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Mesoporous silica-assisted carbon free Li2MnSiO4 cathode nanoparticles for high capacity Li rechargeable batteries† Sue Jin Kim, Jungdon Suk,* Young Jun Yun, Ha-Kyun Jung and Sungho Choi* Porous and spherical Li2MnSiO4 nanoparticles have been synthesized through a facile sol–gel route via a mesoporous silica template. Galvanostatic charge–discharge of the resultant Li2MnSiO4 cathode exhibits
Received 13th August 2013, Accepted 22nd November 2013 DOI: 10.1039/c3cp53436g
enhanced charge–discharge capacity relative to that of particles prepared by the conventional sol–gel process, up to 25% in discharge capacity, even without any particulate process such as milling with conductive agents. The standout electrochemical performance could be attributed to the unique high surface-to-volume ratio, porous geometry and improved accommodation of transformation strains during
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the electrochemical lithiation–delithiation process.
Introduction During the past few decades, one of the most attractive research areas in both science and technology has been rechargeable lithium-based batteries, which are currently regarded as the power source of choice for portable devices and potential power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), etc.1,2 To meet the growing demand for high energy (and power) density, many scientists are constantly searching for new electrode materials with high energy densities and excellent capacity retention. Lithium orthosilicate Li2MSiO4 (where M = Mn2+ and Fe2+) has attracted considerable attention due to its high theoretical capacity (e.g., above 300 mA h g 1), where two lithium ions per formula can be mobile during the electrochemical reactions.3–6 Additionally, silicon, manganese, and iron are relatively safe, abundant, and low-cost, potentially making these silicate materials more sustainable. Recently, many studies have achieved Li-ion batteries with high capacity and good cycle performance using these lithium orthosilicate cathode material.7–11 Despite the advantages mentioned above, lithium orthosilicate still has some drawbacks. Low electrical conductivity (10 12–10 16 S cm 1) has often been pointed out as a major disadvantage compared to other cathode materials: LiCoO2 (10 4 S cm 1), LiMn2O4 (10 6 S cm 1), and LiFePO4 (10 9 S cm 1).6 As widely documented, the electrochemical properties of an electrode material Advanced Battery Materials Research Group, Korea Research Institute of Chemical Technology, Yuseong, 141 Gajeongro, Daejeon 305-600, Republic of Korea. E-mail: [email protected], [email protected]; Fax: +82-42-861-4151; Tel: +82-42-860-7372 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53436g
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are greatly associated with the particle properties, such as size, morphology, specific surface area, and so on. To improve electrochemical performances, nanostructured electrode materials have been applied to these batteries due to their small particle size with large surface area, leading to shorter Li+ insertion–extraction pathways and higher electrode– electrolyte contact area, which is beneficial for achieving high rate capability and good cycling performance.12–16 Thus, designing and tailoring the configuration and morphology of materials from the micron to nanometer scale presents an important challenge to materials scientists. Both the assembly and ordering of nanoparticles into complex structures have been intensively investigated by scientists to carry out the synthesis of materials displaying enhanced properties. Micro-/ meso porous structures of metal oxides are among such complex assemblies and have garnered enormous interest due to their high performance in applications such as batteries, catalysis and photocatalysis, and gas sensors.17 Templateassisted processes based on organic and inorganic templates are by far the most widely used to synthesize micro- and nanostructures of metal oxides.18–21 This approach usually involves surface modification of a template to create electrostatic or chemical interactions capable of driving the growth and selfassembly of a new material on the surface. Spheres of materials such as silica, polystyrene, and colloidal carbon have been successfully used to guide arrangement of the targeted material on their surfaces.22–24 Among these, one of the well-developed and attractive mesoporous materials is silica (SiO2), a chemically inert, thermally stable, harmless, and inexpensive material.25–27 Herein, we report an effective sol–gel route to prepare porous spherical Li2MnSiO4 nanoparticles via nanometer-size controllable pores with a high surface area SiO2 template. We found
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that these porous cathode nanoparticles significantly enhance the specific capacity and rate performance, thereby providing a new approach to increase energy storage in the lithium ion battery field. The information disclosed in this study will benefit the design of high surface area electrode materials in energy storage devices.
