Microwave–Hydrothermal Crystallization of Polymorphic MnO2 for

May 1, 2013 - 5, 160, 10, γ-MnO2 (dominant) + trace β-MnO2, wire + plate, 205.9, 68.0, 23.5 ... Typically, 4.68 mmol of MnCl2•4H2O and 3.12 mmol of KM...
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Microwave−Hydrothermal Crystallization of Polymorphic MnO2 for Electrochemical Energy Storage Kunfeng Chen,†,‡ Young Dong Noh,§ Keyan Li,‡ Sridhar Komarneni,*,§ and Dongfeng Xue*,†,‡ †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China § Materials Research Institute, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: We report a coupled microwave−hydrothermal process to crystallize polymorphs of MnO2 such as α-, β-, and γ-phase samples with plate-, rod-, and wirelike shapes, by a controllable redox reaction in MnCl2−KMnO4 aqueous solution system. MnCl2−KMnO4 redox reaction system was for the first time applied to MnO2 samples under the coupled microwave−hydrothermal conditions, which shows clear advantages such as shorter reaction time, well-crystallized polymorphic MnO2, and good electrochemical performances as electrode materials for lithium ion batteries. For comparison, we also did separate reactions with hydrothermal only and microwave only in our designed MnCl2− KMnO4 aqueous system. The present results indicate that MnCl2− KMnO4 reaction system can selectively lead to α-, β-, and γ-phase MnO2, and the as-crystallized MnO2 samples can show interesting electrochemical performances for both lithium-ion batteries and supercapacitors. Electrochemical measurements show that the as-crystallized MnO2 supercapacitors have Faradaic reactivity sequence α- > γ- > β-MnO2 upon their tunnel structures, the intercalation−deintercalation reactivity of these MnO2 cathodes follows the order γ- > α- > β-phase, and the conversion reactivity of these MnO2 anodes follows the order γ- > α- > β-phase. MnCl2−KMnO4 reaction system can also lead to the mixed-phase MnO2 (β- and γ-MnO2), which can provide better anode performances for lithium-ion batteries. The current work deepens the fundamental understanding of several aspects of physical chemistry, for example, the chemical reaction controllable synthesis, crystal structure selection, electrochemical property improvement, and electrochemical reactivity, as well as their correlations. general electrode materials via various chemical reactions.4 It is quite interesting for us to properly evaluate the as-obtained electrode materials when used as electrochemical energy storage purpose; two stages of chemical reactions are predominant, one is for their preparations and the other is for energy storage. Unfortunately, we still cannot find enough efforts to focus on the crystallization-dependent chemical reactions and the storage-dependent electrochemical reactions of electrode materials. In the current work, we use manganese dioxide (MnO2) as an example to show the reactioncontrollable crystallization and crystallization-dependent electrochemical reactions and energy storage behaviors. MnO2 has attracted significant interest in the development of high-performance electrode materials, owing to its low cost, and environmental compatibility.2,9−11 The existing synthetic processes to crystallize MnO2 include the oxidation of Mn2+ by S2O82−, H2O2, and O2, and the reduction of MnO4− by

1. INTRODUCTION Electrochemical energy storage cells such as rechargeable batteries and electrochemical capacitors have long been highly demanded to meet the various high-tech requirements of sustainable and renewable energy resources.1−4 Different electrochemical reaction mechanisms have been proposed for electrode materials of lithium-ion batteries and supercapacitors, for example, the intercalation−deintercalation reaction, conversion reaction, alloying−dealloying reaction for anodes,5,6 the intercalation−deintercalation reaction for cathodes,7 and the Faradaic reaction for pseudosupercapacitors.2,8 All these chemical reactions belong to redox ones and occur between electrode materials and electrolyte; only those highly reversible ones can deliver high electrochemical capacity,8 therefore, excellent electrochemical properties of electrode materials are strongly dependent on their involved reversible redox reactions. For many years, the phase and morphology studies of general electrode materials have long been a big challenge for electrochemists to effectively improve their electrochemical performances.2 It has been found that during the synthesis process, we can well control the crystallization behaviors of © XXXX American Chemical Society

Received: February 20, 2013 Revised: April 16, 2013

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Scheme 1. Schematic Illustration of MnO2 Crystal Structures and Phase Transformation between γ-MnO2, β-MnO2, and αMnO2, and the Effect of K+ on the Growth of α-MnO2 Process

tunnel-structure, and hollow architecture.24 Even though different phases and morphologies of MnO2 were crystallized in different reaction systems, it is still difficult for us to deeply understand the control strategies in improving their electrochemical properties.2 Therefore, the crystallization of MnO2 with different phases and morphologies in one reaction system can favor such an understanding of the structure−crystallization−property correlation. This work can crystallize different kinds of MnO2 with the use of coupled microwave− hydrothermal method in the proposed MnCl 2 −KMnO 4 reaction system. MnO2 with different crystallographic structures were crystallized by a controllable redox reaction in MnCl2−KMnO4 aqueous solution with the application of microwave−hydrothermal technique. In the designed MnCl2−KMnO4 redox system, both reducing agent and oxidizing agent are from Mn compounds, the occurred chemical reaction is shown in Scheme 2. It is reported that the crystallographic structure of

