Monoclinic-Orthorhombic Na 1.1 Li 2.0 V 2 (PO 4 ... - ACS Publications

Article 1.1

2.0

2

4

3

Monoclinic-Orthorhombic Na Li V(PO)/C Composite Cathode for Na/Li Hybrid-ion Batteries +

+

Joseph Paul Baboo, Jinju Song, Sungjin Kim, Jeonggeun Jo, Sora Baek, Vinod Mathew, Duong Tung Pham, Muhammad Hilmy Alfaruqi, Zhiliang Xiu, Yang-Kook Sun, and Jaekook Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00856 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Monoclinic-Orthorhombic Na1.1Li2.0V2(PO4)3/C Composite Cathode for Na+/Li+ Hybrid-ion Batteries Joseph Paul Baboo†, Jinju Song†, Sungjin Kim†, Jeonggeun Jo†, Sora Baek†, Vinod Mathew†, Duong Tung Pham†, Muhammad Hilmy Alfaruqi†, Zhiliang Xiu†, Yang-Kook Sun⊥, Jaekook Kim†* †

Department of Materials Science and Engineering, Chonnam National University, 300 Yongbongdong, Bukgu, Gwangju 500-757, South Korea ⊥

Energy Storage & Conversion Laboratory, Hanyang University, Seoul 133-791, South Korea

ABSTRACT: Monoclinic Li3V2(PO4)3 (LVP) has been considered a promising cathode material for lithium-ion batteries for the past decade because of its high average potential (> 4.0 V) and specific capacity (197 mAh g-1). In this paper, we report a new monoclinic-orthorhombic Na1.1Li2.0V2(PO4)3/C (NLVP/C) composite cathode synthesized from monoclinic LVP via a soft ion-exchange reaction for use in Na+/Li+ hybrid-ion batteries. High-resolution synchrotron X-ray diffraction (XRD), thermal studies, and electrochemical data confirm room temperature stabilization of the monoclinic-orthorhombic NLVP/C composite phase. Specifically, we report the application of a monoclinic-orthorhombic NLVP/C composite as cathode material in a Na half-cell. The cathode delivered initial discharge capacities of 115 and 145 mAh g-1 at a current density of 7.14 mA g-1 in the 2.5–4 and 2–4.6 V vs. Na/Na+ potential windows, respectively. In the lower potential window (2.5–4 V), the composite electrode demonstrated a two-step voltage plateau during the insertion and extraction of Na+/Li+ ions. Corresponding in-situ synchrotron XRD patterns recorded during initial electrochemical cycling clearly indicate a series of two-phase transitions and confirm the structural stability of the NLVP/C composite cathode during insertion and extraction of the hybrid ions. Under extended cycling, excessive storage of Na-ions resulted in the gradual transformation to the orthorhombic NLVP/C symmetry due to the occupancy of Na-ions in the available orthorhombic sites. Moreover, the estimated average working potential and energy density at the initial cycle for the monoclinic-orthorhombic NLVP/C composite cathode (3.47 V vs. Na/Na+ and 102.5 Wh kg-1, respectively) are higher than those of the pyro-synthesized rhombohedral Na3V2(PO4)3 (3.36 V vs. Na/Na+ and 88.5 Wh kg-1) cathode. Further, the cathode performance of the composite material was significantly higher than that observed with pure monoclinic LVP under the same electrochemical measurement conditions. The present study thus showcases the feasibility of using a soft ion-exchange reaction at 150 °C to facilitate the formation of composite phases suitable for rechargeable hybrid-ion battery applications.

INTRODUCTION The increased demand for high energy/power density materials for electric power storage applications, such as portable electronic devices, electrified transportation, and grid-scale energy storage, has driven the development of new materials for lithium-ion and beyond-lithium-ion batteries.1,2 However, there is significant concern regarding the limited supply and rising price of the lithium required for lithium-ion batteries (LIBs).3 The physical and chemical similarities between lithium and sodium make sodium-based batteries an attractive, less expensive alternative to LIBs.3,4 Room temperature sodium ion batteries (SIBs) have attracted significant attention and have potential applications in large-scale energy storage systems (ESSs). Studies have also been conducted on mixed-ion batteries (Li+/Na+, Mg+/Na+) in order to combine the benefits of two different conducting ions.5–10 Barker et al. introduced a hybrid-ion cell with a lithium-ion electrolyte. The cell used a predominantly lithium-based electrochemical reaction at the anode (graphite, Li4/3Ti5/3O4) and

mixed Li/Na ion insertion mechanisms at the cathodes (Na3V2(PO4)2F3).5,6 Analogous hybrid-ion (Li+/Na+) insertion and extraction using a rhombohedral Na3V2(PO4)3 (NVP) cathode has been recently studied in the Li/NVP half-cell system.7,8 The superionic conductor phase A3M2X3O12 (A = Li, Na; M = Ti, Fe, V; and X = P, Mo, S) has been investigated as an electrode material, as well as for use in solid-state electrolytes for Li-/Na-ion batteries.11–13 Its structure includes a three-dimensional framework of transition metal octahedra and phosphate tetrahedra that share oxygen vertices. Each MO6 octahedron is surrounded by six XO4 tetrahedra, and each XO4 tetrahedron in turn is surrounded by four MO6 octahedra.11 Alkali metals such as Li and Na that are responsible for conduction are situated in the large interstitial space created within the framework by the XO4 tetrahedra, and compensate for the total negative charge of the framework. This open framework is exceedingly advantageous for fast diffusion of mobile ions. Of the sodium super ionic conductor (NASICON) materials

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

available, vanadium based A3V2P3O12 structures have been widely studied as electrode materials over the past two decades because of their structural stability and broad working potential window.14–16 The V2(PO4)3, “lantern” units may be oriented in parallel to produce rhombohedral symmetry or at an angle to one another for monoclinic symmetry.15,16 In general, Na ion-based NVP NASICON structures prepared using direct synthesis are rhombohedral in nature.11 Rhombohedral NVP structures provide two interstitial sites for sodium ions: 6b and 18e in space group R3c. The sites are defined as M1 (one site per formula unit) and M2 (three sites per formula unit), respectively.16–18 In addition, a recent study by Lalere et al. demonstrated the presence of four polymorphs of NVP at temperatures from 30 to 225 °C, while a low temperature sample (monoclinic-NVP) undergoes monoclinic distortion together with ordering of the Na atom.19 The electrochemical properties of both rhombohedral and monoclinic NVP include a flat voltage plateau for reversible insertion and extraction of two Na ions.18,20 In contrast, Li ion-based Li3V2(PO4)3 (LVP) systems form with monoclinic symmetry during direct synthesis due to their thermodynamic stability.21–24 This configuration includes a three-dimensional network with three independent lithium sites.22 In this monoclinic structure, each unit cell has four units of LVP. Within each unit cell, twelve lithium atoms occupy three four-fold crystallographic positions. At room temperature, Li atoms fully (100%) occupy three nonequivalent Wyckoff positions Li1(4e), Li2(4e), and Li3(4e).22,23 The Lil sites are regular tetrahedral, while Li2 and Li3 involve pseudotetrahedral coordination with oxygen atoms. The Li ions in Li3(4e) sites have the highest energies and are the first to be extracted during charging, as determined via bond-sum calculations, neutron diffraction, and 7Li solid-state NMR.22 All three Li ions may be reversibly extracted from monoclinic-LVP over four two-phase electrochemical plateaus to yield a theoretical capacity of 197 mAh g-1.22–24 Monoclinic LVP is isostructural with Li3Sc2(PO4)3 and Li3Fe2(PO4)3.13,25,26 These Li3M2(PO4)3 (M = V, Sc, and Fe) materials undergo several reversible phase transitions between room temperature and 350 ºC, which are accompanied by slight changes in their crystal lattices and probably a disordering of alkali ions.25–28 Bykov et al. presented a detailed structural explanation of the Li-atom arrangements in all the three variants, namely the α, β, and γphases of Li3Sc2(PO4)3 and Li3Fe2(PO4)3.25 Sato et al. used thermal studies to report similar phase transitions in monoclinic LVP.26 As the temperature increases, LVP converts from the monoclinic α-phase (room temperature) to an orthorhombic γ-phase (180 ºC) via a monoclinic β-phase (120 ºC).26 The transitions between α, β, and γphases are caused by changes in lithium distribution among the available sites. Further, the high temperature γ-phase can be stabilized at room temperature by reducing the lithium content, (or) doping of transition metal ions such as Zr4+, Ti4+, Cr3+, or Mg2+ at V3+ sites of monoclinic LVP.26,29–33 In other words, each empty lithium site

