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Sodium-rich ferric pyrophosphate cathode for stationary room-temperature sodium-ion batteries Bolei Shen, Maowen Xu, Yubin Niu, Jin Han, Shi-Yu Lu, Jian Jiang, Yi Li, Chunlong Dai, Linyu Hu, and Chang Ming Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13516 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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ACS Applied Materials & Interfaces

Sodium-Rich Ferric Pyrophosphate Cathode for Stationary Room-Temperature Sodium-Ion Batteries Bolei Shen,†,‡ Maowen Xu*,†,‡ Yubin Niu, †,‡ Jin Han, †,‡ Shiyu Lu, †,‡ Jian Jiang, †,‡ Yi Li, †,‡ Chunlong Dai, †,‡ Linyu Hu, †,‡ Changming Li*,†,‡ † Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing 400715, P. R. China ‡ Chongqing Key Laboratory for Advanced Materials & Technologies of Clean Energies, Chongqing 400715, P. R. China * E-mail: [email protected] and [email protected]. ABSTRACT: In this paper, carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles (～10 nm) were successfully synthesized via a facile sol-gel method and employed as cathode materials for sodium-ion batteries. The results show that the carbon-coated Na3.64Fe2.18(P2O7)2 cathode delivers a high reversible capacity of 99 mAhg-1 at 0.2 C, outstanding cycling life retention of 96 % and high coulomb efficiency of almost 100 % even after 1000 cycles at 10 C. Furthermore, the electrochemical performances of full batteries consisting of the carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles as cathode and commercialized hard carbon as anode are tested. The full batteries exhibit a reversible capacity of 86 mAhg-1 at 0.5 C and capacity retention of 80% after 100 cycles. Therefore, the above cathode is a potential candidate for developing inexpensive sodiumion batteries in large-scale energy storage with long life.

KEYWORDS: Na3.64Fe2.18(P2O7)2; cathode; capacity; long-cycle; half-cell; full-cell

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1. INTRODUCTION Demand for lithium-ion batteries (LIBs) is increasing with the rapid development of portable electronic devices and the popularity of electric vehicles. However, further application of LIBs is restricted by several shortcomings, such as high price and rare resources of lithium. In comparison, sodium is much more abundant than lithium, which means that the cost of sodiumion batteries (SIBs) is lower. Therefore, SIBs are undoubtedly a good alternative to lithium-ion batteries. In recent years, the research of SIBs cathode has tended to be mature. There are usually two main types of SIBs cathode, one is layered transition-metal oxides, and the other is polyanion materials. Layered transition-metal oxides, such as NaxCoO2,1,2 NaxVO2,3 NaxFeO2,4 NaCrO2,5 Na2/3Ni1/3Mn1/2Ti1/6O2,6 and Na0.5Ni0.25Mn0.75O2,7 have a high theoretical capacity and various voltage platforms. However, the cycle stability of these materials is unsatisfactory. Polyanionic materials, such as Na4Mn3(PO4)2(P2O7),8 Na2VOP2O7,9 Na7V4(P2O7)4(PO4),10,11 Na4Co3(PO4)2(P2O7),12 NaVOPO4,13 NaFePO4,14-16 Na2FeP2O7,17-19 Na4Fe3(PO4)2(P2O7),20 Na2Fe2(SO4)3,21 Na2MnP2O7,22,23 Na2CoP2O7,24 Na3V2(PO4)3,

25-28

and so on. Different from

layered metal oxides, these polyanion materials have more stable crystal structures, which lead to long, stable cycles, enhanced safety and adjustable voltages.13 Furthermore, the voltages of these materials are often higher than that of layered transition-metal oxides due to the inductive effect from phosphorus.2 Thus, polyanion materials may be more promising for commercialization of SIBs. Among these polyanion materials, the newly reported ferric pyrophosphate sodium host series Na4- α M2+ α /2(P2O7)229 has gained much attention, some non-stoichiometric compounds, such as Na3.12Fe2.44(P2O7)2

