RGO-Anchored Manganese Hexacyanoferrate with Low Interstitial

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RGO-Anchored Manganese Hexacyanoferrate with Low Interstitial H2O for Superior Sodium-Ion Batteries Hui Wang, Enze Xu, Shimeng Yu, Danting Li, Junjie Quan, Li Xu, Li Wang, and Yang Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11157 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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ACS Applied Materials & Interfaces

RGO-Anchored Manganese Hexacyanoferrate with Low Interstitial H2O for Superior Sodium-Ion Batteries Hui Wanga, Enze Xua, Shimeng Yua, Danting Lia, Junjie Quana, Li Xub, Li Wangb,*, Yang Jianga,* a

School of Materials Science and Engineering, and b School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, 230009, P. R. China.

*

Corresponding authors: Email: [email protected] (Y. J.) and [email protected] (L. W.) Tel. & Fax.: +86 551 62904358.

Abstract: Low cost manganese hexacyanoferrate (NMHCF) possesses many favorable advantages including high theoretical capacity, ease of preparation and robust open channels that enables faster Na+ diffusion kinetics. However, high lattice water and low electronic conductivity are the main bottlenecks to their pragmatic realization. Here, we present a strategy by anchoring NMHCF on RGO to alleviate these problems, featuring a specific discharge capacity of 161/121 mAh g-1 at the current density of 20/200 mA g-1. Moreover, the sodiation process is well revealed by Ex-situ XRD, EIS and car-parrinello molecular dynamics simulations. At a rate of 20 mA g-1, the hard carbon//NMHCF/RGO full cell affords a stable discharge capacity of 84 mAh g−1 (based on the weights of cathode mass) over 50 cycles, thus highlighting NMHCF/RGO an alternative cathode for sodium-ion batteries. Keywords: manganese hexacyanoferrate; sodium cathode; lattice water; sodiation process; full cell

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Introduction Ambient temperature sodium-ion batteries (SIBs) are acquiring considerable interest with respect to exploiting low-cost and sustainable energy storage systems, due to the high abundance of Na and its similar chemistry with Li.1, 2 Although many phosphates and oxides are promising for Na-ion cathode, the development of viable cathode with better performance, durability and low cost calls for continual exploration.3-6 Prussian blue and its analogues (AmM[Fe(CN)6]n･1-n□･xH2O; A, alkaline metal; M，transition metal, such as Fe, Mn, Zn, Cu and Co etc; □, [Fe(CN)6] vacancies occupied by lattice water; 0 < m < 2, n < 1) are surfacing as alternative cathode materials for SIBs.7-9 Among them, Na2MnFe(CN)6 (NMHCF), is being investigated as an representative cathode,10-13 which can be assigned to its robust three-dimensional Na ion diffusion path and many electron intercalation chemistry reaction, along with facile synthetic process.7, 14-17 Nevertheless, the reported NMHCF still subject to poor cycling stability, specific capacity and columbic efficiency, which plagued

its

real

implementations.

For

instance,

Jin

et

al.

found

that

rhombohedral-structured NaxMnFe(CN)6･H2O delivered an initial capacity of 150 mAh g-1 but with poor cycling performance.18 Song et al. improved long cycling life of this cathode through removing interstitial H2O, while it is still essential to push forward its capacity, durability and safety.13 The intrinsic reasons for the poor electrochemical performance of NMHCF can be put as following: (i) NMHCF has a large band gap as clearly evidenced in the 2


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results part, which may deteriorate electrode reaction kinetics, thus leading to large polarization and low cathode utilization;19-21 (ii) the existence of [Fe(CN)6] vacancies and lattice water in NMHCF could induce lattice distortion and even the Mn–N≡C– Fe bridge break during sodiation/disodiation process, hence further sacrificing its electrochemical properties.22-26 In light of above problems, we report a facile solution precipitation routine to fabricate reduced graphene oxide (RGO)-anchored manganese hexacyanoferrate. The RGO provides a reservoir for anchoring NMHCF, which not only significantly suppresses interstitial H2O content, but also promotes electron conduction. The as-synthesized NMHCF/RGO cathode attains a highly reversible specific capacity of 161 mA h g-1 at the current of 20 mA g-1 and 121 mA h g-1 at the current of 200 mA g-1. Moreover, this cathode is also paired with hard carbon anode in full cells featuring a stable discharge capacity of 84 mAh g−1 (based on the weights of cathode mass) at 20 mA g-1 over 50 cycles, thus demonstrating NMHCF/RGO a superior cathode for sodium ion batteries.

