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A core/double-shell structured Na3V2(PO4)2F3@C nano-composite as the high power and long lifespan cathode for sodium-ion batteries Qiang Liu, Xing Meng, Zhixuan Wei, Dongxue Wang, Yu Gao, Yingjin Wei, Fei Du, and Gang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11372 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 4, 2016

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ACS Applied Materials & Interfaces

A core/double-shell structured Na3V2(PO4)2F3@C nano-composite as the high power and long lifespan cathode for sodium-ion batteries

Qiang Liu†, Xing Meng†, Zhixuan Wei†, Dongxue Wang†, Yu Gao†, Yingjin Wei†, Fei Du*†, and Gang Chen†‡

†

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of

Education), College of Physics, Jilin University, Changchun, 130012, People’s Republic of China ‡

State Key Laboratory of Superhard Materials, Jilin University, Changchun, 130012,

People’s Republic of China

*Corresponding author: Prof. Dr. Fei Du Email: [email protected]

1

ACS Paragon Plus Environment


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Abstract NASICON-structured Na3V2(PO4)2F3 is considered as a potentially high-capacity cathode material for Na-ion batteries, however, its poor rate capability and insufficient cyclability remain a challenge for battery applications. To address this issue, we designed and successfully synthesized a core/double-shell structured Na3V2(PO4)2F3@C nanocomposite (Na3V2(PO4)2F3@CD) by in-situ carbon-coating and embedding the Na3V2(PO4)2F3 nanoparticles in ordered mesoporous carbon framework. Benefiting from the sufficient electrochemically available interfaces and abundant electronic/ionic pathways, this Na3V2(PO4)2F3@CD material demonstrated superior Na+-storage performance with a high reversible capacity of 120 mA h g-1 at a moderate current of 1 C, a strong high-rate capability with 63 mA h g-1 at an extremely high rate of 100 C and a long-cycle lifespan with 65% capacity retention over 5000 cycles. These superior electrochemical performances remained stable when the Na3V2(PO4)2F3@CD cathode was used in a full cell, suggesting a promising application of the material for high rate and long lifespan sodium-ion batteries. Moreover, the architectural design and synthetic method developed in this work may provide a new avenue to create high performance Na+-host materials for a wide range of electric energy storage applications.

Keywords：：Na3V2(PO4)2F3, Ordered mesoporous carbon, Nanocrystal, Superior high rate, Ultralong cycle life

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Introduction Rapid expansion of lithium-ion batteries (LIBs) into large scale electric storage markets such as electric vehicles and renewable power stations has now caused severe concerns for the rising cost and sustainable use of Li resources.1,2 Therefore, search for new alternatives to LIBs has stimulated a great interest in the development of sodium ion batteries (SIBs) because of the wide availability and low cost of Na resources.3-10 In the past decade, many attempts have been devoted to identify suitable host materials with acceptable low cost and decent electrochemical performance for Na-insertion reaction and as a result, a large number of different host structures such as layered and 3D-tunneled metal oxides,11-14 Prussian blue compounds,15,16 and NASICON-type phosphates17,18 have demonstrated considerable Na+-storage capacity and cyclability. Among the various Na+-insertion cathode materials, sodium-vanadium fluorophosphates Na3V2O2x(PO4)2F3-2x (0≤x≤1) appear to be an attractive candidate for energetic SIB cathodes because of their high operating voltage of ﹥3.7 V and high theoretical capacity of 128 mA h g-1,19-23 thus enabling them to reach a high energy density of ﹥475 Wh Kg-1, which is close to the LiFePO4 cathode (530 Wh Kg-1) currently used in LIBs.24,25 Nevertheless, these fluorophosphates are usually difficult to realize their potential high capacities even at a slow rate due to their intrinsic low electronic conductivity of 10-12 S cm-1.19,26 To solve this problem, an effective strategy is to encapsulate the fluorophosphates nanoparticles into conductive carbon matrixes such as carbon nanofibers27 and graphene sheets,19 so as to enhance 3



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the electron transport and the electrochemical utilization of the materials. Though the rate performance was improved to some degree, the effect was limited and worse than their sister compound Na3V2(PO4)3@carbon nanocomposite.17 The reason is attributable to the non-uniform size distribution and severe agglomeration of the NVPF nanoparticles led to a poor electrolyte infiltration and an interfacial blockage in the NVPF@carbon nanocomposite,28-30 thus restricting the high power capability of the

material.

