Quaternary Transition Metal Oxide Layered Framework: O3-Type Na

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Quaternary Transition Metal Oxide Layered Framework: O3-type Na[Ni Fe Co Mn ]O Cathode Material for High-Performance Sodium-Ion Batteries 0.32

0.13

0.15

0.40

2

Jang-Yeon Hwang, Seung-Taek Myung, and Yang-Kook Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12140 • Publication Date (Web): 10 Mar 2018 Downloaded from http://pubs.acs.org on March 11, 2018

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The Journal of Physical Chemistry

Quaternary Transition Metal Oxide Layered Framework: O3-type Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 Cathode Material for High-Performance Sodium-Ion Batteries Jang-Yeon Hwang,†∥ Seung-Taek Myung,‡∥and Yang-Kook Sun*,†

†

Department of Energy Engineering, Hanyang University, Seoul 04763, South Korea

‡

Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul,

05006, South Korea

∥: These authors contributed equally to this work.

ABSTRACT

Analogous compounds in lithium-ion batteries (LIBs), various ternary chemical compositions in O3-type layered oxides have been introduced in sodium-ion batteries (SIBs). However, O3-type ternary transition metal oxide cathodes, including the NaNixCoyMnzO2 and NaNixFeyMnzO2 (x+y+z=1) compounds, continue to face several challenges with respect to their low reversible capacity and poor cycle retention owing to their structural instability. Herein, we propose the well-balanced

quaternary

transition

metal

oxide

structure

of

O3-type

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 as cathode materials that having an average composition of both

ACS Paragon Plus Environment

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Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.4Co0.3Mn0.3]O2 compounds. Compare to its respective ternary members, Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode exhibits a higher specific capacity as well as improved cycling stability and rate capability. The postmortem ex-situ X-ray diffraction (XRD) studies of cycled electrode clearly shows that co-existence of quaternary transition metals in a Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode could improve the structural stability. Moreover, quaternary transition metal oxide frameworks effectively prevent the dissolution of transition metals during cycling, thus improving the battery performances. The appealing physical properties and electrochemical performance of this material demonstrate its great promise for high-performance O3-type cathode in sodium-ion batteries.

INTRODUCTION The successful commercialization of lithium-ion batteries (LIBs) in 1991 by Sony shifted the focus from sodium intercalation compounds, which were introduced in the early 1980s.1 However, sodium electrode materials for sodium-ion batteries (SIBs) have recently become of interest again because of the limited available lithium resources, which are unevenly distributed worldwide. A shortage of lithium is expected in the near future if large-format LIBs are fully adopted in earnest for electric vehicles (EVs) and energy-storage systems (ESSs).2-3 The similar battery chemistry of LIBs and SIBs could enable the easy substitution of sodium for lithium, which is expected to result in a decrease in the price of batteries utilized in vehicles as well as in power-backup devices for homes and industries.4 Despite the such merits of sodium resources, equilibrium chemical potential of Na/Na+ is slightly higher than that of Li/Li+ (approximately 0.3 V), leading to lower operating voltage of NIBs compared to that of LIBs with the same cutoff anodic potential for positive electrode.5,6


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Furthermore, the large ionic radius of Na+ (1.02 Å) relative to Li+ (0.76 Å) results in limited insertion in three-dimensional (3D) materials, in particular, spinel compounds. Hence, layered structures with large interslab readily allow the large Na+ ions into and out of the crystalline structure, particularly P2 and O3 types, which differ in their oxygen-stacking sequences. P2-type materials stabilize into a Na-deficient form, such as Na2/3MO2 (M: Ni, Co, Mn, Fe, etc.).7-12 An earlier study by Delmas et al.13 demonstrated that these compounds deliver reversible capacities below 150 mAh g-1. In 2012, Komaba et al.8 proposed a new compound (P2-type Na2/3Fe0.5Mn0.5O2) that delivers 190 mAh g-1, thereby enabling the use of earth-abundant elements. However, it exhibited a cycle retention of only 79% over the course of 30 cycles. Okada et al.12 demonstrated intercalation of Na+ ions into an O3-type NaFeO2 layer structure accompanied by Fe3+/4+ redox reactions in the Na electrolyte. Komaba’s group pointed out the intrinsic problem of structural instability caused by Fe-ion migration during cycling in O3-type NaFeO2 layer structure.15 Later, this drawback has been addressed by replacing the transition metal atoms in TMO2 slabs to form the binary or ternary compounds such as NaCoxFeO2,16 NaFe1-xMnxO2

