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Morphology Processing by Encapsulating GeP5 Nanoparticles into Nanofibers toward Enhanced Thermo/Electrochemical Stability Yaqing Wei, Jiajun Chen, Jun He, Ruihuan Qin, Zhi Zheng, Tianyou Zhai, and Huiqiao Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10462 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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ACS Applied Materials & Interfaces

Morphology Processing by Encapsulating GeP5 Nanoparticles into Nanofibers toward Enhanced Thermo/Electrochemical Stability Yaqing Wei,†,§ Jiajun Chen,†,§ Jun He,†,§ Ruihuan Qin,†,§ Zhi Zheng,† Tianyou Zhai,† and Huiqiao Li*,†,§ †

State Key Laboratory of Material Processing and Die & Mould Technology, School of

Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, Hubei, PR China §

Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen,

518057, Guangdong, PR China * Corresponding author e-mail: [email protected]; [email protected]

KEYWORDS: GeP5, Nanofiber, Electrospinning, Germanium phosphide, Layered structure, Thermostability, Lithium-ion batteries

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ABSTRACT:

Compared with elemental phosphorus, GeP5, with much better thermostability and super higher conductivity, can exhibit a comparable capacity (>2000 mA h g-1) with a much high first coulombic efficiency (95%) for Lithium-ion battery. However, such a high capacity companies with a large volume expansions, leading to fast capacity fading. To improve the cycle stability, fabricating special nanostructure to reduce the volume stress and compositing with carbon matrix to buffer the volume change are highly required. However, nanostructured metal phosphides are rarely reported up to now, since they are difficult to be synthesized via normal wet chemistry method or gas phosphorization due to lack of proper reactants and poor thermostability of phosphides. Herein, we successfully achieve uniform carbon-encapsulated GeP5 nanofibers (GeP5@C-NF) by processing GeP5 nanoparticles into carbon nanofibers via electrospinning. After carbon encapsulation, the thermo-stability of GeP5 can be greatly improved to over 600 °C for higher battery safety. Such a nanofiber structure in which nanosized GeP5 embedded in carbon matrix can greatly accommodate the large volume change during lithiation and provide fast electron transportation, thus contribute a long cycle life (>1000 mAh g-1 after 200 cycles) and high rate performance (803 mA h g-1 at 2000 mA g-1). This morphology processing technique can be easily extended to other metal phosphide anodes which are limited by lack of approiate synthesis methods and poor thermostability.


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INTRODUCTION Recently, the rapid development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) triggers a great exploration of alternative electrode materials with larger capacity to replace the existing graphite anode (limited capacity of only 372 mAh g-1).1-4 The abundant low-cost elemental phosphorus, with a high theoretical capacity up to 2595 mA h g-1, seven time higher than that of graphite, becomes attractive and is expected to serve as the alternative new-type anode for high energy density lithium ion batteries (LIBs).5 However, the elemental phosphorus suffers from an extremely low electronic conductivity (1×10−14 S/cm), thermo-instability (1×10−5 S/cm) and thermo-stability (>300°C).14 Though promising, these transition metal elements are inactive for Li+ storage, which greatly reduces the capacity to less than 1200 mA h g-1. Very recently, Li et al proposed a novel layered phosphorus-like GeP5 as the new anode for LIBs, in which the active Ge element can further alloy with Li and provide additional capacity for Li storage, giving rise to a total capacity up to 2300 mA h g-1.15-16 Compared to elemental P, GeP5 can contribute a comparable capacity with at the same time a much higher first coloumbic efficiency (as high as 95%) and improved thermostability plus conductivity (1×106 S/m), thus is regarded as a much promising new anode materials for LIBs. However, such a high capacity companies with a large volume expansions (>300%), leading to fast capacity fading after a few cycles.15 To buffer the drastic volume change, fabricating special nanostructure to reduce the volume stress and compositing with carbon matrix to buffer the volume change are highly required to improve the cycling performance of such large-capacity


