1 Introduction

Considerable potential has propelled rechargeable zinc-ion batteries (ZIB) to become highly competitive commercial batteries. Their intrinsic safety, abundant zinc reserves, low cost, and ideal specific capacity make them a promising solution, addressing concerns associated with lithium-ion batteries (LIB) Xu, et al. [1]. MnO² is a cost-effective, low-toxicity material for aqueous zinc-ion batteries with a high theoretical capacity, making it a practical choice. Nonetheless, MnO² suffers from low electronic conductivity (10-5 S/cm – 10-6 S/cm), which leads to unsatisfactory performance, such as low capacity and low capability Huang, et al. [2]. Various strategies have been explored to enhance the ionic conductivity of manganese-based materials. These include coating, doping reported by Gunaydin, et al. [3], heterostructure reported by Tu, et al. [4], and altering morphology and measuring temperature. Tiancheng Tu, et al. [4] successfully prepared MnO² by introducing phenylphosphonic acid (H2PP) via a hydrothermal method, and the addition of 0.5% H2PP presented high ionic conductivity and electrochemical kinetics. Gunaydin, et al. [3] noted that doping

materials and altering measuring temperature contributed to improving ionic properties.

In this study, Ni-doped layered-MnO² was synthesized using a hydrothermal process. The resulting Ni-doped layered-MnO² and pure layered-MnO2were utilized as cathode-active materials to assess ionic conductivity and electrochemical behavior. Both Ni-doped layered-MnO² and pure layered-MnO² exhibited high ionic conductivity typical of the hydrothermal process. The Nidoped layered-MnO² showed morphology evolution, enhanced ionic conductivity, and excellent electrochemical performance. These findings suggest the promise of Nickel-doped layered-MnO² in developing cathode materials of zinc ion batteries and for various electrochemical applications.

2 Experimental Section

In short, 2.37 g KMnO⁴ (99.0% Sigma-Aldrich) and 0.363 g Ni(NO3)2.6H2O (98.5% Sigma-Aldrich) were dissolved in 30 ml deionized water at 60°C. The solution was vigorously stirred for 30 minutes, transferred to a 50 ml Teflonlined autoclave, and maintained at 160°C for 10 hours. After reaching room temperature, the resulting product underwent three rounds of rinsing with deionized water and ethanol before drying at 80°C in a vacuum oven. Layered-MnO² was synthesized using a similar procedure, excluding the addition of Ni(NO3)2.6H2O.

The samples were characterized by X-ray diffractometry (XRD, Bruker D8, CuKα irradiation) and scanning electron microscopy (SEM Hitachi SU3500). To evaluate ionic conductivity and electrochemical performance, the samples were fabricated to cathode by blending the samples as active material, C65, and carboxymethyl cellulose (CMC) in a weight ratio of 8:1:1. The resulting slurry was then uniformly coated onto titanium foil. The cathode's thickness, area, and mass loading were 0.5 x 10-2 – 0.6 x 10-2 cm, 2.0114 cm² , and 1-2 mg.cm² , respectively. Pure zinc foil served as an anode, and the electrolyte comprised 2 ZnSO4and 0.2 MnSO4. The ionic conductivity of Ni-doped layered-MnO² and pure layered-MnO2was measured by electrochemical impedance spectroscopy (EIS, Autolab) in the frequency range from 10000 Hz to 0.001 Hz. The cyclic voltammetry and battery performances were tested using Corrtest and Neware battery systems.

