δ-MnO2 nanoflower/graphite cathode for rechargeable aqueous zinc ion batteries

Manganese oxide (MnO2) is one of the most promising intercalation cathode materials for zinc ion batteries (ZIBs). Specifically, a layered type delta manganese dioxide (δ-MnO2) allows reversible insertion/extraction of Zn2+ ions and exhibits high storage capacity of Zn2+ ions. However, a poor conductivity of δ-MnO2, as well as other crystallographic forms, limits its potential applications. This study focuses on δ-MnO2 with nanoflower structure supported on graphite flake, namely MNG, for use as an intercalation host material of rechargeable aqueous ZIBs. Pristine δ-MnO2 nanoflowers and MNG were synthesized and examined using X-ray diffraction, electron spectroscopy, and electrochemical techniques. Also, performances of the batteries with the pristine δ-MnO2 nanoflowers and MNG cathodes were studied in CR2032 coin cells. MNG exhibits a fast insertion/extraction of Zn2+ ions with diffusion scheme and pseudocapacitive behavior. The battery using MNG cathode exhibited a high initial discharge capacity of 235 mAh/g at 200 mA/g specific current density compared to 130 mAh/g which is displayed by the pristine δ-MnO2 cathode at the same specific current density. MNG demonstrated superior electrical conductivity compared to the pristine δ-MnO2. The results obtained pave the way for improving the electrical conductivity of MnO2 by using graphite flake support. The graphite flake support significantly improved performances of ZIBs and made them attractive for use in a wide variety of energy applications.

the charge transport during the electrochemical redox process. In this respect, various carbonaceous materials, namely activated carbon, carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene and graphite, have been integrated with MnO 2 . MnO 2 /graphene nanoflowers were synthesized in the form of sandwich-structured nanoflowers which exhibited excellent super capacitive properties effectively making a very conductive electrode material for high-performance super capacitors 22 . However, it is significant that MnO 2 supported on graphite has not been reported previously in ZIBs application. The crystal structure of graphite consists of parallel planes of carbon atoms which is conductive primarily along its planes 23,24 . In this way, graphite is classified as a semimetal due to its high electrical conductivity 25 . Therefore, MnO 2 supported on graphite is considered as the candidate due to an improvement in electronic conductivity and an increase in the stability of the electrode materials for ZIBs.
The present study reports on δ-MnO 2 nanoflower/graphite as a cathode host material for rechargeable aqueous ZIBs. The δ-MnO 2 nanoflower supported on graphite not only increases the electrical conductivity and discharge capacity of the battery but also improves the insertion/extraction acceleration by increasing the active area of the δ-MnO 2 nanoflower. Accordingly, the electrochemical properties and performances of the batteries which use the MNG as host material cathode are examined and discussed.

Experimental
Chemical and materials. Reagent  solution, and continuous stirring followed for 30 min. Afterward, the mixture was transferred into a 100 mL Teflon autoclave and kept at 160 °C for 24 hr in an oil bath. The product was collected and washed with DI water several times. Then, it was dried at 80 °C for 12 hr. The MNG synthesis was similar to the method reported by Liu et al. 22 with some modifications. Graphite (1.0 g) was mixed in DI water (500 mL) with KMnO 4 (10.0 g). The mixture of graphite and KMnO 4 was stirred for 18 hr. Then, 98% of H 2 SO 4 (5 mL) was added dropwise into the mixture. The solution was continuously stirred for 1 hr and heated to and maintained at 80 °C for 1 hr. After that, the solution was diluted in 1 L of DI water and allowed to stand at room temperature. The solution was filtered using cellulose filter paper (pore size 11 µm). Then, the MNG precipitates were collected and washed by DI water several times until the violet color disappeared. Subsequently, the precipitates were dried at 80 °C overnight.
