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. 2024 May 22;9(22):23597–23602. doi: 10.1021/acsomega.4c01031

Effect of Stirring on Particle Shape and Size in Hydrothermal Synthesis of LiCoO2

Naoki Hamao 1,*, Hiroki Itasaka 1, Ken-ichi Mimura 1, Zheng Liu 1, Koichi Hamamoto 1
PMCID: PMC11154888  PMID: 38854562

Abstract

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Lithium cobalt oxide (LCO) is a highly significant material for the positive electrodes of lithium-ion batteries. Due to the correlation between crystal morphology and electrochemical performance in the layered rock-salt structure, LCO with crystal morphology exhibiting anisotropy demonstrates superior charge–discharge characteristics. In this study, various morphologies of LCO were synthesized via hydrothermal synthesis using a plate-like precursor. Under conditions without agitation, a hexagonal plate-like LCO was synthesized, while a spherical LCO was obtained with agitation during synthesis. The particle morphology was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Furthermore, in the performance evaluation of positive electrode materials for lithium-ion batteries, the hexagonal plate-like LCO exhibited a larger charge–discharge capacity compared to the spherical LCO.

1. Introduction

Lithium cobalt oxide (LCO) functions as a positive electrode material in lithium-ion batteries (LIBs).1,2 Due to its layered rock salt structure, LCO demonstrates anisotropic lithium ion diffusion, establishing a correlation between the crystal morphology and electrochemical performance of LCO. To date, the impact of crystal size and shape on charge–discharge and rate capabilities has been studied using various synthesis methods.3−5 Notably, LCO demonstrates high lithium ion diffusion along the (100) and (010) planes, resulting in numerous studies on the synthesis and charge–discharge characteristics of plate-like LCO.6−10 Furthermore, the reduction in lithium ion diffusion distance achieved through particle size reduction contributes to high-capacity and high-rate charge–discharge behavior.11−13

The particles with oriented planes in the a-b plane direction coincide with the diffusion direction of lithium ions in the crystal structure. Therefore, the particles are expected to exhibit excellent charge–discharge cycle characteristics and rate capabilities. It has been reported that plate-like LCO is synthesized by typical solid-state reaction using a plate-like β-Co(OH)2 precursor and lithium sources at relatively high temperatures.7,14 However, the high-temperature calcination step introduces cracks and distortion during the β-Co(OH)2 to LCO conversion and lithium ion insertion reactions. Xiao et al. suggested that these cracks help to enhance access of the electrolyte to the internal surface of the electrode, contributing to excellent charge–discharge characteristics. Therefore, synthesizing oriented particles without cracks is essential for accurately investigating the impact of particle shape on charge–discharge, particularly for particles oriented along the ab plane. Xia et al. have successfully synthesized single-crystalline, self-supported LCO nanoflake arrays directly grown on carbon cloth through a simple “hydrothermal lithiation” process involving low-temperature (380 °C) annealing.15 Here, by utilizing α-Co(OH)2 as the starting material, they effectively synthesized layered single-crystalline LCO nanoflake arrays without significant morphological changes due to a topological reaction. On the other hand, Liu et al. reported the synthesis of platelet cobalt hydroxide in a solution process.16 We speculated that by utilizing plate-like α-Co(OH)2 material, LCO oriented in the direction of lithium ion conduction would be synthesized due to the topological reactions. Furthermore, we expected to facilitate smoother lithium ion insertion reactions during synthesis by using the wider interlayer α-Co(OH)2 compared to β-Co(OH)2.17

In this study, we demonstrated the low-temperature synthesis of hexagonal plate-like LCO using hexagonal plate-like α-Co(OH)2 precursors. Additionally, we investigated the influence of agitation during the synthesis on the resulting particle shape. By measuring the charge–discharge characteristics of the synthesized LCO particles, we investigated the relationship between the shape of the LCO crystals and their charge–discharge properties.

2. Experimental Section

2.1. Preparation of α-Co(OH)2

The plate-like α-Co(OH)2 was prepared by liquid phase synthesis based on the procedure reported by Liu et al.16 CoCl2·6H2O (99.0%, Wako Pure Chemical Industries Ltd., 20 mmol), NaCl (99.0%, Wako pure Chemical Industries Ltd., 50 mmol), and hexamethylenetetramine (99.0%, Wako Pure Chemical Industries Ltd., 60 mmol) were dissolved in 400 mL of deionized water. The solution was refluxed at 130 °C for 1 h in an ambient atmosphere. After the pink solution turned into a green suspension, the obtained green suspension was filtered and dried overnight at 70 °C.

