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Synthesis of Core-Shell Particles of NCM Hydroxides in Continuous Couette-Taylor Crystallizer Ji-Eun Kim, and Woo-Sik Kim Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00225 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017

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Crystal Growth & Design

Synthesis of Core-Shell Particles of NCM Hydroxides in Continuous Couette-Taylor Crystallizer

Ji-Eun Kim and Woo-Sik Kim*

Functional Crystallization Center Department of Chemical Engineering, Kyung Hee University Seocheon-Dong, Giheung-gu, 446-701 Yongin-Si, Korea

* Corresponding author: Tel +82-31-201-2970, Fax +82-31-273-2971, E-mail [email protected]

ACS Paragon Plus Environment


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Abstract A Taylor vortex flow was used as an effective continuous process for synthesizing uniform and spherical core-shell particles of nickel-manganese-cobalt (NMC) hydroxides, consisting of a (Ni0.90 Mn0.05Co0.05)(OH)2 (Ni-rich hydroxide) core and (Ni0.475Co0.05Mn0.475)(OH)2 (halfhalf hydroxide) shell. Tiny half-half hydroxide particles (primary particles) were initially precipitated and then adhered to core particles of the Ni-rich hydroxide to form core-shell particles via collision and agglomeration between the primary particles and the core particles. pH 10 was determined as the optimum condition for maximizing the interaction between the primary particles and the core particles. The shell-layer formation depended strongly on the operating parameters of the Couette-Taylor crystallizer, including the inner cylinder rotation speed, reactant concentration, and mean residence time. Using those parameters, the shelllayer thickness was controlled from 0.4 mm to 2.0 mm. Plus, a narrow size distribution (coefficient of variation) of 0.16 and high tap density of core-shell particles of 2.06 g/cm3 were obtained. When compared with a continuous MSMPR crystallizer, the toroidal Taylor vortex flow in the CT crystallizer was over 10 times more effective for the core-shell particle synthesis than the random turbulent eddy flow in the MSMPR crystallizer.

Key words: core-shell; shell thickness; agglomeration; Couette-Taylor crystallizer; Taylor vortex


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1. INTRODUCTION Core-shell particles have been receiving considerable attention as they allow the design of new composite materials with unique physical and chemical properties, and broad applications in catalysts, biology, drug delivery, batteries etc. [1-6]. The shell layer is invariably included to protect the core particles from toxic environments and made of a highly functional material. Thus, the shell thickness and its uniformity, and the shape and size distribution of the core-shell particles are important factors when designing and determining the properties of core-shell particles. Ni-Mn-Co ternary transition metal oxide (NMC oxide) has recently become highly attractive as an active cathodic material in the battery industry due to the flexibility in designing its properties [7]. Essentially, the NMC oxide composition determines its electrochemical performance and stability, and the cost-effectiveness of the cathodic material. As such, an NMC oxide with a high Ni and Co composition is favored for a high electrochemical performance, a high Mn composition is required to increase the stability of the NMC oxide, and a low Co composition is preferred to make the cathode more cost-effective, due to the high price of cobalt metal. As a result, a cost-effective Ni-rich (Ni>60%) NMC oxide has been designed with a high electrochemical performance over 200 mAh/g. [7, 8] Plus, an NMC oxide with a half-half (Ni-Mn) composition, in which the Ni and Mn compositions are maximized and the Co composition is minimized, has been suggested for a high structural and thermal stability [8, 9]. Thus, core-shell particles, consisting of the Ni-rich NMC oxide as the core particle and the half-half NMC oxide as the shell, have been developed as active cathodic materials with a high electrochemical performance and stability [10]. For these coreshell particles, the shell thickness is critical for the performance and stability of the particles. If it is too thin, the shell is unable to stabilize the core particles acting as the battery cathode [11-13]. Conversely, if it is too thick, the shell will lower the electric capacity of the core



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particles acting as the battery cathode [13]. Furthermore, a spherical shape and uniform size for the core-shell are important for a high packing density, which increases the electrical capacity of the cathode. [14] When preparing the abovementioned active cathodic materials, the NMC oxide is generally synthesized by precipitating NMC hydroxide in a solution, followed by oxidation in a solid state. In this case, the precipitation process predetermines most of the characteristics of the NMC oxide, including the core-shell structure, particle size distribution, shell thickness, composition, shape, and tap density of particles. Thus, the synthetic process of core-shell particles of NMC hydroxide is very important to produce an active cathodic material with a high performance and high stability. Several synthetic methods, such as a solid state reaction, spray pyrolysis, sol-gel process, and precipitation, have already been suggested for core-shell particles [15]. In particular, precipitation is frequently used for the synthesis of NMC hydroxide particles due to the fabrication of NMC hydroxide with a homogeneous phase and spherical shape [8, 9, 12, 16]. Notwithstanding, most previous studies have only focused on synthesizing NMC oxide coreshell particles with various component compositions for designing high-capacity cathodic materials, whereas a mechanistic investigation of the formation of NMC hydroxide core-shell particles has seldom been reported. A Taylor vortex flow has already been applied to various crystallizations/precipitations, such as the reaction crystallization of NMC hydroxide [17-19], calcium carbonate [20-22], and barium sulfate [23], drowning-out crystallization of GMP (guanisine monophosphate) [24], and calcium lactate [25], and cooling crystallization of sulfamerzine [26] and L-lysine [27]. Plus, in our previous studies, we have demonstrated that a Taylor vortex flow is highly effective for the production of spherical NCM hydroxide particles [17-19]. In those studies, spherical NMC hydroxide particles were formed by agglomeration, and the periodic toroidal


