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Bismuth phosphate: A novel cathode material based on conversion reaction for lithium-ion batteries
Journal of Alloys and Compounds 579 (2013) 18–26
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Bismuth phosphate: A novel cathode material based on conversion reaction for lithium-ion batteries Benan Hu, Xianyou Wang ⇑, Qiliang Wei, Hongbo Shu, Xiukang Yang, Yansong Bai, Hao Wu, Yunfeng Song, Li Liu Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China
a r t i c l e
i n f o
Article history: Received 18 February 2013 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 17 May 2013 Keywords: Lithium-ion batteries Cathode materials Bismuth phosphate Polymorphs Conversion reaction
a b s t r a c t Here, we provide a new scientific insight in the field of lithium-ion batteries, in which BiPO4 is initially reported as a cathode material based on conversion reaction. BiPO4 exists in three polymorphic phases, which have been synthesized using a room-temperature liquid precipitation route followed by heating treatment. The effects of phase transformation on electrochemical performances of BiPO4 have been investigated. Herein the hexagonal BiPO4 exhibits the best reversibility, rate capability and cycling stability. A conversion reaction has also been proved to occur in BiPO4 with reduction of BiPO4 into a metallic Bi0 phase and a Li3PO4 phase upon lithiation, and the reformation of BiPO4 phase during next charge process. Its theoretical specific capacity and theoretical output voltage are 264.5 mA h g1 and 3.14 V, respectively. Especially, its theoretical volumetric energy density is as high as 5253.1 Wh L1, close to twice higher than LiCoO2. Therefore, it is still worthy of further study for lithium-ion batteries. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries have drawn more attention since their commercialization in 1991. However, in the age of automotive technologies, automotive applications require at least a two-fold improvement in energy and power densities. Nowadays, there are three different Li-storage mechanisms to store capacity for electrode materials of lithium-ion batteries, namely, conventional intercalation of Li ions [1,2], alloying with Li [3,4], and conversion reactions [5–7]. Among the three Li-storage mechanisms, because the reversible conversion reactions can make use of all possible oxidation states of the metal cation thus yielding higher specific capacities for lithium-ion batteries [8], the materials accompanying with conversion reactions will be a good alternative to meet the requirements of higher energy and power densities. Since Tarascon et al. [9] firstly reported the reversible Li-storage reaction for transition-metal oxides through the heterogeneous conversion, the reversible conversion reactions have been observed in a large number of compounds, such as metal oxides [10,11], chlorides [12,13], sulfides [14–16], nitrides [17,18], fluorides [19–22], and phosphides [23,24]. The overall reaction for the conversion can be summarized as follows:
⇑ Corresponding author. Tel.: +86 731 58292060; fax: +86 731 58292061. E-mail address: [email protected] (X. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.049
þ
mLi þ me þ MXn
lithiation
! nLim=n X þ M
delithiation
ð1Þ
where M stands for a cation and X an anion [25]. However, for the practical application of the cathode materials, the active materials are required to have the output voltage of more than 2 V. Therefore, for the above compounds, only a few metal oxides [10], chlorides [12,13], sulfides [15,16], and fluorides [19–22] were studied as possible alternatives for the cathode materials of lithium-ion batteries. In recent years, a few metal oxides (CuO) [10], chlorides (CoCl2, AgCl, and CuCl2) [12,13], and sulfides (CuS and Cu2S) [15,16] have been reported as positive electrode materials, but the output voltages of metal oxides and sulfides are only about 2 V, thus further studies on metal oxides and sulfides as the cathode materials have rarely been performed. Besides, because most of metal chlorides are hygroscopic and soluble in liquid organic electrolyte, they are also difficultly served as a solid electrode for practical battery applications. Fortunately, metal fluorides, such as FeF3 [19,21,26], BiF3 [20,25], CoF2 [22,27], and NiF2 [28], have been studied as a promising new class of cathode materials for lithium ion batteries. Among these metal fluorides, FeF3 has recently become one of the most promising cathode materials because of its high capacity and excellent thermal stability. However, the poor kinetic properties of FeF3 cathode material hinder its further development. In order to overcome the above problem, morphology control and size reduction to nanoscale dimension are two effective ways. Therefore,
B. Hu et al. / Journal of Alloys and Compounds 579 (2013) 18–26
many recent studies have focused on synthesizing FeF3 cathode materials with different morphology and particle size, such as three-dimensionally ordered macroporous [26], porous nanospheres [29], highly amorphous and porous microspheres [30], worm-like mesoporous [31]. All these materials exhibited superior electrochemical performances. However, in the age of miniaturization, the portable electronic device needs to further pursue the characteristics of light quality, small size and high specific energy density. There is no denying that the cathode materials of lithium-ion batteries with higher gravimetric energy density and volumetric energy density will play a vital role. Besides, compared with the commercial cathode materials, the output voltage of the FeF3 cathode materials is not enough ideal. Especially, its volumetric energy density is much lower. Therefore, to develop new cathode materials based on conversion reaction will be a very significant work. Bismuth phosphate (BiPO4) is a typical non-transition metal phosphate, which exists in three polymorphic phases, namely hexagonal, low temperature monoclinic and high temperature monoclinic BiPO4 [32–35]. The hexagonal form is generally associated with water of hydration, namely hexagonal BiPO4 (HBP), which can be synthesized at room temperature through a wet chemical method [32]. The most stable form is the low temperature monoclinic monazite-type (LTMBP). The third phase is the high temperature monoclinic structure (HTMBP) which can be obtained via heating HBP or LTMBP at high temperature [32,36]. The crystal structure and the phase transformation of BiPO4 have been studied by many researchers [32,33,37]. Moreover, BiPO4 enables extensive applications in various fields such as separation of radioactive elements [34], microwave dielectric [37], orthophosphate ion sensor [38], and photocatalyst [39]. However, to the best of our knowledge, the study on BiPO4 as positive electrode of lithium ion battery has not been reported. For BiPO4, due to the presence 3 of the polyanion (PO3 4 ), P–O covalent bond of PO4 can stabilize 3+ Bi by the inductive effect of Bi–O–P bond. Thus, BiPO4 will have a higher theoretical output voltage (3.14 V) compared with the corresponding metal oxide (Bi2O3) (2.10 V) which has been reported as the cathode material based on conversion reaction. Thus, theoretically, to render BiPO4 as a positive electrode material has possessed the feasibility. In the present study, we provide a new scientific insight in the field of lithium-ion batteries, where a new class of BiPO4-based cathode material is reported for the first time. We explore whether the conversion reactions are also possible in such systems. Besides,
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the effects of phase transformation on electrochemical properties of BiPO4 have been also examined in detail. 2. Experimental section Hexagonal BiPO4 (HBP) was directly prepared using a chemical liquid precipitation method at room temperature. In a typical synthesis, Bi(NO3)35H2O was dissolved in 10 wt.% acetic acid solution under magnetic stirring, and equimolar (NH4)2HPO4 was dissolved in isopyknic deionized water. Subsequently, Bi(NO3)3 solution was added dropwise to (NH4)2HPO4 solution under vigorous stirring, then a white homogeneous precipitate was observed. The suspension solution was kept on stirring for 2 h. Finally, the product was filtered, sufficiently washed with deionized water and absolute ethyl alcohol, and then dried in an oven at 80 °C for 24 h to obtain HBP. Low-temperature phase (LTMBP) and high temperature phase (HTMBP) were obtained by thermal treatments over HBP at high temperatures. The HBP was sintered at 600 °C and 900 °C for 6 h with a heating rate of 5 °C min1 to obtain LTMBP and HTMBP [36,40], respectively. All samples were obtained after naturally cooling to room temperature. The structural and crystallographic analyses of the samples were performed using powder X-ray diffraction (XRD) techniques (D/max-2550 Rigaku, Japan) using Cu Ka radiation (k = 1.54178 Å) and a graphite monochromator at 40 kV, 20 mA. XRD data were collected at 4° min1 in the 2h range of 10–90°. The surface morphology of the samples was observed using the Hitachi S-3500N scanning electron microscope (SEM) and field emission scanning electron microscope (FE-SEM; LEO1525, Germany). The FT-IR measurements of the samples were carried out via a Fourier transform infrared (FT-IR) spectrometer (Perkin–Elmer Spectrum One). Selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) pictures were taken with JEOL JEM100SX electron microscope. The electrochemical measurements of the samples were carried out using cointype cells (CR2025) assembled in an argon-filled glove box (MIKROUNA 1220/750). The cathodes were made by mixing 80 wt.% active material, 6 wt.% acetylene black, 6 wt.% graphite and 8 wt.% polyvinylidene fluoride (PVDF) binder. The mass of electrode material per 1 cm2 of electrode is 3–4 mg. In all cells, lithium was served as the counter and reference electrodes, Celgard 2400 was used as separator, and the electrolyte was a 1 M LiPF6 solution in ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1, v/v). Galvanostatic discharge–charge measurements were carried out in Neware battery test system (BTS-51, Shenzhen, China) at various current densities between 1.5 and 4.5 V (vs. Li+/Li) at room temperature. The EIS experiments were conducted using a CHI 660a Electrochemical Analyzer (CH Instrument Inc., USA). The ac perturbation signal was ±5 mV, and the frequency range was from 105 to 102 Hz.