Experimental Samples of mesoporous or spherical SiO2 template engaged Li2MnSiO4 nanoparticles were synthesized using a modified sol–gel process. Methods for preparing mesoporous and spherical SiO2 used in this study were suggested by other groups.25,27 First, 0.3 g of mesoporous (or spherical) SiO2 particles dispersed in 30 ml of deionized water was sonicated in an ultrasonic bath for at least 1 h. 1.04 g of lithium acetate dihydrate (LiCH3COO2H2O. Sigma Aldrich, 98%) and 0.87 g of manganese acetate tetrahydrate (Mn(CH3COO)24H2O, Sigma Aldrich, 99%) were dissolved in 50 ml of deionized water. The solution was vigorously stirred at 60 1C followed by the addition of 1.06 g of citric acid (C6H8O7, Sigma Aldrich, 99.5%) and 0.83 ml of ethylene glycol (Sigma Aldrich, 99.8%) as complexation agents. 0.89 g of ascorbic acid (C6H8O6, Sigma Aldrich, 99%) was added to the resultant solution as a reducing agent. Finally, the SiO2 dispersed solution was put into the above solution and maintained at 60 1C under vigorous stirring for 3 h. The mixed solution was evaporated at 90 1C overnight to form a sol. The sol was then transferred to an oven and dried at 100 1C for 24 h. The gel was ground and heated in a reductive atmosphere (5% H2 + 95% N2) at 900 1C for 10 h. Samples of conventional sol–gel prepared Li2MnSiO4 nanoparticles were synthesized using a tetraethylorthosilicate, TEOS (Si(OC2H5)4, Sigma Aldrich, 98%). Proper amounts of lithium acetate dihydrate and manganese acetate tetrahydrate were dissolved in 60 ml of ethanol and the resultant sol was thoroughly heated at 60 1C. 1.14 ml of TEOS was added dropwise to ensure homogeneous mixing. A chelating agent, citric acid, ascorbic acid, and a requisite quantity of HNO3 were then added to maintain pH = 1. The resultant solution was maintained at 60 1C with vigorous stirring for 3 h. The sol was then transferred to an oven and dried at 90 1C for 24 h. The gel was ground and heated in a reductive atmosphere (5% H2 + 95% N2) at 900 1C for 10 h. The resultant product was milled with 20 wt% of carbon (super P) in a planetary ball mill (RETSCH, planetary ball PM100) for 3 h to form a carbon– Li2MnSiO4 composite and the mixture was then dried at 80 1C. X-ray diffraction patterns were measured using a powder diffractometer equipped with a Cu Ka source. The shape and size of as-synthesized nanoparticles were examined by Field Emission Scanning Electron Microscopy (FE-SEM, XL-30S FEG Scanning Electron Microscope, Philips) and Transmission Electron Microscopy (TEM, JEM-2100F, JEOL). Electrochemical measurements were carried out on a CR2032 type coin cell. The working electrode was formulated by mixing 80 wt% of active materials, 10 wt% of carbon black, and 10 wt% of polyvinylidene
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fluoride (PVDF) binder with a minimal amount of NMP (N-methylpyrrolidone) solvent. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate–diethyl carbonate (EC/DEC 1 : 1 v/v). All cells were assembled with lithium metal as the negative electrode and were tested in a voltage range of 1.5–4.8 V at a 0.1 C rate (current density of 33.3 mA g 1). The electrochemical impedance spectroscopy (EIS) was performed by applying an AC signal from 10 mHz to 1 MHz with 5 mVpp using an impedance analyzer (Biologic, Netherland) after the initialization process of the battery.
Results and discussion Fig. 1 shows the powder XRD patterns of the Li2MnSiO4 particles prepared by the mesoporous SiO2 template engaged reaction (named mp-LMS) and the conventional sol–gel method (named c-LMS). The orthorhombic crystal structure Li2MnSiO4 with a space group Pmn21 is favorable in terms of electrochemical properties.28,29 Both diffraction patterns are well matched to that of orthorhombic phase (JCPDS No. 01-0757861), indicating that nanocrystalline Li2MnSiO4 was successfully grown on top of the silica template as well as synthesized by TEOS-using particles. Generally, there could be some additional diffraction peaks positioned at 2y = 20–401 that might be originated from the presence of Li-silicate, Mn-silicate, and MnO, which led to the capacity decline.6,7 Under our synthetic conditions, however, we can not see any noticeable diffraction peaks regarding impurity phases within the corresponding diffraction angle region. In the case of using the spherical SiO2 template (as shown in Fig. S1(b); see ESI†), however, the resultant Li2MnSiO4 samples are partially mixed with MnO, manganese oxide (marked by the circled area in Fig. S1(a); see ESI†). The formation of the MnO phase is due to the dissolution and recrystallization of Mn acetate unreacted with the SiO2 template. Thus, the Li2MnSiO4 samples prepared by the mesoporous silica-assisted sol–gel method showed the lowest level of impurities and thus were selected for further structural and electrochemical characterization. To verify the porous structure of the mp-LMS particles, the specific surface area of as-prepared particles as well as those of the mesoporous SiO2 were characterized via a gas adsorption analysis. Fig. 2(a) and (b) show the gas
Fig. 1 X-ray powder diffraction patterns of Li2MnSiO4 samples prepared by a porous SiO2 template (mp-LMS) and by a conventional sol–gel method (c-LMS). Orthorhombic Li2MnSiO4 (JCPDS No. 01-075-7861) denoted as a reference.
