Ion assisted anchoring Sn nanoparticles on nitrogen-doped graphene as an anode for lithium ion batteries

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 9 1 3 e2 4 9 2 1

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Ion assisted anchoring Sn nanoparticles on nitrogen-doped graphene as an anode for lithium ion batteries Liang Zhan, Xiaosong Zhou, Jin Luo, Xiaomei Ning* School of Chemistry and Chemical Engineering, Key Laboratory of Clean Energy Materials Chemistry of Guangdong Higher Education Institutes, Lingnan Normal University, Zhanjiang, 524048, China

highlights
Sn/N-doped graphene (NGN) was obtained by a simple liquid method with the help of 7,7,8,8-tetracyanoquinodimethane (TCNQ).
Sn/NGN composite exhibits good rate capability and long cycle performance at 1 A g1.
TCNQ∙- anions are avorable for anchoring Sn4þ on graphene oxide and act as a nitrogen source for N doping on graphene.

article info

abstract

Article history:

Though lithium ion batteries are popular in many fields, high performance electrode ma-

Received 11 March 2019

terials with reasonable structure are desired. Herein, Sn/nitrogen-doped graphene (NGN)

Received in revised form

was fabricated with the help of 7,7,8,8-tetracyanoquinodimethane anion (TCNQ∙-). TCNQ∙-

15 July 2019

was used to anchor Sn4þ into the graphene layer with the electrostatic interaction, which

Accepted 20 July 2019

improves the distribution of Sn nanoparticles. Meanwhile, TCNQ∙- acts as a nitrogen

Available online 16 August 2019

source to construct N doped graphene, enhancing the electron conductivity of the composite. Benefiting from the strong structure and good ion/electron conductivity, the Sn/

Keywords:

NGN composite achieved excellent electrochemical battery performance. At high rates of 1

Sn

and 2 A g1, capacities of 433 and 353 mA h g1 were acquired, respectively. Moreover, the

Graphene

Sn/NGN composite outputted a high capacity of 584 mA h g1 at the end of 1000 cycles at

7,7,8,8-tetracyanoquinodimethane

1 A g1.

Lithium ion batteries

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Because of a commercial application in the latest 30 years, lithium ion batteries (LIBs) became a hotspot for energy storing and conversion area [1e5]. In recent years, LIBs are widely employed as electric vehicles batteries, but its energy density is still an obstacle for achieving a long cruising ability [6]. In terms of anode materials, there is an urgent need to explore

high capacity materials instead of the commercial graphite (372 mA h g1) [6e8]. Accompanied with a high theoretical capacity of 992 mA h g1 and more safe operating potential, metallic Sn was regarded as a promising alternative [9,10]. Unfortunately, metallic Sn usually undergoes huge volume change (~260%) during the Liþ insertion/extraction process, bringing about serious pulverization and active material loss, which ultimately results in serious capacity fading [11].

* Corresponding author. E-mail address: [email protected] (X. Ning). https://doi.org/10.1016/j.ijhydene.2019.07.153 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Construction of Sn-M alloy (M ¼ Co [12,13], Cu [14,15], Ni [16e18], Fe [19]) has been confirmed to alleviate the capacity fading problem of Sn materials. In this structure, M is usually an inactive metal to relieve the drastic volume change. However, the output capacity of Sn-based alloy is severely impacted by the presence of inactive metal. Another popular strategy is fabricating nano-sized Sn/carbonaceous composite materials, such as Sn/C [20e27], Sn/carbon nanotube (CNT) [28,29], and Sn/graphene (GN) [9,30e32]. The carbonaceous materials act as a buffer to remit the structure damage and enhance the conductivity of the Sn-based composites. Moreover, carbonaceous materials are active anode materials, which can contribute a part of capacity. Hence, Sn/carbonaceous materials can achieve high capacities and better battery performance than bare Sn materials. However, in order to obtain high performance, constructing simple composite is not enough, how to firmly restrict Sn particles in the carbonaceous materials and improve the distribution of Sn particles is the key issue. In this paper, Sn/N-doped graphene was obtained with the participation of 7,7,8,8-tetracyanoquinodimethane (TCNQ), shown in Fig. 1. TCNQ has been employed as an available molecular to interact with graphitic carbon matrix through pp interaction [33,34]. Besides, TCNQ molecules tend to partially ionize in organic solutions, such as acetonitrile and N, Ndimethylformamide [35]. The formed TCNQ∙- anions are favorable for anchoring Sn4þ on graphene oxide via electrostatic interaction. Moreover, a TCNQ∙- anion has four nitrogen atoms that can be employed as a nitrogen source for graphene N doping (NGN). With the assistance of TCNQ∙-, a unique architecture was fabricated, which can provide several advantages as an anode for LIBs. Firstly, TCNQ∙- anions can be absorbed on graphite oxide and provide more interaction sites for Sn4þ, which improves the distribution of Sn nanoparticles (NPs) on GN. Secondly, Sn NPs supported and intercalated NGN to form a sandwich structure, which effectively prevents the volume increase of Sn NPs during reacting with Liþ,