reducing agents such as N2H4, carbon, ethanol, ascorbic acid, and NO2−.2,10,12 Nanostructured MnO2 with various growth shapes such as those sphere-, rod-, wire-, plate- and flower-like morphologies have been successfully fabricated by the redox reaction between KMnO4 and NaNO2 in aqueous solution.9 Much work has been confirmed that the electrochemical performances of MnO2 strongly depend on its crystal structure, morphology, and particle size.2,10,13,14 Therefore, the development of suitable synthesis methods to prepare well-crystallized MnO2 is important to study the intrinsic correlations between electrochemical property and crystallization of MnO2.15,16 Hydrothermal synthesis is an important technique to prepare materials with controllable structures, sizes, and morphologies.17−19 However, long reaction time is always needed in the traditional hydrothermal synthesis route. Heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the material synthesis, which can dramatically reduce reaction times from days and hours to minutes and seconds.20 In microwave synthesis, the controllable morphologies and phases become difficult through controlling the involved chemical reactions. The coupled microwave−hydrothermal method possesses advantages of both hydrothermal and microwave conditions, which can dramatically reduce the reaction time from 10 h or even several days down to 30 min, keeping the morphology controllability and the ability to produce narrow size distribution particles with high purity.21−23 Therefore, microwave−hydrothermal technique is an effective faster way to crystallize MnO2 with controllable phases and morphologies. MnO2 can crystallize many polymorphic forms such as α-, β-, γ-, δ- and ε-phases in different reaction systems. MnO2 microstructures consist of a series of allotropic forms on the basis of MnO6 octahedra building blocks (Scheme 1). β-MnO2 possesses 1 × 1 tunnel, while γ-MnO2 and α-MnO2 have 1 × 2 (2.3 × 4.6 Å) and 2 × 2 (4.6 × 4.6 Å) tunnels, respectively (Scheme 1). Since β-MnO2 is known as the most stable phase of MnO2, the phase transformation from γ- to β-MnO2 is topologically convenient on the basis of their structural similarity.9,16 Cations such as K+, Na+, H+ can affect the polymorphs of MnO2 during hydrothermal crystallization. Because the different tunnel structures constructed from MnO6 octahedra have different abilities for Li+ (0.68 Å), K+ (1.37 Å), and Na+ (0.98 Å) transfer, polymorphic forms of MnO2 have distinctive electrochemical properties in lithium-ion batteries and supercapacitors.9,14 α-MnO2 nanotubes can exhibit a high reversible capacity and cycling stability when measured as an anode in lithium batteries due to their 2 × 2 tunnels constructed from double chains of octahedral [MnO6] structure.15 Hollow bipyramid β-MnO2 was reported to exhibit the highest specific capacity and the best cyclability, which can be attributed to its unique electrochemical reaction, compact

Scheme 2. Schematic Illustration of Chemical Reaction and Crystallization Process Based on Coupled MicrowaveHydrothermal Synthesis of Polymorphic MnO2 in Designed MnCl2−KMnO4 Reaction System

MnO2 can be controlled by K+ cation, for example, α-MnO2 can be synthesized by adding KCl.26 Our selection of KMnO4 can provide K+, which can thus favor the formation of different crystallographic structures of MnO2. In addition, KCl is an important mineralizing agent during hydrothermal crystallization of inorganic compounds,25 K+ together with Cl− from MnCl2 can form KCl mineralizing agent to favor the crystallization of MnO2 with controllable phase and shape. By using MnCl2 and KMnO4 as reactants, β-MnO2 has been synthesized in hydrothermal conditions.26−28 However, the coupled microwave−hydrothermal technique has not yet been applied to MnCl2−KMnO4 redox reaction system. In this work, B

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Table 1. Conditions of Synthetic Samples and Discharge Capacities of MnO2 Anodes and Cathodes for Lithium-Ion Batteries, and Specific Capacitances of MnO2 Supercapacitor no.

temp. /°C

1

120

60

2

140

60

3

160

60

4 5

170 160

60 10

6

160

20

7

160

8 9

180 180

60(pH = 2) 5 10

10 11

180 180

15 30

time/min

phase

morphology

anode 30th capacity (mAh/g)

cathode 30th capacity (mAh/g)

specific capacitance (F/ g) 1A/g

γ-MnO2(dominant) + trace βMnO2 γ-MnO2(dominant)+trace βMnO2 β-MnO2 (dominant) + trace γMnO2 β-MnO2 γ-MnO2 (dominant) + trace βMnO2 β-MnO2 (dominant) + trace γMnO2 MnO2·H2O0.15, α-MnO2

