Supporting Information Prussian Blue without Coordinated Water as Superior Cathode for Sodium-Ion Batteries

room temperature and then maintained at 80°C for 3 h to form micro-cubic PB particles. The solid product was collected by centrifugation and washed six times with Milli-Q water. The final PB product was then dried in a vacuum oven. The GO was synthesised by a modified Hummer's method. [1] To prepare GOPCs, the GO and PB were stirred together in an aqueous solution to form slurries with different GO and PB contents. Thereafter, the slurries were ultrasonically dispersed for 10 min, and then spray-dried to obtain the GOPCs. The dried GOPCs were heated under an air atmosphere at 220°C for 3 h to form RGOPCs. For a typical coin cell, the cathodes were composed of 70 wt.% active material (PB or RGOPCs), 20 wt.% super P, and 10 wt.% polyvinylidene fluoride (PVDF). The cathode active material loading was 3-4 mg cm-2. The cathodes were incorporated into coin cells (R2016) with sodium metal foil as the anode and 1.0 M NaClO 4 /EC+DMC (1:1, v/v) as the electrolyte in an argon-filled glove box. The morphologies of the samples were analysed by scanning electron microscopy (Sirion 200, FEI Company). The structural characteristic of the samples were measured by the X-ray diffraction (D/max-2200/PC, Rigaku Co., Ltd.) in equal 2θ steps of 0.02° using a Cu Kα radiation. The Mössbauer spectroscopy was performed with a Mössbauer DRIVE UNIT MR-260A with a 57 Co/Rh source matrix as a radiation source. The chemical composition was examined by elemental analysis (Vario-EL Cube, Elementar Analysensysteme) for C and N and by inductively coupled plasma analysis (iCAP 6000 Radial, Thermo Fisher Scientific, Inc.) for Fe and Na. TG analysis was conducted on a DSC/DTA-TG instrument (STA 449 F3 Jupiter, NETZSCH-Gerätebau GmbH) at a heating rate of 5°C/min under an atmospheric environment. FT-IR and Raman spectra were recorded using an FT-IR spectrophotometer Electronic Supplementary Material (ESI) for Chemical Communications.