The Ni–Co–Mn(OH)₂ precursor with an Ni:Co:Mn molar ratio of 0.88:0.06:0.06, synthesized via co-precipitation, was obtained from POSCO. This precursor, Ni_0.88Co_0.06Mn_0.06(OH)₂, was mixed with LiOH·H₂O (Li:TM = 1:1) and calcined under an O₂ atmosphere in a tube under the following conditions: temperatures of 225, 250, 275, 300, 400, 500, and 600°C for 0 h and 700°C for 0–15 h.

The crystal structure was analyzed via X-ray diffraction (XRD; New D8-Advance, Brooker-AXS with Cu Kα radiation) with a scan rate of 2.75°/min in the range of 10–90°. Particle surface morphology was observed via field emission scanning electron microscopy (FE-SEM, JEOL) at 5 kV. Energy-dispersive X-ray spectroscopy (EDS) was conducted at 20 kV using SIGMA 300 (Carl Zeiss) to qualitatively analyze the elemental distribution on the particle surface. Cross-sections of the particles were polished prior to the analysis.

To evaluate the electrochemical performance, the cathode material was mixed with polyvinylidene fluoride (PVDF) binder and Super P conductive carbon black in a weight ratio of 94:3:3. The mixture was then dispersed in n-methylpyrrolidine (NMP) to form a slurry, which was then coated onto an Al foil. After drying in an oven at 120°C for 15 min, coin-type electrodes with a diameter of 10mm (Ø10) were obtained through roll pressing and punching. These electrodes were then dried in a vacuum oven at 120°C for 12h. Subsequently, they were sequentially assembled into 2032 coin-type half-cells with a Li-metal counter electrode. LiPF₆ (1 M), composed of ethylene carbonate and ethyl methyl carbonate (EC:EMC = 3:7 (volume ratio)), was used as the electrolyte. The electrochemical performance of the cell was evaluated by constant current cycling in the voltage range of 3.0–4.3 V vs. Li/Li⁺ at room temperature.

TGA was conducted to observe the compositional changes in the Ni–Co–Mn(OH)₂ precursor and the Ni–Co–Mn(OH)₂ mixed with LiOH·H₂O during calcination. The analysis was performed under flowing O₂ air at a heating rate of 2°C/minfrom 35–800°C as shown in Fig. 1. The TGA curve for the formation of Ni–Co–Mncathode material exhibits three significant weight loss regions: 31–100, 200–250, and 250–500°C. The first weight loss (12.8%) can be attributed to the dehydration of LiOH·H₂O [12]. The second weight loss (11.8%) can be ascribed to the decomposition of Ni–Co–Mn(OH)₂ to (Ni–Co–Mn)O and H₂O. The weight loss (4.3%) is related to the reaction of (Ni–Co–Mn)O with the lithium source and the formation of lithiated nickel-cobalt-manganese oxide (Li(Ni–Co–Mn)O₂). By contrast, the TGA curve of the Ni–Co–Mn(OH)₂ precursor shows a sharp weight decrease only around 250°C, indicating that a calcination temperature above 500°C is required to obtain a layered structure of the NCM cathode material.

XRD analysis was performed to trace the reaction pathway and synthesis mechanism of the Li(Ni_0.88Co_0.06Mn_0.06)O₂ cathode material during calcination. Fig. 2 shows the XRD patterns of the NCM compound synthesized at temperatures ranging from 225–700°C. The synthesized samples were designated based on the temperature; for example, NCM-700 indicates an NCM powder calcined at 700°C. When only the precursor was mixed with the lithium source, the XRD pattern showed peaks corresponding to M(OH)₂ (denoted as M = Ni–Co–Mn) and LiOH·H₂O [13]. The LiOH peak was attributed to the dehydration of LiOH·H₂O synthesized at 225–250°C [14]. At 300°C, the peak of the M(OH)₂ precursor disappeared and the MO peak increased. Around 400°C, the LiOH peaks significantly diminished owing to Li diffusion into the MO structure through a solid-state reaction, causing a decrease in lattice size and transformation to the Li_xM_1–xO phase. At 500°C, only the MO peak remained and gradually shifted to a higher angle. However, the XRD patterns of the powder synthesized at 500°C for 5 h revealed peaks corresponding to LiMO₂, indicating a solid-state reaction with the lithium source. No further phase changes were observed, suggesting that a calcination temperature of at least 500°C with an appropriate dwell time is sufficient to obtain the layered crystal structure. Increasing the temperature to 700°C resulted in sharper peaks than those of NCM-600, indicating crystal growth of the LiMO₂ phase. Additionally, all diffraction peaks were indexed to the α-NaFeO₂ structure with a hexagonal-type of R-3m space group, indicating no crystalline impurities [15]. Sintering NCM-700 for 0, 5, and 15 h further increased the peak, leading to a clear splitting of (108)/(110) peaks, thereby confirming the formation of a well-ordered layered structure in the LiMO₂ cathode material [16]. Additionally, the values of I(003)/I(104) corresponding to the calcination times of 0, 5, and 15 h are 1.28, 1.88, and 1.91, respectively. This observation highlights the importance of a sufficient temperature and retention time to form the layered structure of the Ni-rich NCM cathode material. Furthermore, the (104) peak shifted to a higher angle and the (003) peak appeared (up to 500°C), demonstrating the formation of the R-3m layered phase [17].

