Solution combustion synthesis of porous Sn–C composite as anode material for lithium ion batteries

Solution combustion synthesis of porous Sn–C composite as anode material for lithium ion batteries

Advanced Powder Technology xxx (2016) xxx–xxx Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.co...

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Advanced Powder Technology xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Solution combustion synthesis of porous Sn–C composite as anode material for lithium ion batteries Genki Saito ⇑, Chunyu Zhu, Cheng-Gong Han, Norihito Sakaguchi, Tomohiro Akiyama Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo 060-8628, Japan

a r t i c l e

i n f o

Article history: Received 13 February 2016 Received in revised form 19 May 2016 Accepted 1 June 2016 Available online xxxx Keywords: Tin Composite Solution combustion synthesis Li-ion battery Anode

a b s t r a c t In this study, a Sn–C composite material as anode material for lithium ion batteries was fabricated via MgO template-assisted solution combustion synthesis, in which the starting material was a gel containing Sn(NO3)2, glycine (C2H5O2N) as the carbon source, and Mg(NO3)26H2O for the template. After the combustion reaction, the generated MgO was removed from the carbon, and the Sn nanoparticles were dispersed into a porous carbon structure during the carbon reduction of SnO2 under calcination in N2. The effects of ratios of glycine (n) and MgO (m) on the material phase, morphology, carbon content, and electrochemical properties were mainly investigated. At glycine (n) ratios of 2 and 3, the SnO2 phase was not fully reduced to Sn. With n > 3, a composite material of metallic Sn nanoparticles and carbon was synthesized, in which the ratio of carbon increased with increasing n. With increasing m, the porosity of the particles increased, resulting in enhanced cyclic stability owing to the buffer space provided by the porous structure of carbon. The composite material obtained at n = 4 and m = 4 exhibited the highest reversible capacity of 588 mA h/g after 100 discharge/charge cycles at a current rate of 0.5 A/g as compared to the 269 mA h/g observed for n = 4 and m = 2. Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Rechargeable lithium ion (Li-ion) batteries have been widely used as energy storage devices for portable electronic devices and electric vehicles. As standard anode materials for current Liion batteries, graphite and other carbonaceous materials with a maximum theoretical capacity of 372 mA h/g have been used. Although this capacity is not considered very low, new anode materials with higher capacities are required in order to produce lightweight batteries with high energy densities [1,2]. Recently, Sn anodes have attracted much attention because of their high theoretical capacity of 992 mA h/g to form a Li4.4Sn alloy [3,4]. However, huge inevitable volume changes (>300%) occur during the lithium insertion/extraction process, which leads to pulverization of the tin anode and loss of electric contact to the current collector, resulting in poor cycling performance [5]. Consequently, several strategies like reducing the particle size to nanoscale, fabricating composites of nano-Sn in conductive carbon, and the utilization of porous structures have been proposed to mitigate this volume change issue [6,7]. Actually, various synthesis methods such as

⇑ Corresponding author. Tel./fax: +81 11 706 6766.

hydrothermal synthesis [8], polyacrylonitrile (PAN)-assisted carbothermal reduction [9,10], carbonization using soft-template polymers [11], high-energy mechanical milling [12], aerosol spray pyrolysis [13] and others have been studied for composite materials synthesis. Well organized three-dimensional (3D) nanostructures such as tin nanoparticles encapsulated in hollow elastic carbon spheres [14], few-walled carbon nanotubes encapsulated in Sn composites [15], [email protected] nanoparticles encapsulated in bamboo-like hollow carbon nanofibers [16], etc. have also been fabricated to enhance the electric performance. Although these methods and materials can enhance the electrochemical properties, a facile synthetic method that uses low-cost materials, simple apparatus, and simple processing is still required. Solution combustion synthesis (SCS) is a highly exothermic and self-sustaining process involving heating a homogeneous solution of aqueous metal salts and fuels such as urea, citric acid, glycine acid, or glycine [17,18]. The method enables the homogeneous doping of trace quantities of various elements within a solid matrix. This approach has been used to synthesize a variety of materials, such as SrTiO3 perovskite for photocatalysts [19], brownmillerite-type Ca2AlMnO5 as an oxygen storage material [20], LaMO3 (M = Fe, Co, Mn) perovskite for air cathodes [21], and LiMn2O4-based spinel cathode for Li-ion batteries cathodes [22–

