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LiFeP: A new anode material for lithium ion batteries
Journal of Power Sources 370 (2017) 14e19
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LiFeP: A new anode material for lithium ion batteries Jingjing Luo a, Jianbin Zhou a, Dan Lin b, Yi Ren b, Kaibin Tang a, b, * a b
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A new anode LiFeP was investigated for lithium ion batteries. The layered structure of LiFeP was relatively stable during cycling. A discharge capacity of 507 mA h g1 at 300 mA g1 after 300 cycles was obtained. The reason for the increase in capacity of LiFeP was studied.
a r t i c l e i n f o
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Article history: Received 20 May 2017 Received in revised form 1 September 2017 Accepted 2 September 2017
Transition metal phosphides are promising anode materials for lithium ion batteries because of their abundant natural resources and high theoretical capacities. In this study, the electrochemical properties of LiFeP as an anode material for lithium ion battery were investigated for the first time. LiFeP powders were successfully synthesized by a conventional two-step solid-state reaction method. The results of Xray powder diffraction and selected area electron diffraction revealed that the layered plate-like LiFeP was stacked by the (001) crystal plane. As an electrode material, LiFeP delivered a superior reversible capacity of 507 mA h g1 at a high current density of 300 mA g1 after 300 cycles and excellent rate performance. After cycling, the layered structure can be well maintained, which would be greatly beneficial to the electrochemical performance of LiFeP. The reason for the increase in capacity was also investigated and can be attributed to the high number of conversion reactions of LiFeP and the generation of elemental P during cycling. © 2017 Elsevier B.V. All rights reserved.
Keywords: Transition metal phosphides Anode Lithium ion batteries LiFeP
1. Introduction In the past decades, numerous efforts have been made to explore new anode materials for lithium ion batteries (LIBs) because of the low theoretical capacity of commercial graphite (372 mA h g1). It is well known that stable crystal structure and superior conductivity are crucial to achieve an outstanding * Corresponding author. Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China. E-mail address: [email protected] (K. Tang). https://doi.org/10.1016/j.jpowsour.2017.09.005 0378-7753/© 2017 Elsevier B.V. All rights reserved.
electrochemical performance of electrode materials during cycling. Therefore, many investigations have been focused on materials with high theoretical capacity, stable crystal structure, and high conductivity. Unfortunately, the available materials have limitations for use in high-performance LIBs. For example, silicon anode has an extremely high theoretical capacity of 3600 mA h g1, but the volume expansion is as high as 300% and the crystal structure can be easily destroyed [1]. Titanium oxide (TiO2) [2,3], titanium niobium oxide (TieNbeO) [4e7], and lithium titanate (Li4Ti5O12) [8,9] are typical low volume expansion materials with high cycling stability and remarkably stable crystal structure. However, their
J. Luo et al. / Journal of Power Sources 370 (2017) 14e19
theoretical capacities are less than the desired value. Considering these drawbacks, new anode materials with both high theoretical capacity and a relatively stable structure are urgently needed. Recently, transition metal phosphides have attracted considerable attention because of their abundance in natural resources and high theoretical capacities [10e16]. In particular, iron phosphides such as FeP, FeP4, FeP2, and Fe2P have been investigated as anode materials for LIBs owing to their individual high theoretical capacity of 926, 1789, 1365, and 563 mA h g1, respectively [17e20]. In 2006, Boyanov et al. studied the electrochemical mechanism of FeP and demonstrated a complex conversion reaction among FeP, LixFeP, and Li3P [17]. They further reported a high reversible capacity of ~520 mA h g1 in the first cycle for the FeP4-based LIBs [18]. FeP2based LIBs were investigated by Hall et al. They found that the reversible capacity can reach up to 766 mA h g1 in the first charge/ discharge process and increase to 906 mA h g1 after the 10th cycle [19]. Meanwhile, several disadvantages indeed exist in iron phosphides such as irreversible conversion reactions, large volume changes, low conductivity, and destroyed crystal structure, which seriously affect the electrochemical properties and lead to a rapid capacity fading. To improve the electrochemical performance of iron phosphides, tremendous efforts have been made in exploring morphology engineering and carbon coating strategies [21e23]. However, the structural stability problem of phosphides remains unsolved. Thus, enhanced performances are expected by resolving this challenge. LiFeP, which is an important member of iron phosphides, has been newly developed as a superconductor in the last few years. It crystallizes with space group of P4/nmm and an anti-PbFCl-type structure, which is formed by the alternate stacking of anti-PbOtype FeP layers and double Li planes along the [001] direction. The conductivity of LiFeP is about 2 106 S/m at room temperature [24], which contributes to the charge transfer at relatively high current densities to improve the dynamics of oxidation reaction during cycling processes [25,26]. LiFeP-based lithium ion battery with a theoretical capacity of 572 mA h g1 was investigated in our work for the first time. The special layered crystal structure and high conductivity were expected to provide an outstanding electrochemical performance of LiFeP. In fact, the results showed that LiFeP has a relatively stable layered structure during cycling and, interestingly, delivers an increased reversible capacity at high current density and a remarkable rate performance, thus making it a potential anode material for LIBs.