(x

=

1/2,

1/3),17

Na[Ni1/3Fe1/3Mn1/3]O2,18

Na[Fex(Ni0.5Mn0.5)1-x]O2,19

Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O220. In recent, various strategies have been progressed to investigate the physicochemical properties and reversible phase transformation of layered oxides, providing new insight into the rational design of high-capacity and highly stable cathode materials for SIBs.

21-25

They found that the

charge/discharge process has a direct effect on the bond characteristics of the cathode in the Naion batteries. Although such intensive research efforts have made great progress, the capacity, rate capability and cycling stability still far below those required for successful applications of rechargeable SIBs. The ternary O3-type Na[NixCoyMnz]O2 compound (iron free) also have been


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extensively studied as promising cathodes in sodium batteries because they can eliminate the inferior electrode properties from Fe3+ migration from the transition metal layer to the Na layer during cycling.

26-28

Although the cobalt metal greatly contributes to the structural stability and

rate capability, the ternary O3-type Na[NixCoyMnz]O2 cathode still face their capacity and cycling limitations due to their inherent structural instability and highly reactive Ni4+ ions.26 These results indicated that Fe-rich O3-type cathodes show a high discharge capacity but low rate capability and capacity retentions. Co-rich (without Fe components) O3-type cathodes show relatively low discharge capacities, but they exhibit stable life cycle and good structural stability. To simultaneously address the inherent limitations of capacity and cycling stability, Li et al. proposed

the

quaternary

O3-type

layered

structured

compound

with

composition

Na[Mn0.25Fe0.25Co0.25Ni0.25]O2, with high initial capacity of 180 mAh g-1 and over an average working potential of 3.21 V.29 These merits of quaternary layered structure inspire us to develop them further, through the morphology control and designing them with well-balanced transition metal compositions. Herein, we proposed the high performance O3-type quaternary transition metal oxide with microsphere morphology and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 composition. Based on previous reports, microsphere morphology enhances the density of composite cathodes and ensure the structural stability upon cycling.12 The quaternary [Ni0.32Fe0.13Co0.15Mn0.40](OH)2 precursor was synthesized according to the 1:1 molar ratio between [Ni0.25Fe0.25Mn0.5](OH)2 and [Ni0.4C0.3M0.3](OH)2. Through the calcination step, we obtained the highly pure material comprising microspheres of O3-type Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode. Compare to their respective ternary members, the proposed microspheres O3-type Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode exhibited better electrochemical performances with good structural stability upon cycling. Through the various analysis techniques, we demonstrate the potential importance of


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quaternary systems and microspheres O3-type Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode as viable and promising candidate for advanced sodium-ion battery system. EXPERIMENTAL SECTION Material

The

synthesis.

[Ni0.25Fe0.25Mn0.5](OH)2,

[Ni0.4C0.3M0.3](OH)2

and

[Ni0.32Fe0.13Co0.15Mn0.40](OH)2 precursors were prepared via the co-precipitation route. First, according to the desired stoichiometric amounts of high-purity NiSO4·6H2O, CoSO4·7H2O, FeSO4·9H2O, and MnSO4·H2O (Samchun Chemical, Korea) were uniformly mixed and dissolved (molar ratios of Ni:Fe:Mn = 1:1:2, Ni:Co:Mn = 4:3:3, and

Ni:Co:Fe:Mn =

32:13:15:40) in distilled water at a concentration of 2.0 mol dm-3. This metal solution was pumped into a batch type tank reactor (CSTR, 4 L) under N2 atmosphere.30 Concurrently, a 2 mol dm-3 NaOH solution and an appropriate amount of NH4OH solution as a chelating agent were separately pumped into the reactor. The pH, temperature, and stirring speed of the mixture in the reactor were controlled. After the reaction, the precipitated particles were filtered, washed, and dried at 110°C to remove absorbed water. The obtained hydroxide precursors were thoroughly mixed with NaOH (molar ratio of Na / transition metals = 1.02) and calcined at appropriate temperatures in air for 24 h then quenched at room temperature: 820 oC for Na[Ni0.25Fe0.25Mn0.5]O2,

