vanadium acetylacetonate and phosphoric acid were dissolved in ethanol in the molar ratio 4 :

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1 Supporting Information Na x MV(PO 4 ) 3 (M=Mn, Fe, Ni), Structure and Properties for Sodium Extraction Weidong Zhou, Leigang Xue, Xujie Lü, Hongcai Gao, Yutao Li, * Sen Xin, Gengtao Fu, Zhiming Cui, Ye Zhu, * John B. Goodenough * Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States Earth and Environmental Sciences Division, Los Alamos National Laboratory Los Alamos, NM 87545, USA Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong * yezhu@polyu.edu.hk, lytthu@gmail.com; * jgoodenough@mail.utexas.edu Experimental Section Na 4 MnV(PO 4 ) 3 preparation: The starting precursors of sodium acetate, manganese acetate, vanadium acetylacetonate and phosphoric acid were dissolved in ethanol in the molar ratio 4 : 1 : 1 : 3 (Na : Mn : V : P) and stirred for 2 hours, followed by an evaporation of solvent. Subsequently, the as-prepared powder was annealed at 700 o C for 5 h under argon atmosphere to obtain the carbon mixed network of Na 4 MnV(PO 4 ) 3. Electrochemical analysis: The cathode sample, super P conductive carbon and CMC binder at a weight ratio of 75 : 20 : 5 were mixed in water to make the slurry for the cathode film. Sodium metal, 1 M NaClO 4 in propylene carbonate (PC)/fluoroethylene carbonate (FEC) (10 : 1 v/v), and a glass fiber were used as the anode, electrolyte, and separator for the Na-ion halfcells, respectively. STEM imaging and EELS: STEM observation was performed on a double-aberrationcorrected FEI Titan FEG-TEM/STEM operating at 300 kv at Monash Centre for Electron Microscopy, Monash University. ADF-STEM images were acquired with a convergence semi-angle of 21 mrad and an ADF inner collection angle 29 mrad. EEL spectra were acquired with a Gatan Image Filter (GIF) Tridiem Model 863 P using a 2mm entrance aperture (15 mrad collection angle). Both Na 4 MnV(PO 4 ) 3 and Na 3 FeV(PO 4 ) 3 materials are S1

2 easy to be damaged under high-energy electron irradiation. Therefore STEM image and EELS are only carried out over fresh sample regions to avoid potential artefacts. Synchrotron X-ray diffraction experiments were carried out at Sector 16 (HPCAT BM-D beamline) at Advanced Phonon Source (APS), Argonne National Laboratory (ANL). A monochromatic X-ray beam with wavelength of Å was used for the diffraction experiments. The diffraction data were recorded by a MAR345 image plate, and then the two dimensional images were integrated to one dimensional patterns with the Fit2D program. V-L O-K Mn-L V-L O-K Fe-L Figure S1. EELS spectra of Na 4 MnV(PO 4 ) 3 and Na 3 FeV(PO 4 ) 3. Intensity (a.u.) Experiment Calculation Bragg Difference Theta Theta Figure S2. X-ray diffraction pattern and Rietveld refinement of Na 4 MnV(PO 4 ) 3 and Na 3 FeV(PO 4 ) 3. Table S1. Atomic parameters of the Na 4 MnV(PO 4 ) 3 from the synchrotron radiation. Intensity (a.u.) Experiment Calculation Bragg Difference Atomic parameters Atom Ox. Wyck. Site S.O.F. x/a y/b z/c U [Å 2 ] O1 36f (7) (6) (2) O2 36f (5) (5) (3) P1 18e (3) 0 1/ Na2 18e (5) 0 1/ (3) Mn 12c (10) Na1 6b (3) V 12c (10) S2

3 2 nm (c) (d) 1 nm (e) (f) 2 nm Figure S3. Aberration-corrected STEM images and crystal structure of Na 4 MnV(PO 4 ) 3 viewed from the [001], (c) (d) [110], (e) (f) [42-1] projection. The superimposed atomic array indicates the locations of each atom [Na (green-blue), O (grey), P (red) and Mn/V (green). S3

4 (c) (d) (e) (f) 2 nm Figure S4., Aberration-corrected STEM images of Na3FeV(PO4)3; and crystal structure viewed from the (c) Na3V2(PO4)3 in [-111] (d) Na3Fe2(PO4)3 in [001], (e) Na3V2(PO4)3 in [42-1] (f) Na3Fe2(PO4)3 in [101]. The superimposed atomic array indicating the locations of each atom [Na (green-blue), O (grey), P (red) and Fe/V (yellow)] Current Intensity (a.u.) Current Intensity (a.u.) Voltage (V) Voltage (V) Figure S5. CV curves of Na4MnV(PO4)3 and Na3FeV(PO4)3. S4

5 Voltage (V) st charge 1st discharge 2nd charge Capacity (mah g -1 ) theta Figure S6. The electrochemical profiles and corresponding ex-situ XRD patterns recorded during first charge-discharge of Na 3 FeV(PO 4 ) 3. Charge Discharge Recharge V 2.0V 2.38V 3.0V 3.8V 3.46V 2.6V Na4MnV(PO4)3 Charge to 3.5V Charge to 3.8V Na4MnV(PO4)3 Charge to 3.5V Charge to 3.8V Binding Energy (ev) Binding Energy (ev) Figure S7. XPS spectra of Mn and V in Na 4 MnV(PO 4 ) 3 recorded in different charge voltage Theta Figure S8. XRD pattern of the Na 4 NiV(PO 4 ) 3. S5

6 Na4NiV(PO4)3 Charge to 3.5 Charge to 4.0 Na4NiV(PO4)3 Charge to 3.5 Charge to Binding Energy (ev) Binding energy (ev) Figure S9. XPS spectra of Ni and V in Na 4 NiV(PO 4 ) 3 recorded in different charge voltage. S6