Hollow K0.27MnO2 Nanospheres as Cathode for High-Performance

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Hollow K0.27MnO2 Nanospheres as Cathode for High-Performance Aqueous Sodium-Ion Batteries Yang Liu, Yun Qiao, Xiangdong Lou, Xinhe Zhang, Wuxing Zhang, and Yunhui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03089 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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ACS Applied Materials & Interfaces

Hollow K0.27MnO2 Nanospheres as Cathode for High-Performance

Aqueous

Sodium-Ion

Batteries Yang Liu,† Yun Qiao,∗, †, ǁ Xiangdong Lou,† Xinhe Zhang,& Wuxing Zhang,‡ and Yunhui Huang∗, ‡ †

School of Chemistry and Chemical Engineering, Henan Normal University. Xinxiang 453007,

China. ‡

State Key Laboratory of Material Processing and Die & Mould Technology, School of

Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. ǁ

National and Local Joint Engineering Laboratory of Motive Power and Key Materials, Henan

Normal University, Xinxiang 453007, China. &

McNair New Power CO., LTD. Dongguan 523800, China

ACS Paragon Plus Environment

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ABSTRACT: Hollow K0.27MnO2 nanospheres as cathode material were designed for aqueous sodium-ion batteries (SIBs) using polystyrene (PS) as template. The samples were systematically studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). As cathode materials for aqueous SIBs, the hollow structure can effectively improve the sodium storage property. In the full cell with hollow K0.27MnO2 as cathode and NaTi2(PO4)3 as anode, the coin cell exhibits a specific capacity of 84.9 mA h g–1 at 150 mA g–1, and the capacity of 56.6 mA h g–1 is still maintained at an extremely high current density of 600 mA g–1. Full cell measurement at the current density of 200 mA g–1, 83 % capacity retention also can be attained after 100 cycles. The as-designed hollow K0.27MnO2 nanospheres demonstrate long cyclability as well as high-rate capability, which has a potential application in advanced aqueous SIBs. KEYWORDS: polystyrene, K0.27MnO2, hollow nanospheres, cathode materials, sodium-ion batteries

INTRODUCTION Increasing demands for renewable and clean energy, such as electric vehicles (EVs) and electrical smart grid from wind and solar power, have encouraged great efforts on the investigation of advanced electrochemical energy storage (EES) technologies.1-4 For the practical application of EES devices, the characteristics of low cost, abundant resource, high energy density and long cycling life are especially crucial.5-7 In recent years, lithium-ion batteries (LIBs) have received worldwide application in portable electronics, such as laptop, mobile phone and digital camera, due to the absence of the memory effect, high working voltage, high energy


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density and long life span.8-10 Despite the great success of LIBs up to now, high cost and insufficient lithium resource still restrict the overall practical application on large-scale EES.11-14 Innovation in battery technology is thus highly desired to fulfill the tremendous demands of large-scale EES.15-20 SIBs using non-aqueous electrolytes were investigated, which are similar with commercialized LIBs.21, 22 Sodium layered oxides and Prussian blue analogues as cathode materials are common for promising application in non-aqueous electrolytes SIBs, respectively.23-26 Meanwhile, rechargeable aqueous SIBs as commercial battery also have promising application for large-scale storage of electrical energy.27-29 Recently, considerable efforts in exploring new rechargeable aqueous SIBs system with low cost, safety, and abundant resource.30 Whitacre et al. reported an aqueous energy storage device using Na4Mn9O18 as cathode and activated carbon as anode.31 Whereafter, they successfully boosted the industrialization exploitation of aqueous SIBs.32,

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Yang et al. investigated the

Na2NiFe(CN)6/NaTi2(PO4)3 and Na2CuFe(CN)6/NaTi2(PO4)3 aqueous rechargeable SIBs, both of them showed an excellent sodium storage performance.34,

