Agar Acts as Cathode Microskin to Extend the Cycling Life of Zn//α-MnO2 Batteries

The Zn/MnO2 battery is a promising energy storage system, owing to its high energy density and low cost, but due to the dissolution of the cathode material, its cycle life is limited, which hinders its further development. Therefore, we introduced agar as a microskin for a MnO2 electrode to improve its cycle life and optimize other electrochemical properties. The results showed that the agar-coating layer improved the wettability of the electrode material, thereby promoting the diffusion rate of Zn2+ and reducing the interface impedance of the MnO2 electrode material. Therefore, the Zn/MnO2 battery exhibited outstanding rate performance. In addition, the agar-coating layer promoted the reversibility of the MnO2/Mn2+ reaction and acted as a colloidal physical barrier to prevent the dissolution of Mn2+, so that the Zn/MnO2 battery had a high specific capacity and exhibited excellent cycle stability.


Introduction
In recent years, the lithium-ion battery has permeated our lives and production and is used in both small electronic appliances and large mechanical equipment [1,2]. However, some issues such as long charging time and flammability of electrolyte solvents restrict its wide application, especially in large-scale energy storage systems (ESSs) that require high safety and reliability [3][4][5][6]. Therefore, some researchers shifted their attention to research on aqueous rechargeable batteries [7][8][9][10][11][12]. Among various aqueous battery systems, aqueous Zn-ion battery (ZIB) is very widely studied, owing to its high theoretical specific capacity (820 mAh g −1 ), low redox potential (−0.76 V vs. Standard hydrogen electrode, SHE), and low cost of Zn anodes [13,14]. Although it has these attractive merits, improving its cycle life is still a problem that needs to be solved urgently. As an important part of the ZIB, the cathode material is one of the keys to solving its cycling stability. Until now, candidate cathodes for ZIBs have been based mainly on vanadium-based materials, manganese-based materials, Prussian blue analogues, and other metal oxides or metal chalcogenides [13]. Among them, manganese-based materials, especially tunnel-type α-MnO 2 , are considered promising materials due to their low synthesis cost and high energy density [15]. However, MnO 2 suffers from serious capacity decline in aqueous electrolytes, owing to the dissolution of active materials and structural transformation caused by the disproportionation reaction of Mn 3+ . The addition of MnSO 4 to electrolytes can inhibit this side reaction to a certain extent, because additional Mn 2+ influences the balance between Mn 2+ dissolution and reoxidation [16]. To further improve the cycling performance of MnO 2 , a common strategy is surface coating by conductive carbon materials or polymer materials, which cannot only enhance the electrical conductivity of MnO 2 and ease volume changes, but also prevent Mn dissolution [17]. However, the improvement of cycle life is limited due to the poor reversibility of the MnO 2 /Mn 2+ redox reaction [18]. Recently, Chen's research group used collagen hydrolysate (CH) as an electrode microskin (EMS) to adsorb/confine dissolved Mn 2+ ions, resulting in the reversibility of the redox reaction of MnO 2 /Mn 2+ during the charge and discharge processes, thereby improving cycle performance [19]. This is a simple and effective method to improve the cycle stability of the Zn/MnO 2 battery, which is worthy of further promotion and in-depth research.
Agar is a polysaccharide extracted from seaweed and is widely used in the food industry, pharmaceutical industry, chemical industry, bioengineering, and many other fields. Agar used in food can significantly change the quality of food and improve the quality of food [20]. It can be used as a thickener, a coagulant, a suspending agent, an emulsifier, a preservative, and a stabilizer. In addition, agar is also used in the field of electrochemistry as an electrolyte additive to alleviate metal corrosion, with excellent results [21][22][23][24][25]. Due to the above properties and advantages, agar may be promising for use as a cathode microskin for the Zn/MnO 2 battery.
Here, we report the use of agar as the EMS of an α-MnO 2 electrode to improve the electrochemical performance of a Zn//α-MnO 2 battery. The influences of different thicknesses of an agar coating layer on the wettability and resistance of the α-MnO 2 electrode and the rate performance, specific capacity, and cycling performance of the Zn//α-MnO 2 battery were explored. α-MnO 2 was prepared by a hydrothermal method. Typically, 1 g KMnO 4 (Analytical Reagent, AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) and 0.4 g MnSO 4 (AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were dissolved in 50 mL and 22 mL deionized water, respectively. Then, the two solutions were mixed, and the resulting mixture was stirred for 10 min at room temperature to form a clear solution. Afterwards, the solution was transferred to a 100 mL autoclave (Nanjing Wanqing Chemical Glass Ware & Instrument Co. Ltd., Nanjing, China) and reacted at 160 • C for 12 h. After cooling to room temperature, the brownish black product was collected by vacuum filtration and washed with deionized water followed by absolute ethanol (≥99.7%, Wuxi Yasheng Chemical Co. Ltd., Wuxi, China). The final product was dried in an oven (DGG-9240A, Shanghai Senxin Experimental Instrument Co. Ltd., Shanghai, China) at 80 • C for 12 h before use.

