Electrochemical sodium storage of copper hexacyanoferrate with a well-defined open framework for sodium ion batteries

Abstract

Sodium ion batteries are considered to be a promising low-cost alternative to common lithium batteries. The exploration of new electrode materials is extremely important for developing sodium ion batteries. In this work, copper hexacyanoferrate (CuHCF), a kind of Prussian blue analogue, was synthesized by a chemical co-precipitation method and the feasibility of the electrochemical sodium ion storage reaction in CuHCF nano-particles in aqueous solution was investigated. All the results demonstrate that the sodium ions can be reversibly inserted/extracted into/from CuHCF nano-particles in aqueous solution, and a specific capacity of 46 mA h g⁻¹ is obtained at the current density of 20 mA g⁻¹. The sodium ion insertion and extraction behaviors are controlled by the solid phase diffusion process in the CuHCF electrode. The well-defined open framework of CuHCF ensures it has good cycle stability. The good performance indicates that CuHCF will be a promising candidate for the cathode materials of sodium ion batteries.

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Introduction

Electrical energy storage has attracted wide attention for the electrification of transportation and renewable energy integration.¹ Currently lithium ion batteries are considered one of the most attractive technologies for large-scale energy storage systems in a smart grid due to their high energy density and long lifetime. However, for the devices used in large-scale stationary storage, safety, cost and durability are the most important issues.² For lithium ion batteries, the limited abundance of terrestrial reserves and uneven geographic distribution of lithium, together with the difficulties in recycling lead to their high price and low sustainability. Moreover, the use of organic electrolyte brings some safety risks. All these have raised great concerns about the cost and safety of lithium ion batteries in large scale applications. As a result, great efforts have been made to explore new low-cost and reliable electrochemical energy storage technologies.^1,3–6

For this background, the use of sodium-ion batteries particularly for large-scale energy storage has attracted intense interest recently.^1–3 Sodium could be adopted as a substitute for lithium to meet the demands of rechargeable batteries. Furthermore, the sodium resources are considered to be unlimited for its widespread presence in the sea. Therefore, sodium-ion batteries have the potential to substitute for lithium-ion batteries in the large-scale energy storage system for renewable energy generation as well as smart grid.⁷

In recent years, numerous attempts have been carried out to explore potential cathode and anode materials for sodium ion batteries. Nonetheless, the larger size of sodium ions (0.99 Å) compared to that of lithium ion (0.69 Å), imposes more challenging constraints in the materials design. In other words, the material should have sufficiently large channel structures to accommodate the storage and diffusion of sodium ions.⁸ Therefore, materials with well-established open frameworks have been intensively investigated as sodium ion batteries electrode. In particular, Prussian blue analogues, one class of metal–organic frameworks, have received attentions owing to impressive electrochemical performance as well as easy synthetic procedures.⁸ KMFe(CN)₆, a kind of Prussian blue analogues, has cubic framework with Fe(III) and M(II) on alternate corners of a cube of corner-shared octahedral bridged by linear (–C≡N–) bond (Fig. 1), the low-spin Fe(III) bond only with C atoms, the high-spin M(II) only with N atoms, and (–C≡N–) bond opens the faces of the elementary cubes for inserted ions to move between half-filled body-center positions.⁹ Considering their open frameworks with sufficiently large diffusion channels, KMFe(CN)₆ may be an ideal electrode material, which could effectively accommodate the volume variation during Na⁺ insertion/extraction processes for room-temperature sodium-ion batteries.^10–13

Fig. 1 Framework of Prussian blue analogues.

In this work, hexacyanoferrate (KCuFe(CN)₆, CuHCF) was synthesized by a chemical co-precipitation method and the feasibility of the electrochemical sodium ion storage in CuHCF nano-particles in Na₂SO₄ aqueous solution was investigated for the cathodic material of rechargeable sodium ion batteries. Also, the detailed diffusion behavior of sodium ions at each transition potential was studied using electrochemical impedance spectroscopy and the sodium ion storage mechanism was discussed according to the X-ray photoelectron spectroscopy results.

Experimental

Co-precipitated CuHCF nano-particle was synthesized by simultaneously dropwise addition of 100 mL of 0.2 M CuSO₄ and 100 mL of 0.01 K₃Fe(CN)₆ to 50 mL H₂O during constant stirring. A tawny brown precipitate formed immediately. After sonicating for 30 min, the suspension was allowed to sit for eight hours. Then the precipitate was filtered, washed with water, and dried at 80 °C for 24 h in air.

The structure of synthesized CuHCF nano-particle was characterized by powder X-ray diffraction (XRD) obtained with a Philips X-ray diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å). The angular resolution in 2θ scans was 0.02° over a 2θ range of 10–90°. The morphology of the materials was investigated using scanning electron microscopy (SEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEM-2100, 200 kV).

To prepare electrodes, a mixture of 80% wt/wt CuHCF, 15% wt/wt acetylene black, 5% wt/wt polyvinylidene fluoride and some dimethyl sulfoxide were ground by hand, creating homogeneous, black slurry. The slurry was spread on carbon plate current collectors with an area of 1 cm². The electrodes were dried in air at 80 °C.

