In situ grown MnO₂/graphdiyne oxide hybrid 3D nanoflowers for high-performance aqueous zinc-ion batteries†

Fuhui Wang ^ab, Weiyue Jin ^ab, Zecheng Xiong ^ab and Huibiao Liu *^ab
aBeijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: liuhb@iccas.as.cn
^bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China

First published on 18th May 2021

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Introduction

Rechargeable aqueous zinc-ion batteries (ZIBs) have attracted extensive research activity in recent years because of their resource sustainability, secure electrolyte, simple assembly, eco-friendliness and appreciable energy density.^1,2 The key challenge of ZIBs is to explore suitable cathode materials that possess a high specific capacity, superior rate performance and long-term cycle life, such as manganese-based oxides, vanadium-based oxides and organic compounds.^3,4 Among them, MnO₂ stands out for its high operating potential (approximately 1.4 V vs. Zn/Zn²⁺), large theoretical capacity (308 mA h g⁻¹), inexpensiveness, non-toxicity and easy preparation.^5–7 These virtues make the Zn//MnO₂ system attractive for grid-scale energy-storage deployment. However, the disadvantages of MnO₂ cathodes are their rapid capacity fading, poor rate capability and unsatisfactory cycling stability that arise from their intrinsically poor electrical conductivity, manganese dissolution, phase change and structural damage during repeated discharge–charge processes.^8–10 Therefore, improving the electrochemical activity, inherent stability and suppressing the volumetric change of MnO₂ for a reversible cathode material is an important challenge in aqueous ZIBs based on MnO₂.

There are some strategies for improving the performance of ZIBs based on MnO₂, such as the pre-addition of Mn²⁺ salt in mild aqueous electrolytes for suppressing the dissolution of Mn²⁺ and improving the cycling stability of MnO₂ cathodes,^11,12 developing MnO₂ materials with a vast variety of polymorphs (i.e., α-, β-, γ-, and δ-MnO₂),^13,14 and investigating different morphologies (i.e., nanowires,¹⁵ nanorods,¹⁶ nanospheres¹⁷ and nanoflakes¹⁸) and their hybrids (i.e., MnO₂/graphene,¹⁹ MnO₂/graphite,²⁰ MnO₂/carbon nanotube²¹ and MnO₂/polyaniline²²), which have resulted in preliminary achievements. The nanostructural MnO₂ can provide an enlarged electrolyte/electrode contact area, increase the number active sites and shorten ion-/electron-diffusion pathways for promotion of the capacity and reaction kinetics of MnO₂. Various nanostructures of MnO₂ exhibit good electrochemical performances in either batteries or supercapacitors.^23–25 Hybridizing MnO₂ with carbon materials has been considered as an effective approach to improve its electronic conductivity, structural stability and to accommodate the structural change during the cycling process. Various carbon materials, including graphene, carbon nanotubes and graphite, exhibit superb electrical conductivity when applied in aqueous ZIBs. However, most of the MnO₂/conductive additive composites prepared by physical mixing methods such as ultrasonic mixing,²⁶ ball milling,²⁷ and vacuum filtration^28,29 have an obvious phase-separation interface after repeated discharge–charge cycles, which brings about the loss of good conductive contact and internal structure stability. Therefore, it is imperative to introduce new carbon materials and develop new efficient and reliable methods for the preparation of homogeneous hybrid materials based on MnO₂ and novel carbon materials.

