSiO4 are obviously improved though a KCl–NaCl assisted molten salt process. The inferior electrochemical performance of the sample prepared at 750 ◦ C may be attributed to its large particle size and the presence of Mn2 SiO4 impurity phase. Cyclic voltammetry curves of the Li2 MnSiO4 /C are investigated at a potential scan rate of 0.1 mV s−1 between 1.5 and 5.0 V, and the first three CV curves are shown in Fig. 4a. Great difference between the 1st and the following curves is discerned. For the 1st CV curve,
Fig. 8. Nyqvist plots of the Li2 MnSiO4 /C electrode at the fully discharged state after various cycles.
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Fig. 9. SEM images of the Li2 MnSiO4 /C electrode: before cycling (a), after 10 cycles (b) and after 20 cycles (c); TEM image of the Li2 MnSiO4 /C composite after 30 cycles (d).
a broad peak at 4.4 V corresponds to the oxidation of Mn2+ to Mn3+ [20,21,34]. Another shoulder peak around 4.8 V might be attributed to the conversion from Mn3+ to Mn4+ and irreversible side reactions due to the formation of solid electrolyte interface film [35]. The inconspicuous reduction peaks in the range of 4.0 and 2.5 V are associated with the lithium ions insertion. The oxidation peaks in the 2nd and 3rd CV curves obviously shift toward the low potential at 3.4 and 4.2 V, suggesting a possibility of structural rearrangements during the first cycle. The positions of oxidation/reduction peaks at 3.1/2.8 and 4.2/4.1 V in the 30th CV curve (Fig. 4b) are ascribed to the Mn2+ /Mn3+ and Mn3+ /Mn4+ redox processes. The CV data are in good agreement with the charge-discharge behavior in Fig. 3. To further clarify the charge-discharge characteristics of Li2 MnSiO4 , the dQ/dV vs. potential plot for the 30th charge curve is presented in Fig. 4c, where the peaks correspond to the potential plateaus in the charge-discharge curve. Two oxidation peaks located at 3.1 V and 4.3 V in the charge process can be ascribed to the oxidation of Mn2+ to Mn3+ and Mn3+ to Mn4+ , respectively, which corresponds well with the CV and charge-discharge curves. Fig. 5 exhibits the rate capability of the Li2 MnSiO4 /C composites prepared at various temperatures. The charge rate remains constant at 0.05 C and the discharge rates are varied from 0.05 C to 1 C. The discharge capacities of all samples remarkably decrease with increasing current. The rapid decrease in capacity is possibly due to the poor electronic and ionic conductivity of Li2 MnSiO4. The capacity retentions of the samples prepared at 660 and 700 ◦ C at 0.5 C is 63% and 61% against the capacities at 0.05 C, respectively. In contrast, this value is only 56% for the sample prepared at 750 ◦ C, which can be attributable to long lithium ion diffusion distance within the large particles. Fig. 6 gives a comparison of the cycling performance of three Li2 MnSiO4 /C composites. The discharge capacity of the Li2 MnSiO4 /C composite prepared at 700 ◦ C decreases from 165 to 129 mAh g−1 up to 50 cycles and 78% of the initial capacity can be retained at 0.1 C rate. For the Li2 MnSiO4 /C composites prepared at
660 and 750 ◦ C, the capacity retention are also up to 74% and 70%, respectively, in spite of a relatively lower cycle capacity. The cycling stability of the submicron Li2 MnSiO4 particles prepared by molten salt method is much higher than that of most previously reported Li2 MnSiO4 nanocomposites [18–22,28]. Fig. 7 shows the ex-situ XRD patterns of the Li2 MnSiO4 /C electrode at different charge-discharge stages. XRD peaks associated with the crystal Li2 MnSiO4 phase are observed from the fresh cathode sheet. However, the diffraction peaks intensity of Li2 MnSiO4 decreases significantly when the electrode is charged to 4.5 V. When charged to 4.8 V, the peaks can hardly be distinguished from the background, indicating a transition from crystal Li2 MnSiO4 to an amorphous state after lithium ion extraction in the first charge. Furthermore, no obvious diffraction peaks corresponding to Li2 MnSiO4 are observed when the electrode is discharged to 1.5 V. Most importantly, after the subsequent two cycles, the initial crystal structure is not recovered, which means that the amorphous transition of Li2 MnSiO4 during the first charge process is irreversible and the electrochemical behaviors after the first charge reflect the characteristic of the amorphous Li2 MnSiO4 , in accordance with other reports concerning the crystal structure studies of the Li2 MnSiO4 [12,32,36,37]. The interfacial property between electrode and electrolyte is