3 Result and Discussion

3.1 Structure and Morphology

X-ray diffraction (XRD) analysis is conducted to gather details about a material's structure, chemical composition, and phases. Figure 1a displays the XRD patterns for Ni-layered-MnO² and layered-MnO² prepared through hydrothermal methods. The pattern of Ni-doped layered-MnO² and pure layered-MnO² (Fig. 1a) exhibit two main peaks at 12.5^o and 35.4^o the distinctive peaks indexed to the (001) and (200) planes of layered-MnO² (PDF 01-080- 1098) following literature references Zhang, et al. [5], Xiao, et al. [6], Zhang, et al. [7], each unit comprises MnO⁶ with an octahedral structure Zhu, et al. [8]. The introduction of nickel influences layered-MnO2, evidenced by increased crystallinity, as observed in the higher intensity of the Ni-layered-MnO² diffraction pattern. Furthermore, the XRD pattern indicates the absence of other phases, affirming the sample's purity without impurities and alignment with the database Zhang, et al. [5], Zhang, et al. [7]. Fig. 1b and c show SEM images of Ni-doped layered-MnO² and pure layered-MnO2. The morphology of pure layered-MnO2presents nanoparticles with a diameter of about 200-500 nm and many interconnected nanoflakes on its surface. Ni-doped layered-MnO2shows morphology evolution that forms nanoparticles like flowered (Fig. 1c).

Figure. 1. (a) XRD pattern of Ni-doped layered-MnO² and pure layered-MnO2, (b) SEM image of pure layered-MnO2, (c) SEM image of pure Ni-doped layered-MnO2, and (d) SEM image of pure Ni-doped layered-MnO² at higher resolution.

Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) was used to analyze the morphology and element distribution of the hydrothermal method. The Hydrothermal treatment significantly increased crystallinity. Furthermore, The addition of Ni-layered-MnO² showed a higher level of crystallinity than layered-MnO2. Increasing crystallinity in Ni-layered-MnO² affects changes in morphology, as seen in the SEM images (Fig. 1c and d). Element distribution mapping (Fig. 2b-d) reveals a uniform Mn, O, and Ni distribution throughout the Ni-layered-MnO² structure. Additionally, the SEM-EDS spectrum (Fig. 2e) indicates the presence of potassium (K), carbon (C), and silicon (Si), attributed to sample preparation during synthesis. The silicon (Si) peak is notably high, originating from the silicon wafer used as the sample pressing medium. This distribution confirms the integration of Ni into the Mn-O matrix. The Ni-layered-MnO² structure effectively reinforces the layered-MnO² structure, preventing breakdown during long cycles and enhancing electrical conductivity Zhu, et al. [8], Worku, et al. [9].

Figure 2. (a) SEM image of Ni-doped layered-MnO2, element distribution of (b) Mn, (c) O, (d) Ni, and (e) EDS spectrum.

4 Ionic Conductivity and Electrochemical Performance

The cathode's ionic conductivity (σ) was tested by EIS and calculated using the following equation: Cai, et al. [10]. Fig. 3a and b displayed the Nyquist plot of Ni-doped layered-MnO² and pure layered-MnO2; the bulk resistance was 1.3 Ω and 1.9 Ω, respectively. The corresponding ionic conductivity of Ni-doped layered-MnO² and pure layered-MnO² was 1.91 x 10-3 S/cm and 1.31 x 10-3 S/cm, respectively. It can be seen from these results that the ionic conductivity of Ni-doped layered-MnO2 had a higher value compared to pure layered-MnO2. Notably, pure layered-MnO² in this study demonstrated higher ionic conductivity than reported in the literature Huang, et al. [2]. This improvement is attributed to the enhanced ionic conductivity of nickel Patil, et al. [11].

Figure. 3. (a) Nyquist plot of Ni-doped layered-MnO² and pure layered-MnO2and (b) the bulk resistance Ni-doped layered-MnO² and pure layered-MnO2.

