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Studies on the Kinetic Behaviours of Na Ions Insertion/Extraction in Na2FeSiO4/C Cathode Material at Various Desodiation States Yansong Bai, Xiaoyan Zhang, Ke Tang, Li Yang, Hong Liu, Lei Liu, Qinglan Zhao, Ying Wang, and Xianyou Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10029 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Studies on the Kinetic Behaviours of Na Ions Insertion/Extraction in Na2FeSiO4/C Cathode Material at Various Desodiation States Yansong Bai,† Xiaoyan Zhang,† Ke Tang,† Li Yang,† Hong Liu,† Lei Liu,† Qinglan Zhao,§ Ying Wang,*,§ Xianyou Wang,*,† †National

Base for International Science & Technology Cooperation, National Local Joint

Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan University, Hunan 411105, China §Department

of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong

Kong KEYWORDS: Sodium-ion batteries, Cathode materials, Electrode kinetics, Na ions extraction, Na-ion diffusion coefficient.

ABSTRACT: Na2FeSiO4, as one of the promising cathode materials in sodium-ion batteries, has attracted great interests. However, studies on the kinetic behaviours of Na

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ions insertion/extraction in Na2FeSiO4 composite electrode have been barely considered, until now. Importantly, the specific capacity and rate capability of Na2FeSiO4 cathode material are greatly correlated with the kinetics of Na+ transfer in the host material. Herein, based on the characterizations of microstructure and morphologies (i.e. Rietveld refinement, FESEM, HRTEM, etc.), the electrochemical kinetics of Na ions extraction in Na2FeSiO4/C electrode are firstly studied in detail via two electrochemical techniques (EIS and GITT), establishing the rate-controlling steps of Na+ transport in Na2FeSiO4/C, evaluating series of kinetic parameters as well as calculating the Na+ diffusion coefficient at various state-of-desodiation. Changes of impedance response of Na2FeSiO4/C electrode depending on the different levels of desodiation show that a serial features of electrode process for Na ions migration have tremendous discrepancies, indicating that the kinetics of Na+ extraction from Na2FeSiO4/C electrode are largely influenced by different electrode reaction processes. These results provide useful insight into the inner properties of Na2FeSiO4/C electrode, and it is significant to optimize the electrochemical performance of Na2FeSiO4/C. Moreover, two models of equivalent circuits are also constructed to simulate the electrode process and describe the behaviours of Na ions transfer in Na2FeSiO4/C.

1. INTRODUCTION With the ever-increasing demands for energy in daily life, the large-scale energy storage, especially for solar power, wind energy and tidal energy, become extremely important.1,2 Thus, it is necessary to precisely design energy storage systems (EES) with high conversion efficiency to maximally utilize the renewable energy.3−5 For LIBs, however,


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some issues and challenges such as high costs and insufficient resources greatly limit their applications in large-scale EES and smart grid.1,6 In contrast, sodium-ion batteries (SIBs) based on the cost and resource advantages can fulfil the requirements of large-scale energy storage, making them as one of the most promising alternatives in the rechargeable battery system.2,7 So, many efforts of studies on SIBs have been afforded over the past few decades. The cathode materials, similarly to that of LIBs, play a dominant role in SIBs, and it is crucial to explore the cathode materials with stable crystal structure, high specific capacity and excellent cycle performance for SIBs.8 Owing to the weight and ionic radius of sodium are larger than that of lithium,1,8 the cathode materials employed in SIBs must have an enough space and channels in lattice cell to accommodate and convey sodium ions. Currently, varieties of cathode materials used in SIBs has been widely investigated, especially for the layered transition-metal oxides (NaxMO2, M = Co, Mn, Fe, Ni, Cr, V, etc.9−12) and polyanion-type compounds.13−16 However, the high costs and toxicity to environment of NaxMO2 greatly obstruct their application in SIBs.17 The polyanion-type compounds with the advantages of robust framework, adjustable electrode potential, inherent safety and good thermal stability make themselves form the main-stream cathode materials in SIBs.18,19 To date, lots of Na-based polyanion compounds have been prepared and evaluated by many researchers.20−23 Recently, sodium orthosilicates as one of the important polyanion-type compounds have attracted much attentions increasingly because of their lower cost and more stable crystal structure.19,24 Particularly, the cathode


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material of Na2FeSiO4 with good cost-effectiveness and remarkably stable structure seems like a promising cathode material applied in SIBs, especially in the large-scale energy storage systems (ESSs). But these drawbacks of inferior discharge capacity, unsatisfactory cycle-life and poor rate capability for Na2FeSiO4, which are primarily dominated by the electrochemical kinetics of Na ions insertion/extraction, should be thoroughly solved before they are widely used in SIBs. Of late, many efforts have been carried out to study the Na2FeSiO4 cathode material. For example, Kee et al., for the first time, prepared the metastable P1-Na2FeSiO4,25 but its crystal structure collapsed after cycles. Through the sol-gel route, Li et al. designed the F 4 3m structure of Na2FeSiO4 and a specific capacity of 106 mAh g-1 was achieved.26 To enhance the electrochemical properties of Na2FeSiO4, Ali et al. constructed a hierarchical porous nanospheres of F 4 3m-Na2FeSiO4 modified by CNTs.27 Recently, various polymorphs of Na2FeSiO4, such as C2, C2221, Pna21, Pmn21-cycl, etc., have been also studied in detail by Zhu et al. via PBLD and DFT methods.28 Although the preparation method, electrochemical behaviors, design of polymorphs and theoretical calculations of Na2FeSiO4, just like Li2FeSiO4, have been investigated in detail, the researches on Na2FeSiO4 are far less than that of Li2FeSiO4, especially for the electrochemical kinetics of Na ions insertion/extraction in sodium-based cathodes, and the studies about it have never been systematically conducted before. Generally, the capacity and rate capability of the cathode material in SIBs are largely dependent on the kinetic behaviours of Na+ insertion/extraction in host material. The rate of Na ions


