Synthesis and Electrochemical Reaction of Tin Oxalate-Reduced

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Synthesis and Electrochemical Reaction of Tin Oxalate - Reduced Graphene Oxide Composite Anode for Rechargeable Lithium Batteries Jae-Sang Park, Jae Hyeon Jo, Hitoshi Yashiro, Sung-Soo Kim, Sun-Jae Kim, Yang-Kook Sun, and Seung-Taek Myung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03325 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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ACS Applied Materials & Interfaces

Synthesis and Electrochemical Reaction of Tin Oxalate - Reduced Graphene Oxide Composite Anode for Rechargeable Lithium Batteries

Jae-Sang Parka, Jae-Hyeon Joa, Hitoshi Yashirob, Sung-Soo Kimc, Sun-Jae Kima, YangKook Sund,*, Seung-Taek Myunga,*

a

Department of Nanotechnology and Advanced Materials Engineering & Sejong Battery

Institute, Sejong University, Seoul, 05006, South Korea b

Department of Chemistry and Bioengineering, Iwate University, 4-3-5 Ueda, Morioka,

Iwate 020-8551, Japan c

Graduate School of Green Energy Technology, Chungnam National University, Daejon

34134, South Korea d

Department of Energy Engineering, Hanyang University, Seoul 04763, South Korea

Abstract Unlike SnO2, few studies have reported on the use of SnC2O4 as an anode material for rechargeable lithium batteries. Here, we first introduce a SnC2O4-reduced

*Corresponding authors Tel: 82 2 3408 3454, fax: 82 2 3408 4342, e-mail: [email protected] (S. Myung) Tel: 82 2 2220 0524, fax: 82 2 2282 7329, e-mail: [email protected] (Y. Sun)

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graphene oxide composite produced via hydrothermal reactions followed by a layer-bylayer self-assembly process. The addition of rGO increased the electric conductivity up to ~10-3 S cm-1. As a result, the SnC2O4-reduced graphene oxide electrode exhibited a high charge (oxidation) capacity of ~1166 mAh g-1 at a current of 100 mA g-1 (0.1 Crate) with a good retention delivering approximately 620 mAh g-1 at the 200th cycle. Even at a rate of 10 C (10 A g-1), the composite electrode was able to obtain a charge capacity of 467 mAh g-1. In contrast, the bare SnC2O4 had inferior electrochemical properties relative to the SnC2O4 – reduced graphene oxide composite: ~643 mAh g-1 at the first charge, retaining 192 mAh g-1 at the 200th cycle and 289 mAh g-1 at 10 C. This improvement in electrochemical properties is most likely due to the improvement in electric conductivity, which enables facile electron transfer via simultaneous conversion above 0.75 V and de-/alloy reactions below 0.75 V: SnC2O4 + 2Li+ + 2e- → Sn + Li2C2O4 + xLi+ + xe- → LixSn on discharge (reduction) and vice versa on charge. This was confirmed by systematic studies of ex-situ X-ray diffraction, transmission electron microscopy, and time-of-flight secondary-ion mass spectroscopy. Keywords: Tin oxalate; Reduced graphene oxide; Conversion; Alloy; Anode; Lithium; Battery

1. Introduction Li-ion batteries (LIBs) are a major technology for powering portable electronics and more recently electric vehicles. For automotive applications in particular, electrode materials should have a high capacity with a low cost and reliable safety. So far,

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graphite has commonly been adopted as the anode active material in commercial LIBs; however, more energy is required for longer operation times. Hence, to satisfy the growing power requirements, it is necessary to explore alternative high capacity anode materials to replace graphite, which shows a limited theoretical capacity of 372 mAh g1 1-7

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For these reasons, silicon-based materials operated by de-/alloy reactions have been

widely investigated as a replacement for commercial graphite anodes due to their high capacities of over 1,000 mAh g-1.8 Metal oxides are of interest due to their high capacity; this is because their conversion reactions, in which the metal oxides are converted to metal along with the formation of Li2O, use multiple oxidation states of metal during electrochemical reactions.9-15 Recent studies by Tirado’s group introduced various metal oxalates, MC2O4 (M : Mn, Fe, Co, Ni, Sn, Cu, Zn), as possible alternatives to graphite for use as high capacity anode materials.16-19 The related reaction is also a conversion process via the formation of metal and Li2C2O4 when discharged (reduced): MC2O4 + 2Li+ + 2e- M + Li2C2O4. Very recently, we verified the formation of Li2C2O+ and metal during discharge by means of time-of-flight secondary ion mass spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy [20]. Compared with metal oxides, metal oxalates exhibit a higher capacity and better capacity retention relative to metal oxides, though the reasons are unknown. The addition of reduced graphene oxides (rGO) to NiC2O4·2H2O

