Ni-Containing Electrolytes for Superior Zinc-Ion Aqueous Batteries with Zinc Hexacyanoferrate Cathodes.

A one-step coprecipitation process is designed to synthesize zinc hexacyanoferrate (ZnHCF) cathodes in aqueous zinc-ion batteries (ZIBs). The morphology of the cathode is influenced by the concentration of the precursor solution and valence of iron ions. The rhombohedral ZnHCF sample exhibits high crystallinity on the microscale in the cut-angle cubic structure, whereas Na-rich NaZnHCF contains many interstitial water molecules in the rhombic nanoplates. Both samples show effective insertion of Zn ions in the aqueous ZnSO4 solution. ZnHCF shows a specific capacity of 66.7 mA h g-1, a redox voltage of 1.73 V, and fast decline in a few cycles. On the other hand, NaZnHCF has a lower specific capacity of 48.2 mA h g-1, showing two voltage platforms and robust cycling stability. However, owing to serious side reactions, both samples have low Columbic efficiency. To improve the properties such as Coulombic efficiency, specific capacity, and cycling stability, Ni ions are introduced by adding 10 wt % NiSO4 to the ZnSO4 electrolyte. The ZnHCF cathode in the Ni-containing electrolyte has the best properties such as a high specific capacity of 71.2 mA h g-1 at a current density of 100 mA g-1, 93% retention of the Coulombic efficiency, and a good rate performance manifested by a reversible capacity of 58.2 mA h g-1 at 1 A g-1. The results reveal a strategy to improve the electrochemical properties of aqueous ZIBs by modifying the electrolytes.

Introduction

Aqueous rechargeable batteries with low production costs, nonflammability, environmental friendliness, operating safety, and high ion mobility are desirable for grid-scale energy storage.[1,2] Aqueous rechargeable Zn-ion batteries (ZIBs) have evolved as a new energy storage system that is expected to provide a new option for large-scale energy storage.[3] Zinc is abundant on earth and relatively economical, and the corresponding inorganic salts are nontoxic.[4] Aqueous ZIBs operate by two-electron transfer via transportation of divalent zinc ions to provide a high energy density and fast charging.[5] However, owing to slow Zn2+ diffusion in the host, reversible intercalation/deintercalation of multivalent Zn2+ into/from the cathode is difficult, consequently posing a significant barrier for aqueous ZIBs.[6−9]

Prussian blue analogue (PBA) materials have been explored as cathodes in aqueous ZIBs due to the open framework.[10,11] PBAs, presented as AM[Fe(CN)6]·nH2O (where A denotes an alkali/alkaline metal such as K, Na, and Ca and M indicates a transition metal such as Mn, Fe, Cu, Ni, Co, and Zn), have a simple metal cyanide open framework with interstitial ions in the lattice.[12] Liu et al. prepared ZnHCF for aqueous ZIBs showing a high voltage of ∼1.7 V with a capacity of 65.4 mA h g–1 at 1 C.[5] However, during galvanostatic cycling, ZnHCF undergoes reversible phase transition and electrochemical decomposition causing capacity fading.[13] N-bonded metal modification can strengthen the structure and enhance the cycling life of PBAs.[14,15] Ni et al. introduced a small amount of Mn ions into N-bonded Zn sites of zinc hexacyanoferrate, and the combination of Mn and Zn in PBAs results in good cycling stability.[13] Adding the suitable metal ions to the electrolyte produces in situ doping effects through the redox reaction to alter dissolution and recombination of the active materials to enhance the electrode stability.[16−18] However, the reactions of Zn metal anodes in the aqueous ZnSO4 electrolyte are complex issues involving zinc anode corrosion, hydrogen evolution, zinc dendrite formation, and alkaline zinc sulfate compromising the performance of ZIBs.[19−23] Hence, a suitable electrolyte containing metal ions and the ZnSO4 electrolyte is crucial.[24] Ni in Ni-based hexacyanoferrate is considered electrochemically inert and can stabilize the structure during redox reactions.[25,26] Zinc–nickel plating on metal anodes has been investigated to enhance the corrosion resistance of Zn anodes,[1] and Ni-containing electrolytes can improve the long-term performance of ZIBs.

