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Nitrogen-doped single walled carbon nanohorns enabling effective utilization of Ge nanocrystals for next generation lithium ion batteries
Accepted Manuscript Nitrogen-doped single wall carbon nanohorns enabling effective utilization of Ge nanocrystals for next generation lithium ion batteries Umair Gulzar, Tao Li, Xue Bai, Subrahmanyam Goriparti, Rosaria Brescia, Claudio Capiglia, Remo Proietti Zaccaria PII:
S0013-4686(18)32613-6
DOI:
https://doi.org/10.1016/j.electacta.2018.11.130
Reference:
EA 33131
To appear in:
Electrochimica Acta
Received Date: 17 July 2018 Revised Date:
20 October 2018
Accepted Date: 19 November 2018
Please cite this article as: U. Gulzar, T. Li, X. Bai, S. Goriparti, R. Brescia, C. Capiglia, R.P. Zaccaria, Nitrogen-doped single wall carbon nanohorns enabling effective utilization of Ge nanocrystals for next generation lithium ion batteries, Electrochimica Acta (2018), doi: https://doi.org/10.1016/ j.electacta.2018.11.130. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Nitrogen-doped single wall carbon nanohorns enabling effective utilization of Ge
nanocrystals for next generation lithium ion batteries Umair Gulzar,†‡ Tao Li,†‡
Xue Bai,†§ Subrahmanyam Goriparti†, Rosaria Brescia† Claudio
Capiglia,*#¥ Remo Proietti Zaccaria∥† Istituto Italiano di Tecnologia, via Morego 30, Genova 16163, Italy
‡
DIBRIS, University of Genova, via Opera Pia 13, Genova 16145, Italy
§
Key Laboratory for Liquid−Solid Structural Evolution & Processing of Materials (Ministry of
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†
Education), Shandong University, Jinan 250061, China
Recruit R&D Co., Ltd., Recruit Ginza 8 Bldg. 8-4-17, Ginza Chuo-Ku, Tokyo 104-8001, Japan
¥
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555, Japan
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∥Cixi
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Institute of Biomedical Engineering, Ningbo Institute of Materials Technology and
Engineering, Chinese Academy of Science, Ningbo 315201, China
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*Corresponding author. E-mail: [email protected] ; [email protected].
ACCEPTED MANUSCRIPT ABSTRACT
Among various carbon materials, nitrogen doped single walled carbon nanohorns (N-SWCNHs) has a unique structure of clustered conical cages (2−5 nm in diameter and 40−50 nm in length) arranged in dahlia, bud and seed-like configurations. Each conical cage has five pentagons at their tips which acts as potential reactive site with its own distinct chemistry. We exploited these reactive sites of NSWCNHs by preferentially growing germanium nanocrystals (Ge NCs) onto their conical tips using
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oleylamine as a mild reducing agent. Therefore, Ge decorated N-SWCNHs (Ge@N-SWCNHs) composite was used, for the first time, as active anode material for lithium ion batteries providing high and stable capacity of 1285 mAh/g at 0.1C after 100 cycles. Our results show that preferential growth of Ge Nanocrystals (NCs) on the tips of N-SWCNHs not only allow high utilization of
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active material but prevents the aggregation of Ge NCs after multiple cycling. Finally, we highlight the potential role of N-SWCNHs as cheap and industrially scalable conductive host for next
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generation lithium ion batteries.
Keywords: Nitrogen doped single walled carbon nanohorns, Lithium ion batteries, Anode material, Germanium composites, carbon material, Solvothermal method.
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INTRODUCTION
lithium ion batteries (LIBs) have been providing the electrical power necessary to operate existing portable electronics and hybrid electric vehicles (HEVs)[1,2]. However, the demand for safe batteries with higher power and capacities have started a worldwide hunt of new electrode materials for LIBs. Many researchers from academic and industrial domains have considered Si, Ge and Sn as promising alternative anode materials for next generation LIBs. In fact, Sony has already
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developed its Nexelion battery using an anode material mainly composed of a Sn/Co/C composite[3]. Main attraction of these elements lie in their high theoretical capacities (4200 mAh/g for Si, 1625 mAh/g for Ge, and 994 mAh/g for Sn) which is based on alloy reactions between the respective elements and lithium (LixMy)[4]. Moreover, lower working potentials (0.1-0.6 V vs
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Li/Li+) compare to insertion (TiO2) and conversion (Fe2O3) materials (1.0-2.5V vs Li/Li+) make them even more attractive for full cell configuration with higher energy densities[5].
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Although Sn and Si based electrodes take the lead in terms of cost and production, Ge has also been in the spotlight due to its electronic conductivity (104 times higher than Si)[6] and lithium ion diffusivity (400 times higher than Si)[7]. Contrary to silicon, isotropic lithiation of Ge allows additional structural stability during pulverization of electrode[8]. Therefore, Ge can offer an alternative for the development of durable, high-capacity and high-rate anodes for high value applications like medical and space technologies[6]. Nevertheless, commercialization of Ge based
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electrodes for LIB is mainly hindered by massive volumetric expansion of Ge ( ̴ 250% for Li15Ge4), continuous breakage of SEI layer and electrochemical aggregation of Ge particles during multiple lithiation/de-lithiation cycles[9]. One popular approach to address these problems involves nano
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structuring of Ge particles[10–13] through which mechanical stress and diffusion length can be minimized. Despite the reduction of particle size, Ge based anodes suffer from welding or aggregation effect[14] which can be minimized by anchoring nanoparticles on conductive substrates
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like carbon nanotubes (CNTs)[15], carbon nanofibers (CNFs)[16], graphene/reduced graphene oxide[17] (RGO) and hollow carbon encapsulating active material[12]. These carbon substrates not only provide high electrical conductivity but prevent large volumetric expansion providing high power and long-term stability. However, the ratio between active material (Ge) and carbon has a strong influence on capacity and cycle life as higher ratios may lead to the clustering of Nanocrystal (NCs) due to aggregation effect[17]. Recent efforts have been proposed to improve the electrochemical performance of Ge based anodes involving various carbon-based composites. For example, Fang et al.[18] has prepared GeRGO-CNTs composite comprising of 20-30 nm Ge NPs fixed tightly on the surface of RGO while using CNTs as an efficient electron transport medium. The composite could retain a specific
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discharge capacity of 863.8 mAh/g after 100 cycles with a current density of 100 mA/g. Similarly, Wang et al.[15] developed a free-standing germanium/single-walled CNTs (SWCNTs) composite by trapping different amount of Ge NPs on the entangled network of CNTs. The composite with 50% Ge content gave a stable capacity of 400 mAh/g after 40 cycles at a current density of 25 mA/g. Recently, our group have developed a facile solvothermal method to anchor Germanium nanocrystals (Ge NCs) on the surface of multi-walled carbon nanotubes (MWCNTs). As-prepared
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Ge@MWCNTs composite showed improved electrochemical performance with discharge capacity of 1160 mAh/g after 60 cycles[7]. According to the authors, the remarkable performance Ge@MWCNTs based electrodes can be ascribed to the high conductive properties of MWCNTs, also preventing the aggregation of Ge nanoparticles during multiple lithiation/de-lithiation.
