Introduction

Currently, the growing demand for electric vehicles and portable devices has accelerated the development of the battery industry [1]. Due to the limited resources and safety concerns of the mainstream lithium-ion batteries (LIBs) in the battery industry [2], zinc-ion aqueous batteries (ZIBs) possess extensive research significance to become the successor of LIBs due to their safety and large resource storage capacity [3]. ZIBs are a promising new battery system due to their low cost, high operational safety, and environmental friendliness [4, 5]. Zn anodes with a relatively high capacity of 820 mAh g−1, along with excellent chemical stability in neutral and weakly acidic aqueous electrolytes, are a definite advantage for aqueous ZIBs based on two-electron redox reactions [6,7,8,9,10]. In addition, Zn2+ possesses a small ionic radius that facilitates the dissolved deposition of Zn2+ ions [11].

Among the numerous cathode materials, including manganese-based oxides [12,13,14], vanadium-based oxides [15,16,17,18] and Prussian blue analogues [19,20,21] have been developed as promising candidates for ZIBs. Among these, MnO2 cathode material is the most promising candidate due to the high working voltage (~1.5–1.6 V) and high specific capacity (300–500 mAh/g) of it, achieving the high energy density of the full battery [22]. So far, the reversible Zn2+/H+ insertion mechanism in the MnO2 cathode is widely accepted, and it is inevitable that the MnO2 cathode material will undergo a serious phase transition from the initial structure designed during the charging and discharging process. As a result, these phase transitions during the cycle may cause structural collapse and capacitance decay of the electrode. Various strategic approaches, such as cationic doping (e.g., Mg2+ and V5+), crystal shape regulation (e.g., nanosheets and nanoparticles), and conductive hybridization (e.g., graphene and polyaniline), have been developed to improve storage performance and mitigate capacity decay [23,24,25,26].

Despite these achievements, it is found that there exists a dissolution and redeposition mechanism in the positive electrode of MnO2, and the structure stability of MnO2 suffers from the dissolution and redeposition of it during charge and discharge, which results in the inevitable problem of specific capacity decay [27]. To solve or alleviate these problems, it is necessary to form the overall cathode a porous structure to provide channels for the transfer of the Mn2+ and Zn2+, and also a specific surface area that facilitates the redeposition of MnO2. δ-MnO2 has inherent characteristics such as large interlayer spacing in its layered structure, which is beneficial to the transfer of ions as an active material in batteries. Herein, an accommodational space cathode (named AS cathode) with the structure of accommodation space was constructed by obtaining a simple method of dissolving part of the nano ZnO template.

Experimental

Preparation of MnO2

A concentrated 1 ml of H2SO4 was added drop by drop into the 500 ml 0.02 M KMnO4 solution which then take 0.015 ml of MnSO4 dissolved in 500 ml of distilled water and stir well. Under stirring, the dissolved KMnO4 solution and MnSO4 solution are poured together into a 1 L beaker to react for 2 h and precipitate for 12 h.

Preparation of AS cathode

The obtained MnO2 and nano ZnO, conductive carbon (super P) and PVDF, are ground evenly in a ratio of 7:4:2:1, and the material is spread flat on the steel foil with NMP as dispersant and dried at 120 °C for 12 h to obtain the cathode with template (named as Templated cathode). The nano ZnO particles in Templated cathode were removed partially by immersed and washed by 0.01 mol/L HCl for 6 h. The treated cathode was repeatedly washed with pure water and dried for 12 h to have obtained AS cathode. A control MnO2 cathode the same mass ratio containing MnO2 and no nano ZnO template was obtained using the same procedure as above (named as Regular cathode).

Material characterization

The synthesized MnO2 was characterized and analyzed by an X-ray diffractometer (XRD, Rigaku D). A scanning electron microscope (SEM Hitachi S4800) was used to observe the surface and cross-sectional morphology of the cathode before and after de-template to determine the structural morphological changes. The chemical states of the elements in cathode after cycling were characterized by X-ray electron spectrometry (XPS, Thermo ESCALAB).

Electrical properties characterization

The homemade Swagelok cells which were assembled by the obtained cathodes, nylon mesh separator, zinc foil anode and 2 M ZnSO4 + 0.1 M MnSO4 electrolyte were used to measure the storage capacity of Zn2+ at a temperature of 25 °C for the testing of ZIB cells, respectively. The electrochemical charge/discharge tests were performed at room temperature using a CT 2001A system. The cells were tested with a voltage window of 1.0–1.8 V vs. Zn2+, Zn [28]. An electrochemical workstation (CHI 660E) was used to obtain cyclic voltametric (CV) curves, keeping the test voltage window constant.