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transmission electron microscopy (TEM, Talos F200X), and selected area electron diffraction (SAED, Talos F200X). The surface chemical state was determined by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). 2.3. Electrochemical characterization To investigate the electrochemical behavior of LiFeP as an anode material for LIBs, CR 2016 coin-type half cells were assembled. The cells consisted of the as-synthesized LiFeP as the working electrode, Li metal as the counter electrode, celgard 2400 as the separator, and a 1.0 M LiPF6 solution of ethylene carbonate/diethyl carbonate (volume ratio 1:1) as the electrolyte. The working electrode was prepared by mixing active material with carbon black and polyvinylidene fluoride at a weight ratio of 6:2:2 in N-methyl-2pyrrolidone. The mixture was then coated on a copper foil and dried in a vacuum oven at 110 C for 10 h. The electrochemical properties of electrodes were tested by the galvanostatic charge/ discharge method on a Neware-BTS-TC53 instrument at a potential range of 0.005e3 V. The cyclic voltammograms (CV) were measured on an electrochemical workstation (CHI660E) at a scanning rate of 0.1 mV s1. 3. Results and discussion The powder XRD pattern of the as-synthesized LiFeP is shown in Fig. 1. The diffraction peaks can be well attributed to the tetragonal structure of LiFeP (JCPDS 87e1501) with lattice parameters of a ¼ 3.69 Å and c ¼ 6.03 Å. The typical layered crystal structure of LiFeP is shown in the inset of Fig. 1, and it would enhance the diffusion of Liþ in electrode. The morphology and microstructural features of the assynthesized LiFeP were investigated by SEM, TEM, and SAED. As illustrated in Fig. 2a and b, LiFeP has a typical stacked plate-like microstructure. The thicknesses of the plates range from hundreds of nanometers to several micrometers. Fig. 2c shows a typical TEM image of plate-like LiFeP. The corresponding SAED pattern (Fig. 2d) reveals a single crystal feature, which can be assigned to a tetragonal cell with a ¼ b ¼ 3.7 Å, demonstrating that the plates are stacked by the (001) crystal plane matching well with the results from the XRD pattern. To investigate the electrochemical performance of LiFeP, half cells were assembled and tested by the galvanostatic charge/ discharge method. The CV curves of the LiFeP electrode within a
2. Experimental 2.1. Synthesis of LiFeP LiFeP was synthesized by a conventional two-step solid-state reaction method. First, FeP was obtained by sealing Fe (98%) and P (99.999%) powders (molar ratio 1:1) into an evacuated quartz tube and annealing them at 700 C for 24 h. Later, LiFeP was prepared by mixing stoichiometric amounts of Li (99.9%) and the as-prepared FeP in an evacuated quartz tube and sintering them at 850 C for 24 h. All the processes were performed in a high-purity Ar-protected glovebox. A black product was obtained after the tube was cooled down to room temperature naturally. 2.2. Material characterization The phase characterization of the as-prepared product was performed on a Philips X'pert X-ray diffractometer (Cu Ka radiation, l ¼ 1.54182 Å). The morphology and structural characterization were examined by scanning electron microscopy (SEM, S-4800),
Fig. 1. X-ray diffraction pattern of the as-prepared LiFeP. The LiFeP structural frame is shown in the inset.
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Fig. 2. SEM images (a) and (b) of the as-prepared LiFeP, (c) TEM image and (d) selected area electron diffraction pattern of the as-prepared LiFeP.