3. Results and discussion 3.1. Phase identification To distinguish the crystal structure of three polymorphs of BiPO4, the powder X-ray diffraction techniques were carried out
Fig. 1. XRD patterns of HBP, LTMBP, and HTMBP (b, d and f) and their corresponding standard patterns (a, c and e).
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Table 1 The refined lattice parameters of BiPO4 polymorphs. BiPO4
Unit cell parameters 0
a (Å A)
0
b (Å A)
0
c (Å A)
Vcell
Space group
Z
P3121 P21/n P21/m
3 4 2
q (g cm3)
0
(Å A3) HBP LTMBP HTMBP
6.9843 6.7363 4.8744
6.9843 6.9108 7.0601
6.4738 6.4490 4.6938
273.51 291.70 160.52
5.5360 6.9210 6.2859
0209). When the HBP was calcined at 900 °C for 6 h, as shown in Fig. 1f, all diffraction peaks are consistent with the theoretical diffraction peaks of HTMBP (space group: P21/m (11), PDF#43-0637). The above results of phase transformation are in good agreement with the results reported by Zhao et al. [36]. The refined lattice parameters of HBP, LTMBP, and HTMBP are listed in Table 1. These values are also in good agreement with those reported by MooneySlater [32].
3.2. Morphology characterization on the BiPO4 prepared at room temperature, 600 °C, and 900 °C, respectively. XRD patterns of HBP, LTMBP, and HTMBP and their corresponding standard patterns are shown in Fig. 1. As being seen in Fig. 1b, all diffraction peaks can be indexed to a hexagonal phase (space group: P3121, PDF#15-0766), which indicates that the asprepared BiPO4 is a hexagonal structure. And the diffraction peaks are quite sharp, indicating that HBP has high crystallinity. After the HBP was calcined at 600 °C for 6 h, in Fig. 1d, all diffraction peaks can be readily indexed to LTMBP (space group: P21/n (14), PDF#80-
Surface morphology and particle size could be observed by scanning electron microscopy (SEM) and field emission scanning electron microscope (FE-SEM). SEM images of HBP, LTMBP, and HTMBP are presented in Fig. 2. FE-SEM images of HBP and LTMBP are presented in Fig. 3. As shown in Figs. 2a and 3a, HBP shows rodlike morphology with diameter about 0.8 lm and length ranging from 1.2 to 1.6 lm. When the HBP was calcined at 600 °C, the hexagonal BiPO4 was transformed into LTMBP. As seen from Fig. 2b,
Fig. 2. SEM images of (a) HBP, (b) LTMBP, and (c) HTMBP.
Fig. 3. FE-SEM images of (a) HBP and (b) LTMBP.
B. Hu et al. / Journal of Alloys and Compounds 579 (2013) 18–26
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Fig. 4. Schematic illustration of phase transformation for BiPO4.
LTMBP remains the rod-like morphology, however, it exhibits relatively small particle size. Particularly, after heating treatment at 900 °C, HBP was further converted into HTMBP. In Fig. 2c, it is surprisingly found that HTMBP exists as the axiolitic shape, and shows larger particle size. The possible reason is that higher temperature accelerates the crystal growth. The phase transformation processes of BiPO4 along with the changes of particle size and morphology are described schematically in Fig. 4. 3.3. FT-IR analysis To investigate symmetric changes of PO4 tetrahedron, the FT-IR measurements of HBP, LTMBP, and HTMBP were carried out. Fig. 5 shows the FT-IR spectra of HBP, LTMBP, and HTMBP. For HBP, the absorption bands at 3491 and 1602 cm1 are due to the O–H stretching and H–O–H bending vibrations of crystal water molecules, and the absorption centered at 1026 cm1 corresponds to the m3 stretching vibration of PO4 groups [33]. Owing to the almost identical P–O distances, although the PO4 group has a C2 symmetry, only one band can be seen in this region. Thus, the symmetry becomes pseudo-Td [33]. The two bands centered at 593 and 539 cm1 are assigned to d (O–P–O) and m4 (PO4), respectively [33]. For LTMBP, after calcinating treatment, PO4 tetrahedron is distorted and the symmetry decreases from pseudo-Td to C1. Therefore, the m3 stretching vibration bands of PO4 groups in the region of 1000 cm1 occur to split. Indeed, four bands can be seen at 1072, 1011, 957, and 929 cm1 in this region [33]. For HTMBP, the splitting of the m3 stretching vibration can be observed likewise, and the vibrations of PO4 groups are more complex. In addition, a weak absorption band observed at 3400 cm1 in the spectrum of LTMBP or HTMBP could be attributed to the adsorbed moisture. 3.4. Electrochemical characterization Fig. 6 shows the first discharge/charge and discharge curves of HBP, LTMBP, and HTMBP at a current density of 26.45 mA g1
Fig. 5. FT-IR spectra of HBP, LTMBP, and HTMBP.
