General Synthetic Strategy for Pomegranate-like Transition-Metal

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A General Synthetic Strategy for Pomegranatelike Transition Metal Phosphides@N-doped Carbon Nanostructures with High Lithium Storage Capacity Nana Wang, Zhongchao Bai, Zhiwei Fang, Xiao Zhang, Xun Xu, Yi Du, Lifeng Liu, Shixue Dou, and Guihua Yu ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00216 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Materials Letters

A General Synthetic Strategy for Pomegranate-like Transition Metal Phosphides@N-doped Carbon Nanostructures with High Lithium Storage Capacity Nana Wang, ab Zhongchao Bai, ab* Zhiwei Fang, a Xiao Zhang, a Xun Xu, b Yi Du, b Lifeng Liu, c Shixue Dou, b* Guihua Yua*

a

Materials Science and Engineering Program and Department of Mechanical

Engineering, The University of Texas at Austin, Austin, Texas 78712, United States b

Institute for Superconducting and Electronic Materials, University of Wollongong，

Innovation Campus, Squires Way, Wollongong, New South Wales 2500, Australia c

International Iberian Nanotechnology Laboratory (INL), Avenida Mestre José Veiga,

4715-330 Braga, Portugal

ACS Paragon Plus Environment

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ABSTRACT: Pomegranate-like nanostructured materials promise extensive applications in various fields due to their unique properties derived from special architectures. However, the controlled synthesis of pomegranate-like nanostructured materials remains challenging due to their complicated construction. Here, we develop a versatile synthetic approach to fabricate pomegranate-like transition metal phosphides encapsulated in nitrogen-doped carbon nanospheres (TMP@N-Cs), through a direct vapor-phase leaching reduction (VPL-R) of polydopamine (PDA) coated transition metal phosphates (TMPO@PDA). The distinctive TMP@N-C nanostructure has superior features for their applications in energy storage. For example, pomegranate-like Co2P@N-C when explored as anodes for Li-ion battery, possess faster Li-ion transport kinetics and improved electrical conductivity, demonstrating superior rate capability (discharge capacity of 412 mAh g-1 at a high current density 5 A g-1) and long cycle life (very stable capacity of 632 mAh g-1 over 300 cycles).

Pomegranate-like nanostructured materials, constructed from conductive layers encapsulating active nanoparticles, have been applied in a wide variety of fields, including


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electrochemical energy storage systems, catalysis, magnetics, etc.1-3 Specifically, in the field of energy storage, the pomegranate-like structure has gained more attention, due to its

favorable

characteristics,

which

would

yield

significant

improvements

in

electrochemical performance.4-6 For example, the coating layer could improve the conductivity of the electrode, which would not only be beneficial to good rate performance of batteries, but also enclose all the primary particles to restrict the amount of solidelectrolyte interphase (SEI) formation. The internal void space could buffer the volume expansion during cycling, thus preventing the fracturing of primary nanoparticles. The assembled secondary structure not only maintains the advantages of the nanosized primary features, such as high surface area and high reactivity, but also endows the materials with new characteristics such as high tap density, leading to high energy density. This pomegranate-like nanostructure has already been applied in silicon electrodes, where silicon nanoparticles were encapsulated in a conductive carbon layer through a bottom-up microemulsion approach using SiO2 template.1 In addition, metal oxide@N-doped carbon subunits were used in lithium ion batteries (LIBs),5 and Co


3


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nanoparticles@N-doped carbon subunits were used in Li-S batteries.6 They all achieved better performance than their primary counterparts.