Therefore,

development

of

a

well-designed

NVPF@carbon

nanostructure still remains a challenge for building practically high performance SIBs. In this work, we proposed a new strategy to embed NVPF nanoparticles in ordered mesoporous carbon (CMK-3) framework to form a core/double-shell structured nanocomposite (NVPF@CD), where the high conductivity of the carbon matrix provides good electronic wiring among the electrode-active nanoparticles, meanwhile the well-defined mesopores offer abundant electrolyte pathways, thus creating sufficient electrochemically available interfaces for Na+-insertion reaction. More significantly, the mesoporous carbon framework can not only inhibit the grain growth and aggregation of the NVPF particles, but also act as a flexible buffer to accommodate the volume change of the NVPF lattices during Na+-insertion/extraction cycles, ensuring a structural integrity and cycling stability of the electrode.31 As expected, the NVPF@CD nanocomposite revealed in this work demonstrated a high capacity of 120 mA h g-1 at a high average voltage of 3.75 V, a strong rate capability with 63 mA h g-1 at 100 C rate and a remarkable long-term cyclability with a capacity 4


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retention of 65% over 5000 cycles. These superior Na+-storage performances of the NVPF@CD cathode remain stable when coupled with NaTi2(PO4)3@C anode to construct an all-NASICON full cell, suggesting a practical applicability of this material for high power and long lifespan SIBs.

Experimental CMK-3 was prepared using SBA-15 as the template as described previously.32 Briefly, SBA-15 was impregnated with an aqueous solution obtained by dissolving sucrose and H2SO4. After carbonization by drying and heating, 5 wt% hydrofluoric acid was used to dissolve the silica template. The obtained template-free carbon was filtered, washed with deionized water and ethanol, and finally dried at 100 oC overnight. To prepare the NVPF@CD nanocomposite, 0.2 mmol NH4VO3, 0.3 mmol NaF, and 0.2 mmol NH4H2PO4 powders were first dissolved in deionized water. Citric acid (0.29 g) was then added and constantly stirred at room temperature for 24 hours. Second, 0.13 g CMK-3 and 10 ml ethanol were ultrasonically treated for 2 hours to form a uniform dispersion, and then added into the above solution with stirring at room temperature for another 24 hours. Third, the mixed solution was dried at 60 oC to slowly evaporate the solvent and the gel could be acquired by drying the precursor in an oven at 120 oC over-night. Finally, the NVPF@CD powder sample was acquired after a two-step heat treatment: 300 oC for 4 hours and then 650 oC for 8 hours under a nitrogen atmosphere. In contrast, NVPF/C nanocomposite was prepared following the identical procedure of NVPF@CD without adding CMK-3. When assembling the full

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cell, the NTP@C nanocomposite was employed as the negative electrode, prepared according to the procedure in our previous paper.33 The phase purity of the obtained samples was examined by X-ray diffraction (XRD; RigaKu D/max-2550) with Cu Kα source in the 2θ range of 10 - 80o at a scanning rate of 2o/min. The morphological features were observed by a Hitachi SU8020 scanning electron microscope (SEM) and a FEI Tecnai G2 transmission electron microscope (TEM). Raman spectroscopy was recorded on a Renishaw invia Raman spectroscopy with Ar-ion laser excitation (λ=514.5 nm). Quantitatively, the content of carbon was evaluated using a Mettler-Toledo Thermogravimetric (TG) analyzer. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 instrument. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method and the pore-size distribution (PSD) curves were calculated from the isotherm using the Barrett-Joyner-Halenda (BJH) algorithm. Electrochemical experiments were carried out using 2032-type coin cells. A typical electrode was composed of 70 wt% active material, 20 wt% carbon black conductive additive, and 10 wt% polyvinylidene fluoride binder (PVDF) dissolved in N-methylpyrrolidone (NMP). The obtained slurry mixture was pasted onto aluminum foil and then dried at 120 oC for 12 h in a vacuum oven. After dividing the electrode film into square parts of 0.8 × 0.8 cm2, coin cells were assembled in a glove box (O2 ≤ 0.1 ppm, H2O ≤ 0.1 ppm). The loading mass of the active material in each coin cell is typically 1.0~1.5 mg cm-2. The cathode and anode electrodes were separated by a 6


