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Ultrafine Cobalt Phosphide Nanoparticles Embedded in Nitrogen-Doped Carbon Matrix as a Superior Anode Material for Lithium Ion Batteries Kunjie Zhu, Jun Liu,* Site Li, Liangliang Liu, Linyu Yang, Sailin Liu, Hao Wang, and Tian Xie low energy density and power density. The increasing requests for better LIBs require continuous innovation, namely better power capacity, higher energy density, longer cycle life, more environmental friendliness, and lower cost.[3–5] Countless efforts have been attempted to explore new electrode materials to satisfy these requirements for LIBs. Among all potential anode materials, cobalt-based materials investigated as attractive alternatives anode materials for LIBs have aroused widespread concern due to their higher theoretical capacities than that of the current commercial graphite electrode.[6–9] However, poor electrical conductivity and unavoidable volume expansion during the lithiation/delithiation process restrict their practical application. Numerous explorations have been made to overcome these shortages. Fabricating nanoscale materials by rational design is one effective approach, greatly reducing the diffusion distance of lithium ions and increasing the contact area between the active materials and the electrolyte.[10] Wu et al. synthesized Co3O4 nanoparticles with the size of 10–30 nm, showing a high reversible capacity of 934 mAh g−1 at a current density of 50 mA g−1 after 30 cycles.[11] Jiang et al. synthesized CoO porous nanowire arrays, exhibiting superior rate capability of 464, 312, and 150 mAh g−1 at the current densities of 2, 4, and 6 C (1 C = 716 mA g−1), respectively.[12] Kong et al. synthesized ultrathin CoS nanosheets and achieved excellent electrochemical performance.[13] Doping conductive carbon on electrode materials is another effective strategy, significantly buffering volume expansion during lithiation and improving electrical conductivity. For example, Du et al. fabricated Co3S4/ graphene sheet composite, obtaining excellent electrochemical performance of 672 mAh g−1 after 200 cycles at 500 mA g−1 for LIBs.[14] Yuan et al. used sugar as carbon source to synthesize porous CoO/C composite, obtaining superior capability of 510 mAh g−1 after 50 cycles at 100 mA g−1.[15] Furthermore, to buffer volume expansion, researchers have fabricated hollow structures, such as hollow sphere and multishelled hollow structures. These structures are conducive to increase the contact area between active materials and the electrolyte, as well as providing interior expansion space for active materials. Chen et al. had reported that hierarchical hollow CoS2@C materials exhibited a capacity of 720 mAh g−1 after 200 cycles at
Ultrafine CoP nanoparticles embedded in nitrogen-doped carbon matrix derived from zeolitic imidazolate framework 67 (ZIF-67) template are first obtained. The synthesis strategy only involves a facile method in which ZIF-67 precursor is phosphided under argon atmosphere. Such novel nanostructure consists of ultrafine CoP nanoparticles and N-doped carbon matrix which greatly shorten the transport length of lithium ions, effectively buffer the volume expansion during the lithiation/delithiation process, and improve the electrical conductivity. Owing to their unique architecture characteristics, the active material exhibits a superior specific capacity of 522.6 mAh g−1 after 750 cycles at a current density of 200 mA g−1 and outstanding cycling stability up to 2000 cycles at a high current density of 500 mA g−1. In addition, the active material exhibits superior rate capability.
