Highly Reversible and Superior Li-Storage Characteristics of Layered

Oct 13, 2016 - A layered GeS2 material was assessed as an electrode material in the fabrication of superior rechargeable Li-ion batteries. The electro...
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Highly Reversible and Superior Li-Storage Characteristics of Layered GeS2 and Its Amorphous Composites Geon-Kyu Sung,† Ki-Joon Jeon,*,‡ and Cheol-Min Park*,† †

School of Materials Science and Engineering, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, Gyeongbuk 39177, Republic of Korea ‡ Department of Environmental Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon 22212, Republic of Korea S Supporting Information *

ABSTRACT: A layered GeS2 material was assessed as an electrode material in the fabrication of superior rechargeable Li-ion batteries. The electrochemical Li insertion/extraction behavior of the GeS2 electrode was investigated from extended X-ray absorption measurements as well as by cyclic voltammetry and differential capacity plots to better understand its Li insertion/extraction behavior. Using the Li insertion/extraction reaction mechanism of the GeS2 electrode, an interesting amorphous GeS2-based composite was developed and tested for use as a high-performance electrode. Interestingly, the amorphous GeS2-based composite electrode exhibited highly reversible discharging and charging reactions, which were attributed to a conversion/recombination reaction. The amorphous GeS2-based composite electrode exhibited highly reversible and outstanding electrochemical performances, a highly reversible capacity (first charge capacity: 1298 mAh g−1) with a high first Coulombic efficiency (83.3%), rapid rate capability (ca. 800 mAh g−1 at a high current rate of 700 mA g−1), and long capacity retention over 180 cycles with high capacity (1100 mAh g−1) thanks to its interesting electrochemical reaction mechanism. Overall, this layered GeS2 and its amorphous GeS2/C composite are novel alternative anode materials for the potential mass production of rechargeable Li-ion batteries with excellent performance. KEYWORDS: lithium batteries, anode materials, germanium sulfide, reaction mechanism, amorphous composite nanoparticles,11,12 porous structures,13,14 and honeycomb-like structures,15 and the latter includes Ge-graphene,16 amorphous carbon,17,18 carbon nanotube composites,19,20 and carbon hybrid nanoparticles.21 As compared to the performance shown by pristine Ge, they are still insufficient for commercial utilization. In addition, GeO2-based materials, such as nanostructured GeO2,23−25 GeO2−Ge−C composites,26 and GeO2− SnCoC composites,27 have also been suggested as anode materials with stable cycling behavior. Although GeO2-based materials show good cycling behavior, they exhibit poor initial Coulombic efficiencies (CEs) due to the irreversible formation of an Li2O matrix during Li insertion, similar to other oxide materials.28−30 Currently, S electrodes have been highlighted on account of their high capacity (Li2S: 1672 mAh g−1), abundance of elemental S, cost-effectiveness, and environmental benignity. When S is used as an electrode material, however, several problems arise, such as large volume variations during Li insertion/extraction reaction, low electrical conductivity of S,

1. INTRODUCTION Since the 1990s, the most popular energy storage system has been rechargeable Li-ion batteries (LIBs). Because of their limitations, however, LIBs cannot satisfy the need for rapidly charging and long-lasting electric vehicles (EVs), hybrid EVs, or electronic mobile devices. Therefore, the development of new electrode materials, particularly anode materials, has been investigated intensively because commercial carbon-based anodes have a small capacity (LiC6: 372 mAh g−1) and slow current-rate (C-rate) performance. Si, Sn, P, Ge, and Sb have been suggested to be alternative superior anode materials, which can form alloys containing large amounts of Li ions.1−4 Ge is one of the most fascinating elements as an anode material because it can alloy a large quantity of Li, thereby acquiring a high theoretical capacity (Li3.75Ge: 1385 mAh g−1) at room temperature.5,6 In addition, the Li-ion conductivity and electrical conductivity in Ge are ∼400 and ∼100 times greater, respectively, than those of Si.7 On the other hand, despite these advantages, Ge electrodes suffer from large volume expansion (ca. 370%) in the Li alloying reaction, leading to poor cycling behavior. Therefore, several studies have focused on various morphologically controlled Ge and nanostructured Ge-carbon composites. The former includes Ge nanowires,8,9 nanotubes,10 © XXXX American Chemical Society