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Despite the attractive properties of these carbon materials, higher production cost of CNTs [19] and low quality of graphene[20] limit their industrial use in many applications including lithium ion Nitrogen doped single walled carbon nanohorns (N-SWCNHs) are a kind of carbon
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batteries.
materials that can be produced on a large scale (ton/year). Moreover, this exotic material exist in a clustered arrangement of horn shaped graphitic tubules (2-5 nm diameter and 40-50 nm tube length) which, upon aggregation, form dahlia-like, bud-like and seed-like structures[21,22]. Interestingly, each conical tubules have five pentagons at their tips coupled with heptagons which acts as potential reactive sites having their own distinct chemistry[23]. Moreover, local chemical reactivity
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is also enhanced in regions of higher curvature due to pyramidal distortion of the sp2 carbon bonding [24]. Due to these unique and tunable properties, SWCNHs have found applications in gas storage[25], catalysis[26] and supercapacitors[27]. Although, few reports mention their application as an anode material for lithium ion batteries[28–30], none of them fully characterized their
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electrochemical performance as a conductive substrate for alloying anode material. In an ongoing project of understanding the potential of N-SWCNHs for application in lithium
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ion and post-lithium ion batteries[31], we exploited the reactive sites of N-SWCNHs by preferentially growing germanium nanocrystals (Ge NCs) onto their conical tips using oleylamine as a mild reducing agent. With respect to other popular carbon materials such as graphene or CNTs, N-SWCNHs can be mass produced on an industrial scale. Moreover, nitrogen as a doping agent inside N-SWCNHs is suggested for its high electronegativity, due to C−N bonds characterized by the electric dipoles located at the surface of the nanohorns. This effect might increases the surface chemical reactivity of N-SWCNHs, hence fostering the interaction between nitrogen sites and the surrounding active material. Therefore, the as prepared Ge@SWCNHs composite was used as an anode material for lithium ion battery providing high and stable capacities of 1285 mAh/g after 100 cycles. We believe that the preferential growth of Ge NCs on the tips of SWCNHs not only allow
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high utilization of active material but prevents the aggregation of Ge NCs after multiple cycling. These impressive results show that N-SWCNHs allows the utilization of Ge as suitable anode for LIBs with high energy and high-power applications. 2 2.1
EXPERIMENTAL Synthesis of Ge@N-SWCNHs composites
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N-SWCNHs were mass produced with proprietary industrial process technology and provided by Advanced Technology Partner s.r.l. (ATP). Germanium nanocrystals (Ge NCs) and Ge@NSWCNHs composite were prepared using a facile solvothermal approach in a 25 ml Teflon vessel. In particular, 163mg of GeI2 (concentration, 33 mM) was dissolved into 15 ml of degassed
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oleylamine using a sonication bath. 9 mg of N-SWCNHs were subsequently added to GeI2 solution in an inert argon atmosphere. The final mixture was transferred to a 25 mL Teflon vessel equipped
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with a stainless-steel container and kept in a furnace at 230 oC for 6 hours. Ge@N-SWCNHs composite was obtained by washing the blackish brown product with 1:4 mixture of chloroform and ethanol, respectively. Finally, the ligands were carbonized by thermally treating Ge@N-SWCNHs composite in an argon/hydrogen (95/5%) atmosphere at 500 oC. The final product was transferred into a glovebox to avoid surface oxidation. For comparison, pure Ge-nanocrystals (brown product)
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were also prepared using the same procedure without adding N-SWCNHs. Material Characterization
Microscopic images of N-SWCNHs and Ge@N-SWCNHs composites were acquired using a JEOL JEM 1011 (Jeol, Tokyo, Japan) operated at 100 kV of acceleration voltage, equipped with a
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tungsten thermionic electron source and a FEI TECNAI G2 F20 instrument, equipped with a Schottky field emission gun (FEG), operating at 200 kV acceleration voltage. High resolution
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transmission electron miscopy was performed onto ultrathin carbon/holey carbon-coated Cu grids using JEM-2200FS, image Cs-corrected (Schottky emitter, HT 200 kV), with in-column imaging filter, equipped with Bruker XFlash 5060 SDD system. Scanning electron microscopy was performed using Nova 600 NanoLab microscope (FEI), equipped with a field emission gun and a focused ion beam of gallium ions for milling. XRD patterns were collected on a PANalytical Empyrean X-ray diffractometer equipped with a 1.8 kW CuKα ceramic X-ray tube operating at 45 kV and 40 mA using a PIXcel3D (2 x 2 area) detector. XPS analysis was performed on a Kratos Axis Ultra DLD spectrometer, using a monochromatic Al Kα source (15 kV, 20 mA). High resolution scans were carried out at constant pass energy of 20 eV and steps of 0.1 eV. The pressure in the analysis chamber was maintained below 7×10-9 Torr for data acquisition and photoelectrons were detected at a takeoff angle θ = 0° with respect to the surface normal. Finally, the data were
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converted to VAMAS format and processed using Casa XPS software, version 2.3.16. The binding energy (BE) scale was internally referenced to the C 1s peak (BE for C−C = 284.8 eV). Raman spectroscopy was performed with Renishaw in Via Micro Raman equipped with a laser source of 633 nm using a 50x objective (LEICA N PLAN EPI 50/ 0.75). 2.3
Electrochemical Characterization
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The electrochemical performance of Ge@N-SWCNHs composite was tested on CR2032 coin cells which included lithium chips (15.6 Dia x 0.45t mm; MTI Corporations) as reference and counter electrode, dried glass fibers membrane (What-man GF/D) as separator and Ge@NSWCNHs composite, casted on a copper substrate, as working electrode. Slurries casted on copper
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foil (15 mm Diameter) were composed of 70% weight Ge NPs or Ge@N-SWCNHs composite (containing 70% Germanium by weight), 20% super-P carbon and 10% polyvinylidene difluoride (PVDF) grinded in a mortar using N-methyl-2-pyrrolidone as a solvent. The casted electrodes were
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dried overnight at 120 °C under vacuum and the mass loadings of active material were found in the range of 1-1.2 mg. 250 µl of electrolyte was used in each coin cell which contained 1 M lithium hexafluorophosphate (LiPF6) dissolved in 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), respectively. 5% (v/v) vinylene carbonate (VC) was also used as an additive. After their assembling inside a MBraun glovebox, with H2O and O2 levels below 0.1 ppm, all cells Lab V1.30. 3
RESULT AND DISCUSSION
Synthesis and Characterization of Ge@N-SWCNHs composites
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were electrochemically tested using BioLogic BCS-805 multichannel battery unit controlled by BT
Nitrogen doped single walled carbon nanohorns (N-SWCNHs) typically exist in the form of
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aggregated conical tubules extended by a short cylindrical section (Figure 1a). The tips typically contains five pentagons, however, a sixth pentagon is required for the nanohorn walls to continue parallel to the wall axis[32]. Consequently, a heptagon counteracts the curvature change caused by the additional pentagon. Due to these defects, theoretical calculations have demonstrated a net electron transfer to the pentagonal sites of the CNHs tips[33,34] providing reactive sites for chemical functionalization. Moreover, the local chemical reactivity is also enhanced in the regions of higher curvature due to pyramidal distortion of the sp2 carbon bonding[24]. For both reasons, the chemical reactivity of carbon nanohorns is assumed to be localized around pentagons, heptagons, and other defected sites, toward the nanohorn tip. Our synthetic protocol, depicted in figure 1b, utilizes the reducing properties of pentagonal sites of N-SWCNHs to preferentially grow Ge NCs on the tips N-SWCNHs.