Although pomegranate-like structure has many advantages, its preparation process is still challenging because of its complexity. The template-based synthesis method has been adopted to synthesize some reported pomegranate-like structures, involving hard templates such as silica and the soft templates such as metal-organic frameworks.1-12 The template method often involves the inherent disadvantages of being time-consuming, impurities involved as well as low yield. Worse still, some materials cannot be obtained by the template method owing to the scarcity of suitable precursors, as in the case of transition metal phosphides (TMPs), an important class of materials for energy storage and conversion. Especially, in the field of energy storage, TMPs have gained attention as potential anode candidates for high performance rechargeable batteries, due to their high volumetric and gravimetric capacities, as well as the low potential for lithium storage. In recent years, different methods for the synthesis of TMPs have emerged, including solution-mediated reaction under a hot organic solvent,13 chemical vapor deposition,14


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microwave synthesis,15 and electrospinning preparation,16,17 but they are all accompanied by some critical drawbacks such as low yield, high cost, or the use of highly toxic raw materials. Therefore, new synthetic approaches based on efficient chemical process are highly needed in achieving this effective pomegranate-like nanoarchitecture for TMPs, and therefore to highlight their performances.

Here we present a versatile approach to construct uniform pomegranate-like TMPs@N-C nanostructures through a direct VPL-R process of their solid precursors under Ar/H2 atmosphere. We use Co2P@N-C as a model material to illustrate the pomegranate-like structure formation mechanism. PDA coated hydrated cobalt phosphate nanospheres were firstly prepared, and upon further calcination, the PDA is converted into a nitrogen-doped carbon layer, while the cobalt phosphates decomposed into cobalt phosphides. Time evolution process demonstrated that the coated hydrated cobalt phosphate was gradually reduced into Co2P along with a huge volume loss. Due to the volume loss and surface energy minimization, Co2P nanospheres split into small isolated nanoparticles, confined in the nitrogen-doped carbon layer originated from PDA,


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producing the pomegranate-like Co2P@N-C nanostructures. The designed methodology can be generalized to synthesize other pomegranate-like TMP@N-Cs, such as Ni12P5@N-C, CoNiP@N-C, and Fe2P@N-C. The facile VPL-R approach can avoid the time-consuming post-treatment and high toxic raw materials and benefit scalable production of pomegranate-like nanostructured materials. The as-prepared TMPs@N-Cs exhibit greatly enhanced electrochemical performance when used as anodes in LIBs inspired by the merits of pomegranate-like nanostructure.


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Scheme 1. (a) Schematic illustration of the fabrication of TMPs@N-C samples. (b) Lithiation of TMP nanoparticles, and pomegranate-like TMPs@N-C samples.

The detailed synthetic procedure in the Co2P@N-C protocol is shown in Scheme 1a. First, the precursor, hydrated cobalt phosphate, was synthesized using urea as the pH adjusting agent, sodium dodecyl sulfate (SDS) as the surfactant, and sodium dihydrogen phosphate and cobalt sulfide as the phosphorus source and metal source, respectively.20,21 The plain line in the X-ray diffraction (XRD) pattern indicates that the


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precursor spheres are amorphous (Figure S1a in the supporting information). Their composition was confirmed by Fourier transform infrared spectroscopy (FTIR), as shown in Figure S1b, where the peaks between 900-1200 cm-1 belong to stretching vibrations of PO43-, and the bending mode of O-Co-O bonds is at 400-680 cm-1. In addition, a broad band at around 3400 cm-1 and a weak band at 1600 cm-1 are related to the vibration of H2O/O-H.20-22 Next, a PDA layer was coated on the surface of the precursor in Tris-buffer solution to form the PDA coated hydrated cobalt phosphate, which fully inherits the morphology of hydrated metal phosphates (Figure S1d). Afterwards, the PDA coated hydrated cobalt phosphate was treated under Ar/H2 atmosphere at 600 oC for 2 h, producing the pomegranate- like Co2P@N-C (Figure S2 and S3). The time evolution process of producing this pomegranate-like Co2P@N-C was conducted to investigate its formation mechanism. As shown in Figure S4, a core-shell structure with a solid shell and a solid core can be clearly seen before the reaction. After 15 min reaction, the shell keeps its original morphology, but some pores have appeared in the core (the contrast in the TEM image of the core). Although all the XRD peaks were indexed into Co2P (Figure S5), we can deduce that only part of hydrated cobalt phosphate is reduced because most of


