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carbon hybrid with ultrasmall nanoparticles for increasing lithium storage capacity during cycling
Carbon 99 (2016) 138e147
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Carbon journal homepage: www.elsevier.com/locate/carbon
Crosslinking-derived MnO/carbon hybrid with ultrasmall nanoparticles for increasing lithium storage capacity during cycling Danmiao Kang a, Qinglei Liu a, *, Rui Si b, Jiajun Gu a, Wang Zhang a, Di Zhang a, ** a
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, PR China Shanghai Institute of Applied Physics, Chinese Academy Sciences, Shanghai Synchrotron Radiation Facility, 239 Zhangheng Road, Shanghai, 201204, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2015 Received in revised form 25 November 2015 Accepted 28 November 2015 Available online 9 December 2015
MnO/C hybrid with ultra-small MnO nanoparticles (5.2 nm) embedded in porous carbon is fabricated and applied as the anode material for lithium ion batteries. The MnO/C hybrid electrode exhibits excellent specific capacity of 820 mAh g1 at a current density of 100 mA g1 and also outstanding rate performance with specific capacity of 483 mAh g1 at a current density of 5000 mA g1. After cycling for 1000 times at 1000 mA g1, the specific capacity increases to 1625 mAh g1. The TEM photos show that the particles are broken into 2 nm particles after cycling. We also apply XAFS to detect the final state of the Mn in the hybrid electrodes and find that smaller MnO particles are oxidized to a mixture of Mn2O3 and MnO2. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have attracted increasing attention in the past two decades for its high capacity and widely usages in hybrid electrical vehicle and portable consumers. However, the most studied anode materials for LIBs (such as Si and Sn) are still suffering from limited cycling performance due to the huge volume expansion during the lithiation and delithiation process [1e4]. Instead of the alloying/dealloying mechanism, MnO storages energy by the following conversion reaction: MnO þ 2Li 4 Mn þ Li2O [3] With a high theoretical specific capacity (756 mAh g1), low cost, large abundance (1.06‰ as fraction of Earth's crust) and environmental friendliness, MnO becomes one of the most attractive anode materials for LIBs [4e14]. Since conversion reaction is highly affected by the size of the active materials [15], nano-scale MnO is usually preferred to be used as electrode materials. However, MnO particles tend to aggregate during the chargeedischarge processing, which results
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Zhang).
(Q.
http://dx.doi.org/10.1016/j.carbon.2015.11.068 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
Liu),
[email protected]
in heavy fading of capacity of the electrode. To solve this problem, MnO nanoparticles have been embedded in various carbon matrixes to separate these particles [4e14,16e31]. For example, Jiang et al. embedded MnO in carbon nanotubes to fabricate the MnO/C nanopeapods, which showed high specific capacity of 1119 mAh g1 at 500 mA g1, excellent rate performance of 463 mAh g1 at 5000 mA g1 and long life with 1000 cycles [5]. Sun et al. fabricated MnO/graphene hybrid electrode with conformal nanoparticles. With a current density of 2000 mA g1, the hybrid electrode sustained 400 cycles with only 4% capacity fading [8]. Other than graphene and CNTs, composites of MnO/C could also be fabricated by using biomass template, such as microalgae. The composites with highly developed porous network got a first discharge capacity of 1021.9 mAh g1, and the capacity is maintained at 702.2 mAh g1 after 50 cycles [6]. Carbon nanosheet with 3D structure was used as the carbon matrix for the MnO/C composite and the capacity of the composite reached as high as 1332 mAh g1 at the first discharge cycle and retained at 567 mAh g1 at a current density of 5000 mA g1 [13]. However, in previous studies, most of the MnO/C composites were fabricated by reduction of MnO4 or hydrolysis of the weak acid salt of Mn2þ (such as Mn(CH3COO)2) occurred on the carbon (or carbon precursor) matrix. The sizes of MnO nanoparticles as obtained were usually large than 10 nm, which impeded the fully usage of the active materials. Alginate has a unique character that the guluronic chains could perform crosslinking reaction with polyvalent metal ions to form
D. Kang et al. / Carbon 99 (2016) 138e147
gels [32e34]. Based on this character, alginate was used as the precursor for fabrication of the CoNi alloy nanoparticles and Fe2O3alginate gel [35,36]. Different carbon/metal hybrids could also be fabricated by carbonization of the alginate-derived gels [50]. Here we applied alginate as the building blocks to build a MnO/C hybrid with merits for being used as anode material for LIBs. The ultrasmall particle size (~5 nm) of MnO contributes to capacity of 1080 mAh g1 at the 1st discharge cycle and 820 mAh g1 at the following discharge cycle (100 mA g1). The interconnected porous network ensures the smooth diffusion and penetration of ions into the networks and the hybrid exhibits specific capacity of 483 mAh g1 at a current density of 5000 mA g1. The specific capacity increases during the 1000 cycles at current density of 2000 mA g1 (from 600 to 1280 mAh g1). To investigate the effect of the particle size on the energy storage performance, we also fabricated MnO/C with large particle size of about 100 nm (MnO/C100). It is found that the electrode with large particle size shows low capacity of 630 mAh g1 (100 mA g1) and poor capacity retention of 10% at current density of 5000 mA g1. The structure and chemical composition of these hybrids after cycling are detected to illustrate this phenomenon. It is found that during cycling, small nanoparticles in MnO/C-5 are broken into smaller particle (down to 2 nm), while most of the nanoparticles in MnO/C-100 aggregate into larger particles. Moreover, the Mn element is oxidized to higher state for both of the two hybrid electrodes. For MnO/C-5 hybrid electrode, the chemical state of Mn is composing of Mn(III) and Mn(IV), while in MnO/C-100 hybrid electrode, the chemical state of Mn is composing of Mn(II) and Mn(III).
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Shanghai Synchrotron Radiation Facility (SSRF). Thermogravimetric (TGA) was performed on a TG-DSC analyzer (NETZSCH, STA 449 F3) in air with a temperature ranging from room temperature to 800 C. The contents of MnO in the composites were determined by the following equation:
MnOðwt:%Þ ¼
MðMnOÞ 2 W 100%; MðMn2 O3 Þ
in which M is the molecular weight and W is the weight percent of the residue after heat treatment in air. 2.3. Electrochemical measurement
2. Experimental section
The working electrodes were fabricated by coating slurry containing active materials, super-P and PVDF dissolved in NMP (0.02 g mL1) (8:1:1) onto a copper foil and dried at 60 C for 12 h. The coated copper foil was then cut into disk electrode and pressed at 4 MPa for 1 min. Then the electrodes were dried at 60 C for another 12 h before being transferred into the glove box. The electrochemical performance of the hybrids was examined in 2032 type coin cells. The lithium foil was used as the counter electrode (negative electrode), and Celgard 2500 was used as the separator. Electrolyte was 1 M LiPF6 in mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v ¼ 1:1). The coin cells were assembled in an argon-filled glove box with moisture and oxygen below 0.5 ppm. The chargeedischarge cycles were tested on Land CT2001A at a voltage window of 0.005e3.0 V (vs Li/Liþ). The CV with a scan rate of 0.1 mV/s (0.005e3.0 V) and EIS tests with a frequency range of 30 mHze500 kHz and ac amplitude of 5 mV were performed on VMP3 electrochemical measurement.
2.1. Material synthesis
3. Results and discussion
Alginate was dissolved in deionized water to obtain 1.5 wt. % solution. Mn(NO3)2 solution of 0.34 mol L1 in deionized water was used as crosslink agent. Then the alginate was added into the metal salt solutions with a volume ratio of 2:5 (alginate: metal salt solution). The gels were obtained after 15 h crosslinking and washed before being freeze dried for 24 h. Carbonization of the gels was conducted in nitrogen flow rate of 600 mL min1. The temperature was maintained at 600 C for 1 h. The obtained carbon/MnO hybrid was named as MnO/C-5. To fabricate MnO/C-100, sodium chloride was added to the metal solution (with an atom ratio of Na: Mn to 1:1) before reaction and the following steps were the same as those of MnO/C-5.