5 Electrochemical Performance

Electrochemical performance testing utilized a CR2032 coin-type battery assembled in the open air. The electrode preparation involved mixing the active material, conductive carbon, and carboxymethyl cellulose (CMC) in a weight ratio of 8:1:1, combined in deionized water. The resulting slurry was poured onto a titanium foil current collector and dried at 80°C. The electrode's mass loading (mass to electrode area) is approximately 1-2 mg/cm² . Electrolyte solutions consisted of 2 molar ZnSO⁴ and 0.2 molar MnSO4. The anode was composed of zinc, with a Whatman sheet (GF/A) serving as a separator between the anode and cathode. Cyclic voltammetry (CV) covered a voltage range of 0.8-1.9 V at scan rates 0.2 mV.s-1 , and electrochemical impedance spectroscopy (EIS) spanned a frequency range from 10,000 Hz to 0.001 Hz using Corrtest and Autolab (EIS) devices. The GCD charging/discharging measurements were conducted on the Autolab Battery Tester. In Fig. 4a, the cyclic voltammetry (CV) curve for Ni-doped layered-MnO² exhibits two sets of redox peaks during both anodic and cathodic sweeps, indicating efficient intercalation-extraction of Zn2+/H⁺ . The well-maintained shapes of the CV curves affirm the reversible redox reaction of the Ni-doped layered-MnO² cathode Wu, et al. [12], Fang, et al. [13]. The cycling performance and coulombic efficiency of Ni-doped layered-MnO² and pure layered-MnO2cathode in the voltage range of 0.8 – 1.8

V at current density 100 mA. g -1 are depicted in Fig. 4b. The Specific capacity calculations are based on the mass of Ni-doped layered-MnO² and pure layered-MnO² cathode. In the initial cycle, the Ni-layered-MnO² exhibits slightly lower capacity than layered-MnO² (351 mAh.g-1 ), observed in the first 15 cycles (Fig.4b). This is attributed to H⁺ and Zn2+ insertion reactions in the layered-MnO² electrode, which do not occur in the first cycle of the Ni-layered-MnO² electrode, where only Zn2+ reaction takes place Lee, et al. [14]. Additionally, the higher percentage of Mn2+ in layered-MnO² than Ni-layered-MnO² contributes to decreased electron storage ability. After 15 cycles, the active material on the Ni-layered-MnO² electrode is effectively activated, which is evident in the increased capacity (311 mAh.g-1 ) compared to layered-MnO² (287 mAh.g-1 ). In contrast, the layered-MnO² electrode experiences dissolution of the active material due to the John Taller effect Zhang, et al. [15]. Throughout the entire cycle, coulumbic efficiency remains at 96%. The Ni-layered-MnO² electrode exhibits a capacity retention of 80.4% after 50 cycles, with a capacity of 194 mAh.g-1 . In contrast, layered-MnO² experiences a significant capacity decrease, retaining only 38.7% after the 50th cycle, with a capacity of 136 mAh. g -1 . This disparity is attributed to the dissolution of Mn ions, leading to the breakdown of the interlayer circuit in the layered-MnO² structure Zhang, et al. [15]. The exceptional electrochemical performance is primarily attributed to the high ionic conductivity resulting from the intercalation of Ni2+ in layered-MnO² structure and enhanced structural stability. This structure can facilitate both ion intercalation and de-intercalation during the repeating cycle with high ion diffusion. These results promise cathode materials in zinc ion batteries and various electrochemical applications.

Fig. 4. (a) CV Curves at scan rates 0.2 mV.s-1 of Ni-doped layered-MnO² and (b) cycle performance at 0.1 A/g of Ni-doped layered-MnO² compared to layered-MnO2.

6 Conclusion

In summary, Ni-doped layered-MnO² was synthesized through a simple hydrothermal process, enhancing ionic conductivity through Ni2+ intercalation and morphology evolution. The intercalation of Ni2+ at a ratio of 6:0.5 does not alter the crystal structure but substantially improves the electrochemical performance of layered-MnO² electrodes. This enhancement is evident in increased capacity and ion transfer speed. The Ni2+ intercalation in layered-MnO² also results in a more stable structure than pure layered-MnO2. The Electrochemical assessments confirm that Ni2+ incorporation significantly enhances the material's performance by forming robust ionic bonds with oxygen atoms.

7 Acknowledgement

I thank Dr. Afriyanti Sumboja for her valuable technical assistance and insightful discussions. This study received support from the Indonesian Endowment Fund for Education (LPDP) from the Republic of Indonesia.

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