material for high performance Li-ion batteries

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1 Supporting information for Low defect FeFe(CN) 6 framework as stable host material for high performance Li-ion batteries Xianyong Wu, Miaomiao Shao, Chenghao Wu, Jiangfeng Qian,* Yuliang Cao, Xinping Ai, Hanxi Yang* College of Chemistry and Molecular Science, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan , China. * Address correspondence to jfqian@whu.edu.cn; hxyang@whu.edu.cn ; S-1

2 1. Preparation and possible crystallization mechanism of FeFe(CN) 6 nanocrystals Figure S1. Schematic representation the crystallization process of FeFe(CN) 6 nanocubes. Figure S1 illustrates the possible synthetic mechanism of this single iron-source method. Firstly, the Fe(CN) -3 6 anions tend to decompose to Fe 3+ and CN - ions with the addition of HCl. Then, the Fe 3+ ions will coordinate with undecomposed Fe(CN) -3 6 anions to form Prussian yellow Fe +3 Fe +3 (CN) 6. Due to its very strong oxidation ability, FeFe(CN) 6 is reduced and then generates some part of Fe(CN) 6-4 ions, which initiates the nucleation formation and the subsequent crystal growth. As the reaction goes on, large amounts of FeFe(CN) 6 nanocubes slowly precipiate with a chemical formula of Fe +3 [Fe (CN) 6 ] 1-y y (Fe : a mixed state between +2 and +3 valance; 0<y<0.25). S-2

3 2. Morphological features and EDS of FeFe(CN) 6 Figure S2. (a) SEM images and (b) EDS analyses, the inset in (a) is a photograph of FeFe(CN) 6 sample. As shown in Figure S2a inset, the FeFe(CN) 6 sample appears as dark green in color, agreeing well with its common name Berlin green. Energy dispersive spectra (Figure S2b) only detect the Fe, C, N, and O elements but not any K and Cl elements present in the as-prepared FeFe(CN) 6 framework, suggesting a high purity in the chemical composition of the sample. S-3

4 3. Elemental compositions and physical characterizations of FeFe(CN) 6 Table S1. Elemental contents of FeFe(CN) 6 sample (weight percentage) Sample K Fe C N H 2 O FeFe(CN) 6 0% 38.3% 23.8% 27.8% 10.1% Figure S3. Physical characterizations of FeFe(CN) 6 sample: (a) TG curve; (b) FT-IR spectrum; (c) XPS characterizations and (d) 57 Fe Mössbauer spectrum. By ICP-AES calibration of K and Fe ions, elemental analyses of C and N element (Table S1) and TG determination (Figure S3a) of H 2 O molecules, the chemical composition of the FeFe(CN) 6 sample can be determined as Fe[Fe(CN) 6 ] H 2 O. This FeFe(CN) 6 material S-4