3.1. The structure characterization of the MnO/C hybrids
2.2. Material characterization Materials were analyzed by scanning electron microscopy (SEM, FEI Quanta FEM 250), transmission electron microscopy (TEM, JOEL 2100 F) and high resolution TEM (HRTEM). All the TEM samples were dispersed in ethanol and treated with ultra-sound for 40 min and then a drop of dispersion was loaded on copper grid supported with ultrathin carbon film. X-ray diffraction (XRD) was conducted on a Rigaku D/Max-2500 with Cu-Ka radiation. The porous structure was characterized by mean of N2 adsorption-desorption at 77.4 K (Micrometritics ASAP 2020). Before the gas adsorption measurements, the samples were degassed at 200 C for 6 h in vacuum. X-ray photoelectron spectroscopy (XPS) spectra were recorded on AXIS UltraDLD spectrometer from Kratos using 120 W Al Ka radiation with a chamber pressure of 5 109 Torr. All the peak positions detected by XPS were calibrated based on sp2 carbon peak located at 284.8 eV. Ex-situ measurement of X-ray absorption fine structure (XAFS) was performed at the beamline (BL14W1) in
As shown is Fig. 1, the MnO/C hybrids are fabricated by carbonization of self-assembled gels formed by crosslinking of alginate and Mn2þ ions. The microstructure of the as fabricated MnO/C-5 is shown in Fig. 2. It has a fine nanostructure with ultrasmall MnO particles uniformly dispersed on a partial graphitic porous carbon matrix. As shown in Fig. 1a, the carbon matrix in MnO/C hybrid is a porous network. Fig. 1b and c shows that nanoparticles with size of around 5 nm are homogenously distributed on the carbon matrix. HRTEM (Fig. 2d) shows that nanoparticles are tightly surrounded by the partial graphitic carbon layers. This nanostructure is derived from the crosslinked zones formed during the spontaneous reaction between Mn2þ ions and guluronic acid units. There are around 20 ions in a specific crosslinked zone [32e34]. After freeze drying and carbonization, these ions melt into nanoparticles with size around 5 nm. The carbon chains are broken down during the heat treatment and the carbon chains are squeezed to the edges of the nanoparticles and thus form the carbon layers tightly surrounding these nanoparticles [50]. The pore structures of the MnO/C hybrids were detected by N2 adsorption-desorption. As indicated by the isotherms of MnO/C hybrids (Fig. 3), which show type-IV curves with obvious increase absorbed nitrogen and hysteria loops at moderate relative pressure, there are large amount of mesopores in the hybrids. The pore size distributions demonstrate the interconnected mesopore structures of MnO/C hybrids, especially the small mesopores with size around 2e4 nm. As concluded in Table 1, the MnO/C-5 has a BET specific surface area of 180 m2 g1, and the ratio of mesopore volume to the micropore volume is 9.50, while the values of MnO/C-100 are 158 m2 g1 and 1.75, respectively. It means the surface areas of the
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Fig. 1. The fabrication of the MnO/C hybrid by crosslinking of alginate with Mn2þ ions. (A colour version of this figure can be viewed online.)
Fig. 2. SEM photo (a) and (b), TEM (c) and HRTEM (d) photos of MnO/C-5. (A colour version of this figure can be viewed online.)
two composites are comparable, while there are more small mesopores in MnO/C-5 than in MnO/C-100. And both of the hybrids have hierarchical porous structure, which is vital for ion transportation when they are applied in energy storage.
3.2. The chemical composition of MnO/C hybrids Fig. 4 shows the X-ray diffraction (XRD) spectra of the MnO/C hybrids as obtained. The diffraction peaks could be indexed to the cubic MnO (JCPDS card no. 07-0230) and there is no other impurity in the obtained hybrids. For both of the hybrids, the MnO particles show high crystallinity with a carbonization temperature of 600 C. The sizes of the MnO nanoparticles calculated by Scherer formula are 5.2 nm and 36.0 nm for MnO/C-5 and MnO/C-100, respectively. Since the nanoparticles in MnO/C-100 are larger than 100 nm as seen in TEM (Fig. S2), the Scherer formula is not suitable for
measuring the particle size any more. The weight percent of MnO in the hybrids was confirmed by TGA in air with a temperature range from room temperature (20 C) to 900 C with final product as Mn2O3 (Fig. 5). The MnO contents in MnO/C-5 and MnO/C-100 are calculated to be 66.9 wt. % and 46.7 wt. %, respectively. It is interesting that the DTG curves of the hybrids show different temperatures at which the weight starts to drop and the weight drops most heavily (346.7 C and 432.1 C for MnO/C-5, 308.5 C and 421.7 C for MnO/C-100). Since these two temperatures reflect the carbon consumption during heat treatment (C þ O2/CO2), we attribute this phenomenon to the different graphitic degrees of these two carbon matrixes. That is, the carbon matrix for MnO/C-5 has higher graphitic degree, less defects and less oxygen functional groups than that of MnO/C-100. Thus the oxidation of the carbon in MnO/C-5 requires more energy and higher reaction temperature. This assumption is also supported by
D. Kang et al. / Carbon 99 (2016) 138e147
Fig. 3. The nitrogen isotherm plots of MnO/C-5 and MnO/C-100 and the pore size distribution (inset) of MnO/C-5 and MnO/C-100. (A colour version of this figure can be viewed online.)
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Fig. 5. TGA and DTG curves of MnO/C-5 and MnO/C-100 in air. (A colour version of this figure can be viewed online.)
Table 1 The porous structure of MnO/C hybrids.
MnO/C-5 MnO/C-100 a b c d e f
SBETa [m2 g1]
Smib [m2 g1]
Smec [m2 g1]
Vtd [cm3 g1]
Vmie [cm3 g1]
Vmef [cm3 g1]
179.50 158.90
47.34 79.96
132.16 78.94
0.21 0.11
0.02 0.04
0.19 0.07
SBET: specific surface area calculated by BrunauereEmmetteTeller model. Smi: specific surface area of the micropores. Sme: specific surface area of the mesopores. Vt: the total pore volume. Vmi: the pore volume of micropores. Vme: the pore volume of mesopores.
Fig. 4. The XRD spectra of MnO/C-5 and MnO/C-100. (A colour version of this figure can be viewed online.)
the Raman spectra of the hybrids. As shown in Fig. 6, the MnO/C hybrids show characteristic peaks of manganese oxide and carbon matrix. The G band (at 1594 cm1) is assigned to the E2g phonon of sp2 carbon atoms and D band (1337 cm1) is related with the local defects and disorders of the carbon matrix. The Raman spectrum of MnO/C-5 show higher IG/ID ratio (0.98) than that of MnO/C-100 (0.60), which also confirms the existence of considerable partial graphitic carbon in MnO/C-5.
Fig. 6. Raman spectra of MnO/C-5 and MnO/C-100 hybrids. The inset pictures show the details of the spectra at characteristic band range of manganese oxide and carbon. The D band (~1337 cm1) is related with the local defects and disorders of the carbon matrix and G band (~1594 cm1) is assigned to the E2g phonon of sp2 carbon atoms. (A colour version of this figure can be viewed online.)
Since MnO could easily transform to Mn3O4 under a laser irradiation in short time, the peaks at around 650 cm1 could be due to the Mn3O4 rather than MnO [37e39]. Moreover, there is a Raman shift from 656 cm1 to 640 cm1 when the size of MnO nanoparticle decreases to 5 nm. There are two possible reasons to explain this shift, that is, the size effect and the chemical bonding between
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carbon and the manganese oxide particles. The spectrum of MnO/ C-5 shows slight asymmetry of the peak at 640 cm1 and the peak in MnO/C-100 spectrum at 656 cm1 shows perfect symmetry, indicating a smaller particle size of MnO/C-5 than MnO/C-100. The bonding between the carbon matrix and manganese oxide could also induces a shift of the peak position, which could be further confirmed by XPS tests. As shown in Fig. 7a, characteristic peaks of Mn, C, O, N were detected without any other impurities. High resolution XPS of C 1s was further analysed in Fig. 7b. The high amount of CeC carbon (284.6 eV) demonstrates the high graphitic degree of the carbon matrix. The oxygen state is analysed in Fig. 7b, and CeO(286.2 eV), C]O(287.4 eV) and CeC]O(288.8 eV) make up 46.6%, 26.5% and 26.9% of the oxygen functional groups on carbon matrix. The Mn 2p3/2 peak at 642.2 eV and the energy difference of 11.6 eV between peaks of Mn 2p3/2 and Mn 2p1/2 indicate that the Mn element exist as MnO in MnO/C-5 [40e43]. The slightly shift of the binding energy compared with pure MnO indicates the bonding exists between the carbon matrix and MnO. There is also some amount of nitrogen in MnO/C hybrids (Fig. 7d). The nitrogen functional groups on the carbon matrix have proven to be effectively when used in LIBs [44]. For example, graphitic nitrogen could effectively enhance the conductivity of the carbon matrix. In this specific case, pyrridinic N (399.0 eV), pyrrolic N (400.5 eV) and graphitic N (401.7 eV) compose 35.2%, 47.7% and 17.1% of the nitrogen functional groups on the carbon matrix, respectively. 3.3. The electrochemical performance of the MnO/C hybrids As descried above, the MnO/C hybrids show some outstanding advantages as the potential electrode materials for LIBs. The electrochemical property is detected by CV test as shown in Fig. 8a and
b. Both of the 1st discharge cycles of MnO/C-5 and MnO/C-100 electrodes exhibit small reduction peaks at around 1.80 V and 1.40 V, which could be due to the partially reduction of the oxidized MnO particles on the surface. These peaks disappear in the following cycles, implying the irreversibility of this reaction. The peaks at around 0.10 V is attributed to the reduction of MnO to metallic Mn (MnOþ2Li4Mn þ Li2O) and the formation of solid electrolyte interface (SEI) film. The peak then shifts to around 0.25 V at the 2nd discharge cycles, which is resulted from the improved kinetics of the reaction. And the peak positions shift further in the following cycles. For MnO/C-5 electrode, the peak shifts from 0.25 V to 0.27 V at the 6th discharge cycle, while the peak of MnO/C-100 electrode shifts from 0.29 V to 0.26 V. In the charge cycles, similar phenomenon is observed. The oxidation peak of the MnO/C-5 electrode shift from 1.32 V to 1.27 V after six cycles while the oxidation peak of MnO/C-100 electrode shift from 1.35 V to 1.37 V. The opposite trends could be explained by the different kinetics of reactions caused by the different particle sizes. Smaller particles provide more active sizes for reactions while large particles provide limited exposed surface for reactions. As the reaction processes, the large particles tend to aggregate while the small particles separated by the carbon matrix are broken into smaller nanoparticles [45]. These smaller particles further improved the reaction kinetics. Thus the potential for oxidation is reduced and the potential for reduction is enhanced. Except for the small shifts of peak positions, the voltage profiles of MnO/C-5 electrode shows high overlapping, indicating the reversible electrode process over cycles [46]. The GCD curves (Fig. 8c and d) also provide the information of the changes of kinetics occur during cycling. In the GCDs of MnO/C hybrid electrode, the discharge voltage plateaus at 0.25 V shift to around 0.50 V after the 2nd cycles. This potential remains constant
Fig. 7. (a)XPS spectrum of MnO/C-5. High resolution XPS spectra of C 1s(b), Mn 2p(c) and N 1s(d). (A colour version of this figure can be viewed online.)
