Deintercalation in Two-Dimensional

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Redox Active Cation Intercalation/Deintercalation in Two-Dimensional Layered MnO2 Nanostructures for High-Rate Electrochemical Energy Storage Pan Xiong, Renzhi Ma, Nobuyuki Sakai, Xueyin Bai, Shen Li, and Takayoshi Sasaki ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14612 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

Redox Active Cation Intercalation/Deintercalation in Two-Dimensional Layered MnO2 Nanostructures for High-Rate Electrochemical Energy Storage Pan Xiong,† Renzhi Ma,† Nobuyuki Sakai,† Xueyin Bai,† Shen Li,§ Takayoshi Sasaki*† †

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan §

Department of Materials Science & Engineering, Royal Institute of Technology, SE-100 44

Stockholm, Sweden KEYWORDS Two-dimensional layered nanostructures, cation intercalation/deintercalation, interlayer spacing, metal ion-based energy storage, rate capability

ABSTRACT Two-dimensional (2D) layered materials with high intercalation pseudocapacitance have long been investigated for Li+-ion-based electrochemical energy storage. By contrast, the exploration of guest ions other than Li+ has been limited, although promising. The present study investigate the intercalation/deintercalation behaviors of various metal ions in 2D layered MnO2 with various interlayer distances, K-birnessite nanobelt (K-MnO2), its protonated form (HMnO2), and a freeze-dried sample of exfoliated nanosheets. Series of metal ions, such as

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monovalent Li+, Na+ and K+ and divalent Mg2+, exhibit reversible intercalation during charge/discharge cycling, delivering high-rate pseudocapacitances. In particular, the freeze-dried MnO2 of exfoliated nanosheets restacked with the largest interlayer spacing and a less com-pact 3D network exhibits the best rate capability and a stable cyclability over 5000 cycles. Both theoretical calculation and kinetic analysis reveal that increased interlayer distance facilitates the fast diffusion of cations in layered MnO2 host. The results presented herein provide a basis for the controllable synthesis of layered nanostructures for high-rate electrochemical energy storage using various single- and multivalent ions.

INTRODUCTION The fast-growing demand for portable electronic devices calls for the urgent development of advanced electrochemical energy storage technologies to store/deliver more energy at fast charge/discharge rates with low cost, high safety and long-term stability.1–3 Over the past few decades, lithium-ion batteries (LIBs) have been widely used in our daily life as one of the main power sources for portable electronic devices such as cell phones, laptop computers, and digital cameras, etc.. However, suffering from low power density, a LIB-equipped mobile phone or laptop usually requires hours to be fully charged. High-power supercapacitors, known as electricdouble-layer capacitors (EDLCs), are able to store/deliver energy in less than 10 seconds because of their rapid charge/discharge rates, but are beset with limited energy density of 5–10 W h/kg.4,5 Between the regimes where EDLCs and LIBs exhibit their best performance is a time domain in which pseudocapacitors appear to have the ability to strike a balance between energy density and power capability.6–8


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Generally, pseudocapacitors store charge through a faradaic process that involves surface or near-surface redox reactions, which provide much greater charge storage capability than EDLCs based on ion adsorption at the electrode/electrolyte interface. Transition metal oxides and conducting polymers with specific capacitance values exceeding 1000 F/g are common electrode materials for pseudocapacitors. However, they usually suffer from short cyclic lifetimes and low rate capabilities due to a detrimental phase change involving sluggish rate-limited faradaic processes.7–9 Recently, a new pseudocapacitive charge storage mechanism called cation intercalation pseudocapacitance has been proposed.6,10 In particular, the charge storage via pseudocapacitive intercalation is not a diffusion-controlled process. Thus, more charge can be stored not only at the surfaces but also in the bulk of the electrode materials in a short time without a crystallographic phase change, leading to superior high-rate performance and cyclability. Up to now, the intercalation pseudocapacitive mechanism has been widely explored in Li+-ion-based electrochemical energy storage.10–13 Unlike Li+ intercalation/deintercalation that occurs in batteries, the pseudocapacitive charging and discharging processes occur on the order of seconds and minutes. Therefore, both high energy densities and high power densities are possible to be achieved. The intercalation pseudocapacitive mechanism has been extended to technologies beyond Li+-ion-based energy storage, such as Na+ and Mg2+ ion batteries, due to the growing cost and limited availability of Li.14–18 For instance, a graphene-coupled TiO2 hybrid with high Na+-ion intercalation pseudocapacitance has been explored for high-rate Na+-ion batteries.16 Despite such advances, the present electrode materials still exhibit unsatisfactory capacity, sluggish rate capability or poor cycling stability in other metal-ion-based energy storage systems because of the distinct differences in terms of size and valence between Li+ ion and other metal ions. Therefore, breakthroughs in electrochemical energy storage technology


