Enhanced Cycling Stability and Rate Capability in a La-Doped Na3V2

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Enhanced Cycling Stability and Rate Capability in La-doped Na3V2(PO4)3/C Cathode for High-Performance Sodium Ion Batteries Linnan Bi, Xiaoyan Li, Xiaoqin Liu, Qiaoji Zheng, and Dunmin Lin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06385 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Sustainable Chemistry & Engineering

Enhanced Cycling Stability and Rate Capability in La-doped Na3V2(PO4)3/C Cathode for High-Performance Sodium Ion Batteries

Linnan Bi, Xiaoyan Li, Xiaoqin Liu, Qiaoji Zheng, Dunmin Lin

College of Chemistry and Materials Science, Sichuan Normal University, No. 5 Jingan Road, Jinjiang District, Chengdu 610066, China

ABSTRACT In recent years, phosphate Na3V2(PO4)3 (NVP) has attracted considerable attention as a promising cathode for high-performance sodium ion batteries owing to its open 3D framework structure. However, low rate capacity of the material greatly hampers the practical application due to its poor conductivity. Herein, La-doped Na3V2-xLax(PO4)3/C materials were prepared by a combination process of sol-gel and carbon-thermal reduction methods. All materials possess NASICON-type structure with the R3C space group and no impurity can be detected, and a thin carbon layer is coated on the surface of the materials. The doping of La3+ significantly reduces internal resistance and enhances fast Na+ mobility. As a result, the material with the addition of 2 % La3+ exhibits excellent cyclic performance and rate capability: a high initial reversible capacity of 105.4 mA h g-1 at 0.2 C, capacity retentions of 96.5 % at 1 C after 200 cycles

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Corresponding author: Email:[email protected] (Dunmin Lin); Fax: +86 28 84760802 Tel: +86 28 84760802

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and excellent rate performance of 92.6 mA h g-1 at 30 C. Under at high current density of 20 C for 3000 cycles, it can still deliver a superior reversible capacity of 73.5 mA h g-1 with a high capacity retention of 93.5 %. Even at 50 C, the Na3V1.98La0.02(PO4)3/C electrode can release a satisfactory initial capacity of 79.9 mA h g-1 with an average coulombic efficiency of 99.3 % after 8000 cycles. Our work demonstrates that La3+doped Na3V2(PO4)3/C may be a promising candidate of cathode for high performance sodium ion batteries.

KEYWORDS: Na3V2(PO4)3; Sodium ion batteries; La3+ doping; Electrochemical properties

INTRODUCTION Electrochemical energy storage systems have developed rapidly over the past decades.1 Among various storage devices, secondary batteries have evoked enormous significant attention in large-scale energy storage.2,3 It has been proved that the application of lithium ion batteries (LIBs) in electrics vehicles (EVs) and electronic products is very successful due to their high power/energy density.4 However, with the widespread application of LIBs, lithium resources become extremely scarce and expensive, which makes it difficult to meet market demand.5 Therefore, it is urgent to explore a low-cost, high-power density and long lifetime cathode based on abundant resources as an alternative to LIBs.6 In recent years, sodium ion batteries (SIBs) have drawn more interests due to wide 2 ACS Paragon Plus Environment

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availability and low cost of Na resource.7,8 Many efforts have been devoted to investigating active materials for SIBs, especially cathodes. For example, layered transition metal (TM) oxides (e.g. NaNi1/3Mn1/3Co1/3O2,9 P3-NaxCoO210), tunnel-type oxides (e.g. Na0.44MnO2,11 α-NaMnO212), phosphates and derivatives (e.g. NaFePO4,13 Na3.32Fe2.34 (P2O7)2,14 Na3V2(PO4)2F315) have been extensively studied as promising candidates of SIBs cathode materials. Among various cathode materials, Na+ superionic conductor (NASICON) of Na3V2(PO4) (NVP) has attracted much attention for the cathode of SIBs owing to its open 3D framework structure, which is beneficial to quick reversible inversion/extraction of Na+.16,17 The NVP possesses NASICON-type structure with the R3C space group and is composed of repeated V2(PO3)4 units framework, in which octahedral VO6 shares all its corners with PO4 tetrahedra and Na occupies two different positions (M1 and M2) in the lattices. Therefore, based on the extraction of three Na+ ions from the NVP lattices, the theoretical capacity of NVP can achieve a high value of 176 mA h g-1. However, the extraction of the third Na+ is kinetically difficult owing to the significantly low electronic conductivity of the endmember of V2(PO4)3 in each space unit and low Na-occupancy energy of the M1 sites.18,19 It has been demonstrated that NVP possesses a highly stable operating voltage at ~3.4V by the V4+/V3+ redox couples, impressive theoretical energy density (400 Wh/kg) as well as good thermal stability.18,20 However, low rate capacity of the material greatly hampers the practical application due to its intrinsically poor electrical conductivity, especially.21 Among various strategies for improving the electrochemical properties for NVP materials, carbon coating and metallic ion doping are the two most 3 ACS Paragon Plus Environment


