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Nanocomb Architecture Design using Germanium Selenide as High-Performance Lithium Storage Material Hyungki Kim, Yeonguk Son, Jinho Lee, Minkyung Lee, Seungkyu Park, Jaephil Cho, and Hee Cheul Choi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02016 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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Chemistry of Materials

Nanocomb Architecture Design using Germanium Selenide as HighPerformance Lithium Storage Material Hyungki Kim,‡,†,§ Yeonguk Son,‡,∥ Jinho Lee,†,§ Minkyung Lee,†,§ Seungkyu Park,∥ Jaephil Cho,*,∥ and Hee Cheul Choi,*,†,§ †

Center for Artificial Low Dimensional Electronic System, Institute for Basic Science (IBS), 77 Cheongam-Ro, Nam-Gu, Pohang, 790-784 Korea § Department of Chemistry, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang, 790-784 Korea ∥

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798 Korea

ABSTRACT: A key to improve the electrochemical performance of anode materials is to exploit the rational nanostructure designing beneficial for structural toughness and high rate capability. As a nanostructure design in accordance with this criterion, we introduced GeSe nanocomb architecture with well-developed nanocomb teeth on the backbone. In this unique nanocomb architecture, the free space between nanocomb teeth effectively alleviates tremendous volume expansion during lithiation and anisotropic structure with short Li+ diffusion length of tens of nanometer scale guarantees the favorable lithiation/delithiation kinetics. These structural advantages of GeSe nanocomb architecture lead to significantly improved electrochemical performance compared to GeSe nanopowder counterpart. This GeSe nanocomb architecture exhibits electrochemical performance with the reversible capacity of 726 mAhg-1, showing superior capacity retention of 89 % even after 1000 cycles at 1.0 C (1.01 A·g-1).

1. INTRODUCTION Improvements in existing lithium ion batteries (LIB) based on graphite anodes have nearly reached their limit due to the low theoretical capacity of graphite.1 To overcome this limitation, high-energy density anode materials based on group IV elements, i.e., single elements,2-4 binary alloys5 and chalcogenide compounds,6-10 have emerged as replacements for graphite. Unfortunately, group IV elements undergo complete structural change that causes tremendous volume expansion11 (e.g., 280 % for Li3.75Si, 261 % for Li3.75Ge) when Li is stored by alloy reaction with them. This causes mechanical pulverization of them and their electrical isolation from the conductive network; as a result the capacity fades within a few cycles. To avoid these problems that result from tremendous volume expansion, group IV chalcogenide compounds (MX, M = Si, Ge, Sn, X = S, Se) have been investigated because of their intrinsic advantages to mitigate volume expansion and facilitate Li+ diffusion.6-10 During the first lithiation, group IV chalcogenides decompose to form an active M phase and an inactive and irreversible Li2X phase. The Li2X phase serves as buffer matrix to alleviate the volume expansion of M phase and serves as fast ion conducting phase to facilitate Li+ diffusion. As the anode in an LIB, germanium selenide (GeSe) is the most attractive group IV chalcogenide, because it is robust lithium storage ability with isotropic lithiation kinetics and high rate capability. Considering that the formation of cracks is attributed to concentrated hoop tensile stress due to anisotropic lithiation kinetics,12,13 the isotropic volume expansion of Ge is advantageous