strengthening the structure stability. Thirdly, the NGN can offer an electron transport channel, which increases the conductivity of the composite and enhances the rate capability. Therefore, as-obtained Sn/NGN electrode exhibits a high capacity of 584 mA h g1 after cycling 1000 times at 1 A g1.

Experimental section Synthesis of Sn/NGN and Sn/GN Graphene oxide (GO) was obtained by using modified hummers method [36]. GO suspension (2 mg mL1) was obtained through ultrasonically dispersing GO powder in the solvent of N,N-Dimethylformamide (DMF) and deionized water with the volume ratio of 9:1. The 7,7,8,8-tetracyanoquinodimethane acetonitrile solution (10 mg mL1) was dropwise added to above GO suspension with stirring and ultrasonication for 30 min. Then, a certain amount of SnCl4$5H2O dissolved in ethanol (47.5 mg mL1) was further added into the GO suspension drop by drop with magnetic stirring and ultrasonication within 1 h. Subsequently, hydrazine hydrate (1.25 mL for 1 mg GO) was added to the suspension, and continuously stirred for 2 h at 80  C. After naturally cooled, the mixed solution was centrifuged, then the precipitate was washed using distilled water and ethanol, then the cleaned product was dried at 70  C overnight in a vacuum oven. Lastly, the ground solid was further reduced in 7 v % H2/Ar at 550  C for 3 h. The obtained sample is marked as Sn/NGN. Contrastively, the sample of Sn/GN was synthesized with the same process of Sn/NGN, however, without procedure for adding 7,7,8,8-tetracyanoquinodimethane anion acetonitrile solution to the GO suspension. The prepared Sn/NGN and Sn/GN were dispersed into 6 M HCl and stirred at room temperature for 1.5 h. After filtration and vacuum drying at 70  C for 12 h, the obtained samples were denoted as Sn/NGN(H) and Sn/GN(H), respectively.

Fig. 1 e Schematic diagram to illustrate the frabrication of Sn/NGN composite.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 2 4 9 1 3 e2 4 9 2 1

Materials characterizations Raman spectrometer (Raman, LabRAM Aramis) and X-ray diffraction (XRD, Bruker D8) were used to analyze the composition of samples. Sn contents of samples were acquired by electron probe microanalysis (EPMA, EPMA-1600, Shimadzu). Xray photoelectron spectroscopy (XPS) measurements were operated on an ESCALAB 250Xi X-ray photoelectron spectrometer. C1s peak with binding energy of 284.6 eV was employed as calibration. The morphology and element distribution of asobtained samples were detected using transmission electron microscopy (TEM, JEM-2010, 200 kV) and scanning electron microscopy (SEM, JSM-7610F, 5 kV). The average Sn NPs size (dð  Þ) P P was calculated as the formula of nid3i / nid2i , herein, ni is the Sn NPs number with the diameter of di.

Electrochemical measurements The Sn/GN and Sn/NGN composite were used as working electrode within the following steps: Sn/NGN or Sn/GN powder, carbon black, and PVDF binder were mixed (wt.%, 8:1:1). The obtained mixture was covered on copper foil and transferred to an oven with 80  C to make it dry. Then the treated copper foil was cutted into a circle electrode with the diameter of 12 mm. Mass loading of each electrode is 0.9e1.2 mg cm3. A half-button battery was assembled in a glove box with an argon atmosphere (Mikrouna, Super) to test the battery performance of Sn/GN and Sn/NGN electrode. The separator was Celgard 2400, and the counter electrode was Li foil. The electrolyte was LiPF6 (1 M) in ethylene carbonate/diethyl carbonate (vol%, 1:1). A LAND battery-test system (CT2001A) was used for galvanostatic chargedischarge tests within 0.01 Ve3.0 V. Cyclic voltammetry (CV) tests were operated on a CHI660D electrochemical workstation, with the test voltage range of 0.01 Ve3.0 V and the sweeping speed of 0.2 mV s1. Electrochemical impedance spectroscopy (EIS, PGSTAT l00N) was conducted in a frequency region of 0.01 Hze100 kHz with the amplitude of 5 mV. Before EIS testing, coin battery was cycled for 3 times at 0.1 A g1. The specific capacity is calculated based on the total mass of Sn/GN and Sn/NGN.