potential window of 0.005e3 V are shown in Fig. 3a. The oxidation and reduction peaks correspond to the charge/discharge profiles in Fig. 3c. The peaks located at 1.16 V in the first discharge can be assigned to a simple lithium ion insertion reaction (Eq. (1)) [27]. The irreversible reduction peak at 0.72 V can be attributed to the side reaction between active materials and electrolyte for the formation of solid electrolyte interface (SEI) membrane. A following cathode peak at 0.35 V can be associated with the reduction of partial LiFeP phase into Fe0 and Li3P (Eq. (2)) [28]. During the following charge process, the two oxidation peaks at 0.11 and 1.43 V, respectively, can be ascribed to a two-step lithium ion deintercalation reaction from the interlamination of Li1þxFeP phase (Eq. (3) and Eq. (5)), while the oxidation peak at 0.92 V can be associated with the transformation of Li3P and Fe0 to LiFeP (Eq. (4)). The reaction processes in the first cycle accord with the changes in the XRD patterns of LiFeP at different potentials (Fig. S1, Supplementary Information). In the subsequent two scanning cycles, the oxidation peaks and the reduction peaks at 1.49, 1.16, and 0.35 V repeat well, indicating good reversible electrochemical reactions of LiFeP in Li half cells. In addition, a new weak peak at around 1.06 V emerged initially during the second charge process and remained in the following charge processes, which can be ascribed to the delithiation from LiFeP to form a small quantity of FeP (Eq. (6)). From these observations, the theoretical reaction processes of LiFeP can be described using the following equations. Discharge processes
LiFeP þ xLi/Li1þx FeP
(1)
LiFeP þ 2Li/Li3 P þ Fe0
(2)
Charge processes
Li1þx FeP/yLi þ Li1þxy FeP
(3)
Li3 P þ Fe0 /LiFeP þ 2Li
(4)
Li1þxy FeP/LiFeP þ ðx yÞLi
(5)
LiFeP/Li þ FeP
(6)
Fig. 3b shows the cycling performance of LiFeP at a current density of 300 mA g1. The first discharge and charge capacities were 282 and 218 mA h g1, respectively. From Fig. 3a and b, the low discharge capacity indicates that the insertion/de-insertion processes were dominant, and the conversion reaction was limited during the first cycling. The corresponding Coulombic efficiency was 77.3%. The capacity lost in the first cycle can be attributed to some irreversible reactions such as the formation of SEI membrane. After 30 cycles, the reversible discharge capacity was stable at 222 mA h g1. Then, the discharge capacity slowly increased to 507 mA h g1 after 300 cycles. It is worth noting that the corresponding Coulombic efficiency was maintained over 98% from the second cycle, indicating the highly reversible lithium storage ability of LiFeP anode. In addition, the XRD pattern and SEM images of LiFeP anode after 300 cycles were also collected. As shown in Fig. 3e, the diffraction peak at 14.95 remained visible, suggesting that the layered structure along the [001] direction of LiFeP is very stable after cycling. Meanwhile, the quite smooth and complete film coated over the surface of electrode (Fig. 3g) indicates that the SEI membrane of LiFeP electrode is highly stable, which is very helpful to improve the cycling stability. Furthermore, LiFeP maintained part of the initial plate-like microstructure even after 300 cycles (Fig. 3h), which is in agreement with the result from Fig. 3e. By combining this result with the cycle performance, it
J. Luo et al. / Journal of Power Sources 370 (2017) 14e19
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Fig. 3. Electrochemical performance of LiFeP: (a) initial CV curves of LiFeP at a voltage range of 0.005e3 V at a scanning rate 0.1 mV s1; (b) the cycling performance and corresponding Coulombic efficiency at a current density of 300 mA g1; (c) the charge and discharge curves at a current density of 300 mA g1 between 0.005 and 3 V for the 1st, 2nd, 50th, and 300th cycles; (d) the rate capacity curves at current densities of 300, 500, 600, 800, 1000, and 300 mA g1; (e) XRD pattern of the LiFeP material after 300 cycles; (f) CV curves of LiFeP after 300 cycles at 300 mA g1 between 0.005 and 3 V; SEM images of (g) the top view of LiFeP electrode after 300 cycles, (h) LiFeP material after 300 cycles.
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Table 1 Comparison of the electrochemical performance among the as-obtained LiFeP in this work, decorated FeP, and other transition metal phosphides. Material
Reversible capacity (mA h g1)
Bulk FeP Mesoporous FeP FeP FeP@C FeP@C nanoplates MnP CoP Ni2P CuP2 LiFeP
310 355 334 480 610 287 510 296 554 507
(50th cycle) (30th cycle) (50th cycle) (200th cycle) (400th cycle) (50th cycle) (400th cycle) (50th cycle) (50th cycle) (300th cycle)
Current density (mA g1)
Ref.