used methods. Carbon coating can effectively improve the surface electron conductivity of NVP-based cathode materials, while the doping of metallic ions may enhance bulk phase intrinsic electron conductivity of the materials.22-24 It has been reported that the Na-ion channels in the Na3V2(PO4)3 can be adjusted by replacing Na-ions with pillar ions such as K+ and Li+.25-27 In addition, other metallic ions with similar ion radius to V3+ have also been widely reported as effective dopants to improve the electrochemical properties of Na3V2-xMx(PO4)3 materials (M = Al3+,28 Mg2+,29 Y3+,30 Ce3+,31 Mo6+,24 etc.) through doping. As well know, rare earth element lanthanum (La) has many interesting features, such as large radium, high self-polarization ability and high electric charge, which are favor of energy storage. It has been reported that the incorporated ions that are acted as pillars in the lattice could effectively improve the cycle stability by preventing the lattice from collapsing and suppressing undesirable phase transformations. For instance, the transition metal based lanthanide-silicates, like M2OLn2O3-2SiO2 (M = Li, Na, K, Rb, Cs; Ln = La, Nd, Sm, Cd, Dy, Y, Er, Yb) in which the anions occupy the interconnected LnO6 octahedra and SiO4 tetrahedra sites to form a 3D network, may lead to an easy ion passage and thereby enhances ion conducting properties.32-34 Moreover, La3+ is inactive within voltage range 2-4 V, which can not only play the role of a pillar in the NVP lattice, but also can buffer the deformation of NVP framework during charging and discharging process, thus enhancing the structure stability.31 However, to the best of our knowledge, La-doped NVP materials is hardly reported. Herein, Na3V2-xLax(PO4)3/C (NVP/C-La(x)) with NASICON-type structure have been synthesized by simultaneous modifications of La substitution for V and 4 ACS Paragon Plus Environment

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carbon coating via a facile process of sol-gel and carbon-thermal reduction. The structure, micromorphology and electrochemical properties of the NVP/C-La(x) were investigated in detail. Our results show that the addition of 2 % La3+ can effectively improve the rate capability (92.6 mA h g-1 at 30 C), long-term cycling life performance (48.5 mA h g-1 at 50 C after 8000 cycles) for NVP/C-La(0.02) cathode and the corresponding mechanism has been proposed.

EXPERIMENTAL SECTION Materials Synthesis. NVP/C-La(x) materials were synthesized by a combination process of sol-gel and carbon-thermal reduction methods. The analytical reagents of NaHCO3, NH4VO3, La(NO3)3·6H2O, and NH4H2PO4 were utilized as raw materials in a molar ratio of 3:(2-x):x:3. Citric acid not only was used as the reducing regent but also acted as the source of carbon to coat on the surface of materials. In a typical synthesis, citric acid, NH4VO3 and La(NO3)3·6H2O were firstly dissolved in deionized water with violent stirring at 70 oC for 30 min to form a light blue solution. After that, the stoichiometric amount of NH4H2PO4 and NaHCO3 were added, and then continuously stirred at 70 oC for several hours to form a viscous dark green gel. After this, the gel was dried overnight at 120 oC to form a light green foam with rich porous precursor in a vacuum oven. Finally, the ground precursor was preheated to expel H2O and NH3 at 350 oC for 4 h, followed by annealing at 800 oC for 6 h in Ar-H2 (v/v, 95:5) atmosphere with a heating rate of 5 oC·min-1. The as-prepared samples were recorded as NVP/CLa(x). 5 ACS Paragon Plus Environment


