Supporting information. Carbon Matrix: An Ultrafast Na-Storage Cathode with. the Potential of Outperforming Li-Cathodes

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1 Supporting information Carbon-Coated Na 3 V 2 (PO 4 ) 3 Embedded in Porous Carbon Matrix: An Ultrafast Na-Storage Cathode with the Potential of Outperforming Li-Cathodes By Changbao Zhu, Kepeng Song, Peter A. van Aken, Joachim Maier and Yan Yu * [*]Prof. Yan Yu CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei , Anhui, P. R. China. yanyumse@ustc.edu.cn Dr. Changbao Zhu, Prof. Yan Yu and Prof. Joachim Maier Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart, 70569, Germany Kepeng Song, Prof. Peter A. van Aken Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, Stuttgart, 70569, Germany (i) Experimental section Preparation of Na 3 V 2 (PO 4 ) 3 : a stoichiometric ratio (3:2:3) of sodium acetate (Na- CH 3 COO), vanadium (III) acetylacetonate (VO(C 5 H 7 O 2 ) 2 ) and ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ) were added in a tetraethylene glycol (TEG). The solution was 1

2 stirred overnight at room temperature in order to obtain a homogenous solution. The solution was heated at 320 C (note that the temperature is controlled by the thermocouple dipped in the solution, and the system is heated by sand bath) in a roundbottom flask with magnetic stirring attached to a refluxing condenser. The resulting powders were collected by repeated washing and centrifugation with ethanol and acetone for 3 times respectively, followed by drying in a vacuum oven at 80 C overnight. The post heat-treatment was performed as follows: the as-prepared powder was annealed at 650 C for 6 hours in Ar/H 2 (95:5) and followed by heated in Ar atmosphere at 800 C for 6 hours. Preparation of LiFePO 4 : a stoichiometric equimolar ratio of Li-CH 3 COO, Fe- (CH 3 COO) 2, and NH 4 H 2 PO 4 was added in a tetraethylene glycol (TEG). The solution was heated at 320 C (controlled by the thermocouple dipped in the solution) in a roundbottom flask with magnetic stirring attached to a refluxing condenser. The resultant LiFePO 4 nanorods were collected by repeated washing and centrifugation with ethanol and acetone for 3 times respectively, followed by drying in a vacuum oven at 80 C for 20 hours. The as-prepared LiFePO 4 was annealed at 700 C for 2 hours in Ar/H 2 atmosphere before electrochemical performance texting. XRD measurements were carried out with a Philips PW 3020 diffractometer using Cu- K α radiation. SEM was performed using JEOL 6300F field-emission scanning electron microscopy (JEOL, Tokyo, Japan) operated at 15 kv. HRTEM was performed on a JEOL 4000FX transmission electron microscope (JEOL, Tokyo, Japan) operated at 400kV. Raman spectrum is measured by Labram V010 with a 532 nm diode laser. The packing density of Na 3 V 2 (PO 4 ) 3 /C composite (0.9 g/cm 3 ), is measured by a tap density testing 2

3 machine (HNT301, Beijing haonate Company). The carbon content is measured by carbon sulfur determinator (ELTRA, CS800). The electrode preparation for sodium batteries: Na 3 V 2 (PO 4 ) 3 (70 wt. %), carbon black (20 wt. %, Super-P, Timcal), and poly(vinylidene fluoride) binder (10 wt. %, Aldrich) in N-methylpyrrolidone were mixed. The obtained homogenous slurry was pasted on an Al foil, followed by drying in a vacuum oven for 12 hours at 80 C. Electrochemical test cells (Swagelok-type) were assembled in an argon-filled glove box (O ppm, H 2 O 1 ppm) with the coated Al foil as working electrode, sodium metal foil as the counter/reference electrode, and 1 M solution of NaClO 4 in the propylene carbonate (PC) as the electrolyte. Glass fiber (Whatman) was used as separator. The typical loading mass of active material is ~ 1 mg/cm 2. This mass is certainly up-scalable, but is definitely enough to exclude supercapacitive effects. The batteries were charged and discharged galvanostatically in the fixed voltage window between 2.3 V to 3.9 V using an Arbin MSTAT battery tester at room temperature. In this work, C rates are used for charactering the current rate, where 1C equals to 110 ma/g. The capacities are calculated based on the mass of Na 3 V 2 (PO 4 ) 3 and charge and discharge rates are always identical. Cyclic voltammetry was carried out with a Voltalab system (D21V032, Radiometer Analytical SAS, France) on Swagelok-type cells. The electrode preparation for lithium batteries: LiFePO 4 (70 wt. %), carbon black (20 wt. %, Super-P, Timcal), and poly(vinylidene fluoride) binder (10 wt. %, Aldrich) in N-methylpyrrolidone were mixed. The obtained homogenous slurry was pasted on an Al foil, followed by drying in a vacuum oven for 12 hours at 80 C. Electrochemical test cells (Swagelok-type) were assembled in an argon-filled glove box (O ppm, H 2 O 3