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migration in cathodes attaches to the properties of host materials such as the diffusion of Na+ within the bulk phase, electronic conductivity and stability of structure, etc. As the repeated cycling continues for electrode in SIBs, rearrangement of crystal structure, transformation of polymorph (if any) together with changes in the micro-morphology of active nano-particles will inevitably bring about the fundamental changes of kinetic properties for Na ions transfer. Lately, our group have reported a novel polymorph of orthorhombic crystal with Pb21a structure for Na2FeSiO4,29 and the tremendous change of diffusion rate of Na ions within bulk material during cycling process, which is mainly caused by the phase conversion, has also supported the views above. Therefore, studies on the process which controls electrode kinetics, determination of sodiation/desodiation rates as well as investigation of changes in kinetic parameters of electrode during cycling process would provide very useful insight into the properties of Na2FeSiO4/C electrode, which can broaden the understanding of inner performance of sodium iron-based orthosilicates, and are vital and significant for promoting the development and industrialization of Na2FeSiO4/C. Herein, this work is to precisely focuses on the electrochemical kinetics of Na ions extraction in Na2FeSiO4 composite electrode during the first and second desodiation process. Through a series of characterizations, such as the X-ray diffraction, FESEM, HRTEM and Rietveld refinement, etc., the kinetic behaviours of Na+ insertion/extraction in Na2FeSiO4/C are clarified clearly by electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) that is indeed widely applied in


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intercalation compounds for LIBs. According to investigate various kinetic parameters and calculate the Na+ diffusion coefficient (DNa+) of Na2FeSiO4/C, the factors which control the electrode reaction process at various desodiation states have been established. Importantly, we also construct two theoretical EIS models to analyse the impedance response at different levels of desodiation for Na2FeSiO4/C electrode.

2. EXPERIMENTAL SECTION 2.1. Preparation of Na2FeSiO4/C. The cathode material of Na2FeSiO4/C is prepared via the sol-gel method assisted by citric acid. Fe(NO3)3·9H2O (AR, ≧98.5%), Tetraethyl orthosilicate (TEOS, SiO2 ≧28%) and CH3COONa (AR, ≧99%) are used as the raw materials, and they are weighted in the molar ratio of Na:Fe:Si = 2:1:1. In order to prepare the quasi-spherical nanoparticles for Na2FeSiO4/C, the spherical nanoparticles of SiO2 are firstly synthesized by the hydrolysis of TEOS under alkaline conditions. Firstly, 3ml TEOS was dissolved in water-ethanol (1:10, vol.%) solution, and 2 mL NH3·H2O as catalyst was added. The solution was stirred for 3 h at room temperature to make the TEOS hydrolyze completely. Then, a kind of milky solution appeared. Next, 5.4368 g Fe(NO3)3·9H2O, 2.2187 g CH3COONa and 5.6558 g citric acid were dissolved in the water-ethanol (1:5, vol.%) solution, respectively, and they were added into the above solution drop by drop. Secondly, the mixed solution was transferred into a three-necked round-bottom flask (250 mL) and refluxed for 24 h at 80 oC under mechanical agitation. During the reflux process in flask, the colour of the solution changed from deep rose to


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light khaki gradually. Subsequently, the solution was evaporated at 80 oC in oil bath until viscous gels turned up. Thirdly, the gels were dried at 100 oC in the blast air oven to obtain xerogels. Then, the xerogels were milled into powders, calcinating at 350 oC for 3 h and 600 oC for 10 h under the argon atmosphere. The Na2FeSiO4/C samples were finally obtained. 2.1. Material Characterizations. The crystal structural of the as-prepared samples were analyzed by X-ray diffraction (D/Max-2500, Rigaku) using Cu Kα radiation source (λ = 1.54178 Å,) at a generator of 60 kV, 300 mA. The sweep range (2θ) was settled from 10o to 90o and the scanning rate was 2o min-1. The emission scanning electron microscopy (FESEM, Tescan Mira3, operating at 20 kV) and transmission electron microscopy (TEM, JEM-2100F) were introduced to examine the microstructures and morphologies of the samples. The carbon contents of sample were measured by the elemental analyzer (Vario EL III, Germany). 2.3. EIS and GITT Measurements. In order to evaluate the electrochemical properties and obtain the precise data of EIS and GITT, a three-electrode coin cells were designed.30 Coin cells were constructed with Na auxiliary electrode ‖ electrolyte ‖ separator ‖ Na reference electrode ‖ electrolyte ‖ separator ‖ Na cathode electrode systems. The cathode electrode of samples was prepared by blending with 80 wt% Na2FeSiO4/C, 5 wt% graphite, 5 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrodidene (NMP) solvent. After stirring them for 3 h at room temperature, the homogeneous slurries were uniformly spread on the current collector of