Modification of α-MnO 2 Electrode by Agar Coating
To prepare the α-MnO 2 electrode, α-MnO 2 , acetylene black (AR, Aladdin, Shanghai, China), and poly(vinylidene fluoride) (PVDF) (AR, Aladdin, Shanghai, China) were mixed with a mass ratio of 7:2:1 in an appropriate amount of in N-methyl-2-pyrrolidinone (AR, Macklin Biochemical Co. Ltd. Shanghai, China), and the mixture was stirred for 3 h. Then, the stirred slurry was coated on a stainless-steel foil (thickness: 0.01 mm, Dongguan Pingjie Metal Material Co., Ltd. Dongguan, China) and dried in a vacuum drying oven (DZG-6050, Shanghai Senxin Experimental Instrument Co. Ltd., Shanghai, China) at 80 • C for 12 h. Finally, the foil was cut into a disc with a diameter of 5 mm for use. The active substance loaded per disc was about 2 mg.
Agar solutions with agar mass fractions of 1% and 2.5% were obtained by mixing agar (AR, Macklin Biochemical Co. Ltd. Shanghai, China) with ultra-pure water at mass ratios of 1:99 and 2.5:97.5, respectively, and the mixtures were heated at 90 • C for 1 h.
Ten microliters of each agar solution were dripped onto the cut α-MnO 2 electrodes, which were dried on a hot plate at 60 • C to prepare agar-modified α-MnO 2 electrodes. The electrodes coated with 1% and 2.5% agar solutions were denoted as MnO 2 -1 and MnO 2 -2, respectively.

Electrochemical Measurements
Both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured using a Chenhua 660 E electrochemical workstation (CH Instruments Inc, Shanghai, China). The rate and cycling performances were tested on a LAND 2001A battery test system (Wuhan LAND Electronic Co. Ltd., Wuhan, China). A 0.8 mm commercial zinc foil (Kunshan Yizhongtian New Material Co. Ltd., Suzhou, China), a 2 M ZnSO 4 + 0.5 M MnSO 4 solution, and a glass fiber membrane (Whatman GF/A, Maidstone, England) were used as an anode, an electrolyte, and a diaphragm, respectively. A button battery-packaging machine (Shenzhen Kejing Star Technology Co. Ltd. Shenzhen, China) was used to package CR2025 button batteries. After standing for 1 h, the electrochemical test was performed.

Characterization of the Physical Properties of the Prepared α-MnO 2
The physical properties of the prepared MnO 2 are illustrated in Figure 1. The MnO 2 nanorods, with a length of 2-3 µm and a thickness of 100-150 nm, can be clearly observed (Figure 1a-c). The TEM is shown in Figure 1d. A lattice spacing of 0.308 nm was found to match well with the (310) crystal planes of α-MnO 2 . The XRD patterns conformed to a pure α-MnO 2 phase (JCPDS No. 44-0141) [26].

