Black Phosphorus and its Composite for Lithium Rechargeable Batteries**

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DOI: 10.1002/adma.200602592

Black Phosphorus and its Composite for Lithium Rechargeable Batteries**

By Cheol-Min Park and Hun-Joon Sohn*

Lithium-ion rechargeable batteries are used as portable power sources for a wide variety of electronic devices, such as cellular phones, notebook computers, and camcorders. Inten- sive research efforts have been made over the past decade to increase the gravimetric and volumetric energy density of lithium ion batteries. At present, graphite (372 mA h g^–1) is used as an anode material for lithium ion batteries, but higher capacity alternatives are being actively pursued. Among the many possible alternatives, a lot of work has been devoted to Sn-based oxide,^[1–3] Si-based composite,^[4,5] transition metal oxide,^[6,7] metal nitride^[8,9]and metal phosphide^[10–14]systems, due to their ability to react reversibly with large amounts of Li per formula unit. Although alloy-based systems have a higher energy density, they suffer from poor capacity reten- tion, since a large volume change occurs during charge/discharge.

Among these alternatives, a concept based on the quasi-to- potactic intercalation mechanism was proposed, in which lithium is inserted into monoclinic binary MnP4to form the cubic ternary Li7MnP4phase.^[10]Since then, Li insertion/extraction in transition metal phosphides has been investigated as a possible candidate for the anode material in lithium ion batteries.^[10–14] In these systems, commercial red P and transition metals were used to synthesize metal phosphides, but the energy density is reduced due to the heavy transition metals employed. If phosphorus were used for electrode materials, it would have a good energy density, but little is known about its electrochemical behavior, since commercial red P has an amorphous structure with a poor bulk conductivity and poor cyclability.

Phosphorus, an element of the fifth group in the periodic table, has three main allotropes: white, red, and black.^[15]

Among these modifications of allotropes,^[16–19] black phosphorus is thermodynamically the most stable, insoluble in most solvents, practically non-flammable, and chemically the

least reactive form, and exists in three known crystalline modifications (orthorhombic, rhombohedral, and simple cubic), as well as in an amorphous form.^[20–23]Since orthorhombic black phosphorus was obtained from white phosphorus at 200 C and 1.2 GPa,^[24] many studies designed to synthesize black phosphorus have been reported.^[25–27]However, the basic concept of a high temperature and high pressure being required has not been changed, and black phosphorus still remains dif- ficult to synthesize, and has the lowest commercial value of the three forms.

Considering that orthorhombic black phosphorus exhibits a layer structure^[20,21] similar to that of graphite, which is cur- rently used as an anode material for Li ion batteries, we developed a simple method of transforming commercially available amorphous red phosphorus into orthorhombic black phosphorus using a high energy mechanical milling (HEMM) technique at ambient temperature and pressure. It is known that the temperature during HEMM can rise above 200 C and the pressure generated can be of the order of 6 GPa.^[28,29]These conditions should be sufficient to transform red P into its high-pressure allotrope, the black phosphorus phase, at ambient temperature and pressure.

Figure 1a shows the color photo image, XRD pattern, and TEM electron diffraction pattern showing a diffuse ring of red P, which confirms the amorphous nature of the red P. The sample prepared by HEMM corresponds to orthorhombic black P according to the XRD data and color photo image (Fig. 1b), and was also identified by high resolution TEM electron diffraction, and by its lattice spacing.

Figure 2a and 2b^[14b]show the voltage profiles of red P and black P, respectively. Their electrochemical behaviors are very different from each other. The discharge and charge capacities of red P are 1692 and 67 mA h g^–1, respectively, and it cannot be used as an anode material since its charge capacity is negli- gible. Although black P shows an increased charge capacity of 1279 mA h g^–1, the first cycle efficiency is only 57 %. The electrochemical performance of Si as an anode material for Li ion batteries can be much improved using Si–carbon composites.^[4,5]Black P also has a low electronic conductivity inher- ited from its characteristic as a semiconductor. The electrochemical behaviors of the black P-carbon composite during the discharge/charge reaction with Li were excellent compared with the above two cases, as shown in Figure 2c. The first discharge and charge capacities are 2010 and 1814 mA h g^–1, respectively, and the first cycle efficiency is about 90 %, which is one of the highest reported. The good coulombic efficiency of the black P–carbon composite for the

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[*] Prof. H.-J. Sohn, C.-M. Park

School of Materials Science and Engineering Research Center for Energy Conversion and Storage Seoul National University, Seoul 151-742 (Korea) E-mail: hjsohn@snu.ac.kr

[**] The authors thank Dr. J.-H. Jung for the valuable discussion on the crystal structure of the LiP phase. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Research Center for Energy Conversion and Storage at Seoul Na- tional University (Grant No. R11-2002-102-02001-0).