Material Characterization. The crystalline structure of NVP/C-La(x) were examined by X-ray diffraction (XRD) with a Cu-Kα (λ=1.540598 Å, Smart Lab) source in the range (2θ) from 10-60°. Rietveld refinements were carried out using the GSAS programs with EXPGUI interface.24,35 The morphologies of the as-obtained materials were observed by a field-emission scanning electron microscope (SEM, FEI-Quanta 250). The transmission electron microscopy (TEM) images and the selected area electron diffraction (SEAD) pattern were recorded using a transmission electron microscope (TEM, JEOL JEM-2100HR). The element distribution can be confirmed by EDS mapping. The amounts of the La3+ were determined by inductively coupled plasma (Agilent ICPOES 720). Thermal gravimetric (TG) analysis was performed between room temperature to 600 oC under air atmosphere flow with a heating rate of 10 oC·min−1 by SDT Q600 apparatus. The surface composition and state of the NVP/CLa(x) was probed through X-ray photoelectron spectroscopy system (XPS, Thermo ESCALAB 250XI). The different properties of carbon for these obtained samples were performed by Raman spectroscopy. Electrochemical Performance Measurements. Electrochemical measurements were carried out based on the standard CR2032 half-cells, which were assembled in a glovebox filled with high-purity Ar. The working electrodes were prepared by blending 70 wt% of NVP/C-La(x), 20 wt% of acetylene black and 10 wt% of the polyvinylidene difluoride (PVDF) binder in an N-methyl-pyrrolidone (NMP) solvent and then coating the mixed slurry onto aluminous foil. The prepared electrode film was dried at 120 oC in a vacuum oven overnight, and then cut into wafers with a diameter of 14 mm. The 6 ACS Paragon Plus Environment

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loading of cathode active mass was ~1.1 mg·cm-2. The solution of 1 M NaClO4 dissolved in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 by vol.) with 5 wt% annexing agent FEC (Fluoroethylene Carbonate) was used as the electrolyte. A selfmade sodium metal foils were selected as the counter electrode and a glass fiber (GF/A, Whatman, China) was used as the separator. Galvanostatic charge/discharge tests were performed at room temperature (25 oC) and conducted an automatic battery test system (LAND, CT2100A, Wuhan, China) in the potential range of 2.0-4.0 V (vs. Na/Na+). To further investigate the electrochemical behavior, the cyclic voltammetry (CV) tests were conducted in the potential rang of 2.4-3.8 V (vs. Na/Na+) at a scanning rate of 0.1 mv·s-1 on an electrochemical working station (CHI660E, Shanghai, China). The electrochemical impedance spectrums (EIS) were recorded at the frequency range of 0.01 to 105 Hz and fitted using Z-View program.

RESULTS AND DISCUSSION Structure Characterization. The crystal schematic illustration of the NVP with R3C space group at different axis and graphical representations of La3+ doping in the NVP crystal lattices are shown in Figure 1. The three-dimensional (3D) NASICON framework structure and constituents of NVP are described well. It can be seen that a basic unit of V2(PO4)3 is built by [VO6] octahedral and [PO4] tetrahedral interlinked via corner-sharing. Specially, each [VO6] unit can connect with six [PO4] units, and a large interstitial space is generated, which is in favor of accommodating sodium ions. Owing to the difference in bond length data for Na-O, there are two types of sodium ions.18 7 ACS Paragon Plus Environment