Received: August 31, 2016 Accepted: October 13, 2016

A

DOI: 10.1021/acsami.6b10994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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black (Denka) conducting agent (10 wt %), and polyvinylidene fluoride binder (10 wt %) dissolved in N-methyl-2-pyrrolidone solvent. The coated slurries were vacuum-dried at 120 °C for 3 h. Each electrode was then pressed and punched into discs, 1.2 cm in diameter. In a glovebox with Ar environment, coin-typed electrochemical cells were assembled using Li foil (counter and reference electrodes), separator (Celgard 2400), and electrolyte (1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 by volume), Panax STARLYTE). The GeS2 and GeS2/C composite electrodes had loadings of 3.55 and 3.12 mg cm−2, respectively. With the exception of the electrochemical C-rate performance tests, all electrochemical cells were examined galvanostatically (current density, 100 mA g−1; voltage range, 0−3.0 V vs Li+/Li) using an automated battery cycling tester (Series 4000, Maccor), in which Li had been inserted into (discharging) and extracted from (charging) the working electrode, respectively. On the basis of the weight of the active materials, the gravimetric capacity was calculated. CV of the GeS2 and GeS2/C composite electrodes was performed using a potentiostat (SP-240, Biologic) with a voltage range of 0−3.0 V (vs Li+/Li) at a scanning rate of 0.1 mV s−1. To investigate the electrochemical reaction resistance of the electrodes, electrochemical impedance spectroscopy (EIS) was conducted using an impedance analyzer (ZIVE MP2A, WonATech), and the impedance patterns were recorded (frequency range, 105−10−2 Hz; amplitude, 5 mV).

and high solubility of Li-polysulfides in organic-based electrolytes, which lead to poor electrochemical performance.31,32 To overcome these problems, S electrodes have been modified by many researchers using different approaches, such as the fabrication of S−C composites incorporating a range of C sources,31−36 synthesis of metal sulfide compounds,34−39 and the use of proper electrolytes containing a range of additives.39,40 Although these approaches have been shown to enhance the electrochemical performance of S electrodes, the improvement is insufficient for the practical applications of S electrodes. To overcome the problems of elemental Ge and S electrode materials for LIBs, a GeS2 compound was synthesized in thinfilm and nanoparticle form and assessed as electrodes for LIBs.41,42 On the other hand, their electrochemical performance still has considerable room for improvement. Therefore, in this study, a GeS2 compound was synthesized using a facile synthetic method and assessed for use as LIBs electrode material. The Li insertion/extraction mechanism of GeS2 was verified using extended X-ray absorption fine structure (EXAFS) and high-resolution transmission electron microscopy (HRTEM) based on the results of cyclic voltammetry (CV) and differential capacity plots (DCPs). Using the determined reaction mechanisms of GeS2, an interesting, amorphous GeS2/ C composite (a-GeS2/C) was prepared and tested. The aGeS2/C composite exhibited very high reversibility and excellent capacity retention with Li.

3. RESULTS AND DISCUSSION Figure 1a shows the crystalline structure of layered GeS2, which crystallized in the monoclinic with the space group P21/c. The structure was layered and comprised of both corner- and edgesharing tetrahedra. The corner-sharing chains of the tetrahedra