because it can distribute the hoop tensile stress uniformly.14 Furthermore, Se is much more polarizable than S, and therefore conducts Li+ much better, than does Li2S.6, 9,10 Despite these advantages of GeSe, sustaining the structural stability of GeSe over numerous lithiation/delithiation cycles is on the premise of exploiting rational nanostructure designing. Ideal nanostructure designing is to give the structural robustness while it guarantees the fast lithiation/delithiation kinetics. As a strategy to meet these two objectives, various nanostructure designs of group IV compound to introduce a void inside nanostructure, increase surface area, and accommodate the volume expansion have been reported.15-19 Although these nanostructure designs exhibit the remarkable progress of the electrochemical performance, most require multistep processes including etching of sacrificial templates to introduce void space, except the few cases where Kirkendall effect is exploited.20,21 By contrast, morphology control of GeSe, such as nanosheets and microtubes architecture22,23, has been reported by using simple bottom-up process. However, there have been only few papers which deal with battery research depending on various GeSe morphologies. Herein, we propose nanocomb architecture as new nanostructure design beneficial for structural stability and fast lithiation/delithiation kinetics. We use a simple vapor-solid (VS) process to synthesize unique self-assembled GeSe nanostructures that consist of nanocombs with teeth of tens of nanometers dimension on a backbone. In addition to the intrinsic advantages resulting from use of GeSe as building block for nanostructure, this nanostructure designing provides 1) free space between nanocomb teeth to accommodate volume expansion, 2) toughness of nanocombs supported by the backbone, 3) short Li+ diffusion length by thin structure with tens of

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nanometer scale and 4) high accessibility of Li+ by well-developed teeth in nanocomb (Figure 1a). This synergy effect significantly improved the electrochemical performance of GeSe. GeSe nanocombs had the reversible capacity of 726 mAhg-1 with the outstanding cycling durability of 89 % even after 1000 cycles at 1.0 C (1.016 A·g-1). Further, such a unique nanostructure demonstrated the high rate capability of 331 mA·h·g-1 at 20 C (20.32 A·g-1). To the best of our knowledge, this is the first use of GeSe nanocomb architecture in the anode material of an LIB.

2. EXPERIMENTAL SECTION Synthesis of GeSe nanocomb architecture. GeSe nanocomb architectures were synthesized using a VS process. An alumina crucible containing 0.34 mmol GeSe powder (American Elements) as source material was placed at the center of a quartz tube in a furnace, and Si substrate was placed 16 cm away from the alumina crucible to collect the product. Before increasing the temperature, the quartz tube was flushed using Ar carrier gas for 10 min at room temperature. The reaction was performed at 570 °C for 30 min at designated Ar flow rates to synthesize the GeSe nanocomb architecture. Structure characterization. Morphologies of GeSe nanocomb architecture and GeSe nanopowder were characterized using a scanning electron microscope (JEOL, JSM-7410F). Powder Xray diffraction (XRD, RIGAKU, D/MAX-250/PC) and transmission electron microscopy (JEOL, JEM-2200FS with image Cs-corrector) were used to analyze the crystal structure of GeSe nanocomb architecture. The vibration modes of GeSe nanocomb architecture were characterized using Raman spectroscopy (WITEC Alpha 300R). Electrochemical measurement. The slurry was prepared by mixing the GeSe nanocomb architecture, super-P as conductive additive and carboxymethyl cellulose (CMC)/polyacrylic acid (PAA) as polymeric binder (weight ratio 8: 1: (0.5:0.5)). This slurry was homogenously coated on Cu foil as a current collector by using the doctor blade method. The slurry of GeSe nanopowder was prepared by the same method. Before assembling the half cells, the slurry was annealed at 150 °C in a vacuum condition. The half cells with Li reference electrode were assembled in a glove box. As an electrolyte, 1.3 M LiPF6 in ethylene carbonate/diethyl carbonate (volume ratio 3:7) with 10 % fluoroethylene carbonate (Panax Starlyte) is used. All half cells were cycled at 0.05 C during the first cycle.

3. RESULTS AND DISCUSSION GeSe nanocombs were easily synthesized using the VS method. An alumina crucible containing GeSe nanopowder was placed in the center of a quartz tube and Si substrate was placed at the end of quartz tube to collect the product. At the optimized reaction condition (temperature: 570 °C, flow rate of Ar: 2000 sccm, reaction time: 30 min), well-developed GeSe nanocombs were successfully synthesized with high yield, confirmed as representative scanning electron microscopy (SEM) image (Figure 1b and 1c). Each tooth of the nanocomb had a rectangular cross section with thin thickness of average 30 nm (Figure S1 in Supporting Information). The GeSe in the nanocombs had orthorhombic unit cells (Pcmn space group, lattice parameters: a = 4.390, b = 3.827, c = 10.824, = 90°), as confirmed by the good match of XRD patterns with the GeSe nanopowder used as source material (Figure 1d). They consisted of 49.69: 50.31