1. Introduction With the increasing decrease of fossil energy and the intensification of environmental pollution caused by traditional nonrenewable energy, renewable energy has attracted much attention recently.[1] Since commercially applied, lithium ion batteries (LIBs) play a key role in energy storage system. LIBs possess an enormous market in the portable electronic devices and great potential for application in the electric vehicle.[2] However, the commercial graphite electrode delivers a low theoretical specific capacity (372 mAh g−1), resulting in its relatively Dr. K. J. Zhu, Dr. S. T. Li, Dr. S. L. Liu, Dr. H. Wang, Dr. T. Xie School of Materials Science and Engineering Central South University Changsha, Hunan 410083, China Prof. J. Liu School of Materials Science and Engineering Central South University Changsha, Hunan 410083, China E-mail:
[email protected] Dr. L. L. Liu School of Advanced Material Peking University Shenzhen Graduate School Shenzhen, Guangdong 518055, China Dr. L. Y. Yang School of Physics and Technology Xin Jiang University Urumqi, Xinjiang 83000, China
DOI: 10.1002/admi.201700377
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200 mA g−1.[16] Wang et al. designed hollow multishelled Co3O4 materials and the electrode possesses excellent performance for LIBs.[17] Thus, the electrochemical performance of cobalt-based materials for LIBs can be improved by (1) developing the cobaltbased materials into nanoscale; (2) coating or doping with conductive carbon; (3) designing and fabricating hollow sphere or multishelled hollow structures. Recently, metal–organic Frameworks (MOFs) and their derivatives are of great interest to researchers for their superior capability when used as LIB electrodes. MOFs possess a series of advantages, such as large Figure 1. Schematic of the three-step formation process to synthesis CoP/NC. surface area, high porosity, 3D carbon skeleton, and controllable morphology.[18,19] Therefore, the unique previous reports with no detected impurity.[24] The morphologarchitecture can effectively enhance electrochemical perforical characterizations of ZIF-67 are shown in Figure S2 (Supmance by shortening the diffusion distance of ions, increasing porting Information). Figure S2a (Supporting Information) the contact area between active materials and the electrolyte, clearly shows that the precursor displays homogenous polyheimproving the electrical conductivity, and providing interior dral with a size of about 1.5 µm. The single polyhedral magnifiexpansion space for active materials. For instance, Wang et al. cation scanning electron microscope (SEM) image (Figure S2b, used zeolitic imidazolate framework 67 (ZIF-67) as precursor Supporting Information) reveals that it has a smooth surface to synthesize CoO as a high-performance anode material for without visible nanoparticles. The ZIF-67 rhombic dodecaLIBs.[20] Similarly, cobalt-based sulfide and selenide have been hedral is solid as intuitively confirmed by transmission electron microscope (TEM) observation (Figure S3, Supporting prepared and applied in LIBs by employing ZIF-67 as temInformation). plate.[21–23] Interestingly, although cobalt-based phosphide has As depicted in Figure 2, the XRD patterns of Co/NC nanobeen obtained, to our best knowledge, there is no report about particles and CoP/NC nanoparticles can be indexed to the cobalt-based phosphides (CoP) derived from ZIF-67 template standard card JCPDS# 15-0806 and JCPDS# 29-0497, respecused as anode material for LIBs. Hence, it is meaningful of tively. Through additional comparisons with the pure phases, being explored. the broad diffraction peaks at around 26° existing in their XRD Herein, CoP embedded in nitrogen-doped carbon (CoP/NC) patterns are in accordance with amorphous carbon.[25,26] That is derived from ZIF-67 template was first obtained and used as LIB anode material. The synthesis strategy involves a novel to say, 2-methylimidazole was carbonized during the thermal method in which ZIF-67 is fabricated as precursor and then treatment in argon atmosphere at 600 °C. Furthermore, the phosphided by sodium hypophosphite under argon atmosphere. The unique structure consists of ultrasmall CoP nanoparticles and N-doped 3D carbon matrix, greatly shortening the transport length of lithium ions and effectively buffering volume expansion during the lithiation/delithiation process and improving electrical conductivity. Owing to their unique architecture characteristics, the active material exhibits a high specific capacity, superior rate capability, and excellent cycling stability.
2. Results and Discussion Figure 1 shows the schematic illustration of the three-step formation process to synthesis CoP/NC. During the process, ZIF-67 template was first synthesized with a typical method. Subsequently, the precursor was heated to 600 °C under argon atmosphere to obtain N-doped carbon Co (Co/NC) nanoparticles. In the final process, CoP/NC nanoparticles were synthesized in a low-temperature phosphidation process where Co/ NC nanoparticles were phosphided by sodium hypophosphite under argon atmosphere. Figure S1 (Supporting Information) clearly shows the X-ray diffraction (XRD) pattern of ZIF-67 crystals, matching well with
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Figure 2. X-ray diffraction (XRD) patterns of a) Co/NC, b) CoP/NC.
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Figure 3. XPS spectra for the CoP/NC: a) survey spectrum, and the high-resolution spectra of b) Co 2p, c) P 2p, d) C 1s, and e) N 1s.