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flow motion of a Taylor vortex was shown to be about 10 time more effective for the continuous precipitation of spherical NMC hydroxide particles when compared with a random turbulent eddy. Accordingly, the present study focused on the use of a Taylor vortex flow for the efficient continuous synthesis of spherical NMC hydroxide core-shell particles, and also investigated the formation mechanism of the core-shell particles. Thus, a shell layer of (Ni0.47505Mn0.475Co0.05)(OH)2 (half-half hydroxide) was formed on (Ni0.90Co0.05Mn0.05)(OH)2 (Ni-rich hydroxide) core particles in a continuous Couette-Taylor crystallizer when varying the operating parameters: pH, ammonia flow rate, Taylor vortex flow, reactant concentration, and reactant flow rate (mean residence time). The characteristics of the core-shell particles were then evaluated in terms of the shape, shell thickness, coefficient of variation (CV) of the particles, and tap density.

2. EXPERIMENT Transition metal sulfates of nickel sulfate hexahydrate (>98.5%, Samchun Co., Korea), cobalt sulfate heptahydrate(>98%, Samchun Co., Korea), and manganese sulfate monohydrate (>98.5%, Daejung Co., Korea) were used to synthesize the NMC hydroxide core-shell particles. NaOH (99%, Samchun Co., Korea) was also used as a reactant and NH4OH (32%, Samchun Co., Korea) applied as the chelating agent for agglomeration. First, the Ni-rich hydroxide core particles ((Ni0.9Mn0.05Co0.05)(OH)2) were prepared via continuous precipitation in a Couette-Taylor (CT) crystallizer, as reported in our previous studies [17, 19]. Using the metal sulfates (NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O), an aqueous reactant solution of NiSO4, CoSO4, and MnSO4 was prepared with a molar ratio of Ni:Co:Mn=90:5:5. Here, the total concentration of Ni, Co, and Mn in the reactant solution was fixed at 3mol/L.



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Similarly, a reactant solution of NaOH was prepared at 6.0 mol/L for an equilmolar reaction. NH4OH was used as the chelating agent for the agglomeration of the Ni-rich hydroxide ((Ni0.90Co0.05Mn0.05)(OH)2). For the formation of spherical Ni-rich hydroxide particles, the precipitation in the CT crystallizer was always run at a reaction temperature of 50 °C, mean residence time of 60 min, inner cylinder rotation speed of 1500 rpm, and pH of 12.5. The resulting spherical Ni-rich hydroxide particles were then used as the core particles. Meanwhile, the half-half hydroxide ((Ni0.475Mn0.475Co0.05)(OH)2) was precipitated in the continuous CT crystallizer to form the core-shell particles, as illustrated in Fig. 1. The CT crystallizer consisted of two coaxial cylinders, where the radius of the inner and outer cylinder was 60 mm and 64 mm, respectively, and the working volume was 0.32 L. Both cylinders were equipped with a heating jacket to control the reaction temperature. To generate a Taylor vortex flow in the gap between the inner and outer cylinders, the inner cylinder was rotated while the outer cylinder remained stationary. The molar ratio of the metal reactant solution was fixed at Ni:Mn:Co=0.475:0.475:0.05. The total concentration of the metal reactant solution was adjusted from 0.5M to 2.5M. The NaOH reactant concentration was correspondingly determined to create an equimolar reaction with the metal reactant. During the operation of the continuous CT crystallizer, the feed flow rate of NaOH was slightly adjusted to control the pH in the crystallizer. The NH4OH solution was used as the chelating agent to form the shell layer on the core particles, and its concentration was fixed at 15.0 mol/L. The CT crystallizer was initially filled with distilled water, followed by the reactant solutions (NMC and NaOH), and the ammonia solution and Ni-rich hydroxide core particle suspension were then simultaneously fed into the CT crystallizer for the precipitation of the half-half hydroxide, as shown in Fig. 2. The mean residence time in the CT crystallizer was varied from 30min to 120min by changing the flow rates of the reactant solutions. That is, the


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flow rates of the metal reactant solution, NaOH reactant solution, and Ni-rich hydroxide core suspension were all varied under an equi-flow rate for an equimolar reaction. The flow rate of the NH4OH solution was controlled at 1/20 of the metal reactant flow rate. The hydrodynamic condition in the CT crystallizer was changed by varying the rotation speed of the inner cylinder from 300 rpm to 1500rpm. The pH in the crystallizer was varied from 9.5 to 11. Furthermore, a continuous MSMPR (Mix Suspension Mixed Product Removal) crystallizer, designed based on a standard Rushton mixing tank, was also used for the synthesis of NMC hydroxide core-shell particles. It was made of glass and equipped with four baffles and a heating jacket on the wall of the reactor. A three-blade turbine impeller was used for agitation. A sample suspension was taken from the crystallizer at a steady state, filtered using a 5µmpore filter paper (HYUNDAI Micro Munktell, Korea), washed 3-4 times with distilled water, and then dried in a convection oven (OV-11, Jeio Tech, Korea) at 50°C for 24hr. The particle morphology and shell thickness were analyzed using a field emission scanning electron microscope (FE-SEM, Carl Zeiss, Merlin, Germany). Energy dispersive spectroscopy (EDS, Stereoscan 440(Leica Cambridge) England) was used to analyze the metal composition of the core-shell particles. The tap density of the core-shell particles was measured using a Campbell Tapped Density Apparatus (Mode, TDA-2).

