Improved cycling performance of prussian blue cathode for sodium ion batteries by controlling operation voltage range

Accepted Manuscript Title: Improved cycling performance of prussian blue cathode for sodium ion batteries by controlling operation voltage range Author: Xiaomin Yan YangYang Ershuai Liu Liqi Sun Hong Wang Xiao-Zhen Liao Yushi He Zi-Feng Ma PII: DOI: Reference:

S0013-4686(16)32680-9 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.121 EA 28593

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

26-10-2016 28-11-2016 20-12-2016

Please cite this article as: Xiaomin Yan, YangYang, Ershuai Liu, Liqi Sun, Hong Wang, Xiao-Zhen Liao, Yushi He, Zi-Feng Ma, Improved cycling performance of prussian blue cathode for sodium ion batteries by controlling operation voltage range, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.121 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved cycling performance of prussian blue cathode for sodium ion batteries by controlling operation voltage range

Xiaomin Yan, YangYang, Ershuai Liu, Liqi Sun, Hong Wang, Xiao-Zhen Liao*, Yushi He, Zi-Feng Ma* Department of Chemical Engineering, Shanghai Electrochemical Energy Devices Research Center, Shanghai Jiao Tong University, Shanghai 200240, China

To whom correspondence should be addressed: Fax: +86-21-54747717 Tel:+86-21-54742894 E-mail: [email protected], [email protected],

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Abstract Prussian blue and its analogues have been intensively studied as potential electrode materials for sodium ion batteries. In this work, a prussian blue sample was prepared by a simple precipitation method and the electrochemical performance of the prussian blue (PB) cathode was investigated at various temperatures and in different voltage ranges. When cycled in the voltage range of 2.0 - 4.0 V vs. Na/Na+, the PB cathode exhibited initial discharge capacities of 135.4 mAh g -1 (55℃), 128.5 mAh g-1 (25℃) and 99.2 mAh g-1 (-10℃) at 10 mA g-1. The PB cathode showed good cycling stability at low temperature, however the cycling capacity degraded remarkably at elevated temperature. CV test indicated that the capacity fade was mainly due to the side reactions on the electrode-electrolyte interface at near 4.0 V vs. Na/Na+. XPS and FTIR analyses revealed several types of compounds formed on the cathode surface which indicated the complexity of the side reactions. Controlling the high cut-off voltage at 3.8 V vs. Na/Na+ significantly improved the cycling stability of the PB electrode. The PB cathode delivered 1 C rate reversible capacity of 100.0 mAh g -1 in 2.0 - 3.8 V vs. Na/Na+ with outstanding capacity retention of 98.2 % after 300 cycles.

Keywords: prussian blue, cathode, sodium ion batteries

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1. Introduction Rechargeable sodium ion batteries (SIBs) have recently attracted growing interest as appealing alternative of lithium ion batteries due to the low cost and abundant resource of sodium. The large radius of sodium ion Na+ (1.02 Å) requires sodium storage materials with large ionic channels. Various cathode materials for SIBs have been developed such as layered transition metal oxides NaxMO2 (M = Mn, Fe, Ni, Co, Cr, etc)[1-5], tunnel-structured spinel oxides[6, 7], poly-anionic phosphates and fluorophosphates[8, 9], and prussian blue analogues (PBAs)[10-22]. Among the Na-insertion compounds, PBAs with the general formula of NaxM[M’(CN)6]y·nH2O (M,M’ = transition metals ) have been intensively investigated owing to their low cost, ease of synthesis, and open framework structure with the advantage of fast Na + insertion/extraction. Recently, much effort has been focused on developing high performance PBA materials: (1) Exploring new synthesis methods to optimize the crystallization process[10, 11]. Guo et al.[10] prepared low defect prussian blue crystals with superior electrochemical performance by a single iron-source method. Qian et al.[11] synthesized vacancy-free Na2CoFe(CN)6 with superior cycleability by a controlled crystallization reaction using sodium citrate as the chelating agent. (2) Optimizing the composition and framework structure of NaxM1M2(CN)6[12-15]. Cui’s group[12] reported a manganese hexacyanomanganate Na2Mnп[Mnп(CN)6] with a monoclinic structure. The open-framework structure of MnHCMn showed sufficient space for