Characterization and electrochemical measurement. X-ray Diffraction (XRD, Bruker AXS Model D8 Discover) of the powder samples was carried out with Cu Kα radiation at a scanning range of 5-80°. Field Emission Scanning Electron Microscope (FESEM, JEOL JSM-7610F, Tokyo, Japan) was used to take the morphology image and nanoflower size of MNG.
The cathode using MNG was prepared by mixing together 70% wt. of MNG, 20% wt. of carbon black (CB), and 10% wt. of cellulose acetate binder. Alternatively, the cathode using pristine δ-MnO 2 was prepared by mixing together 70% wt. of the pristine δ-MnO 2 , 20% wt. of carbon black (CB), and 10% wt. of cellulose acetate binder. Acetone was used to adjust the viscosity of the slurries. Each mixed slurry was coated on graphite foil using a lab coating machine (AOT-FCM-250, AOT Electronic Technology Co., LTD) and dried at 70 °C under vacuum. The thickness of the cathode material deposited was 25 µm. The zinc anode was prepared by electrodeposition of zinc from ZnSO 4 (0.5 M) aqueous solution onto Ni-foam using zinc sheet as a counter electrode at the current density of 60 mA/cm 2 . The amount of zinc deposited was 20 mg/cm 2 . Both cathode and anode were punched into a 15 mm diameter disk. The filter paper was punched into a 19 mm disk and used as the separator. Then, 0.3 mL of ZnSO 4 (1 M) was added to the cell. The testing cells were fabricated as a coin cell (CR2032).
Electrochemical measurements were carried out using a CR2032 coin cell. Cyclic Voltammetry (CV) was performed by Potentiostat (VersaStat3, Princeton Applied Research) at a scan rate of 0.5 mV/s in the voltage range 1.0-1.8 V versus Zn 2+ /Zn. A battery testing system (BTS-5V10mA, Neware, China) was used to investigate the performance of the battery. The charge-transfer resistance, as illustrated by the Nyquist plots for the cathode, was carried out using an Electrochemical Impedance Spectroscopy (EIS) technique using an amplitude of 10 mV in the frequency range of 1-100,000 Hz.

Results and discussion
In this study, δ-MnO 2 nanoflower/graphite (MNG) was prepared by a modified method of Liu et al. 22 . In Fig. 1, the XRD patterns of δ-MnO 2 and graphite are shown. Thus, it can be seen that both patterns, i.e. δ-MnO 2 (JSPDS card no. 80-1098) 26,27 and graphite (JSPDS card no. 41-1487) 28 29 . The high-intensity diffraction peak of MNG at 26.5° indicates the high crystallinity of graphite. δ-MnO 2 possesses a planar layered-structure as illustrated by the inset of Fig. 1. This structure suggests that the interlayer gap is easy to insert/extract foreign cations and thereby can be useful for energy www.nature.com/scientificreports www.nature.com/scientificreports/ storage applications 27 . Further, the XRD analysis confirmed that the delta (δ) phase was presented in the pristine δ-MnO 2 sample (see Supplementary Information Fig. S1).
In Fig. 2(a), the FESEM image of MNG is shown. It is observed that the numerous MnO 2 nanoflowers constructed on the graphite surface reveal a flake-like sample. Figure 2(b) shows the higher magnification image of MNG which indicates that many petals can interconnect forming micropores about 50 nm in diameter size. The MNG having micropores among petals will assist in increasing the contact area between the electrolyte and cathode material as well as ensure fast ion transfer in the charge/discharge process 22 .