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require not only a deep fundamental understanding of the charge storage mechanisms for different metal ions but also the development of novel electrode materials with high performance, low cost, and environmentally friendly production that can be used in various metal ion-based systems.19,20 The cations are inserted through the tunnels or interlayers of the electrode materials during the pseudocapacitive intercalation process. For example, in a Nb2O5 nanocrystal, the vacant sites between the (001) planes provide open tunnels for rapid Li+ ion transport.21,22 An open channel was formed at the interface of a graphene-coupled TiO2 system, which enabled rapid Na+ ion intercalation/deintercalation.16 In this regard, layered materials with 2D interlayer galleries should be promising candidates for intercalation pseudocapacitive energy storage. Birnessite MnO2 has a layered structure consisting of edge-shared MnO6 octahedral layers, guest cations and water molecules. The 2D interlayers can be used for the fast intercalation of cations.23–27 For example, K-birnessite MnO2 has been used as cathode materials for supercapacitors in Li+-, Na+-, or K+-ion-based electrolytes.28–31 In addition, layered K-birnessite MnO2 can also be used as cathodes for Li+-ion batteries or aqueous Na+-ion batteries.30,32 Our group reported the exfoliation of a layered birnessite MnO2 into single-layer 2D nanosheets.33,34 Then, layered Li+-, Na+-, or K+-incorporated MnO2 has been synthesized via flocculation of delaminated MnO2 nanosheets with the corresponding metal ions.35,36 These flocculated MnO2 products were further employed as electrode materials for supercapacitors, and promising performance was demonstrated.36 However, most of the previous reports focused on the intercalation behavior of a single type of cation. Recently, various cations, including Na+, K+, NH4+, Mg2+, and Al3+, were intercalated electrochemically into layered titanium carbide, resulting in a capacitance greater than 300 F/cm3.37 Therefore, it is of great interest to investigate


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the intercalation performance of various cations in layered MnO2 nanostructures. Moreover, the intercalation/deintercalation mechanism of different metal ions with different size and valences, which may strongly affect the intercalation performance, has rarely been reported. On the other hand, most electrochemical reactions are generally assumed to take place at the electrode/electrolyte interface.38 From this viewpoint, the electrochemical intercalation performance of layered nanostructures is critically related to the accessible surface area within the interlayers, which can be tuned by controlling the interlayer geometry. The interlayer cations of birnessite MnO2 are exchangeable with a range of cations, including H+, K+, Na+, Mg2+ and Ca2+, introducing the possibility of controlling the interlayer distance. Nazar’s group studied the ion intercalation behavior in a layered Mg-birnessite MnO2.18 Huang’s group reported that (K, Na)-co-incorporated birnessite MnO2 exhibited better performance than the K- or Na-forms.39 Our group exfoliated layered birnessite MnO2 into single-layer 2D nanosheets with a dramatically increased accessible surface area, which supposed to host more guest cations.33,34 However, ion intercalation/deintercalation in layered MnO2 nanostructures with different accessible surface areas has rarely been studied. In our previous reports, 2D layered MnO2 in various nanoarchitectures had been synthesized facilely.34,35 These 2D layered MnO2 nanostructures with controllable interlayer spacing and high crystallinity are promising platforms for studying the effects of interlayer geometry on the intercalation charge storage mechanism. Herein, 2D layered MnO2 nanostructures, such as K-MnO2 nanobelt, its protonated form, and a restacked MnO2 composed of exfoliated nanosheets in varied morphologies with different interlayer distances, were synthesized and used as electrode materials for metal-ion-based electrochemical energy storage. A series of metal ions, including monovalent Li+, Na+ and K+ and divalent Mg2+ were intercalated into these 2D layered MnO2 nanostructures, showing large