Experimental section All chemical raw materials utilized were analytically pure and bought from Sigma-Aldrich without any further treatment. Firstly, graphene oxide (GO) was prepared by a modified Hummers’ method.27 And the reduced GO (RGO) was obtained via the thermal dilution method through maintaining GO in a muffle furnace at 1000℃ for one minute. After that, 0.1 g RGO was dispersed in 100 ml deionized 3



water by means of sonication for 3h, 0.05 M Na4Fe(CN)6 and 14 g sodium chloride were then added into above solution. Next, 0.2 M MnCl2 and 0.2 g PVP dissolved in 50 ml of deionized water were added drop by drop under vigorous stirring. And the whole mixture was further whisked for another 12h and aged for 10h at ambient temperature. Finally, the NMHCF/RGO was retained by centrifugation and washed several times with deionized water and ethanol. The composite was annealed at 110℃ for 24h under N2 protection. For comparison, NMHCF powder was also prepared via the same method without RGO. The X-ray diffraction (XRD) data of the sample was collected by X-ray diffractometer (D/MAX2500V, Rigaku, Japan) using Cu Kα radiation. X-ray photoelectron spectra (XPS) was obtained via Thermo Scientific spectrometer (ESCALAB250Xi, Thermo, USA). The Fe, Mn, Na elements were determined utilizing inductively coupled plasma (ICP) mass spectroscopy with a Vista-MPX ICP atomic emission spectrometer. And C , N was examined by the elemental analysis (Flash EA 1112). The thermogravimetric analyzer (TG) was conducted by TGA Q500 thermogravimetric analyzer (STA449F3, Netzsch, Germany) in argon with a heating rate of 10℃ min-1 in the temperature range of 50-500℃. Fourier transformation infrared (FT-IR) was observed by NICOLET AVATAR 360 FT-IR spectrometer with KBr pellet. Raman analysis was performed on a micro-Raman spectrometer (HR Evolution, HORIBA JOBIV YVON, JAPAN) using a 532 nm laser. The morphological features of the synthesized materials were characterized by field-emission scanning electron microscopy (FESEM) (SU8020, Hitachi, Japan) 4


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together with transmission electron microscopy (HRTEM) (JEM-2100F, JEOL, JAPAN). Electrochemical performances of NMHCF/RGO were evaluated with CR2032 coin cells. A black mixture was made of active materials (80%), acetylene black (10%), and polyvinylidene difluoride (PVDF) (10%) binder; the slurry of the mixture was dissolved in an appropriate amount of N-methyl-2-pyrrolidone (NMP) and uniformly casted to an Al foil and was then dried overnight in a vacuum oven for 12 h at 100℃. A piece of Na-foil and glass fiber were acted as the counter electrode and separator. 1 M NaClO4 in a volume ratio of 1:1 of ethylene carbonate/diethyl carbonate (EC/DEC) containing 5wt % fluoroethylene carbonate (FEC enables a much more stable SEI and prevents the decomposition of EC/DEC28) was served as the electrolyte. The ratio of electrolyte/electrode in NMHCF/RGO-Na half-cell is around 5.17 ul mg-1. The typical mass loading was 3.48 mg cm-2 for the NMHCF/RGO electrode. As to full cells, an anode of 80% hard carbon, 10% acetylene black and 10% PVDF binder was employed, the corresponding mass loading for the NMHCF/RGO cathode and hard carbon anode are 3.48 mg cm-2 and 2.16 mg cm-2 respectively. The ratio of electrolyte/electrode in NMHCF/RGO-hard carbon full-cell is around 8.26 ul mg-1. The charge and discharge measurements were conducted on a NEWWARE battery test system (Shenzhen, China) under different current densities within the potential window of 2.0-4.0 V. Electrochemical Impedance spectroscopy (EIS) was carried out on CHI600D electrochemical workstation within a frequency range of 100 KHZ to 0.01 HZ. 5



