A Simple and Cost-Effective Approach to Dramatically Enhance the

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A Simple and Cost-Effective Approach to Dramatically Enhance the Durability and Capability of a Layered Delta-MnO2 Based Electrode for Pseudocapacitors: A Practical Electrochemical Test and Mechanistic Revealing Minghai Yao, Xu Ji, Tsung-Fu Chou, Shuang Cheng, Lufeng Yang, Peng Wu, Haowei Luo, Yuanyuan Zhu, Lujie Tang, Jeng-Han Wang, and Meilin Liu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00075 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Energy Materials

A Simple and Cost-Effective Approach to Dramatically Enhance the Durability and Capability of a Layered Delta-MnO2 Based Electrode for Pseudocapacitors: A Practical Electrochemical Test and Mechanistic Revealing Minghai Yao,† Xu Ji,† Tsung-Fu Chou,‡ Shuang Cheng,*,† Lufeng Yang,§ Peng Wu,† Haowei Luo,† Yuanyuan Zhu,† Lujie Tang,† Jenghan Wang*,‡ and Meilin Liu*,§

†Guangzhou

Key Laboratory for Surface Chemistry of Energy Materials, New Energy

Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China. Email: [email protected] ‡Department

of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan.

Email: [email protected] §School

of Materials Science and Engineering, Georgia Institute of Technology, Atlanta,

ACS Paragon Plus Environment

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GA 30332-0245, USA. Email: [email protected]

ABSTRACT: Inadequate capacity and poor durability of MnO2 based pseudo-capacitive electrodes have long been stumbling blocks on the way of their commercial use. Though layered δ-MnO2 has higher potential to be used due to its proton-free energy storage reactions, its durability is still far away from carbon-based electrodes associated with structure deformation caused by interlayer spacing change and Jahn-teller effect. Here we report an effective approach to dramatically enhance not only the stability but also the capacity of δ-MnO2 based electrode through a simple incorporation of exotic cations, hydrated Zn2+, in the tunnel of the material. Even at a very fast charge/discharge rate (50 A g-1), the capacity of the electrode is gradually increased from 268 to 348 F g-1 after ~3,000 cycles and then remains relatively constant in the subsequent ~17,000 cycles, which means ~128 % of the initial capacity is maintained after 20,000 cycles. In contrast, the capacity of bare δ-MnO2 electrode without modification is degraded gradually along cycling, retaining only ~74% of the initial value after 20,000 cycles. To reveal the basic chemistry between, synchrotron X-ray diffraction and Raman spectroscopy were


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performed to explore structural evolution of the modified δ-MnO2 during cycling, DFT computation was used to estimate the energetics and vibration modes associated with the hydrated Zn2+. The performance enhancement is attributed largely to the preaccommodation of [Zn(H2O)n]2+, which effectively suppresses the interlayer spacing change during cycling and thus benefits the stability. KEYWORDS:

pseudo-capacitor,

layered

δ-MnO2/Na0.55Mn2O4,

tunnel

structure

modification/pre-accommodation of exotic ions, in-situ Raman, DFT computations

1. INTRODUCTION Fast, efficient and green electric energy storage is needed for many emerging new technologies, from portable electronics to electrical vehicles and smart grids. Supercapacitors (SCs), including electric double layer capacitors (EDLCs) and pseudocapacitors (PSCs), seem to be candidates of the best choices due to their distinctive property of high power density as well as considerable energy stability.1 In recent decades, a wide variety of materials have been studied as electrodes for supercapacitors.2-4 While carbon based electrodes have high power density, their


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capacities are still inadequate for many applications. Accordingly, transition metal based compounds, including oxides,5-12 carbides,13-15 nitrides,16-18 and hydroxides,19,