Graphdiyne (GDY) is a novel 2D carbon allotrope with benzene rings connected by butadiyne linkages (–CC–CC–) to form 18-C hexagons and has displayed excellent electrical, catalytic and optical properties.^30–32 The extended existence of sp- and sp²-hybridized carbon atoms endows GDY with a 2D planar structure, uniformly distributed pores, excellent electrical conductivity, high chemical stability and so on.^33–35 Graphdiyne oxide (GDYO) is one of the important derivatives of GDY, and is prepared by surface oxidation with an oxidizing agent. GDYO preserves the integrity of the skeletal 2D structure of GDY (CC, CC) to ensure a rich pore structure with uniform distribution, and forms abundant hydrophilic oxygen-containing functional groups (CO) on its basal planes and edges, which are beneficial to guest metal ions, chelating them firmly. Herein, we develop an in situ growth strategy to construct MnO₂@GDYO hybrid 3D nanoflowers using GDYO nanosheets. With abundant nanopores in each GDYO skeleton plus the porous network composed of the interconnected GDYO nanosheets, the integrated MnO₂@GDYO hybrid 3D nanoflowers provide numerous channels for the transport of ions/electrons, enhanced charge-transfer kinetics and structural stability. In addition, the 3D nanoflower is capable of greatly enlarging the electrolyte/electrode contact area, increasing the number of active sites and shortening the ion-/electron-diffusion pathways. Furthermore, the size, surface area and morphologies of MnO₂@GDYO hybrid 3D nanoflowers can be tuned by controlling the conditions of in situ growth, which results in the controllable performance of aqueous ZIBs. The as-prepared MnO₂@50GDYO hybrid 3D nanoflowers used as a cathode for aqueous ZIBs exhibit a superior rate capability of 80.6 mA h g⁻¹ at 10C and an excellent cycling stability of up to 1000 cycles with a capacity retention of 77.6%, a coulombic efficiency approaching 100% and a high capacity of 253.7 mA h g⁻¹ at 1C. This work brings new prospects for the design of GDYO hybrids for high-performance rechargeable aqueous ZIBs.

Experimental section

Preparation of GDYO

Graphdiyne (GDY) was synthesized on the surface of copper foil via a cross-coupling reaction using hexaethynylbenzene (HEB) precursors as previously described.³⁰ Graphdiyne oxide (GDYO) was prepared from GDY using a concentrated H₂SO₄/H₂O₂ mixture (volume ratio, 2:3) as the oxidizing agent. Briefly, GDY powder (100 mg) was mixed carefully with concentrated H₂SO₄ (1.2 mL) in an ice bath for 10 min. Then hydrogen peroxide (30% H₂O₂, 1.8 mL) was added dropwise into the mixture slowly and stirred vigorously for 12 h at room temperature. For further stripping and oxidation, the mixture was added to distilled water (20 mL) drop by drop under ultrasonication for 4 h. The suspension was centrifuged at 8000 rpm for 5 min and washed several times with distilled water until the supernatant was neutral. Finally, the GDYO powder was obtained by freeze-drying.

Preparation of MnO₂

The synthetic prcedure for MnO₂ is similar to previously reported work.³⁶ In a typical experiment, 4.0 g of (NH₄)₂S₂O₈ was added to 10 mL of H₂O. After vigorous stirring for 20 min, 4.5 g of TMAOH (tetramethylammonium hydroxide, (CH₃)₄NOH, 25% wt in H₂O) was added to the mixed solution, followed by stirring for another 20 min and the addition of water to 20 mL, giving a homogeneous solution. The obtained solution was then added to 10 mL of 0.3 M MnCl₂ solution slowly over 10 min under vigorous stirring. The dark brown MnO₂ sample was obtained after vigorous stirring overnight in the ambient atmosphere at room temperature. Afterward, the solid sample was collected through vacuum filtration and washed with deionized water and ethanol. Finally, the desired products were obtained after being dried under vacuum at 60 °C. The product was labeled as MnO₂.

Preparation of MnO₂@GDYO hybrid 3D nanoflowers

Firstly, 50 mg of the as-prepared GDYO powder was ultrasonically dispersed in 10 mL of deionized water. Then, 0.3 M MnCl₂ was added to the GDYO dispersion solution. After vigorous stirring for 20 min, the MnCl₂ was totally dissolved, giving a homogeneous solution. At the same time, 4.0 g of (NH₄)₂S₂O₈ was added to another 10 mL of H₂O. After vigorous stirring for 20 min, 4.5 g of TMAOH was added to the mixed solution, followed by another 20 min of stirring and the addition of water to 20 mL. The above solution was then added to the GDYO dispersion solution containing 0.3 M MnCl₂ slowly over 10 min under vigorous stirring. The dark brown MnO₂@GDYO hybrid 3D nanoflowers were obtained after vigorous stirring overnight in the ambient atmosphere at room temperature. The product was vacuum filtered, washed three times with deionized water and ethanol, and thoroughly dried under vacuum at 60 °C. The product was labeled as MnO₂@50GDYO. Different ratio MnO₂@GDYO hybrids were synthesized via controlling the mass of the GDYO in the mixtures, where the MnO₂@GDYO hybrids with GDYO loadings of 10 mg, 20 mg and 100 mg are named MnO₂@10GDYO, MnO₂@20GDYO and MnO₂@100GDYO, respectively.