D. Kang et al. / Carbon 99 (2016) 138e147
143
Fig. 8. (a) and (b) CV curves of MnO/C-5 and MnO/C-100. (c) and (d) GCD curves of MnO/C-5 and MnO/C-100 electrodes. (A colour version of this figure can be viewed online.)
during the following cycles, which indicates a heterogeneous reduction process [47]. The specific capacity of MnO/C-5 electrode at the 1st discharge cycle is calculated to be 1080 mAh g1, and then dropped to 820 mAh g1 at the 2nd discharge cycle. This capacity fading is due to irreversible consumption of electrolyte and the formation of SEI film. Similar capacity fading is observed for MnO/ C-100 electrode, and the irreversible capacity is even larger than that of MnO/C-5 electrode. As the cycle number increases, the MnO/ C-5 electrode tends to has higher capacity while the capacity of MnO/C-100 electrode continues fading. It is worth of mention that both in the CV curves and GCD curves, MnO/C-5 electrode shows a oxidation reaction at around 2.10 V, which is resulted from the further oxidization of Mn(II) to higher state. There is no obvious peak or plateau in the CV curves and GCD curves of MnO/C-100 electrode at around 2.10 V. This implies that the further oxidization of the MnO to higher state may be related with the size of the nanoparticles. The long cycle test was thus performed to investigate the changes during the reactions. As shown in Fig. 9a, the rate performance of MnO/C-5 electrode is compared with that of MnO/C-100 electrode as current density ranges from 100 mA g1 to 5000 mA g1. The MnO/C-5 electrode shows better capacity than MnO/C-100 electrode at every current density. Even at a current density of 5000 mA g1, the specific capacity of MnO/C-5 electrode retains at 483 mAh g1, while for MnO/C-100 electrode, the specific capacity drops to less than 100 mAh g1. When cycling at 2000 mAh g1 for 1000 cycles, the two hybrids show opposite changes of capacity. MnO/C-5 electrode shows an increase of capacity from 540 mAh g1 to 1200 mAh g1 while MnO/C-100 electrode shows a capacity decrease from 170 mAh g1 to 100 mAh g1. The increase of capacity of MnO/C-5 electrode could be due to the oxidation of MnO to manganese oxide, in which
manganese has higher chemical state. It is worth of mention that the capacity performance and cycling performance of MnO/C-5 are outstanding when compared with the best results as reported, as indicated in Table 2. For example, MnO/ carbon nanopeapods with 84.4 wt. % MnO shows similar specific capacity (845 mAh g1) [5] with MnO/C-5 (819 mAh g1) at 100 mA g1. However, when the current density increases to 5000 mA g1, the specific capacity of MnO/carbon nanopeapods drops to 436 mAh g1 while the value of MnO/C-5 remains at 483 mAh g1. Another character of MnO/C-5 as anode material is the long cycle life. Most of the MnO-based anode materials could be charged-discharged for hundreds of cycles [6e13], while for MnO/ C-5, even after 1000 cycles (1000 mA g1), the specific capacity is maintained at 1625 mAh g1. Compared with hybrid contains similar content of MnO [7], MnO/C-5 shows higher specific capacity, better rate performance and longer cycle life. In conclusion, MnO/C-5 exhibits its high applicability being used as anode materials for LIBs. The electrochemical impedance spectra of the hybrid electrodes are shown and fitted based on the modified equivalent circuit in Fig. 10 and the detailed kinetic parameters are listed in Table 3. Rs is the internal resistance of the cell, and the high frequency sections of the plots are semicircle, indicating the impedance of the Rf, which is corresponding to the resistance of the SEI film. The medium-frequency semicircle corresponds to the Rct, which is associated with charge-transfer resistance and constant phase. It is obvious that the charge transfer resistance in MnO/C-100 electrode is much harder than that of MnO/C-5 electrode (the fitted Rct for MnO/C-5 electrode and MnO/C-100 electrode is 40.09 U and 70.11 U, respectively). Moreover, the Rf of MnO/C-100 electrode is 2.58 U, while the Rf of MnO/C-5 electrode is only 1.38 U, indicating less SEI film resistance in MnO/C-5 electrode. At low frequency,
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Fig. 9. (a) Rate performance of MnO/C-5 and MnO/C-100 electrodes with current density ranging from 100 mA g1 to 5000 mA g1. (b) Cycling performance of MnO/C-5 and MnO/ C-100 electrodes with a current density of 2000 mA g1. The dash line in (a) shows the theoretical specific capacity of commercial graphite. (A colour version of this figure can be viewed online.)
Table 2 The performance comparison of MnO/C (with 46.7 wt.% loading of MnO of ca. 5.2 nm) with the best ones reported very recently. (Theoretical specific capacity of MnO: 756 mAh g1). The result of this work is in bold. Materials(carbon content)
Rate performance
Cycling performance
MnO/C-5 (53.3 wt. %) MnO/Carbon nanopeapods (15.6 wt. %) MnO/Carbon microalgea (12.3 wt. %) MnOx/Carbon (61.0 wt. %) MnO/graphene (17.4 wt. %) Carbon nanofiber@MnO (24.0 wt. %) MnO@N-doped carbon (23.7 wt. %) Carbon-coated MnO (14.9 wt. %) MnO nanowire/graphene (35.6 wt. %) MnO/carbon nanosheets (18.8 wt. %)
100 mA g1 819 mA h g1 /5000 mA g1 483 mA h g1 100 mA g1 845 mAh g1 /5000 mA g1 436 mAh g1 100 mA g1 702.2 mAh g1 /3000 mA g1 234.7 mAh g1 200 mA g1 650 mAh g1 /800 mA g1 500 mAh g1 200 mA g1 890.7 mAh g1 /3000 mA g1 625.8 mAh g1 200 mA g1 936 mAh g1 /1600 mA g1 310 mAh g1 50 mA g1 893 mAh g1 /10 A g1 386 mAh g1 100 mA g1 590.6 mAh g1 /800 mA g1 238.2 mAh g1 50 mA g1 790 mAh g1 /1000 mA g1 300 mAh g1 100 mA g1 1332 mAh g1 /20,000 mA g1 285 mAh g1
1000 mA g1 1000 cycles 1625 mA h g1 2000 mA g1 1000 cycles 525 mAh g1 100 mA g1 50 cycles 705 mAh g1 200 mA g1 130 cycles 650 mAh g1 2000 mA g1 400 cycles 843.4 mAh g1 200 mA g1 150 cycles 750 mAh g1 1000 mA g1 700 cycles 1268 mAh g1 100 mA g1 100 cycles 525 mAh g1 500 mA g1 500 cycles ~950 mAh g1 2000 mA g1 500 cycles ~950 mAh g1
Ref. This work 5
6
7
8
9
10
11
12
13
Table 3 The Kinetic parameters of the MnO/C electrodes. Rs [U]a
MnO/C-5 MnO/C-100 a b c
Fig. 10. Nyquist plots and the fitting plots of MnO/C electrodes after 1st CV cycle. (A colour version of this figure can be viewed online.)
