On the Mechanism of Crystal Water Insertion during Anomalous Spinel

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On the Mechanism of Crystal Water Insertion during Anomalous Spinel-to-Birnessite Phase Transition Sangryun Kim, Soyeon Lee, Kwan Woo Nam, Jaeho Shin, Soo Yeon Lim, Woosuk Cho, Kota Suzuki, Yoshifumi Oshima, Masaaki Hirayama, Ryoji Kanno, and Jang Wook Choi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02083 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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Chemistry of Materials

On the Mechanism of Crystal Water Insertion during Anomalous Spinel-to-Birnessite Phase Transition Sangryun Kim,†,‡ Soyeon Lee,‡ Kwan Woo Nam,† Jaeho Shin,† Soo Yeon Lim,† Woosuk Cho,§ Kota Suzuki,‡ Yoshifumi Oshima,⊥ Masaaki Hirayama,‡ Ryoji Kanno,‡ and Jang Wook Choi*,† †

Graduate School of Energy, Environment, Water, and Sustainability (EEWS) and KAIST Institute NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ‡ Department of Chemical Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan § Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam 463-816, Republic of Korea ⊥School

of Materials Science I, Japan Advanced Institute of Science and Technology, Ishikawa 923-1293, Japan

ABSTRACT: Hydrated materials contain crystal water within their crystal frameworks and can exhibit extraordinary properties as a result. However, a detailed understanding of the mechanism involved in the hydration process is largely lacking, because the overall synthesis process is very difficult to monitor. Here, we elucidate how the insertion of crystal water mediates an anomalous spinel-to-Birnessite phase transition during electrochemical cycling in aqueous media. We find that at the initial stage of the phase transition, crystal water is inserted into the interlayer space between MnO6 layers in the form of a hydronium ion (H3O+). The H3O+ insertion is chemically driven in the reverse (reducing) direction to the applied anodic (oxidizing) electric field, stabilizing the structure and recovering the charge balance following the deinsertion of Mn2+. A comparative investigation using various electrolyte solutions revealed that the H3O+ insertion competes with the insertion of other ionic charge carriers (Li+, Na+, and Mg2+), and the overall efficiency of the phase transition is determined by this competition. This understanding of crystal water insertion offers an insight into strategies to synthesize hydrated materials.

1. INTRODUCTION Certain crystal structures can contain water molecules within their frameworks, and these water molecules are often called ‘crystal water’. Materials containing crystal water, called hydrated materials, can exhibit abnormal properties, such as superconductivity,1-4 high capacitive charge storage,5-8 facile ionic diffusion kinetics,9-12 and superior structural stability.1316 Crystal water in a structural framework usually originates from synthesis environments involving aqueous media, when the formation of crystal water-containing structures is energetically preferred. A variety of synthetic routes have been reported for the production of hydrated materials, and chemical redox reaction,1 calcination,17 and photochemical,18 solgel,19-21 hydrothermal22-24 and electrochemical processes25-27 are most representative. Since hydrated materials are being adopted for a wide range of applications, including superconductors, electric double layer capacitors, and rechargeable batteries, investigation of the detailed mechanisms involved in the synthesis of crystal water-containing materials is important and can be helpful in the design of similar materials in the same category. However, obtaining a fundamental understanding of how water molecules are contained in a given framework is difficult because capturing the critical step that promotes hydration is nontrivial. Structural complexes, which involve interactions among the crystal water and other charge carriers and the crystal host, as well as the low crystallinity of the framework, further complicate efforts to clarify the hydration

