Unique crystallization behavior of sodium manganese ...

Journal of Asian Ceramic Societies 5 (2017) 209–215

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Unique crystallization behavior of sodium manganese pyrophosphate Na2 MnP2 O7 glass and its electrochemical properties Morito Tanabe, Tsuyoshi Honma ∗ , Takayuki Komatsu Department of Materials Science and Technology, Nagaoka University of Technology, Kamitomioka-cho 1603-1, Nagaoka Niigata 940-2188, Japan

a r t i c l e

i n f o

Article history: Received 3 March 2017 Received in revised form 27 April 2017 Accepted 30 April 2017 Available online 17 May 2017 Keywords: Sodium ion batteries Sodium manganese phosphate Glass-ceramics Electrochemical properties

a b s t r a c t Crystallization behavior of Na2 MnP2 O7 precursor glass was examined. Layered type Na2 MnP2 O7 was formed at 461 ◦ C for 3 h in N2 filled electric furnace. Irreversible phase change was confirmed from layered Na2 MnP2 O7 to ␤-Na2 MnP2 O7 over 600 ◦ C. At 650 ◦ C crystallized phase was completely changed to ␤phase. By means of charge and discharge testing it is found that layered Na2 MnP2 O7 is also active as cathode in sodium ion batteries. We found glass-ceramics technology is one of the suitable process for the synthesis of layered Na2 MnP2 O7 cathode without any complicate process. © 2017 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

1. Introduction As expanding of lithium ion batteries for large scale electrical storage usage, the study of alternative post-lithium ion batteries is drawing attention in recently. In especially, the study on sodium ion batteries become active because of its materials abundance and low cost as well. Poly-anion type cathode has high structure stability which is made from phosphate, silicate, borate and sulfate systems. Variety of cathode materials have been found rather than anode materials. Even in sodium cell, poly-anion based phosphates and sulphates are well known as principal category of cathode active materials after the discovery of LiFePO4 [1]. Table 1 lists the typical phosphates which exhibits good electrochemical performance for sodium cell [2–13]. We proposed Na2 FeP2 O7 which is prepared by crystallization of precursor glass and reported its electrochemical properties [2,3]. Na2 FeP2 O7 and the other pyrophosphates are also reported as promising cathode active materials [14–19]. Na2 FeP2 O7 is more environmentally friendly compared with Na2 FePO4 F and Na3 V2 (PO4 )3 . Each FeO6 octahedral units are forming Fe2 O11 units and polymerized P2 O7 units are corner shared with Fe2 O11 . Due to the three dimensional Na+ ion diffusion mobility of Na+ is relative high. Relative amount of Na ion substitute Fe sites, in generally, chemical formula such materials are expressed as Na2−x M1+x/2 P2 O7 (Na4−x M2+x/2 (P2 O7 )2 ) and M is divalent ions such as Mg [20,21], Mn [22], Co [23], Ni [21]. Their

∗ Corresponding author. Fax: +81 258 47 9300. E-mail address: [email protected] (T. Honma).

crystal structure varies widely with the type of M species. It is noted that battery property sensitively depends on structure of active materials. There are several scientific novelties if we found new structure in Na2 MP2 O7 system from a view point of crystallization behavior of glass precursor as well as electrochemical properties. For instance, the other NaMP2 O7 , which is already cleared its electrochemical properties, is listed in Table 2 [3,4,22]. Crystal structure of Na2 Fe2 P2 O7 and Na2 CoP2 O7 is also shown in Fig. 1. Na2 MnP2 O7 is more interesting to increase discharge voltage rather than Na2 FeP2 O7 (Fig. 2). Barpanda et al. reported electrochemical properties of ␤-Na2 MnP2 O7 which is same as the structure of Na2 FeP2 O7 , as 3.6 V (Mn2+ /Mn3+ vs sodium anode) for 80 mAh/g [22]. The other groups also reported electrochemical properties in ␤-Na2 MnP2 O7 [24–28]. On the other hand, some literatures are suggesting that ␤-Na2 MnP2 O7 is electrochemically inactive rather than Na2 FeP2 O7 [24,26]. There is another structure in Na2 MnP2 O7 . Layered Na2 MnP2 O7 [29] is almost identical with Na2 ZrSi2 O7 [30], Na2 PdP2 O7 [31] and Na2 Cu2 P2 O7 [32] phase. In this study we examined crystallization behavior as well as electrochemical properties of Na2 MP2 O7 glass and glass-ceramics. 2. Experimental procedure 2.1. Preparation of Na2 MnP2 O7 precursor glass Precursor glass of Na2 MnP2 O7 was prepared by conventional melt-quenching method. Sodium dihydrogen phosphate (98% NaH2 PO4 , Nakarai Tesque Co., Japan) and manganese monoxide (99.9% MnO, Kojyundo chemicals Co., Japan) was used as start-