E-mail address: [email protected] (G. Saito). http://dx.doi.org/10.1016/j.apt.2016.06.004 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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25], where added fuel such as glycine acts as a complexing agent. Recently, SCS of nanosized amorphous iron oxide incorporated in a carbon matrix was reported, in which glycine acted not only as a complexing agent but also as the carbon source [26]. However, SCS of porous Sn–C composites has not been reported. Herein, we report the synthesis of a Sn–C composite material via SCS that is suitable for large-scale application. To design the porous structure, nanosized MgO was used as a template. The effects of the ratios of glycine and MgO on the materials phase, morphology, carbon content, and electrochemical properties were mainly investigated.

2. Experimental 2.1. Synthesis of Sn/C composite As the starting materials, the commercial reagents SnCl22H2O, Mg(NO3)26H2O, glycine (C2H5O2N), and HNO3 were used. In a typical process, SnCl22H2O was dissolved in ammonia water (28 mass %) to precipitate Sn(OH)2. The obtained Sn(OH)2 was rinsed several times with deionized water, after which it was dissolved in an HNO3 solution to form a Sn(NO3)2 solution. Mg(NO3)26H2O and C2H5O2N, which were weighted as described in Eq. (1), were added to the HNO3 solution. The resulting solution was then dried in a ceramic crucible to form a transparent gel at 90 °C.

nð1 þ mÞ  C2 H5 O2 N þ SnðNO3 Þ2 þ m  MgðNO3 Þ2  6H2 O þ 34  HNO3 ! SnO2 þ m  MgO þ a  CO2 þ b  NO2 þ c  H2 O þ d  C þ Q J

ð1Þ

The fuel ratio, n, is the ratio between the total valences of the fuels and the total valences of the nitrates based on the assumption described by Jain et al. [27]. The MgO ratio, m, is the molar ratio of Sn(NO3)2 and Mg(NO3)26H2O. In the experiment, n was varied from 2 to 5, whereas m was changed from 2 to 4. For comparison, porous carbon without Sn was also prepared. Table S1 supplies the detailed mass ratio of raw materials, as supporting information. The prepared gel was transferred to a lab-built apparatus for combustion synthesis [22,23]. The reactor consisted of a stainless-steel bin with a long vertical stainless-steel mesh chimney. The crucible was transferred to a heater, which was preheated and maintained at 400 °C. Upon reaching 200–300 °C, the gel quickly combusted, releasing a large amount of gases; at this point, power to the heater was immediately turned off to avoid burning the carbon. After SCS, the obtained particles were washed with 0.1 M HNO3 solution and with deionized water to remove the MgO phase. After drying, the sample was calcined at 700 °C for 1 h in N2 atmosphere. 2.2. Materials characterization The obtained particles at the end of each step were analyzed via an X-ray diffractometer (XRD; Miniflex II, Rigaku, Japan) employing Cu Ka radiation (k = 1.5418 Å). The microstructure of the samples was observed using a field-emission scanning electron microscope (SEM; JSM-7001FA, JEOL), in which the inner structure of the porous particle was observed by the ion-milling technique using a cross-sectional polisher (CP, IB-19510CP, JEOL). A transmission electron microscope (TEM, JEOL JEM-2010F) equipped with energy-dispersive X-ray spectroscopy (EDX) and electron energyloss spectroscopy (EELS) components were used. The stability of the as-synthesized sample was characterized using a thermogravimetric (TG, Mettler Toledo) analyzer with quadrupole mass analyzer (MS, Thermofisher) at a heating rate of 10 °C/min under a 100 mL/min flow of dry air or Ar. Sample weight loss recorded during TG analysis was used to determine the carbon content of the

sample. Sn and carbon in the composites were oxidized to SnO2 and CO2, respectively, in air, according to the following reactions:

Sn þ O2 ðgÞ ! SnO2

ð2Þ

C þ O2 ðgÞ ! CO2 ðgÞ

ð3Þ

2.3. Electrochemical measurement The electrochemical characterizations were carried out in twoelectrode Swagelok-type cells [28,29]. The working electrode consisted of an active material, conductive carbon (acetylene black), and a polymer binder of sodium carboxymethyl cellulose (CMC) and poly(acrylic acid) (PAA) in the weight ratio of 75:15:5:5. The well-blended solution-based slurry was pasted onto a copper foil and dried at 60 °C for 3 h in vacuum. The dried electrode was punched into a 14-mm-diameter disc with a mass load of 2– 3 mg. A 15-mm-diameter metallic lithium disc was used as the counter and reference electrode. The cells were assembled in an Ar-filled glove box (UNICO), using a solution of 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 v/v) as the electrolyte and a polypropylene membrane as the separator. The cells were galvanostatically cycled from 0.01 to 3.0 V versus Li/Li+ at 0.5 A/g in the constant current mode using a battery charge/discharge system (Hokuto Denko HJ1020mSD8) at a constant temperature of 25 °C. Cycling voltammograms (CV) were measured at a scanning rate of 0.2 mV/s using a potentiostat (PGSTAT 128N, Metrohm Autolab).

3. Results and discussion The samples prepared at different n and m conditions were characterized by XRD and TG analysis for optimizing synthetic conditions. Fig. 1 shows the XRD patterns of the sample synthesized at n (fuel ratio) = 4 and m (MgO ratio) = 2. After SCS at 400 °C, broad SnO2 peaks appeared in Fig. 1(a), in which the MgO (peak data: JCPDS No. 01-075-0444) and carbon peaks were not detected due to amorphous structure. Similar behavior has been noted in previous reports of the sol–gel synthesis of SnO2–MgO [30,31]; in these, the crystallization of MgO required higher temperatures. However, here, crystalline MgO was generated after calcination at 700 °C without washing. Then, to clarify the structure and elemental distribution of the sample synthesized at n = 4 and m = 4, characterization using SEM–EDS was carried out. Fig. 2(a) shows a secondary electron image of the powder cross-section. As shown in Fig. 2(a), pores have been introduced to the particles due to the release of CO2 and NOx gases during SCS (see Eq. (1)). Fig. 2(b) shows a compositional map determined from backscattered electrons. In this image, some particles and surface layers have greater contrast (see arrows) compared to other areas, indicating the presence of materials consisting of relatively heavy elements like Sn and Mg. EDS analysis (Fig. 2(c)–(e)) revealed that Mg and Sn are widely distributed throughout the carbon matrix and some SnO2–MgO particles or layers can also be seen. Based on macroscopic observation, SnO2 and MgO are not clearly separated. Therefore, the SnO2 and MgO particles are small, and these two phases are well mixed together. To remove MgO as the template, the synthesized particles were washed with 0.1 M HNO3 solution for 20 min. The SnO2 peaks of the washed sample shown in Fig. 1(b) became sharper than that of as SCS samples, which might indicate the removal of the amorphous MgO phase. In the washing process, any remaining glycine or other soluble unreacted materials were also eliminated. Finally, the sample was calcined at 700 °C for 1 h in N2 atmosphere. In this

Please cite this article in press as: G. Saito et al., Solution combustion synthesis of porous Sn–C composite as anode material for lithium ion batteries, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.06.004

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n = 4, m = 2

SnO2

Sn

Carbon

MgO-SnO2

Gases

Intensity (arb. units)

(a) As SCSed SnO2/MgO/C composite (a)

(b) HNO3 wash

(c) 700 ºC in N2

MgO

SnO2/C composite

MgO Sn/C composite

2θ (degree) Fig. 1. XRD patterns of the samples after (a) SCS, (b) HNO3 washing, and (c) heat treatment at 700 °C for 1 h under N2 flow. Referential peaks of MgO (JCPDS No. 01-075-0444) are shown in the bottom. Inset figures are the corresponding illustrations of the samples at each step.