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. GeS2 was synthesized by a simple mechanical milling (MM) route. Similar to our previous work,28,34,38 stoichiometric amounts of Ge (Kojundo, 99.9%, mean size: ca. 20 μm) and S (Sigma-Aldrich, 99.98%, mean size: ca. 50 μm) powders were premixed by high-energy MM (Spex-8000) for 6 h and heated to 800 °C in a quartz tube furnace (heating rate: 5 °C min−1) under an Ar atmosphere. Additional high-energy MM was conducted to produce the GeS2/C composite sample, as follows. The GeS2 powder, carbon (Super P), and balls (stainless-steel, diameter: 3/8 and 3/16 in.) were put into a vial (hardened-steel, volume: 80 cm3) in Ar environment to give a ball-to-powder weight ratio (20:1). High-energy MM was then carried out on the sample for 6 h. The mean sizes of the GeS2 and GeS2/C composite particles were ca. 8 and 5 μm, respectively. The optimal amounts of GeS2 and C in the GeS2/C composite were 70% and 30% by weight, respectively, based on the electrochemical performance results. In addition, Ge/C and S/C composite samples with the same composition were also prepared using the same highenergy MM method. 2.2. Material Characterization. The crystallinity, morphology, and elemental composition of the samples of GeS2 and GeS2/C composite were examined by X-ray diffraction (XRD, DMAX2500-PC, Rigaku, operating at 40 kV and 40 mA), HRTEM (FEI F20, operating at acceleration voltage of 200 kV), and energy-dispersive spectroscopy (EDS), which was EDS unit attached to the HRTEM. In addition, EXAFS, ex situ XRD, and HRTEM analyses were performed to monitor the changes of phase occurring in the GeS2 and GeS2/C composites during Li insertion/extraction. For ex situ XRD analyses of the electrodes at the chosen potentials, the samples were detached from the cells in an Ar environment, and then passivated using polyimide tape (Kapton) to protect them from undesirable reactions. The K-edge EXAFS spectra of Ge were characterized at the 8C (Nano XAFS, 3.0 GeV) beamline at the Pohang Light Source in South Korea. 2.3. Electrochemical Measurements. The electrodes of GeS2 and GeS2/C composite samples were prepared by coating slurries onto Cu foil substrates for an electrochemical evaluation. Similar to our previous work,28,34,38 the slurries were comprised of the active materials (GeS2 and GeS2/C composite) powder (80 wt %), carbon

Figure 1. Crystalline structure and synthesis of GeS2. (a) Crystalline structure of layered monoclinic GeS2 and (b) XRD powder pattern of synthesized GeS2. B

DOI: 10.1021/acsami.6b10994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Comparison of the electrochemical performances of Ge, S, and GeS2 electrodes (cycling rate: 100 mA g−1). (a) Voltage profile of the Ge electrode, (b) the S electrode, (c) the GeS2 electrode, and (d) cycling performances of the Ge, S, and GeS2 electrodes.

Figure 3. Electrochemical Li insertion/extraction mechanism of GeS2 electrode. (a) DCP for the first and second cycles, (b) Ge K-edge EXAFS spectra during the first cycle, and (c) schematic diagram of phase transition mechanism during the electrochemical Li insertion/extraction in the GeS2 electrode.

were bridged via edge-sharing tetrahedra along the a-axis, perpendicular to the chains. The interesting layered structure of GeS2 enables facile Li insertion/extraction and diffusion. Therefore, to utilize this interesting structure, GeS2 was synthesized using a simple solid-state synthetic route. As

confirmed by Figure 1b, all of the XRD patterns of the synthesized GeS2 were well matched to the standard GeS2 (JCPDS no. 71-0366); no other phases were detected. Figure 2a−c presents the voltage profiles (first, second, fifth, and 10th cycles) of Ge, S, and GeS2 electrodes, respectively. C