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Figure 1: a) TEM image of N-SWCNHs. b) Schematic illustration for synthesis of Ge@N-SWCNHs composite. TEM images of c) Ge@N-SWCNHs composite d) Ge NCs. e) XRD patterns of Ge NCs (Red), Ge@N-SWCNHs (Blue) and Ge@N-SWCNHs (black) after exposed to air. f) Raman spectra of N-SWCNHs (Red) and Ge@NSWCNHs composite (Blue)
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Instead of using strong reducing agents like alkali metals or organoalkali reagents[35–37] for the
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solution phase synthesis of Ge NCs, our protocol employs a mild reducing agent like oleylamine to synthesize crystalline Ge NCs where not only it acts as a solvent but also a mild reducing agent [38]. Moreover, the mild reducing power of oleylamine allows the reduction of Ge NCs to be dictated by the active sites of N-SWCNHs. As expected, TEM images of Ge@N-SWCNHs showed preferential arrangement of 4-10 nm Ge NCs at the conical tips of N-SWCNHs (Figure 1c). We believe that the preferential growth is the result of enhance reactivity of conical tips of N-SWCNHs due to the heptagons[23] providing a nucleation site of Ge NCs to grow[24]. The preferential growth of Ge NCs not only provides void space to accommodate volume changes, but also prevents aggregation of Ge NCs during lithiation/de-lithiation process. Ge NCs without N-SWCNHs were also prepared for comparison (Figure 1d) showing 4-10 nm Ge NCs intrinsically capable of sustaining large volumetric expansion. Homogenous distribution of Ge NCs inside the carbon
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matrix of N-SWCNHs was confirmed using scanning electron microscopy/Energy Dispersive XRay Spectroscopy (SEM/EDS) analysis (Figure S1). Moreover, ED’s analysis showed the Ge content inside Ge@N-SWCNHs composite to be 55% which was also confirmed by ICP/MS analysis. Crystallinity of Ge NCs and Ge@N-SWCNHs composite was confirmed using X-ray diffraction (XRD) analysis (Figure 1e). In particular, XRD spectra of freshly prepared Ge NCs and Ge@N-SWCNHs composite show characteristic peaks of cubic germanium at 2θ values of 27.32º,
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45.22º, 53.76º, 65.92º and 72.83º (JCPDS card No. 03-065-0333)[16,39]. The average particle size of Ge NCs obtained using Scherrer equation was found to be 8.5 nm matching well with our TEM observations.
Detailed characterization of N-SWCNHs and Ge@N-SWCNHs composite was performed using
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Raman spectroscopy. Figure 1f shows prominent D and G peaks which can be attributed to disordered sp2 carbon and the vibrational modes of graphitic carbons[40], respectively. Due to the
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presence of pentagons and heptagons in N-SWCNHs, these D and G peaks appears at 1348 and 1590 cm-1, respectively[41]. ID/IG ratios calculated for both spectra showed values close to unity, explaining the defected structure associated to N-SWCNHs[41]. Additional peaks at 2953 cm−1 (2D) and 3225 cm−1 (D+G) are the overtones of D and G peaks, respectively[42].. Finally, the peak
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of Ge appears close to 300 cm-1 demonstrating its presence in Ge@N-SWCNHs composites.
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Figure 2: XPS analysis of a) N-SWCNHs. High resolution spectra of b) C1s and c) N1s peaks showing the presence of nitrogen on N-SWCNHs
To confirm the presence of nitrogen doping X-ray Photoelectron Spectroscopy (XPS) was
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performed on N-SWCNHs. Figure 2a shows a wide range spectrum of N-SWCNHs depicting C 1s, N 1s and O 1s peaks. The C 1s peak (Figure 2b) shows the typical shape related to graphitic carbon[43] shown by an asymmetric peak at 284.4±0.2 eV (associated to sp2 carbon atoms) accompanied by a low intensity shake-up feature at 290.8±0.2 eV, related to π→π* transition. The best fit of the C 1s profile was obtained using other low intensity carbon peaks[44] centred
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at 284.9±0.2 eV, 286.4±0.2 eV, 288.0±0.2 eV and 289.7±0.2 eV. These peaks are generally assigned to sp3-hybridized carbon atoms, C-O, C=O and O-C=O species, respectively. However, the peak at 286.4±0.2 eV can be associated to C-N species (corresponding to the N-sp2 C bonds) which might be originated by the addition of N atoms in the carbon network of the N-SWCNHs[45]. This
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assignment is also supported by N 1s broad peak appeared between 396 and 406 eV (Figure 2c) which can be split into three components centred at 399.0±0.2 eV, 400.8±0.2 eV, and 402.2±0.2 eV.
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Each component corresponds to pyridinic, pyrrolic, and graphitic nitrogen, respectively[46]. Finally, the atomic percentage of carbon, nitrogen and oxygen were found to be 97.8, 1.1 and 1.1%, respectively. 3.2
Electrochemical performance of Ge@N-SWCNHs composite In order to utilize N-SWCNHs as a conductive substrate for Ge NCs and its catalytic
properties for electrolyte decomposition, cyclic voltammetry of pure N-SWCNHs, Ge NCs and Ge@N-SWCNHs based electrodes was performed using different combinations of electrolyte additive (VC). The choice of the electrolyte additive was based on the fact that it facilitate the formation of mechanically stable solid electrolyte interface (SEI) which acts as a passivating layer to inhibit further electrolyte reduction[47]. Moreover, VC is the most common electrolyte additive
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used to improve the stability of SEI in Si, Ge and Sn based anode materials[48]. For comparison, initial tests were performed on pure N-SWCNHs using electrolyte consist of 1M LiPF6 in EC:DMC with 5% of VC as an additive. In order to test the stability of N-SWCNHs, cyclic voltammetry was performed at a wide potential range of 0.005-3V, confirming no oxidative decomposition at higher potentials. However, four prominent reduction peaks at 1.61V, 1.1V, 0.60V (minimized after 1st
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cycle) and 0.005 V (Figure 3a) were observed during the first reduction cycle.
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Figure 3: Cyclic voltammograms of a) Pure N-SWCNHs, b) Ge NCs without additives, c) Ge@NSWCNHs without additive, d) Ge@N-SWCNHs with 5% VC. † represents SEI peaks related to electrolyte decomposition, ǂ represents SEI peaks related to electrolyte additive (VC), ¥ represents LixGex alloying/de-alloying peaks while # represent peaks related to adsorption/intercalation of lithium inside N-SWCNHs.
Peaks at 1.61V and 1.1V can be associated to the reductive decomposition of VC while the peak at 0.60V can associated to the decomposition of EC. We believe that all three peaks are responsible to form a solid electrolyte interface (SEI)[49]. Finally, the reduction peak at 0.005V can be ascribed to adsorption/intercalation of lithium inside N-SWCNHs[29]. During anodic scan, peak at 0.21V describes the de-intercalation of lithium from N-SWCNHs. The low intensity of anodic peak compare to its cathodic counterpart suggests the irreversible intercalation/de-intercalation of NSWCNHs which might explains the lower columbic efficiency of Ge@N-SWCNHs at low C-rates (described later). Similarly, cyclic voltammetry performed on Ge NCs and Ge@N-SWCNHs based electrodes using base electrolyte (1M LiPF6 in EC:DMC) showed a first cycle reduction peak at
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0.75V and 0.61V, respectively (Figure 3b,c) which can be ascribed to the decomposition of EC. Additional reduction peaks from 0.31 to 0.05V can be associated to lithium insertion into equipotential sites and the formation of Ge-alloys, i.e. Li9Ge4 and Li7Ge4 followed by Li15Ge4 and Li22Ge4 [10,12,39,50].
The anodic (de-lithiation) scan of the Ge NCs and Ge–N-SWCNHs
composite based electrodes show oxidative peaks, located in the range of 0.42-0.72V suggesting dealloying process of Ge–Li composites. Similar alloying and de-alloying peaks related to Ge were
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observed in Ge@N-SWCNHs composites, however, an additional broad peak starting from 1.61 V was observed due to the decomposition of VC, as observed in the case of pristine N-SWCNHs (Figure 3d).
