Growth Manner of Octahedral-Shaped Li(Ni1 ... - ACS Publications

Mar 24, 2016 - ing these crystals showed less gas evolution at high voltages compared to those of ... Received: December 6, 2015. Revised: March 10, 2...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/crystal

Growth Manner of Octahedral-Shaped Li(Ni1/3Co1/3Mn1/3)O2 Single Crystals in Molten Na2SO4 Takeshi Kimijima,† Nobuyuki Zettsu,†,‡ and Katsuya Teshima*,†,‡ †

Department of Environmental Science and Technology, Faculty of Engineering and ‡Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan S Supporting Information *

ABSTRACT: The synthesis of shape-controlled crystals has been a highly attractive research topic in modern materials chemistry. In this work, the growth of Li(Ni1/3Co1/3Mn1/3)O2 (NCM) crystals in molten sulfate or carbonate salts (flux) at 1000 °C was systematically studied under various conditions. In situ X-ray diffraction during the growth and thermogravimetry-differential thermal analysis revealed that the growth of NCM crystals in the flux was controlled by liquid-phase sintering according to the Ostwald ripening principle. We studied the effect of Na+ in the flux on the crystal shapes and found that Na+ was critical in forming octahedral crystals with well-developed facets. Single crystals with well-developed facets were obtained homogeneously from Na2SO4, while truncated polyhedral crystals of smaller size were obtained from Li2SO4. The shape-controlled NCM crystals showed discharge capacities approaching 160 mAh g−1 in the operating voltage range of 2.8−4.4 V vs Li/Li+ under a low current density of 0.1 C, independent of flux composition. This suggests that the Li+ and transition-metal ions in the individual NCM crystals were highly ordered into hexagonal arrangements belonging to the R3̅m space group, without cation mixing.

■

dispersed primary particles with median diameters of 1−5 μm are critical for maintaining the C-rate capabilities and tap densities achieved by conventional systems of secondaryaggregated particles.12 The synthesis of well-dispersed primary particles with such small median diameters remains challenging. Many researchers have used sol−gel methods for shapecontrolled syntheses, as these methods can provide submicronsized crystals. This class of methods requires an additional postheating process to promote crystallization in many cases; however, crystals naturally aggregate into irregular secondary particles at high temperatures. On the basis of the above, we have explored the flux growth of battery materials; the liquid-phase crystal growth technique can produce high-quality idiomorphic crystals with no thermal strains.6,13−15 Our group has reported the flux growth of idiomorphic crystals of LiCoO2, Li4Ti5O12, LiMn2O4, LiFePO4, NCM, Li5La3Nb2O12, Li7La3Zr2O12, and Li5La3Ta2O12, all of which have potential as Li+-conducting materials.16−23 The growth mechanisms of these crystals in fluxes of molten salts can be classified into three categories. First, supersaturation by the evaporation of flux is a strong driving force for crystal growth. The holding time at a designated temperature was found to be a crucial parameter in determining the final morphology of the material.24 Second, supersaturation driven by the temperature of the reactants is another possible driving force for crystal growth. The cooling rate strongly contributes to the size and shape of crystals. The third possible mechanism

INTRODUCTION The synthesis of size- and shape-controlled crystals is a promising way to understand the intrinsic physicochemical properties of inorganic materials.1−5 Recently, in the field of Liion secondary battery (LIB) research, morphology-dependent high energy density, high-C rate capability, and cycling capability have been realized thanks to the development of synthetic routes for size- and shape-controlled active materials.6−9 Kim reported the flux growth of polyhedral and octahedral Li(Ni0.8Co0.1Mn0.1)O2 crystals; electrodes comprising these crystals showed less gas evolution at high voltages compared to those of conventionally accepted secondaryaggregated spherical particles. Choi et al.7 demonstrated the shape-controlled syntheses of three differently shaped types of LiMn2O4 crystals and realized that the truncated octahedral shape exhibited better rate and cycling capabilities than regular octahedral. Fu et al. demonstrated the synthesis of hexagonal nanobrick-shaped Li(Ni1/3Co1/3Mn1/3)O2 (NCM) crystals with a high percentage of exposed {010} facets; these showed a particularly high rate performance.10 Dixit et al. reported that the amount of crystalline disorder depends on the facet, and the (11̅2) facet of Li1.2Ni0.175Mn0.525Co0.1O2 had higher concentrations of antisite pairs than the (001) facet. These results have all suggested that active materials capable of forming multiple crystal systems have the most favorable crystal surface facets for Li+ transfer.11 Therefore, understanding the effect of growth mechanism on the precise size and shape of crystalline active materials is important for the development of high-performance LIBs. Regarding the energy density of LIBs, the tap density of active electrode materials must also be considered. Well© 2016 American Chemical Society