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Fig. 2. (a) Secondary electron image, (b) compositional map determined using a backscattered electron detector of powder treated by SCS with n = 4 and m = 4 without washing or heating treatment, and (c–e) corresponding EDS mapping.

process, carbothermal reduction occurred for synthesizing the composite of metallic Sn and carbon as per the following reactions:

SnO2 þ C ¼ Sn þ CO2 ðgÞ

ð4Þ

SnO2 þ 2C ¼ Sn þ 2CO ðgÞ

ð5Þ

According to thermodynamic calculations using the Outokumpu HSC 5.11 software, these reactions can occur at temperatures over 592 °C and 641 °C for Eqs. (4) and (5), respectively. Actually, TG mass analysis detected CO2 generation during heating under Ar flow, as shown in Fig. S1(a) (Supporting Information). In the n = 4 and m = 2 condition, the SnO2 phase was completely reduced to Sn, as shown in Fig. 1, where a broad graphite (0 0 2)

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reflection was also detected. Generally, graphitization requires higher temperature over 1000 °C. Additionally, the synthesized carbon contained nitrogen [Fig. S1(b) in Supporting Information], resulting in disarrangement of the carbon structure. It has been reported that nitrogen-doped carbon can enhance the electronic conductivity of carbon-based materials [32,33]. Thus, the obtained carbon was expected to serve as an effective matrix of Sn nanoparticles. The inset schemes in Fig. 1 show the corresponding illustrations of the samples at each step. Table S1 shows a summary of the sample compositions after calcination. When fuel ratio n was below 3, the SnO2 phase was not completely reduced to Sn because of the lack of enough carbon for reduction. At n values over 4, a composite material of metallic Sn nanoparticles and carbon was synthesized, in which the ratio of carbon increased with increasing n. In our study, the fuel ratio n was determined by the total valences of Sn(NO3)2 and Mg(NO3)26H2O. Thus, the increased m resulted in increased carbon content because of the relative decrease of the SnO2 phase against MgO. To fabricate the porous structure, Mg(NO3)26H2O as a template was added to the gel raw materials with the MgO ratio m varied from 2 to 4. To clarify the inner structure of the obtained particles, the cross-sectional SEM sample was prepared by using ion milling with a rotation holder after mixing with the binder and conductive carbon. The mixture was then pasted onto copper foil. Fig. 3 shows the cross-sectional SEM images of the electrode with different m values. It is clearly found that the porosity of the particles increased significantly owing to the increase of MgO amount. As shown in the image, spherical particles are attached to the porous carbon frame. Fig. 4(a) shows the TEM image of a sample with n = 4 and m = 4. The EDS analysis shown in Fig. 4(b) indicates that the spherical particles are metallic Sn. Since the melting point of Sn is 231.9 °C, liquid Sn is generated during the carbothermal reduction; this liquid Sn then agglomerates with surrounding liquid Sn, resulting in the formation of spherical Sn particles on cooling.

(a) n = 4, m = 2

Additionally, small black dots can be seen uniformly dispersed throughout the carbon matrix (see arrows). From EDS and EELS analysis, the film like structure in the area marked 2 in Fig. 4(a) was found to contain not only carbon but also Sn and N. The doping of nitrogen in graphite occurred because glycine was used as a precursor. Consequently, small Sn particles remained on graphite after reduction, and these particles did not agglomerate with the surrounding Sn. At m = 2, shown in Fig. 3(a), coarse Sn particles over 100 nm in size were observed. In contrast, owing to the highly porous structure, the size of Sn particles generated at n = 4 and m = 4 decreased to less than 100 nm. In addition, the number density of Sn particles attached to the carbon matrix decreased at high m. This result might be useful to accommodate the volume expansion of Sn due to the large distances between particles. The electrochemical properties of the synthesized composite were investigated using a Li disk as a reference and counter electrode at a current density of 0.5 A/g. Fig. 5(a) shows the cyclic performance of the synthesized materials with different m values under a fixed n value of 4. At m = 2, initial discharge capacity was 783 mA h/g. It then decreased dramatically upon fracture of the electrode, which occurred during the charge/discharge cycle because the dense carbon structure could not accommodate the volume expansion of Sn. When m = 4, the initial discharge capacity was 1121 mA h/g, which was higher than that of m = 2. Generally, irreversible reactions such as the formation of a solid electrolyte interface layer and solvent decomposition in the electrolyte can occur [15]. With increasing m, the surface area of the carbon might increase, resulting in an increase in the formation of a solid electrolyte interface layer. To investigate the origin of irreversible capacities, porous carbon without Sn addition was also produced via SCS. As compared to the initial discharge capacity of 258 mA h/g in commercial graphite, the porous carbon produced without Sn exhibited an initial discharge capacity of 676 mA h/g (see Fig. S2 in Supporting Information). Thus, the porous carbon

(b) n = 4, m = 4

Copper foil

Fig. 3. Cross-sectional SEM images of the electrode with different m values.