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aluminum foil, drying them at 120 oC for 24 h in vacuum oven. Then, the electrode (the thickness is about 0.24 mm) was tailored into the small circles with the diameter of 9.8 mm. Typically, the active materials of 2.1-2.4 mg were loaded on each electrode. Sodium metal was served as auxiliary electrode and reference electrode. A glass fiber (GF/A, whatman) was used as separator. The electrolyte was 1M NaClO4 in ethylene carbonate (EC)-dimethyl carbonate (DMC)-ethyl methyl carbonate (EMC) (1:1:1 by volume, battery grade) solvent and 5 vol.% fluoroethylene carbonate (FEC) as an additive was used. The coin cells were assembled in an argon-filled glove box with oxygen and moisture level below 1 ppm. The EIS measurements of the Na2FeSiO4/C composite electrode at different desodiation states were carried out by an electrochemical workstation (Princeton, VersaSTAT3, USA). The range of frequency applied in the electrochemical workstation was range from 105 Hz to 10–2 Hz and the amplitude of the AC signal was kept at 5 mV. For getting the impedance response of Na2FeSiO4/C electrode, the coin cells were firstly charged at 0.05 C for 1.5 h, and let them leave for 4 h to make the open-circuit voltage (OCV) of cell reach to the quasi-equilibrium state (the fluctuation of OCV was 5 mV within 0.5 h), repeating the above procedures in sequence. The data of EIS response were fitted by Zview2 software. The GITT measurement of cathode electrode at different desodiation states was carried out by the devices of Neware test system. The cells were firstly charged at a constant current flux (Io = 15 µA) for 1.5 h and left for 4 h, repeating the procedures until reaching 4.5 V for the voltage of cell. Before testing, all the fresh


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cells were left standing for 24 h. Unless otherwise specified, all the measurements were performed at room temperature. 3. RESULTS AND DISCUSSION Sodium iron-based orthosilicates, Na2FeSiO4/C, was successfully prepared via the sol-gel method. In order to determine the crystallographic structure of the as-prepared sample, the measurement of X-ray diffraction (XRD) was carried out. Figure 1 shows the XRD pattern of Na2FeSiO4/C sample. The inset shows the close-up view of diffraction peaks between 15o and 45o in 2. It is observed that the diffraction peaks of sample are sharp, illustrating that the sample has good crystallinity after sintering at 600 oC for 10 h. Up to now, many polymorphs of Na2FeSiO4/C such as triclinic phase and cubic phase have been reported in literatures,26,31 and they can be also assigned to varieties of space group, including F 4 3m, Pmn21, C2221, P213, C2, etc. Based on the analysis of Rietveld refinement (as indicated in Figure 1), the calculated results fit well with the observed data and all the diffraction lines of the as-prepared sample can be indexed to an orthorhombic cell with C2221 space group. No uncharted peaks are detected. The values of reliability (Rwp = 7.85% and Rp = 5.34%, < 10%) is relatively small, indicating the reasonable refinement results. The lattice parameters refined from the data of Na2FeSiO4/C are a = 7.53424 (4), b = 7.22703 (6) and c = 7.03267(3) Å, corresponding to a volume of V = 382.93 Å3. The diffraction peaks of Na2FeSiO4/C at 21.18o, 30.61o, 24.38o, 34.87o, 35.55o and 62.81o in 2 are associated with the planes of (111), (112), (220), (202), (022) and


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(242), respectively. Besides, in order to evaluate the effect of carbon doping on the crystal structure, the XRD peaks of pristine Na2FeSiO4 and Na2FeSiO4/C were compared and the results were presented in Figure S1. Clearly, both of them have nearly identical reflections and well-crystallized structure in the range of 10o-80o in 2. All the diffraction peaks for them can be ascribed to the orthorhombic unit cell with C2221 space group and no changes in the position of peaks, indicating that the carbon coating has not influence on the polymorph of Na2FeSiO4.

Figure 1. XRD patterns and the analysis of Rietveld refinement of Na2FeSiO4/C sample. The black, red and purple lines denote the observed, calculated and difference profiles, respectively. The black vertical bars indicate the position of diffraction peaks of C2221 phase for Na2FeSiO4/C. The inset shows the close-up view of diffraction peaks between 15o and 45o in 2. Figure 2 shows the morphologies of Na2FeSiO4/C sample characterized by the field


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emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). In Figure 2a, the Na2FeSiO4/C sample prepared from the sol-gel method has the morphologies of quasi-spherical nanoparticles with the diameter of 100-200 nm, and the nanoparticles are slightly agglomerated. The results show that the nano-spherical silica is formed and provides an effect of “template” when TEOS is completely hydrolyzed under ammonia as catalyst, finally causing the formation of quasi-spherical morphologies for Na2FeSiO4 after calcination at 600 oC for 10 h. Figure 2b presents the TEM image of the as-prepared sample, and the nanoparticles with about 200 nm can be observed clearly. In the same time, the samples have a good sphericity and are well coated by the in-situ carbon which are formed by the carbonization of citric acid during the sintered process. The carbon content measured by the elemental analyzer is about 14.8%. The high-resolution TEM image of Na2FeSiO4/C sample can be seen in Figure 2c. Clearly, a thin layer of carbon at the nano-scale has been formed on the surface of the active particles. The incorporation of carbon on the surface of active materials is an effective strategy to improve the electrochemical properties, especially for the polyanion-type compounds with low ionic and electronic conductivity.32 It probably enhances the electrochemical kinetics of Na+ insertion/extraction in Na2FeSiO4/C composite electrode to some extent. Furthermore, two kinds of lattice fringes appear in Figure 2c, which can be attributed to the (111) and (202) planes (as shown in Figure 2d), corresponding to the diffraction peaks of Na2FeSiO4/C at 21.18o and 34.87o in 2. The d-spacing of (111) and (202) planes measured by the Digital Micrograph software are 0.463 nm and 0.262 nm,


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respectively.

(a)

(b)

Carbon

Carbon

Nanoparticles

(c)

(d)

Carbon layer d(111)=0.463 nm

(202) (111) d(202)=0.262 nm

(111) (202)

Figure 2. Microscopy structure investigation of the as-prepared Na2FeSiO4/C sample sintered at 600 oC for 10 h. (a) FE-SEM image; (b) TEM image; (c) High-resolution TEM image; (d) The image of fast Fourier transform (FFT).

