C Composites as an Anode Material for K-Ion Batteries | ACS

Jun 4, 2019 - (21−23) Using metal phosphides as the anode is an effective way to ... Figure 2c shows the transmission electron microscopy (TEM) imag...
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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22364−22370

FeP/C Composites as an Anode Material for K‑Ion Batteries Wenting Li,†,‡,§ Baijun Yan,† Hongwei Fan,§ Chao Zhang,† Hanying Xu,‡ Xiaolu Cheng,‡ Zelin Li,§ Guixiao Jia,*,§ Shengli An,*,†,§ and Xinping Qiu*,‡ †

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China § School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China Downloaded via BUFFALO STATE on July 17, 2019 at 07:54:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Owing to their natural abundance, the low potential, and the low cost of potassium, potassium-ion batteries are regarded as one of the alternatives to lithium-ion batteries. In this work, we successfully fabricated a FeP/C composite, a novel electrode material for PIBs, through a simple and productive high-energy ball-milling method. The electrode delivers a reversible capacity of 288.9 mA·h·g−1 (2nd) at a discharge rate of 50 mA g−1, which can meet the future energy storage requirements. Density functional theory calculations suggest a lower diffusion barrier energy of K+ than Na+, which allows faster K+ diffusion in FeP.

KEYWORDS: FeP/C, anode material, KIBs, high-energy ball-milling method, electrochemical performance, DFT calculations

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INTRODUCTION Over decades, renewable energy, such as solar energy and wind energy, has been one of the most important energy sources in the world.1,2 However, the renewable energy is limited by time and space.3 Hence, it is crucial to develop electric energy storage systems to improve the stability and efficiency of the electricity supply system. Lithium-ion batteries (LIBs), as the most dominant energy storage devices, have made remarkable progress in recent years.4−6 However, the uneven distribution and scarcity (only 0.0017%) of lithium resource in the earth’s crust impedes its application.7−9 Therefore, sodium-ion batteries, potassium-ion batteries, and K−S batteries have been studied as low cost and material-abundant alternatives to LIBs.10−14 KIBs can offer a higher voltage plateau and energy density than SIBs because its electrochemical standard potential (−2.93 V vs E°) is lower than that of Na/Na+ (−2.71 V vs E°) and closer to Li/Li+ (−3.04 V vs E°).15 Up to now, anode materials of KIBs have been well investigated, such as graphene,16 metal phosphides,17 carbon,18 Ti-based compounds,19 mental sulfide,20 and so forth. Among them, phosphorus is an attractive choice because of its high theoretical capacity (2594 mA·h·g−1). However, its poor conductivity and large volume changes during cycling hinder its application.21−23 Using metal phosphides as the anode is an effective way to alleviate this problem, which has been demonstrated in LIBs and SIBs.24,25 Zhang et al. investigated the electrochemical performance of the Sn4P3 anode in KIBs.17 A reversible capacity of 385 mA·h·g−1 was achieved at 50 mA· g−1, where the K-storage mechanism was characterized as a combination of the conversion reaction (P to K3P11, K3P) and © 2019 American Chemical Society

alloying (Sn to KSn) reaction. However, a fast fade of capacity was also observed after 40 cycles because of the large volume change during charge/discharge processes. In order to alleviate the volume change, a kind of core−shell-like cobalt phosphide nanoparticles embedded in porous carbon sheets was synthesized.26 Because of the superiority of the microstructures and advantageous electronic and ionic transport within the composite, core−shell-like cobalt phosphide nanoparticles showed outstanding electrochemical performances in KIBs. The GeP5 compound was also used as an anode material for PIBs, which had a reversible high specific capacity over 500 mA·h·g−1.27 The reaction mechanism of GeP5 was also considered as a combination of conversion-type (P to K4P3) and alloy-type (Ge to KGe) reactions. Although these phosphides have demonstrated promising electrochemical performance, their high cost still limits the large-scale applications. Iron (Fe) has the benefit of low cost, and its content in the earth’s crust is about 5%.28 Hence, iron phosphide (FeP) would be an ideal anode material to meet the request of a large-format battery. Here, we prepared FeP via a simple ball milling method and studied its electrochemical behaviors in KIBs. After mixing FeP with carbon, the as-prepared FeP/C composite exhibited good cyclic stability and rate capabilities. Received: March 17, 2019 Accepted: June 4, 2019 Published: June 4, 2019 22364

DOI: 10.1021/acsami.9b04774 ACS Appl. Mater. Interfaces 2019, 11, 22364−22370

Research Article

ACS Applied Materials & Interfaces

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RESULTS AND DISCUSSION The crystal structures of FeP and FeP/C composites were detected by X-ray diffraction (XRD) in Figure 1a. All the

Figure 1. (a) XRD patterns of FeP and FeP/C composites; (b) Raman spectra of Super P carbon black, FeP, and FeP/C composite (c) TGA profiles of FeP and FeP/C composites.