750

o

C

for

Na[Ni0.4Fe0.3Mn0.3]O2

and

780

o

C

for

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2. Physicochemical analysis. The crystalline phase of the synthesized materials was characterized by powder X-ray diffraction (XRD, Rint-2000, Rigaku, Japan) measurements using Cu-Kα radiation. For ex-situ XRD measurement, the electrodes collected from cycled cells were washed for several times with DMC solvent and dried in glove box. All the XRD data were obtained in the 2θ range between 10° and 80° with a step size of 0.03°. Particle morphologies of


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the precursor and as-synthesized powders were observed by scanning electron microscopy (SEM, JSM 6400, JEOL Ltd., Japan). In order to monitor the localized composition of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode, cross-sectioning of selected particles was performed by a focused-ion beam (FIB). Chemical compositions of the prepared powders were analyzed by atomic absorption spectroscopy (Vario 6, Analyticjena). Transition metal dissolution test procedure was divided into three main steps; First, the Na[Ni0.25Fe0.25Mn0.5]O2, and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode were initially charged to 4.3 V. recovered from half-cell using Li as anode. Second, both electrodes at charge-end state (4.3 V) immersed in electrolyte solution (0.5 M NaPF6 in 98:2 volumetric mixture of PC and FEC) and stored in a Teflon beaker during four weeks. Finally, these electrolytes were analyzed by inductively coupled plasma (ICP) to measure concentration of dissolved transition metals. Electrochemical tests. Electrochemical testing was performed in a 2032 coin-type cell using Na metal (Alfa Aesar, USA) as an anode. Electrode were fabricated by blending the prepared cathode powders (85 wt%), carbon black (7.5 wt%), and polyvinylidene fluoride (7.5 wt%) in NMethyl-2-pyrrolidone (Daejung Chem, Korea). We make slurry by using an agate mortar (mixing time: 30 minutes). The slurry was then cast on aluminum foil (Hohsen Corp., Japan) and pre-dried at 110 oC in oven. Then, electrode further dried at 110 oC for 12 h in a vacuum oven, and then disks were punched out of the foil. The electrolyte solution was 0.5 M NaPF6 (Tokyo Chemical Industry, Japan) in a 98:2 volumetric mixture of propylene carbonate (Tokyo Chemical Industry Japan) and fluoro ethylene carbonate (Tokyo Chemical Industry, Japan). All cells were prepared in an Ar-filled glove box (MBRAUN, German). The fabricated cathodes and sodium metal anodes were separated by a glass fiber (Advantec, USA) to prevent short circuitng. Cells


6

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were cycled in a constant-current mode at a C/10 rate within a voltage range of 1.5-4.3 V versus Na/Na+, respectively (where 1 C = 150 mAh g-1).


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RESULTS AND DISCUSSION

Figure 1. SEM images of as-prepared hydroxide precursors: (a) [Ni0.25Fe0.25Mn0.5](OH)2, (c) [Ni0.4Co0.3Mn0.3](OH)2, and (e) [Ni0.32Fe0.13Co0.15Mn0.40](OH)2. SEM image of thermally sodiated:

(b)

Na[Ni0.25Fe0.25Mn0.5]O2,

(d)

Na[Ni0.4Co0.3Mn0.3]O2,

and

(f)

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2. The

hydroxide

precursors

of

[Ni0.25Fe0.25Mn0.5](OH)2,

[Ni0.4Co0.3Mn0.3](OH)2,

and

[Ni0.32Fe0.13Co0.15Mn0.40](OH)2 were first synthesized via co-precipitation, and the resultant material comprised microscale spherical particles agglomerated with needle-shaped primary particles (Figure 1a-c). After heat treatments at high temperature, all samples retained their spherical morphology and had an average diameter of around ~7 µm (Figure 1d- f). The microsphere cathode by co-precipitation route is very unique methodology compared to other group and provide an additional advantage of the high tap density (~1.9 g cm-3), which is very welcome property for practical use of SIBs (in Figure S1). The desired chemical composition of all products was confirmed by atomic absorption spectroscopy (AAS).