3. RESULTS and DISCUSSION The Ni-rich hydroxide ((Ni0.9Mn0.05Co0.05)(OH)2) core particles obtained from the continuous conical Couette-Taylor crystallizer are shown in Fig. 3. The core particles were spherical and uniform, where the mean size and coefficient of variation (CV) were about 12.5µm and 0.15, respectively (Fig. 3(a)). Thus, the tap density of the core particles was 2.26 g/ml, representing the best value in literature. As shown in Fig. 3(b), the core particles were formed



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by the agglomeration of plate-like individual particles aligned in the c-direction, as in our previous study [18]. Using these core particles, the NMC hydroxide core-shell particles were synthesized in the continuous Couette-Taylor (CT) crystallizer. First, the optimum pH condition was examined, as shown in Fig. 4, where pH in the CT crystallizer was varied from 9.5 to 11, as summarized in Table 1. Here, the mean residence time and rotation speed of the CT crystallizer were fixed at 60 min and 600 rom, respectively. At a low pH of 9.5 (Run 1 in Table 1), particles with various sizes and shapes were obtained (Fig. 4(a)). The large spherical particles seemingly originated from the Ni-rich hydroxide core particles, while small irregular-shaped particles were

newly

formed

by

the

precipitation

of

the

half-half

hydroxide

((Ni0.475Mn0.047Co0.05)(OH)2). The microscopic images showed that the surface of the large spherical particles was covered with agglomerates of flake-like particles, implying the formation of a shell layer on the core particles (Fig. 4(b)). However, confirming the exact thickness of the shell layer on the core particles was difficult as it was too thin in the particle cross-section images (Fig. 4(c)). At pH 10 (Run 2 in Table 1), there were many large spherical particles and a few small irregular ones (Fig. 4(d)). The particle cross-section images clearly confirmed the formation of a shell layer on the core particle (Fig. 4(f)). When increasing the pH to 11 (Run 3 in Table 1), the precipitation generated many irregular particles (Fig. 4(g)) and the particle cross-section images revealed almost no shell layer on the large spherical particles (Fig. (4(i)). According to the synthetic mechanism of the core-shell particles, as shown in Fig. 5, the reaction precipitation in the CT crystallizer produced tiny primary particles of the half-half hydroxide ((Ni0.475Mn0.475Co0.05)(OH)2), which were then used in two ways. The first was to form a half-half hydroxide shell layer on the Ni-rich hydroxide core particles, producing NMC hydroxide core-shell particles. The second was to form a homo-agglomerate of the


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half-half particles, resulting in a mixture of the core particles and the homo-agglomerates. The former occurred when the interaction between the primary particles and the core particles (interaction of primary-core particles) was higher than the interaction between the primary particles (interaction of primary-primary particles). Meanwhile, the latter occurred when the intra-action of primary-primary particles was favored over the interaction of primary-core particles. In general, the interaction between particles in a solution depends directly on the particle surface charge. Thus, the solution pH was the most critical factor in determining the above pathways for the primary particles. The NMC hydroxide was synthesized based on the following reactions; Ni + Co + Mn + nNH OH → [NiCoMn(NH ) ]

(1)

[NiCoMn(NH ) ] + 2OH → NiCoMn(OH) (s) + nNH

(2)

In this synthetic reaction, the optimum pH condition for NMC hydroxide particles varied according to the composition of Ni, Mn, and Co, as the pH condition for the formation of a single metal-ammonia or complex metal-ammonia complex varied according to the metal species. Thus, an Ni-ammonia complex was formed with a high pH, whereas an Mnammonia complex and Co-ammonia complex were favored with a low pH. Thus, pH 12 was the optimum for the formation of spherical particles of (Ni0.9Mn0.05Co0.05)(OH)2, whereas pH 11 was the optimum pH for spherical particles of (Ni0.33Mn0.33Co0.33)(OH)2 [17-19]. Here, it should be noted that previous studies have already investigated the pH condition to optimize the interaction between the same composite NMC hydroxide particles for spherical agglomerates. However, the present study focused on the pH condition to optimize the interaction between different composite NMC hydroxide particles, such as half-half hydroxide particles and Ni-rich hydroxide particles, for spherical core-shell particles and to minimize the interaction between half-half hydroxide particles to prevent homo-