mechanism. Moreover, crystal water can take different forms28,29 including H2O, H3O+, and OH- depending on the local environment, making precise analysis even more difficult. Recently, we observed a phase transformation, from spinel Mn3O4 to crystal water-containing layered Birnessite MnO2, during an electrochemical process in aqueous electrolyte solutions.14,30 The intercalation of crystal water from the aqueous electrolytes was directly captured using a scanning transmission electron microscope (STEM). This phase transition was quite striking because it occurred in the opposite direction of the well-known spontaneous spinel-to-layered phase transitions,31-34 and the crystal water insertion turned out to be the origin of the phase transition.30 However, understanding how the given phase transition takes place, and how crystal water is inserted, is still shallow. Herein, we report that during the aqueous electrochemical synthesis, crystal water is inserted in the form of H3O+, and its insertion competes with charge carrier insertion. The experimental results clarify that the given cationic insertions occur in the opposite direction to the anodic electric field applied, serving to stabilize the host structure, and that suppressing charge carrier insertion is critical for crystal water insertion, and the consequent anomalous phase transition.

2. EXPERIMENTAL SECTION Sample synthesis. Mn3O4: All of the chemicals used were purchased from Sig

ACS Paragon Plus Environment


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Figure 1. (a) The charge-discharge profiles in 1 M MgSO4 aqueous electrolyte solution. (b) SEM images showing the morphology change from spinel Mn3O4 to layered Birnessite MnO2 after 4 cycles in the aqueous electrolyte solution. ma-Aldrich. Spinel Mn3O4 (~100 nm in dimension) was synthesized based on the following process. First, 10 mM of potassium permanganate (KMnO4) was dissolved in 30 mL of deionized (DI) water. Next, 40 mL of polyethylene glycol (PEG, MW~200) was added to the aqueous solution, and the solution was vigorously stirred until the solution changed to a dark-brown suspension. The obtained suspension was then transferred into a Teflon-lined stainless-steel autoclave (100 mL), and a hydrothermal reaction was conducted for 8 h at 160 °C. Then, the precipitant was collected and washed using water and ethanol (9:1 in volume) four times. The particles were dried by a freeze-dryer to keep the particle morphology. Birnessite: To generate the Birnessite, a composite electrode was first fabricated by making a slurry in which 60 wt% spinel Mn3O4, 20 wt% acetylene black, and 20 wt% poly(vinylidene difluoride) (PVDF) were dispersed in 1methyl-2-pyrrolidinon (NMP). The slurry was then cast onto stainless steel (SUS 304) foil, followed by drying for 12 h at 70 °C in a vacuum oven. The mass loading of the active material was ~2.0 mg cm-2. A flooded three-electrode beaker cell was prepared by engaging the spinel Mn3O4 working electrode (~1 cm2), a counter electrode with an excessive amount of spinel Mn3O4 (~10 cm2), and an Ag/AgCl reference electrode. The counter electrode is simply to ensure the charge balance between both electrodes. A flooded cell was chosen to facilitate the phase transition since it takes advantage of the large volume of electrolyte, which is more efficient in inducing the Mn2+ deinsertion and crystal water insertion. However, other cell types (pouch, coin, etc.) must produce the same result. The electrodes were galvanostatically cycled at 50 mA g-1 in the potential range of -0.2 V ~ 0.9 V (vs. Ag/AgCl). All of the cells were aged for 1 h before any electrochemical processes to facilitate good soaking of the electrolyte into the electrode. To compare the phase transition in various monovalent- and divalent-based aque-