Page 2 of 14

in the monoclinic symmetry corresponds to orthorhombic symmetry. For example, the monoclinic Li1 and Li1s sites is related to orthorhombic Li1o site while Li2, Li2s is attributed to Li2o site and monoclinic Li3 and Li3s sites represent orthorhombic Li3o site.26 The earlier studies by Bykov on the γ-phase demonstrated that the lithium atoms of the unit cell are redistributed over three eight-fold crystallographic positions, only one of which is fully occupied.25 Two other positions are occupied by lithium atoms at only 25% density. The relative coordinates of the transition metal and phosphorous atoms remain nearly unchanged during these phase transitions.25 On the other hand, based on neutron refinement data, Sato et. al, demonstrated that lithium locations are partially occupied over all the available Li-sites in both high temperature and room temperature stabilized orthorhombic phases of Li3Sc2(PO4)3 and LVP systems.26,34,35 In recent years, several approaches have been used to develop fundamental knowledge and solve practical challenges regarding monoclinic LVP. These approaches include synthesizing composites with carbon, doping, and performing structural design-based studies.24 Doping of Li-sites with various alkali/alkali earth metals such as Na+, K+ and Ca2+ has been studied.36–43 Na+ doping of the LVP structure is the most widely studied. In several studies, monoclinic NaxLi3-xV2(PO4)3 (0 ≤ x ≤ 0.1) samples were made via sol-gel/solid state synthesis and tested as cathode materials for LIBs.36–41 These doped Na ions acted as pillars for conduction of Li+ and thereby enhanced the electrochemical performance of the monoclinic NaxLi324 + xV2(PO4)3 electrode. With regard to excess doping of Na (x ≥ 0.2), Tang et al. reported on the solid state synthesis of a Li2NaV2(PO4)3 composite cathode which consists of a mixture of monoclinic-LVP, rhombohedral-LVP, and rhombohedral-NVP phases in a 10:59:31 molar ratio. This study included direct synthesis of the rhombohedral LVP phase.40 Shao et al. reported that the monoclinic LVP (NaxLi(3-x)V2(PO4)3) structure was maintained with lower Na+ concentrations (x ≤ 0.2). However, further increases in the Na ion concentration after doping lead to partial or complete transformation to the rhombohedral phase (space group, R3c).41 Our group has focused on preparing transition metal oxides and Li/Na transition metal phosphates using a polyol-assisted pyro-synthesis technique.44–47 In our earlier reports, we studied the structural and electrochemical properties of monoclinic LVP/C and rhombohedral NVP/C samples prepared using pyro-synthesis, followed by annealing at 800 °C in argon.46,47 Compared to the well-known rhombohedral NVP NASICON structure used in Na ion battery applications, the monoclinic LVP-based structure can provide insertion and extraction of more than two Na ions, a wider average working potential and higher energy densities. In order to use the benefits of the monoclinic structure in Na ion battery applications, we report the synthesis of a new monoclinic-orthorhombic Na1.1Li2.0V2(PO4)3/C (NLVP/C) composite phase made from monoclinic LVP/C via soft ion-exchange


reactions.48,49 The resulting NLVP/C composite material was studied as a cathode material for Na+/Li+ hybrid-ion batteries.

ing homogeneous solution was poured uniformly onto a hot-plate maintained at 200 °C. The polyol precursor solution was ignited with a torch to induce a selfextinguishing combustion process. The resulting powders were annealed at 800 °C in argon to produce a highly crystalline monoclinic LVP/C powder. Ion-exchange was performed at 150 °C by heating the monoclinic LVP/C precursor sample under reflux for 10 h in a solution of sodium chloride (0.01 mol NaCl) in hexanol. After refluxing the sample in hexanol at 150 °C for 10 h, the product was filtered and then washed with water several times in order to remove excess Na salts. The final product was dried overnight at 120 °C in vacuum.

EXPERIMENTAL SECTION

Intensity (a.u.)

Intensity (a.u.)

A detailed description of the synthesis of precursor samples (monoclinic LVP/C) is provided in our previous publication.46 Typically, stoichiometric amounts (1.5:1:1.5) of Lithium acetate (C2H3LiO2, ≥99% - Aldrich), vanadium acety-acetonate (C15H21O6V, 97% - Aldrich) and phosphoric acid (H3PO4, ≥85% - Daejung) were dissolved in 80 ml of ethylene glycol (C2H6O2, ≥99% - Daejung). The result-

# #

22

23

24

25

# #

28

#

30

2θ (deg.)

32

#

34

#

#

43

44

2θ (deg.)

46

47

48

2θ (deg.)

# - New peaks

Intensity (a.u.) 10

20

30

40

(5-12)

(2-3-3)

(2-40) (1-16)

Monoclinic LVP/C

(3-12)

(301) / (2-13) (2-22) (13-1)

(220) (2-2-1) (004)

(1-2-2)

(103)

(11-3)

(02-1)

(1-11)

(2-10)

(11-2)

(10-3)

NLVP/C

(11-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

Page 3 of 14

50

60

2θ (deg.) Figure 1. Synchrotron XRD patterns of monoclinic LVP/C and NLVP/C composite samples. Structural characterization of the powder samples and ex-situ electrodes prepared at different electrochemical states were performed using a synchrotron X-ray diffractometer (SXRD) from the 9B station beam line of the Pohang light source (PLS, South Korea). The incident Xrays were vertically collimated using a mirror and monochromated to a wavelength of 1.5184 Å using a doublecrystal Si (111) monochromator. The detector arm of the vertical scan diffractometer was composed of seven sets of Soller slits, flat Ge (111) crystal analyzers, antiscatter baffles, and scintillation detectors. Each set was separated by 20°. The samples were rotated about the normal to the

surface during the measurement to increase the statistical validity of the results. A step scan was performed in 0.02° increments at room temperature. Particle surface morphologies were observed using field-emission scanning electron microscopy (FE-SEM, S-4700 Hitachi). Highresolution transmission electron microscopy (HR-TEM) images were observed using a FEI Tecnai F20 at 200 kV. The elemental compositions of the samples were determined via ICP analysis, using a Perkin Elmer 4300 DV analyzer. Differential scanning calorimetry studies were performed using a TGA/DTA Analyzer (SDT Q600, TA instruments, New Castle, DE USA). Samples were heated


Chemistry of Materials and cooled between 30 and 300 °C at 5 °C /min under nitrogen. Electrochemical measurements were performed using a 2032 coin cell composed of sodium metal as the anode, our NLVP/C composite material as cathode, a polymer membrane as the separator and 1 M NaPF6 in ethylene carbonate/propylene carbonate with 2% fluoroethylene carbonate as the electrolyte. Cathode materials were prepared by mixing the NLVP/C composite (70 wt%) with ketjen black (20 wt%) and a polytetrafluoroethylene

binder (10 wt%). The above mixture was made into a thin film, pressed onto a 2 cm2 stainless steel mesh, and dried under vacuum at 120 °C for 24 h. The mass loading of the electrode was 3.5 mg cm−2. The cells were assembled in an argon-filled glove box and tested at room temperature using a Battery Tester System (2004H instrument, Nagano, Japan). Galvanostatic charge/discharge studies were performed at various current densities. The charging current density was maintained at 7.14 mA g-1 and the discharge current densities varied from 7.14 to 1428 mA g-1.

Rex: 3.37, Rwp: 4.55, S: 1.9 Yobs Ycal Y(obs-cal) Orthorhombic Monoclinic

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 14

(121) (112) (310)

(002) (211) (020)

(020) (200) (-112)(112)

10

20

30

40

50

60 19.5

20.0

20.5 23.0

2θ (deg.)

2θ (deg.)

(-112)(-211)(121)(211)(013)

23.5

24.0

2θ (deg.)

Figure 2. Synchrotron XRD profile of the NLVP/C composite sample.