185.2 144 100 30 400 50 400 54.2 200 300
17 23 27 22 21 29 30 16 28 This work
can be demonstrated that a high number of conversion reactions of LiFeP occur during cycling, which can result in the part fading of the crystal structure and the increase in capacity. Nevertheless, LiFeP has a relatively stable layered structure compared with other phosphides during cycling, which would considerably enhance the diffusion of Liþ in active materials for the improvement of cycling and rate performance. The rate performance of LiFeP at different current densities is shown in Fig. 3d. With the increase in current density from 300 to 500, 600, 800, and 1000 mA g1, the observed discharge capacities were 270, 218, 206, 195, and 188 mA h g1, respectively. In particular, the capacity recovered to 258 mA h g1 when the current density was decreased to initial value, i.e., 300 mA g1, which is approximately 96% of the initial capacity during the first 10 cycles. The Coulombic efficiency was always over 98% for different current densities. The excellent rate performance of LiFeP can be ascribed to the stable layered structure. Herein, a comparison of the electrochemical performance was made among the as-obtained LiFeP, decorated FeP [17,21e23,29], and other pure transition metal phosphides [16,30e32]. As presented in Table 1, LiFeP shows a much better electrochemical performance than most of these anode materials. In particular, the LiFeP electrode can possess a remarkable reversible capacity of 507 mA h g1 even at a high current density of 300 mA g1. Therefore, it can be concluded that LiFeP has a superior capability to overcome the capacity fading and maintains a good reversible capacity even at a high current density. The increase in capacity during cycling has been observed in various anode materials. From the above investigations, it is known that the occurrence of a high number of conversion reactions of LiFeP during cycling can lead to the increase in capacity. To investigate other possible reasons for the increase in capacity, CV curves for LiFeP electrode after 300 cycles in a potential range of
0.005e3 V were measured. As displayed in Fig. 3f, the oxidation peak at 0.11 V and broad reduction peak at about 1.03 V can be attributed to the intercalation/de-intercalation processes of lithium ions from the interlamination of LiFeP, which match with the initial CV curves, indicating the outstanding cycling stability and cycle life. However, a new oxidation peak at 0.61 V and two new reduction peaks at 1.02 and 1.20 V were observed, as shown in Fig. 3f, which imply the different electrochemical reactions compared with the initial ones (Fig. 3a). Regarding the previous report on P anodes [33], the new redox peaks at 0.61, 1.02, and 1.20 V can be attributed to the lithiation/de-lithiation platform of elemental P. In fact, a broader and higher peak of Fe2P than the one in initial compounds emerged after 300 cycles, as shown in Fig. 3e (Eq. (7)). Therefore, elemental P could be generated according to the law of conservation of mass. This phenomenon indicates that other conversion reactions appear during the charge/discharge processes, which result in the generation of elemental P and increase in capacity. The conversion processes could be described as follows:
Li3 P þ 2Fe0 4Fe2 P þ 3Li Li3 P43Li þ P
(7) (8)
Furthermore, the XPS of LiFeP electrode before and after 300 cycles was also evaluated to verify the generation of elemental P. The high-resolution P 2p spectrum of the as-prepared LiFeP is shown in Fig. 4a. The two main peaks with binding energy (BE) values of 128.4 and 129.3 eV correspond to P 2p3/2 and 2p1/2 of LiFeP, respectively [34,35]. The peaks at 132.7 and 133.7 eV may be associated with P 2p3/2 and 2p1/2 caused by the oxidation on the surface of LiFeP [36]. After 300 cycles, the peaks at 128.7 and 129.3 eV can be ascribed to the P 2p3/2 and 2p1/2 of LiFeP, respectively (Fig. 4b). The P 2p peaks (129.2 and 130.1 eV) can be attributed to the Fe-P bond [21]. In addition, the peaks located at 129.8 and 130.5 eV can be ascribed to elemental P [37], suggesting the generation of P during the charge/discharge processes. Furthermore, the two couple peaks at 133.4/134.3 eV and 132.5/133.5 eV can be associated with different oxidation states of phosphorus [36,38]. 4. Conclusions In summary, the plate-like LiFeP was successfully synthesized by the traditional two-step solid-state method and investigated as an anode material for LIBs for the first time. LiFeP exhibited excellent electrochemical properties with a highly reversible cycling capacity of 507 mA h g1 at 300 mA g1 after 300 cycles and outstanding rate capacity (188 mA h g1 at 1000 mA g1). This excellent electrochemical behavior can be attributed to the special preserved
Fig. 4. XPS spectra for the P 2p of (a) the as-prepared LiFeP and (b) LiFeP material after 300 cycles.
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layered crystal structure and remarkable conductivity, which could considerably enhance the diffusion of lithium ions. In addition, the reason for the increase in capacity of LiFeP was also investigated and can be attributed to the high number of conversion reactions of LiFeP and the generation of elemental P during cycling. This result is different from those of previous reports and might explain the reason for the slowly increasing capacity of other phosphides. Considering the above lithium storage performances, LiFeP has a considerable potential application as an anode material for LIBs. Acknowledgments
[17]
[18]
[19]
[20]
This work was supported by the National Natural Science Foundation of China (no. 21671182). The author would like to thank Prof. Maosheng Ma for the assistance in XPS spectrum fitting.