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two Na ions within a subunit cell in its fully discharged form. This sample delivered a surprisingly high specific capacity up to 209 mAh g -1. Our previous work demonstrated a structural optimized Na1.76Ni0.12Mn0.88[Fe(CN)6]0.98 with excellent cycling performance[13]. Y. Moritomo et al.[14] reported the enhanced rate and cycling performance of Mn-HCF by partial substitution of Fe, Co, and Ni for Mn. (3) Preparing high Na content materials by reacting in Na-rich environment and preventing M(п) from being oxidized to M(ш)[16-18]. Compared to Na-poor PBAs, Na-rich PBAs with less structure defects exhibited higher capacity and better cycling stability[16]. (4) Modifying the particle surface with conductive materials to improve the electrochemical performance[23-26]. Polypyrrole coating has been demonstrated to be a promising strategy to promote the cycleability and rate capability of PBA cathodes[23, 24]. Jiang et al.[25] reported an in-situ synthesized PB@C with PB cubes grown directly on carbon chains. The PB@C composite achieved outstanding rate capability and cycling stability. However, to push forward the practical application of prussian blue analogue materials in sodium ion batteries, further research work need to be performed. The electrochemical behavior of PBAs at different operation conditions and environment need to be better understood. Till now, most of the literature works were carried out at room temperature. The electrochemical behavior of PBAs at low or elevated temperatures has been rarely reported. In this work, we prepared a prussian blue sample using the most simple precipitation method which is easy to scale up for large scale production. We investigated the electrochemical behavior of the prussian blue

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sample at various temperatures. To further understand the possible side reactions on the electrode/electrolyte interface, the cycled PB cathode was performed ex-situ surface analysis by XPS and FTIR techniques. The accelerated fading mechanism of the PB electrode at elevated temperature was studied. The influence of the operation voltage range on the cycling performances was investigated. By controlling upper cut-off voltage, outstanding cycling stability of the PB electrode was obtained. These research results can provide reference for the practical application of prussian blue analogue materials.

2. Experimental The prussian blue sample was synthesized using a simple precipitation method at room temperature by mixing 50 mL of 0.2 M Na4Fe(CN)6 solution and 50 mL of 0.2 M FeCl2 solution dropwise to 200 mL of 0.5 M NaCl solution under continuous stirring. The mixture was stirred for 3 hours at room temperature. Then the obtained precipitate was filtered, washed with deionized water and then dried in vacuum at 120℃. The crystallographic structure of the as-prepared sample was characterized by powder X-ray diffraction (XRD, D/max-2200/PC, Rigaku Co., Ltd.) with nickel filtered Cu Kα radiation. The morphology was probed by field emission scanning electron microscopy (Nova NanoSEM 450, FEI Company) and transmission electron microscope (JEM 2100F). Chemical composition of the sample was determined by elemental analysis (EA, Vario-EL Cube, Elementar Analysensysteme) for C and N elements, and by inductively coupled plasma analysis (ICP, iCAP 6000 Radial, Thermo Fisher Scientific Inc.) for Fe and Na elements. The thermo gravimetric (TG) 5

analysis was conducted on a DSC/DTA-TG instrument (TG, STA 449 F3 Jupiter, NETZSCH-Gerätebau GmbH) at 2 ℃ min−1 heating rate under N2 environment to measure the H2O content in the sample. For a typical coin cell fabrication, the cathodes were prepared by slurrying 70 wt.% active material, 10 wt.% super P, 10 wt.% ketjen black, and 10 wt.% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP), and then casting the mixture onto an aluminum foil. After drying at 100℃ for about 2 h, the electrode disks (14 mm) were punched. After further drying under vacuum at 120℃ for 12 h, the cathodes were incorporated into coin cells (R2016) with sodium metal foil and 1.0 M NaPF6/PC+EMC+FEC (50:48:2, v/v/v) electrolyte in an argon filled glove box. The mass loading of active materials on cathodes was 2.5 - 2.6 mg cm-2. The galvanostatic charge-discharge tests of the coin cells were conducted using a battery test system (Land CT2001A model, Wuhan Jinnuo Electronics Co., Ltd.). The cyclic voltammetric experiments were conducted using a CHI electrochemical workstation (CHI 670D, CHI Instrument Co.). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the surface layer on the cycled PB cathodes. The cell after 100 cycles at 1C rate, 55℃ was disassembled in the Ar filled glove box. The PB cathode disc was extracted from the cell and dried in the glove box at room temperature. The PB disc was divided into three parts for XPS, FTIR analysis and SEM observation. An air-isolating sample holder was used to transfer the sample to the sample chamber of XPS equipment under Argon atmosphere. The XPS