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core is still solid (hydrated cobalt phosphate is in amorphous state). On prolonging the reaction time to 30 min during VPL-R process, the core became more porous because of the further reduction of hydrated cobalt phosphate and oxygen leaching. Yet, the material was still in the interconnected state as an entirety. When the reaction was continued (60 min), some nanoparticles fallen off from porous structure due to the huge volume loss and surface energy minimization. Further prolonging the reaction time leads to the pomegranate-like Co2P@N-C microspheres obtained (at 2 h). Based on the above analysis, the formation mechanism of pomegranate-like Co2P@N-C is a process of vaporphase leaching of oxygen and part of phosphorus. The volume loss in the process is more than 76% (because of the hydrate precursor) based on the density of Co2P and Co3(PO4)2. This huge volume loss makes Co2P cannot maintain a continuous porous structure and thereby collapse into small isolated nanoparticles, forming the pomegranate-like [email protected],24 This result is further confirmed by the control experiment, in which the porous structure assembled by nanoparticles was obtained without the carbon shell protection (Figure S6). Some of Co2P permeated into the polymer shell during the collapse, and finally embedded in the carbon shell. Moreover, this


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synthetic methodology can be generalized to obtain other pomegranate-like structured materials such as Ni12P5@N-C, Fe2P@N-C and CoNiP@N-C. Furthermore, as shown in Scheme 1b, this pomegranate-like structure could avoid the whole-electrode-level cracking and the excessive formation of SEI layer when used as anodes for LIBs. In addition, the internal void space can accommodate volume expansion after lithiation, and the spatially confined SEI can improve cycle stability. Therefore, these pomegranate-like structures are expected to achieve superior battery performance.


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Figure 1. Structural characterization of Co2P@N-C sample. (a-c) Scanning electron microscopy (SEM) images and (d, f) TEM images of Co2P@N-C sample. (e) Energy dispersive X-ray spectroscopy (EDS) mapping area and corresponding elemental distributions of C, N, P, and Co for the Co2P@N-C sample.

The structure of Co2P@N-C was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 1a, the sample is composed of uniform nanospheres with a diameter of around 200 nm. Close-up SEM observation clearly demonstrates that two kinds of nanoparticles with different sizes are distributed inside the carbon shell (Figure 1b and c). The large particle size is about 20 nm, and the small one is just a few several nanometers. This result is further proved by the TEM analysis (Figure 1d). The small nanoparticles are embedded in the carbon shell, but the larger nanoparticles are confined in the carbon shell (Figure 1f and 1g). It is interesting that the inner nanoparticles are all isolated without aggregation, even they are in contact with each other (Figure 1f), just like a pomegranate. This structure can not only accommodate the volume expansion in lithium ion insertion/extraction process, but also


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help improve the electronic conductivity of the composite, and thus leading to good electrochemical performance. The high magnification TEM image of the carbon shell (Figure 1g-1i) reveals that the small Co2P nanoparticles with an average diameter of 5 nm are evenly distributed in the carbon matrix due to the diffusion of Co2P in the high temperature. The energy dispersive X-ray spectroscopy (EDS) elemental mapping demonstrates that phosphorus and cobalt are distributed inside the nitrogen-doped carbon shells (Figure 1e and 1h). The high resolution TEM images further confirm that both the large and the small nanoparticles are Co2P. Thermogravimetric analysis (TGA) was carried out to determine the carbon content of Co2P@N-C. As shown in Figure S7, the carbon content in this sample is 20.19% based on the XRD and TGA results.


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Figure 2. Electrochemical properties of Co2P@N-C sample for Li-ion storage. (a) Galvanostatic charge-discharge curves of Co2P@N-C sample at a current density of 100 mA g-1. (b) CV curves of Co2P@N-C sample at a sweep rate of 0.1 mV s-1. (c) Cycling performance of Co2P@N-C sample. (d) Nyquist plots in the frequency range of 10 kHz to 0.1 Hz. (e, f) Rate performance of Co2P@N-C sample and comparison of the performance with literature values.