[21]
Appendix A. Supplementary data
[22]
Supplementary data related to this article can be found at https://doi.org/10.1016/j.jpowsour.2017.09.005.
[23]
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LiFeP: A new anode material for lithium ion batteries Jingjing Luo a, Jianbin Zhou a, Dan Lin b, Yi Ren b, Kaibin Tang a, b, * a b
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A new anode LiFeP was investigated for lithium ion batteries. The layered structure of LiFeP was relatively stable during cycling. A discharge capacity of 507 mA h g1 at 300 mA g1 after 300 cycles was obtained. The reason for the increase in capacity of LiFeP was studied.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2017 Received in revised form 1 September 2017 Accepted 2 September 2017
Transition metal phosphides are promising anode materials for lithium ion batteries because of their abundant natural resources and high theoretical capacities. In this study, the electrochemical properties of LiFeP as an anode material for lithium ion battery were investigated for the first time. LiFeP powders were successfully synthesized by a conventional two-step solid-state reaction method. The results of Xray powder diffraction and selected area electron diffraction revealed that the layered plate-like LiFeP was stacked by the (001) crystal plane. As an electrode material, LiFeP delivered a superior reversible capacity of 507 mA h g1 at a high current density of 300 mA g1 after 300 cycles and excellent rate performance. After cycling, the layered structure can be well maintained, which would be greatly beneficial to the electrochemical performance of LiFeP. The reason for the increase in capacity was also investigated and can be attributed to the high number of conversion reactions of LiFeP and the generation of elemental P during cycling. © 2017 Elsevier B.V. All rights reserved.
Keywords: Transition metal phosphides Anode Lithium ion batteries LiFeP
1. Introduction In the past decades, numerous efforts have been made to explore new anode materials for lithium ion batteries (LIBs) because of the low theoretical capacity of commercial graphite (372 mA h g1). It is well known that stable crystal structure and superior conductivity are crucial to achieve an outstanding * Corresponding author. Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026, China. E-mail address: [email protected] (K. Tang). https://doi.org/10.1016/j.jpowsour.2017.09.005 0378-7753/© 2017 Elsevier B.V. All rights reserved.
electrochemical performance of electrode materials during cycling. Therefore, many investigations have been focused on materials with high theoretical capacity, stable crystal structure, and high conductivity. Unfortunately, the available materials have limitations for use in high-performance LIBs. For example, silicon anode has an extremely high theoretical capacity of 3600 mA h g1, but the volume expansion is as high as 300% and the crystal structure can be easily destroyed [1]. Titanium oxide (TiO2) [2,3], titanium niobium oxide (TieNbeO) [4e7], and lithium titanate (Li4Ti5O12) [8,9] are typical low volume expansion materials with high cycling stability and remarkably stable crystal structure. However, their
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theoretical capacities are less than the desired value. Considering these drawbacks, new anode materials with both high theoretical capacity and a relatively stable structure are urgently needed. Recently, transition metal phosphides have attracted considerable attention because of their abundance in natural resources and high theoretical capacities [10e16]. In particular, iron phosphides such as FeP, FeP4, FeP2, and Fe2P have been investigated as anode materials for LIBs owing to their individual high theoretical capacity of 926, 1789, 1365, and 563 mA h g1, respectively [17e20]. In 2006, Boyanov et al. studied the electrochemical mechanism of FeP and demonstrated a complex conversion reaction among FeP, LixFeP, and Li3P [17]. They further reported a high reversible capacity of ~520 mA h g1 in the first cycle for the FeP4-based LIBs [18]. FeP2based LIBs were investigated by Hall et al. They found that the reversible capacity can reach up to 766 mA h g1 in the first charge/ discharge process and increase to 906 mA h g1 after the 10th cycle [19]. Meanwhile, several disadvantages indeed exist in iron phosphides such as irreversible conversion reactions, large volume changes, low conductivity, and destroyed crystal structure, which seriously affect the electrochemical properties and lead to a rapid capacity fading. To improve the electrochemical performance of iron phosphides, tremendous efforts have been made in exploring morphology engineering and carbon coating strategies [21e23]. However, the structural stability problem of phosphides remains unsolved. Thus, enhanced performances are expected by resolving this challenge. LiFeP, which is an important member of iron phosphides, has been newly developed as a superconductor in the last few years. It crystallizes with space group of P4/nmm and an anti-PbFCl-type structure, which is formed by the alternate stacking of anti-PbOtype FeP layers and double Li planes along the [001] direction. The conductivity of LiFeP is about 2 106 S/m at room temperature [24], which contributes to the charge transfer at relatively high current densities to improve the dynamics of oxidation reaction during cycling processes [25,26]. LiFeP-based lithium ion battery with a theoretical capacity of 572 mA h g1 was investigated in our work for the first time. The special layered crystal structure and high conductivity were expected to provide an outstanding electrochemical performance of LiFeP. In fact, the results showed that LiFeP has a relatively stable layered structure during cycling and, interestingly, delivers an increased reversible capacity at high current density and a remarkable rate performance, thus making it a potential anode material for LIBs.