Cycling performance of electrodes based on pristine N-SWCNHs were initially tested at 0.1C under
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the potential window of 0.005-2.5V using galvanostatic measurements showing stable capacity of 87 mAh/g after 100 cycles (Figure S4). In order to avoid the involvement from lithiation/de-
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lithiation of N-SWCNHs, galvanostatic charge discharge measurements were performed in the
Figure 4: a) First lithiation profiles of Ge NCs and Ge@N-SWCNHs with and without additive. b) Comparison between 5th lithiation/de-lithiation curves of Ge NC and Ge@N-SWCNHs with VC additive. c) Comparison between
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lithiation/de-lithiation curves of the 5th and 100th cycles for Ge@N-SWCNHs composite with VC additive.
potential range of 1.5 and 0.01 V vs. Li/Li+ at 0.1C. Figure 4a compare the 1st lithiation curves
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(starting from OCV) of Ge NCs and Ge@N-SWCNHs composite in the presence and absence of VC additive at 0.1C (1C =1600 mAh/g). Indeed the initial lithiation gravimetric capacities achieved for Ge@N-SWCNHs composites (2449 mAh/g) are higher than Ge NCs (1595 mAh/g) based electrodes, however, after 5 cycles these capacities stabilized to 1435 and 1046 mAh/g, respectively. Interestingly, the decomposition of electrolyte additive or formation of SEI is more pronounced in Ge@N-SWCNHs based electrodes, suggesting the enhanced catalytic properties of N-SWCNHs[26]. Moreover, the 5th cycle voltage profile of Ge@N-SWCNHs using 5% VC additive appeared at lower activation potentials compare to the voltage profiles of Ge NCs (Figure 4b). We believe that higher capacities obtained for Ge@N-SWCNHs composites are associated to the lower activation energies allowing complete
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utilization of active material in a given potential range of 0.01-1.5V. Moreover, the comparison between 5th and 100th charge discharge profile of Ge@N-SWCNHs composite electrode with 5% VC show minimal activation barrier and the loss of capacity is mainly due to the concentration overpotential created at the electrode surface (Figure 4c). Finally, the capacity retention of pristine Ge NCs and Ge@N-SWCNHs composite in the presence of VC was tested for over 100 cycles. Pristine Ge NCs and Ge@N-SWCNHs composites show
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initial discharge capacities of 1595 and 2449 mAh/g which stabilized at 811 and 1285 mAh/g after 100 cycles, respectively (Figure 3 a, b). It is interesting and important to notice that the majority of
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the de-lithiation capacity in Ge@N-SWCNHs composite was lost during the first 10 cycles (6 %)
while only 2.5 % loss was observed for the remaining 90 cycles.
For comparison, electrodes consisted of Ge NCs, Ge@N-SWCNHs composite operated in base electrolyte (1M LiPF6 in EC:DMC) without VC additive showed a rapid decay in capacity confirming the importance of VC for long term stability (Figure 5a, b). The remarkable storage
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Figure 5: Cycling performance of a) Ge NCs and b) Ge@N-SWCNHs composite with and without VC at 0.1C. ( )
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Blue and ( ) red spheres depicts lithiation and de-lithiation, respectively while ( ) black and ( ) dark red spheres represents columbic efficiencies with and without additive, respectively. Rate performance of c) Ge NCs and d) Ge@N-
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SWCNHs. ( ) Black and ( ) red spheres correspond to lithiation and de-lithiation, respectively ( ) while blue spheres represents columbic efficacies with VC as an electrolyte additive.
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capacities observed for Ge@N-SWCNHs using VC as an electrolyte additive were comparatively higher to our previously reported method where carbon nanotubes were used a conductive host for Ge NCs[7] Therefore, these results suggest an intimate contact between Ge NCs and N-SWCNHs allowing high utilization of the active material. The rate performance of Ge NCs and Ge@SWCNHs composite was analyzed at different charge discharge rates. Ge NCs show de-lithiation capacities of 908, 762, 599, 462, and 271 mAh/g at
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0.1C, 0.4C, 1C, 2C and 5C respectively while regaining the capacity of 850 mAh/g at 0.1C. (Figure 5c) One the other hand, Ge@N-SWCNHs composite based electrodes showed de-lithiation capacities of 1420, 1193, 935, 866, 611, 366 mAh/g
at 0.1C, 0.4C, 0.8C, 1C, 2C and 5C
respectively (Figure 5d). Finally, the rate performance of pristine N-SWCNHs showed negligible
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capacities of 48, 36, 26, 15 and 9 mAh/g at 0.2C, 0.4C, 0.6C, 0.8C and 1C respectively.(Figure S4b) Superior performance of Ge@N-SWCNHs composite can be realized by the fact that electrode
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material regained its original capacity of 1413 mAh/g after been cycled at high scan rates. The comparison of Ge@N-SWCNHs composite with recently reported Ge-carbon composites shown in Table S1 suggest its suitability as an attractive and cheap conductive substrate for applications in high energy and high power LIBs. Moreover, large scale and cost effective production of industrial grade N-SWCNHs along with as-reported performance of Ge@N-SWCNHs composite provide this
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electrode material an edge over graphene and carbon nanotube based composites.
CONCLUSION
We have reported a facile solvothermal synthesis of 5-10 nm germanium nanocrystal
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decorated on the tips of nitrogen doped single wall carbon nanohorns. The as-prepared Ge@NSWCNHs composite was electrochemical tested as anode material for lithium ion battery showing
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remarkable capacities of 1285 mAh/g after 100 cycles. Moreover, the electrochemical performance of Ge@N-SWCNHs composites was compared with bare Ge-NC with and without electrolyte additives. Our results suggest that Ge@N-SWCNHs show improved performance in the presence of electrolyte additive VC, possibly due to the formation of a stable SEI. Due to the cheap production cost and scalability, N-SWCNHs can be a promising candidate as a supporting conducting agent for high storage active materials in lithium ion batteries.
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AUTHOR INFORMATION
Corresponding Authors *E-mail: [email protected]; [email protected]
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Present Address
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555, Japan. Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
Authors would like to acknowledge the support from the 3315 project Ningbo (nr. Y70001DL01),
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China. Authors would like to thank Advanced Technology Partner S.r.l. to provide the N-SWCNHs samples,
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([email protected]).
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SUPPLEMENTARY INFORMATION
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Figure S1: EDS analysis of Ge@N-SWCNHs showing a) SEM image b) Carbon and c) Germanium mapping
Figure S2: High Resolution TEM images and elemental mapping of Ge@N-SWCNHs composite showing Ge cubic phase [110] and partially oxidized Ge NC [021].
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•
•
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Figure S4: a) Cycling performance of pristine N-SWCNHs using 5%VC. b) Rate performance of N-SWCNHs. ( ) Red
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and ( ) black balls representing lithiation and de-lithiation while ( ) blue stars represents columbic efficiency.