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Results and discussion The crystal form and composition of Sn/GN and Sn/NGN were investigated by XRD and Raman in Fig. 2. For both samples with the XRD data (Fig. 2a), the main diffraction peaks correspond to that of tetragonal Sn. Two unconspicuous peaks assigned to SnO2 can be detected, indicating that there is still a small amount of SnO2 in the Sn/NGN composite. No obvious peak belongs to GN can be scanned. To further confirm the existence of GN, Raman spectra of GN, Sn/GN and Sn/NGN are shown in Fig. 2b. For all the three samples, there are two main peaks around 1350 cm1 (D) and 1605 cm1 (G), which belong to the disordered and graphitic sp2 bonding carbon, respectively [9,37]. It should be noted that the ID/IG intensity ratios of GN, Sn/GN, Sn/NGN are 0.91, 1.04, and 1.09, respectively. The increased disorder structure of Sn/GN and Sn/NGN conposites can be attributed to the partial insertion of Sn into GN [29]. According to the EPMA tests, Sn contents in the Sn/GN and Sn/ NGN composites were 45.3 wt % and 42.2 wt %, respectively. The EPMA result shows that Sn content in two samples is similar. The Sn 3d and N 1s XPS spectra of Sn/NGN composite were exhibited in Fig. 3. After peak separation, the Sn 3d in Fig. 3a contains four peaks, peaks at 495.1 eV and 487.1 eV belong to Sn 3d3/2 and Sn 3d5/2 of SnO2 [38]. Peaks at 493.5 eV and 485.1 eV belong to Sn 3d3/2 and Sn 3d5/2 of metallic Sn [39]. Because the detective depth of XPS is a few nanometers, the XPS result of Sn indicates that the surface Sn is easily oxidized in air. Besides, a layer of SnO2 on Sn particles can prevent further oxidation and remain the main existence of metallic Sn, which coincides with the XRD results. The N1s curve in Fig. 3b can be broke up to five peaks, the peaks at 398.4 eV, 400 eV, 401.3 eV, 403.1 eV, and 406.5 eV belong to the pyridinic N, pyrrolic N, graphitic N, oxidized N, and chemisorbed N, respectively [40e42]. The N content measured by XPS is 2.83 at. %. In the synthesis process, TCNQ and TCNQ∙- anions on graphene act as nitrogen sources to form N-doped graphene through thermal decomposition. Therefore, N doped both on the surface of graphene and in the graphene layer. The N1s distribution of N-doped graphene in our paper due to

Fig. 2 e (a) XRD patterns of Sn/GN and Sn/NGN. (b) Raman spectra of GN, Sn/GN, and Sn/NGN.

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Fig. 3 e XPS spectra of Sn/NGN composite: (a) Sn 3d and (b) N 1s.

Fig. 4 e SEM images of (a, b) Sn/GN and (d, e) Sn/NGN. TEM images of (c) Sn/GN and (f) Sn/NGN, the inset are particle size distribution graphs of Sn/GN and Sn/NGN. Elemental mapping images of Sn/NGN: (g) C, (h) N, (i) Sn.

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the thermal decomposition was varied compared with N1s of original TCNQ [43]. Previous researches have proved that N doping can improve the electron conductivity of graphene [44,45]. Hence, N doping can improve the electron conductivity of Sn/NGN composite. As shown in Fig. 4, the morphological characteristics of Sn/ GN and Sn/NGN composites were observed by SEM and TEM. By contrast, SEM images (Fig. 4a and d) of Sn/GN and Sn/NGN composites have no obvious difference, both materials show a structure of Sn NPs tightly wrapped in the graphene sheets. In the HRSEM images (Fig. 4b and e), graphene displays a wrinkled paper-like structure [46]. Few significantly brighter particles exist on the surface of GN and NGN, which is possibly due to the Sn particles with larger size. The TEM image (Fig. 4f) exhibits that Sn NPs are well distributed on NGN without obvious aggregation. However, Sn NPs on GN have obvious conjunction and agglomeration (Fig. 4c). Beyond that, the Sn NPs on GN had a wider size distribution (3 nm ~ 10 nm) than that of Sn NPs on NGN (0.5 nm ~ 5.5 nm). Sn NPs on GN are mainly distributed between 4.5 nme6 nm, while Sn NPs on NGN are mainly distributed between 1.5 nme2.5 nm. Hence, the average diameter of Sn NPs in Sn/NGN (3.3 nm, Fig. 4f) is much smaller than that of Sn/GN (6.2 nm, Fig. 4c). Additionally, the TEM element mapping images in Fig. 4gei reveal that the elements of C, N, and Sn are existed and uniformly distributed in the Sn/NGN material. Electrochemical performance of the Sn/GN and Sn/NGN composites were tested, seen in Fig. 5. The initial three CV splines of Sn/NGN were displayed in Fig. 5a. In the initial reduction scanning, there are three peaks around 0.88 V, 0.62 V, and 0.30 V, relating to the formation of a solid electrolyte interface (SEI) film and LixSn alloys [47]. In the following oxidation process, there are four peaks