Received: December 6, 2015 Revised: March 10, 2016 Published: March 24, 2016 2618

DOI: 10.1021/acs.cgd.5b01723 Cryst. Growth Des. 2016, 16, 2618−2623

Crystal Growth & Design

Article

growth mechanism of idiomorphic NCM crystals. The obtained crystal products were observed by scanning electron microscopy (SEM; JEOL, JCM-5700) at an acceleration voltage of 15 kV. The elemental compositions were determined by energy-dispersive spectroscopy (EDS) with field-emission scanning electron microscopy (JEOL, JSM7600F). The crystal phases were also identified using ex situ XRD (Rigaku, SmartLab) with Cu Kα radiation (λ = 0.154 nm) at 40 kV and 30 mA. The Li occupancy was evaluated by Rietveld refinement using RIETAN-FP software.38 The elemental compositions of the crystals were evaluated by using inductively coupled plasma optical emission spectroscopy (ICP-OES, SII, SPS5510). The electrochemical properties of the NCM crystals were observed in galvanostatic charge−discharge tests using a R2032 coin-type cell. The NCM-based working electrodes were prepared on Al foils, with slurries composed of the NCM powders (90 wt %), acetylene black (5 wt %), and polyvinylidene fluoride (5 wt %) mixed with N-methylpyrrolidone. The electrodes were dried under a vacuum at 120 °C for 10 h prior to cell assembly. Metallic Li foils and polypropylene films were used as the counter electrode and separator, respectively. The electrolyte was 1 M LiPF6 containing a mixed solution of ethylene carbonate (30 vol %) and dimethyl carbonate (70 vol %). The cells were assembled in an Arfilled glovebox. The charge−discharge tests were performed using a galvanostat/potentiostat (Hokuto Denko HJ1005SD8) with a cutoff voltage of 2.8−4.4 V (vs Li/Li+) at 25 °C with a current density of 20 mA g−1. In addition, in situ XRD measurement was performed to gain a better understanding of the growth mechanism, using a diffractometer (Rigaku, SmartLab) equipped with an infrared heating system. The diffractometer was operated at 45 kV and 200 mA with Cu Kα radiation (λ = 0.154 nm). The mixture of NCM(OH)−LiOH· H2O−flux was placed on an Al2O3 plate, which was heated to 1000 °C at a rate of 1000 °C h−1 and held at this temperature for 1 h. The spectrum was continuously measured during heating, and each scan (2θ = 10−70°) required ∼3 min. Thermogravimetry-differential thermal analysis (TG-DTA, Thermo Plus EVO II TG8120, Rigaku) was also performed to understand the growth mechanism.

is liquid-phase sintering with the assistance of Ostwald ripening in molten salts of lower solubility of the target materials. During heating, the solute repeatedly dissolves and reprecipitates at the surface, resulting in the narrowing of the crystal size distribution with increased time by the assistance of the Gibbs−Thomson effect.25−28 This indicates that the evaporation rate and solubility of the flux are critical in determining the crystal growth mechanism. In situ X-ray diffraction (XRD) measurement may be a powerful tool to study their formation reaction and crystal growth mechanism.29,30 However, few studies have focused on the direct observation of reactions in the flux growth process.31 This difficulty may be due to the high molar ratio of flux to target material. Liquid-state salt may impede the detection of X-ray diffractions from the solute crystals. In this study, we examined the growth of idiomorphic NCM crystals in multiple sulfate salt fluxes (as well as one carbonate species) and discussed plausible growth mechanisms occurring in the systems. Sulfates have many advantages for use as fluxes compared to other salts, including halides, nitrates, and oxides. The sulfate can be removed easily from the products by washing with hot water. It is low in cost, and the flue gas is not a concern. Furthermore, the solubility of oxides, such as ferrites, in molten sulfates is higher than that in halides,32 meaning that a small amount of sulfate flux is sufficient to dissolve oxides. This offers the opportunity for clear observations of reactions in the fluxes by in situ XRD. The target material of NCM is considered the most promising candidate for the cathode material of LIBs. For instance, NCM has greater specific capacity and thermal stability than LiCoO2, which is historically used cathode material in commercialized LIBs.33−37 Furthermore, NCM shows great advantage in small volumetric change during the Li insertion/extraction reaction. Yabuuchi et al. realizes zero-volume change in the Li1−xNi1/3Co1/3Mn1/3O2 lattice until x = 0.67 (200 mAh·g−1)35 and its significantly high cyclability. However, only two prior studies reported on the flux growth of NCM crystals. We believe that, therefore, the new findings here on the size and shape control of NCM crystals may contribute significantly to the development of high-performance NCM cathode-based LIBs.