The electrochemical properties of Na2FeSiO4/C sample are shown in Figure 3. The first and second charge-discharge profiles at 0.1 C in the voltage range of 1.5-4.5 V are presented in Figure 3a. The charge and discharge capacity of Na2FeSiO4/C sample during the initial cycle are 131 mAh g-1 and 123 mAh g-1, respectively, corresponding to the


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coulombic efficiency (CE) of 93.89%. The second charge and discharge capacity are 118 mAh g-1 and 115 mAh g-1, respectively, corresponding to the CE of 97.45%. In addition, note that there is an obvious difference in the curves between the first and second charge process, and the phenomenon can be also observed by previous works.26,27,31 Generally, the distinction of curves in the first and second charging process can be ascribed to the rearrangement of crystal structure,33 which will inevitably lead to the variation of the kinetic behaviours during the electrode reaction process. Figure 3b shows the cyclic performance of Na2FeSiO4/C. After 100 cycles at 0.1 C, the reversible capacity of Na2FeSiO4/C electrode is 89.55 mAh g-1 with 72.8% retention and the CE is 99%. To evaluate the rate capacity of Na2FeSiO4/C, the electrochemical performance of Na2FeSiO4/C at different current density are studied and the results are shown in Figure 3c. The reversible capacity of Na2FeSiO4/C electrode are 121.42, 97.68, 70.57, 50.71 and 36.05 mAh g-1 at 0.1, 0.2, 0.5, 1 and 2 C, respectively. Apparently, the reversible capacity of Na2FeSiO4/C drops obviously as the discharge rate increasing, especially at high rates. Although the phenomenon can be usually observed in the cathode material, the attenuation of capacity for Na2FeSiO4/C should be probably suffered from not only its inferior electronic conductivity but also the sluggish electrochemical kinetics. Therefore, it is necessary to systematically study the kinetics properties of Na2FeSiO4/C electrode for Na ions transport.


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(b)

(a)

(c)

Figure 3. Electrochemical properties of Na2FeSiO4/C electrodes in the range of 1.5-4.5 V. (a) The first and second charge-discharge profiles at 0.1 C; (b) Cycling performance and corresponding coulombic efficiencies vs. cycle number curves; (c) The rate capacity from 0.1 C to 2 C for 6 cycles. As known to all, the electrochemical properties of the cathode materials are greatly affected by the electrochemical kinetics of ions insertion/extraction in electrode. But the work evaluating the kinetics properties of Na+ insertion/extraction in Na2FeSiO4 has not been found until now. To date, varieties of electroanalytical techniques that have been widely applied in LIBs can be employed to analyze the kinetics properties of Na ions insertion/extraction in Na2FeSiO4/C electrode. The EIS technique, which use the current


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of small-amplitude sine wave as disturbing signal to collected the impedance response of electrode, has been proven to be a powerful tool for studying electrode kinetics.34 In general, the impedance response based on the Nyquist plots of SIBs, analogous to LIBs, can be regarded as the multi-step reaction processes of Na ions insertion/extraction in composite electrode,35 i.e. Na ions migration through the solid electrolyte films (SEI films), charge transfer among the electrolyte-electrode and the solid-state diffusion of Na ions in the bulk material. By constructing the equivalent circuits composed by circuit elements (Ri and Ci), series of the physical process of electrode reaction can be exactly simulated. Thus, it is possibly to interpret the kinetic behaviours of Na ions intercalation/deintercalation in Na2FeSiO4/C composite electrodes. Figure 4 shows a family of Nyquist plots measured from Na2FeSiO4/C composite electrode at various potentials (open circuit voltage, OCV) during the first and second charging process, and the insets display a close-up view of impedance response in high-to-medium frequency. As the electrode potential varies (from fresh cell to 3.642 V, during the first desodiation process), the diameter of the depressed semicircle in impedance spectrum substantially reduces on starting charging (corresponding to the desodiation states, i.e. Na content x in Na2-xFeSiO4/C), achieving its minimum in voltage range of 3.35-3.40 V. Whereafter, the semicircle of impedance response presents an increased tendency with the increasing of voltage. As for the second desodiation process, two semicircles can be observed. Nevertheless, The impedance response of Na2FeSiO4/C electrode is mainly dominated by a low-frequency semicircle in the initial charging stages, and it shows the trend to close


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to the real axis until the appearance of the sloped straight line as the electrode potential increases (i.e. increase in the degrees of desodiation for Na2FeSiO4/C). In addition, the high-frequency semicircle similarly changes with the variation of electrode potentials. The characteristics of potential correlation for impedance response spectrum can be also observed in Figure S2, which shows the relation curves of the frequency with impedance (imaginary part) for Na2FeSiO4/C composite electrode during the first and second desodiation process. As may be seen from Figure S2, several peaks (see the arrows in the Figure S2a and Figure S2b) that correspond to the diameter of the depressed semicircle (e.g. Rsf, Rct) in impedance spectrum are distinguishable. Apparently, the diameter of impedance response is potential dependent. The evolution of the impedance response at different charging states illustrate that a series of kinetic properties for Na ions extraction in Na2FeSiO4/C electrode have changed significantly. In Figure 4a-b, the shape of impedance spectra are similar, and they are comprised of a depressed semicircle in high-to-medium frequency domain (frequency range from 100 kHz to 2 kHz) and a sloped straight line (slope of approximately 45o) in low frequency domain, which are considered as the charge transfer among the electrolyte-electrode and the solid-state diffusion of Na ions in the bulk material, respectively.36 After applying current on Na2FeSiO4/C electrode, the diameter of semicircle in high-to-medium frequency region decreases dramatically, varying with the different level of desodiation. The phenomenon may be ascribed to the activating effect of current flux on the electrode surface. An analogous variation trend can be also found in Figure 4c-d, but two