plate + wire

167.8

67.4

26.63

plate + wire

120.1

77.0

23.5

rod

150.2

26.4

10.4

rod wire + plate

94.7 205.9

13.2 68.0

6.33 23.5

rod + wire + plate wire

220.7

43.3

16.4

116.9

33.8

32.75

β-MnO2 (dominant) + γ-MnO2 β-MnO2 (dominant) + γ-MnO2

rod+wire+ plate rod + wire + plate rod rod

189 97.3

49.1 29.9

17.5 11.43

62.1 88.8

18.3 4.8

12.7 5.45

β-MnO2 (dominant) + γ-MnO2 β-MnO2

α-, β-, and γ-type MnO2 can be crystallized in MnCl2−KMnO4 aqueous solution under microwave−hydrothermal conditions. The microwave-driven redox reaction and hydrothermal-driven crystallization process in the coupled microwave−hydrothermal method were effectively used to control the crystallization and electrochemical performances of MnO2. The electrochemical performances of MnO2 with different structures for both lithium-ion batteries and supercapacitors were systematically investigated.

Microwave Synthesis of MnO2. A 4.68 mmol sample of MnCl2•4H2O and 3.12 mmol of KMnO4 were dissolved in 40 mL of H2O. Then, the reaction solution was placed in the microwave oven with power set to 100% of 800 W, which was fluctuated with a hold time of 3−10 min. The resulting products were filtered, washed thoroughly with deionized water, and dried at 60 °C for 5 h. Powder XRD patterns were collected using a PAN X’Pert MPD X-ray diffractometer operated at 45 kV voltage and 40 mA current with CuKα radiation and the phases were identified by matching with Joint Committee on Powder Diffraction Standards (JCPDS) patterns. Crystal size and morphology were observed using a field-emission scanning electron microscope (FESEM, Hitachi-S4800). 2.2. Electrochemical Measurement. For the electrochemical properties as lithium-ion battery electrodes, coin cells (CR2025) were fabricated using MnO2 as the working electrode and lithium metal as the counter electrode. Working electrodes were prepared by pressing a mixture of MnO2 sample, acetylene black, and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10. The electrolyte was 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 vol %). Charge−discharge tests were performed using a CT2001A cell tester (LAND Electronic Co.). For supercapacitor applications, the working electrode was prepared by mixing 75 wt % of the synthesized MnO2 powder, 20 wt % acetylene black, and 5 wt % polytetrafluoroethylene (PTFE). Briefly, the resulting paste was pressed on a sheet of nickel foam (1 × 1 cm) at 10 MPa. All experiments were performed in a three-electrode glass cell in 0.5 M Na2SO4 at 20 °C under normal atmosphere. The MnO2/nickel foam was used as the working electrode, saturated calomel electrode (SCE) as the reference electrode, and the Pt wire as a counter electrode. The cyclic voltammetry (CV), and galvanostatic charge−discharge measurements were carried out by an electrochemical workstation (CHI 660D). Experiments to obtain CV curves at different scan rates and galvanostatic charge−discharge measurements at different current densities were performed between 0 and 0.8 V.

2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization. Microwave− Hydrothermal Synthesis of MnO2. We used a novel combined oxidation−reduction process using MnCl2 and KMnO4 to synthesize MnO2 (Table 1). In a typical synthesis of MnO2, 4.68 mmol of MnCl2•4H2O and 3.12 mmol of KMnO4 were stirred in 40 mL H2O. The solution pH can be changed by adding given concentration of KOH and HCl. After stirring for 30 min, the precursor solution was microwave−hydrothermally heated at different temperatures and times. The microwaveassisted experiments were performed using a MARS5 (CEM Corp., Matthews, NC) microwave digestion system. In this system, the microwaves operate at a frequency of 2.45 GHz with a maximum power of 1200W. The experiments here were carried out with 300 W power in double-walled digestion vessels having an inner nonreactive Teflon PFA liner in which the solutions were present and an outer Ultem Polyetherimide shell of high mechanical strength. Temperature and pressure probes allow the reaction to be controlled by monitoring the temperature or pressure within a control vessel. The maximum operating temperature and pressure for the system are 240 °C and 350 psi, respectively. Hydrothermal Synthesis of MnO2. Typically, 4.68 mmol of MnCl2•4H2O and 3.12 mmol of KMnO4 were stirred in 40 mL of H2O. The above solution was then transferred into a Teflonlined stainless steel autoclave. The autoclave was heated to 160 °C and maintained at this temperature for 1−7 h. After cooling down to room temperature, the resulting products were collected and washed. The final products were dried at 60 °C for further characterizations. C

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3. RESULTS AND DISCUSSION MnO2 can be achieved by the controllable redox reaction in MnCl2−KMnO4 aqueous solution as shown in Scheme 2. The redox reaction comprises two half reactions Mn 2 + + 2H 2O → MnO2 + 4H+ + 2e−

(E° = 1.23V) (1)

MnO4 − + 4H+ + 3e− → MnO2 + 2H 2O

(E° = 1.68V) (2)