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Fig. 2 N2 adsorption–desorption isotherm plots of (a) mesoporous SiO2 and (b) mp-LMS samples. Inset shows the corresponding microscopy image of the samples. (c) SEM image of carbon mixed Li2MnSiO4 samples prepared by the conventional sol–gel process. (d) BET surface area obtained from the absorption–desorption isotherms.
adsorption and desorption properties of the given particles, mesoporous SiO2 and mp-LMS. The porous SiO2 used as the template exhibits an ideal mesoporous behaviour with comparable pore size just like the previous work27 and that of the mpLMS particles is quite similar to the porous template. Thus, the resultant mp-LMS nanoparticles still preserve the high specific surface area with spherical shaped silica template properties. The measured specific surface area (BET) of the corresponding nanoparticles is presented in Fig. 2(d) and those of the c-LMS particles are also provided. From the figure, we can realize that both the porous SiO2 template and the as-prepared mp-LMS have a greater specific surface area than that of the c-LMS ball-milled with conductive carbons; 584.5 m2 g 1 and 235.1 m2 g 1, respectively. Note that the BET of the as-prepared LMS particles via the spherical SiO2 template substantially decreased as well to the same degree: about 40% of the SiO2 template (data not shown here). Nevertheless, the as-prepared LMS via the mesoporous SiO2 template (mp-LMS) still has a high specific surface area, which is expected to facilitate Li ion insertion–extraction during the electrochemical reaction and provide short pathways for rapid lithium-ion and electron conduction within the Li-ion battery electrode. The morphology of the corresponding particles was analyzed via electron microscopy and the results are also presented in Fig. 2. From the transmission electron microscopy (TEM) image shown in Fig. 2(a), spherical shaped, nearly monodisperse SiO2 particles with a diameter of 50–70 nm were presented and used as the mp-LMS template. The as-prepared mp-LMS resembles an embossed wallpaper. Importantly, the spherical SiO2 morphology is well retained, as expected, during this templateengaged sol–gel process. In contrast, the particle morphology of c-LMS is substantially different, as the resultant particles show aggregation even after pulverization followed by ball milling (Fig. 2(c)). It is well known that tailoring the particle morphology in TEOS-based sol–gel synthesis is quite complicated due to the use of gelating agents followed by high temperature postannealing, which results in irregular particulate morphology
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(similar results under our synthetic conditions are shown in Fig. S2; see ESI†).30 Note that no additional procedure such as milling with carbons to form a conductive electrode was employed for any of the mp-LMS cathode particles in this context (we have conducted an electrochemical cell test of the as-prepared mp-LMS particles followed by milling and forming a carbon–Li2MnSiO4 mixture, which leads to a poor charge capacity with agglomerated particle morphology). There can be a scenario to support the enhanced cathodic performance of mp-LMS even without any additional carbonaceous procedures; uniform carbon layers and/or homogeneous carbonaceous particles during reductive heating. Actually, we already have conducted the elemental analysis, using an Energy Dispersive Spectrometer (EDS), on both the c-LMS and mp-LMS, which led to no appreciable difference in carbon concentration as a result. Thus, as of now, the enhanced cathode performance can be attributed to the unique high specific surface area and improved accommodation of transformation strains during the electrochemical lithiation–delithiation process rather than the more uniform carbon coating onto the porous Li2MnSiO4 nanoparticles during the sol–gel process. The electrochemical property of the corresponding nanoparticles was evaluated by galvanostatic charge–discharge cycling (Fig. 3). The initial discharge capacity of the mp-LMS electrode is about 150 mA h g 1, being higher than those of the c-LMS particles (about 120 mA h g 1 and 47 mA h g 1 for c-LMS with/without milling with conductive carbons, respectively). This can be understood by the enhanced electrochemical reaction of Li+ ions and electronic conduction at a high surface/ volume with three dimensional porous cathode nanoparticles, mp-LMS. The charge–discharge plots show a sloping profile and the sol–gel prepared Li2MnSiO4 had a large irreversible capacity after the 1st charge–discharge, consistent with most previously reported studies; electronic exchange with oxygen may partially account for the observed large irreversible capacity loss found for Li, because the Li ions deintercalated during
Fig. 3 Charge–discharge profiles of (a) pristine mp-LMS, (b) c-LMS after ball milling with carbon and (c) c-LMS without ball milling with carbon. (d) Cycle retention of specific discharge capacity for the corresponding samples.
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charging cannot be intercalated into the material lattice during discharge and hence are lost from the system.5,7 Nevertheless, porous and carbon-free Li2MnSiO4 cathode nanoparticles exhibit stable charge–discharge profiles (Fig. 3(a)) which are more comparable to that of the sample prepared using the conventional sol–gel method followed by carbon coating, Fig. 3(b) and (c). The porous framework can offer an increased specific surface area and a shorten diffusion length for Li+ ions even without any particulate process. Improved electrochemical performance of cathodes by using unique nanoparticle morphology has been reported elsewhere.9,21 Additionally, the unique structure could accommodate volume expansion, provide more reaction sites on the surface, and provide a short length for Li+ insertion, which could enhance charge transfer and electron conduction within the template framework. This is analogous to the mesoporous SiO2 template-engaged Li2MnSiO4 in this context. Fig. 3(d) shows the cycling performance of resultant Li2MnSiO4 cathode materials carried out at a rate of 0.1 C at room temperature. The discharge capacities of the three materials all decrease gradually with the cycling number except for c-LMS without any particulate modification. The discharge capacities of the porous SiO2-supported samples decreased rapidly with an increasing number of cycles from the first cycle, while the discharge capacity stabilized in the subsequent cycles irrespective of the template-engaged reactions. Additional conductive agents as well as optimization of the synthesis conditions are likely to lead to enhanced cycle properties. Further studies are needed to explore the relationship between nanoparticle morphology and Li intercalation activity regarding the degree of charge–discharge capacity of low electrical conductive cathode materials. The impedance analysis can probe and detect Li+ ion migration within cathode Li2MnSiO4 lattices and resistance components at various charge–discharge states of the battery.4,31 Fig. 4 shows Nyquist plots obtained from the coin cell with the cathode by using mp-LMS and c-LMS after ball milling with carbon, respectively, which shows the similar discharge capacity (Fig. 3).