The Na2 at the M2 sites are more easily extracted than those at M1 sites due to the relatively weaker bonding. When the V3+ (~64 pm) sites are replaced in the NVP lattices by larger La3+ (~103.2 pm), lattice expansion in the structure would be created, which favors Na+ transportation. The XRD patterns of the NVP/C-La(x) are shown in Figure 2a. All NVP/C-La(x) samples show a series of similar sharp diffraction peaks, which can be indexed to NASICON-type structure (JCPDS No. 53-0018) with rhombohedral symmetry with R3 C space group. It can be seen that there is no obvious change in the crystal structure of the NVP materials after La3+ doping. Figure 2b and Figure S1 display the Rietveld refinements of the NVP/C-La(x) on the basis of the powder XRD data, while the detailed lattice parameters and refined factors are listed in Table S1. It can be seen that there is obvious change in the crystal structure of the NVP materials after La3+ doping. For all refinements, the final factors of Rwp, Rp and χ2 are not larger than 7.65%, 5.95% and 1.93 for all samples, respectively, suggesting that the observed and calculated patterns are well matched. In addition, the contents of La3+ for the materials with x = 0.01, 0.02 and 0.03 are 0.34 %, 0.49%, 0.80 %, respectively, which are close to the stoichiometric ratios of the materials. With the increasing of the doping content of La3+, an obvious expansion of the cell volume can be found, confirming that La3+ are successfully introduced into the crystal lattices. Aforementioned expanded cell size of NVP/C-La(x) means larger transportation channels for Na+ which is beneficial to improving rate performance. The Raman spectra of the NVP/C-La(x) materials are shown in Figure 2c. There are 8 ACS Paragon Plus Environment

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three characteristic bands for all materials in the range of 500 cm-1 to 3500 cm-1. The peaks located around 1367, 1598 and 2760 cm-1 are attributed to D-bond (sp3-typed), G-bond (sp2-typed) and D + G modes of carbon, respectively.36 The D-band is assigned to the disorder carbon including edge defects, dangling bonds, vacancies and so on, while the G-band is associated with graphitic ordered carbon.37 The ratios of ID/IG are 0.894, 0.935, 0.955, 0.948 for NVP/C-La(0), NVP/C-La(0.01), NVP/C-La(0.02), NVP/C-La(0.03), respectively. It can be seen that the material with x= 0.02 possesses the largest ID/IG, suggesting the highest degree of graphitization for carbon layer, which is conductive to the electron transfer. To confirm the carbon content of the materials, the thermal gravimetric analysis (TGA) curves of NVP/C-La(x) composites are collected and shown in Figure 2d. For the NVP/C-La(0), NVP/C-La(0.01), NVP/CLa(0.02) and NVP/C-La(0.03) materials, carbon contents are 4.11 wt%, 4.47 wt%, 3.59 wt% and 4.23 wt%, respectively. It is noteworthy that the TGA curves of the NVP/CLa(x) composites exhibit a weak rise0 at ~570 oC, which is due to the oxidation of V3+ to V4+ and V5+.19 To further investigate the surface chemical compositions and bonds of NVP/CLa(0.02), X-ray photoelectron spectroscopy (XPS) was collected. The survey spectrum in Figure 3a illustrates the presence of Na, V, P, O and C, which are consistent with the elemental mapping results in Figure 4(g, h) and Figure S2(b, c, d).38 From the highresolution spectrum of C 1s in Figure 3b, the peak with the binding energy of 284.1 eV can be ascribed to the C-C bond, while the other peaks at 285.4 and 286.6 eV are attributed to the oxygen-containing functional groups of C-O and C=O, respectively.39 The high-resolution spectrum of O 1s is fitted into two peaks located at 530.3 eV and 531.9 eV (Figure 3c), which are ascribed to the O-P and O=C of NVP.40 The high9 ACS Paragon Plus Environment