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semicircles in Nyquist plots appear. One associating with the high-frequency domain can be assigned to Na ions migration through SEI films and the other involving with the medium-frequency region can be assigned to the charge transfer among the electrolyte-electrode.8,34 The difference of impedance response between the first and second desodiation process is mainly due to the solid electrolyte films (SEI films) which has well developed and formed on the surface of Na2FeSiO4/C electrode after the initial cycle.36,37 Although the formation of SEI films is achieved at the expense of capacity loss, it can prevent the electrode-electrolyte side reactions from taking place, playing an important role on the cycling performance and safety of electrode.38 In order to analyze the electrochemical kinetics of Na ions extraction in Na2FeSiO4/C electrode and simulate various physical processes of Na2FeSiO4/C electrode during the first and second charging process, two kinds of equivalent circuits based on the impedance response of Na2FeSiO4/C electrode, as shown in Figure 4e, are proposed. Generally, the circuit elements of Rs, Rsf, Rct, Wo and CPE are related to the ohmic resistance of cell, the resistance of SEI films, the charge transfer resistance among the electrolyte-electrode, Na+ diffusion in the bulk material (Warburg impedance) and the constant-phase element (non-ideal capacitance of interface), respectively.39 Given the discrepancy of electrode reaction process at different desodiation stages for Na2FeSiO4/C, the equivalent circuits of model (Ⅰ) and model (Ⅱ) are employed to simulate the first and second desodiation process, respectively. Usually, there is a certain deviation between the observed data and fitting data when the impedance response is fitted via the Zview2 software. Therefore, the


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degree of deviation, i.e. the reliable factor, can be identified by the Chi-Square during the fitting by the Zview2 software. The smaller values that the Chi-Square has, the better results are going to be. Table S1–S4 present the reliable factors acquired from the fitting impedance response during the first and second desodiation process. All the values of Chi-Square obtained from the fitting experiment data by the Zview2 software are relatively small and can be acceptable, indicating that the fitting results of EIS response are reliable. Figure 4f presents the typical fitting result of Nyquist plots. The linear fitting results are highly consistent with the experimental data at 3.351 V and 3.693 V, showing that the equivalent circuits of model (Ⅰ) and (Ⅱ) can appropriately describe the case of Na ions extraction in Na2FeSiO4/C electrode for the first and second desodiated process. Furthermore, there are obviously discrepant in the impedance response between the Figure 4f (1) and Figure 4f (2), implying the dynamical changes of kinetic behaviours for Na2FeSiO4/C electrode at various states of desodiation.


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(a)

(b)

(c)

(d)

(e)

(f)

(I)

(II)

Figure 4. Nyquist plots of Na2FeSiO4/C composite electrode and the fitting results at various potentials vs. Na/Na+ during the first and second desodiation process. (a) Fresh cell and at 3.178-3.351 V; (b) At 3.354-3.642 V; (c) At 1.819-3.693 V; (d) At 3.705-4.119 V; (e) The equivalent circuits of model (Ⅰ) and model (Ⅱ) used to simulate the first and second desodiation process, respectively; (f) Typical of the fitting example, at 3.351 V in the first desodiation process and 3.693 V in the second desodiation process. Frequency range from 100 kHz to 1 mHz. The insets display a close-up view of spectra in


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high-to-medium frequency. Figure 5 shows the profiles of Ri and Ci vs. voltage during the first and second desodiation process. The parameters of Rs, Rsf, Rct, Csf and Cdl at different desodiation states are deduced by fitting the impedance spectrum using the equivalent circuits (as shown in Figure 4e). Obviously, all the values of Ri and Ci vary with the change of the voltage (i.e. the charging states), but their change trends in the range of 1.5-4.5 V are different. The ohmic resistance of cell, Rs (6.0-7.5 Ω), is very small and little change during the charging process. Thus, its contribution to the total impedance of Na2FeSiO4/C electrode can be ignored, assuming invariableness of values during the cycling process. On the starting desodiation (the initial charging process), the values of Rct (2810 Ω at 2.545 V), as shown in Figure 5a, are relatively large, indicating the badly sluggish kinetics of electrode reaction process. The impedance of Na2FeSiO4/C electrode is mainly contributed by the charge transfer resistance at this moment. The result is consistent with the profile of the initial charge process, as shown in Figure 3a, which has a large electrochemical polarization. As the desodiation continues, the values of Rct decrease dramatically (from 2810 Ω to 585 Ω), suggesting a rapid migration of electric charge between electrolyte and electrode, and leading to the vigorous electrode kinetics.40 When the Na2FeSiO4/C electrode approaches to the full-desodiation state, the values of Rct increase slightly. It can be ascribed to the high concentration polarization at the end of charge transfer process.41 The values of double-layer capacitance (Cdl, Figure 5b) related to the charge transfer between electrolyte and electrode show a monotonic increasing