both of the electrodes perform mix-controlled process. The higher slope of the plot of MnO/C-5 electrode than that of MnO/C-100
1.756 2.204
Rf [U]b
1.38 2.53
Rct [U]c
41.09 70.11
CPE1
CPE2
g
n
g
n
0.173e-3 0.312e-3
0.74 0.63
4.66e-6 27.09e-6
0.47 0.34
Rs: internal resistance. Rf: resistance of the SEI film. Rct: charge-transfer resistance.
electrode indicates the less activation controlled character in the MnO/C-5 electrode with smaller nanoparticles. The charge transferring in MnO/C-5 electrode suffers less impedance during diffusion and reactions, which also explains the high rate performance of MnO/C-5 electrode. We further investigate the cycling performance of MnO/C-5 electrode at different current densities. As shown in Fig. 11, the capacity increase is observed at each current density. The cycle life of the half battery at a current density of 1000 mA g1 is about 1200 cycles. The MnO/C electrode shows more stable capacity after large current density cycling, such as 2000 mA g1 and 5000 mA g1, the capacity is stable at 1250 and 800 mAh g1 after 1000 and 2000
D. Kang et al. / Carbon 99 (2016) 138e147
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Fig. 11. The cycling performance of MnO/C-5 at different current densities. (A colour version of this figure can be viewed online.)
cycles, respectively. Similar phenomenon was observed in previous studies, and the reasons for the enhanced capacity were assigned to the microstructure change and increase of chemical state of Mn element [9]. However, only small amount of ultra-small nanoparticles were observed in previous study. Moreover, it is difficult to define the chemical state of Mn in manganese oxide since the binding energy positions of Mn in different manganese oxides are always overlapped [40e43]. We thus perform our study by applying TEM, XPS and XAFS to investigate the exact reason that induces the capacity increase during cycling. 3.4. The structure and chemical composition of MnO/C hybrids after cycling As described above, the particle size and additional oxidization reaction may result in the increasing capacity. The microstructures of the MnO/C-5 and MnO/C-100 after 1000 cycles were detected by TEM as shown in Fig. 12. The MnO particles in MnO/C-5 have been
broken into smaller particles with size around 2 nm, while the particles in MnO/C-100 aggregate to larger particles. It implies that the capacity enhancement and capacity decrease are related with change of particle size in the hybrids. Previous work has observed that, NiO particles could be separated by the products generated during the battery process with a “Finger” mode [45]. In this work, the MnO nanoparticles are also broken into smaller ones. Smaller particle provide more active sites for reactions, and for a conversion reaction, this is a decisive factor to improve kinetics. The chemical state of Mn element in the MnO/C hybrids after cycling was detected by high resolution XPS (Fig. S4). Both of the MnO in MnO/C-5 and MnO/C-100 was oxidized to higher state (MnO/C-5 as MnO2, and MnO/C-100 as Mn2O3) [40e43,48,49] This could be explained by the CV results and GCD results. The oxidation of MnO to higher chemical state is observed in MnO/C-5 at a potential around 2.10 V. However, there is also some part of MnO in MnO/C-100 which has been oxidized to higher state though there is
Fig. 12. (a) TEM and SAED images, (b) HRTEM photos of MnO/C-5 after cycling for 1000 cycles at 2000 mA g1 (c) and (d) TEM photos of MnO/C-100 after cycling. The red circles refer to the nanoparticles in the hybrids. (A colour version of this figure can be viewed online.)
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D. Kang et al. / Carbon 99 (2016) 138e147
Fig. 13. (a) Normalized Mn K-edge XANES and EXAFS spectra for the MnO/C hybrids charged to 3 V. (b) The K-edge Fourier transform magnitudes of k3 weighted EXAFS spectra of the MnO/C hybrids charged to 3 V. (A colour version of this figure can be viewed online.)
only small peak at potential of 2.10 V in the CV curves of the 1st and 2nd cycles. Since XPS has limited resolution to distinguish different chemical states of manganese oxides, we further perform XAFS to detect the final state of the Mn in MnO/C hybrid electrodes. Fig. 13a shows the normalized M k-edge X-ray absorption nearedge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) for the MnO/C-5 and MnO/C-100 hybrid electrodes after 1000 GCD cycles and then recharged to 3 V. It can be seen that the EXAFS profiles of the charged electrodes are similar to that of Mn2O3 and the peak position (6559.3 eV) is close to that of Mn2O3 (6559.0 eV). This slightly shift of peak position may be due to the higher chemical state of the MnO in the electrode, that is, the MnO inside the electrode may be partially oxidized to MnO2. And this suspect could be demonstrated by the Mn K-edge Fourier transform (FT) magnitudes of k3 weighted EXAFS spectrum of MnO/ C-5 electrode at 1.73 Å and 2.77 Å, which are close to those peaks of MnO2 at 1.78 Å and 2.76 Å. Also, the FT peaks of Mn2O3 are at 1.70 Å and 3.07 Å, which are also close to those of MnO/C-5 electrode. Thus we imply the chemical state of the Mn in MnO/C-5 electrode at 3 V is a mixture of Mn(III) and Mn(IV). For Mn in MnO/C-100 electrode, the EXAFS profile is similar to that of Mn3O4 (6558.9 eV) with a peak position at 6558.7 eV. The FT peaks of MnO/ C-100 electrode at 1.77 Å and 2.56 Å are also close to those peaks of Mn3O4 at 1.74 Å and 2.60 Å. However, Mn2O3 shows peak at 6559.0 eV in its EXAFS profile and FT peak at 1.70 Å, which are close to those of MnO/C-100 electrodes, it is hard to determine what the final product of the oxidization reaction is. MnO/C-100 exhibits FT peaks at 3.94 Å, 4.40 Å and 4.93 Å, being close to that of Mn2O3 at 3.91 Å, Mn3O4 at 4.40 Å and 4.94 Å. We thus speculate that the MnO in MnO/C-100 is oxidized to a mixture of Mn2O3 and Mn3O4. That is, after charging and discharging for 1000 cycles, the oxidization of Mn(II) to higher oxidization state occurs both in MnO/C-5 and MnO/C-100 and the chemical state of the Mn element in MnO/C100 electrode is mixture of Mn(II) and (III). We attribute this difference in chemical state of the final products to the different particle sizes. Since the smaller particles required lower surface activation energy for reaction, the MnO particles in MnO/C-5 with size around 5 nm could be easily oxidized to Mn2O3. Moreover, as the reaction proceeding, the broken nanoparticles with smaller particles (down to 2 nm) could be further oxidized to MnO2. Thus in the electrochemical energy storage of MnO, the particle size exhibits as the decisive factor for the increasing capacity. Smaller particle provides higher surface for reaction and the
reaction results in even smaller nanoparticles to introduce more active sites for reaction. This mutual effect then reduces the surface activation energy for oxidization of the nanoparticles and further increases the capacity. 4. Conclusion In conclusion, a MnO/C hybrid with ultra-small particles, partial graphitic porous carbon matrix and strong chemical bonding between particles and carbon is prepared and shows outstanding energy storage capacity. There is an increase in specific capacity after long cycling, and the reason is studied in depth. We found the fragmenting of the nanoparticles and oxidation of MnO to higher state both induced the results. However, the compared experiment shows that the MnO/C hybrid with larger particles also exhibits an increase of chemical valence after cycling with lower oxidized degree. The particle size directly affects the difficulty of oxidization reaction, and the reaction further refines the microstructure. This mutual effect results in the increasing capacity. We thus conclude the fragmenting of nanoparticles is the decisive reason of the enhancement of capacity. Acknowledgements This work was supported by Shanghai Science and Technology Committee (15ZR1422400, 14JC1403300 and 14520710100), Research Fund for the Doctoral Program of Higher Education of China (20120073130001). J. Gu greatly thanks the support from Program for New Century Excellent Talents in University, Ministry of Education, China. Great thanks for Q. Wang in Tianjin University for designing the schematic. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.11.068. References [1] D. Larcher, J. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (1) (2015) 19e29. [2] M. Obrovac, V. Chevrier, Alloy negative electrodes for Li-ion batteries, Chem. Rev. 114 (23) (2014) 11444e11502. [3] M. Reddy, G. Subba Rao, B. Chowdari, Metal oxides and oxysalts as anode materials for Li ion batteries, Chem. Rev. 113 (7) (2013) 5364e5457.