20

have

been explored due to their potential for high capacity or energy density. For example, battery-type NiCo2O4,21, 22 Ni(OH)2 23, 24 and pseudo-capacitive type RuOx 25, 26

electrodes have been well studied to achieve high capacities. However, their broad

commercialization is limited by the narrow potential window, high cost, or eco-unfriendly electrolyte used (such as strong base or acid). Thus, other suitable material systems or effective solutions are urgently needed. One of the well-known pseudocapacitive electrode materials, MnO2, still attracts much attention due to its natural abundance, ecofriendly property and high theoretical specific capacitance (1,109 F g-1) in a wide potential window of ~1 V.27 To date, however, the commercialization of MnO2 based electrodes is still obstructed by their inadequate capacity and stability, as well as low power density. The low power density, attributed to intrinsic poor conductivity, can be mitigated by incorporating with highly conductive materials or doping at a cost of complicated synthesis procedure. The poor cycling stability, induced by the dissolution of MnO2 active materials that is associated with proton involved energy storage process, can be improved through


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proper selection from the α,28 β,28, 29 γ, δ, ε, λ, σ phases with different crystal or tunnel structures.30 According to previous studys,31-33 δ-MnO2, a layered structure linked by edge-shared [MnO6] octahedron, is a good choice due to its suitable structure with low proton adsorption energy (and hence unfavorable proton storage behavior). However, the durability of δ-MnO2 based electrodes is still out of expectation due to structural deformation induced by the interlayer spacing variation and Jahn-Teller effect associated with intercalation/deintercalation of electrolyte ions during discharge/charge processes. Besides, because of the low mobility of large hydrated electrolyte cations such as hydrated [Na(H2O)n]+, the practical capacity of δ-MnO2 is always insufficient unless a very thin film of δ-MnO2 is deposited on a highly conductive substrate.6, 34 Very thin δ-MnO2 films of several atomic layers (or even a single layer) have been synthesized by some groups,35-37 who indeed demonstrated the better tolerate of structural deformation. Yet, packing volume always will be increased due to the low tap density of electrodes with high specific surface area. Meanwhile, Jahn-Teller effect is still able to influence the structure and hence the stability. Therefore, the cycling stability and capacity of the δMnO2 electrode must be significantly enhanced to be commercially viable.


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In this study, a simple and cost-effective way for tunnel structure modification is developed to improve the stability, capacity as well as the power density of the δ-MnO2 electrode. The interlayer of [MnO6] slab that ended with O2- can also be described as a 2D tunnel with a large spacing of about 7 Å. It can be easily occupied by cations of proper sizes. Hydrated [Zn(H2O)n]2+ complex is used as exotic cation here due to its unique structure, which can be absorbed in the tunnel and form strong bond with O (e.g., the O of a Mn(IV) vacancy site in the birnessite minerals as reported). 38 Successful insertion of Zn2+ in the interlayer of δ-MnO2 has also been confirmed in recent Zn batteries concerned reports.39 After the modification, the capacity of the δ-MnO2 electrode is gradually enhanced to 348 F g-1 along with cycling at 50 A g-1 and ~ 400 F g-1 from 359 F g-1 at a smaller current of 1 A g-1. Most importantly, the high capacity is remained even after 20,000 cycles, ~128% of its initial value, which is comparable with carbon based electrodes. Without modification, the capacity decays gradually and only 74% of the initial capacitance is left after long time cycling. Synchrotron X-ray diffraction was employed to explore the crystal structure of the samples with and without incorporation of ions. Ex-situ and operando Raman measurements were performed to explore the change of energy


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storage behavior after modification and observed that band shifts from the electrolyte cations’ intercalation/deintercalation are suppressed obviously for the modified sample. DFT calculations examined the energetics of hydrated Zn2+ adsorption and its replacement with Na+ and indicated that the intercalation/deintercalation of hydrated [Zn(H2O)n]2+ are energetically unfavorable during discharging/charging processes on the incorporated sample. Additionally, the DFT calculations analyzed the vibrations induced by [Zn(H2O)n]2+ adsorption to confirm the Raman results. The experimental and computational efforts clarify the stability enhancement corresponds to a decline of the structural deformation. Besides, the stability of an electrode with the same modification process by hydrated Ga3+ and Fe3+ is also improved, while the capacity has a smaller enhancement for Ga3+ and an obvious fast decline for Fe3+, implying that the modification has ionic selectivity. Furthermore, it is reasonable to speculate that the materials with tunnels or holes can be easily modified through a similar approach only if a proper exotic ion is chosen.