5 has a greatly reduced Fe(CN) 6 vacancies (6%) and water molecules (10%) than conventional PB frameworks (Table S2). FT-IR spectrum, XPS characterizations and 57 Fe Mössbauer spectrum were utilized to analyze the chemical state of Fe atoms in the FeFe(CN) 6 lattice. According to previous spectroelectrochemical studies on PB electrode, 1-2 the strong peak at 2176 and 2085 cm -1 can be attributed to the stretching vibration of Fe +3 -CN-Fe +3 and Fe +2 -CN-Fe +3 respectively, confirming the co-existence of Fe(CN) -3 6 and Fe(CN) -4 6 in FeFe(CN) 6 structure. The 3443 and 1636 cm -1 peaks are originated from the stretching and in-plane deformation of O-H (crystal water) respectively. 1-2 XPS analysis shown in Figure S3c provides detailed information on valance states of these two Fe atoms. The electron binding energies at and ev belong to Fe +2 2p 3/2 and Fe +2 2p 1/2 in [Fe +2 (CN) 6 ] -4 framework, while the peak at and ev can be assigned to the Fe +3 2p 3/2 and Fe +3 2p 1/2 in [Fe +3 (CN) 6 ] -3 group. 3-4 Other two peaks at and ev correspond to the Fe +3 2p 3/2 and Fe +3 2p 1/2 in nitrogen coordinated Fe +3 atoms. 3-4 Furthermore, 57 Fe Mössbauer spectrum was carried out to further confirm the Fe +3 /Fe +2 ratio in FeFe(CN) 6 framework. Based on the fitting and integral analysis of Mössbauer spectra, 5 the weight percentage of Fe +3 and Fe +2 atoms can be determined to be 91.4% and 8.6% respectively, giving a atomic ratio of Fe +3 /Fe +2 = The average oxidation state of Fe element can be confirmed as Fe +3 [Fe +2.8 (CN) 6 ] At the same time, we observe that the amount of C-Fe 2+ observed by XPS is higher than that detected by Mössbauer spectroscopy. We think this phenomenon could be explained by the instability of FeFe(CN) 6 during XPS analysis. As we know, FeFe(CN) 6 is highly oxidative with an open circuit potential of 3.7 V vs. Li + /Li, 3 so the Fe 3+ can be partially reduced to Fe 2+ under strong X-ray irradiation in XPS tests. Similar reduction behavior of C-Fe 3+ to C-Fe 2+ has been also observed in some other Prussian blue researches. 6 In contrast, Mössbauer spectroscopy is recognized to be a non-destructive and effective characterization tool for analyzing the chemical state of Fe, 5 thus avoiding reduction of this FeFe(CN) 6 material. Therefore, it s more accurate to determine the chemical state of Fe elements in this oxidative FeFe(CN) 6 sample by Mössbauer spectroscopy. S-5

6 4. Comparison of reported Prussian blue cathodes for Li-ion batteries Table S2. A comparison of FeFe(CN) 6 with other PB cathodes Material M(CN) 6 vacancies Water content Reversible capacity Capacity utilization Rate performance Capacity retention K 0.1 Cu[Fe(CN) 6 ] Mn 0.5 Cu 0.5 [Fe(CN) 6 ] Fe 4 [Fe(CN) 6 ] % 24% 119 mah g -1 74% %@4 th 30% 23.4% 94 mah g -1 59% 32% at 10C 74%@50 th 25% 22.7% 110 mah g -1 65% %@10 th K 0.1 Mn[Fe(CN) 6 ] % 27% 60 mah g -1 37% %@100 th K 1.72 Mn[Mn(CN) 6 ] % 2.7% 197 mah g % %@10 th FeFe(CN) % 140 mah g -1 77% 60% at 3C 70%@50 th LiPB-PPy-PSS mah g % at 3C 80%@200 th KNi[Fe(CN) 6 ] mah g %@50 th Our Fe[Fe(CN) 6 ] % 10% 160 mah g -1 90% 65% at 24C 90%@300 th S-6

7 5. Rietveld refinement results of Fe[Fe(CN) 6 ] H 2 O material. Table S3. Rietveld refinement results of this Fe[Fe(CN) 6 ] H 2 O material. The Fe(CN) 6 vacancies, coordinated water molecules (O1) and zeolitic water molecules (O2) have all been taken into account. Space group = Fm-3m, a= (34) Å, χ 2 = 1.54, R wp = 1.24%. Atom Site x y z Occ Beq Fe1 4a (56) Fe2 4b (56) C 24e (18) (56) N 24e (22) (56) O1 24e 0.261(34) (56) O2 8c (36) 1.769(56) S-7

8 6. Ex-situ XRD patterns of the FeFe(CN) 6 electrodes measured at different charge/discharge depths Figure S4. Ex-situ XRD patterns of FeFe(CN) 6 electrodes: (a) Typical charge/discharge profile at current density of 25 ma g -1 ; (b) and (c) XRD patterns at selected charge or discharge state. Table S4. The lattice parameter of FeFe(CN) 6 during charge/discharge process. Process Potential (V) Li content (x) Lattice parameter (Å) Li intercalation (discharge) Li de-intercalation (charge) To clarify the structural evolution during Li intercalation and de-intercalation reaction, ex situ XRD patterns of the FeFe(CN) 6 electrode were recorded at different depths of charge/discharge states. As shown in Figure S4, the FeFe(CN) 6 electrode could sustain its typical face-centered cubic structure during the whole 2-Li intercalation, indicating the single-phase reaction and agreeing well with the observed S-shaped sloping voltage profiles. When FeFe(CN) 6 S-8