35

Kumar et al. confirmed that the

Na3V2O2x(PO4)2F3-2x/MWCNT composite as a cathode showed a long-term stability for aqueous SIBs.36 In order to support the electrical energy grid, nickel hexacyanoferrate as aqueous SIB electrode for energy storage has been deeply investigated by Cui et al.37 Qian et al. fabricated NaMnO2/NaTi2(PO4)3 hybrid system as coin cell for stationary energy storage.38 Meanwhile, Deng et al. tested the sodium storage performance of Na0.44MnO2/Na2V6O16 in coin cell.39 In our previous work, Binessite-type AxMnO2 (A = Na+, K+)/NaTi2(PO4)3 coin cell has been fabricated and investigated, which demonstrated a specific capacity of 74.6 mA h g–1 at 150 mA g–1.40 NaTi2(PO4)3 with NASICON-type framework structure as anode material, demonstrated a potential application in aqueous SIBs. Binessite-type AxMnO2 is a promising cathode for


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aqueous SIBs, because they own layered structure consisting of edge-sharing MnO6 octahedral with a plane spacing of ~7 Å, which allows fast mobility of interlayer cations, excellent reaction kinetics, and easy access of ions to the electrode/electrolyte interface.40, 41 The electrode materials are significant for the future development of aqueous SIBs with highperformance, especially cathode. The electrochemical property of the cathode can be promoted through designing nanostructures, assigning to its short diffusion distance and enlarged surface/volume ratio, which can increase the kinetics of ion and electron transport. Hollow nanostructures have been received considerable attentions due to their large surface area, low density, and good loading capacity, which is a promising application in SIBs. Our previous works also confirm that the synthesis method and microstructure are also important for the design of K0.27MnO2 to achieve high electrochemical performance. Herein, hollow K0.27MnO2 nanospheres have been prepared using nanospherical polystyrene (PS) as template to further improve the sodium storage performance in SIBs. Briefly, the PVPcapped PS templates were prepared via emulsifier-free emulsion polymerization. Then core-shell PS@K-δ-MnO2 flower-like spheres were designed via a hydrothermal process. Finally, the coreshell samples were sintered at 500 °C under air to obtain hollow K0.27MnO2 nanosphere materials. The structure and morphology of the samples at different stages have been characterized. Coin cell also has been assembled using NaTi2(PO4)3 as anode and K0.27MnO2 as cathode. The full cell with hollow K0.27MnO2 nanospheres as cathode exhibits high rate performance and stable cycling performance. EXPERIMENTAL SECTION Synthesis. The analytical grade chemicals were directly used without purification. Deionized water as solvent was adopted in the synthetic process.


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Synthesis of PS spheres: PS spheres used in this work were prepared by a modified polymerization reaction.42 In a typical preparation process, styrene was washed with NaOH aqueous solution (5 wt.%) to remove the inhibitor. Then, 10 mL styrene and 1.5 g PVP were dissolved in deionized water (90 mL). The solution was heated to 80 °C and stirred for 10 min. Sodium persulfate (78 mg) was added to the above solution drop by drop. The reaction solution maintained at 80 °C under vigorous stirring for another 5 h. Finally, the PS spheres could be directly obtained. Synthesis of hollow K0.27MnO2: 0.5 mol MnSO4 and 0.5 mol KMnO4 were dispersed in 25 mL H2O, respectively. After stirring for 30 min, the solutions were mixed with 2 mL PS solution and transferred into 100 mL Teflon-lined stainless autoclave and heated to 160 °C for 1 h in an electric oven. The resulting precipitate was obtained after centrifuging and washing by deionized water and ethanol thoroughly. The as-obtained composite after drying at 80 °C was labeled as PS@K-δ-MnO2. The composite was heat treated at 500 °C in N2 atmosphere for 2 h. The samples were thoroughly washed with H2O and ethanol, and then dried at 80 °C. The PS@K-δMnO2 precursor was converted into hollow K0.27MnO2. Characterizations. The morphology of the samples was performed on a JSM 7600F field emission scanning electron microscope (FE-SEM, Japan), and the elemental composition was detected on Inca X-Max 50 (Oxford instrument) equipment with an energy-dispersive spectrometer (EDS). X-ray diffraction (XRD) pattern was collected using an X’pert PRO X-Ray Diffractometer (Cu Kα radiation, Holland). The fine morphology and microstructure of the samples were analyzed on JEM-2010 electron microscope. Raman and fourier transform infrared spectrometer (FTIR) spectra were measured on a Bruker VERTEX 70 equipment. Electrochemical Measurements. The cathode and anode electrodes were made by mixing the as-prepared sample, super P, polyvinylidene fluoride (80:10:10, wt%) and coating on stainless steel net. The active material loading of the electrode was about 1.2 mg cm–2. Before test, the electrode was vacuum dried in an electric oven for 12 h at 80 °C. Three-electrode system and coin cell were used to measure the electrochemical measurements. The electrochemical properties of the electrodes were collected in a three-electrode system, where a platinum foil as counter electrode, 1 mol L–1 Na2SO4 aqueous solution as electrolyte, and standard calomel electrode (SCE) as the reference electrode. Chronopotentiometry measurements and cyclic voltammetry were conducted on an electrochemical workstation (CHI 660E). The galvanostatic


