Tailoring Three-Dimensional Composite Architecture for Advanced

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Tailoring Three-Dimensional Composite Architecture for Advanced Zinc-ion Batteries Yang Liu, Xiaoming Zhou, Rong Liu, Xiaolong Li, Yang Bai, Huanhao Xiao, Yuanming Wang, and Guohui Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04583 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Materials & Interfaces

Tailoring Three-Dimensional Composite Architecture for Advanced Zinc-ion Batteries Yang Liu, †, # Xiaoming Zhou, †, # Rong Liu, *, ‡ Xiaolong Li, † Yang Bai, † Huanhao Xiao, † Yuanming Wang, † and Guohui Yuan*, † †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School

of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. ‡

Key Laboratory of Functional Inorganic Material Chemistry Ministry of Education of the People’s

Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China.

ABSTRACT: Rechargeable aqueous Zn-ion batteries (ZIBs) are of considerable interest for future energy storage. Their main limitation, however, is how to develop suitable cathode materials capable of sustaining the Zn2+ repeated intercalation/deintercalation. Herein, a three-dimensional polypyrrole (PPy) encapsuled Mn2O3 composite architecture is developed for advanced ZIBs. The engineering can be easily realized via in situ phase transformation of MnCO3 microboxes with subsequent self-initiated polymerization of PPy. The abundant open-up pores (~30nm) throughout the construction accelerate the ion migration and provide more active interface for Zn2+ storage in the Mn2O3@PPy bulk electrode. Meanwhile, the PPy skin uniformly wrapped on the Mn2O3 microbox not only guarantees good conductive network for faster electron transport but also inhibits the dissolution of Mn2O3 and protects the integrity of electrode from structural damage. As results, Mn2O3@PPy electrode can operate at reversible capacity exceeding that of most other cathode materials, but still provide longer lifetime (no capacity decay over 2000 cycles at 0.4 A g-1) and higher rate performance than others. Furthermore, theoretical studies show the H+ and Zn2+ co-insertion storage mechanism and reaction dynamics. The results show that this threedimensional Mn2O3@PPy architecture is a promising cathode material for high performance ZIBs.

KEYWORDS: three-dimensional architecture; PPy encapsulated Mn2O3; cathode materials; zinc storage mechanism; aqueous zinc-ion batteries

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INTRODUCTION Li-ion batteries with high energy density and long cycling life take over the current power market ranging from various intelligent devices to aerospace applications. Nonetheless, limited lithium resources, financial cost and safety problems of Li-ion batteries trigger researchers to effort alternative energy system on abundant natural elements and aqueous electrolytes.

1-4

Rechargeable aqueous Zn-ion batteries are

emerging as the frontrunner to complement or even replace Li-ion batteries. 5-7 On the one hand, Zn metal features plentiful global stockpile and chemical stability, allows divalent charge transport, and has a low redox potential (-0.763 V versus the standard hydrogen electrode) and a high theoretical capacity (820 mAh g-1).

8-10

On the other hand, compared with the organic electrolyte, the aqueous electrolyte possesses

more reliable security and higher electrical conductive as well as less the environmental issue.

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However, despite these advantages, the great limitation that has impeded the development of ZIBs lies in exploiting the suitable cathode materials for Zn ions storage. Many cathode materials have been attempted to accommodate the troublesome Zn2+. Despite metal vanadates and Prussian blue analogues exhibit acceptable rechargeability, the quite low operating voltage and/or limited capacity seriously hinder their further application. 13-15 In practice, Mn-based materials are the most promising candidate due to abundant resources, decent operation voltage (>1.2 V) and output capacity (>200 mAh g-1).

16, 17

However, the Mn-based hosts still suffer from the low electrical/ionic

conductivity and severe structural collapse during cycling, resulting in sluggish reaction kinetics, fast capacity fading and inferior rate capability. 18, 19 Nano technology has been extensively developed to solve these problem, including α-MnO2 nanofiber, MnOx nanorods coated by N-doped carbon nanowalls, and spinel ZnMn2O4 nanograins.

