Morphology and structure
The morphologies of the carbon matrix and C materials were studied by SEM. It is obvious that the morphologies of the two materials are very different. Figure 1a-c presents the SEM images of the homemade carbon matrix, and its overall appearance is similar to that of corals formed by spherical aggregation with a smooth surface. The coral-like particles, which are gathered by approximately 200 nm microspheres, may be more conducive to electron transfer and structural stability. The prepared CCS@MnO2 microsphere has a spherical nucleus of approximately 800 nm, which is similar to the nucleus of the carbon matrix in its overall morphology but at the same time very different. Figure 1d-f shows a coral-like structure formed by multiple particles but with a large number of nanosheets on its surface. This is most likely because MnO2 nanosheets have grown on the surface of the carbon matrix.
Figure 2a and 2b present the TEM images of CCS@MnO2, the distinct carbon skeleton and spherical local structure with a diameter of about 600 nm of the carbon skeleton is can be clearly observed and the nanosheets are interconnected to form into a lamellar architecture on its surface[3, 18]. Figure 1b further confirm that δ-MnO2 particles with the diameter of ~ 150 nmare uniformly complexed with homemade CS [34]. The HRTEM image in Fig. 2c shows that the lattice spacing of 0.718 nm is corresponding to (003) crystal plane of δ-MnO2, a typical lattice spacing of 0.359 nm is corresponding to (006) crystal plane of δ-MnO2, 0.241 nm is fitting with the (012) crystal plane of δ-MnO2, 0.143 nm is corresponding to the (113) crystal plane of δ-MnO2, further confirmed the layered characteristics of the crystal material is indeed the δ-MnO2. Figure 2d further investigate the element distribution reveals the EDS mapping of O, Mn and C elements, the final results shows that the Mnand O elements are covered on the surface of the C, which proves that MnO2 is indeed uniformly coated on the surface of CCS [35].
In Fig. 3a, the unique structure and composition of the final sample are examined by XRD, which is indexed in JCPDS 30–0820 at the 001, 002, 111, and 312 planes, and each diffraction peak clearly points to CCS@MnO2, which has a lamellar structure[3, 21, 22]. The TG and DTG curves of CCS@MnO2 in Fig. 3b show that the percent weight loss is 12.501%, and the residue percent is 82.900%[27]. Figure 3c displays the Fourier transform infrared (FT-IR) spectra of CCS@MnO2. In the picture, several groups can be observed, such as -OH, -C = C, -C-H, and the bands of 1288 and 1614. Figure 3d presents the adsorption-desorption isotherm and pore size distribution. This type of curve implies that the material has a clear mesoporous structure, in which the remarkable hysteresis loop in range and the corresponding pore size distribution are shown in the illustration. The specific surface area is 59.0203 m2g− 1, the average pore diameter is 14.2423 nm, and the pore volume is 0.197977 cm3g− 1. Therefore, it is concluded that the mesoporous structure of MnO2 and the interlayer tunnel have great advantages in the diffusion process of the entire electrode. Its large surface area and abundant pore structure can effectively promote the diffusion of zinc ions[3].
To further study the elemental composition and electronic structure of the CCS@MnO2 material, it was systematically analyzed by X-ray photoelectron spectroscopy (XPS). Figure 4a shows a high-energy XPS full spectrum measurement spectrum, and the sample shows the presence of C, O and Mn without other impurities, which is consistent with the XRD result. It is worth noting that Fig. 4b shows the high-resolution Mn 2p spectrum in which two typical peaks appear, Mn 2p 3/2 and Mn 2p 1/2, with binding energies of 656.80 eV and 654.27 eV, respectively. As shown in Fig. 4c, it can be seen from the O1s XPS spectrum that the peaks at binding energies O1s 531.75 eV and 525.18 eV correspond to Mn-O-Mn and H-O-H bonds, respectively, and the presence of Mn-O-H bonds is associated with the hydrothermal reaction. The C1s XPS spectra are displayed in Fig. 4d, and different chemical states of carbon atoms in three peaks are associated with O-C = O (284.45 eV), C-O (285.62 eV) and C-C (290.23 eV) bonds[24, 25, 33].
