Zero-Strain Na2FeSiO4 as Novel Cathode Material ... - ACS Publications

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A zero-strain Na2FeSiO4 as novel cathode material for sodium ion batteries Shouding Li, Jianghuai Guo, Zhuo Ye, Xin Zhao, Shunqing Wu, Jin-Xiao Mi, Cai-Zhuang Wang, Zhengliang Gong, Matthew J McDonald, Zizhong Zhu, Kai-Ming Ho, and Yong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03969 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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ACS Applied Materials & Interfaces

A Zero-Strain Na2FeSiO4 as Novel Cathode Material for Sodium Ion Batteries Shouding Lia,‡, Jianghuai Guoa ‡, Zhuo Yeb, Xin Zhaob, Shunqing Wuc, Jin-Xiao Mid, Cai-Zhuang Wangb, Zhengliang Gonge, Matthew J McDonalda, Zizhong Zhuc, Kai-Ming Hob,f, Yong Yanga,e* a

Collaborative Innovation Center of Chemistry for Energy Materials, State Key Lab of Physical

Chemistry of Solid Surface and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. b

Ames Laboratory - US DOE and Department of Physics and Astronomy, Iowa State University,

Ames, Iowa 50011, USA. c

Department of Physics and Collaborative Innovation Center for Optoelectronic Semiconductors

and Efficient Devices, Xiamen University, Xiamen 361005, China. d

Department of Material Science and Engineering, Xiamen University, Xiamen, 361005, China.

e

College of Energy, Xiamen University, Xiamen, 361005, China.

f

International Center for Quantum Design of Functional Materials (ICQD), Hefei National

Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China. KEYWORDS: Na2FeSiO4; Nanocrystallites; Zero-strain; Cathode; Sodium ion batteries

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ABSTRACT: A new cubic polymorph of sodium iron silicate, Na2FeSiO4, is reported for the first time as a cathode material for Na-ion batteries. It adopts an unprecedented cubic rigid tetrahedral open framework structure i.e. F4̅3m, leading to a polyanion cathode material without apparent cell volume change during the charge/discharge processes. This cathode shows a reversible capacity of 106 mAh g-1, and a capacity retention of 96 % at 5mA g-1 after 20 cycles.

1. Introduction Sodium ion batteries (NIBs) are considered to be the most attractive alternative to lithium ion batteries (LIBs) as the next generation of rechargeable battery systems, due to their advantages of resource abundance, low cost and environmental friendliness.1,2 In contrast to lithium, sodium is one of the most common elements on the earth and sodium resources are practically unlimited nearly everywhere.3,4 Na is located below Li in the periodic table and they share similar chemical properties, including ionic charge, electronegativity and electrochemical reactivity. Currently focused cathode materials for NIBs mainly include layered transition oxides (NaMO2, M = Ni, Fe, Co, Mn, V) 5-10 and phosphate polyanionic compounds (NaFePO4, Na3V2(PO4)3, etc.).11-19 Polyanionic cathode materials have a stable polyhedral framework, thus exhibiting excellent cycling performance and safety characteristics, which make them an attractive type of cathode materials for sodium ion batteries.20, 21 However, cathode materials with tetrahedral open framework structure have not been reported yet to date. The expected high structure stability will improve stability of cathode materials in turn. Orthosilicates (A2MSiO4, A = Li, Na; M = Fe, Mn) are one type of promising polyanionic cathode material, due to their properties of low cost and high theoretical specific capacity, showing two


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electrons exchanged per formula unit.22,23 In particular, sodium Fe-based silicates should be one of the cheapest cathode materials available, considering the abundance of Na, Fe and Si resources. These advantages make silicates have a great potential for use in large-scale energy storage systems such as smart electric grid applications. In this study, we explored the synthesis, structure and electrochemical properties of cubic Na2FeSiO4 for the first time. Various methods were used to synthesize the compound, and its structure was successfully simulated by first-principles calculations using a cubic cell (F4̅3m). In addition, the electrochemical performance of Na2FeSiO4 in NIBs was also evaluated. From the experimental data together with theoretical calculations, it can be observed that the Na2FeSiO4 cell volume did not change during charge and discharge processes.