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glass fiber filter (Whatman GF/C). The electrolyte was 1 M NaClO4 dissolved in a solvent of ethylene carbonate (EC) and propylene carbonate (PC) (1 : 1 v/v) and 5% fluoroethylene carbonate (FEC). Galvanostatic charge-discharge tests were carried out at a voltage window of 2.0-4.3 V on a Land-2001A (Wuhan, China) automatic battery tester at room temperature. Here, C rate was used to characterize the current rate; 1C equaled 128 mA g-1. The specific capacity was calculated on the basis of the active cathode material after subtracting the carbon content. Data of Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were recorded on a VSP multichannel potentiostatic–galvanostatic system (Bio-logic, France). The test conditions of EIS were in the frequency range from 1 MHz to 1 mHz with an ac voltage of 5 mV.

Results and discussion The synthetic process and charge/electron transport pathways of the NVPF@CD material are schematically illustrated in Figure 1. In this work, CMK-3 carbon was selected as a conductive mesoporous framework because of its appropriate pore size (r = 4.8 nm) and well-ordered pore structure. The NVPF@CD powders were synthesized by impregnating the CMK-3 with a NVPF precursor mixture containing Na+, VO3-, PO43-, and F- ions and then heat-treating the impregnated carbon. To prevent the growth of NVPF particles, citric acid was also used as a carbon precursor to form a thin layer of carbon coating on the NVPF nanoparticles during heat treatment. The experimental procedures are described in more detail in the Experimental Section. As

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demonstrated

in

Figure

1,

the

as-prepared

NVPF@CD

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material

has

a

core/double-shell architecture, in which electrons can flow through the carbon framework to arrive at each nanoparticle, while Na+ ions can diffuse through the mesopores into the deep inside of the composite material, ensuring a full utilization of the material for Na+-insertion reaction.

Figure 1. Schematical representation of the fabrication of NVPF@CD nanocomposite and magnified representation of the electronic and ionic transport within the CMK-3 channels.

Figure 2a shows the XRD pattern of the NVPF@CD nanocomposite, compared with that of simply carbon-coated NVPF nanoparticles (denoted as NVPF/C). All the diffraction peaks can be well indexed to a tetragonal NASICON-type lattice with the space group of P42/mnm (PDF #97-008-8808). No diffraction peaks due to impurity phases were detected, suggesting that CMK-3 carbon had no influence on the phase formation of NVPF. The lattice parameters of both NVPF@CD and NVPF/C calculated by the least-square method (Table S1) showed no significant difference, 8


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further confirming the phase-pure NASICON structure of the NVPF particles in the mesoporous carbon matrix. Furthermore, no additional diffractions of carbon can be observed in the XRD pattern, possibly owing to the amorphous structure of the carbon matrix. Raman spectroscopy also revealed the disordered state of carbon in the NVPF@CD and NVPF/C nanocomposites. As displayed in Figure S1, two typical Raman

signals

appeared

at

~1342

and

~1588

cm-1,

corresponding

to

D-band(disordered carbon, sp3-coordinated behavior) and G-band (crystalline graphitized carbon, sp2-coordinated behavior),

respectively. The relative intensity

ratio (ID/IG) is usually used as a criterion to evaluate the degree of disorder and defects in carbonaceous materials. A higher ID/IG value indicates a greater disorder of carbon.17,34 When the carbon matrix was changed from amorphous carbon to ordered mesoporous carbon, the ID/IG value decreased from 1.084 (NVPF/C) to 0.991 (NVPF@CD), suggesting that the mesoporous carbon framework has a low degree of disorder and a better crystallinity. Measured from the weight losses in thermogravimetric spectra (TG) as shown in Figure S2, the carbon content of the NVPF@CD sample was determined to be 15.0 wt%. Detailed microstructure of the composite was examined using the nitrogen adsorption-desorption isotherms. As presented in Figure 2b, the NVPF/C samples showed a IV-type isotherm with a BET surface area of 47 m2 g-1. Though the NVPF@CD (Figure 2c) displayed a similar IV-type isotherm with a mesoporous structure,35 its BET surface area was significantly enlarged to 195 m2 g-1, suggesting more abundant surface areas available for Na+-insertion reaction. However, it is 9