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or

functionalized graphene oxides (FGO) to Cu1-xCoxC2O4 and Co1-xCuxC2O4·2H2O 21 was also promising for retaining the high capacity and rate capability upon cycling. Alcántara et al.22 showed that SnC2O4 reacted according to an 11 Li per unit formula, yielding a reversible capacity of ~600 mAh g-1, although the electrode underwent gradual capacity fading during cycling testing. Kataoka et al.23 employed SnC2O4 to 3



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modify the surface of Si anode material. Their SnC2O4 exhibited a discharge capacity of approximately 500 mAh g-1. Kim et al.24 introduced nanostructured SnC2O4 using chemical anodization of Sn metal discs, although they did not test the SnC2O4 nanowires for battery applications. The above-mentioned metal oxalates are produced via several synthetic methods such as spray pyrolysis25, solvothermal reaction26, sol-gel methods27,28, hydrothermal reaction29, and microwave-assisted solution approaches30. Nacimiento et al.31 employed polyacrylonitrile (PAN) to improve the dispersion of SnC2O4 particles. According to our prior work20, the attachment of NiC2O4·2H2O nanorods onto rGO sheets effectively prevented agglomeration and improved the electric conductivity, which is attributed to the intimate contact between oxalate materials and electro-conducting rGO sheets. In this study, we synthesized phase-pure SnC2O4 via a hydrothermal method at low temperatures. Provided that such dual functionalities are achieved by the formation of SnC2O4/rGO composite, the higher capacity of over 600 mAh g-1 reported by Alcántara et al.22 can be obtained. For this reason, we sandwiched the hydrothermally synthesized SnC2O4 particles between rGO sheets using a layer-by-layer self-assembly (SA-LBL) process (schemed in Figure 1). In this paper, we report a synthetic procedure for forming SnC2O4/rGO composite, which shows dramatic improvement in the electrochemical performances with help of electro-conducting rGO sheets. Also, we first unveil the reaction mechanism of SnC2O4/rGO composite electrode progressed via consecutive reaction; namely, conversion to Sn metal accompanying formation of Li2C2O4 and LixSn alloy formation on discharge (reduction) and on charge (oxidation) vice versa by means of ex-situ X-ray diffraction, transmission electron microscopy, and time-of-flight secondary-ion mass spectroscopy. 4


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2. Experimental 2.1. Synthesis of SnC2O4 and rGO/SnO2 composite via SA-LBL process. Tin oxalate, i.e., SnC2O4, was synthesized via a hydrothermal reaction of SnCl2 · 6H2O (0.001 M), Na2C2O4 (0.001 M), distilled water (9 ml), and ethylene glycol (EG, 16 ml) in a Teflon-lined autoclave (100 ml capacity) in the temperature range of 100 – 200 oC for 12 h. After the reaction, the precipitated products were filtered and washed with de-ionized water and then dried at 80 oC overnight in a vacuum oven. The rGO/SnO2 composite was produced via SA-LBL process (rGO produced via chemical vapor deposition method, IDT International). Details of the SA-LBL process for synthesis of rGO/SnO2 composite are described in our earlier work. 20

2.2. Characterization Crystal structures of the products were identified by X-Ray diffraction (XRD, Rigaku, D-max 2500) using Cu Kα radiation with a scan rate of 0.03o (2θ) per second. The above products were observed by scanning electron microscopy (SEM, Hitachi, S4700) and transmission electron microscopy (TEM, Hitachi, H-800). Raman spectroscopy (Renishow, inVia Raman Microscope) was used to confirm the presence of rGO in the rGO/SnC2O4 composite. These two compounds were compressed to pellets (diameter: 10 mm, thickness: 5 mm), contacted with a four probe, to measure the DC electrical conductivity using a direct volt-ampere method (CMT-SR1000, AIT). Elemental analyzer (CHN elemental analyzer EA110, CE Instrument) was used to

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determine the carbon content in the SnC2O4 and SnC2O4/rGO composite, of which the subtracted value was referred as the amount of rGO in the SnC2O4/rGO composite. 2032 coin type cells were employed to measure electrochemical properties of SnC2O4 and SnC2O4/rGO composite. The prepared SnC2O4 powders (85 wt%), ketjen black (5 wt%), super-P (5 wt%), and polyacrylic acid (5 wt%) were mixed in distilled water. For the SnC2O4/rGO composite electrode, the prepared SnC2O4/rGO powders (89.8 wt%) was comprised of rGO (4.8 wt%), ketjen black (2.6 wt%), super-P (2.6 wt%), and polyacrylic acid (5 wt%) in distilled water. These slurries were then coated onto copper foil and dried in a vacuum oven at 80 oC for 1 h. Details of electrode fabrication process are found in our earlier work.