Material characterization

Scanning electron microscope (SEM) images were collected using a Hitachi S-4800 microscope, combined with energy dispersive X-ray spectroscopy (EDS) for the determination of elemental composition. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) images and elemental mapping results were obtained using a JEM-2100F electron microscope operating at 200 kV. X-Ray diffraction (XRD) measurement was performed using a Japan Rigaku D/max-2500 rotation anode X-ray diffractometer and graphite-monochromated Cu Kα radiation (λ = 1.54178 Å) with a scan rate of 5° min⁻¹ from 5° to 90° (2θ). Raman spectra were recorded using an NT-MDT NTEGRA Spectra system, with excitation from an Ar laser at 473 nm. X-Ray photoelectron spectroscopy (XPS) was performed using a VG Scientific ESCALab220i-XL X-ray photoelectron spectrometer, with Mg or Al Kα radiation as the excitation source. Thermogravimetric analysis (TGA) was carried out using Mettler-Toledo TGA/SDTA851 apparatus under a flow of air at a heating rate of 5 °C min⁻¹ from 30 to 800 °C. Nitrogen adsorption and desorption measurements were performed using a Micromeritics ASAP 2020HD88 model instrument (Norcross, GA, USA). The specific surface area of the hybrids were determined using the Brunauer–Emmett–Teller (BET) method.

Preparation of cathode electrodes

To prepare the cathodes, the synthesized MnO₂ and MnO₂@GDYO hybrid 3D nanoflowers (70 wt%) were mixed with a conductive additive (carbon black, 20 wt%) and a binder (polyvinylidene fluoride, PVDF, 10 wt%) in a solvent of N-methyl-2-pyrrolidone (NMP), respectively, mixing with constant stirring until a homogeneous slurry was obtained. Next, the resulting slurry was cast onto Ti foils using the doctor blading method and dried under vacuum at 80 °C for 6 h. Finally, the electrodes were cut into round discs with a diameter of 10 mm for further use. The average mass loading of the active materials was about 1 mg cm⁻².

Electrochemical characterization

Zinc foil and filter paper were used as the anode and separator, respectively, and 2 M ZnSO₄ with 0.1 M MnSO₄ additive solution was employed as the electrolyte. CR 2032-type coin cells were assembled in air to evaluate the electrochemical performance. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured using a CHI 660E electrochemical workstation. The electrochemical properties, including the specific capacity, rate capacity and cycling performance, were evaluated using the galvanostatic charge–discharge method on a LAND battery testing system (Land CT 2001A) at different current densities. All tests were carried out at room temperature.

Results and discussion

The MnO₂@GDYO hybrid 3D nanoflowers were prepared through an in situ induced growth strategy. Briefly, the homogeneous (NH₄)₂S₂O₈ and (CH₃)₄NOH aqueous solution was dripped into MnCl₂ aqueous solution containing uniformly dispersed GDYO nanosheets in aqueous solution. The mixture reacted under constant vigorous stirring at room temperature for 12 hours. The MnO₂@GDYO hybrid 3D nanoflowers were obtained after washing and drying (details are in the Experimental section). The size, surface area and morphologies of the MnO₂@GDYO hybrid 3D nanoflowers can be tuned by controlling the conditions of in situ growth. The growth process of the MnO₂@GDYO hybrid 3D nanoflowers is proposed as depicted in Fig. 1. The Mn²⁺ species were anchored on the surface of the GDYO nanosheets (Fig. S1, ESI†) through their carboxyl groups in the mixed solution. After the addition of (NH₄)₂S₂O₈ and (CH₃)₄NOH aqueous solution, the MnO₂ nuclei began to be formed on the surface of GDYO nanosheets and grew in size. Subsequently, 2D MnO₂ nanosheets gradually formed under the induction of the GDYO nanosheets. Then, the obtained 2D MnO₂ nanosheets crosslinked with the GDYO nanosheets through the carboxyl functional groups of GDYO to produce the MnO₂@GDYO hybrid 3D nanoflowers. In this in situ growth process, the amount of GDYO nanosheets is a key factor for the size, surface area and morphology of the MnO₂@GDYO hybrid 3D nanoflowers. An excess of GDYO nanosheets promotes the agglomeration of 3D nanoflowers, while a lack of GDYO nanosheets results in a hybrid material with a low degree of cross-linking, and only an appropriate amount of GDYO nanosheets can induce 3D nanoflowers with a perfect structure.