Experimental results and discussion

Morphology and structure

The schematic diagram of the preparation process of MnO2 and AS cathode is shown in Fig. 1. This method of preparing Nano-to-micron-sized MnO2 under normal temperature and pressure conditions is very simple and can produce MnO2 in large quantities. The way to prepare the AS cathode is to directly dissolve the template on the conventional coated cathode sheet, which is an attempt based on the process of industrial production of batteries

Fig. 1
figure 1

The schematic diagram of the preparation process of MnO2 and AS cathode

Full size image

Figure 2a shows the composition and lattice morphology of the obtained MnO2 as observed by XRD. Notably, distinct broad diffraction peaks appear at 2θ = 12.2°, 24.8°, 37.4°, 42.2°, 66.9°, which match well with the (003), (006), (101), (015) and (110) facets of JCPDF no.86-0666, indicating the generation of single-phase δ-MnO2. Figure 2b, c show the SEM images of the surface of cathode before and after de-template. It can be observed that compared to the Templated cathode, the nano-sized ZnO can no longer be observed and many pore structures clearly appear in the surface of the AS cathode. It should be noted that the de-template does not collapse the original structure, and a solid structure with a high specific surface area of pore cathode material appears. SEM images show an obvious intricate cavity structure in the cross-section of the AS cathode, and at the same time, there are also dense and non-porous flat planes caused by cutting the cathode. The EDS mapping (Fig. 2d, e) reveals that Mn and C are uniformly distributed, which also contain traces of Zn residues (only 0.34% of the total electrode mass).

Fig. 2
figure 2

a XRD curves; b, c SEM images before and after de-template; d SEM images of the cross-section and corresponding mapping images of the AS cathode; e EDS of the AS cathode

Full size image

Electrochemical performance

The CV curves of the AS cathode at a scan rate of 0.1 mV/s for 5 cycles were studied. As shown in Fig. 3a, the same anodic peak at 1.56 V was presented in all 5 cycles. The cathode was observed to show two reduction peaks around 1.25 V and 1.38 V during the scans, which are mainly considered as the dissolution of MnO2 i.e. Mn4+ (s) → Mn 2+(l) [16]. The CV plot is a stepwise redox process, which is typical of a MnO2 cathode. Of particular interest is the clear indication of increasing voltametric current as the redox peak was cycled, indicating a gradual electrochemical activation. The specific capacity versus voltage at different current densities is shown in Fig. 3b, which can provide a reversible capacity of 456 mAh g−1 at 0.2 A g−1. As the current density increases to 0.4 A g−1, 0.6 A g−1, 0.8 A g−1, 1.0 A g−1, and 2.0 A g−1, the specific capacity is maintained at 309.1 mAh g−1, 236.8 mAh g−1, 190.1 mAh g−1, 159.3 mAh g−1, and 99.7 mAh g−1, respectively, showing excellent rate performance.

Fig. 3
figure 3

a CV curves; b Constant current charge/discharge curves at different current densities of the AS cathode

Full size image

Figure 4a shows the discharge-specific capacities of the AS cathode, the Regular cathode, and the Templated cathode at different rates. At a current rate of 0.2 A g−1, the discharge specific capacity of the AS cathode reached 540.6 mAh g−1, which is much higher than that of the Regular cathode (406.2 mAh g−1) and Templated cathode (250 mAh g−1). And, the discharge-specific capacity of the AS cathode was generally higher than that of the other cathodes at different rates. Figure 4b shows the cycling performance of these three cathodes at a high circuit density of 1.0 A g−1. The specifical capacities of Regular cathode and Templated cathode rapid decay from 243.1 mAh g−1 to 87.3 mAh g−1 and 189.3 mAh g−1 to 62.6 mAh g−1in the first 100 cycles respectively; in contrast, the cycle performance of the AS cathode decreases from 289.5 mAh g−1 to 190 mAh g−1, which is obviously better than the data for the other two cathodes. This indicates that the accommodation space has a significant effect on improving the discharge-specific capacity and cycle performance of the MnO2 cathode for ZIBs. It is worth noting that the positive effect of ZnO on water zinc ion battery is excluded by the multiplier and circulation of Template cathode.

Fig. 4
figure 4

a Rate performance, and b long-term cycling stability at 1.0 A g−1of the AS cathode

Full size image

The AS cathode after charge/discharge cycles was further investigated by XPS to determine the elemental composition of the cathode material. Figure 5a evaluates the full spectrum, the peaks corresponding to elements of Mn, O, C and Zn can be clearly observed. The Mn 2p spectrum is shown in Fig. 5b, and the two curves in Mn 2p 3/2 and Mn 2p 1/2 indicate the presence of trivalent Mn ions and tetravalent Mn ions, respectively. The two peaks appearing in the O 1s spectrum (Fig. 5c) confirm the presence of Mn-O-Mn and H-O-H, respectively [29, 30]. Figure 5d shows the Zn 2p spectrum with two peaks attributed to Zn 2p 3/2 and Zn 2p 1/2 respectively, has shown that the Zn2+ enters the cathode material during the charging and discharging process.