Figure 1. a) Schematic illustration of GeSe nanocomb architecture showing synergy with intrinsic advantages of GeSe as building block. (b-c) SEM images of GeSe nanocomb architecture. d) XRD patterns of GeSe nanopowder (black) and GeSe nanocomb architecture (red). e) Raman spectrum of GeSe nanocomb architecture. at % Ge:Se (Figure S2); i.e., essentially 1:1 Ge:Se stoichiometry, as was observed in the GeSe nanopowder source material. The presence of two characteristic peaks22 in Raman spectroscopy, which correspond to the B2u mode at 150 cm-1 and the Ag mode at 188 cm-1, suggests that the Ge and Se in the nanocombs are chemically bonded with each other (Figure 1e). High resolution transmission electron spectroscopy (HRTEM) provides the detailed structural information of GeSe nanocomb architecture. Representative HRTEM image showed several well-developed teeth with length of ~3.8 m on a backbone of the length of 400 nm (Figure 2a-c). The clear lattice pattern and corresponding selected-area electron diffraction (SAED) pattern represent that both nanocomb teeth and backbone were highly crystalline feature with preferred growth direction of [020] (d110 = 0.190 nm) (Figure 2b and 2c). The entire GeSe nanocomb architecture has single crystalline nature as evidenced by SAED pattern of nanocomb teeth identical to that of the backbone. Elemental mapping using energy-filtered TEM demonstrated that Ge and Se are both homogeneously distributed throughout the structure (Figure S3).


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Figure 2. a) Low-magnification TEM image of GeSe nanocomb architecture. HRTEM images of b) backbone and c) tooth in nanocomb architecture. Insets: SAED patterns from each area.

Figure 3. a) Voltage profiles of GeSe nanocomb architecture. b) Cyclic voltammograms of GeSe nanocomb architecture at scan rate of 0.01 mV·s-1. c) Electrochemical performance of GeSe nanocomb architecture at 1.0 C.

The morphological evolution of GeSe structure was significantly affected by the flow rate of Ar (Figure S4). The trend of morphological evolution is that thin nanocomb architecture by anisotropic 2D growth is commonly observed and teeth part in nanocomb architecture is gradually dominant over backbone part as flow rate of Ar increased. Considering the crystal structure of GeSe with orthorhombic unit cell, thin nanocomb architecture by anisotropic 2D growth of GeSe can be intuitively understood because in-plane growth with strong covalent bonding with three neighbors is preferred to out-of-plane growth with weak van der Waals forces.24 The driving force of the formation of teeth on the backbone is attributed to the generation of valley-shaped defects that form naturally during crystal growth as reported in our previous paper about P-type semiconducting GeSe comb microstructure.25 Threading dislocations which are the origin of valley-shaped defects places mechanical stress by pulling the structure downward, then surface tension induces the flocking of GeSe between neighboring valley-shaped defects.25,26 This flocking results in supersaturation and the ensuing growth of teeth in nanocombs. As the flow rate of Ar increased, increase in supersaturation ratio induces the rapid crystallization because impingement of a number of GeSe molecules on the surface plane doesn’t give enough time to rearrange the crystal structure. Therefore, increase of flow rate caused valley-shaped defects to appear at a progressively earlier stage of crystal growth; as a result the teeth part dominated the backbone part as flow rate of Ar increased (Figure S4). To investigate the electrochemical characteristics of as-prepared GeSe nanocomb architecture, they were fabricated into Li half cells (2016R coin cell). The electrodes were prepared by homogenously mixing GeSe nanocombs, super-P and CMC/PAA in a weight ratio of 8: 1: 1. This slurry was coated on Cu foil using the doctor blade method. (See Experimental Section for the details of cell fabrication). After the first cycle at the slow rate of 0.05 C to encourage formation of the smooth solid electrolyte interphase (SEI), the half cells were charged (lithiation)/discharged (delithiation) in the voltage window between 1.5 V and 0.01 V at 1.0 C (1.016 A·g-1). Figure 3a shows each voltage profile of GeSe nanocomb architecture at the first cycle and the second cycle. During the first cycle, GeSe nanocomb architecture showed three voltage plateaus at 1.3 V, 1.0 V and 0.1 V (Figure 3a), and the peaks that correspond to these

