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A microsphere Na0.65[Ni0.17Co0.11Mn0.72]O2 Cathode material for High Performance Sodium-Ion Batteries Tae-Yeon Yu, Jang-Yeon Hwang, Doron Aurbach, and Yang-Kook Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15267 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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ACS Applied Materials & Interfaces

A microsphere Na0.65[Ni0.17Co0.11Mn0.72]O2 Cathode material for High Performance Sodium-Ion Batteries Tae-YeonYu,†Jang-Yeon Hwang,†Doron Aurbach,‡and Yang-Kook Sun*,† †

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

‡

Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel

KEYWORDS: sodium-ion battery, P2-type, cathode material, high capacity, co-precipitation, layered structure, spherical morphology

ABSTRACT: P2-type layered oxides have been considered promising candidates as cathodes for sodium-ion batteries owing to their high capacity and high rate capability. However, due to the difficulty involved in forming hierarchical microstructures, it remains challenging to develop high energy density P2-type layered oxides with good electrochemical performance and high electrode density. In this study, we demonstrate the feasibility of P2-type Na0.65[Ni0.17Co0.11Mn0.72]O2 as a very efficient cathode material for high energy density sodium-ion batteries by synthesizing a micron-sized hierarchical structure via the co-precipitation route. The as-prepared P2-type microsphere cathode constructed from nano-scale primary particles provides a sufficient interface between the electrodes and the electrolyte solution, which enables to shorten the transport pathways for Na+-ions and electrons. Simultaneously, the hierarchical microstructure enhances the structural stability and high tap density (~1.18 g cm-3). 1 ACS Paragon Plus Environment

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Benefiting from these merits, the proposed P2-type microsphere Na0.65[Ni0.17Co0.11Mn0.72]O2 displays a high discharge capacity of 187 mAh g-1 at 12 mA g-1 and an exceptional cycle retention of 74.7% after 500 cycles, even at the high current density of 600 mA g-1. In addition, the high tap density of this P2type microsphere enhances the density of composite cathodes, which translates to a high volumetric energy density of 340 Wh L-1 based on the overall volume of the cathode active mass and the aluminum foil current collector.

1. INTRODUCTION Fossil fuels have been the most widely used energy resources in daily life for a long period of time. However, increasing concerns about fossil-fuel depletion and environmental issues are driving the search for eco-friendly and sustainable energy sources.1,2 Due to public demands, the market for rechargeable batteries has grown rapidly.3 Currently, about one billion lithium-ion batteries (LIBs) are produced every year for consumer devices, including electronic devices and electric vehicles. However, the demand for lithium is rising steadily and sharply, and a shortage appears to be imminent.4 To mitigate this situation, various types of other rechargeable battery systems have been proposed.1,5-8 Among them, sodium-ion batteries (SIBs) have attracted tremendous attention owing to their cost effectiveness and the geographical distribution of sodium.9 The development of cathode materials with high capacity and operating potential and good cycling stability is crucial for practical SIBs. Analogous to LIBs, insertion materials have been extensively investigated over the past few years for use as cathodes in SIBs.1,2 After the Delmas group defined layered sodium oxides as NaxMO2 (A: alkali metal; M: transition metals), many research groups developed and explored the application of cathode materials with layered structures for SIBs,