Characterization of the Agar-Modified Cathodes
The FTIR spectrum of agar is shown in Figure 2a. The absorption band at 3429.89 cm −1 corresponds to the stretching vibration of the hydroxyl groups (O-H) in agar, which

Characterization of the Agar-Modified Cathodes
The FTIR spectrum of agar is shown in Figure 2a. The absorption band at 3429.89 cm −1 corresponds to the stretching vibration of the hydroxyl groups (O-H) in agar, which participate in the formation of intermolecular and intramolecular hydrogen bonds; the weak absorption band at 1623.49 cm −1 is allocated to the C=O stretching vibration; the band around 1405.69 cm −1 can be attributed to the sulfate ester group in agar; the absorption band at 1053.38 cm −1 corresponds to several C-O-H bending modes; the absorption band at 963.7 cm −1 is attributed to the C-O-C bending mode in 3,6-anhydrogalactose [22]. The FTIR spectra of ager coated MnO 2 electrode is shown in Figure S1, which is consistent with that of pure agar. It has been shown that electrode wettability is closely related to the capacity and cycle life of the battery. Generally speaking, improving the wettability of electrode materials can shorten the wetting time, make full use of the electrode capacity and provide better electrochemical performance [27,28]. The surface hydrophilicity tests results of the α-MnO2 electrode and electrodes coated with agar are shown in Figure 3. It can be seen that the contact angle was reduced from 99° to 52° with increasing amounts of agar on the cathode. This means that agar coating may increase the diffusion rate of Zn 2+ on the MnO2 electrode surface, reduce the interface impedance and improve the rate performance.

Electrochemical Performance of Zn//α-MnO2 Batteries
Zn//α-MnO2 batteries were assembled using a 2 M ZnSO4 + 0.5 M MnSO4 aqueous It has been shown that electrode wettability is closely related to the capacity and cycle life of the battery. Generally speaking, improving the wettability of electrode materials can shorten the wetting time, make full use of the electrode capacity and provide better electrochemical performance [27,28]. The surface hydrophilicity tests results of the α-MnO 2 electrode and electrodes coated with agar are shown in Figure 3. It can be seen that the contact angle was reduced from 99 • to 52 • with increasing amounts of agar on the cathode. This means that agar coating may increase the diffusion rate of Zn 2+ on the MnO 2 electrode surface, reduce the interface impedance and improve the rate performance.
can shorten the wetting time, make full use of the electrode capacity and provide better electrochemical performance [27,28]. The surface hydrophilicity tests results of the α-MnO2 electrode and electrodes coated with agar are shown in Figure 3. It can be seen that the contact angle was reduced from 99° to 52° with increasing amounts of agar on the cathode. This means that agar coating may increase the diffusion rate of Zn 2+ on the MnO2 electrode surface, reduce the interface impedance and improve the rate performance.

Electrochemical Performance of Zn//α-MnO2 Batteries
Zn//α-MnO2 batteries were assembled using a 2 M ZnSO4 + 0.5 M MnSO4 aqueous solution as an electrolyte. In order to explore the effect of agar as a cathode microskin on the electrochemical performance of Zn//α-MnO2 batteries, its EIS spectrum was first measured. As shown in Figure 4a, the Zn/MnO2-1 battery showed the lowest charge transfer resistance in the high-frequency area, which may be attributed to the decrease of the crosssectional impedance of the electrode due to the improvement of its wettability [29,30]. The charge transfer resistance of the Zn/MnO2-2 battery came second, because although the cathode had the best wettability, the agar microskin coating was thicker, which increased the interface resistance. The slope of the low-frequency parts was calculated, as shown in Figure S2. The slopes for the Zn/MnO2 battery, Zn/MnO2-1 battery, and Zn/MnO2-2 battery are 1.01, 2.78, and 1.54, respectively. The slope of the Zn/MnO2-1 battery was the

