High-stability monoclinic nickel hexacyanoferrate cathode materials for ultrafast aqueous sodium ion battery

Journal Pre-proofs High-stability Monoclinic Nickel Hexacyanoferrate Cathode Materials for Ultrafast Aqueous Sodium Ion Battery Liuxue Shen, Yu Jiang, Yuefeng Liu, Junlin Ma, Tongrui Sun, Nan Zhu PII: DOI: Reference:

S1385-8947(20)30219-9 https://doi.org/10.1016/j.cej.2020.124228 CEJ 124228

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Chemical Engineering Journal

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3 December 2019 9 January 2020 25 January 2020

Please cite this article as: L. Shen, Y. Jiang, Y. Liu, J. Ma, T. Sun, N. Zhu, High-stability Monoclinic Nickel Hexacyanoferrate Cathode Materials for Ultrafast Aqueous Sodium Ion Battery, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124228

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High-stability Monoclinic Nickel Hexacyanoferrate Cathode Materials for Ultrafast Aqueous Sodium Ion Battery Liuxue Shena, Yu Jianga, Yuefeng Liub, Junlin Maa, Tongrui Suna and Nan Zhua* a

Zhang Dayu School of Chemistry, Dalian University of Technology, Dalian 116024, China

b Dalian

National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical Physics,

Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China * Corresponding Author: [email protected] (Nan Zhu)

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Abstract Prussian blue and its analogues are regarded as superior cathode materials for sodium ion batteries (SIBs) owing to low cost, open framework and large interstitial spaces for the insertion/extraction of sodium ion. Herein, a highly stable monoclinic sodium rich nickel hexacyanoferrate (II) nanocube (m-NiHCF) has been synthesized via a facile coprecipitation method with the aid of chelating agent and surfactant. It delivers a high specific capacity of 70.1 mAh g-1 and maintains 97.1% capacity retention after 8000 cycles. Even at a high current density of 2000 mA g-1, an impressive capacity of 53.2 mAh g-1 is obtained. The fast kinetics of m-NiHCF is mainly benefited from the capacitive-controlled domination under the charge storage process. Meanwhile, ex-situ X-ray diffraction together with ex-situ X-ray photoelectron spectroscope and ex-situ Raman and ex-situ Fourier transform infrared analysis have revealed the reversible phase transition between monoclinic and cubic phases with the reaction of carbon coordinated FeII/FeIII redox-active site during extraction and insertion of sodium ion in mNiHCF framework. Furthermore, a high voltage aqueous SIB full cell assembled with NaTi2(PO4)3@C anode achieves a high energy density of 86 Wh kg-1 with capacity retention of 83% after 600 cycles, showing great prospects in the grid-scale energy storage application. Keywords: Prussian blue analogues, nickel hexacyanoferrate (II), monoclinic phase, aqueous sodium ion batteries, ultralong life 1. Introduction The scarcity of fossil energy and the aggravation of environmental issue trigger the booming development of renewable energy technologies [1-4]. To integrate renewable energy into the electric grid, it is vital to elaborate efficient large-scale electric energy storage systems 2

(EESs) [5-7]. Currently, lithium ion batteries (LIBs), as a primary energy storage device, is widely used in portable electronics, electric vehicles and smart grids [8-11]. However, the high cost of lithium resources and potential safety hazard of LIBs hinder using LIBs for EESs [1215]. Sodium ion batteries (SIBs) have attracted increasing attentions in recent years due to the abundant of sodium resources and the similar electrochemical behaviors comparing to LIBs [16-21]. In particular, aqueous sodium ion batteries (ASIBs) are regarded as one of the most promising candidates for EESs in term of their low cost, high safety and environment-friendly properties that arise from the use of nonflammable and nontoxic aqueous electrolytes [22-24]. Until now, a number of insertion materials including transition-metal oxides and polyanionic compounds have demonstrated stable electrochemical performance in ASIBs applications [24]. Nevertheless, these materials generally exhibited lower reversible capacity in ASIBs, which are limited to the potential range of aqueous electrolyte [25]. For instance, Huang and co-works prepared tunnel structured Ti-substituted Na0.44MnO2, delivering a high specific capacity of 110 mAh g-1 in non-aqueous electrolyte; while displaying a lower specific capacity of 37 mAh g-1 in aqueous electrolyte [26]. Goodenough and co-works reported NASICONstructured Na3MnTi(PO4)3, exhibiting a specific capacity of 80 and 58.4 mAh g-1 in nonaqueous and aqueous electrolyte, respectively [27-28]. Therefore, it is urgent to develop an ideal candidate of cathode materials for the demand of high capacity and cycling stability in ASIBs. Prussian blue (PB) and its analogues (PBAs), a large family of hexacyanometallates with chemical formula of AxM [Mˊ(CN)6]y·nH2O (A is an alkaline metal; M and Mˊ are transition metals; 0≤x≤2; 0＜y≤1), are composed of