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experiments were carried out on a KratosTM Axis Ultra DLD surface analysis instrument with Al K radiation (h = 1486.6 eV). The binding energy scale was calibrated with the C 1s peak (284.8 eV) of adventitious carbon on the sample surface. Before transferring the samples to the sample holders for SEM observation and FTIR analysis, the PB cathode pieces were kept in the air isolating containers filled with Ar. FTIR spectra (Spectrum 100, Perkin Elmer Inc.) were conducted to analyze the chemical group of the surface products. 3. Results and discussion Powder XRD pattern of the prepared PB product is shown in Fig.1a. The peaks can be indexed to the face-centered cubic structure (FCC, space group Fm 3 m) similar to the standard XRD diffraction pattern of prussian blue Fe4[Fe(CN)6]3 (JCPDS No. 52-1907). The morphology and particle size of sample were observed by field scanning electron microscopy as shown in Fig. 1b. The PB particles show irregular shape with particle diameters in the range of 50 – 200 nm. HRTEM image of a PB particle in Fig.1c shows clear lattice fringe indicating a high crystallinity of the particles. The chemical composition of the as-prepared PB material was analyzed using CNS elemental analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The formula was calculated to be Na1.59Fe[Fe(CN)6]0.95□0.05. Water content in the PB sample was 14.1 wt.%, which was measured using TG analysis as shown in Fig. 1d. Electrochemical performance of the prepared PB cathode was measured at various temperatures in 2.0 - 4.0 V vs. Na/Na+ using a constant current density of 10 mA g-1 for the first ten cycles and then 100 mA g -1 for the subsequent cycles. The charge and 7

discharge profiles at low current density 10 mA g -1 were shown in Fig. 2a. The voltage profiles show two plateaus attributed to the redox reactions of Fe3+(C)/Fe2+(C) couple at high potential and Fe3+(N)/Fe2+(N) couple at low potential[27, 28]. The initial charge/discharge capacities were 131.4 mAh g -1 / 135.4 mAh g-1 (55℃), 105.7 mAh g-1 / 128.5 mAh g-1 (25℃), and 72.6 mAh g-1 / 99.2 mAh g-1 (-10℃), respectively. It can be seen that the PB electrode showed deteriorated cycle stability at elevated temperature compared with the results at room and low temperatures. The first 10 cycles discharge profiles at 55℃ showed obvious capacity decrease from 135.4 mAh g-1 to 128.4 mAh g-1 at 10 mA g-1. Furthermore, the PB electrode exhibited low coulombic efficiency at elevated temperature (Fig. 2b). The second cycle charge and discharge capacities at 55℃ were 160.3 mAh g-1 and 134.8 mAh g-1, respectively. The coulombic efficiency was only 84.1 % though it increased to 90.8 % in the subsequent cycles. On the other hand, the coulombic efficiencies in first 10 cycles at 25℃ were 94 - 97 %. The discharge profiles at 25℃ showed slightly higher operation voltage compared with those profiles at 55℃ with discharge capacities of 128.5 - 131.4 mAh g-1. And the PB electrode showed high coulombic efficiency of 99.3% at low temperature, although the discharge capacities at -10℃ were only 98.8 - 100.7 mAh g-1. The above results indicated that the elevated temperature might accelerate the side reactions resulting in obvious decline of the columbic efficiency[10, 29]. Fig. 2c shows the long term cycling results of the PB cathode at 1 C rate (100 mA g -1). It can be seen that the PB electrode showed very good cycle stability at low temperature with discharge capacity of 87.3 mAh g-1 and 100 % capacity retention after 200 cycles. When cycled at room temperature, PB electrode exhibited a higher capacity of 122.3 mAh g-1, however the capacity retention was only 73.4 % after 200 cycles. It is obvious that the PB electrode exhibited the worst cycle behavior at 55℃. The 8

