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Spinel Nickel Cobaltite Mesostructures Assembled from Ultrathin Nanosheets for High-Performance Electrochemical Energy Storage Hongwei Lai, Longmei Shang, Qiang Wu, Lijun Yang, Jin Zhao, He Li, Zhiyang Lyu, Xizhang Wang, and Zheng Hu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00178 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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ACS Applied Energy Materials

Spinel Nickel Cobaltite Mesostructures Assembled from Ultrathin Nanosheets for High-Performance Electrochemical Energy Storage Hongwei Lai,†,‡ Longmei Shang,† Qiang Wu,*,† Lijun Yang,† Jin Zhao,† He Li,† Zhiyang Lyu,† Xizhang Wang,† and Zheng Hu*,† †

Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu Provincial Laboratory for

Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡

Jilin Medical University, Jilin 132013, China

KEYWORDS: Mesostructures; Nickel cobaltite; Electrochemical energy storage; Electrode engineering; Synergistic effect

ACS Paragon Plus Environment

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ABSTRACT

Transition metal oxides (TMOs) are promising electrode materials for advanced electrochemical energy storage (EES) due to their high theoretical capacities, but usually exhibit quite poor practical performance. There is a pressing need to boost their EES performance by electrode engineering directed with well-defined structure-performance relationship. Herein, we report an efficient approach to improve the specific capacitance and high-rate capability of spinel nickel cobaltite by constructing three-dimensional (3D) hierarchical porous mesostructures. The optimal Ni1.4Co1.6O4 mesostructures assembled from ultrathin nanosheets exhibit high capacitance (2282 F g-1 at 1 A g-1), excellent high-rate capability (1234 F g-1 @ 50 A g-1) and good cycling performance, much superior to the counterparts of Co3O4 mesostructures, Ni1.4Co1.6O4 mesostructures assembled from nanowires and randomly packed Ni1.4Co1.6O4 nanosheets. The excellent performance is attributed to the stable hierarchical porous architecture which enables large electroactive area and synergistically enhanced electrolyte access, solid-state ion diffusion and electron transfer. This tactic of constructing 3D mesostructured electrode with enhanced charge transport can be generalized to other TMOs for improving their EES performances.

INTRODUCTION The demand for high-performance electrochemical energy storage (EES) is ever-growing due to the expanding markets for portable electronics and electric transportation as well as the emerging application in efficient use of renewable energies. The key is to develop advanced electrode materials combining high energy density with large power density.1,2 Compared electrical double layer capacitive materials based on electrostatic adsorption, transition metal oxides (TMOs)


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show more promising prospects in advanced EES system since they can theoretically offer much higher energy densities by Faradaic redox reactions. However, TMOs usually present poor practical performance with low capacity far below the theoretical value and inferior high-rate capability,3-5 which is mainly due to their low electroactive areas and slow charge transport kinetics during the redox reaction. In general, the redox reaction in a TMO electrode depends on three charge transport processes, i.e., 1) ion diffusion in electrolyte between anode and cathode and also throughout the electrode material, 2) solid-state diffusion of reaction-involved ions (e.g. OH-) between the electrolyte/material interface and the sites where Faradaic reaction takes place, 3) electron transfer between the reaction sites and the current collector. In an ideal case, all these processes should run unimpeded, otherwise the EES performance will be suppressed to some extent. To improve the EES performance of TMOs, electrode engineering should focus not only on the high exposure of electroactive sites to ensure ample space for Faradaic reaction, but also on the design of optimum porous framework to allow fast electrolyte access, ion solid-state diffusion and electron transfer.2,6 In the long efforts to improve EES performance of TMOs, the common strategies such as nanostructuring and electrodeposition demonstrate their merits but with accompanied drawbacks.7-11 Specifically, the nanostructuring can increase surface area and shorten solid-state ion diffusion length, but is harmful to electrolyte penetration due to the aggregation of nanoscale blocks.7,8 The electrodeposition on conductive substrates boosts the electron transfer but is beset with the inherently low amount of deposited TMOs as well as the extra volume/weight increase to devices.9-11 In recent years, mesostructured materials assembled by nanoscale building blocks have attracted increasing attention in various fields especially in energy storage owing to their three-dimensional (3D) hierarchical structures and large surface areas.12-20 Very recently, we