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particularly O3-type and P2-type layered compounds.10 O3-type layered oxides appear attractive based on the success achieved by using layered LiMO2 cathodes in commercial LIBs. However, due to the large ionic size of Na+ (Li: 0.76 Å, Na: 1.02 Å), O3-type cathodes have a high energy barrier for facile sodium diffusion, which results in complex phase transformations expressed electrochemically as several voltage plateaus during the sodiation/desodiation processes. For this reason, O3-type cathodes usually demonstrated poor capacity and cycling stability.11 On the other hand, P2-type compounds are more stable than O3-type ones during the de/intercalation processes. Formation of prismatic sites is achieved by the gliding of TMO2 slabs without breaking TM-O bonds. Compared to a series of slab glides for an O3-type structure, the P2-type framework has an open path for Na+ diffusion, which is expected to present a lower diffusion barrier.1,12 Thus, Na+-ions diffusion occurs readily in the P2-type structure thus delivering a higher discharge capacity and rate capability relative to the O3-type structure.1,13 Therefore, numerous studies on P2-type layered oxide compounds have been explored as promising cathodes for rechargeable SIBs.14-24 The P2-type Na0.7MnO2 cathode is one of the most widely studied electrode materials for SIBs.25 It was shown to deliver a high capacity of ~160 mAh g-1 in sodium ion cells, but its capacity and cycling stability needs to be further improved for practical application. Substituting part of the manganese by nickel, for instance forming Na0.67Ni0.33Mn0.67O2, increases the discharge capacity through the

Ni2+/4+ redox coupling, which is a double-electron

process.26 A partial addition of other metals (Al, Fe, and Co) further enhances the electrochemical activities of P2-type NaxNi0.3Mn0.7O2 layered oxides.21,23,26,27 In particular, addition of

Co3+ ions

significantly improves the structural stability and enhances the cycling stability.21 In this regard, various Nax[Ni0.2Co0.1Mn0.7]O2 (x ≤ 2/3) compounds have been extensively investigated by various groups.18-21 In spite of such research efforts, their poor capacity retention and rate performance and low electrode density limit their practical application. This is because such compounds were mostly synthesized using the solid-state or sol-gel methods without considering the particles morphology and structural design. 3 ACS Paragon Plus Environment


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Looking back through the history of LIBs, it is clear that suitable choices of synthesis methods for preparing electrode materials having a robust structure and pure crystal phase has been important for enhancing the electrochemical performance.28-30 Thus, in order to mitigate these shortcomings, development of suitable synthesis methods is imperative. In this study, we propose microsphere Na0.65[Ni0.17Co0.11Mn0.72]O2 as a highly efficient cathode material for SIBs. First, we successfully synthesized [Ni0.17Co0.11Mn0.72]3O4 precursor in morphology of microspheres via the co-precipitation method by using a batch-type reactor, which exhibited a homogeneous particle-size distribution. Through the calcination step, we obtained a highly pure material comprising microspheres of Na0.65[Ni0.17Co0.11Mn0.72]O2. The as-prepared microspheres show advantage in both long cycle life of the nano-scale primary particles and the high tap density (1.2 g cm-3) of the micro intertexture. Compared to the same chemical composition synthesized by the typical solid-state method, the proposed microspheres of Na0.65[Ni0.17Co0.11Mn0.72]O2 exhibited better electrochemical performances with good structural stability upon cycling as well as higher electrodes’ density. The measured performance was superior to that of previously reported similar materials. Thus we propose this cathode material, namely microspheres of Na0.65[Ni0.17Co0.11Mn0.72]O2, as a viable and promising candidate for powerful sodium-ion based energy storage systems.

2. EXPERIMENTAL SECTION Synthesis of cathode materials. The spherical precursors [Ni0.17Co0.11Mn0.72]3O4 were synthesized by the co-precipitation method. Appropriate amounts of NiSO4, CoSO4·7H2O, and MnSO4·H2O were used as starting materials. An aqueous metal salts solution with a concentration of 2.0 mol dm-3 was pumped into a batch-type reactor under an air atmosphere.31,32 At the same time, NaOH solution (aq.) of 4.0 mol dm-3 and an appropriate amount of NH4OH solution (aq.) as chelating agent were separately fed into the reactor. The pH, temperature, and stirring speed in the reactor were carefully controlled. After the 4 ACS Paragon Plus Environment