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charge-discharge cycling were carried out on CT2001A (Land, China) from 0.0 to 1.6 V at various current densities in the coin cell.

Scheme 1. Schematic illustration of the fabrication of the hollow K0.27MnO2 nanospheres. RESULTS AND DISCUSSION The synthetic strategy for hollow K0.27MnO2 nanospheres based on PS template and heat treatment is schematically depicted in Scheme 1. Firstly, PS colloids as template were synthesized via a modified polymerization reaction.42 Subsequently, K-δ-MnO2 was coated on PS nanospheres by a simple hydrothermal reaction. After a thermal treatment process, the coreshell PS@K-δ-MnO2 can be converted into hollow K0.27MnO2 nanospheres. Using (NH4)2S2O8 as the cationic initiator and PVP stabilizer, the PS colloids were prepared by emulsifier-free emulsion polymerization. The size and morphology of PS spheres were firstly characterized by FE-SEM after drying in absolute ethanol under infrared lamp. Typical FE-SEM images of such colloidal PS nanospheres are linked together, as PVP molecules are desorbed on the surface of PS spheres (Figure 1). For these homogeneous sphere, the average diameter of is about 200 nm. PVP as an amphiphilic surfactant is beneficial to the dispersion of PS nanospheres in water. In addition, Mn ions can be coordinated with its pyrrolidone groups,

which is

beneficial for the homogeneous coating of K-δ-MnO2 shell on the surface of PS spheres.


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Figure 1. (a, b) FE-SEM images of PS nanospheres.

Flower-like PS@K-δ-MnO2 spheres with core-shell structure were obtained after a hydrothermal process, using mono-dispersed PS as template. Figure 2a shows the typical XRD pattern of as-prepared core-shell PS@K-δ-MnO2. The broad XRD pattern reveals that the shells are mainly in the layered structure. The δ-MnO2 with large interlayer make fraction of guest K ions incorporate into distinct interlayer structure.43,

44

Therefore, K ions as pillar commonly

occupy the alternating layers of δ-MnO2. The EDS spectrum in Figure 2b shows that K, Mn, O, and C elements are distributed in the sample, suggesting that K ions are embedded into the interlayer space of δ-MnO2. The typical low-magnification FE-SEM image in Figure 2c demonstrates that flower-like spheres are well-dispersed and homogeneous. The core-shell structure of flower-like PS@K-δ-MnO2 spheres is clearly observed from the low-magnification TEM image, in agreement with the above FE-SEM results (Figure 2d). The inner PS also can be observed through the cracked segment. The HR-TEM image obviously demonstrates that the asprepared flower-like spheres are about 300 nm in diameter and the shell is about 100 nm in thickness (Figure 2e). The HR-TEM image in Figure 2f shows the plane spacing of ~0.7 nm is attributed to the (100) plane of layered MnO2. The polycrystallinity is confirmed by SAED


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pattern (insert in Figure 2f). These results confirm that the core-shell PS@K-δ-MnO2 flower-like spheres can be obtained by a simple hydrothermal method. (001) Intensity (a.u.)