The first principle calculations were performed based on the density functional theory implemented in the quantum expresso software package.29 Effective U values of 4.3 eV for Fe coordinated by C and 5.0 eV for Mn coordinated by N.30 Ultrasoft pseudopotentials were employed for Mn, Fe, C and N with a cut-off energy of 600 eV. The k-point of Brillouin zone in the reciprocal space was 6 × 6 × 6. The atomic coordinates and cell parameters were completely released up to the remnant forces were smaller than 0.001 eV Å in each species and the energy convergence accuracy of total energy was approximate 10-6 eV for each atom. Car-parrinello molecular dynamics simulation was performed at 500 K with a time step of 2 fs for 2000 ps to illustrate Na+ diffusion properties.

Results and discussion As schematically indicated in Fig. 1a, the NMHCF/RGO cathode was prepared by virtue of a facile solution precipitation method. As also depicted in Fig. 1b, the bare NMHCF consisted of uniform cubes in the range of 180 to 450 nm, which can also be attested by the TEM images (Fig. S1). As to NMHCF/RGO, it was noteworthy that homogeneous NMHCF cubes were uniformly anchored to the conductive RGO matrix (Fig. 1c and 1d), assuring intimate electronic interaction between the NMHCF cubes and RGO. The elemental mapping images presented in Fig. 1e demonstrate the uniform distributions of Na 、 C 、 N 、 Fe and Mn components throughout the NMHCF/RGO composite. It is therefore anticipated that the NMHCF/RGO composite

6


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would display lower interstitial H2O and better electronic conductivity than pure NMHCF.31 Fig. 2a shows the crystal structure of NMHCF, the three dimensional (3D) framework was constructed by –Mn–N≡C–Fe– chains with Na+ or lattice water in the 8c sites. And this open 3D channels were expected to facilitate sodium ion diffusion. However, as shown in Fig. 2b, the density of states indicates that NMHCF has a large band gap (around 2.02 eV), thus would result in poor electrode reaction kinetics, large polarization and low cathode utilization. Figure 2c demonstrates the powder X-ray diffraction pattern (PXRD) of the as-synthesized NMHCF and NMHCF/RGO. It was noted that the diffraction peaks of pure NMHCF were indexed to the monoclinic structure with the p21/n space group. Interestingly, the NMHCF/RGO displayed the rhombohedral structure with the R3 space group, meaning the presence of RGO greatly reduced the interstitial H2O content.18, 25, 32 The measured lattice parameters of RGO-anchored NMHCF are a = b = 6.5801 Å, c =18.9294 Å, α = β = 90°, γ =120°, and these results were consistent well with those reported previously13. Fig. 3a shows the Raman spectra of RGO, NMHCF and NMHCF/RGO between 1200 and 2250 cm-1. The broad peaks at 1338 and 1602 cm-1 are attributed to the RGO. It was clearly that two peaks at 2091 and 2131 cm-1 were observed for NMHCF, which could be related to the Fe Ⅱ -CN-Mn Ⅱ and Fe Ⅱ -CN-MnⅢ, respectively. By contrast, there was only peak located at 2091 cm-1 corresponding to FeⅡ-CN-MnⅡ, which indicating the nearly absence of lattice water.33-35 And this can be further 7