In-situ SXRD measurements were performed at beamline 1D at the Pohang Accelerator Laboratory (PAL) using a MAR345-image plate detector operating at 2.5 GeV with a maximum storage current of 200 mA. The X-ray beam was focused using a toroidal mirror and monochromatized to 12.4016 keV (0.9997 Å) by a double bounce Si(111) monochromator. The Si(111) monochromator and a Si(111) analyzer crystal were used to provide a high-resolution configuration in reciprocal space. The patterns were recorded based on a wavelength of 0.999 Å. However, the XRD patterns displayed in the present study were plotted after re-calculating the 2θ values based on conventional Cu Kα radiation (λ=1.5406 Å). During preparation of the in-situ cell, the electrode active material was mixed with carbon black and the TAB binder in the ratio mentioned earlier. The mixture was then cast onto a stainless steel mesh and assembled in a spectro-electrochemical cell. The cell was cycled between fully charged and discharged

states using a portable potentiostat with a constant current density of 0.014 A g-1. Kapton tape was applied to the openings on the outer cases of the test cells. XRD measurements for ex-situ electrodes prepared at different electrochemical states were also performed using 3D High Resolution X-Ray Diffractometer using CuKα radiation over the range 10° ≤ 2θ ≤ 80° under slow scan for 1 h.

RESULTS AND DISCUSSION In this study, we synthesized NASICON-structured NLVP/C via a soft ion-exchange reaction.48 In brief, monoclinic LVP/C was mixed with 0.01 mol NaC1 and refluxed in n-hexanol at 150 °C for 10 h. The ICP results from the synthesized powder samples shown in Table S1 (Li : Na : V is 2.0 : 1.1 : 2.0) indicate that the (1/3) Li+ per formula unit in the monoclinic LVP structure is replaced with one Na+ ion. Thus, the ion-exchange reaction with monoclinic LVP/C results in the formation of a mixed-alkali ion conducting NLVP/C compound. The high resolution SXRD pattern recorded for the prepared NLVP/C composite is


Page 5 of 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


compared to that of a pure monoclinic LVP/C in Fig. 1. The intense diffraction peaks of NLVP/C indicate that the crystallinity of the precursor sample is maintained even after ion-exchange. The intensities of the (10-3), (1-2-2), (301), and (3-12) planes increase relative to those in the monoclinic LVP/C samples. This indicates that a majority of Na+ ions are exchanged for Li+ in the specified planes. Interestingly, a majority of the characteristic Bragg peak positions for the prepared sample are slightly shifted to

lower scanning angles (2θ), as observed from the pattern of the monoclinic LVP/C. This shift can be related to the corresponding increase in inter-planar spacing due to the partial exchange of Na+ and Li+ ions in the monoclinic structure. Upon closer observation, the NLVP/C pattern reveals new peaks (2θ = 22.9, 24.03, 29.04, 29.96, 32.60, 33.39, 43.67, and 46.7 degree). Moreover, the gradual shift towards lower scanning angles (2θ) can also indicate the presence of solid solution phases. Table 1. Cell parameters of LVP/C, NLVP/C composite powder sample and ex-situ electrode after the 25th discharged state. Monoclinic LVP/C

Monoclinic NLVP/C

Orthorhombic NLVP/C

NLVP/C

a (Å)

8.6118

8.6658

12.2009

12.2205

b (Å)

8.5987

8.7067

8.6166

8.6187

c (Å)

12.052

12.0321

8.6547

8.6688

β (deg.)

90.5441

90.515

892.418

907.7816

909.8666

913.0330

P 21/n (14)

P 21/n (14)

Pbcn (60)

Pbcn (60)

Unit 3 (Ǻ )

cell

Space group

volume

To precisely identify the structural phases formed after ion-exchange reaction, profile matching was performed by Fullprof Suite program (shown in Fig. 2).50 The profile matching was performed based on a monoclinic symmetry with space group P121/c1 (no. 14) and for orthorhombic symmetry with Pbcn (no.60). Also, the reliability value of profile matching from the fitting parameters S (Rwp/Rexp where, Rexp is expected R factor, Rwp is the weighted profile R-factor and S is goodness of fit) was determined to be 1.9 and thus indicates good agreement between the simulated and observed SXRD patterns. Thus, the obtained results confirmed that the crystal structure of SXRD pattern is well indexed to monoclinic and orthorhombic phases. Finally, the magnified images in Fig. 2 (around 2θ = 20 and 23.5 degree), clearly confirmed that the indexed peaks and planes correspond to monoclinic and orthorhombic phases. In fact, as mentioned earlier, the monoclinic cell results from a small distortion of the orthorhombic symmetry framework, which is caused by lithium ion ordering at room temperature.28 The refined lattice parameters for monoclinic phase (a = 8.6658 Ǻ, b = 8.7067 Ǻ, c = 12.0321 Ǻ, β = 90.515) and orthorhombic phases (a = 12.2009 Ǻ, b = 8.6166 Ǻ, c = 8.6547 Ǻ) are compared in Table 1. The increase in the lattice parameter values of monoclinic–orthorhombic NLVP/C sample can be attributed to the partial exchange of Li+ ions (r = 0.068 nm) in the monoclinic-LVP/C structure for larger Na+ ions (r = 0.097 nm). Based on the above profile matching results, Rietveld refinement was performed by assigning atomic position in the monoclinic and orthorhombic sites. Although the observed S(Rwp/Rexp) value is around 3.3, but from the refinement results, we expect the phase fraction to be an equimolar ratio of monoclinic and orthorhombic phases in the NLVP sample. Furthermore, previous studies have demonstrated that excessive doping

25th discharge

with Na ions (x ≥ 0.2) may lead to transformation from a monoclinic to a rhombohedral NaxLi(3-x)V2(PO4)3 phase.40,41 However, in the present work, although nearly one sodium ion replaces (1/3) Li per formula unit of monoclinic LVP, the observed XRD pattern of NLVP/C in Fig. 2 is significantly different from those of the pure monoclinic or rhombohedral NVP phases.19,47 Therefore, the XRD studies clearly indicate that the ion-exchanged sample is a composite that includes both monoclinic and orthorhombic phases of NLVP/C. The thermal behaviors of the pure monoclinic LVP/C and NLVP/C composite samples were examined via differential scanning calorimetry (DSC) analysis between 30−300 °C under nitrogen. The results are shown in Fig. 3. The DSC curve from the LVP/C sample in Fig. 3 (a) exhibits endothermic peaks in the 100−225 °C region, which can be ascribed to transitions between α, β, and then γ phases. The transition peaks are not completely separate. They form a broad, endothermic effect that ranges from 100 to 250 °C with two maxima near 110 and 208 °C. The irreversibility of this phase transition was observed during cooling. However, with NASICON materials, the appearance of a phase transition during heating and its reversibility during cooling may depend on the synthesis method used and the subsequent sintering conditions.20,51 On the other hand, the DSC curve from the NLVP/C composite sample in Fig. 3(b) indicates the absence of significant phase transitions during heating and cooling. More importantly, the slanted curve observed during heating and cooling corresponds to the thermal behaviors reported with orthorhombic LVP phase-based materials.26 Since the ionexchange reactions in the present work were performed at 150 °C, which is between the temperatures at which the α to β and β to γ phase transitions occur, redistribution of


Chemistry of Materials Na/Li between available sites in the β and γ phases can take place.26 Hence, after ion-exchange, the final product (NLVP/C) tends to crystallize with a mixture of monoclinic and orthorhombic symmetry. (a) -0.5

-1.0 α −β

Cooling

-1.5

β −γ

Heating

Heat Flow (W/g)

Monoclinic LVP/C

-2.0

-2.5 100

200

300

ο

Temperature ( C) (b) 0

Heat Flow (W/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 14

NLVP/C

-2 -4 -6

Cooling Heating

-8 -10 100

200

300

ο

Temperature ( C)

Figure 3. Differential scanning calorimetry curves of (a) monoclinic LVP/C and (b) NLVP/C composite samples. Based on the above XRD and DSC results, we deduce that the following processes occur simultaneously during ion-exchange reaction: (i) because of the reaction temperature (150 °C), the existing Li ions in the LVP structure may mainly occupy the Li(1) and Li(1s) sites, while also being partially distributed over the remaining Li(2), Li(2s), Li(3) and Li(3s) sites, and (ii) Na ions are partially exchanged for Li ions from all of the above mentioned sites. However, a part of the orthorhombic sites remain occupied during cooling process, whereas remaining sites transform back to the Na ion-exchanged monoclinic phase. This may be possible because the ion exchange reaction is performed at 150 °C, which lies between the temperatures at which the α to β and β to γ phase transitions occur.