Figure 2. (a) SEM images of FeP and (b) the FeP/C composite; (c) TEM image, (d) HRTEM image, and (e) SAED pattern of the FeP/C composite; (f) EDS mapping of constituent elements.

The X-ray photoelectron spectroscopy (XPS) spectrum was carried out further to identify the surface elemental compositions and chemical bonding states of the FeP/C composite. The survey spectrum in Figure 3a verifies the existence of Fe, P, and C elements on the surface of the sample in accordance with the above EDS elemental mapping results. The O element comes from surface oxidation of the FeP/C composite due to its exposure to air during the sample transfer. Figure 3b presents a high-resolution XPS spectrum of Fe 2p. Typical peaks, located at 707.6 and 720.1 eV, correspond to Fe 2p3/2 and Fe 2p1/2 of FeP, while the other broader peaks at 712.3 and 725.2 eV may be caused by the inevitable surface oxidation of FeP.30 The high-resolution XPS spectrum of P 2p was fitted with three components (Figure 3c). The peaks at 130 and 130.9 eV are corresponding to the binding energy of P 2p3/2 and P 2p1/2, which can belong to the P−Fe species in FeP. While the peak at 133.9 eV can belong to the partial surface oxidation of P, when the sample was exposed to an air atmosphere.31 For the high-resolution C 1s spectra, four fitted peaks are observed at around 284.7, 285.3, 286.1, and 289.1 eV, which can be assigned to CC/C−C, C−O, CO, and O−CO bonds (Figure 3d). In order to investigate the electrochemical performance of the FeP/C composite, coin-type half-cells were assembled and the electrochemical measurements were performed at room temperature. Figure 4a shows the cyclic voltammetry (CV) curves obtained at a scan rate of 0.1 mV·s−1 in a potential range of 0.01−3 V for 3 cycles. In the first scan, a distinct cathodic peak can be found at about 0.34 V, which is caused by the formation of the solid electrolyte interphase (SEI) film on the surface of the electrode. During the following scanning, redox peaks at about 0.8/1.8 V, which also appear in the subsequent scans, are associated with potassiation and depotassiation processes of the FeP/C composite. After the first cycle, the CV curves tend to overlap with each other, indicating the high reversibility of the FeP/C composite. The galvanostatic charge

samples are well indexed to the orthorhombic Pnma FeP (JCPDS card no. 65-2595) without Fe or P residues. The crystal size of FeP, calculated based on the Debye−Scherrer formula, is about 11.9 nm. Furthermore, the existence of C in the FeP/C composite was confirmed by the Raman spectra in Figure 1b. There are no obvious peaks in FeP, and two characteristic peaks located at around 1340 and 1380 cm−1 can be observed for the FeP/C composite, corresponding to the D band (sp3-type disordered carbon) and G band (sp2-type ordered graphitic carbon) of carbon.29 The thermogravimetry analysis (TGA) was performed (Figure 1c) to obtain the actual content of carbon. From the TGA analysis results of FeP and FeP/C composites based on the oxidation reaction, the carbon content of FeP/C is 19.53%, consistent with that of the actual additional amount (20%), indicating no reaction of carbon with phosphorus materials. Figure 2a,b show the morphology of FeP and FeP/C composites analyzed by scanning electron microscopy (SEM). The particles of the FeP/C composite are irregular agglomerates with a size of 0.1−1 μm, which are smaller than that of FeP. Figure 2c shows the transmission electron microscopy (TEM) image of the FeP/C composite. It can be clearly observed that FeP particles with a diameter of 10−30 nm are distributed homogeneously in the amorphous carbon matrix. Here, carbon serves not only as a conductive additive for electrons but also as buffer to suppress the volume changes of FeP during the charge−discharge. The high-resolution TEM (HRTEM) image (Figure 2d) shows that the interlayer spacing in FeP is about 0.273 nm, which corresponds to the d(011) spacing. The corresponding selected area electron diffraction (SAED) pattern (Figure 2e) displays four diffraction rings, which can be assigned to (011), (111), (211), and (203) planes of FeP. EDS elemental mapping (Figure 2f) exhibits a uniform distribution of the elements of C, Fe, and P in the FeP/C composite. 22365

DOI: 10.1021/acsami.9b04774 ACS Appl. Mater. Interfaces 2019, 11, 22364−22370

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ACS Applied Materials & Interfaces

Figure 3. XPS spectra of the FeP/C composite: (a) survey, (b) Fe 2p, (c) P 2p and (d) C 1s.