broad peak at around 26° in Figure 2b is representative evidence that the amorphous carbon was present in the CoP sample. The diffraction peaks located at 44.2°, 51.5°, and 75.8° in Figure 2a can be assigned to the (111), (200), and (220) planes of metallic Co (JCPDS# 15-0806). Similarly, The diffractions located at 31.6°, 36.3°, 46.2°, 48.1°, and 56.7° can be identified as the (011), (111), (112), (211), and (301) planes of CoP (JCPDS# 29-0497), respectively.[27,28] Additionally, no diffraction peaks of any Co phases can be observed in the XRD patterns of CoP sample, indicating the complete transformation of the Co/NC sample to the CoP/NC sample during the thermal treatment and the high purity of the CoP/NC sample. For further determining the valence state of each element, X-ray photoelectron spectroscopy (XPS) test was conducted on the CoP/NC sample as exhibited in Figure 3. The peaks of Co 2p, P 2p, C 1s, and N 1s can be clearly observed from the survey spectrum in Figure 3a. From Figure 3b, the peaks located at 778.8 and 782.3 eV are assigned to Co 2p3/2. In addition, the peaks located at 786.8 and 803.2 eV are satellite peaks and the peak located at 798.3 eV is assigned to Co 2p1/2.[29,30] One main peak located at 133.7 eV can be seen in Figure 3c, which corresponds to P 2p species in CoP.[31] From Figure 3d, the spectra of C 1s reveals two peaks at 284.3 and 285.7 eV.[32,33] Furthermore, four spectral lines can be observed from in the high-resolution XPS spectra of N 1s in Figure 3e, namely, pyrrolic-N (397.8 and 400.1 eV), Co–N (399.1 eV), and graphitic-N (401.8 eV).[29,34,35] Similar results were reported by Wang et al. and Zhang et al.[36,37]
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The morphology characterizations and structure details of Co/NC sample are shown in Figure 4. Uniform rhombic dodecahedral and well distributed morphology of Co/NC sample can be clearly observed from Figure 4a, which were well consistent with the structure and morphology of the ZIF-67 template (Figure S2a, Supporting Information). Particularly, from the high-magnification SEM image (Figure 4b), the innerinvaginated surface of polyhedral is observed. Surface variation was the result of the organic carbon pyrolysis during the thermal treatment. Interestingly, the stable 3D carbon skeleton was formed during the process. TEM was used to investigate the detailed structure of the Co/NC sample. As shown in Figure 4c, the microscopic morphology of the Co/NC sample is an approximate regular hexagon, in agreement with the ZIF-67 template. The difference is Co/NC nanoparticles are clearly visible at the edge. The high resolution TEM (HRTEM) image as shown in Figure 4d reveals that the Co/NC nanoparticles are surrounded by amorphous carbon, which is well consistent with our design strategy (Figure 1b). Interlayer distances of around 0.205 and 0.182 nm observed in magnified HRTEM image (Figure 4e) are consistent with the (111) and (200) planes of metallic Co, in agreement with the XRD results (Figure 2). Similarly, SEM and TEM were used to further investigate the morphology and structure details of the CoP/NC sample. Obviously, as can be seen from Figure 5a–c, the morphology of the CoP/NC sample is similar to the Co/NC sample. More specifically, the inner-invaginated surface of rhombic dodecahedral with a size of around 1.5 µm can be observed. It is worth to
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Figure 4. a,b) SEM images of Co/NC. c–e) TEM, HRTEM, and magnified HRTEM images of Co/NC.
mention that the surface roughness still remained unchanged after the second thermal treatment due to its stable 3D carbon skeleton. Additionally, the corresponding elemental mapping of Co, P, C, and N elements are displayed in Figure 5e–h, respectively, clearly indicating the uniform distribution of each element. Figure 5i shows the TEM images of the CoP/NC sample. Similar to the Co/NC sample, the CoP/NC nanoparticles are clearly visible at the edge surrounded by N-doped carbon. Obviously the HRTEM results in Figure 5j implied the design purpose that the CoP nanoparticles are embedded in the 3D conductive carbon framework (Figure 1c) was reached. Figure 5k shows the magnified HRTEM image of the CoP/NC sample. The lattice spacing of 0.183 nm is corresponding to the (211) plane of the CoP/NC sample, consistent with the XRD results in Figure 3. Energy dispersive X-ray spectroscopy analysis from Figure S4 (Supporting Information) indicates that the content of the Co element in CoP/NC is ≈44.96%, in agreement with the thermogravimetric result (Figure S5, Supporting Information). To investigate the fundamental electrochemical behaviors of the CoP/NC sample for LIBs, electrochemical characterizations have been measured in half-cell batteries with Li metal as the counter electrode. Figure 6 shows the cyclic voltammetry (CV) curves of the CoP/NC electrode at a scan rate of 0.05 mV s−1 in the voltage range of 0.01–3 V. As depicted in Figure 6, the reduction peak appearing at 1.0 V in the first discharge cycle is related to the irreversible reaction of CoP + 3Li + + 3e − → Co + Li 3P