PCCP Table 1 Fitting parameters obtained from the electrochemical impedance spectroscopy (EIS) data to the equivalent circuits shown in Fig. 4
Sample mp-LMS c-LMS
RS (O) 1.22 0.69
Cdl (F) 1.73 10 1.37 10
5 5
Rct (O)
WR (O)
Cint (F)
275 367
15.2 149.8
0.001 0.01
The equivalent circuit model used for fitting those impedance data during cycling after 5 cycles is also shown and the resultant parameters are listed in Table 1. The symbols RS, Rct, Cdl, WR and Cint imply the electrolyte resistance, the charge transfer resistance, the double layer capacitance, the Warburg resistance and the intercalation capacitance, respectively.31 The depressed semicircle is ascribed to the charge transfer process and the slope line is related to the diffusion of lithium ions. In the middle frequency region, electrodes with mp-LMS exhibit a charge transfer resistance of 275 O, which is smaller than 367 O for even c-LMS. It seems to be a marked decrease in impedance of mp-LMS especially on WR and Cint as well. This tendency implied that the porous structured nanoparticles can increase the electrode conductivity followed by enhancing the charge transfer during the electrochemical reaction of Li2MnSiO4 even without any particulate process such as milling with conductive carbons. Nevertheless, we need to consider other ways to evaluate our template-originated enhanced electrode performance. Generally, the additional conductive carbon will be effective on the overall electrode performance of Li+ ion rechargeable batteries, notably in high impedance oxide nanoparticles. In this work, we have utilized the high energy ball milling process mixing with the conductive carbon, thus the final particle morphology might be substantially changed (broken to pieces and/or agglomeration) compared to the well-defined original spherical and porous structure one. As a result, the origin of the enhanced discharge–charge capacity induced by the porousstructured particles will be more ambiguous. More controlled experiments for forming the uniform conductive layer on the cathode nanoparticles to retain the unique template structure are needed.
Conclusions
Fig. 4 Impedance spectra of the coin cell assembly with the c-LMS after ball milling with carbon (black) and mp-LMS (red). Frequency range is from 10 mHz to 1 MHz. Inset is the equivalent circuits of EIS.
2088 | Phys. Chem. Chem. Phys., 2014, 16, 2085--2089
Mesoporous SiO2-supported Li2MnSiO4 cathode nanoparticles were synthesized through an effective sol–gel route. The prepared Li2MnSiO4 materials with densely packed spherical nanoparticles show substantially increased specific charge– discharge capacity even without any carbonaceous conductive additives or milling processes. The outstanding electrochemical performance is ascribed to the unique pore geometry and improved accommodation of transformation strains during cycling, which allow for short electron transport pathways while offering sufficient lithium-ion storage sites during Li+ insertion–extraction. Although the electrochemical performance of mesoporous template engaged Li2MnSiO4 cathodes must be improved, especially cycle retention, our results suggest a new
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strategy for preparing Li2MnSiO4 cathode nanoparticles that may lead to enhanced charge–discharge capacity.