Figure 4. Electrochemical performance of the FeP/C composite for PIBs: (a) CV curves of Fe/C at a rate of 0.1 mV·s−1 between 0.01 and 3.0 V; (b) discharge/charge profiles of Fe/C at a current density of 50 mA·g−1; (c) cycling performance of Fe/C at a current density of 50 mA·g−1; (d) rate performance of FeP/C and FeP in PIBs at various current densities from 50 to 1000 mA·g−1.

and discharge profiles of the FeP/C composite at 50 mA·g−1 from 0.01 to 3.0 V are presented in Figure 4b. In the first cycle, the discharge capacity is 483.79 mA·h·g−1 and the charge capacity is only 218.26 mA·h·g−1, corresponding to a Coulombic efficiency of 45.11%. The irreversible capacity loss may result from the formation of the SEI layer and electrolyte decomposition. In the following cycles, the Coulombic efficiency rapidly increases and the curves almost overlapped with each other, consistent with the CV curves. The cycle performance of the FeP/C composite was investigated and compared with FeP between 0.01 and 3.0 V

at 50 mA·g−1 over 50 cycles. Figure 4c shows that the second discharge capacity of the FeP/C composite is 288.9 mA·h·g−1, and it still remains of 63.2% after 50 cycles, much higher than that of FeP. The FeP/C composite achieves a better cycling stability due to the addition of carbon, which can improve the electronic conductivity and suppress the volume change of FeP effectively. The presence of carbon can also improve the rate performance of the material. As depicted in Figure 4d, the rate performances of FeP and FeP/C composites were examined at various current densities, ranging from 50 to 1000 mA·g−1 and then returned to 50 mA·g−1. The FeP/C composite shows a 22366

DOI: 10.1021/acsami.9b04774 ACS Appl. Mater. Interfaces 2019, 11, 22364−22370

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Figure 5. (a) Nyquist plots of FeP and FeP/C composites before cycling (the inset shows the relevant equivalent circuit); (b) the linear fits of the Z′ vs ω−1/2 in the low frequency region.

Figure 6. (a) CV curve of the FeP/C composite at different scan rates; (b) relationship between the peak currents and scan rates in logarithmic format; (c) capacitive contribution (red region) to the charge storage at 0.1 mV·s−1; (d) diffusion-controlled CV curve of the FeP/C composite; (e) CV curve of diffusion controlled part of the FeP/C composite; (f) ex situ XRD patterns of the FeP/C composite at different states.

plots of the FeP and FeP/C composite at the frequency range from 0.1 Hz to 100 kHz, and the inset shows the equivalent circuit. The FeP/C composite presents a lower charge-transfer resistance (1293 Ω) than FeP (1580 Ω) before the charge/ discharge cycling test, suggesting a higher interfacial electron transfer rate. The K+ diffusion coefficient in the electrodes can be determined by plotting the linear of the real part of impedance (Z′) versus the square root of the angular frequency (ω−1/2) in a low frequency region32 (Figure 5b). The slope of the FeP/C composite is much smaller than FeP,

better rate performance than FeP, delivering discharge capacities of 232.21, 185.82, 156.29, 112.52, 89.13, and 78.69 mA·h·g−1 at the rates of 50, 100, 200, 500, 800, and 1000 mA·g−1, respectively. In addition, when the current density returns back to 50 mA·g−1, the reversible discharge capacity of the FeP/C composite is still higher than that of FeP. The influence of carbon on the charge transfer and diffusion kinetics of FeP was investigated by electrochemical impedance spectra (EIS) measurements. Figure 5a shows the Nyquist 22367

DOI: 10.1021/acsami.9b04774 ACS Appl. Mater. Interfaces 2019, 11, 22364−22370

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Figure 7. (a) Diffusion pathway of the Na+/K+ ions in the FeP structure; (b) diffusion barrier energy of Na+ ions and K+ ions along the diffusion pathway of FeP; (c) rate performance of the FeP/C composite in PIBs and SIBs at various current densities from 50 to 1000 mA·g−1.