The morphology and structure of the MnO₂@GDYO hybrid 3D nanoflowers were characterized by SEM and TEM (Fig. 2 and Fig. S2–S4, ESI†). From the SEM images, the evolution of the MnO₂@GDYO nanostructures indicates that the morphologies are well controlled. With an increase in the amount of GDYO, the formation of thinner and finer 3D nanoflower structures of MnO₂@GDYO hybrids are promoted compared with bare MnO₂ (Fig. S5, ESI†). Fig. 2 shows the typical morphology of MnO₂@50GDYO, which is composed of agglomerated nanoflowers with ample wrinkles and folds. There was no obvious exposure of GDYO nanosheets, which confirmed that all MnO₂ grew on the surface of GDYO, covering it completely, and there was no separated phase interface as illustrated in Fig. S6 (ESI†). Such an interconnected network not only ensures the electrochemical activity but also enhances the integral mechanical stability. More detailed structural information can be obtained by TEM, and all of the MnO₂@GDYO hybrids display a thin sheet-like morphology which folds up into complex 3D nanoflower structures. When the content of GDYO is too low, the morphology of the MnO₂@10GDYO hybrid is close to that of MnO₂, which has a thicker and larger sheet aggregated structure. With the increase of GDYO content, the degree of cross-linking between MnO₂ and GDYO increases, gradually forming the ultra-thin sheet aggregate structure. However, when the amount of GDYO is too high, the MnO₂@100GDYO hybrid aggregates severely, covering the exposed surface of the ultrafine structure, and resulting in a decrease in the specific surface area and pore structure. The TEM images of the MnO₂@50GDYO hybrid indicate that the flower-like structures are constructed from nanosheets, consistent with the SEM observations. High-resolution TEM (HR-TEM) characterization of the MnO₂@50GDYO hybrid is shown in Fig. 2f, and the lattice spacings of 0.24 nm and 0.48 nm are found to match well with the (006) and (004) crystal planes, respectively, of birnessite-MnO₂. Moreover, energy dispersive X-ray spectroscopy (EDS) mapping images (Fig. 2g) with respect to Mn, O and C show that these elements are abundant and well-distributed throughout the nanoflower, suggesting that the MnO₂ grows uniformly on the GDYO surface. Similar results were obtained in the MnO₂@10GDYO, MnO₂@20GDYO, and MnO₂@100GDYO hybrids.

The crystallographic structures of the MnO₂@GDYO hybrid 3D nanoflowers were confirmed by XRD. As shown in Fig. 3a and Fig. S7 (ESI†), all of the diffraction peaks can be well indexed to pure phase birnessite-MnO₂ (JCPDS 18-0802) and GDYO. The peaks at 12.1°, 24.7°, 36.5°, 53.2° and 65.6° are related to the (002), (004), (006), (301) and (119) planes of birnessite-MnO₂, respectively. GDYO exhibits a broad diffraction peak around 23°, which is ascribed to the typical interlayer distance of the GDY material.^37,38 In addition, the XRD patterns for the MnO₂@GDYO hybrid 3D nanoflowers present less noticeable peaks for MnO₂ and a high peak for GDYO, showing the combination of MnO₂ and GDYO. It is noticeable that the peaks with a broad feature and low intensity are indicative of the nanoscale structure and low crystallinity of the MnO₂@GDYO hybrid 3D nanoflowers, which is in line with the HR-TEM observations. In particular, when the GDYO content reached 100 mg, the corresponding (002) and (004) diffraction peaks in the XRD pattern almost disappeared, and the lattice spacing of 0.48 nm was not observed in the HR-TEM image (Fig. S4f, ESI†).

To further characterize the composites, Raman analysis was carried out. As shown in Fig. 3b and Fig. S8 (ESI†), a sharp peak is seen at 648 cm⁻¹, which can be attributed to the symmetric Mn–O stretching vibrations of the MnO₆ octahedron in MnO₂, similar to those previously reported.³⁹ In addition, in the spectrum of the MnO₂@GDYO hybrid 3D nanoflowers, the prominent peaks of GDYO are seen at 1378 and 1589 cm⁻¹, corresponding to the defect-induced (D band) and graphitic-induced (G band) carbon structures, respectively, demonstrating the presence of MnO₂ and GDYO.

