Article pubs.acs.org/JPCC
Electrochemical Activity of Black Phosphorus as an Anode Material for Lithium-Ion Batteries Li-Qun Sun,†,‡ Ming-Juan Li,†,‡ Kai Sun,†,‡ Shi-Hua Yu,†,‡ Rong-Shun Wang,*,†,‡ and Hai-Ming Xie*,†,‡ †
Institute of Functional Materials, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, P.R. China LIB Engineering Laboratory, Materials Science and Technology Center, Changchun, Jilin 130024, P.R. China
‡
ABSTRACT: Black phosphorus (black P), which is a promising candidate as an anode material for lithium-ion batteries, was synthesized by a high-pressure and high-temperature (HPHT) method from white and red phosphorus. The study revealed the electrochemical activity of pure black P under different pressures and temperatures systematically. The sample shows higher crystallinity and purity by the HPHT method. Lithium-ion batteries containing black phosphorus as anode materials exhibited a high specific capacity and excellent cycling performance. Black phosphorus obtained from white phosphorus exhibited the highest first discharge and charge capacities of 2505 and 1354 mAh·g−1 at 4 GPa and 400 °C and that obtained from red phosphorus exhibited the highest first discharge and charge capacities of 2649 and 1425 mAh·g−1 at 4.5 GPa and 800 °C. Black P was characterized by X-ray diffraction, Raman microscopy, scanning electron microscopy, and high-resolution transmission electron microscopy.
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milling (HEMM) method.20 Their sample showed a high charge capacity of 1279 mAh·g−1, but this decreased to about 220 mAh·g−1 after 30 cycles. Although they synthesized black P at ambient pressure and temperature, the conditions actually can reach pressures of 6 GPa and temperatures of 200 °C in the reaction vessel using HEMM technique. Thus, high pressures and temperatures are crucial for obtaining black P. However, it is difficult to control reaction conditions accurately using HEMM. Therefore, a detailed systematic study is required to determine if black P obtained under different pressures and temperatures show varying electrochemical activity, and which conditions produce the best black P for use as an anode material in Li-ion batteries. Here, we synthesized black P by a high-pressure and high-temperature (HPHT) method using white and red P separately as starting materials. The electrochemical activities of the samples were investigated.
INTRODUCTION Rechargeable lithium-ion (Li-ion) batteries with high energy and power density are urgently needed to meet the demands of portable electronic devices, and electrical and hybrid vehicles. However, the development of Li-ion batteries is limited by the low theoretical capacity (372 mAh·g−1) of commercial anode materials such as graphite. Therefore, great effort is being expended on finding alternative anode materials. Many alternatives with higher capacity have been studied recently, and among them transition-metal phosphides are promising anode materials because of their high gravimetric, and low polarization and volumetric capacities.1−6 However, the energy densities of the anodes were low because of the heavy transition metals employed. Direct use of phosphorus as an anode material for lithium-ion batteries was considered as an alternative to improve energy density. Like carbon, phosphorus exists in a number of allotropic forms. Black phosphorus (black P) is the most stable form and is very like graphite, with a black, flaky appearance and a similar layered structure. Black P has a similar structure to graphite with larger interlayer spacing, so it is believed to possess similar lithium-storage properties to graphite. Black P was first produced by Bridgeman in 1914 under high pressure (1.2 GPa) at 200 °C using white phosphorus as the starting material.7 A number of its physical properties8−19 were determined by Bridgeman and other researchers but its electrochemical performance was not investigated because the field of lithium-ion batteries was unknown at that time. Park and Sohn first used black P as an anode material for lithium-ion batteries in 2007 by a high-energy mechanical © 2012 American Chemical Society
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EXPERIMENTAL SECTION Orthorhombic black P was prepared by a HTHP method in a cubic anvil high-pressure apparatus (SPD-6_1200) with a cubic sample chamber (23 mm edges) under pressures of 2−5.0 GPa and temperatures of 200−800 °C for 15 min. A block of white P and red P powder were shaped into cylindrical capsules about 3 mm thick and 10 mm in diameter in a chamber made of sintered boron nitride. The capsule was embedded in the center of a cube-shaped pressure-transmitting medium, as shown in a Received: March 8, 2012 Revised: June 14, 2012 Published: June 18, 2012 14772
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Figure 1. Schematic diagram of the experimental setup used to produce black P.