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transmission electron microscopy (TEM, Talos F200X), and selected area electron diffraction (SAED, Talos F200X). The surface chemical state was determined by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). 2.3. Electrochemical characterization To investigate the electrochemical behavior of LiFeP as an anode material for LIBs, CR 2016 coin-type half cells were assembled. The cells consisted of the as-synthesized LiFeP as the working electrode, Li metal as the counter electrode, celgard 2400 as the separator, and a 1.0 M LiPF6 solution of ethylene carbonate/diethyl carbonate (volume ratio 1:1) as the electrolyte. The working electrode was prepared by mixing active material with carbon black and polyvinylidene fluoride at a weight ratio of 6:2:2 in N-methyl-2pyrrolidone. The mixture was then coated on a copper foil and dried in a vacuum oven at 110 C for 10 h. The electrochemical properties of electrodes were tested by the galvanostatic charge/ discharge method on a Neware-BTS-TC53 instrument at a potential range of 0.005e3 V. The cyclic voltammograms (CV) were measured on an electrochemical workstation (CHI660E) at a scanning rate of 0.1 mV s1. 3. Results and discussion The powder XRD pattern of the as-synthesized LiFeP is shown in Fig. 1. The diffraction peaks can be well attributed to the tetragonal structure of LiFeP (JCPDS 87e1501) with lattice parameters of a ¼ 3.69 Å and c ¼ 6.03 Å. The typical layered crystal structure of LiFeP is shown in the inset of Fig. 1, and it would enhance the diffusion of Liþ in electrode. The morphology and microstructural features of the assynthesized LiFeP were investigated by SEM, TEM, and SAED. As illustrated in Fig. 2a and b, LiFeP has a typical stacked plate-like microstructure. The thicknesses of the plates range from hundreds of nanometers to several micrometers. Fig. 2c shows a typical TEM image of plate-like LiFeP. The corresponding SAED pattern (Fig. 2d) reveals a single crystal feature, which can be assigned to a tetragonal cell with a ¼ b ¼ 3.7 Å, demonstrating that the plates are stacked by the (001) crystal plane matching well with the results from the XRD pattern. To investigate the electrochemical performance of LiFeP, half cells were assembled and tested by the galvanostatic charge/ discharge method. The CV curves of the LiFeP electrode within a
2. Experimental 2.1. Synthesis of LiFeP LiFeP was synthesized by a conventional two-step solid-state reaction method. First, FeP was obtained by sealing Fe (98%) and P (99.999%) powders (molar ratio 1:1) into an evacuated quartz tube and annealing them at 700 C for 24 h. Later, LiFeP was prepared by mixing stoichiometric amounts of Li (99.9%) and the as-prepared FeP in an evacuated quartz tube and sintering them at 850 C for 24 h. All the processes were performed in a high-purity Ar-protected glovebox. A black product was obtained after the tube was cooled down to room temperature naturally. 2.2. Material characterization The phase characterization of the as-prepared product was performed on a Philips X'pert X-ray diffractometer (Cu Ka radiation, l ¼ 1.54182 Å). The morphology and structural characterization were examined by scanning electron microscopy (SEM, S-4800),
Fig. 1. X-ray diffraction pattern of the as-prepared LiFeP. The LiFeP structural frame is shown in the inset.
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Fig. 2. SEM images (a) and (b) of the as-prepared LiFeP, (c) TEM image and (d) selected area electron diffraction pattern of the as-prepared LiFeP.