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found that the EES performance of NiO could be boosted to its theoretical limit by constructing a novel mesostructured NiO/Ni composites, which consisted of the hetero-NiO/Ni components at nanoscale while displayed the 3D porous architecture at mesoscale.21 For such unique NiO/Ni composites, the metallic Ni is necessary to enhance the conductivity, and thereby boost the EES performance of NiO. However, the coexisting metallic Ni itself is electrochemically inert to Faradaic reaction in alkaline electrolyte, which has the side effect to decrease the capacity in terms of the total mass of NiO/Ni composites. In this regard, the mesostructured TMOs with high intrinsic conductivity are highly desirable for EES. As known, spinel nickel cobaltite with high intrinsic conductivity has been intensively studied as electrodes for supercapacitors22-45 and lithium-ion batteries.45-48 The hierarchical flower-like and urchin-like NiCo2O4 mesostructures exhibited quite good capacitances of 550-1650 F g-1,33-39 but still far below the theoretical limit of 3008 F g-1 assuming a potential window of 0.4 V with a 3-electron process.2 In addition, even similar geometries displayed much disparate capacitances (Table S1), showing ill-defined structure-performance relationship from an overall perspective. This situation implies there is great space from the EES potential of nickel cobaltite, which could be fulfilled by engineering the electrode structure under the direction of well-defined structureperformance relationship. Herein four spinel NixCo3-xO4 architectures with tunable morphologies, crystallite sizes and conductivities have been designed, and their EES performances have been compared systematically. Among them, the structurally stable Ni1.4Co1.6O4 mesostructures assembled from ultrathin nanosheets exhibit the best performance owing to the synergism of facilitated electrolyte accessibility, short solid-state ions diffusion length and high conductivity, while the other counterparts only present moderate capacitances owing to the weakness in at least one of the three charge transport processes. This study


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demonstrates an efficient approach to improve the EES performance of nickel cobaltite by constructing suitable 3D mesostructures, which could be generalized to other TMOs for exploring advanced EES electrode materials. EXPERIMENTAL SECTION Synthesis of NixCo3-xO4 Architectures. Four NixCo3-xO4 electrode materials were synthesized via a convenient coprecipitation-calcination route, i.e. fabricating Ni/Co-based precursors by a coprecipitation method followed by calcination in air, with some modifications on our recently developed method.21,49,50 Taking the synthesis of nickel cobaltite mesostructures assembled from ultrathin nanosheets (NCO-mNS) as an example, nickel(II) nitrate hexahydrate (6.7 mmol) and cobalt(II) nitrate hexahydrate (13.3 mmol) were dissolved into distilled water (150 mL) under stirring,

and

then

an

aqueous

solution

(50

mL)

containing

the

precipitators

hexamethylenetetramine (HMT, 20 mmol) and oxalic acid dihydrate (OA, 1.0 mmol) was added dropwise. The mixed solution was heated to 95 ºC and refluxed for 6 h. The precipitate was collected, and then calcined at 350 ºC in air for 2 h. This route was applied to synthesize Co3O4 mesostructures assembled from nanosheets (Co3O4-mNS) by using sole cobalt(II) salts. Changing the used precipitators can regulate the microscopic morphology of the Co/Ni precursors, and thereby the NixCo3-xO4 product. Specifically, randomly packed nickel cobaltite nanosheets (NCO-rNS) was obtained by using HMT (20 mmol) as the sole precipitator, and nickel cobaltite mesostructures assembled from nanowires (NCO-mNW) was prepared by using urea (20 mmol) and OA (1.0 mmol) as combined precipitators under the modified synthetic conditions.


