Binder-Free α-MnO2 Nanowires on Carbon Cloth as Cathode Material for Zinc-Ion Batteries

Recently, rechargeable zinc-ion batteries (ZIBs) have gained a considerable amount of attention due to their high safety, low toxicity, abundance, and low cost. Traditionally, a composite manganese oxide (MnO2) and a conductive carbon having a polymeric binder are used as a positive electrode. In general, a binder is employed to bond all materials together and to prevent detachment and dissolution of the active materials. Herein, the synthesis of α-MnO2 nanowires on carbon cloth via a simple one-step hydrothermal process and its electrochemical performance, as a binder-free cathode in aqueous and nonaqueous-based ZIBs, is duly reported. Morphological and elemental analyses reveal a single crystal α-MnO2 having homogeneous nanowire morphology with preferential growth along {001}. It is significant that analysis of the electrochemical performance of the α-MnO2 nanowires demonstrates more stable capacity and superior cyclability in a dimethyl sulfoxide (DMSO) electrolyte ZIB than in an aqueous electrolyte system. This is because DMSO can prevent irreversible proton insertion as well as unfavorable dendritic zinc deposition. The application of the binder-free α-MnO2 nanowires cathode in DMSO can promote follow-up research on the high cyclability of ZIBs.


Introduction
Nowadays, due to the increasing use of energy in modern society and intensifying degrees of electrification, rechargeable batteries are in great demand. In particular, lithium-ion batteries (LIBs) are versatile and have a wide range of applications, proving them to be the market leaders. However, LIBs have several shortcomings, such as safety issues, recycling, and especially their high cost and limited resources [1][2][3]. Therefore, alternative battery technologies using cheap and abundant materials such as sodium (Na) [4,5], aluminum (Al) [6,7], magnesium (Mg) [8][9][10], and zinc (Zn) [11][12][13] for electrodes are actively sought after in order to address these concerns. area and redox reaction sites in comparison to its bulk counterpart. Therefore, it is expected that MnO 2 with its one-dimensional (1D) nanostructure will show enhanced electrochemical performance.

Electrode and Battery Fabrication
The carbon cloth was surface treated with 1.0 M H 2 SO 4 for 1 h, washed with DI water several times, and vacuum dried at 60 • C for 2 h before usage.
In a typical experiment, 0.1264 g KMnO 4 and 0.0428 g (NH 4 ) 2 SO 4 were dissolved and mixed in 40 mL DI water. Then, the resulting solution was sonicated for 1 h and hydrothermally synthesized at 180 • C for seven days using a Teflon-lined autoclave decorated with carbon cloth on its inner wall. Next, the carbon cloth, deposited with MnO 2 particles, was washed with DI water several times, rinsed with IPA, and vacuum dried for at least 4 h before usage.
In a typical experiment, pre-cut Zn foil (Shandong AME Energy Co. Ltd., China) 15 mm in diameter and 0.08 mm thick was ultrasonicated in acetone for 30 min. Then, it was washed with distilled water several times and rinsed with IPA before vacuum drying for 2 h. Next, 0.1 M and 3.0 M aqueous Zn(OTf) 2 and 0.1 M Zn(OTf) 2 in DMSO were used as electrolytes to investigate the electrochemical performance of the nanowire α-MnO 2 -carbon fiber composite.
A CR2032 cell was used to assemble the ZIB. The assembled battery consisted of an anode (made up of Zn foil), a cathode (hydrothermally grown MnO 2 on carbon cloth), and an electrolyte (Zn(OTf) 2 dissolved in DI water or DMSO). The electrodes were separated with a glass microfiber (Whatman, Sigma-Aldrich, St. Louis, MO, USA), punched into a disc which was 19 mm in diameter, and enclosed within circular metal cases. The mass loading of α-MnO 2 on circular carbon cloth is 1.3 mg/cm 2 .

Characterization
To evaluate the electrochemical performance of the fabricated Zn/MnO 2 battery, the following electrochemical tests were conducted: cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge tests. CV tests were conducted using VersaSTAT 3F (AMETEK, Berwyn, PA, USA) within the voltage range of 0.7-2.1 V vs. Zn/Zn 2+ with scan rates of 0.5, 2.0, 5.0, and 10.0 mV/s. The tests were carried out using the CR2032 cell with a two-electrode configuration. In this configuration, the positive electrode of the cell was used as the working electrode. The negative electrode of the cell was used as both counter and reference electrode.