The electrochemical performance of the material was evaluated on a CHI760D workstation. All these electrochemical experiments were carried out in a three electrodes set-up. The above prepared CuHCF electrode was used as the working electrode. The counter and reference electrodes were graphite and saturated calomel electrode (SCE), respectively. 1 mol L⁻¹ Na₂SO₄ aqueous solution was used as the electrolyte. Cyclic voltammetries (CV) were measured at different potential scan rates from 2–100 mV s⁻¹. Galvanostatic electrochemical charge/discharge analysis was carried out by a half cell under the potential window of 0–1.1 V vs. SCE at a current density of 20 mA g⁻¹ using graphite as the counter electrode. Electrochemical impedance spectra (EIS) were measured at different charging/discharging potentials to have an insight into the electrochemical insertion/extraction reactions. To verify the feasibility of the sodium ion insertion, the surface state of the sample after the sodium ion insertion was further analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi (Thermo Fisher Scientific Inc.)).

Results and discussion

The XRD pattern in Fig. 2(a) shows that the synthesized CuHCF has a hexagonal lattice as has been reported.^8–13 As the XRD pattern obtained is in good agreement with the standard JCPDS pattern, the synthesized CuHCF is considered to be phase-pure. As shown in the SEM and TEM images (Fig. 2(b and c)), the synthesized CuHCF consists of large agglomerations of 50–100 nm nano-cubic particles.

Fig. 2 XRD pattern, SEM and TEM images of the synthesized CuHCF nano-particles used for the Na-ion secondary battery cathode.

To evaluate the feasibility of the synthesized CuHCF nano-powder for sodium ion battery application, its electrochemical properties were examined by cyclic voltammetry analyses. Fig. 3(a) shows the CVs of the CuHCF electrode in aqueous Na₂SO₄ electrolyte at a scanning rate of 5 mV s⁻¹. It can be clearly observed that the CVs exhibit a cathodic peak at ∼0.5 V and a corresponding anodic peak at ∼0.7 V respectively, which may be attributed to the insertion/extraction of sodium into/from the face-centered cubic Prussian blue crystal lattice of CuHCF nano-particles. To further evaluate the sodium ion insertion and extraction mechanism, the CVs at different scanning rates were conducted, and the relationship between the peak current density and scanning rate was investigated, which are presented in Fig. 3(c). It is well known that the relationship between the peak current density and potential scanning rate indicates two different electrochemical reaction characteristics, i.e. solid-phase controlled processes or surface confined charge-transfer processes.^9,14–18 Here, the dependence of the cathodic and anodic peak current densities on the square root of the potential scanning rates confirms that the solid-phase diffusion process in the sodium ion insertion and extraction process is dominant for the reaction in CuHCF electrode.

Fig. 3 (a) Typical cyclic voltammograms at 1 mV s⁻¹; (b) CVs at different potential scanning rates, (c) relationship between peak current densities and potential scanning rates, of the nano CuHCF electrode in Na₂SO₄ aqueous solution.

To illustrate the concept of sodium ion insertion/extraction reaction applied in electrochemical energy storage devices, the typical charge/discharge curves and electrochemical cycle performance in 1 mol L⁻¹ Na₂SO₄ aqueous solution are shown in Fig. 4. As anticipated, the synthesized nano CuHCF electrode shows a flat charging plateau between 0.5∼0.8 V and two flat discharging plateaus at the potential range of 0.8–0.5 V and 0.3–0.2 V vs. SCE (highly dependent on the current density). The maximum discharging capacity of 46 mA h g⁻¹ is obtained at the current density of 20 mA g⁻¹. Additionally, as shown in Fig. 4(b), after 50 cycles, the discharging capacity is still about 37 mA h g⁻¹, which is about 81.8% of the original discharging capacity. It demonstrates that CuHCF has good electrochemical stability as the sodium ion battery electrode material.

Fig. 4 (a) Voltage vs. sp. capacity and (b) sp. capacity vs. cycle number of nano CuHCF electrode in aqueous solution under the potential window 0–1.1 V and at a constant current density of 20 mA g⁻¹.