plateaus were clearly observed in cyclic voltammograms (Figure 3b). During the second cycle, the cathodic peaks that correspond to the first (1.3 V) and second (1.0 V) plateaus were absent; i.e., irreversible reaction with Li had occurred, as a result of irreversible decomposition of GeSe to Ge and Li2Se in the nanocomb architecture.8,9 The cathodic peak at 0.1 V after the two irreversible cathodic peaks is attributed to reversible formation of LixGe (0 < x ≤ 3.75) from Ge. Ex-situ XRD patterns shows crystalline GeSe totally converted into amorphous Ge after 1st cycle (Figure S5). Together with the presence of Li2Se to serve as buffer matrix and Li+ pathway with high ionic conductivity, the designing of GeSe to nanocomb architecture creates the great synergy for high electrochemical performance. GeSe nanocomb architecture exhibited exceptionally stable electrochemical stability during 1000 cycles at 1.0 C (1.016 A·g-1) (Figure 3c). Even after 1000 cycles, the capacity remained stable at > 566.1 mA·h·g-1 and corresponding capacity retention was as high as 89 %, which indicates that the average capacity loss per cycle was only 0.012 %. Notice that voltage profiles of GeSe nanocomb architecture at each cycle are shown in Figure S6. Considering that rapid capacity fading results from the material pulverization and the morphology change of the electrode due to large volume expansion, our strategy seems to accommodate the volume expansion of the active Ge phase effectively, because 1) free space between nanocomb teeth can minimize the mechanical stress that is generated when neighboring comb teeth push against each other during lithiation and 2) the presence of inactive Li2Se surrounding active Ge phase can function as buffer matrix for its volume expansion. Ex-situ SEM measurement of GeSe nanocomb architectures supports that our nanostructure design effectively suppresses the mechanical pulverization of GeSe, showing they still maintained the integrity of their nanocombs after 100 cycles (Figure 4). Notice that the aggregation between teeth in nanocomb architecture was not observed during lithiation-mediated volume expansion. Although the initial coulombic efficiency (CE), an indicator of the reversibility of electrode reaction was only 62.97 % at the first cycle due to irreversible formation of Li2Se (charge capacity: 1107 mA·h·g1 , discharge capacity: 726 mA·h·g-1), our nanocomb architecture has remarkably high reversibility in lithiation/delithiation, and