XPS measurement was carried out to investigate the chemical composition and the electronic states of the MnO₂ and MnO₂@GDYO hybrid 3D nanoflowers. The existence of elemental C, O, and Mn signals are obvious from the full-scan XPS spectrum (Fig. 3c), suggesting the presence of manganese oxide and carbon materials. The C 1s peak of the MnO₂@50GDYO hybrid 3D nanoflowers can be deconvoluted into four subpeaks of CC (sp²), CC (sp), C–O, and CO at 284.4, 285.0, 286.2, and 288.8 eV, respectively (Fig. 3d), consistent with the expected chemical composition of GDYO.⁴⁰ The deconvoluted O 1s spectrum shows that five components appear at 529.65, 530.77, 531.8, 532.51 and 533.5 eV (Fig. 3e), which are in turn indexed to Mn–O–Mn bonds for the tetravalent oxide, Mn–O–H bonds for a hydrated trivalent oxide, CO bonds for the carboxyl groups on the surface of GDYO, H–O–H bonds for residual water and –OH bonds for the hydroxyl groups on the surface of GDYO, respectively. The peak splitting of the doublet in the Mn 3s core-level spectrum is about 5.04 eV, suggesting that Mn⁴⁺ is dominant in the MnO₂@50GDYO hybrid 3D nanoflowers. Moreover, the shoulder peaks of Mn 2p in Fig. 3g are characteristic of Mn 2p_1/2 (653.9 eV) and Mn 2p_3/2 (642.2 eV), agreeing well with the typical spin energy separation of 11.7 eV for MnO₂.⁴¹ The same results for MnO₂@10GDYO, MnO₂@20GDYO and MnO₂@100GDYO have been obtained and are presented in Fig. S9 (ESI†). The results clearly confirm that the MnO₂@GDYO hybrid 3D nanoflowers were successfully prepared.

TGA was used to study the content of GDYO in the MnO₂@GDYO hybrid 3D nanoflowers (Fig. 3h and Fig. S10, ESI†). The weight loss below 240 °C is derived from the removal of adsorbed water, and the subsequent decrease originates from the combustion of GDYO to CO₂.¹⁶ For MnO₂, a weight loss of about 5% is observed in the range of 240–500 °C, which is attributed to the phase transformation of MnO₂.⁴² By calculation, we estimated that the mass percentage of GDYO in the MnO₂@50GDYO hybrid 3D nanoflowers is about 10.53%. The results for the GDYO mass percentage obtained in the MnO₂@10GDYO, MnO₂@20GDYO and MnO₂@100GDYO hybrid 3D nanoflowers are about 3.79%, 5.91%, 16.92%, respectively.

Fig. 3i shows the nitrogen adsorption–desorption isotherm of the MnO₂@50GDYO hybrid 3D nanoflowers, in which the remarkable hysteresis loop in the intermediate P/P₀ range illustrates a unique mesoporous texture and the BET specific surface area is as high as 204.83 m² g⁻¹, exceeding that of the pure MnO₂ (77.93 m² g⁻¹). The measured value is higher than those reported for MnO₂ prepared using other synthetic routes (Table S1, ESI†). The samples with different GDYO contents all showed similar hysteresis loops with mesoporous dominant structural characteristics, and the BET specific surface areas are calculated as 81.85 (MnO₂@10GDYO), 130.53 (MnO₂@20GDYO), and 128.30 m² g⁻¹ (MnO₂@100GDYO) (Fig. S11 and S12, ESI†). In particular, the use of excessive GDYO resulted in agglomeration, a smaller specific surface area and a larger volume expansion for MnO₂@100GDYO (Fig. S13, ESI†). It is believed that the mesoporous architecture and the large specific surface area of MnO₂ can not only promote the swift diffusion, transportation, and intercalation of electrolyte ions throughout the electrode, but also accommodate the volume change of MnO₂ during the charging–discharging processes.⁴³