30,31

and Na3.32Fe2.34(P2O7)2.32 have been reported. In these ferric


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pyrophosphate family members, sodium-rich cathode may promote a better cycle stability. Hence, we try to design and synthesize a kind high sodium ratios compound of ferric pyrophosphate. In this work, a new sodium-rich Na3.64Fe2.18(P2O7)2 compound as cathode materials for SIBs is explored for the first time. Considering that the polyanion materials are suffering from the poor conductivity and atmosphere sensitivity,29 nano-sized and carbon-coated Na3.64Fe2.18(P2O7)2 was designed and synthesized by a simple sol-gel method. The as-prepared Na3.64Fe2.18(P2O7)2 shows excellent electrochemical performances.

2. EXPERIMENTAL SECTION Materials preparation All chemical reagents were analytical grade and used without further purification. Sol-gel and pre-oxidation methods were combined to achieve both carbon-coated Na3.64Fe2.18(P2O7)2

nanoparticles

and

pristine

Na3.64Fe2.18(P2O7)2.

Firstly,

1.7614

g

Fe(NO3)3·9H2O was dissolved into 30 mL distilled water to obtain a brown solution, which was followed by a slow addition of 0.3858 g Vitamin C. 2.2362 g citric acid, as a chelating agent, 0.3858 g Na2CO3, and 0.9202 g NH4H2PO4 were then added and dissolved in the solution. Continuous stirring was performed until a clear solution was achieved. The solution was heated to 80 °C with continuous stirring for 8 h, and then dried at 120 °C in a vacuum to obtain a dry gel. To obtain the carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles, the dry gel was heated in H2/Ar (10:90) atmosphere at 600 °C for 24 h. The color of the as-synthesized sample was black (Figure S1). The pristine Na3.64Fe2.18(P2O7)2 were synthesized by pre-oxidation method. The dry gel was firstly heated in air at 600 °C for 8 h to eliminate organic species and carbon. After annealing,



the sample was heated in H2/Ar (10:90) atmosphere at 600 °C for 48 h, the color of the asobtained sample was deep green (Figure S1). Materials characterization Powder X-ray diffraction (PXRD, MAXima-X XRD-7000) using Cu Kα radiation (λ = 1.5416 Å) was used to analysis the crystalline structure of the material. The test was performed in step mode with a fixed time of 12 s and a step size of 0.02º. An inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000) was employed to measure the content of Na and Fe. Thermogravimetric analysis (TGA, Q50) in flowing nitrogen and air was used to monitor the reaction process and measure the carbon content of the sample. The specific surface area of samples was analyzed by a Quadrasorb evo 2QDS-MP-30 surface area analyzer (Quantachrome Instruments, USA). The morphologies and structures of the samples were observed with a field-emission scanning electron microscopy (FESEM; JEOL6300F) and transmission electron microscopy (TEM; JEOL-2100). The chemical element distribution was determined by an energy dispersive spectroscopy (EDS; JEOL-6300F). Finally, X-ray photoelectron spectroscopy (XPS) was used to analyze the elemental at different states using a Thermo Scientific ESCALAB 250Xi electron spectrometer Electrochemical measurements The working electrode consisted of as-synthesized cathode materials, conductor (AB), and binder (PVDF) in a weight ratio of 8:1:1 with NMP as the solvent. The mixed slurry was evenly spread on pristine Al foil and dried in vacuum for 12 hours at 120 °C. The resulting mass loading of the working electrode is 1.5 mg/cm2. A sodium foil was employed as reference and counter electrode. Celgard 2400 was employed as separator to separate the working electrode and counter electrode, and 1 M NaClO4 dissolved in a mixture of EC and DEC with 5 wt% FEC as an additive was used as the electrolyte. The as-prepared electrode, separator, and electrolyte were fabricated in CR2032-type coin cells in a high-purity 4 Environment ACS Paragon Plus

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Ar-filled glove box with both the moisture and oxygen content below 1 ppm. A LAND cycler (Wuhan Land Electronics Co., Ltd. China) was used to test the galvanostatic discharge and charge cycling of the batteries at different rates. The electrochemical capacity measurements were tested in the voltage range of 1.6-4.0 V. The cyclic voltammetry (CV) measurements were carried out by an Arbin instruments testing system at 0.1 mV s-1.