Electrochemical Performance of Zn//α-MnO 2 Batteries
Zn//α-MnO 2 batteries were assembled using a 2 M ZnSO 4 + 0.5 M MnSO 4 aqueous solution as an electrolyte. In order to explore the effect of agar as a cathode microskin on the electrochemical performance of Zn//α-MnO 2 batteries, its EIS spectrum was first measured. As shown in Figure 4a, the Zn/MnO 2 -1 battery showed the lowest charge transfer resistance in the high-frequency area, which may be attributed to the decrease of the cross-sectional impedance of the electrode due to the improvement of its wettability [29,30]. The charge transfer resistance of the Zn/MnO 2 -2 battery came second, because although the cathode had the best wettability, the agar microskin coating was thicker, which increased the interface resistance. The slope of the low-frequency parts was calculated, as shown in Figure S2. The slopes for the Zn/MnO 2 battery, Zn/MnO 2 -1 battery, and Zn/MnO 2 -2 battery are 1.01, 2.78, and 1.54, respectively. The slope of the Zn/MnO 2 -1 battery was the highest, indicating that the diffusion rate of Zn 2+ was the highest in the MnO 2 -1 electrode. In Figure 4b, the CV curves of the three batteries at a scan rate of 0.3 mV s −1 over the potential range of 0.1-1.95 V (vs. Zn 2+ /Zn) are shown. The CV curves of these three batteries had similar shapes, with two reduction peaks and two oxidation peaks, which can be attributed to the redox processes of Mn 4+ /Mn 3+ caused by the intercalation of Zn 2+ into the MnO 2 host and the extraction of Zn 2+ from the MnO 2 host, respectively. Compared with the Zn/MnO 2 battery and the Zn/MnO 2 -2 battery, the Zn/MnO 2 -1 battery had a higher peak current density, indicating that it had the highest electrochemical activity and capacity [31]. In addition, the Zn/MnO 2 -1 battery had the lowest polarization, meaning that it had the lowest interface impedance, which is consistent with the EIS test results. highest, indicating that the diffusion rate of Zn 2+ was the highest in the MnO2-1 electrode.
In Figure 4b, the CV curves of the three batteries at a scan rate of 0.3 mV s −1 over the potential range of 0.1-1.95 V (vs. Zn 2+ /Zn) are shown. The CV curves of these three batteries had similar shapes, with two reduction peaks and two oxidation peaks, which can be attributed to the redox processes of Mn 4+ /Mn 3+ caused by the intercalation of Zn 2+ into the MnO2 host and the extraction of Zn 2+ from the MnO2 host, respectively. Compared with the Zn/MnO2 battery and the Zn/MnO2-2 battery, the Zn/MnO2-1 battery had a higher peak current density, indicating that it had the highest electrochemical activity and capacity [31]. In addition, the Zn/MnO2-1 battery had the lowest polarization, meaning that it had the lowest interface impedance, which is consistent with the EIS test results. The CV curves of the Zn/MnO2-1 battery at various scan rates from 0.3 to 1 mV s −1 are shown in Figure 5a. With the increasing scan rate, it can be seen clearly that the shapes of the CV curves are consistent, but the peaks in the CV gradually grow wider. According to the equation: log(i) = log(a) + blog(v), where the b values were calculated as 0.43 (R1), 0.67 (R2), 0.43 (O1), and 0.56 (O2) by fitting the linear relationship between log(i) and log(v) (Figure 5b). This clearly shows that the ion diffusion process and surface capacitive effects both contributed to the electrochemical kinetics of the MnO2-1. Figure 5c records the rate performances of the Zn/MnO2 battery, the Zn/MnO2-1 battery, and the Zn/MnO2-2 battery at various current densities. The results showed that the rate performance of the Zn/MnO2-1 battery was the best, followed by that of the Zn/MnO2-2 battery, while the Zn/MnO2 battery performed the worst. The Zn/MnO2-1 battery delivered a capacity of 384.7, 232.8, 179.3, and 133.3 mAh g −1 at current densities of 0.25, 0.5, 1.0, and 2 A g −1 , respectively. The corresponding charge and discharge curves are shown in Figure 5d. It The CV curves of the Zn/MnO 2 -1 battery at various scan rates from 0.3 to 1 mV s −1 are shown in Figure 5a. With the increasing scan rate, it can be seen clearly that the shapes of the CV curves are consistent, but the peaks in the CV gradually grow wider. According to the equation: log(i) = log(a) + blog(v), where the b values were calculated as 0.43 (R1), 0.67 (R2), 0.43 (O1), and 0.56 (O2) by fitting the linear relationship between log(i) and log(v) (Figure 5b). This clearly shows that the ion diffusion process and surface capacitive effects both contributed to the electrochemical kinetics of the MnO 2 -1. Figure 5c records the rate performances of the Zn/MnO 2 battery, the Zn/MnO 2 -1 battery, and the Zn/MnO 2 -2 battery at various current densities. The results showed that the rate performance of the Zn/MnO 2 -1 battery was the best, followed by that of the Zn/MnO 2 -2 battery, while the Zn/MnO 2 battery performed the worst. The Zn/MnO 2 -1 battery delivered a capacity of 384.7, 232.8, 179.3, and 133.3 mAh g −1 at current densities of 0.25, 0.5, 1.0, and 2 A g −1 , respectively. The corresponding charge and discharge curves are shown in Figure 5d. It can be seen that when the current density was 0.25 A g −1 , there were two plateaus on the charge-discharge curve, which are consistent with the CV curve. It is worth noting that as the current density increased, the second plateau gradually disappeared. This may be due to the slow diffusion of Zn 2+ at high current density; its capacity contribution mainly comes from H + insertion/extraction [32]. The specific capacity of the Zn/MnO 2 battery at the corresponding current density was much lower than that of the Zn/MnO 2 -1 battery ( Figure