In Fig. 3, the battery configuration in this study which is composed of the MNG cathode, zinc anode, and ZnSO 4 aqueous electrolyte is shown. During discharging, anodic zinc is dissolved in the form of Zn 2+ ions into an aqueous electrolyte, containing Zn 2+ ions and rapidly solvate in the form of solvated Zn 2+ ion. Then, they diffuse and pass through the separator to the MNG cathode. The solvated Zn 2+ ions are de-solvated in the form of Zn 2+ ions and intercalate into δ-MnO 2 structure 30,31 as illustrated by the inset of Fig. 3. Further, an electron current starts to flow in the electrical loop from the electrical conduction of graphite. These three processes can be reversed by (1) the de-intercalation of Zn 2+ ions from MNG cathode; then (2) solvated species are formed and lastly (3) Zn 2+ ions are reduced to Zn and deposited back on the zinc anode, respectively. The electrochemical reaction may be expressed as in Eq. (1) anode reaction and Eq. (2) cathode reaction:   www.nature.com/scientificreports www.nature.com/scientificreports/ To investigate the kinetics of the MNG electrode, cyclic voltammograms using sweep rates of 0.5 to 32 mV/s in the voltage range 1.0-1.8 V versus Zn 2+ /Zn were measured using CR2032 coin cells. As shown in Fig. 4(a), a dominating pair of redox peaks exhibits increasing currents when the sweep rates increase, which do not display rectangular-shape and symmetrical voltammograms even at high scan rates, compared to MnO 2 /activated carbon composite for supercapacitors 32 . It is noted that the MNG electrode does not present the capacitive behavior of the electrode 33,34 . The capacitive effect is characterized by analyzing the cyclic voltammetry data at different sweep rates as in Eq. (3): where i is the peak specific current, v is a potential sweep rate and a, b are adjustable parameters. The redox reaction is limited by the diffusion-controlled behavior; the peak current i varies as v 1/2 . Although the capacitance contribution suggests that the peak current i varies as v 35,36 . Equation (3) can be taken with logarithm and can be expressed according to Eq. (4): The b value denotes the slope of the plot of lni versus lnv. When b value is close to 1, the system is mainly controlled by capacitance; when b value is close to 0.5, the Zn 2+ ion insertion process dominates. Figure 4  www.nature.com/scientificreports www.nature.com/scientificreports/ is a transition metal oxide that typically displays the pseudocapacitance behavior 37 . The capacitive-controlled process occurs only on the surface. However, in the case of MNG cathode, the characteristic of Zn 2+ ions insertion/extraction deviates from capacitive-controlled process towards the diffusion-controlled process. That is, the insertion/extraction of Zn 2+ ions occur not only on the surface but also the pores inside. The result shows good agreement of a fast Zn 2+ ion insertion/extraction or high rate property for the battery 35 .
In order to compare the improved electrochemical properties of MNG as the cathode for ZIB, the pristine δ-MnO 2 is used as a comparable cathode. Figure 5 (a) shows the CV profiles of the pristine δ-MnO 2 and MNG electrodes at a scan rate of 0.5 mV/s over the potential range 1.0-1.8 V for the initial three cycles. During the first cycle, two distinct peaks are observed at 1.20 and 1.57 V for the pristine δ-MnO 2 electrode and at 1.18 and 1.54 V for MNG. The peaks in the low potential region appear at 1.20 and 1.18 V which can be attributed to Zn 2+ ion insertion into the δ-MnO 2 host structure. In the higher potential region, two oxidation peaks can be seen at 1.57 and 1.54 V for δ-MnO 2 and MNG, respectively, which correspond to the extraction of Zn 2+ ions from the δ-MnO 2 host structure. The results suggest that, the oxidation state of Mn is reduced to Mn 3+ states and is oxidized back to Mn 4+ states, during Zn 2+ ion insertion and extraction, respectively 13,14,18,38 . On subsequent cycling, two distinct peaks appear at 1.37 and 1.21 V for Zn 2+ ion insertion into δ-MnO 2 whereas the peaks at 1.38 and 1.20 V were observed for MNG in the low voltage region. In the high voltage region, the peak at 1.57 and shoulder at 1.62 V can be clearly seen for the pristine δ-MnO 2 . Likewise, the peak at 1.53 V and shoulder at 1.60 V can be