(0.1C) in the voltage range of 1.5–4.5 V at room temperature. As seen from Fig. 6a, HBP, LTMBP, and HTMBP electrodes exhibit an initial discharge capacity of 252.0, 260.1, and 241.4 mA h g1, and a charge capacity of 188.3, 176.6, and 122.2 mA h g1, respectively. Since smaller grain size is in favor of the lithium-ion mobility in the particles by reducing the ion-diffusion pathway, the results are well consistent with the results of SEM images (Fig. 2). However, their irreversible capacities are 63.7, 83.5, and 118.8 mA h g1, respectively. Therefore, among three polymorphic BiPO4, HBP electrode not only reveals a high discharge capacity, but also shows the best reversibility. This further manifests that HBP electrode has the best electrochemical activity. The highest kinetics of HBP is also evidenced by the decrease in polarization of the voltage curve. As shown in Fig. 6b–d, HBP, LTMBP, and HTMBP electrodes all display two discharge plateaus. The discharge plateaus of HBP are positioned at 2.37 and 2.20 V, LTMBP has two plateaus at 2.16 and 2.11 V, and the plateaus of HTMBP are 2.15 and 1.97 V. In the crystal structure of BiPO4, Bi atom is directly coordinated with oxygen atom. The results from FT-IR analysis (Fig. 6) show that PO4 tetrahedron is distorted and the symmetry decreases after calcinating treatment, which may lead to the difference of Bi–O bonds among the three BiPO4 polymorphs. For HBP, each Bi atom is surrounded by eight oxygen atoms, in which four Bi–O bonds 0 have a distance of 2.33 Å A , and another four bonds are at a longer 0 distance of 2.66 Å A. For LTMBP, Bi atom has an irregular coordination with eight0 oxygen atoms. Four short bonds are given as a dis0 tance of 2.46 Å A, and four longer bonds are at a distance of 2.66 Å A. For HTMBP, the values are fallen into two groups: six bonds are 0 0 close to 2.47 Å A, and two bonds average at 2.78 Å A [32]. In the lithiation process, lithium may break the Bi–O bonds. Therefore, for the three electrodes, this may interpret why two different discharge plateaus present to the lithiation process. Similar situations have also been observed in BiOxF32x/C nanocomposites [8], FePO4 H2O [41], and Li2x(Fe1yMny)P2O7 [42]. Due to the shortest Bi–O bond, HBP shows the highest discharge plateau than LTMBP and HTMBP. The initial discharge profiles of HBP, LTMBP, and HTMBP at increasing current densities (13.23–529.04 mA g1) between 1.5 and 4.5 V at room temperature are shown in Fig. 7. As shown in Fig. 7, HBP exhibits an excellent rate capability. For HBP, the degree of polarization is much less than that of LTMBP and HTMBP, and even at a high current density of 529.04 mA g1 the approximate output voltage at the first plateau is 2.22 V, however, for LTMBP and HTMBP, the output voltage is only 1.94 and 1.90 V, respectively. In addition, the discharge capacities of HBP at different discharge current densities are 259.1 (13.23 mA g1), 252.0 (26.45 mA g1), 250.5 (52.90 mA g1), 249.0 (132.26 mA g1), 248.7 (264.52 mA g1), and 236.7 mA h g1 (529.04 mA g1), respectively. At relatively low current densities (626.45 mA g1), the specific capacity of LTMBP is 260.0 mA h g1, however, when the current density is increased to 529.04 mA g1, it still reveals a specific capacity of 225.8 mA h g1. For HTMBP, a lower specific capacity (180.6 mA h g1) is yielded with increasing current density to 529.04 mA g1. Overall, HBP exhibits the best rate capability.
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Fig. 6. (a) The initial discharge/charge curves of HBP, LTMBP, and HTMBP; the initial discharge curves of (b) HBP, (c) LTMBP, and (d) HTMBP a current density of 26.45 mA g1.
Fig. 7. The initial discharge profiles of (a) HBP, (b) LTMBP, and (c) HTMBP at increasing current densities (13.23–529.04 mA g1).
Fig. 8a–c shows the voltage-composition profiles of HBP, LTMBP, and HTMBP electrodes for the first 15 cycles at a current density of 26.45 mA g1 between 1.5 and 4.5 V. The discharge plateaus in the discharge/charge curves can be attributed to the electrochemical reactions of the three electrodes with lithium. In
Fig. 8a, the first discharge capacity of HBP is 252.0 mA h g1 and the discharge plateau is about 2.37 V. The subsequent discharge plateaus positioned at about 2.7–2.5 V are at higher values than the initial discharge plateau. This may be due to the polarization, lithium-driven structure and morphological modifications. In
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Fig. 8. Voltage-composition profiles of (a) HBP, (b) LTMBP, and (c) HTMBP electrodes for the first 15 cycles at a current density of 26.45 mA g1, and (d) cycling stability curves of HBP, LTMBP, and HTMBP electrodes.
Fig. 8b and c, LTMBP and HTMBP electrodes similarly deliver higher discharge plateaus (2.7–2.5 V) in the subsequent cycles. After 15 cycles, HBP electrode reveals a specific capacity of 92.4 mA h g1, however, LTMBP and HTMBP electrodes only exhibit specific capacity of 60.8 and 56.5 mA h g1, respectively. Fig. 8d shows the specific capacity as a function of cycle number of the three electrodes for the first 40 cycles. As shown in Fig. 8d, HBP electrode shows the best cycling stability, and its specific capacity reaches to 77.4 mA h g1 after 40 cycles. However, the specific capacity of LTMBP and HTMBP electrodes after 40 cycles is only 41.2 and 32.0 mA h g1, respectively. These electrodes are all subjected to a significant capacity fading. One of main reasons is that during the discharge and charge process the active materials will undergo large volume changes which cause the loss of electronic contacts between the electrode material and current collector, which could be solved by decreasing particle size and coating. The other two reasons are large voltage hysteresis and poor transport properties for electrons and ions within the active materials like FePO4 or LiFePO4, which could be also solved by decreasing particle size and improving the intrinsic electronic conductivity via hetero atom doping or adding conductive agents. Therefore, although the electrochemical properties of BiPO4 need to be further improved, note that such electrode activities are confirmed for a large particle size and an electrode with no special effort for electron percolation such as carbon coating, indicating that significant improvement is possible by further optimization. To further compare the performance of HBP, LTMBP, and HTMBP, electrochemical impedance measurements were performed in the frequency range from 105 to 102 Hz. The Nyquist plots of coin cells with HBP, LTMBP, and HTMBP cathodes are illustrated in Fig. 9a. The impedance spectra of the three cathodes both consist of one semicircle in the high frequency region and a sloping line in the low frequency region, which indicates the double-layer response at the electrode/sample interface and the diffusion of
Fig. 9. (a) Nyquist plots of HBP, LTMBP, and HTMBP, and (b) Equivalent circuit for the impedance spectra.
Table 2 Impedance parameters of HBP, LTMBP and HTMBP.
R1 (X) R2 (X)
HBP
LTMBP
HTMBP
2.034 94.38
1.816 133.5
1.824 158.5
lithium ions in the solid matrix. The Nyquist plots are fitted using the equivalent circuit model (See Fig. 9b), and the fitted impedance parameters are listed in Table 2. The equivalent circuit model
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B. Hu et al. / Journal of Alloys and Compounds 579 (2013) 18–26
Fig. 10. Ex situ XRD of hexagonal BiPO4 during the initial discharge/charge process.