To investigate the electrochemical performance of the MnO₂@GDYO hybrid 3D nanoflowers as a cathode for aqueous ZIBs, coin-type cells were assembled using Zn foil as the anode. Fig. 4a exhibits the CV curves of MnO₂ and MnO₂@GDYO electrodes at a scan rate of 0.1 mV s⁻¹. There are two couples of redox reactions, indicating that the ZIBs undergo a two-step electrochemical process. It should be noted that GDYO makes no capacity contribution to zinc ion storage as shown in Fig. S14a and b (ESI†). Fig. 4b presents the rate performances tested at different current densities, and corresponding cycle profiles are given in Fig. 4d and Fig. S14c–f (ESI†). In addition to MnO₂@100GDYO, MnO₂ and other MnO₂@GDYO electrodes have an approximate specific capacity at 0.1C, indicating that the excess GDYO actually reduces the specific capacity of the battery. Although MnO₂@100GDYO cathode makes a higher capacity retention rate, it comes at the cost of specific capacity. In the process of developing electrode materials, the specific capacity should be considered as well as the rate performance. When the GDYO amount is increased to 50 mg, both the specific capacity and the rate capability stand out among the five samples. Therefore, the optimized hybrid MnO₂@50GDYO is used for the following study. As shown in Fig. 4b, the MnO₂@50GDYO cathode delivers the capacities of 265.1, 272.3, 273.8, 243.0, 204.3, 127.4, and 80.6 mA h g⁻¹ at discharge–charge rates of 0.1, 0.2, 0.5, 1, 2, 5 and 10C (based on the active mass of MnO₂), respectively. More importantly, when the current rate switches back to 1C, the capacity recovers to 253.7 mA h g⁻¹, which is 95.7% of the value at 0.1C. The high-rate capability is not only superior to the corresponding values for the control electrodes as displayed in Fig. 4c, but is also comparable to those of known MnO₂ cathode materials reported previously (Table S2, ESI†). This outstanding rate performance can be understood based on the EIS results of MnO₂ and MnO₂@GDYO electrodes, and the resulting Nyquist plots were fitted by the inset equivalent circuit (Fig. S15a, ESI†). The high frequency semi-circle corresponds to the charge-transfer resistance (R_ct), the low-frequency inclined line stands for the Warburg impedance (Z_w) and R₀ is the series resistance of the electrochemical system. The detailed EIS information is shown in Table S3 (ESI†). According to the results, the MnO₂@GDYO electrodes exhibit significantly smaller R₀ and R_ct values, and a lower Warburg diffusion resistance (the steeper slope). As a consequence, the introduction of the GDYO in the in situ grown MnO₂ can effectively enhance the charge-transfer ability, and thereby accelerate the electrode reaction kinetics for a high rate performance. The excellent rate performance also can be attributed to the 3D flower-like MnO₂@GDYO nanostructures, which provide a high specific surface area for more active sites and a unique mesoporous nanotexture for ion-diffusion channels as well as the porous nature of GDYO. The specific capacity could return to the initial level when the current density is switched back to a current rate of 1C, again confirming the high reactivity and reversibility of the MnO₂@GDYO cathodes. In addition to the excellent rate capability, the MnO₂@GDYO cathodes also possesses remarkable cyclability. Fig. 4e compares the cycling performance of MnO₂ and MnO₂@GDYO cathodes at 1C. Dramatic capacity fading can be clearly observed from the cycling curves of MnO₂ in the initial cycles. The MnO₂@GDYO cathodes all have an excellent cycle stability, but the presence of excessive GDYO will reduce its specific capacity, for example, the specific capacity of MnO₂@100GDYO hybrid is only 215 mA h g⁻¹ at 1C. The higher specific capacity of the MnO₂@GDYO cathodes is due to the larger specific surface area and higher number of active sites of the MnO₂@GDYO hybrid 3D nanoflower material. Serious aggregation of MnO₂@100GDYO hybrid leads to a decrease in specific surface area, which in turn leads to a decrease in the specific capacity. The structural stability of the MnO₂@GDYO cathodes was strengthened by the ingenious in situ induction growth strategy, and the cycling and rate properties of the MnO₂@GDYO cathodes were significantly improved. Of the five electrodes, the MnO₂@50GDYO hybrid clearly exhibits the best cycling performance (239 mA h g⁻¹ after 50 cycles, a capacity retention ratio of 99.2% at 1C), even after cycling for over 150 cycles (Fig. S16, ESI†). The long-term cycling stability with quite a high current density of 5C over 1000 cycles shows a remarkable capacity retention of 77.6% and coulombic efficiency approaching 100% (Fig. 4f). After the discharge–charge testing, postmortem analysis shows that the electrode retains the nanoflower morphology, indicating the strong structural stability of the MnO₂@GDYO hybrids (Fig. S17, ESI†).