Table S1: Comparison of the performance metrics between Ge @N-SWCNHs and other carbon based Ge composites
Ge Composites
Synthetic route
Capacities (mAh/g)
Current Densities (mA/g)
Cycles
Ref
Magnetic sputtering coupled with 980 plasma reactors
2000
100
[51]
Ge-N-RGO
Hydrolysis to GeO2 and annealed 700 back to Ge
5000
200
[52]
Ge-RGO
Colloidal conditions
1600
600
[53]
Ge-RGOCNT
Colloidal method in inert condition
863.8
100
100
[18]
Hydrolysis followed by annealing
1232
C/2
1000
[10]
Hydrolysis to GeO2
417
25
40
[15]
Ge-hollow carbon
Hydrolysis to GeO2
1000
C/10
100
[12]
GeMWCNTs
Solvothermal
1060
160
60
[7]
Ge@NSWCNHs
Solvothermal
1285
120
100
This work
Ge-CNT paper
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GRAPHICAL ABSTRACT
S0013-4686(18)32613-6
DOI:
https://doi.org/10.1016/j.electacta.2018.11.130
Reference:
EA 33131
To appear in:
Electrochimica Acta
Received Date: 17 July 2018 Revised Date:
20 October 2018
Accepted Date: 19 November 2018
Please cite this article as: U. Gulzar, T. Li, X. Bai, S. Goriparti, R. Brescia, C. Capiglia, R.P. Zaccaria, Nitrogen-doped single wall carbon nanohorns enabling effective utilization of Ge nanocrystals for next generation lithium ion batteries, Electrochimica Acta (2018), doi: https://doi.org/10.1016/ j.electacta.2018.11.130. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Nitrogen-doped single wall carbon nanohorns enabling effective utilization of Ge
nanocrystals for next generation lithium ion batteries Umair Gulzar,†‡ Tao Li,†‡
Xue Bai,†§ Subrahmanyam Goriparti†, Rosaria Brescia† Claudio
Capiglia,*#¥ Remo Proietti Zaccaria∥† Istituto Italiano di Tecnologia, via Morego 30, Genova 16163, Italy
‡
DIBRIS, University of Genova, via Opera Pia 13, Genova 16145, Italy
§
Key Laboratory for Liquid−Solid Structural Evolution & Processing of Materials (Ministry of
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†
Education), Shandong University, Jinan 250061, China
Recruit R&D Co., Ltd., Recruit Ginza 8 Bldg. 8-4-17, Ginza Chuo-Ku, Tokyo 104-8001, Japan
¥
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555, Japan
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∥Cixi
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#
Institute of Biomedical Engineering, Ningbo Institute of Materials Technology and
Engineering, Chinese Academy of Science, Ningbo 315201, China
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*Corresponding author. E-mail: [email protected] ; [email protected].
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Among various carbon materials, nitrogen doped single walled carbon nanohorns (N-SWCNHs) has a unique structure of clustered conical cages (2−5 nm in diameter and 40−50 nm in length) arranged in dahlia, bud and seed-like configurations. Each conical cage has five pentagons at their tips which acts as potential reactive site with its own distinct chemistry. We exploited these reactive sites of NSWCNHs by preferentially growing germanium nanocrystals (Ge NCs) onto their conical tips using
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oleylamine as a mild reducing agent. Therefore, Ge decorated N-SWCNHs (Ge@N-SWCNHs) composite was used, for the first time, as active anode material for lithium ion batteries providing high and stable capacity of 1285 mAh/g at 0.1C after 100 cycles. Our results show that preferential growth of Ge Nanocrystals (NCs) on the tips of N-SWCNHs not only allow high utilization of
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active material but prevents the aggregation of Ge NCs after multiple cycling. Finally, we highlight the potential role of N-SWCNHs as cheap and industrially scalable conductive host for next
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generation lithium ion batteries.
Keywords: Nitrogen doped single walled carbon nanohorns, Lithium ion batteries, Anode material, Germanium composites, carbon material, Solvothermal method.
1
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INTRODUCTION
lithium ion batteries (LIBs) have been providing the electrical power necessary to operate existing portable electronics and hybrid electric vehicles (HEVs)[1,2]. However, the demand for safe batteries with higher power and capacities have started a worldwide hunt of new electrode materials for LIBs. Many researchers from academic and industrial domains have considered Si, Ge and Sn as promising alternative anode materials for next generation LIBs. In fact, Sony has already
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developed its Nexelion battery using an anode material mainly composed of a Sn/Co/C composite[3]. Main attraction of these elements lie in their high theoretical capacities (4200 mAh/g for Si, 1625 mAh/g for Ge, and 994 mAh/g for Sn) which is based on alloy reactions between the respective elements and lithium (LixMy)[4]. Moreover, lower working potentials (0.1-0.6 V vs
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Li/Li+) compare to insertion (TiO2) and conversion (Fe2O3) materials (1.0-2.5V vs Li/Li+) make them even more attractive for full cell configuration with higher energy densities[5].
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Although Sn and Si based electrodes take the lead in terms of cost and production, Ge has also been in the spotlight due to its electronic conductivity (104 times higher than Si)[6] and lithium ion diffusivity (400 times higher than Si)[7]. Contrary to silicon, isotropic lithiation of Ge allows additional structural stability during pulverization of electrode[8]. Therefore, Ge can offer an alternative for the development of durable, high-capacity and high-rate anodes for high value applications like medical and space technologies[6]. Nevertheless, commercialization of Ge based
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electrodes for LIB is mainly hindered by massive volumetric expansion of Ge ( ̴ 250% for Li15Ge4), continuous breakage of SEI layer and electrochemical aggregation of Ge particles during multiple lithiation/de-lithiation cycles[9]. One popular approach to address these problems involves nano
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structuring of Ge particles[10–13] through which mechanical stress and diffusion length can be minimized. Despite the reduction of particle size, Ge based anodes suffer from welding or aggregation effect[14] which can be minimized by anchoring nanoparticles on conductive substrates
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like carbon nanotubes (CNTs)[15], carbon nanofibers (CNFs)[16], graphene/reduced graphene oxide[17] (RGO) and hollow carbon encapsulating active material[12]. These carbon substrates not only provide high electrical conductivity but prevent large volumetric expansion providing high power and long-term stability. However, the ratio between active material (Ge) and carbon has a strong influence on capacity and cycle life as higher ratios may lead to the clustering of Nanocrystal (NCs) due to aggregation effect[17]. Recent efforts have been proposed to improve the electrochemical performance of Ge based anodes involving various carbon-based composites. For example, Fang et al.[18] has prepared GeRGO-CNTs composite comprising of 20-30 nm Ge NPs fixed tightly on the surface of RGO while using CNTs as an efficient electron transport medium. The composite could retain a specific
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discharge capacity of 863.8 mAh/g after 100 cycles with a current density of 100 mA/g. Similarly, Wang et al.[15] developed a free-standing germanium/single-walled CNTs (SWCNTs) composite by trapping different amount of Ge NPs on the entangled network of CNTs. The composite with 50% Ge content gave a stable capacity of 400 mAh/g after 40 cycles at a current density of 25 mA/g. Recently, our group have developed a facile solvothermal method to anchor Germanium nanocrystals (Ge NCs) on the surface of multi-walled carbon nanotubes (MWCNTs). As-prepared
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Ge@MWCNTs composite showed improved electrochemical performance with discharge capacity of 1160 mAh/g after 60 cycles[7]. According to the authors, the remarkable performance Ge@MWCNTs based electrodes can be ascribed to the high conductive properties of MWCNTs, also preventing the aggregation of Ge nanoparticles during multiple lithiation/de-lithiation.