On the basis of the values of E°, the standard Gibbs free energy change ΔG° of redox reaction can be estimated to be −261 kJ mol−1, (ΔG° = −zFE), implying a very strong tendency for redox reaction to progress toward the right-hand side.14 The redox reaction between MnCl2 and KMnO4 under suitable microwave−hydrothermal conditions can result in the formation of thermodynamic-control α-, β-, and γ-phase MnO2 and kinetic-control growth morphologies (Scheme 2). The reaction processes include two stages, the microwave-driven redox reaction and the hydrothermal-driven crystallization. On the basis of our experimental results, microwave can mainly accelerate the redox reaction between MnCl2 and KMnO4 by “microwave flash heating”; MnO2 products can be synthesized within 3 min with the use of microwave only, while only poorly crystallized MnO2 can be obtained. However, the hydrothermal method is an effective way to synthesize highly crystalline MnO2 and mainly controls the crystallization phases and morphologies. The use of coupled microwave−hydrothermal method can synthesize highly crystalline MnO2 with controllable phase and morphology in a short reaction time. Microwave−hydrothermal synthesis has inherent advantages, including rapid volumetric heating, good homogeneity, high yield, morphology controllability, and the ability to produce narrow size distribution particles with high purity. The MnO2 tends to grow homogeneously in a short period of time because MnO4− and Mn2+ are selectively heated by microwave irradiation due to their large dipole moments.29 Figure 1 shows the XRD patterns of MnO2 obtained by microwave−hydrothermal synthesis. The characteristic peaks can be readily indexed to α-, β-, and γ-type MnO2 using standard files of PDF#97-009-9418, PDF#98-000-0365, and PDF#04-007-8867, respectively. It should be noted that the γMnO2 includes a trace amount of β-MnO2 crystallized at low temperature and short reaction time (Figure 1a). Figure 1b,c shows that the well-defined peaks are corresponding to βMnO2 and trace amount of γ-MnO2 with d = 3.98 Å in XRD patterns are present with higher reaction temperature and longer reaction time. With the reaction solution pH was changed to 2, α-MnO2 was obtained as shown in Figure 1d. The solution pH can change the redox potential of reaction 1 and MnO4− shows higher oxidation ability at acidic condition, thus the chemical reaction and crystallization processes were changed. In addition, K+ can serve as the support of (2 × 2) tunnel structure, which favors the formation of α-MnO2. The large size of K+ cation with the effective hydrated ionic radius of 137 pm leads to the creation of α-MnO2 successfully. The role of H+ is believed to impede the support action of K+ during the formation of α-MnO2; high H+ concentration makes K+ ions more difficult to occupy tunnel sites.26 The results prove that α-, β-, and γ-type MnO2 can be selectively obtained by the microwave−hydrothermal method in MnCl2−KMnO4 aqueous solution.

Figure 1. XRD patterns of different MnO2 samples obtained by microwave−hydrothermal synthesis: (a) mainly shows peaks of ramsdellite, γ-MnO2 (PDF#04-007-8867) with a trace amount of βMnO2; (b) mainly shows peaks of pyrolucite, β-MnO2 (PDF#98-0000365) with a small amount of γ-MnO2; (c) mainly shows peaks of pyrolucite, β-MnO2 (PDF#98-000-0365); (d) shows peaks of α-MnO2 (PDF#97-009-9418). Inset in Figure 2 shows low-angle peaks of αMnO2.

Figure 2 shows typical morphologies of α-, β-, and γ-MnO2 with the plate-, rod-, and wirelike shapes by the microwave−

Figure 2. SEM images of as-obtained MnO2 samples obtained by microwave−hydrothermal synthesis. γ-MnO2 + trace β-MnO2 (a), βMnO2 + trace γ-MnO2 (b), β-MnO2 (c), and α-MnO2 (d) as shown in Figure 1.

hydrothermal method in MnCl2−KMnO4 reaction system, which prove that the morphology of MnO2 is strongly dependent on their crystallographic structures. SEM observations clearly indicate that α-MnO2 has a wire shape and βMnO2 has a rodlike shape. Platelike structures dominate in γMnO2, while rodlike and platelike structures are present in the mix-phase of β-MnO 2 (dominate) and γ-MnO 2 . The polymorphic MnO2 can be selectively crystallized by control the microwave−hydrothermal reaction time and temperature (Table 1). The reaction times of the crystallization of MnO2 with the usage of this microwave−hydrothermal method were D

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always less than 1 h compared with 10 h or even several days with the traditional hydrothermal method. Figure 3 shows SEM images of MnO2 materials prepared at 160 °C by microwave−hydrothermal method in MnCl2−

Figure 4. SEM images of MnO2 synthesized by microwave− hydrothermal synthesis at different reaction temperatures for the reaction of 60 min.