Results and Discussion
The investigation commenced by optimizing different parameters such as hydrothermal synthesis time and temperature. Figure S1 shows SEM images at different hydrothermal synthesis time, at T = 140 • C. Thus, it is observed that the sample obtained is a mixed morphology of nanoflowers and nanowires, even if synthesis time was prolonged from one to seven days. However, when the hydrothermal processing temperature and time was set to 110 • C and 24 h, respectively, the nanoflower morphology without the presence of nanowires was readily obtained, as shown in Figure S2. On the other hand, when temperature increased from 140 • C to 180 • C while keeping the synthesis time for seven days, the nanowire morphology without the presence of nanoflowers was obtained. Figure 1 displays the representative image of MnO 2 nanowires produced at 180 • C for seven days. The nanowires have a diameter 26 ± 5 nm, where the observed thick wires of~100 nm are bundles of wires. As shown in Figure S2, EDS analysis depicts the elements Mn, O, C, and K, verifying the possible existence of the MnO 2 -carbon fiber composite. To determine the structure and phase of these MnO 2 nanowires, XRD analysis was conducted. 1400, 100 kV. The crystalline and phase structure was determined using X-ray diffraction (XRD), Bruker (Billerica, MA, USA) D8-Advance, Cu Kα radiation, λ = 1.5418 Å, operating at 40 kV and 40 mA within 2θ range of 5 to 90 degrees.