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Mn (002)

(100)

K (110)

O C

10

20

30

40 ο 50 2θ ( )

60

70

80

0

Mn 1

2

3 4 5 Energy (keV)

6

7

Figure 2. (a) XRD pattern, (b) EDS pattern, (c) FE-SEM image, (d, e) TEM images, (f) HRTEM image (with inset SAED pattern ) of PS@K-δ-MnO2.

After heat treating in air at 500 °C, the crystalline structure of the as-obtained sample was analyzed by XRD. It can be clearly observed that all diffraction peaks in Figure 3a are assigned to rhombohedral K0.27MnO2 (JCPDS: 86-0666; space group: R-3m, a=b=0.2849 nm, c=2.1536 nm). No diffraction peaks of residues or impurities have been detected, demonstrating the high quality of the product. Therefore, the PS template can be removed from sample after the heating process. As shown in Figure 3b, EDS analysis further reflects the distribution of K, Mn, and O in the sample, meaning that K ions are commonly occupied the sites within the layered structure of


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hollow K0.27MnO2. The FTIR spectrum of the hollow K0.27MnO2 is presented in Figure 3c. Stretching and bending vibrations are visible at 3300 cm–1 and are ascribed to the water molecules or hydroxyl groups in the sample. The weak band around 1630 cm–1 is normally recognized as the Mn3+-OH vibration, which confirms that Mn3+ ions are in the interlayer and/or in the octahedral layers.45 The peaks below 600 cm–1 are assigned to the MnO6 octahedral structure. The Mn-O stretching vibrations are identified from the peaks at 530 and 480 cm–1, indicating that water and hydroxyl groups are coordinated with Mn ions in the samples.45 In order to further obtain molecular and crystal vibrational properties, Raman spectroscopic investigation was also conducted, because it is a sensitive technique to study the local structure of hollow K0.27MnO2 and also can replenish the FTIR result. Figure 3d presents the Raman spectrum of the hollow K0.27MnO2. In the present research, the sample shows two broad bands located at 570 and 640 cm–1. In detail, the strong peak centered at ~570 cm−1 is attributed to the stretching vibration of Mn-O-Mn bond in the basal plane of MnO6 octahedral lattice, confirming the high ratio of Mn (IV) in the as-obtained sample. The symmetric stretching vibration of Mn-O in MnO6 groups is clarified by the weak peak at around 640 cm–1.


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(a)

(b)

Intensity (a.u.)

(003)

O

Mn

(006)

K C

10

20

30

40ο 50 2θ ( )

60

70

80

(c)

0

Mn Mn 1

2

3 4 5 Energy (keV)

6

500

600 700 800 900 −1 Wavenumber (cm )

7

(d) Intensity (a. u)

Intensity (a. u)

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−1

1630 cm

−1

530 cm −1 480 cm

−1

3300 cm

500 1000 1500 2000 2500 3000 3500 4000 −1 Wavenumber (cm )

400

1000

Figure 3. (a) XRD pattern, (b) EDS pattern, (c) FTIR spectrum, and (d) Raman spectrum of the as-obtained hollow K0.27MnO2.