In this work, Na-free ZnHCF and Na-rich NaZnHCF cathodes are prepared for aqueous ZIBs. Different concentration ratios of ZnSO4 and NiSO4 are investigated, and the ZnHCF cathode shows the best results in the 2 mol L–1 ZnSO4 electrolyte with 10 wt % NiSO4 such as a high specific capacity of 71.2 mA h g–1 at a current density of 100 mA g–1, a retention of ∼93% Coulombic efficiency, a capacity of 58 mA h g–1 at 1 A g–1, good rate performance, as well as no obvious attenuation after 150 cycles. The efficient and cost-effective aqueous electrolytes are demonstrated to improve the electrochemical properties for aqueous ZIBs.

Results and Discussion

Structure and Morphology

Synthesis of ZnHCF and NaZnHCF, as shown in Figure a proceeds by simple coprecipitation without chelator assistance or temperature control. x mmol ZnSO4 was dissolved in 200 mL of H2O, and y mmol Fe(CN)63+/4+ was dissolved in 200 mL of H2O, and then, they were mixed together. The ZnHCF sample with a high crystallinity and regular cut-angle cubic morphology has dimensions ranging from 700 nm to 2 μm (Figure b) [x = 2; y = 2; K3Fe(CN)6]. As shown in Figure c, the NaZnHCF sample shows well-defined rhombic nanoplates with an average particle size of 500 nm [x = 2; y = 2; Na4Fe(CN)6]. The morphology variation is attributed to different concentrations in the solution as shown in Figure S1. A higher concentration leads to faster nucleation, producing irregular particles.[27]Figure S1c shows that ZnHCF has a single-particle morphology when the concentration ratio is small (x/y = 1:1 or 1:2), but at higher concentrations, the particles agglomerate and become irregular. NaZnHCF exhibits a mixed morphology of rhombic particles and nanoplates, and at a high concentration ratio (x/y = 3:2), there are more irregular nanoparticles with different sizes. The different morphologies can be attributed to nucleation and framework evolution at different concentrations.

Figure 1

(a) Schematic illustration of the synthesis process and scanning electron microscopy (SEM) images of (b) ZnHCF and (c) NaZnHCF.

All the X-ray diffraction (XRD) patterns exhibit sharp and intense peaks, reflecting the crystalline rhombohedral structure,[28] and a larger concentration does not impact the XRD spectra, as shown in Figures S1a,b. The local structures of ZnHCF and NaZnHCF are evaluated by the Rietveld refinement using the GSAS software as shown in Figure a,b. The pattern of ZnHCF synthesized without Na ions (Figure c) shows peaks at 13.42, 14.00, 16.16, 19.46, 21.42, and 21.64°, corresponding to the (104), (110), (113), (024), (116), and (211) planes of the rhombohedral phase Zn3[Fe(CN)6]2 (JCPDS no. 38-0688), respectively. The structure belongs to the group of R3̅c with lattice parameters of a = 12.718 Å and c = 32.364 Å.[13] The crystal structure of NaZnHCF synthesized with Na ions (Figure d) can be indexed to Na2Zn3[Fe(CN)6]2 (JCPDS no. 36-0539) also with good crystallinity (a = 12.502 Å, c = 32.867 Å, and Rp = 2.76%), showing a rhombohedral structure with the R3̅c space group. Zn-coordinated cyanide changes NaZnHCF/ZnHCF into a different space group known for PBAs which generally have a face-centered cubic structure belonging to the Fm3m space group,[29] in which the FeC6 octahedra and ZnN4 tetrahedra are linked by cyanide ligands. The three-dimensional porous structure with large interstitial pores is integrated irregularly with inserted ions and water molecules,[13] which facilitate electrochemical insertion/extraction of different cations including Zn ions via a quick and reversible mechanism.[30] ZnHCF provides empty frames, while the NaZnHCF framework is occupied with Na ions and zeolite water due to the charge balance between the initial Fe(CN)64– reactants and hydrated Na ions in the precursors.[31]

Figure 2

Local structure of (a) ZnHCF and (b) NaZnHCF obtained by the Rietveld refinement using the GSAS software. XRD patterns of (c) ZnHCF and (d) NaZnHCF with the red symbols (×) representing the acquired data, black lines denoting the calculated patterns, light green bars showing the position of Bragg reflections, and blue patterns denoting the difference between the experimental and calculated data. (e) Raman scattering spectra and (f) thermogravimetric analysis (TGA) curves of the as-prepared samples.