discharge capacity decreased from 120.5 mAh g -1 to 65.9 mAh g-1 over 200 cycles with capacity retention of 54.7 %. To understand the capacity degradation mechanism of PB electrode at elevated temperature, the CV curves at a scan rate of 0.2 mV S -1 were measured between 2.0 V and 4.0 V at 55 ℃. Fig. 3a shows the CV curves of first 20 cycles. The curves present two couples of redox peaks corresponding to oxidation/reduction of Fe п(C)/Feш(C) at high potential and Feп(N)/Feш(N) at low potential, respectively. In the first two cycles, the peak potentials were 3.00/2.80 V vs. Na/Na+ and 3.67/3.30 V vs. Na/Na+, respectively. In the subsequent cycles the voltage separation between the cathodic and anodic peaks gradually increased and the peak current densities decreased. At 20 th cycle the peak potentials were 3.11/2.68 V vs. Na/Na+ and 3.63/3.23 V vs. Na/Na+, respectively. The obviously increased overpotential within cycling process may be related to side reactions. On the first cycle CV curve, a large oxidation peak appeared at 3.95 V vs. Na/Na+ while no corresponding reduction peak was observed. This irreversible oxidic peak was attributed to side reactions. The side reaction products might cause polarization of the electrode reaction as observed in the CV profiles. To confirm this speculation, the CV curves were measured in the voltage range of 2.0 3.8 V vs. Na/Na+. As shown in Fig. 3b, the CV profiles in 2.0 - 3.8 V vs. Na/Na+ show better reproducibility compared with those in 2.0 - 4.0 V vs. Na/Na+. This result demonstrated the lower upper cut-off potential could alleviate the side reactions and improve cycling stability of PB electrode. To analyze the side reaction products, the PB electrode was extracted from the cell after 100 cycles at 1 C rate, 55℃. It can be clearly seen in Fig. 4 that a SEI film formed on the surface of cycled PB cathode. X-ray photoelectron spectroscopy (XPS) was conducted to analyze the composition of the surface products. Fig. 5 compares

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the XPS spectra of the cycled and uncycled PB electrodes. Considering the possible influence of adsorbed electrolyte on the cycled electrode, the uncycled PB electrode was soaked in electrolyte and dried in the glove box before measurement. The C1s XPS spectrum of the cycled PB electrode showed five peaks. The peaks at 290.8, 284.8, and 279.2 eV, which also appeared on uncycled PB electrode, could be assigned to the C-F bond (PVDF), C-C/C-H bond (PVDF), and carbon black / -C≡N, respectively. The other two peaks at 288.6 and 286.7 eV appeared only on the cycled electrode might indicate the existence of the side reaction products Na 2CO3 / ROCO2Na and RONa, respectively. These results indicated that the organic solvents were decomposed on the surface of PB electrode. The F1s spectrum showed two peaks. The peak at 687.9 eV corresponded to PVDF binder and possible traces of NaPF6, the second peak at 685.0 eV might be assigned to NaF. It is clear that The Fe2p and N1s spectra of the cycled electrode were noticeably changed compared with those of uncycled electrode. In Fe2p spectrum, The Fe2p 3/2 peak move from 709.1 eV to 711.2 eV indicated that on the surface layer the iron element was mainly in the Fe (Ш) state. Furthermore, a new peak appeared at the high binding energy 714.2 eV, which might indicate the forming of Fe-O due to the decomposition reaction related to interstitial H2O. The N1s spectrum of cycled electrode also showed a new peak at 400.3 eV. This might indicate the breaking of Fe-N bond. Trace amount of PB might involve in the surface side reactions occurred at voltage above 3.8 V vs. Na/Na+. This phenomenon is also reported in literature [19]. J. Sottmann et al. performed ex situ XRD studies of Na1.32Mn[Fe(CN)6]0.83·z H2O in 1 M NaClO4/EC+DEC electrolyte. They revealed the presence of NaMnCl3 on the electrode surface which was caused by the irreversible side reaction occurred at voltage above 3.8 V vs. Na/Na+, and the formation of NaMnCl3 seems to be favoured by the presence of water in the crystal