■

■

RESULTS AND DISCUSSION An SEM image and XRD pattern of the NCM(OH) used here as a precursor are shown in Figure S1. We first investigated the effects of Na2SO4 flux on the morphology of NCM crystals. Figure 1a−d shows SEM images of the crystals grown at the solute concentrations of 100 (no flux) and 80 mol % at 1000 °C for 10 h. Irregular and inhomogeneous crystals are formed in the absence of flux. This morphology is similar to that of the precursor, which agrees well with one previous study.37 This suggested that a lithiation of NCM(OH) with Li2O and/or LiOH occurred under solid-state reaction conditions. In sharp contrast, well-defined octahedral crystals are formed when Na2SO4 flux is used. The obtained crystals are individually dispersed with no agglomeration, indicating that Na2SO4 flux promotes the growth of idiomorphic crystals and prevents the aggregation of particles. The average size of the NCM crystals was 3.4 ± 1.5 μm. The XRD patterns from the NCM crystals are shown in Figure 1e. All diffraction peaks can be assigned to Li(Ni1/3Co1/3Mn1/3)O2 in the R3m ̅ space group (ICDD PDF 56-0147). Notably, the XRD pattern of the NCM contains two strong peaks centered at 2θ values of 64.3° and 65.0°, correlating to the (108) and (110) planes. These peaks imply that a highly ordered NCM lattice, without disordered atomic arrangements via cation mixing of Li+ and Ni2+, is formed.35,37 The elemental distribution in the flux-grown crystals was studied by EDS mapping. As shown in Figure S2, Ni, Co, and Mn were homogeneously distributed in all individual crystals. In addition, ICP-OES analyses were performed to determine the elemental compositions of the crystals. The atomic ratios of Li, Ni, Co, Mn, and Na were 1.04,

EXPERIMENTAL SECTION

Reagent-grade NiSO4·6H2O, CoSO4·7H2O, MnSO4·H2O (SigmaAldrich), LiOH·H2O, and 2.0 M aqueous NaOH (Wako Pure Chemical Industries Ltd.) were used for the growth of NCM crystals. A typical synthetic procedure was as follows: first, {NCM(OH)} precursor was prepared by a conventional coprecipitation method. The 2.0 M NaOH solution (45 mL) was added dropwise to a homogeneous aqueous solution of NiSO4·6H2O, CoSO4·7H2O, MnSO4·H2O (45 mL, [Ni2+] = [Co2+] = [Mn2+] = 0.33 M) while stirring at 400 rpm. The precipitates were collected via filtration and washing in distilled water, and then dried in an oven at 65 °C. Next, idiomorphic NCM crystals were grown by the sulfate flux method. Briefly, the precursor powders were mixed thoroughly with a stoichiometric amount of LiOH·H2O, to which the sulfate powder was added. At this point, the solute concentration was adjusted to 80 mol %. A 2.0-g sample of well-mixed powder was loaded into an Al2O3 crucible, which was placed in an electric furnace. The crucible was heated to 1000 °C at a rate of 1000 °C h−1 and held at this temperature for 10 h. The crucible was subsequently cooled to 400 °C at a rate of 200 °C h−1 using a cooling program. The products were separated from the remaining flux by washing with warm water. The flux compositions (Na2SO4, Li2SO4, and Na2CO3), solute concentration (20−100 mol %), holding temperature (800−1000 °C), and holding time (0−10 h) were varied to better understand the 2619

DOI: 10.1021/acs.cgd.5b01723 Cryst. Growth Des. 2016, 16, 2618−2623

Crystal Growth & Design

Article

Figure 2. Chages in in situ XRD profiles of the NCM precursor mixed with LiOH·H2O and Na2SO4 flux during heating at (a) 27 °C, (b) 112 °C, (c) 314 °C, (d) 497 °C, (e) 715 °C, (f) 840 °C, (g) 880 °C, and (h) 1000 °C.