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materials.17-18 However, the current reported synthetic methods for metal phosphides are almost limited to high-energy ball milling, whose products are hardly to directly realize a smart morphology design.5, 19-20 Due to the lack of appropriate soluble reactant resources, it is difficult to synthesize the phosphide by normal solution-based methods such as sol-gel or hydrothermal process.21 In addition, the reaction of Ge with P need to be run in a tetrahedral anvil press at conditions ranging from 600-1300 °C and 15-65 kbars.22-23 Gas-phase phosphorization method seems also hard to get these phosphides because most of the phosphides are easily decomposed upon heating but phosphating the oxides needs to be operated at a high temperature and ultrahigh pressure for a much long time on the contrary.24-25 Limited by the dilemma in synthesis , metal phosphides with specific nanomorphology and good carbon compositing or encapsulation in easy and scalable way have not been reported up to now. Herein, we successfully obtain uniform carbon-encapsulated one dimentional (1D) GeP5 nanofibers on a large scale (denoted as GeP5@C-NF) by firstly synthesizing GeP5 nanoparticles and then processing them into nanofibers via electrospinning. After encapsulation into carbon nanofiber, the thermo-stability of GeP5 can be greatly improved to over 600 °C, in comparison with the 180 °C for red P and 400 °C for pure GeP5 powder, which is vitally important as an electrode for battery safety. On the other hand, such a nanofiber structure in which the GeP5 nanoparticles well embedded in carbon matrix can provide ultrafast electron transportations and shorten the Li+ diffusion length greatly.26-31 When served as the anode for LIBs, the GeP5@C-NF electrode delivers a reversible capacity high up to 1330 mA h g-1 (2000 mAh/g based on GeP5 only) with superior cycle stability (>1000 mAh g-1 remained after 200 cycles). Even at a high current density of 2000 mA g-1, a large capacity of 803 mA h g-1 can still be obtained. This morphology processing technique can be easily extended to other metal phosphide anodes which


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are hard to synthesis by wet-chemistry, and the improvement of thermostability can largely increae the flexbility of heat treatments on metal phosphides. EXPERIMENTAL ESCTION Synthetic Procedures. The pure GeP5 powder was synthesized by a facile high-energy mechanical milling method. Specifically, the Ge and P powder in a molar ratio of 1:5 were ball milled (Fritsch, Pulverisette-6) at a rotation speed of 400 rpm for 10 h. The synthesized nanosized GeP5 powder was then added into the solution of N, N-dimethylformamide (DMF) and Polyacrylonitrile (PAN, Mw = 150,000, Aldrich), which was dispersed and stirred strongly overnight. Then the obtained dispersion was transfered into a injection syringe and further spuned into nanofiber at an operation voltage of 13 kV. The distance between the nozzle and aluminum foil collector was 15 cm and the flow rate of solution was controlled at 0.02 ml/min. Afterwards, the synthesized electrospun nanofibers were heated at 200 °C for 1 h to stabilize the morphology and 450 °C for 6 h to carbonize the PAN in a tube furnace under the Ar atmosphere (heating rate of 5 °C /min). The obtained GeP5 carbon nanofiber was denoted as GeP5@C-NF. Materials Characterization. The synthesized GeP5 powder and GeP5@C-NF were characterized by X-ray diffraction (XRD, PANalytical X’pert PRO-DY2198 with Cu-Kα radiation) and thermogravimetric analysis (TG PerkinElmer). The morphology and microstructure were measured by field-emission scanning electron microscope (FESEM, FEI Quanta650) and transmission electron microscope (TEM, JEOL JEM 2100). The surface structure analysis for the samples were carried out by the X-ray photoelectron spectroscopy (XPS) technique (Thermo Fisher ESCALab250 with monochromatic 150 W AlKα radiation), confocal Raman spectrometer (Raman, WITec ALPHA300 with a 532 nm excitation laser) , and Brunauer, Emmett, and Teller (BET) N2 adsorption/desorption (Quantachrome autosorb iQ). The


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pore size was analyzed based on a quenched solid density functional theory (QSDFT) kernel applied to the adsorption branch using a slit pore model. Electrochemical Measurement. The synthesized GeP5@C-NF nanofiber was mixed with conductive carbon black (C45) and poly acrylic acid binder (PAA) in a mass ratio of 7:2:1 to get a slurry, which was further coated onto a copper foil and dried for 12 h in vacuum chamber at 80 °C. For comparison, the pure GeP5 powder was also used to prepare electrode in the same procedure as above. The loading mass of materials in our study was 3.5 ± 0.2 mg cm−2. The obtained GeP5@C-NF electrode was assembled into a 2032 coin-type cell in a glove box under Ar atmosphere (H2O, O2 300%) of the GeP5 particles during lithiation, which would trigger large stress-strain on the surface of the lithiated GeP5 particle and facilitate the formation of cracks. After continuous lithiation/de-lithiation during


