## 3 Results and Discussion  
### Morphology and Structure of Carbon Black Additives  
To derive the effect of different SSA carbon blacks on the performance of VRLA batteries, three kinds of carbon black materials were characterized by different physical properties (such as BET surface area, Raman, SEM and TEM). The SSA and pore volume of the carbon black additives were analyzed by using N\({}_{2}\) adsorption/desorption isotherms, as shown in Figure 2a. The isotherms of carbon black samples exhibit typical IV isotherms (The IV isotherm shows a significant inflection point in the lower region of the relative pressure (P/P\({}_{0}\)), and hysteresis loops occur under higher pressure due to the phenomenon of capillary condensation.) with a significant hysteresis loop, indicating the presence of a typical mesoporous structure in all samples [25]. The BET surface areas of carbon black 1 (CB1), CB2 and CB3 were 156 m\({}^{2}\) g\({}^{-1}\), 234 m\({}^{2}\) g\({}^{-1}\) and 1378 m\({}^{2}\) g\({}^{-1}\), with BJH adsorption cumulative volumes of pores of 0.387 cm\({}^{3}\) g\({}^{-1}\), 1.100 cm\({}^{3}\) g\({}^{-1}\) and 1.129 cm\({}^{3}\) g\({}^{-1}\), respectively. It is apparent that the CB3 sample possesses a larger surface area and a higher pore volume. Figure 2b shows the pore size distribution of the three carbon black samples. There is only one mesopore peak in the curve of CB1 and CB3, which is mainly centered at 2.5 nm and 10 nm, respectively. However, the CB2 sample has two pore-size distribution peaks and the peaks of mesopore and macropore were about 2.5 nm and 50 nm, respectively. Carbon black with suitable mesopore distribution facilitates rapid diffusion of the acid electrolyte and promotes ion diffusion during the electrochemical processes [26]. Therefore, the mesoporous structure is conducive to improving the charge acceptance of VRLA batteries for e-bikes in the deep cycle duty. As seen in Figure 2c, the Raman spectra of carbon black exhibits two main characteristic peaks: the D band (1350 cm\({}^{-1}\)) is known as the disordered band and is caused by defects in the lattice of carbon atom, and the G band (1568 cm\({}^{-1}\)) reflects the crystalline and graphitic structure of the carbon materials [27]. The structural defects and the indication of disorder of the carbon black material can be characterized by the intensity ratio of \(\rm I_{D}/I_{G}\)[28]. \(\rm I_{D}/I_{G}\) is the ratio of the intensity of the D peak to that of the G peak. This ratio generally indicates the degree of defect density of the carbon material, and the larger the ratio of \(\rm I_{D}/I_{G}\), the greater the degree of defect of the material. Therefore, in the Raman spectra, the \(\rm I_{D}/I_{G}\) values of CB1, CB2 and CB3 are calculated to be 0.80, 0.89 and 0.95, respectively, indicating that the CB3 sample has more defective sites. Accordingly, with the increase of SSA and pore volume, the defect degree of the samples is also increased. Meanwhile, the ratio of \(\rm I_{D}/I_{G}\) is inversely proportional to the grain size, which is confirmed by TEM observations. The morphology and microstructure of three samples with different SSAs of carbon black is studied by SEM, TEM and high-resolution TEM (HRTEM), as shown in Figure 3. From the SEM images, it can be seen that the CB1 (Figure 3a), CB2 (Figure 3b) and CB3 (Figure 3c) particles have a spherical structure. The mean size of CB1 particles is about 38 nm and they exhibited a certain degree of agglomeration. The average particle size of CB3 (about 4 nm) is significantly smaller than that of CB2 (about 25 nm) and the aggregation of the small particles of CB3 is more severe than CB2. Furthermore, the CB1 particle size is obviously larger than that of the other two particles. As the particle size decreases, the SSA of carbon black increases. Figure 3d-g further verify that CB1, CB2 and CB3 particles are irregular spherical structures whose particle size is basically consistent with the structure range obtained from the SEM images.