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A typical carbonate solution, 1 M

LiPF6 in a 3 : 7 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), was used as the electrode. The fabricated cells were tested in a voltage range of 0 - 3 V at a current of 100 mA g-1 at 25 oC. The typical loading level of the active material was approximately 8 mg on the Cu disk (φ: 1.6 cm). The experimental apparatus for single particle microelectrodes was reported elsewhere.32 In a typical measurement, a Cu coated micro electrode, which aims to minimize the background current, was attached to an active material particle in the electrolyte using a micromanipulator under optical microscope observation. Then, electrochemical charge and discharge tests were performed with the single particles under observation. Li metal was used as the counter electrode. One of the aim of the present work is to investigate reaction mechanism for both SnC2O4 and SnC2O4/rGO composite in Li cells, so that XRD, TEM, X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5600) with a monochromatic Al-Kα 6


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source, and time-of-flight secondary ion mass spectroscopy (ToF-SIMS, PHI TRIFT V nanoTOF, ULVAC-PHI, Kanagawa, Japan) surface analyzer were employed to investigate the crystal structure, chemical and surface states of those active materials before and after electrochemical tests. Details of experimental procedure and sample fabrication methods are reported in our prior work. 20

3. Result and discussion To synthesize SnC2O4/rGO composite, SnC2O4 was first produced via a hydrothermal reaction of SnCl2·6H2O (0.001 M) and Na2C2O4 (0.001 M) at 100 oC (Figure 2a), as follows. SnCl2·6H2O + Na2C2O4 SnC2O4 + 2NaCl + 6H2O

(1).

The reaction does not require oxidant to obtain SnC2O4 because the oxidation state of Sn for starting SnCl2·6H2O is 2+. Hence, the simple hydrothermal reaction yielded a crystalline SnC2O4 product without impurities. Long stick-like particles were produced in the synthetic condition (Figure 2b-1). Increasing the reaction temperature to 140 oC did not alter the crystal structure of the SnC2O4. (Figure 2a), and the resulting particle morphology was altered into an irregular shape while the observed particle size became smaller with increasing reaction temperature (Figure 2b-2). Note that there is no change in the crystal structure compared to the resultant synthesized at 100 oC. However, the reaction performed at 170 oC decreased the relative intensity of the XRD pattern (Figure 2a), and the products exhibited a further decrease in particle size (Figure 2b-3). At 200 o

C, the SnC2O4 phase was decomposed into SnO2 and SnO (Figure 2a), and spherical

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particles were found in the products (Figure 2b-4). Due to the high crystallinity and reduced particle size, we decided the optimal condition to be 140 oC. The SnC2O4 synthesized at 140 oC was modified by rGO via the SA-LBL process. The resulting XRD pattern indicates that the SA-LBL treatment did not cause the formation of impurities in the final products (Figure 3a). TEM images of the SnC2O4 synthesized at 140 oC demonstrate that the particle size of the produced SnC2O4 ranged from 20 – 30 nm in diameter (Figure 3b). High resolution TEM images further display that the SnC2O4 particle is composed of several nanometer-sized crystalline primary particles (Figure 3c). From microscopic studies, it is concluded that the SnC2O4 particles are attached on the surface of rGO sheets (hereafter referred as to be SnC2O4/rGO). The presence of rGO is also evident from the Raman spectra comparison of rGO with SnC2O4/rGO (Figure 4a). There is no appearance of D and G bands for the assynthesized SnC2O4. The band observed at 1600 cm-1 stems from the C=C symmetrical vibration, which is due to the formation of a crystalline π-bond sp3 character. In comparison with the as-received rGO, relative intensity of the G band slightly increased for the SA-LBL-treated SnC2O4/rGO composite. The presence of SnC2O4 was also noticed in the spectrum though the resulting intensity is very small (see inset of Figure 4). To confirm the reason why there was such variation in the D and G bands, we first dispersed only rGO sheets in distilled water (100 ml) for 10 min to produce a homogeneous rGO suspension and subsequently added poly(diallyldimethylammonium chloride) (PDDA, 15 ml). The SA-LBL-treated rGO also exhibited the similar Raman spectrum to the SnC2O4/rGO composite in D and G bands. This suggests that the SA-