verified by the Mn 2p XPS spectrum as illustrated in Fig.2e, 2f, S2a, S2b, S2c and S2d. On the other hand, the lattice water content of NMHCF could also be determined qualitatively through thermogravimetric analysis (TGA). As shown in Fig. 2g, it was worth noting that NMHCF/RGO exhibits much lower weight loss between 120 and 190°C, corresponding to the elimination of lattice water,36 which was consistent with above results. As also suggested in Fig. 2h, FT-IR spectra exhibits a sharp absorption at 2067 cm-1 which is assigned to the (C≡N)-1 stretching mode.37 It was interestingly noted that NMHCF displays two absorption peaks located at around 1619 and 3443 cm-1, which are associated with the O-H stretching arising from interstitial water and free surface water. While there is no obvious absorption peaks for NMHCF/RGO, again implying the removal of interstitial water by RGO. Furthermore, as verified by ICP analysis (table S1), the formula of the NMHCF wrapped with RGO is determined to be Na1.89Mn[Fe(CN)6]0.98﹒□0.02﹒0.16H2O, and the formula of NMHCF without RGO is measured to be Na1.67Mn[Fe(CN)6]0.9﹒□0.1﹒1.47H2O, again attesting above analysis. A possible mechanism that RGO could aid in the elimination of H2O is illustrated in Fig.S3. As evidenced in the I process, the nucleation and growth of NMHCF occurs immediately, thus leading to the non-uniform crystal with many Fe(CN)6 defects and thus lattice water.32 As to in the II process, however, the RGO (contains many negative defects and oxide groups) could serve as a reservoir to slowly release Mn2+ ions and even absorbed on the surface of NMHCF cubic, thus suppressing the nucleation and growth rate of NMHCF and reducing crystal defects.31 Electrochemical performances of the NMHCF and NMHCF/RGO were 8


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evaluated with 2032 coin cells. It was observed that the pure NMHCF delivered two redox centers (3.52/3.11 V, 3.75/3.40 V) (Fig. 3b), with a specific discharge capacity of 132.5 mAh g-1 (20 mA g-1) as shown in Fig. 3a. Interestingly, the NMHCF/RGO yielded only one redox center (3.56/3.44 V) (Fig. 3d), together with a much improved discharge capacity of 161 mAh g-1 as illustrated in Fig. 3c. And this remarkable difference can be ascribed to the diverse intercalation chemistry as discussed later. As further evidenced in Fig. 3e and 3f, NMHCF/RGO cathode was able to endow a discharge capacity of 151.2 mAh g-1 (20 mA g-1) withstand 100 cycles, corresponding to the high capacity retention of 92.8%; while the pure NMHCF with high lattice water defects displayed much inferior performance with only 96.5 mAh g-1 (20 mA g-1) after 100 cycles. To assess the rate performance, the specific rate current was programmed to vary from 20 mA g-1 up to 1000 mA g-1 rate, and then drastically returning back to 20 mA g-1 rate. As presented in Fig. 3g, the NMHCF/RGO cathode displays striking rate capability, with reversible capacities of 162, 156, 139, 121, 104, and 90 mAh g-1, respectively, at 20 mA g-1, 50 mA g-1, 100 mA g-1, 200 mA g-1, 500 mA g-1 and 1000 mA g-1 rates. Upon changing the current rate from 1000 mA g-1 to 20 mA g-1 rate, 158 mAh g-1 was still retained, meaning good capacity retention, which is better than the reported performance of NMHCF for SIBs as evidenced in Table S2. By stark contrast, the pure NMHCF showed much lower discharge capacity as clearly observed in Fig. 3h. Moreover, the NMHCF/RGO electrode was also capable of yielding an attractive capacity of 121 mAh g-1 at 200 mA g-1 over 250 cycles (Fig. 3g), which largely benefiting from intimate contact with RGO and low interstitial H2O 9



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content. Ex situ XRD measurements were performed to reveal the structural evolution during the Na+ deinsertion/insertion process for NMHCF and NMHCF/RGO samples. During Na+ extraction from NMHCF, it was noted that a peak at 14.7° was vanished and the split peaks gradually transformed into a peak (2θ = 23.6°, 37.9°), suggesting the phase transformation from monoclinic phase to cubic (Fig. 4a and 4b). After charging to 4 V, the corresponding XRD pattern was taken to tetragonal symmetry (a = b =10.1182 Å and c = 10.5409 Å), and this can be associated with a cooperative Jahn−Teller distortion of Mn3+, which is in line with previous results.13 Although the single monoclinic phase was reverted as the cathode was discharged to 2 V, notably, there was obvious peaks shift during the Na ions insertion process. And this indicates relative large volume variation as verified in Fig. S4b and stress−strain, which may bring about the collapse of the crystal framework and thus poor performance. As to the NMHCF/RGO sample, the XRD patterns changed from Na rich rhombohedral phase to Na poor tetragonal phase and then was reversed to a well-developed rhombohedral phase (Fig.4c and 4d), indicating its superior structural stability (Fig. S4a) and a much more reversible discharge/charge process.38 According to thermodynamics, voltage curves are linearly related to the slope of the free energy of the electrode materials.39 Therefore, it was generally expected that one and two first-order phase transformations existed for NMHCF/RGO, NMHCF, respectively, thus resulting in one and two voltage plateaus as shown in schematic graph Fig.S5. Car-parrinello