(1) +
corresponding to the intercalation of Li into the active material.[38,39] The weak peak located at around 0.6 V subsequently is
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assigned to the further Li+ ions insertion to form LiP and Co, namely the corresponding reaction is CoP + Li + + e − → LiP + Co
(2)
The following broad peak located at around 0.1 V is attributed to the inevitable formation of a stable solid electrolyte interface (SEI) layer on the surface of the CoP/NC electrode. In the first charge process, two obvious anode peaks located at 1.1 and 1.3 V can be observed, which are ascribed to the decomposition of Li3P, namely Li 3P → LiP + 2Li + + 2e −
(3)
For the following cycles, the redox peaks are almost identical, implying the highly reversible lithium ion insertion and extraction processes in the CoP/NC electrode. The CV results indicated that the CoP/NC sample possesses an excellent reversibility of the electrochemical reaction. Thus, it can be concluded that the reversible mechanism of lithium storage in CoP/NC electrode is Li 3P ←→ LiP + 2Li + + 2e −
(4)
Figure 7a illustrates the electrochemical cycling performance of CoP/NC electrode used as LIB anode material at a current density of 200 mA g−1 in the voltage range of 0.01–3.0 V versus Li/Li+. The initial discharge specific capacity and charge specific capacity of the active material can reach 1045.4 and 648.6 mAh g−1, respectively, with a Coulombic
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Figure 5. a–c) SEM images of CoP/NC. d–h) SEM image and the corresponding elemental mapping of Co, P, C, and N in the N-doped 3D carbon skeleton CoP nanoparticles. i–k) TEM, HRTEM, and magnified HRTEM images of CoP/NC.
efficiency of 61.5%. It should be noted that the irreversible capacity mainly resulted from the formation of SEI layer and the irreversible reaction (1), in agreement with the previous CV analysis, as reported in the literatures.[38,39] In the next four electrochemical charge/discharge cycles, the discharge capacity gradually tended to be stable with a slight decline and it can be intuitively observed that the CoP/NC electrode almost showed no obvious decline from 5th cycle to 750th cycle. Remarkably, the CoP/NC electrode still maintained a discharge capacity of
Figure 6. Cyclic voltammetry curves of the electrode of the CoP/NC electrode for LIBs at a scan rate of 0.05 mV s−1 from 0 to 3.1 V.
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522.6 mAh g−1 after 750 cycles, nearly more than 1.4 times the theoretical specific capacity of current commercial graphite electrode (372 mAh g−1). The discharge capacity retention was 96.7% calculated from 5th to 750th cycle. Meanwhile, the galvanostatic discharge/charge profiles of CoP/NC electrode was shown in Figure 7b. One obvious discharge platform around 1.0 V was only observed in the first curve, corresponding to the irreversible formation of Co, which is supported by the CV results. Discharge plateau at about 0.6 V as well as charge plateaus at about 1.1 and 1.3 V were consistent with the reversible lithium insertion and extraction process, respectively. Apparently, the galvanostatic charge/discharge voltage-specific capacity curves of the 10th, 30th, 50th, 100th, and 200th cycles almost overlap together, also indicating that the CoP/NC electrode delivers superior reversibility and cycling stability. To further investigate the electrochemical performance of the CoP/NC electrode, rate capacity was also detected at various current densities, namely, 50, 100, 200, 300, 500, and 1000 mA g−1. The rate capacity test results were shown in Figure 7c. Specifically, when tested at extremely large current density of 1000 mA g−1, the CoP/NC electrode can still deliver a reversible capacity of 314.7 mAh g−1. Notably, when the current density returned to 50 from 1000 mA g−1, the discharge capacity can be recovered to 569.5 mAh g−1, with a capacity retention of nearly 100% calculated from its initial cycle. Figure 7d shows the CoP/NC electrode has excellent performance of long-term cycling stability at a high current density of 500 mA g−1. After 2000 cycles, a stable discharge capacity of
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Figure 7. Electrochemical performance of the electrode of the CoP/NC electrode for LIBs. a) Cyclic performances of the CoP/NC electrode at 200 mA g−1. b) The charge/discharge voltage–specific capacity curves of the 1st, 10th, 30th, 50th, 100th, and 200th cycles. c) Rate performances of the CoP/NC electrode from 50 to 1000 mA g−1. d) Cycling performance of the electrode at a high current density 500 mA g−1.