20-22

Although this structure can shorten electron/ion transport route and

alleviate the volume effect to a certain extent, the crack, exfoliation and aggregation of nanomaterials would be inevitable in the long-term cycles. 23 Besides, the synthetic method of nanomaterials is normally a complicated process and need advanced techniques, which is hard to the large-scale production to practical ZIB systems. Therefore, it is highly desirable to develop an efficient strategy for obtaining versatile cathode materials in ZIBs. Recently, constructing micron-sized materials with nano-porosity have been introduced as an emerging but significant strategy to enhance the performance of electrode materials.

24-26

Breaking from the

conventional nanomaterials, porous microstructured materials overcome the stubborn ionic migration of the


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bulk electrode enabling fast reaction kinetics for charge storage. 27 Additionally, porous microstructure with extraordinary mechanical robustness can buffer the volume changes during guest ions insertion/deinsertion from hosts.

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So far, porous microstructure has been applied to a variety of electrode materials. For

instance, favorable ion/electron transfer has been demonstrated in the 3D ordered macro-porous carbonconfined Co9S8 quantum dots for high-performance sodium storage.

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Due to good structural stability,

ZnO-CuO oriented macroporous spheres presented long cycle life and remarkable rate capability for lithium-ion batteries.

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Attributing to the synergistic effects, pancake-like hexagonal WO3 with ordered

mesopores exhibited the highest specific capacitance compared with that of WO3 electrode materials. 31 In this regard, we envisage that manipulating electrode materials with porous micrometre-structure would be one possible approach to achieve the motif of high-performance cathodes for ZIBs. Herein, inspired by previous research, hierarchical porous Mn2O3 microboxes-encapsulated PPy skin are firstly designed and synthesized as a new potential cathode material for advanced rechargeable aqueous ZIBs. Highly opening-up Mn2O3@PPy microboxes are fabricated by a viable approach containing in situ phase transformation of MnCO3 microboxes companied with subsequent self-initiated polymerization of PPy. The abundant holes throughout the microarchitecture afford fast ion migration and mass transport, while the wrapped PPy skin serves as a protective film to endow the electrode with both great electrical conductivity and structural integrity during prolonged electrochemical reactions. Thanks to these structural advantages, the Mn2O3@PPy cathode exhibits remarkably enhanced electrochemical performance including a high capacity of 255 mAh g-1 at current densities of 0.1 A g-1, impressive cycling durability (up to 2000 cycles) and good rate capability (75.6 mAh g-1 at current densities of 2.0 A g-1). In addition, the H+ and Zn2+ co-insertion mechanism and reaction kinetics are further demonstrated by detailed investigation and discussion. This study will open a new avenue to developing high-performance cathode materials for ZIBs.

RESULTS AND DISCUSSION The three-dimensional Mn2O3@PPy architecture was prepared via a three-step process as depicted in Figure 1a. Briefly, uniform MnCO3 microboxes as the starting precursor were synthesized through a simple co-precipitation method without any high temperature and pressure, surfactants or sacrificial templates. After a simple in situ phase transformation of the MnCO3 precursor, Mn2O3 microboxes with hierarchical porous architecture were produced. In this process, MnCO3 microboxes underwent a valence transformation ACS Paragon Plus Environment


Figure 1. (a) Schematic illustration of the preparation process of the Mn2O3@PPy microbox. (b) XRD patterns of the MnCO3, Mn2O3 and Mn2O3@PPy microbox. SEM images of (c) MnCO3, (d) Mn2O3 and (e) Mn2O3@PPy microbox. (f) and (g) TEM and HRTEM images of the Mn2O3@PPy microbox.

from Mn2+ to Mn3+, and large amounts of carbon were removed at the same time resulting in the generation of honeycomb-like holes (the detailed mass reduction as analyzed by thermogravimetric measurement (TGA), Figure S1). Finally, the PPy skin was built on the surface of Mn2O3 microboxes through a selfinitiated polymerization. It is worth noting that Mn2O3, for the first time, sacrificially serves as the oxidizer to induce pyrrole molecules into PPy by reducing superficial Mn2O3 (the detailed chemical reaction as shown in Figure S2). The phase structure and purity of as-prepared samples were evidenced by the X-ray powder diffraction (XRD) measurements. As revealed in Figure 1b, all typical diffraction peaks of Mn2O3 microboxes are well indexed to the cubic Mn2O3 phase (JCPDS No. 41-1442),