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15 Z. Qin, X. Zhou, Y. Xia, C. Tang and Z. Liu, J. Mater. Chem., 2012, 22, 21144–21153. 16 M. K. Devaraju and I. Honma, Adv. Energy Mater., 2012, 2, 284–297. 17 I. E. Wachs, Catal. Today, 2005, 100, 79–94. 18 N. Du, H. Zhang, J. Chen, J. Y. Sun, B. D. Chen and D. R. Yang, J. Phys. Chem. B, 2008, 112, 14836–14842. 19 H. M. Galindo, Y. Carvajal, E. Njagi, R. A. Ristau and S. L. Suib, Langmuir, 2010, 26(16), 13677–13683. 20 J. Yu, W. Liu and H. Yu, Cryst. Growth Des., 2008, 8, 930–934. 21 Y. L. Ding, J. Xie, G. S. Cao, T. J. Zhu, H. M. Yu and X. B. Zhao, Adv. Funct. Mater., 2011, 21, 348–355. 22 H. Cheng and K. Scott, J. Power Sources, 2010, 195, 1370–1374. 23 X. Song and L. Gao, J. Phys. Chem. C, 2007, 111, 8180–8187. 24 X. Wang, P. Hu, Y. F. Li and L. J. Yu, J. Phys. Chem. C, 2007, 111, 6706–6712. 25 W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, 62–69. 26 L. Li, J. Ding and J. M. Xue, Chem. Mater., 2009, 21(15), 3629–3637. 27 A. B. D. Nandiyanto, S. G. Kim, F. Iskandar and K. Okuyama, Microporous Mesoporous Mater., 2009, 120, 447–453. 28 M. E. Arroyo-deDompablo, R. Dominko, J. M. Gallardo-Amores, L. Dupont, G. Mali, H. Ehrenberg, J. Jamnik and E. Moran, Chem. Mater., 2008, 20, 5574–5584. 29 V. V. Politaev, A. A. Petrenko, V. B. Nalbandyan, B. S. Medvedev and E. S. Shvetsova, J. Solid State Chem., 2007, 180(3), 1045–1050. 30 R. Dominiko, M. Bele, A. Kokalj, M. Gaberscek and J. Jamnik, J. Power Sources, 2007, 174, 457–461. 31 F. Wang, J. Chen, C. Wang and B. Yi, J. Electroanal. Chem., 2013, 688, 123–129.
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Mesoporous silica-assisted carbon free Li2MnSiO4 cathode nanoparticles for high capacity Li rechargeable batteries† Sue Jin Kim, Jungdon Suk,* Young Jun Yun, Ha-Kyun Jung and Sungho Choi* Porous and spherical Li2MnSiO4 nanoparticles have been synthesized through a facile sol–gel route via a mesoporous silica template. Galvanostatic charge–discharge of the resultant Li2MnSiO4 cathode exhibits
Received 13th August 2013, Accepted 22nd November 2013 DOI: 10.1039/c3cp53436g
enhanced charge–discharge capacity relative to that of particles prepared by the conventional sol–gel process, up to 25% in discharge capacity, even without any particulate process such as milling with conductive agents. The standout electrochemical performance could be attributed to the unique high surface-to-volume ratio, porous geometry and improved accommodation of transformation strains during
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the electrochemical lithiation–delithiation process.
Introduction During the past few decades, one of the most attractive research areas in both science and technology has been rechargeable lithium-based batteries, which are currently regarded as the power source of choice for portable devices and potential power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), etc.1,2 To meet the growing demand for high energy (and power) density, many scientists are constantly searching for new electrode materials with high energy densities and excellent capacity retention. Lithium orthosilicate Li2MSiO4 (where M = Mn2+ and Fe2+) has attracted considerable attention due to its high theoretical capacity (e.g., above 300 mA h g 1), where two lithium ions per formula can be mobile during the electrochemical reactions.3–6 Additionally, silicon, manganese, and iron are relatively safe, abundant, and low-cost, potentially making these silicate materials more sustainable. Recently, many studies have achieved Li-ion batteries with high capacity and good cycle performance using these lithium orthosilicate cathode material.7–11 Despite the advantages mentioned above, lithium orthosilicate still has some drawbacks. Low electrical conductivity (10 12–10 16 S cm 1) has often been pointed out as a major disadvantage compared to other cathode materials: LiCoO2 (10 4 S cm 1), LiMn2O4 (10 6 S cm 1), and LiFePO4 (10 9 S cm 1).6 As widely documented, the electrochemical properties of an electrode material Advanced Battery Materials Research Group, Korea Research Institute of Chemical Technology, Yuseong, 141 Gajeongro, Daejeon 305-600, Republic of Korea. E-mail: [email protected], [email protected]; Fax: +82-42-861-4151; Tel: +82-42-860-7372 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53436g
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are greatly associated with the particle properties, such as size, morphology, specific surface area, and so on. To improve electrochemical performances, nanostructured electrode materials have been applied to these batteries due to their small particle size with large surface area, leading to shorter Li+ insertion–extraction pathways and higher electrode– electrolyte contact area, which is beneficial for achieving high rate capability and good cycling performance.12–16 Thus, designing and tailoring the configuration and morphology of materials from the micron to nanometer scale presents an important challenge to materials scientists. Both the assembly and ordering of nanoparticles into complex structures have been intensively investigated by scientists to carry out the synthesis of materials displaying enhanced properties. Micro-/ meso porous structures of metal oxides are among such complex assemblies and have garnered enormous interest due to their high performance in applications such as batteries, catalysis and photocatalysis, and gas sensors.17 Templateassisted processes based on organic and inorganic templates are by far the most widely used to synthesize micro- and nanostructures of metal oxides.18–21 This approach usually involves surface modification of a template to create electrostatic or chemical interactions capable of driving the growth and selfassembly of a new material on the surface. Spheres of materials such as silica, polystyrene, and colloidal carbon have been successfully used to guide arrangement of the targeted material on their surfaces.22–24 Among these, one of the well-developed and attractive mesoporous materials is silica (SiO2), a chemically inert, thermally stable, harmless, and inexpensive material.25–27 Herein, we report an effective sol–gel route to prepare porous spherical Li2MnSiO4 nanoparticles via nanometer-size controllable pores with a high surface area SiO2 template. We found
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that these porous cathode nanoparticles significantly enhance the specific capacity and rate performance, thereby providing a new approach to increase energy storage in the lithium ion battery field. The information disclosed in this study will benefit the design of high surface area electrode materials in energy storage devices.