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D ,QWHQVLW\ DX 

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Area 2

Area 1

Sn-L

C C

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Sn-L

(QHUJ\ NH9 

C-K

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(QHUJ\ORVV H9 Fig. 4. (a) TEM image of n = 4 and m = 4 sample. (b and c) Energy dispersive X-ray spectrometry (EDS) analysis of area 1 and area 2, as indicated in image (a). (d) Electron energy-loss spectroscopy (EELS) analysis of area 2.

Specific capacity (mAh/g)

Fuel ratio, n = 4 0.5 A/g

MgO ratio m =4 m =3 m =2

(b) Specific capacity (mAh/g)

(a)

MgO ratio, m = 4 0.5 A/g Fuel ratio n=3 n=4

n=5

Graphite

Cycle Number ( -)

Cycle Number (-)

Fig. 5. Cycling performance of Sn/C composite anode materials at 0.5 A/g ranging from 0.01 to 3.00 V. (a) Effect of m at n = 4 and (b) effect of n at m = 4.

(a) n = 4, m = 2

Copper foil

(b) n = 4, m = 4

Fig. 6. Cross-sectional SEM images of the electrode after 150 cycling measurements at a current density of 0.5 A/g. In the case of m = 2, cracks were generated on the samples (see arrows).

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Fig. 7. Cyclic voltammograms of the initial three cycles scanned between 0 and 3 V at a rate of 0.2 mV/s of (a) porous carbon without Sn addition and (b) n = 4 and m = 4 sample. (c) Charge/discharge profiles at 0.5 A/g of n = 4 and m = 4 sample. (d) Rate capacity of n = 4 and m = 4 sample.

structure increased the initial discharge capacity owing to its higher surface area. This tendency agrees with that of other carbon nanostructures [9,15]. The capacity of m = 4 after 100 cycles showed a discharge capacity of 588 mA h/g, which is much higher than that of graphite. The Sn addition and highly porous structure increased the capacity and cycle stability. Fig. 6 shows the crosssectional SEM images of the electrode after 150 cycling measurements under a current density of 0.5 A/g. In the case of m = 2, cracks clearly appeared in the electrode, which might have occurred because of the huge volume change of Sn during the lithium insertion/extraction process. In contrast, cracks did not form in the m = 4 sample. The high structural stability of the por-

ous Sn–C composite yielded excellent cyclic performance with high capacity. Fig. 5(b) shows the cycling performance for samples at different fuel ratios n and a fixed m value of 4. The Sn contents were 48, 35, and 20 mass% at n = 3, 4, and 5, respectively. The n = 3 sample exhibited a higher capacity at an early stage; however, the capacity gradually decreased because the higher Sn content concurrently increased the capacity and decreased the stability for volume expansion. In contrast, the n = 5 sample had a lower capacity of 512 mA h/g after 100 cycles owing to the small amount of Sn. Table 1 summarizes the typical values for the electrochemical properties. From these results, it was found that n = 4 and m = 4 are the optimum conditions.

Table 1 Capacity (Q) and capacity retention with different n and m values at the current density of 0.5 A/g. n

m

Q1st (mA h/g), discharge/charge; Coulombic efficiency

Q50th discharge (mA h/g); Coulombic efficiency

Q100th discharge (mA h/g)

Capacity retention; Q100th/Q1st (%)

3

4

1328/822; 61.9%

570; 99.5%

524

39.5

4 4 4

2 3 4

783/297; 38.0% 1012/735; 72.6% 1121/770; 68.7%

269; 99.9% 651; 98.7% 606; 99.6%

270 569 588

34.4 56.2 52.5

5 5 5

2 3 4

1187/593; 50.0% 1229/845; 68.7% 1018/529; 51.9%

427; 99.7% 572; 99.6% 560; 99.6%

409 544 512

34.4 44.2 50.9

3 Graphite

Sn:0%

676/308; 45.5% 258/195; 75.5%

301; 98.7% 216; 99.6%

284 240

42.3 93.4

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G. Saito et al. / Advanced Powder Technology xxx (2016) xxx–xxx Table 2 Comparison of electrochemical performance for composite of Sn and carbon. Synthesized material

Electrochemical performance +

Sn–C composite (This study) Sn–C composite Sn-in-CNTs Sn-NPs encapsulated in hollow carbon Nano-Sn/C composite Hierarchical tin/carbon composite Carbon coating on Sn

Ref.