3

the high-spin (HS) MN6 and low-spin

(LS) MˊC6 octahedral bridged by cyanide groups (-C≡N-) to form a double-perovskite framework with interconnected channels for the diffusion of alkali ions [29-30]. Many researches have focused on sodium storage performance of PBAs in aqueous electrolyte because of their large ionic channels, abundant interstitial sites and tunable redox potentials [31-33]. Although recent researches reported reasonable performance based on PBAs in ASIBs, most of them suffer from performance deterioration including low specific capacity, poor cycling stability, and low coulombic efficiency due to the low Na content caused by high Fe(CN)6 vacancies and coordinated water in the crystal framework [34]. To solve this problem, Yang and co-works synthesized low-defect and well-crystallized cobalt hexacyanoferrate (CoHCF) nanocrystals by citrate-assisted controlled crystallization method, demonstrating impressive electrochemical performance [35]. Goodenough and co-works reported a dehydrated rhombohedral Na2−δMn[Fe(CN)6] by removing interstitial water, delivering a high capacity of 150 mAh g-1 with flat charge-discharge plateaus at 3.5 V vs. Na/Na+ [36]. Prussian white (Na3.1Fe4[Fe(CN)6]3) hierarchical nanotubes were successfully prepared via a selftemplate method by Zhao and co-workers, and exhibited an outstanding rate capability and cycling stability [37]. In this work, we synthesized a monoclinic sodium-rich nickel hexacyanoferrate nanocubes (m-NiHCF) through a chelating agent and surfactant co-assisted coprecipitation method. As a result, the as-obtained m-NiHCF as cathode material for ASIBs delivered outstanding performance including high specific capacity (70.1 mAh g-1 at 100 mA g-1), ultrahigh rate capability (76% capacity retention at 2000 mA g-1), and stable cycling performance (97.1%

4

capacity retention over 8000 cycles). Fast kinetics of m-NiHCF cathode have been discussed in detail, which mainly benefited from the capacitive-controlled domination under the charge storage process. And ex-situ X-ray diffraction (XRD), ex-situ X-ray photoelectron (XPS) spectroscope, ex-situ Raman spectroscopy, and ex-situ Fourier transform infrared (FTIR) spectrum techniques have been utilized to further uncover the sodium storage mechanism of mNiHCF cathode. More importantly, the combination of the m-NiHCF cathode, NaTi2(PO4)3@C (NTP@C) anode, and aqueous electrolyte enabled the assembly of the m-NiHCF//5M NaClO4//NTP@C ASIB full cell, which exhibited superior rate and cycling performance. Further demonstrating the potential of this m-NiHCF material for practical storage energy applications. 2. Experimental Section 2.1. Synthesis of m-NiHCF In a typical procedure, solution A was formed by dissolving 5 mmol nickel (II) chloride hexahydrate and 25 mmol trisodium citrate dihydrate in 50 mL deionized water; solution B was formed by dissolving 5 mmol sodium ferrocyanide decahydrate in 50 mL deionized water; and solution C was formed by dissolving 5 g polyvinylpyrrolidone K-30 and 10 g sodium chloride in 100 mL deionized water. Then, the solution A and B were simultaneously dropwise added into solution C by peristaltic pump at the rate of 0.5 mL min-1. After continuous stirring for 5 h and aged for 24 h, the precipitate was centrifuged, washed with deionized water and alcohol for several times, and dried overnight in a vacuum oven at 60 °C. The cubic nickel