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LBL process is a possible reason for the change in the D and G bands. XPS spectra of both SnC2O4 and SnC2O4/rGO composite show that there is no change in the binding energy of Sn 3d orbital, in which the oxidation state of Sn is stabilized as 2+ (Figure 4b). A distinct C-C binding energy originated from the rGO is observed in the XPS spectra for the SnC2O4/rGO composite, and the appearance of C=O binding energy is ascribed to the bond between carbon and oxygen in the SnC2O4 (Figure 4c). The analyzed carbon content in the SnC2O4/rGO composite was approximately 4.8 wt. %, which agrees with the added amount of rGO (initially at 5 wt. % for SnC2O4) during the SA-LBL process. Therefore, the presence of electro-conducting rGO substantially improved the electric conductivity to approximately 8 x 10-3 S cm-1 for the rGO composite material, while that of SnC2O4 was approximately 1 x 10-8 S cm-1. The reduced contact resistance achieved with help of rGO is also evidenced in ac-impedance spectra (Figure S1). The synthesized SnC2O4 and SnC2O4/rGO composite were tested in the voltage range of 0.01 to 3.0 V by applying a constant current of 100 mA g-1 at 25 oC. Both electrodes exhibited similar voltage profiles on discharge (reduction), though they had different discharge capacities: 1394 mAh g-1 for SnC2O4 and 1763 mAh g-1 for SnC2O4/rGO (Figure 5a). The delivered charge (oxidation) capacity was approximately 643 mAh g-1 for the SnC2O4 (46% coulombic efficiency), while a higher capacity (1166 mAh g-1) was seen for the SnC2O4/rGO electrode (66% coulombic efficiency at the first cycle). The only difference in both materials is the presence of rGO with SnC2O4. As a result, this significant improvement in capacity is most likely related to the enhanced electric conductivity of the SnC2O4/rGO electrode. In addition, the smaller addition of nanosized conducting agents such as ketjen black (2.6 wt%) and super-P (2.6 wt%) relative to the SnC2O4 electrode with ketjen black (5 wt%) and super-P (5 wt%) is 9



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responsible for the decreased reductive decomposition of the electrolyte, which is reflected to capacity on discharge. The present high capacity is remarkable compared with an earlier report of SnC2O4/PAN by Alcántara et al.22: approximately 620 mAh g-1 on charge with a 47% coulombic efficiency at the first cycle. Cyclic voltametric curves of the first cycle for both SnC2O4 and SnC2O4/rGO electrodes show two distinct reactions at around 0.7 V and 0.1 V on discharge and 0.7 V and 1.15 V on charge (Figure 5b). These reversible reactions are related to the conversion to metal at high voltages and alloy formation at low voltages. It is evident that the SnC2O4/rGO electrode had a higher capacity than that of the SnC2O4 and SnC2O4/PAN (polyacrylonitrile) composite, which had about 40% retention in the voltage range of 0 – 2 V during 40 cycles.22 The presence of SnC2O4/rGO electrode, which was assisted by the electro-conducting rGO sheets, enabled a delivery of approximately 620 mAh g-1 at the 200th cycle (Figures 5c and d). The rate capability also demonstrates the availability of the SnC2O4/rGO at high rates. The cell was charged and discharged at a rate of 0.2 C (200 mA g-1) to 10 C (Figure 5e). At the 10 C-rate (10 A g-1), the delivered charge capacity was approximately 467 mAh g-1, which is significantly greater than that of the capacity observed in bare SnC2O4, 289 mAh g-1. Furthermore, the charge capacity recovered to 925 mAh g-1 when tested at a current of 200 mA g-1, marked as buffer in Figure 5e. Continuous cycling at rates of 5C and 10 C was also possible for the SnC2O4/rGO composite electrode even at a rate of 10C (Figure 5f). We believe that this improvement in the electrode performance of SnC2O4/rGO is ascribed to the enhanced electrical conductivity, which is due to the presence of the rGO sheets on which the SnC2O4 nanoparticles were loaded. Direct contact between the active materials and rGO sheets 10