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Carbon journal homepage: www.elsevier.com/locate/carbon
Crosslinking-derived MnO/carbon hybrid with ultrasmall nanoparticles for increasing lithium storage capacity during cycling Danmiao Kang a, Qinglei Liu a, *, Rui Si b, Jiajun Gu a, Wang Zhang a, Di Zhang a, ** a
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, PR China Shanghai Institute of Applied Physics, Chinese Academy Sciences, Shanghai Synchrotron Radiation Facility, 239 Zhangheng Road, Shanghai, 201204, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2015 Received in revised form 25 November 2015 Accepted 28 November 2015 Available online 9 December 2015
MnO/C hybrid with ultra-small MnO nanoparticles (5.2 nm) embedded in porous carbon is fabricated and applied as the anode material for lithium ion batteries. The MnO/C hybrid electrode exhibits excellent specific capacity of 820 mAh g1 at a current density of 100 mA g1 and also outstanding rate performance with specific capacity of 483 mAh g1 at a current density of 5000 mA g1. After cycling for 1000 times at 1000 mA g1, the specific capacity increases to 1625 mAh g1. The TEM photos show that the particles are broken into 2 nm particles after cycling. We also apply XAFS to detect the final state of the Mn in the hybrid electrodes and find that smaller MnO particles are oxidized to a mixture of Mn2O3 and MnO2. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have attracted increasing attention in the past two decades for its high capacity and widely usages in hybrid electrical vehicle and portable consumers. However, the most studied anode materials for LIBs (such as Si and Sn) are still suffering from limited cycling performance due to the huge volume expansion during the lithiation and delithiation process [1e4]. Instead of the alloying/dealloying mechanism, MnO storages energy by the following conversion reaction: MnO þ 2Li 4 Mn þ Li2O [3] With a high theoretical specific capacity (756 mAh g1), low cost, large abundance (1.06‰ as fraction of Earth's crust) and environmental friendliness, MnO becomes one of the most attractive anode materials for LIBs [4e14]. Since conversion reaction is highly affected by the size of the active materials [15], nano-scale MnO is usually preferred to be used as electrode materials. However, MnO particles tend to aggregate during the chargeedischarge processing, which results
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Zhang).
(Q.
http://dx.doi.org/10.1016/j.carbon.2015.11.068 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
Liu),
[email protected]
in heavy fading of capacity of the electrode. To solve this problem, MnO nanoparticles have been embedded in various carbon matrixes to separate these particles [4e14,16e31]. For example, Jiang et al. embedded MnO in carbon nanotubes to fabricate the MnO/C nanopeapods, which showed high specific capacity of 1119 mAh g1 at 500 mA g1, excellent rate performance of 463 mAh g1 at 5000 mA g1 and long life with 1000 cycles [5]. Sun et al. fabricated MnO/graphene hybrid electrode with conformal nanoparticles. With a current density of 2000 mA g1, the hybrid electrode sustained 400 cycles with only 4% capacity fading [8]. Other than graphene and CNTs, composites of MnO/C could also be fabricated by using biomass template, such as microalgae. The composites with highly developed porous network got a first discharge capacity of 1021.9 mAh g1, and the capacity is maintained at 702.2 mAh g1 after 50 cycles [6]. Carbon nanosheet with 3D structure was used as the carbon matrix for the MnO/C composite and the capacity of the composite reached as high as 1332 mAh g1 at the first discharge cycle and retained at 567 mAh g1 at a current density of 5000 mA g1 [13]. However, in previous studies, most of the MnO/C composites were fabricated by reduction of MnO4 or hydrolysis of the weak acid salt of Mn2þ (such as Mn(CH3COO)2) occurred on the carbon (or carbon precursor) matrix. The sizes of MnO nanoparticles as obtained were usually large than 10 nm, which impeded the fully usage of the active materials. Alginate has a unique character that the guluronic chains could perform crosslinking reaction with polyvalent metal ions to form
D. Kang et al. / Carbon 99 (2016) 138e147
gels [32e34]. Based on this character, alginate was used as the precursor for fabrication of the CoNi alloy nanoparticles and Fe2O3alginate gel [35,36]. Different carbon/metal hybrids could also be fabricated by carbonization of the alginate-derived gels [50]. Here we applied alginate as the building blocks to build a MnO/C hybrid with merits for being used as anode material for LIBs. The ultrasmall particle size (~5 nm) of MnO contributes to capacity of 1080 mAh g1 at the 1st discharge cycle and 820 mAh g1 at the following discharge cycle (100 mA g1). The interconnected porous network ensures the smooth diffusion and penetration of ions into the networks and the hybrid exhibits specific capacity of 483 mAh g1 at a current density of 5000 mA g1. The specific capacity increases during the 1000 cycles at current density of 2000 mA g1 (from 600 to 1280 mAh g1). To investigate the effect of the particle size on the energy storage performance, we also fabricated MnO/C with large particle size of about 100 nm (MnO/C100). It is found that the electrode with large particle size shows low capacity of 630 mAh g1 (100 mA g1) and poor capacity retention of 10% at current density of 5000 mA g1. The structure and chemical composition of these hybrids after cycling are detected to illustrate this phenomenon. It is found that during cycling, small nanoparticles in MnO/C-5 are broken into smaller particle (down to 2 nm), while most of the nanoparticles in MnO/C-100 aggregate into larger particles. Moreover, the Mn element is oxidized to higher state for both of the two hybrid electrodes. For MnO/C-5 hybrid electrode, the chemical state of Mn is composing of Mn(III) and Mn(IV), while in MnO/C-100 hybrid electrode, the chemical state of Mn is composing of Mn(II) and Mn(III).
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Shanghai Synchrotron Radiation Facility (SSRF). Thermogravimetric (TGA) was performed on a TG-DSC analyzer (NETZSCH, STA 449 F3) in air with a temperature ranging from room temperature to 800 C. The contents of MnO in the composites were determined by the following equation:
MnOðwt:%Þ ¼
MðMnOÞ 2 W 100%; MðMn2 O3 Þ
in which M is the molecular weight and W is the weight percent of the residue after heat treatment in air. 2.3. Electrochemical measurement
2. Experimental section
The working electrodes were fabricated by coating slurry containing active materials, super-P and PVDF dissolved in NMP (0.02 g mL1) (8:1:1) onto a copper foil and dried at 60 C for 12 h. The coated copper foil was then cut into disk electrode and pressed at 4 MPa for 1 min. Then the electrodes were dried at 60 C for another 12 h before being transferred into the glove box. The electrochemical performance of the hybrids was examined in 2032 type coin cells. The lithium foil was used as the counter electrode (negative electrode), and Celgard 2500 was used as the separator. Electrolyte was 1 M LiPF6 in mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v ¼ 1:1). The coin cells were assembled in an argon-filled glove box with moisture and oxygen below 0.5 ppm. The chargeedischarge cycles were tested on Land CT2001A at a voltage window of 0.005e3.0 V (vs Li/Liþ). The CV with a scan rate of 0.1 mV/s (0.005e3.0 V) and EIS tests with a frequency range of 30 mHze500 kHz and ac amplitude of 5 mV were performed on VMP3 electrochemical measurement.
2.1. Material synthesis
3. Results and discussion
Alginate was dissolved in deionized water to obtain 1.5 wt. % solution. Mn(NO3)2 solution of 0.34 mol L1 in deionized water was used as crosslink agent. Then the alginate was added into the metal salt solutions with a volume ratio of 2:5 (alginate: metal salt solution). The gels were obtained after 15 h crosslinking and washed before being freeze dried for 24 h. Carbonization of the gels was conducted in nitrogen flow rate of 600 mL min1. The temperature was maintained at 600 C for 1 h. The obtained carbon/MnO hybrid was named as MnO/C-5. To fabricate MnO/C-100, sodium chloride was added to the metal solution (with an atom ratio of Na: Mn to 1:1) before reaction and the following steps were the same as those of MnO/C-5.