Figure 1. Low- and high-magnification SEM images of the crystals grown at different solute concentrations of (a, b) 100 mol % and (c, d) 80 mol %. (e) XRD profile of the crystals grown at a solute concentration of 80 mol %. (f) Li(Ni0.333Co0.333Mn0.333)O2 (ICDD PDF 56-0147).

corresponding diffraction pattern does not show any change, while the heating temperature is held at 1000 °C for 1 h. These results suggest that the NCM crystal did not dissolve fully in a molten Na2SO4. We consider that the crystal growth is probably not controlled primary by supersaturation driven by the cooling process. Supersaturation driven by condensation of flux through evaporation is considered to be possible driving force for the NCM crystal growth in a molten Na2SO4. In order to realize above validation, we performed TG-DTA analysis. As shown in Figure 4S, three endothermic peaks were observed at 100, 260, and 410 °C, respectively. The weight loss at 70 to 120 °C was 10.5%, which accorded with the calculated value for H2O desorption from LiOH·H2O (10.6%). The second endothermic peak at 260 °C was assigned to the dehydration of NCM(OH). Because no weight loss occurred at 410 °C, the third endothermic peak was attributed to the melting of LiOH and/or formation of the NCM phase. In addition, 2.3% weight loss due to evaporation of absorbed water was observed during the heating from room temperature to 70 °C. On the basis of these results, the total weight loss was found to be 23.1%. This was 1.2% higher than the amount theoretically estimated (21.9%) by the following reactions:

0.337, 0.324, 0.339, and 0.003, respectively. The amount of substituted Na+ ions in the NCM lattice is considered negligible because of the minimum detection limits of our devices. We performed Rietveld refinement in order to estimate Li+ occupancy in the R3m ̅ lattice (see Figure S3). The occupancy of Li+ at 3b sites in the NCM lattice was 0.9811, while Ni2+ occupancy of Li+ sites was 0.0189. These values approached those of crystals grown by conventional solid-sate reactions.39,40 The above results strongly suggest that the Na2SO4 flux significantly influences the formation of octahedral NCM crystals with the rock-salt structure of the R3̅m space group. To understand the growth mechanism of the NCM crystals, we performed both TG-DTA analysis and time-dependent in situ XRD, ex situ XRD, and ex situ SEM observations. Figure 2 shows the changes of the in situ XRD profiles of the mixture with increasing temperature. All diffraction pattern features can be assigned to NCM(OH), LiOH·H2O, or Na2SO4. The broad peak centered at 2θ = 18° in the as-prepared mixture is assigned to NCM(OH) (Figure 2a), while sharp peaks are assigned as LiOH·H2O and Na2SO4. The diffraction peaks from LiOH· H2O disappear at the temperature of 112 °C, indicating a structural change in LiOH·H2O by H2O desorption. The orthorhombic Na2SO4 phase changes to the hexagonal Na2SO4 phase at ∼314 °C; this peak disappears at 840 °C. The diffraction lines attributed to NCM phase is observed at around 314 °C; meanwhile the diffraction line intensity of the precursor became weaker. Note that diffraction patterns from LiOH peak still remained at the corresponding temperature, indicating that solid-state lithiation reaction of the precursors with LiOH was initiated prior to melting LiOH. The diffraction intensity of the NCM phase was continuously increased as the temperature increased until 1000 °C. Furthermore, the

(Ni1/3Co1/3Mn1/3)(OH)2 + LiOH· H 2O + Na 2SO4 1 + O2 → Li(Ni1/3Co1/3Mn1/3)O2 + Na 2SO4 4 5 + H 2O (1) 2 We consider that the loss of Na2SO4 via evaporation is primarily responsible for the discrepancy in the weight loss of 1.2% between the experimental value and theoretical one. Based on the above results, excluding the unlikely possibility, 2620

DOI: 10.1021/acs.cgd.5b01723 Cryst. Growth Des. 2016, 16, 2618−2623

Crystal Growth & Design

Article

the NCM crystal growth mechanism is considered to be driven by liquid-phase sintering with the assistance of Ostwald ripening.25−28 The ex situ SEM observation of the effects of solute concentration on the crystal size strongly suggested one plausible NCM crystal growth mechanism, supported by the results of in situ XRD analysis. The solute concentration correlated directly with the diffusion-distance dependence of the solute in the flux, which correlated with the crystal size. Figure 3 shows SEM images of NCM crystals grown from the