further examined by EIS measurement. Fig. 8 shows the Nyqvist plots of Li2 MnSiO4 /C at the fully discharged state of the 1st , 10th , 20th and 30th cycles. All Nyquist plots contain a semicircle in the high-to-medium frequency region and a slope line in the low frequency region. The semicircle can be attributed to the charge transfer impedance between the electrolyte and electrode material, and the slope line is related to the Li+ diffusion in the electrode material. In the inset of Fig. 8, an equivalent circuit is proposed to model the Nyquist plot. The symbols Re, Rct, CPE, and Zw represent the ohmic resistance of the cell, charge transfer resistance, double layer capacitance, and Warburg diffusion impedance, respectively. It is seen that the Li2 MnSiO4 /C electrode exhibits a relatively huge
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charge transfer resistance of 315 after the 1st cycle, which could due to the formation of sold electrolyte interface film on the surface of Li2 MnSiO4 /C composite [32]. The values of Rct decrease with the increase of the cycle number, up to 116 after the 30th cycle. Moreover, the value of Rct after the 30th cycle is close to that after the 10th cycle, suggesting that the passivating film becomes very stable after ten charge-discharge cycles and offers high ion conductivity. Fig. 9 shows the SEM and TEM images of the Li2 MnSiO4 /C electrode before cycling and after different charge-discharge cycles. Well-shaped Li2 MnSiO4 particles dispersed among Super P conductive carbon can be clearly observed from the SEM image of the fresh electrode (Fig. 9a). As shown in Fig. 9b-d, Li2 MnSiO4 particles basically retain the original spherical morphology and particle size after 10, 20 and 30 cycles, respectively. This result confirms that the crystal structure transition during cycling does not make submicron Li2 MnSiO4 break down into smaller particles. Based on the literature, the poor capacity retention of Li2 MnSiO4 /C is probably ascribed to the structural instability of the delithiated phase and manganese dissolution in the electrolyte [20,22]. According to the charge-discharge results (Fig. 3 and Fig. 4b), the amorphous Li2 MnSiO4 displays fairly good electrochemical activity and reversibility during cycling. Benefiting from the liquid reaction environment, smooth-surfaced Li2 MnSiO4 products with a larger particle size can be successfully prepared in the KCl–NaCl molten phase. The smooth surface and relatively low specific surface are beneficial to decreasing the side reactions at the electrode/electrolyte interfaces and forming stable interfacial film, which contributes to the suppression of manganese dissolution into the electrolyte. In summary, the improved electrochemical performance of Li2 MnSiO4 /C composite can be mainly ascribed to the perfect surface morphology and stable electrode/electrolyte interfacial property during charge-discharge process. 4. Conclusions In conclusion, the well-dispersed Li2 MnSiO4 particles have been successfully prepared by the KCl–NaCl molten salt method with short reaction time of 3 h. The Li2 MnSiO4 particles are coated with a relatively uniform carbon layer using sucrose and acetylene black as carbon source. The ex-situ XRD analysis indicates the amorphization of Li2 MnSiO4 during the first charge process. Additionally, the existence of two redox couples determined by CV together with the dQ/dV analysis further confirms two lithium insertion/extraction processes. At a low rate of 0.05 C, the Li2 MnSiO4 /C composite prepared at 700 ◦ C delivers an initial discharge capacity of 194 mAh g−1 . Furthermore, about 78% of the initial discharge capacity is retained after 50 cycles at 0.1 C rate. The smooth surface morphology and stable electrode/electrolyte interfacial property are favorable for improving the reversible capacity and cyclability of Li2 MnSiO4 /C composites. Acknowledgement This work was supported by Anhui Provincial Natural Science Foundation, China (No. 1308085QB41) and Special Foundation for Outstanding Young Scientists of Anhui Province, China (No. 2012SQRL226ZD). References [1] Y.-K. Sun, C.S. Yoon, S.-T. Myung, I. Belharouak, K. Amine, Role of AlF3 coating on LiCoO2 particles during cycling to cutoff voltage above 4.5 V, J. Electrochem. Soc. 156 (2009) A1005. [2] Y. Chen, K. Xie, Y. Pan, C. Zheng, Nano-sized LiMn2 O4 spinel cathode materials exhibiting high rate discharge capability for lithium-ion batteries, J. Power Sources 196 (2011) 6493.
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