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reaction, the precursors were filtered, washed with deionized water, and dried in vacuum at 110 °C in order to remove adsorbed water. The obtained spherical precursors were mixed with Na2CO3 at a Na:[Ni+Co+Mn] molar ratio of 0.65:1 and calcined at 970 °C for 12 h in an O2 atmosphere. The resultant microspheres are denoted hereafter as MS-Na0.65[Ni0.17Co0.11Mn0.72]O2. As a reference, samples of Na0.65[Ni0.17Co0.11Mn0.72]O2 were synthesized through the typical solid-state method. The solid-state route involved mixing together the precursors viz. Na2CO3, NiO, Mn2O3, and Co3O4 in appropriate stoichiometric ratios. Then, the mixed powders were calcined at 970 °C for 12 h in an O2 atmosphere. The resultant reference material is denoted hereafter as R-Na0.65[Ni0.17Co0.11Mn0.72]O2. Material characterization. The particles morphology of the as-prepared powders was observed using scanning electron microscopy (SEM, JSM-6340F, JEOL). The crystalline phases of the synthesized and cycled materials were identified by powder X-ray diffraction (XRD, Rigaku, Rint-2000) using Cu Kα radiation. XRD data were obtained in the 2θ range between 10° and 70° with a step size of 0.02°. The chemical composition of the prepared powders was determined by inductively coupled plasma (ICP, OPIMA 8300, Perkin Elmer). The particle-size distribution was measured using a particle-size analyzer (CILAS 1090).In order to monitor the electrode density, the cross-sectioning of selected areas of electrodes was performed by a focused-ion beam (FIB). Electrochemical testing. Electrochemical testing was performed in R2032 coin-type cells, adopting Na metal (Alfa Aesar, USA) as the anode for the half-cell tests. The electrodes were fabricated by blending the as-synthesized cathode material (85 wt.%), carbon black (10 wt.%), and polyvinylidene fluoride (5 wt.%). The resulting slurry was applied to aluminum foil and dried at 110 °C for 3 h in a vacuum oven. The fabricated cathode and sodium metal anode were separated using glass fiber (ADVENTEC) to prevent short-circuiting. Electrochemical characterization was conducted using a 2032 coin-type cell in a 0.5 mol dm-3 NaPF6 solution in propylene carbonate and fluoroethylene carbonate (98:2 by volume). All cells were prepared in an argon-filled glove box. The loading amount of 5 ACS Paragon Plus Environment


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active material on all electrodes was ~1.5 mg cm-2 for the R-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathode and ~ 3.0 mg cm-2 for the MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathode; all the electrochemical tests were conducted with the same electrodes’ mass loading. The cells were typically cycled within the voltage range of 1.5–4.3 V versus Na/Na+ at 30 °C.

3. RESULTS We successfully synthesized spherical [Ni0.17Co0.11Mn0.72]3O4 precursors via the co-precipitation method using a batch-type reactor. The scanning electron microscope (SEM) images in Figure 1a (with magnified images in the insets) show that the as-prepared [Ni0.17Co0.11Mn0.72]3O4 precursor was densely packed with nano-scale primary particles, and had a spherical morphology with average diameters ranging from 7 to 9 µm. As shown in Figure 1b, we observe that the XRD pattern of the synthesized powder is consistent with that of the typical M3O4 structure.32 The thermal sodiation of the [Ni0.17Co0.11Mn0.72]3O4 precursor with sodium carbonate (Na: [Ni+Co+Mn]=0.65:1 molar ratio) at a high temperature resulted in highly crystalline layered-oxide compounds of MS-Na0.65[Ni0.17Co0.11Mn0.72]O2. After heat treatments at high temperature, the MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 retained its spherical morphology and had an average diameter of around 8 µm, which can be attributed to the interconnected secondary units that ensure the structural stability of the microspheres (see magnified inset image in Figure 1c). Such micron-sized hierarchical structure constructed with nano-scale primary particles is beneficial for both electrochemical activities and electrode density.33-36 The XRD patterns in Figure 1d exhibit the typical response of P2-phase layered metal-oxide form, and all of the peaks could be well indexed to the planes of the pure rhombohedral structure belonging to the P63/mmc space group.37 For comparison, P2-type R-Na0.65[Ni0.17Co0.11Mn0.72]O2 was synthesized by the typical solid-state route; although this regular cathode exhibits a similar P2-type layered oxide structure (see Figure 1f), it appears to be formed by irregular aggregates of micrometric particles, as 6 ACS Paragon Plus Environment

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shown by SEM in Figure 1e. The chemical composition of Ni, Co, and Mn elements in MS- and RNa0.65[Ni0.17Co0.11Mn0.72]O2 cathodes were confirmed by ICP-OES, which demonstrates that the analyzed data agrees with the as-designated compositions (in Table 1).