Results and Discussion
The investigation commenced by optimizing different parameters such as hydrothermal synthesis time and temperature. Figure S1 shows SEM images at different hydrothermal synthesis time, at T = 140 °C. Thus, it is observed that the sample obtained is a mixed morphology of nanoflowers and nanowires, even if synthesis time was prolonged from one to seven days. However, when the hydrothermal processing temperature and time was set to 110 °C and 24 h, respectively, the nanoflower morphology without the presence of nanowires was readily obtained, as shown in Figure S2. On the other hand, when temperature increased from 140 °C to 180 °C while keeping the synthesis time for seven days, the nanowire morphology without the presence of nanoflowers was obtained. Figure 1 displays the representative image of MnO2 nanowires produced at 180 °C for seven days. The nanowires have a diameter 26 ± 5 nm, where the observed thick wires of ~100 nm are bundles of wires. As shown in Figure S2, EDS analysis depicts the elements Mn, O, C, and K, verifying the possible existence of the MnO2-carbon fiber composite. To determine the structure and phase of these MnO2 nanowires, XRD analysis was conducted.   (541), respectively, the produced MnO2 had an α phase. On the other hand, the diffraction peak around 2 ≃ 26.0° is associated with the carbon fiber. In Figure 2b, the simulated crystal structure showing the tunnel configuration is presented. Typically, α-MnO2 consists of double chains of edge-sharing MnO6 octahedra, which are linked at the corners to form 1D 2 × 2 and 1 × 1 tunnels in the tetragonal unit cell. The size of the 2 × 2 tunnel is 4.6 Å, which is a large tunnel for insertion/extraction of cations. Moreover, based on the TEM with selected area electron diffraction (SAED) pattern (Figure 2c,d), the synthesized α-MnO2 is a single crystal, as can be inferred from the absence of a ring, which is commonly observed for polycrystalline materials. Distinct spots   (541), respectively, the produced MnO 2 had an α phase. On the other hand, the diffraction peak around 2θ 26.0 • is associated with the carbon fiber. In Figure 2b, the simulated crystal structure showing the tunnel configuration is presented. Typically, α-MnO 2 consists of double chains of edge-sharing MnO 6 octahedra, which are linked at the corners to form 1D 2 × 2 and 1 × 1 tunnels in the tetragonal unit cell. The size of the 2 × 2 tunnel is 4.6 Å, which is a large tunnel for insertion/extraction of cations. Moreover, based on the TEM with selected area electron diffraction (SAED) pattern (Figure 2c,d), the synthesized α-MnO 2 is a single crystal, as can be inferred from the absence of a ring, which is commonly observed for polycrystalline materials. Distinct spots determined via SAED, which formed hexagonal patterns, are assigned to the family of planes {001}, {00 , as can be inferred from the measured angles and distances among these planes. The preferred growth of the crystal is perpendicular to the {001} planes, owing to its high surface energy [53]. Based on the EDS result in Figure S3, the produced α-MnO 2 nanowires still contain partial amount of K + , i.e., 1.32 at %. 1}, as can be inferred from the measured angles and distances among these planes. The preferred growth of the crystal is perpendicular to the {001} planes, owing to its high surface energy [53]. Based on the EDS result in Figure S3, the produced α-MnO2 nanowires still contain partial amount of K + , i.e., 1.32 at %. As shown in Figure 3a, CVs at a scan rate of 0.5 mV/s revealed broad redox peaks around 1.2 and 1.6 V vs. Zn/Zn 2+ for the sample 0.1 M aqueous Zn(OTf)2 electrolyte. The ill-resolved redox peaks for the sample 0.1 M aqueous Zn(OTf)2 electrolyte can be attributed to the capacitive behavior of α-MnO2 on neutral aqueous electrolyte, i.e., at low salt concentration [54]. Interestingly, when concentration increased to 3.0 M, using the same aqueous electrolyte, well-defined redox peaks were observed at 1.35 and 1.65 V, which were at higher potentials in comparison to those observed at lower concentration. Typically, redox peaks are attributed to Zn 2+ intercalation/deintercalation, which vary depending on the charge state of Mn [46,52,53]. On the other hand, using 0.1 M Zn(OTf)2 electrolyte As shown in Figure 3a, CVs at a scan rate of 0.5 mV/s revealed broad redox peaks around 1.2 and 1.6 V vs. Zn/Zn 2+ for the sample 0.1 M aqueous Zn(OTf) 2 electrolyte. The ill-resolved redox peaks for the sample 0.1 M aqueous Zn(OTf) 2 electrolyte can be attributed to the capacitive behavior of α-MnO 2 on neutral aqueous electrolyte, i.e., at low salt concentration [54]. Interestingly, when concentration increased to 3.0 M, using the same aqueous electrolyte, well-defined redox peaks were observed at 1.35 and 1.65 V, which were at higher potentials in comparison to those observed at lower concentration.
Typically, redox peaks are attributed to Zn 2+ intercalation/deintercalation, which vary depending on the charge state of Mn [46,52,53]. On the other hand, using 0.1 M Zn(OTf) 2 electrolyte in DMSO, the CV displayed distinct reversible redox peaks situated around 1.15 and 1.7 V. Since this report is the first which deals with α-MnO 2 in DMSO-based ZIBs, the charge storage mechanism of the cathode at different scan rates of 0.5, 2.0, 5.0, and 10.0 mV/s were examined, as shown in Figure 3b-d, according to Equations (1) and (2) [55]: The i and v in Equation (1) correspond to the peak current and scan rate, respectively. The b-value can be calculated using the slope of log (v) vs. log (i): if the b-value is close to 0.5, the electrochemical behavior is controlled by the diffusion process, while the b-value close to 1.0 is based on capacitive behavior. As shown in Figure 3c, the calculated b-value is 0.69. It can be inferred that at the considered scan rates, the charge storage mechanism is dominantly diffusion-controlled in 0.1 M Zn(TOf) 2 DMSO. To understand the behavior with respect to each applied current density, Equation (2) was used, where the k 1 v term refers to the capacitive process and the k 2 v 1/2 term corresponds to the diffusion-controlled process. The values of k 1 and k 2 were obtained by using the slope and y-intercept of Equation (3). It is noted that the capacitive contribution of the storage mechanism in DMSO-based ZIBs increased at an incremental scan rate, as shown in Figure 3d. Specifically, the diffusion-controlled process turned out to be the predominant role under lower current densities whilst the capacitive process dominated at higher current densities.
The galvanostatic charge-discharge test revealed that the battery using 0.1 M aqueous Zn(OTf) 2 electrolyte demonstrated higher capacity in comparison to both the 3.0 M aqueous Zn(OTf) 2 and 0.1 M Zn(OTf) 2 DMSO for all current densities (50, 100, 150, and 200 mA g −1 ). However, after 56 cycles, the battery eventually failed. In an aqueous system, cycling performance is commonly attributed to the following: (1) dendritic Zn deposition [56], (2) irreversible surface passivation on the Zn anode [57], and (3) dissolution and irreversible phase transformation of the MnO 2 cathode leading to capacity fading [58]. It is observed that when electrolyte concentration increased to 3.0 M Zn(OTf) 2 , long-term cyclability significantly improved and even reached up to 1000 cycles, although both capacity and capacity retention were found to be relatively lower in comparison to the more dilute aqueous electrolyte system. Based on this result, it is noted that when a concentrated electrolyte was used, anode passivation and dendritic Zn formation could be prevented, but capacity fading remained a critical issue. Hence, replacing the aqueous electrolyte system with a DMSO electrolyte proved to be beneficial in solving the problems of irreversible reactions in the anode and cathode.
In Figure 4, both the galvanostatic charge-discharge test and the cycling performance of the nonaqueous α-MnO 2 -based ZIB with 0.1 M Zn(OTf) 2 DMSO electrolyte are indicated. For the first 300 cycles, results demonstrated stable capacity of around 60 mAh g −1 . Thereafter, it stabilized at a capacity of around 50 mAh g −1 up to 2000 cycles, at current density of 100 mA g −1 . It is recognized that the electrochemical performance and cyclability of a ZIB is highly dependent on the stability and reversibility of the reaction, occurring in both anode and cathode. In an aqueous system, efforts are made to avoid the formation of a passivation layer and other irreversible reactions. This is achieved with the use of a mild acidic and neutral pH electrolyte system. However, long-term stability and cyclability are still an imminent issue for an aqueous system due to corrosion and hydrogen evolution problems, which are unavoidable owing to the presence of water. An attempt to address this issue was demonstrated in previous reports [46,52], such as the use of DES utilizing δ-MnO 2 as cathode material in both unannealed and annealed conditions. The obtained capacity was much lower than that obtained in an aqueous system because of inferior ionic conductivity. However, a stable capacity up to 150 cycles was achieved, which served as the benchmark for this newly explored nonaqueous system. Unfortunately, long-term stability and cyclability beyond 150 cycles was not attained using the DES electrolyte system. This was probably due to an inferior choice of cathode material: δ-MnO 2 , which has a physiosorbed interlayer and structurally bonded water that can affect the reversibility of the reaction in the cathode. Hence, to avoid this problem, α-MnO 2 , having no bonded water, was synthesized and investigated in a novel nonaqueous electrolyte system using DMSO as an electrolyte. In the present case, it can be argued that the long-term cyclability and stability of MnO 2 -based ZIB is not only highly dependent on the cathode and the anode, but also on the electrolyte used. For instance, when α-MnO 2 was used in an aqueous system, the battery instantly failed after only a few cycles. However, this did not occur when DMSO was used as the electrolyte, even at dilute Zn(OTf) 2 . As shown in Figure S4, both Nyquist and Bode plots of EIS depicted lower impedance value in the anode in the case of the DMSO-based ZIBs compared to the aqueous ones. This implied that DMSO played a significant role in the prevention of Zn passivation, dendritic Zn deposition, and irreversible phase transformation of MnO 2 , leading to better cyclability and electrochemical performance as shown in Figure S5.  . However, after 56 cycles, the battery eventually failed. In an aqueous system, cycling performance is commonly attributed to the following: 1) dendritic Zn deposition [56], 2) irreversible surface passivation on the Zn anode [57], and 3) dissolution and irreversible phase transformation of the MnO2 cathode leading to capacity