Figure 4. XRD profiles of the crystals grown at different solute concentrations of (a) 90 mol %, (b) 70 mol %, (c) 50 mol %, and (d) 20 mol %. (e) Li(Ni0.333Co0.333Mn0.333)O2 (ICDD PDF 56-0147).

by cation mixing induced lattice expansion. The conditions of decreased solute concentration promoted the mixing of transition-metal cations site with Li+ occupied at 3b sites in the NCM lattice. We also studied the effect of holding time on the crystal growth, because the reaction time is a significant parameter in determining the individual particle size with the Ostwald ripening mechanism.41 Figure 5 shows SEM images of NCM

Figure 3. SEM images of the crystals grown at different solute concentrations of (a) 90 mol %, (b) 70 mol %, (c) 50 mol %, and (d) 20 mol %.

Na2SO4 flux at solute concentrations from 20 to 90 mol %. The holding temperature and time are fixed at 1000 °C and 10 h, respectively. The crystals primarily form from the flux as octahedra with well-defined facets at concentrations between 50 and 90 mol %. The average crystal size tends to increase with increasing solute concentration ranging from 1.1 to 1.9 μm. Notably, a diphasic morphology distribution is observed from the flux growth with 20 mol % of solute, with larger crystals possessing highly developed facets and smaller crystals with irregular shapes. The effects of concentration can be interpreted in terms of the diffusion distance of solute species in the molten Na2SO4 at temperatures up to 1000 °C. In the reaction systems in which the solute is not completely dissolved in the flux, the solute concentration can be represented by the average distance between neighboring solid solute particles. Decreasing the solute concentration increases the diffusion distance: partially dissolved NCM must travel long distances in the flux to attach to the surfaces of solid NCM crystals for further growth, which slows the growth rate. The average diameter of the NCM crystals decreases from 3.4 to 1.1 μm as the solute concentration decreases from 80 to 50 mol %. However, the crystals grown at 90 mol % have smaller sizes than those at 80 mol %. This conflict can be understood as follows: the amount of liquid flux was insufficient to completely cover the NCM crystal surfaces, which slowed crystal growth and reduced the average size. Corresponding XRD profiles are shown in Figure 4. All diffraction pattern features from crystals grown at solute concentrations of 50 to 90 mol % are attributed to single-phase NCM. In addition, a distinct peak splitting of the (108)/(110) planes, centered at ∼65° is clearly observed, indicating the highly ordered arrangement of transition-metal ions into the hexagonal NCM lattice belonging to the R3m ̅ space group. In contrast, the corresponding diffraction line is shifted to 63.5° and is split broadly when the crystals are grown with a solute concentration of 20 mol %. This shift is deduced to be driven

Figure 5. SEM images of the crystals grown at different conditions of (a) 1000 °C, 0 h, and (b) 1000 °C, 5 h. The inset in (a) is a highmagnification image.

crystals grown at 1000 °C during holding times of 0 and 5 h, respectively. Octahedral crystals with well-developed facets are obtained at all holding times. The average sizes of the crystals are 0.28 ± 0.06 and 2.0 ± 1.1 μm, respectively. In addition, the crystal size was increased to 3.4 ± 1.5 μm with a further increase in holding time to 10 h, as shown in Figure 1d. These results demonstrate that the growth mechanism of NCM crystals from the Na2SO4 flux was controlled by liquid-phase sintering with the assistance of Ostwald ripening. We found that the recrystallization of the NCM powders was useful for shape control by post-flux growth treatment. A mixture containing irregular-shaped NCM crystals with diameters of 2.1 ± 0.81 μm, synthesized by a solid-state reaction (Figure 1a), and Na2SO4 flux was heated at 1000 °C for 10 h. The concentration was controlled at 80 mol %. As shown in Figure S5, the crystal shape was drastically changed to regular octahedral, and the average size increased to 3.4 μm. XRD measurement revealed the formation of the NCM phase with enhanced hexagonal ordering of transition metal ions. This supported our proposed growth mechanism and the crystal grows by the Ostwald ripening process, dissolution, and reprecipitation mechanism during heating. Finally, we studied the effects of alkali cations and counteranions in the flux used on the NCM growth. Li2SO4 and Na2CO3 were used instead of Na2SO4 (Figure S6). All other 2621