The morphology and microstructure of the sample were further analyzed (Figure 4). As shown in Figure 4a, FE-SEM investigation clearly demonstrates that a large amount of hollow K0.27MnO2 nanospheres with a size of about 300 nm in diameter are connect with each other after heat treatment. The hollow structure can be observed in the breakage nanospheres. The TEM image of K0.27MnO2 nanospheres shows that the sample owns hollow and hierarchical structure composed with stacked multilayer nanosheets, in accordance with the above FE-SEM result (Figure 4b). The higher magnification TEM image demonstrates that hollow structure of the nanosphere is assembled with sheet-like crystals, and the typical thickness of the shell is about 150 nm (Figure 4c). The HR-TEM image of K0.27MnO2 in Figure 4d demonstrates that the sample is a typical layered structure. The plane spacing in layered K0.27MnO2 was measured to be 7 Å, corresponding to the (003) direction. The corresponding SAED pattern is demonstrated


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in Figure 4d inset, indicating the polycrystalline nature of K0.27MnO2. For comprising, K0.27MnO2 nanoparticles were prepared under the same conditions without PS template (Figure 4e). The XRD pattern confirms the layered Birnessite structure (Figure 4f).

(003)

Intensity (a.u.)

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(006)

10

20

30

40 o 50 2θ ( )

60

70

80

Figure 4. (a) FE-SEM image, (b, c) TEM images, and (d) HR-TEM image (with inset the SAED pattern) of hollow K0.27MnO2. (e) FE-SEM image and (f) XRD pattern of the as-prepared K0.27MnO2 nanoparticles without PS template.

The electrochemical performances of hollow K0.27MnO2 were evaluated using a three-electrode experimental setup. Figure 5a shows the typical CV curves within a potential range from 0.0 to 1.0 V vs. SCE at various scan rates in 1 mol L−1 Na2SO4. The mostly symmetric CV curves in each cycle represent typical behaviors of the battery, whereas one pair of redox peaks are


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ascribed to the insertion/extraction of alkali-metal ions accompanied by the reversible reaction between Mn3+ and Mn4+ ions. The overall CV shape is maintained well as the scan rate increasing to 1.5 mV s–1 from 0.2 mV s–1. The voltage gap between the reduction and oxidation peaks is attributed to the electrode polarization, closely correlating with the conductivity of the active material.46 The almost linear relationship between the square root of the scan rate (v1/2) and peak current (ip) is demonstrated in Figure 5b, indicating that the crucial procedure in electrochemical reaction is the diffusion process.47 However, the electrode also performs characteristic capacitance during the electrochemical process, due to the whole active surface area and the internal pores of the electrode.48 Therefore, the capacitive contribution has been evaluated in a three-electrode system, using hollow K0.27MnO2 as aqueous SIBs electrode.

-0.2

mV s

−1

(b)

1.5 1.2 1.0

0.05

-0.1 0.0 0.1

Anodic peak

0.00

Cathodic peak

-0.05 -0.10 -0.15

0.3

KMO 0.8

-0.04

-0.2

-0.02

-0.1

0.00

0.0

0.02

0.1

KMO 0.04 0.2

0.4 0.6 Potential vs. SCE (V)

0.8

-0.20 0.4

1.0

1.0

KMO 0.6

0.8 1.0 1/2 −1/2 (V s )

1.2

1/2

ν

(d)

1 st 2 nd 3 rd

-1.0

Current (mA)

0.4 0.6 Potential vs. SCE (V)

Current (mA)

0.2

(c)

0.2 0.0

y= 0.08465x−0.02749

y= −0.16363x+0.04577

0.2

0.0

0.10

−1

Current (mA)

-0.3

0.8 0.6 0.4 0.2

Current (A g )

(a)-0.4

Current (mA)

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-0.5

1 mV s

−1

0.0

0.5

NaTi2(PO4)3

-1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 Potential vs. SCE (V)

Figure 5. (a) CV curves of hollow K0.27MnO2 at all scan rates from 0.2 to 1.5 mV s–1; (b) the linear dependence of i/ν1/2 on ν1/2; (c) CV curve of K0.27MnO2 and the estimated capacitive contribution (shaded region) and (d) CV curves of NaTi2(PO4)3 at a scan rate of 1.0 mV s–1.