molecular

dynamics

and

electrochemical

10


impedance

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spectroscopy (EIS) measurements were performed to elucidate the Na+ diffusion properties of NMHCF (Fig.5). As shown in Fig. 5c and 5d, the energy and temperature variation satisfy the predefined convergence criteria, implying the NVT (Number of atoms, volume of the supercell and temperature are fixed) system reached the equilibrium state after 2000 ps. It was clear that NMHCF provides three open channel for Na+ diffusion (Fig. 5a and S6) and the corresponding DNa+ was 8.37*10-10 cm2 s-1 at 500 K (Fig.5b), which was better than Na ion superionic conductor (Na3M2(XO4)3, X = Si4+,P5+, S6+, Mo6+, As5+)

40

, thus denoting fast Na+ diffusion

properties. EIS measurements of NMHCF/RGO and NMHCF electrodes were then carried out before and after the 1st cycle from 0.01 Hz to 10 MHz. As shown in Fig. 5e and S7, the Nyquist plot is composed of a flat semicircle in the high-frequency region and a linear sloping region in the low-frequency, which are ascribed to the charge-transfer process (Rct) and diffusion resistance of sodium ions, respectively. The fitting of Nyquist plots and the corresponding equivalent circuit are inset Fig. 5e and S7. Accordingly, the impedance parameters were provided in Table S3. The NMHCF/RGO yields an equivalent series resistance (Re, Rct) of 5.22/112.62 Ω and 7.84/132.71Ω before and after initial cycle. By contrast, the pure NMHCF presents much larger (Re, Rct) of 7.32/234.21 Ω and 11.64/392.43 Ω. As further corroborated in Fig. S8a and S8b, the smaller solution and charge-transfer resistance changes of NMHCF/RGO imply much more stable electrode interface and faster electrode reaction kinetics, thus associated with better rate performance. The corresponding 11



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diffusion coefficient of sodium-ion (DNa+) can be calculated by the diagonal of the low-frequency region in EIS using the following formula:41 =

.

(1)

Here, respectively, n is the number of electrons (1.9 according to specific capacity), S is the actual effective surface area of the cathode (0.98 cm-2), T indicates absolute temperature, R represents gas constant, F represents the Faraday constant, C represents the Na ions concentration in the active material, which is probably 1.142 × 10-3 mol cm-3, and σ is Warburg constant related to Zreal (Fig.5f). According to Eq. (1), the DNa of the NMHCF/RGO electrode before and after the 1th cycle is calculated to be 2.17 × 10-11 cm2 s-1and 1.14 × 10-11 cm2 s-1. Since the CPMD calculation was carried out at much higher temperature and no lattice defects was taken into account, therefore, it was reasonably to deem that the CMPD results was consistent with our simulation results, thus corroborating fast sodium ion diffusion kinetics behavior and making NMHCF/RGO suitable for large-scale energy storage.. To manifest the feasibility of NMHCF/RGO in a sodium metal free battery, we constructed sodium ion full cells by coupling the NMHCF/RGO with hard carbon anode. Fig. 6a indicates the schematic graph of the full sodium-ion battery with hard carbon//MHCF pair. It was clearly observed that the hard carbon yields a stable discharge/charge capacities of 252/245 mAh g-1 (Fig. 6b). On the basis of NMHCF/RGO, the full cells could give rise to the high initial discharge/charge capacities of 102/108 mAh g-1 (based on the weights of cathode electrode mass) ranging from 2 V to 3.6 V (Fig. 6c). According to the half-cell results, the 12


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carbon/NMHCF-rGO full-cell should be able to yield a specific capacity of 128.8 mAh g-1 (161 mAh g-1×0.8). This discrepancy may be ascribed to the SEI and some irreversible trapping of Na within the carbon pores.42 Notably, a reversible capacity of 84 mAh g−1 was retained over 50 cycles (Fig. 6d), thus making NMHCF/RGO promising for future energy storage.