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pseudocapacitances, high rate capabilities as well as good long-term cyclability. Theoretical calculation and intercalation kinetics analyses were carried out, and a possible charge storage mechanism is proposed. RESULTS AND DISCUSSION Material synthesis and characterization The synthetic processes of 2D layered MnO2 nanostructures are illustrated in Figure 1a. The KMnO2 was first synthesized via a hydrothermal method. X-ray diffraction (XRD) data (Figure S1a) showed a monoclinic structure with unit-cell parameters of a = 0.5141(2) nm, b = 0.2839(3) nm, c = 0.7163(1) nm, and β = 100.28(1)°, which agree well with previous studies.34 The sharp diffraction peaks indicate its highly crystalline nature. In particular, two sharp basal peaks from the (001) and (002) planes of layered K-MnO2 gave an interlayer spacing of 0.69 nm (see the layer structure of K-MnO2 in Figure 1a). Scanning electron microscopy (SEM) images of the resultant K-MnO2 (Figure 1b) showed the 2D nanobelt morphology with a length of several tens of micrometers and a width of several hundreds of nanometers. The K-MnO2 nanobelts were subsequently treated with an (NH4)2S2O8 aqueous solution, which behaves as a weak acid with strong oxidizing power. This is different from the common protonation method involving HCl, which may not only induce the exchange of interlayer K+ but also cause disproportionation of Mn3+ into Mn2+ and Mn4+, resulting in deteriorated crystallinity and quality.40,41 In contrast, the moderate oxidative acid-exchange in the present route produced H-MnO2 with well-maintained crystallinity (Figure S1b) and morphology (Figure 1c). As shown in Figure S1b, all the diffraction peaks were indexed to a hexagonal structure with refined lattice parameters of a = 0.2835(1) nm and c = 0.7288(1) nm, which are also consistent with the


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previous reports.34 The basal spacing was expanded to 0.72 nm after protonation (see the layer structure of H-MnO2 in Figure 1a).

Figure 1. (a) Schematic of layered structures and synthetic processes. (b-c) SEM images of KMnO2 and H-MnO2 nanobelts. (d) TEM image of exfoliated MnO2 nanosheets. (e) SEM image of the freeze-dried product of the aqueous suspension of MnO2 nanosheets. (f) UV-vis absorption spectrum of the MnO2 nanosheet suspension. The inset shows the Tyndall light-scattering effect, suggesting the uniform dispersion of the nanosheets. (g) AFM image and corresponding height profile of MnO2 nanosheets deposited on a Si wafer. (h) XRD pattern of the freeze-dried sample. Finally, MnO2 nanosheets were obtained by a two-step delamination process according to our previously reported procedure.34 A transmission electron microscopy (TEM) image revealed a very thin sheet-like morphology (Figure 1d), confirming the lamellar nature of the delaminated nanosheets. The UV-vis absorption spectrum of the diluted MnO2 nanosheet suspension (Figure 1f) showed a broad absorption band centered at approximately 380 nm, which is consistent with


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the results of a previous report.33 Atomic force microscopy (AFM) images obtained after drying the suspension showed a large number of sheet-like objects with a height of approximately 0.8 nm (Figure 1g), providing direct evidence for the successful preparation of single-layer MnO2 nanosheets. The suspension was further freeze-dried to produce a lamellar aggregate of the exfoliated MnO2 nanosheets. An SEM image (Figure 1e) showed a 3D sponge-like network with open pores ranging from hundreds of nanometers to tens of micrometers. XRD data depicted in Figure 1h confirm the restacking of MnO2 nanosheets at a separation of 0.93 nm, as indicated by the two obvious diffraction peaks at around 9.5° and 18.9°. The other two peaks at approximately 36.5° and 65.4° are ascribable to in-plane (10) and (11) diffractions from the turbostatically stacked nanosheets, respectively.42 These features indicate that the MnO2 nanosheets are restacked without sheet-to-sheet registry, accommodating tetramethylammonium ions (TMA+) in the interlayer galleries (see the layer structure of the restacked MnO2 nanosheets in Figure 1a). In addition, we note that the restacked MnO2 sample exhibited the largest full-width at halfmaximum of the (001) peak, indicating the restacked MnO2 sample has a limited number of MnO2 nanosheets loosely stacked along the c-axis compared to the K- and H-MnO2 nanobelts. The characterization results above indicate that three kinds of 2D layered MnO2 nanostructures with various morphologies and different interlayer spacing were prepared. These layered nanostructures are promising hosts, possessing 2D open spaces for rapid ion intercalation/deintercalation. Intercalation/deintercalation mechanism