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renders the electron transport facile during the electrochemical reaction. To follow the reaction process of the SnC2O4/rGO electrode, the crystal structure was investigated via ex-situ XRD and TEM with a selected-area diffraction (SAD) pattern at the first cycle (Figure 6a). The Cu current collector was used as an internal standard for the calibration of the XRD patterns (Figure 6b). During lithiation (reduction) at 0.75 V, it is notable that the original SnC2O4 structure was no longer present, but low crystalline Sn metal and Li2C2O4 were dominant in the XRD pattern. In the TEM images (Figure 7a,b), it is clear that the nanosized particles merged due to lithiation. Also, the agglomerate was surrounded by a Li2C2O4 matrix. Sn metal and Li2C2O4 were found as polycrystalline patterns, which are in accordance with the XRD pattern (Figure 6b). To further confirm the formation of Li2C2O4, the electrodes were analyzed with ToF-SIMS (Figure 8a). It is clear that there are no LiCO+ (m = 35.01) fragments for the fresh electrode. The LiCO+ fragment appeared at 1.5 V, and the fragment was further intensified by lithiation to 0.75 V. These data demonstrate that the lithiation reaction is related to conversion: SnC2O4 + 2Li+ + 2e- → Li2C2O4 + Sn (reaction 1). Further lithiation to 0 V induced the development of a Sn metal peak though the appearance of broadened Li22Sn5 alloy in the XRD pattern (Figure 6b). The development of a Sn metal peak indicates progressive conversion toward Sn metal following reaction 1. The Sn metal simultaneously reacts with Li on discharge, and the resulting signature of Li22Sn5 can be explained by the alloy formation of Sn with Li. These are demonstrated in the TEM images in Figure 7c, exhibiting the formation of large plate-like particles due to the volume expansion resulting from Li-Sn alloy 11



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formation. Compared with the data obtained at 0.75 V, the LiCO+ fragment did not vary when discharged to 0 V. The LiSn+ (m = 126.92) fragment is clearly notable at 0 V, although the presence of the LiSn+ fragment was not obvious at 0.75 V. This suggests that the Li-Sn alloy formation is dominant below 0.75 V as follows: Sn + xLi+ + xe- → LixSn (reaction 2). Upon charge (oxidation) to 3 V, the electrode was seemingly composed of the SnC2O4 phase, though showed a low crystallinity in the XRD pattern (Figure 6b). The large particle observed at the end of discharge became smaller, but the nanosized SnC2O4 particles were no longer seen at the end of the charge (Figure 7d-1). Furthermore, the resulting SAD pattern showed the coexistence of Sn metal and Li2C2O4 (which could not be converted to SnC2O4) together with SnC2O4, but a LixSn alloy was not detected (Figure 7d-2). The low crystalline or amorphous-like Sn metal and Li2C2O4 explain why they were not found in the XRD pattern for SnC2O4/rGO (Figure 6b). In ToF-SIMS, it is obvious that the relative intensity of the LiCO+ fragment was gradually lowered on charge, and that of the LiSn+ fragment decreased significantly at the end of charging (Figure 8b). This indicates that a large portion of Sn metal and LixSn alloy recovered to SnC2O4 on charge, although the resultant exhibited low crystallinity. The de-alloying process seems to be reversible because of the significant reduction in intensity of the LiSn+ fragment (Figure 8b). The reaction, in particular oxidative conversion reaction as follow; Sn + Li2C2O4 SnC2O4 + 2Li+, seemingly accompanies an additional voltage plateau at approximately 2.5 V (Figure 5c). However, the conversion from Sn metal to SnC2O4 does not properly occur on charge because Sn metal and Li2C2O4 are still observed in the SAD pattern of TEM and ToF-

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SIMS. Apart from the reductive decomposition of the electrolyte on discharge, the irreversible capacity is ascribed to the incomplete conversion from Sn metal to SnC2O4 rather than the de-/alloy reactions. For the bare SnC2O4 electrode, however, the conversion and consecutive de-/alloy reactions seem less likely to occur in the reverse direction relative to the SnC2O4/rGO because Sn metal was still observed after the charge to 3 V (Figure 6b). This causes the large irreversible capacity at the first cycle for bare SnC2O4, shown in Figure 5a. In an attempt to highlight the superiority of the present SnC2O4/rGO composite, both SnC2O4 and SnC2O4/rGO single agglomerates ( > 10 µm in diameter) were tested electrochemically in electrolyte solution without conducting agent and binder under optical objective lens (Figure 9a). For the measurement, the discharge current was fixed to 0.5 nA, however, the charge current varied from 0.5 to 5 nA in the voltage range of 0 – 2.0 V. The SnC2O4 single agglomerate could not be properly discharge or charged but, even worse, the morphology was crumbled at the current (Figure 9b-3). Hence, it is thought lithiation probably induces stress at the currents, which implies that the current was too high, so that the particle damage would occur. As a result, there was almost no capacity delivered at the current (Figure 9b-1). Repetition of volume changes in these particles is likely to induce disruption of the secondary particles as evidenced in Figure 9b-3. The rupture of particles consequently causes loss of electric contact among the particles and the capacity fade gradually occurs during cycling. Surprisingly, the SnC2O4/rGO single agglomerate delivered capacity at 5 nA (Figure 9c-1), showing no serious damage in the particle morphology (Figure 9c-3) presumably due to anchoring of SnC2O4 particles on the rGO sheets. Since these tests were carried out using single agglomerates without additional binder or a current collector, de-/lithiation would be the 13



