## 3 Results and Discussion
NCA with higher pH values reacts readily with CO2 and atmospheric H2O and results in a degraded electrochemical performance. Therefore, unmodified NCA is detrimental to long term storage. Figure 4 shows the cycle performance of the pristine NCA sample and the NCA-0.25 samples at different calcination temperatures from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C. Table 2 shows that when the molar ratio of Al(NO3)3-9H2O and residual Li2 CO3 of the NCA surface is 0.25, the pH value and residual alkaline of materials is reduced, which is beneficial to storage performance. LiAlO2 modified NCA shows no improvements in first discharge capacity. However, the discharge capacity of LiAlO2 modified NCA is significantly higher than the raw materials at 1 C rate. Furthermore, capacity retention of LiAlO2 modified NCA is higher than the raw materials. The results indicate that LiAlO2 modification can improve the electrochemical properties and physical properties of the cathode materials. The NCA-0.25-400 \({}^{\circ}\)C exhibits an initial discharge capacity of 166.3 mAh/g at 1 C and capacity retention of 88.03% after 100 cycles. When the calcination temperature of NCA-0.25 is 500 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, the initial discharge capacity is 172.3 and 168.6 mAh/g at 1 C, and its capacity retention is 94.67% and 92.88%, respectively. Based on XRD analysis, the optimum electrochemical performance is when the calcination temperature of NCA-0.25 is 500 \({}^{\circ}\)C. At the same time, in order to avoid the uncertainty of the data caused by the accident of one measurement, we take the average value of each parallel test three times, the results are appropriate, seeing Table S1 and Figure S1. Figure 4: Cyclic performance of the pristine NCA sample and NCA-0.25 samples at different calcination temperatures from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C.  
Figure 3: (a) XRD patterns of the pristine NCA sample and NCA-0.25 samples at different calcination temperature from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C, and (b) the magnified patterns of (a) between 33\({}^{\circ}\) and 50\({}^{\circ}\). The TEM and EDS images of pristine NCA and NCA-0.25 are shown in Figure 5. The TEM images show that there is a significant difference in the particle surface before and after coating with a uniform coating layer of about 5.3 nm after LiAlO\({}_{2}\) modification (Figure 5b). The results indicate that the Al(NO\({}_{3}\))\({}_{3}\)\(\cdot\)9H\({}_{2}\)O reacted with the surface lithium compound residues and generated LiAlO\({}_{2}\), which can suppress the reactions between the electrode and the electrolyte, and facilitated the charge transfer process. The composition of NCA was examined by EDS, and it was found that NCA had typical diffraction peaks of O, Ni, Co, Al. The molar ratio of Ni, Co and Al was 82.4:14.8:2.8, which is close to the NCA molecular formula. Because Al(OH)\({}_{3}\) is a bisexual compound, it is difficult to co-precipitate Al with Ni and Co, and the actual Al content is smaller than 5%. Furthermore, the content of aluminum on the particle surface of NCA increased to 3.8% after LiAlO\({}_{2}\) modification [11]. Electrochemical measurements were taken to investigate the influence of LiAlO\({}_{2}\) on the electrochemical performances of NCA. Figure 6a shows the initial charge/discharge curves of NCA and NCA-0.25-500 \({}^{\circ}\)C at 0.2 C. The initial coulombic efficiencies of NCA and NCA-0.25-500 \({}^{\circ}\)C are 88.6% and 83.8%, respectively. Despite the initial coulombic efficiencies, they declined after LiAlO\({}_{2}\) modification, however the difference is small. When cells were cycled for 100 cycles at 1 C, the discharge capacities of raw NCA and NCA-0.25-500 \({}^{\circ}\)C were 154.5 and 163.1 mAh/g (Figure 6b), respectively, indicating that LiAlO\({}_{2}\) can improve the cycling stability of NCA [19].