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worthy to note that CMK-3 template displayed a larger BET surface of 950 m2 g-1 (Figure S3c-d). The decrease in the specific surface area of NVPF@CD can be attributed to the filling of mesopores by the NVPF/C nanoparticles. Pore size analysis (Figure S3) showed that there still existed a large portion of free pore space with average size of 4.1 nm in the NVPF@CD compositewhich facilitates the infiltration of electrolyte to form ionic transport channels.31 Apparently, such a high surface area and abundant nanopores of the NVPF@CD composite provide sufficient electrochemical interfaces with fast electronic and ionic pathways, enabling high capacity and high power delivery of the material.

Figure 2. (a) X-ray diffraction pattern of the NVPF@CD and NVPF/C nanocomposites; the Nitrogen adsorption-desorption isotherms of the NVPF/C (b) and NVPF@CD (c), respectively.

The morphological and structural features of the NVPF@CD nanocomposite can be visualized by the scanning (SEM) and transmission electron microscopy (TEM). As shown in Figure 3a and 3b, the NVPF@CD material displayed the 10


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micrometer-sized rod-like particles with a worm-like mesoporous structure, similar to the as-prepared CMK-3 template (Figure S4c-d) but different from the agglomerated NVPF particles in the carbon matrix (Figure S4a-b). HRTEM image of the NVPF@CD composite in Figure 3c and 3d showed a hierarchical carbon structure, in which each NVPF particle was coated with a thin layer of carbon (≈2 nm) formed during the pyrolysis of citric acid precursor. Both the surface-coated carbon layer and the mesoporous carbon matrix can no doubt prevent the aggregation of NVPF nanoparticles and greatly enhance electronic transport. The Fourier filtering image in Figure 3e acquired using the in-plane lattice fringes exhibited basal distances of 0.318, 0.344 and 0.275 nm, corresponding to the (220), (202) and (222) planes of tetragonal NVPF, respectively. In addition, the element mappings (Figure 3f-l) demonstrated a uniform distribution of Na, V, P, O, F and C elements in the NVPF@CD samples.

Figure 3. SEM (a) and TEM (b) images of NVPF@CD; high-magnification TEM images of 11



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NVPF@CD (c and d), and FFT image (e) of NVPF@CD nanocomposite; (f)-(l) represent the HAADF-STEM image and elemental mapping of Na, V, P, O, F and C, respectively.

The electrochemical Na+-storage properties of the NVPF@CD material as a cathode of SIBs were evaluated using coin-type cells with sodium metal anode. Figure 4a shows the cyclic voltammograms (CV) of the NVPF@CD material at a scan rate of 0.1 mV s-1. The main features of the CV curves were three pairs of symmetric redox bands at 3.44/3.32, 3.73/3.6 and 4.22/4.14 V, respectively. The first two pairs were attributed to two-stepped extraction/insertion reactions of the Na(2) ions, whereas the latter one was due to the extraction/insertion of Na+ from Na(1) sites. 28,29 The initial anodic peaks showed a tiny voltage shift relative to subsequent ones as a consequence of the lattice stress/strain change, similarly as observed in the previous studies of NASICON-type NaTi2(PO4)3.33 From the 2nd cycle, all the CV curves remained stable in the peak positions and intensities during repeated cycles, implying a high reversibility of Na+-insertion process and structural stability of the material. In comparison, the redox polarization of the NVPF@CD electrode was much lower than that of NVPF/C (Figure S5), suggesting facile electronic and ionic transport kinetics when the mesoporous carbon was used as the electrode backbones. To reveal its cathodic Na+-storage performance, the NVPF@CD electrode was evaluated by galvanostatic charge/discharge profiles. All the experimental capacities were calculated on the basis of the mass weight of NVPF, because all the carbons in the composite could not accommodate Na+ ions in the positive voltage region of 2.0 12