8

*

10

20

30

40

50

60

70

80

10

20

30

2θ / degree

Figure

2.

XRD

patterns

[Ni0.4Co0.3Mn0.3](OH)2,

and

Na[Ni0.4Co0.3Mn0.3]O2

40

50

60

(116) (201) (202)

(113) (021)

(108)

(110)

(107)

Na[Ni0.32Fe0.13Co0.15Mn0.4]O2

(105)

(111)

(110)(003)

(102)

(100)

(011)(002)

[Ni0.32Fe0.13Co0.15Mn0.4](OH)2

Na[Ni0.25Fe0.25Mn0.5]O2

(006)(102)

Intensity / arb. unit

(001)

[Ni0.4Co0.3Mn0.3](OH)2

(104)

(b)

[Ni0.25Fe0.25Mn0.5](OH)2

(101)

(a)

Intensity / arb. unit

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


(003)

Page 9 of 27

70

80

2θ / degree

of

(a)

precursor

powders

[Ni0.32Fe0.13Co0.15Mn0.40](OH)2,

of

[Ni0.25Fe0.25Mn0.5](OH)2,

(b)

sodiated

powders

of

Na[Ni0.25Fe0.25Mn0.5]O2, Na[Ni0.4Co0.3Mn0.3]O2, and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 .

Table 1. Calculated lattice parameters of Na[Ni0.25Co0.25Mn0.5]O2, Na[Ni0.4Co0.3Mn0.3]O2, and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 materials. Samples

Lattice Parameters

Rwp

a/Å

c /Å

%

Na[Ni0.25Co0.25Mn0.5]O2

2.9387(4)

16.2706(2)

13.1


2.9190(3)

16.1756(5)

12.4

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2

2.9267(2)

16.1593(6)

12.8

The resulting XRD patterns illustrated the typical layered hydroxides form in Figure 2a. The splitting of the XRD peaks is in [Ni0.32Fe0.13Co0.15Mn0.40](OH)2 (blue line) owing to the presence of two

hydroxides

from

both

the [Ni0.25Fe0.25Mn0.5](OH)2

(black

line) and

of

[Ni0.4Co0.3Mn0.3](OH)2 (red line) compounds. The sodiated products had an pure X-ray diffraction (XRD) pattern of an α-NaFeO2 typical layer structure with space group R-3m (Figure


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2b).19,31 Two layer phases that appeared in the hydroxide (Figure 2a) were not observed in the final product, which exhibits a O3-phase structure without impurities in the XRD patterns. Further, we calculated the lattice parameters of Na[Ni0.25Fe0.25Mn0.5]O2, Na[Ni0.4Co0.3Mn0.3]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathodes based on the XRD results in Figure 2b. They have almost similar lattice parameters because the resultant products were formed with the same crystalline structure (in Table 1). Note that the Na[Ni0.25Fe0.25Mn0.5]O2, Na[Ni0.4Co0.3Mn0.3]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 samples were well crystallized into same O3-type layered structure, shift of several reflections in XRD patterns was observed due to their different chemical composition.

Figure 3. (a) SEM image, (b) cross-sectional SEM image and (c-f) corresponding EDX mapping data of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode. To characterize the distributions of transition metals in Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 particle, cross-sectional SEM EDX mapping of sodiated particles was performed in Figure 3. The ~8 µm


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spherical particles samples were cross-sectioned for analysis by focused ion beam milling (Figure 3 a and b). As we designated, the transition metals of Ni (orange color), Fe (blue color), Co (green color) and Mn (yellow color) were well distributed through the whole