Raman scattering is performed, and as shown in Figure e, the peaks of ZnHCF located at 2180 and 2191 cm–1 correspond to the ν(CN) stretching mode of CN– coordinated with Fe(III).[32,33] ν(CN) of NaZnHCF shifts gradually to smaller wavenumbers of 2117 and 2163 cm–1, indicating reduced average valance states of Fe, consistent with the chemical composition.[34] The FTIR spectrum of ZnHCF in Figure S2 shows bands at 2097 and 2187 cm–1 associated with the stretching of cyanide CN.[35] The H–O–H bending mode at 1620 cm–1 and O–H stretching at 3617 cm–1 of H2O are not observed, implying negligible water adsorption and coordination in the ZnHCF framework. TGA is performed to determine the water content and material degradation, as shown in Figure f. ZnHCF shows a moderate weight loss of ∼2.5 wt % below 380 °C, indicating a small water content. At 400 °C, the weight loss stems from the collapse of the framework.[36] NaZnHCF shows 15.3 wt % weight loss due to adsorption and zeolite water content below 200 °C.[37] NaZnHCF shows a weight loss platform between 200 and 500 °C, indicating better thermal stability than that of sodium-free ZnHCF.[38] The compositions of ZnHCF and NaZnHCF determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) are listed in Table and the formulas of ZnHCF and NaZnHCF are determined to be Zn3[Fe(CN)6]2 and Na1.48Zn3[Fe(CN)6]1.824·6.18H2O according to ICP and TGA.

Table 1

Composition of ZnHCF and NaZnHCF Determined by ICP-AES

samples	elements	weight (mg kg–1)	atomic (mol kg–1)
ZnHCF	K	3946.7	0.1
	Zn	408295.9	6.245
	Fe	228159.1	4.085
NaZnHCF	Na	63989.2	2.784
	Zn	368099.5	5.630
	Fe	191168.5	3.423
ZnHCF–Zn3[Fe(CN)6]2		NaZnHCF–Na1.48Zn3[Fe(CN)6]1.824·6.18H2O

Electrochemical Properties

The electrochemical properties are determined at voltages between 0.9 and 1.95 V versus Zn/Zn2+ at a constant current of 300 mA g–1 in the 2 M ZnSO4 electrolyte as shown in Figure . Owing to the narrow stable electrochemical window of the ZnSO4 electrolyte, both ZnHCF and NaZnHCF have low initial Coulombic efficiencies of 67.4 and 63.1% and first discharging capacities of 40.5 and 43.9 mA h g–1, respectively. During cycling, the discharging capacity of ZnHCF increases rapidly to 60.7 mA h g–1 within 10 cycles and then gradually to 66.7 mA h g–1 in the following 50 cycles (Figure a). The capacity escalation can be ascribed to electrode galvanization in the initial charging–discharging phase.[13] In the following 100 cycles, the Zn storage capacity decreases sharply and is only 11.3 mA h g–1 in the 150th cycle. Although dissolution of ZnHCF in the ZnSO4 electrolyte is generally inhibited due to the solubility equilibrium, a small volume of free [Fe(CN)6]3– may transfer to the anode to cause unsuitable side reactions or recombination with free Zn2+ in the electrolyte.[13] NaZnHCF also exhibits a relatively low Coulombic efficiency in the first cycle. The charging and discharging capacities are obviously smaller than those of ZnHCF with a limited discharge capacity of 48.2 mA h g–1 (Figure b), but there is good consistency after 200 cycles. NaZnHCF contains fewer electrochemically active sites which cause around 20 mA g–1 capacity disparity. The standard structure of Zn3[Fe(CN)6]2 with Na/Zn insertion is shown in Figure c. With regard to ZnHCF, the electrolyte without Ni exhibits higher plateaus at 1.9/1.7 V in the charging/discharging process, respectively, and the low Coulombic efficiency stems from the side reactions.