The electrochemical properties of the prepared Co2P@N-Cs were comprehensively evaluated as an anode for LIBs, and the results are shown in Figure 2. The Co2P@N-C sample delivered an initial discharge capacity of 995 mAh g-1 and a first charge capacity


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of 812 mAh g-1, corresponding to a high initial coulombic efficiency of 81.6% (Figure 2a). The multi-step lithiation of Co2P is reflected by the peaks in the cyclic voltammetry (CV) curves.25-28 The excess area of the first cycle curve over the second cycle one is due to the irreversible formation of the SEI layer.25-30 The subsequent CV curves are almost identical, indicating the highly reversibility of the electrode. The Co2P@N-C nanospheres delivered a stable capacity of 632 mAh g-1 after 300 cycles at a current density of 100 mA g-1, which is much higher than for the control sample of porous Co2P particles (Figure 2c). The electrochemical impedance spectrum (EIS) demonstrated a much lower charge transfer resistance (Rct) at the electrode-electrolyte interface in the Co2P@N-C electrode than that for the control sample (Figure 2d). Even at a high current density of 5 A g-1, the Co2P@N-C nanospheres could deliver a capacity of 412 mAh g-1 (Figure 2e), demonstrating extraordinary rate capability, and much better than previous reports (Figure 2f).16,18,19,27,31-33 This superior electrochemical performance of Co2P@N-C sample is attributed to some interdependent characteristics of the pomegranate-like nanostructure: (i) shortened Li-ion diffusion path because of its nanometer size of primary particles; (ii) enhanced electrical conductivity and fast charge transfer provided by


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nitrogen doped carbon layer and metallic properties of metal phosphides; (iii) accommodated volume expansion after lithiation by internal void space; (iv) formation of thin, stable and spatially confined SEI layer.

Figure 3. Kinetics analysis and post-cycling characterization of Co2P@N-C sample for Liion storage. (a) CV curves at various sweep rates from 0.1 to 100 mV s−1. (b) b-value determined by the relationship between the response current and sweep rate. (c) Separation of the capacitive and diffusion currents at a scan rate of 0.7 mV s−1. (d) Contribution ratio of the capacitive and diffusion-controlled charge at various scan rates.


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(e) TEM images and (f) corresponding elemental distributions of C, N, P, and Co for Co2P@N-C nanospheres after 200 cycles at 100 mA g-1.

To better understand the origin of the excellent battery performance of Co2P@N-C sample, the underlying Li-ion storage mechanism was investigated by sweeping CV. The similar CV shapes collected at different sweep rates are related to the characteristic pseudocapacitive features (Figure 3a), which are favorable to good cycling stability and rate performance. The peaks are nearly identical in the range of 0.1-2 mV s–1, whereas the peak separation increases at high sweep rates due to the polarization related to higher overpotential. The measured current at the sweep rate (v) is the sum of the current related to the capacitive like behavior (icap) and diffusion-controlled process (idiff), obeying a power-law relationship:

i (v) = icap + idiff = avb

in which a b-value of 1 indicates a capacitive behavior, whereas 0.5 denotes a diffusioncontrolled process.34-36 The b values for cathodic and anodic peaks of Co2P@N-C were 0.79 and 0.91, suggesting surface-dominant reactions and capacitive like fast kinetics


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(Figure 3b). A decrease of b-value to 0.58 at a sweep rate >2 mV s-1, is ascribed to an increase of ohmic resistance and diffusion limitations at high sweep rate. Furthermore, a general expression can be used to quantitatively distinguish diffusion-controlled process (k1v) and capacitive behavior (k2v1/2):

i (V) = icap + idiff = k1v + k2v1/2

Based on the quantification, at a scan rate of 0.7 mV s−1, 77.8% of capacities is originated from the capacitive process of Co2P@N-C (Figure 3c). Figure 3d shows the contribution ratios between the two different processes at various sweep rates. It is observed that the capacitive process contribution improves with the increasing sweep rate and can reach a value of 91.1% at 5 mV s-1. This kinetics analysis demonstrates that pomegranate-like Co2P@N-C nanospheres possess fast transport of Li+ owing to the unique architecture, thus leading to the superior battery performance. Significantly, the unique pomegranatelike nanostructure of Co2P@N-C was maintained after cycling, as revealed by TEM images (Figure 3e, 3f), indicating excellent stability of this structure.


