## 3 Results and Discussion  
Figure 1 shows the XRD diffraction profiles of the NCA with different coating contents of LiAlO\({}_{2}\). As can be seen, the diffraction patterns of all samples are well-indexed to the hexagonal \(\alpha\)-NaFeO\({}_{2}\) structure with a space group \(R\overline{3}m\). All samples display clear diffraction peaks without any discernable secondary phase. The clear splitting of (006)/(012) and (018)/(110) peak pairs for NCA, NCA-0.25 and NCA-0.5 suggests the formation of highly ordered hexagonal layered structure [1; 17]. The results suggest that the crystal structure of NCA is not affected by a small amount of LiAlO\({}_{2}\). As seen in the diffraction patterns of NCA-2, the separation of (006)/(012) and (018)/(110) peak pairs is not clear, indicating that increasing amounts of Al(NO\({}_{3}\))\({}_{3}\cdot\)9H\({}_{2}\)O will influence the crystalline structure of NCA. Figure 1: XRD patterns of four samples, NCA, NCA-0.25, NCA-0.5 and NCA-2. Table 1 presents the values of a, c, c/a, \(I_{003}/I_{104}\) calculated from XRD patterns of the samples. The ionic radius of Ni\({}^{2+}\) (r = 70 pm) is close to Li\({}^{+}\) (r = 74 pm), which results in the migration of some of the Ni\({}^{2+}\) ions to Li\({}^{+}\) sites. The extent of cation mixing is usually evaluated by the intensity ratio of (003)/(104). Higher values of \(I_{003}/I_{104}\) correspond to a low extent of cation mixing. It can be seen from Table 1 that the value of \(I_{003}/I_{104}\) of all samples are higher than 1.2, which indicates that the samples have a low degree of cation mixing [17]. It is noted that the lattice parameter increases with the addition of Al(NO\({}_{3}\))\({}_{3}\cdot\)9H\({}_{2}\)O, but the value of c/a is not reduced. Larger ratios of c/a bring greater degrees of crystallinity and stability of the NCA with layered structure [18]. The SEM images of NCA, NCA-0.25, NCA-0.5, and NCA-2 are shown in Figure 2 and it can be seen that the surface of the pristine NCA is smooth. After LiAlO\({}_{2}\) modification, a layer appears on the surface of NCA and the surface becomes rough, which increases with increasing amount of Al(NO\({}_{3}\))\({}_{3}\)[18; 1]. From Figure 2b, it can be seen that there are some floccules on the particle surface and between the particle clearances. When the molar ratio of Al(NO\({}_{3}\))\({}_{3}\cdot\)9H\({}_{2}\)O and residual Li\({}_{2}\)CO\({}_{3}\) of the NCA surface is 0.5:1, more floccules are present between the particles and some floccules are agglomerated. Even at the molar ratio of Al(NO\({}_{3}\))\({}_{3}\cdot\)9H\({}_{2}\)O and residual Li\({}_{2}\)CO\({}_{3}\) is 2:1, the agglomerated floccules appear on the surface of NCA. In combination with the XRD, the layered structure of NCA is optimal at a molar ratio of Al(NO\({}_{3}\))\({}_{3}\cdot\)9H\({}_{2}\)O and residual Li\({}_{2}\)CO\({}_{3}\) of 0.25. Figure 3 displays the XRD patterns of NCA-0.25 under different calcination temperatures. The diffraction patterns of samples correspond to the hexagonal \(\alpha\)-NaFeO\({}_{2}\) structure with a space group \(R\bar{3}m\). When the calcination temperature is 400 \({}^{\circ}\)C (Figure 3b), the position of all peaks shifts slightly toward the lower degree. At the calcination temperature of 500 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, there are no peaks of the impurity phase, indicating that the structure of the sample is not destroyed. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **a (\(\pm\)0.0003 Å)** & **c (\(\pm\)0.0004 Å)** & **c/a** & **I003/I04** \\ \hline NCA & 2.8576 & 14.0508 & 4.92 & 2.35 \\ NCA-0.25 & 2.8601 & 14.0340 & 4.91 & 2.19 \\ NCA-0.5 & 2.8677 & 14.2335 & 4.96 & 1.90 \\ NCA-2 & 2.8648 & 14.0131 & 4.89 & 2.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Lattice parameters and the values of \(I_{003}/I_{104}\) for NCA, NCA-0.25, NCA-0.5 and NCA-2. Figure 2: SEM images of the as-prepared samples (**a**) NCA, (**b**) NCA-0.25, (**c**) NCA-0.5, (**d**) NCA-2.