3. RESULTS AND DISCUSSION The carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles were formed via the facile sol-gel method. As schematically illustrated in Figure 1, all chemical reagents were initially distributed evenly in the precursor. The decomposition of the organic species led to the formation of the porous structure of the nanoparticles, and the extended annealing process prompted crystal growth. Because the chemical reagents were evenly distributed in the framework of porous carbon, the growth of crystal was limited and could only grow along a plane leading to the formation of the nanoparticles product. As for pristine Na3.64Fe2.18(P2O7)2, the nanoparticles will agglomerate to form large bulks if without porous carbon. The porous structure of materials can allow the electrolyte to easily penetrate every corner, which is an advantage for Na+ and electron migration. Meanwhile, the well-coated carbon can protect the crystal structure of Na3.64Fe2.18(P2O7)2 during Na+ de/intercalation, which supports the stability of the cycle.

Figure 1 Schematic illustration for the formation of the carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles.



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Figure 2a displays the whole pattern fitting (WPF) refinement XRD profile of the carbon-coated Na3.64Fe2.18(P2O7)2 product, and the XRD pattern of Na3.64Fe2.18(P2O7)2 without carbon-coat is shown in Figure S2. The WPF refinement was carried out and achieved results with a = 6.39641 Å, b = 9.41633 Å, c = 10.93129 Å, α = 64.935°, β = 80.186°, γ = 73.104°, V = 569.8 Å3. All peaks agree well with those of isostructural Na3.64M2.18(P2O7)2(M=Ni and Mg),33 and no extra peaks from impurities were observed, demonstrating high purity of the Na3.64Fe2.18(P2O7)2. The inset displays the crystal structure

of

Na3.64Fe2.18(P2O7)2,

which

reveals

that

the

crystal

structure

of

Na3.64Fe2.18(P2O7)2 contains PO4 tetrahedra and FeO6 octahedra. FeO6 and PO4 are linked together to form a three-dimensional network structure and the forming large tunnel is conducive to Na+ mobility. It is worth noting that the crystal structures of Na3.64Fe2.18(P2O7)2 and Na3.12Fe2.44(P2O7)2 are very similar, but there also are some differences (the crystal structures comparison of these materials are shown in Figure S3). The Fe3 atoms in Na3.12Fe2.44(P2O7)2 were replaced by two Na atoms (Na4, Na5) in Na3.64Fe2.18(P2O7)2. And in the Na3.64Fe2.18(P2O7)2 crystal, there are more sodium atoms instead of iron atoms (see Figure S3). It is known that the Na-ions will overextract at high voltage, which leads to a structural degradation.34 The extra Na-ions can alleviate this phenomenon. Furthermore, during the charging process, more Na-ions can stay in the crystal structure. The extra Na-ions can stabilize the crystal structure which leading to a better cycle performance. Figure 2b shows the wide range scanning spectrum of Na3.64Fe2.18(P2O7)2/C and it can be seen from the results that the sample is combined by Na, Fe, P, O and C elements. Figure 2c displays the XPS narrow spectra of the Fe 2p and it is obvious that the Fe 2p signal consists of two components due to spin-orbit coupling


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(Fe 2p3/2 and Fe 2p1/2), which can confirm the +2 valence state of Fe in this sample, suggesting that the sample consists of only one valence state of iron atoms. Furthermore, the 2p3/2 peak shows a distinct satellite around 719.0 eV. This satellite is caused by a dynamic charge-transfer during the photoemission process (these charge-transfer satellites appears in the spectra of most transition-metal compounds).35-37

Figure 2 (a) PXRD pattern of the as-prepared product. The inset represents its crystal structure. (b) XPS wide range scanning spectra of the Na3.64Fe2.18(P2O7)2/C. (c) XPS narrow spectra of the Fe 2p.