Conclusions In summary, we have developed a NMHCF/RGO composite with high Na content by means of a facile precipitation routine. The presence of RGO not only improves electron conductivity, but also aids in the elimination of the interstitial H2O. As the cathode, NMFCF/RGO yields a high initial specific capacity of 161 mAh g-1 at a current density of 20 mA g-1. And even at 1000 mA g-1, the cell could still deliver a stable discharge capacity of 83 mAh g-1. In addition, sodium-ions insertion and extraction process are clearly complemented by Ex-situ XRD, EIS and CPMD simulations. Furthermore, the as-fabricated NMHCF/RGO//hard carbon full cells are able to give highly reversible discharge capacities of 84 mAh g−1 over 50 cycles, thus enabling NMHCF/RGO competitive among various sodium cathodes pursued.

Supporting Information Supporting Information is available free of charge on the ACS website. TEM, XPS, EIS and CPMD simulation results of NMHCF and NMHCF/RGO.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant 13



Nos. U1632151 and 61076040) and the Key Research and Development Project of Anhui Province of China (Grant No. 1704a0902023).

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Tuned

Prussian-Blue

Wrapped

with

RGO:

a

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Figures

Fig. 1. (a) The synthesis process of highly uniform NMHCF/RGO. SEM images of (b) NMHCF and (c) NMHCF/RGO samples. TEM images of (d) NMHCF/RGO. (e) Elemental mapping images of the NMHCF/RGO.

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Fig. 2. (a) Crystal structure of NMHCF. (b) density of states of NMHCF. (c) XRD pattern for the NMHCF. (d) Raman spectra for NMHCF/RGO and NMHCF. (e) and (f) are the Mn 2p XPS spectrum of NMHCF and NMHCF/RGO. (g) and (h) are TG and FT-IR curves of NMHCF and NMHCF/RGO.

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Fig. 3. (a) and (b) are dQ/dV curves and charge-discharge profiles of NMHCF at the current density of 20mA g-1 within the voltage range of 2.0- 4.0 V vs. Na+/Na. (c) and (d) dQ/dV curves and charge-discharge profiles of NMHCF at the current density of 20mA g-1 within the voltage range of 2.0- 4.0 V vs. Na+/Na. (e) and (f) Cycling performance and Coulombic efficiency of NMHCF/RGO at the current density of 20mA g-1. (g) Long cycling performance of NMHCF/RGO at 200mA g-1. (h) Rate performances of NMHCF/RGO and NMHCF.

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Fig. 4. Schematic illustration of structural evolution of (a) monoclinic NMHCF and (c) rhombohedral NMHCF during Na+ extraction and insertion. Ex situ XRD patterns of (b) monoclinic NMHCF and (d) rhombohedral NMHCF/NMHCF at different states for the first cycle.

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Fig. 5. (a) The illustration of sodium ion migration path. (b) Sodium ion diffusion coefficient at 500 K. (c) and (d) the energy and temperature vibration virus molecular dynamics simulation time. (e) The electrochemical impedance spectra before and after the initial cycle of NMHCF/RGO; the inset is the corresponding equivalent circuit. (f) The linear fitting of the Zreal versus ω-1/2 curves in the frequency area.

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Fig. 6. (a) Schematic illustration of the full sodium-ion battery with NMHCF/RGO//hard carbon couple. (b) Discharge-charge curves of the hard carbon-Na half-cell at 20 mA g-1 after initial cycle. (c) Discharge-charge curves of the NMHCF/RGO//hard carbon full-cell at 20 mA g-1 after initial cycle (normalized to the weight of cathode mass). (d) Cycling performances of the NMHCF/RGO//hard carbon at 20 mA g-1.

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Table of contents

Dehydrated NMHCF cube/RGO//hard cabon full cell yields a stable capacity of 83 mAh g-1 over 50 cycles.

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RGO-Anchored Manganese Hexacyanoferrate with Low Interstitial

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