5

hexacyanoferrate (c-NiHCF) were synthesized through a similar process without chelating agent and surfactant. 2.2. Material Characterizations XRD patterns were measured by using an X-ray diffractometer (SmartLab, Rigaku) with Cu Kα radiation. SEM images and corresponding elemental mappings were obtained using a field emission scanning electron microscope (FE-SEM, SU8220, Hitachi) equipped with an energy dispersive X-ray spectrometer (EDS). TEM images and corresponding elemental mappings were acquired using an environmental transmission electron microscope (ETEM, Titan Themis G3, Thermofisher) equipped with EDS. XPS measurement was performed on an X-ray Photoelectron Spectrometer (ESCALAB XI+, Thermo). FTIR spectrum was detected by NIXLET 6700 (Thermofisher). Raman spectrum was collected with DXR Microscope (Thermofisher) with an excitation wavelength of 532 nm. TG measurement was performed on SDT-Q 600 (TA Instruments). Elemental compositions of Na, Ni, and Fe were determined by ICP-OES (Optima2000DV, Perkinelmer). 2.3. Electrochemical Measurement The working electrodes were fabricated by compressing 70 wt% active material, 20 wt% ketjen black, and 10 wt% poly(tetrafluoroethylene) (60% PTFE dispersion) on stainless steel mesh. Electrochemical behaviors of working electrode were performed at room temperature by a three-electrode configuration using a large piece of active carbon (YP-80F, Kurary) as the counter electrode, a Ag/AgCl electrode as the reference electrode, and 5 M sodium perchlorate (NaClO4) solution as the aqueous electrolyte. Electrochemical tests of ASIBs full cell were carried out using CR2032 coin cells, with m-NiHCF as the cathode, NTP@C as the anode, 5M NaClO4 solution as the aqueous electrolyte, and Grade F glass fiber (Whatman®) as the separator. CV and EIS tests were performed by an electrochemical workstation (CHI 660E). 6

The EIS measurements were performed by applying at frequency ranging from 0.01 Hz to 10,000 Hz with a potential amplitude of 5 mV. GCD and GITT measurements were accomplished by a multichannel battery testing system (LAND CT2001A) at room temperature. The GITT measurements were performed by applying at a current pulse of 20 mA g-1 for 15 min and then left on open circuit for 30 min

3. Results and Discussion 3.1. Structure Analysis and Chemical Properties The chemical formula of as-prepared NiHCF samples were confirmed by inductively coupled plasma optical emission spectrometer (ICP-OES) and thermogravimetric analysis (TGA). According to the results of ICP-OES analysis (Table S1 and S2), the Na/Fe/Ni molar ratio of m-NiHCF and c-NiHCF can be determined as 1.45:0.87:1.00 and 1.21:0.86:1.00, respectively. TGA results showed that the mass loss below 300 °C can be assigned to the elimination of adsorbed water and interstitial water (Fig. S1) [38]. Combining the results of ICP-OES and TGA, the chemical formula of as-prepared NiHCF samples are listed in Table 1. Comparing with c-NiHCF, the m-NiHCF contained higher Na content, lower Fe(CN)6 vacancies, and lower coordinated water. The Rietveld-refined XRD patterns were carried out to determine the crystal structure of as-prepared NiHCF samples [39]. The refinement result of c-NiHCF was well coincided with the typical cubic phase with a space group of Fm-3m (Fig. 1a). However, the structure distortion caused by introducing more sodium in m-NiHCF crystal framework, and the XRD pattern of m-NiHCF showed peak splitting located at 24.5, 39.5, 50.3, and 56.5°, exhibiting a monoclinic phase with a space group of P21/n and lattice parameters of a= 10.27155 Å, b=7.33525 Å, c= 7.19637 Å with β= 92.080° (Fig. 1b) [38, 40]. The crystal structures of two as-prepared NiHCF samples both displayed a 3D open structure with HS NiN6 and LS FeC6 octahedral bridged by 7