To further verify the stoichiometric ratio of the material, ICP was conducted. The result was displayed in Table. S1, the concentrations of Na, Fe are 0.526 mg L-1 and 0.765 mg L-1, respectively. The theoretically mass ratio of Na to Fe is 0.6873, and the measured mass ratio of Na to Fe is 0.6876. The results indicate that high purity Na3.64Fe2.18(P2O7)2 was successfully synthesized.



As suggested by Figure 3a, the surface of Na3.64Fe2.18(P2O7)2/C is very rough, and different sizes of pores are randomly connected with each other to form the porous framework, which is beneficial for electrolyte penetration and Na+ migration. By comparison, the pristine Na3.64Fe2.18(P2O7)2 is consisted of large size and no porous structure (see Figure S4). In the low resolution of TEM image (Figure 3b), it can be seen that about 10 nm nanoparticles are uniformly dispersed in the carbon framework. The high resolution of TEM image is displayed in Figure S5. The interplanar distance of 0.307 nm corresponds well to the (2 0 0) lattice plane. The SEAD pattern is displayed in Figure S6. To obtain more detailed surface information of the two samples, Brunauer-EmmettTeller (BET) measurements were taken and shown in Figure 3c. According to the results, the surface area and pore volume of Na3.64Fe2.18(P2O7)2/C is 74 m2 g-1 and 0.119 cm3 g-1, respectively. Furthermore, the pore diameters are concentrated to approximately 2 nm. Figure S7 shows the isotherms of the pristine Na3.64Fe2.18(P2O7)2, which has a surface area of only 1.89 m2 g-1. TGA were used to analysis the carbon content of the Na3.64Fe2.18(P2O7)2/C and the result was displayed in Figure 3d. After heating in air, a white product was obtained. According to the graphic in Figure 3d, the weight of asprepared product remained ~89% after heating, which means that the as-prepared Na3.64Fe2.18(P2O7)2/C has a carbon content of ~11%. The formation process of the asprepared products based on TGA was displayed in Figure S8. According to the results, the sample formation temperature was around 460 °C. To obtain the best crystallinity and make sure that the residual organic species were completely decomposed, 600 °C of the heating temperature for 24 h was maintained. Furthermore, EDS element mapping analysis was employed to identify the element distribution of the Na3.64Fe2.18(P2O7)2, all


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elements are homogeneously distributed throughout the whole selected Na3.64Fe2.18(P2O7)2 plates from Figure S9.

Figure 3 (a) FESEM and (b) TEM images of the carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles. (c) Nitrogen adsorption and desorption isotherms of the Na3.64Fe2.18(P2O7)2/C. The inset is the pore size distribution of the Na3.64Fe2.18(P2O7)2/C. (d) Carbon content of the as-prepared product based on thermogravimetric analysis (TGA).

The theoretical capacity of Na3.64Fe2.18(P2O7)2 is 106 mAhg-1, 2.18 Na-ion is reversibly de/intercalated by the oxidation and reduction of 2.18 Fe2+/Fe3+, the electrochemical behaviors of the Na3.64Fe2.18(P2O7)2/C were further studied by CV at 0.1 mV s-1 (Figure 4a). Four pairs of redox peaks were observed (2.5/2.45, 2.98/2.89, 3.24/3.20, and 3.42/3.32 V) with equilibrium potentials of 2.475, 2.935, 3.22, and 3.37 V, respectively, corresponding to the phase transition during the Na+ insertion/extraction reactions. The total potentials were around 3 V, which correspond well with the galvanostatic charge-