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implying that MnCO3 microboxes are

successfully evolved into Mn2O3 by annealing treatment in air. No peaks correlated with other manganese oxides or metallic Mn can be captured, demonstrating the high purity of Mn2O3. After the coating process, the broad peak around 22° is detected originating from the amorphous PPy skin on the surface of Mn2O3 microboxes. X-ray photoelectron spectroscopy (XPS) technique was further carried out to characterize the


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chemical composition and surface electronic states of the Mn2O3@PPy sample. The XPS spectrum shown in Figure S3a confirms the signatures of Mn, O, C and N elements in the Mn2O3@PPy composites. In the high resolution spectra for Mn 2p (Figure S3b), the peaks at 640.4 eV and 652.2 eV belong to Mn 2p3/2 and Mn 2p1/2, respectively. The energy separation of 11.8 eV between these two peaks reveals the observation that Mn2O3 in prepared composites is pure phase. 33 Figure S4 gives the fourier transformed infrared (FTIR) spectroscopy of the Mn2O3@PPy sample, in which the characteristic PPy peaks can be identified, such as the antisymmetric and symmetric ring vibration (1537 cm-1 and 1449 cm-1), in-plane vibration of C-H bond (1028 cm-1) and the C-N stretching mode (1292 cm-1). 34, 35 In comparison to the FTIR spectra of PPy, the peaks for the Mn2O3@PPy sample slightly shift due to the influence of Mn2O3. The PPY ratio in Mn2O3@PPy composites is calculated to be 13.5 wt% based on the TGA analysis (Figure S5). All above results demonstrate the successful synthesis of Mn2O3@PPy composites. The morphologies of the synthesized composites at different stage were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM). The FESEM images (Figure 1c and Figure S6) reveal the uniform cubic geometry of MnCO3 microbox with an average particle size of ca. 2.5 um. Attributing to high homogeneity and appropriate size, these MnCO3 microboxes serve as precursor for the generation of Mn2O3 microboxes via an annealing treatment. As shown in Figure 1d, the Mn2O3 product manifests a honeycomb-like disordered porous structure and meanwhile, monodisperse microboxes are well preserved. Figure 1e and inset show the SEM images of Mn2O3@PPy composites, where can be seen that the introduction of PPy has no impact on morphology of Mn2O3 microboxes and the interconnected porous framework is still visible. The over-lapped elemental mappings (Figure S7) present the homogeneous distribution of Mn, O, C and N elements, further indicating the uniform PPy skin. Furthermore, a panoramic TEM image (Figure 1f) clearly elucidates the hierarchical porous microstructure of Mn2O3@PPy microboxes. From a close observation of Mn2O3@PPy core-shell structure (Figure 1g), an intimate PPy skin with a thickness of 10 nm is observed on the Mn2O3 core. The lattice distance (inset in Figure 1g) in the surface layer of Mn2O3 is 0.38nm (index to Mn2O3 (211) plane) and the corresponding selected area electron diffraction pattern reveals high crystalline nature of the obtained Mn2O3 (Figure S8), which are well consistent with the aforementioned XRD analysis. As verified in SEM and TEM images, a honeycomb-like Mn2O3@PPy microbox reveals three-order hierarchy structure, which are (i) the external profile of the original MnCO3 precursor, (ii) the uniform PPy skin, and (iii) numerous disordered pores throughout the construction. Moreover, in virtue of hierarchical microstructure and abundant open pores, ACS Paragon Plus Environment


Figure 2. (a) Representative CV curves of the Mn2O3@PPy microboxes at a scan rate of 0.1 mV s-1. (b) Galvanostatic chargedischarge profiles of Mn2O3@PPy microboxes at a current density of 100 mA g-1. (c) Rate performance of Mn2O3@PPy microboxes. (d) The GCD profiles at various current rates of the Mn2O3@PPy electrode. (e) The capacity retention rate of the Mn2O3@PPy microboxes at different current densities. (f) Cycling performance of Mn2O3@PPy microboxes at a current density of 0.1 A g-1. (g) Cycling performance of Mn2O3@PPy microboxes at a current density of 0.4 A g-1.