DOI: 10.1021/acsami.6b10994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces The Ge electrode demonstrated a high initial discharge/charge capacity of 1387/1194 mAh g−1 but very low capacity retention, which is ca. 30% of the first charge capacity after the 10th cycle (Figure 2a). The severe decline in capacity of the Ge electrode was originated from a drastic volume variation, which occurred due to the formation of Li3.75Ge. Figure 2b presents the electrochemical performance of a S electrode. The S electrode exhibited a very small first discharge/charge capacity of 649/ 320 mAh g−1. Although the theoretical capacity of a S electrode is quite high (Li2S: 1672 mAh g−1), the poor reversibility between S and Li can be explained by the poor electrical conductivity of S. In contrast, the GeS2 electrode showed a large initial discharge/charge capacity of 1773/719 mAh g−1 (Figure 2c). The GeS2 electrode showed a large discharge capacity corresponding to its theoretical value, but it exhibited a low initial CE of 41%. Interestingly, the GeS2 electrode showed relatively good capacity retention (charge capacity: ca. 450 mAh g−1) after the 100th cycle (Figure 2d). Figures 3a−c, S1, and S2 show the electrochemical Li insertion/extraction mechanism of the GeS2. The DCP and CV of the initial two cycles for the GeS2 electrode exhibited two main peaks and several small peaks during discharging and a large peak during charging (Figures 3a and S1). During discharging, the large peaks of ∼1.1 and ∼0.1 V coincided with the reaction potentials of Li insertion into S and Ge, respectively, whereas the large peak of ∼0.4 V during charging coincided with that of Li extraction from Li−Ge alloying.12−22,31−33 On the basis of the DCP and CV results, ex situ XRD was also characterized at the chosen potentials. On the other hand, the XRD peaks during Li insertion and extraction were broad and noisy, which is indicative of an amorphous material, as shown in Figure S2. Therefore, Ge Kedge EXAFS of the GeS2 were examined at the chosen potentials determined by the DCP and CV data, which are shown in Figure 3b. The main peak in the EXAFS spectrum of the GeS2 electrode was associated with a Ge−S (∼1.78 Å) bond, whereas the other peaks were too small to compare, which was caused by the background absorption. When the electrode was discharged to 0.7 V (Figures 3a, t1), the Ge−S (∼1.78 Å) peak of GeS2 disappeared, and a Ge−Ge (∼2.15 Å) peak, arising from the Ge metal, appeared (Figure 3b, t1), confirming that GeS2 had been transformed to Li2S and Ge. When the electrode was at 0.0 V (fully discharged state, Figure 3a, t2), the Ge−Ge peak disappeared and a new peak for Li−Ge bonds (∼1.35 and ∼2.05 Å) arose (Figure 3b, t2). The Li−Ge bonds coincided with those of the fully discharged state of the pristine Ge electrode (Figure S3), which confirms that the fully discharged phase of Ge in GeS2 electrode is a Li3.75Ge phase. These results show that GeS2 was converted completely to Li2S and Li3.75Ge phases when the electrode was fully discharged. In contrast, upon charging, the peak arising from the Ge−Ge (∼2.15 Å) bonds of the Ge metal reappeared at 0.7 V (Figure 3b, t3). In addition, the EXAFS peak did not change when the electrode was fully charged to 3.0 V (Figure 3b, t4). On the basis of the EXAFS results, the following electrochemical Li insertion/extraction mechanism of the GeS2 is suggested:

during initial charge: Li 2S + Li3.75Ge → Li 2S(major amount) + S(minor amount) + Ge subsequent cycles: Li 2S(major amount) + S(minor amount) + Ge ↔ Li 2S + Li3.75Ge

(3)

The above reaction mechanism shows that the GeS2 electrode undergoes a conversion reaction during discharge (eq 1), which is represented schematically in Figure 3c. The poor initial CE of the GeS2 electrode originated from the formation of a highly irreversible Li2S phase (eq 2), which is similar to that suggested by other researchers.41,42 In addition, the highly irreversible Li2S acted as a matrix during cycling (eq 3), contributing to the good cycling behavior of the GeS2 electrode, which was confirmed by ex situ HRTEM (Figure S4). After the first cycle, the GeS2 transformed to nanocrystalline Ge, Li2S, and amorphous S (Figure S4a), which was retained after the 10th cycle (Figure S4b). Recently, there have been many reports showing that nanostructured composites modified with C, which are produced by a range of synthetic tools, are potentially superior electrode materials.1,43−47 Among the various synthetic methods, high-energy MM is considered to be an alternative method for the preparation of nanostructured composites because it produces finely dispersed and nanosized crystallites, such as oxide, carbide, metal, or alloy, in the carbon.48 Therefore, a GeS2/C composite sample was produced in the present study using the high-energy MM technique from the presynthesized GeS2 and amorphous C (Super P) powders. XRD showed that the prepared GeS2/C composite was amorphous (Figure 4a). Amorphization of the crystalline solid occurs easily due to the layered crystal structure of GeS2 because layered materials have relatively small slip systems. Therefore, the high pressure (∼6 GPa) generated during high-energy MM affects the amorphization of GeS2 within the composite.48 On the other hand, the EXAFS peaks in the spectrum of the amorphous GeS2/C (a-GeS2/C) composite matched those of crystalline GeS2, confirming that the amorphous composite still has a GeS2 structure (Figure 4b). Figure 5a−c shows the bright-field TEM, HRTEM with selected-area electron diffraction (SAED) patterns, verifying that the composite was amorphous. EDS mapping confirmed that the a-GeS2/C composite comprised amorphous GeS2 dispersed evenly throughout the amorphous carbon matrix (Figure 5d). Several Li-alloy-based amorphous composite materials showed good electrochemical performances, which was made possible by the accommodation of large volume variation that the amorphous materials can reduce the strain occurring during electrochemically repeated cycling. Figure 6 shows the voltage profile of the a-GeS2/C composite. The a-GeS2/C composite electrode exhibited a high initial discharge/charge capacity of 1558/1298 mAh g−1 with a high initial CE of 83.3%. Considering the irreversible capacities of the high-energy MM-treated amorphous carbon (30 wt %) present in the a-GeS2/C composite (Figure S5) and the formation of electrode−electrolyte interphase layer, a-GeS2 in the composite underwent a fully reversible reaction with Li. Because a-GeS 2 was dispersed evenly throughout the composite, excellent reversibility between the a-GeS2 and Li,

during initial discharge: GeS2 → Li 2S + Ge → Li 2S + Li3.75Ge

(2)