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discharge profile in Figure 4b. In Figure 4b, during the first charge cycle, the voltage increased to 4 V from open circuit voltage, resulting in a capacity of 108.3 mAhg-1. The capacity

of

initial cycle

was

little

higher

than

the

theoretical

capacity

of

Na3.64Fe2.18(P2O7)2 (106 mAhg-1). This phenomenon may be due to the slightly oxidize of Na3.64Fe2.18(P2O7)2/C, which forms a layer of Na2CO3 on the surface29. The tiny amount of Na2CO3 will react with electrolyte, which leads to irreversible capacity.30 The peak at 3.12 V in CV test may correspond to the reaction. Then, the voltage decreased to 1.6 V, resulting in a capacity of 99 mAhg-1. Two obvious discharge platforms can be also observed at 3 and 2.4 V, which correspond well with the equilibrium potentials mentioned in Figure 4a. Three pairs of redox peaks (2.98/2.89, 3.24/3.20, and 3.42/3.32 V) were combined to produce a 3 V equilibrium potential, and the redox peak at 2.5/2.45 V corresponds with an equilibrium potential of 2.4 V. To characterize the recovery performance of the material, various current densities (0.2, 1, 2, 5, and 10 C) were employed to evaluate the rate capability; the results are shown in Figure 4c. The charge-discharge curves at different rates are shown in Figure 4d and suggest that even under the drastic changes in current, the carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles display a good recovery performance. Importantly, at 10 C, the material exhibits excellent stability. Figure 4e displays the long cycling performance of the carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles, which showed a discharge specific capacity of 101 and 88 mAhg-1 when the current rates were 0.2 and 1 C, respectively. After 900 cycles, the discharge capacity was 80 mAhg-1 with capacity retention of 90.9%. Further, when the current rate was 2 C, the material exhibited a discharge specific capacity of 66.0 mAhg-1, which increased to 74 mAhg-1. Even after


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1000 cycles, the discharge capacity remained 66 mAhg-1, revealing good stability of the material. Most importantly, when the current rate reached 10 C, the discharge capacity was remained 40 mAhg-1 with a high capacity retention of 96% after 1000 cycles. The excellent properties of the material can be attributed to the porous structure and the unique nano size of Na3.64Fe2.18(P2O7)2, which increase the conductivity and the diffusion coefficient of the Na-ion. Furthermore, the cycle performance of the Na3.64Fe2.18(P2O7)2/C was significantly better than other ferric pyrophosphates family members (Figure 4f). The electrochemical performances of the pristine Na3.64Fe2.18(P2O7)2 are displayed in Figure S10. As can been seen from Figure S10, the electrochemical properties of the pristine Na3.64Fe2.18(P2O7)2 are not good. The discharge capacity only achieves 61 mAhg-1 at 0.1 C. And according to the CV, the polarization of the pristine Na3.64Fe2.18(P2O7)2 is very serious. This phenomenon could be attribute to the poor conductivity and the large size of the pristine Na3.64Fe2.18(P2O7)2. However, the pristine Na3.64Fe2.18(P2O7)2 has very good cycling performance, it may be due to

the extra Na-ions can stabilize the crystal

structure.



Figure 4 Electrochemical properties of the carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles. (a) CV at the scan rate of 0.1 mV/s. (b) Galvanostatic charge–discharge profiles at 0.2 C. (c) Rate capability. (d) Discharge curves of at 0.2 C～10 C (e) Long cycling performance. (f) The comparison with other works.