the Brunauer-Emmett-Teller (BET) surface area of Mn2O3@PPy microboxes is 56.03 m2g-1 based on the nitrogen absorption-desorption isotherm (Figure S9a). In the Barrett-Joyner-Halenda (BJH) pore size distribution plot (Figure S9b), there are two peaks at 30.394 nm and 4.295 nm in the range of 3-120 nm, indicating the characteristic of mesoporous structure in the samples, which coincides with the SEM and TEM analysis. Due to the unique porous structure and PPy-reinforced conductive framework, synthesized Mn2O3@PPy microboxes are expected to be a remarkable cathode material for ZIBs. The electrochemical performance of the Mn2O3@PPy microboxes cathode was investigated using Zn foil as an anode and the aqueous 2 M ZnSO4/0.1 M MnSO4 solution as electrolyte. It has been proved that the MnSO4 as an electrolyte additive can partly inhibit the Mn2+ dissolution of Mn-based cathodes. 36 Figure 2a shows the cyclic voltammetry (CV) curves for the Mn2O3@PPy cathode in the voltage window of 1.0-1.8 V (vs. Zn/Zn2+) at a scan rate of 0.1 mV s-1 (the voltage of water splitting is 1.87 V (vs. Zn/Zn2+), Figure S10). In the cathodic scan, two separate peaks at 1.35 V and 1.25 V are associated with the different


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insertion stages of H+ and/or Zn2+ (the mechanism will be further investigated in following). And the peaks at 1.58 V and 1.65 V in the anodic scan are attributed to the ionic extraction process. In addition, the first five CV curves are almost overlapped, demonstrating the stable electrochemical reaction and good reversibility of Mn2O3@PPy cathode. The galvanostatic charge-discharge (GCD) profiles of Mn2O3@PPy electrode are presented in Figure 2b. The charge-discharge platforms at 1.52/1.28 V and 1.58/1.40 V are related to the extraction and insertion of cations (H+/Zn2+) in the Mn2O3@PPy host. The Mn2O3@PPy electrode exhibits the initial discharge capacity of 216.6 mAh g-1 at the current density of 0.1 A g-1 and in the second cycle, the high reversible capacity of 255.7 mAh g-1 is achieved. Note that the capacity of the natural PPy is 6.5 mAh g-1 (Figure S11), showing negligible contribution to the overall capacity. The rate performance as a critical parameter for grid storage applications of ZIBs was assessed at various current densities (Figure 2c). The Mn2O3@PPy cathode delivers average specific capacities of 255, 211, 174, 130, and 117 mAh g-1 at the current rates of 0.1, 0.2, 0.4, 0.8 and 1.0 A g-1, respectively. The high reversible capacity of 75.6 mAh g-1 is still remained even at a high current density of 2.0 A g-1, revealing the excellent rate performance. Impressively, when the current density is set back to 0.1 A g-1, the reversible capacity of 229 mAh g-1 is recovered. The clear-cut GCD profiles and capacity retention at various current rates (Figure 2d and e) demonstrate the high rate capability of Mn2O3@PPy microboxes. As contrast, a low capacity retention rate (26.6%, Figure S12) is observed for the Mn2O3 electrode when the current density is increased to 1.6 A g-1. To further evaluate cycle stability, the Mn2O3@PPy was cycled at the current density of 0.1 and 0.4 A g-1. As shown in Figure 2f, the Mn2O3@PPy electrode delivers 230 mAh g-1 after 130 cycles at the current density of 0.1 A g-1, which corresponds to 90.1% of its maximum capacity. Moreover, the discharge capacity of 178 mAh g-1 is maintained after 2000 cycles at the current density of 0.4 A g-1 (the corresponding average coulombic efficiency of 97.94 %, Figure 2g). The declined specific capacity after the initial cycles should be ascribed to the irreversible capacity loss in the initial stage of reaction. And the increase of the capacity in the subsequent cycles is attributed to the fact that the electrolyte gradually penetrated into the porous space of Mn2O3@PPy microboxes.