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4.3 V.31 As shown in Figure 4b, the charge-discharge profiles displayed three well-defined plateaus, which agreed well with the potential positions of the CV peaks. The initial reversible capacity of the NVPF@CD electrode was ca. 120 mA h g-1, very close to the theoretical two Na+ -insertion capacity of NVPF (128 mA h g-1), indicating a full utilization of the material for Na+-storage reaction. In a sharp contrast to the NVPF/C material (Figure S6 and S7), the NVPF@CD nanocomposite showed much improved cycling stability with only a small voltage drop of 0.08 V and a small capacity decrease of 10 mA h g-1 over 200 cycles. This strong cycle stability is most likely resulted from the well-ordered carbon framework, which confines the carbon coated NVPF nanoparticles in its mesoporous pores and prevents them from aggregation. In addition to its high capacity utilization and strong cyclability, the NVPF@CD cathode also demonstrated a superior high-rate capability. As displayed in Figure 4c, the cathode can deliver reversible capacities of 125, 123, 121, 116, 100, 92 and 84 mA h g-1 at current densities of 0.5, 1, 5, 10, 20, 30 and 50 C, respectively. Even cycled at an extremely high current rate of 100 C (12.8 A g-1), the NVPF@CD cathode can still give a high reversible capacity of 63 mA h g-1, corresponding to a full discharge in 36 s. To the best of our knowledge, such a high-rate performance has never been reported for the sodium-vanadium fluorophosphates (see Table S2) and is comparable to that of supercapacitors.36 Particularly, when the current density returned to 0.5 C after 110 cycles, the NVPF@CD cathode recovered its full initial capacity (120 mA h g-1), indicating a strong tolerance for high-rate cycles due to its structural integrity of the 13



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electrode. To account for the extraordinary high-rate performance, electrochemical impedance spectra (EIS) of the NVPF@CD cells were recorded and given in Figure 4d. The EIS spectra of both NVPF@CD and NVPF/C cells were composed of a depressed semicircle in the high frequency-to-medium frequency region characteristic of the charge-transfer reaction and one straight line in the low frequency region reflecting the sodium-diffusion process in the bulk phase.31 Based on the equivalent circuit (Figure S8) and quantitative fitting of the EIS spectra, it was found that the charge-transfer resistance (Rct) of the NVPF@CD cell was almost 25% lower and much more stable than that of the NVPF/C cell during all the charge/discharge stages (see Table S3). The decreased Rct value and enhanced power performance are most likely benefited from the core/double-shell architecture of the NVPF@CD nanocomposite, which provides a number of advantages for the Na+-insertion process. Firstly, the NVPF@CD nanocomposite consists of well-ordered mesoporore that functions as electrolyte channels to facilitate the fast transport of Na+ ions. Secondly, its mesoporous structure can effectively prevent the growth and agglomeration of NVPF particles, thus to maintain the structural stability of the electrode. Moreover, the addition of CMK-3 could greatly improve the electronic conductivity of the composite electrodes. As expected, the electronic conductivity of NVPF@CD was measured as 9.8×10-6 S cm-1 at ambient temperature, much higher than that of NVPF/C sample (6.9×10-8 S cm-1), which enable fast charge transport and therefore promote the charge-discharge rates. 14


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The long-term cycle stability of the NVPF@CD cathode was examined by galvanostatic cycling at a high rate of 50 C after 10 cycles at 1 C rate for initial activation, as shown in Figure 4e. Over 5000 cycles, this cathode can still release a discharge capacity of 62 mA h g-1, corresponding to 65% capacity retention. To get a better understanding of the strong cyclability, XRD pattern of the cathode cycled over 5000 cycles were recorded and given Figure S9. Compared to the pristine electrode, no any discernible changes could be detected in the diffraction peaks, indicating an excellent structural stability of the NASICON-lattice.18,33 In addition, SEM image (Figure S10) also revealed that the NVPF@CD nanocomposite maintained its rod-like morphology with worm-like mesoporous structure after the prolonged cycling, suggesting that the NVPF particles nanoconfined in the mesoporous structure could mitigate the strain change during Na+ insertion/extraction to keep effective electrical contract between the carbon framework and NVPF nanoparticles.31