Figure 4. (a) HR-SEM images of monoclinic LVP/C, (b) HR-SEM, and (c) HR-TEM images of NLVP/C composite samples. Further, for comparison purposes, we performed Naion exchange reaction with monoclinic LVP using different sodium salts (NaCl/NaNO3) and appropriate solvents (such as, water (100 °C), ethylene glycol (195 °C), and diethylene glycol (240 °C)) under similar reflux conditions and prepared three different samples. A comparison of the corresponding XRD patterns and galvanostatic charge/discharge curves (in Fig S1 (a&b)) reveal relatively low structural stability and poor electrochemical reactivity than in the present NLVP/C sample prepared at 150 °C in hexanol medium. Hence the reaction temperature pursued in the present study can facilitate the transport of Na ions to all possible Li sites (even the lowest energy sites Li(1) and Li(2)), resulting in the formation of a composite of monoclinic-orthorhombic NLVP/C phases.


Page 7 of 14

SEM images of the pristine monoclinic LVP/C and NLVP/C composite samples are shown in Fig. 4 (a) and (b). The pyro-synthesized precursor sample (monoclinic LVP/C) contains a mixture of spherical and rod-like particles nanometers in size. On the other hand, the ionexchanged NLVP/C sample also exhibits particle aggregation in addition to nano-rod morphology. The sizes and morphologies of the ion-exchanged particles are similar those found in the precursor. This indicates that the overall particle shapes are maintained after the ion-exchange reaction. Furthermore, the HR-TEM image in Fig. 4(c) reveals dark regions that correspond to the carbon coated NLVP/C particles. Contrast in the image tends to confirm

the presence of a carbon network between the NLVP/C particles. Energy dispersive X-ray (EDX) elemental maps for Na, V, C, O, and P are shown in Fig. S2 (a-f). Importantly, the uniform distribution of Na ions over the particles indicates that ions are exchanged across the whole particle. The amorphous carbon coatings and networks on the surfaces of the precursor particles (LVP/C) were maintained after ion-exchange in hexanol.46 In addition, the uniform distribution of vanadium and phosphorous across the region indicate that the monoclinic LVP structure and V/P stoichiometric ratio is maintained after ion-exchange.

(b) 4.0

(c)

Rex: 1.53, Rwp: 2.21, S : 1.4 Yobs Ycal Y(obs-cal) Orthorhombic

2000

3.0

d(Q)/dV

1000

2.5

0

3.5

Intensity (a.u.)

3.5

600

3.0 300

d(Q)/dV

+

+

Voltage (V vs. Na/Na )

(a) 4.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5

0

#

# - Substrate

-300

-1000 2.5

3.0

3.5

4.0

NLVP/C Rhombohedral NVP/C

-600 2.5

3.0

3.5

4.0 +

+



2.0

2.0 0

20

40

60

80

100

120

140

0

20

-1

40

60

80

100

-1

Capacity (mAh g )

Capacity (mAh g )

120

10

20

30

40

50

60

2θ (deg.)

Figure 5. Galvanostatic charge/discharge curve of NLVP/C composite cathode in Na-half cell within the potential windows of 2.5–4 V vs. Na/Na+ (a) 1st cycle (inset: corresponding dQ/dV plot ) and rhombohedral NVP/C cathode, (b) 25th cycle and (c) Ex-situ XRD pattern of the NLVP/C composite cathode after the 25th discharged state. The galvanostatic charge/discharge curve, corresponding dQ/dV plot, cycling stability, and rate performance of the NLVP/C composite cathode within the 2.5–4 V range vs. Na/Na+ at current density of 7.14 mA g-1 are shown in Fig. 5 and 6. To test the hybrid-ion storage properties of the ion-exchanged NLVP/C cathode, metallic sodium was used as the anode and 1 M NaPF6 dissolved in an equimolar mixture of ethylene carbonate and propylene carbonate (EC/PC) with 2 % FEC was used as the electrolyte. All of the electrochemical potentials in the following discussions are stated with respect to Na/Na+, unless otherwise mentioned. Although the present system uses a Na ion electrolyte, the initial electrochemical cycles involve contributions from both Li and Na ions. The NLVP/C composite cathode delivers an initial discharge capacity of 115 mAh g-1 within the 2.5–4 V potential window at current density of 7.14 mA g-1. The initial charging curve in the 2.5–4 V electrochemical window (Fig. 5 (a)) exhibits two flat plateaus around 3.37 and 3.85 V. Interestingly, the flat voltage plateau in the charge/discharge curve and the corresponding single oxidation/reduction peak in the differential capacity (dQ/dV) plot near 3.37/3.2 V for the present monoclinic-orthorhombic NLVP/C composite sample in a sodium half-cell (Fig. 5(a)) is distinctively different from the two characteristic voltage plateaus (∼ 3.6/3.55 and ∼3.68/3.64 V vs. Li/Li+) observed using a typical monoclinic-LVP phase in a Li half-cell.46 In general, the voltage plateaus in the charging curve of LixV2(PO4)3 (x = 3.0, 2.5, 2.0, 1.0, and 0.0) correspond to phase transi-

tions between single phases. The main features of charging plateau are the steps at x = 2.5, 2 and 1. Yin et al. observed that the factors of charge ordering and lithium site ordering drive these phase transitions.22,23 The existence of a stable orthorhombic phase Li2.5V2(PO4)3 with medium lithium content (x=2.5, lying between 2 & 3) leads to a small voltage jump during lithium extraction from Li(3) site.29 However, when V3+ in LVP is substituted by Ti4+, Zr4+, Cr3+ or Mg2+, the first two charge plateaus were gradually merged into one and became inclined further.26,30–33 The disappearance of two-plateau boundary in the capacity–voltage profiles can be ascribed to the disorder of Li ions that result in the formation of orthorhombic LVP phase.26,30–33 As mentioned earlier, the crucial difference between monoclinic and orthorhombic phases is the distribution of lithium ions. The study by Sato. et. al, explained only three kinds of sites (Li(1), Li(2) and Li(3) sites) among six available lithium sites are fully occupied for the monoclinic structure, whereas, Li(1o), Li(2o) and Li(3o) sites are partially occupied for the orthorhombic structure.26,35 Considering that the present NLVP sample consists of an equimolar mixture of both monoclinic and orthorhombic phase and hybrid-ions (Na+/Li+), the observed SXRD pattern (in Fig. 2) clearly confirms the existence of Na-ions in both monoclinic and orthorhombic NLVP phase. And ex-situ ICP results at different state of charge and discharge indicated the partial distribution of Na-ions in all the available sites. Further, the initial charging curve near 3.37 V involves the extraction of hybridions (Na+/Li+) from the monoclinic Li(3) and orthorhom-



On the other hand, the voltage profile of the discharge curve includes three stages, 3.8–3.6 V (stage I), 3.6–3.3 V (stage II), and 3.3–3.0 V (stage III). In contrast to the electrochemical curve that describes Li ion insertion in LVP within LIBs, the additional small plateau region in stage II of the monoclinic-orthorhombic NLVP/C composite curve may be due to the insertion of a combination of Na+ and Li+ into the structure. In fact, Barker et al. and Song et al. also reported similar phenomena when they observed the voltage profiles of other cathode materials during insertion and extraction of hybrid-ions.5,6 This may occur because, although the electrolyte is dominated by an excess of Na ions, a small number of Li ions extracted from the NLVP/C cathode during the initial charging process are present near the electrode/electrolyte interface. These extracted Li ions also intercalate and deintercalate into the cathode during the initial stages of the electrochemical cycles. Fig. S4(a) shows the electrochemical curves with respect to increase in cycle numbers. As the number of cycles increased, the plateau region at stage-I (shown in Fig. S4(a)) gradually decreased and on the other hand, the plateau potential around 3.45 V (stage II) increased proportionally. A closer examination of the charge/discharge curve of the 1st and 25th cycles reveal that the plateau region near 3.6 (stage-II) becomes dominant while stage-I is almost diminished. On the contrary, the voltage plateau around 3.2 V (stage-III) gets gradually shifted towards higher voltage region of 3.3 V with respect to increase in the electrochemical cycling. The shapes of the charge/discharge curves beyond 10th cycles are quite reproducible and clearly indicate the merging of voltage plateaus around 3.37 and 3.85 V into a slopping curve. Specifically, the charge/discharge curve of 25th cycle in Fig. 5(b) clearly demonstrate the characteristic behavior similar to the room temperature stabilized orthorhombic phase reported for transition metal ion (Ti4+ or Zr4+) doped at vanadium sites of LVP structure.26,30,31 These transition metal ion (Ti4+ or Zr4+) doping causes disorder of Li-ion in the structure which leads to the formation of the solid solution regimes.26,30,31 Further, compared to the

sharp dQ/dV peaks at the first charge/discharge cycle (inset Fig. 5(a)), the broad, featureless peaks (inset Fig. 5(b)) generally are characteristic of an insertion reaction where alkali-ion disorder is present (i.e., solid solution behavior).30 In comparison to the Na:Li ratio of the NLVP powder sample, the ICP values of ex-situ electrodes at 15th and 25th discharged state (given in Table S1) were increased and almost saturated.