rodlike shapes was synthesized after reaction at 170 °C. On the basis of the temperature-dependent and time-dependent studies, β-MnO2 crystallites were the stable phase at higher reaction temperatures or longer reaction times as observed in the present study (Figures 3 and 4). Because the β-MnO2 is known as the most stable MnO2 phase, γ-MnO2 is slowly transformed to β-MnO2.15 During the hydrothermal process, the platelike γ-MnO2 can redissolve into the solution phase and regrow in the form of rodlike β-MnO2. Thus, the phase transformation from γ-MnO2 to β-MnO2 can be considered as the growth of β-MnO2 domains within a ramsdellite, γ-MnO2 matrix, which involves a collapse of the 1 × 2 framework and a subsequent short-range rearrangement of the MnO6 octahedra.16 At the initial synthesis process, chemical reactions can control the crystallization of MnO2 by the microwave− hydrothermal method as shown in reactions 1 and 2 and Scheme 2. When used as electrode materials for lithium-ion battery, different electrochemical reactions can occur. With MnO2 as lithium-ion battery anode materials, the electrochemical conversion reaction can be formulated as30

Figure 3. SEM images of MnO2 synthesized by microwave− hydrothermal synthesis at 160 °C for different reaction times and solution conditions.

KMnO4 reaction system for different reaction times. The early stage of the reaction yields platelike structures and XRD indicates that the γ-MnO2 dominates (Figures 3a and 1a). Some initial rodlike structures are formed at the reaction of 20 min and the plate is present at the surface of rod (Figure 3b). The rodlike structures and β-MnO2 dominate (Figures 1c and 3c) with the reaction time of 60 min. These results show that the phase transformation from γ- to β-MnO2 couples with the shape change from platelike to rodlike structures. The results prove that in the microwave−hydrothermal MnCl2−KMnO4 reaction system the phase change can occur in a short reaction time. When the reaction temperature was 180 °C, the phase transformation from γ- to β-MnO2 couples with the shape change from platelike to rodlike structures also occurred. The phase transformation can be explained by their crystallographic structures. Because the β-MnO2 is known as the most stable MnO2 phase, γ-MnO2 can be slowly transformed to β-MnO2.15 On the basis of their structural similarity, the phase transformation from γ-MnO2 to β-MnO2 is topologically convenient.9 γ-MnO2 (1 × 2) can be considered as a random intergrowth of β-MnO2 (1 × 1) blocks within a ramsdellite, γMnO2 matrix (Scheme 1).16 Thus, β-MnO2 can be formed by a short-range rearrangement of [MnO6] with higher temperature and longer reaction time. Therefore, the formation of β-MnO2 can include two different processes, that is, nucleation and growth of γ-MnO2 crystallites, and subsequent phase transformation from γ-MnO2 to β-MnO2. When the solution pH was adjusted to 2, MnO2 with wire shapes were obtained and XRD proved that the wirelike products are α-type MnO2 (Figures 3d and 1d). In addition to the reaction time, crystallization temperature can also adjust the crystal structure of MnO2 in the microwave−hydrothermal MnCl2−KMnO4 reaction system. Figure 4 shows SEM images of MnO2 synthesized at different temperatures with a constant holding time of 60 min. It can be seen that at 120 and 140 °C, the products tend to form platelike γ-MnO2. At 160 °C, rodlike β-MnO2 dominates with a little amount of platelike γ-MnO2. The pure β-MnO2 with

MnO2 + 4Li+ + 4e− → Mn + 2Li 2O

(3)

The reversible formation of Mn and MnO2 delivers the capacity of lithium-ion battery anodes. Electrochemical performances of MnO2 electrodes as lithium-ion battery anodes are shown in Figure 5 and Supporting Information Figures S1−S2. Figure 5 shows the discharge/charge profiles and cycling performances of the MnO2 anodes at a current density of 100 mA g−1 in the voltage range from 0.01 to 3 V. Figure 5a−d shows discharge/charge curves of β- and γ-type MnO2 anodes at 1, 2, 10, 20, 30 cycles. The discharge/charge profiles and cycling performances of other MnO2 electrodes are shown in Supporting Information Figure S1. The voltage plateau is present at around 0.4 V in the first discharge cycle, which can be ascribed to the reduction of MnO2 to Mn (reaction 3) and the formation of amorphous Li2O. The first discharge capacities are larger than 1000 mAh g−1 and the capacities decrease after subsequent discharge/ charge cycles. The reversible capacities are 600 and 620 mAh g−1 for pure γ-MnO2 and γ-MnO2 together with a little amount of β-MnO2, respectively (Figure 5a,c). The reversible capacities are 280 and 550 mAh g−1 for pure β-MnO2 and β-MnO2 E

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Figure 5. Electrochemical performances of MnO2 anodes obtained by microwave−hydrothermal synthesis. Voltage profiles (a−d) and cycling retention (e,f) of MnO2 were measured as Li-ion battery anode materials at a rate of 100 mA/g between 0.01 and 3.0 V. Insets show crystallographic structures of β-MnO2 and γ-MnO2.