3.1. The structure characterization of the MnO/C hybrids
2.2. Material characterization Materials were analyzed by scanning electron microscopy (SEM, FEI Quanta FEM 250), transmission electron microscopy (TEM, JOEL 2100 F) and high resolution TEM (HRTEM). All the TEM samples were dispersed in ethanol and treated with ultra-sound for 40 min and then a drop of dispersion was loaded on copper grid supported with ultrathin carbon film. X-ray diffraction (XRD) was conducted on a Rigaku D/Max-2500 with Cu-Ka radiation. The porous structure was characterized by mean of N2 adsorption-desorption at 77.4 K (Micrometritics ASAP 2020). Before the gas adsorption measurements, the samples were degassed at 200 C for 6 h in vacuum. X-ray photoelectron spectroscopy (XPS) spectra were recorded on AXIS UltraDLD spectrometer from Kratos using 120 W Al Ka radiation with a chamber pressure of 5 109 Torr. All the peak positions detected by XPS were calibrated based on sp2 carbon peak located at 284.8 eV. Ex-situ measurement of X-ray absorption fine structure (XAFS) was performed at the beamline (BL14W1) in
As shown is Fig. 1, the MnO/C hybrids are fabricated by carbonization of self-assembled gels formed by crosslinking of alginate and Mn2þ ions. The microstructure of the as fabricated MnO/C-5 is shown in Fig. 2. It has a fine nanostructure with ultrasmall MnO particles uniformly dispersed on a partial graphitic porous carbon matrix. As shown in Fig. 1a, the carbon matrix in MnO/C hybrid is a porous network. Fig. 1b and c shows that nanoparticles with size of around 5 nm are homogenously distributed on the carbon matrix. HRTEM (Fig. 2d) shows that nanoparticles are tightly surrounded by the partial graphitic carbon layers. This nanostructure is derived from the crosslinked zones formed during the spontaneous reaction between Mn2þ ions and guluronic acid units. There are around 20 ions in a specific crosslinked zone [32e34]. After freeze drying and carbonization, these ions melt into nanoparticles with size around 5 nm. The carbon chains are broken down during the heat treatment and the carbon chains are squeezed to the edges of the nanoparticles and thus form the carbon layers tightly surrounding these nanoparticles [50]. The pore structures of the MnO/C hybrids were detected by N2 adsorption-desorption. As indicated by the isotherms of MnO/C hybrids (Fig. 3), which show type-IV curves with obvious increase absorbed nitrogen and hysteria loops at moderate relative pressure, there are large amount of mesopores in the hybrids. The pore size distributions demonstrate the interconnected mesopore structures of MnO/C hybrids, especially the small mesopores with size around 2e4 nm. As concluded in Table 1, the MnO/C-5 has a BET specific surface area of 180 m2 g1, and the ratio of mesopore volume to the micropore volume is 9.50, while the values of MnO/C-100 are 158 m2 g1 and 1.75, respectively. It means the surface areas of the
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D. Kang et al. / Carbon 99 (2016) 138e147
Fig. 1. The fabrication of the MnO/C hybrid by crosslinking of alginate with Mn2þ ions. (A colour version of this figure can be viewed online.)
Fig. 2. SEM photo (a) and (b), TEM (c) and HRTEM (d) photos of MnO/C-5. (A colour version of this figure can be viewed online.)
two composites are comparable, while there are more small mesopores in MnO/C-5 than in MnO/C-100. And both of the hybrids have hierarchical porous structure, which is vital for ion transportation when they are applied in energy storage.
3.2. The chemical composition of MnO/C hybrids Fig. 4 shows the X-ray diffraction (XRD) spectra of the MnO/C hybrids as obtained. The diffraction peaks could be indexed to the cubic MnO (JCPDS card no. 07-0230) and there is no other impurity in the obtained hybrids. For both of the hybrids, the MnO particles show high crystallinity with a carbonization temperature of 600 C. The sizes of the MnO nanoparticles calculated by Scherer formula are 5.2 nm and 36.0 nm for MnO/C-5 and MnO/C-100, respectively. Since the nanoparticles in MnO/C-100 are larger than 100 nm as seen in TEM (Fig. S2), the Scherer formula is not suitable for
measuring the particle size any more. The weight percent of MnO in the hybrids was confirmed by TGA in air with a temperature range from room temperature (20 C) to 900 C with final product as Mn2O3 (Fig. 5). The MnO contents in MnO/C-5 and MnO/C-100 are calculated to be 66.9 wt. % and 46.7 wt. %, respectively. It is interesting that the DTG curves of the hybrids show different temperatures at which the weight starts to drop and the weight drops most heavily (346.7 C and 432.1 C for MnO/C-5, 308.5 C and 421.7 C for MnO/C-100). Since these two temperatures reflect the carbon consumption during heat treatment (C þ O2/CO2), we attribute this phenomenon to the different graphitic degrees of these two carbon matrixes. That is, the carbon matrix for MnO/C-5 has higher graphitic degree, less defects and less oxygen functional groups than that of MnO/C-100. Thus the oxidation of the carbon in MnO/C-5 requires more energy and higher reaction temperature. This assumption is also supported by
D. Kang et al. / Carbon 99 (2016) 138e147
Fig. 3. The nitrogen isotherm plots of MnO/C-5 and MnO/C-100 and the pore size distribution (inset) of MnO/C-5 and MnO/C-100. (A colour version of this figure can be viewed online.)
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Fig. 5. TGA and DTG curves of MnO/C-5 and MnO/C-100 in air. (A colour version of this figure can be viewed online.)
Table 1 The porous structure of MnO/C hybrids.
MnO/C-5 MnO/C-100 a b c d e f
SBETa [m2 g1]
Smib [m2 g1]
Smec [m2 g1]
Vtd [cm3 g1]
Vmie [cm3 g1]
Vmef [cm3 g1]
179.50 158.90
47.34 79.96
132.16 78.94
0.21 0.11
0.02 0.04
0.19 0.07
SBET: specific surface area calculated by BrunauereEmmetteTeller model. Smi: specific surface area of the micropores. Sme: specific surface area of the mesopores. Vt: the total pore volume. Vmi: the pore volume of micropores. Vme: the pore volume of mesopores.
Fig. 4. The XRD spectra of MnO/C-5 and MnO/C-100. (A colour version of this figure can be viewed online.)
the Raman spectra of the hybrids. As shown in Fig. 6, the MnO/C hybrids show characteristic peaks of manganese oxide and carbon matrix. The G band (at 1594 cm1) is assigned to the E2g phonon of sp2 carbon atoms and D band (1337 cm1) is related with the local defects and disorders of the carbon matrix. The Raman spectrum of MnO/C-5 show higher IG/ID ratio (0.98) than that of MnO/C-100 (0.60), which also confirms the existence of considerable partial graphitic carbon in MnO/C-5.
Fig. 6. Raman spectra of MnO/C-5 and MnO/C-100 hybrids. The inset pictures show the details of the spectra at characteristic band range of manganese oxide and carbon. The D band (~1337 cm1) is related with the local defects and disorders of the carbon matrix and G band (~1594 cm1) is assigned to the E2g phonon of sp2 carbon atoms. (A colour version of this figure can be viewed online.)