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first cycle shows that the electrochemical reaction between black P (initial phase) and Li3P (final phase) is almost revers- ible, considering the irreversible capacity of the carbon present, which would be approximately 150 mA h g^–1.

The differential capacity plot (Fig. 3a) shows several peaks during discharge along with one very large peak during

When the potential is lowered to 0.78 from 2 V, several small peaks can be observed in this region, believed to be due to various LixP phases,^[30]such as LiP7,^[30a] LiP5[30b]

and Li3P7,^[30c] which are formed in sequence, although no characteristic X-ray peaks corresponding to any of these phases can be identified. Auger electron spectroscopy (AES) analyses were conducted to confirm the composition, but the Auger spectra were also too weak to resolve the above mentioned phases.

When the potential reaches 0.78 V (Fig. 3b(2)), corresponding to one mole of Li per mole of P, no peaks can be observed. The TEM image with the lattice spacing shown in Figure 3c indicates that the LiP phase is formed at this potential. At 0.63 V (Fig. 3b(3)), corresponding to two moles of Li per mole of P, a new peak (denoted as *) appears. AES analyses were also performed to confirm the composition of the unknown phase, and the molar ratio between Li and P was found to be about 1.93:1. As the potential is lowered to 0 V (Fig. 3b(4)), the XRD pattern shows the presence of Li3P phase only. Li3P has a hexagonal layered structure (S.G.

P63/mmc, a = 4.394 and c = 7.581 ) consisting of alternate Li and Li2P layers.^[31] So, the X-ray diffraction peak at 0.63 V can be assumed to be the metastable Li2P phase, although this phase has never been reported before.

During charging, the Li3P phase disappeared and the Li2P phase reappeared again at 1.12 V (Fig. 3b(5)). In the fully charged state (2 V, Fig. 3b(6)), no apparent diffraction peaks can be observed. Based on the above argument, the reactions involved during the first discharge would be as follows:

Black PfiLixPfiLiPfiLi2PfiLi3P (1)

Comparisons of the cycle performances were made for the red P (voltage range: 0.0–2.0 V) and black P–carbon composite (voltage range: 0.0–2.0 V and 0.78–2.0 V) electrodes. As shown in Figure 4, the cycle performance of the red P is very poor. When the black P–carbon composite is cycled within the voltage range between 0.0 and 2.0 V, its capacity decreases drastically. This is caused by the mechanical cracking and crumbling due to the large volume change originating from the formation of the Li3P phase. Surprisingly, in the case of the voltage range between 0.78 and 2.0 V (corresponding to the LiP and black P phase, respectively), the test electrode shows excellent cycle performance with a large capacity of more than 600 mA h g^–1(or 1600 mA h cm^–3) over 100 cycles.

To compare the structures of black P and LiP, the crystalline structure diagrams of these two phases are shown in Figure 5.

Orthorhombic black P has a layered structure consisting of puckered layers, as shown in Figure 5a,^[20,21,32] where the P atoms are located close together along the a-axis and form an accordion structure along the c-axis, in which each layer is

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2 θθ (degrees) 2 θ (degrees)

Figure 1. X-ray diffraction patterns with color photo images and TEM images with corresponding lattice spacing: a) Red phosphorus; b) black phosphorus.

a)

b)

c)

Figure 2. Electrochemical behaviors of various types of phosphorus:

a) Voltage profile of red phosphorus for the first cycle; b) Voltage profile of black phosphorus for the first cycle; c) Voltage profile of black phosphorus–carbon composite for the first cycle.

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weakly coupled by van der Waals forces. In the monoclinic LiP (Fig. 5b),^[33] the P atoms are connected in chains along the b-axis, and cleaved along the a-axis by Li atoms during discharge. In each unit cell, the length of the b-axis of orthorhombic black P^[32]and that of the c-axis of LiP^[33]are almost the same. This crystalline structural relationship and relatively low volume expansion compared with that between black P and Li3P can probably be attributed to the good cyclability between black P and LiP during cycling. By controlling the voltage range, the safety hazard problems associated with the formation of the Li dendrite when the potential is lowered to 0.0 V during discharge can be avoided.

In conclusion, we have developed a simple method of transforming amorphous red phosphorus into orthorhombic black phosphorus, using a high energy mechanical milling technique at ambient temperature and pressure. Further modification with carbon produced a black P–carbon composite which, when applied as an anode material for Li rechargeable batteries, showed enhanced electrochemical discharge/charge behavior with a high coulombic efficiency during the first cycle.

Moreover, excellent cyclic performance can be obtained by carefully controlling the voltage range, which also prohibits the formation of metallic Li. The large specific capacity and good cyclability of the black P–carbon composite electrode makes it a new alternative anode material for Li-ion batteries.