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Characterization. The morphology and composition of the NixCo3-xO4 materials were characterized by SEM (Hitachi S-4800), HRTEM (JEM-2100), XRD (Bruker D8 Advances A25, Co Kα, λ=1.78897 Å), XPS (VG ESCALAB MKII) and ICP-MS (Optima 5300DV). N2 adsorption-desorption isotherms of the powder samples were measured at 77 K (Thermo Fisher). The specific surface area is calculated by using the BET (Brunauer-Emmett-Teller) method based on the adsorption data in the relative pressure (p/p0) range of 0.05 to 0.3. The total pore volume was measured from the amount of nitrogen adsorbed at a relative pressure (p/p0) of 0.97. The pore size distribution was calculated by using the BJH (Barrett-Joyner-Halenda) method from the adsorption branch of N2 isotherms. The electrical conductivity of the pressed samples was measured by a four-probe method using a source measure unit (Keithley 6430) as detailed in our recent work.18,19 Electrochemical Evaluation. The electrochemical properties of all the electrodes were evaluated on an electrochemical workstation (Biologic VMP3) using a three-electrode configuration in 2 mol L-1 KOH solution. To prepare working electrodes, a slurry was firstly made by stirring the NixCo3-xO4 materials, carbon black and polyvinylidene fluoride (PVDF) with weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solvent for 24 h, and then the slurry was spread on a nickel foam with mass loading of ~1.0 mg cm-2 for the active materials. The electrode was dried in a vacuum oven at 100 ºC for 12 h, and then pressed at 5 MPa. A platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. Cyclic voltammetry (CV) measurements were conducted in the range of 0‒0.5V and chronopotentiometry (CP) tests were performed in the range of 0‒0.4V for avoiding the electrolysis of H2O. All the electrochemical results were calculated by subtracting the contribution of Ni foam and reducing the substrate effect to the lowest level. The mean specific


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capacitance (Cm, F g-1) was calculated based on the CP curves by using the equation Cm = I·∆t/(∆V·m), where I, ∆t are the discharge current (A) and discharge time (s), m is the mass of active material, and ∆V is the discharge potential range (V). RESULTS AND DISCUSSION Figure 1 shows the different morphologies of four NixCo3-xO4 products, which inherited the morphologies of corresponding precursors (Figure S1). Specifically, porous spherical mesostructures assembled from thin nanosheets are obtained with the combined precipitators of hexamethylenetetramine (HMT) and oxalic acid (OA) (Figure 1a,b,e,f), the urchin-like mesostructures assembled from chain-like nanowires are obtained with the combined precipitators of urea and OA (Figure 1c,g), and the randomly packed nanosheets are obtained with the sole precipitator HMT (Figure 1d,h). All NixCo3-xO4 products have spinel structures with the compositions depending on the used metal salts, i.e., existing as NixCo3-xO4 from the mixed cobalt(II) and nickel(II) salts, and as Co3O4 (x=0) from the sole Co(II) source as revealed by XRD characterization. As revealed by detailed microscopic observations, the nanosheet-assembled mesostructures have the size of 2-4 micrometers, containing numerous meso-/macropores enclosed by the flexible and intercrossed nanosheets (Figure 1a,b). Such a porous morphology endows them with large exposed surface areas and pore volumes as evidenced by N2 adsorption-desorption result (Figure 2c, Table S2). For the Co3O4-mNS, the nanosheets are 100-300 nm in width and 8-20 nm in thickness, with polycrystalline structure comprising of interconnected nanoparticles (Figure 1a,e, Figure S1h, Figure S2). For the NCO-mNS, the nanosheets have ultrathin thickness in the range of 3-7 nm (Figure. 1b, Figure S3), obviously thinner than that of Co3O4-mNS. The