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between 0.50 V and 0.8 V, which is attributed to the taking off of Liþ from the LixSn alloys [20,47]. In the following two cycles, reduction peaks appear at 0.64 V and 0.36 V, and the oxide peaks are still unchanged. The CV splines of the second cycle coincide well with the third cycle, indicating good electrochemical invertibility of the Sn/ NGN composite. Fig. 5b showed galvanostatic charge/discharge curves of Sn/NGN in the 1st, 2nd and 3rd cycles at 100 mA g1. The 1st cycle outputs discharge and charge capacities of 1251 mA h g1 and 802 mA h g1, respectively. The coulombic efficiency is 64.1%. The capacity loss involves electrolyte decomposing to form SEI film [48]. Cycle performance of Sn/GN and Sn/NGN composites at 0.1 A g1 were shown in Fig. 5c. Sn/NGN displays nearly 800 mA h g1 in the early several cycles and delivers a capacity of 1100 mA h g1 at the end of 200 cycles. The Sn/GN composite has a stable cyclic performance, but the output capacity is only 400 mA h g1 in the initial few cycles and 500 mA h g1 after 200 cycles, which was only half that of Sn/NGN. Rate performances of Sn/GN and Sn/NGN composites were tested between 0.1 A g1 and 2 A g1, the results were displayed in Fig. 5d. Sn/NGN composite exhibits capacities of 690 mA h g1, 606 mA h g1, 506 mA h g1, 433 mA h g1, and 353 mA h g1 at 0.1 A g1, 0.2 A g1, 0.5 A g1, 1 A g1, and 2 A g1, respectively. By comparison, Sn/GN composite outputs capacities of 437 mA h g1, 362 mA h g1, 302 mA h g1, 266 mA h g1, and 222 mA h g1 at the same rates, respectively. Obviously, Sn/NGN composite achieves a better rate capability than the Sn/GN composite. The function of TCNQ∙- contributes most to this performance gap, which constructs the Ndoped structure to improve the electron conductivity and provides more active sites.

Fig. 5 e (a) Cyclic voltammogram of Sn/NGN. (b) The initial three charge and discharge curves of Sn/NGN. (c) Cycling performance of Sn/GN and Sn/NGN at 0.1 A g¡1. (d) Rate capability of Sn/GN and Sn/NGN from 0.1 to 2 A g¡1. (e) Cycling performance of Sn/GN and Sn/NGN at 1 A g¡1.

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Fig. 6 e Nyquist impedance and fitting curves of Sn/GN and Sn/NGN.

Fig. 5e exhibits the cycle performance of Sn/GN and Sn/ NGN composites at 1 A g1. Sn/NGN electrode displays capacities of 450 mA h g1 and 584 mA h g1 at 1st and 1000th cycles, respectively, exhibiting good cycle stability. For comparison, Sn/GN electrode exhibits 316 mA h g1 for the 1st cycle, only 261 mA h g1 is achieved at 1000th cycles, which delivers a worse performance than the Sn/NGN composite.

The cycle performance indicates that the Sn/NGN electrode can offer more capacity due to more active sites and higher electron conductivity. EIS testing were carried out to confirm the electron transport kinetics of Sn/GN and Sn/NGN composites. As shown in Fig. 6, both EIS data include a semicircle and a sloping line. An equivalent circuit model (the inset in Fig. 6) was used to obtain fitting curves. The fitting result shows that the Sn/NGNS composite has a smaller charge transfer resistance (110 U) than the Sn/NG composite (154 U), exhibiting a better conductivity. The Sn/GN and Sn/NGN electrodes after 200 cycles at 0.1 A g1 were tested by SEM and XPS, the results were shown in Fig. 7. The Sn/GN electrode showed an integrated structure with some cracks (Fig. 7a). The Sn/NGN electrode exhibited an integrated structure without any crack (Fig. 7b), showing better structural stability. The XPS spectra of Sn/GN and Sn/ NGN after cycling (Fig. 7c) showed both samples remained active Sn, which was covered with the decomposition product of the electrolyte. Two peaks at 495.1 and 487.0 eV (Fig. 7d) belong to Sn 3d3/2 and Sn 3d5/2, corresponding to the XPS results of fresh Sn/NGN in Fig. 3a. To better clarify why Sn/NGN has better electrochemical performance than Sn/GN, more detailed analysis and discussions are carried out. We have contrasted the CV curves of Sn/ GN and Sn/NGN after several cycles (Fig. 8a), the reduction and oxidation peaks appear at the same voltage position,