To further demonstrate the prospective of applying, a coin type sodium-ion full battery and a soft-package sodium-ion full battery with Na3.64Fe2.18(P2O7)2/C as cathode and presodiated hard carbon as anode were assembled (the capacity ratio of cathode to anode is 1 : 1.25). The electrochemical performances of the hard carbon are displayed in Fig. S11. Figure 5a illustrates the galvanostatic charge/discharge profiles at 0.5 C. A voltage platform of 2.9 V and capacity of 86 mAhg-1 can be observed. Figure 5b displays the cycling performance of the full battery at 0.5 C. More than 80% capacity remains after 100 cycles. The rate performance shown in Figure 5c proves that the as-prepared full battery can be stably cycled at various rates from 0.2 C to 5 C. The discharge capacity is from 85.2 mAhg-1 at 0.2 C to 65.1 mAhg-1 at 1 C. Even at 2C, the discharge capacity


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remains at 45.3 mAhg-1, nearly 53% of the capacity at 0.2 C. The corresponding discharge curves were displayed at Figure 5d. Furthermore, Figure S12 shows a commercial red LED powered by the assembled soft-package full batteries, further indicating the application prospects of the Na3.64Fe2.18(P2O7)2/C//hard carbon system.

Figure 5 Electrochemical properties of the Na3.64Fe2.18(P2O7)2//hard carbon full batteries. (a) Galvanostatic charge–discharge profiles at 0.5 C. (b) Long cycling performance at 0.5 C. (c) Rate capability of full batteries. (d) Discharge curves of at 0.2 C～5 C. (The capacity ratio of cathode to anode is 1 : 1.25.).

4. CONCLUSIONS In summary, a carbon-coated Na3.64Fe2.18(P2O7)2 nanoparticles are fabricated to investigate sodium de/intercalation behavior for the first time. We found that the as-



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prepared sodium-rich Na3.64Fe2.18(P2O7)2 exhibits outstanding cycle capability. Such excellent cycle stability is attributed to the supporting effect of extra sodium-ion. The cycle performance of Na3.64Fe2.18(P2O7)2 is obviously better in the ferric pyrophosphate family

members.

Meanwhile,

the

unique

porous

network

structure

provides

interconnected open pores and effective carbon coating, which allows electrolyte penetration and improves the electronic conductivity of uniform nanoparticles. Because of the low price, simple synthesis method, non-toxic, and excellent electrochemical performances of the as-prepared product, we believe this material is very promising in future applications.

ASSOCIATED CONTENT

Supporting Information The concentration and mass ratio of Na and Fe based on ICP; Color difference between Na3.64Fe2.18(P2O7)2/C and pristine Na3.64Fe2.18(P2O7)2; XRD pattern of pristine Na3.64Fe2.18(P2O7)2; Comparison

of

the

crystal

structure

of

Na3.12Fe2.44(P2O7)2,

Na3.32Fe2.34(P2O7)2

and

Na3.64Fe2.18(P2O7)2; FESEM images of pristine Na3.64Fe2.18(P2O7)2; The HRTEM image of Na3.64Fe2.18(P2O7)2/C; The SEAD pattern of Na3.64Fe2.18(P2O7)2/C; Nitrogen adsorption and desorption isotherms of pristine Na3.64Fe2.18(P2O7)2. The inset shows the pore size distribution of pristine Na3.64Fe2.18(P2O7)2; Formation mechanism of as-prepared product based on thermogravimetric analysis (TGA); EDS element mappings of Na3.64Fe2.18(P2O7)2/C; Electrochemical properties of the pristine Na3.64Fe2.18(P2O7)2; Electrochemical properties of hard carbon; A commercial red LED powered by the soft-package full batteries; The XRD pattern of Na3.64Fe2.18(P2O7)2/C after TGA test.

AUTHOR INFORMATION

Corresponding Author


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* E-mail: [email protected] and [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work is financially supported by grants from the National Natural Science Foundation of China (No. 21773188), Basic and frontier research project of Chongqing (cstc2015jcyjA50031) and Fundamental Research Funds for the Central Universities (XDJK2017A002 ， XDJK2017B055，XDJK2017B048) and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011).



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M.-M.,

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Na0.5Ni0.25Mn0.75O2, as a high performance cathode material for sodium-ion batteries.


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