Fig. 11. (a and c) Ex situ HRTEM and (b and d) SAED of the hexagonal BiPO4 electrode discharged to 1.5 V and recharged to 4.5 V.
includes ohmic resistance R1, charge transfer and bulk diffusion resistance R2, a constant phase element (CPE) associated with the interfacial resistance, and Warburg impedance W1. It can be seen that although HBP has the largest R1 value, it shows the smallest R2 value (94.38 X). According to the studies on EIS of lithium-ion cells by Chen et al. [43], the cell impedance is mainly attributed to cathode impedance, especially charge-transfer resistance. Therefore, it can be concluded that the electrochemical impedance of HBP is less than LTMBP and HTMBP, which indicates that HBP has the highest conductivity. This result is consistent with the results shown in Fig. 6. 3.5. Reaction mechanism To explore the discharge/charge mechanism of BiPO4, ex situ XRD, HRTEM and SAED measurements were conducted on the
front and the backside of discharged electrodes. Here, the hexagonal BiPO4 electrode is selected as an example to examine the mechanism. As seen from Fig. 10, before cycling, BiPO4 is hexagonal phase. At the end of the first discharge at 1.5 V, two new phases of Bi0 (PDF#85-1331) and orthorhombic Li3PO4 with space group of Pmn21 (PDF#71-1528) can be found in the pattern. Particles of product can be found to be in the nanometer ranges in the HRTEM image (Fig. 11a). Besides, the Bi0 and Li3PO4 have been confirmed by ex SAED as shown in Fig. 11b. All d-spacings derived from the SAED spectra are shown in Table 3 and agree well with that of the standard Bi0 and Li3PO4. After recharging to 4.5 V, the ex situ XRD pattern is still the characteristic of hexagonal BiPO4. The ex SAED and HRTEM (Fig. 11c) have also proved the presence of nanometer hexagonal BiPO4. The resulting d-spacings derived from Fig. 11d are consistent with that of P3121 hexagonal BiPO4. Besides, as shown in Fig. 10, BiPO4 can be still retained when recharging to
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B. Hu et al. / Journal of Alloys and Compounds 579 (2013) 18–26 Table 3 The d-spacings derived from the SAED spectra and the corresponding standard data of discharged and recharged products during the initial discharge/charge process. 1.5 V (Å)
Bi0-R3m (Å)
1.5 V (Å)
Li3PO4-Pmn21 (Å)
4.5 V (Å)
BiPO4-P3121 (Å)
3.2701 2.3599 2.2412 1.8595 1.6322 1.4813 a = 4.545 ± 0.005 c = 11.77 ± 0.03
3.2680 (012) 2.3579 (104) 2.2665 (110) 1.8624 (202) 1.6340 (024) 1.4852 (116) a = 4.553 c = 11.79
3.7922 2.6411 2.4282 1.5153 1.3804 1.3771 a = 6.108 ± 0.005 b = 5.243 ± 0.002 c = 4.859 ± 0.001
3.8025 (101) 2.6407 (210) 2.4277 (002) 1.5165 (230) 1.3799 (213) 1.3768 (023) a = 6.115 b = 5.239 c = 4.855
6.0581 4.4188 3.4898 2.8580 2.1544 1.8725 a = 6.984 ± 0.002 c = 6.474 ± 0.007
6.0600 (100) 4.4210 (101) 3.4940 (110) 2.8540 (102) 2.1560 (211) 1.8670 (212) a = 6.982 c = 6.476
4.5 V after 15 cycles. However, due to the presence of volume changes during the charge–discharge processes, Bi0 and Li3PO4 do not be completely reconverted into BiPO4, thus two phases of Bi0 and Li3PO4 can be found in the pattern. The fact that bismuth is not a transition metal and has very stable intermediate oxidation states may render BiPO4 less prone to undergo initial intercalation reaction like as it has been observed in FePO4 and other transition metal phosphates, and similar situation has been also demonstrated by the first principles study of phase diagram [44]. Therefore, combined with ex situ XRD, HRTEM and SAED, it can be drawn a conclusion that the conversion reactions in BiPO4 are also possible. Therefore, the overall reactions can be summarized as follows: lithiation þ 0 ! BiPO4 ðP31 21Þ þ 3Li þ 3e Bi ðR3mÞ þ Li3 PO4 ðPmn2Þ delithiation
ð2Þ Besides, the conversion reactions have been also demonstrated in other bismuth compounds such as Bi2O3, BiOxF32x, and BiF3 [8,25,45]. Thus, based on the conversion reaction, the theoretical specific capacity (264.5 mA h g1) and theoretical output voltage (3.14 V) of BiPO4 can be calculated according to the Nernst equation. In addition, the theoretical gravimetric energy density of BiPO4 is 830.5 W h kg1, and the volumetric energy density is as high as 5253.1 Wh L1, close to twice higher than LiCoO2. For HBP, at a current density of 26.45 mA g1, the practical gravimetric energy density and volumetric energy density are still 597.2 Wh kg1 and 3777.3 Wh L1. Therefore, it is still worthy of further study for lithium-ion batteries. 4. Conclusions Bismuth phosphate, for the first time, is served as a novel cathode material based on conversion reaction for lithium-ion batteries. This finding proves that conversion reactions for lithium-ion batteries can be expanded to metal phosphates. For BiPO4, it is converted into metallic Bi0 and Li3PO4 during lithiation, and undergoes reconversion upon delithiation into BiPO4. Besides, the hexagonal BiPO4 (HBP) shows the best electrochemical performances with the best reversibility, rate capability, and cycling stability compared to the low temperature monoclinic BiPO4 (LTMBP) and high temperature monoclinic BiPO4 (HTMBP). HBP exhibits an initial discharge capacity of 252.0 mA h g1 and an acceptable reversible capacity of 188.3 mA h g1 with an irreversible capacity of 63.7 mA h g1 at a rate of 26.45 mA g1 in the voltage range of 1.5–4.5 V. However, the irreversible capacities of LTMBP and HTMBP are as high as 83.5 and 118.8 mA h g1, respectively. Also, HBP shows the highest initial discharge plateau and the lowest polarization. And HBP still retains a specific capacity of 236.7 mA h g1 when the rate was elevated to 529.04 mA g1, while, the specific capacities of LTMBP and HTMBP are only 225.8 and 180.6 mA h g1, respectively. Although the electrochem-
ical properties of BiPO4 need to be further improved, due to its high theoretical specific capacity of 264.5 mA h g1 and theoretical volumetric energy density of 5253.1 Wh L1 (closed to twice higher than LiCoO2), it is still worthy of further study for lithium-ion batteries. Acknowledgements This work was financially supported by the National Natural Science Foundation of China under Project Nos. 51272221 and 51202209, Joint Fund of Natural Science of Hunan Province and Xiangtan City under Project No. 09BG005, Industrial Project of Colleges and Universities of Hunan Province under Project No. 10CY005, Project of Condition Research of Hunan Province under Project No. 2010TC2004 and Colleges and Universities in Hunan Province plans to graduate research and innovation under Project No. CX2009B133. References [1] M.R. Palacín, Chem. Soc. Rev. 38 (2009) 2565–2575. [2] Sv. Ivanova, E. Zhecheva, D. Nihtianova, Ml. Mladenov, R. Stoyanova, J. Alloys Comp. 561 (2013) 252–261. [3] C.M. Park, J.H. Kim, H. Kim, H.J. Sohn, Chem. Soc. Rev. 39 (2010) 3115–3141. [4] C. Liang, M.I. Gao, H. Pan, Y. Liu, M. Yan, J. Alloys Comp. (2013). http:// dx.doi.org/10.1016/j.jallcom.2013.04.001. [5] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Adv. Mater. 