together with a little amount of γ-MnO2, respectively (Figure 5b,d). β-MnO2 together with a little amount of γ-MnO2 shows high capacity and can be attributed to the effect of the mixphase MnO2, which can provide more grain and phase boundaries for Li+ transfer.11 The large irreversible capacity can be mainly due to the partially irreversible MnO2 conversion reaction with Li, the structure collapses with the volume change, and the formation of solid electrolyte interface (SEI) in the initial lithiation and delithiation cycles.15,31 The capacities of pure α- and β-MnO2 decrease fast than that of the mixture of β- and γ-type MnO2 (Figure 5e,f and Supporting Information Figure S2). The electrochemical performances indicate that the mix-phase of β- and γ-type MnO2 show the best discharge capacities, such as 220.7, 205.9, and 167.8 mAh g−1 at different synthesis conditions after 30 cycles as shown in Table 1. The discharge capacities of pure β-MnO2 are less than 100 mA h g−1 after 30 cycles, while the capacity of α-MnO2 is 116.9 mAh g−1. The present results prove that the conversion reactivity of these MnO2 anodes follows the order γ- > α- > β-phase, and the mixed-phase β- and γ-MnO2 can provide better anode performances for lithium-ion batteries. Except for the unique electrochemical reaction mechanism, the different tunnel-structures of MnO2 account for the different electrochemical performances. It is known that the β-MnO2, γ-MnO2, and α-MnO2 have 1 × 1, 1 × 2, and 2 × 2

tunnel-structures, respectively. The loose tunnel structures can decrease the effective diffusion path and increased the effective space for insertion and extraction of Li. In addition, the tunnels can effectively relieve structural collapse and local volumetric variation during the charge/discharge process.15 Although the compact (1 × 1) tunnels with size of 0.189 nm are larger than the radius of Li+ (0.68 Å), the incorporated other ions may block the transportation of Li ions.16 Therefore, the large tunnels are feasible for Li+ insertion/extraction. Therefore, γMnO2 has high electrochemical capacity than that of β-MnO2. However, for α-MnO2, the two-tunnel structure is not stable and the incorporated K+ ions (from KMnO4) may block the transportation of Li ions, resulting in low capacity and poor cyclability.16 In addition to the conversion reaction for anode, MnO2 can serve as cathode material based on the intercalation− deintercalation reaction. The reaction mechanism of MnO2 is the insertion of lithium ions into the lattice of MnO2 during the insertion/extraction process. The process can be described as the following reaction27,32 MnO2 + x Li+ + x e− → LixMnO2

(4)

where x is the number of moles of Li ions and electrons participating in the reaction system. The reaction can be viewed as the intercalation of Li+ ions from the electrolyte into the F

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Figure S4). However, they have low specific capacitance values no more than 33 F/g as shown in Table 1. The specific capacitances of α-MnO2 (32.8 F/g) and γ-MnO2 (24 F/g) are higher than that of β-MnO2 (5.5 F/g). The as-crystallized MnO2 supercapacitors have Faradaic reactivity sequence α- > γ> β-MnO2 upon their tunnel structures. The results prove that the large tunnels of α-MnO2 (2 × 2, 4.6 × 4.6 Å) and γ-MnO2 (1 × 2, 2.3 × 4.6 Å) are feasible for the transport of Na+ ions, while β-MnO2 is with a narrow tunnel (1 × 1, 2.3 × 2.3 Å). Our results show that the as-obtained MnO2 by microwave− hydrothermal method is more suitable for the lithium-ion battery than that of supercapacitor due to their different electrochemical reaction mechanisms, such as Faradaic reaction for MnO2 supercapacitor, conversion reaction (redox) for MnO2 lithium-ion battery anode, and intercalation−deintercalation reaction for MnO2 lithium-ion battery cathode. For comparison, we did separate reactions with hydrothermal only and microwave only in our designed MnCl2−KMnO4 aqueous solution. Figures 6 and 7 show the as-obtained MnO2