2.2. Synthesis of LiCoO2

LiCoO2 was synthesized by a hydrothermal method. The plate-like α-Co(OH)2 powder (0.1 g), lithium hydroxide monohydrate (98.0%∼ , Wako pure Chemical Industries Ltd., 2.51778 g, 60 mmol), and 30 mL of deionized water were placed in an autoclave with carbon fiber lining (100 mL capacity, Sanai Kagaku Co., Ltd.). The autoclave was then set in a hot stirrer reaction decomposition device (model RDV-TMS, Sanai Kagaku Co., Ltd.). The hydrothermal synthesis was carried out at 240 °C for 24 h. After synthesis, the precipitate was collected by centrifugation and washed with H2O four times. Lithium cobaltite was then obtained by vacuum drying at 70 °C.

2.3. Characterization

The X-ray diffraction (XRD, SmartLab, RIGAKU) patterns of the samples were recorded using CuKα radiation with 2θ step of 0.02° at room temperature. The morphology of the samples was observed using SEM (JSM-6000Plus, JEOL) at an acceleration voltage of 10 kV. Furthermore, dynamic light scattering (Ultra model ZSU5700, Malvern) is also used to assess the particle size of the synthesis sample. Transmission electron microscopy (TEM) images were taken for the samples directly dispersed on a TEM grid using a JEM-ARM200F (cold FEG) equipped with CEOS double Cs correctors, operated at 200 kV.

2.4. Cell Performance Measurement

Electrochemical measurements were conducted using a coin-type cell (CR2032) assembled inside an argon-filled glovebox. The working electrode was fabricated by using 5 mg of active material, 5 mg of acetylene black (AB) as a conductive additive, and 1 mg of polytetrafluoroethylene (PTFE) as a binder. The working electrode was prepared by mixing the active material, AB, and PTFE in a weight ratio of 90.5:6.5:3.0. Moreover, for charge–discharge rate measurements, the components were mixed in a weight ratio of 80:17.5:2.5, emphasizing a higher ratio of conductive additive. An aluminum mesh served as the current collector, and the electrode diameter was set at 15 mm. The separator was a microporous polypropylene sheet. A lithium foil with a diameter of 16 mm was used as the counter electrode. The electrolyte was LiPF6 dissolved in ethylene carbonate(EC)/diethyl carbonate(DMC) (1:1). The charge–discharge cycles were performed in constant current mode with the voltage range of 2.5 to 4.3 V at a 1/50 C rate. During the assessment of rate characteristics, the cycling protocol involved five cycles at each of the following rates: 0.05, 0.1, 0.5, and 1C. Cyclic voltammetry (CV) measurements were carried out in the voltage range between 2.5 and 4.5 V at a scan rate of 0.1 mVs–1 using Potentiogalvanostat (SP-300-AH-2CH, Biologic).

3. Results and Discussion

3.1. Phase Identification and Crystal Morphology of α-Co(OH)2

Figure 1 shows (a) the XRD pattern and (b) the SEM image of α-Co(OH)2. All diffraction peaks were attributed to rhombohedral symmetry, with a space group in R-3m. The lattice parameters were calculated as a = 0.31492(1) nm and c = 2.4058(1) nm. These values are in excellent accordance with previously reported results and references (JCPDS: file no. 46-0605). The SEM image unveils a platelike crystal morphology of α-Co(OH)2. As observed from the image, a myriad of crystals exhibiting a hexagonal shape was identified, reminiscent of the crystal morphology documented by Liu et al.16

Figure 1.

Figure 1

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(a) XRD pattern and (b) SEM image of a α-Co(OH)2 precursor synthesized from reflux reaction of CoCl2·6H2O, NaOH, and HMT at 130 °C for 1 h.