Electrochemical performance
Figure 5a shows the first five cycles of the CV curve at 0.1 mVs− 1, which shows the cyclic voltammetry and indicates that the electrochemical cycle maintains a certain structural stability. In the 1st cycle, only one oxidation peak appeared at 1.61 V when forward scanned starting from the open circuit voltage. When negatively scanned, two reduction peaks appear at 1.35 V and 1.28 V, respectively. In the following cycles, a reduction peak at ~ 1.3 V and an oxidation peak at ~ 1.62 V are observed. Remarkably, the plateau at 1.5 V was observed. To further study the reaction kinetics of the discharge platform. Figure 5b shows CV execution at different scanning rates (0.1, 0.2, 0.5, 1.0 and 2.0 mVs− 1) in the voltage range of 1.0-1.8 V. With a pair of characteristic oxidation/reduction peaks at approximately 1.7/1.3 V. The reduction peak at 1.3 V gradually strengthened as the scanning rate increased. The cyclic curve is complete, which shows that the reaction has good reversibility and stability. At the same time, the CV curves remain similar, the peak current increases obviously, and the oxidation/reduction peak is wider than the other peaks. It is obvious that the reduction peak gradually moves to low potential, and the oxidation peak gradually moves to high potential. This is because the diffusion resistance increases as the scanning rate increases. However, it is noteworthy that the peak value is suppressed at high potential sweep rates, such as 2.0 mVs− 1. According to previous studies, there is a relationship between the peak current and sweep rate of ln (i) = ln (a) + b ln (ν). The value range of b is 0.5-1.0. When b is 0.5, the reaction is a diffusion control process and insertion/extraction reaction. When b is 1.0, the reaction is a surface control capacitor response process. Figure 5c shows the fitting curve of the cathode current peak and anode current peak[3, 26, 33]. Slope b is 0.41 and 0.37, respectively, approximately 0.5, indicating a very typical diffusion control process. Figure 5d shows the constant-current charge‒discharge distribution curves at different current densities (0.2-2.0 Ag− 1). This result corresponds to the CV redox voltage platform at different scanning rates, but it is only obvious at low current densities and is almost invisible at high current densities, indicating that different redox reactions occur during the whole reaction process. The image of voltage variation in charge and discharge distribution is shown in Fig. 5e. Figure 5f shows the corresponding diffusion coefficient of Zn2+. The value of diffusion coefficient D ranges from 10 − 11 cm2 s− 1 to 10 − 14 cm2 s− 1. To further study the electrochemical reaction kinetics, Zn was selected as a more suitable cathode material, and the dynamic behavior of CCS@MnO2 was studied by constant current batch titration (GITT). [4, 28–30, 32].
To analyze the electrochemical impedance of CCS@MnO2, Nyquist diagrams of MnO2 and CCS@MnO2 are compared, as shown in Fig. 6a. The charge transfer resistance (RCT) in the electrochemical process belongs to the high-frequency region, while the ion diffusion in the electrochemical process belongs to the low-frequency region. By comparing the curve radius of the two samples in the high-frequency curve, it can be seen that the electrode of MnO2 is lower than that of CCS@MnO2 and has better performance. As shown in Fig. 6b, when the current density is 0.2 mA h g− 1, the specific capacity of CCS@MnO2 is 392 mA h g− 1, showing a high reversibility. Moreover, it has relatively good rate performance with capacities of 392, 343, 258, 202, 165 and 75 mA h g− 1 at 0.2, 0.4, 0.6, 0.8, 1.0 and 2.0 A g− 1, respectively. This indicates that CCS@MnO2 has better capacity retention than MnO2 and delivers a large specific capacity and excellent rate performance. Figure 6c shows the sample cycling performance of 500 cycles at a current density of 1.0 A g− 1 for the two cathodes. CCS@MnO2 drops significantly in the first cycle and becomes stable in the later cycle. The battery shows high structural stability, lasting cycling stability and no obvious capacity attenuation. The Coulombic efficiency is approximately 100%. The specific discharge capacity of MnO2 is reduced from 82 mA h g− 1 to 30 mA h g− 1, keeping the capacity at 63%. In contrast, the specific discharge capacity of CCS@MnO2 dropped from 170 mA h g− 1 to 70 mA h g− 1, maintaining 58% of the capacity. The main advantages of CCS@MnO2 electrochemical performance are its high theoretical specific capacity, high specific surface area, and its ability to maintain approximately 100% of the initial capacity[21, 22, 30–32].