Since MnO could easily transform to Mn3O4 under a laser irradiation in short time, the peaks at around 650 cm1 could be due to the Mn3O4 rather than MnO [37e39]. Moreover, there is a Raman shift from 656 cm1 to 640 cm1 when the size of MnO nanoparticle decreases to 5 nm. There are two possible reasons to explain this shift, that is, the size effect and the chemical bonding between
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carbon and the manganese oxide particles. The spectrum of MnO/ C-5 shows slight asymmetry of the peak at 640 cm1 and the peak in MnO/C-100 spectrum at 656 cm1 shows perfect symmetry, indicating a smaller particle size of MnO/C-5 than MnO/C-100. The bonding between the carbon matrix and manganese oxide could also induces a shift of the peak position, which could be further confirmed by XPS tests. As shown in Fig. 7a, characteristic peaks of Mn, C, O, N were detected without any other impurities. High resolution XPS of C 1s was further analysed in Fig. 7b. The high amount of CeC carbon (284.6 eV) demonstrates the high graphitic degree of the carbon matrix. The oxygen state is analysed in Fig. 7b, and CeO(286.2 eV), C]O(287.4 eV) and CeC]O(288.8 eV) make up 46.6%, 26.5% and 26.9% of the oxygen functional groups on carbon matrix. The Mn 2p3/2 peak at 642.2 eV and the energy difference of 11.6 eV between peaks of Mn 2p3/2 and Mn 2p1/2 indicate that the Mn element exist as MnO in MnO/C-5 [40e43]. The slightly shift of the binding energy compared with pure MnO indicates the bonding exists between the carbon matrix and MnO. There is also some amount of nitrogen in MnO/C hybrids (Fig. 7d). The nitrogen functional groups on the carbon matrix have proven to be effectively when used in LIBs [44]. For example, graphitic nitrogen could effectively enhance the conductivity of the carbon matrix. In this specific case, pyrridinic N (399.0 eV), pyrrolic N (400.5 eV) and graphitic N (401.7 eV) compose 35.2%, 47.7% and 17.1% of the nitrogen functional groups on the carbon matrix, respectively. 3.3. The electrochemical performance of the MnO/C hybrids As descried above, the MnO/C hybrids show some outstanding advantages as the potential electrode materials for LIBs. The electrochemical property is detected by CV test as shown in Fig. 8a and
b. Both of the 1st discharge cycles of MnO/C-5 and MnO/C-100 electrodes exhibit small reduction peaks at around 1.80 V and 1.40 V, which could be due to the partially reduction of the oxidized MnO particles on the surface. These peaks disappear in the following cycles, implying the irreversibility of this reaction. The peaks at around 0.10 V is attributed to the reduction of MnO to metallic Mn (MnOþ2Li4Mn þ Li2O) and the formation of solid electrolyte interface (SEI) film. The peak then shifts to around 0.25 V at the 2nd discharge cycles, which is resulted from the improved kinetics of the reaction. And the peak positions shift further in the following cycles. For MnO/C-5 electrode, the peak shifts from 0.25 V to 0.27 V at the 6th discharge cycle, while the peak of MnO/C-100 electrode shifts from 0.29 V to 0.26 V. In the charge cycles, similar phenomenon is observed. The oxidation peak of the MnO/C-5 electrode shift from 1.32 V to 1.27 V after six cycles while the oxidation peak of MnO/C-100 electrode shift from 1.35 V to 1.37 V. The opposite trends could be explained by the different kinetics of reactions caused by the different particle sizes. Smaller particles provide more active sizes for reactions while large particles provide limited exposed surface for reactions. As the reaction processes, the large particles tend to aggregate while the small particles separated by the carbon matrix are broken into smaller nanoparticles [45]. These smaller particles further improved the reaction kinetics. Thus the potential for oxidation is reduced and the potential for reduction is enhanced. Except for the small shifts of peak positions, the voltage profiles of MnO/C-5 electrode shows high overlapping, indicating the reversible electrode process over cycles [46]. The GCD curves (Fig. 8c and d) also provide the information of the changes of kinetics occur during cycling. In the GCDs of MnO/C hybrid electrode, the discharge voltage plateaus at 0.25 V shift to around 0.50 V after the 2nd cycles. This potential remains constant
Fig. 7. (a)XPS spectrum of MnO/C-5. High resolution XPS spectra of C 1s(b), Mn 2p(c) and N 1s(d). (A colour version of this figure can be viewed online.)
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Fig. 8. (a) and (b) CV curves of MnO/C-5 and MnO/C-100. (c) and (d) GCD curves of MnO/C-5 and MnO/C-100 electrodes. (A colour version of this figure can be viewed online.)
during the following cycles, which indicates a heterogeneous reduction process [47]. The specific capacity of MnO/C-5 electrode at the 1st discharge cycle is calculated to be 1080 mAh g1, and then dropped to 820 mAh g1 at the 2nd discharge cycle. This capacity fading is due to irreversible consumption of electrolyte and the formation of SEI film. Similar capacity fading is observed for MnO/ C-100 electrode, and the irreversible capacity is even larger than that of MnO/C-5 electrode. As the cycle number increases, the MnO/ C-5 electrode tends to has higher capacity while the capacity of MnO/C-100 electrode continues fading. It is worth of mention that both in the CV curves and GCD curves, MnO/C-5 electrode shows a oxidation reaction at around 2.10 V, which is resulted from the further oxidization of Mn(II) to higher state. There is no obvious peak or plateau in the CV curves and GCD curves of MnO/C-100 electrode at around 2.10 V. This implies that the further oxidization of the MnO to higher state may be related with the size of the nanoparticles. The long cycle test was thus performed to investigate the changes during the reactions. As shown in Fig. 9a, the rate performance of MnO/C-5 electrode is compared with that of MnO/C-100 electrode as current density ranges from 100 mA g1 to 5000 mA g1. The MnO/C-5 electrode shows better capacity than MnO/C-100 electrode at every current density. Even at a current density of 5000 mA g1, the specific capacity of MnO/C-5 electrode retains at 483 mAh g1, while for MnO/C-100 electrode, the specific capacity drops to less than 100 mAh g1. When cycling at 2000 mAh g1 for 1000 cycles, the two hybrids show opposite changes of capacity. MnO/C-5 electrode shows an increase of capacity from 540 mAh g1 to 1200 mAh g1 while MnO/C-100 electrode shows a capacity decrease from 170 mAh g1 to 100 mAh g1. The increase of capacity of MnO/C-5 electrode could be due to the oxidation of MnO to manganese oxide, in which
manganese has higher chemical state. It is worth of mention that the capacity performance and cycling performance of MnO/C-5 are outstanding when compared with the best results as reported, as indicated in Table 2. For example, MnO/ carbon nanopeapods with 84.4 wt. % MnO shows similar specific capacity (845 mAh g1) [5] with MnO/C-5 (819 mAh g1) at 100 mA g1. However, when the current density increases to 5000 mA g1, the specific capacity of MnO/carbon nanopeapods drops to 436 mAh g1 while the value of MnO/C-5 remains at 483 mAh g1. Another character of MnO/C-5 as anode material is the long cycle life. Most of the MnO-based anode materials could be charged-discharged for hundreds of cycles [6e13], while for MnO/ C-5, even after 1000 cycles (1000 mA g1), the specific capacity is maintained at 1625 mAh g1. Compared with hybrid contains similar content of MnO [7], MnO/C-5 shows higher specific capacity, better rate performance and longer cycle life. In conclusion, MnO/C-5 exhibits its high applicability being used as anode materials for LIBs. The electrochemical impedance spectra of the hybrid electrodes are shown and fitted based on the modified equivalent circuit in Fig. 10 and the detailed kinetic parameters are listed in Table 3. Rs is the internal resistance of the cell, and the high frequency sections of the plots are semicircle, indicating the impedance of the Rf, which is corresponding to the resistance of the SEI film. The medium-frequency semicircle corresponds to the Rct, which is associated with charge-transfer resistance and constant phase. It is obvious that the charge transfer resistance in MnO/C-100 electrode is much harder than that of MnO/C-5 electrode (the fitted Rct for MnO/C-5 electrode and MnO/C-100 electrode is 40.09 U and 70.11 U, respectively). Moreover, the Rf of MnO/C-100 electrode is 2.58 U, while the Rf of MnO/C-5 electrode is only 1.38 U, indicating less SEI film resistance in MnO/C-5 electrode. At low frequency,
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Fig. 9. (a) Rate performance of MnO/C-5 and MnO/C-100 electrodes with current density ranging from 100 mA g1 to 5000 mA g1. (b) Cycling performance of MnO/C-5 and MnO/ C-100 electrodes with a current density of 2000 mA g1. The dash line in (a) shows the theoretical specific capacity of commercial graphite. (A colour version of this figure can be viewed online.)
Table 2 The performance comparison of MnO/C (with 46.7 wt.% loading of MnO of ca. 5.2 nm) with the best ones reported very recently. (Theoretical specific capacity of MnO: 756 mAh g1). The result of this work is in bold. Materials(carbon content)
Rate performance
Cycling performance
MnO/C-5 (53.3 wt. %) MnO/Carbon nanopeapods (15.6 wt. %) MnO/Carbon microalgea (12.3 wt. %) MnOx/Carbon (61.0 wt. %) MnO/graphene (17.4 wt. %) Carbon nanofiber@MnO (24.0 wt. %) MnO@N-doped carbon (23.7 wt. %) Carbon-coated MnO (14.9 wt. %) MnO nanowire/graphene (35.6 wt. %) MnO/carbon nanosheets (18.8 wt. %)
100 mA g1 819 mA h g1 /5000 mA g1 483 mA h g1 100 mA g1 845 mAh g1 /5000 mA g1 436 mAh g1 100 mA g1 702.2 mAh g1 /3000 mA g1 234.7 mAh g1 200 mA g1 650 mAh g1 /800 mA g1 500 mAh g1 200 mA g1 890.7 mAh g1 /3000 mA g1 625.8 mAh g1 200 mA g1 936 mAh g1 /1600 mA g1 310 mAh g1 50 mA g1 893 mAh g1 /10 A g1 386 mAh g1 100 mA g1 590.6 mAh g1 /800 mA g1 238.2 mAh g1 50 mA g1 790 mAh g1 /1000 mA g1 300 mAh g1 100 mA g1 1332 mAh g1 /20,000 mA g1 285 mAh g1
1000 mA g1 1000 cycles 1625 mA h g1 2000 mA g1 1000 cycles 525 mAh g1 100 mA g1 50 cycles 705 mAh g1 200 mA g1 130 cycles 650 mAh g1 2000 mA g1 400 cycles 843.4 mAh g1 200 mA g1 150 cycles 750 mAh g1 1000 mA g1 700 cycles 1268 mAh g1 100 mA g1 100 cycles 525 mAh g1 500 mA g1 500 cycles ~950 mAh g1 2000 mA g1 500 cycles ~950 mAh g1
Ref. This work 5
6
7
8
9
10
11
12
13
Table 3 The Kinetic parameters of the MnO/C electrodes. Rs [U]a
MnO/C-5 MnO/C-100 a b c
Fig. 10. Nyquist plots and the fitting plots of MnO/C electrodes after 1st CV cycle. (A colour version of this figure can be viewed online.)