37, 38

For comparison, the Mn2O3 electrode

only retains a specific capacity of 60 mAh g-1 after 200 cycles (at the current density of 0.4 A g-1, Figure S13), confirming the key role of PPy skin on Mn2O3 microboxes. The sustainable cycle life of as-prepared Mn2O3@PPy microboxes is also superior or competitive to that of other reported Mn-based cathodes (Table S1 for detailed comparison). ACS Paragon Plus Environment

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Figure 3. Schematic diagram of aqueous ZIB comprising of Zn anode and Mn2O3@PPy cathode and an enlarged part of the Mn2O3@PPy microbox with superior electrochemical performance.

As we expect, the Mn2O3@PPy microboxes exhibit satisfactory electrochemical performance (high specific capacity, rate capability and cycle life), which can be attributed to the synergistic effect of the honeycomb-like microbox and uniform PPy skin. As displayed in Figure 3, the open framework with free space boosts the affnity between electrolyte and electrode, thus rendering the fast kinetic for Zn2+ storage. The mesoporous structure contributes to promote electrochemical utilization by offering abundant electroactive sites, enabling improved storage capacity. Additionally, the “PPy gauze” not only serves as an electron percolation path to ameliorate the poor electronic conductivity of Mn2O3 microboxes, but also protects structural integrity during prolonged ions insertion/deinsertion and avoids detrimental reactions generated from direct exposure to electrolyte (the SEM images of the cycled Mn2O3@PPy microboxes and Zn anode are shown in Figure S14 and S15, respectively). In order to understand the origin behind remarkable electrochemical performance of Mn2O3@PPy microboxes, the storage behavior was investigated by cyclic voltammetry at various scan rates. As shown in Figure 4a, the anodic and cathodic peaks become broader and slightly shift with the increased scan rates from 0.1 to 1.0 mV s-1. According to calculation principles in the literatures, the relationship between the peak current (i) and sweep rate (v) can be described using the following equation: 41 i = avb


(1)

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Figure 4. (a) The CV curves of the Mn2O3@PPy electrode at various scan rates. (b) Determination of the b value according to log i and log v plots at specific peak currents. (c) The contribution ratios of capacitive-like and diffusion controlled capacities at different scan rates. (d) CV curve with the capacitive-like fraction at a scan rate of 1.0 mV s-1. (e) Nyquist plots of the Mn2O3@PPy and Mn2O3 electrodes before and after cycles at 0.4 A g-1. (f) The relationship between Z′ and ω-1/2 of the electrodes in the low-frequency region. (g) The CV curves of the Mn2O3 electrode at different scan rates. (h) The linear behavior of v1/2 vs Ip. (i) The slope values of v1/2 vs Ip for the Mn2O3@PPy and Mn2O3 electrodes.

where a and b are adjustable parameters. The b value can be determined by the slope of log i versus log v, and the b value of 0.5 and 1 is related to the diffusion controlled process and capacitive-like behavior, 42 respectively. From fitting results shown in Figure 4b, the b values of the four peaks are 0.67, 0.56, 0.59, and 0.65, indicating that the electrochemical behavior of the Mn2O3@PPy electrode is controlled by both the diffusion process and capacitive-like reaction contribution. To further quantitatively separate above two process, in general, the current response (i) at a certain potential (V) is assumed to comprise of k1v (capacitive-like contribution) and k2v1/2 (diffusion process), 43 as following described: i(V) = k1v + k2v1/2

(2)

Figure 4c displays the contribution ratios of capacitive-like process at scan rates from 0.1 to 1.0 mV s-1. Clearly, when the scan rates are 0.1 mV s-1 and 0.2 mV s-1, the mobile-ion diffusion occupies a main storage mechanism. As the scan rates increased, the solid-solution reaction ratios stepwise rise to the