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structure. FTIR spectra in Fig. 6 confirm the existence of carbonate on the surface layer of the cycled electrode. The peak at approximately 2061.8 cm-1 corresponds to the stretching vibrations of -C≡N of PB material. The intensive peak at 1069.3 cm-1 is related to -C-O stretching vibrations, and the peak at 1772.9 cm-1 is related to -C=O stretching vibration. The peak in the region between 3000 and 3600cm-1 is associated with the stretching mode of O-H indicating the probably existence of Fe-O-H. XPS and FTIR results may indicate that the interstitial H2O might be decomposed to form intermediate oxygen species which might cause the Fe-N bond broken and forming of O-Fe, Fe-O-H and N-O as shown in XPS spectra. The side reactions on the PB electrode at near 4.0 V were complicated and the surface layer composed of several compounds. To clarify the structure stability of the PB cathode on the cycling process, ex situ XRD patterns of the PB cathodes cycled at 55℃ were recorded and shown in Fig. 7. The XRD analyses were conducted at charge and discharge states of the first cycle and 100th cycle electrode samples. The XRD patterns of the charge state electrodes were characterized as cubic phase. After Na+ insertion, new phase peaks appeared in the XRD patterns of discharge state (2.0 V vs. Na/Na+) electrodes. The XRD patterns of 100th cycle were similar to those of the first cycle. It is clear that the phase transition during sodium insertion/extraction is reversible. This result shows no evidence for structure degradation of the PB framework. The cycle lifetime is one of the most important metrics for an electrode material in practical application. In order to reduce the negative influence of the side reactions at high voltage range, cycling behavior of PB electrode was estimated in the voltage range of 2.0 - 3.8 V vs. Na/Na+. Fig. 7a&b show the rate performance of PB electrode 11

at various temperatures. Although the operation voltage range was narrower, the PB cathode still present attractive specific capacities with initial discharge capacity of 83.7 mAh g-1 (-10℃), 100.0 mAh g-1 (25℃) and 106.6 mAh g-1 (55℃) at 1 C rate. In Fig. 7a PB electrodes showed remarkable cycling stability in 2.0 - 3.8 V vs. Na/Na+. at low and room temperatures. The discharge capacities at room temperature were 100.0 mAh g-1 (1 C), 89.7 mAh g-1 (2 C) and 85.2 mAh g-1 (5 C) with capacity retention of over 99 % after 200 cycles, respectively. Fig. 7b show the cycling performance PB electrode at 55℃ in 2.0 - 3.8 V. The PB electrode showed good rate performance at elevated temperature with the discharge capacities reached up to 106.6 mAh g-1 (1 C), 102.1 mAh g-1 (2 C), 97.4 mAh g-1 (5C). It can also be seen that although the cycling stability in 2.0 - 3.8 V vs. Na/Na+ at 55℃ was obviously improved compared with those in 2.0 - 4.0 V vs. Na/Na+, the capacity degradation was still noticeable, which might be due to the electrolyte decomposition at elevated temperature. Fig.7C compares the 1 C rate cycling performance of PB electrodes in different operation voltage ranges in room temperature. Although the PB electrode exhibited higher initial discharge capacity of about 122.3 mAh g-1 in 2.0 - 4.0 V vs. Na/Na+, the capacity decreased gradually to 68.9 mAh g -1 at 300th cycle with capacity retention of 56.3 %. On the other hand, when cycled in 2.0 - 3.8 V vs. Na/Na+, the PB electrode delivered lower capacity of about 100.0 mAh g-1, however, the electrode displays outstanding cycling stability, and still delivered a discharge capacity of 98.2 mAh g-1 after 300 cycles with capacity retention of 98.2 %. The above results indicate that the PB sample exhibit excellent electrochemical performance in the controlled