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At a fixed potential, the overall current response is assumed to two parts, one from the surface capacitive effect and one from the diffusion controlled insertion process, which can be calculated as follows:49 i(V) = k1v + k2v1/2

(1)

It can be written as i(V)/ v1/2 = k1v1/2 + k2

(2)

where k1ν is the current contribution from surface capacitive effects, k2ν1/2 is the diffusion controlled insertion process. As shown in Figure 5b and equation (2), the linear dependence of i/ν1/2 on the scan rate of ν1/2 is used to determine the slop (k1) and the intercept (k2) at each fixed potential. This means that the capacitive effects can be estimated at a certain voltage. Figure 5c clearly demonstrates the estimated capacitive contribution in shaded region at a scan rate of 1 mV s–1. The capacitive charge storage accounts for 5% of the overall charge storage via the enclosed area. On the basis of our previous work, the NaTi2(PO4)3 anode was prepared by a solid state process.40 In Figure 5d, a pair of redox peaks at around -0.9 V and 0.7 V can be observed in a wide voltage range from -1.1 to -0.3 V, which is lower than potential window limit for the aqueous electrolyte.28 In order to further investigate the electrochemical performance, the coin cell has been assembled using hollow K0.27MnO2 as cathode, NaTi2(PO4)3 as anode, 1 mol L–1 Na2SO4 aqueous solution as electrolyte. Galvanostatic cycling measurements were used to characterize the hollow K0.27MnO2 electrode using a coin cell at a current density of 200 mA g−1 within the cut-off voltage range of 0–1.6 V. Figure 6a demonstrates that the cell exhibits two plateaus at ~1.2 and ~0.6 V during the cycling process, which is consistent with the extraction of alkalimetal ions from electrodes. The first charge and discharge capacity in the coin cell are 98.3 and


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83.0 mA h g−1, respectively. The charge and discharge curves have no obvious variation from the second cycle, indicating the structure of the electrode is stable during the cycling process. After 100 charge and discharge cycles, the corresponding Coulombic efficiency is 94.8%.

(b)140

0.4

40 60 −1 Capacity (mA h g )

(c)140

80

100

120

80

100

−1

200 mA g

80 60

Discharge

60 40

40 20

20 0

0

−1

200 300

60

200 mA g 400

500 600

40 20 0

100

0

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20

30 40 50 60 Cycle number

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0 10 20 30 40 50 60 70 80 90 100 Cycle number

90 100

120 −1

20

Capacity (mA h g )

0

100 150 80

80

100 60

80 −1

60 200 mA g 40

Discharge

20 0

0

10 20 30 40 50 60 70 80 90 Cycle number

40 20 0

Coulombic efficiency (%)

−1

Capacity (mA h g )

0.8

0.0

−1

120

1 st 2 nd 5 th

1.2

Coulombic efficiency (%)

Cell voltage (V)

(a) 1.6

Capacity (mA h g )

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Figure 6. Electrochemical performances of NaTi2(PO4)3/K0.27MnO2 hybrid coin cells: (a) Galvanostatic cycling profiles at 200 mA g–1, (b) Rate capacity at various current densities, (c) Long-term cyclability of hollow K0.27MnO2 at a current density of 200 mA g–1 . (d) Cycling performances and corresponding Columbic efficiencies of K0.27MnO2 without PS template in hybrid full cell at a current density of 200 mA g–1.