Experimental Samples of mesoporous or spherical SiO2 template engaged Li2MnSiO4 nanoparticles were synthesized using a modified sol–gel process. Methods for preparing mesoporous and spherical SiO2 used in this study were suggested by other groups.25,27 First, 0.3 g of mesoporous (or spherical) SiO2 particles dispersed in 30 ml of deionized water was sonicated in an ultrasonic bath for at least 1 h. 1.04 g of lithium acetate dihydrate (LiCH3COO2H2O. Sigma Aldrich, 98%) and 0.87 g of manganese acetate tetrahydrate (Mn(CH3COO)24H2O, Sigma Aldrich, 99%) were dissolved in 50 ml of deionized water. The solution was vigorously stirred at 60 1C followed by the addition of 1.06 g of citric acid (C6H8O7, Sigma Aldrich, 99.5%) and 0.83 ml of ethylene glycol (Sigma Aldrich, 99.8%) as complexation agents. 0.89 g of ascorbic acid (C6H8O6, Sigma Aldrich, 99%) was added to the resultant solution as a reducing agent. Finally, the SiO2 dispersed solution was put into the above solution and maintained at 60 1C under vigorous stirring for 3 h. The mixed solution was evaporated at 90 1C overnight to form a sol. The sol was then transferred to an oven and dried at 100 1C for 24 h. The gel was ground and heated in a reductive atmosphere (5% H2 + 95% N2) at 900 1C for 10 h. Samples of conventional sol–gel prepared Li2MnSiO4 nanoparticles were synthesized using a tetraethylorthosilicate, TEOS (Si(OC2H5)4, Sigma Aldrich, 98%). Proper amounts of lithium acetate dihydrate and manganese acetate tetrahydrate were dissolved in 60 ml of ethanol and the resultant sol was thoroughly heated at 60 1C. 1.14 ml of TEOS was added dropwise to ensure homogeneous mixing. A chelating agent, citric acid, ascorbic acid, and a requisite quantity of HNO3 were then added to maintain pH = 1. The resultant solution was maintained at 60 1C with vigorous stirring for 3 h. The sol was then transferred to an oven and dried at 90 1C for 24 h. The gel was ground and heated in a reductive atmosphere (5% H2 + 95% N2) at 900 1C for 10 h. The resultant product was milled with 20 wt% of carbon (super P) in a planetary ball mill (RETSCH, planetary ball PM100) for 3 h to form a carbon– Li2MnSiO4 composite and the mixture was then dried at 80 1C. X-ray diffraction patterns were measured using a powder diffractometer equipped with a Cu Ka source. The shape and size of as-synthesized nanoparticles were examined by Field Emission Scanning Electron Microscopy (FE-SEM, XL-30S FEG Scanning Electron Microscope, Philips) and Transmission Electron Microscopy (TEM, JEM-2100F, JEOL). Electrochemical measurements were carried out on a CR2032 type coin cell. The working electrode was formulated by mixing 80 wt% of active materials, 10 wt% of carbon black, and 10 wt% of polyvinylidene
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fluoride (PVDF) binder with a minimal amount of NMP (N-methylpyrrolidone) solvent. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate–diethyl carbonate (EC/DEC 1 : 1 v/v). All cells were assembled with lithium metal as the negative electrode and were tested in a voltage range of 1.5–4.8 V at a 0.1 C rate (current density of 33.3 mA g 1). The electrochemical impedance spectroscopy (EIS) was performed by applying an AC signal from 10 mHz to 1 MHz with 5 mVpp using an impedance analyzer (Biologic, Netherland) after the initialization process of the battery.
Results and discussion Fig. 1 shows the powder XRD patterns of the Li2MnSiO4 particles prepared by the mesoporous SiO2 template engaged reaction (named mp-LMS) and the conventional sol–gel method (named c-LMS). The orthorhombic crystal structure Li2MnSiO4 with a space group Pmn21 is favorable in terms of electrochemical properties.28,29 Both diffraction patterns are well matched to that of orthorhombic phase (JCPDS No. 01-0757861), indicating that nanocrystalline Li2MnSiO4 was successfully grown on top of the silica template as well as synthesized by TEOS-using particles. Generally, there could be some additional diffraction peaks positioned at 2y = 20–401 that might be originated from the presence of Li-silicate, Mn-silicate, and MnO, which led to the capacity decline.6,7 Under our synthetic conditions, however, we can not see any noticeable diffraction peaks regarding impurity phases within the corresponding diffraction angle region. In the case of using the spherical SiO2 template (as shown in Fig. S1(b); see ESI†), however, the resultant Li2MnSiO4 samples are partially mixed with MnO, manganese oxide (marked by the circled area in Fig. S1(a); see ESI†). The formation of the MnO phase is due to the dissolution and recrystallization of Mn acetate unreacted with the SiO2 template. Thus, the Li2MnSiO4 samples prepared by the mesoporous silica-assisted sol–gel method showed the lowest level of impurities and thus were selected for further structural and electrochemical characterization. To verify the porous structure of the mp-LMS particles, the specific surface area of as-prepared particles as well as those of the mesoporous SiO2 were characterized via a gas adsorption analysis. Fig. 2(a) and (b) show the gas
Fig. 1 X-ray powder diffraction patterns of Li2MnSiO4 samples prepared by a porous SiO2 template (mp-LMS) and by a conventional sol–gel method (c-LMS). Orthorhombic Li2MnSiO4 (JCPDS No. 01-075-7861) denoted as a reference.
