Figure 1 shows XRD plots (a) and magnified views of peaks (b) (003), (c) (104) peaks, (d) (006)/(102) peaks and (e) (108)/ (110) peaks. All peaks correspond to NCA with α-NaFeO₂ layered structure, belonging to the hexagonal system (space group R-3 m). Moreover, it was confirmed that there are no additional peaks, indicating a secondary phase, are not observed^24.25. The Rietveld refinement is measured to confirm the lattice parameters and the I(003)/I(104) ratio, as shown in Fig. 2. All samples with an I(003)I(104) ratio greater than 1.2 show low cationic disorder. Moreover, the obvious division of I(006)/I(102) and I(108)/I(110) demonstrates that all samples except 660°C are a well-formed ordered layer structure. As shown in Table 1, the intensity ratio of I(003)/I(104) increased as the temperature increased from 660 to 720°C. However, it can be confirmed that the intensity ratio of I(003)/I(104) is negatively decreased at the sintering temperature above 720°C. It can be concluded that excessive sintering temperature deteriorates the structural stability of NCA.

Results of the Reitveld refinement of the sintered NCA at (a) 660°C, (b) 690°C, (vs) 720°C, (D) 750°C, (e) 780°C and (F) 810°C.

FE-SEM images of sintered NCA at different sintering temperatures [(a) 660 °C, (b) 690 °C, (c) 720 °C, (d) 750 °C, (e) 780 °C and (f) 810 °C] are illustrated in Fig. 3. From SEM images, all samples exhibit spherical morphology with an average particle size of 10~20um, which is composed of many primary particles (400~800nm). Notably, the average primary particle size increases as the sintering temperature increases from 660 to 810°C. The average primary particle size sintered at 660°C is about 450 nm, much smaller than that of NCA (800 nm) sintered at 810°C. Moreover, the clear Ni, Co and Al peaks were observed in the EDS mapping without any impurity peaks, as shown in Fig. 3g. This indicates that the desired composition of NCA has been successfully synthesized.

FE-SEM images of NCA sintered to (a) 660°C, (b) 690°C, (vs) 720°C, (D) 750°C, (e) 780°C and (F) 810°C (g) EDS mapping of NCA sintered at 720 °C.

Figure 4a shows the initial charge-discharge curves of NCA with different sintering temperatures at a current density of 0.5 A/g in the voltage range of 3.0 to 4.3 V. The voltage plateau around of 4.2 V is due to the high concentration of Ni ions in the NCA.²⁶. We can confirm that the capacity of all samples except 660℃ and 810°C is not significantly different. Therefore, the sintering temperature does not significantly affect the initial charge-discharge capabilities in the range of 690-780°C. The sample sintered at 720°C has the highest discharge capacity of 217.48 mAh g⁻¹ with an excellent Coulomb efficiency of 87.84%. This is due to the low mixing of cations and the well-crystallized layered structure, as mentioned earlier (Fig. 1). Figure 4b shows the rate performance of NCA with different C rates (0.1, 0.5, 1.0, and 2.0 C) over a voltage range of 3.0 to 4.3 V. All performance Rates were measured at a fixed charge current density of 0.5 C while discharge current density was measured at different current densities. The cell was cycled at each rate and then again at 0.5 C. The rate capacities of all samples are decreased with increasing C rate. However, it can be seen that the difference between the capacity values ​​of the samples increases as the C rate increases. Compared to the other cases, the NCA sintered at 720°C retains the highest capacities regardless of the C-rates. More importantly, the NCA sintered at 720°C still has the highest capacity of 195.3 mA hg⁻¹ when the C rate is reduced to 0.5 C, indicating greater reversibility. Indeed, a well-defined layered structure via optimized sintering temperature enables electrochemically active {010} planes of lithium ions and electronic conduction despite the fast charge/discharge rate, resulting from high cationic ordering and a highly crystallized layered structure.

(a) Initial charge-discharge curves and (b) evaluate the performance of NCA.

Figure 5 shows the cycle performance of NCA samples at 0.5 C in the voltage range of 3.0 to 4.3 V. No obvious capacitance fade is observed for any samples until 45 cycles. However, there is a difference in the rate of decline for each sample after 45 cycles. Among samples at different sintering temperatures, NCA sintered at 720°C has the highest capacity with a capacity retention of 95.4% after 80 cycles. This can be explained not only by the rapid/sustainable electrochemical kinetics but also by the interfacial stability between electrode/electrolyte. On the other hand, the NCA sintered at 660°C and 810°C shows lower cyclabilities compared to the others. The samples sintered at 660°C and 810°C show a strong decrease in capacity during cycling. This is due to lower crystallization (660 °C), disordered layered structure (660 °C and 810 °C), and longer Li ion transfer channels (810 °C). Such drawbacks cause a sudden drop in capacitance during cycling as they could destroy the structural integrity of the NCA, resulting in reduced reactivity of the active material. Hydrogen fluoride (HF), has been reported to come from the reaction of water and LiPF₆, is one of the most important factors of performance degradation. Indeed, the HF elutes the transition metal ions in the NCA, resulting in a deformation of the layered structure. The optimal sintering temperature can suppress the negative effects on the NCA by increasing the structural stability.

NCA cycling performance.

To better understand the electrochemical performance, EIS tests of sintered NCA at 660°C and 720°C are carried out. Nyquist plots for the two samples after the 1st and 80th cycles are shown in Figs. 6a and b. It is well known that Nyquist plots are composed of three components: electrolytic resistance (R_s) in the high frequency, the charge transfer resistance (R_side) at medium frequency and the Warburg impedance at low frequency^26,27,28. Among three components, the R_side can be considered a key parameter for cathodic impedance, affecting electrochemical behavior. The R_s the values ​​of the two samples are almost identical since they use the same electrolyte. However, there is a significant difference in R_side values ​​between 1 and 80 cycles for both samples. Among them, the increase in R_side The value of NCA sintered at 660°C (174.4 Ω to 295.9 Ω) is much larger than that of NCA sintered at 720°C as shown in Table 2. It can be inferred that the layered NCA highly crystallized offers enlarged exposed active planes for Li ions. The R_side The value of NCA sintered at 720°C is approximately two-thirds of that of NCA sintered at 660°C, resulting from enhanced transport of charge carriers to the surface of the NCA. Therefore, it can be concluded that the optimized sintering temperature is beneficial for suppressing capacitor discoloration during cycling based on the structural stability of NCA.

Nyquist plots of NCA sintered to (a) 660°C and (b) 720°C.

Figure 7 shows the HCl titration curves of NCA with different sintered samples. Residual lithium compounds (Li₂CO₃ and LiOH) is derived from (i) moisture absorption and (ii) spontaneous reduction of Ni³⁺ in Nor²⁺, accompanied by the release of oxygen. The amount of HCl used in HCl titration to pH 4 of sintered NCA at 660°C is higher than that at 720°C as shown in Table 3. The amount of Li₂CO₃ and LiOH can be calculated via the following equations^29.30:

HCI NCA titration curves with different sintering temperatures.

The Li₂CO₃ is produced on the surface of LiOH by reaction with CO₂then some moisture remains, as follows:

Also, the CO₂ and POF₃ gases are generated by reaction with Li₂CO₃ and LiPF₆. It can be represented by the following equation:

Some of the lithium comes from within the NCA with the enhanced cation disorder. Therefore, the amount of residual lithium on the surface of a sample sintered at 720°C is small due to the optimum sintering temperature³¹. This indicates that proper sintering temperature can effectively suppress gassing and performance degradation due to smooth surface chemistry without unwanted materials.