resolution spectrum of V 2p in Figure 3d shows two characteristic peaks at 516.3 eV and 523.74 eV that correspond to V 2p3/2 and V 2p1/2, indicating the presence of V3+ species in the NVP/C-La(0.02) samples.41 It should be noted that other states of V 2p are not detected, demonstrating the successful synthesis of the pure NVP/C-La(x). Morphology Characterization. The detailed morphologies and microstructures of the NVP/C-La(x) materials were characterized by SEM and TEM. From Figure 4a-d, the addition of La3+ leads to significant structure evolution. The NVP/C-La(0) consists of inhomogeneous micron-sized bulk layers with an average sizes of 2-3 μm. As well known, larger size particles enlarge the diffusion path of Na+ during the insertion/extraction reactions and thus degrade electrochemical properties of the materials. Interestingly, with the increasing of the doping level of La3+, the grain size of the NVP/C-La(x) significantly decreases. The porous particles are constituted by several small bulk layers, which is conductive to the contact of active material with electrolyte and thus enhancing the Na+ migration rate. The TEM and high-resolution TEM (HRTEM) images are shown in Figure 4e, f. The HRTEM image and SEAD pattern of the material with x = 0.02 confirm good crystal characteristic. From Figure 4f, the clear lattice fingers (d=0.31nm) can be seen, corresponding to the (024) planes of NVP in the XRD spectrum (Figure 2a). Furthermore, thin amorphous carbon layers with the thickness of ~3.16 nm cover the surface of NVP/C-La(0.02) particles, forming a typical core-shell structure of carbon coating. The core-shell structure is not only beneficial for quickly transporting of electrons but also retards the volume change during the charge/discharge process.21 In addition, thin carbon coating layer may inhibit the grain growth of NVP/C-La(0.02) during sintering. The elemental mapping images (Figure 4g-i and Figure S2) reveal that the elements of sodium (Na), vanadium (V), lanthanum (La), phosphorus (P), oxygen (O) and carbon (C) are uniformly distributed 10 ACS Paragon Plus Environment

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in the NVP/C-La(0.02) material. Electrochemical Performance. The electrochemical performances of the NVP/C-La(x) cathodes are exhibited in Figure 5. The initial charge/discharge profiles of the NVP/CLa(x) cathodes in the voltage window of 2 - 4 V (vs. Na/Na+) at 0.2 C (1 C=117 mA g-1) are shown in Figure 5a. A pair of apparent voltage platforms are found, corresponding charge/discharge plateaus at ~3.4 V, which are attributed to the V3+/V4+ oxidation and reduction. The NVP/C-La(0.02) electrode delivers the highest reversible capacity of 105.4 mA h g-1 at 0.2 C, in comparison with NVP/C-La(0.03), (102.1 mA h g-1), NVP/C-La(0.01) (102.9 mA h g-1) and NVP/C-La(0) (75.1 mA h g-1). Obviously, the discharge capacity is effectively improved after the introduction of La3+. In addition, all La3+ doped samples show smaller voltage polarization than the un-doped sample, indicating enhanced electrochemical performance. This suggests that La3+ doped NVP/C-La(x) materials are more stable during the charge/discharge process. The cycling performance of all electrodes and galvanostatic charge/discharge curves of NVP/C-La(0.02) cathode at 1 C are demonstrated in Figure 5b and Figure S3, respectively. After activated at 0.2 C, the NVP/C-La(0.02) cathode delivers the reversible capacity of 104.5 mA h g-1 at 1 C. After 200 cycles, the NVP/C-La(0.02) cathode still displays a high capacity of 100.8 mA h g-1 , giving a high retention of ~ 96.5 %. However, the reversible capacities of NVP/C-La(0), NVP/C-La(0.01) and NVP/C-La(0.03) cathodes are only 63.8, 75.2 and 84.3 mA h g-1 after 200 cycles, respectively, which suggest the excellent cycling performance of NVP/C-La(0.02) at small current density of 1 C. Figure 5c displays the rate performance of the NVP/C11 ACS Paragon Plus Environment