Figure 1. SEM images and XRD patterns of powders of (a),(b) [Ni0.17Co0.11Mn0.72]3O4 precursor, (c),(d) sodiated MS-Na0.65[Ni0.17Co0.11Mn0.72]O2, and (e),(f) R-Na0.65[Ni0.17Co0.11Mn0.72]O2. Insets demonstrate high magnification images of each particle. 7 ACS Paragon Plus Environment


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Table 1. Chemical compositions of the prepared MS- and R-Na0.65[Ni0.17Co0.11Mn0.72]O2. Metal stoichiometry determined by ICP-OES Formula

Ni

Co

Mn

Prepared material

MS-Na0.65[Ni0.20Co0.10Mn0.70]O2

17.09

11.21

71.70

Na0.65[Ni0.1709Co0.1121Mn0.7170]O2

R –Na0.65[Ni0.20Co0.10Mn0.70]O2

17.64

10.48

71.88

Na0.65[Ni0.1764Co0.1048Mn0.7188]O2

The particle size analyzer (PSA) data demonstrated that the MS- cathode material exhibited a narrow size distribution with average diameters of 7.69 µm (Figure 2a), while the reference cathode material revealed a broad and random size distribution (Figure 2b). As expected from the SEM image and PSA results, the MS-cathode exhibited a tap density nearly two times higher than that of the regular cathode (~1.18 g cm-3 for the microsphere cathode and ~0.68 g cm-3 for the regular cathode). The cross-sectional SEM images of the MS- and R-Na0.65[Ni0.17Co0.11Mn0.72]O2 electrodes (Figure 2, panels c and d) provide further evidence of the morphological advantages of the proposed microsphere particles for the electrode density. The ability to load larger amounts of active material within the same volume (2.45 mg cm-2 for the micro-spherical cathode material vs. 1.46 mg cm-2 for the reference cathode material) indicates that the spherical morphology is beneficial for improving the volumetric energy density. 38 When we loaded the same active mass in both electrodes (2.45 mg cm-2), the MSelectrodes delivered a 1.6 times higher volumetric energy density of 340 Wh L-1 than the reference electrodes (see Table 2, the calculated values are based on the overall volume of the cathode and the Alfoil (thickness : 20 µm after roll pressing)).


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Figure 2. Particle size analysis of (a) MS- and (b) R-Na0.65[Ni0.17Co0.11Mn0.72]O2 powders; cross sectional SEM images of the respective electrode films are shown below in panels (c) and (d).

Table 2. Detailed parameters for calculating the volumetric energy density of the MS- and RNa0.65[Ni0.17Co0.11Mn0.72]O2 electrodes. Active mass loading

Average potential

Specific capacity

Electrode area

Height

Energy density

[mg cm-2]

[V]

[mAh g-1]

[cm2]

[µm]

[Wh L-1]