C≡N bond (Fig. 1c and d). In the monoclinic phase, Ni-N≡C angles were shifted from typical 180° to 169.69° and 164.36°, and Fe-C≡N-Ni bond distances lengthened from 5.1347 Å in cubic phase to 5.1736 Å and 5.1961 Å, thus the monoclinic structure has a larger volume than those of the cubic phase. The scanning electron microscopy (SEM) images of c-NiHCF (inset of Fig. 1a, Fig. S2) showed that the nanoparticles were randomly aggregated to form irregular block shape with edge length of 4-10 μm. When using trisodium citrate as chelating agent and polyvinylpyrrolidone as surfactant during the coprecipitation process, the morphology of NiHCF was converted from block structure into uniform well-packed nanocubes with edge length of ~300 nm (inset of Fig. 1b, Fig. 2a), which benefited from chelating agent and surfactant co-assisted crystallization mechanism for significantly decreasing nucleation kinetics and inhibiting crystal aggregation. Energy dispersive X-ray spectroscopy (EDS) mapping images demonstrated uniform dispersion of Na, Ni, Fe, C, and N elements in the m-NiHCF composite (Fig. 2b), which achieved agreement with wide-scan XPS spectrum (Fig. 2c). In addition, high-resolution XPS measurements were utilized to investigate the valance states of Ni and Fe in m-NiHCF composite. The high-resolution Ni 2p XPS spectra displayed the peaks at 874.3 and 856.6 eV, assigned to NiII 2p1/2 and NiII 2p3/2, respectively (Fig. 2d) [41]. The highresolution Fe 2p XPS spectra showed that the peaks at 721.5 and 708.7 eV corresponded to FeII 2p1/2 and FeII 2p3/2, respectively (Fig. 2e) [42]. Moreover, as revealed by FTIR spectrum (Fig. 2f), a broad peak at ~3200 cm-1 and a sharp peak at 1620 cm-1 were assigned to O-H stretching mode and O-H bending modes of H2O, respectively [43-44]. A strong sharp peak at 2094 cm-1 was assigned to the stretching vibrations of the C≡N bond of cyanide-coordinated FeII and NiII [45], the detail of cyanide-coordinated FeII and NiII was further confirmed by Raman spectrum (Fig. S3), The two peaks located at 2105 cm-1 and 2140 cm-1 are corresponding to the vibrations

8

of CN group in FeII-C≡N-NiII, and no FeIII-C≡N-NiII peak is observed [46]. This results were in good agreement with the discussed above.

3.2. Half-Cell Performance The cyclic voltammetry (CV) curves of NiHCF electrodes were measured in Fig. 3a. A pair of redox peaks between 0.4 and 0.7 V appeared in both samples, which can be attributed to reversible reactions of carbon-coordinated FeII/FeIII redox couple during extraction and insertion of sodium ion in NiHCF lattice [47]. Moreover, the m-NiHCF showed a broader peak and a larger curve area compared with c-NiHCF, demonstrating higher specific capacity of mNiHCF. The corresponding galvanostatic charge-discharge (GCD) profiles of NiHCF electrodes were presented in Fig. 3b. In accordance with the CV curves, the both electrodes exhibited a well-defined voltage plateaus at a range of 0.4-0.7 V, where the voltage hysteresis of m-NiHCF was smaller than c-NiHCF, indicating fast kinetics and high energy efficiency of m-NiHCF. Expectedly, the specific capacity and coulombic efficiency of m-NiHCF (70.1 mAh g-1, 88.9%) were both higher than c-NiHCF (60.8 mAh g-1, 72.8%). The rate performance of NiHCF electrodes at various current densities was shown in Fig. 3c and Fig. S4. The m-NiHCF delivered specific capacities of 68.5, 66.3, 61.7, and 53.2 mAh g-1 at the current densities of 500, 1000, 1500, and 2000 mA g-1, respectively. While the cNiHCF delivered 55.6 mAh g-1 at 500 mA g-1, but only retained 18.2 mAh g-1 capacity at 2000 mA g-1. Furthermore, long-term cycling stability of the NiHCF was investigated at the current density of 500 mA g-1. As shown in Fig. 3d, the c-NiHCF synthesized by traditional coprecipitation method showed an obvious performance degradation with only 79.3% capacity retention after 1000 cycles. In contrast, the m-NiHCF exhibited outstanding cycling performance with 97.1% capacity retention over 8000 cycles, moreover, the morphology of mNiHCF shows barely change after cycling tests, indicating remarkable structure stability of m-