ous electrolytes, 1 M lithium sulfate (Li2SO4), 1 M sodium sulfate (Na2SO4) and 1 M magnesium sulfate (MgSO4) aqueous electrolyte solutions were used. Characterization. The crystal structure was characterized by X-ray diffraction (XRD, Rigaku) by scanning in the 2θ range of 10°−60° with a scan step of 0.02° and an acquisition time of 2 sec for each step. The specimens used for ex-situ XRD measurements were sealed in a home-made holder inside an Ar-filled glove box to avoid hydration from moisture in the air. The morphology before and after electrochemical cycling was analyzed using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi) and a field emission transmission electron microscope (FE-TEM, Technai G2 F30 S-Twin, FEI). Annular bright field (ABF) imaging of the scanning transmission electron microscope (STEM) was carried out using an R005 microscope35 at an acceleration voltage of 200 kV. The inner-outer semi-angle of the ABF detector was 11 - 23 mrad, and the electron probe current was 10 pA. The dwell time per pixel was 38 µsec. The size of pixel was 16.5 × 16.5 pm2. To investigate the water content, thermogravimetric analysis (TGA, Netzsch) was carried out from 25 °C to 300 °C at a ramping rate of 5 °C min-1 under an Ar flow. Time-offlight secondary ion mass spectrometry (TOF-SIMS) analysis was performed using a TOF-SIMS 5 spectrometer (IONTOF GmbH). A primary 30 keV Bi+ beam with a raster size of 250 × 250 µm2 was employed for spectral analysis. Depth profiles were attained using a 0.25 keV Cs+ sputter beam with a raster size of 300 × 300 µm2 in the non-interlaced mode (longer sputter and data acquisition cycles) to avoid charging. The sputtering rate was estimated to be ~0.55 Å s-1 from a control experiment with Mn3O4 thin film. The measurements were performed at a pressure of 10-9 Pa. All spectra were collected in positive ion mode. The molar ratios of cation (Li, Na, and Mg) to Mn were obtained by using inductively coupled plasma (ICP, Hewlett Packard) measure


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Figure 2. (a) TEM and (b) ABF-STEM images after 4 electrochemical cycles in 1 M MgSO4 aqueous electrolyte solution. Inset of (b): FFT pattern from (b). ments. For the impedance measurements, flooded beaker cells in which identical Mn3O4 electrodes were symmetrically assembled were tested using a BioLogic VSP multi potentiostat over the frequency range of 0.01 Hz ~ 1 MHz with 10 mV in amplitude. For the characterization during the aqueous electrochemical cycling, the cells were disassembled, rinsed with DI water, and then dried inside a glove box.

3. RESULTS AND DISCUSSION The electrochemical cycling was first conducted in a Mgbased aqueous electrolyte solution (1 M MgSO4) in the galvanostatic mode. With increasing cycle number, the specific capacity increased (Figure 1a). This capacity change was accompanied by a phase transition; the XRD peaks assigned to spinel Mn3O4 gradually weakened, and peaks corresponding to Birnessite continuously grew (Figure S1). After the electrochemical cycling, the morphology of the Mn3O4 changed from an initial octahedron to a lamellar structure consisting of 2D nano-sheets with a thickness of ~10 nm, as shown in the FE-SEM (Figure 1b) and FE-TEM (Figures 2a and S2) images. The ABF-STEM image directly captured the presence of crystal water between the MnO6 octahedral layers in the generated Birnessite (see green balls in Figure 2b). The Mn-to-Mn (7.2 Å) and Mn-to-water (3.6 Å) interlayer distances estimated by a fast Fourier transform (FFT) pattern (Figure 2b inset) matched well with those ob-

tained in the XRD results (Figure S1). For reference, it is noted that the oxygen atoms in the MnO6 octahedral layers are unlikely to represent the observed Mn-to-water distance because the distance from the oxygen atoms in the MnO6 octahedral layers would be shorter. In order to elucidate the water insertion process, the phase transition was compared for aqueous Li-, Na-, and Mg-based electrolyte solutions containing 1 M Li2SO4, 1 M Na2SO4, and 1 M MgSO4, respectively. In all cases, the specific charge capacity, which is directly related to the degree of the phase transition,30 increased continuously during cycling (Figures 3a and S3). However, the three cells exhibited distinct phase transition efficiencies (Table 1); under the same electrochemical condition, the increase in specific capacity was more significant in the order of Li-, Na-, and Mg-based electrolyte solutions. This distinctive transition efficiency was also confirmed with XRD results (Figure 3b). The degree of phase transition estimated by the intensity ratio between the Birnessite (001) and Mn3O4 (101) peaks (Table 1) followed the same trend as the change in the specific capacity. Based on these results, the Mg-based electrolyte solution was found to be most efficient at promoting the phase transition. Once again, the main driving force of the anomalous phase transition is the crystal water insertion. As direct evidence,

Figure 3. (a) Specific charge capacity evolution when measured at a rate of 50 mA g-1 using aqueous electrolyte solutions containing 1 M Li2SO4, 1 M Na2SO4, and 1 M MgSO4. (b) XRD patterns after 4 cycles for the same three cases in (a).