Figure 5. Structure evolution of the Mn2O3@PPy microboxes cathode during cycles. (a) Ex-situ XRD patterns of Mn2O3@PPy electrodes at the selected charge and discharge states (sample 1 to 14). (b) The XRD patterns of sample 1, 8 and 14 from a. (c-h) SEM images for the morphological development of the Mn2O3@PPy electrode. (i) The element mapping of Mn, Zn and S for the full discharging electrode. (j) SEM image of full discharging Mn2O3@PPy microbox and (k) the corresponding EDS analysis. (l) SEM image of full charging Mn2O3@PPy microbox and (m) the corresponding EDS analysis.

dominant position. Specifically, as shown in Figure 4d, the fraction of capacitive-like contribution (shaded region) accounts for 76.87% at 1.0 mV s-1. This impressive capacitive-like behavior may be ascribed to the fast ions/electrons migration derived from porous structure, which gives the high rate capacity for Mn2O3@PPy electrodes. The electrochemical impedance spectroscopy (EIS) measurements were carried out to insight into the reaction kinetics. Figure 4e shows the Nyquist plots of the Mn2O3@PPy and Mn2O3 electrode before and after cycles. The resistance values are determined by fitting data with a suitable equivalent circuit model (listed in Table S2). The charge-transfer resistance (Rct) value of Mn2O3@PPy cathodes is 75.83 Ω before cycles, and in contrast, that of Mn2O3 cathodes is 146.45 Ω. Such results suggest that Mn2O3@PPy


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cathodes hold a faster reaction kinetics benefited from the PPy skin. Moreover, the Zn-ion diffusion coefficients can be estimated from the low-frequency region by using the Warburg factor 44, 45 (Equation S1 and S2). To obtain the Warburg factor, the liner relationships between Z′ and ω

-1/2

are fitted in Figure 4f.

The zinc-ion diffusion coefficient of Mn2O3@PPY cathodes is 2-3 times higher than that of Mn2O3 cathodes, which suggests a well-ameliorative interface kinetics in the Mn2O3@PPy composite. This improved diffusion kinetic of Mn2O3@PPy electrode is further proved by the CV measurements at different scan rates (Figure 4g-i, S16 and Equation S3). The reaction mechanism of the Mn2O3@PPy microboxes cathode was explored through monitoring the structural evolution during cycles. Figure 5a shows the ex-situ XRD patterns of Mn2O3@PPy electrodes at the selected charge and discharge states (at the current density of 100 mA g-1). At the first discharge platform around 1.4 V (stage I), the diffraction peaks at 23.1°, 32.9°, 55.2°and 65.8° are well-indexed to Mn2O3, indicating that the crystal structure of Mn2O3@PPy microboxes has no significant variation. However, during the second discharge platform (stage II), some new peaks at 16.2°, 24.4° and 58.4° start to arise and show increased intensity along with discharging to 1.0 V. Then, these arisen peaks gradually reduce their intensities and disappear completely in the subsequence charging process from 1.0 V to 1.5 V (stage III). After the end of stage IV, only the diffraction peaks of Mn2O3 can be detected, which are similar to that obtained in stage I. This observation shows that the Mn2O3@PPy electrode is reversible during cycling. Figure 5b shows the XRD patterns of sample 1, 8 and 14, where the emerging peaks on sample 8 are assigned to zinc hydroxide sulfate hydrate ((Zn(OH)2)3(ZnSO4)(H2O)5, JCPDS:39-0688). 46 It has been demonstrated in the recent literatures that owing to the consumption of H+ in discharging process, accumulated OH- induces the generation of zinc hydroxide sulfate hydrate on the cathode.

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In this

regard, it suggests the Mn2O3@PPy electrode occurs H+ insertion/extraction during cycles. The morphological development of the Mn2O3@PPy electrode was further imaged with SEM. The electrode shows a flat surface and has no obvious change at stage I (Figure 5c). As the discharging progressed from 1.3 V to 1.0 V, large nanoflakes with the diameter of ca. 4 um start to precipitate and even cover the electrode as shown in Figure 5d and 5e. Upon charging, the above nanoflakes gradually dissolve and vanish, and the electrode backs to the state like in stage I (Figure 5f-h). Such a self-regulating morphological evolution of Mn2O3@PPy electrodes is well consistent with the aforementioned ex-situ XRD analysis. Figure 5i shows the element mapping of discharging products, confirming that the precipitated nanoflakes comprised of Zn and S are zinc hydroxide sulfate hydrate. From Figure 5i, it can ACS Paragon Plus Environment