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cycles, the sustained volume expansion and contraction resulted in serious pulverization and exfoliation from the current collector finally. In comparison, after encapsulating GeP5 nanoparticles into carbon nanofiber for the GeP5@C-NF electreode, the carbon nanofiber could effectively buffer the volume change of GeP5 particles during lithiation by a typical confinement effect shown in Figure S7. On the one hand, the nanoparticle size of GeP5 was decreased within 50 nm, helping it to get more favourable kinetics and restrain the mechanical degradation to some extent.41 On the other hand, such a nanofiber structure in which the GeP5 nanoparticles well embedded in carbon matrix can effectively restrain the particle growth and agglomeration. Thus, there was no pulverization or destruction for the GeP5@C-NF electrode as shown in Figure 6e. Besides, the existence of carbon nanofiber was able to provide a direct current pathway to promote ultrafast electron transport, which greatly relieved the mechanical stress and improves the conductivity of the electrode. Therefore, the GeP5@C-NF electrode exhibits more superior cycle stability and better rate performance than the pure GeP5 powder. CONCLUTION In summary, the GeP5@C-NF is synthesized by a two-step synthesis method through firstly simple ball milling to synthesize GeP5 nanoparticles and then processing into nanofiber via electrospinning technique. After morphology processing, the GeP5 nanoparticles are uniformly encapsulated into the 1D carbon nanofiber. Attributed to this unique morphology design, the GeP5@C-NF exhibits much more superior thermo-stability (>600°C) than phosphorus and GeP5 powder, which favors promoted battery safety. Besides, the carbon nanowires are able to accommodate the large volume change and restrain the mechanical degradation during lithiation/de-lithiation, which contribute to a long cycle life for LIBs. In addition, such a nanofiber structure in which the GeP5 nanoparticles well embedded in carbon matrix can provide


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fast electron transportations and shorten the Li+ diffusion length, enhancing the rate performance as well. Consequently, the GeP5@C-NF electrode delivers a large reversible capacity of 1330 mA h g-1 with superior cycle stability (>1000 mAh g-1 after 200 cycles). At a current desity high up to 2000 mA g-1, large reversible capacity of 803 mA h g-1 can still be obtained (capacity retention of 66%). Thus, the GeP5@C-NF is expected to be served as a promising anode for the next generation high energy LIBs. And morphology processing of encapsulating GeP5 nanoparticles into carbon nanofiber may open an avenue to develop series of high performance phosphide anodes for energy storage applications.

AUTHOR INFORMATION Corresponding Author * E-mail: Prof. H.Q. Li, [email protected]. [email protected] ACKNOWLEDGMENT We acknowledge the National Natural Science Foundation of China (21571073, 51772115), Ministry of Science and Technology of China (2015CB932600), Hubei Provincial Natural Science Foundation of China (2016CFA031), the Shenzhen Science and Technology Project (JCYJ20170307154129933) and the Fundamental Research Funds for the Central University. The authors also thank the Analytical and Testing Center of HUST for the measurements.


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Figure 1. Schematic illustration of the preparation process of GeP5@C-NF. the pure GeP5 powder was synthesized via a traditional high energy ball milling method, then the as prepared GeP5 powder was added into the DMF solution containing PAN to carry out the morphology processing via a typical electrospining process. The GeP5@C-NF can be obtained after teating the as synthesized nanofiber at 450 °C for 6 h to carbonize the PAN under Ar atmosphere.


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Figure 2. The SEM images of GeP5@C-NF: without heating (a) and after heat treatment (b) (c); the HRTEM images of GeP5@C-NF (d) (e); the elemental mapping of image (c) of GeP5@CNF(f-i); the XPS spectra of the GeP5@C-NF: (j) survey, (k) C 1s and (l) N 1s.


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Figure 3. (a) The XRD pattern of GeP5 powder, PAN, GeP5@PAN without heating and GeP5@C-NF after calcining at 450 °C for 6 h; (b) the thermos-gravimetric (TG) curves of PAN, GeP5 powder and GeP5@C-NF.


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0 1 st 2 nd 3 rd 4 th

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Figure 4. The discharge/charge profiles and corresponding dQ/dV curves of GeP5 powder (a) (c) and GeP5@C-NF (b) (d).


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Figure 5. (a) The discharge/charge profiles at different current densities of GeP5@C-NF; comparison of the rate performances (b) and cycle stability (c) of GeP5 powder and GeP5@CNF; (d) the discharge/charge curves of the GeP5@C-NF//LiCoO2 full cell; (e) the picture of GeP5@C-NF//LiCoO2 full cell lightening the 14 LED red bulbs for several hours.


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Figure 6. Morphologic changes and EIS measurement upon the cell tests: (a) (b) (c) flesh GeP5 electrode and that after 50 cycles, respectively; (d) (e) (f) GeP5@C-NF electrode and that after 50 cycles, respectively. The insets are their corresponding high magnification pictures.


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Figure 7. Schematic illustration of lithiation-delithiation process of pure GeP5 electrode (a) and GeP5@C-NF (b) electrode during cycles.


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Morphology Processing by Encapsulating GeP5 ... - ACS Publications

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