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2. EXPERIMENTAL SECTION 2.1 Preparation of Manganese Oxide (MnO2) Electrode The electrode samples were prepared via an electrochemical plating process.31 Active material was deposited directly on a porous and highly conductive carbon fiber paper (CFP; Hesen, Shanghai Electric. Co.). Before deposition, the CFP was cleaned with ethanol accompanied by ultrasonic for 10 mins, 0.1 M HCl and acetone successively, then dried at 70 °C for 2 h. The plating was conducted in a three-electrode system, with CFP substrate as the working electrode, Pt mesh as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Aqueous solution of 0.1 M Mn(CH3COO)2 mixed with 0.1 M Na2SO4 was used as precursor solution. A potentiostatic deposition process was performed on a CHI 660E (Chenhua, shanghai, China): a bias of 0.44 V (vs. Ag/AgCl) for 40 s and then 0.49 V for 600 s was applied. The reaction formula is suggested as follow: Anode reaction: Mn2+ +2H2O → MnO2 + 4H+ + 2e-; Cathode reaction: 2H+ + 2e- → H2 ↑; Total reaction: Mn(CH3COO)2 + 2H2O → MnO2 + 2H(CH3COO) + H2 ↑.


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Finally, the as-obtained sample was washed with deionized water, dried at 140 ºC in air for 8 h, and recorded as MnO2. The MnO2 electrodes after modification through dipping in 0.1 M ZnSO4, Fe(NO3)3 and Ga(NO3)3 solution for 2 mins are recorded as Zn-MnO2, Fe-MnO2 and Ga-MnO2, respectively. 2.2 Material Characterization The morphology and microstructure of the samples were examined using a field emission scanning electron microscope (FESEM; Hitachi LEO 1530) equipped with an energydispersive X-ray spectroscopy (EDX) detector. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEM-2100F system (JEOL) operated at 200 kV. Synchrotron x-ray diffraction (SXRD) was carried out with the X-ray beam line (λ = 0.24125 Å) at the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory (BNL). Ex-situ and in-situ Raman measurements were performed on a Horiba LabRAM HR Evolution system with an Ar ion laser (wavelength λ = 514.5 nm). Ex-situ Raman was implemented without applying of any voltage on the working electrode. In-

situ Raman signals were captured at time mapping mode (one spectrum in every 100 s) while the sample was tested with cyclic voltammetry (CV) scan at 1 mV s-1 in a special


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three-electrode configuration designed for Raman. 2.3 Electrochemical Measurement The electrochemical measurements, including CV, galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS), were performed using a CHI 660E electrochemical workstation in a three-electrode configuration with the sample as working electrode, a platinum mesh (1.0 cm2) as counter electrode, and Ag/AgCl (in saturated KCl) as reference electrode. 3. RESULTS AND DISCUSSION 3.1 Structural and morphological properties. To explore the influence of decorated ions to the electrode in-depth, detailed characterization was performed to the before and after modified samples in Figure 1. Firstly, synchrotron X-ray diffraction (SXRD) was employed to detect possible crystal structure change induced by the modification, as shown in Figure 1b. For the sample MnO2, all the reflection peaks can be indexed to a monoclinic lattice with a space group of C2/m(12) (birnessite Na0.55Mn2O4·nH2O, JCPDS: 43-1456), which is a layered structure of δ-MnO2 composed of [MnO6] slabs that linked by edge-shared [MnO6]


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octahedral with [Na(H2O)n]+ occupying the middle of the interlayer. The interlayer spacing (d001) was calculated to be 0.71 nm from the diffraction angle of the (001) peak, implying the electrostatic interaction of the [Na(H2O)n]+ accommodated in; the d001 is 0.72 nm for neutral H2O incorporated MnO2.40, 41 For the sample after modification with hydrated Zn2+, there is almost no change for all the diffraction peaks except the weakening intensity of most peaks and a relative stronger intensity of (002) peak for the crystal plane located at the middle of (001) interlayer. Relative intensity enhancement of this peak (inset of Figure 1b, prepared by normalizing the strongest (001) peak) indicates the increase of incorporated hydrated cations and the successful accommodation of Zn2+ between the [MnO6] slabs without changing the lattice structure of the interlayer spacing of d001 (the increasing of electrostatic interaction upon incorporated cations may be buffered by the larger size of [Zn(H2O)n]2+,42, 43 Figure S2 and Table S1), which coincides with the recent reports about δ-MnO2 based Zn batteries.39 Lattice structural model of the sample is illustrated accordingly in Figure 1a.