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The intercalation behaviors of different metal ions in these layered nanostructures were examined. The associated ex situ XRD patterns were recorded at the end of each charge and discharge cycle (Figure 2a and Figure S2). Two diffraction peaks corresponding to the (001) and (002) planes of layered K-MnO2 were observed during CV cycles in all four kinds of electrolytes, confirming the preservation of the layered structure. More importantly, down- and upshifts of these two diffraction peaks were observed reversibly in every charge and discharge cycle. The reversible variation of interlayer distance is shown in Figure 2b. During the charge process, the expansion of interlayer spacing was observed in all four kinds of electrolytes, whereas discharge process resulted in contraction. This observation can be explained by the cations intercalated into K-MnO2 (discharging process) increasing the electrostatic attraction between negatively charged MnO2 layers, resulting in the observed spacing contraction. By contrast, the extraction of cations during the charge process leads to interlayer expansion as a result of the weakened attraction between the layers.43 Energy-dispersive X-ray spectroscopy (EDX) analysis during CV cycles was also carried out (Table S1). After the first charge process, the K/Mn ratios in all tested electrolytes decreased, suggesting that K+ ions were oxidatively extracted. The detection of Na and Mg after first charge may be due to the physical adsorption of Na+ and Mg2+ ions onto the surfaces of the K-MnO2 nanobelts. The Li could not be detected because of the detection limit. A reversible change of the K content was observed in the K2SO4 electrolyte. The content of K gradually decreased in the Li2SO4, Na2SO4, and MgSO4 electrolytes, and reversible changes in the Na and Mg contents were detected in the corresponding electrolytes, suggesting the replacement of K+ for Na+ and Mg2+ present in large excesses in the electrolyte solutions. Although no obvious redox peaks


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were observed in the aqueous MgSO4 electrolyte, the reversible changes of interlayer distance and Mg content imply the intercalation/deintercalation of Mg2+.


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Figure 2. (a) Ex situ XRD patterns of K-MnO2 nanobelts during the initial three charge/discharge cycles in Li2SO4 electrolyte. (b) Variation of the interlayer distance of K-MnO2 nanobelts during charge/discharge cycles in various aqueous electrolytes. (c) Schematic of the reversible intercalation/deintercalation of Li+, Na+, K+, and Mg2+ ions in layered K-MnO2. Notably, the interlayer spacings were similar to each other for the appreciable range of ionic sizes (0.068 nm for Li+ to 0.133 nm for K+). Furthermore, their changing magnitudes upon charge/discharge were rather small: 0.02 nm for Li+ and even smaller for the others. These facts may be understood by a monolayer hydrate structure that accommodates guest cations and H2O molecules in a monolayer configuration (Figure 2c). The amount of interlayer water was quantified to be ~ 5.5% by thermogravimetric (TG) analysis (Figure S3). In such a structure, the interlayer spacing is primarily governed by H2O, which is larger than Li+, Na+, and Mg2+ and comparable to the size of K+. The above results demonstrate the reversible electrochemical intercalation/deintercalation of a series of metal ions, including Li+, Na+, K+ and Mg2+ ions, in layered K-MnO2 without a major structural change. The intercalation behaviors of these metal ions in layered H-MnO2 and restacked MnO2 nanosheets were examined under the identical conditions. The ex situ XRD patterns of H-MnO2 nanobelts are displayed in Figure S4. Associated diffraction peak shifts reversibly appeared upon charge/discharge cycling (Figure S5). During the first charge process, the interlayer distance slightly changed from an initial value of 0.711 nm to 0.715 nm. This observation is attributed to the extraction of H3O+. The interlayer spacing then decreased during the first discharge cycle, which can be ascribed to the intercalation of Na+, K+, and Mg2+. The Na, K, and Mg contents decreased after the charge processes and increased during the subsequent discharge cycles in the corresponding aqueous electrolytes (Table S2).


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Similarly, the structural features of restacked MnO2 nanosheets were followed via the ex situ XRD method during the initial three CV cycles (Figure S6). Two diffraction peaks corresponding to the (001) and (002) planes of restacked MnO2 nanosheets were observed in all tested aqueous electrolytes. Unlike the cases of the K-MnO2 and H-MnO2 nanobelts, significant interlayer contraction from 0.928 nm to 0.712–0.716 nm occurred during the initial charge process (Figure S7), which can be explained by the extraction of the TMA+ ions.44 The interlayer distance exhibited reversible but small changes during the subsequent charge/discharge cycles in all of the tested aqueous electrolytes (Figure S7), which again coincided with the change in metal-ion content (Table S3). All of these results again indicate the reversible electrochemical intercalation/deintercalation of Li+, Na+, K+ and Mg2+ in the H-MnO2 nanobelts and restacked MnO2 nanosheets. On the basis of the aforementioned results, the intercalation process can be summarized using layered K-MnO2 as an example: During the first charge process, most of the K+ ions are extracted from the interlayer space of K-MnO2, resulting in the expansion of the interlayer distance. The reversible intercalation and deintercalation of Li+, Na+, K+, and Mg2+ ions subsequently occurs in the corresponding electrolytes (Figure 2c). Electrochemical performance Figure 3a compares the typical CV curves of K-MnO2 nanobelts, H-MnO2 nanobelts, and restacked MnO2 nanosheets at a scan rate of 5 mV/s in the aqueous K2SO4 electrolyte. Different from the previous work on MnO2 electrodes, where a rectangular CV curve was reported,29,45–47 the as-prepared layered MnO2 electrodes exhibited redox peaks (anodic peaks at approximately 0.25–0.45 V and cathodic peaks at approximately 0.50–0.60 V) in the aqueous K2SO4 electrolyte. These peaks are likely associated with the faradic pseudocapacitive process of MnO2, involving the reversible intercalation/deintercalation of K+ ions, accompanied by electrochemical