In order to have a deep insight into the kinetics of the sodium ion insertion or extraction behaviors, EIS were measured at different charging/discharging potentials. Firstly, the electrode was charged to 0.6, 0.7, 0.8, 0.9, 1.0 V, and then discharged to 0 V versus SCE, as shown in Fig. 5(a). At the same time, the open circuit potentials (OCPs) of the electrode were measured (shown as Fig. 5(b)) and EIS tests were carried out at these OCPs. It can be found from Fig. 5(c) that all EIS have a depressed semicircle in the high frequency region and a straight line in the low frequency region. The high frequency result reflects the charge transfer process and the low frequency result indicates the diffusion process of reactants and products.¹⁹ The appearance of the very long straight line in EIS indicates that the solid-phase diffusion process in the sodium ion insertion and extraction process is dominant for the electrochemical reaction on the electrode. A simplified equivalent circuit model shown in Fig. 5(d) was employed to analyze the EIS results, where R_s is the solution resistance; Y_o is the constant phase element which expresses the double layer capacitance; R_c is the charge transfer resistance of the electrochemical reaction and W indicates the Warburg diffusion impedance.¹⁹ The reaction resistance and Warburg resistance of CuHCF at different charging/discharging potentials obtained by fitting the experimental spectra are shown in Fig. 5(e). It can be seen that the reaction resistance increases with the rise of the charging potential while the Warburg resistance has a non-monotonic change, i.e. firstly increasing and then decreasing with the rise of the charging potential. As the potential is more positive than the charging/discharging plateau, the sodium ion extraction behavior is dominant in the electrode process. For the gradual depletion of the sodium ions in the lattice of the electrode materials, the kinetic resistance of the sodium ion extraction behavior will increase with the rise of charging potentials. The Warburg resistance has the maximum value at the potentials around the charging/discharging plateau. It is attributed to the fact that at the potentials around the charging/discharging plateau, numbers of sodium ions will diffuse out from the CuHCF lattice. Due to the limited passageway in the CuHCF lattice, the Warburg resistance achieves the maximum value.

Fig. 5 (a) The charging/discharging curves to different potentials, (b) the open circuit potential of the CuHCF electrodes after charged/discharged, (c) electrochemical impedance spectra and enlarged view of the part in the circle, (d) equivalent electrical circuit for CuHCF electrodes after charged/discharged and (e) the reaction resistance and Warburg resistance of CuHCF at different charging/discharging potentials vs. SCE.

To further explore the charge storage mechanism, the nature of the surface species of the electrodes with the original state and after discharging to 0 V was investigated by XPS. According to Fig. 6(a), there is no apparent change in the XPS spectra of Cu in Na-inserted sample compared to those in the original sample. It indicates that the copper elements in the samples have no variation in valence state. As shown in Fig. 6(b), there is no signal of Na element in the XPS spectra of the original sample and the existence of sodium is verified on the subsurface in the Na-inserted sample. The center of the Na 1s peak appears at 1072.0 eV, which is assigned to Na⁺. It is also shown that the spectra of Fe element in the original sample were deconvoluted into three peaks within 700–730 eV in Fig. 6(c). All these peaks are assigned to Fe³⁺ and no signal about Fe²⁺ appears in the XPS spectra of the original sample. However, for the Na-inserted sample, it is found that the Fe³⁺ and Fe²⁺ coexist due to the charge compensation after the insertion of sodium ion into CuHCF lattice according to Fig. 6(d).^20,21 Therefore, the XPS analyses demonstrate that the redox at Fe³⁺/Fe²⁺ is responsible for the capacity contribution during sodium insertion/extraction processes.

Fig. 6 XPS spectra of the original and Na-inserted CuHCF electrode (a) Cu in original and Na-inserted sample; (b) Na in original and Na-inserted sample; (c) Fe in original sample; (d) Fe in Na-inserted sample; (e) K in original and Na-inserted samples.

Since the solid-phase diffusion process is the dominant procedure in the sodium ion insertion and extraction behaviors in the nano CuHCF electrode, the diffusion mechanism of the sodium ion needs to be studied to have a deep insight into how to design materials for accelerating the solid-phase diffusion process. According to the XPS spectra of K element shown in Fig. 6(e), the peak shifts a little to higher binding energy in the Na-inserted sample and there is no other apparent change in its pattern. Therefore, the diffusion of sodium ion in the CuHCF materials is considered to be in line with the direct interstitial mechanism, rather than the knock-off or other mechanisms. In the direct interstitial mechanism shown in Fig. 7(a), the guest ions diffuse in the interstitial position of the crystal lattice. Although the guest ion does not knock off the ions at the lattice position, it also can change the binding energy of the other ions by the electrostatic force, which is verified by Fig. 6(e). As shown in Fig. 7(b), the sodium ion takes the center of 1/8 sub-cube of CuHCF lattice and it can diffuse from a sub-cube to another in the contiguous lattice to realize ion diffusion in the solid electrode materials.

Fig. 7 (a) Direct interstitial mechanism of diffusion and (b) schematic diagram of the sodium ion diffusion mechanism in the electrode materials.

Conclusions

Nano copper hexacyanoferrate, a kind of Prussian blue analogues, was synthesized by a chemical co-precipitation method and the feasibility of the electrochemical sodium ion storage in CuHCF nano-particles in aqueous solution was investigated. It is demonstrated that the sodium ions can be reversibly inserted/extracted into/from CuHCF nano-particles in aqueous solution, and a specific capacity of 46 mA h g⁻¹ is obtained at the current density of 20 mA g⁻¹. The sodium ion insertion and extraction behaviors are controlled by the solid phase diffusion process in the CuHCF electrode. The well-defined open framework of CuHCF ensures it has good cycle stability. All the good performance exhibits CuHCF will be a promising candidate for the cathode materials of sodium ion batteries.

Acknowledgements

The authors are grateful for the financial support by One Hundred Talent Program of Chinese Academy of Sciences, as well as by the NSFC (51302264) of China.

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