Figure 4. (a) Cyclic voltammograms curves of the NVPF@CD electrode for the initial 5 cycles at

a scanning rate of 0.1 mV s-1 vs. Na+/Na in the voltage range of 2.0 - 4.3 V; (b) The charge-discharge profiles of the NVPF@CD electrode; (c) Rate performance of the NVPF@CD 15



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and NVPF/C electrodes at various current densities; (d) Nyquist plots for the NVPF@CD and NVPF/C electrodes at different cycles; (e) long-term cycle stability of the NVPF@CD electrode tested at 50 C rate for 5000 cycles.

To further demonstrate its practical feasibility for SIBs, the NVPF@CD cathode was

coupled

with

a

NaTi2(PO4)3@C

anode

to

construct

a

prototype

all-NASICON-structured full cell. The charge-discharge profiles (Figure S11) and cycle stability (Figure S12) of NaTi2(PO4)3@C nanocomposite between 1.0 and 2.8 V were evaluated firstly with comparable performance in the previous literatures.33 The mass balance between the positive and negative electrodes was fixed as 1:1.5 to ensure full activation of NVPF during the initial charging process. The electrochemical performances of this cell are shown in Figure 5. As shown in Figure 5a, four LED lamps could be lit successfully by one coin-type cell, indicative of great practical prospect of this cell. The initial charge and discharge capacity were 176 and 123 mA h g-1, corresponding to a first coulombic efficiency of 70%. This low coulombic efficiency was attributed to the formation of solid-state interfacial (SEI) film by the irreversible interfacial storage. Encouragingly, the coulombic efficiency was greatly improved above 95% at the 2nd cycle and the discharge capacity reached 111 mA h g-1 with a capacity retention of 91% after 100 cycles (Figure 5b). Also, the cell demonstrated strong rate capability with a high reversible capacity of 85 mA h g-1 delivered at a high rate of 20 C (Figure 5c). When the current density returned to 1 C rate after 60 cycles, the reversible capacity was nearly recovered to the initial value, due to the structural reversibility of both of the electrodes. Cycled at a high rate of 10 16


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C, this cell displayed 71% capacity retention over 1000 cycles (Figure 5d), further confirming the high rate capability and long-term cyclability of the NVPF@CD material.

Figure 5. Electrochemical properties of NVPF@CD as the cathode material in a prototype

all-NASICON-structured full cell coupled with NTP@C as the anode; (a) The lighted LED lamps driven by the full cell; (b) Charge-discharge profile of the NVPF@CD//NTP@C full cell in the voltage range of 1.0 - 3.0 V at 1 C rate; (c) Rate performance of the NVPF@CD//NTP@C full cell; (d) Long-term cycle stability and coulombic efficiency of the NVPF@CD//NTP@C full cell for 1000 cycles at 10 C rate.

Conclusions In summary, a core/double-shell structured NVPF@C nanocomposite was successfully synthesized by in-situ carbon-coating and embedding the NVPF nanoparticles in ordered mesoporous carbon framework. Benefiting from the 17



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sufficient electrochemical interfaces and abundant electronic/ionic pathways, this NVPF@CD material demonstrated superior Na+-storage performance in terms of a high capacity utilization with 120 mA h g-1 at a moderate high-rate of 1 C, a strong high-rate capability with 63 mA h g-1 at an extremly high rate of 100 C and a long cycle lifespan with 65% capacity retention over 5000 cycles. These superior electrochemical performances were further evidenced when the NVPF@CD cathode was used in a full cell, suggesting a practical applicability of the material for high rate and long lifespan SIBs. More significantly, the nanoarchitectural strategy and synthetic method developed in this work may provide a new avenue to create high performance Na+-host materials for a vide range of electric energy storage applications.

Supporting Information Raman spectrum and Thermogravimetry patterns of NVPF@CD and NVPF/C samples, pore size distribution, SEM and TEM images, Cyclic voltammograms, Cycling performance, Ex situ XRD and SEM, and charge-discharge profile.