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Despite the attractive properties of these carbon materials, higher production cost of CNTs [19] and low quality of graphene[20] limit their industrial use in many applications including lithium ion Nitrogen doped single walled carbon nanohorns (N-SWCNHs) are a kind of carbon
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batteries.
materials that can be produced on a large scale (ton/year). Moreover, this exotic material exist in a clustered arrangement of horn shaped graphitic tubules (2-5 nm diameter and 40-50 nm tube length) which, upon aggregation, form dahlia-like, bud-like and seed-like structures[21,22]. Interestingly, each conical tubules have five pentagons at their tips coupled with heptagons which acts as potential reactive sites having their own distinct chemistry[23]. Moreover, local chemical reactivity
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is also enhanced in regions of higher curvature due to pyramidal distortion of the sp2 carbon bonding [24]. Due to these unique and tunable properties, SWCNHs have found applications in gas storage[25], catalysis[26] and supercapacitors[27]. Although, few reports mention their application as an anode material for lithium ion batteries[28–30], none of them fully characterized their
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electrochemical performance as a conductive substrate for alloying anode material. In an ongoing project of understanding the potential of N-SWCNHs for application in lithium
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ion and post-lithium ion batteries[31], we exploited the reactive sites of N-SWCNHs by preferentially growing germanium nanocrystals (Ge NCs) onto their conical tips using oleylamine as a mild reducing agent. With respect to other popular carbon materials such as graphene or CNTs, N-SWCNHs can be mass produced on an industrial scale. Moreover, nitrogen as a doping agent inside N-SWCNHs is suggested for its high electronegativity, due to C−N bonds characterized by the electric dipoles located at the surface of the nanohorns. This effect might increases the surface chemical reactivity of N-SWCNHs, hence fostering the interaction between nitrogen sites and the surrounding active material. Therefore, the as prepared Ge@SWCNHs composite was used as an anode material for lithium ion battery providing high and stable capacities of 1285 mAh/g after 100 cycles. We believe that the preferential growth of Ge NCs on the tips of SWCNHs not only allow
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high utilization of active material but prevents the aggregation of Ge NCs after multiple cycling. These impressive results show that N-SWCNHs allows the utilization of Ge as suitable anode for LIBs with high energy and high-power applications. 2 2.1
EXPERIMENTAL Synthesis of Ge@N-SWCNHs composites
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N-SWCNHs were mass produced with proprietary industrial process technology and provided by Advanced Technology Partner s.r.l. (ATP). Germanium nanocrystals (Ge NCs) and Ge@NSWCNHs composite were prepared using a facile solvothermal approach in a 25 ml Teflon vessel. In particular, 163mg of GeI2 (concentration, 33 mM) was dissolved into 15 ml of degassed
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oleylamine using a sonication bath. 9 mg of N-SWCNHs were subsequently added to GeI2 solution in an inert argon atmosphere. The final mixture was transferred to a 25 mL Teflon vessel equipped
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with a stainless-steel container and kept in a furnace at 230 oC for 6 hours. Ge@N-SWCNHs composite was obtained by washing the blackish brown product with 1:4 mixture of chloroform and ethanol, respectively. Finally, the ligands were carbonized by thermally treating Ge@N-SWCNHs composite in an argon/hydrogen (95/5%) atmosphere at 500 oC. The final product was transferred into a glovebox to avoid surface oxidation. For comparison, pure Ge-nanocrystals (brown product)
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were also prepared using the same procedure without adding N-SWCNHs. Material Characterization
Microscopic images of N-SWCNHs and Ge@N-SWCNHs composites were acquired using a JEOL JEM 1011 (Jeol, Tokyo, Japan) operated at 100 kV of acceleration voltage, equipped with a
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tungsten thermionic electron source and a FEI TECNAI G2 F20 instrument, equipped with a Schottky field emission gun (FEG), operating at 200 kV acceleration voltage. High resolution
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transmission electron miscopy was performed onto ultrathin carbon/holey carbon-coated Cu grids using JEM-2200FS, image Cs-corrected (Schottky emitter, HT 200 kV), with in-column imaging filter, equipped with Bruker XFlash 5060 SDD system. Scanning electron microscopy was performed using Nova 600 NanoLab microscope (FEI), equipped with a field emission gun and a focused ion beam of gallium ions for milling. XRD patterns were collected on a PANalytical Empyrean X-ray diffractometer equipped with a 1.8 kW CuKα ceramic X-ray tube operating at 45 kV and 40 mA using a PIXcel3D (2 x 2 area) detector. XPS analysis was performed on a Kratos Axis Ultra DLD spectrometer, using a monochromatic Al Kα source (15 kV, 20 mA). High resolution scans were carried out at constant pass energy of 20 eV and steps of 0.1 eV. The pressure in the analysis chamber was maintained below 7×10-9 Torr for data acquisition and photoelectrons were detected at a takeoff angle θ = 0° with respect to the surface normal. Finally, the data were
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converted to VAMAS format and processed using Casa XPS software, version 2.3.16. The binding energy (BE) scale was internally referenced to the C 1s peak (BE for C−C = 284.8 eV). Raman spectroscopy was performed with Renishaw in Via Micro Raman equipped with a laser source of 633 nm using a 50x objective (LEICA N PLAN EPI 50/ 0.75). 2.3
Electrochemical Characterization
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The electrochemical performance of Ge@N-SWCNHs composite was tested on CR2032 coin cells which included lithium chips (15.6 Dia x 0.45t mm; MTI Corporations) as reference and counter electrode, dried glass fibers membrane (What-man GF/D) as separator and Ge@NSWCNHs composite, casted on a copper substrate, as working electrode. Slurries casted on copper
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foil (15 mm Diameter) were composed of 70% weight Ge NPs or Ge@N-SWCNHs composite (containing 70% Germanium by weight), 20% super-P carbon and 10% polyvinylidene difluoride (PVDF) grinded in a mortar using N-methyl-2-pyrrolidone as a solvent. The casted electrodes were
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dried overnight at 120 °C under vacuum and the mass loadings of active material were found in the range of 1-1.2 mg. 250 µl of electrolyte was used in each coin cell which contained 1 M lithium hexafluorophosphate (LiPF6) dissolved in 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), respectively. 5% (v/v) vinylene carbonate (VC) was also used as an additive. After their assembling inside a MBraun glovebox, with H2O and O2 levels below 0.1 ppm, all cells Lab V1.30. 3
RESULT AND DISCUSSION
Synthesis and Characterization of Ge@N-SWCNHs composites
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were electrochemically tested using BioLogic BCS-805 multichannel battery unit controlled by BT
Nitrogen doped single walled carbon nanohorns (N-SWCNHs) typically exist in the form of
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aggregated conical tubules extended by a short cylindrical section (Figure 1a). The tips typically contains five pentagons, however, a sixth pentagon is required for the nanohorn walls to continue parallel to the wall axis[32]. Consequently, a heptagon counteracts the curvature change caused by the additional pentagon. Due to these defects, theoretical calculations have demonstrated a net electron transfer to the pentagonal sites of the CNHs tips[33,34] providing reactive sites for chemical functionalization. Moreover, the local chemical reactivity is also enhanced in the regions of higher curvature due to pyramidal distortion of the sp2 carbon bonding[24]. For both reasons, the chemical reactivity of carbon nanohorns is assumed to be localized around pentagons, heptagons, and other defected sites, toward the nanohorn tip. Our synthetic protocol, depicted in figure 1b, utilizes the reducing properties of pentagonal sites of N-SWCNHs to preferentially grow Ge NCs on the tips N-SWCNHs.
