Prussian Blue Embedded into Three-Dimensional Graphene Network Prepared by a Simplified Hydrothermal Method and Its Performance as Superior Sodium-Ion Battery Cathode

GONG Chun

(Wuhan University of Technology, Wuhan 430070, China)

Abstract： Enhancing the electronic conductivity of Prussian blue (PB) by graphene decoration is an effective method to improve its sodium storage performance. However, this method suffers from the extremely poor thermal stability of PB nanocubes. For this purpose, Prussian blue embedded in three-dimensional graphene network (PB-rGO) has been synthesized via a facile, low temperature hydrothermal method. The ascorbic acid was used as reducing agent in the low temperature hydrothermal synthesis process and the reduced graphene associated with Prussian blue nanocubes were self-assembled into a black hydrogel. After freeze-drying treatment, the graphene aerogel with Prussian blue nanocubes wrapped displays unique 3D structure SEM results, which provide a continuous electron conductive network. When it utilized as the cathode of sodium ion battery (SIB), the PB-rGO exhibits enhanced performance with a high specific capacity of 61 mAh/g at the current density of 50 C. It even shows excellent cyclability with 85.1% capacity retention over 1000 cycles at the current density of 5 C.

Key words：Prussian blue; sodium ion battery; hydrothermal method; 3D graphene network

To meet the immensely critical issue for the storage and utilization of clean energy like solar and wind energy, developing smart electrical grids with cost-effective and long lifespan energy storage systems is urgently needed[1-2]. Among numerous energy storage systems, sodium ion battery (SIB) has been considered as one of the most promising candidates in last decades owing to the low cost and large abundance of sodium resources[3]. However, SIBs are known to suffer from relatively low specific capacity, short cycle life and poor rate capability, which substantially limited the commercialization of SIBs[4]. Very recently, NaxFeFe(CN)·nH2O, a typical kind of Prussian blue (PB), has attracted increasing interest due to its open framework structure, large interstitial sites and ease of synthesis[5]. Although PB-based cathode materials have been wildly considered as a promising candidate for SIBs, they still face the critical problem of rapid capacity fading and unsatisfied rate performance due to the relatively low electronic conductivity, which extremely hinders their applications[6]. Various matrixes have been applied to enhance the electronic conductivity of PB, like conducing polymer[5], carbon nanotubes (CNT)[7], carbon[4], etc.

However, few works had focused on the utilization of reduced graphene oxide (rGO). There are many issues when choosing rGO as matrixes especially the conflict between the poor thermal stability of PB and the completely reduction of graphene oxide (GO)[1,8]. Yang et al.[9]reported PB-rGO composites via spray-dried and heated at 220℃ to reduce GO, which is close to the decomposition temperature of PB, but the rate performance was still unsatisfactory. Wang et al.[6]successfully synthesized K0.33FeFe(CN)6/rGO with enhanced cycle performance and rate performance by preparing rGO solution in advance, but the rGO solution is not easy to store, which restricts the application of this method.

Herein, we first reported asimplified hydrothermal method to synthesize a graphene aerogel with Prussian blue cubes embedded in three-dimensional graphene network. The formation process involves the reduction of the GO sheets in the presence of ascorbic acid and self-assembly of rGO sheets into three-dimensional network, the as-prepared PB cubes is attached to GO sheets under the effect of active site on the GO sheets. In this way, the 3D network formed by almost completely reduced graphene sheets can provide a continuous electron conductive network, which leads to improved electrochemical performance, and electrochemical tests clearly demonstrate an enhanced rate performance of a high specific capacity of 61 mAh/g at the current density of 50C.

1 Experimental section

1.1 Raw material and reagent

Hydrochloric acid (37%), polyvineypirrolydone (PVP, K30,MW=40000, analytical grade), Na4Fe(CN)6·10H2O
(analytical grade), graphite
(analytical grade), ascorbic acid
(analytical grade) are all purchased from the Sinopharm Chemical Reagent Co., Ltd.

1.2 Materials synthesis of PB-rGO and PB-bare

Graphene oxide was prepared via a modified Hummers’ method. In a typical synthesis process, 1 mL hydrochloric acid (37%) were added to 100 mL deionized water to obtain a homogeneous solution, then 2 mmol Na4Fe(CN)6·10H2O and 5 g polyvineypirrolydone were dissolved in the obtained solution. The mixture was maintained at 65℃ for 6 h under vigorous stirring to obtain a dark blue precipitate followed with filtered, washed with deionized water and dried in vacuum oven at 80℃ for 8 h to obtain the bare PB nanocubes (PB-bare). For the preparation of PB-rGO, the as-prepared PB nanocubes (100 mg) and ascorbic acid (0.2 g) were added to 20 mL GO solution (0.5 mg/mL), followed by vigorous ultrasonic treatment to form a homogeneous solution. Then the mixture was maintain at 95℃ for 1 h to obtain a black hydrogel. The hydrogel was washed and then treated by freeze-drying to obtain the PB-rGO composite finally.