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only parameter influencing the physical strength of the particles. During the reaction, volume expansion is mitigated by the presence of rGO sheets, which also plays another role to provide conducting path. Therefore, the SnC2O4/rGO could be durable without serious damage in their morphology (Figure 9c). The only difference between SnC2O4 and the SnC2O4/rGO composite is the presence of rGO, which renders electron transport facile due to its electro-conducting character. With the help of rGO, the electric conductivity of the SnC2O4/rGO composite could increase to 8 x 10-3 S cm-1, which is significantly higher than that of the bare SnC2O4, 1 x 10-8 S cm-1. According to the ex-situ XRD, TEM, and ToF-SIMS studies, we found that SnC2O4 underwent two simultaneous reactions, namely, conversion (reaction 1) followed by de-/alloy (reaction 2) reactions. The conversion reaction was dominant above 0.75 V, below which the main reaction was related to the alloy process on discharge and vice versa on charge. During this reaction, the observed large capacity difference at the first cycle is ascribed to the irreversible reactions on the conversion, the de-/alloy process, and more seriously reductive decomposition of electrolyte to form a solid electrolyte interface (SEI) film. Therefore, improvement in the reversibility in the reaction 1 and reaction 2 is substantially important to realize the high irreversibility. As seen in CV curves (Figure 5b), the SnC2O4/rGO composite electrode delivered more capacities in both conversion and de-/alloy reactions; in particular, the improved cathodic reversibility is notable during the de-alloy reaction in the range of 0.25 – 0.75 V. Another words, the presence of conductive rGO sheets enabled the alloy to de-alloy reaction facile in the voltage range, which spontaneously improves the coulombic efficiency at the first cycle. In comparison with the earlier report of SnC2O4/PAN (47% coulombic efficiency) by Alcántara et al.22 and our bare SnC2O4 the present 14


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achievement by SnC2O4/rGO composite is worthy of mentioning because of its improvement in coulombic efficiency at the first cycle to 66% with help of the electroconducting rGO sheets. This value is sufficiently high in comparison recent high capacity conversion-alloy anode materials.33-37 The repetitive conversion and de/-alloy reactions lead to self-pulverization of the active materials due to continuous expansion and contraction followed by conversion and de-/alloy reactions. Figure 7e shows a TEM image of the SnC2O4/rGO composite after extensive cycles, and the resulting SAD pattern demonstrated low crystallinity or an amorphous-like structure, in accordance with the XRD data. The pulverized active materials can lose the electric contact supported by the carbon conducting agent or can be isolated, and the loss of active material is reflected in the drastic capacity fading present during cycling. For the SnC2O4/rGO composite, on the other hand, the pulverized active materials can be restacked on the rGO sheets, as seen in Figure 7e, such that the pulverized SnC2O4 particles were connected with the parent SnC2O4 particles; this allowed for the rGO sheets to act as a current collector. Hence, even the pulverized particles can continuously participate in the electrochemical reaction and contribute to the improved capacity. This is all schematically illustrated in Figure 10. This pulverization was caused by formation of Li-Sn and it cause volume expansion. As mentioned in Introduction, it is the first report on SnC2O4/rGO composite and the resulting capacity and its retention of our SnC2O4/rGO composite is believed to rank as the highest level even in comparison with recent conversion – de-/alloy SnO2/carbon compounds.33-37 We also tested the SnC2O4/rGO composite in the Na cells in the same voltage range (Figure S2). Since the electrode was activated by the simultaneous conversion and de-/alloy reaction in Li cells, a similar behavior was expected in Na cells. Also, SnO2 is 15



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known as a high capacity anode material in rechargeable sodium batteries. Nonetheless, the electrode did not exhibit electrochemical activity in Na cells. At present, we have no direct evidence of the formation of Na2C2O4 for conversion in Na cells, but it is hypothesized that an oxalated-based conversion reaction is more preferred in Li systems rather than in Na systems. Another possibility is that the large ionic radius of Na+ (1.02 Å)38 relative to Li+ (0.76 Å)38 may delay the formation of Na2C2O4. Shortening of the diffusion path might be necessary for electrochemical activity in Na cells to occur. Unless Na2C2O4 is produced, Sn metal is not formed from SnC2O4 due to progressive conversion. Provided that Sn metal is formed via conversion, the alloy formation of NaSn readily occurs. Therefore, the suggested conversion reaction is suitable for Li systems, and the addition of rGO enables a stable SnC2O4/rGO composite electrode cycle for an extended period of time, even at high rates. Recent several works also highlight the importance of electron conduction path provided by carbons in lithium and sodium cells.39-42