Fig. 5
figure 5

a XPS spectra of pored cathode, b Mn 2p; c O 1s and d Zn 2p after cycles

Full size image

Figure 6a shows the CV results at different rates. The current characteristic curve of the redox peak (i, mA) and the scan rate (v, mV s−1) at a certain potential conform to the following equation [18, 31].

$$i={av}^b$$
(1)
Fig. 6
figure 6

a CV curves at different scan rates, and b linear fit of log (i) versus log (v) to the redox peaks of the AS cathode

Full size image

Furthermore, the relationship i = aνb can be divided into two parts including capacitive (k1v) and diffusion-limited effects (k2v1/2), as described below

$$i={k}_1v+{k}_2{v}^{1/2}$$
(2)
$$i/{v}^{1/2}={k}_1{v}^{1/2}+{k}_2$$
(3)

where a and b are empirical values, while b = 0.5 indicates the diffusion control electrochemical behavior and b = 1.0 indicates surface capacitance process control. Figure 6b shows a linear fit of log(i) versus log(v) for the redox peaks. Of note is the high b value of 0.65 for the cathode peak, indicating a joint control of diffusion and surface capacitance process, this is due to the adsorption of SO42− on the surface of the cathode while oxidizing Mn2+ during the cathodic process. The anode peak has a b-value of 0.48, which indicates diffusion-controlled behavior.

The ion diffusion coefficients of redox reactions were investigated using GITT [32, 33]. Figure 7a, b shows the voltage characteristic curves and the corresponding ion diffusion coefficients of the different battery, respectively. The chemical diffusion coefficient of Zn2+ can be calculated based on the following equation:

$${D}_{GITT}=\frac{4}{\pi }{\left(\frac{m_B{V}_m}{m_BS}\right)}^2{\left(\frac{\Delta {E}_s}{\uptau \left({dE}_{\uptau}/d\sqrt{\uptau}\right)}\right)}^2$$
(4)
$$\approx \frac{4}{\pi \uptau}{\left(\frac{n_m{V}_m}{S}\right)}^2{\left(\frac{\Delta {E}_s}{\Delta {E}_{\uptau}}\right)}^2\left(\uptau \ll \frac{L^2}{D_{GITT}}\right)$$
(5)

where τ is the relaxation time, mB is the mass of the MnO2, MB, Vm and nm are shorts for the mass (g), molecular weight (g/mol), molar volume (cm3/mol) and molal number, respectively. The S is the actual area of the cathode, \(\frac{dE_{\uptau}}{d\sqrt{\uptau}}\) is the slope of the linearized region of the potential Eτ during the current pulse of duration time τ, ΔEs is the difference in the open circuit voltage measured at the end of the relaxation period for two successive steps, and L is the thickness of the cathode.

Fig. 7
figure 7

a GITT curves of different electrodes in discharge state, and b corresponding ion diffusion coefficients

Full size image

Clearly, it is verified that the reaction kinetics of AS cathode is not better than that of Regular cathode and Templated cathode. This is mainly since the MnO2 used in the three electrodes is the same. The GITT results prove that the charge-discharge performance of the AS cathode is better than that of the other two cathodes, mainly because the space created after de-template provides an additional place for the deposition of Mn2+. It has been pointed out earlier that, the MnO2 cathode has a Mn2+ dissolution-deposition mechanism during cycling, especially at the back end of charging and discharging. And the Mn2+ originally contained in the electrolyte will be oxidized to MnO2 during charging, which contributes to the improvement of the capacity. The innovation point of our work is that, since Mn2+ will eventually be deposited on the surface of the cathode material, the cathode considers the role played by conductive carbon in the charging and discharging process of the battery. On the other hand, the overall design of porous structure for the cathode material increases the specific surface area of the cathode material, which increases the utilization rate of Mn2+ while improving the channel for the dissolution redeposition process of it. Thus, the effect of increasing the specific capacity of battery discharge and mitigating the decay of cycle performance is achieved.

Conclusion

In summary, AS electrodes were successfully prepared by template method, which have been successfully prepared for use as an excellent strategy in MnO2 cathode ZIBs. AS electrodes which were constructed for ionic dissolution deposition channel have high discharge capacity and good rate capacity at 0.2 A g−1. More importantly, the cell demonstrated excellent cycle durability after 500 cycles, outperforming conventional and template electrodes in hybrid electrolyte of 2.0 M ZnSO4 + 0.1 M MnSO4. It is believed that the current work will provide a new design strategy for ZIBs cathodes for commercial production of high capacity and long life ZIBs.