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agglomeration. According to the above experimental results, the optimum pH for the NMC core-shell particles was determined as 10. Plus, NH4OH had an influence on the formation of the NMC hydroxide particles, as it is also directly involved in the metal-ammonia complex, playing an important role in preventing the phase separation of NMC hydroxide and its agglomeration. Thus, a high concentration of NH4OH is usually preferred for spherical NMC hydroxide agglomerates. However, in the present continuous process, a high flow rate of the NH4OH solution inhibited the formation of a shell layer on the core particles due to the promotion of homo-agglomeration. As a result, irregular homo-agglomerates of the half-half hydroxide were increasingly formed when increasing the flow rate of the NH4OH solution (Run 4 in Table 1, Fig. 4(j)-(l)). The formation of a shell layer on the core particles was also confirmed using energy dispersive spectroscopy (EDS), as summarized in Table 2. The metal compositions of the surfaces of particle 1 (P1) and particle 2 (P2) in Fig. 4(a) and particle 3 (P3) and particle 4 (P4) in Fig. 4(d) were similar to the composition of the half-half hydroxide. These results indicate that Run 1 and Run 2 produced core-shell particles, where the half-half hydroxide was layered on the Ni-rich hydroxide core particles, although the shell thickness of particle 1 was not measurable in the particle cross-section image (Fig. 4(c)). Meanwhile, particle 2 and particle 3 originated from the homo-agglomeration of the half-half hydroxide. However, the metal compositions of the surfaces of the particles (P5 and P6) in Fig. 4(g) differed from each other. While the metal composition of the surface of P6 was similar to that of the half-half hydroxide, the metal composition of the surface of P5 was closer to that of the Ni-rich hydroxide. Similar analysis results were also obtained for the particles (P7 and P8) in Fig. 4(j). Therefore, these results indicate that the half-half hydroxide synthesized in Run 3 and Run 4 was not used for a shell layer on the core particles, but for homo-agglomerates.


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EDS was used to measure the metal composition profile of the core-shell particles, as shown in Fig. 6. According to the microscopic particle cross-section images, a shell layer with a thickness of 1~2 µm was formed on the core particles (Fig. 6(a)). The EDS analysis also confirmed that the core particles were the Ni-rich hydroxide, while the shells were the halfhalf hydroxide. Furthermore, the metal composition sharply changed at the interface between the shell and the core particle (Fig. 6(b)). The shell formation process in the continuous CT crystallizer was monitored using various precipitation conditions, as shown in Fig. 7. The shell thickness increased monotonically along the axial direction of the CT crystallizer under all the precipitation conditions. At a high rotation speed of 1200 rpm, the initial shell thickness of 0.4 mm at port 1 increased to about 0.7 mm at the outlet stream (port 4). This increment in the shell thickness was increased when reducing the rotation speed of the CT crystallizer to 600 rpm, and further increased when increasing the reactant concentration to 1.3 mol/L. These experimental results imply that the formation of the shell layer on the core particles depended on the precipitation parameters of the hydrodynamics (Taylor vortex flow), the reactant concentration, and the time. The influence of the Taylor vortex flow on the formation of the shell layer is shown in Fig. 8. As mentioned above, the formation of the shell layer on the core particles occurred via the interaction of the primary-core particles. That is, the primary particles adhered to the core particles via collision and agglomerated to form the shell layer. According to this mechanism, the collision of the primary particles and core particles (collision of primary-core particles) is directly dictated by the fluid motion. When increasing the fluid motion, the collision frequency of the primary-core particles increases, thereby promoting the agglomeration of the primary particles on the core particles. At the same time, the fluid shear of the fluid motion breaks and re-disperses the agglomerated primary particles from the core particles. As such,



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the fluid motion simultaneously exerts two opposite influences on the shell layer formation. Therefore, the shell-layer formation is inhibited when the fluid motion promotes the breakage/re-dispersion of the agglomerated primary particles more than the collision of the primary-core particles. Conversely, the shell layer formation is enhanced when the fluid motion promotes the collision of the primary-core particles more than the breakage/redispersion of the agglomerated primary particles [19]. In the present study, the breakage/redispersion of the agglomerated primary particles was seemingly promoted over the collision of the primary-core particles due to the high shear rate of the Taylor vortex flow. As a result, the shell-layer thickness decreased when increasing the rotation speed of the CT crystallizer. These experimental results are confirmed in the cross-section images of the core-shell particles in Fig. 9. The shell layer was also shown to be well developed under all the Taylor vortex flow conditions. The tap density and coefficient of variation (CV) of the core-shell particles according to the rotation speed of the CT crystallizer are shown in Fig 10. Under a fixed reactant concentration and amount of core particles, the reduction of the shell-layer thickness (Figs. 8 and 9) was directly connected to an increase in the homo-agglomerates. That is, when a smaller portion of the half-half hydroxide was used for the shell layer, a larger portion was used for the homo-agglomerates. Therefore, when increasing the rotation speed, the population of irregular homo-agglomerates of the half-half hydroxide increased. The particlesize distribution was also broadened (increase of CV), which resulted in a decreased tap density. The influence of the reactant concentration on the formation of the shell layer is shown in Figs. 11 and 12. The reactant concentration was directly related to the amount of primary particles of the half-half hydroxide generated in the crystallizer. The amount of primary