potential window of 0.005e3 V are shown in Fig. 3a. The oxidation and reduction peaks correspond to the charge/discharge profiles in Fig. 3c. The peaks located at 1.16 V in the first discharge can be assigned to a simple lithium ion insertion reaction (Eq. (1)) [27]. The irreversible reduction peak at 0.72 V can be attributed to the side reaction between active materials and electrolyte for the formation of solid electrolyte interface (SEI) membrane. A following cathode peak at 0.35 V can be associated with the reduction of partial LiFeP phase into Fe0 and Li3P (Eq. (2)) [28]. During the following charge process, the two oxidation peaks at 0.11 and 1.43 V, respectively, can be ascribed to a two-step lithium ion deintercalation reaction from the interlamination of Li1þxFeP phase (Eq. (3) and Eq. (5)), while the oxidation peak at 0.92 V can be associated with the transformation of Li3P and Fe0 to LiFeP (Eq. (4)). The reaction processes in the first cycle accord with the changes in the XRD patterns of LiFeP at different potentials (Fig. S1, Supplementary Information). In the subsequent two scanning cycles, the oxidation peaks and the reduction peaks at 1.49, 1.16, and 0.35 V repeat well, indicating good reversible electrochemical reactions of LiFeP in Li half cells. In addition, a new weak peak at around 1.06 V emerged initially during the second charge process and remained in the following charge processes, which can be ascribed to the delithiation from LiFeP to form a small quantity of FeP (Eq. (6)). From these observations, the theoretical reaction processes of LiFeP can be described using the following equations. Discharge processes
LiFeP þ xLi/Li1þx FeP
(1)
LiFeP þ 2Li/Li3 P þ Fe0
(2)
Charge processes
Li1þx FeP/yLi þ Li1þxy FeP
(3)
Li3 P þ Fe0 /LiFeP þ 2Li
(4)
Li1þxy FeP/LiFeP þ ðx yÞLi
(5)
LiFeP/Li þ FeP
(6)
Fig. 3b shows the cycling performance of LiFeP at a current density of 300 mA g1. The first discharge and charge capacities were 282 and 218 mA h g1, respectively. From Fig. 3a and b, the low discharge capacity indicates that the insertion/de-insertion processes were dominant, and the conversion reaction was limited during the first cycling. The corresponding Coulombic efficiency was 77.3%. The capacity lost in the first cycle can be attributed to some irreversible reactions such as the formation of SEI membrane. After 30 cycles, the reversible discharge capacity was stable at 222 mA h g1. Then, the discharge capacity slowly increased to 507 mA h g1 after 300 cycles. It is worth noting that the corresponding Coulombic efficiency was maintained over 98% from the second cycle, indicating the highly reversible lithium storage ability of LiFeP anode. In addition, the XRD pattern and SEM images of LiFeP anode after 300 cycles were also collected. As shown in Fig. 3e, the diffraction peak at 14.95 remained visible, suggesting that the layered structure along the [001] direction of LiFeP is very stable after cycling. Meanwhile, the quite smooth and complete film coated over the surface of electrode (Fig. 3g) indicates that the SEI membrane of LiFeP electrode is highly stable, which is very helpful to improve the cycling stability. Furthermore, LiFeP maintained part of the initial plate-like microstructure even after 300 cycles (Fig. 3h), which is in agreement with the result from Fig. 3e. By combining this result with the cycle performance, it
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Fig. 3. Electrochemical performance of LiFeP: (a) initial CV curves of LiFeP at a voltage range of 0.005e3 V at a scanning rate 0.1 mV s1; (b) the cycling performance and corresponding Coulombic efficiency at a current density of 300 mA g1; (c) the charge and discharge curves at a current density of 300 mA g1 between 0.005 and 3 V for the 1st, 2nd, 50th, and 300th cycles; (d) the rate capacity curves at current densities of 300, 500, 600, 800, 1000, and 300 mA g1; (e) XRD pattern of the LiFeP material after 300 cycles; (f) CV curves of LiFeP after 300 cycles at 300 mA g1 between 0.005 and 3 V; SEM images of (g) the top view of LiFeP electrode after 300 cycles, (h) LiFeP material after 300 cycles.
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Table 1 Comparison of the electrochemical performance among the as-obtained LiFeP in this work, decorated FeP, and other transition metal phosphides. Material
Reversible capacity (mA h g1)
Bulk FeP Mesoporous FeP FeP FeP@C FeP@C nanoplates MnP CoP Ni2P CuP2 LiFeP
310 355 334 480 610 287 510 296 554 507
(50th cycle) (30th cycle) (50th cycle) (200th cycle) (400th cycle) (50th cycle) (400th cycle) (50th cycle) (50th cycle) (300th cycle)
Current density (mA g1)
Ref.