(a) Discharge -1 capacity (mAh g )

bic Li(3o) sites. However, this plateau gradually transforms to a sloping curve on prolonged cycling (ie. Increase in the insertion/extraction of more Na-ion). The electrochemical mechanism at this potential is thus complex involving several factors such as extraction of hybridions (Na+/Li+), existence of disordered and ordered sites of orthorhombic and monoclinic phases of NLVP. Moreover, the voltage profile of the monoclinic LVP/C in sodium half-cell (Fig. S3, Supporting Information) also indicate the slight merging of two plateaus around 3.4 V. Hence based on the above observations, we speculate that the plateau in the electrochemical curve around 3.37 V could be attributed to the combined effect of extraction of different-ions (Na+/Li+) from both the orthorhombic and monoclinic phases. Also, it is feasible that there can be extraction of Na-ions from the orthorhombic phase in the initial stages and extraction of Li-ions from the monoclinic phase at the later stage although more experiments are needed to confirm this finding.

120

90

60

30

0 0

5

10

15

20

25

30

Cycle number

(b)

7.14 14.3

28.6

57

114

229

457

120

Discharge -1 capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 14

914 1429 -1

Unit : mA.g

90 60 30 0 0

3

6

9

12

15

18

21

24

27

Cycle number Figure 6. (a) cycling stability and (b) discharge capacity at various current densities of the NLVP/C composite cathode in Na-half cell within the potential windows of 2.5–4 V vs. Na/Na+. In addition, the ICP values of ex-situ electrodes of 15th and 25th discharged state (given in Table S1) indicates that the Na:Li ratio of the NLVP powder sample (1.1:2.0) increased to around 2.1:0.7 after cycling. Hence, in the course of long-term cycling, the electrochemical reactions on the NLVP cathode is dominated by the Na-ions that are present in excess in the electrolyte over the Li ions extracted from the cathode during the initial charging process. The small quantity of Li-ion (∼0.7) in the ex-situ sample is due to the presence of Li-ion in the lowest energy site (Li2/Li2o) which is not involved in electrochemical reaction in this specific potential window (2.5 − 4 V). However, those Li-ions were removed after charging the electrodes to 4.6 V vs. Na/Na+. Furthermore, when the charged electrodes (4.6 V vs. Na/Na+) are replaced with


Page 9 of 14

fresh electrolyte leads to complete Na-ion battery system. Corresponding ICP values after replacing with fresh electrolyte are given in Table. S1.

4.5

+

Voltage(V vs. Na/Na )

4.0

6000

3.5 d(Q)/dV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000

2000

3.0 0 2.5

3.0

3.5

4.0

4.5

Voltage (V vs. Na/Na+)

2.5 0

50

100

150

200

250

300

-1

Capacity(mAh g )

Figure 7. Electrochemical Performance of NLVP/C composite cathode in Na-half cell within the potential windows of 2.5–4.6 V vs. Na/Na+. To identify structural phases after cycling, HR-XRD and profile matching were performed by Fullprof Suite program.50 The reliability value of profile matching was calculated from the fitting parameters S(Rwp/Rexp) to be 1.4 and thus indicates good agreement between the simulated and observed XRD patterns. The results clearly revealed that the crystal structure of XRD patterns were well indexed to orthorhombic phase without any other impurity peaks. The lattice parameters of orthorhombic phase were calculated to be a = 12.2205 Ǻ, b = 8.6187 Ǻ, c = 8.6688 Ǻ. The marginal increase in the lattice parameter values of orthorhombic phase after 25th discharge clearly indicates that more of the Li-ions in the structure are replaced with Na-ions. These results clearly indicate that the composite of monoclinic and orthorhombic phase at the initial state is now transformed to single orthorhombic phase after cycling. For a better understanding, after first charging the NLVP/C electrode, we reassembled a new Na-coin cell using fresh anode and electrolyte. The corresponding charge/discharge curves are shown in Fig. S4(b). This clearly shows that the removal of Li-ions in the electrolyte resulted in the insertion/extraction of only Na-ions. These curves resemble with the charge/discharge curves of 25th cycle in original NLVP/C hybrid ions system. This clearly demonstrates the insertion/extraction of Na-ion into the orthorhombic sites of NLVP/C sample. Although lower than the theoretical capacity (130 mAh g-1) for the insertion of 2 Li ions into monoclinic LVP in LIBs, the discharge capacity delivered by the NLVP/C composite cathode of a hybrid-ion battery is still better than that attained by the monoclinic LVP/C cathode (Fig. S3, Supporting Information). We examined the reversible intercalation of hybrid ions (Na+/Li+) in a pure monoclinic LVP/C cathode by measuring the galvanostatic

charge/discharge curves and the corresponding dQ/dV plots within the 2.5−4 V range. The initial voltage profile of the monoclinic LVP/C sample (Fig. S3, Supporting Information) is similar to that of the chemically ionexchanged NLVP/C sample in Fig. 5(a). However, after the second charge, Na+ can access only the Li(3) site of the α-LVP structure and thus delivers inferior reversible capacities (~ 67 mAh g-1) during successive cycles. The corresponding dQ/dV plots indicate characteristic charge ordering in the monoclinic structure. Generally, ionexchange reactions performed using 1 M LiPF6 EC/DMC in galvanostatic conditions are milder than the chemical ion-exchange reaction. However, the solvent reflux approach to ion exchange with monoclinic LVP has sufficient energy to produce partial exchange of Li for Na. Furthermore, the voltage profiles measured within the 2.5−4.0 V range vs. Na/Na+ at a current density of 7.14 mA g-1 for the easily available rhombohedral Na3V2(PO4)3/C cathode prepared via pyro-synthesis are compared in Fig. 5(a).47 As expected, the voltage profiles of the rhombohedral NVP/C cathode indicate a characteristic single voltage plateau near 3.39/3.36 V involving the extraction and insertion of two sodium ions and a delivered discharge capacity of 109 mAh g-1.47 The specific energy density of each cathode was calculated using the area under the discharge curve with a lower cutoff voltage of 2.5 V. The pyro-synthesized rhombohedral NVP/C cathode exhibits an energy density of 88.5 Wh kg-1 within the specified potential window (2.5 − 4.0 V) for the initial curve. On the other hand, the initial discharge curve of monoclinicorthorhombic NLVP/C composite sample demonstrates a higher average working potential of 3.47 V and energy density of 102.5 Wh kg-1. Nevertheless, the capacity retention curve of the NLVP/C composite cathode in Fig. 6(a) shows that 86% of the initial capacity was retained after 30 cycles. However, the energy densities of the NLVP/C electrode over cycling are higher than the energy density of NVP/C electrodes. Gradual capacity fading in the initial electrochemical cycles may be attributed to a relative increase in the insertion/extraction of Na+ (rather than Li+). More energy is required to move Na ions due to their larger atomic masses and ionic radii. The discharge capacities of the NLVP/C electrode at current densities from 7.14 to 1429 mA g-1 are displayed in Fig. 6(b). The capacity decreases gradually as the current density increases. Impressively, the NLVP/C electrode delivers average discharge capacities of 71, 62, 50, and 40 mAh g-1 at higher current densities (0.229, 0.457, 0.914, and 1.429 A g-1 respectively). This demonstrates the feasibility of intercalation and deintercalation of hybrid ions (Na+/Li+) into the ion-exchanged NLVP/C composite electrode at higher current densities. The electrochemical performance of the NLVP/C composite electrode within a wider potential widow (2.5–4.6 V) is shown in Fig. 7. Increasing the upper cut-off voltage to 4.6 V leads to an additional plateau-like region in the higher potential range (≥4.3 V). This is ascribed to the kinetically difficult extraction of Li/Na ions from the third site (Li2) of the monoclinic LVP structure.16,21–24 The material delivers an initial discharge capac-


Chemistry of Materials ity of about 145 mAh g-1. In contrast, the discharge curve exhibits characteristic solid solution behavior during insertion of the first two Li/Na ions and a plateau-like region during insertion of the third ion. ICP studies conducted after charging to 4.6 V indicate that there are few Li/Na ions (0.02/0.03) in the structure (Table S1, Supporting Information). A successive discharge curve shows solid solution phase behavior that is characteristic of monoclinic LVP. The excess charging capacity observed during the initial charging process may be attributed to decom-

23

17 -1

Capacity (mAh g )

(13-1)

(3-12)

120 (2-22)

(220)

(1-2-2)

(10-3)

(2-10)

(301) / (2-13)

150

(11-2)

(1-11)

position of the electrolyte in the higher voltage region. During consecutive charge/discharge cycles, the discharge capacity fades as the number of cycles increases. Fig. S5(a) shows that approximately 50% of the initial capacity is retained after 50 cycles. The ex-situ SXRD patterns of the NLVP/C composite electrode in Fig. S5(b) indicate that this poor cycling performance may be due to deformation/re-formation of the structure after charge/discharge cycle.