(1) D

DOI: 10.1021/acsami.6b10994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Voltage profile of the a-GeS2/C composite electrode in the potential range, 0−3.0 V (current density: 100 mA g−1).

electrical conductivity of the GeS2 and a-GeS2/C composite was compared using the impedance analyses results (Figure S6). The a-GeS2/C composite showed a much smaller semicircle than GeS2, confirming the much higher electrical conductivity in the a-GeS2/C composite electrode. In addition, the electrochemical reaction resistance in the a-GeS2/C composite did not change, even after the 10th cycle, due to the uniform dispersion of the amorphous GeS2 within the conducing carbon matrix. The highly reversible electrochemical behaviors of the aGeS2/C composite electrode based on the electrochemical Li insertion/extraction mechanism were examined using the electrochemical measurements shown in Figures 7a and b and S7. The DCP and CV results during the initial two cycles of the a-GeS2/C composite electrode showed that this electrode had slightly different behavior from that shown by the GeS2 electrode, which occurred due to the large overpotential resulting from the low electrical conductivity of

Figure 4. Synthesis and characterization of the a-GeS2/C composite. (a) XRD pattern and (b) Ge K-edge EXAFS spectra of the synthesized GeS, GeS2, and a-GeS2/C composite.

and enhanced electrical conductivity, were observed due to the employment of conducting amorphous carbon matrix. The

Figure 5. TEM images of the a-GeS2/C composite. (a) TEM bright-field, (b) HRTEM, (c) SAED patterns corresponding to the HRTEM, and (d) scanning TEM and its EDS mapping images. E

DOI: 10.1021/acsami.6b10994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. Electrochemical Li insertion/extraction mechanism of the a-GeS2/C composite electrode. (a) DCP for the first and second cycles, (b) Ge K-edge EXAFS spectra during the first cycle, and (c) schematic diagram of phase transition mechanism during the electrochemical Li insertion/ extraction in the a-GeS2/C composite electrode.

are then held throughout following cycles due to repeated conversion/recombination reactions. In addition, several compound materials, such as SnSb,44 Sb2S3,49 Cu6Sn5,50 ZnSb,51 ZnSe,52 and nanosized metal oxides,28−30,53 show similar conversion/recombination reactions after discharge/ charge reactions. HRTEM was performed on the samples obtained after the first and 10th cycles to determine if there were any changes to the crystallite size of the a-GeS2/C composite after cycling (Figure S8). The HRTEM images and their corresponding SAED and FT patterns confirmed the presence of an extremely small and amorphous GeS2 (crystallite size: sub 3 nm), after the first cycle, which retained the same size after the 10th cycle. Figure 8 shows the electrochemical performance of the aGeS2/C composite electrode. Figure 8a compares the cycling behaviors of the Ge, S, GeS2, a-GeS2/C composite, and commercial meso-carbon microbead (MCMB) graphite electrodes (cycling rate: 100 mA g−1). The capacity retentions of the Ge and S electrode were poor due to the huge volume variations by the formation of Li3.75Ge and Li2S phases, respectively. Although the GeS2 electrode showed enhanced cycling behavior as compared to that shown by the Ge and S electrodes, it exhibited the poor first CE due to the formation of an inactive Li2S phase. In contrast, the a-GeS2/C composite showed excellent cycling performance with highly reversible capacity (ca. 1100 mAh g−1, ca. 85% of the first charge capacity) over 180 cycles, which was superior to those of the Ge/C composite and S/C composite electrodes (Figure S9). This excellent and long cycling performance was credited to the uniformly distributed a-GeS2 throughout the carbon, which plays a role as a buffer matrix, and their recombination reactions (formation of amorphous GeS2 within carbon) after repeated cycling. In addition, the C-rate performance of the aGeS2/C composite electrode was tested, as shown in Figure 8b. At rapid C-rates of 500 and 700 mA g−1, the a-GeS2/C