MS

2.45

3.63

186.9

1.54

32

340

R

2.45

3.68

189.3

1.54

54

206

Sample type



A cyclic voltammetry (CV) test was initially performed on both MS- and R-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes to obtain a general electrochemical response within the voltage range of 1.5–4.3 V. The CV plots in Figure 3a and b reveal a series of peaks associated with the electrochemical processes, which are involved in the redox couples and phase transformations during the overall charge-discharge process. Upon cycling, the reversible anodic/cathodic peaks at 4.25/4.1 V are attributable to the redox reactions of Ni2+/Ni4+ and the reversible MO2 sheet shift, which takes place for low sodium contents in the trigonal prismatic coordination, respectively.18,20 The pair of peaks at the low voltage (~2.3 V vs. Na/Na+)

relates to the redox reactions of the Mn3+/Mn4+.19,24 Interestingly, the intensity of each

reduction peak at 1.7–1.8 V gradually decreases, and a new peak appears at 2.0–2.1 V during cycling, which is attributed to the improved reversibility of the Mn3+/Mn4+ redox reaction during the Na+ ion insertion/extraction process.20 Co3+/Co4+ redox reaction peaksare also observed at around 3.7/3.65 V and in the CV curves.19,20

st

0.5

(a)

1 nd 2 rd 3

2+

st

4+

3+

4+

Co /Co : ~3.65V 3+ 4+ Mn /Mn : ~2.3V

0.0

-0.5

1 nd 2 rd 3

Ni /Ni : ~4.25V

Current / mA

0.5

Current / mA

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

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2.0

2.5

3.0

3.5

4+

3+

4+

Mn /Mn : ~2.3V 3+ 4+ Co /Co : ~3.65V

0.0

MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 1.5

(b) 2+

Ni /Ni : ~4.25V

R-Na0.65[Ni0.17Co0.11Mn0.72]O2 4.0

4.5

-0.5

1.5

2.0

2.5

3.0

3.5

4.0

Voltage / V

Voltage / V

Figure 3. Cyclic voltammogram of (a) MS- and (b) R-Na0.65[Ni0.17Co0.11Mn0.72]O2 electrodes.


4.5

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Figure 4 illustrates the comprehensive electrochemical properties of both MS- and RNa0.65[Ni0.17Co0.11Mn0.72]O2 cathodes in the voltage range of 1.5–4.3 V (vs. Na/Na+) at 30 °C. Figure 4a exhibits the initial charge-discharge volt-age profiles of both types of cathodes at a current density of 12 mA g-1. Based on the typical oxidation/reduction reaction presented above in the CV results, both MSand R-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes deliver high discharge capacities of 187 and 190 mAh g-1, respectively. The cycling performances of both cathodes were measured at 60 mA g-1, as shown in Figure 4b. Although the MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes delivered a slightly lower discharge capacity of 170 mAh g-1 compared to the reference cathodes at the initial cycles, the MSNa0.65[Ni0.17Co0.11Mn0.72]O2 cathodes demonstrated a better capacity retention of 91% after 100 cycles, whereas the reference cathodes maintained only 55% of their initial discharge capacity. Furthermore, the rate capabilities of both cathodes were tested, as shown in Figure 4c. All cells were charged to 4.3 V at a constant-current density of 12 mA g-1, and were then discharged to 1.5 V at different rates ranging from 12 to 840 mA g-1. As the current density increases, the MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes displays better rate capabilities than the reference cathodes. In particular, a significant difference in the discharge capacity was shown above 120 mA g-1 with 139.3 mAh g-1 for the MS-cathodes and 130.8 mAh g-1 for the reference cathodes at 840 mA g-1. Electrochemical impedance spectroscopic measurements

provide

further

evidence

for

the

better

cycling

stability

of

the

MS-

Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes, as shown in Figure 5a and b. The Nyquist plots for both electrodes show two semicircles, one in the high-to-medium frequency region, which is ascribed to the surface film resistance (Rsf), and the other in the low-frequency region, which is associated with the charge-transfer resistance (Rct). After 50 cycles, the high-frequency semicircles in the impedance spectra reflect lower surface resistance of the MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes.