Figure 3

Local charging–discharging curves of (a) ZnHCF and (b) NaZnHCF for different cycles with both samples showing low Coulombic efficiency. (c) Schematic illustration of the primitive cell of Zn3[Fe(CN)6]2 with Na/Zn insertion simulated by the GSAS software, the pie charts represent the percentage of occupancy; zinc hexacyanoferrate is present as a trigonal R3̅c phase.

The electrochemical reactions are monitored in the presence of 10 wt % NiSO4 as shown in Figure . For ZnHCF, the electrolyte with Ni gives rise to a higher Coulombic efficiency, a larger specific capacity of 69.7 mA h g–1 for a current density of 300 mA g–1, and a lower redox potential region from 1.4 to 1.8 V. The capacity increases in the first dozen cycles, and good cyclic stability with 100% capacity retention is observed in the 150th cycle, which is much better than that without Ni. The assembled ZIB exhibits a sudden drop in the following cycles due to battery damage arising from the side reactions in the R2032 coin cell. As for NaZnHCF, the electrolyte containing Ni yields a higher Coulombic efficiency, a higher specific capacity increase from 42.2 to 51.2 mA h g–1, a long sloping redox platform with a higher average potential, and stable cyclic characteristics. The results confirm that the addition of Ni to the electrolyte improves the specific capacity, Coulombic efficiency, and cycling stability.

Figure 4

(a,c) Charge–discharge curves of the zinc hexacyanoferrate samples (red for ZnHCF samples and blue for NaZnHCF samples) in a 2 mol L–1 ZnSO4 solution (dashed line/small dots) and ZnSO4 solution with 10 wt % NiSO4 (solid line/big dots) at a current density of 300 mA g–1. (b,d) Cyclic stability of zinc hexacyanoferrate samples at a current density of 300 mA g–1 (red for ZnHCF samples and blue for NaZnHCF samples; solid dots for specific capacity and hollow circles for Coulombic efficiency). The electrolyte containing Ni ions leads to a better specific capacity, Coulombic efficiency, and cyclic stability.

To investigate the underlying mechanism, linear sweep voltammetry (LSV) is performed on the Zn metal symmetrical batteries at a scanning rate of 5 mV s–1 and a voltage range of 0–2.7 V. Figure a shows that in the pure 2 mol L–1 ZnSO4 solution, the larger current density in the oxygen evolution reaction (OER) indicates enhanced water splitting due to uneven Zn plating.[39] In the 1 mol L–1 NiSO4 solution (Figure b), the current density at a high voltage decreases with the number of cycles on account of inhibition of corrosion with the participation of Ni ions.[4] Artificial electrolyte interphase protective layers have been widely applied to protect metal anodes to enhance metal anode stability.[40] The cyclic voltammetry (CV) curves obtained from the cells with the Zn3[Fe(CN)6]2 cathode and Zn metal anode in Figure c confirm that the redox voltage shifts to a lower potential in the presence of Ni and avoidance of the serious side reaction at a high voltage. There is slight insertion of Ni ions into the Zn3[Fe(CN)6]2 cathode. Both Zn2+ and Ni2+ are inserted into the ZnHCF structure during discharging in the Zn2+/Ni2+ solution to increase the capacities. The XRD pattern of the Zn anode in the electrolyte of 2 mol L–1 ZnSO4 and 10 wt % NiSO4 after five cycles is shown in Figure d, and codeposition of the zinc–nickel alloy is detected to benefit the corrosion resistance of the Zn metal anode in the ZnSO4 solution.[1] As a result, the Ni-assisted redox reaction avoids the serious water splitting side reaction at a high voltage during charging and improves the Zn corrosion resistance during discharging, resulting in an improved electrochemical performance of the Zn hexacyanoferrate cathode and zinc foil anode.