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Fig. 2 N2 adsorption–desorption isotherm plots of (a) mesoporous SiO2 and (b) mp-LMS samples. Inset shows the corresponding microscopy image of the samples. (c) SEM image of carbon mixed Li2MnSiO4 samples prepared by the conventional sol–gel process. (d) BET surface area obtained from the absorption–desorption isotherms.
adsorption and desorption properties of the given particles, mesoporous SiO2 and mp-LMS. The porous SiO2 used as the template exhibits an ideal mesoporous behaviour with comparable pore size just like the previous work27 and that of the mpLMS particles is quite similar to the porous template. Thus, the resultant mp-LMS nanoparticles still preserve the high specific surface area with spherical shaped silica template properties. The measured specific surface area (BET) of the corresponding nanoparticles is presented in Fig. 2(d) and those of the c-LMS particles are also provided. From the figure, we can realize that both the porous SiO2 template and the as-prepared mp-LMS have a greater specific surface area than that of the c-LMS ball-milled with conductive carbons; 584.5 m2 g 1 and 235.1 m2 g 1, respectively. Note that the BET of the as-prepared LMS particles via the spherical SiO2 template substantially decreased as well to the same degree: about 40% of the SiO2 template (data not shown here). Nevertheless, the as-prepared LMS via the mesoporous SiO2 template (mp-LMS) still has a high specific surface area, which is expected to facilitate Li ion insertion–extraction during the electrochemical reaction and provide short pathways for rapid lithium-ion and electron conduction within the Li-ion battery electrode. The morphology of the corresponding particles was analyzed via electron microscopy and the results are also presented in Fig. 2. From the transmission electron microscopy (TEM) image shown in Fig. 2(a), spherical shaped, nearly monodisperse SiO2 particles with a diameter of 50–70 nm were presented and used as the mp-LMS template. The as-prepared mp-LMS resembles an embossed wallpaper. Importantly, the spherical SiO2 morphology is well retained, as expected, during this templateengaged sol–gel process. In contrast, the particle morphology of c-LMS is substantially different, as the resultant particles show aggregation even after pulverization followed by ball milling (Fig. 2(c)). It is well known that tailoring the particle morphology in TEOS-based sol–gel synthesis is quite complicated due to the use of gelating agents followed by high temperature postannealing, which results in irregular particulate morphology
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(similar results under our synthetic conditions are shown in Fig. S2; see ESI†).30 Note that no additional procedure such as milling with carbons to form a conductive electrode was employed for any of the mp-LMS cathode particles in this context (we have conducted an electrochemical cell test of the as-prepared mp-LMS particles followed by milling and forming a carbon–Li2MnSiO4 mixture, which leads to a poor charge capacity with agglomerated particle morphology). There can be a scenario to support the enhanced cathodic performance of mp-LMS even without any additional carbonaceous procedures; uniform carbon layers and/or homogeneous carbonaceous particles during reductive heating. Actually, we already have conducted the elemental analysis, using an Energy Dispersive Spectrometer (EDS), on both the c-LMS and mp-LMS, which led to no appreciable difference in carbon concentration as a result. Thus, as of now, the enhanced cathode performance can be attributed to the unique high specific surface area and improved accommodation of transformation strains during the electrochemical lithiation–delithiation process rather than the more uniform carbon coating onto the porous Li2MnSiO4 nanoparticles during the sol–gel process. The electrochemical property of the corresponding nanoparticles was evaluated by galvanostatic charge–discharge cycling (Fig. 3). The initial discharge capacity of the mp-LMS electrode is about 150 mA h g 1, being higher than those of the c-LMS particles (about 120 mA h g 1 and 47 mA h g 1 for c-LMS with/without milling with conductive carbons, respectively). This can be understood by the enhanced electrochemical reaction of Li+ ions and electronic conduction at a high surface/ volume with three dimensional porous cathode nanoparticles, mp-LMS. The charge–discharge plots show a sloping profile and the sol–gel prepared Li2MnSiO4 had a large irreversible capacity after the 1st charge–discharge, consistent with most previously reported studies; electronic exchange with oxygen may partially account for the observed large irreversible capacity loss found for Li, because the Li ions deintercalated during
Fig. 3 Charge–discharge profiles of (a) pristine mp-LMS, (b) c-LMS after ball milling with carbon and (c) c-LMS without ball milling with carbon. (d) Cycle retention of specific discharge capacity for the corresponding samples.