4 1 ppm) with the coated Al foil as working electrode, lithium metal foil as the counter/reference electrode, and 1 M solution of LiPF 6 in the mixture of ethylene carbonate and diethyl carbonate (volume ratio 1:1) as the electrolyte (Novolyte technologies). Glass fiber (Whatman) was used as separator. The typical loading mass of active material is ~ 1 mg/cm 2. The batteries were charged and discharged galvanostatically in the fixed voltage window between 2.5 V to 4.3 V using an Arbin MSTAT battery tester at room temperature. (ii) Relation between the electrochemical performance and parameters for post-heat treatment The results in this work clearly show the significance of optimizing temperatures and atmospheres of the post heat-treatment. For (C@NVP)@pC annealed at 650 C, in spite of smaller particle sizes, only a fair specific capacity can be obtained (supporting information, Figure S4) due to the low electronic conductivity of carbon. (C@NVP)@pC, however, annealed at higher temperature (900 C in Ar) shows increased conductivity of the carbon, yet at the cost of large amounts of NaV 3 (PO 4 ) 3 and NaV 2 O 5 impurities phases, which are detrimental for the specific capacity (even without charge-discharge plateaus) as shown in Figure S5 (supporting information). Hence we developed an optimized procedure as follows: (C@NVP)@pC were annealed at 650 C in Ar/H 2 followed by annealing at 800 C in Ar. The material so-prepared showed the excellent performance we refer to in this paper. 4

5 (iii) Figures and captions: Figure S1. DSC curves of vanadium (III) acetylacetonate. 5

6 Figure S2. The XRD diffraction with Rietveld refinement for Na 3 V 2 (PO 4 ) 3 annealed at 650 C in Ar/H 2 and followed by annealing at 800 C in Ar. (Black, blue and red curves are corresponding to observed curve, calculated curve and difference between them.) Figure S3. Raman spectrum of (C@NVP)@pC. 6

7 Figure S4. Charge and discharge profiles of Na 3 V 2 (PO 4 ) 3 annealed at 650 C in Ar/H 2 at 0.5C rate. 7

8 Figure S5. Charge and discharge profiles of Na 3 V 2 (PO 4 ) 3 heat treated at even higher temperature at 0.5C rate. (650 C in Ar/H C in Ar+ 900 C in Ar) Figure S6. Comparison of XRD patterns before and after cycling. 8

9 Figure S7. Comparison of high rate performance of our with some of best reported high-rate Li cathodes: a) capacities vs. C-rate; b) capacity retention (to the low C-rate) vs. C rate. (LiCoO 2-45% carbon, LiMn 2 O 4-45% carbon, LiFePO 4-30% carbon, LiFePO 4-15% carbon are corresponding to the references 37, 40, 35, 39.) 9

10 Figure S8. TEM and HRTEM images of homemade carbon-coated LiFePO 4. The carbon content is 6wt% measured by elemental analysis. Figure S9. Comparison of high rate performances between NVP/C and LFP/C with different carbon content. 10

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12 Figure S10. (a) Cyclic voltammetry curves of Na 3 V 2 (PO 4 ) 3 annealed at 650 C in Ar/H 2 and followed by annealing at 800 C in Ar for various scan rates; (b) Cathodic peak current I p as a function of square root of scan rate; (c) Anodic peak current I p as a function of square root of scan rate. 12