(a)

1 Cycle

4.0 3.5 3.0

12 mA g

-1

2.5 2.0 R-Na0.65[Ni0.17Co0.11Mn0.72]O2

1.5 1.0

MS-Na0.65[Ni0.17Co0.11Mn0.72]O2

0

50

100

150

-1

200

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250

100

(b)

200 60 mA g

-1

80

150 60 100 40

50 0

R-Na0.65[Ni0.17Co0.11Mn0.72]O2 MS-Na0.65[Ni0.17Co0.11Mn0.72]O2

0

20 10 20 30 40 50 60 70 80 90 100

Specific capacity / mAh g

Discharge Capacity / mAh g

-1

Coulombic Efficiency (%)

st

Discharge capacity / mAh g

4.5

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

-1


Number of Cycle

220

12

200

(c) 24 60

180

120 360

160

600 840

140

Unit: mA g

120 100

-1

R-Na0.65[Ni0.17Co0.11Mn0.72]O2 MS-Na0.65[Ni0.17Co0.11Mn0.72]O2

0

2

4

6

8

10

12

14

16

Number of Cycle

Figure 4. Comprehensive electrochemical performances of MS- and R-Na0.65[Ni0.17Co0.11Mn0.72]O2 electrodes. (a) Initial charge-discharge curves at 12 mA g-1, (b) cycle life test at 60 mA g-1 during 100 cycles, (c) capacity retentions at various current densities from 12 to 840 mA g-1. All cells were tested in the voltage range of 1.5-4.3 V at 30 oC.

This can be mainly attributed to the lower surface reactivity of the active mass with the solution species. In these impedance spectra, the low-frequency semicircle (corresponding to various chargetransfer processes) is more than three times smaller for the MS-cathodes. These electrochemical advantages are well explained by the superior microstructure that is constructed from densely packed nano-scale primary particles, which can contribute to facile Na+-ion transport. This microstructure also minimizes unwanted side reactions due to the morphology of compact particles.39 The better cycling stability and rate capability of the MS- cathodes may be attributed to its unique microstructure. 12 ACS Paragon Plus Environment

MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 100

63.2 Hz

60 40

1000

(a)

- Im(Z) / Ohm

80

1200

95.5 Hz

0 0

800

20

40

60

80

100

After 1 Cycle th After 50 Cycle

100

1 mHz 1.74 Hz

400

57.3 mHz

200 0

60 40

Re(Z) / Ohm

600

80

1200

st

20

R-Na0.65[Ni0.17Co0.11Mn0.72]O2

1400

- Im(Z) / Ohm

1400

- Im(Z) / Ohm

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


1000

(b)

- Im(Z) / Ohm

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47.8 Hz 95.5 Hz

st

After 1 Cycle th After 50 Cycle

20 0 0

800

20

40

60

80

100

Re(Z) / Ohm

600

1 mHz 7.94 mHz

1.32 Hz 400 200

0

200

400

600

800

1000

1200

1400

0

0

200

Re(Z) / Ohm

400

600

800

1000

1200

1400

Re(Z) / Ohm

Figure 5. Electrochemical impedance spectra of (a) MS- electrodes and (b) R-electrodes after cycling.

The highly compact structure of microspheres constructed from nano-primary particles not only enhances the stability of such an active mass and subsequently leads to satisfactory cycling performance, but also facilitates the Na+-ions and electrons transport, ensuring the excellent rate capability. In an attempt to highlight the superiority of these MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes, long-term cycling tests were performed at a high current density of 600 mA g-1, and the results are shown in Figure 6. As expected from the fundamental electrochemical data in Figure 4 and 5, the MS-cathodes exhibit a superior cycling stability retention of 74.7 % after 500 cycles. In contrast, the reference cathodes revealed a continuous capacity fading per cycle, leading to a low discharge capacity of around 25 mAh g-1 after 500 cycles. The corresponding dQ/dV plots and charge-discharge curves provide further evidence for the excellent cycling stability of the MS-cathodes upon prolonged cycling (in Figure 6b and d). As the cycling proceeded, the redox peaks for the MS-cathodes remain very stable and thus exhibiting fully reversible series of Na+-ion insertion/extraction processes. However, with the reference cathodes the redox peaks changes upon cycling to more polarized states and are shifted further apart of



each other. In particular, all of the oxidation and reduction peaks for the reference cathodes disappear after 500 cycles. Such a poor cycling stability of the R-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes is mainly attributed to the random particle distribution in the composite electrodes, leading to an increase in the