Fig. 3–7 show the morphologies and surface chemistries of the powders calcined at 225, 300, 400, and 500°C for 0 h and 700°C for 15 h, as analyzed via SEM and EDS mapping. In the powder prepared at 225°C (Fig. 3), a dark region circled in yellow was identified in the SEM image. EDS mapping showed that these regions lacked Ni, Co, and Mn signals. Moreover, numerous non-spherical, nano-sized, and irregular particles were observed, as depicted in Fig. S1. These findings suggest that the lithium source did not react with the NCM hydroxide precursor and remained on the particle surface.

Fig. 4 indicates that dark regions are present even in the sample calcined at 300°C, implying that although the precursor reacted with oxygen to form the (Ni–Co–Mn)O phase, the LiOH particles still remained on the surface.

The powder synthesized at 400°C also shows the same LiOH particles (Fig. 5); however, a new rod-shaped phase that was not observed at lower temperatures was detected as shown in Fig. S3. This suggests that some LiOH started reacting with (Ni–Co–Mn)O to form a lithiated NCM (Lix(Ni–Co–Mn)_1–xO) phase through a solid-state reaction. Increasing the temperature to 500°C eliminated the dark regions (Fig. 6), probably because the melting point of LiOH is approximately 462°C. This allowed some lithium to directly diffuse into the NCM structure, leading to complete synthesis and gradual transformation into a layered phase, consistent with the XRD results. Additionally, most secondary particles became spherical after calcination (Fig. S4)

Thus, it was confirmed that increasing the temperature and time (700°C for 15 h) noticeably reduced the submicron particles compared to that at low temperatures, and the secondary particles were composed of primary particles with a layered phase (Fig. 7 and S5).

Fig. 8 illustrates the synthesis mechanism of Ni-rich NCM cathode material at various calcination temperatures, highlighting the key structural changes. The overall reaction can be summarized as follows:

1. At 100°C, lithium hydroxide monohydrate undergoes dehydration:

LiOH·H₂O → LiOH + H₂O (31–100°C)

2. At approximately 275°C, M(OH)₂ undergoes dehydration, producing an MO phase:

M(OH)₂ → MO + H₂O (200–275°C)

3. As the melting point of LiOH is approximately 462°C, LiOH directly reacts with MO in the solid phase to form a LixNi1–xO phase between 275 and 470°C:

MO + LiOH + O₂ → Li_xM_1–xO + H₂O (275–470°C)

4. Above 470°C, the produced Li_xM_1–xO reacts completely with LiOH in a solid-state reaction to form a LiMO₂ layered phase:

Li_xM_1–xO + LiOH + O₂ → LiMO₂ (>470°C)

To investigate the effect of different calcination holding times on the electrochemical performance of the LiNi_0.88Co_0.06Mn_0.06O₂ cathode, 2023 coin-type half-cells with Li foil were assembled as the counter electrode. The cathode materials were initially calcined at 480°C for 2 h and then at 700°C for 5, 10, and 15 h, as shown in Fig. 8. Fig. 9a–c shows the charge–discharge curves in a voltage range of 3.0–4.3 V at 1 C for the 1st, 25th, and 50th cycles. Evidently, all samples exhibit a voltage plateau near 4.08 V; however, for the sample calcined for 15 h, it disappears near 3.5 V after 50 cycles. According to the results presented in Table S1, residual Li compounds decrease as the calcination time increases. The initial discharge capacities of the samples calcined for 5, 10, and 15 h are 193.54, 202.98, and 201.47 mAh/g, respectively. The sample calcined at 700°C for 15 h exhibits the highest capacity retention, which is attributed to a reduction in residual lithium and increased particle size with longer holding times.

Fig. 10 shows the rate capability and c-rate efficiency at various current densities (0.1, 0.2, 0.5, 1, 2, and 4 C) and in the voltage range of 3.0–4.3 V. All samples demonstrate similar specific capacities and c-rate efficiencies at all current densities. However, the sample calcined at 700°C for 5 h exhibits the highest rate efficiency, primarily owing to its smaller particle size than the other samples. Typically, small particle sizes in NCM cathode materials enhance their power characteristics.