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Figure 4. Ex-situ SEM measurement of GeSe nanocomb architecture. Representative SEM images of GeSe nanocomb architecture (a-b) before cycling, (c-d) after 1st cycle and (e-f) after 100th cycle. CE increased to 99.5 % within 12 cycles and maintained average CE as high as 99.63 % throughout 1000 cycles (Figure 3c). The charge capacity at the first cycle is higher than theoretical capacity (1016 mA·h·g-1) of GeSe; the excess is attributed to the formation of an SEI at the surface of the anode material.27 We also testified electrochemical properties of GeSe nanocomb architecture at the voltage window between 0.01-3.0 V to investigate whether irreversible Li2Se can be activated at the higher cut-off voltage. In the previous report about activation of irreversible Li2S, the high potential on Li2S cathode induces the activation of irreversible Li2S.28 At the voltage window between 0.01-3.0 V, charge and discharge capacities were 1110 and 887 mA·h·g-1, this increase of discharge capacity (726 vs 887 mA·h·g-1) indicates that Li2Se can be activated, and both Ge and Se function as active element in reversible mechanism (Figure S7a). However, the cycling performance at this voltage window exhibited the fluctuations of capacity due to dissolution of LixSe in electrolyte (Figure S7b). The further study such as optimization of electrolyte on the condition where Ge and Se function as active material will be required to investigate the usefulness of Li2Se activation in view of electrochemical performance. To confirm that the uniqueness of nanocomb architecture indeed contributes to improved electrochemical performance of GeSe anode material, we fabricated a half cells using GeSe nanopowder (average size: 293.2 nm; Figure S8) to have only the intrinsic advantages of the presence of Li2Se. The measurement of electrochemical performance of GeSe nanopowder was conducted in the same conditions used in the measurements with GeSe nanocomb architecture (Figure 5a and Figure S9). The initial capacity of 507.5 mA·h·g-1 at the second cycle increased to the maximum capacity of 587.5 mA·h·g-1 after 95 cycles. The slower diffusion kinetics than provided by the nanocomb architecture caused increase in the time required to activate the whole electrode when powder was compared to when nanocombs were used. Capacity faded rapidly after 250 cycles and the capacity retention dropped below 50 % after 400 cycles, which is a striking contrast to the stable electrochemical performance of GeSe nanocomb architecture (Figure 5a). In addition, the rate capability of each structure was measured by increasing C-rates stepwise (0.2, 0.5, 1, 2, 5, 10, 20 C) and then returning to 0.2 C again. (Figure 5b and Figure S10). GeSe nanocomb architecture exhibited superior rate capability compared to GeSe nanopowder, as confirmed in the trend where the magnitude of capacity loss owing to increased overpotential as C-rate increases is much lower than that of GeSe nanopowder. Especially, at the high rate of 20 C (20.32 A·g-1), GeSe nanocomb

Figure 5. (a) Cycling performance of GeSe nanocomb architecture and GeSe nanopowder at 1.0 C. Notice that electrochemical performance of GeSe nanocomb architecture in Figure 3a is presented in Figure 5a again for direct comparison with GeSe nanopowder. (b) Rate capability of GeSe nanocomb architecture and GeSe nanopowder. (c) Cyclic voltammograms of GeSe nanocomb architecture and GeSe nanopowder at scan rates of 0.01 mV·s-1. (d) Relationship between cathodic peak current and square root of scan rate. (e) Voltage profiles of each nanostructure by GITT. (f) Overpotentials depending on Li stoichiometry in each nanostructure. architecture exhibited a capacity of 331 mA·h·g-1, which is comparable to the theoretical capacity (372 mA·h·g-1) of commercialized graphite anodes, whereas the capacity of GeSe nanopowder is rapidly decayed as C-rate increased, and decreased to as little as 16 mA·h·g-1 at 20 C. Furthermore, GeSe nanocomb architecture exhibited more stable cycling performance at fast charging/discharging rate of 5.0 C (Figure S11). At the 3000th cycle, GeSe nanocombs maintained the capacity retention of 42.83 % with regard to maximum capacity, whereas GeSe nanopowders maintained only 11.15 %. Additionally, to verify the structural advantages of our GeSe nanocomb architecture, we compared electrochemical performance of our nanostructure to previously reported GeSe nanoparticle system (average size: 20 nm).8 Even though we used the half of conductive additive in comparison with previous report (our case: 10 wt % vs GeSe nanoparticles: 20 wt %), the rate capability of our nanocomb architecture is comparable to that of GeSe nanoparticles. These results indicate that our designing of nanocomb architecture plays an essential role in improvement of electrochemical performance. We scrutinized electrochemically-active surface area and overpotential of each structure by using cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) to identify the mechanism by which of fast charging/discharging occurred. The lithiation/delithiation of GeSe nanopowder and GeSe nanocomb architecture were similar in CV (Figure 5c). The electrochemical surface area that corresponds to each structure was estimated by the relationship between the peak


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current and scan rate in CV (Figure 5d and Figure S12). The slope in Figure 5d is proportional to the square root of Li+ diffusivity multiplied by surface area according to following peak current equation29 2.69