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Figure 1 (a) The corresponding crystal structure of δ-MnO2 with (001) and (002) planes exposed, which is speculated from the SXRD results. [M(H2O)n]m+ stands for hydrated metal cations. (b) Synchrotron x-ray diffraction (SXRD) of MnO2 and Zn-MnO2. Insert: relative intensity profile of (001) and (002) peaks by normalizing the strongest (001) peak. TEM image of MnO2 exfoliated from the CFP substrate: (c) large sheets with some well crystallized wire-like structure interspersed in and (d) enlarged lattice-resolved HRTEM image of the area marked with red rectangle in (c), showing a clear lattice plane with a distance of 0.35 nm for the wire-like structure, attributed to the (002) plane. SEM images of MnO2 (e) before and (f) after long-term cycling.


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Detailed morphology and structure information was further confirmed by Transmission Electron Microscope (TEM) and High-resolution TEM (HRTEM), as presented in Figure 1c and d. A very thin and large irregular sheet can be observed from TEM image (Figure 1c). Besides, some line-like structures scattered in the sheet can be seen clearly from the image. In a larger HRTEM view of Figure 1c, it is found that the lines are in fact better crystallized structures compared with the amorphous structures around. The interlayer distance of the lines can be evaluated to be ~0.35 nm (Figure 1d), attributed to the (002) plane of Na0.55Mn2O4·nH2O, consisting with the SXRD results (Figure 1b) discussed above. The HRTEM result implies that the sheets are most likely grown along the [001] direction in plane and expose their large tunnels to the electrolyte. No obvious difference can be detected from the HRTEM between the samples MnO2 and Zn-MnO2. Furthermore, the morphologies of the sample before and after electrochemical testing are recorded by SEM. Sheet-like structures perpendicular to the substrate surface were observed for the sample MnO2, like a piece of grass due to the top convergence of several sheets nearby (Figure 2e). After the modification of hydrated Zn2+, though the trace of Zn can be clearly detected by EDX (Figure S3) and SXRD, no obvious morphology change


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can be observed, hence only the image of the MnO2 is presented here. After long-term cycling, the previous irregular sheets changed to be thicker and larger petaloid-like sheets, a typical looking of layered MnO2 (Figure 1f). According to previous reports,44, 45 it is suspected that the larger sheets should be formed through an oriented attachment process: free [MnO6] groups are produced due to the high surface energy and the existence of H+ at high potential range and then reattach onto the small sheets through oriented attachment during cycling; the large interlayer spacing is stabilized by the abundant electrolyte ions of Na+, while the existence of Zn2+ should also be helpful for the stabilization. The morphology experienced a similar but faster evolution process for the sample Zn-MnO2. For example, it needs ~2,000 cycles for the sample MnO2 but only ~1,000 cycles for ZnMnO2 to evolve into a similar feature, a possible reflection of higher amount of electrolyte ions that involve the energy storage and hence higher or faster energy storage ability for Zn-MnO2.


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3.2 Electrochemical performance

Figure 2 (a) Cyclic voltammetry (CV) curves of MnO2 and Zn-MnO2 at initial cycle and the 10,000 th cycle at a scan rate of 50 mV s-1. (b) Galvanostatic charge and discharge (GCD) curves of MnO2 and Zn-MnO2 after long-time running collected at different current densities. (c) Dependence of specific capacitance on current density as determined from the galvanostatic charge/discharge curves. (d) Capacitance retention of the MnO2 and


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Zn-MnO2 electrode as a function of discharge current of 5 A g-1 and 50 A g-1.