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Figure 1: a) TEM image of N-SWCNHs. b) Schematic illustration for synthesis of Ge@N-SWCNHs composite. TEM images of c) Ge@N-SWCNHs composite d) Ge NCs. e) XRD patterns of Ge NCs (Red), Ge@N-SWCNHs (Blue) and Ge@N-SWCNHs (black) after exposed to air. f) Raman spectra of N-SWCNHs (Red) and Ge@NSWCNHs composite (Blue)
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Instead of using strong reducing agents like alkali metals or organoalkali reagents[35–37] for the
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solution phase synthesis of Ge NCs, our protocol employs a mild reducing agent like oleylamine to synthesize crystalline Ge NCs where not only it acts as a solvent but also a mild reducing agent [38]. Moreover, the mild reducing power of oleylamine allows the reduction of Ge NCs to be dictated by the active sites of N-SWCNHs. As expected, TEM images of Ge@N-SWCNHs showed preferential arrangement of 4-10 nm Ge NCs at the conical tips of N-SWCNHs (Figure 1c). We believe that the preferential growth is the result of enhance reactivity of conical tips of N-SWCNHs due to the heptagons[23] providing a nucleation site of Ge NCs to grow[24]. The preferential growth of Ge NCs not only provides void space to accommodate volume changes, but also prevents aggregation of Ge NCs during lithiation/de-lithiation process. Ge NCs without N-SWCNHs were also prepared for comparison (Figure 1d) showing 4-10 nm Ge NCs intrinsically capable of sustaining large volumetric expansion. Homogenous distribution of Ge NCs inside the carbon
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matrix of N-SWCNHs was confirmed using scanning electron microscopy/Energy Dispersive XRay Spectroscopy (SEM/EDS) analysis (Figure S1). Moreover, ED’s analysis showed the Ge content inside Ge@N-SWCNHs composite to be 55% which was also confirmed by ICP/MS analysis. Crystallinity of Ge NCs and Ge@N-SWCNHs composite was confirmed using X-ray diffraction (XRD) analysis (Figure 1e). In particular, XRD spectra of freshly prepared Ge NCs and Ge@N-SWCNHs composite show characteristic peaks of cubic germanium at 2θ values of 27.32º,
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45.22º, 53.76º, 65.92º and 72.83º (JCPDS card No. 03-065-0333)[16,39]. The average particle size of Ge NCs obtained using Scherrer equation was found to be 8.5 nm matching well with our TEM observations.
Detailed characterization of N-SWCNHs and Ge@N-SWCNHs composite was performed using
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Raman spectroscopy. Figure 1f shows prominent D and G peaks which can be attributed to disordered sp2 carbon and the vibrational modes of graphitic carbons[40], respectively. Due to the
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presence of pentagons and heptagons in N-SWCNHs, these D and G peaks appears at 1348 and 1590 cm-1, respectively[41]. ID/IG ratios calculated for both spectra showed values close to unity, explaining the defected structure associated to N-SWCNHs[41]. Additional peaks at 2953 cm−1 (2D) and 3225 cm−1 (D+G) are the overtones of D and G peaks, respectively[42].. Finally, the peak
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of Ge appears close to 300 cm-1 demonstrating its presence in Ge@N-SWCNHs composites.
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Figure 2: XPS analysis of a) N-SWCNHs. High resolution spectra of b) C1s and c) N1s peaks showing the presence of nitrogen on N-SWCNHs
To confirm the presence of nitrogen doping X-ray Photoelectron Spectroscopy (XPS) was
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performed on N-SWCNHs. Figure 2a shows a wide range spectrum of N-SWCNHs depicting C 1s, N 1s and O 1s peaks. The C 1s peak (Figure 2b) shows the typical shape related to graphitic carbon[43] shown by an asymmetric peak at 284.4±0.2 eV (associated to sp2 carbon atoms) accompanied by a low intensity shake-up feature at 290.8±0.2 eV, related to π→π* transition. The best fit of the C 1s profile was obtained using other low intensity carbon peaks[44] centred
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at 284.9±0.2 eV, 286.4±0.2 eV, 288.0±0.2 eV and 289.7±0.2 eV. These peaks are generally assigned to sp3-hybridized carbon atoms, C-O, C=O and O-C=O species, respectively. However, the peak at 286.4±0.2 eV can be associated to C-N species (corresponding to the N-sp2 C bonds) which might be originated by the addition of N atoms in the carbon network of the N-SWCNHs[45]. This
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assignment is also supported by N 1s broad peak appeared between 396 and 406 eV (Figure 2c) which can be split into three components centred at 399.0±0.2 eV, 400.8±0.2 eV, and 402.2±0.2 eV.
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Each component corresponds to pyridinic, pyrrolic, and graphitic nitrogen, respectively[46]. Finally, the atomic percentage of carbon, nitrogen and oxygen were found to be 97.8, 1.1 and 1.1%, respectively. 3.2
Electrochemical performance of Ge@N-SWCNHs composite In order to utilize N-SWCNHs as a conductive substrate for Ge NCs and its catalytic
properties for electrolyte decomposition, cyclic voltammetry of pure N-SWCNHs, Ge NCs and Ge@N-SWCNHs based electrodes was performed using different combinations of electrolyte additive (VC). The choice of the electrolyte additive was based on the fact that it facilitate the formation of mechanically stable solid electrolyte interface (SEI) which acts as a passivating layer to inhibit further electrolyte reduction[47]. Moreover, VC is the most common electrolyte additive
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used to improve the stability of SEI in Si, Ge and Sn based anode materials[48]. For comparison, initial tests were performed on pure N-SWCNHs using electrolyte consist of 1M LiPF6 in EC:DMC with 5% of VC as an additive. In order to test the stability of N-SWCNHs, cyclic voltammetry was performed at a wide potential range of 0.005-3V, confirming no oxidative decomposition at higher potentials. However, four prominent reduction peaks at 1.61V, 1.1V, 0.60V (minimized after 1st
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cycle) and 0.005 V (Figure 3a) were observed during the first reduction cycle.
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Figure 3: Cyclic voltammograms of a) Pure N-SWCNHs, b) Ge NCs without additives, c) Ge@NSWCNHs without additive, d) Ge@N-SWCNHs with 5% VC. † represents SEI peaks related to electrolyte decomposition, ǂ represents SEI peaks related to electrolyte additive (VC), ¥ represents LixGex alloying/de-alloying peaks while # represent peaks related to adsorption/intercalation of lithium inside N-SWCNHs.
Peaks at 1.61V and 1.1V can be associated to the reductive decomposition of VC while the peak at 0.60V can associated to the decomposition of EC. We believe that all three peaks are responsible to form a solid electrolyte interface (SEI)[49]. Finally, the reduction peak at 0.005V can be ascribed to adsorption/intercalation of lithium inside N-SWCNHs[29]. During anodic scan, peak at 0.21V describes the de-intercalation of lithium from N-SWCNHs. The low intensity of anodic peak compare to its cathodic counterpart suggests the irreversible intercalation/de-intercalation of NSWCNHs which might explains the lower columbic efficiency of Ge@N-SWCNHs at low C-rates (described later). Similarly, cyclic voltammetry performed on Ge NCs and Ge@N-SWCNHs based electrodes using base electrolyte (1M LiPF6 in EC:DMC) showed a first cycle reduction peak at
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0.75V and 0.61V, respectively (Figure 3b,c) which can be ascribed to the decomposition of EC. Additional reduction peaks from 0.31 to 0.05V can be associated to lithium insertion into equipotential sites and the formation of Ge-alloys, i.e. Li9Ge4 and Li7Ge4 followed by Li15Ge4 and Li22Ge4 [10,12,39,50].
The anodic (de-lithiation) scan of the Ge NCs and Ge–N-SWCNHs
composite based electrodes show oxidative peaks, located in the range of 0.42-0.72V suggesting dealloying process of Ge–Li composites. Similar alloying and de-alloying peaks related to Ge were
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observed in Ge@N-SWCNHs composites, however, an additional broad peak starting from 1.61 V was observed due to the decomposition of VC, as observed in the case of pristine N-SWCNHs (Figure 3d).