We anticipate that this simple transformation method from red P to black P will allow for many other applications of phosphorus in the electronics industry, due to the semiconducting nature of black P with its uncommon layered structure.

Experimental

Preparation of Black P and Composite: Black P (average size: 3.3 lm) powder was prepared by means of high energy mechanical milling technique at ambient temperature and pressure. Red P powder (High Purity Chemicals,

>99 %, average size: 15 lm) and stainless steel balls (di- ameter: 3/8 and 3/16 in.) were put into a hardened steel vial having a capacity of 80 cm³with a ball to powder ratio of 20:1, and the HEMM process was conducted under an Ar atmosphere for 54 h. We employed the same milling technique for 12 h. to produce black P–carbon (Super P) composites, as described above. Preliminary studies showed that the optimum composition was 70 wt % black P to 30 wt % carbon.

Materials Characterization: The red P and black P sam- ples were identified by color photography, X-ray diffraction (XRD, Rigaku, D-MAX2500-PC) and high resolution transmission electron microscopy (HRTEM, JEOL 3000F) operating at 300 kV. For the TEM observation, a dilute suspension was dropped onto a carbon-coated TEM grid, and dried. The lithiated electrode materials were observed by ex situ XRD, Auger Electron Spectroscopy (AES, spot size: 10–100 nm, Perkin-Elmer, Model 660) and HRTEM. Ex situ XRD methods were used to observe the structural changes occurring in the active material.

The electrodes were detached from the cell and coated with Kapton tape as a protective film. The compositions of phases formed during Li insertion were analyzed using AES. After washing the electrode with diethyl carbonate

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Figure 3. The black phosphorus–carbon composite for the first cycle:

a) Differential capacity plot; b) X-ray diffraction patterns (numbers corre- spond to the voltage point indicated in Fig. 3a); c) TEM image and corresponding lattice spacing of LiP phase formed at 0.78 V during the first discharge.

Figure 4. Comparison of cycle performances for red phosphorus and black phosphorus–carbon composite. (Inset graphs: Voltage profiles of black phosphorus–carbon composite cycled between 0.78 and 2 V for the 1st, 5th, 50th, and 100th cycles, respectively, showing the same behaviors during discharge and charge, except in the 1st cycle).

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(DEC, Merck) solvent, a glove bag (Aldrich) was attached to the mouth of the preparation chamber in the AES equipment to prevent the electrodes from contamination by air and moisture, and high purity Ar gas was purged prior to introduction into a ultra-high vacuum chamber. Before measurements, Ar ion sputtering was performed to remove any oxidized surface layer. The phase not seen by XRD was also analyzed by means of HRTEM. The lithiated electrode sample scratched from the Cu substrate was introduced into a glass vial containing DEC solution. After ultrasonic treatments, a droplet of DEC containing the dispersed lithiated black P–carbon composite particles was placed on a Cu grid coated with a carbon film. The Cu grid was sealed in a glass vial for the TEM analyses in an Ar-filled glove box.

Electrochemical Measurements: For the electrochemical evaluation of the black P, electrodes were prepared by coating slurries containing the active material (70 wt %), carbon black (Super P, 15 wt %) as a conductor, and polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone (NMP) as a binder (15 wt %) on copper foil substrates and pressed and dried at 120 C for four hours under a vacuum (electrode; thickness: ca. 0.045 mm, area: 0.79 cm², weight of active material: ca. 2 mg). Laboratory-made coin-type electrochemical cells were assembled in an Ar-filled glove box using Celgard 2400 as a separator, Li foil as the counter and reference electrodes, and 1MLiPF₆in ethyl- ene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume, Sam-

extracted from the working electrode, while, during discharge, Li was inserted into the electrode.

Received: November 15, 2006 Revised: March 6, 2007 Published online: j

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b)

Figure 5. Ball-stick models of crystal structures of orthorhombic black phosphorus and monoclinic LiP: a) Orthorhombic black phosphorus phase; b) Monoclinic LiP phase. Regions enclosed by solid and dot lines show the same regions between black phosphorus and LiP phase, respectively.

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Lithium-Ion Batteries

C.-M. Park, H.-J. Sohn* ... j – j Black Phosphorus and its Composite for Lithium Rechargeable Batteries

a) b)

2θθ (degrees) 2θ (degrees)

The transformation of amorphous red phosphorus into orthorhombic black phosphorus is demonstrated by a simple method. The method uses a high-energy mechanical milling technique at ambient temperature and pressure.

Modification with carbon produces a composite that shows enhanced electrochemical discharge/charge behavior when used as an anode material for lithium-ion rechargeable batteries.