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 14

90

23

17

60 8

8

30

Monoclinic Li3V2P3O12 ICSD No: 81589

1

1

0

16

2θ /

18 20 ο

21

2θ /

ο

22

24

2θ /

26

ο

28 ο 2θ /

30

32

2θ /

34 ο

36 38 ο 2θ /

4

3

Voltage (V)

Figure 8. In-situ SXRD patterns along with the corresponding charge curve of NLVP/C composite cathode during charge process in Na-half cell within the potential windows of 2.5–4 V vs. Na/Na+. (Note: the numbers on the right side of the SXRD figure (1, 8, 17 and 23) specifies the related SOCs in the electrochemical curve). To gain further insight regarding the electrochemical mechanism and material performance, in-situ SXRD patterns were recorded for the NLVP/C composite sample at various states of charge (SOCs) and depths of discharge (DODs) in the 2.5−4.0 V potential range at a current density of 0.014 A g-1. The SXRD patterns and charge/discharge curves with corresponding voltage positions are shown in Figs. 8 and 9. The diffraction lines (111), (11-2), (2-10), (10-3), (1-2-2), (220), and (3-12) of the monoclinic LVP lattice system were chosen to identify the phase changes with respect to de-intercalation and intercalation of hybrid ions. Generally, the voltage plateaus in the monoclinic LVP structure charging curve correspond to the extraction of three lithium ions over the sequence of transitions between the phases of LixV2(PO4)3 (where, x = 3.0, 2.5, 2.0, 1.0, and 0.0) within the voltage range of 3−4.8 V vs. Li/Li+.52 In-situ XRD studies by Morcrette et al. confirmed the sequence of two-phase transitions during charging.52 When a smaller potential window (3−4.3 V vs. Li/Li+) was used, the phase transition was restricted to the first four phases. In addition, Nazar et al. conducted an extensive study of the pseudo-orthorhombic phase (x = 2.5) which lies between the starting (x = 3) and charge-

ordered (x = 2) phases.29 In the present NLVP/C composite system, we observe only two intermediate phases within the 2.5 to 4.0 V potential window. No significant shifts in the peak position are observed as Li/Na ions deintercalate from the cathode material. However, new peaks related to phase II emerge as we charge the electrode beyond ∼3.39 V. Further charging increases the intensity of the second phase at the expense of first phase. Subsequent extraction of hybrid ions leads to the emergence of a third phase at ∼3.8 V, which then gradually increases at the expense of initial two phases. Finally, the third phase (phase III) dominates at the end of the charging process. Altogether, the system undergoes series of two-phase transitions between single phases of Na(1.1a)Li(2.0-b)V2(PO4)3 (a + b = x = 0, 1 and 2) that correspond with the voltage plateaus at around 3.37 and 3.85 V during extraction of Li/Na ions. The in-situ SXRD patterns in Fig. 9 demonstrate the reversibility of the phase change during the discharge process. Furthermore, the SXRD patterns measured in-situ during the two-phase transitions of the present monoclinic-orthorhombic NLVP/C composite during charging and discharging demonstrate behavior characteristic of the monoclinic/orthorhombic


Page 11 of 14

NASICON structure, and show that no rhombohedral phase is formed during the Na ion exchange reaction. Furthermore the crystallinity and structure of the NLVP/C sample are preserved even after de-intercalation and intercalation of larger Na ions into the structure. The SXRD pattern of the initial sample is nearly identical to

-1

41

35

60

35

30

28

28

Monoclinic Li3V2P3O12 ICSD No: 81589 24

16

18 20

2θ /

ο

21

2θ /

ο

22

24 ο 2θ /

26

28 ο 2θ /

30

32

2θ /

34 ο

Capacity (mAh g )

(3-12)

(13-1)

90 (2-22)

(220)

(301) / (2-13)

41

(1-2-2)

(2-10)

(10-3)

120

(11-2)

(1-11)

one measured after one electrochemical cycle. However the final sample exhibits a minor peak position shift that may be attributed to differences in the extent of Li/Na ion deinsertion and reinsertion during electrochemical cycling.

Intensity / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 24

36

3 38 4 2θ / Voltage (V) ο

Figure 9. In-situ SXRD patterns along with the corresponding discharge curve of NLVP/C composite cathode during discharge process in Na-half cell within the potential windows of 2.5–4 V vs. Na/Na+. (Note: the numbers on the right side of the SXRD figure (1, 8, 17 and 23) specifies DODs in the electrochemical curve). To understand the distribution of Na ions across different sites in the ion-exchanged monoclinic LVP structure, we performed ICP studies on ex-situ electrodes in various DOD and SOC conditions. ICP data from the powder samples indicates that approximately one Na ion is exchanged for one Li ion per formula unit in the monoclinic LVP structure. It is well known from previous studies that the Li(3) site has higher energy than to the Li(1) and Li(2) sites.16,22,24 High-energy sites are most favored during the ion-exchange reaction. Interestingly, ICP results of the exsitu samples at 4 V in Table S1 indicate that 0.25 of a Na ion is exchanged for Li ions at the Li(2) site, which has the lowest energy. Additional ex-situ ICP results measured at different charge/discharge states in Table S1 clearly indicate that the Na ion is partially exchanged for Li ions at all three sites (Li(1), Li(2), and Li(3)) in the monoclinic LVP structure. Thus, electrochemical, in-situ XRD and ex-situ ICP studies confirm that the two-step voltage plateaus of the present monoclinic-orthorhombic NLVP/C composite correspond to the replacement of almost one Na ion in the monoclinic LVP structure without transformation to a rhombohedral NASICON structure.40,41 The ICP studies performed at different electrochemical stages indicate that the partially exchanged Na ions are distributed across all available sites in the monoclinic and orthorhombic phases of LVP.

The strategy of using a soft Li/Na-ion exchange reaction with monoclinic LVP/C produces a new monoclinicorthorhombic NLVP/C composite phase that is a promising cathode material for hybrid (Li+/Na+) storage applications. Usually, the high-temperature orthorhombic γphases is stabilized at room temperature by introducing additional vacancies into the lithium sites in monoclinic LVP via transition metal doping at vanadium sites (Zr4+, Ti4+, Cr3+, or Mg2+).26,29–33 However, the present chemical ion-exchange reaction facilitates the formation of this new alkali-ion conducting NASICON polymorph. While the ion-exchange reaction did not transform the monoclinic LVP to the usual rhombohedral structure, the prepared monoclinic-orthorhombic NLVP/C composite possesses monoclinic sites with three different energies thereby providing opportunities for insertion/extraction of guest-ions. Within the lower potential window (2.5 – 4V), the initial electrochemical reaction of monoclinicorthorhombic NLVP/C composite cathode involved combination of both Li and Na-ions. Under prolonged cycle rates, the NLVP/C cathode facilitates excessive storage of Na-ions while simultaneously undergoing the transformation from a monoclinic-orthorhombic NLVP/C composite phase to a complete orthorhombic NLVP/C phase. This could be attributed to the occupancy of Na-ions in the available orthorhombic sites. Therefore, shedding light on the phase behavior of such composites will help



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in the practical applications of these phosphates as electrodes. Nevertheless, widening the potential window to 2.5–4.6 V contributed to a high energy density; albeit the structural properties require to be tuned. The limitation commonly known to rhombohedral NVP/C structure is that only 2 Na–ion can be inserted/extracted whereas the use of this composite will help to visualize a high average potential (>3.4) and the storing of more than two alkali ions.