Figure 5

LSV results obtained from the symmetrical cell with Zn metal electrodes in a (a) 2 mol L–1 ZnSO4 solution and (b) 1 mol L–1 NiSO4 solution. (c) Typical CV curves of the cells with the Zn3[Fe(CN)6]2 cathode and Zn metal anode in different electrolytes. (d) XRD pattern of the Zn anode in the hybrid electrolyte containing 2 mol L–1 ZnSO4 and 10 wt % NiSO4.

The electrochemical properties of the electrolytes with 2 mol L–1 ZnSO4 and different concentrations of NiSO4 (0, 2, 5, 10, and 20 wt %) are shown in Figure a. Ni improves the Coulombic efficiency by over 90% and enhances the cyclic stability for more than 150 cycles without apparent decay. The electrolyte with 10 wt % NiSO4 yields the highest specific capacity of 71.2 mA h g–1. In each system, Ni gives rise to a capacity increase for 150 cycles, indicating that activation is required to improve the kinetics and discharging capacity.[3] The electrolyte without Ni2+ leads to reduced capacities within 150 cycles, and the capacity fades to 0 as shown in Figure b in addition to very poor Coulombic efficiency. The 20 wt % Ni2+ electrolyte shows a stable capacity of 62 mA h g–1 until the 107th cycle, but the cell is damaged afterward.

Figure 6

(a) Cyclic characteristics of the ZnHCF cathode and Zn metal anode in electrolytes with different concentrations of NiSO4 at a current density of 300 mA g–1. (Solid dots for specific capacity and hollow circles for Coulombic efficiency.) (b) Charging–discharging curves of the ZnHCF coin cell for different cycles in the electrolyte comprising 2 mol L–1 ZnSO4 and 10 wt % NiSO4 showing excellent cycling stability. (c) Rate characteristics of the ZnHCF cathode and Zn metal anode in 2 mol L–1 ZnSO4 with (red) and without (blue) 10 wt % NiSO4 (solid dots for specific capacity and hollow circles for Coulombic efficiency). (d) Charging–discharging curves of the ZnHCF coin cell at various rates in the electrolyte with 10 wt % NiSO4.

The rate performance is shown in Figure c. Compared to the electrolyte without NiSO4 (blue dots), 2 mol L–1 ZnSO4 with 10 wt % NiSO4 (red dots) yields higher average capacities of 71.2, 69.5, 65.8, and 58.2 mA h g–1 at current densities of 100, 200, 500, and 1000 mA g–1, respectively. When the current density is reverted back to 100 mA g–1, the reversible capacity recovers to 71 mA h g–1, implying excellent rate capability in the presence of Ni ions in the electrolyte. The charging–discharging curves of ZnHCF at different current densities are shown in Figures d and S3. In the presence of 10 wt % NiSO4, the ZnHCF coin cell displays a discharging capacity of 71.2 mA h g–1 at a lower current density of 100 mA g–1 and a Coulombic efficiency of 90.8% that increases with current densities. The Coulombic efficiency rises to ∼98% at a current density of 1 A g–1. However, without NiSO4, the Coulombic efficiency is only about 60% at low current densities, and serious side reactions occur during the process.

Our study demonstrates that the Ni2+-containing electrolyte (2 mol L–1 ZnSO4 + 10 wt % NiSO4) balances Zn loss in the reactions. Both Zn2+ and Ni2+ are inserted into the ZnHCF structure during discharging in the Zn2+/Ni2+ electrolyte, leading to larger capacities. When Ni2+ ions are cointercalated with Zn2+ ions during discharging, the deintercalated Ni2+ ions dissolve back into the electrolyte and deposit onto the Zn anode during charging. As a result, the reduction/oxidation peaks with shoulders reflect the insertion/extraction processes of both Zn2+ and Ni2+ cations.