100

200

(a)

150

600 mA g

80

-1

60

100 50 0

40

R-Na 0.65[Ni0.17Co 0.11Mn 0.72]O 2 MS-Na 0.65[Ni0.17Co 0.11Mn 0.72]O 2 0

100

200

300

20 500

400

Coulombic Efficiency (%)

Discharge capacity / mAh g

-1

barriers for Na+-ion and electron transfer and to structural degradation upon cycling.39

Number of Cycle MS-Na0.65[Ni0.17Co0.11Mn0.72]O2

0.2

(b) dQ/dV

dQ/dV

(c)

0.1

0.0 th

50 Cycle th 100 Cycle th 300 Cycle th 500 Cycle

-0.1 -1

1.5 - 4.3 V, 600 mA g

-0.2

R-Na0.65[Ni0.17Co0.11Mn0.72]O2

0.2

0.1

1.5

2.0

2.5

3.0

3.5

4.0

0.0 th

-1

1.5 - 4.3 V, 600 mA g

-0.2

4.5

1.5

2.0

(d)

-1

1.5 - 4.3 V, 600 mA g

4.0 3.5

th


3.0 2.5

4.5

1.0

3.0

3.5

4.0

4.5

(e)

-1

1.5 - 4.3 V, 600 mA g

4.0 3.5

th


3.0 2.5 2.0

2.0 1.5

2.5

Voltage / V

Voltage / V

4.5


-0.1

Voltage / V

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

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1.5

MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 0

30

60

90

120

-1

150


1.0

R-Na0.65[Ni0.17Co0.11Mn0.72]O2 0

30

60

90

120

-1

150


Figure 6. Long-term cycling tests at 600 mA g-1 of MS- and R-Na0.65[Ni0.17Co0.11Mn0.72]O2 electrodes (a) and corresponding dQ dV-1 profiles (b, c) and charge-discharge curves (d, e). All cells were tested in the voltage range of 1.5-4.3 V at 30 oC. 14 ACS Paragon Plus Environment

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MS-Na0.65[Ni0.17Co0.11Mn0.72]O2

12

14

16

18

20

30

40

(110)

(104)

50

# (112)

(106)

(103)

(102)

*

as prepared th After 500 Cycled

(100)

(004)

* : Carbon # : Aluminium

(002)

Intensity / arb. unit

(a)

60

70

2θ / degree R-Na0.65[Ni0.17Co0.11Mn0.72]O2 * : Carbon # : Aluminium

12

14

16

18

20

30

(110)

40

50

(106)

60

(112)

#

(104)

(103)

*

as prepared th After 500 Cycled

(004) (100) P3(101) (102)

(002)

(b)

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


70

2θ / degree

Figure 7. XRD patterns after 500 cycles of (a) MS- and (b) R-Na0.65[Ni0.17Co0.11Mn0.72]O2 electrodes.

In order to confirm the structural stability, we also evaluated the changes in the crystal structures of both cathodes before and after cycling by performing ex-situ XRD measurements, as shown in Figure 7a and b. Surprisingly, after 500 cycles, the MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathodes maintain their original P2 phase structure with all major peaks appearing well in the XRD patterns. In contrast, for the reference cathodes a shift of the P2 (002) peak in the XRD patterns towards a lower angle was observed for cycled cathodes and a minor impurity P3 (101) phase was observed as well. These indicate a loss of sodium ions from the host structure as well as a structural instability of this cathode material.40,41 15 ACS Paragon Plus Environment


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Table 3. Comparison table of electrochemical performances of P2-tpye NaxNiCoMnO2 cathode. Sample

Method

Cut-off

Capacity

co-

2.1–

145 mAh g-1

Retention

Ref.