2. Results and discussion As described in the experimental details in the supporting information, two methods (i.e. the SS and SG methods) were used to prepare carbon free Na2FeSiO4 and carbon coated Na2FeSiO4/C samples. XRD diffraction patterns (Figure 1) indicate that the samples from the SS method had better crystallinity than those from the SG method. The results show that the reaction conditions such as temperatures had a significant influence on material purity (Figure S1). The XRD pattern from the SS sample can be indexed to a cubic cell with a = 7.330(3) Å and a space group of F4̅3m, excepting a few weak reflections from impurities. As shown in Figure S2, the XRD pattern from the SS method can be well fitted by the Pawley refinement method with a Rp’ of 16.4%. The indices, observed and calculated positions of reflections are given in Table S4.



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Figure 1 (a) XRD patterns of Na2FeSiO4 prepared with solid state (SS) and Na2FeSiO4/C prepared with sol-gel (SG) methods, vertical bars indicate the diffraction peak positions for a cubic cell of a = 7.330(3) Å with space group of F4̅ 3m. (b) The crystal structure of Na2FeSiO4, which is topologically isostructural to MgCu2 (the Laves phase, Figure S4). Sodium atoms (green) form a diamond framework highlighted with yellow bonds. The FeO4 and SiO4 tetrahedra are shown in blue and red, respectively. The crystal structure of cubic Na2FeSiO4 may be isostructural to Na2CaSiO4 (Fm-3m).24 Unfortunately, discovering the exact structure has been a major challenge which could not be overcome by the use of current X-ray techniques such as Rietveld refinement. The positions of the cations Ca2+(Fe2+), Si4+, and Na+ in the unit cell are relatively clear. However, the anions could not be properly arranged in the cell with an order mode, which led to the oxygen positions have not been determined yet until now. Moreover, even if the atomic sites were determined, the low resolution of X-ray diffraction patterns (e.g., few and significantly broadened peaks) could not allow the refinement of the crystal structure via the Rietveld method. Thus, we turned to firstprinciples calculations in order to determine the crystal structure of Na2FeSiO4. The relevant details will be discussed later in this paper.


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SEM analysis shows that the obtained Na2FeSiO4/C composite synthesized by the sol-gel method consists of micro-scaled secondary particles, which are in turn composed of smaller particles 3050 nm in size (Figure 2 a and b). TEM image confirms that the primary particles are on the order of tens of nanometers in diameter. The HRTEM image suggests that the composite consists of Na2FeSiO4 nanocrystallites with a thin layer of amorphous carbon. The SG product is a black powder with a carbon content of about 16 wt. % by elemental analysis. The SS product is a light yellow powder, which is carbon free. The small particle size implies short sodium pathways and a large electrochemical surface area25 while the uniform carbon network in the nanocomposite greatly enhances the electronic conductivity of the material.26 Both of them synergistically result in the impressive electrochemical performance of the observed materials.

Figure 2 (a) SEM image of Na2FeSiO4/C synthesized by the sol-gel method. (b) Magnification of a secondary particle. (c) TEM image of a secondary particle. (d) HRTEM image.



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The voltage profiles of the initial three cycles of Na2FeSiO4/C at a rate of C/16 and at 30 oC are plotted in Figure 3a. The composite could deliver a first discharge capacity of 106 mAh g-1 without obvious capacity fading in the subsequent cycles. A plateau at 1.9V can be observed during the charge/discharge processes, which is corresponding to the Fe2+/Fe3+ redox reaction and being accompanied by the insertion/extraction of sodium ions. A differential capacity plot (Figure 3b) shows the high similarity of the oxidation and reduction peaks, indicating an excellent reversibility with a small polarization in the electrochemical process, which can be attributed to the good stability of the cubic structure of Na2FeSiO4 during Na+ insertion/extraction. The electrochemical performance of the material under different cutoff conditions upon charging (sodium extraction) was also investigated, with results visible in Figure 3c. The reversible capacity clearly decreases when charged to 3.5 V, indicating that the capacity beyond 3.5 V still corresponds to amounts of sodium ions being extracted from the crystal lattice. The cycling performance of Na2FeSiO4/C composite at different current densities at 30 oC is shown in Figure 3d. The composite exhibits impressive cycling stability, with capacity retention of 95 %, 96 %, 91 % and 94 % obtained after 20 cycles when the current density is 10, 50, 100, and 200 mA g-1, respectively. However, relatively poor rate capability is also observed in the profile, as when the current density goes up from 10 to 200 mA g-1, the discharge capacity goes down from around 100 to 40 mA g-1. The poor rate capability of the composite may be ascribed to the internal resistance caused by the relatively large ionic radii of Na+ (1.02 Å vs the 0.76 Å of Li+).