Electrochemical performance of the electrodes was evaluated in 1.0 M Na2SO4 aqueous electrolyte, as shown in Figure 2 (MnO2 and Zn-MnO2) and Figure S1b (GaMnO2 and Fe-MnO2). Since the cyclic voltammetry (CV) curves for different electrodes in the early stage of cycling are similar, only one curve at a sweep rate of 50 mV s-1 is presented in Figure 2a to represent the initial state. After thousands of cycles, however, the CV profiles for the different electrodes evolve differently, as shown in Figure 2a. For example, the CV area (or the area enclosed in the CV curve) for the as-prepared MnO2 is obviously reduced after 10,000 cycles, corresponding to a capacity drop from 243 F g1

to 185 F g-1 (calculated from the CV area, and the calculation method is offered in SI).

For the electrode incorporated with Zn-MnO2, in contrast, the CV area increased to 325 F g-1 after 10,000 cycles, which can be attributed to the lower diffusion resistance (Figure S1d) caused by the more and more uniform distribution of Zn2+ (including the surface and interlayer pre-adsorption cations) during the rearrangement process discussed above during cycling. Meanwhile, the shape of the CV curve changed to be closer to a


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rectangular, indicating an ideal capacitive response. Accordantly, the galvanostatic charge/discharge (GCD) profiles, tested at current densities varied from 1 A g-1 to 50 A g-1, become linear with negligible voltage drop (Figure 2b). The Zn-MnO2 electrode with hydrated Zn2+ modification exhibits longer discharge period than that for MnO2 at the same discharge current density, implying larger capacitance, as reflected in the specific capacitances calculated from the GCD curves (Figure 2c). Capacitance can achieve a high value of 400 F g-1 at 1 A g-1 for Zn-MnO2 after cycling, while it is only 340 F g-1 for MnO2. Though the capacitance obtained here is obviously improved, it is still much smaller than its theoretical value (1230 F g-1 in a potential window of 0.9 V) due to the fast surface/subsurface reaction. Intriguingly, not only the capacity, but also the capacity retention enhanced for the Zn-MnO2 electrode after cycling. Only 73% of the capacitance is remained for initial MnO2 and worse after long-time cycling when the discharge current increases 50 times to 50 A g-1 from 1 A g-1. While for Zn-MnO2, 87% of the capacity is remained, implying the much enhanced power density. Durability of the raw and modified electrodes is shown in Figure 2d. At a high current density of 50 A g-1, the capacitance of Zn-MnO2 was gradually increased to 348 F g-1 after


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the first 3,000 cycles and remained relatively constant in the subsequent cycles. No obvious fading can be observed even after 20,000 cycles. At a lower discharge current, the capacitance experienced a similar change tendency along with cycling. The stability of the modified sample Zn-MnO2 is comparable to that of carbon based electrodes, but its volumetric and gravimetric capacity are much higher, achieving ~400 F g-1 at 1 A g-1 in a neutral aqueous electrolyte. In contrast, ~26 % fading was observed for the MnO2 electrode after 20,000 cycles at 50 A g-1, due likely to structural deformation associated with interlayer spacing change and Jahn-teller effect mentioned above. Besides, a similar capacity fading tendency was observed at a lower current of 5 A g-1. Moreover, cycling stability of the Ga3+ and Fe3+ modified sample, Ga-MnO2 and Fe-MnO2, was also investigated: the durability is obviously improved for Ga-MnO2 while it is largely reduced for Fe-MnO2, as shown in Figure S1b, implying an influence of different decorated ions. Briefly, not only the capacity (or energy density) and the charge/discharge rate (or power density), but also the durability of δ-MnO2 are obviously improved after a facile modification process.


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3.3 Relaxation analysis Raman spectroscopy that is sensitive to trace amount of chemical bonds is further employed, as shown in Figure 3. All the Raman bands can be indexed to layered MnO2 for the as-obtained sample (blue curve in Figure 3b), consistent with prior reports.46, 47 In particular, five bands in the range 300-700 cm-1 can be detected and marked as 𝑣1 to 𝑣5. The band 𝑣1 located at ~627 cm-1 can be assigned to the asymmetric stretching vibration of Mn-O band in the [MnO6] octahedral whereas the 𝑣2 band located at ~573 cm-1 can be assigned to the symmetric stretching vibration in the basal plane of the [MnO6] slabs. These two bands have been proved to be highly concerned with the d001 spacing that affects the phonon properties as well as the Raman bands’ shifts, intensities and widths; e.g., 𝑣1 band experiences a red shift while 𝑣2 band undergoes a blue shift along with the reduced interlayer spacing.47 Therefore, we can evaluate the charge storage process from Raman signals that are highly concerned with the interlayer spacing change.