4. Conclusions SnC2O4 was successfully synthesized using a hydrothermal method. Although SnC2O4 is a newly proposed electrode material for use as anodes in LIBs, it suffers from a large irreversibility during conversion and de-/alloy reactions, presumably due the small electric conductivity. The present SA-LBL process effectively helped synthesize the SnC2O4/rGO composite, and the use of rGO allowed for dramatic improvement in the electric conductivity of the SnC2O4/rGO composite to ~10-3 S cm-1, compared to ~10-8 S cm-1 for SnC2O4. As a result, the SnC2O4/rGO composite could have 16


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significantly improved electrochemical performances. It is clear that the high electric conductivity supported by the rGO sheets enabled the reversible lithiation-delithiation reaction process followed by the conversion reaction; this is accomplished through both the transformation into Sn metal with the formation of LiC2O4 and the alloy reaction that produced the Li-Sn alloy on discharge. On charge, the simultaneous de-alloy and conversion resulted in recovery of the original SnC2O4 structure in the presence of rGO sheets, as shown by XRD, TEM, and ToF-SIMS. Further elaboration is required to activate the present SnC2O4/rGO composite in Na cells.

5. Acknowledgements The authors would like to thank Miwa Watanabe, Iwate University, for her assistance in the experimental work. This research was supported by the basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology of Korea (NRF2014R1A2A1A11051197) and by the National Research Foundation of Korea funded by the Korean government (MEST) (NRF-2015M3D1A1069713). This work was also partly supported by a grant from the KETEP (Grant 20164010201070).

Notes The authors declare no competing financial interest.

Supporting Information 17



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Supporting Information Available: Ac-impedance spectra of SnC2O4 and SnC2O4/rGO electrodes in Li cells; electrochemical test of SnC2O4 in Na cell

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23



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Figure 1. Schematic illustration of the synthesized SnC2O4/rGO via a self-assembly layer-by-layer process.

24


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a

O : SnO2 O : SnO

O O

Intensity / A.U.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

O

O

200 C O

O O

O

O OO O

o

170 C o

140 C o

100 C JCPDS# : 51-0614

10

20

30

40

50

60

70

CuKα 2θ / degree Figure 2. (a) Phase evolution of hydrothermally synthesized SnC2O4 (Bragg peak position: JCPDS Card number 51-0614) and (b) the resulting SEM images.

25



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Figure 3. (a)XRD results of as-synthesized SnC2O4 (bottom) and SnC2O4/rGO (top) (Bragg peak position: JCPDS Card number 51-0614); (b-1) TEM and (b-2) SEM image of SnC2O4/rGO that show embedment of SnC2O4 onto rGO sheets, (b-3) TEM image of as-synthesized SnC2O4, and (b-4) crystal structure of SnC2O4; (c-1) high resolution TEM image (scale bar: 20 nm) and (c-2) its magnified view of SnC2O4/rGO composite (scale bar: 10 nm).

26


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Intensity / A.U.

As-synthesized SnC2O4

a

As-received rGO SA-LBL-treated rGO SA-LBL-treated SnC2O4/rGO

500

500

750

1000

1500

2000

2500 -1

3000

Raman shift / cm

b

Sn 3d orbital Sn 3d5/2

SnC2O4/rGO Sn 3d3/2

500

495

490

C1s orbital

485

480

Binding energy / eV

c

C-C

SnC2O4

Intensity / A.U.

SnC2O4

Intensity / A.U.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SnC2O4/rGO C-O C=O

294

292

290

288

286

284

282

Binding energy / eV

Figure 4. (a) Raman spectra of as-synthesized SnC2O4, as-received rGO, SA-LBLtreated rGO, and SnC2O4/rGO composites (inset: magnified spectrum of SnC2O4/rGO in the range of 500 – 800 cm-1); XPS photoelectron spectra of (b) Sn3d orbital and (c) C1s orbital for as-synthesized SnC2O4 and SnC2O4/rGO composites.