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Figure 3 (a) Charge/discharge curves of the initial three cycles of Na2FeSiO4/C (synthesized by the sol-gel method) at a current density of 5 mA g-1 and in a voltage range of 1.5–4.0 V. (b) Differential capacity plots of the initial three cycles. (c) Comparison of charge/discharge curves in the initial cycles between the voltage ranges 1.5–3.5 V and 1.5–4.0 V at a current density of 5 mA g-1. (d) Cycling performance of Na2FeSiO4/C at various current densities: 10 mA g-1, 50 mA g-1, 100 mA g-1 and 200 mA g-1, respectively. Ex-situ XRD analysis was also carried out to investigate the structural evolution of Na2FeSiO4/C upon electrochemical cycling. Figure 4a gives the XRD patterns of the electrode material when charged to different voltages. The magnified graph of [111] and [220] peaks shows that the peak positions shift a very small amount to higher angles when charging to high voltages, corresponding to the shrinking of the crystal lattice with the extraction of sodium ions. Compared to the pristine



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material, when charging to 2.5 V, 3.5 V and 4.0 V, volume shrinking of only 0.5 %, 0.6 % and 0.9 % respectively is evident. Compared to other polyanion type materials, the volume change is about 6.7 % for olivine LiFePO4 27, about 10.4 % for LiFeSO4F28, about 4 % for Na4Fe3(PO4)2(P2O7)29 and about 2 % for LiFeBO330 , which is much bigger than Na2FeSiO4. The major peaks of the XRD pattern almost do not shift during the charging and discharging process (Figure 4a), which are further confirmed by the XRD patterns of Na2FeSiO4/C after undergoing different numbers of cycles, as seen in Figure S5. There, the two major peaks shift almost no amount after 100 cycles, indicating the high stability of the structure and zero-strain characteristics31,

32

in polyanion family. However, a thorough understanding of the structural

stability is not possible without the identification of the crystal structure. Using structure search methods and the lattice parameters proposed above, we were able to find several structure models that explain the experiment observations. When we restrained our search for (Na2FeSiO4)4 with the F4̅3m (No. 216), we did not find any low energy structures. However, when we lifted the space group restraint to primitive cubic, we found a low energy structure with P213 (No. 198) as shown in Figure S3 (a). When we further relaxed the restraint and enforced no symmetry32, we found another two lower energy structures with space group C2 (No. 5) and C2221(No. 20) as shown in Figure S3 (b) and (c). To study the evolution of the crystal structures in the charging and discharging process, we constructed a 2×2×2 supercell of the P213 structure and randomly removed 0.25, 1, and 1.75 sodium ions per formula unit from the parent (Na2FeSiO4)32 structure. The resulting structures were then fully relaxed using GGA+U. Computational details of our calculations can be found in supporting information. Figure 4(b) shows their XRD patterns, along with the atomic structures. The two major peaks do not shift as sodium ions are being extracted, which is consistent with


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experiment XRD measurements. The 2nd major peak is broadened or split into two small peaks, which is also consistent with experiments (Figure 4a).