Acknowledgements This work was supported by funding from “973” project (2015CB251103), NSFC (51472104 and 51572107), the development program of science and technology of Jilin Province (No. 20150312002ZG) and graduate innovation fund of 18


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Jilin University (Project 2016062).

Notes and references (1) Kim, S.-W.; Seo, D.-H.; Ma, X.; Ceder, G.; Kang, K., Electrode Materials for Rechargeable Sodium-ion Batteries: Potential Alternatives to Current Lithium-ion Batteries. Adv. Energy Mater. 2012, 2, 710-721.

(2) Raju, V.; Rains, J.; Gates, C.; Luo, W.; Wang, X.; Stickle, W. F.; Stucky, G. D.; Ji, X., Superior Cathode of Sodium-ion Batteries: Orthorhombic V(2)O(5) Nanoparticles Generated in Nanoporous Carbon by Ambient Hydrolysis Deposition. Nano Lett. 2014, 14, 4119-4124.

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(4) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F., The Emerging Chemistry of Sodium ion Batteries for Electrochemical Energy Storage. Angew. Chem. Int. Ed. Engl. 2015, 54, 3431-3448.

(5) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S., Research Development on Sodium-ion Batteries. Chem. Rev. 2014, 114, 11636-11682.

(6) Wen, Z.; Hu, Y.; Wu, X.; Han, J.; Gu, Z., Main Challenges for High Performance NAS Battery: Materials and Interfaces. Adv. Funct. Mater. 2013, 23, 1005-1018.

(7) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-González, J.; Rojo, T., Na-ion Batteries, Recent Advances and Present Challenges to Become Low 19



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(9) Li, Y.; Mu, L.; Hu, Y.; Li, H.; Chen, L.; Huang, X., Pitch-Derived Amorphous Carbon as High Performance Anode for Sodium-ion Batteries. Energy Storage Materials 2016, 2, 139-145.

(10) Li, Y.; Hu, Y.; Qi, X.; Rong, X.; Li, H.; Huang, X.; Chen, L., Advanced Sodium-ion Batteries Using Superior Low Cost Pyrolyzed Anthracite Anode: Towards Practical Applications. Energy Storage Materials 2016, 5, 191-197.

(11) Mu, L.; Xu, S.; Li, Y.; Hu, Y.; Li, H.; Chen, L.; Huang, X., Prototype Sodium-ion Batteries Using an Air-Stable and Co/Ni-Free O3-Layered Metal Oxide Cathode. Adv. Mater. 2015, 27, 6928-6933.

(12) Wang, P.; You, Y.; Yin, Y.; Wang, Y.; Wan, L.; Gu, L.; Guo, Y., Suppressing the P2–O2 Phase Transition of Na0.67Mn0.67Ni0.33O2 by Magnesium Substitution for Improved Sodium-ion Batteries. Angew. Chem. 2016, 128, 7571-7575.

(13) Yuan D.; Liang X.; Wu L.; Cao Y.; Ai X.; Feng J.; Yang H., A Honeycomb-Layered Na3Ni2SbO6: A High-Rate and Cycle-Stable Cathode for Sodium-ion Batteries. Adv. Mater. 2014, 26, 6301-6306. 20


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(16) Li, W.-J.; Chou, S.-L.; Wang, J.-Z.; Kang, Y.-M.; Wang, J.-L.; Liu, Y.; Gu, Q.-F.; Liu, H.-K.; Dou, S.-X., Facile Method To Synthesize Na-Enriched Na1+xFeFe(CN)6 Frameworks as Cathode with Superior Electrochemical Performance for Sodium-ion Batteries. Chem. Mater. 2015, 27, 1997-2003.

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(19) Xiang, X.; Lu, Q.; Han, M.; Chen, J., Superior High-Rate Capability of Na3(VO(0.5))2(PO4)2F2 Nanoparticles Embedded in Porous Graphene through the Pseudocapacitive Effect. Chem. Commun. 2016, 52, 3653-3656. 21



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(22) Peng, M.; Li, B.; Yan, H.; Zhang, D.; Wang, X.; Xia, D.; Guo, G., Ruthenium-oxide-Coated

Sodium

Vanadium

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Double-Shell Structured Na - ACS Publications - American

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