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RESULTS AND DISCUSSION Black P possesses a puckered double-layer structure along the (0k0) plane (Figure 2a),8−10 which provides a 2D interstitial space for Li insertion/extraction (Figure 2b). It has the largest space for Li ion intercalation along the a-axis in the (020) crystal plane and the optimal space for Li ion intercalation in the (040) crystal plane along either the a-axis or c-axis as shown in the projection of the spaces between three neighboring layers on the yz and xy plane (Figure 2c). Black P can intercalate three Li ions in the fully discharged state (Li3P) and therefore possesses a high theoretical specific capacity of 2596 mAh·g−1 upon lithiation. This renders black P a potential anode material for Li-ion batteries with high gravimetric and volumetric energy density. The successful synthesis of WBP (black P obtained from white P) under different conditions was identified from its appearance, which is a metallic luster and dark gray color (Figure 3a). Despite their varying size and shape, the samples exhibit a lamellar morphology (Figure 3b−d) and elemental mapping by EDX spectroscopy (inset in Figure 3c) confirmed that the samples were pure black P. A puckered layer structure with lattice fringes is obvious in an HRTEM image of a WBP sample (Figure 3e). Two different lattice spacings of 0.524 and 0.262 nm are observed, which are consistent with the (020) and (040) faces of the orthorhombic phase of P, respectively (JCPDS 73-1358, space group Cmca(64)). A SAED pattern obtained for WBP sample contains several concentric diffraction rings and some irregular diffraction spots, indicating that the sample is polycrystalline. The XRD patterns of the WBP samples depended on the preparation conditions (Figure 4a). We first tried to synthesize black P at 2 GPa and 200 °C using white P as the starting material. However, only a broadband was present in the XRD pattern of the black product, suggesting that sample W1 was amorphous. The characteristic (020), (040), and (060) peaks of the layered structure do not appear until the temperature reaches 300 °C at 2 GPa. At 3 GPa, the peaks are present at both 200 and 300 °C. This apparent difference indicates
schematic diagram of the experimental setup (Figure 1). Pressure was applied with six tungsten carbide anvils to the cube containing the sample and a heater. Black P samples were identified by photography, and phase analyses of the samples were carried out using X-ray powder diffraction and Raman microscopy (HR-800). X-ray diffractometer (Rigaku, D-MAX 2200-PC) is equipped with Cu Kα radiation (λ = 1.5406 Å). Data were recorded over a 2θ range of 10−80 °C, with a step size of 0.04°. The lithiated electrode materials were observed by ex situ XRD. The samples for ex situ XRD were recovered by disassembling cycled batteries in an argon-filled glovebox, and the powder mixture was scraped from the copper disks and coated with Kapton tape as a protective film. The morphologies of black P were observed by scanning electron microscopy (SEM with energy-dispersive Xray (EDX) analysis; Hitachi S-3500 V). High-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) patterns were recorded using a JEOL-2100F microscope operating at 200 kV. The electronic conductivity of the samples was measured using a four-point probe meter (SDY-5, Research Institute of Semiconducting Materials of Guangzhou in China). FTIR absorption spectra were recorded with a Fourier transform interferometer (D/MAX-IIIC) over the wavenumber range 500−4000 cm−1. Electrochemical measurements were performed in CR2025type coin cells consisting of metallic lithium as the counter electrode and an electrolyte comprised of 1 M LiPF6 in a 1:1 volume fraction of ethylene carbonate (EC) and diethyl carbonate (DEC). Celgard 2400 was used as separator. The working electrode was fabricated by mixing the active material (black P, 80 wt %) with acetylene black (5 wt %) and polyvinylidene fluoride (15 wt %) in N-methyl-2-pyrrolidone. The resulting slurry was pasted onto copper foil substrates, dried at 393 K for 4 h in a vacuum oven, and then pressed under a pressure of 200 kg·cm−2. Galvanostatic cycling tests of the assembled cells were carried out on a Land battery test system between 0.0 and 2.0 V vs Li+/Li at a current density of 50 mA·g−1. 14773