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particles then had a simultaneous influence on the collisions of both the primary-core particles and the primary-primary particles. As demonstrated above (Figs. 4 and 5), the present precipitation conditions, including the pH and NH4OH, were set make the interaction of the primary-core particles predominant over the interaction of the primary-primary particles. As a result, the shell-layer formation via the collision of the primary-core particles was fast enough to consume most of the primary particles below a certain amount. However, when the amount of primary particles exceeded the consumption for the shell layer, the excess primary particles homo-agglomerated by themselves, and this only accelerated with more excess primary particles. Thus, the shell-layer thickness increased with a reactant concentration below 1.3 mol/L, and then decreased when increasing the reactant concentration further. This homo-agglomeration according to the reactant concentration was directly reflected in the tap density and CV of the particles, as shown in Fig. 13. The CV was reduced when increasing the reactant concentration up to around 1.3 mol/L, and then increased when increasing the reactant concentration further due to the increase of irregular homo-agglomerates. Conversely, the tap density was initially enhanced and then reduced when increasing the reactant concentration. The mean residence time in the CT crystallizer was varied to control the shell-layer thickness of the core-shell particles, as shown in Figs. 14 and 15. During the continuous process, increasing the mean residence time meant decreasing the reactant flow rates. As a result, the generation rate of primary particles was reduced when decreasing the reactant flow rate (increasing the mean residence time). With a high reactant flow rate, the generation rate of primary particles was faster than the consumption rate for shell-layer formation, resulting in the production of homo-agglomerates of the primary particles. Moreover, a high reactant flow rate did not provide enough time for the formation of the shell layer in the continuous crystallizer. However, when decreasing the reactant flow rate, the consumption for the shell



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layer was adequate to consume most of the primary particles generated in the crystallizer, preventing any homo-agglomeration of the primary particles. Furthermore, the residence time of the particles in the crystallizer was enough for the shell-layer formation. Consequently, when increasing the mean residence time, the shell-layer thickness was increased and the core-shell particle size (CV) was narrowed, thereby increasing the tap density of core-shell particles. The influence of the mean residence time on the shell-layer thickness was confirmed by cross-section images of the core-shell particles, as shown in the Supporting Information (Fig. S1). The synthesis of the core-shell particles in the continuous CT crystallizer was compared to that in a continuous mixed-suspension mixed-product removal (MSMPR) crystallizer, as shown in Figs. 16 and 17. In the MSMPR crystallizer, the random turbulent eddy flow was generated by a six-paddle impeller. Actually, a long mean residence time of over 8 hrs was required to synthesize the spherical core-shell particles in the MSMPR crystallizer. When increasing the mean residence time from 8 hrs to 12 hrs, the shell-layer thickness increased from 1.3 µm to 1.6 µm and the tap density of core-shell particles was enhanced from 2.0 g/cm3 to 2.15 g/cm3. However, the process time for the synthesis of the core-shell particles in the MSMPR crystallizer was much longer than that in the CT crystallizer, where 1.0 hr or 1.5 hrs was a sufficient mean residence time to produce uniform and spherical core-shell particles with a shell-layer thicknesses of 1.4 µm and 2.1 µm, respectively, and tap density of around 2.06 g/cm3. Thus, when comparing the core-shell particles, the continuous CT crystallizer was about ten-fold more efficient for synthesizing the NMC core-shell particles than the continuous MSMPR crystallizer due to the more effective fluid motion of the toroidal Taylor vortex flow for the shell-layer formation on the core particles.


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CONCLUSION A Couette-Taylor crystallizer was used as a highly effective continuous process for the synthesis of core-shell particles consisting of a Ni-rich hydroxide ((Ni0.9Mn0.05Co0.05)(OH)2) core particles and half-half hydroxide ((Ni0.475Mn0.475Co0.05)(OH)2) shell. Primary particles of the half-half hydroxide were initially precipitated, and then adhered to the core particles via collisions between the primary particles and the core particles (collision of primary-core particles), resulting in core-shell particles. Simultaneously, the primary particles also adhered to each other via collisions between the primary particles (collision of primary-primary particles), resulting in homo-agglomerates. Thus, to prevent such homo-agglomerates and promote the core-shell, the pH condition was optimized at 10. When the pH was above 11, irregular-shaped homo-agglomerates were produced without the formation of any core-shell particles. However, when the pH was below 9.5, the synthesis of the primary particles of the half-half hydroxide was incomplete. The rotation of the inner cylinder, reactant concentration, and mean residence time in the CT crystallizer were identified as the most critical operating parameters for the formation of the core-shell particles. The high fluid shear of the Taylor vortex flow hindered the adhesion of the primary particles to the core particles, and also re-dispersed the primary particles adhered to the core particles. As a result, the shell-layer thickness was reduced when increasing the rotation speed of the CT crystallizer, plus the tap density of particles was lowered due to the increase of homo-agglomerates. The amount of primary particles generated during the precipitation had a simultaneous influence on the formation of the core-shell particles and homo-agglomerates. With a reactant concentration below 1.3 mol/L, the primary particles were predominantly consumed for the shell-layer formation. Thus, the shell-layer thickness increased with the reactant concentration. However, when increasing the reactant concentration above 1.3 mol/L, the shell-layer formation was not fast enough to consume the



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primary particles, resulting in homo-agglomeration and a reduced shell-layer thickness. Based on the same mechanism, the shell-layer thickness increased when increasing the mean residence time. When compared with an MSMPR crystallizer, the CT crystallizer was about 10 times more productive in synthesizing the core-shell particles due to the effective fluid motion of the toroidal Taylor vortex flow. Thus, the proposed synthetic process using a Taylor vortex flow can be highly effective for industrial application.