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switching between Mn3+ and Mn4+.10,28,44 The contribution to the electrochemical response from the bare Ni foam substrate and carbon black additive is negligible, giving much smaller current densities (Figure S8). The smallest separation between the anodic and cathodic peaks of restacked MnO2 nanosheets suggests the more favorable intercalation/deintercalation of K+ ions.10,28

Figure 3. Comparison of the electrochemical performance of K-MnO2 nanobelts, H-MnO2 nanobelts, and restacked MnO2 nanosheets. (a) CV curves at a scan rate of 5 mV/s in K2SO4 electrolyte. (b) Rate capability in the K2SO4 electrolyte at current densities ranging from 0.2 to


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10 A/g. (c) Charge/discharge curves at current density of 0.2 A/g in the K2SO4 electrolyte. (d) Charge/discharge curves at current density of 5 A/g in the K2SO4 electrolyte. These layered MnO2 nanostructures were subsequently charged and discharged at various current densities ranging from 0.2 to 10 A/g (Figure S9). Charge/discharge curves deviated from the ideal straight lines in all cases, consistent with the pseudocapacitive nature. The rate performances of K-MnO2 nanobelts, H-MnO2 nanobelts, and restacked MnO2 nanosheets in the K2SO4 electrolyte are compared in Figure 3b. Comparable specific capacities of approximately 140, 150, and 160 F/g were obtained, respectively, at a low current density of 0.2 A/g (Figure 3c). As the current density was increased, differences in the electrochemical performances of these layered nanostructures became obvious. As shown in Figure 3d, the symmetric charge/discharge curves were retained at a current density of 5 A/g in all layered MnO2 nanostructures, whereas a voltage drop (IRdrop) was clearly observed and increased in the order of restacked MnO2 nanosheets < H-MnO2 nanobelts < K-MnO2 nanobelts. The lowest voltage drop implies the smallest intrinsic series resistance inside the electrodes, endowing the restacked MnO2 nanosheets with superior pseudocapacitor properties.30 Figure S10 shows the electrochemical impedance spectra (EIS) of K-MnO2, H-MnO2 and restacked MnO2 nanosheets, further implying the smallest equivalent series resistance (Rs) and charge-transfer resistance (Rct) of restacked MnO2 nanosheets (2.32(2) and 10.44(3) Ω) compared to those of K-MnO2 (7.45(1) and 20.23(4) Ω) and H-MnO2 (6.32(2) and 18.86(2) Ω), respectively. At a high current density of 10 A/g, the reversible capacity of the restacked MnO2 nanosheets remained as high as 110 F/g, with 70% capacity retention. By contrast, in the cases of the K- and H-MnO2 nanobelts, only 50% and 55% capacity retentions were obtained, respectively, as the current density was increased from 0.2 to 10 A/g. The coulombic efficiency of K-MnO2, H-MnO2, and restacked


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MnO2 at current densities ranging from 0.2 to 10 A/g was shown in Figure S11. The restacked MnO2 showed relative low coulombic efficiency at the low current density compare to K-MnO2 and H-MnO2. As the current density increased, larger coulombic efficiency than that of K-MnO2 and H-MnO2 was observed for restacked MnO2. The variations of average areal energy densities vs power densities of K-MnO2, H-MnO2, and restacked MnO2 are compared in Figure S12, where restacked MnO2 showed the best performances. A maximum areal energy density of ~ 0.30 mWh/cm2 at a power density of ~ 1.4 mW/cm2 and an areal energy density of ~ 0.13 mWh/cm2 at a power density of ~ 55 mW/cm2 were obtained, respectively, superior to those of K-MnO2 and H-MnO2. The highest specific capacity and best rate capability of the restacked MnO2 nanosheets may be due to the largest interlayer spacing, as well as an open 3D network and less compact structure, which may offer a more accessible diffusion path for improved electrochemical processes.48,49 To further explain the best electrochemical performances of restacked MnO2 nanosheets, density functional theory (DFT) simulations were performed to analyze the diffusion behavior of K ion in layered MnO2 with varying interlayer distances. Energetically favorable AB stacking of bilayer MnO2 is considered here. Two sites for the K ion adsorbed between two MnO2 layers were considered according to previous report.50 One is the octahedral site (O-site), where K locates at the center of a Mn–O hexagonal ring. The other is the tetrahedral site (T-site), where K locates at the top site above a Mn atom (Figure S13). As shown in Figure 4a, the K ion migrated from one most energetically favorable O-site to another adjacent one via the metastable T-site. This is in agreement with previous observation of Li within MnO2 layers.50 At a small interlayer distance of d = 0.60 nm, a diffusion energy barrier of 0.135 eV was observed (Figure 4b). As the interlayer distance increases from 0.60 to 0.80 nm, a slight shift of K ion was observed at the O-