http://dx.doi.org/10.1016/j.jascer.2017.04.009 2187-0764/© 2017 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Table 1 Typical cathode candidates in phosphate system for Na cell. Red-ox pair 2+

Na2 FeP2 O7 [2,3] Na4 Fe3 (PO4 )2 (P2 O7 ) [5] Na3 V2 (PO4 )3 [9] Na2 FePO4 F [10] Na3 V2 (PO4 )2 F3 [13]

3+

M /M M2+ /M3+ M3+ /M4+ M2+ /M3+ M3+ /M4+

Voltage (V vs. Na+ /Na)

Capacity (mA h/g)

M

3.0 3.2 3.3 3.06 and 2.91 4.1 and 3.6

90 100 120 110 120

Mn [22], Co [4] Mn [6], Co [7], Co2.4 Mn0.3 Ni0.3 [8] Mn [11], Co [12]

Table 2 List of Na2 MP2 O7 and their electrochemical properties.

Na2 FeP2 O7 [3] ˇ-Na2 MnP2 O7 [22] Na2 CoP2 O7 [4]

Space group

Structure

M–O polyhedra

Voltage (V vs. Na+ /Na)

Capacity (mAh/g)

Triclinic (P−1 ) Triclinic (P1 ) Orthorhombic (Pna21 )

Three dimensional Three dimensional Layered Melilite

Octahedra Octahedra tetrahedra

3.0 3.6 3.0

90 80 80

Fig. 1. Crystal structure of (a) Na2 FeP2 O7 and (b) Na2 CoP2 O7 .

ing reagent. The mixture of glass batch of 10 g was melted in graphite crucible with a lid under N2 flow at 1050 ◦ C for 30 min. The melts were poured onto steel plate then pressed another iron plate quickly to obtain bulk shape. After mirror polishing we obtained the glass plate with 0.3 mm thick. A Shimadzu UV-3100 UV–vis–NIR recording spectrometer was utilized to record the absorption spectra in the wavelength range of 300–2000 nm. Differential thermal analysis (DTA) were performed using a TG-DTA system (Thermoplus EVO TG-8120, RIGAKU Corp., Japan) to evaluate the thermal properties of the precursor samples. N2 flow rate and temperature scanning rate were set to 70 ml min−1 and 10 K min−1 , respectively. 2.2. Crystallization process The glass powder kept on an alumina boat and introduced N2 filled tubular furnace and heat treatment was performed from 475 ◦ C to 700 ◦ C. In order to avoid overshoot of heater heating rate was changed from 10 K/min to 1 K/min before 30 ◦ C of heat treatment temperature. In order to confirm glass formation and to characterize crystalline phase in-glass ceramics, XRD patterns of all samples were

obtained on Rigaku Ultima IV X-ray diffractometer (Rigaku, Japan) with D/tex 1D high-speed detector, which was operated at 40 kV, 40 mA with Cu-K␣ radiation (␭ = 0.154056 nm). All the measurements were carried out at room temperature under atmospheric air. Raman spectra were measured at 1 cm−1 resolution, using a dispersive Raman spectrometer (Jasco, model NRS-7200) with excitation wavelength of 785.36 nm to avoid photoluminescence from Mn2+ [33]. 2.3. Electrochemical properties In order to improve electronic conductivity, both carbon coating process and pulverize finely [34–37]. Therefore, glass/carbon (designate hereafter as glass/CB) and glass-ceramics/carbon (designate hereafter as glass-ceramics/CB) powders were obtained by means of ball-milling process. A mixture of glass and acetylene black (Denka Black) for 2 g batch was placed in a ZrO2 pot (45 cm3 ) together with 50 g balls (3 mm) under an Air atmosphere and treatment was carried out using a planetary ball mill (Fritsch Pulverisette 7) at a rotation speed of 600 rpm for 1 h. Then post anneal process was performed at 465–650 ◦ C again to recover crystallinity. Cathode electrodes were fabricated from a mixture