10

,

GeSe nanocomb architecture, SEM images of GeSe nanocomb architecture depending on flow rate of Ar, Voltage profiles of GeSe nanocomb architecture and GeSe nanopowder at various condition, Cyclic voltammograms of each nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

+

where Ip [A] is the peak current , n [1 for Li ] is the number of transferred electrons, A [cm2] is the surface area, D [cm2·s-1] is the diffusion coefficient of Li+, C0 is the bulk concentration of Li+, and v [V·s-1] is the scan rate. The slopes in the graph of v1/2 vs. Ip were 9.06 x 10-4 for nanocomb architecture and 4.82 x 104 for nanopowder. Because the intrinsic diffusivities of GeSe should be independent of morphology, we speculate that GeSe nanocomb architecture has a larger electrochemically-active surface area than does GeSe nanopowder. Considering the presence of well-developed teeth in nanocomb architecture, it provide high surface area through which Li+ can diffuse into the nanocomb architecture, this result is consistent with our experimental intuition. The overpotential of each structure was evaluated for the comparison of internal resistance by using GITT during the first lithiation (Figure 5e). Each voltage profile was obtained by applying pulse current of 0.1 C (101.6 mA·g-1, 24 °C) for 10 min followed by a resting time of 1 h. The overpotential of each structure was plotted as a function of Li stoichiometry in LixGeSe (Figure 5f). In both the nanocomb architecture and the nanopowder, the overpotential initially decreased greatly as Li+ was inserted into GeSe, then increased slowly with further addition. This trend results from the phase transformation of amorphous LixGe to crystalline Li3.75Ge (Figure 5f).29,30 At all Li+ contents, GeSe nanocomb architecture had slightly lower overpotential than did GeSe nanopowder; this difference means that GeSe nanocomb architecture has lower internal resistance, including the electrolyte, SEI, charge transfer, and diffusion resistance, than did the GeSe nanopowder. The difference occurs because the very thin (average 30 nm) GeSe nanocomb architecture guarantees easier Li+ diffusion than occurs in the nanopowder. The IR drop of each structure was also evaluated by using GITT and it exhibited similar tendency with the overpotential (Figure S13). The value x in of LixGeSe exceeded 3.75 at the fully-lithiated state of Ge because we included Li+ consumed by the irreversible formation of SEI and Li2Se at the first lithiation.

4. CONCLUSION In summary, we synthesized unique GeSe nanocomb architecture with well-devoloped teeth on a backbone using simple VS method. This GeSe nanocomb architecture exhibited great synergy with intrinsic structural advantages of GeSe material as a basic unit of nanostructure. As a result the designing of nanocombs architecture gave structural robustness to an the anode material, so that it could withstand tremendous volume expansion; they also facilitated lithiation/delithiation to guarantee high rate capability. GeSe nanocomb architecture accomplished the remarkable cycling performance with high capacity retention >89 % after 1000 cycles at 1.0 C, and exhibited high rate capability of 331 mA·h·g-1 at 20 C.

ASSOCIATED CONTENT Supporting Information. SEM image of GeSe nanopowder, EDX data of GeSe nanocomb architecture, Elemental mapping data of

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT This work was supported by IBS-R014-G2, NRF2013K1A3A1A32035430, and the IT R&D program of MOTIE/KEIT (Development of Li-rich Cathode and Carbon-free Anode Materials for High Capacity/High Rate Lithium Secondary Batteries, 10046306). Authors specially thank to Hyun Jin Park in NINT for TEM analysis.

ABBREVIATIONS LIB, lithium ion batteries; GeSe, germanium selenide; VS, vaporsolid; CMC, carboxymethyl; PAA, cellulose polyacrylic acid; XRD, X-ray diffraction; SEM, scanning electron microscopy; HRTEM, High resolution transmission electron spectroscopy; SAED, selected-area electron diffraction; SEI, solid electrolyte interphase; CE, coulombic efficiency; CV, cyclic voltammetry; GITT, galvanostatic intermittent titration technique

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