20

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tendency during the first charging process. It can be attributed to the effect of accumulation for charge on the boundary of mass transfer, finally creating more double-layer capacitors.35 Figure 5c shows the values of Rs, Rsf and Rct obtained from the linear fitting via the equivalent circuit of model (Ⅱ) during the second desodiation process. Although the resistance of Rsf presents a descending trend on the second desodiation process, the changes of value are little. So, presumably, the physical properties and chemical compositions of SEI films formed on the surface of Na2FeSiO4/C electrode remain unchanged during the second charging process. The values of Rct are extremely large on the starting desodiation and the entire impedance of Na2FeSiO4/C electrode are mostly contributed by Rct. The electrode kinetics of Na ions extraction in Na2FeSiO4/C electrode is mainly dominated by the rate of charge transfer between electrolyte and electrode. The result is in accordance with the initial stages of Na ions extraction in Na2FeSiO4/C electrode during the first charging process. Moreover, the values of Rct gradually increase with further desodiation. But they decrease sharply when the sloped straight line in low-frequency domain (i.e. Warburg impedance) begin to appear (as shown in Figure 4c), achieving to the minimum (76 Ω) at 3.53 V. Consequently, the kinetics of Na2FeSiO4/C electrode reaction is primarily dominated by the solid-state diffusion of Na+ in the host material. In Figure 5d, the highly potential-dependent values of Csf (10-6 order of magnitude, typical of the capacitance of surface films34) connected with the SEI films show an increasing tendency on the starting desodiation and they reach to the maximum


21


Page 22 of 44

of 4.27 μF at 3.53 V. It is probable that when the Na2FeSiO4/C electrode reaches the transient equilibrium on one level of desodiation state or by-passing channel of the migration of Na ions arises due to the slight fluctuation on the thickness of SEI films, the most intensive ionic fluxes will appear, resulting in the maximum of Csf during the second desodiation process.36 Moreover, the values of Cdl (1.4-1.8 mF, the early stage of desodiation over a narrow range of 1.8-2.3 V) associated with the charge transfer resistance are larger than that of in Figure 5b, and its variation trend relying on the potential accords with the Rct (as shown in Figure 5c). A possible explanation for the feature of Cdl during the second desodiation is that the massive accumulation of electrons on the surface of the active nanoparticles arises and the flow of them are difficult at the beginning stages of desodiation. Subsequently, the Cdl values, just like Rct, also diminish rapidly after the emergence of the sloped straight line in low-frequency region, implying that the rate-controlling step of electrochemical kinetics for Na2FeSiO4/C electrode changes gradually.


22

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(a)

(b)

(c)

(d)

Figure 5. The curves of Ri and Ci vs. voltage during the first and second desodiation process. The values of Rs, Rsf, Rct, Csf and Cdl at different desodiation state are deduced via equivalent circuits. (a) Rs and Rct; (b) Cdl; (c) Rs, Rsf and Rct; (d) Csf and Cdl. As known to all, the diffusion coefficient of Na ions (DNa+) reflects a series of features of Na ions migration in the host material. To date, however, few data of the kinetic parameters are available for the cathode material of Na2FeSiO4. Given the structural rearrangement and phase conversion of C2221-Na2FeSiO4/C during the first cycle, it is necessary to calculate the DNa+ of Na2FeSiO4/C to optimize its electrochemical performance. As mentioned above, the sloped straight line corresponding to the Warburg contribution in low frequency domain denotes the Na+ diffusion in the host material.


23


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Therefore, the DNa+ of Na2FeSiO4/C at different desodiation states can be deduced by utilizing the part of the low frequency region in the impedance spectrum. In the case of the semi-infinite diffusion, the expression of Warburg impedance is determined as follows:42 Zw =  w

1



- j w

1



(1)

=Z' - jZ"

where ω is the radial frequency, j =

1 , and σw is the Warburg coefficient factor. It is

observed from Eq. (1) that the real or imaginary parts (Z′ or –Z″) in impedance spectrum is proportional to ω–1/2, and the slop of line is σw. Thus, the values of σw at various desodiation states can be deduced by linear fitting of the curves of Z′/–Z″ vs. ω–1/2 over a limited time period (in the range of 0-100 s). Figure 6a-b shows the typical curves of Z′/–Z″ vs. ω–1/2 during the first and second desodiation process. Clearly, the results of linear fitting for both of them show a very well parallel feature. Figure 6c shows the plots of σw vs. x in Na2-xFeSiO4/C during the first and second desodiation process. It should be note that the variation trends for σw as a function of x in Na2-xFeSiO4/C are very analogous to that of Rct in Figure 5a and Figure 5c, respectively.


24

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(b)

(a)

(c)

Figure 6. The profiles of real and imaginary parts vs. ω−1/2, and the curves of σw against x in Na2-xFeSiO4/C. (a) At 3.056 V for the first desodiation process; (b) At 3.825 V for the second desodiation process; (c) The profiles of σw vs. x in Na2-xFeSiO4/C at various desoidation stages during the first and second charging process. Besides, based on the Fick's laws and Butler-Volmer equation, the expression of σw can be also expressed as follows:42 dE dx w = zFA DNa 2 Vm

(2)

Thus, the equation of DNa+ can be deduced by Eq. (2) and written as follows:42,43 DNa

1  Vm   dE   =    2  FA w   dx  

2


(3)

25


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where Vm (about 76.84 cm3 mol-1) is the molar volume of Na2FeSiO4/C, which is deduced from the data of crystal (i.e. Vm = NA×Vcell/3, where NA is Avogadro constant and Vcell is the volume of unit cell) and can be assumed to remain invariability approximatively during the desodiation process,44 F is the Faraday constant, A is the active area of Na2FeSiO4/C electrode and dE/dx is the slope of the open-circuit voltage of Na2FeSiO4/C electrode vs. x in Na2-xFeSiO4/C. Thus, the values of DNa+ for Na2FeSiO4/C during the first and second charging process can be calculated from Eq. (2) and Eq. (3). Figure 7 shows the profiles of the diffusion coefficient of Na ions vs. x in Na2-xFeSiO4/C during the first and second desodiation process. In Figure 7a, the values of DNa+(EIS) (values from 3.57 × 10-14 cm2 s-1 to 2.49 × 10-13 cm2 s-1, the first desodiation process) decrease remarkably with the onset of charging, and its change trend is inconsistent with the previous reports on the diffusion coefficient of Li ions in layered metal oxides.41,45–47 The reasons for the changing features may be that the structural rearrangement and phase transformation of Na2FeSiO4/C on the starting desodiation induce extremely high kinetics barriers. The homologous case has also been detected by Rui et al. and Hjelm et al. in other materials of LIBs such as Li3V2(PO4)3 and LiMn2O4.48,49 As the desodiation continues, a relatively stable values of DNa+(EIS) are observed at x = 0.2-0.6 in Na2-xFeSiO4/C, which is probably due to the appearance of the two-phase region (i.e. the solid-solution behaviour), and the phenomenon has been widely verified in the cathode materials of LIBs.39,45 Towards the end of the desodiation process (x = 0.6-0.8 in Na2-xFeSiO4/C), the values of DNa+(EIS) for Na2FeSiO4/C rise