3.2. Phase Identification and Crystal Morphology of LiCoO2 Prepared by Hydrothermal Method

The XRD patterns and SEM images of LCO, synthesized by using the hydrothermal method with varying stirring speeds, are presented in Figures 2 and 3, respectively. All observed diffraction peaks were attributed to LiCoO2. However, a small amount of Na0.70CoO2 remained as impurities (2θ = 17°).18 This suggests that the cleaning process during the precursor synthesis was incomplete. The stirring speed during the hydrothermal synthesis influenced the diffraction patterns, causing peak broadening and a shift toward lower angles. As depicted in the enlarged view of Figure 2, the diffraction peaks distinctly broaden with the increase in stirring speed. This indicates that the stirring speed during synthesis influences the crystal size. The calculated lattice constants from the XRD pattern are summarized in Table 1, showing a slight increase with increasing stirring speed. The SEM images show that the stirring conditions significantly influenced the morphology of LCO. Hexagonal plate-shaped crystals were obtained without stirring (Figure 3a), with clear facets. At 240 rpm, irregularly shaped crystals and fine crystals of less than 150 nm were observed (Figure 3b). When the stirring speed was set to 1000 rpm, the crystal morphology of LCO was not plate-like LCO (Figure 3c). Furthermore, the increase in stirring speed is associated with a reduction in the crystal size, consistent with the broadening of XRD diffraction peaks. In the relatively high-temperature sintering process, numerous cracks appeared on the plate-like LCO due to degassing and Li-ion insertion reactions.7,14 However, in this study, the generation of cracks on the crystal surface was effectively suppressed through utilization of the relatively low-temperature synthesis method. In general, agitation during hydrothermal synthesis effects on the increase in the probability of spontaneous nucleation and the growth rate.19,20 Moreover, the obtained size of the crystals is determined by the equilibrium between the nucleation rate and growth rate. In our study, it is considered that due to the dominance of nucleation rate over the crystal growth rate, the crystal size decreased with increasing stirring speed.

Figure 2.

Figure 2

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X-ray diffraction patterns of LiCoO2 synthesized at stirring speeds of (a) without stirring, (b) 240 rpm, and (c) 1000 rpm.

Figure 3.

Figure 3

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(a) Low and (b) high-magnification SEM images of LiCoO2 particles synthesized at stirring speed without stirring. High-magnification SEM image of LiCoO2 particles synthesized at stirring speeds of (c) 240 and (d) 1000 rpm.

Table 1. X-ray Diffraction Patterns of LiCoO2 Synthesized without Stirring and at Stirring Speeds of 240 and 1000 rpm.

sample a (nm) c (nm)
without stirring 0.28163(1) 1.4056(9)
240 rpm 0.28171(1) 1.4061(9)
1000 rpm 0.28178(1) 1.4071(9)
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The left panel in Figure 4 shows a low-magnification TEM image of a hexagonal LCO plate, and the right panel displays the corresponding selected-area electron diffraction (SAED) pattern obtained from the yellow-circled region in the TEM image. The reflection indices, based the LCO crystal lattice parameters, are labeled in the SAED pattern. SAED observations across the entire LCO plate indicate that it is a single crystal. The normal direction of this LCO plate aligns with the <001> direction, and each terminal edge plane of this hexagonal plate belongs to {100}. Figure 5 shows the STEM-EDS analysis, revealing the uniform diffusion of the Co element within the plate-like crystals.

Figure 4.

Figure 4

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(left) TEM image of LCO without stirring and (right) SAED pattern derived from the TEM image.

Figure 5.

Figure 5

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(a) STEM image of LCO without stirring. (b) Co and (c) O of EDX elemental mapping images of (a).

3.3. Electrochemical Performance

The electrochemical behavior of the prepared LCO was investigated by using a coin-type cell, and the results are depicted in Figure 6. Each sample exhibited an initial charge capacity of over 170 mAh/g, while the initial discharge capacities for LCO synthesized without stirring, and with stirring at 240 and 1000 rpm, were 143, 135, and 137 mAh/g, respectively. The plate-like LCO synthesized without stirring showed a slight decrease in capacity after 20 cycles. However, the plate-like LCO maintained a discharge capacity of over 130 mAh/g even after 20 cycles, demonstrated a superior stability in charge–discharge capacity at low rates compared to LCO synthesized under other conditions. On the other hand, the LCO synthesized at stirring speeds of 240 and 1000 rpm exhibited a decrease in charge–discharge capacity with each cycle. Additionally, a small plateau at 3.8 V was observed in the charge curve of the second cycle of LCO synthesized at stirring speeds of 240 and 1000 rpm, indicating the occurrence of phase transitions during the charge–discharge process.

Figure 6.

Figure 6

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Charge and discharge characteristics for LiCoO2 synthesized at stirring speeds of (a) without stirring, (b) 240 rpm, and (c) 1000 rpm. The first 20 cycle at 0.02C rate.