22 (2010) E170– E192. [6] B. Das, M.V. Reddy, B.V.R. Chowdari, J. Alloys Comp. 565 (2013) 90–96. [7] M. Zhang, M. Jia, J. Alloys Comp. 551 (2013) 53–60. [8] M. Bervas, L.C. Klein, G.G. Amatucci, J. Electrochem. Soc. 153 (2006) A159– A170. [9] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496–499. [10] T. Li, X.P. Ai, H.X. Yang, J. Phys. Chem. C 115 (2011) 6167–6174. [11] M.C. López, G.F. Ortiz, P. Lavela, R. Alcántara, J.L. Tirado, J. Alloys Comp. 557 (2013) 82–90. [12] J.-L. Liu, W.-J. Cui, C.-X. Wang, Y.Y. Xia, Electrochem. Commun. 13 (2011) 269– 271. [13] T. Li, Z.X. Chen, Y.L. Cao, X.P. Ai, H.X. Yang, Electrochim. Acta 68 (2012) 202– 205. [14] P. Poizot, S. Laruelle, S. Grugeon, J.M. Tarascon, J. Electrochem. Soc. 149 (2002) A1212–A1217. [15] C.H. Lai, K.W. Huang, J.H. Cheng, C.Y. Lee, B.J. Hwang, L.J. Chen, J. Mater. Chem. 20 (2010) 6638–6645. [16] A. Débart, L. Dupont, R. Patrice, J.-M. Tarascon, Solid State Sci. 8 (2006) 640– 651. [17] N. Pereira, L.C. Klein, G.G. Amatucci, J. Electrochem. Soc. 149 (2002) A262– A271. [18] N. Pereira, L. Dupont, J.M. Tarascon, L.C. Klein, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1273–A1280. [19] F. Badway, N. Pereira, F. Cosandey, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1209–A1218. [20] B.A. Hu, X.Y. Wang, Y.P. Wang, Q.L. Wei, Y.F. Song, H.B. Shu, X.K. Yang, J. Power Sources 218 (2012) 204–211. [21] W. Wu, Y. Wang, X.Y. Wang, Q.Q. Chen, X. Wang, S.Y. Yang, X.M. Liu, J. Guo, Z.H. Yang, J. Alloys Comp. 486 (2009) 93–96. [22] C. Wall, R. Prakash, C. Kübel, H. Hahn, M. Fichtner, J. Alloys Comp. 530 (2012) 121–126. [23] M.S. Chandrasekar, S. Mitra, Electrochim. Acta 92 (2013) 47–54. [24] Y.H. Cui, M.-Z. Xue, Z.-W. Fu, X.-L. Wang, X.-J. Liu, J. Alloys Comp. 555 (2013) 283–290.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Bismuth phosphate: A novel cathode material based on conversion reaction for lithium-ion batteries Benan Hu, Xianyou Wang ⇑, Qiliang Wei, Hongbo Shu, Xiukang Yang, Yansong Bai, Hao Wu, Yunfeng Song, Li Liu Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China
a r t i c l e
i n f o
Article history: Received 18 February 2013 Received in revised form 7 May 2013 Accepted 8 May 2013 Available online 17 May 2013 Keywords: Lithium-ion batteries Cathode materials Bismuth phosphate Polymorphs Conversion reaction
a b s t r a c t Here, we provide a new scientific insight in the field of lithium-ion batteries, in which BiPO4 is initially reported as a cathode material based on conversion reaction. BiPO4 exists in three polymorphic phases, which have been synthesized using a room-temperature liquid precipitation route followed by heating treatment. The effects of phase transformation on electrochemical performances of BiPO4 have been investigated. Herein the hexagonal BiPO4 exhibits the best reversibility, rate capability and cycling stability. A conversion reaction has also been proved to occur in BiPO4 with reduction of BiPO4 into a metallic Bi0 phase and a Li3PO4 phase upon lithiation, and the reformation of BiPO4 phase during next charge process. Its theoretical specific capacity and theoretical output voltage are 264.5 mA h g1 and 3.14 V, respectively. Especially, its theoretical volumetric energy density is as high as 5253.1 Wh L1, close to twice higher than LiCoO2. Therefore, it is still worthy of further study for lithium-ion batteries. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries have drawn more attention since their commercialization in 1991. However, in the age of automotive technologies, automotive applications require at least a two-fold improvement in energy and power densities. Nowadays, there are three different Li-storage mechanisms to store capacity for electrode materials of lithium-ion batteries, namely, conventional intercalation of Li ions [1,2], alloying with Li [3,4], and conversion reactions [5–7]. Among the three Li-storage mechanisms, because the reversible conversion reactions can make use of all possible oxidation states of the metal cation thus yielding higher specific capacities for lithium-ion batteries [8], the materials accompanying with conversion reactions will be a good alternative to meet the requirements of higher energy and power densities. Since Tarascon et al. [9] firstly reported the reversible Li-storage reaction for transition-metal oxides through the heterogeneous conversion, the reversible conversion reactions have been observed in a large number of compounds, such as metal oxides [10,11], chlorides [12,13], sulfides [14–16], nitrides [17,18], fluorides [19–22], and phosphides [23,24]. The overall reaction for the conversion can be summarized as follows:
⇑ Corresponding author. Tel.: +86 731 58292060; fax: +86 731 58292061. E-mail address: [email protected] (X. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.049
þ
mLi þ me þ MXn
lithiation
! nLim=n X þ M
delithiation
ð1Þ
where M stands for a cation and X an anion [25]. However, for the practical application of the cathode materials, the active materials are required to have the output voltage of more than 2 V. Therefore, for the above compounds, only a few metal oxides [10], chlorides [12,13], sulfides [15,16], and fluorides [19–22] were studied as possible alternatives for the cathode materials of lithium-ion batteries. In recent years, a few metal oxides (CuO) [10], chlorides (CoCl2, AgCl, and CuCl2) [12,13], and sulfides (CuS and Cu2S) [15,16] have been reported as positive electrode materials, but the output voltages of metal oxides and sulfides are only about 2 V, thus further studies on metal oxides and sulfides as the cathode materials have rarely been performed. Besides, because most of metal chlorides are hygroscopic and soluble in liquid organic electrolyte, they are also difficultly served as a solid electrode for practical battery applications. Fortunately, metal fluorides, such as FeF3 [19,21,26], BiF3 [20,25], CoF2 [22,27], and NiF2 [28], have been studied as a promising new class of cathode materials for lithium ion batteries. Among these metal fluorides, FeF3 has recently become one of the most promising cathode materials because of its high capacity and excellent thermal stability. However, the poor kinetic properties of FeF3 cathode material hinder its further development. In order to overcome the above problem, morphology control and size reduction to nanoscale dimension are two effective ways. Therefore,