3

2

Na[Ni0.25Fe0.25Mn0.5]O2 Na[Ni0.4Co0.3Mn0.3]O2

1

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 0

30

60

90

120

150

180

210

240

180 0.5C

150

90

120 80

90 60

Na[Ni0.25Fe0.25Mn0.5]O2

30


0

-1

70

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2

0

Specific capacity / mAh g

10

20

30

40

50

60

3.5 3.0 2.5 o

30 C o 0C o -10 C

2.0 1.5 0

30

60

90

120

150

180

210 -1

Specific capacity / mAh g

0.1C

0.2C

180

240

210

(e) 100

0.2C

180 0.5C

90

150 120

80

90 70

o

30 C o 0C o -10 C

60 30

0

10

20

30

(c) 0.5C

1C

2C

3C

5C

150 120 90 60

Na[Ni0.25Fe0.25Mn0.5]O2

30


0

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 0

5

10

15

Number of Cycle

-1

4.0

Discharge capacity / mAh g

(d)

4.5

210

Number of Cycle

5.0

1.0

100

60

Number of Cycle

Coulombic Efficiency / % -1 Discharge capacity / mAh g

Voltage / V

4

(b)

210 0.2C

Coulombic Efficiency / %

(a)



-1

5

-1

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 particle (white dot circle).

Voltage / V

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


210 0.1C 180

(f) 0.2C

0.5C

1C

2C

3C 5C

150 120 o

30 C o 0C o -10 C

90 60 0

5

10

15

Number of Cycle

Figure 4. (a) First charge-discharge curves at 0.2 C-rate (30 mA g-1), (b) cycle life test at 0.5 Crate (75 mA g-1), and (c) rate capabilities test of Na[Ni0.25Fe0.25Mn0.5]O2, Na[Ni0.4Co0.3Mn0.3]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathodes in the voltage range of 1.5-4.3 V at 30 oC. (d) First charge-discharge curves at 0.2 C-rate (30 mA g-1), (e) cycle life test at 0.5 C-rate (75 mA g-1), and (f) rate capabilities of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathodes at various operating temperatures from -10°C to 30 oC. For rate capability test, all cells were charged to 4.3 V with a constant C-rate of 0.1 C (15 mA g-1) and then discharged to 1.5 V with a different C-rate ranging from 0.1 C (15 mA g-1) to 5 C (750 mA g-1).


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Figure 4 exhibited the comprehensive electrochemical performances of Na[Ni0.25Fe0.25Mn0.5]O2, Na[Ni0.4Co0.3Mn0.3]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathodes in the voltage range of 1.5– 4.3 V. The Na[Ni0.25Fe0.25Mn0.5]O2 cathode delivered the highest discharge capacity of 192.7 mAh g-1 at 0.2 C-rate (30 mA g-1) which was associated with the electrochemical reaction related to the Ni2+/4+, Fe3+/4+ and partial Mn3+/4+ redox couples.18 In contrast, the Na[Ni0.4Co0.3Mn0.3]O2 cathode exhibited lower discharge capacities of 156.8 mAh g-1 at 0.2 C-rate (30 mA g-1) because Co composition does not significantly contribute to the redox reaction in O3-type Na[NixCoyMnz]O2 compound (Figure 4a). The Na[Ni0.25Fe0.25Mn0.5]O2 electrode had the highest efficiency of 84.7%, whereas the Na[Ni0.4Co0.3Mn0.3]O2 electrode had the lowest efficiency of 74.1% at the first cycle due to highly reactive Ni4+ ions.26 However, this low efficiency was increased at the second cycle and maintained during cycling (Figure 4a). Although a higher capacity was obtained in Na[Ni0.25Fe0.25Mn0.5]O2 cathode with the high electrochemical activity of Ni2+/4+ and Fe3+/4+, better capacity retentions were obtained in Na[Ni0.4Co0.3Mn0.3]O2

cathode

(Figure

4b):

Na[Ni0.25Fe0.25Mn0.5]O2 cathode:

40

%

Na[Ni0.4Co0.3Mn0.3]O2 cathode: 67 % after 50 cycles at 0.5 C-rate (75 mA g-1). As discussed in the introduction part, iron-containing materials have an intrinsic structural collapse problem, which is caused by iron migration to sodium sites in the desodiated state at high voltage.15 In contrast, the existence of cobalt in crystal structure improve the structural stability and facilitates the

reversible

intercalation

of

Na+

during

cycling.