La(x) electrodes. Firstly, the NVP/C-La(0.02) electrode is activated at the low current density of 0.1 C, delivering the discharge capacity of 107.8 mA h g-1, corresponding to an electrode activation process. Subsequently, with the increase of current density, the capacities appear a significant increase before 25 cycles compared with those at 0.1 C. From 0.2 to 2 C, the significantly enhanced discharge capacity of the NVP/C-La(0.02) cathode almost remains constant (~110.3 mA h g-1). Moreover, the capacity is slightly decayed to 103.3 and 98.1 mA h g-1 at 10 C and 20 C, respectively. Even at ultrahigh rate of 30 C, the reversible capacity still maintains at 92.6 mA h g-1, corresponding that average coulombic efficiency is 99.6 % and the capacity is quickly utilized within 95 seconds. However, the other cathodes exhibit the inferior rate performance, only possessing 51.3, 72.1 and 75.7 mA h g-1 for NVP/C-La(0), NVP/C-La(0.01) and NVP/C-La(0.03) at 30 C. When the current density is reset back to 0.1 C, the specific capacity of the NVP/C-La(0.02) electrode recovers to 105.3 mA hg-1, which is much higher than those of the NVP/C-La(0), NVP/C-La(0.01) and NVP/C-La(0.03) cathodes. The correlation among the energy density, various C rates (from 0.1 C to 30 C) and the La3+ doping amount shown in Figure 5d. It can be clearly observed that the NVP/CLa(0.02) cathode exhibits the excellent energy density of ~352 Wh /kg at 0.1 C, higher than that of the NVP/C-La(0.03) (328 Wh /kg), NVP/C-La(0.01) (335 Wh /kg) and NVP/C-La(0) (251 Wh /kg) electrodes. Even at high current density of 30 C, the NVP/C-La(0.02) cathode still can hold the energy density of 279 Wh /kg, but the NVP/C-La(0.03), NVP/C-La(0.01) and NVP/C-La(0) electrodes exhibit the degraded values of 221, 224 and 150 Wh /kg, respectively. All of these results indicate that La3+ 12 ACS Paragon Plus Environment

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doping can effectively improve the energy density of the NVP/C-La(x) electrodes, and the NVP/C-La(0.02) cathode exhibits excellent rate capability. In addition, a series of long-term cycles for La3+ doped NVP cathodes were tested at 20 C for 3000 cycles. As shown in the Figure 6a and Figure S6, the NVP/C-La(0.02) cathode delivers an excellent capacity of 73.5 mAh g-1 after 3000 cycles at 20 C with a high capacity retention of 93.5 %. However, the NVP/C-La(0.01) and NVP/C-La(0.03) only hold the inferior capacities of 54.4 and 61.4 mAh g-1 after 3000 cycles, respectively. Besides, a ultra-long cycling measurement of the NVP/C-La(0.02) cathode at 50 C is also performed to further value its advantages and the result is in Figure 6b. The NVP/CLa(0.02) cathode achieves a satisfying reversible capacity of 79.9 mA h g-1 at 50 C, and delivers the capacity of 67.7 mA h g-1 with a capacity retention of 77.3 % of the original capacity after 5000 cycles. Even after 8000 cycles, the NVP/C-La(0.02) electrode can still release the capacity of 48.5 mA h g-1, indicating the outstanding cycling property. However, as shown in Figure S5, the NVP/C-La(0) electrode only displays a capacity of 82.1 mA h g-1 at 5 C and exhibits a relatively poor cycling performance (77.8 mA h g-1 at 5 C after 300 cycles). Therefore, It may be concluded that the NVP/C-La(0.02) electrode may be a promising cathode candidate for sodium ion batteries due to its excellent cycling performance and rate capacity. Figure 6a exhibits the CV profiles of NVP/C-La(0.00) and NVP/C-La(0.02) with a scan rate of 0.1 mV s-1 in the potential region of 2.4-3.8V (vs Na/Na+). It can be found that the NVP/C-La(0.02) cathode shows the relatively sharp and symmetric redox peaks, which reveling the excellent reversibity and very low electrochemical polarization 13 ACS Paragon Plus Environment