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voltage range of 2.0 - 3.8 V vs. Na/Na+. Conclusions In this work, prussian blue with the formula of Na1.59Fe[Fe(CN)6]0.95□0.05 was prepared by a simple precipitation method. Electrochemical behaviors of the PB material at different operation temperatures (-10℃, 25℃, 55℃) were studied. Cycled in the voltage range of 2.0 - 4.0 V vs. Na/Na+, the PB cathode showed 1 C rate initial reversible capacities of 87.3 mAh g -1 (-10℃), 122.3 mAh g-1 (25℃) and 120.5 mAh g-1 (55℃), with capacity retention of 100% (-10℃), 73.4% (25℃), and 54.7% (55℃) after 200 cycles, respectively. Cyclic voltammetry demonstrated the existence of side reactions at near 4.0 V, which might cause the increase of elecrtrochemical impedance during cycling process. XPS and FTIR analyses indicated the complicate compositions of the side reaction products. XPS results also indicated the possible breaking of Fe-N bond which signified the involvement of PB in the side reactions despite the XRD analysis still show the structure stability of PB electrode. By limiting the upper cut-off voltage to 3.8 V vs. Na/Na+, the PB electrode showed outstanding cycling stability with 1 C rate reversible capacity of 100.0 mAh g-1 and capacity retention of 98.2 % over 300 cycles at room temperature.

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Acknowledgements The authors gratefully acknowledge financial support from the Major State Basic Research Development Program of China (2014CB239703, 2016YFB0901505), the National Natural Science Foundation of China (21573147, 21506123 and 21336003) and the Science and Technology Commission of Shanghai Municipality (15ZR1422300, 14DZ2250800).

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Figure captions Fig. 1 (a) Powder XRD pattern of the prepared prussian blue sample; (b) FESEM image of the prepared PB powder; (3) TEM image of a PB particle; (d) TG curve of the prepared PB sample. Fig. 2 (a) The first 10 cycle charge-discharge profiles of the PB cathode in 2.0 - 4.0 V vs. Na/Na+ at a constant current density of 10 mA g -1 ; (b) The first ten cycle charge and discharge capacities of the PB cathode in 2.0 - 4.0 V vs. Na/Na+ at 10 mA g-1 ; (c) Cycle stability of PB cathode at various temperatures in 2.0 4.0 V vs. Na/Na+ with current density of 100 mAh g-1. Fig. 3 Cyclic voltammograms of the Na||PB cells scanned at 0.2 mV s-1, 55℃: (a)

in

2.0 - 4.0 V vs. Na/Na+; (b) in 2.0 - 3.8 V vs. Na/Na+. Fig. 4 SEM image of the cross-section of the PB cathode after 100 cycles at 1 C rate, 55℃ Fig. 5 C1s, F1s, Fe2p and N1s XPS spectra of the PB electrode after 100 cycles at 1C rate, 55℃ Fig 6. FTIR spectra of the uncyled PB electrode and the PB electrode after 100cycles at 1C rate, 55℃. Fig. 7 Ex situ XRD patterns of the PB electrodes. Fig.8 Electrochemical performance of the PB cathode in the voltage range of 2.0 - 3.8 V vs. Na/Na+: (a) Cycling performance of PB cathode at 25℃ and -10℃; (b) Cycling performance of PB cathode at 55℃; (c) Comparison of the 1C rate cycling stability of the PB cathode in different voltage ranges at room temperature.

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Fig.1a

Fig. 1b

Fig. 1c

Fig. 1d

Fig. 2a

Fig. 2b

Fig. 2c

Fig. 3

Fig. 4

Fig. 5 (C1s)

Fig. 5 (F1s)

Fig. 5 (Fe2p)

Fig. 5 (N1s)

Fig. 6

Fig. 7

Fig. 8a

Fig. 8b

Fig. 8c