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NiHCF (Fig. S5). The excellent electrochemical performance of m-NiHCF could be ascribed to the following factors: (1) The higher Na content in m-NiHCF lattice with large volume can deliver higher specific capacities. (2) The uniform nanocube morphology can provide larger contact areas between electrode and electrolyte interface, which can significantly shorten ion transport path. (3) The well-crystallized open framework structure possessed large interstitial sites, which would facilitate fast and stable sodium ion extraction/insertion reactions.

3.3. Electrochemical Mechanisms To understand the reason of the enhanced performance of m-NiHCF during electrochemical process, the CV measurement at various scan rates from 0.1 to 1.0 mV s-1 was performed to investigate the storage mechanism and reaction kinetics process. Redox peaks showed the similar shape with the increasing of scan rate (Fig. 4a), indicating a low voltage polarization of the m-NiHCF. The obtained peaks current density (ip, A g-1) obeyed the powerlaw relationship with scan rate (ν, mV s−1) as following [48-49] ip = aνb

(1)

where a and b are adjustable parameters, and the b-value can be determined by the slope of the log (ν)-log (i) plots. Typically, the b-values of 0.5 and 1 revealed diffusion-controlled process and capacitive-controlled process, respectively. As linearly fitted of redox peaks in Fig. 4b, the b-values of anodic peak and cathodic peak were 0.81 and 0.93, respectively. It confirmed that the kinetics were dominated by capacitive-controlled process. Moreover, the current response at a specified potential can be quantified by surface capacitive reactions and diffusioncontrolled intercalation process according to the following equations [50-51] Q = ∫idt = ∫i ν

dE

(2)

i = k1ν + k2ν1/2

(3)

10

dE

Q = ∫(k1ν + k2ν1/2) ν =

∫k2dE ν1/2

(4)

+ ∫k1dE

where Q is integral capacity based on the area of CV curves (mAh g-1), i is current density (A g-1), E is voltage window (V), ν is scan rate (V s-1), k1 and k2 are two potential-dependent constants. As shown in Fig. 4c, a linear fitting can be obtained for the curve of Q versus ν-1/2. As a result, the ratio of the slope (∫k2dE) to the square root of scan rate (ν1/2) was the diffusioncontrolled capacity, while the intercept (∫k1dE) was the capacitive-controlled capacity. The proportions of the capacitive-controlled contribution at different scan rate from 0.1 to 1.0 mV s−1 were concluded in Fig. 4d. In agreement with the b-values calculated above, a dominating capacitive contribution of 63.9% was obtained at a scan rate of 0.1 mV s-1, and the proportions of capacitive controlled gradually enlarged with the scan rate increases up to 84.0% (1.0 mV s-1). To further investigate the kinetic process, galvanostatic intermittent titration (GITT) technique was employed to evaluate the sodium ion diffusion coefficient of NiHCF electrodes. The diffusion coefficient (DGITT) can be calculated by Fick’s second law with the following equation [51-52] 4 mBVM 2 ∆Es 2 M BS ∆Eτ

(

DGITT = πτ

)( )

(5)

where τ is the current pulse time (s), mB, MB, and VM are the mass (g), the molar mass (mol g1),

and the molar volume (cm3 mol-1) of the active material, respectively. S is electrode-

electrolyte interface area (cm2). ΔEs is the voltage difference during the open circuit period, and ΔEτ is the change of voltage during a constant current pulse excluding IR drop. Expectedly, the calculated DGITT of m-NiHCF (10-8-10-11 cm2 s-1) was higher than that of c-NiHCF (10-8-10-14 cm2 s-1) during the whole process (Fig. 4e, Fig. S6). Meanwhile, a similar decreasing trend of DGITT at full charging state was observed for both samples, and the downtrend of c-NiHCF was more serious than m-NiHCF, indicating the faster kinetics of m-NiHCF. In addition, 11