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Figure 4. (a) TGA profiles after the 1st charge using 1 M Li2SO4, 1 M Na2SO4, and 1 M MgSO4 aqueous electrolyte solutions. (b) TOF-SIMS spectrum at 0.72 V during the 1st charge in the 1 M MgSO4 aqueous electrolyte solution. (c) A depth profile of the H3O+ detected in (b). (d) Atomic ratios between charge carriers and Mn after the 1st charge for the same electrolyte cases. Table 1. Specific charge capacity, intensity ratio between Birnessite (001) and Mn3O4 (101) peaks, crystal water content, and atomic ratio between charge carrier and Mn at different points during the electrochemical cycling.

Capacity (mAh g-1) IBirnessite (001) / IMn3O4 (101) Water content (%) Atomic ratio (charge carrier / Mn)

state

Li

Na

Mg

st

1 charged

36.4

56.2

110.7

th

77.6

140.9

221.8

th

2.95

6.64

34.3

st

2.00

2.22

3.31

th

4 charged

7.29

7.70

12.01

pristine

0

0

0

0.18

0.15

0

4 charged 4 charged 1 charged

st

1 charged

TGA data indicated that the Mg-based electrolyte solution permits a higher content of crystal water than the Li- and Nabased electrolyte solutions (Mn > Na > Li) (Figures 4a and S4, and Table 1). In the case of the Mg-based sample measured at 0.72 V (vs. Ag/AgCl) in the 1st charge, TOF-SIMS analysis confirms the presence of crystal water at the surface of the transient phase (Figure 4b), and that the crystal water can exist in the form of H3O+. From the depth profile of the TOF-SIMS, H3O+ was detected at a depth of ~5 Å from the surface (Figure 4c), corresponding to approximately two MnO6 octahedral layers. By contrast, the pristine state (Mn3O4) displayed no H3O+ signals (Figure S5), reconfirm-

ing the critical role of the electrochemical cycling in the aqueous electrolyte solution for the crystal water insertion. It was found30 that the formation of Birnessite takes place mainly during charging. Since the material is oxidized during charging,36 it is expected that cations are being extracted. In this sense, the H3O+ insertion observed during the charge cycle is quite surprising because it would drive the redox reaction in the opposite (reducing) direction. In an effort to understand this peculiar H3O+ insertion, we first carried out a control experiment in which pristine Mn3O4 was immersed in the electrolyte solutions without electrochemical bias. This control test did not trigger any structural change, according to XRD analysis (Figure S6), verifying that the electrochemical bias is essential for the phase transition. Meanwhile, it was found that the spinel-tolayered phase transition is initiated with Mn2+ deinsertion, accompanied by the oxidation of Mn3+ and its rearrangement in the electrochemical charge process.30 The Mn2+ deinsertion can be understood in the same manner as the charging process of typical cathode materials, where cations are extracted during the application of an anodic bias.35 This Mn2+ deinsertion gives rise to substantial lattice evolutions, a drastic change in the oxidation state of Mn3+, and strong electrostatic repulsion between the oxygen ions (Figure S7), which produces a chemical (rather than electrochemical) insertion of H3O+ that is preferable to stabilize the transient phase. In fact, ionic charge carriers also turned out to be inserted together during the charge process according to ICP (Figure