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Figure 4. Phase identification and structure characterization of MxPy@N-C samples. (a) XRD patterns, (b) SEM images, (c) TEM images, and (d) EDS elemental mapping of C, N, P, and M (M= Fe, Co, Ni) for the MxPy@N-C samples.

Inspired by the pomegranate-like nanostructure of Co2P@N-C, other TMP@N-C samples (Fe2P@N-C, Ni12P5@N-C, CoNiP@N-C) were prepared following the same method. Figure S8 displays SEM images of the hydrated metal phosphates. The precursors have similar spherical morphologies but with different sizes, and the XRD patterns indicate their amorphous nature (Figure S9). All the FTIR spectra in Figure S10


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involve a broad band at around 3400 cm-1 and a weak band at 1600 cm-1, which are related to the vibrations of H2O/O-H, indicating that these nanoparticles are all the hydrated metal phosphates.20-22,37 After the PDA coating and annealing at high temperature, the phase identification of the synthesized metal phosphides was conducted by XRD, and the patterns are shown in Figure 4a. The corresponding SEM and TEM images demonstrate that the metal phosphide nanoparticles are embedded in the N doped carbon shells (Figure 4b, 4c). These series of TMP@N-C samples are similar, but the specific morphology is not the same. In the Ni12P5@N-C and CoNiP@N-C samples, isolated nanoparticles are dispersed in the N-C shell, which is like the Co2P@N-C sample. Although pomegranate-like Fe2P@N-C was also obtained, the nanoparticles inside are bigger and fewer than those of Co2P@N-C. This is because Fe2P can only be achieved at higher temperature, which will lead to serious aggregation. The EDS elemental mappings also confirm the compositions of the metal phosphides and that C, N, P, as well as the metal ions are evenly distributed in the whole nanospheres (Figure 4d). The valence states of elements were investigated in detail by X-ray photoelectron spectroscopy (XPS) (Figure S11). In these TMPs@N-C, the nitrogen doped carbon layers


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enhance electronic conductivity and guarantee fast ion/electron transfer, internal voids accommodate the large volume expansion during cycling, and the metallic properties of the metal phosphides also ensure better conductivity, all of which contribute to their good electrochemical performance in LIBs.29-30

In conclusion, we have developed a versatile synthetic approach to prepare a series of pomegranate-like metal phosphides embedded in N-doped carbon shell, through the vapor phase leaching reduction of PDA coated metal phosphates. This novel pathway involves the first stage of forming a porous structure and followed by the collapse of the newly born porous structure into isolated nanoparticles, which can be readily generalized to prepare a series of pomegranate-like TMP@N-Cs, such as Co2P@N-C, Fe2P@N-C, Ni12P5@N-C, and CoNiP@N-C. The designed nanoarchitecture is useful for LIBs, wherein N-C layer can facilitate faster ion/electron transfer and internal void space accommodate volume expansion. Co2P@N-C sample as a model electrode material presents excellent cyclability with high capacity. This work not only provides a new


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synthetic protocol in achieving controllable pomegranate-like TMP@N-Cs, but also highlights their advantages in LIBs.

ASSOCIATED CONTENT

Supporting Information.

Experimental section, XRD patterns, TEM images, SEM images, TGA curve, XPS spectra, FTIR spectra, electrochemical performance.

The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION

Corresponding Author *[email protected] (G. Yu) *[email protected] (S. Dou) *[email protected] (Z. Bai) ACKNOWLEDGMENT


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G.Y. acknowledges the financial support from the UT Austin-Portuguese Foundation of Science & Technology (FCT) Collaborative programme, and the Welch Foundation award (F-1861). S.D. acknowledges the financial support from the Australian Research Council (ARC)

through

Discovery

Projects

(DP160102627),

and

a

Linkage

Project

(LP160100273). Z.B. acknowledges the financial support from Research Project from Shanxi Scholarship Council of China (No. 2015-034), the Natural Science Foundation of Shanxi Province of China (201701D221077). The authors thank Dr. Tania Silver for helpful discussions.

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General Synthetic Strategy for Pomegranate-like Transition-Metal

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