Figure 4 (a) XRD patterns of Na2FeSiO4/C when charged to different voltages. (b) Calculated XRD patterns of 2×2×2 supercell structures of NaxFeSiO4 (x=0.25, 1, 1.75). (c) and (d) Crystal structures of Na0.25FeSiO4 and Na1.75FeSiO4. A close look at the parent Na2FeSiO4 structures indicates that in all three structures, the Na and Fe-Si cations form 2 penetrating diamond sub-lattices (Figure S3). The different arrangements of oxygen ions result in different orientations of the diamond sub-lattices. Besides, similar to the delithium transition-metal silicates found so far, all the cations locate in tetrahedral sites. And the SiO4 and FeO4 tetrahedra are alternately combined together by sharing all O-vertices to form a



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three dimensional diamond-like open framework. Sodium atoms are located within the channel of the framework of [FeSiO4], which is topologically isostructural to MgCu2 (Figure S4). It is not surprising that the experiment XRD can be approximately indexed as space group F4̅3m as a result from the Fe-Si diamond sub-lattice. The realistic structure (NaxFeSiO4)4 (x=0.25~1.75) can be regarded as an ensemble average of numerous unit cells with sodium ions randomly extracted from the parent structure (Na2FeSiO4)4. The order of Na ions is broken, as well as oxygen ions. However, the diamond Fe-Si sub-lattice which characterizes all 3 structures is robust against sodium ion insertion/deinsertion (Figure 4 (c) and (d)). As a consequence, the material attains a high structural stability resulting in a nearly zero volume change during the charging/discharging process. Here we note that the space group F4̅3m is an ensemble average symmetry. The broadening 2nd major peak in Figure 4(a) also indicates a perturbation of the cubic symmetry, which is consistent with our analysis. Different processing methods may result in slightly different structures due to the arrangements of oxygen ions. Experiment XRD patterns in Figure 1 indicate that the sample synthesized by sol-gel method is likely to be P213 structure as the 2nd major peak is broadened, and that the sample prepared by solid state method is likely to be C2 (No. 5) structure as the 2nd peak is as sharp as the 1st peak. The detailed arrangement of oxygen ions cannot be determined unless XRD patterns with very high resolution are acquired.

3. Conclusion In conclusion, Na2FeSiO4 was successfully synthesized via both the solid state method and sol-gel method for the first time. The new compound was indexed as a cubic structure with the space group F4̅3m by both DFT calculations and XRD results. A reversible capacity of 106 mAhg-1 was


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obtained when cycled between 1.5 to 4.0 V at 30 oC. Moreover, this material exhibited impressive cycling performance due to its structural stability. This ultrahigh structural stability can be attributed to its rigid 3D open framework, composed of corner-sharing tetrahedra. In addition, this compound is also a polyanion-type cathode material that does not show appreciable cell volume change during charge/discharge processes. Thus, this work provides a new direction to search for novel, high-stability cathode materials that will make up part of the next generation of rechargeable battery systems. Although there still exists issues such as a relatively low work voltage, this is the first report of a Fe-based silicate as a cathode material for Na batteries, and many strategies could be pursued to improve the voltage, such as substituting a portion of the Fe for Mn or Co. A low cost and remarkable stability make Na2FeSiO4 a cheap potential cathode material for large scale energy storage systems with great promise. Remarking: We found a paper with the title “Investigation of metastable Na2FeSiO4 as a cathode material for Na-ion secondary battery”, published recently in the journal “Materials Chemistry and Physics” during our submission process.33 Its structure was metastable and very likely to be in the P1 phase with two unavoidable impurities. In addition, their Na2FeSiO4 underwent irreversible crystal lattice breakdown and became amorphous during the initial charge process. In comparison, our material was indexed as a cubic structure with the space group F4̅3m and exhibited impressive cycling performance, indicating the high stability of the structure and its zero-strain characteristics. We therefore believe that our material shows more promise as a viable cathode material for Naion batteries. ASSOCIATED CONTENT Supporting Information.