during electron transfer and Na+ insertion/extraction reaction.42 On the other hand, the potential interval between the cathodic and anodic peaks of NVP/C-La(0.02) and NVP/C-La(0.00) are 270 mV and 300 mV, respectively. The small value of the NVP/CLa(0.02) cathode can attributed to the much lower electrochemical polarization than NVP/C-La(0) electrode. Furthermore, the peak current of the NVP/C-La(0.02) is obviously higher than NVP/C-La(0.00) cathode, demonstrating the faster reaction kinetics induced by the optimized ion/electron conduction.43 Electronic impedance spectra (EIS) can also affirm the improved electrochemical behaviors of La3+-doped material, as illustrated in Figure 6b. Nyquist plots of the NVP/C-La(0.00) and NVP/CLa(0.02) were performed at the frequency range of 0.01 to 105 Hz after 2 charge/discharge tests. The EIS spectra consist of a depressed semicircle at high frequency region, which is ascribed to charge transfer resistance (Rct) and double-layer capacitance at interface of electrode-electrolyte (SEI), and an oblique line at low frequency range, corresponding to the diffusion of Na+ (Warburg impedance).44 Specially, an almost invisible small intercept on the axis in high frequency region represents the compound resistance (Re) of electrolyte, separator and electrode. Quantitatively, based on the fitted equivalent circuit (Figure 6c), the Rct value for the NVP/C-La(0.02) is simulated to be ~ 215Ω, while the NVP/C-La(0.00) is much higher (~ 461 Ω ), suggesting a fast charge transfer kinetics for sodium ion intercalation in La3+-doped NVP/C-La(0.02) cathode. Generally, the diffusion coefficient of Na+ (𝐷𝑁𝑎 + ) can be calculated according to the sloping line at low-frequency region using the following two equations:17 14 ACS Paragon Plus Environment

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0.5𝑅2𝑇2

𝐷𝑁𝑎 + = 𝑆2𝑛4𝐹4𝑐2𝜎2

(1)

𝑍𝑟𝑒 = 𝑅𝑒 + 𝑅𝑐𝑡 +𝜎𝜔1/2

(2)

Where R is the gas constant, T is the absolute temperature, S is the surface area of the active cathode, n is the number of electrons per molecule during oxidation-reduction process, F is the Faraday constant, C is the concentration of sodium ions and σ is the Warburg factor that is related to Zre.43 Figure 6d displays linear fits of Zre vs ω-1/2 curves for the NVP/C-La(0) and NVP/C-La(0.02) cathodes. As presented in Table. S2, the 𝐷𝑁𝑎 + of the NVP/C-La(0) and NVP/C-La(0.02) are 2.09×10-13 and 6.66×10-13, respectively. Obviously, the 𝐷𝑁𝑎 + of NVP/C-La(0.02) is much higher than that of the material with x = 0. It is mainly due to the fact that La3+-doped NVP/C-La(0.02) cathode is easy to contact with electrolyte and shortens the diffusion path of Na+.45 In addition, the obtained 𝐷𝑁𝑎 + value for the NVP/C-La(0.02) is also much higher than that of olivine NaFePO4 (8.63×10−17),46 indicating that La3+-doped NVP/C-La(0.02) could be a promising cathodic material for SIBs.

CONCLUSIONS To summarize, a series of La3+ doped Na3V2-xLax(PO4)3/C (x=0, 0.01, 0.02, 0.03) cathode materials were prepared by a combination process of sol-gel and carbonthermal reduction methods. It has been found that the doping of La3+ significantly reduces internal resistance and enhances fast Na+ ion mobility, resulting in a noticeable improvement of electrochemical performance. The material with the addition of 2 % La3+ exhibits excellent cyclic performance and rate capability: a high initial reversible 15 ACS Paragon Plus Environment


capacity of 105.4 mA h g-1 at 0.2 C, capacity retentions of 96.5 % at 1 C for 200 cycles and excellent rate performance of 92.6 mA h g-1 at 30 C. Under at high current density of 20 C for 3000 cycles, it can still deliver a superior reversible capacity of 73.5 mA h g-1 with a high capacity retention of 93.5 %. Even at 50 C, the Na3V1.98La0.02(PO4)3/C can release a satisfactory initial capacity of 79.9 mA h g-1 with an average coulombic efficiency of 99.3 % after 8000 cycles. Our work demonstrates that La3+-doped Na3V2(PO4)/C may be a promising candidate of cathode for high performance sodium ion batteries. Supporting Information The Rietveld-refined XRD patterns of NVP/C-La(x) and elemental mapping of NVP/C-La (0.02); charge/ discharge curves of NVP/C-La(0.02) at 1 C for 200 cycles and Charge/ discharge curves of NVP/C-La(x) at different rate; Cycle performance of NVP/C-La(0) sample for 300 cycles at 5 C; Cycle performance of NVP/C-La(0.01) and NVP/C-La(0.03) cathodes for 3000 cycles at 20 C; Unit cell parameters of NVP/C-La(x), EIS and Crystallographic data.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by Sichuan Science and Technology Program (2018JY0447).