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site (Figure S14). Notably, the diffusion energy barrier decreased remarkably to a much lower value of 0.034 eV. As the interlayer distance increased to 1.00 nm, the diffusion energy barrier further decreased to 0.028 eV. The decrease in diffusion energy barrier should be the gradually weaker interactions between positively charged K ions and the negatively charged MnO2 layers as the interlayer distance increased. The theoretical simulation suggests that increased interlayer distance provides improved diffusion kinetics of cations in layered MnO2, further explaining the best electrochemical performances of restacked MnO2 nanosheets.

Figure 4. Theoretical simulation of K diffusion in layered MnO2 with different interlayer distances. (a) Calculated K diffusion path in layered MnO2. (b) Potential energy diagram for K migration at interlayer distance of d = 0.60, 0.80, and 1.00 nm. The electrochemical performance of the restacked MnO2 nanosheets in various aqueous electrolytes was examined in depth (Figure 5a). Symmetric CV curves with anodic and cathodic peaks were observed in the Li2SO4, Na2SO4, and K2SO4 electrolytes. By contrast, no obvious redox peaks were observed in the MgSO4 electrolyte, which is similar to previously reported


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results for layered MnO2.18,51 Figure S15 shows the charge/discharge curves of restacked MnO2 nanosheets measured in different aqueous electrolytes. Symmetric triangular charge/discharge curves were observed at various current densities. Notably, the restacked MnO2 nanosheets delivered a specific capacity of approximately 280 and 250 F/g at 0.2 A/g in the Li2SO4 and MgSO4 electrolytes, respectively; these capacities are larger than those achieved in the Na2SO4 and K2SO4 electrolytes (approximately 220 and 160 F/g) (Figure 5b). At the lower current density, the reversible intercalation/deintercalation behavior dominates the charge storage process. In this regard, Li+ and Mg2+ ions with smaller radii are more favorable to intercalate into layered K-MnO2.45 When the current density increased, the effective utilization of the redox reaction is restrained to some extent, thus resulting in decreased capacity in all of the tested electrolytes. Large specific capacity of approximately 90 and 110 F/g was still obtained at a high current density of 10 A/g in Na2SO4 and K2SO4, respectively. By contrast, poor capacitance of approximately 40 and 40 F/g at 10 A/g was obtained in Li2SO4 and MgSO4, respectively (Figure 5b). At high current densities, the insertion of some ions into the interior part of the electrode materials will be limited by insufficient charge/discharge time. At this point, the Na2SO4 and K2SO4 electrolytes with higher ionic conductivities are more beneficial for fast charge transport, leading to superior rate capability.45 Figure S16 shows the electrochemical impedance spectra (EIS) of restacked MnO2 nanosheets in different electrolytes, in which smaller electrolyte resistances of Na2SO4 and K2SO4 were observed. These results contribute to the better rate performances of restacked MnO2 nanosheets in the Na2SO4 and K2SO4 electrolytes.


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Figure 5. Electrochemical performance of restacked MnO2 nanosheets in various aqueous electrolytes. (a) CV curves at a scan rate of 5 mV/s. (b) Rate capability at current density ranging from 0.2 to 10 A/g. (c) Specific capacity (Cs) at 0.5 A/g over 100 cycles and corresponding