Potential range vs. Li/Li (V)

Current density

Initial discharge capacity (mA h/g)

Capacity retention (mA h/g)

3.0–0.01 2.0–0.01 2.5–0.01 3.0–0.05 3.0–0.02 2.5–0.01 3.0–0.01

500 mA/g 50 mA/g 50 mA/g 0.2 C 0.26 C (200 mA/g) 3000 mA/g 0.1 C

1121 848.8 1340.9 1500 1029 – 1000

588, 100 cycles 630, 100 cycles 639.7, 170 cycles 550, 100 cycles 710, 130 cycles 537, 1000 cycles 730, 200 cycles

Fig. 7(a) and (b) shows the cyclic voltammograms (CVs) of porous carbon without Sn and the n = 4, m = 4 sample, respectively, in the 1st, 2nd, and 3rd scanning cycles at 0.2 mV/s. In Fig. 7(a), the reduction peak range of 0.4–1.0 V (vs. Li/Li+) is attributed to the formation of a solid electrolyte interface film on the carbon surface [9], which disappeared in the 2nd and 3rd cycles. A similar peak was observed for the n = 4 and m = 4 sample shown in Fig. 7(b). The reduction peak at approximately 0.01–0.4 V corresponds to the lithium alloying process in Sn particles and the insertion of Li+ into the carbon matrix. Oxidation peaks at 0.64, 0.74, and 0.81 V are associated with lithium extraction from LixSn alloys in the subsequent steps [13–15], described in Eq. (6). The broad anodic peak at 1.15 V represents lithium extraction from carbon [34]. þ

Sn þ xLi þ xe $ Lix Sn ð0 5 x 5 4:4Þ

ð6Þ

The voltage profiles of the Sn–C composite anodes at cycle 1, 2, 10, and 100 are shown in Fig. 7(c). The first discharge capacity was 1121 mA h/g and 68.7% of the inserted Li could be reversibly de-lithiated, resulting in an initial charge capacity of 770 mA h/g. The capacity loss of 351 mA h/g in the first discharge/charge process is mainly caused by the formation of a solid electrolyte interface film. The obtained charge/discharge curves after the first cycle demonstrated good cycling stability. The rate capacity of the obtained composite at different current densities is shown in Fig. 7(d). The reversible capacity decreased with increasing current density from approximately 800 mA h/g at 0.1 A/g to approximately 450 mA h/g at 1.0 A/g. Below a current density of 1.0 A/g, the capacities remained stable. Table 2 summarizes the electrochemical performance for this study and reported data for comparison. Even though the measurement conditions were different, the obtained results show sufficient capacity at a relatively high current density of 500 mA/g. This increased capacity is ascribed not only to the addition of Sn but also to the increased capacity of carbon with its highly porous structure. The highly porous structure of carbon caused an increase in the initial discharge capacity and a decrease in voltage plateau stability. In a future work, this issue should be resolved, in which a decrease in the amount of carbon and modification of the carbon surface and the electrolyte are possible. 4. Conclusions In this study, a composite material of Sn nanoparticles and porous carbon was synthesized by glycine–nitrate-based solution combustion synthesis using a MgO template. To optimize the synthesis conditions, the effects of fuel ratio n and MgO ratio m on the structure, morphology, size, and electrochemical performance were systematically investigated. At n values over 4, a composite material of metallic Sn nanoparticles and carbon was synthesized, in which the ratio of carbon increased with increasing n. With increasing m, the porosity of the particles increased, resulting in enhanced cyclic stability owing to the buffer space provided by the porous structure of the carbon. The composite material

– [9] [15] [14] [13] [34] [35]

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