electrode is thin, the diffraction peaks of the current collector (Al foil) are obvious and the characteristic peaks of iron phosphide are weak. When the electrode is discharged to 0.01 V, no distinctive characteristic peaks of Fe and K3P can be observed, which may be due to the small size of the particles of the final products. Peaks of FeP appear again when the potential is further charged to 3.0 V, indicating a good reversibility of FeP. FeP was also applied as an anode material in SIBs.29,37,38 To illustrate the differences between sodiation and potassiation of FeP, we performed a density functional theory (DFT) calculation to obtain the diffusion behavior of Na+ and K+ in FeP. A 3a × 3b × 1c supercell of orthorhombic FeP (space group Pnma(62), a = 1.544 nm, b = 0.917 nm and c = 0.575 nm) was built, and we found the most stable doped position of Na+ and K+. Figure 7a shows the diffusion pathway of K+ and Na+, where the most stable doped structure and the neighboring equivalent one served as the reactant and product. The calculated K+ ion adsorption energy is −1.6 eV, while the calculated Na+ ion adsorption energy is −1.2 eV. This indicates that K−FeP adsorption interactions are more preferable than the Na−FeP ones because K has more electrons and a larger radius than Na. Figure 7b shows diffusion barrier energies along the diffusion pathway, it is clearly seen that the diffusion barrier energies are 2.99 eV for K+ ions and 5.53 eV for Na+ ions, respectively, meaning that K+ can diffuse faster than Na+ in FeP. The adsorption energy difference (0.4 eV) between K− FeP and Na−FeP interactions is much smaller than the diffusion barrier energy difference (2.54 eV) of K+/Na+ in FeP, indicating that ion diffusion barrier energy is a crucial parameter for battery performance. In all, PIBs have a better performance than SIBs, matched by the experimental results. In order to confirm this result, the rate capability of the FeP/ C composite in SIBs was also tested. Figure 7c compares the rate performance of the FeP/C anode material in PIBs and SIBs. The specific capacity of the FeP/C composite in SIBs is close to those in PIBs at low rates of 50−200 mA·g−1, while

indicating that the electrochemical kinetics of FeP can be enhanced by mixing with carbon. Because the FeP/C composite has better rate capability and cycle performance, it is necessary to study its electrochemical kinetics to separate the two contributions of the capacitive and diffusion-controlled process by CV measurement. Figure 6a shows the CV curves of the FeP/C composite at scan rates from 0.1 to 1.0 mV·s−1 over 0.01−3.0 V. The capacity contribution of capacitive- and diffusion-controlled processes was distinguished by the eq 1.33,34 i = avb

(1)

By plotting log(i) versus log(v), the b value can be obtained from the slope of the profile. Based on the CV curves of 0.1− 1.0 mV·s−1, the b value at the anodic and cathodic peaks were determined to be 0.825 and 0.915, respectively (Figure 6b), which implies that the capacitance is the dominant electrochemical process. The capacity ratio between the two contributions can be quantified at a fixed voltage. i(V ) = k1v + k 2v1/2

(2)

The capacitive contribution was represented as k1v and the diffusion controlled part was represented as k2v1/2. As shown in Figure 6c, the capacity ratio of capacitive contribution (red region) is 50.74% at 0.1 mV·s−1, and gradually increases to 77.28% with the increase of the scan rate to 1.0 mV·s−1 (Figure 6d). The CV curve of the diffusion controlled part of the FeP/ C composite is shown in Figure 6e. A cathodic peak can be observed at about 0.84 V and an anodic peak at 1.78 V with a large voltage hysteresis of 0.94 V, a typical characteristic of conversion reaction.35,36 Combining with previous studies on LIBs and sodium ion batteries,37,38 the potassiation reaction of FeP can be deduced as the following FeP + 3K+ + 3e− ↔ K3P + Fe

(3)

Further confirmation of the potassiation mechanism of FeP was carried out through ex situ XRD of the electrode at different SOC (Figure 6f). Because the coating layer on the 22368

DOI: 10.1021/acsami.9b04774 ACS Appl. Mater. Interfaces 2019, 11, 22364−22370

Research Article

ACS Applied Materials & Interfaces exhibits 44.48 mA·h·g−1 at a rate of 1000 mA·g−1, much lower than that in PIBs.

obtain the transition states using the five images along the reaction coordinate.