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of glass/CB and glass-ceramic/CB powder, polyvinylidenefluoride (PVDF) and conductive carbon black in a weight ratio of 85:5:10. N-Methylpyrrolidone (NMP) was used to make slurry of their mixtures. After homogenization, slurry was coated on a thin aluminum foil and dried at 100 ◦ C for 10 h in a vacuum oven. Electrodes were then pressed and disks were punched out as 16 mm␾. Electro-chemical cells were prepared using 2032-coin type cells. Sodium metal foils were used as anode, and glass filter papers (Advantec Co., GA-100) were used as separator. Test cells were assembled in an argon-filled glove box. The dew point of Ar atmosphere in the glove box was kept as −86 ◦ C. The oxygen content was less than 0.33 ppm. The solution of 1 M-NaPF6 (Tokyo Kasei Co.) in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v/v, Kishida Chemicals Co.) was used as electrolyte. Cells were examined by using a battery testing system (Hokuto-denko Co.) at the charge/discharge current density of 1/100C (0.08 mA cm−2 ) or 1/10C for the theoretical capacity for one-electron reaction between 1.5 and 4.5 V. Morphologies of glass-ceramics/CB composites were observed by scanning electron microscope (SEM, Keyence VE-8800).

Absorption coefficient (cm-1)

Fig. 2. Crystal structure of (a) ␤-Na2 MnP2 O7 and (b) layered-Na2 MnP2 O7.

60

Melted in an Air

50 Mn3+

40

Melted in a N2

30 20 10 0

500

1000 1500 Wavelength (nm)

2000

Fig. 3. Optical absorption spectra of Na2 MnP2 O7 precursor glasses those are melted under air or N2 flow in electric furnace.

3.2. Crystallization behavior 3. Results and discussion 3.1. Optical absorption spectra of precursor glass Fig. 3 shows the appearance of the sample and optical absorption spectra of quenched Na2 MnP2 O7 glasses those melted under atmosphere and N2 gas respectively. For the sample which melted air condition, relative amount of Mn3+ is being in glass matrix. On the other hand, transparent Mn2+ rich glass was obtained successfully by N2 gas flow during melting process. For the cathode materials it is suitable to keep as Mn2+ at initial state [38], therefore, we decided N2 flow atmosphere to keep Mn2+ rich state in glass melts during melting process.

The DTA patterns for the as-quenched sample are shown in Fig. 4. The endothermic dips due to the glass transition (390 ◦ C) and exothermic peak (465 ◦ C) due to the crystallization are clearly observed. Compared to the Na2 FeP2 O7 glass [2,39] both glass transition and crystallization temperature shift much lower temperature. It is noteworthy that endothermic peaks are observed from 650 to 750 ◦ C in Na2 MnP2 O7 glass. This endothermic peak is not melting temperature. In order to check the crystallization behavior more in detail, heat treatment was performed in various temperature above glass transition temperature and the results of X-ray diffraction patterns are shown in Fig. 5. At first diffractions derived from layered-Na2 MnP2 O7 [29] as sin-

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Na2MnP2O7

Tg: 460°C

Tp: 590°C Na2FeP2O7

Reheated β-Na2MnP2O7 (465° C)

Intensity (arb. units)

Tg: 390°C

Endo.

Exo.

Tp: 465°C

β-Na2MnP2O7 (650° C)

Simulated β-Na2MnP2O7 ICSD 187790

300

400 500 600 700 Temperature (°C)

800

10

20

30 40 50 2θ (deg)

Fig. 4. Differential thermal curves of NaMnP2 O7 glass and Na2 FeP2 O7 glass.