electrochemical impedance spectroscopy (EIS) spectra was performed to uncover the charge transfer properties of NiHCF [51, 53]. The corresponding Nyquist plots were revealed similar shape with a semicircle at high-medium frequency region and an inclined line at low-frequency region (Fig. S7a). Moreover, the EIS data can be well fitted by a typical equivalent electrical circuit (Fig. S7b), while the charge transfer resistance (Rct) values of m-NiHCF and c-NiHCF were 3.11 and 12.18 Ω, respectively. The smaller Rct value of m-NiHCF indicated faster electronic conductivity between the current collector to the interior of NiHCF. To uncover the sodium storage mechanism of m-NiHCF cathode, ex-situ XRD tests were carried out to investigate the structural evolution of m-NiHCF at different charge/discharge states (Fig. 5a). When charging from 0.0 to 1.0 V vs. Ag/AgCl, the XRD patterns showed that doublets merged into a single sharp peak at 24.5°, indicating the phase transition occurred from monoclinic to cubic induced by sodium ion extraction from the lattice. During discharge process, the monoclinic phase was regained with the sharp single peak gradually splitting into doublets, demonstrating the reversible phase transition from monoclinic to cubic phases. Moreover, exsitu XPS spectra collected at different charge/ discharge states was utilized to probe the valance state change of metal elements during Na+ ions extraction/insertion, At the charged state, Fe 2p peaks shift to higher energy, which reveals FeII has been oxidized to FeIII. And all the peaks recovered to the initial state after discharging, suggesting the full reduced of FeIII to FeII (Fig. 5b). In contrast, NiIII-related peak intensities did not appear at the charging and discharging process, indicating Ni always exists in the form of NiII (Fig. 5c) [45]. Meanwhile, ex-situ Raman analyses were employed to further confirm redox-active site (Fig. 5d). At the initial state, two peaks located at 2105 and 2140 cm-1 were corresponding to the vibrations of C≡N group in FeIIC≡N-NiII [46, 54]. Upon charging to the full charge state, a spectrum corresponding to that of FeIII-C≡N-NiII was observed with a new peak at 2183 cm−1 emerging, while peaks located at 2105 and 2140 cm-1 disappeared [37, 45]. And all the peaks recovered to the initial state after 12

discharging, suggesting the full reduced of FeIII to FeII. The same trend was also observed in the ex-situ FTIR. The broad peak of C≡N stretching vibrations located around 2094 cm-1 shift to 2154 cm-1 (full charged state) and back to the initial state (full discharged state), corresponding to that the carbon-coordinated FeII/FeIII redox reaction during charge and discharge process (Fig. S8) [43]. In view of above results, it can be concluded that the carboncoordinated FeII/FeIII redox couple was demonstrated as the redox-active site of m-NiHCF. Thus, the total charge/discharge processes of m-NiHCF can be ascribed as (Fig. 5e) Na1.45NiII[FeII(CN)6]0.87↔Na1.45 - xNiII[FeII(CN)6]0.87 - x[FeIII(CN)6]x + xe - + xNa +

3.4. Full-Cell Performance In the practical application, a 1.4 V high-voltage ASIBs full cell was assembled with a mNiHCF cathode, an NTP@C anode (Fig. S9-S11), and 5 M NaClO4 aqueous electrolyte (Fig. 6a). Fig. 6b showed CV curves of the ASIBs full cell at various scan rates from 0.1 to 1.0 mV s-1.

A couple of redox peaks around 1.44/1.25 V were in consistent with the voltage

differences between the FeII/FeIII redox couple in m-NiHCF cathode and the TiIV/TiIII redox couple in the NTP@C anode. The corresponding GCD profiles in Fig. 6c showed that the ASIBs full cell can stably deliver a high capacity of 61.4 mAh g-1 (based on the mass of cathode material) at 100 mA g-1, there still maintained a substantial capacity of 56.4 mA h g−1 even at the high current density of 1000 mA g-1. As shown in the Ragone plot, the ASIBs full cell showed an energy density of 86 Wh kg-1 at a power density of 141 W kg-1 and still retained 79 Wh kg-1 at a high power of 1316 W kg-1, which was greatly higher than other reported PBA materials in the ASIBs (Fig. 6d) [55-59]. Moreover, this ASIBs full cell exhibited remarkable cycling stability, with capacity retention of 83% over 600 cycles at 100 mA g-1 (Fig. 6e). It is clear that the proposed m-NiHCF//5M NaClO4//NTP@C ASIB full cell showed excellent

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performance in terms of rate capability, energy density, and cycling stability, enabling a promising candidate for practical application in EESs.