S3), which indicated that the agar microskin improved the reversibility of the MnO 2 /Mn 2+ reaction [19]. The specific capacity of the Zn/MnO 2 -2 battery was higher than that of the Zn/MnO 2 battery, but lower than that of the Zn/MnO 2 -2 battery. This is mainly because the agar microskin of the MnO 2 -2 electrode was thicker than that of the MnO 2 -1 electrode, which hindered the transmission speed of Zn 2+ to a certain extent. The cycling performance was studied by continually charging and discharging at 0.5 A g −1 . Figure 6a compares the cycling performance of the Zn//α-MnO2 batteries with that of the Zn/MnO2-1 battery exhibiting the best cycling performance with a capacity retention of 95.4% after 500 cycles and with the highest specific capacity, which was substantially higher than the values for most Zn/MnO2 batteries reported to date (Table 1). This superior performance can be attributed mostly to the agar coating. During the charge and discharge processes, agar acted as a colloidal physical barrier to inhibit the dissolution of Mn 2+ , hence improving the cycling performance of the Zn//α-MnO2 battery. Moreover, due to the improvement of the reversibility of the MnO2/Mn 2+ reaction by agar coating, the specific capacities of the Zn/MnO2-1 battery and the Zn/MnO2-2 battery were higher than that of the Zn/MnO2 battery. However, due to the thick coating layer of MnO2-2, the migration rate of Zn 2+ in this battery was hindered to a certain extent, and the interface impedance was increased, so that the specific capacity of the Zn/MnO2-2 battery was lower than that of the Zn/MnO2-1 battery. It should be noted that both the Zn/MnO2-1 battery and the Zn/MnO2-2 battery had an activation process during cycling and the specific capacity reached a maximum after about 20 cycles, after which the capacity remained relatively stable. The specific capacity of the Zn/MnO2 battery decayed rapidly over the first 100 cycles and then more slowly. After 500 cycles, the capacity retention rate was only 51.6%. The charge and discharge curves of the Zn/MnO2-1 battery over different cycles are The cycling performance was studied by continually charging and discharging at 0.5 A g −1 . Figure 6a compares the cycling performance of the Zn//α-MnO 2 batteries with that of the Zn/MnO 2 -1 battery exhibiting the best cycling performance with a capacity retention of 95.4% after 500 cycles and with the highest specific capacity, which was substantially higher than the values for most Zn/MnO 2 batteries reported to date (Table 1). This superior performance can be attributed mostly to the agar coating. During the charge and discharge processes, agar acted as a colloidal physical barrier to inhibit the dissolution of Mn 2+ , hence improving the cycling performance of the Zn//α-MnO 2 battery. Moreover, due to the improvement of the reversibility of the MnO 2 /Mn 2+ reaction by agar coating, the specific capacities of the Zn/MnO 2 -1 battery and the Zn/MnO 2 -2 battery were higher than that of the Zn/MnO 2 battery. However, due to the thick coating layer of MnO 2 -2, the migration rate of Zn 2+ in this battery was hindered to a certain extent, and the interface impedance was increased, so that the specific capacity of the Zn/MnO 2 -2 battery was lower than that of the Zn/MnO 2 -1 battery. It should be noted that both the Zn/MnO 2 -1 battery and the Zn/MnO 2 -2 battery had an activation process during cycling and the specific capacity reached a maximum after about 20 cycles, after which the capacity remained Materials 2021, 14, 4895 7 of 10 relatively stable. The specific capacity of the Zn/MnO 2 battery decayed rapidly over the first 100 cycles and then more slowly. After 500 cycles, the capacity retention rate was only 51.6%. The charge and discharge curves of the Zn/MnO 2 -1 battery over different cycles are shown in Figure 6b. The charge and discharge curves remained stable over 500 cycles, which also means that the structure of the MnO 2 -1 electrode remained stable, indicating that the agar coating inhibited the dissolution of Mn 2+ and maintained the structural stability of the α-MnO 2 . Compared with that of the Zn/MnO 2 battery, the cycling stability of the Zn/MnO 2 -2 battery was also greatly improved ( Figure S4b). In contrast, the chargedischarge curves of the Zn/MnO 2 battery varied greatly during different charge-discharge cycles ( Figure S4a). After 100 cycles, the second discharge plateau disappeared, which means that the structure of the α-MnO 2 was destroyed.  To further explore the working principle of the MnO2-1 electrode, we investigated the ex situ XRD of the materials in different charge and discharge states during the first cycle (as shown in Figure 7b,c). During the discharge process, new peaks at 33° and 34.7°, which are the characteristic peaks of tunnel-type ZnMn2O4, were observed. During the charge process, the two peaks gradually disappear and return to the initial state. Therefore, the working principle of the MnO2-1 electrode during the charge and discharge processes can be expressed as [44]:  To further explore the working principle of the MnO 2 -1 electrode, we investigated the ex situ XRD of the materials in different charge and discharge states during the first cycle (as shown in Figure 7b,c). During the discharge process, new peaks at 33 • and 34.7 • , which are the characteristic peaks of tunnel-type ZnMn 2 O 4 , were observed. During the charge process, the two peaks gradually disappear and return to the initial state. Therefore, the working principle of the MnO 2 -1 electrode during the charge and discharge processes can be expressed as [44]: MnO 2 + Zn 2+ + x × e Zn x MnO 2 (0 < x < 1). (1)