MnO 2 matrix upon the discharging process and the deintercalation of Li+ ions from the MnO2 matrix into the electrolyte upon the charging process. The voltage profiles and cycling retention of MnO2 are shown in Supporting Information Figure S3, which were measured as Li-ion battery cathode materials at a rate of 50 mA/g between 2 and 4.5 V. The first discharge capacity of MnO2 obtained at 160 °C can reach 150 mAh/g. The discharge capacities of MnO2 obtained at 160 °C for 10, 20, and 60 min are 68.0, 43.3, and 26.4 mAh/g after 30 cycles, respectively (Table 1). The discharge capacities of MnO2 after 30 cycles obtained at 120, 140, 160, 170, 180 °C are 67.4, 77.0, 26.4, 13.2, and 4.8 mAh/g, respectively. It is suggested that after the initial discharge, the electrode active materials transform into the newly developed LixMnO2 phase. Such a LixMnO2 phase needs a part of the lithium ions to remain to stabilize its structure, which is the cause of the irreversible capacity for the initial electrochemical process.27 Therefore, the low capacity of MnO2 cathode is due to the occurrence of irreversible phase transformation. The present results prove that the γ-type MnO2 shows high capacity than those of α- and β-MnO2 as shown in Table 1. The intercalation−deintercalation reactivity of these MnO2 cathodes follows the order γ- > α- > β-phase. Results show that α- and δ-MnO2 can always offer large capacity as cathode materials for rechargeable lithium batteries because they can accommodate significant lithium ions in their structures. Such a narrow tunnel makes β-MnO2 difficult for lithium ions to diffuse into bulk upon electrochemically intercalating. As a result, conventional β-MnO2 with high crystallinity usually exhibits very low electrochemical activity.33 The high capacity of γ-type MnO2 with trace amounts of βMnO2 originates from the effect of the mix-phase of MnO2, which can provide more grain and phase boundaries for Li+ transfer.11 In addition, these results prove that the ascrystallized MnO2 can be served as both cathode and anode for lithium-ion battery, which is a potential candidate to fabricate homogeneous materials for lithium-ion battery. The pseudocapacitance of electrochemical supercapacitors is a Faradaic process, which is based on the redox reactions that occur in the electrode materials. MnO2 is a kind of potential electrode materials for supercapacitors based on pseudocapacitors mechanism and different MnO2 polymorphy can affect its capacitance.10 It has been reported that the specific capacitance values of MnO2 samples qualitatively decrease in the order α >δ > γ > β.10 In neutral electrolyte (Na2SO4 in this case), the charge storage mechanism of MnO2 materials can be explained by the following redox reactions:33

Figure 6. SEM images (a−d) and XRD patterns (e) of MnO2 obtained by hydrothermal only synthesis at 160 °C for different reaction times. XRD patterns show the products are γ-MnO2 at the reaction time of 1, 3, 5, and 7 h. Scale bars in Figure 6d are 200 nm.

MnO2(surface) + Na + + e− ↔ (MnOO−Na +)(surface) surface absorption

(5)

in MnCl2−KMnO4 aqueous solution by the hydrothermal only and microwave only methods. Platelike MnO2 is dominated in traditional hydrothermal method with different reaction times (1, 3, 5, and 7 h). At the same time, the wire-structure can be found in the samples obtained at the reaction time of 1, 3, and 5 h (Figure 6a−c). When the reaction time was increased to 7 h, the plate-, rod-, and wire-structured MnO2 were obtained as shown in Figure 6d. XRD patterns prove that the platelike structures are γ-MnO2, which match the standard file PDF#14644. The broader peak indicates small size and poor crystalline γ-MnO2. In coupled microwave−hydrothermal method, platestructured and well-crystallized MnO2 can be synthesized at 10 min. Figure 7a,b shows that spherical MnO2 was obtained in

MnO2 + Na + + e− ↔ MnOO−Na + bulk ion intercalation

(6)

The pseudocapacitance of MnO2 was mainly contributed from the redox reactions that occurred on the outermost surface layer of MnO2 in contact with the aqueous Na2SO4 electrolyte. Rapid transfer of Na+ cation and electrons to MnO2 simultaneously is the key to completing the charge storage reaction. The CV curves of MnO2 electrodes at 20 mV s−1 exhibit rectangular shapes, demonstrate that MnO2 was charged and discharged at pseudoconstant rate (Supporting Information G

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Table 3. Specific Capacitances of MnO2 Samples Obtained by Hydrothermal Only, Microwave Only, and Coupled Microwave-Hydrothermal Method in Our Designed MnCl2− KMnO4 Aqueous System as Supercapacitors reaction conditions

composition

hydrothermal only at 160 °C for 1h microwave only for 10 min microwave−hydrothermal at 160 °C for 10 min microwave−hydrothermal at 160 °C for 1h

specific capacitance (F/g) at the current rate of 1A/g

γ

16.5

γ γ (dominant)

68 23.5

β

10.4 (dominant)

MnO2 synthesized with hydrothermal only, microwave only, and microwave−hydrothermal method (Table 2 and Supporting Information Figure S5). The high-specific capacitance of MnO2 electrode with microwave only can be originated from its poor crystallinity, which is much better than those wellcrystallized MnO2.2 Compared with electrochemical property of MnO2 synthesized by hydrothermal only and microwave only methods, the well-crystallized MnO2 by coupled microwave−hydrothermal method shows higher anode capacity and similar cathode capacity. The present results indicate that microwave−hydrothermal method can effectively improve the lithium-ion battery anode property of the well-crystallized MnO2 in a short reaction time.