Acknowledgement This work was supported by the Engineering Research Center of the Excellence Program of the Korean Ministry of Science, ICT & Future Planning (MSIP)/National Research Foundation of Korea (NRF) (Grant NRF-2014R1A5A1009799)

Appendix A. Supporting Information Supporting information related to this article is available.


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References (1) Ghosh Chaudhuri, R.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373-2433. (2) Radtchenko, I. L.; Sukhorukov, G. B.; Gaponik, N.; Kornowski, A.; Rogach, A. L.; Möhwald, H. Core–Shell Structures Formed by the Solvent-Controlled Precipitation of Luminescent CdTe Nanocrystals on Latex Spheres. Adv. Mater. 2001, 13, 1684-1687 (3) Gill, I.; Ballesteros, A. Encapsulation of Biologicals within Silicate, Siloxane, and Hybrid Sol−Gel Polymers: An Efficient and Generic Approach. J. Am. Chem. Soc. 1998, 120, 8587-8598. (4) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Magnetic Nanocomposite Particles and Hollow Spheres Constructed by a Sequential Layering Approach. Chem. Mater. 2000, 13, 109-116. (5) Suryanarayanan, V.; Nair, A. S.; Tom, R. T.; Pradeep, T. Porosity of Core-Shell Nanoparticles. J. Mater. Chem. 2004, 14, 2661-2666. (6) Lee, J. W.; Kong, S.; Kim,W. S.; Kim. J. Preparation and Characterization of SiO2/TiO2 Core-Shell Particles with Controlled Shell Thickness. Mater. Chem. Phys. 2007, 106, 3944. (7) Chen, Z.; Qin, Y.; Amine, K.; Sun, Y. K. Role of surface coating on cathode materials for lithium-ion batteries. J. Mater. Chem. 2010, 20, 7606-7612. (8) Sun, Y. K.; Myung, S. T.; Shin, H .S.; Bae, Y. C.; Yoon, C.S. Novel Core-Shell-Structured Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 via Coprecipitation as Positive Electrode Material for Lithium Secondary Batteries. J. Phys. Chem. B. 2006, 110, 6810-6815. (9) Sun, Y. K.; Myung, S. T.; Kim, M. H.; Prakash J.; Amine, K.; Amine, J. Synthesis and Characterization of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 with the Microscale



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Core-Shell Structure as the Positive Electrode Material for Lithium Batteries. AM. CHEM. SOC. 2005, 127, 13411-13418. (10) Lee, E. J.; Noh, H. J.; Yoon, C. S.; Sun, Y. K. Effect of outer layer thickness on full concentration gradient layered cathode material for lithium-ion batteries. J.Power Sources. 2015, 273, 663–669. (11) Myung, S. T.; Lee, K. S.; Kim, D. W.; S,Bruno.; Sun, Y.K. Spherical core-shell Li[(Li0.05Mn0.95)0.8(Ni0.25Mn0.75)0.2]2O4 spinels as high performance cathodes for lithium batteries. Energy Environ. Sci. 2011, 4, 935. (12) Camardese, J.; McCalla, J. E.; Abarbanel, D. W.; Dahn, J. R. Determination of Shell Thickness of Spherical Core-Shell NixMn1-x(OH)2 Particles via Absorption Calculations of X-Ray Diffraction Patterns. J.Electrochem. Soc. 2014, 161, A814-A820 (13) Sun, Y. K.; Myung, S. T.; Kim, M .H.; Kim, J. H. Microscale Core-Shell Structured Li(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2)O2 as Positive Electrode Material for Lithium Batteries. Electrochem. Solid-State Lett. 2006, 9, A171-A174. (14) Lee, M. H.; Kang, Y. J.; Myung, S. T.; Sun, Y. K. Synthetic optimization of Li[Ni1/3Co1/3Mn1/3]O2 via co-precipiation, Electrochim. Acta. 2004, 50, 939-948. (15) Choi, N. S.; Chen, Z.; Stefan A, F.; Ji, X.; Sun, Y. K.; Amine, K.; Gleb. Y.; Nazar, L. F. Cho, J., Bruce, P. G. Challenges Facing Lithium Batteries and Electrical Double-Layer Capacitors. Angew. Chem. Int. Ed. 2012, 51, 9994 – 10024. (16) Myung, S. T.; Noh, H. J.; Yoon, S. J.; Lee, E. J.; Sun, Y. K. Progress in High-Capacity Core−Shell Cathode Materials for Rechargeable Lithium Batteries. J. Phys. Chem. Lett. 2014, 5, 671−679. (17) Thai, D. K.; Mayra, Q. P.; Kim, W. S. Agglomeration of Ni-rich hydroxide crystals in Taylor vortex flow. Powder Technol. 2015, 274, 5-13.