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many recent studies have focused on synthesizing FeF3 cathode materials with different morphology and particle size, such as three-dimensionally ordered macroporous [26], porous nanospheres [29], highly amorphous and porous microspheres [30], worm-like mesoporous [31]. All these materials exhibited superior electrochemical performances. However, in the age of miniaturization, the portable electronic device needs to further pursue the characteristics of light quality, small size and high specific energy density. There is no denying that the cathode materials of lithium-ion batteries with higher gravimetric energy density and volumetric energy density will play a vital role. Besides, compared with the commercial cathode materials, the output voltage of the FeF3 cathode materials is not enough ideal. Especially, its volumetric energy density is much lower. Therefore, to develop new cathode materials based on conversion reaction will be a very significant work. Bismuth phosphate (BiPO4) is a typical non-transition metal phosphate, which exists in three polymorphic phases, namely hexagonal, low temperature monoclinic and high temperature monoclinic BiPO4 [32–35]. The hexagonal form is generally associated with water of hydration, namely hexagonal BiPO4 (HBP), which can be synthesized at room temperature through a wet chemical method [32]. The most stable form is the low temperature monoclinic monazite-type (LTMBP). The third phase is the high temperature monoclinic structure (HTMBP) which can be obtained via heating HBP or LTMBP at high temperature [32,36]. The crystal structure and the phase transformation of BiPO4 have been studied by many researchers [32,33,37]. Moreover, BiPO4 enables extensive applications in various fields such as separation of radioactive elements [34], microwave dielectric [37], orthophosphate ion sensor [38], and photocatalyst [39]. However, to the best of our knowledge, the study on BiPO4 as positive electrode of lithium ion battery has not been reported. For BiPO4, due to the presence 3 of the polyanion (PO3 4 ), P–O covalent bond of PO4 can stabilize 3+ Bi by the inductive effect of Bi–O–P bond. Thus, BiPO4 will have a higher theoretical output voltage (3.14 V) compared with the corresponding metal oxide (Bi2O3) (2.10 V) which has been reported as the cathode material based on conversion reaction. Thus, theoretically, to render BiPO4 as a positive electrode material has possessed the feasibility. In the present study, we provide a new scientific insight in the field of lithium-ion batteries, where a new class of BiPO4-based cathode material is reported for the first time. We explore whether the conversion reactions are also possible in such systems. Besides,
19
the effects of phase transformation on electrochemical properties of BiPO4 have been also examined in detail. 2. Experimental section Hexagonal BiPO4 (HBP) was directly prepared using a chemical liquid precipitation method at room temperature. In a typical synthesis, Bi(NO3)35H2O was dissolved in 10 wt.% acetic acid solution under magnetic stirring, and equimolar (NH4)2HPO4 was dissolved in isopyknic deionized water. Subsequently, Bi(NO3)3 solution was added dropwise to (NH4)2HPO4 solution under vigorous stirring, then a white homogeneous precipitate was observed. The suspension solution was kept on stirring for 2 h. Finally, the product was filtered, sufficiently washed with deionized water and absolute ethyl alcohol, and then dried in an oven at 80 °C for 24 h to obtain HBP. Low-temperature phase (LTMBP) and high temperature phase (HTMBP) were obtained by thermal treatments over HBP at high temperatures. The HBP was sintered at 600 °C and 900 °C for 6 h with a heating rate of 5 °C min1 to obtain LTMBP and HTMBP [36,40], respectively. All samples were obtained after naturally cooling to room temperature. The structural and crystallographic analyses of the samples were performed using powder X-ray diffraction (XRD) techniques (D/max-2550 Rigaku, Japan) using Cu Ka radiation (k = 1.54178 Å) and a graphite monochromator at 40 kV, 20 mA. XRD data were collected at 4° min1 in the 2h range of 10–90°. The surface morphology of the samples was observed using the Hitachi S-3500N scanning electron microscope (SEM) and field emission scanning electron microscope (FE-SEM; LEO1525, Germany). The FT-IR measurements of the samples were carried out via a Fourier transform infrared (FT-IR) spectrometer (Perkin–Elmer Spectrum One). Selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) pictures were taken with JEOL JEM100SX electron microscope. The electrochemical measurements of the samples were carried out using cointype cells (CR2025) assembled in an argon-filled glove box (MIKROUNA 1220/750). The cathodes were made by mixing 80 wt.% active material, 6 wt.% acetylene black, 6 wt.% graphite and 8 wt.% polyvinylidene fluoride (PVDF) binder. The mass of electrode material per 1 cm2 of electrode is 3–4 mg. In all cells, lithium was served as the counter and reference electrodes, Celgard 2400 was used as separator, and the electrolyte was a 1 M LiPF6 solution in ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1, v/v). Galvanostatic discharge–charge measurements were carried out in Neware battery test system (BTS-51, Shenzhen, China) at various current densities between 1.5 and 4.5 V (vs. Li+/Li) at room temperature. The EIS experiments were conducted using a CHI 660a Electrochemical Analyzer (CH Instrument Inc., USA). The ac perturbation signal was ±5 mV, and the frequency range was from 105 to 102 Hz.
3. Results and discussion 3.1. Phase identification To distinguish the crystal structure of three polymorphs of BiPO4, the powder X-ray diffraction techniques were carried out
Fig. 1. XRD patterns of HBP, LTMBP, and HTMBP (b, d and f) and their corresponding standard patterns (a, c and e).
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Table 1 The refined lattice parameters of BiPO4 polymorphs. BiPO4
Unit cell parameters 0
a (Å A)
0
b (Å A)
0
c (Å A)
Vcell
Space group
Z
P3121 P21/n P21/m
3 4 2
q (g cm3)
0
(Å A3) HBP LTMBP HTMBP
6.9843 6.7363 4.8744
6.9843 6.9108 7.0601
6.4738 6.4490 4.6938
273.51 291.70 160.52
5.5360 6.9210 6.2859
0209). When the HBP was calcined at 900 °C for 6 h, as shown in Fig. 1f, all diffraction peaks are consistent with the theoretical diffraction peaks of HTMBP (space group: P21/m (11), PDF#43-0637). The above results of phase transformation are in good agreement with the results reported by Zhao et al. [36]. The refined lattice parameters of HBP, LTMBP, and HTMBP are listed in Table 1. These values are also in good agreement with those reported by MooneySlater [32].
3.2. Morphology characterization on the BiPO4 prepared at room temperature, 600 °C, and 900 °C, respectively. XRD patterns of HBP, LTMBP, and HTMBP and their corresponding standard patterns are shown in Fig. 1. As being seen in Fig. 1b, all diffraction peaks can be indexed to a hexagonal phase (space group: P3121, PDF#15-0766), which indicates that the asprepared BiPO4 is a hexagonal structure. And the diffraction peaks are quite sharp, indicating that HBP has high crystallinity. After the HBP was calcined at 600 °C for 6 h, in Fig. 1d, all diffraction peaks can be readily indexed to LTMBP (space group: P21/n (14), PDF#80-
Surface morphology and particle size could be observed by scanning electron microscopy (SEM) and field emission scanning electron microscope (FE-SEM). SEM images of HBP, LTMBP, and HTMBP are presented in Fig. 2. FE-SEM images of HBP and LTMBP are presented in Fig. 3. As shown in Figs. 2a and 3a, HBP shows rodlike morphology with diameter about 0.8 lm and length ranging from 1.2 to 1.6 lm. When the HBP was calcined at 600 °C, the hexagonal BiPO4 was transformed into LTMBP. As seen from Fig. 2b,
Fig. 2. SEM images of (a) HBP, (b) LTMBP, and (c) HTMBP.
Fig. 3. FE-SEM images of (a) HBP and (b) LTMBP.
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Fig. 4. Schematic illustration of phase transformation for BiPO4.