clearly seen for the MNG electrodes. The cyclic voltammogram, having two peaks during discharge and having a peak with shoulder during charge, exhibits typical characteristics of the electrochemical insertion/extraction of Zn 2+ ions in MnO 2 structure 12,13,39-41 . These results with two peaks during discharge may be more clearly described by the two-step reaction of Zn 2+ ion insertion in electrochemical reaction 12,38 and spinel-type ZnMn 2 O 4 transformation 33 . In the following scan cycles, the peaks at 1.37 V for δ-MnO 2 and at 1.38 for MNG increase gradually during discharge indicating an activation process 38 . The CV curve of MNG exhibits a higher peak intensity and a larger enclosed area when compared with the pristine δ-MnO 2 indicating improved electrochemical performance and fast Zn 2+ ion insertion/extraction 42 .  13 , the initial discharge capacity can deliver only 98 mAh/g (at 50 mA/g) and 122 mAh/g (at 83 mA/g), respectively. It is clear that δ-MnO 2 supported on graphite samples can accommodate more numbers of Zn 2+ ions than the www.nature.com/scientificreports www.nature.com/scientificreports/ unsupported δ-MnO 2 (pristine δ-MnO 2 ). It appears that the nanoflower δ-MnO 2 in the structure of MNG tend to enhance the electrode/electrolyte contact area, thereby favoring Zn 2+ ion insertion 41 . In addition, MNG shows a longer horizontal discharge curve than that of δ-MnO 2 , suggesting a more stable Zn 2+ ion insertion into the MNG than in the pristine δ-MnO 2 . On the subsequent cycle, during the continuous discharge, the voltage profiles present two distinct plateaus at 1.45 and 1.25 V for MNG and 1.4 and 1.2 V for δ-MnO 2 . These characteristics were also observed for MnO 2 electrodes in aqueous ZIB systems 14,43 . It can be implied that Zn 2+ ions can insert into the layered δ-MnO 2 39 which is in agreement with the two distinct peaks during discharge, as shown in Fig. 5(a). Figure 5(c) displays the cycling behavior and corresponding coulombic efficiency of the pristine δ-MnO 2 and MNG electrodes, under the specific current density of 400 mA/g. At the 100 th cycle, the discharge and charge capacities registered by δ-MnO 2 were 63.3 and 63.8 mAh/g, respectively while that of MNG were 113.4 and 114.2 mAh/g, respectively. The lower capacity retention of δ-MnO 2 may result from low intrinsic electronic conductivity because of the appearance of unstable Mn 3+ states during zinc-ion insertion 10 . The higher capacity retention of MNG may indicate that the electronic conductivity of MNG is improved. In the initial cycles, gradual capacity fade was observed for the pristine δ-MnO 2 electrode, but fast capacity fade was observed for the MNG electrode. The formation of ZnMn 2 O 4 with the Jahn-Teller Mn 3+ ion may contribute to electrode degradation and hence lead to the fast capacity fade 13 . However, MNG demonstrates higher capacity than the pristine δ-MnO 2 . Over long-term cycling, the coulombic efficiency of both host material electrodes is maintained at around 100%. Thus, this clearly demonstrates that no irreversible capacity losses occurred 41 .
In Fig. 5(d), the rate performances of the pristine δ-MnO 2 and MNG host material cathodes are shown. Cycling takes place at various specific current densities of 200, 400, 800 and 1600 mA/g, namely 5 times for each rate. The rate performance of MNG is significantly higher than those of the pristine δ-MnO 2 . It is indicated that nanoscale morphology of δ-MnO 2 nanoflowers on graphite increases the contact area between the electrode and the electrolyte and provides more electrochemically active sites for ion-insertion 13 . Graphite not only improves the electronic conductivity of the MNG electrode but also tends to disperse the δ-MnO 2 nanoflower sites. The MNG cathode can be charged and discharged at different rates; a high rate of 1600 mA/g leads to a discharge and charge capacity of 76 and 64 mAh/g, respectively. When cycled at a specific current density of 200 mA/g, the MNG cathode can deliver a discharge and charge capacity of 181 and 179 mAh/g, respectively. This behavior indicates that the MNG cathode can well be considered for the Zn 2+ ion storage material 10 . It is clear therefore that MNG can improve not only the cycling performance but also the rate performance for ZIBs.