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It also can be seen from Figure 4a that the crystal orientation in the sample is eliminated when the pressure is increased to 4 GPa. The intensity of the (040) peak suddenly decreases and the ratio of (040)/(111) changes from more than 1 at 3 GPa to less than 1 at 4 GPa, which is consistent with the standard XRD pattern of orthorhombic black P. The peak intensity fluctuates a little as the temperature is increased at a constant pressure of 4 GPa, and sample W6 shows a higher relative peak intensity at 4 GPa and 400 °C, indicating higher crystallinity. The diffraction patterns weakened and broadened when the pressure was increased to 4.5 GPa at 600 °C. These changes probably result from phase transformation. It has been reported by Jamieson11 and others that black P shows a structural phase transformation from the semiconducting orthorhombic (A11) phase to the semimetallic rhombohedral (A7) phase at about 5.0 GPa and ambient temperature, which is also observed in the Raman spectrum shown in Figure 4c. Five Raman active modes at 465.9, 438.9, 361.7, 227.7, and 192.4 cm−1 are observed and are assigned to the A1g, B2g, A2g , B1g, and B13g modes, respectively.22 The B1g mode is obviously weaker and almost vanishes as the pressure increases. The B13g mode also weakens under increased pressure like the B1g mode. According to the results of the atomic displacements analyzed by Sugai et al.,23 the B13g and B1g modes correspond to the displacement between the zigzag chains parallel to the c-axis and a-axis, respectively. The decrease intensity of the B13g and B1g modes corresponds to the band overlap metallization, which is saturated above 4 GPa. At 4.5 GPa, the bands showed a sudden decrease in intensity. This behavior was ascribed to the transition from orthorhombic to rhombohedral structure. Before the black P samples containing puckered layers were used as an anode material in Li-ion batteries, their electronic conductivity was investigated. Black P is a p-type narrow-gap semiconductor with a band gap of about 0.33 eV. It has been reported that pure black P has a low electronic conductivity caused by its semiconducting nature.20 Here, except for sample W1, high conductivity is observed for the samples irrespective of the reaction conditions (Figure 5a). Such high conductivity will increase the rate of electron transport in the crystals and improve the electrochemical activity of black P. However, the difference in Li insertion behavior among these samples is evident, as shown in Figure 5b. For samples W2, W3, and W4 synthesized at relatively low pressure and temperature, the discharge−charge curves showed a sloping profile rather than a flat plateau, and only a small degree of Li (0.24−0.5 mol) insertion into black P was observed even though they all exhibit high electronic conductivity. The low specific capacity of these samples might derive mainly from lithium storage on the sample surface. In contrast, for other samples synthesized at relatively high temperature and pressure, a significantly larger amount of Li (1.4−2.8 mol of Li/mol of black P) can be inserted in the first discharge process. Sample W6 shows the highest discharge capacity of 2505 mAh·g−1 and charge capacity of 1354 mAh·g−1. The improved properties result from the relatively high crystallinity and small crystallite size of the sample. A higher degree of crystallinity provided a clear pathway for Li+ ion migration, and the relatively small crystallite size could not only reduce the diffusion length for lithium insertion but also decrease the charge-transfer resistance of the electrodes. Therefore, as mentioned above, an appropriate degree of crystallinity and small crystallite size are required to improve the electrochemical activity of black P.
Figure 2. (a) The crystal structure of black P. (b) Transport channels of Li ions in black P. (c) Projections along the a-axis and c-axis of black P.
pressure is more significant to the transformation than temperature. It can be observed that the positions of the characteristic peaks of all of the products are in good agreement with the orthorhombic phase of black P (JCPDS number 731358), while the peak intensity differs from one material to another. Figure 4a shows that the intensities of the (0k0) diffraction peaks of the samples W2, W3, and W4 are unusually high compared with those of the other samples, probably owing to the grains adopting a preferred orientation.21 The sharper (0k0) peaks indicate that black P with higher crystallinity and larger particle size forms at 2−3 GPa. The size of polycrystalline particles, D, can be calculated using the Scherrer equation D = kλ/β cos θ. The parameters k, λ, β, and θ correspond to the shape factor (normally assigned a value of 0.89), the wavelength of X-ray irradiation (0.15406 nm), the half-peak width, and the Bragg angle, respectively. The degree of crystallinity was estimated from the peak areas of XRD patterns. The crystallite size and crystallinity of samples W2, W3, and W4 are all higher than those of the other samples (Figure 4b). This can result in a longer diffusion distance for Li+ in black P crystals and decrease the mobility of Li+ ions. An appropriate degree of crystallinity and small crystallite size are required to improve the electrochemical activity of black P. 14774
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Figure 3. (a) Photograph of the WBP sample. (b,c) SEM and TEM images of the WBP. (d,e) SAED pattern and HRTEM images of WBP with corresponding lattice spacing.
Figure 4. (a) Changes in the XRD patterns of WBP prepared under different conditions. (b) The crystallite size and crystallinity of WBP. (c) Typical Raman spectra of WBP synthesized under various conditions.