both of the electrodes perform mix-controlled process. The higher slope of the plot of MnO/C-5 electrode than that of MnO/C-100
1.756 2.204
Rf [U]b
1.38 2.53
Rct [U]c
41.09 70.11
CPE1
CPE2
g
n
g
n
0.173e-3 0.312e-3
0.74 0.63
4.66e-6 27.09e-6
0.47 0.34
Rs: internal resistance. Rf: resistance of the SEI film. Rct: charge-transfer resistance.
electrode indicates the less activation controlled character in the MnO/C-5 electrode with smaller nanoparticles. The charge transferring in MnO/C-5 electrode suffers less impedance during diffusion and reactions, which also explains the high rate performance of MnO/C-5 electrode. We further investigate the cycling performance of MnO/C-5 electrode at different current densities. As shown in Fig. 11, the capacity increase is observed at each current density. The cycle life of the half battery at a current density of 1000 mA g1 is about 1200 cycles. The MnO/C electrode shows more stable capacity after large current density cycling, such as 2000 mA g1 and 5000 mA g1, the capacity is stable at 1250 and 800 mAh g1 after 1000 and 2000
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Fig. 11. The cycling performance of MnO/C-5 at different current densities. (A colour version of this figure can be viewed online.)
cycles, respectively. Similar phenomenon was observed in previous studies, and the reasons for the enhanced capacity were assigned to the microstructure change and increase of chemical state of Mn element [9]. However, only small amount of ultra-small nanoparticles were observed in previous study. Moreover, it is difficult to define the chemical state of Mn in manganese oxide since the binding energy positions of Mn in different manganese oxides are always overlapped [40e43]. We thus perform our study by applying TEM, XPS and XAFS to investigate the exact reason that induces the capacity increase during cycling. 3.4. The structure and chemical composition of MnO/C hybrids after cycling As described above, the particle size and additional oxidization reaction may result in the increasing capacity. The microstructures of the MnO/C-5 and MnO/C-100 after 1000 cycles were detected by TEM as shown in Fig. 12. The MnO particles in MnO/C-5 have been
broken into smaller particles with size around 2 nm, while the particles in MnO/C-100 aggregate to larger particles. It implies that the capacity enhancement and capacity decrease are related with change of particle size in the hybrids. Previous work has observed that, NiO particles could be separated by the products generated during the battery process with a “Finger” mode [45]. In this work, the MnO nanoparticles are also broken into smaller ones. Smaller particle provide more active sites for reactions, and for a conversion reaction, this is a decisive factor to improve kinetics. The chemical state of Mn element in the MnO/C hybrids after cycling was detected by high resolution XPS (Fig. S4). Both of the MnO in MnO/C-5 and MnO/C-100 was oxidized to higher state (MnO/C-5 as MnO2, and MnO/C-100 as Mn2O3) [40e43,48,49] This could be explained by the CV results and GCD results. The oxidation of MnO to higher chemical state is observed in MnO/C-5 at a potential around 2.10 V. However, there is also some part of MnO in MnO/C-100 which has been oxidized to higher state though there is
Fig. 12. (a) TEM and SAED images, (b) HRTEM photos of MnO/C-5 after cycling for 1000 cycles at 2000 mA g1 (c) and (d) TEM photos of MnO/C-100 after cycling. The red circles refer to the nanoparticles in the hybrids. (A colour version of this figure can be viewed online.)
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Fig. 13. (a) Normalized Mn K-edge XANES and EXAFS spectra for the MnO/C hybrids charged to 3 V. (b) The K-edge Fourier transform magnitudes of k3 weighted EXAFS spectra of the MnO/C hybrids charged to 3 V. (A colour version of this figure can be viewed online.)
only small peak at potential of 2.10 V in the CV curves of the 1st and 2nd cycles. Since XPS has limited resolution to distinguish different chemical states of manganese oxides, we further perform XAFS to detect the final state of the Mn in MnO/C hybrid electrodes. Fig. 13a shows the normalized M k-edge X-ray absorption nearedge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) for the MnO/C-5 and MnO/C-100 hybrid electrodes after 1000 GCD cycles and then recharged to 3 V. It can be seen that the EXAFS profiles of the charged electrodes are similar to that of Mn2O3 and the peak position (6559.3 eV) is close to that of Mn2O3 (6559.0 eV). This slightly shift of peak position may be due to the higher chemical state of the MnO in the electrode, that is, the MnO inside the electrode may be partially oxidized to MnO2. And this suspect could be demonstrated by the Mn K-edge Fourier transform (FT) magnitudes of k3 weighted EXAFS spectrum of MnO/ C-5 electrode at 1.73 Å and 2.77 Å, which are close to those peaks of MnO2 at 1.78 Å and 2.76 Å. Also, the FT peaks of Mn2O3 are at 1.70 Å and 3.07 Å, which are also close to those of MnO/C-5 electrode. Thus we imply the chemical state of the Mn in MnO/C-5 electrode at 3 V is a mixture of Mn(III) and Mn(IV). For Mn in MnO/C-100 electrode, the EXAFS profile is similar to that of Mn3O4 (6558.9 eV) with a peak position at 6558.7 eV. The FT peaks of MnO/ C-100 electrode at 1.77 Å and 2.56 Å are also close to those peaks of Mn3O4 at 1.74 Å and 2.60 Å. However, Mn2O3 shows peak at 6559.0 eV in its EXAFS profile and FT peak at 1.70 Å, which are close to those of MnO/C-100 electrodes, it is hard to determine what the final product of the oxidization reaction is. MnO/C-100 exhibits FT peaks at 3.94 Å, 4.40 Å and 4.93 Å, being close to that of Mn2O3 at 3.91 Å, Mn3O4 at 4.40 Å and 4.94 Å. We thus speculate that the MnO in MnO/C-100 is oxidized to a mixture of Mn2O3 and Mn3O4. That is, after charging and discharging for 1000 cycles, the oxidization of Mn(II) to higher oxidization state occurs both in MnO/C-5 and MnO/C-100 and the chemical state of the Mn element in MnO/C100 electrode is mixture of Mn(II) and (III). We attribute this difference in chemical state of the final products to the different particle sizes. Since the smaller particles required lower surface activation energy for reaction, the MnO particles in MnO/C-5 with size around 5 nm could be easily oxidized to Mn2O3. Moreover, as the reaction proceeding, the broken nanoparticles with smaller particles (down to 2 nm) could be further oxidized to MnO2. Thus in the electrochemical energy storage of MnO, the particle size exhibits as the decisive factor for the increasing capacity. Smaller particle provides higher surface for reaction and the
reaction results in even smaller nanoparticles to introduce more active sites for reaction. This mutual effect then reduces the surface activation energy for oxidization of the nanoparticles and further increases the capacity. 4. Conclusion In conclusion, a MnO/C hybrid with ultra-small particles, partial graphitic porous carbon matrix and strong chemical bonding between particles and carbon is prepared and shows outstanding energy storage capacity. There is an increase in specific capacity after long cycling, and the reason is studied in depth. We found the fragmenting of the nanoparticles and oxidation of MnO to higher state both induced the results. However, the compared experiment shows that the MnO/C hybrid with larger particles also exhibits an increase of chemical valence after cycling with lower oxidized degree. The particle size directly affects the difficulty of oxidization reaction, and the reaction further refines the microstructure. This mutual effect results in the increasing capacity. We thus conclude the fragmenting of nanoparticles is the decisive reason of the enhancement of capacity. Acknowledgements This work was supported by Shanghai Science and Technology Committee (15ZR1422400, 14JC1403300 and 14520710100), Research Fund for the Doctoral Program of Higher Education of China (20120073130001). J. Gu greatly thanks the support from Program for New Century Excellent Talents in University, Ministry of Education, China. Great thanks for Q. Wang in Tianjin University for designing the schematic. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.11.068. References [1] D. Larcher, J. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (1) (2015) 19e29. [2] M. Obrovac, V. Chevrier, Alloy negative electrodes for Li-ion batteries, Chem. Rev. 114 (23) (2014) 11444e11502. [3] M. Reddy, G. Subba Rao, B. Chowdari, Metal oxides and oxysalts as anode materials for Li ion batteries, Chem. Rev. 113 (7) (2013) 5364e5457.
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