On

the

other

hand,

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode was designed the mixed chemical composition of both Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.4Co0.3Mn0.3]O2 cathodes, but electrochemical performances were very interesting. Indeed, for designing the Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode, we carefully designed and optimized the appropriate composition based on experimental data. For


12

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ternary Fe-rich cathode, we first selected the Na[Ni0.25Fe0.25Mn0.5]O2 cathode which can deliver an high discharge capacity around 192.7 mAh g-1 at 0.2 C-rate in the voltage range of 1.5-4.3 V. However, this Fe-rich cathode exhibited the poor cycling stability and rate capability due to the structural instability resulting from Fe3+ migration from the transition metal layer to the Na layer upon cycling.15 In general, cobalt (Co) composition contributed to the structural stability and rate capability of the O3-type layered cathodes.16,26 Thus, we postulate that the cycling stability and rate capability can be improved by Co substitution into Na[Ni0.25Fe0.25Mn0.5]O2 cathode. Based on our previous report,26 three kinds of Na[NiFeCoMn]O2 cathodes consisting of combination of Na[NiCoMn]O2

and

Na[NiFeMn]O2

cathodes

were

fabricated

and

evaluated:

1)

Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni1/3Co1/3Mn1/3]O2 for Na[Ni0.29Fe0.13Co0.17Mn0.41]O2, 2) Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.4Co0.3Mn0.3]O2 for Na[Ni0.32Fe0.13Co0.15Mn0.40]O2, 3) Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.5Co0.2Mn0.3]O2 for Na[Ni0.37Fe0.13Co0.10Mn0.40]O2. For preliminary cycle life test (in Figure S2), it is evident that the cycling stability of all the quaternary Na[NiFeCoMn]O2 cathodes was improved by substitution of Co content. The delivered capacity increased proportionally with increasing Ni content due to the electrochemical activity of Ni2+/3+/4+; however, better capacity retentions were obtained with low Ni compositions. In summary, Na[Ni0.32Fe0.13Co0.15Mn0.4]O2 cathode exhibited the relatively high discharge capacity and good cycling stability. Therefore, we can suggest that combination of ternary Na[Ni0.25Fe0.25Mn.0.5]O2 and Na[Ni0.4Co0.3Mn0.3]O2 compounds are logical for designing an optimal quaternary transition metal cathode in this study. Combined

with

advantages

of

its

respective

ternary

members,

the

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode exhibited the high discharge capacity of 182 mAh g-1 at 0.2 C-rate (30 mA g-1) and superior cycling stability of 85 % after 50 cycles at 0.5 C-rate (75 mA


13


Page 14 of 27

g-1). The rate capabilities of the Na[Ni0.25Fe0.25Mn0.5]O2, Na[Ni0.4Co0.3Mn0.3]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathodes were tested in Figure 4c. The Na[Ni0.25Fe0.25Mn0.5]O2 electrode exhibited a poor rate performance with only 54 mAh g-1 at a 5 C-rate (750 mA g-1), whereas the Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode delivered 182 mAh g-1 at a 0.2 C-rate, 160.0 mAh g-1 at a 1 C-rate, and 137.4 mAh g-1 at a 5 C-rate (750 mA g-1). The Na[Ni0.4Co0.3Mn0.3]O2 also shows a good rate performance, even at the 5 C-rate (750 mA g-1). Generally, the Co transition metal led to an increase in the electronic conductivity and high diffusion value in the O3-type layered structure.16,32,33 The improved electronic conductivity by Co substitution (in Table 2) provides further evidence for better rate capability of Na[Ni0.32Fe0.13Co0.15Mn0.4]O2 cathode. From above results, we can confirm that the high capacity, excellent cycling stability and rate capability of the Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode was originated from the well balanced quaternary transition metals in O3-type crystal structure. In addition, the lowtemperature properties highlight the superiority of quaternary Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode

in

Figure

4

d

and

e.