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capacity retention. (d) XRD patterns after cycling at 0.5 A/g over 100 cycles. (e) Long-term cycling stability at 5 A/g over 5000 cycles. Furthermore, the restacked MnO2 nanosheets were charged and discharged at 0.5 A/g over 100 cycles in all tested aqueous electrolytes (Figure S17). Good cycling stability without obvious capacity decay was observed (Figure 5c). The layer structure of restacked MnO2 nanosheets after cycling was maintained in all tested aqueous electrolytes, as confirmed by the XRD data (Figure 5d) and SEM observations (Figure S18). Long-term cyclability is critical but challenging for electrochemical energy storage. As little crystal structure change was taken place during the pseudocapacitive intercalation/deintercalation processes, high cycling stability is possible. In this regard, the long-term cycling performance of restacked MnO2 nanosheets was evaluated at a current density of 5 A/g over 5000 cycles (Figure 5e). Capacity retention of approximately 85%, 95%, 100%, and 80% at the end of the 5000th cycle were obtained in the aqueous Li2SO4, Na2SO4, K2SO4, and MgSO4 electrolytes, respectively. The electrochemical performances of K-MnO2 and H-MnO2 nanobelts in various aqueous electrolytes were examined under the same conditions (Figure S19 and S20). Similarly, better rate capability was observed in the Na2SO4 and K2SO4 electrolytes for both K-MnO2 and HMnO2 nanobelts (Figure S21). Because of the expanded interlayer spacing, the H-MnO2 nanobelts exhibited a larger specific capacity and better rate performance than those of K-MnO2 in all tested aqueous electrolytes. Moreover, high cyclability and structural stability of K-MnO2 (Figure S22-S24) and H-MnO2 nanobelts (Figure S25-S27) in all four electrolytes were observed, again indicating negligible structure change during the reversible electrochemical intercalation/deintercalation processes of these cations.


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Kinetics analysis The kinetics analysis of restacked MnO2 nanosheets in different electrolytes was carried out to gain further insight into the intercalation/deintercalation processes (see Supporting Information for the detailed calculations, Figure S28 and S29). The b-values in these aqueous electrolytes are summarized in Table S4. The larger b-value for the restacked MnO2 nanosheet electrodes in the Na2SO4 and K2SO4 electrolytes suggests superior intercalation kinetics of Na+ and K+ ions, which is consistent with the observed high rate capability in the Na2SO4 and K2SO4 electrolytes. In addition, the capacitive contribution at a scan rate of 5 mV/s was quantified to be approximately 85% of the total current, namely, the capacity (Figure 6a).

Figure 6. Kinetic analysis. (a) Separation of capacitive and diffusion current of restacked MnO2 nanosheets at a scan rate of 5 mV/s in the K2SO4 electrolyte. (b) Determination of the infinite sweep rate capacitance of K-MnO2 nanobelts, H-MnO2 nanobelts and restacked MnO2 nanosheets. The charge storage processes of these layered nanostructures were studied through analysis of the relationship between capacity and sweep rate (see Supporting Information for the detailed


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calculations). As shown in Figure 6b, the Ccapacitive of K-MnO2 nanobelts, H-MnO2 nanobelts, and restacked MnO2 nanosheets were approximately 120, 125, and 140 F/g, respectively, resulting in approximately 86%, 84%, and 90% portions of the total capacitance at 0.2 A/g, respectively. The higher capacitance and portion indicate a superior capacitive behavior of the restacked MnO2 nanosheets. CONCLUSION Three types of 2D layered MnO2 nanostructures were explored for aqueous metal-ion-based energy storage. A reversible intercalation/deintercalation of Li+, Na+, K+, and Mg2+ with minor structural changes was demonstrated in these 2D layered MnO2 nanostructures, resulting in high rate capability and long-term cycling stability. In particular, the restacked MnO2 nanosheets with the largest interlayer spacing and open 3D architecture displayed the highest capacitances and the best rate performances. Moreover, a stable, long-term cycling durability at a current density of 5 A/g over 5000 cycles was observed in all of the tested aqueous electrolytes. These results demonstrate that these 2D layered nanostructures with controllable interlayer distances are not only promising candidates for high-rate electrochemical energy storage through pseudocapacitive intercalation of various metal ions but also ideal platforms for investigating the effect of interlayer geometry on the intercalation charge storage mechanism. However, the limited specific capacities restricted by the intrinsic poor conductivity of MnO2 should be elucidated and will be the subject of our subsequent research. The demonstrated intercalation mechanism may be applicable to other layered materials and could therefore open new opportunities for developing improved intercalation energy storage systems using a large variety of ions.