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(18) Kim, J. M.; Chang, S. M.; Chang, J. H.; Kim, W. K. Agglomeration of nickel/cobalt/manganese hydroxide crystals in Couette–Taylor crystallizer. Colloids Surf., A. 2011, 384, 31-39. (19) Mayra, Q. P.; Kim, W. S. Agglomeration of Ni-rich hydroxide in reaction crystallization: Effect of Taylor Vortex dimension and intensity. Cryst. Growth Des. 2015, 15, 1726-1734. (20) Jung, T. S.; Kim, W. S.; Choi, C. K. Effect of non-stoichiometry of calcium carbonate precipitation in a Couette-Taylor Reactor. Cryst. Growth Des. 2004, 4, 491-495. (21) Kang, S. H.; Lee, S. G.; Jung, W. M.; Kim, M. C.; Kim, W. S.; Choi, C. K.; Feigelson, R. S. Effect of Taylor Vortices on Calcium Carbonate Crystallization by Gas-Liquid Reaction. J. Crystal Growth. 2003, 254, 196-205. (22) Jung, W. M.; Kang, S. H.; Kim, W. S.; Choi, C. K. Particle Morphology of Calcium Carbonate by Gas-Liquid Reaction Precipitation in a Couette-Taylor Reactor. Chem. Eng. Sci. 2000, 55, 733-747. (23) Mohammad, F. A.; Ruo, A. C.; Park, J. H.; Bader, N.; Kim, W. S.; Joo, Y. L. Effect of Flow Structure at the Onset of Instability on Barium Sulfate Precipitation in TaylorCouette Crystallizers. J. Crystal Growth. 2013, 373, 20–31. (24) Nguyen A. T.; Kim, J. M.; Chang, S. M.; Kim, W. S. Taylor Vortex Effect on Phase Transformation of Guanosine 5-Monophosphate in Drowning-Out Crystallization. Ind. Eng. Chem. Res. 2010, 49, 4865-4872. (25) Lee, S.; Lee, C. H.; Kim, W. S. Taylor Vortex Effect on Flocculation of Hairy Crystals of Calcium Lactate in Anti-Solvent Crystallization. J. Crystal Growth. 2013, 373, 32-37. (26) Park, S. A.; Lee S.; Kim, W. S. Polymorphic Crystallization of Sulfamerazine in Taylor Vortex Flow: Polymorphic Nucleation and Phase Transformation. Cryst. Growth Des. 2015, 15, 3617–3627. (27) Wu, Z.; Seok, S. H.; Kim, D. H.; Kim, W. S. Control of Crystal Size Distribution using



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Non-Isothermal Taylor Vortex. Cryst. Growth Des. 2015, 15, 5675-5684.


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Table 1. Experimental run conditions of continuous CT crystallizer Flow ratio of Run number

pH

Common conditions NH4OH/Metal reactant

Run 1

9.5

0.05

Run 2

10

0.05

Rotation speed=600rpm Mean residence time=60min

Run 3

11

0.05

Run 4

10

1.0

Concentr. of metal reactant=1.0M

Table 2. Element analysis of particles. “Run number” indicated the experimental conditions in Table 1, and “Position” indicated the positions of EDS analysis on the samples which were noted in Fig. 4. Atomic ratio (%) Run number

Tap density

Position

Ni/Mn Ni

Co

Mn

P1

49.80

6.09

44.11

1.13

P2

47.25

6.48

46.27

1.02

P3

46.52

5.96

47.52

0.97

P4

47.47

4.63

47.91

0.99

P5

89.78

5.00

5.22

17.20

P6

46.23

5.51

48.25

0.96

P7

89.82

5.34

4.84

18.56

P8

46.46

5.16

48.39

0.96

Run 1

(g/cm3)

1.66

Run 2

2.04

Run 3

1.61

Run 4

1.54



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(Ni0.475Co0.05Mn0.475)SO4

Page 22 of 39

(Ni0.475Co0.05Mn0.475)(OH)2

Core

Core-shell NaOH ri

ro

Figure 1. Conceptual drawing of core-shell particle formation in the continuous Couette-Taylor crystallizer


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 3 2

4

Port1 Port2 Port3 Port4

1

10 7 9

5 6

2 2

Out

In 50°

RW-0525G

8

Figure 2. Schematic diagram of experimental system used for synthesis of core-shell particles; 1. DC motor; 2. Peristaltic pump; 3. Metal reactant solution; 4. Core particle suspension; 5. Ammonia solution; 6. Sodium hydroxide solution; 7. Couette-Taylor crystallizer; 8. Heater; 9. Outlet stream; 10. pH meter



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Page 24 of 39

2μm

(b)

(a)

Figure 3. The morphology of Ni-rich hydroxide core particles [(Ni0.9Co0.05Mn0.05)(OH)2] used for synthesis of core-shell particles. (a) core particle shape, (b) surface of core particle: mean diameter of core particle size was 12μm, coefficient of variation was 0.15, and tap-density of particles was 2.26g/ml.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) particle morphology obtained by Run 1, (b) surface of particles obtained by Run 1, (c) cross-section image of particle obtained Run 1, (d) particle morphology obtained by Run 2, (e) surface of particles obtained by Run 2, (f) cross-section image of particle obtained Run 2, (g) particle morphology obtained by Run 3, (h) surface of particles obtained by Run 3, (i) crosssection image of particle obtained Run 3, (j) particle morphology obtained by Run 4, (k) surface of particles obtained by Run 4, (c) cross-section image of particle obtained Run 4.