27


280

ACS Applied Materials & Interfaces 3.5

a

-1

1166 mAh g

3.0

0.4

Current / mA

2.5 2.0 1.5 1.0 0.5

0.2 0.0 -0.2 -0.4 SnC 2O 4

SnC2O4

0.0 0

500

SnC 2O 4/rGO

-0.6

SnC2O4/rGO

1000

1500

-1

-0.5

2000

0.0

3.0 2.5 2.0 1.5 1.0 0.5 0.0

2nd 5th 10th 30th 50th 100th 150th 200th

SnC2O4/rGO

500

1000

2.0

2.5

3.0

+

3.5

100

1200

SnC2O4 Coulombic efficiency

80

SnC2O4/rGO Coulombic efficiency

900 60 600

40 SnC2O4 SnC2O4/rGO

300 0

0

1.5

d -1

SnC2O4

1.0

1500

c Capacity / mAh g

Voltage / V

3.0 2.5 2.0 1.5 1.0 0.5 0.0

0.5

Potential / V vs. Li /Li

Capacity / mAh (g-SnC2O4)

0

1500

40

20

80

120

0 200

160

-1

Capacity / mAh g

Buffer 0.5C 1C 3C 5C 10C

SnC2O4/rGO

0

500

1000

-1

e

Capacity / mAh g

3.0 SnC2O4 2.5 2.0 1.5 1.0 0.5 0.0 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Number of cycle 1200 1000

SnC2O4 0.2

SnC2O4/rGO 0.5

800 1 3

600

5C

10C

400 200 0

1500

f

0.2

0

10

20

30

40

50

60

-1

Capacity / mAh g

Number of cycle

Figure 5. (a) First discharge and charge curves, (b) the cyclic voltammogram curves, (c) continuous discharge and charge curves for 200 cycles, and (d) cyclability results for 200 cycles obtained in the range of 0 - 3 V, applying a constant current of 100 mAg-1 at 25 oC; (e) rate capability of SnC2O4-/rGO electrode discharged and charged at a rate of 0.2 C (200 mA g-1) to 10 C (10 A g-1) and (f) the resulting cyclability at 5C and 10C where 0.2C denotes buffer.

28


70

Efficiency / %

Voltage / V

b

-1

643 mAh g

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Voltage / V

3.5

a

3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

500

1000

1500

2000

-1


b

Cu

Li2C2O4 Li4.4Sn Sn SnC2O4

Cu

Intensity / A.U.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Extensively cycled

Charged to 3 V Bare SnC2O4 Charged to 3 V

Discharged to 0 V

Discharged to 0.75 V

As-synthesized SnC2O4/rGO

20

30

40

50

60

70

Cu Kα 2θ / degree Figure 6. (a) First discharge and charge curves with black dots, which indicate that XRD measurements were performed; (b) ex-situ XRD patterns of SnC2O4/rGO as well as after extensive cycles, in which the bare SnC2O4 electrode after 1st cycle was added for comparison.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Figure 7. TEM images of SnC2O4/rGO during discharge, charge, and after extensive cycles and as well as SAD patterns; (a-1) TEM image and (b-1) the resulting SAD pattern for the as-synthesized SnC2O4/rGO; (a-2) TEM image and (b-2) the resulting SAD pattern for the SnC2O4/rGO electrode discharged to 0.75 V; (a-3) TEM image and (b-3) the resulting SAD pattern for the SnC2O4/rGO electrode discharged to 0 V; (a-4) TEM image and (b-4) the resulting SAD pattern for the SnC2O4/rGO electrode charged to 3 V; (a-5) TEM image and (b-5) the resulting SAD pattern for the SnC2O4/rGO electrode after extensive cycles.

30


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31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Single particle measurement: (a) schematic illustration of measurements apparatus; the corresponding test results and particle images for (b) SnC2O4, red scale bar (15.221 µm) and (c) SnC2O4/rGO, red scale bar (11.976 µm).

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Voltage / V

Page 33 of 34

3.5

: SnC2O4

3.0

: Sn : Li22Sn5

2.5 2.0 1.5

SnC2O4 + 2Li+ + 2e- ↔ Sn + Li2C2O4

1.0 5Sn + 22Li+ + 22e- ↔ Li22Sn5

0.5 0.0 0

500

1000

1500

-1

2000


Figure 10. Schematic electrochemical reaction of the SnC2O4/rGO composite during cycles.

33



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Page 34 of 34

Table of Content THE SNC2O4/RGO

USE OF RGO ALLOWED THE HIGH ELECTRIC CONDUCTIVITY SUPPORTED BY THE RGO SH

EETS PROMOTED THE REVERSIBLE LITHIATION-DELITHIATION REACTION PROCESS ASSOCIATED WITH THE CON VERSION REACTION; THIS IS ACCOMPLISHED THROUGH BOTH THE TRANSFORMATION INTO HE FORMATION OF

LIC2O 4

AND THE ALLOY REACTION THAT PRODUCED THE

LI-SN

SN

METAL WITH T

ALLOY ON DISCHARGE.

Keywords: Tin oxalate; Reduced graphene oxide; Conversion; Alloy; Anode; Lithium; Battery

34


Synthesis and Electrochemical Reaction of Tin Oxalate-Reduced

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