Figure 6. (a) The typical discharge profile of Mn2O3@PPy electrode at current density of 0.1 A g-1 with 2 M ZnSO4/0.1 M MnSO4 electrolyte. (b) Nyquist plots measured at the selected states. (c) Z′ and ω-1/2plots at the low frequency region. (d) GITT curve and the corresponding (ΔEs/ΔEτ)2. (e) enlarged t vs E curves at the two platforms. (f) The linear relationship of ∆Eτ and τ1/2 at discharge process.

also be observed that the content of Zn is more than that of S on the Mn2O3@PPy microboxes. To inspect whether Zn2+ insertion, the discharging electrode washed away zinc hydroxide sulfate hydrate was further investigated by energy-dispersive X-ray spectroscopy (EDS). As revealed in Figure 5j-m, the element Zn presents on the surface of discharged Mn2O3@PPy microboxes, while on the charging product, Zn is completely absent, indicating the reversible insertion/extraction of Zn2+ in Mn2O3@PPy host. Moreover, XPS spectra of Zn 2p (Figure S17) arises two peaks corresponding to Zn 2p1/2 (1021.8 eV) and Zn 2p3/2 (1044.9 eV) on the discharged product, which confirms the appearance of Zn2+. On base of above analysis, the Mn2O3@PPy electrode experiences H+ and Zn2+ co-insertion during discharging process. To further clarify this mechanism, the discharge behaviors of Mn2O3@PPy cathode in 2 M ZnSO4 /0.1 M MnSO4 electrolyte, 2 M ZnSO4 electrolyte and 0.1 M MnSO4 electrolyte are investigated (Figure S18), respectively. As known, the discharge profile shows two plateaus (1.4 V and 1.3 V) with the electrolyte contained of Zn2+. However, only a slope-like plateau is observed in 0.1 M MnSO4 electrolyte. The second discharge plateau disappears with the absence of Zn2+, demonstrating that the Zn2+ insertion happens on the Mn2O3@PPy cathode during discharging. The diffusion kinetics were analyzed at two discharge platforms related to H+/Zn2+ insertion. Nyquist plots and the linear relationships of Z′ and ω1/2

are shown in Figure 6a-d. The ion diffusion coefficient in the first platform is ~30 times greater than that


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in the later platform. To directly show the diffusion coefficients during discharge process, the galvanostatic intermittent titration technique (GITT) is further performed 47 (Equation S4 and S5). The different diffusion coefficients between two discharge platforms are also observed from Figure 6d-f. Thus, the turning point on the discharge profile locate at around 1.3 V could be attributed to the sudden slowing down of the ion diffusion coefficient.

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And due to the slow diffusion kinetic, the inserted ion at the second voltage

platform reduces gradually with the increasing of current density, leading to the turning point disappears gradually with the increasing of current density (Figure S19). The significantly different diffusion coefficients suggest that the cation insertion in two discharge platforms are different. Given the smaller size of H+ compared with Zn2+,

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the first voltage plateau should be attributed to H+ insertion and the second

voltage plateau involves Zn2+ insertion. In the first discharge plateau, the H+ insertion results in a gradual increase of OH- concentration on the electrode surface. Because zinc hydroxide sulfate hydrate is formed during the second plateau, it can be concluded that the H+ insertion also happens in the second plateau. Therefore, the H+ and Zn2+ co-insertion mechanism is confirmed on the Mn2O3@PPy microboxes cathode.

CONCLUSION In summary, three-dimensional Mn2O3@PPy architecture is successfully fabricated via an in situ phase transformation method followed by self-initiated polymerization. The as-prepared Mn2O3@PPy microboxes constructed with unique honeycomb-like microstructure and polymer-reinforced skin remarkably facilitate the electronic-ionic transfer kinetics for charge storage. More importantly, the PPy protective layers prevent the dissolution of the active material and improve the structural and electrical integrity. When used as a cathode material for state-of-the-art aqueous ZIBs, the Mn2O3@PPy microboxes deliver a high specific capacity of 255 mAh g-1 at current densities of 0.1 A g-1, excellent cycling stability and rate capability. Furthermore, the storage mechanism of H+ and Zn2+ co-insertion on the Mn2O3@PPy microboxes cathode is preliminarily elucidated through ex-situ analyses and electrochemical technology. We believe that this structural engineering strategy of hierarchical porous Mn2O3@PPy microboxes will open a new pathway to light up high-performance rechargeable aqueous ZIBs.