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nanosheets are composed of interconnected nanoparticles, and a great amount of inter-particle holes with sizes NCO-rNS > NCO-mNW > Co3O4-mNS, and the Coulombic efficiencies are 99.0%, 98.0%, 96.7% and 96.4%, respectively. The specific capacitances at different current densities are calculated from the corresponding charge/discharge curves (Figure 3c, Figure S6). NCO-mNS delivers an ultrahigh capacitance of 2282 F g-1 at the current density of 1 A g-1, much superior to NCO-rNS (872 F g-1), NCO-mNW (782 F g-1) and Co3O4-mNS (491 F g-1). The intrinsically conductive NCO-mNS electrode indeed presents much higher specific capacitance than the mesostructured NiO/Ni composites as expected,21 which is also superior to various nickel cobaltite electrode materials reported in the literature (Table S1). 22-40


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By increasing the current densities to 2, 5, 10 and 20 A g-1, the specific capacitances of the NCO-mNS are 2117, 1933, 1787 and 1599 F g-1, respectively. The capacitance retention is ~70% when the current density is increased from 1 A g-1 to 20 A g-1. At a high current density of 50 A g-1, the NCO-mNS materials still deliver a capacitance of 1234 F g-1 (within a short discharge time of 9.9 s), showing the superb high-rate capability. The corresponding Ragone plots show that the NCO-mNS can release the large electric energy of 50.7 Wh kg-1 at 0.2 kW kg-1, and deliver 27.4 Wh kg-1 energy at a high power of 10 kW kg-1, much superior to the other three NixCo3-xO4 materials (Figure S7). The cycling stability of the four electrode materials are evaluated at a high current density of 20 A g-1, as depicted in Figure 3d. After 4000 cycles, about 82.5% and 93.1% of capacitances can be retained for NCO-mNS and Co3O4-mNS, respectively. Correspondingly, the capacitances of NCO-rNS and NCO-mNW decay quickly to the low retentions of 75.8 % and 77.8 % after 2000 cycles. Obviously, the Co3O4-mNS electrode presents the best cycling stability, and the NCO-mNS exhibits superior cycling performance to the NCOrNS and NCO-mNW.


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NCO-mNS NCO-rNS NCO-mNW Co3O4-mNS

60 30

10 mV s

-1

(b)

0 -30

-1

5Ag

0.4 Potential (V vs. SCE)

-1

Current density (A g )

(a) 90

0.3 0.2 NCO-mNS NCO-rNS NCO-mNW Co3O4-mNS

0.1 0.0

-60 0.0

0.1

0.2

0.3

0.4

0

0.5

40

-1

-1

1600

NCO-mNS NCO-rNS NCO-mNW

900

Co3O4-mNS

2282 F g

2000

600

0 10

20

30

160

80

NCO-mNS NCO-rNS NCO-mNW Co3O4-mNS

40

300

0

120

(d) 120 20 A g-1 Retention (%)

(c) 2400

80 Time (s)

Potential (V vs. SCE) Specific capacitance (F 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

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40

50

0 0

1000

-1


2000 3000 Cycle number

4000

Figure 3. Electrochemical performances of the spinel NixCo3-xO4 samples. (a) CVs. (b) galvanostatic charging/discharging curves. (c) specific capacitance as a function of current density. (d) cycling performance at 20 A g-1. The preceding results indicate that the mesostructured NCO-mNS presents excellent EES performance, much better than the Co3O4-mNS with a similar mesostructure or the two nickel cobaltite counterparts with different morphologies. The relationship between electrode structure and EES performance of these materials could be understood as follows. For the Co3O4-mNS, the electronic conductivity is ~4 orders of magnitude lower than that for Ni1.4Co1.6O4, and the average crystallite size of ~14.0 nm is larger than the optimal sizes (~5 nm) for supercapacitor applications.22 Hence, the poor capacitive performance of the Co3O4-mNS is mainly attributed to the limited electron transfer and long solid-stated diffusion length for OH‒ ions. For the three Ni1.4Co1.6O4 architectures, the electron conductivities are comparable at a high level owing to