Fig. 7 e SEM images of Sn/GN (a) and Sn/NGN (b) after 200 cycles at 0.1 A g¡1. (c) XPS spectrum of Sn/NGN after 200 cycles at 0.1 A g¡1. (d) High-resolution XPS spectrum of Sn 3d.

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Fig. 8 e (a) CV curves of Sn/GN and Sn/NGN, (b) XRD patterns of Sn/GN and Sn/NGN after HCl etching.

indicating the same reaction mechanism. The better electrochemical performance is mainly related to the following three reasons. Firstly, smaller particle size and uniform distribution of Sn in Sn/NGN improve the electrochemical performance. According to the TEM images, Sn NPs are well distributed on NGN without obvious aggregation, the average diameter of Sn NPs in Sn/NGN (3.3 nm) is much smaller than that of Sn/GN (6.2 nm). The smaller particle size and uniform distribution of Sn in Sn/NGN can improve the utilization of Sn and enhance the conductivity, finally delivers higher lithium storage ability. Secondly, N-doped graphene contributes greatly to the capacity of Sn/NGN composite. According to EPMA data, the proportions of GN and NGN in the Sn/GN and Sn/NGN composites are about 54.7 and 57.8 wt %. Therefore, except the capacity of Sn, GN and NGN contribute nonnegligible capacities to the Sn/GN and Sn/NGN composites, respectively. The N content in Sn/NGN measured by XPS is 2.83 at. %. Previous research indicated that N-doped graphene obtained with TCNQ can achieve higher capacity and rate performance than bare graphene [49]. Hence, NGN can help Sn/NGN to obtain a better battery performance. Thirdly, sandwich structure of Sn/ NGN constructed with the assistance of TCNQ improves the battery performance. TCNQ is partially ionized in acetonitrile solution to form TCNQ∙-. TCNQ∙- plays a key role to achieve the sandwich structure, it can anchoring Sn4þ into inter-layer of graphene. To prove this, Sn/GN and Sn/NGN composites were treated with 6 M HCl at room temperature for 1.5 h, the obtained samples were denoted as Sn/NGN(H) and Sn/GN(H), respectively. The XRD results of Sn/NGN(H) and Sn/GN(H) are shown in Fig. 8b. It is obvious that a little SnO2 still exists in the Sn/NGN(H). Because the existed SnO2 locates deep in the graphene layer, it cannot be reduced by H2 at the annealing process and corroded by HCl, indicating TCNQ can anchor Sn4þ deeply in the graphene layer to construct a strong sandwich structure. For the Sn/GN composite, Sn and SnO2 cannot be detected after HCl etching, indicating Sn4þ cannot be anchored deeply in the graphene layer. The stronger sandwich structure helps Sn/NGN achieving better battery performance.

Conclusion In conclusion, a Sn/NGN composite was synthesized with the assistance of TCNQ∙- and worked as an anode for LIBs. In this synthesizing procedure, TCNQ∙- anion was used as an anchoring agent and a nitrogen source, which was conductive to achieve well-dispersed Sn NPs on the N-doped graphene layer. The designed architecture can strengthen the structural stability and enhance the ion/electron transfer channel, leading to a good electrochemical performance. The Sn/NGN composite exhibited a high capacity of 584 mA h g1 after cycling for 1000 times at 1 A g1. Additionally, the Sn/NGN electrode delivered a capacity of 353 mA h g1 at a high rate of 2 A g1. The good battery performance indicates that asprepared Sn/NGN composite is a possible material using for high performance LIBs.

Acknowledgements Thanks for the support by National Natural Science Foundation of China (No. 21706110, No. 21703093), Nature Science Foundation of Guangdong (No. 2017A030310638, No. 2017A030310600), Youth Innovation Talent Project for the Universities of Guangdong (No. 2016KQNCX095), and Special Talent Fund of Lingnan Normal University (No. ZL1804, No. ZL1805).

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