82.1% @ 12 mA g-1

Na0.45[Ni0.22Co0.11Mn0.66]O2 precipitation

4.3V

18

-1

@ 12 mA g

(100 cycles) 88.6% 2.0– Na0.67[Ni0.15Co0.20Mn0.65]O2

141 mAh g-1 @ 20 mA g-1

sol-gel 4.4V

19

-1

@ 20 mA g

(50 cycles) 79.5% 1.5– Na0.67[Ni0.20Co0.15Mn0.65]O2

155 mAh g-1 @ 120 mA g-1 (100

sol-gel 4.2V

20

-1

@ 12 mA g

cycles) 74.0% 1.5– Na0.70[Ni0.20Co0.10Mn0.70]O2

154 mAh g-1 @ 240 mA g-1

sol-gel 4.2V

21

-1

@ 12 mA g

(100 cycles) 91% / 75% co-

1.5–

185 mAh g-1

This @ 60 / 600 mA g-1

Na0.65[Ni0.17Co0.11Mn0.72]O2 precipitation

4.3V

@ 12 mA g-1

work (100 / 500 cycles)

4. SUMMARY AND CONCLUSION In summary, we successfully synthesized P2-type MS-Na0.65[Ni0.17Co0.11Mn0.72]O2 cathode material with a morphology of compact microspheres, using the co-precipitation method. Compared to an irregular-shaped reference Na0.65[Ni0.17Co0.11Mn0.72]O2 cathode material synthesized by a solid-state reaction, the MS-cathodes provide the following advantages: 1) the intertexture structure of microspheres comprising secondary nano-sized sub particles bound compactly together, enhances the 16 ACS Paragon Plus Environment

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structural stability of the active mass, 2) the hierarchical architecture with spherical morphology enabled shorter transport pathways for electrons and ions. As a result, the microspheres based cathodes exhibit better cycling stability and higher rate capability compared to cathodes comprising the reference active mass. In particular, at a high current density of 600 mA g-1, the MS-cathodes could demonstrate a superior cycling stability with 75% retention and structural stability after 500 cycles. Compared with similar previously reported P2-type nickel-cobalt-manganese layered oxide compounds, the present microspheres based cathodes showed good performance in full SIB application Table 3 provides comparison between the performance of the cathodes presented in our work and that of different cathode materials for SIBs that were reported before. Moreover, the nearly two times higher tap density (~ 1.18 g cm-3) of the MS-cathodes enables us to fabricate high-density electrodes, thereby achieving high volumetric energy density of 340 Wh L-1(calculated based on the cathodes only). This should be compared with a much lower value of 206 Wh L-1 for the reference cathode material of the same composition which was tested herein. We believe that the proposed P2-type Na0.65[Ni0.17Co0.11Mn0.72]O2 material with microspheres morphology synthesized by the co-precipitation method is very promising as a sustainable and low-cost cathode material for high energy density SIBs.



Page 18 of 24

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID *Yang-Kook Sun: 0000-0002-0117-0170 Notes The authors declare no competing financial interest. 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) Hwang, J. Y.; Myung, S. T.; Sun, Y.-K. Sodium-ion Batteries: Present and Future, Chem. Soc. Rev., 2017, 46, 3529-3614. (2) Llave, E. D. L.; Borgel, V.; Park, K. J.; Hwang, J. Y.; Sun, Y.-K.; Hartmann, P.; Chesneau, F.;Aurbach D. Comparison between Na-Ion and Li-Ion Cells: Understanding the Critical Role of the Cathodes Stability and the Anodes Pretreatment on the Cells Behavior, ACS Appl. Mater. Interfaces, 2016, 8, 1867-1875. 18 ACS Paragon Plus Environment

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Table of Contents

The P2-type Na0.65[Ni0.17Co0.11Mn0.72]O2 cathode material with a morphology of compact microspheres was successfully synthesized via co-precipitation route. The proposed materials with hierarchical microstructure enhances the structural stability and high tap density. Consequently, the microspheres based cathodes exhibit the superior cycling stability compared to irregular shaped bulk cathodes.

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