LTMBP remains the rod-like morphology, however, it exhibits relatively small particle size. Particularly, after heating treatment at 900 °C, HBP was further converted into HTMBP. In Fig. 2c, it is surprisingly found that HTMBP exists as the axiolitic shape, and shows larger particle size. The possible reason is that higher temperature accelerates the crystal growth. The phase transformation processes of BiPO4 along with the changes of particle size and morphology are described schematically in Fig. 4. 3.3. FT-IR analysis To investigate symmetric changes of PO4 tetrahedron, the FT-IR measurements of HBP, LTMBP, and HTMBP were carried out. Fig. 5 shows the FT-IR spectra of HBP, LTMBP, and HTMBP. For HBP, the absorption bands at 3491 and 1602 cm1 are due to the O–H stretching and H–O–H bending vibrations of crystal water molecules, and the absorption centered at 1026 cm1 corresponds to the m3 stretching vibration of PO4 groups [33]. Owing to the almost identical P–O distances, although the PO4 group has a C2 symmetry, only one band can be seen in this region. Thus, the symmetry becomes pseudo-Td [33]. The two bands centered at 593 and 539 cm1 are assigned to d (O–P–O) and m4 (PO4), respectively [33]. For LTMBP, after calcinating treatment, PO4 tetrahedron is distorted and the symmetry decreases from pseudo-Td to C1. Therefore, the m3 stretching vibration bands of PO4 groups in the region of 1000 cm1 occur to split. Indeed, four bands can be seen at 1072, 1011, 957, and 929 cm1 in this region [33]. For HTMBP, the splitting of the m3 stretching vibration can be observed likewise, and the vibrations of PO4 groups are more complex. In addition, a weak absorption band observed at 3400 cm1 in the spectrum of LTMBP or HTMBP could be attributed to the adsorbed moisture. 3.4. Electrochemical characterization Fig. 6 shows the first discharge/charge and discharge curves of HBP, LTMBP, and HTMBP at a current density of 26.45 mA g1
Fig. 5. FT-IR spectra of HBP, LTMBP, and HTMBP.
(0.1C) in the voltage range of 1.5–4.5 V at room temperature. As seen from Fig. 6a, HBP, LTMBP, and HTMBP electrodes exhibit an initial discharge capacity of 252.0, 260.1, and 241.4 mA h g1, and a charge capacity of 188.3, 176.6, and 122.2 mA h g1, respectively. Since smaller grain size is in favor of the lithium-ion mobility in the particles by reducing the ion-diffusion pathway, the results are well consistent with the results of SEM images (Fig. 2). However, their irreversible capacities are 63.7, 83.5, and 118.8 mA h g1, respectively. Therefore, among three polymorphic BiPO4, HBP electrode not only reveals a high discharge capacity, but also shows the best reversibility. This further manifests that HBP electrode has the best electrochemical activity. The highest kinetics of HBP is also evidenced by the decrease in polarization of the voltage curve. As shown in Fig. 6b–d, HBP, LTMBP, and HTMBP electrodes all display two discharge plateaus. The discharge plateaus of HBP are positioned at 2.37 and 2.20 V, LTMBP has two plateaus at 2.16 and 2.11 V, and the plateaus of HTMBP are 2.15 and 1.97 V. In the crystal structure of BiPO4, Bi atom is directly coordinated with oxygen atom. The results from FT-IR analysis (Fig. 6) show that PO4 tetrahedron is distorted and the symmetry decreases after calcinating treatment, which may lead to the difference of Bi–O bonds among the three BiPO4 polymorphs. For HBP, each Bi atom is surrounded by eight oxygen atoms, in which four Bi–O bonds 0 have a distance of 2.33 Å A , and another four bonds are at a longer 0 distance of 2.66 Å A. For LTMBP, Bi atom has an irregular coordination with eight0 oxygen atoms. Four short bonds are given as a dis0 tance of 2.46 Å A, and four longer bonds are at a distance of 2.66 Å A. For HTMBP, the values are fallen into two groups: six bonds are 0 0 close to 2.47 Å A, and two bonds average at 2.78 Å A [32]. In the lithiation process, lithium may break the Bi–O bonds. Therefore, for the three electrodes, this may interpret why two different discharge plateaus present to the lithiation process. Similar situations have also been observed in BiOxF32x/C nanocomposites [8], FePO4 H2O [41], and Li2x(Fe1yMny)P2O7 [42]. Due to the shortest Bi–O bond, HBP shows the highest discharge plateau than LTMBP and HTMBP. The initial discharge profiles of HBP, LTMBP, and HTMBP at increasing current densities (13.23–529.04 mA g1) between 1.5 and 4.5 V at room temperature are shown in Fig. 7. As shown in Fig. 7, HBP exhibits an excellent rate capability. For HBP, the degree of polarization is much less than that of LTMBP and HTMBP, and even at a high current density of 529.04 mA g1 the approximate output voltage at the first plateau is 2.22 V, however, for LTMBP and HTMBP, the output voltage is only 1.94 and 1.90 V, respectively. In addition, the discharge capacities of HBP at different discharge current densities are 259.1 (13.23 mA g1), 252.0 (26.45 mA g1), 250.5 (52.90 mA g1), 249.0 (132.26 mA g1), 248.7 (264.52 mA g1), and 236.7 mA h g1 (529.04 mA g1), respectively. At relatively low current densities (626.45 mA g1), the specific capacity of LTMBP is 260.0 mA h g1, however, when the current density is increased to 529.04 mA g1, it still reveals a specific capacity of 225.8 mA h g1. For HTMBP, a lower specific capacity (180.6 mA h g1) is yielded with increasing current density to 529.04 mA g1. Overall, HBP exhibits the best rate capability.
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Fig. 6. (a) The initial discharge/charge curves of HBP, LTMBP, and HTMBP; the initial discharge curves of (b) HBP, (c) LTMBP, and (d) HTMBP a current density of 26.45 mA g1.
Fig. 7. The initial discharge profiles of (a) HBP, (b) LTMBP, and (c) HTMBP at increasing current densities (13.23–529.04 mA g1).
Fig. 8a–c shows the voltage-composition profiles of HBP, LTMBP, and HTMBP electrodes for the first 15 cycles at a current density of 26.45 mA g1 between 1.5 and 4.5 V. The discharge plateaus in the discharge/charge curves can be attributed to the electrochemical reactions of the three electrodes with lithium. In
Fig. 8a, the first discharge capacity of HBP is 252.0 mA h g1 and the discharge plateau is about 2.37 V. The subsequent discharge plateaus positioned at about 2.7–2.5 V are at higher values than the initial discharge plateau. This may be due to the polarization, lithium-driven structure and morphological modifications. In
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Fig. 8. Voltage-composition profiles of (a) HBP, (b) LTMBP, and (c) HTMBP electrodes for the first 15 cycles at a current density of 26.45 mA g1, and (d) cycling stability curves of HBP, LTMBP, and HTMBP electrodes.
Fig. 8b and c, LTMBP and HTMBP electrodes similarly deliver higher discharge plateaus (2.7–2.5 V) in the subsequent cycles. After 15 cycles, HBP electrode reveals a specific capacity of 92.4 mA h g1, however, LTMBP and HTMBP electrodes only exhibit specific capacity of 60.8 and 56.5 mA h g1, respectively. Fig. 8d shows the specific capacity as a function of cycle number of the three electrodes for the first 40 cycles. As shown in Fig. 8d, HBP electrode shows the best cycling stability, and its specific capacity reaches to 77.4 mA h g1 after 40 cycles. However, the specific capacity of LTMBP and HTMBP electrodes after 40 cycles is only 41.2 and 32.0 mA h g1, respectively. These electrodes are all subjected to a significant capacity fading. One of main reasons is that during the discharge and charge process the active materials will undergo large volume changes which cause the loss of electronic contacts between the electrode material and current collector, which could be solved by decreasing particle size and coating. The other two reasons are large voltage hysteresis and poor transport properties for electrons and ions within the active materials like FePO4 or LiFePO4, which could be also solved by decreasing particle size and improving the intrinsic electronic conductivity via hetero atom doping or adding conductive agents. Therefore, although the electrochemical properties of BiPO4 need to be further improved, note that such electrode activities are confirmed for a large particle size and an electrode with no special effort for electron percolation such as carbon coating, indicating that significant improvement is possible by further optimization. To further compare the performance of HBP, LTMBP, and HTMBP, electrochemical impedance measurements were performed in the frequency range from 105 to 102 Hz. The Nyquist plots of coin cells with HBP, LTMBP, and HTMBP cathodes are illustrated in Fig. 9a. The impedance spectra of the three cathodes both consist of one semicircle in the high frequency region and a sloping line in the low frequency region, which indicates the double-layer response at the electrode/sample interface and the diffusion of
Fig. 9. (a) Nyquist plots of HBP, LTMBP, and HTMBP, and (b) Equivalent circuit for the impedance spectra.