1.3 Morphology and structure characterization of PB-rGO and PB-bare

XRD measurement was performed to investigate the crystallographic information using a D8 Advance X-ray diffractometer with CuKαX-ray source (U.S.A., Bruker Daltonics Inc.). Field emission scanning electron microscopy (FESEM) images was collected with a JEOL JSM-7100F at an acceleration voltage of 10 kV (Japan, JEOL Ltd.). Raman spectra were obtained using a Renishaw INVIA micro-Raman spectroscopy system (U.K., Renishaw changers Inc). Inductively coupled plasma (ICP) tests were carried out using an Optima 4300 DV (U.S.A., Perkin-Elmer Inc).

1.4 Electrochemical measurements of PB-rGO and PB-bare

The electrochemical measurements were tested with 2016 coin cells assembled in a glove box filled with pure argon gas. For sodium battery, sodium foil was used as the anode, a 1 mol/L solution of NaClO4in ethylene carbon (EC)-dimethyl carbonate (DMC) (m(EC)/m(DMC)=1/1) and 5% fluoroethylene carbonate (FEC) was used as the electrolyte, and a Whatman Glass Microfibre Filter (Grade GF/F) was used as the separator.The cathode electrode was produced with 65% (mass fraction) active material, 25% Ketjen black and 10% poly (tetrauoroethylene) (PTFE). The mass loading of the active material was 2-3 mg/cm. The battery was aged for 12 h before test to ensure full absorption of the electrolyte into the electrodes. Galvanostatic charge/discharge measurements were performed with a LAND CT2001A multichannel battery testing system (China, Wuhan Land Inc.). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed with an Autolab PGSTAT302N electrochemical workstation (Switzerland,Metrohm Inc).

2 Results and discussion

The PB-rGO was synthesised via a simplified hydrothermal method (Fig.1), the as-prepared PB nanocubes and GO were dispersed under strong ultrasonic conditions, then in the process of hydrothermal treatment, GO sheets were reduced in the presence of ascorbic acid[10]followed with a self-assembly process, the as-prepared PB nanocubes were embeddedby GO sheets under the action of the active site on the GO sheets[11]. Thereby, 3D hierarchical graphene hydrogel wrapped PB nanocubes was prepared. After freezing-drying and vacuum drying overnight, the final PB-rGO was prepared. The inset picture in Fig.1 shows clearly that a black graphene aerogel was eventually obtained.

Fig.1 Schematic diagrams of the synthesis process of PB-rGO

The morphology characterization of PB-bare and PB-rGO is provided in Fig.2, the high resolution scanning electron microscopy (HRSEM) image of PB-bare (Fig.2(a)) shows that the PB nanocubes are relatively uniform with an average size of 1 μm. The HRSEM image of PB-rGO is showed in Fig.2(b), it is clear enough that the morphology of PB nanocubes does not change in the hydrothermal process due to the weak acidity of ascorbic acid[12], and the nanocubes are wrapped in the 3D porous graphene network. The Fig.2(c) is the HRSEM image of the pure graphene obtained by etching PB-bare with hydrofluoric acid, indicating that the network of graphene is obviously a 3D structure. The phase characterization of PB-bare and PB-rGO was verified by powder X-ray diffraction (Fig.2(d)), both of them feature strong peaks with face-centered cubic phases on account of the good crystallinity, and the shape of the two samples is approximately the same, indicating that the hydrothermal process did not influence the crystal structure of PB nanocubes[12]. From the results from inductively coupled plasma (ICP) analysis, we can know that the mass fraction of Na and Fein PB-rGO are 9.14% and 65.45% respectively, which of PB-bare are 7.12% and 63.76% respectively, combining with thermogravimetric analysis (TGA) results displayed in Fig.2(e), we could figure out that the as-synthesized PB-rGO and PB-bare are determined to be Na0.649Fe[Fe(CN)6]0.900.10·0.6H2O and Na0.506Fe[Fe(CN)6]0.860.14·1.4H2O (is defined as defect in the PB crystals), the decrease of residual water may due to the reduction process of GO[9]. The Raman spectrum of PB-rGO (Fig.2(f)) shows two characteristic bands located at 1340 cm-1and 1595 cm-1, which can be identified as D-band (disorder-induced phonon mode) and G-band (graphite), respectively. The peak intensity ratio of the two bandsID/IGis 0.98, which indicates a high-degree reduction of GO[13].