Conclusions

A simple coprecipitation one-step technique is employed to synthesize zinc-based hexacyanoferrate materials for ZIBs. The morphology of the materials is influenced by the concentration of the precursor solution and valence of iron ions. The rhombohedral ZnHCF structure has high crystallinity with diameters ranging from 700 nm to 2 μm. The Na-rich sample, NaZnHCF, consists of rhombic nanoplates with a large number of interstitial water molecules. Both samples have Zn-ion storage capability. ZnHCF has a large capacity of 66.7 mA h g–1 but poor cyclic stability as it decomposes during cycling. NaZnHCF shows two zinc-embedding platforms with high cycling stability, but the capacity is only 48.2 mA h g–1. The electrochemical properties of both samples can be enhanced by introducing a small amount of NiSO4 (10 wt %) to the ZnSO4 electrolyte. In the electrolyte containing Ni, the capacity of ZnHCF increases to 71.2 mA h g–1 at a current density of 100 mA g–1 together with 93% Coulombic efficiency and no noticeable capacity decline after 150 cycles. On the Zn metal anode, codeposition of the zinc–nickel alloy improves the corrosion resistance in the aqueous ZnSO4 electrolyte. The Ni-assisted redox reaction avoids the serious water splitting side reaction at a high voltage during charging, boding well for the Zn corrosion resistance during discharging, leading to an improved electrochemical performance of the Zn hexacyanoferrate cathode and zinc anode systems. This study reveals a promising route to enhance the properties of zinc-based Prussian blue cathodes in aqueous ZIBs by optimizing cost-effective electrolytes.

Experimental Details

Synthesis of ZnHCF and NaZnHCF

The ZnHCF particles were synthesized by a simple coprecipitation technique at room temperature. To prepare solution A, 2 mmol zinc sulfate (ZnSO4·7H2O, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) was dissolved in 200 mL of deionized water, and 2 mmol potassium ferricyanide (K3Fe(CN)6, Aladdin Industrial Inc, Shanghai, China) was also dissolved in 200 mL of deionized water to obtain solution B. After vigorous stirring, solution A was added to solution B at ambient temperature. After stirring for a while, the suspension was left to stand for 24 h at room temperature. The sediment was collected by centrifugation, rinsed with deionized water and ethanol twice, and dried at 70 °C for 24 h to obtain cut-angle cubic ZnHCF. The NaZnHCF nanoplates were synthesized by a similar precipitation method, and the only difference was that the hexacyanoferrate source was replaced by Na4Fe(CN)6.

Material Characterization

The XRD patterns were obtained using the Panalytical X’pert PRO MRD (Holland) with Cu Kα radiation, and the SEM images were acquired using the TESCAN VEGA3. TGA was performed using the thermogravimetric analyzer, Netzsch STA 449 F3, in an argon atmosphere from room temperature to 800 °C at a heating rate of 10 °C min–1. The Raman scattering spectra were acquired using an Ar ion laser with a wavelength of 532 nm (LabRAM HR800, Horiba, France). The elements were identified by elemental analysis (Vario Micro), and the concentrations of K, Zn, Fe, and Na were determined by ICP-OES (IRIS Intrepid II XSP, Thermo Elemental, USA).

Electrical Assessment

The working electrodes were designed by mixing the active ingredient (ZnHCF or NaZnHCF), conductive carbon black, and a polytetrafluoroethylene binder with a weight ratio of 7:2:1. The materials were ground using an agate mortar by hand to form a homogeneous mixture, pressed into a thin film, and vacuum-dried at 70 °C for 12 h. Afterward, it was pressed onto a stainless steel mesh (15 μm). The galvanostatic charging–discharging measurements were performed in a regular coin cell (CR2032) consisting of a Zn metal foil (a diameter of 12 mm and a thickness of 30 μm) as the counter electrode and Whatman glass fibers (GF/A, Whatman) as the separator. Five electrolytes containing 2 mol L–1 ZnSO4 and different concentrations of NiSO4 (0%, 2%, 5%, 10%, and 20% wt %) were tested. Each cell was prepared under ambient conditions. CV was conducted on an electrochemical workstation (CHI 760E) at 0.1 V s–1, and the galvanostatic charging–discharging experiments were carried out using the battery testing system (Neware BTS 3000n) from 0.9 to 1.95 V (vs Zn/Zn2+) at 25 °C.