As displayed in Fig. 6(a), the difference in electrochemical conductivity before cycling between the pristine δ-MnO 2 and MNG host cathodes was examined using EIS. The curves of the pristine δ-MnO 2 and MNG host cathodes consist of depressed semicircles and diffusion drift which can be perfectly fitted using Randles www.nature.com/scientificreports www.nature.com/scientificreports/ equivalent circuit (see Inset Fig. 6(a)). Hence, in the equivalent circuit, R s is the solution resistance, R ct is the charge-transfer resistance at the interfaces and Z w is the Warburg impedance related to the diffusion of Zn 2+ ions. The R ct value for the pristine δ-MnO 2 is 5.9 Ω and the value reduces to 4.8 Ω when supported on graphite (MNG). The result indicates that the electrical conductivity of the MNG sample is improved by constructing a conductive support using the graphite. The relationship between real impedance (Z′) and angular frequency (ω) in the low frequency region can be expressed accordingly by Eq. (5) 44 : where σ is the Warburg factor which is relative to Z′-ω obtained from the slope of the lines in Fig. 6(b). The diffusion coefficient of zinc ion can be calculated as in Eq. (6) 45 : where R is the gas constant, T is the absolute temperature, n is the number of electrons per molecule oxidized, A is the surface area, F is Faraday's constant, C is the concentration and D is the diffusion coefficient. As shown in Fig. 6(b), the slope which is the σ value of MNG (3.94) host electrode is lower than that of the pristine δ-MnO 2 (9.01) indicating that the diffusion coefficient of MNG is higher than that of the pristine δ-MnO 2 , as in Eq. (6). It is clear that the MNG host material electrode can enhance the diffusion coefficient of Zn 2+ ion, highlighting the electrical conductivity improvement 44,46 .
In Fig. 6(c), the MNG cathode which is a δ-MnO 2 structure supported on graphite is drawn and illustrated. It is highly possible that Zn 2+ ions can insert into the δ-MnO 2 nanoflower structure having short diffusion length. Thereby, electrochemical performance can be improved during cycling at high rate 47 .
In Fig. 6(d), after charge/discharge cycling, the EIS measurements of MNG are shown. After the 1 st , 10 th , 20 th , and 30 th cycles, the MNG host cathode exhibits the R ct values, namely 6.51, 10.9, 13.7 and 18.2 Ω, respectively. After the first cycle, the charge-transfer resistance increases which indicates that the intercalation of Zn 2+ ions into the δ-MnO 2 structure becomes more difficult. The conduction of ions before intercalation depends not only on the cathode material but also on the electrolyte access into the cathode. The porosity of the cathode material is an important factor which can affect the electrolyte access. The SEM image of MNG and the pristine δ-MnO 2 compound is displayed in the Supplementary Information Fig. S2.

Conclusion
In summary, δ-MnO 2 nanoflower supported on graphite flake was synthesized and used as an intercalation host material for a rechargeable ZIB. While the XRD studies confirmed the crystallographic structure, the FESEM studies revealed that the sample showed a nanoflower-like morphology having micropores about 50 nm in diameter. This study demonstrates that a diffusion process controlled the electrochemical reactions of the MNG cathode. When tested in ZIB, the MNG sample registered a prominent discharge capacity of 235 mAh/g, which was higher than the discharge capacity of pristine δ-MnO 2 (130 mAh/g). After the 100 th cycle, the discharge capacity registered by the δ-MnO 2 was only 63.3 mAh/g whereas the MNG delivered a discharge capacity of 113.4 mAh/g. Moreover, the conductivity of the supporting graphite improved the conductivity by reducing the charge transfer resistance of the cathode materials. The present results concluded that the supporting graphite not only improved the electrical conductivity but also enhanced the specific capacity and the cycling performance of the pristine δ-MnO 2 . Thus, MNG is one of promising candidate cathode materials for ZIBs.