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Synthesis, characterization and electrochemistry of Na2FeSiO4/Na cell；XRD patterns, pawley refinement and crystal structures of Na2FeSiO4；Computational details of our calculations. AUTHOR INFORMATION Corresponding Author Yong Yang, E-mail: [email protected], Tel: 0086-592-2185753. Author Contributions ‡ These authors contributed equally. ACKNOWLEDGMENT This work was supported by Financial support from the National Basic Research Program of China (973 program, Grant No. 2011CB935903), the National Natural Science Foundation of China (Grants No. 21233004, 21473148 and 21021002, and in part 21428303) and the Natural Science Foundation of Fujian Province of China (Grant No. 2015J01030) are gratefully acknowledged. Work at Ames Laboratory was supported by the US Department of Energy, Basic Energy Sciences, Division of Materials Science and Engineering, under Contract No. DE-AC02-07CH11358, including a grant of computer time at the National Energy Research Scientific Computing Center (NERSC) in Berkeley, CA. REFERENCES (1) Palomares, V.; Casas-Cabanas, M.; Castillo-Martinez, E.; Han, M. H.; Rojo, T. Update on Nabased Battery Materials. A Growing Research Path. Energy Environ. Sci. 2013, 6, 2312-2337. (2) Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-Ion Batteries. Adv. Funct. Mater. 2013, 23, 947-958.


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(3) Pan, H. L.; Hu, Y. S.; Chen, L. Q. Room-temperature Stationary Sodium-ion Batteries for Large-scale Electric Energy Storage. Energy Environ. Sci. 2013, 6, 2338-2360. (4) Luo, W.; Allen, M.; Raju, V.; Ji, X. L. An Organic Pigment as a High-Performance Cathode for Sodium-Ion Batteries. Adv. Energy Mater. 2014, 4, 1400554. (5) Wang, X. F.; Liu, G. D.; Iwao, T.; Okubo, M.; Yamada, A. Role of Ligand-to-Metal Charge Transfer in O3-Type NaFeO2-NaNiO2 Solid Solution for Enhanced Electrochemical Properties. J. Phys. Chem. C 2014, 118, 2970-2976. (6) Zhao, J.; Zhao, L. W.; Dimov, N.; Okada, S.; Nishida, T. Electrochemical and Thermal Properties of alpha-NaFeO2 Cathode for Na-Ion Batteries. J. Electrochem. Soc. 2013, 160, A3077A3081. (7) Berthelot, R.; Carlier, D.; Delmas, C. Electrochemical Investigation of the P2-NaxCoO2 Phase Diagram. Nat. Mater. 2011, 10, 74-U3. (8) Ma, X. H.; Chen, H. L.; Ceder, G. Electrochemical Properties of Monoclinic NaMnO2. J. Electrochem. Soc. 2011, 158, A1307-A1312. (9) Guignard, M.; Didier, C.; Darriet, J.; Bordet, P.; Elkaim, E.; Delmas, C. P2-NaxVO2 System as Electrodes for Batteries and Electron-correlated Materials. Nat. Mater. 2013, 12, 74-80. (10) Kalluri, S.; Seng, K. H.; Pang, W. K.; Guo, Z. P.; Chen, Z. X.; Liu, H. K.; Dou, S. X. Electrospun P2-type Na2/3(Fe1/2Mn1/2)O2 Hierarchical Nanofibers as Cathode Material for SodiumIon Batteries. ACS Appl. Mater. Interfaces 2014, 6, 8953-8958.



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(11) Wu, X. B.; Zheng, J. M.; Gong, Z. L.; Yang, Y. Sol-gel Synthesis and Electrochemical Properties of Fluorophosphates Na2Fe1-xMnxPO4F/C (x=0, 0.1, 0.3, 0.7, 1) Composite as Cathode Materials for Lithium Ion Battery. J. Mater. Chem. 2011, 21, 18630-18637. (12) Oh, S. M.; Myung, S. T.; Hassoun, J.; Scrosati, B.; Sun, Y. K. Reversible NaFePO4 Electrode for Sodium Secondary Batteries. Electrochem. Commun. 2012, 22, 149-152. (13) Lee, I. K.; Shim, I. B.; Kim, C. S. Phase Transition Studies of Sodium Deintercalated Na2xFePO4F

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Zero-Strain Na2FeSiO4 as Novel Cathode Material ... - ACS Publications

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