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EXPERIMENTAL SECTION Sample preparation: K-MnO2, H-MnO2 nanobelts, and restacked MnO2 nanosheets were synthesized according to our previously reported method with some modifications.34 The KMnO2 nanobelts were synthesized via a hydrothermal method. Briefly, 5.75 mmol of KMnO4 and 3 mol of KOH were dissolved into 140 cm3 of H2O. Then, 10 cm3 of 1.15 M MnCl2 aqueous solution was added immediately. After cooling to room temperature, the resulting mixture was transferred into a Teflon-lined stainless steel autoclave and maintained at 205 °C for 2 days. The resulting precipitate was collected by filtration, washed several times with water, and then air dried. To obtain H-MnO2 nanobelts, 1 g of the as-prepared K-MnO2 was dispersed into 250 cm3 of a 0.5 M (NH4)2S2O8 aqueous solution and stirred at 60 °C for 3 h. After filtration, the solid was treated again with the (NH4)2S2O8 aqueous solution. The final product was collected by filtration, washed with water, and then air dried. Finally, 0.1 g of the as-prepared H-MnO2 was immersed in 25 cm3 of aqueous tetramethylammonium hydroxide (TMAOH) solution, where the molar ratio between TMA+ and the exchangeable protons in the H-MnO2 (formulated as H0.08MnO2·H2O) was equal to 30. After shaking for 2 days, the mixture was centrifuged (6000 rpm) and washed with water repeatedly until the supernatant solution became neutral. The sediment was then dispersed in 25 cm3 of an aqueous tetrabutylammonium hydroxide (TBAOH) solution. The concentration of TBA+ was the same as that of TMA+ in the TMAOH solution. The resulting slurry was shaken at 80 rpm for 10 days. The MnO2 nanosheet suspension was collected by centrifugation (6000 rpm) to remove the unexfoliated portion. The obtained MnO2 nanosheet suspension was further centrifuged (20,000


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rpm) and washed with water repeatedly until the supernatant solution became neutral. The obtained sediment was freeze-dried and named as restacked MnO2 nanosheets. Electrochemical characterization: Electrochemical measurements were carried out at room temperature in a conventional three-electrode system. Platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The working electrode was prepared by casting a slurry of active material (K-MnO2, H-MnO2 nanobelts, or restacked MnO2 nanosheets), carbon black, and polytetrafluoroethylene in a weight ratio of 80:15:5 onto Ni foam (1.6 mm thick, 0.45 g/cm3 bulk density, 95% porosity, purchased from Sigma-Aldrich). The electrodes were dried at 60 °C for at least 24 h. The mass loading of the active material was approximately 3–5 mg/cm2. CV and galvanostatic charge/discharge testing were performed on a Solartron SI 1287 electrochemical workstation. EIS was performed at the open-circuit potential using a 10 mV AC amplitude and frequencies ranging from 10 mHz to 20 kHz. Aqueous solutions of 0.5 M Li2SO4, Na2SO4, and K2SO4 and 1.0 M MgSO4 with the same pH of approximately 6.5 were used as electrolytes. Structural characterization: Powder XRD data were recorded on a Rigaku Rint-2200 diffractometer equipped with a monochromatic Cu Kα radiation (λ = 0.15405 nm). The morphologies of the as-prepared products were examined with a JEOL JSM-6700F scanning electron microscope and a JEOL JEM-3000F transmission electron microscope. EDX analysis was measured for the electrodes. Before EDX analysis, the electrodes after charged or discharged were washed with DI water and then air dried. UV–vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. A Seiko SPA 400 atomic force microscope was used to examine the topography of nanosheets deposited onto a Si wafer substrate precoated


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with polyethylenimine (PEI). TG analysis was performed on a Rigaku TGA-8120 instrument at a heating rate of 10 °C/min in air. Computational methods: The calculations were conducted by means of DFT as implemented in Vienna ab initio simulation package (VASP) program package,52,53 where the basis set is described by the projector augmented wave method54 and exchange-correlation potential is described by Perdew-Burke-Ernzerhof (PBE) approximation.55 The MnO2 bilayer were modeled using a 4 × 4 MnO2 supercells in plane (with lattices of 11.13 Å × 11.13 Å) and a vacuum region with thickness 15 Å perpendicular to the MnO2 bilayer. The cutoff energy for the basis set was set to 520 eV and the Brillouin zone integration was performed with a 4 × 4 × 1 Γ-centered Monkhorst-Pack k-mesh. The ion positions were relaxed to a convergence of 1 meV/Å for the forces of each atom. To study the diffusion of K ion in the MnO2 bilayer, the nudged elastic band (NEB) method56 was used to determine the minimum energy path and energy barrier between given initial and final positions. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Characterizations of K-MnO2 and H-MnO2 nanobelts. Electrochemical measurements, structural and morphology analysis of K-MnO2, H-MnO2 nanobelts, and restacked MnO2 nanosheets. Theoretical calculation results (PDF) AUTHOR INFORMATION


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Table of Contents


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Deintercalation in Two-Dimensional

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