Conclusion
It is evident that binder-free α-MnO2 nanowires on carbon cloth was successfully prepared via the hydrothermal method. In both aqueous and DMSO-based electrolytes, the characteristics and electrochemical performances of the cathode were examined. It was found that the ZIB using 0.1 M Zn(OTf)2 DMSO electrolyte demonstrated exceptional cycling performance in comparison to the 0.1 M and 3.0 M Zn(OTf)2 aqueous electrolytes. In the DMSO electrolyte, even in the absence of a polymeric binder, the cathode proved to be very stable. Overall, higher cyclability and stability of the MnO2-based ZIBs could be attained with the Zn(OTf)2 DMSO electrolyte.
Supplementary Materials: Supplementary materials can be found at www.mdpi.com/xxx/s1. Figure  S1. Morphology of MnO2-carbon fiber composite synthesized at 140 °C with respect to synthesis time:

Conclusions
It is evident that binder-free α-MnO 2 nanowires on carbon cloth was successfully prepared via the hydrothermal method. In both aqueous and DMSO-based electrolytes, the characteristics and electrochemical performances of the cathode were examined. It was found that the ZIB using 0.1 M Zn(OTf) 2 DMSO electrolyte demonstrated exceptional cycling performance in comparison to the 0.1 M and 3.0 M Zn(OTf) 2 aqueous electrolytes. In the DMSO electrolyte, even in the absence of a polymeric binder, the cathode proved to be very stable. Overall, higher cyclability and stability of the MnO 2 -based ZIBs could be attained with the Zn(OTf) 2 DMSO electrolyte.  Acknowledgments: The authors thank Sigurd Skogestad for his comments and the support from Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University.

Conflicts of Interest:
The authors declare no conflict of interest.