Furthermore, the discharge capacity of these LCO samples drastically decreased to 115 mAh/g after 20 cycles, suggesting the influence of resistance layers in the charge–discharge process. Additionally, Figure 7 shows the charge–discharge curves of the synthesized LCO samples at different rates. For all LCO samples, an initial irreversible capacity was observed from the first few cycles, which is believed to be primarily attributed to the formation of the solid electrolyte interphase (SEI) at the interface between the electrolyte and the electrode. At relatively low rates of 0.05C and 0.1C, the charge–discharge capacities decreased with each cycle, with the 10th cycle discharge capacities for LCO synthesized without stirring, and with stirring at 240 and 1000 rpm being 146, 133, and 137 mAh/g, respectively. However, when the charge–discharge rate was increased to 1C, the discharge capacities of both samples significantly decreased. Specifically, the discharge capacities for LCO synthesized without stirring and with stirring at 240 and 1000 rpm were 83, 52, and 68 mAh/g, respectively. In contrast to this study, bulk LCO in other reports maintained a discharge capacity of 130 mAh/g even at 10C rate.14 This significant decrease in charge–discharge capacity could be attributed to the relatively low quantity of conductive additives, leading to increased internal resistance. The charge–discharge capacities of these LCO synthesized via low-temperature methods in this study were found to be larger than that of LCO synthesized without any post-treatments at high temperature.21 In comparison to LCO subjected to thermal treatment, the charge–discharge capacity and rate performance of the LCO in this study are noticeably lower.4,7,14 Hence, for potential applications, we are also considering improvements in the characteristics through thermal treatment.

Figure 7.

Figure 7

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Rate capability plots of LiCoO2 synthesized at stirring speeds of (a) without stirring, (b) 240 rpm, and (c) 1000 rpm (■, charge capacity; ● (red circle), discharge capacity).

To investigate the influence of particle shape on the phase transitions, cyclic voltammetry measurements were conducted. Figure 8 shows the cyclic voltammetry curves of the plate-like and the spherical LCO electrodes.

Figure 8.

Figure 8

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Scanning cyclic voltammetry curves of; LiCoO2 synthesized at stirring speeds of (a) without stirring and (b) 1000 rpm. Scan rate: 0.1 mV s–1.

It is known that LCO undergoes structural changes during charge and discharge reactions, initially transforming to the monoclinic phase and subsequently to the hexagonal phase.22 As evident from Figure 8, a significant shift in the oxidation–reduction peaks is observed, indicating ongoing phase transitions in each cycle. However, Figure 8 reveals that the peak shift attributed to phase transitions is smaller in spherical LCO compared to the plate-like LCO. Additionally, It has been reported that the phase transitions during charge and discharge are suppressed in LCO with excess lithium incorporation.23 From the XRD results, it is evident that the lattice constants differ between the plate-like and the spherical LCO. We speculated that these initial differences in crystal structure influenced the structures during charge and discharge. Crystal structure analysis after charge–discharge testing is important for clarification of the reaction mechanisms. However, due to the inclusion of multiple additives in this study, obtaining precise crystal structure information necessitates access to advanced experimental facilities, such as synchrotron radiation facilities like SPring-8. We acknowledge the significance of these data and plan to address these methodological challenges in future investigations.

4. Conclusion

In this study, a various morphology LCO was synthesized via low-temperature synthesis using α-Co(OH)2 precursors via hydrothermal synthesis. Plate-like LCO was synthesized at low temperatures by employing plate-like α-Co(OH)2 precursors, and further, spherical LCO particles were synthesized by controlling the stirring speed during synthesis. Analysis such as XRD patterns and SEM images confirmed the hexagonal crystal morphology of LCO with distinct facets. Electrochemical performance evaluation showed relatively stable charge–discharge capacity at low rates, with the hexagonal plate-like LCO exhibiting significant capacity retention even after 20 cycles compared to the spherical LCO. However, at high charge–discharge rates, both samples showed a notable decrease in charge–discharge capacity. Results from CV measurements indicated that phase transitions were suppressed in the plate-like LCO compared to the spherical LCO.

Acknowledgments

This research was supported by NEDO, grant number JPNP20005, Japan.

Author Contributions

N.H. contributed to Conceptualization, Formal analysis, Investigation, and Writing-Original Draft. H.I. contributed to Formal analysis and Writing-Review & Editing. K.-i.M. contributed to Conceptualization and Writing-Review & Editing. Z.L. contributed to Formal analysis and Investigation. K.H. contributed to Conceptualization, Writing-Review & Editing, and Visualization.

The authors declare no competing financial interest.

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