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Figure 5. (a) Electronic conductivity of WBP prepared under different conditions. (b) Voltage profiles of WBP on the first cycle.
Figure 6. (a) XRD patterns of RBP prepared under different conditions. (b) Comparison of the initial discharge−charge profiles of RBP prepared under different conditions.
In the case of black P prepared from red phosphorus (RBP), no preferred orientation was found at both low and high pressures (Figure 6a). The transformation from red to black P did not take place until the reaction conditions reached 3 GPa and 300 °C, at which only the (020), (040), and (111) peaks appeared. Moreover, there was a broad region at 20−30° (as shown in Figure 6a), meaning the sample R1 has low crystallinity. Other peaks appeared gradually as the pressure was increased. Both the positions and intensities of diffraction peaks were consistent with the standard spectrum of black P. Sample R6 shows the highest relative peak intensity at 4.5 GPa and 800 °C. Its high purity and crystallinity were confirmed (Figure 7). The diffraction patterns weakened and broadened when the pressure was increased to 5.0 GPa at 800 °C. According to the above analysis, the reaction conditions for WBP are milder than those of RBP because of the different bond energies in white and red P. Compared with red P, white P has a tetrahedral configuration with unstable P−P σ bonds, which lowers its bond energy. However, we preferred to use red P as a precursor because red P is more stable in air at room temperature so it is easier to handle. The SAED pattern shown as an inset in Figure 7b indicates that sample R6 has a polycrystalline structure. In the electron diffraction ring pattern, each ring represents the electron diffraction from a different lattice plane. The lattice planes were indexed by matching the calculated interplanar spacing, d, with the standard d-spacing from powder diffraction file cards. The d-spacing corresponding to a ring can be calculated from the formula Rd = Lλ, where R is the radius of a ring, d is the d-spacing to be calculated, and L
and λ are the camera length and electron wavelength, respectively. The electrochemical behavior of the RBP samples is shown in Figure 6b. As indicated from the XRD patterns, the RBP crystals grow incompletely at 3 GPa, which is unfavorable for the migration of Li ions and results in a low specific capacity. Other samples prepared at higher pressure (4−4.5 GPa) show much higher capacity and voltage platforms at 0.7−1.0 V corresponding to LiP and 0.18 V corresponding to Li3P become apparent. The specific capacity increases as both the pressure and temperature are increased. The highest discharge capacity of 2649 mAh·g−1 and charge capacity of 1425 mAh·g−1 were obtained at 4.5 GPa, which is higher than WBP, suggesting that the electrochemical activity of RBP is superior to that of WBP. Large irreversible capacities were observed for the samples in the first discharge−charge cycle. It is reasonable to consider that a passivation layer, called the solid electrolyte interphase (SEI), is formed on the electrode, as reported for graphite negative electrodes.24−26 The absorption peaks at 1795 (νCO), 1648 (νCO), 1454 (δCH2), 1317 (νCO), and 1077 cm−1 (νC−O) in Figure 8 indicate the presence of (CH2OCOOLi)2, and a characteristic peak at 872 cm−1 is attributed to Li2CO3 formed in an SEI film. In addition, at the end of the charge process, some LixP phases and Li3PO4 were also observed in the ex situ XRD pattern except for the phases of black P (Figure 9). These results indicate that the initial low Coulombic efficiency is caused by the irreversible consumption of lithium 14776
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Figure 9. Ex situ XRD patterns of black P at the end of charge.
Figure 7. (a) SEM, (b) HRTEM, (inset of a) EDX, and (inset of b) SAED images of black P synthesized by red phosphorus.
Figure 8. FTIR spectrum of the passive film formed on the electrode after 10 cycles.
Figure 10. Discharge−charge capacity profiles of (a) sample W6 and (b) sample R6 from 0 to 2 V (vs. Li+/Li). The insets show discharge− charge profiles for later cycles.
in the cell. However, after the fifth cycle, the Coulombic efficiency is greater than 95% as shown in Figure 10. It also can be seen from the cycling performance in Figure 10 that sample R6 has a specific capacity that is higher than 900 mAh·g−1 in the first 5 cycles and higher than 703 mAh·g−1 after 60 cycles (Figure 10b). The specific capacity of sample R6 is always higher than the corresponding value of sample W6, which has a specific capacity higher than 750 mAh·g−1 in the first 5 cycles and higher than 475 mAh·g−1 after 60 cycles (Figure 10a). It is supposed that the capacity decreasing is caused by the mechanical cracking and crumbling due to the large volume change originating from the formation of the Li3P phase.