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CONCLUSIONS In summary, we fabricated FeP and FeP/C composites via a facile and inexpensive high-energy ball-milling method. After mixing with carbon, the cycle performance and rate performance of the FeP/C composite are greatly improved. The FeP/ C composite shows a discharge capacity of 288.9 mA·h·g−1 at 50 mA·g−1 and still has 112.52 mA·h·g−1 at a high current density of 500 mA·g−1, which is much better than that of pure FeP. The capacity was still 178.56 mA·h·g−1 after 50 cycles. By comparing with that in SIBs, the FeP/C composite presents a better rate performance in KIBs. We attribute this to the lower diffusion barrier energy of K+ in the FeP structure than that of Na+. This work suggests that the FeP/C composite can be expected to be a potential candidate of anode materials for KIBs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.J.). *E-mail: [email protected] (S.A.). *E-mail: [email protected] (X.Q.). ORCID

Hanying Xu: 0000-0002-7102-8428 Xinping Qiu: 0000-0001-5291-7943 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate financial support from Project from the National Key Research and Development Program (2016YFB0901703), Beijing Municipal Science and Technology Commission (Z181100004718006), National Natural Science Foundation of China (U1664256, 51474133), China-US Electric Vehicle Project (S2016G9004), and National Key Project on Fundamental Research Program (2015CB251104).

EXPERIMENTAL SECTION

Material Preparation. Fe powder (≥99%) was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China), and red phosphorus powder (98.5%) from Energy Chemical (Shanghai, China). All the other chemicals were used as received without further purification. FeP powder was prepared by mechanical ball milling of the mixture of Fe (1.07 g) and red phosphorus (1.93 g) powder for 40 h, then the mixture of the FeP powder and Super P carbon black with a weight ratio of 8:2 was further ball milled for another 10 h. The ball mill process was carried out under an argon atmosphere. The weight ratio of balls to powder was 20:1 and the rotation speed was 450 rpm. Material Characterization. The crystal structure of the products was characterized by XRD on a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα irradiation with a step of 5 min−1 over a 2θ range of 10°−80°. SEM (Zeiss MERLIN) and TEM (JEM 2100F) were employed to observe the morphology and elemental composition of the products. The Raman spectrum was performed by a HORIBA LabRAM HR Evolution with 532 nm laser excitation. The TGA (METTLER TOLEDO) was conducted from room temperature up to 800 °C under flowing air. XPS was carried out using an ULVAC PHI Quantro SXM. Electrochemical Measurements. Working electrodes were prepared by mixing the active material, Super P carbon black, and sodium alginate binder with deionized water in the weight ratio of 8:1:1 to form a homogeneous slurry. Then, the obtained slurry was coated on Al foil and dried at 80 °C or 12 h in a vacuum oven. The electrolyte was 0.8 M solution of KPF6 in ethylene carbonate/diethyl carbonate (1:1, by volume). Celgard 2325 and a glass microfiber filter (Whatman GF/D) were used as separators and K metal foil was used as the counter electrode. CR2032 coin cells were assembled in an argon-filled dry box. Galvanostatic measurements were tested using a Neware BTS battery test system (Neware Co., Ltd., Shenzhen, China) in the potential range of 0.01−3.00 V (vs K/K+) at room temperature. CV and EIS were conducted with a PARSTAT 2273 electrochemical workstation (AMETEK). CV curves were collected at a scan rate of 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 mV·s−1. EIS measurements were conducted in the frequency range from 0.1 Hz to 100 kHz with a 5 mV amplitude. Computational Details. All the DFT calculations in this work were performed using the Vienna Ab initio Simulation Package. A spin-polarized generalized gradient approximation with the Perdew− Burke−Ernzerhof exchange−correlation functional was employed to study the diffusion of Na/K in FeP. All the geometric structures were fully relaxed. Total energy convergence criterion is 10−4 eV. Valence electrons Fe 3s23p63d64s2, P 3s23p3 and Na 3s13p0, and K 3s23p64s0 were expanded in plane waves with a cutoff energy of 400 eV. A climbing image nudged elastic band algorithm was employed to

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DOI: 10.1021/acsami.9b04774 ACS Appl. Mater. Interfaces 2019, 11, 22364−22370

Research Article

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DOI: 10.1021/acsami.9b04774 ACS Appl. Mater. Interfaces 2019, 11, 22364−22370