(3)

(4) (5) (6)

CONCLUSION We synthesized a monoclinic-orthorhombic NLVP/C + + composite phase as a new cathode material for (Li /Na ) hybrid ion batteries via a chemical ion-exchange reaction with monoclinic LVP/C. The formation of a monoclinicorthorhombic NLVP/C composite phase was confirmed based on synchrotron XRD patterns (profile matching) and an absence of significant phase transitions related to transformations between α, β, and γ phases in thermal studies. The HR-SEM and HR-TEM images clearly indicate that the morphology and carbon coating of the pristine sample (monoclinic LVP/C) are maintained after ion-exchange. Encouragingly, when placed in a Na half-cell, this monoclinicorthorhombic NLVP/C composite cathode delivered a dis-1 charge capacity of 115 mAh g with an average discharge potential of 3.47 V. The estimated energy density of 102.5 Wh -1 kg for the ion-exchanged electrode within the selected potential window (2.5–4 V) of the initial cycle is significantly higher than that of the pyro-synthesized rhombohedral NVP cathode in the 2.5−4.0 V voltage range.

(7)

(8)

(9) (10)

(11)

(12) (13)

ASSOCIATED CONTENT Supporting Information. The Na-ion exchange reaction under different solvent medium, elemental mapping(HRTEM), charge/discharge curve (monoclinic LVP/C, NLVP/C), cycleability data, ex-situ XRD, and ex-situ ICP data information. This material is available free of charge via the Internet at http://pubs.acs.org.

(14)

(15)

AUTHOR INFORMATION Corresponding Author

(16)

*E-mail: [email protected]

Notes

The authors declare no competing interests.

ACKNOWLEDGMENT

(17)

This work was supported by (i) National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2014R1A2A1A10050821), and (ii) the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078875) or (2013-073298).

(18)

(19)

REFERENCES (1) (2)

Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451 (7179), 652–657. Crabtree, G.; Kócs, E.; Trahey, L. The Energy-Storage Frontier: Lithium-Ion Batteries and beyond. MRS Bull. 2015, 40 (12), 1067–1078.

(20)

Page 12 of 14

Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage. Angew. Chemie - Int. Ed. 2015, 54 (11), 3432– 3448. Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114 (23), 11636–11682. Barker, J.; Gover, R. K. B.; Burns, P.; Bryan, A. J. Hybrid-Ion A Lithium-Ion Cell Based on a Sodium Insertion Material. Electrochem. Solid-State Lett. 2006, 9 (4), A190–A192. Barker, J.; Gover, R. K. B.; Burns, P.; Bryan, a. J. Na3V2(PO4)2F3: An Example of a HybridLi[4/3]Ti[5/3]O4∥ Ion Cell Using a Non-Graphitic Anode. J. Electrochem. Soc. 2007, 154 (9), A882–A887. Song, W.; Ji, X.; Pan, C.; Zhu, Y.; Chen, Q.; Banks, C. E. A Na3V2(PO4)3 Cathode Material for Use in Hybrid Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2013, 15 (34), 14357– 14363. Song, W.; Ji, X.; Yao, Y.; Zhu, H.; Chen, Q.; Sun, Q.; Banks, C. E. A Promising Na3V2(PO4)3 Cathode for Use in the Construction of High Energy Batteries. Phys. Chem. Chem. Phys. 2014, 16 (7), 3055–3061. Walter, M.; Kravchyk, K. V.; Ibáñez, M.; Kovalenko, M. V. Efficient and Inexpensive Sodium-Magnesium Hybrid Battery. Chem. Mater. 2015, 27 (21), 7452–7458. Nose, M.; Nobuhara, K.; Shiotani, S.; Nakayama, H.; Nakanishi, S.; Iba, H. Electrochemical Li+ Insertion Capabilities of Na4−xCo3(PO4)2P2O7 and Its Application to Novel Hybrid-Ion Batteries. RSC Adv. 2014, 4 (18), 9044. Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113 (8), 6552–6591. Goodenough, J. B.; Hong, H. .-P.; Kafalas, J. A. Fast Na+-Ion Transport in Skeleton Structures. Mater. Res. Bull. 1976, 11 (2), 203–220. Anantharamulu, N.; Koteswara Rao, K.; Rambabu, G.; Vijaya Kumar, B.; Radha, V.; Vithal, M. A Wide-Ranging Review on Nasicon Type Materials. J. Mater. Sci. 2011, 46 (9), 2821–2837. Gopalakrishnan, J.; Rangan, K. K. Vanadium Phosphate (V2(PO4)3): A Novel NASICON-Type Vanadium Phosphate Synthesized by Oxidative Deintercalation of Sodium from Sodium Vanadium Phosphate (Na3V2(PO4)3). Chem. Mater. 1992, 4 (4), 745–747. Gaubicher, J.; Wurm, C.; Goward, G.; Masquelier, C.; Nazar, L. Rhombohedral Form of Li3V2(PO4)3 as a Cathode in LiIon Batteries. Chem. Mater. 2000, 12 (11), 3240–3242. Morgan, D.; Ceder, G.; Saïdi; Barker, J.; Swoyer, J.; Huang, H.; Adamson, G. Experimental and Computational Study of the Structure and Electrochemical Properties of LixM2(PO4)3Compounds with the Monoclinic and Rhombohedral Structure. Chem. Mater. 2002, 14 (11), 4684– 4693. Lim, S. Y.; Kim, H.; Shakoor, R. a.; Jung, Y.; Choi, J. W. Electrochemical and Thermal Properties of NASICON Structured Na3V2(PO4)3 as a Sodium Rechargeable Battery Cathode: A Combined Experimental and Theoretical Study. J. Electrochem. Soc. 2012, 159 (9), A1393–A1397. Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. The First Report on Excellent Cycling Stability and Superior Rate Capability of Na3V2(PO4)3 for Sodium Ion Batteries. Adv. Energy Mater. 2013, 3 (4), 444–450. Chotard, J.-N.; Rousse, G.; David, R.; Mentré, O.; Courty, M.; Masquelier, C. Discovery of a Sodium-Ordered Form of Na3V2(PO4)3 below Ambient Temperature. Chem. Mater. 2015, 27 (17), 5982–5987. Lalère, F.; Leriche, J. B.; Courty, M.; Boulineau, S.; Viallet, V.; Masquelier, C.; Seznec, V. An All-Solid State NASICON Sodium Battery Operating at 200 C. J. Power Sources 2014, 247, 975–980.


Page 13 of 14 (21)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(22)

(23)

(24) (25)

(26)

(27)

(28)

(29)

(30)

(31) (32)

(33)

(34)

(35) (36)