To further confirm the formation of the Li3P, ex situ XRD analysis of black P was performed when using different discharge termination voltages of 0 and 0.7 V after 30 cycles, as shown in Figure 11. It can be seen that the diffraction peaks of black P nearly disappear. As the potential is lowered to 0 V, the Li3P was the major component, although various LixP phases such as LiP5, LiP, and Li3P were also observed (Figure 11a). Li3P has a hexagonal layered structure (P63/mmc) with a lithium intercalation potential of 0.18 V. When the discharge termination voltage is held at 0.7 V, the characteristic peaks of 14777
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temperature, the entire synthetic procedure is simple and rapid. The synthetic conditions can be accurately controlled by regulating the parameters of the high-pressure apparatus. The highest initial charge capacity of RBP is approximately 3.8 times higher than the theoretical specific capacity of graphite. RBP retained a highly stable capacity of 703 mAh·g−1 after 60 cycles, which makes it a promising anode material for high-energy Liion batteries. These favorable results are attributed to the high crystallinity and purity of the as-prepared black P by HPHT, which may improve the dynamic mechanism of Li-ion intercalation/deintercalation. Further investigations on black P to improve its overall electrochemical performance (capacity, cyclability, Coulombic efficiency, and voltage profile) will be carried out, including the electrode structure design, controlling cutoff potential, the electrolyte choice, the surface-coating treatment, etc. These results will be reported in a future publication.
Figure 11. X-ray diffraction patterns of the RBP (a) at the end of the 30th discharge, and (b) discharged to 0.7 V at the 30th cycle.
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Li3P are almost not present and the major component is LiP phase (Figure 11b). Further galvanostatic discharge/charge measurements were conducted for the RBP electrode. Figure 12 shows the initial
AUTHOR INFORMATION
Corresponding Author
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[email protected]. Tel.: +86-431-85099511. Fax: +86-431-85099511. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a project issued by the National Key Technologies R&D Program of China (Grant No. 2009BAG19B00) and National High Technology Research and Development Program of China (863 Program, No. SS2012AA110301).
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REFERENCES
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Figure 12. Discharge−charge curves of RBP as a function of C rate between 0 and 2 V (vs Li+/Li). The inset shows a cyclability graph at various rates.
discharge−charge profiles of the RBP electrode measured at various current rates (0.2, 0.5, 1, and 2 C) between 0 and 2 V (vs Li+/Li). Initial specific capacities of 852 and 732 mAh·g−1 were obtained at 0.2 and 0.5 C, respectively. A reversible capacity of 453 mAh·g−1 was achieved at a rate of 2 C. In the case of the pure black P sample, these results are better than those previously reported.20 The cycling performance of RBP at various rates is shown in the inset of Figure 12. Relatively good cycling performance was observed for each rate, which can be attributed to the higher crystallinity and relatively low volume expansion of the electrode.
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CONCLUSION In this work, pure black P was obtained by a HPHT method. To study the potential electrochemical activity of black P, none of the samples were modified with conductive materials, and the cutoff potential was not controlled during discharge−charge cycles to inhibit the formmation of unstable Li3P. WBP and RBP samples all exhibited much higher electrochemical activity toward Li insertion than black P samples prepared by HEMM. Although the reaction conditions involved high pressure and 14778
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(20) Park, C. M.; Sohn, H. J. Adv. Mater. 2007, 19, 2465−2468. (21) Kikegawa, T.; Iwasaki, H.; Fujimura, T.; Endo, S.; Akahama, Y.; Akai, T.; Shimomura, O.; Yagi, T.; Akimoto, S.; Shirotani, I. J. Appl. Crystallogr. 1987, 20, 406−410. (22) Akahama, Y.; Kobayashi, M.; Kawamura, H. Solid State Commun. 1997, 104 (6), 311−315. (23) Sugai, S.; Ueda, T.; Murase, K. J. Phys. Soc. Jpn. 1981, 50 (10), 3356−3361. (24) Jeong, S. K.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Power Sources 2003, 119−121, 555−560. (25) Jeong, S. K.; Inaba, M.; Mogi, R.; Iriyama, Y.; Abe, T.; Ogumi, Z. Langmuir 2001, 17 (26), 8281−8286. (26) Inaba, M.; Tomiyasu, H.; Tasaka, A.; Jeong, S. K.; Ogumi, Z. Langmuir 2004, 20 (4), 1348−1355.
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