9 electrode was discharged from 4.3 V to middle 3.1 V, its lattice parameter a decreased from Å to Å possibly due to the smaller radius of [Fe II (CN) 6 ] 4- than that of [Fe III (CN) 6 ] ,14 Upon continuously discharging from 3.1 V to terminal 2.0 V, its lattice parameter a increased from Å to Å, implying higher Li content in PB lattice would cause cell expansion. Once reversed to charge, the XRD pattern of this cathode could restore to its initial state, suggesting a reversible structural evolution. S-9

10 7. Elemental compositions and physical characterizations of two defective PB samples, Fe[Fe(CN) 6 ] H 2 O and Fe[Fe(CN) 6 ] H 2 O. Table S5. Elemental contents of Fe[Fe(CN) 6 ] H 2 O and Fe[Fe(CN) 6 ] H 2 O sample Sample K Fe C N H 2 O Fe[Fe(CN) 6 ] % 33.1% 17.7% 20.6% 28.6% Fe[Fe(CN) 6 ] % 35.4% 21.1% 24.5% 19.0% Figure S5. Physical characterizations of two defective PB samples Fe[Fe(CN) 6 ] H 2 O and Fe[Fe(CN) 6 ] H 2 O. (a) XRD patterns; (b) TG curves measured in N 2 atmosphere; (c) FT-IR spectra; and (d) XPS characterizations of Fe 2p spectra. S-10

11 The chemical formula of two defective PB samples can be determined to be Fe[Fe(CN) 6 ] H 2 O and Fe[Fe(CN) 6 ] H 2 O respectively. The synthesis of Fe[Fe(CN) 6 ] H 2 O is based on a rapid crystallization process between FeCl 3 and K 4 Fe(CN) 6 at room temperature, 9 while the preparation of Fe[Fe(CN) 6 ] H 2 O is based on a slow precipitation method between FeCl 3 and K 3 Fe(CN) 6 at 60 C. 3 As shown in Figure S5a, all these two samples exhibit a typical face-center cubic lattice. Thermogravimetric analysis in Figure S5b reveals the water contents in these two frameworks are 28.6% and 19.0% respectively. FT-IR (Figure S5c) and XPS tests (Figure S5d) together confirmed the valance states of Fe ions are Fe +2 -CN-Fe +3 and Fe mixed -CN-Fe +3 for Fe[Fe(CN) 6 ] H 2 O and Fe[Fe(CN) 6 ] H 2 O respectively, agreeing very with their chemical formulas. S-11

12 8. Photos and EDS elemental analysis of the Li anode in the cycled cells Figure S6. Photos and EDS elemental analysis of Li anodes taken from the cells after 5 cycles at 25 ma g -1 : (a) and (b) from the cell using the Fe[Fe(CN) 6 ] H 2 O cathode; (c) and (d) from the cell using low defect Fe[Fe(CN) 6 ] 0.94 cathode. References: [1] Kulesza, P. J.; Malik, M. A.; Denca, A.; Strojek, J. In Situ Ft-Ir/Atr Spectroelectrochemistry of Prussian Blue in the Solid State. Anal. Chem. 1996, 68, [2] Pajerowski, D. M.; Watanabe, T.; Yamamoto, T.; Einaga, Y. Electronic Conductivity in Berlin Green and Prussian Blue. Phys. Rev. B 2011, 83, [3] Shen, L.; Wang, Z.; Chen, L. Prussian Blues as a Cathode Material for Lithium Ion Batteries. Chemistry 2014, 20, [4] Qian, J.; Zhou, M.; Cao, Y.; Ai, X.; Yang, H. Nanosized Na 4 Fe(CN) 6 /C Composite as a Low- Cost and High-Rate Cathode Material for Sodium-Ion Batteries. Adv. Energy Mater. 2012, 2, [5] Kumar, A.; Yusuf, S. M.; Keller, L. Structural and Magnetic Properties of Fe[Fe(CN) 6 ] 4H 2 O. Phys. Rev. B 2005, 71, S-12

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