4. Conclusion In summary, a sodium rich m-NiHCF cathode with superior electrochemical performance has been synthesized through a controlled crystallization approach.

It exhibited a high

specific capacity of 70.1 mAh g-1, an extremely high rate capability of 53.2 mAh g-1 at 2000 mA g-1, and the impressive cycle life over 8000 times. We further demonstrated the capacitivecontrolled domination under the charge storage process of m-NiHCF cathode. Ex-situ XRD, ex-situ Raman, and ex-situ FTIR spectroscopy indicated that m-NiHCF achieved reversible phase transition from monoclinic to cubic phases with the reaction of carbon coordinated FeIII/FeII redox-active site during sodium ion extraction and insertion processes. Moreover, the ASIBs full cell base on m-NiHCF cathode and NTP@C anode was fabricated, which showed a high energy density of 86 Wh kg-1 with capacity retention of 83% after 600 cycles. This work proposed the sodium rich m-NiHCF cathode-based ASIBs, and it would be as a desirable candidate for grid-scale energy storage. Acknowledgements We are grateful for the financial support from National Natural Science Foundation of China [Grant No. 21705014], the Fundamental Research Funds for Central Universities [Grant No. DUT18LK56], Natural Science Foundation of Liaoning Province, China [Grant No. 20180510060], Dalian Science and Technology Bureau, China [Grant No. 2019J12SN54], and Zhang Dayu School of Chemistry, Dalian University of Technology, China.

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23

Figure Captions Fig. 1. Structural characterization and analysis of the as-prepared NiHCF samples. XRD Rietveld refinement patterns of (a) c-NiHCF and (b) m-NiHCF, the inset are corresponding SEM images. Refined crystal structure of (c) c-NiHCF and (d) m-NiHCF. Fig. 2. Characterization of the m-NiHCF composite.

(a) TEM image. (b) High angle annular

dark field-scanning transmission electron microscope (HAADF-STEM) images and corresponding elemental mappings. (c) Wide-scan XPS spectrum. (d) High-resolution Fe 2p XPS spectrum. (e) High-resolution Ni 2p XPS spectrum. (f) FTIR spectrum. Fig. 3. Electrochemical properties of c-NiHCF and m-NiHCF electrodes. (a) CV curves at the scan rate of 0.4 mV s-1 with the potential range of 0.0-1.0 V vs. Ag/AgCl. (b) Corresponding galvanostatic charge-discharge curves at a current density of 100 mA g-1. (c) Rate performance at various current densities from 100 mA g-1 to 2000 mA g-1. (d)

Long-term cycling

performance at a current density of 500 mA g-1. Fig. 4. Kinetic analysis of sodium storage for the m-NiHCF. (a) CV curves at various scan rates from 0.1 mV s-1 to 1.0 mV s-1. (b) The power law relationship between logarithm peak current densities and logarithm scan rates. (c) The linear fitting of integral capacity versus reciprocal of square root of scan rate. (d) Normalized contribution ratio of the capacitive and diffusioncontrolled capacity at different scan rates. (e) GITT curve and corresponding diffusion coefficients of m-NiHCF during charge and discharge processes. Fig. 5. The structural evolution of the m-NiHCF during charge and discharge process. (a) Exsitu XRD patterns and corresponding charge-discharge curve. Ex-situ XPS of (b) Fe 2p and (c)

24

Ni 2p spectra. (d) Ex-situ Raman spectra. (e) Schematic illustration of sodium storage mechanism in the m-NiHCF cathode electrode. Fig. 6. Electrochemical properties of the m-NiHCF//5 M NaClO4//NTP@C full cell. (a) Schematic illustration of full cell. (b) CV curves at various scan rates from 0.1 mV s-1 to 1.0 mV s-1 with the voltage range of 0.2-1.6 V.