185.2 144 100 30 400 50 400 54.2 200 300
17 23 27 22 21 29 30 16 28 This work
can be demonstrated that a high number of conversion reactions of LiFeP occur during cycling, which can result in the part fading of the crystal structure and the increase in capacity. Nevertheless, LiFeP has a relatively stable layered structure compared with other phosphides during cycling, which would considerably enhance the diffusion of Liþ in active materials for the improvement of cycling and rate performance. The rate performance of LiFeP at different current densities is shown in Fig. 3d. With the increase in current density from 300 to 500, 600, 800, and 1000 mA g1, the observed discharge capacities were 270, 218, 206, 195, and 188 mA h g1, respectively. In particular, the capacity recovered to 258 mA h g1 when the current density was decreased to initial value, i.e., 300 mA g1, which is approximately 96% of the initial capacity during the first 10 cycles. The Coulombic efficiency was always over 98% for different current densities. The excellent rate performance of LiFeP can be ascribed to the stable layered structure. Herein, a comparison of the electrochemical performance was made among the as-obtained LiFeP, decorated FeP [17,21e23,29], and other pure transition metal phosphides [16,30e32]. As presented in Table 1, LiFeP shows a much better electrochemical performance than most of these anode materials. In particular, the LiFeP electrode can possess a remarkable reversible capacity of 507 mA h g1 even at a high current density of 300 mA g1. Therefore, it can be concluded that LiFeP has a superior capability to overcome the capacity fading and maintains a good reversible capacity even at a high current density. The increase in capacity during cycling has been observed in various anode materials. From the above investigations, it is known that the occurrence of a high number of conversion reactions of LiFeP during cycling can lead to the increase in capacity. To investigate other possible reasons for the increase in capacity, CV curves for LiFeP electrode after 300 cycles in a potential range of
0.005e3 V were measured. As displayed in Fig. 3f, the oxidation peak at 0.11 V and broad reduction peak at about 1.03 V can be attributed to the intercalation/de-intercalation processes of lithium ions from the interlamination of LiFeP, which match with the initial CV curves, indicating the outstanding cycling stability and cycle life. However, a new oxidation peak at 0.61 V and two new reduction peaks at 1.02 and 1.20 V were observed, as shown in Fig. 3f, which imply the different electrochemical reactions compared with the initial ones (Fig. 3a). Regarding the previous report on P anodes [33], the new redox peaks at 0.61, 1.02, and 1.20 V can be attributed to the lithiation/de-lithiation platform of elemental P. In fact, a broader and higher peak of Fe2P than the one in initial compounds emerged after 300 cycles, as shown in Fig. 3e (Eq. (7)). Therefore, elemental P could be generated according to the law of conservation of mass. This phenomenon indicates that other conversion reactions appear during the charge/discharge processes, which result in the generation of elemental P and increase in capacity. The conversion processes could be described as follows:
Li3 P þ 2Fe0 4Fe2 P þ 3Li Li3 P43Li þ P
(7) (8)
Furthermore, the XPS of LiFeP electrode before and after 300 cycles was also evaluated to verify the generation of elemental P. The high-resolution P 2p spectrum of the as-prepared LiFeP is shown in Fig. 4a. The two main peaks with binding energy (BE) values of 128.4 and 129.3 eV correspond to P 2p3/2 and 2p1/2 of LiFeP, respectively [34,35]. The peaks at 132.7 and 133.7 eV may be associated with P 2p3/2 and 2p1/2 caused by the oxidation on the surface of LiFeP [36]. After 300 cycles, the peaks at 128.7 and 129.3 eV can be ascribed to the P 2p3/2 and 2p1/2 of LiFeP, respectively (Fig. 4b). The P 2p peaks (129.2 and 130.1 eV) can be attributed to the Fe-P bond [21]. In addition, the peaks located at 129.8 and 130.5 eV can be ascribed to elemental P [37], suggesting the generation of P during the charge/discharge processes. Furthermore, the two couple peaks at 133.4/134.3 eV and 132.5/133.5 eV can be associated with different oxidation states of phosphorus [36,38]. 4. Conclusions In summary, the plate-like LiFeP was successfully synthesized by the traditional two-step solid-state method and investigated as an anode material for LIBs for the first time. LiFeP exhibited excellent electrochemical properties with a highly reversible cycling capacity of 507 mA h g1 at 300 mA g1 after 300 cycles and outstanding rate capacity (188 mA h g1 at 1000 mA g1). This excellent electrochemical behavior can be attributed to the special preserved
Fig. 4. XPS spectra for the P 2p of (a) the as-prepared LiFeP and (b) LiFeP material after 300 cycles.
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layered crystal structure and remarkable conductivity, which could considerably enhance the diffusion of lithium ions. In addition, the reason for the increase in capacity of LiFeP was also investigated and can be attributed to the high number of conversion reactions of LiFeP and the generation of elemental P during cycling. This result is different from those of previous reports and might explain the reason for the slowly increasing capacity of other phosphides. Considering the above lithium storage performances, LiFeP has a considerable potential application as an anode material for LIBs. Acknowledgments
[17]
[18]
[19]
[20]
This work was supported by the National Natural Science Foundation of China (no. 21671182). The author would like to thank Prof. Maosheng Ma for the assistance in XPS spectrum fitting.
[21]
Appendix A. Supplementary data
[22]
Supplementary data related to this article can be found at https://doi.org/10.1016/j.jpowsour.2017.09.005.
[23]
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