Simulated β-Na2MnP2O7

60

70

Fig. 6. Powdered XRD patterns of reheated ␤-Na2 MnP2 O7 glass ceramics at 465 ◦ C and 650 ◦ C.

νs (P–O–P)

700° C

ICDD 01-089-5448

10 20 30 40 50 60 70 2θ (deg) Fig. 5. Powdered XRD patterns after heat treatment in various temperature of Na2 MnP2 O7 glass-ceramics.

gle phase without any impurities. Crystallized phase was changed from layered Na2 MnP2 O7 to ␤-Na2 MnP2 O7 with increasing heat-treatment temperature. The product prepared at 460C is well-crystallized and can be indexed to layeredNa2 MnP2 O7 with the defined cell parameters: a = 0.53093(7) nm, b = 0.65871(8) nm, c = 9.4255(6) nm, ␣ = 109.610◦ , ␤ = 95.134◦ and ␥ = 106.302◦ ; space group: P-1(2) [40]. On the other hand, for the ␤-Na2 MnP2 O7 at 700 ◦ C, cell parameters are defined cell parameters: a = 0.99112(8) nm, b = 1.10823(7) nm, c = 1.24657(1) nm, ␣ = 148.415◦ , ␤ = 121.952◦ and ␥ = 68.411◦ : space group: P1(1) [22]. For the ␤-Na2 MnP2 O7 glass-ceramics which heated at 650 ◦ C was reheated at 465 ◦ C for 3 h and XRD pattern is shown in Fig. 6. After all, crystalline phase never changed. Fig. 7 shows the microRaman scattering spectra of as quenched glass, layered Na2 MnP2 O7 (465 ◦ C) and ␤-Na2 MnP2 O7 (650 ◦ C). Structural reconstructions of phosphates units during both crystallizations from glass to layered structure as well as phase transition to ␤-Na2 MnP2 O7 respectively. As progress of crystallization form precursor glass, rearrangement

400

1316

1252

1052 1099

1005

1242 1241

1094 1111

1158 1175

1042 946

747

712

Glass Layered Na2MnP2O7

742

439 491 520 557 592

glass

1040

Tp= 465° C

737

550° C

Layered Na2MnP2O7 (465°C ) 424 463 523 547 946

600° C

β-Na2MnP2O7 (650°C )

493

650° C

νas (PO3) and νs (PO3) νas (P–O–P)



ICSD 187790

600 800 1000 1200 1400 Raman shift (cm-1)

Fig. 7. Raman scattering spectra of as quenched glass, layered- and b-phase.

of [P2 O7 ]4− units confirmed. In addition, such rearrangement of [P2 O7 ]4− requires so long time, hence we observed irreversible phase transition phenomena from layered to ␤ phase. [41–44] In the case of Na2 CuP2 O7 , it is reported that irreversible phase transition from ␣- to ␤-phase at 600 ◦ C [32]. We also clarified irreversible phase transition from ␤Na2 MnP2 O7 to layered Na2 MnP2 O7 by post annealing at 465 ◦ C for 10 h. Therefore, it is found that temperature selective crystallization is available for the preparation of Na2 MnP2 O7 glass-ceramics in this study. 3.3. Electrochemical performance We successfully prepared single phase of layered Na2 MnP2 O7 glass-ceramics. As we mentioned, there is no literature of layered Na2 MnP2 O7 and glassy state as well for electro-chemical reaction. It is interesting to clear electro-chemical activity of layeredNa2 MnP2 O7 as well as glassy state. The charge and discharge profiles of glassy state, layered and ␤-Na2 MnP2 O7 glass-ceramic cathodes are shown in Fig. 8. We also show the dQ/dV plots. For the glassy state, initial electric discharge capacity was 36 mAh/g (37% for theoretical capacity) but, clear plateau voltage due to


60

(a)

4.0

40 20

3.5

dQ/dV

Voltage (V vs Na/Na+)

4.5

3.0 2.5

0

-20

1st

2.0

0

10

20

30

40

50

-60 1.5

60

2nd

2.0 2.5 3.0 3.5 4.0 Voltage (V vs. Na/Na+)