Cycling performance of electrodes based on pristine N-SWCNHs were initially tested at 0.1C under
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the potential window of 0.005-2.5V using galvanostatic measurements showing stable capacity of 87 mAh/g after 100 cycles (Figure S4). In order to avoid the involvement from lithiation/de-
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lithiation of N-SWCNHs, galvanostatic charge discharge measurements were performed in the
Figure 4: a) First lithiation profiles of Ge NCs and Ge@N-SWCNHs with and without additive. b) Comparison between 5th lithiation/de-lithiation curves of Ge NC and Ge@N-SWCNHs with VC additive. c) Comparison between
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lithiation/de-lithiation curves of the 5th and 100th cycles for Ge@N-SWCNHs composite with VC additive.
potential range of 1.5 and 0.01 V vs. Li/Li+ at 0.1C. Figure 4a compare the 1st lithiation curves
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(starting from OCV) of Ge NCs and Ge@N-SWCNHs composite in the presence and absence of VC additive at 0.1C (1C =1600 mAh/g). Indeed the initial lithiation gravimetric capacities achieved for Ge@N-SWCNHs composites (2449 mAh/g) are higher than Ge NCs (1595 mAh/g) based electrodes, however, after 5 cycles these capacities stabilized to 1435 and 1046 mAh/g, respectively. Interestingly, the decomposition of electrolyte additive or formation of SEI is more pronounced in Ge@N-SWCNHs based electrodes, suggesting the enhanced catalytic properties of N-SWCNHs[26]. Moreover, the 5th cycle voltage profile of Ge@N-SWCNHs using 5% VC additive appeared at lower activation potentials compare to the voltage profiles of Ge NCs (Figure 4b). We believe that higher capacities obtained for Ge@N-SWCNHs composites are associated to the lower activation energies allowing complete
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utilization of active material in a given potential range of 0.01-1.5V. Moreover, the comparison between 5th and 100th charge discharge profile of Ge@N-SWCNHs composite electrode with 5% VC show minimal activation barrier and the loss of capacity is mainly due to the concentration overpotential created at the electrode surface (Figure 4c). Finally, the capacity retention of pristine Ge NCs and Ge@N-SWCNHs composite in the presence of VC was tested for over 100 cycles. Pristine Ge NCs and Ge@N-SWCNHs composites show
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initial discharge capacities of 1595 and 2449 mAh/g which stabilized at 811 and 1285 mAh/g after 100 cycles, respectively (Figure 3 a, b). It is interesting and important to notice that the majority of
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the de-lithiation capacity in Ge@N-SWCNHs composite was lost during the first 10 cycles (6 %)
while only 2.5 % loss was observed for the remaining 90 cycles.
For comparison, electrodes consisted of Ge NCs, Ge@N-SWCNHs composite operated in base electrolyte (1M LiPF6 in EC:DMC) without VC additive showed a rapid decay in capacity confirming the importance of VC for long term stability (Figure 5a, b). The remarkable storage
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Figure 5: Cycling performance of a) Ge NCs and b) Ge@N-SWCNHs composite with and without VC at 0.1C. ( )
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•
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Blue and ( ) red spheres depicts lithiation and de-lithiation, respectively while ( ) black and ( ) dark red spheres represents columbic efficiencies with and without additive, respectively. Rate performance of c) Ge NCs and d) Ge@N-
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•
•
SWCNHs. ( ) Black and ( ) red spheres correspond to lithiation and de-lithiation, respectively ( ) while blue spheres represents columbic efficacies with VC as an electrolyte additive.
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capacities observed for Ge@N-SWCNHs using VC as an electrolyte additive were comparatively higher to our previously reported method where carbon nanotubes were used a conductive host for Ge NCs[7] Therefore, these results suggest an intimate contact between Ge NCs and N-SWCNHs allowing high utilization of the active material. The rate performance of Ge NCs and Ge@SWCNHs composite was analyzed at different charge discharge rates. Ge NCs show de-lithiation capacities of 908, 762, 599, 462, and 271 mAh/g at
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0.1C, 0.4C, 1C, 2C and 5C respectively while regaining the capacity of 850 mAh/g at 0.1C. (Figure 5c) One the other hand, Ge@N-SWCNHs composite based electrodes showed de-lithiation capacities of 1420, 1193, 935, 866, 611, 366 mAh/g
at 0.1C, 0.4C, 0.8C, 1C, 2C and 5C
respectively (Figure 5d). Finally, the rate performance of pristine N-SWCNHs showed negligible
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capacities of 48, 36, 26, 15 and 9 mAh/g at 0.2C, 0.4C, 0.6C, 0.8C and 1C respectively.(Figure S4b) Superior performance of Ge@N-SWCNHs composite can be realized by the fact that electrode
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material regained its original capacity of 1413 mAh/g after been cycled at high scan rates. The comparison of Ge@N-SWCNHs composite with recently reported Ge-carbon composites shown in Table S1 suggest its suitability as an attractive and cheap conductive substrate for applications in high energy and high power LIBs. Moreover, large scale and cost effective production of industrial grade N-SWCNHs along with as-reported performance of Ge@N-SWCNHs composite provide this
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electrode material an edge over graphene and carbon nanotube based composites.
CONCLUSION
We have reported a facile solvothermal synthesis of 5-10 nm germanium nanocrystal
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decorated on the tips of nitrogen doped single wall carbon nanohorns. The as-prepared Ge@NSWCNHs composite was electrochemical tested as anode material for lithium ion battery showing
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remarkable capacities of 1285 mAh/g after 100 cycles. Moreover, the electrochemical performance of Ge@N-SWCNHs composites was compared with bare Ge-NC with and without electrolyte additives. Our results suggest that Ge@N-SWCNHs show improved performance in the presence of electrolyte additive VC, possibly due to the formation of a stable SEI. Due to the cheap production cost and scalability, N-SWCNHs can be a promising candidate as a supporting conducting agent for high storage active materials in lithium ion batteries.
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AUTHOR INFORMATION
Corresponding Authors *E-mail: [email protected]; [email protected]
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Present Address
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, 466-8555, Japan. Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
Authors would like to acknowledge the support from the 3315 project Ningbo (nr. Y70001DL01),
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China. Authors would like to thank Advanced Technology Partner S.r.l. to provide the N-SWCNHs samples,
7 [1]
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([email protected]).
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Figure S1: EDS analysis of Ge@N-SWCNHs showing a) SEM image b) Carbon and c) Germanium mapping
Figure S2: High Resolution TEM images and elemental mapping of Ge@N-SWCNHs composite showing Ge cubic phase [110] and partially oxidized Ge NC [021].
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•
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Figure S4: a) Cycling performance of pristine N-SWCNHs using 5%VC. b) Rate performance of N-SWCNHs. ( ) Red
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and ( ) black balls representing lithiation and de-lithiation while ( ) blue stars represents columbic efficiency.
Table S1: Comparison of the performance metrics between Ge @N-SWCNHs and other carbon based Ge composites
Ge Composites
Synthetic route
Capacities (mAh/g)
Current Densities (mA/g)
Cycles
Ref
Magnetic sputtering coupled with 980 plasma reactors
2000
100
[51]
Ge-N-RGO
Hydrolysis to GeO2 and annealed 700 back to Ge
5000
200
[52]
Ge-RGO
Colloidal conditions
1600
600
[53]
Ge-RGOCNT
Colloidal method in inert condition
863.8
100
100
[18]
Hydrolysis followed by annealing
1232
C/2
1000
[10]
Hydrolysis to GeO2
417
25
40
[15]
Ge-hollow carbon
Hydrolysis to GeO2
1000
C/10
100
[12]
GeMWCNTs
Solvothermal
1060
160
60
[7]
Ge@NSWCNHs
Solvothermal
1285
120
100
This work
Ge-CNT paper
in
inert 1350
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synthesis
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Ge-C
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Ge-C
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GRAPHICAL ABSTRACT