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gradually. The main reason for the appearance is probably that a new phase for Na2FeSiO4 appears at this stage, which is beneficial to Na ions insertion/extraction in the host material. Moreover, the phase conversion of C2221-Na2FeSiO4/C during the first cycle inevitably incurs the changes in the lattice parameters of Na2FeSiO4/C, inducing microcrack formation at the boundaries of nano-particles.50 Therefore, the more double-layer capacitors of the interface between the active nanoparticles and electrolyte will emerge, finally causing the increase in the values of Cdl, as shown in Figure 5b. As for the second desodiation process, as presented in Figure 7b, a valley value of DNa+(EIS), 7.23 × 10-13 cm2 s-1, can be observed at x = 0.2-0.3 in Na2-xFeSiO4/C, which is in accordance with the voltage-plateau region of the charging process. In addition, much higher values of DNa+(EIS) appear before and after the valley value. It could be ascribed to the strong interaction between the Na ions and unite cell of the host material during the two-phase transition reaction process, and a similar phenomenon has also been found in the

other

cathode

materials,

such

as

Li(NixMnyCoz)O2,

Na3V2(PO4)3/C

and

Li2.8(V0.9Ge0.1)2(PO4)3, etc.51−53 Compared with the first desodiation process, the values of DNa+(EIS) for the second desodiation process are larger, implying that the kinetics of Na ions extraction in Na2FeSiO4/C electrode are enhanced after the first cycle. Meanwhile, both of the DNa+(EIS) values at the end of the first and second desodiation process show a declining trend, suggesting that the rate of Na ions diffusion for Na2FeSiO4/C cathode material is largely affected by the “Na-rare” states in the bulk material.


27


(a)

Page 28 of 44

(b)

Figure 7. The profiles of the diffusion coefficient of Na ions vs. x in Na2-xFeSiO4/C during the first and second desodiation process. The DNa+ of Na2FeSiO4/C can be also determined by other electrochemical techniques, for example, potentiostatic intermittent titration technique (PITT), galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV).44,54 GITT based on the chronopotentiometry at quasi thermodynamic equilibrium, which is firstly introduced and applied in LIBs by Weppner, is a dependable electroanalytical technique to calculate the highly resolved data of DNa+ for sodium-based cathode mateial.41,55,56 The typical profile of a single GITT titration at 2.29 V is shown in Figure 8a. When a constant current pulse (Io = 15 µA) for τ = 1.5 h is applied on the Na2FeSiO4/C electrode, the potential of electrode will increase from the quasi-equilibrium state (E0 = 2.29 V) to a new value (Eτ). Then, the current is interrupted and the electrode is left for 4 h under open-circuit voltage (OCV) to reach a new quasi-equilibrium potential (Es), and the change of the quasi-equilibrium potential (ΔEs = Es - Eo) is determined. Figure 8b shows the GITT curves of Na2FeSiO4/C electrode in the first and second desodiation process.


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Obviously, the overpotential of the first desodiation process is larger than the second desodiation process, implying a severe electrode polarization during the initial charging process (This case is in line with the profile of the first charging in Figure 3a). Besides, an increasing overpotential at the end of the second desodiation process indicates a sluggish kinetics of Na ions extraction in Na2FeSiO4/C electrode. The curves of the quasi-equilibrium potential (Es) vs. x in Na2-xFeSiO4/C, as shown in Figure 8c, is very similar to the profile of voltage vs. capacities in Figure 3a, which has a voltage plateau in the range of x = 0.2 ~ 0.4 in Na2-xFeSiO4/C. The profile of differential factor (dE/dx) vs. x in Na2-xFeSiO4/C is presented at Figure 8d. Specifically, the variation of values for dE/dx in the voltage plateau region is relatively little and the values of dE/dx should be zero theoretically. However, no zero is detected during the first and second charging process. It is probably that the incomplete relaxation during the desodiation process or other factors such as the thermodynamics, strain and surface energy of material, etc.57


29


(a)

(b)

(c)

(d)

Page 30 of 44

Figure 8. (a) The profile of voltage vs. time for a single GITT titration; (b) The curves of GITT for Na2FeSiO4/C electrode during the first and second desodiation process; (c) The plots of quasi-equilibrium potential (Es) vs. x in Na2-xFeSiO4/C; (d) The correlation curves of dE/dx vs. x in Na2-xFeSiO4/C. According to the GITT measurement and a series of assumptions (i.e. one-dimensional diffusion mode, semi-infinite diffusion and Fick's second law), the DNa+ of Na2FeSiO4/C can be determined by the following formula:52 2

2

4  V   dE / dx  L2 DNa =  I o m   ， t =   FA   dE / d t  DNa

(4)