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2  3fh  Mn with a zoomed-in view of phonon Figure 3. (a) The structure of [Zn(H 2 O)3 ]ad

2  3fh  O from the side vibration for Raman bands at 605 and 686 cm-1 and [Zn(H 2 O)3 ]ad

and top view. (b) Ex-situ Raman spectra of MnO2 and Zn-MnO2 at open-circuit voltage.

In-situ Raman spectra of (c) MnO2 and (d) Zn-MnO2 captured during two CV cycles in a potential range of 0 ~0.9 V (vs Ag/AgCl) in 1 M Na2SO4 at a scan rate of 1 mV/s. (e) Raman shift evolution of the ν2 band (located at ~573 cm-1) of MnO2 (blue curve) and ZnMnO2 (red curve) as a function of potential (black line) approximately.


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For the modified sample, Zn-MnO2, a broad and large band appeared (red curve), in which a clear peak centered at ~605 cm-1 together with a shoulder band centered at ~686 cm-1 can be distinguished. The features are neither belongs to MnO2 nor ZnO and should be aroused from the accommodation of hydrated [Zn(H2O)n]2+. To further confirm this, spin-polarized DFT computations were conducted at the GGA-PW91 level.48,

49

The

valence electrons were expanded by plane waves with 600-eV cutoff energy and the core electrons were simulated by the cost-effective pseudopotentials, the PAW method.50, 51 The Brillouin-Zone integration was sampled at 0.05 × 2 (1/Å) interval in the reciprocal space by Monkhorst-Pack scheme.52 The very thin MnO2 flake was modeled by a twolayer δ-MnO2 slab with the surface area of 10 × 10 Å and interlayer separation of 7 Å (Figure S4). The modeled surface was utilized to examine the adsorption of hydrated [Zn(H2O)n]2+ for the modified sample Zn-MnO2 and analyze the vibrations for the Raman spectrum assignment. According to previous report,38 both tetrahedral [Zn(H2O)4]2+ and octahedral [Zn(H2O)6]2+ complexes can stably adsorbed on layer-type MnO2 surfaces as Zn2+ bonds with three surface oxygen and three waters dissociate from the complexes in the


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2 2 formation of [Zn(H 2 O)]ad and [Zn(H 2 O)3 ]ad , respectively, chelating on the MnO2 surface

2 (Figure S5). Additionally, our computation found that the adsorption of [Zn(H 2 O)3 ]ad from

2 octahedral complex is about 1 eV more stable than that of [Zn(H 2 O)]ad from tetrahedral

2 complex (Table S2). The optimized structures of the most stable [Zn(H 2 O)3 ]ad are shown

in Figure 3a. The related adsorption energies (Eads), listed in Table 1, were calculated according to the adsorption reaction in the following equations at open circuit potential (OCP); the energetic trends are unchanged upon applied voltages as the energies of electron shift concurrently for all adsorptions. (1)

2 [Zn(H 2 O)6 ]2  [Zn(H 2 O)3 ]ad  3H 2 O

E ads  E[Zn(H O) ]2  3  E H2O  E[Zn(H O) 2

E[Zn(H O) 2

2 3 ]ad

3 ad

2

(2)

2 6]

, 3  E H 2O and E[Zn(H O) 2

2 6]

correspond to the DFT computed energies of

2 [Zn(H 2 O)3 ]ad , the three dissociated H2O from the octahedral complex and the free

octahedral complex, respectively. Also, we calculated the replacement energy (Erep), 2  listed in Table 1, by replacing adsorbed [Zn(H 2 O)3 ]ad with Na ad to examine the stability in

the discharge/charge processes, according to the following reaction and equation.