Chemistry of Materials Huang, H.; Yin, S. C.; Kerr, T.; Taylor, N.; Nazar, L. F. Nanostructured Composites: A High Capacity, Fast Rate Li3V2(PO4)3/carbon Cathode for Rechargeable Lithium Batteries. Adv. Mater. 2002, 14 (21), 1525–1528. Yin, S.-C.; Grondey, H.; Strobel, P.; Anne, M.; Nazar, L. F. Electrochemical Property: Structure Relationships in Monoclinic Li3-yV2(PO4)3. J. Am. Chem. Soc. 2003, 125 (34), 10402–10411. Yin, S. C.; Grondey, H.; Strobel, P.; Huang, H.; Nazar, L. F. Charge Ordering in Lithium Vanadium Phosphates: Electrode Materials for Lithium-Ion Batteries. J. Am. Chem. Soc. 2003, 125 (2), 326–327. Liu, C.; Massé, R.; Nan, X.; Cao, G. A Promising Cathode for Li-Ion Batteries: Li3V2(PO4)3. Energy Storage Mater. 2016, 4, 15–58. Bykov, A. B.; Chirkin, A. P.; Demyanets, L. N.; Doronin, S. N.; Genkina, E. A.; Ivanov-shits, A. K.; Kondratyuk, I. P.; Maksimov, B. A.; Mel’nikov, O. K.; Muradyan, L. N.; Simonov, V. I.; Timofeeva, V. A. Superionic Conductors Li3M2(PO4)3 (MFe, Sc, Cr): Synthesis, Structure and Electrophysical Properties. Solid State Ionics 1990, 38 (1), 31–52. Sato, M.; Ohkawa, H.; Yoshida, K.; Saito, M.; Uematsu, K. Enhancement of Discharge Capacity of Li3V2(PO4)3 by Stabilizing the Orthorhombic Phase at Room Temperature. 2000, 135, 137–142. de la Rochère, M.; d’Yvoire, F.; Collin, G.; Comès, R.; Boilot, J. P. NASICON Type Materials - Na3M2(PO4)3 (M=Sc, Cr, Fe): Na+-Na+ Correlations and Phase Transitions. Solid State Ionics 1983, 9–10 (PART 2), 825–828. Masquelier, C.; Padhi, A.; Nanjundaswamy, K.; Goodenough, J. New Cathode Materials for Rechargeable Lithium Batteries: The 3-D Framework Structures Li3Fe2(XO4)3 (X=P, As). J. Solid State Chem. 1998, 135 (2), 228–234. Yin, S.-C.; Strobel, P. S.; Grondey, H.; Nazar, L. F. Li2.5V2(PO4)3: A Room-Temperature Analogue to the Fast-Ion Conducting High-Temperature γ-Phase of Li3V2(PO4)3. Chem. Mater. 2004, 16 (8), 1456–1465. Deng, C.; Zhang, S.; Yang, S. Y.; Gao, Y.; Wu, B.; Ma, L.; Fu, B. L.; Wu, Q.; Liu, F. L. Effects of Ti and Mg Codoping on the Electrochemical Performance of Li 3 V 2 (PO 4 ) 3 Cathode Material for Lithium Ion Batteries. J. Phys. Chem. C 2011, 115, 15048–15056. LIU, S.; LI, S.; HUANG, K.; CHEN, Z. Effect of Doping Ti4+ on the Structure and Performances of Li3V2(PO4)3. Acta Physico-Chimica Sin. 2007, 23 (4), 537–542. Chen, Y.; Zhao, Y.; An, X.; Liu, J.; Dong, Y.; Chen, L. Preparation and Electrochemical Performance Studies on Cr-Doped Li3V2(PO4)3 as Cathode Materials for LithiumIon Batteries. Electrochim. Acta 2009, 54 (24), 5844–5850. Huang, J. S.; Yang, L.; Liu, K. Y.; Tang, Y. F. Synthesis and Characterization of Li3V(2 - 2x/3)Mgx(PO4)3/C Cathode Material for Lithium-Ion Batteries. J. Power Sources 2010, 195 (15), 5013–5018. Suzuki, T.; Yoshida, K.; Uematsu, K.; Kodama, T.; Toda, K.; Ye, Z.-G.; Sato, M. Stabilization of Superionic Conduction Phase in Li3Sc2(PO4)3. Solid State Ionics 1997, 104 (1–2), 27– 33. Suzuki, T.; Yoshida, K.; Uematsu, K.; Kodama, T.; Toda, K. Structure Refinement of Lithium Ion Conductors Li 3 Sc 2 ( PO 4 ) 3 and Diffraction. Solid State Ionics 1998, 115, 89–96. Kuang, Q.; Zhao, Y.; Liang, Z. Synthesis and Electrochemical Properties of Na-Doped Li3V2(PO4)3 Cathode Materials for Li-Ion Batteries. J. Power Sources

(37)

(38)

(39)

(40)

(41)

(42)

(43)

(44) (45)

(46)

(47)

(48) (49)

(50) (51) (52)

2011, 196 (23), 10169–10175. Yan, J.; Yuan, W.; Xie, H.; Tang, Z. Y.; Liu, F. J.; Mao, W. F.; Xu, Q.; Zhang, X. H. Preparation and Electrochemical Performance of Na-Doped Li3V2(PO4)3/C Cathode Material. J. Solid State Electrochem. 2012, 16 (10), 3201–3206. Chen, Q.; Qiao, X.; Wang, Y.; Zhang, T.; Peng, C.; Yin, W.; Liu, L. Electrochemical Performance of Li3−xNaxV2(PO4)3/C Composite Cathode Materials for Lithium Ion Batteries. J. Power Sources 2012, 201, 267–273. Wang, R.; Xiao, S.; Li, X.; Wang, J.; Guo, H.; Zhong, F. Structural and Electrochemical Performance of Na-Doped Li3V2(PO4)3/C Cathode Materials for Lithium-Ion Batteries via Rheological Phase Reaction. J. Alloys Compd. 2013, 575 (December 2015), 268–272. Tang, Y.; Wang, C.; Zhou, J.; Bi, Y.; Liu, Y.; Wang, D.; Shi, S.; Li, G. Li2NaV2(PO4)3: A Novel Composite Cathode Material with High Ratio of Rhombohedral Phase. J. Power Sources 2013, 227, 199–203. Shao, L.; Shu, J.; Tang, Y.; Li, P.; Lin, X.; Shui, M.; Long, N.; Wang, D. Phase Diagram and Electrochemical Behavior of Lithium Sodium Vanadium Phosphates Cathode Materials for Lithium Ion Batteries. Ceram. Int. 2015, 41 (3), 5164–5171. Mateyshina, Y. G.; Uvarov, N. F. Electrochemical Behavior of Li3−xM′ xV2−yM″ y(PO4)3 (M′ =K, M″ =Sc, Mg+Ti)/C Composite Cathode Material for Lithium-Ion Batteries. J. Power Sources 2011, 196 (3), 1494–1497. Sun, C.; Rajasekhara, S.; Dong, Y.; Goodenough, J. B. Hydrothermal Synthesis and Electrochemical Properties of Li3V2(PO4)3/C-Based Composites for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2011, 3 (9), 3772–3776. Gim, J.; Mathew, V.; Lim, J.; Song, J.; Baek, S.; Kang, J.; Ahn, D.; Song, S.-J.; Yoon, H.; Kim, J. Pyro-Synthesis of Functional Nanocrystals. Sci. Rep. 2012, 2, 946. Kang, J.; Song, J.; Kim, S.; Gim, J.; Jo, J.; Mathew, V.; Han, J.; Kim, J. A High Voltage LiMnPO4–LiMn2O4 Nanocomposite Cathode Synthesized by a One-Pot Pyro Synthesis for Li-Ion Batteries. RSC Adv. 2013, 3 (48), 25640– 25643. Kang, J.; Mathew, V.; Gim, J.; Kim, S.; Song, J.; Im, W. Bin; Han, J.; Lee, J. Y.; Kim, J. Pyro-Synthesis of a High Rate Nano-Li3V2(PO4)3/C Cathode with Mixed Morphology for Advanced Li-Ion Batteries. Sci. Rep. 2014, 4, 4047. Song, J.; Park, S.; Mathew, V.; Gim, J.; Kim, S.; Jo, J.; Kim, S.; Alfaruqi, M. H.; Baboo, J. P.; Kim, I.-H.; Song, S.-J.; Kim, J. An Enhanced High-Rate Na3V2(PO4)3-Ni2P Nanocomposite Cathode with Stable Lifetime for SodiumIon Batteries. ACS Appl. Mater. Interfaces 2016, 8 (51), 35235–35242. Clearfield, A. Role of Ion Exchange in Solid-State Chemistry. Chem. Rev. 1988, 88 (1), 125–148. Park, Y.-U.; Seo, D.-H.; Kim, B.; Hong, K.-P.; Kim, H.; Lee, S.; Shakoor, R. a; Miyasaka, K.; Tarascon, J.-M.; Kang, K. Tailoring a Fluorophosphate as a Novel 4 V Cathode for Lithium-Ion Batteries. Sci. Rep. 2012, 2 (i), 704. Rodríguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Phys. B Condens. Matter 1993, 192 (1–2), 55–69. Boilot, J.; Colomban, P.;Collin, G. Nasicon : Amorphous to Crystalline Compounds. Solid State Ionics 1986, 18–19, 974– 980. Morcrette, M.; Leriche, J.-B.; Patoux, S.; Wurm, C.; Masquelier, C. In Situ X-Ray Diffraction during Lithium Extraction from Rhombohedral and Monoclinic Li3 V 2 ( PO 4 ) 3. Electrochem. Solid-State Lett. 2003, 6 (5), A80– A84.



SYNOPSIS TOC. Prolonged Cycling

Monoclinic-orthorhombic NLVP

Orthorhombic NLVP 4.0

3.5

2000

3.0

d(Q)/dV

1000

2.5

0

-1000 2.5

3.0

3.5

+

3.5

600

3.0 d(Q)/dV

+

+


4.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 14

2.5

300 0

-300 -600 2.5

4.0

3.0

3.5

4.0 +



2.0

2.0 0

20

40

60

80

100

120

140

0

20

-1

40

60

80 -1

Capacity (mAh g )

Capacity (mAh g )


100

120

Monoclinic-Orthorhombic Na 1.1 Li 2.0 V 2 (PO 4 ... - ACS Publications

Recommend Documents