Where Vm is the molar volume of Na2FeSiO4/C, Io is the constant current pulse, F is the Faraday constant, A (2.75 m2 g-1) is the interfacial area of electrode-electrolyte, which is


30

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generally replaced by the specific area of electrode and acquired from BET method, dE/dx is the instantaneous slope of the titration profile, L is the thickness of the electrode and dE/dt1/2 is the slop of E vs. t1/2 plot. Note that the variation of the quasi-equilibrium potential (Es) against t1/2 (Figure 9a-b) during a short period of current pulse (10-100 s) shows a well linear behaviour, representing the diffusion process of Na+ in Na2FeSiO4/C. Figure 9c shows the profile of dE/dt1/2 vs. x in Na2-xFeSiO4/C. Because of the same current pulse for each procedure during the GITT titration process, the values of dE/dt1/2 depending on the composition of x in Na2-xFeSiO4/C reveal the variation of impedance for Na2FeSiO4/C electrode to some extent.57

(b)

(a)

(c)

Figure 9. (a) Linear relationship of E vs. t1/2 during the first desodiation process; (b) Linear relationship of E vs. t1/2 during the secong desodiation process; (c) The profile of


31


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dE/dt1/2 vs. x in Na2-xFeSiO4/C. On the base of the values of dE/dx and dE/dt1/2, the DNa+ of Na2FeSiO4/C can be calculated by Eq. (4) and the profiles of DNa+(GITT) vs. x in Na2-xFeSiO4/C are shown in Figure 10. As is seen, the variation trends of DNa+(GITT) during the first and second desodiation process is in line with the DNa+(EIS). For the first desodiation process, however, the values of DNa+(EIS) ranging from 2.60 × 10-14 to 2.49 × 10-13 cm2 s-1 are larger by one or two order of magnitude than DNa+(GITT) ranging from 1.22 × 10-15 to 3.71 × 10-14 cm2 s-1. Similar differences can be also observed during the second desodiation process. Compared to the EIS technique, one prominent advantage of GITT is that it can minimize the impact of the disturbance of polarization, eliminating the resistance polarizations (for example, ohmic resistance) readily.58 Furthermore, the technique of GITT identifies a more precise value on τ (diffusion time constant). Thus, the values of DNa+(GITT) are more precise than that of DNa+(EIS), and the result had been proved by Wen et al. and Levi et. al. 54,59 Moreover, some evidences can be also found in the open literatures. For example, the diffusion coefficients of Li[Ni1/3Co1/3Mn1/3]O2 obtained from GITT were compared with EIS by Shaju et al., and the results presented that GITT provided more accurate diffusion coefficients than EIS.52 Chowdari and co-workers calculated the diffusion coefficient of Li(M1/6Mn11/6)O4 (M = Mn, Co, CoAl) via GITT and EIS, showing that the diffusion coefficient determined by GITT is more reliable.60 Therefore, the diffusion coefficient of Na ions in Na2FeSiO4/C acquired from GITT is more precise than that of EIS.


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(a)

(b)

Figure 10. The curves of diffusion coefficient of Na ions vs. x in Na2-xFeSiO4/C during the first and second desodiation process.

4. CONCLUSIONS The electrochemical kinetics of Na ions extraction in Na2FeSiO4/C electrode during the first and second desodiation process has been firstly study in detail by EIS and GITT techniques. The discrepancies of EIS response between the initial and second desodiation process are mainly attributed to the differences of electrode reaction process. The kinetics parameters deduced from the equivalent circuits will change with the different level of the desodiation in Na2FeSiO4/C. On the starting desodiation, the sluggish kinetics of Na2FeSiO4/C electrode is primarily ascribed to the slow rate of charge transfer among the electrolyte-electrode. When the desodiation continues during the first charging process, the values of Rct decrease sharply, reaching to the minimum at 3.38 V, and the kinetics of Na2FeSiO4/C electrode turns to be determined by the Na+ diffusion in Na2FeSiO4/C.


33


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However, the values of Rct increase gradually after starting desodiation during the second process, reaching the maximum at the region of voltage plateaus, and the electrode kinetics of Na2FeSiO4/C is mainly controlled by the charge-transfer process. After the first cycle, the kinetics of Na ions extraction in Na2FeSiO4/C electrode is enhanced because of the rearrangement and phase transformation in the crystal structure of Na2FeSiO4/C, and the diffusion coefficient of Na ions (10-13–10-12 order of magnitude) calculated from EIS and GITT for the second desodiation process is larger by one or two order magnitude than the first desodiation process (10-14–10-13 order of magnitude). Although a promoted diffusion of Na+ in Na2FeSiO4/C can be observed after the initial charging process, the rate capability and cycling properties of Na2FeSiO4/C have not been significantly improved. Further studies are needed to overcome these drawbacks before the large-scale application of Na2FeSiO4/C, and we hope that this work can serve as a call for more endeavours into the development of Na2FeSiO4 with excellent electrochemical properties. ASSOCIATED CONTENT Supporting Information Comparison of XRD pattern between pristine Na2FeSiO4 and Na2FeSiO4/C; The relation curves of the frequency with impedance; The values of Chi-Square at various state of deisodiation. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] (X. Wang) *E-mail: [email protected] (Y. Wang) Phone: +86 731 58293377 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was carried out under the financial support from the National Natural Science Foundation of China (No. 51472211), the Natural Science Foundation of Hunan Province (No. 2016JJ2126). REFERENCES (1) Zhang, X.; Zhang, Z.; Yao, S.; Chen, A.; Zhao, X.; Zhou, Z. An Effective Method to Screen Sodium-Based Layered Materials for Sodium Ion Batteries. npj Comput. Mater. 2018, 13, 1–6. (2) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636–11682.


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