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2  Na   [Zn(H 2 O)3 ]ad  3H 2 O  Na ad  [Zn(H 2 O)6 ]2

(3)

E rep  E Na   E[Zn(H O)

(4)

ad

2

2 6]

 E Na   E[Zn(H O) 2

2 3 ]ad

 3  E H2O

Energies of E Na  and E Na  correspond to the energies of the adsorbed and free Na+, ad

respectively. 2 Table 1 Adsorption energies (Eads) of [Zn(H 2 O)3 ]ad at two possible locations in MnO2 (001)

 slabs at open circuit potential and its replacing energy by Na ad .

Eads(eV) 2 [Zn(H 2 O)3 ]ad

3fh  Mn

2 [Zn(H 2 O)3 ]ad

3fh  O

-5.85

-5.85

Erep(eV)

1.79

1.74

Configuration

Bonding

In 3-fold-hollow site of 3O while Mn in

3 × Zn-(OH2) = 2.03 Å

the hollow; coordinated Zn has 3 bonds to H2O and 3 to surface O

3 × Zn-O = 2.26 Å

In 3-fold-hollow site of 3O while O in

3 × Zn-(OH2) = 2.09 Å

the hollow; coordinated Zn has 3 bonds to H2O and 3 to surface O

3 × Zn-O = 2.28 Å

2 The computed results show that the two stable adsorptions of [Zn(H 2 O)3 ]ad located at

2  3fh  Mn ) and O 3-fold-hollow site of three surface O with subsurface Mn ( [Zn(H 2 O)3 ]ad

2  3fh  O ) in the hollow were found; the related configuration and bonding of ( [Zn(H 2 O)3 ]ad

the adsorptions are listed in Table 1, coincident with the XRD result. Both of them have


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equal and strong Eads, indicating that the hydrated Zn2+ complex can tightly bond with MnO2 substrate and the Zn-MnO2 sample is easily formed and stable. Also, Eads for 2   [Zn(H 2 O)3 ]ad is much larger than that of Na ad and proton ( H ad )

31,

indicating that the

adsorbed Zn2+ complex can be hardly replaced and the Zn-MnO2 sample is intact during 2 charging/discharging process. The strong adsorption make the [Zn(H 2 O)3 ]ad stay at the

2  interlayer even at a high potential after the deintercalation of Na ad ( [Zn(H 2 O) n ]ad

accommodation related Raman bands still can be observed clearly at a high charge potential of 0.9 V, as presented in Figure S6), which is consistent with the reports of δ-MnO2 based Zn batteries where large polarization voltage (low Zn2+ insertion potential and high deintercalation potential) is observed.39 The computational results are also coincident with the Raman results that confirm the strong accommodation of hydrated Zn2+ complex at the interlayer. All the possible Raman 2 vibration modes that induced by the [Zn(H 2 O)3 ]ad accommodation were computed in

Figure S7. The vibrations include the bending and wigging of the hydrated H2O in the adsorbed complex at low frequencies (< 300 cm-1) and the stretching of Zn-O bond in the adsorbed interlayer at high frequencies (> 500 cm-1). The low frequency vibrations largely


24

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mix with the phonons of MnO2 flake and are hard distinct from the Raman spectra. On the other hand, the high frequency vibrations have strong intensity from the stretching modes and are observable from the spectra. The asymmetric stretching modes of the adsorbed complex at 605 and 686 cm-1, as shown in Figure 3a, are well accordance with the wide Raman band centered at ~605 cm-1 together with the shoulder band at 686 cm1

in Figure 3b for Zn-MnO2. It can be concluded that hydrated Zn2+ complex can successfully accommodate at the

interlayer of the [MnO6] slaps without changing the layered structure of δ-MnO2 but taking an obvious impact to the electrochemical performance. Herein, operando Raman was further used to evaluate the influence to the charge storage behavior. Raman signal evolution upon charge/discharge of layered δ-MnO2 has been well elaborated in our prior reports,31,