To investigate the electrochemical performance of the sample, we fabricated the sodium half cells. Cyclicvoltammetry (CV) curve of PB-rGO (Fig.3(a)) at a scan rate of 0.1 mV/s displays two pair sharpen redox peaks at 2.61/2.81 V and 2.83/3.00 V respectively, and a series of small redox peaks at higher potentials. The exiting peak separation may due to the differences in the surrounding crystal structure of the Fe1 site with or without vacancy defects[8]. Fig.3(b) is the charge/discharge profiles of PB-rGO at a current density of 20 mA/g (0.2 C, 1 C=100 mA/g) over a potential window of 2.0-4.0 V. There is a relatively flat platform in the curve between 2.5-3.1 V, which corresponds to the two redox pairs and the reaction of FeHS(N); and there is also an inconspicuous platform at high potential, which corresponds to the reaction of FeLS(C)[4]. The rate performance of PB-bare and PB-rGO are showed in Fig.3(c). The PB-rGO showed excellent capacity retention, varying from 120 to 105, 101, 97, 94, and 61 mAh/g as the current density increased from 1 C to 50 C, however, the specific capacity of PB-bare at high current density (50C) is unsatisfactory with 18.7 mAh/g only. The long-term cycle performance of these samples was also investigated with galvanostatic charge-discharge measurement at 5 C (Fig.3(e)), the capacity retention of PB-rGO is 85.31% over 1000 cycles compared to 39.78% of PB-bare. Both of the two test indicated that the PB-rGO exists much better electrochemical performance than PB-bare. To investigate the mechanism of the optimized performance, electrochemical impedance spectroscopy (EIS) was applied (Fig.3(e)). Before the EIS test, the cells were charged tocycles compared to 39.78% of PB-bare. Both of the two test indicated that the PB-rGO exists much better electrochemical performance than PB-bare. To investigate the mechanism of the optimized performance, electrochemical impedance spectroscopy (EIS) was applied (Fig.3(e)). Before the EIS test, the cells were charged to 4.0 V and then kept for a period of time to reach a stable state. The results of EIS display a much smaller charge transfer resistance (Rct) of the PB-rGO (406.2 Ω)compared to this of PB-bare (646.9 Ω), demonstrating a better electronic conductivity of PB-rGO, The suppression ofRctfurther confirms that the 3D graphene network structure indeed supplies continuous electronic pathway and improves the charge transfer kinetics. The Ex-situ SEM images of PB-rGO after 300 cycles with the current density of 5C is presented in Fig.3(f). We can see it clearly that the PB nanocubes was maintained with the protection of rGO, and the Ketjen black was dispersed uniformly in the rGO.This result demonstrates the well morphology integrity stability of PB-rGO, which is in accord with the good electrochemical stability.

Fig.2 Morphology and structure characterization of PB-rGO and PB-bare(a) FESEM images of the PB-bare; (b) FESEM images of PB-rGO; (c) FESEM images of pure 3D graphene networks; (d) XRD pattern of the PB-bare and PB-rGO; (e) TGA profiles of the PB-bare and PB-rGO; (f) Raman spectrum of PB-rGO, D-band is defined as disorder-induced phonon mode and G-band isdefined as graphite

Fig.3 The electrochemical performance of the PB-rGO and PB-bare(a) The CV curves of PB-rGO at a scan rate of 0.1 mV/s from 2.0-4.0 V, the mentioned potential is vs. Na+/Na; (b) Discharge-charge curves of PB-rGO at a rate of 0.2C, the mentioned potential is vs. Na+/Na; (c) Rate performances of the PB-bare and PB-rGO, ( PB-rGO, PB-bare); (d) Cycling performances of the PB-bare and PB-rGO, ( PB-rGO, PB-bare, the testing current density is 5C); (e) EIS results of desodiated state PB-rGO and PB-bare, ( PB-rGO, PB-bare, Z′—Real part of electrical impedance, Z″—Imaginary part of electrical impedance); (f) The SEM image of PB-rGO after 300 cycles with the current density of 5C

3 Conclusion

(1) To achieve a facile, low temperature, effective decoration of PB nanocubes with graphene for conductivity improving, we delivered a simplified hydrothermal method to synthesis a graphene aerogel with Prussian blue cubes embedded in three-dimensional graphene network (PB-rGO).

(2) Structural and morphology characterization indicated the as-prepared sample is pure cubic phases Prussian blue with 3D continuous graphene network structure.

(3) Electrochemical performance of the sample has been investigated,and enhanced obviously electrochemical performance with a high specific capacity of 61 mAh/g at the current density of 50C and excellent cyclability with 85.1% capacity retention over 1000 cycles at 5 C.