Capacity (mAh/g)

4.5

40

4.5

(b)

30

4.0

20 10

3.5

dQ/dV

V (V vs Na/Na+)

1st

(d)

-40

2nd

1.5

3.0

0 -10

2.5

-20

1st 2nd

2.0 0

10

20

30

40

50

60

-30

4.5

(c)

4.0

-40 2.0

50 40 30 20 10 0 -10 -20 -30 -40 -50 2.0

2nd

2.5 3.0 3.5 4.0 Voltage (V vs. Na+/Na)

4.5

dQ/dV

3.5

1st

(e)

Capacity (mAh/g)

Voltage (V vs Na/Na+)

213

3.0 2.5

1st 2nd

2.0 0

10

20

30

40

50

60

Capacity (mAh/g)

1st

(f) 2.5

2nd

3.0

3.5

4.0

4.5

+

V ( vs. Na/Na )

Fig. 8. The 1st and the 2nd charge and discharge curves (0.1C) of (a) glassy state, (b) layered phase and (c) ␤ phase. And the 1st and the 2nd differential capacity (dQ/dV) curves versus voltage (d)–(f) are also shown.

Mn2+ /Mn3+ redox did not appear. To comparing with glassy Na2 FeP2 O7 [39], reversible capacity is less than Na2 FeP2 O7 that mean electrochemical activity is poor. After crystallization, both of crystalline formation depress reversible capacity as 20 mAh/g (for layered-Na2 MnP2 O7 ) and 11 mAh/g (for ␤-Na2 MnP2 O7 ). Eventually, the discharge capacity decreased in order of glassy state, layered-phase and b-phase and all of them is poor electrochemical activity even C/10 rate. These results are in contrast with Na2 FeP2 O7 that is allowable at higher current density up to 10C. In generally, manganese oxide based materials involving with Mn2+ /Mn3+ redox state exhibits poor activity due to lower elec-

tronic conduction in bulk, and difficulty of structural relaxation from Mn2+ to Mn3+ oxide polyhedra so called Jahn-Teller effect [26] [45]. On the other hand, glassy state exhibit larger capacity rather than glass-ceramics after crystallization. Due to the existence of free volume and corner-shared PO4 , P2 O7 and MnO6 those involve lower density, three-dimensional isotropic ionic conduction. A presence of large-scale particle is also considered as one of the reasons for poor electro-chemical activity in Na2 MnP2 O7 . As show in Fig. 9, most of all grains have less than 1 ␮m size. The condition of carbon coating is considered as another factor. In this study we did not perform any carbon coating process by heat-treatment.

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4. Conclusion In conclusion, crystallization behavior of Na2 MnP2 O7 glass and its electrochemical activity was characterized. Layered Na2 MnP2 O7 single phase was successfully synthesized by conventional heattreatment of precursor glass. Due to the difficulty of carbon coating and poor electrical conductivity in comparison with Na2 FeP2 O7 , unfortunately, good electrochemical performance was not able to obtained from Na2 MnP2 O7 . However, such concerns will be overcome by optimization of grain size and carbon coating process in the future. Acknowledgements This study was supported by the Grant-in-Aid for Scientific Research From the Ministry of Education, Science, Sport, Culture, and Technology, Japan (Grant No. 25288105), Research Corroboration Project to develop high performance secondary battery materials between Nagaoka University Technology and Nippon Electric Glass Co. Ltd. References

Fig. 9. SEM images of (a) Na2 MnP2 O7 glass with 10 wt% carbon black, (b) heattreatment at 465 ◦ C and (c) heat-treatment at 650 ◦ C.

To improve electronic conduction conductive carbon was mixed during ball-milling process. Because capacity was dropped in a heat-treatment sample, added carbon may be break away from Na2 MnP2 O7 grain. Hence added carbon deactivated as electronic conductive agent. Therefore, enhancement of electronic conduction is necessary for activation of Na2 MnP2 O7 . However, it is found that glassy state and layered Na2 MnP2 O7 is superior than that of ␤-Na2 MnP2 O7 in this study.

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