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Page 26 of 39

Half-half hydroxide shell (Ni0.475Mn0.475Co0.05)(OH)2 Half-half reactants (Ni:Mn:Co=0.475:0.475:0.05)

core

core

core

core

core

core

Core-shell formation core

core

Ni-rich hydroxide particles (Ni0.9Mn0.05Co0.05)(OH)2 Homo-agglomerate of half-half hydroxide Homo-agglomeration

Figure 5. Mechanistic concept of core-shell particle formation.


Page 27 of 39

(a)

1μm

100

(b) Metal Fraction in Particle [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

Ni Co Mn

40

20

0 0

1

2

3

4

5

6

7

8

Radial Position in Particle [µm]

Figure 6. Composition analysis of core-shell particle along to radial position using energy dispersive spectroscopic (EDS). Core-shell particle was obtained at rotation speed of 600rpm, mean residence time of 60min, metal reactant concentration ofr 1M and pH of 10



2.0

Shell Layer Thickness [µm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Rotation

Metal

speed

reactant

Page 28 of 39

600rpm, 1.0mol/L 600rpm, 1.3mol/L 1200rpm, 1.0mol/L

1.6

1.2

0.8

0.4

0.0 0.25

0.50

0.75

1.00

(Port 1)

(Port 2)

(Port 3)

(Port 4)

Axial Position [x/L]

Figure 7. Axial profile of shell thickness of core-shell particles at various precipitation condition. pH and mean residence time in the continuous Couette-Taylor crystallizer were fixed at 10 and 60 min, respectively. Mean core particle diameter was 12μm.


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1.6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.2

0.8

0.4

200

400

600

800

1000

1200

1400

1600

Rotation Speed [rpm]

Figure 8. Influence of inner cylinder rotation speed on shell layer thickness. (mean residence time of 60min and metal reactant concentration of 1M in the continuous Couette-Taylor crystallizer and mean core particle diameter of 12μm).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Cross-section image of core-shell particles produced with various rotation speeds of (a) 300 rpm, (b) 600 rpm,

(c) 900 rpm, (d) 1200 rpm, (e) 1500 rpm.


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0.4

2.1

2.0

0.3

1.9

1.8

0.2

1.7

Coefficient of Variation

Tap Density of Particles [g/ml]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6

1.5 200

400

600

800

1000

1200

1400

0.1 1600

Rotation Speed [rpm]

Figure 10. Influence of inner cylinder rotation speed on tap density and coefficient of variation of core-shell particles



2.4

2.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

1.6

1.2

0.8

0.4

0.0 0.5

1.0

1.5

2.0

2.5

Metal Reactant Concentration [mol/L]

Figure 11. Influence of metal reactant concentration on shell layer thickness (mean residence time of 60min, rotation speed of 600rpm in continuous Couette-Taylor crystallizer, mean core particle diameter of 12μm).


Page 33 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12. Cross-section image of core-shell particles produced with various metal reactant concentrations of (a) 0.5 mol/L, (b) 0.8 mol/L, (c) 1.0 mol/L, (d) 1.1 mol/L, (e) 1.3 mol/L, (f) 2.0 mol/L.



0.4

2.2

0.3

2.0

1.9

1.8

0.2


2.1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 39

1.7

1.6

0.1 0.5

1.0

1.5

2.0

2.5

Metal Reactant Concentration [mol/L]

Figure 13. Influence of metal reactant concentration on tap density and coefficient of variation of core-shell particles.


Page 35 of 39

2.8

2.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0

1.6

1.2

0.8

0.4 20

40

60

80

100

120

Mean Residence Time [min]

Figure 14. Influence of mean residence time on shell layer thickness (rotation speed of 600 rpm, metal reactant concentration of 1M, mean core particle diameter of 12μm).



0.4 2.1

2.0 0.3 1.9

1.8 0.2

1.7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 39

1.6

1.5

0.1 20

40

60

80

100

120

Mean Residence Time [min]

Figure 15. Influence of mean residence time on tap density and coefficient of variation of core-shell particles.


Page 37 of 39

600rpm-90min 2.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6

1000rpm-12hr 600rpm-60min 1000rpm-8hr

1.2

0.8

0.4

0.0

CT

CT

MSMPR

MSMPR

Figure 16. Comparison of shell layer thickness formed in continuous CT and MSMPR crystallizers.



Mean Residence Time in MSMPR Crystallizer [hr] 4

6

8

10

12

2.2

CT crystallizer MSMPR crystallizer Tap Density of Particles [g/ml]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 39

2.1

2.0

1.9

1.8

1.0

1.5

2.0

Mean Residence Time in CT Crystallizer [hr]

Figure 17. Comparison of tap density of core-shell particles synthesized in continuous CT and MSMPR crystallizers (rotation speed of CT crystallizer=600 rpm and agitation speed of MSMPR crystallizer=1000 rpm)


Page 39 of 39

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Graphical Abstract

For Table of Contents Use Only

Title: Synthesis of Core-Shell Particles of NMC Hydroxides in Continuous Couette-Taylor Crystallizer Authors: Ji-Eun Kim, Woo-Sik Kim*

Taylor vortex flow was applied for synthesis of Ni-rich hydroxide core- half-half hydroxide shell particles. Toroidal fluid motion of Taylor vortex was highly effective for formation of uniform core-shell particles. Synthetic process of core-shell particles using Taylor vortex flow was about 10 times more productive than that using random turbulent eddy flow.


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