Conclusions
In conclusion, agar was introduced as an EMS for an α-MnO2 electrode. After coating with agar, the wettability of the MnO2 electrode was greatly improved; this increased the diffusion rate of Zn 2+ on the surface of the MnO2 electrode and reduced the interface impedance. The impedance was the lowest, when the thickness of the coating layer was 4.94 μm (MnO2-1), so that the assembled Zn/MnO2-1 battery showed excellent rate performance. In addition, because the agar-coating layer promoted the reversibility of the MnO2/Mn 2+ reaction, the Zn/MnO2-1 battery showed the highest specific discharge capacity of 384.7 mAh g −1 at a current density of 0.25 A g −1 . Moreover, the agar EMS acted as a colloidal physical barrier on the surface, inhibited the dissolution of Mn 2+ and maintained the stability of the structure of α-MnO2, so that the Zn/MnO2-1 battery had excellent cycling stability with a high capacity retention of 85.6% after 500 cycles at a current density of 0.5 A g −1 .

Conclusions
In conclusion, agar was introduced as an EMS for an α-MnO 2 electrode. After coating with agar, the wettability of the MnO 2 electrode was greatly improved; this increased the diffusion rate of Zn 2+ on the surface of the MnO 2 electrode and reduced the interface impedance. The impedance was the lowest, when the thickness of the coating layer was 4.94 µm (MnO 2 -1), so that the assembled Zn/MnO 2 -1 battery showed excellent rate performance. In addition, because the agar-coating layer promoted the reversibility of the MnO 2 /Mn 2+ reaction, the Zn/MnO 2 -1 battery showed the highest specific discharge capacity of 384.7 mAh g −1 at a current density of 0.25 A g −1 . Moreover, the agar EMS acted as a colloidal physical barrier on the surface, inhibited the dissolution of Mn 2+ and maintained the stability of the structure of α-MnO 2 , so that the Zn/MnO 2 -1 battery had excellent cycling stability with a high capacity retention of 85.6% after 500 cycles at a current density of 0.5 A g −1 .