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their close compositions, while the NCO-mNW and NCO-rNS present much lower capacitances than the NCO-mNS. Intuitively, the larger crystallite size (~18.9 nm) could be a reasonable cause for the much inferior capacitive performance of NCO-mNW compared to the NCO-mNS; however, the crystallite size is not responsible for the different capacitances of NCO-rNS and NCO-mNS owing to their similar grain sizes of ~5 nm. By looking into the microstructures of these materials after electrode preparation under the pressure of 5 MPa, it is found that the electrolyte access and ionic transport are quite different throughout these three Ni1.4Co1.6O4 architectures. Specifically, the nanosheet-assembled NCOmNS mesostructures almost remain the hierarchical porous architecture owing to the structural rigidity of the densely assembled nanosheets, and additional macropores are accumulated among these microspheres (Figure 4a). Thanks to the multiscale pore structure, the NCO-mNS electrode is beneficial for electrolyte penetration and thereby high exposure of the electroactive area. In contrast, the nanosheets of NCO-rNS are compressed into a compact film as “nanobricks”, leaving less interlayer space (Figure 4b), and the NCO-mNW mesostructures collapse under pressure to form a tightly accumulated electrode with partially broken nanowires (Figure 4c). The NCO-rNS and NCO-mNW possess loose original architectures (Figure 1c,d), and the poor structural rigidity under pressure resulted in such morphologic transformation. As a consequence, the NCO-mNS electrode with multiscale porous architecture shows facilitated ions transport through electrolyte, while the NCO-rNS and NCO-mNW electrodes with reduced pore sizes present obviously suppressed electrolyte penetration and ionic transport. This is reflected by the electrochemical impedance spectra of these electrodes (Figure S8). The NCO-mNS electrode presented the smallest charge transfer resistance (Rct) and Warburg resistance (RW), while the NCO-rNS and NCO-mNW electrodes showed larger Rct and RW values owing to the suppressed


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electrolyte access throughout the compact structures. Hence, the inferior EES performance of the NCO-rNS and NCO-mNW is also correlated with the much suppressed electrolyte access and the retarded ionic transfer to the electroactive surface. Thanks to the small crystallite sizes (~5.5 nm) for short solid-state ion diffusion, good conductivity for fast electron transfer, and the structurally stable porous structure for good wettability and transport of electrolytes, the NCOmNS is vested with excellent capability of fast charge transport, and therefore superior EES performance. The different electrode microstructure, along with the different energy storage mechanisms for nickel cobaltite and Co3O4, results in the different cycling stabilities of these four materials (Figure 3d). As known the cycling stability closely correlates with the structural stability of electrode materials upon cycling. It has been revealed that the charge/discharge mainly stems from the surface redox reaction for Co3O4 with intrinsic pseudocapacitive property, and from both surface and bulk redox reactions for nickel cobaltite materials.2 The capacitance of Co3O4mNS is quite low due to the low participation of active material, but can be largely retained owing to the slight volume change during cycling, showing the excellent cycling stability. For the nickel cobaltite materials, the redox reactions are accompanied by big volume changes, which can be well accommodated for the NCO-mNS electrode owing to its porous structure, while may cause severe blocking of the ion transport channels for the NCO-rNS/NCO-mNW electrodes owing to their compact arrangement with much less pores (Figure 4b,c). This makes NCO-mNS superior to NCO-rNS and NCO-mNW in the cycling stability (Figure 3d).


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a

2 µm

b

1 µm

200 nm

c

1 µm

Figure 4. Cross-section SEM images of three NixCo3-xO4 architectures compressed under the working pressure of 5 MPa. (a) NCO-mNS, inset is the structural illustration. (b) NCO-rNS. (c) NCO-mNS, white arrows mark the broken nanowires. Insets in (b,c) are the magnified images. For the four NixCo3-xO4 electrode materials, the relationships between the structural features correlative to charge transport and their EES performances are summarized in Table 1. The