Even

at

low

temperatures

0

and

-10°C,

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode had high discharge capacities at 0.5 (0 oC: 177.5 mAh g1

, -10 oC: 134.1 mAh g-1) and good capacity retention at 0.5 C-rate (75 mA g-1) during 30 cycles

(0 oC: 77 %,-10 oC: 86 %). Surprisingly, the present electrode was capable of operation at low temperatures at such high rates (Figure 4f). With respect to the good performances under various environments, the advanced quaternary transition metal oxide clearly ensures high chargedischarge reversibility over a wide temperature range from -10 °C to 30 °C, even at high rates. Based on above results, we can conclude that the excellent electrochemical performances of the Na[Ni0.32Co0.13Fe0.15Mn0.40]O2 electrode resulted from the synergetic effects of i) the high discharge capacity by Fe composition, ii) structural stabilization via prevention of Fe migration


14

Page 15 of 27 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


by Co substitution. Compared to previous work on Fe-based quaternary O3-type layered oxide cathodes,29,34 our Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode can achieve the better capacity, rate capability up to 5 C-rate (750 mA g-1) and cycling stability at similar test condition (Table 3). Moreover, we also highlighted the advanced electrochemical properties of O3-type quaternary layered oxide of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode at low temperatures test condition. Table

2.

Comparison

of

electronic

conductivities

of

Na[Ni0.25Fe0.25Mn0.5]O2,

Na[Ni0.4Co0.3Mn0.3]O2, and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 powders.

Samples

Electronic conductivity / S cm-1

Na[Ni0.25Fe0.25Mn0.5]O2

9.73 (± 0.07) × 10-8


4.32 (± 0.08) × 10-7

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2

2.14 (± 0.07) × 10-7

Table 3. Comparison of electrochemical performances in Fe-based quaternary O3-type layered oxide cathode. Cathode

O3-type Na[Mn0.25Fe0.25Co0.25Ni0.25]O2

Cut-Off Potential

Initial

Capacity

Rate

Discharge Capacity

Retention

Capability

180 mAh g-1

~ 90% (± 1%) at 20th

1.9-4.3 V

O3-type

(0.1 C)

(0.05 C)

163 mAh g-1

~ 90% (± 1%) at 20th

2-4.2 V Na[Ti0.25Fe0.25Co0.25Ni0.25]O2 O3-type Na[Ni0.32Fe0.13Co0.15Mn0.4]O2

(0.05 C)

(0.05 C)

182 mAh g-1

85 % (± 1%) at 50th

Refere nce

Unknown

Ref. 29

125 mAh g-1 (4C)

Ref. 34

137.4 mAh g-1

1.5-4.3 V 1

(0.2C, 30 mA g- )

(0.5C, 75 mA g-1)

-1

(5C, 750 mA g )

This work


15


(b)

P: P3

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2

35

40

45

30

110 110

107

P

P

40

50

Al

113 021

108 108

105

P

105

104

101

20

006 102

003

P

O3-hex

35

107

104

45

Na[Ni0.25Fe0.25Mn0.5]O2

P

O3-hex

40

006 102

P

P3-mon

P3-mon

As-prepsred

101

O3-hex

O3-hex 4.3 V charge

Gr.

1.5 V Discahrge

30

Al

003

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2

P

P

60

113 021

(a)

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 16 of 27

70

Cu Kα 2θ / degree

Cu Kα 2θ / degree

Figure 5. (a) Ex-situ XRD patterns of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode at different state of charge at initial cycle in the voltage range of 1.5 – 4.3 V. (b) Comparison of XRD patterns of Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode after 50 cycles. The

significant

improvement

in

the

electrochemical

performance

of

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 could be related to the reversibility of the crystalline structure with respect to large Na+ ions upon multiple sodiation/desodiation process. The original O3 layer structure was transformed to the monoclinic P3 phase for desodiated electrodes when charged (oxidized) to 4.3 V, and this was followed by the gliding of the transition metal layer slab, which induced the formation of the P3 phase from the O3 phase, as proposed by Delmas et al.13 and Komaba et al.35,36 This structural change was clearly observed at different state of charge at initial cycle (as-prepared nd 4.3 V charge) for Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode in Figure 5a. After full discharge at 1.5 V, the O3-phase Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 recovered its original O3 phase structure. This result that the Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode highly reversible during insertion/extraction process of Na+ ion. The XRD patterns of the cycled electrode provide further evidence of reversibility of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode in Figure 5b. For comparison, both Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode were cycled at 0.2 C-rate for 2 cycles and 0.5 C-rate for 50 cycles in the voltage range


16

Page 17 of 27

of 1.5 - 4.3 V at 30 oC. The original O3 phase is maintained as a major phase for the extensively cycled (50 calyces) Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode, while the presence of a P3 phase for the Na[Ni0.25Fe0.25Mn0.5]O2 would provide evidence of the loss of Na ion (continuous migration of Fe) from the host structure during the 50th cycling, which comes from structural instability of materials.