Figure 7. SEM images (a−c) and XRD patterns (d) of the as-obtained γ-MnO2 by microwave only synthesis with the reaction time of 3, 5, and 10 min.

microwave only for less than 5 min. After 10 min microwave synthesis, the wirelike structures were present at the surface of spherical MnO2. XRD patterns also prove that the products are γ-MnO2, which match the standard file PDF#14-644. The broader peak indicates small size and poor crystalline of γMnO2 (Figure 7d). The microwave only method can reduce the reaction time to 3 min. However, the well-crystallized MnO2 with controllable morphologies, such as the plate-, rod-, and wirelike ones, is difficult to obtain. The present results prove that hydrothermal only and microwave only methods have clear limitations such as poorly crystallized γ-MnO2 and few kinds of morphologies and phases. It is further proved that the coupled microwave−hydrothermal method can provide a fast and controllable way to the synthesis of MnO2 with the multiphase and well-crystallized morphology in MnCl2− KMnO4 aqueous solution. The electrochemical properties of MnO2 samples synthesized with the use of hydrothermal only and microwave only method were also evaluated as lithium-ion batteries and supercapacitors. The electrochemical measurement results are shown in Tables 2, 3, and Supporting Information Tables S2 and S3 and Figure S5. As lithium-ion battery anodes, the discharge capacities of MnO2 electrodes are 84.5 and 26.9 mAh g−1 by hydrothermal only and microwave only, respectively, while the discharge capacities are 286.7 and 196.0 mAh g−1 for microwave− hydrothermal synthesis at 10 min and 1 h after 15 cycles (Table 2). When served as lithium-ion battery cathodes, the capacities of the as-obtained γ-MnO2 show with hydrothermal only, microwave only, and microwave−hydrothermal synthesis are 88.7, 129.2, and 77.6 mAh g−1 after 15 cycles, which proves that they have similar cathode reaction reactivity. The specific capacitances of supercapacitors are 16.5, 68, and 23.5 F/g for γ-

4. CONCLUSIONS In summary, various MnO2 crystallographic structures, such as α-, β-, and γ-phase with the plate-, rod-, and wirelike shapes, were crystallized by the controllable redox reaction with coupled microwave−hydrothermal technique in MnCl2− KMnO4 aqueous solution. It is for the first time to apply microwave−hydrothermal method to MnCl2−KMnO4 redox reaction system. The coupled microwave−hydrothermal method can increase the kinetics of chemical reaction, adjust the crystallization morphologies and phases by a controllable reaction route. Chemical reactions and crystallization processes were systematically adjusted to synthesize α-, β-, and γ-type MnO2. Higher reaction temperature and longer reaction time favor the crystallization of β-MnO2, while γ-MnO2 formed at low temperature and short reaction time. The phase transformation from γ-MnO2 to β-MnO2 is based on their similar structures, that is, γ-MnO2 can be considered as a random intergrowth of β-MnO2 blocks within a ramsdellite, γ-MnO2 matrix. Electrochemical measurements show that the ascrystallized MnO2 supercapacitors have Faradaic reactivity sequence α- > γ- > β-MnO2 upon their tunnel structures, the intercalation−deintercalation reactivity of these MnO2 cathodes follows the order γ- > α- > β-phase, the conversion reactivity of these MnO2 anodes follows the order γ- > α- > β-phase. The

Table 2. Discharge Capacities of MnO2 Samples Obtained by Hydrothermal Only, Microwave Only, and Coupled MicrowaveHydrothermal Method in Our Designed MnCl2−KMnO4 Aqueous System as Lithium-Ion Batteries anode discharge capacity/mAh g−1

cathode discharge capacity/mAh g−1

reaction conditions

composition

1st

2nd

15th

1st

2nd

15th

hydrothermal only at 160 °C for 1h microwave only for 10 min microwave−hydrothermal at 160 °C for 10 min microwave−hydrothermal at 160 °C for 1h

γ γ γ (dominant) β (dominant)

1259.4 1244.2 1379.7 1505.4

354.4 256.4 636.3 446.9

84.5 26.9 286.7 196.0

168.3 181.9 151.1 57.9

137.9 175.1 111.9 39.5

88.7 129.2 77.6 28.3

H

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present results demonstrate that MnO2 with broad tunnel among the tunnel-type MnO2 family shows better electrochemical performances. In addition, the mix-phase MnO2 (βand γ-MnO2) can deliver high capacity. For comparison, the separate reactions with hydrothermal only and microwave only were carried out in our designed MnCl2−KMnO4 aqueous system. The microwave−hydrothermal method is shown as a facile and efficient way to synthesize MnO2 electrode materials for good performance lithium-ion battery anode. The current work deepens the fundamental understanding of several aspects of physical chemistry, for example, the chemical reaction controllable synthesis, crystal structure selection, electrochemical property improvement, and electrochemical reactivity, as well as their correlations.

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ASSOCIATED CONTENT

* Supporting Information S

Electrochemical performances of MnO2 anodes and cathodes. Specific capacitances of different MnO2 electrodes as supercapacitors. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (D.X.) [email protected]; (S.K.) komarneni@psu. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant nos. 50872016, 20973033, and 51125009), the National Natural Science Foundation for Creative Research Group (Grant 21221061), and the Hundred Talents Program of the Chinese Academy of Sciences is acknowledged.

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REFERENCES

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