Table 2 Impedance parameters of HBP, LTMBP and HTMBP.
R1 (X) R2 (X)
HBP
LTMBP
HTMBP
2.034 94.38
1.816 133.5
1.824 158.5
lithium ions in the solid matrix. The Nyquist plots are fitted using the equivalent circuit model (See Fig. 9b), and the fitted impedance parameters are listed in Table 2. The equivalent circuit model
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Fig. 10. Ex situ XRD of hexagonal BiPO4 during the initial discharge/charge process.
Fig. 11. (a and c) Ex situ HRTEM and (b and d) SAED of the hexagonal BiPO4 electrode discharged to 1.5 V and recharged to 4.5 V.
includes ohmic resistance R1, charge transfer and bulk diffusion resistance R2, a constant phase element (CPE) associated with the interfacial resistance, and Warburg impedance W1. It can be seen that although HBP has the largest R1 value, it shows the smallest R2 value (94.38 X). According to the studies on EIS of lithium-ion cells by Chen et al. [43], the cell impedance is mainly attributed to cathode impedance, especially charge-transfer resistance. Therefore, it can be concluded that the electrochemical impedance of HBP is less than LTMBP and HTMBP, which indicates that HBP has the highest conductivity. This result is consistent with the results shown in Fig. 6. 3.5. Reaction mechanism To explore the discharge/charge mechanism of BiPO4, ex situ XRD, HRTEM and SAED measurements were conducted on the
front and the backside of discharged electrodes. Here, the hexagonal BiPO4 electrode is selected as an example to examine the mechanism. As seen from Fig. 10, before cycling, BiPO4 is hexagonal phase. At the end of the first discharge at 1.5 V, two new phases of Bi0 (PDF#85-1331) and orthorhombic Li3PO4 with space group of Pmn21 (PDF#71-1528) can be found in the pattern. Particles of product can be found to be in the nanometer ranges in the HRTEM image (Fig. 11a). Besides, the Bi0 and Li3PO4 have been confirmed by ex SAED as shown in Fig. 11b. All d-spacings derived from the SAED spectra are shown in Table 3 and agree well with that of the standard Bi0 and Li3PO4. After recharging to 4.5 V, the ex situ XRD pattern is still the characteristic of hexagonal BiPO4. The ex SAED and HRTEM (Fig. 11c) have also proved the presence of nanometer hexagonal BiPO4. The resulting d-spacings derived from Fig. 11d are consistent with that of P3121 hexagonal BiPO4. Besides, as shown in Fig. 10, BiPO4 can be still retained when recharging to
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B. Hu et al. / Journal of Alloys and Compounds 579 (2013) 18–26 Table 3 The d-spacings derived from the SAED spectra and the corresponding standard data of discharged and recharged products during the initial discharge/charge process. 1.5 V (Å)
Bi0-R3m (Å)
1.5 V (Å)
Li3PO4-Pmn21 (Å)
4.5 V (Å)
BiPO4-P3121 (Å)
3.2701 2.3599 2.2412 1.8595 1.6322 1.4813 a = 4.545 ± 0.005 c = 11.77 ± 0.03
3.2680 (012) 2.3579 (104) 2.2665 (110) 1.8624 (202) 1.6340 (024) 1.4852 (116) a = 4.553 c = 11.79
3.7922 2.6411 2.4282 1.5153 1.3804 1.3771 a = 6.108 ± 0.005 b = 5.243 ± 0.002 c = 4.859 ± 0.001
3.8025 (101) 2.6407 (210) 2.4277 (002) 1.5165 (230) 1.3799 (213) 1.3768 (023) a = 6.115 b = 5.239 c = 4.855
6.0581 4.4188 3.4898 2.8580 2.1544 1.8725 a = 6.984 ± 0.002 c = 6.474 ± 0.007
6.0600 (100) 4.4210 (101) 3.4940 (110) 2.8540 (102) 2.1560 (211) 1.8670 (212) a = 6.982 c = 6.476
4.5 V after 15 cycles. However, due to the presence of volume changes during the charge–discharge processes, Bi0 and Li3PO4 do not be completely reconverted into BiPO4, thus two phases of Bi0 and Li3PO4 can be found in the pattern. The fact that bismuth is not a transition metal and has very stable intermediate oxidation states may render BiPO4 less prone to undergo initial intercalation reaction like as it has been observed in FePO4 and other transition metal phosphates, and similar situation has been also demonstrated by the first principles study of phase diagram [44]. Therefore, combined with ex situ XRD, HRTEM and SAED, it can be drawn a conclusion that the conversion reactions in BiPO4 are also possible. Therefore, the overall reactions can be summarized as follows: lithiation þ 0 ! BiPO4 ðP31 21Þ þ 3Li þ 3e Bi ðR3mÞ þ Li3 PO4 ðPmn2Þ delithiation
ð2Þ Besides, the conversion reactions have been also demonstrated in other bismuth compounds such as Bi2O3, BiOxF32x, and BiF3 [8,25,45]. Thus, based on the conversion reaction, the theoretical specific capacity (264.5 mA h g1) and theoretical output voltage (3.14 V) of BiPO4 can be calculated according to the Nernst equation. In addition, the theoretical gravimetric energy density of BiPO4 is 830.5 W h kg1, and the volumetric energy density is as high as 5253.1 Wh L1, close to twice higher than LiCoO2. For HBP, at a current density of 26.45 mA g1, the practical gravimetric energy density and volumetric energy density are still 597.2 Wh kg1 and 3777.3 Wh L1. Therefore, it is still worthy of further study for lithium-ion batteries. 4. Conclusions Bismuth phosphate, for the first time, is served as a novel cathode material based on conversion reaction for lithium-ion batteries. This finding proves that conversion reactions for lithium-ion batteries can be expanded to metal phosphates. For BiPO4, it is converted into metallic Bi0 and Li3PO4 during lithiation, and undergoes reconversion upon delithiation into BiPO4. Besides, the hexagonal BiPO4 (HBP) shows the best electrochemical performances with the best reversibility, rate capability, and cycling stability compared to the low temperature monoclinic BiPO4 (LTMBP) and high temperature monoclinic BiPO4 (HTMBP). HBP exhibits an initial discharge capacity of 252.0 mA h g1 and an acceptable reversible capacity of 188.3 mA h g1 with an irreversible capacity of 63.7 mA h g1 at a rate of 26.45 mA g1 in the voltage range of 1.5–4.5 V. However, the irreversible capacities of LTMBP and HTMBP are as high as 83.5 and 118.8 mA h g1, respectively. Also, HBP shows the highest initial discharge plateau and the lowest polarization. And HBP still retains a specific capacity of 236.7 mA h g1 when the rate was elevated to 529.04 mA g1, while, the specific capacities of LTMBP and HTMBP are only 225.8 and 180.6 mA h g1, respectively. Although the electrochem-
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