The rate capability of the coin cell was further tested at different current densities (Figure 6b). The discharge capacities of the coin cell are 84.9, 80.1, 71.0, 66.0, and 60.8 mA h g–1 when cycled at 150, 200, 300, 400 and 500 mA g–1, respectively. While the current density increased to 600 mA g–1, a discharge capacity of 56.6 mA h g–1 can still be maintained. This indicates that the electrode demonstrates excellent rate performance. Moreover, the specific capacity of coin cell can recover to the original value as the current density is switched to 200 mA g–1. Cycling


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performance further reflects the cyclic stability of the hollow K0.27MnO2 electrode. Figure 6c shows the discharge capacity and the corresponding Coulombic efficiency at 200 mA g−1. The first Coulombic efficiency is about 80%, and it gradually increases to 87% in the fifth cycle. The reversible discharge capacity of 68.7 mA h g–1 can still be retained after 100 cycles, inducing to a comparatively low irreversible capacity loss (17% compared with the first cycle). As a contrast, the cycling performance of K0.27MnO2 without PS template is showed in Figure 6d. A low discharge capacity of 50 mA h g−1 is observed at 200 mA g−1. The morphology and structure of hollow K0.27MnO2 electrode after 100 cycles have been further characterized. As presented in Figure 7a, FE-SEM image clearly shows the nanosphere morphology. Meanwhile, TEM image in Figure 7b further confirms the hollow structure can be retained. Moreover, HR-TEM image and SAED pattern show the layered and polycrystalline structure of the hollow K0.27MnO2 after 100 cycles (Figure 7c and Figure7d). All these results prove the structure stability of the electrode. It is obvious that the hollow K0.27MnO2 in the coin cell exhibits outstanding rate capability and highly stable cycling performance, attributing to these two aspects: (1) nanospheres with hollow structure enable short diffusion distance and enlarge the contact area with electrolyte, thus can facilitate the insertion and extraction of alkalimetal ions; (2) the stacked multilayer nanosheets can enlarge the stability of layered structure and promote the electrical conductivity, which is favorable for the charge transport and cycling stability of the electrode.


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Figure 7. (a) FE-SEM image, (b) TEM image, (c) HR-TEM image, and (d) SAED pattern of hollow K0.27MnO2 electrode after 100 cycles. CONCLUSIONS Hollow K0.27MnO2 nanospheres have been designed using nanospherical PS as template. Such hollow architecture greatly facilitates the electron/ion transport kinetics of K0.27MnO2, leading to high electrochemical performance, according to the excellent charge/discharge capacities, high rate capability and stable cyclability. This work indicates that hierarchical K0.27MnO2 nanospheres with multilayer nanosheets and hollow architecture have a great potential for largescale application of high-performance aqueous SIBs in large-scale energy storage. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; *E-mail: [email protected]. ACKNOWLEDGMENTS


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This work was supported by the Natural Science Foundation of China (Grant No. 21503071 and 21501049), the Fund of Department of Education of Henan Province (No. 14B150030 and 15A150014), the Fund of Key Scientific and Technological Project of Henan Province (No. 152102210286). REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. (2) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. (3) Amin, M. Energy: The Smart-Grid Solution. Nature 2013, 499, 145-147. (4) Simon, P.; Gogotsi, Y.; Dunn, B. Where do Batteries End and Supercapacitors Begin? Science 2014, 343, 1210-1211. (5) Choi, N. S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y. K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges Facing Lithium Batteries and Electrical DoubleLayer Capacitors. Angew. Chem. Int. Ed. 2012, 51, 9994-10024. (6) Wang, K.; Jiang, K.; Chung, B.; Ouchi, T.; Burke, P. J.; Boysen, D. A.; Bradwell, D. J.; Kim, H.; Muecke, U.; Sadoway, D. R. Lithium-Antimony-Lead Liquid Metal Battery for Grid-Level Energy Storage. Nature 2014, 514, 348-350. (7) Yang, Z.; Zhang, J.; Kintner Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577-3613. (8) Qiao, Y.; Hu, X. L.; Liu, Y.; Liang, G.; Croft, M.; Huang, Y. H. Surface Modification of MoOxSy on Porous TiO2 Nanospheres as an Anode Material with Highly Reversible and Ultra-Fast Lithium Storage Properties. J. Mater. Chem. A 2013, 1, 15128-15134.


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The Table of Contents (TOC)


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Hollow K0.27MnO2 Nanospheres as Cathode for High-Performance

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