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NCO-mNS electrodes with simultaneously boosted electron conductivity, solid-state ion diffusion and electrolyte accessibility present excellent EEs performance, while any mismatch of these key factors will lead to remarkable performance degradation (Table 1). It is also indicated that, other than the commonly concerned high surface area, good conductivity and small crystallite size, the structural stability of the hierarchical porous structure is very crucial to the EES performance of electrode materials, which was usually unnoticed previously. Actually, the structural stability could be a main cause for the much disparate capacitances of the flower-like and urchin-like NiCo2O4 architectures in literatures (Figure S1),33-39 since those architectures have distinct compression resistances to remain their electrolyte accessible surface area after compression. Table 1. The structure-performance relationship of four NixCo3-xO4 electrode materials. Key factors of charge transport Samples

EES performance

Electron conductivity

Solid-state ion diffusion

Electrolyte accessibility

Capacitance @1 A g-1[F g-1]

Energy density @10 kW kg-1 [Wh kg-1]

2282

27.4

NCO-rNS

872

15.0

NCO-mNW

782

12.9

Co3O4-mNS

491

6.64

NCO-mNS

The preceding results suggest that a promising TMO electrode structure for high-performance EES should integrate the advantages of (i) a high specific surface area to ensure the high exposure of electroactive sites, (ii) small crystallite size (~5 nm)22 to shorten the solid-state ion diffusion length for increasing rate capability and TMO utilization, (iii) good electron


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conductivity to ensure high rate capability, and (iv) a stable multiscale porous structure to improve electrolyte accessibility and cycling stability. These requirements could serve as guidelines for exploring advanced TMO electrode materials for EES with combined high energy and power density, and the 3D porous mesostructures that meet these requirements are highly promising. CONCLUSIONS In summary, we have obtained the unique 3D mesostructured Ni1.4Co1.6O4 electrode materials assembled from ultrathin nanosheets, which integrate the advantages of large surface area, high conductivity, small crystallite size and stable multiscale porous structure. These characteristics enable the large electroactive area and synergistically boosted charge transport kinetics, leading to the excellent EES performance including ultrahigh specific capacitance (2282 F g-1 @ 1 A g-1), excellent high-rate capability (1234 F g-1 @ 50 A g-1) and superior cycling stability. The mesostructured Ni1.4Co1.6O4 electrode presents much higher capacitance than the mesostructured NiO/Ni composites as expected owing to the intrinsic high conductivity of nickel cobaltite, and is also superior to most of the nickel cobaltite electrodes in literatures. Together with the facile synthesis, the NCO-mNS shows great promise as advanced EES materials with combined high energy and power densities. This study also suggests an efficient tactic that can be generalized to other TMOs for improving their EES performance by constructing 3D mesostructured electrodes with high charge transport kinetics.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxx Morphological evolution during the post calcination, additional SEM and TEM images, Ragone plots and electrochemical impedance spectra of the four electrode materials, Co2p and Ni2p XPS spectra of NCO-mNS, Specific capacitances of NCO-mNS calculated by the segmenting method, the supercapacitive performances of the nickel cobaltite electrodes in literatures and in this study, the structural parameters of the electrode materials (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Q. W.); [email protected] (Z. H.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was jointly supported by NSFC (51232003, 21373108 and 21473089), ‘973’ program (2013CB932902), Suzhou Program (ZXG2013025) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. HWL thanks the support from Traditional Chinese Medicine Science and Technology Program of Jilin Province (2017102), Project of Jilin Province Health and Family Planning Commission (2016Q054, 2017ZC034). REFERENCES


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SYNOPSIS Spinel Nickel Cobaltite Mesostructures Assembled from Ultrathin Nanosheets for HighPerformance Electrochemical Energy Storage

Hongwei Lai, Longmei Shang, Qiang Wu,* Lijun Yang, Jin Zhao, He Li, Zhiyang Lyu, Xizhang Wang, and Zheng Hu*

-1

Crystallite size: 5.5 nm

-1

500 nm

Specific capacitance (F g )

2400

Conductiivty: 9.23 S m

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


2282 F g

2000

NCO-mNS

1600 900 600 300

NCO-rNS Co3O4-mNS

0

NCO-mNS

-1

0

10

20

NCO-mNW 30

40

-1

50



27

Spinel Nickel Cobaltite Mesostructures Assembled ... - ACS Publications

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