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charging cannot be intercalated into the material lattice during discharge and hence are lost from the system.5,7 Nevertheless, porous and carbon-free Li2MnSiO4 cathode nanoparticles exhibit stable charge–discharge profiles (Fig. 3(a)) which are more comparable to that of the sample prepared using the conventional sol–gel method followed by carbon coating, Fig. 3(b) and (c). The porous framework can offer an increased specific surface area and a shorten diffusion length for Li+ ions even without any particulate process. Improved electrochemical performance of cathodes by using unique nanoparticle morphology has been reported elsewhere.9,21 Additionally, the unique structure could accommodate volume expansion, provide more reaction sites on the surface, and provide a short length for Li+ insertion, which could enhance charge transfer and electron conduction within the template framework. This is analogous to the mesoporous SiO2 template-engaged Li2MnSiO4 in this context. Fig. 3(d) shows the cycling performance of resultant Li2MnSiO4 cathode materials carried out at a rate of 0.1 C at room temperature. The discharge capacities of the three materials all decrease gradually with the cycling number except for c-LMS without any particulate modification. The discharge capacities of the porous SiO2-supported samples decreased rapidly with an increasing number of cycles from the first cycle, while the discharge capacity stabilized in the subsequent cycles irrespective of the template-engaged reactions. Additional conductive agents as well as optimization of the synthesis conditions are likely to lead to enhanced cycle properties. Further studies are needed to explore the relationship between nanoparticle morphology and Li intercalation activity regarding the degree of charge–discharge capacity of low electrical conductive cathode materials. The impedance analysis can probe and detect Li+ ion migration within cathode Li2MnSiO4 lattices and resistance components at various charge–discharge states of the battery.4,31 Fig. 4 shows Nyquist plots obtained from the coin cell with the cathode by using mp-LMS and c-LMS after ball milling with carbon, respectively, which shows the similar discharge capacity (Fig. 3).
PCCP Table 1 Fitting parameters obtained from the electrochemical impedance spectroscopy (EIS) data to the equivalent circuits shown in Fig. 4
Sample mp-LMS c-LMS
RS (O) 1.22 0.69
Cdl (F) 1.73 10 1.37 10
5 5
Rct (O)
WR (O)
Cint (F)
275 367
15.2 149.8
0.001 0.01
The equivalent circuit model used for fitting those impedance data during cycling after 5 cycles is also shown and the resultant parameters are listed in Table 1. The symbols RS, Rct, Cdl, WR and Cint imply the electrolyte resistance, the charge transfer resistance, the double layer capacitance, the Warburg resistance and the intercalation capacitance, respectively.31 The depressed semicircle is ascribed to the charge transfer process and the slope line is related to the diffusion of lithium ions. In the middle frequency region, electrodes with mp-LMS exhibit a charge transfer resistance of 275 O, which is smaller than 367 O for even c-LMS. It seems to be a marked decrease in impedance of mp-LMS especially on WR and Cint as well. This tendency implied that the porous structured nanoparticles can increase the electrode conductivity followed by enhancing the charge transfer during the electrochemical reaction of Li2MnSiO4 even without any particulate process such as milling with conductive carbons. Nevertheless, we need to consider other ways to evaluate our template-originated enhanced electrode performance. Generally, the additional conductive carbon will be effective on the overall electrode performance of Li+ ion rechargeable batteries, notably in high impedance oxide nanoparticles. In this work, we have utilized the high energy ball milling process mixing with the conductive carbon, thus the final particle morphology might be substantially changed (broken to pieces and/or agglomeration) compared to the well-defined original spherical and porous structure one. As a result, the origin of the enhanced discharge–charge capacity induced by the porousstructured particles will be more ambiguous. More controlled experiments for forming the uniform conductive layer on the cathode nanoparticles to retain the unique template structure are needed.
Conclusions
Fig. 4 Impedance spectra of the coin cell assembly with the c-LMS after ball milling with carbon (black) and mp-LMS (red). Frequency range is from 10 mHz to 1 MHz. Inset is the equivalent circuits of EIS.
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Mesoporous SiO2-supported Li2MnSiO4 cathode nanoparticles were synthesized through an effective sol–gel route. The prepared Li2MnSiO4 materials with densely packed spherical nanoparticles show substantially increased specific charge– discharge capacity even without any carbonaceous conductive additives or milling processes. The outstanding electrochemical performance is ascribed to the unique pore geometry and improved accommodation of transformation strains during cycling, which allow for short electron transport pathways while offering sufficient lithium-ion storage sites during Li+ insertion–extraction. Although the electrochemical performance of mesoporous template engaged Li2MnSiO4 cathodes must be improved, especially cycle retention, our results suggest a new
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strategy for preparing Li2MnSiO4 cathode nanoparticles that may lead to enhanced charge–discharge capacity.
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