The fundamental energy-storage mechanism of the MnO₂@GDYO cathodes, using MnO₂@50GDYO as an example, during electrochemical cycling is investigated by ex situ XRD, SEM and XPS measurements to unveil the phase and microstructural evolution. Fig. 5a shows the typical charge–discharge curves for the initial two cycles marked with different states from A to I. During the initial discharging (A–B–C), the corresponding XRD patterns exhibit diffraction peaks at 19° and 35°, which belong to MnOOH (JPCDS 18-0804) (Fig. 5b and Fig. S18, ESI†), formed because of the reaction of proton insertion into manganese dioxide, H⁺ + MnO₂ + e⁻ → MnOOH. The gradual depletion of H⁺ would lead to an enrichment of OH⁻ surrounding the cathode and thus the subsequent OH⁻ react with ZnSO₄ and H₂O in the aqueous electrolyte, 6OH⁻ + 4Zn²⁺ + SO₄²⁻ + 3H₂O → Zn₄SO₄(OH)₆·3H₂O.^44–46 From this, a new phase with characteristic 2θ peaks at around 10°, 21°, 22°, 26°, 28°, and 59° appear in the discharge plateau, which can be assigned to zinc sulfate hydroxide hydrate (Zn₄SO₄(OH)₆·3H₂O, JPCDS 39-0689). With deep discharging to 1.0 V (C and G), the peak intensity of Zn₄SO₄(OH)₆·3H₂O increases, indicating the growth of the Zn₄SO₄(OH)₆·3H₂O structure. As displayed in Fig. 5c and Fig. S18 (ESI†), the ultrathin nanosheets evolve into large size micro-sheets and a white substance can be observed on the electrode surface. Compared with the original state, a new diffraction peak emerges at 33°, which depicts the formation of ZnMn₂O₄ (JCPDS 24-1133). This can be attributed to the insertion of Zn²⁺ into the MnO₂ tunnels, accompanied by the valence decrease of positive quadrivalent manganese (Fig. 5d and Fig. S19, ESI†), Zn²⁺ + 2MnO₂ + 2e⁻ → ZnMn₂O₄. HR-TEM EDS mapping of the MnO₂@50GDYO cathode in a fully discharged state offers further proof of the insertion of Zn²⁺, while the Zn signal is seen uniformly everywhere (Fig. 5e). In the reverse charging process (C–D–E, G–H–I), the extraction of H⁺ and Zn²⁺ from the electrode could result in the dissolution of the Zn₄SO₄(OH)₆·3H₂O phase. Both the diffraction peaks of the Zn₄SO₄(OH)₆·3H₂O phase and the micro-sheets disappear, revealing a reversible precipitation–dissolution of the Zn₄SO₄(OH)₆·3H₂O structure. A reversible precipitation–dissolution process can be maintained even after long-term cycle testing, as shown in Fig. S20 (ESI†). Therefore, the energy-storage mechanism is a co-insertion/extraction process of Zn²⁺ and H⁺ into MnO₂ accompanied by the precipitation–dissolution of the Zn₄SO₄(OH)₆·3H₂O phase.

Conclusions

In summary, a novel in situ induced growth strategy has been developed to construct 3D nanoflower MnO₂@GDYO hybrids, which not only have the advantages of the 2D GDYO porous structure but also show characteristics of the 3D nanostructure. The combined properties of large surface area, enhanced charge-transfer kinetics and superior structural integrity are achieved in the MnO₂@GDYO hybrid 3D nanoflowers. The MnO₂@GDYO hybrid 3D nanoflowers have been fabricated as cathodes in aqueous ZIBs. The aqueous ZIBs of MnO₂@50GDYO exhibit a high reversible capacity of 265.1 mA h g⁻¹ at 0.1C, an excellent rate capability of 253.7 mA h g⁻¹ at 1C and 80.6 mA h g⁻¹ at 10C, and decent cycling stability over 1000 cycles at 5C (with a capacity retention of 77.6% and a coulombic efficiency approaching 100%). The results demonstrate a novel perspective and idea on constructing advanced hybrid materials for high-performance aqueous zinc-ion batteries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the National Key Research and Development Project of China (2016YFA0200104 and 2018YFA0703501) and the National Nature Science Foundation of China (21790050, 21790053, 21875258, 22071251).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00548k

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