411.5 mAh g−1 was obtained, with a slight decline in several initial cycles. The results implied that the CoP/NC electrode can stand quick charge and discharge under high current density. Compared with recently reported CoP/C anodes for LIBs, it can be seen that although the capacity is not perfect, our work is the first report of CoP nanoparticles embedded in nitrogendoped carbon matrix derived from ZIF-67 template used for LIBs. Remarkably, the CoP/NC electrode exhibits a better long-term cycling performance. Therefore, our work may shed some light on the development of advanced anode materials for LIBs. The comparisons are listed in Table S1 (Supporting Information). The excellent electrochemical performance for LIBs of the CoP/NC electrode can be resulted from a series of aspects as follows: (1) uniform CoP nanoparticles effectively reduce
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the lithium transport length during insertion/extraction. (2) 3D carbon matrix improves the electrical conductivity of the active material. (3) Nitrogen elements doping can enhance the stability of the frame and increase the probability of contact between the active materials and lithium ions.
3. Conclusions In summary, ultrafine CoP nanoparticles embedded in nitrogen-doped carbon matrix derived from ZIF-67 templates were first synthesized and used as an anode material for LIBs. The results show that the unique architecture possesses a series of advantages, such as nanostructure and 3D carbon skeleton, effectively improving the electrical conductivity of
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the active material and reducing the lithium ions diffusion distance during insertion/extraction. Thus, the CoP/NC electrode delivers excellent electrochemical performance in terms of a high reversible specific capacity, good rate capability, and superior long-term cycling stability, reversibility. The results demonstrate that the CoP/NC sample is one advanced anode material for next-generation LIBs.
4. Experimental Section
Conflict of Interest The authors declare no conflict of interest.
Keywords
Synthesis of ZIF-67 Nanocrystal Template: All chemicals and solvents were purchased from commercial sources and used without purification. In a typical synthesis process of ZIF-67,[21] 5 mmol Co(NO3)2·6H2O and 20 mmol 2-methylimidazole were dissolved into 30 mL methanol solution under vigorous stirring, respectively. Then, the above two solutions were mixed and aged for 1 d. The pink precursor was obtained by centrifugation, washed with methanol three times, and finally dried in air at 50 °C. Synthesis of Co/NC: The obtained pink precursor was put into a quartz boat and then transferred into a tube furnace. The reactor was heated to 600 °C and maintained for 2 h under the argon flow of 150 sccm, and naturally cooled to room temperature. Then, the Co/NC product was obtained. Preparation of CoP/NC: Co/NC and NaH2PO2 were put into two separate quartz boats and placed in a quartz tube reactor with NaH2PO2 at the furnace upstream side. Then, the reactor was heated to 300 °C maintained for 2 h under argon flow, and naturally cooled to ambient temperature. Finally, the CoP/NC product was obtained. Structural Characterization: The collected CoP/NC product was characterized by XRD on a Rigaku D/max 2500 XRD diffractometer (CuKα radiation, λ = 1.15178 Å). SEM was characterized using FEI Nova Nano SEM 230 scanning electron microscope. TEM (JEOLJEM-2100) examination was used to investigate the structure of CoP/NC nanostructure. XPS measurement was carried out with a Thermo Fisher K-Alpha 1063, UK spectrophotometer. Thermogravimetric analyses (Q500) were carried out in the temperature range of 35–1000 °C at a rate of 10 °C min−1 under air atmosphere. Electrochemical Measurements: The anode electrodes were prepared by mixing the CoP/NC sample, conductive agent (Super P), and binder (Polyacrylic acid) together in a weight ratio of 7:2:1 in a mortar. Then, the mixture was dissolved in N-methyl-2-pyrrolidinone and magnetically stirred for 24 h. Subsequently, the obtained slurry was coated on Cu foils, and then dried at 100 °C in vacuum oven for 12 h to evaporate the dispersant. The coin cells were laboratory-assembled by a CR2016 press in a glove box (Mbraun, Germany) filled with ultrahigh purity argon. As for LIBs, polyethylene membrane was used as separator and lithium foil was used as the anode. The electrolyte used for LIBs was 1 m LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 in volume). The electrochemical measurements were conducted by a Land Battery Tester (Land CT 2001A, Wuhan, China) at room temperature. CV measurements were carried out on a Chi604e electrochemical workstation at a scan rate of 0.05 mV s−1.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Natural Science Foundation of China (grant Nos. 51472271, 61376018, and 51174233), the Project
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of Innovation-driven Plan in Central South University (2016CX002), and the National Basic Research Program of China (973 Program) grant No. 2013CB932901.
cobalt phosphide, lithium ions frameworks, nitrogen-doped carbon
batteries,
metal–organic
Received: March 29, 2017 Revised: June 20, 2017 Published online:
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