the GeS2 electrode (Figures 7a and S7). Therefore, Ge K-edge EXAFS analysis of the a-GeS2/C composite electrode was also carried out at the chosen potentials with reference to the DCP result (Figure 7b). During the discharge reaction of the aGeS2/C composite electrode (Figure 7a, t1 and t2), the EXAFS results were similar to those of the GeS2 electrode (Figure 7b, t1 and t2), which confirmed that a-GeS2 in the composite had been converted completely to Li2S and Li3.75Ge. In contrast, at a charged state at 0.7 V (Figure 7a, t3), the Ge−Ge (∼2.15 Å) peak appeared due to the presence of elemental Ge (Figure 7b, t3). At the completely charged state (3.0 V, Figure 7a, t4), the main EXAFS peak (∼1.78 Å), which was assigned to the Ge−S bond of GeS2, reappeared (Figure 7b, t4), confirming that amorphous GeS2 in the composite had recombined completely during charging, unlike the nonrecombinant reaction of the GeS2 electrode. Therefore, on the basis of the DCP, CV, and EXAFS results, the following Li insertion/extraction mechanism of the a-GeS2/C composite is suggested: during discharge:

GeS2 (in composite) → Li 2S + Ge

→ Li 2S + Li3.75Ge

during charge:

(4)

Li 2S + Li3.75Ge → Li 2S + Ge

→ GeS2(in composite)

(5)

Figure 7c presents schematically the conversion and recombination reactions of the a-GeS2/C composite electrode (eqs 4 and 5). The recombination reaction of amorphous GeS2 in the composite was attributed to amorphous (or extremely small) GeS2 and the enhanced electrical conductivity when it was incorporated with conducting amorphous carbon. Generally, recombination reactions of compound electrodes are advantageous in terms of the electrochemical behavior because the compound crystallites in the composite decrease gradually to 2−3 nm after several electrochemical cycling, and these sizes F

DOI: 10.1021/acsami.6b10994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10994. Additional cyclic voltammogram, ex situ XRD, ex situ HRTEM, and electrochemical impedance results of GeS2 and a-GeS2/C composite electrodes, DCP, ex situ XRD, and EXAFS spectra of Ge electrode, electrochemical performances of ball milled C, and voltage profiles of Ge/C and S/C composite electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-32-860-7509. E-mail: [email protected]. *Tel.: +82-54-478-7746. Fax: +82-54-478-7769. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2014R1A2A1A11053057).

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REFERENCES

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Figure 8. Electrochemical performance of the a-GeS2/C composite electrode. (a) Cycling performance of the Ge, S, GeS2, graphite (MCMB), and a-GeS2/C composite electrodes (cycling rate: 100 mA g−1) and (b) C-rate performance of the a-GeS2/C composite electrode at the various current densities (100, 300, 500, and 700 mA g−1).

composite electrode showed highly reversible capacities of ca. 960 and 800 mAh g−1, respectively, with balanced cycling performance. The relatively rapid C-rate performance of the aGeS2/C composite electrode was attributed to the short diffusion lengths caused by the formation of uniformly distributed amorphous GeS2 within the conducting carbon matrix.

4. CONCLUSION GeS2 was simply synthesized for use as an electrode material for rechargeable LIBs and the electrochemical Li insertion/ extraction behaviors during cycling were investigated. Using the reaction mechanism determined for the GeS2 electrode, a GeS2-based amorphous composite was prepared by a simple high-energy MM method and assessed as an electrode for rechargeable LIBs. The a-GeS2/C composite electrode exhibited highly reversible and excellent electrochemical performance (high first charge capacity, 1298 mAh g−1; excellent first CE, 83.3%; fast rate capability, ca. 800 mAh g−1 at a high C-rate of 700 mA g−1; and long cycling behavior, a high capacity of 1100 mAh g−1 over 180 cycles). This superior electrochemical performance was credited to the amorphous GeS2 distributed uniformly in the conducting carbon matrix and its repeated conversion/recombination reactions during electrochemical cycling. Overall, the a-GeS2/C composite is a suitable alternative electrode material for LIBs. G

DOI: 10.1021/acsami.6b10994 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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