(c) Galvanostatic charge-discharge curves at

various current densities from 100 mA g-1 to 1000 mA g-1. (d) Ragone plots of PBA cathodes reported in ASIBs. (e) Cycling stability at a current density of 100 mA g-1.

25

Table 1. Chemical formulas (determined by ICP and TGA results) and lattice parameters (calculated by the XRD refinement results) of the as-prepared samples. the unit volume

Material

Chemical formula

a (Å)

b (Å)

c (Å)

β (°)

m-NiHCF

Na1.45Ni[Fe(CN)6]0.87·3.02H2O

10.27155

7.33525

7.19637

92.080

270.925

c-NiHCF

Na1.21Ni[Fe(CN)6]0.86·3.21H2O

10.26955

10.26955

10.26955

90.000

264.512

26

per formula [Å3]

Fig. 1. Structural characterization and analysis of the as-prepared NiHCF samples. XRD Rietveld refinement patterns of (a) c-NiHCF and (b) m-NiHCF, the inset are corresponding SEM images. Refined crystal structure of (c) c-NiHCF and (d) m-NiHCF.

27

Fig. 2. Characterization of the m-NiHCF composite. (a) TEM image. (b) High angle annular dark field-scanning transmission electron microscope (HAADF-STEM) images and corresponding elemental mappings. (c) Wide-scan XPS spectrum. (d) High-resolution Fe 2p XPS spectrum. (e) High-resolution Ni 2p XPS spectrum. (f) FTIR spectrum.

28

Fig. 3. Electrochemical properties of m-NiHCF and c-NiHCF electrodes. (a) CV curves at the scan rate of 0.4 mV s-1 with the potential range of 0.0-1.0 V vs. Ag/AgCl. (b) Corresponding galvanostatic charge-discharge curves at a current density of 100 mA g-1. (c) Rate performance at various current densities from 100 mA g-1 to 2000 mA g-1. (d) Long-term cycling performance at a current density of 500 mA g-1.

29

Fig. 4. Kinetic analysis of sodium storage for the m-NiHCF. (a) CV curves at various scan rates from 0.1 mV s-1 to 1.0 mV s-1. (b) The power law relationship between logarithm peak current densities and logarithm scan rates. (c) The linear fitting of integral capacity versus reciprocal of square root of scan rate. (d) Normalized contribution ratio of the capacitive and diffusioncontrolled capacity at different scan rates. (e) GITT curve and corresponding diffusion coefficients of m-NiHCF during charge and discharge processes.

30

Fig. 5. The structural evolution of the m-NiHCF during charge and discharge process. (a) Exsitu XRD patterns and corresponding charge-discharge curve. Ex-situ XPS of (b) Fe 2p and (c) Ni 2p spectra. (d) Ex-situ Raman spectra. (e) Schematic illustration of sodium storage mechanism in the m-NiHCF cathode electrode.

31

Fig. 6. Electrochemical properties of the m-NiHCF//5 M NaClO4//NTP@C full cell. (a) Schematic illustration of full cell. (b) CV curves at various scan rates from 0.1 mV s-1 to 1.0 mV s-1 with the voltage range of 0.2-1.6 V. (c) Galvanostatic charge-discharge curves at various current densities from 100 mA g-1 to 1000 mA g-1. (d) Ragone plots of PBA cathodes reported in ASIBs. (e) Cycling stability at a current density of 100 mA g-1.

32

High-stability Monoclinic Nickel Hexacyanoferrate Cathode Materials for Ultrafast Aqueous Sodium Ion Battery Liuxue Shen, Yu Jiang, Yuefeng Liu, Junlin Ma, Tongrui Sun and Nan Zhu*

Graphical Abstract Monoclinic nickel hexacyanoferrate (m-NiHCF) as aqueous sodium ion battery cathode materials exhibits remarkable electrochemical performance.

33

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

34

Highlights  Monoclinic nickel hexacyanoferrate (m-NiHCF) nanocubes are synthesized  Long-term stability (over 8000 cycles) for aqueous sodium ion battery is realized  Ultrafast kinetics benefits from the capacitive-controlled charge storage process  The sodium storage mechanism of m-NiHCF is revealed by ex-situ techniques

35