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Figure captions Figure 1. Graphical representations of NVP crystal structure viewed at different axis. Figure 2. a) XRD patterns for NVP/C-La(x) samples; b) Rietveld Refinement pattern of NVP/C-La(0.02); c) Raman spectrum and d) TGA curves of NVP/C-La(x) samples. Figure 3. XPS spectrum of NVP/C-La(0.02) sample. a) Survey XPS spectrum; b) C 1s; c) O 1s and d) V 2p of high-resolution XPS spectra. Figure 4. SEM image for NVP/C-La(x) samples with different La contents. a) x=0; b) x=0.01; c) x=0.02; d) x=0.03 and e) TEM images of NVP/C-La(0.02); (f) HRTEM image of NVP/CLa(0.02) and corresponding elemental mapping of NVP/C-La(0.02); g) sodium (Na); h) vanadium (V) and i) lanthanum (La). Figure 5. a) Initial charge/discharge curves of the NVP/C-La(x) at 0.2 C; b) Cycling performance of the NVP/C-La(x) at 1 C for 200 cycles; c) rate capability of the NVP/C-La(x) at different C rates; d) Relation of energy density, C rate and doping amount of La3+. Figure 6. a) Long cycles of the NVP/C-La(0.02) electrode for 3000 cycles at 20 C; b) Ultralong cycles of the NVP/C-La(0.02) electrode for 8000 cycles at 50 C. Figure 7. a) CV plots of the NVP/C-La(0) and NVP/C-La(0.02) at the scanning of 0.1 mV s−1; b) Nyquist plots of the NVP/C-La(0) and NVP/C-La(0.02) electrodes with an equivalent circuit shown in c); d) linear fittings of Zreal vs. ω−1/2 curves of the NVP/C-La(0) and NVP/CLa(0.02).



Figure 1. Graphical representations of NVP crystal structure viewed at different axis.

Figure 2. a) XRD patterns for NVP/C-La(x) samples; b) Rietveld Refinement pattern of NVP/CLa(0.02); c) Raman spectrum and d) TGA curves of NVP/C-La(x) samples.

Figure 3. XPS spectrum of NVP/C-La(0.02) sample; a) Survey XPS spectrum; b) C 1s; c) O 1s, and d) V 2p of high-resolution XPS spectra.


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Figure 4. SEM image for NVP/C-La(x) samples with different La contents. a) x=0; b) x=0.01; c) x=0.02; d) x=0.03 and e) TEM images of NVP/C-La(0.02); (f) HRTEM image of NVP/C-La(0.02) and corresponding elemental mapping of NVP/C-La(0.02); g) sodium (Na); h) vanadium (V) and i) lanthanum (La).

Figure 5. a) Initial charge/discharge curves of the NVP/C-La(x) at 0.2 C; b) Cycling performance of the NVP/C-La(x) at 1 C for 200 cycles; c) rate capability of the NVP/C-La(x) at different C rates; d)Relation of energy density, C rate and doping amount of La3+.

Figure 6. a) Long cycles of the NVP/C-La(0.02) electrode for 3000 cycles at 20 C; b) Ultra-long cycles of the NVP/C-La(0.02) electrode for 8000 cycles at 50 C.



Figure 7. a) CV plots of the NVP/C-La(0) and NVP/C-La(0.02) at the scanning of 0.1 mV s−1; b) Nyquist plots of the NVP/C-La(0) and NVP/C-La(0.02) electrodes with an equivalent circuit shown in c); d) linear fittings of Zreal vs. ω−1/2 curves of the NVP/C-La(0) and NVP/C-La(0.02).


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For Table of Contents Use Only

La3+-doped Na3V2-xLax(PO4)3/C cathode materials were synthesized and exhibited superior electrochemical performance, which contributes to the sustainability of secondary battery for storing energy.


Enhanced Cycling Stability and Rate Capability in a La-Doped Na3V2

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