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Figure 5. A graphical illustration of the formation process of Birnessite and the relevant synthesis principle. 4d). These insertions can be understood in the same way as the insertion of H3O+; the charge carrier insertion compensates the structural destabilization and the imbalanced charge, which both originate with the Mn2+ deinsertion. The degree of charge carrier insertion was in the opposite order (Li > Na > Mg) of the H3O+ insertion (Table 1); in the case of the Mgbased electrolyte solution, the Mg/Mn ratio was almost zero after the 1st charge, indicating that the inserted Mg content was negligible. The observed trend for the different charge carriers suggests a competition between H3O+ insertion and charge carrier insertion; the details are discussed in the following paragraph. It was also confirmed that a simple soaking in the electrolyte solution was not sufficient to insert the charge carriers, in all three charge carrier cases (Figures S8 and S9). Taking the combined results of TGA, TOF-SIMS, ICP, XRD, and impedance into consideration, it is concluded that a high crystal water content and a low charge carrier content are preferred for the formation of Birnessite. Considering the final structure of Birnessite with its enlarged interlayer spacing, a key step for the transition to Birnessite is crystal water (H3O+) insertion. In this regard, since the inserted charge carriers are located at the same interlayer sites as H3O+, between the MnO6 octahedral layers, 37 the insertion of both cations creates a competing situation, and thus the insertion of charge carriers serves as a barrier to the phase transition. Following this logic, suppressing the charge carrier insertion is expected to promote the selective insertion of H3O+ and therefore the phase transition to Birnessite. The suppression of Mg insertion must be associated with its divalent charge. It is well known38 that the bivalency causes strong electrostatic interactions with the anions in the host framework, leading to sluggish kinetics. Furthermore, compared to monovalent cases, the bivalency results in stronger solvation by the solvent molecules in the electrolyte,

imposing a higher energy penalty on desolvation at the electrode/electrolyte interface.10,39 Based on this, it can be concluded that the negligible insertion of Mg ions is a result of their unfavorable insertion characteristics, suggesting that the use of high-valence charge carriers in aqueous electrolyte solutions is a general strategy for efficient crystal water insertion (and consequently, the phase transition to Birnessite). Apparently, considering these two aspects, monovalent charge carriers are generally more favorable for insertion. Between the Li ion and the Na ion, it is known that the Na ion has a lower desolvation energy and better migration kinetics.27,39 However, in our experiment, the inserted charge carrier content for the Na-based electrolyte solution was lower than that of the Li-based electrolyte solution. This observation can be explained by the preferred crystallographic sites of the Li ion; the Li ion can be stabilized at the tetrahedral sites in the spinel structure because of its small ionic radius,40 and therefore Li ion insertion would tend to impede the phase transition from the spinel structure. In contrast, the Na ion prefers the octahedral or prismatic sites over the tetrahedral sites,40 which is in line with the phase transition to the layered Birnessite structure. The formation mechanism of Birnessite is summarized in Figure 5.

4. CONCLUSIONS In summary, the mechanism involved in crystal water insertion, which is critical to the anomalous spinel-to-layered phase transition observed during aqueous electrochemical cycling, was determined to correlate with the competition between the insertion of H3O+ and charge carriers. The insertion of both types of cations mainly occurs during Mn2+ deinsertion, when it stabilizes the structure and recovers the charge balance. For this reason, the insertion of those cations is chemically driven when an external bias is applied in the opposite direction. From a broader perspective, understand-



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ing the competition between H3O+ and charge carries provides a useful insight into strategies that can be applied to synthesize hydrated materials, such as pH control.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional characterization (XRD, TEM, electrochemical cycling, TGA, TOF-SIMS, ICP, and impedance) and the formation mechanism of Birnessite (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT J.W.C. acknowledges the support by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2014R1A4A1003712 and NRF2015R1A2A1A05001737). This work was also supported by

the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which is granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (20152020104870).

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Page 8 of 8

On the Mechanism of Crystal Water Insertion during Anomalous Spinel

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