32

as seen in Figure S8. Mapping mode of two continuous CV cycles was

exhibited in Figure 3c. As reported previously, clear reversible intensity evolution can be observed, which should be attributed to the polarizability change induced by electron density change upon potential; meanwhile, the main band centered at 573 cm-1 (asprepared MnO2 at OCP in air) that is corresponding to the stretching vibration in the basal


25


Page 26 of 36

plane of the [MnO6] slabs experienced a regular and reversible shift upon cycling due to the [MnO6] slabs’ expansion/contraction. A maximum blue shift of ~12 cm-1 from 572 to 584 cm-1 was observed for the MnO2 electrode during the cathodic scan from 0.9 V to 0 V, as schematized in Figure 3e. For the electrode of Zn-MnO2 (Figure 3d), though the reversible intensity evolution is similar with MnO2, the shift of the main band is largely suppressed and a maximum value of only ~4 cm-1 upon scanning is detected (Figure 3e), much smaller than that of MnO2. As discussed above, the Raman shift is highly concerned with interlayer spacing d001. The largely suppression of Raman shift of Zn-MnO2 should result from the decrease of interlayer spacing change, indicating a much smaller volume change of Zn2+ complex modified sample during charge/discharge, and should be the main reason for the significantly enhanced durability. Even after long-term cycling, the Raman shift for Zn-MnO2 is still much smaller that of MnO2 (Figure S9), indicating a high stability of the modification. 4. CONCLUSIONS In summary, a simple and effective approach to enhance dramatically the electrochemical performance of a δ-MnO2 based electrode has been developed. Through a simple pre-


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accommodation of hydrated Zn2+ complex, the capacity of the active electrode material was gradually increased from an initial value of 268 F g-1 to ~348 F g-1 after several thousand cycles, and then maintained relatively constant during the subsequent cycles (even after 20,000 cycles) at a high cycling rate (50 A g-1). In contrast, the capacity of a bare δ-MnO2 electrode without the modification faded gradually, retaining only ~74% of the initial capacitance (about 200 F g-1) after ~20,000 cycles. Synchrotron X-ray diffraction and operando Raman measurements were performed to explore the structural evolution during cycling. DFT computation was used to gain insight into the mechanism of the performance enhancement from the modification, and to analyze the vibration modes probed by Raman spectroscopy. Operando Raman results suggest that the band shifts were obviously suppressed for the modified sample, implying the less structure deformation and Jahn-teller distortion induced by intercalation/deintercalation of electrolyte ions during cycling. The suppressed Raman shifts, thus, resolve the main reason for the stability enhancement.



ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acsaem.XXX.

Electrochemical performance of the initial MnO2 and Ga-MnO2 and Fe-MnO2, EDX data of the Zn-MnO2, in-situ/ex-situ Raman data of Zn-MnO2 and MnO2, 2 2 computation results of relative shrink of [Zn(H 2 O)3 ]ad and [Zn(H 2 O)]ad at the two possible

locations, computation results of all possible Raman vibration modes that induced by the 2 [Zn(H 2 O)3 ]ad accommodation, DFT calculation data of possible locations and bonds of

2 Na/Zn adsorption at the interlayer of δ-MnO2 slabs and adsorption energies of [Zn(H 2 O)]ad

at two possible locations in MnO2 slabs at open circuit potential as well as its replacing energy by Na+ (PDF)



AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Shuang Cheng), [email protected] (Jenghan Wang) and [email protected] (Meilin Liu) ORCID Shuang Cheng: 0000-0001-6301-175X


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Jenghan Wang: 0000-0002-3465-4067 Meilin Liu: 0000-0002-6188-2372 Notes The authors declare no competing financial interest. Minghai Yao and Xu Ji contributed equally.



ACKNOWLEDGMENTS

This work was supported by the Fundamental Research Funds for Central Universities of SCUT, China (no. 2018ZD20), the National Science Foundation for Young Scientists of China (Grant 21403073), the National Science Foundation for Key Support Major Research project of China (No. 91745203), Guangdong Innovative and Entrepreneurial Research Team Program (Grant 2014ZT05N200), and Guangzhou Science and Technology Program (No. 20181002SF0115).

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A Simple and Cost-Effective Approach to Dramatically Enhance the

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