EXPERIMENTAI SECTION Preparation of MnCO3 microboxes. The MnCO3 microboxes were prepared according to the reported method. 50 Briefly, 0.6 g of MnSO4·H2O, 0.6 g of (NH4)2SO4, and 35 mL of ethanol were dissolved in 350



mL de-ionized water (DI water) and stirred for 30 min. Then, 350 mL 0.14 mol L-1 NH4HCO3 was poured into the above solution under stirring. Subsequently, the mixture was maintained at 50 °C for 10 h. The obtained MnCO3 precipitate was collected by centrifugation, and washed with DI water for several times, and then dried at 50 °C. Preparation of Mn2O3 microboxes and Mn2O3@PPy microboxes. The Mn2O3 microboxes were obtained by annealing MnCO3 microboxes at 600 °C for 2 h with a ramping rate of 2 °C min-1 in the presence of air. Afterward, the PPy skin was covered on Mn2O3 microboxes by a self-initiated polymerization route without any other oxidizer. 0.1 g Mn2O3 powder was dispersed in 20 mL DI water by ultrasonication for 30 min. Then, 50 uL pyrrole monomer was dropwise added into the Mn2O3 dispersion and stirred at room temperature for 2 h. Finally, the synthesized Mn2O3@PPy microboxes was filtered, washed thoroughly with DI water, and dried in a vacuum at 50 ºC. Materials characterization. The XRD patterns of as-synthesized samples were recorded by Rigaku D/max IIIA X-Ray Diffractometer (Cu Kα radiation) with the 2θ range of 10-80°. XPS was detected by an ESCALab220i-XL electron spectrometer (VG Scientific, 300W Al Kα radiation). The functional groups of the prepared samples were characterized by FTIR spectra (Bruker IFS-85 spectrometer). TGA curves were obtained by a Perkin-Elmer TGA 4000 analyzer with air atmosphere at a 10 °C ramp rate from 25 °C to 800 °C. FESEM images were imaged on Carl Zeiss MERLIN Compact at 15 kV and the elemental distribution was analyzed by EDS. TEM images and select area electron diffraction were acquired on a JEOL-2010 microscope. Electrochemical Characterizations. The Mn2O3 microboxes and Mn2O3@PPy microboxes cathodes were prepared by casting slurries of active materials (80 wt%), acetylene black (10 wt%) and polytetraﬂuoroethylene (10 wt%). Then the mixed slurry was tableted on the titanium grid (200 mesh) and cut into the disks of 14 mm diameter (The mass loading of Mn2O3@PPy microboxes is 2-3 mg cm-2). CR2025 coin cells were assembled in air using the prepared cathode, Zinc foil as the anode (99.99% purity, Sinopharm Chemical Reagent Co., Ltd.), glass fiber membrane as the separator and the electrolyte containing of 2 mol L-1 zinc sulfate and 0.1 mol L-1 manganese sulfate (pH = 4). The negligible selfdischarge phenomenon in this Zn/Mn2O3@PPy battery was demonstrated in Figure S20. The galvanostatic charge-discharge was carried out by a battery tester (Land CT2001A system). The CV curves and EIS were performed with an electrochemical workstation (CHI660e, Chenhua). All electrochemical tests were performed at room temperature. ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional XRD, SEM, BET, FTIR, TGA and electrochemical performance (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected].

ORCID Xiaoming Zhou: 0000-0002-9843-9710 Guohui Yuan: 0000-0003-3662-2875

Author Contributions # Y. Liu and X. M. Zhou contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Key Technology R&D Program (Grant No. 2017YFB1401805).

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Tailoring Three-Dimensional Composite Architecture for Advanced

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