Dissolved metal amounts / ppm

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


300 250

263.5

Na[Ni0.25Fe0.25Mn0.5]O2 Na[Ni0.32Fe0.13Co0.15Mn0.4]O2 194.5

200 150

130.1

100 50

42.1

44.2

53.8 2.8

0

Co

Ni

Mn

Fe

Figure 6. Transition metal dissolution for the electrolyte of the fully charged (to 4.3 V) Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrodes after four weeks at 55°C. Relative standard deviation (RSD) values: 0.821 wt. % for Ni, 0.675 wt. % for Fe, 0.829 wt. % for Co and 0.540 wt.% for Mn. The observed structural degradation may also be related to transition metal dissolution into the electrolyte during cycling.37-39 In particular, in iron-based cathodes, Fe migration phenomena are critical issues that affect the structural stability. The high operation voltage up to 4.3 V vs. Na/Na+ is likely to accelerate the breakdown of the electrolyte salt, NaPF6, and the produced


17


Page 18 of 27

byproduct, HF, can decompose active materials. Hence, both electrodes (Na[Ni0.25Fe0.25Mn0.5]O2 and Na[Ni0.32Fe0.13Co0.15Mn0.40]O2) charged up to 4.3 V were aged at 55°C for four weeks in fresh electrolytes (in Figure 6). For the desodiated Na[Ni0.25Fe0.25Mn0.5]O2 electrode, a large amount of metal dissolution was observed, particular that of Fe ions, whereas the desodiated Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 electrode showed less transition metal dissolution, which may be due to the presence of Co metal in the transition metal site instead of Fe metal, which provides structural stability. Therefore, the dissolution of trivalent Fe and Mn can be significantly reduced. CONCLUSION In summary, the well-balanced quaternary transition metal structure of O3-type Na[Ni0.32Co0.13Fe0.15Mn0.4]O2 cathode materials was successfully synthesized using a coprecipitation method. By performing various analysis techniques, we evaluated and confirmed the superiority of quaternary transition metal oxide frameworks. EDX mapping images reveal that the quaternary transition metals of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 were well distributed throughout the whole particle. The Fe composition contributed to a higher discharge capacity with Fe3+/4+ redox couples, however, fast capacity fading was observed due to detrimental Fe-ion migration during cycling. The existence of Co metal in structure resulted in a slight decrease in the discharge capacity; however, by enhancing the structural stability and improving the cycling stability, we can resolve this minor issue. Combine such merits, Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode delivered the high discharge capacities of 182 mAh g-1 at 0.2 C rate (30 mA g-1) and 137.4 mAh g-1 at 5 C-rate (750 mA g-1). In addition, Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode exhibited good cycling stability at various operation temperatures (30, 0 and -10 oC). Based on these results, we propose that well-balanced quaternary transition metal oxides have the potential


18

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for development as advanced cathode materials that have improved structural stability and high energy density.


19


Page 20 of 27

ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publications website. Tap density measurement result of Na[Ni0.32Fe0.13Co0.15Mn0.40]O2 cathode, and cycle life test of Na[Ni0.25Fe0.25Mn0.5]O2,

Na[Ni0.29Fe0.13Co0.17Mn0.41]O2,

Na[Ni0.32Fe0.13Co0.15Mn0.40]O2

and

Na[Ni0.37Fe0.13Co0.10Mn0.40]O2 cathodes. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program (2013M3A6B1078875) of the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning, and by a Human Resources Development Program (No. 20154010200840) grant from the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which is funded by the Korean Ministry of Trade, Industry and Energy. REFERENCES (1)

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TOC Graphic


27

Quaternary Transition Metal Oxide Layered Framework: O3-Type Na

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