DOI: 10.1021/acs.cgd.5b01723 Cryst. Growth Des. 2016, 16, 2618−2623

Crystal Growth & Design

Article

drastically enhanced from 142 mAh g−1 to 160 mAh g−1 by the annealing process. This approached the theoretical specific capacity of conventional NCM electrodes under operation at the cutoff voltage of 2.8−4.4 V (∼160 mAh g−1),37 which suggested that the Li+ and transition-metal ions were highly ordered in hexagonal arrangements within the NCM lattice in the R3̅m space group.40 We found no great distinctions in the initial galvanostatic tests of NCN crystals grown from Li2SO4 or Na2CO3 fluxes. All NCM crystals of varied morphologies showed discharge capacities approaching 160 mAh g−1, independent of flux composition. Systematic studies of the morphology-dependent LIB characteristics of these NCM crystals, including rate, cycle, high-voltage, and high-temperature capabilities will be published soon in the near feature.

experimental conditions were maintained with a holding temperature of 1000 °C, time of 10 h, and solute concentration of 80 mol %. Compared to crystals synthesized in solid-state reactions (Figure 1a,b), all flux species resulted in somewhat flat surfaces on individual crystals. Notably, Li2SO4 promoted decreases in the mean size and blurred the morphological nature of the NCM crystals, indicating that the solubility was greatly enhanced through a common ion effect. In contrast, the Na2CO3 flux promoted the formation of octahedral with sharp corners. Because the Na/Li molar ratio in the NCM crystals was evaluated at ∼0.003 by ICP-OES analysis, Na+ ions were not incorporated into the NCM crystal lattice, resulting in lattice distortion and morphological deformation. Interestingly, similar trends were observed in the literature.6 Lee et al. reported that Li(Ni0.8Co0.1Mn0.1)O2 crystals grown from NaCl flux formed octahedra with well-developed facets which were bounded by {003}, {101}, and {111} faces. Furthermore, the crystals grown from other halide fluxes resulted in polyhedra. In view of these common trends in the cation effects for the flux growth of NCM crystals, Na+ in the flux significantly promotes the formation of octahedral-shaped NCM crystals. We have not yet had direct evidence; we deduce the conclusion that the interaction of specific facets on NCM crystals with Na+ ions inhibits the crystal growth in directions perpendicular to the faces. According to the above results, the growth mechanism of NCM crystals from Na2SO4 flux can be summarized as follows. First, H2O desorption and melting of LiOH·H2O occur at temperatures below 500 °C; the NCM phase is then formed through the lithiation of NCM(OH) with LiOH and/or Li2O at ∼500 °C. The formation of polyhedral NCM crystals and their subsequent growth occur just after the melting of the Na2SO4 flux at 1000 °C through the liquid-state sintering process assisted by Ostwald ripening. The crystal growth reaction is completed during the heating process. The cooling rate does not affect the morphological features of the crystals. Importantly, Na+ ions may act as capping agents to form octahedra bounded by {003}, {101}, and {111} faces. The primitive galvanostatic charge−discharge test of the halfcell electrode composed of the NCM crystals flux-grown at 1000 °C was performed at 25 °C. Before preparing the composite electrodes, the NCM crystals were annealed at 700 °C for 5 h. No visible changes occurred in the morphological characteristics (see Figure S4). As reported previously, the excess flux was removed with warm water, which caused degradation of the NCM crystal surface.20 This promoted the formation of a resistant layer as well as fading in capacity. As shown in Figure 6, the discharge capacity of the first cycle was

■

CONCLUSION The growth mechanism of octahedral NCM crystals was systematically investigated by in situ XRD and TG-DTA and ex situ SEM observations. The crystallization of NCM started during the heating process via inhomogeneous nucleation in molten sulfate or carbonate salts. The subsequent growth process was dominated by liquid-phase sintering assisted by Ostwald ripening. The supersaturation driven by the cooling and evaporation of the flux contributed very little to the growth mechanism. Furthermore, the presence of Na+ ions in the flux used for growth was critical in the formation of octahedral crystals. We believe that the findings of this study will aid in the shape-controlled growth of single crystals within layered rocksalt systems, which are widely studied in battery research.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01723. SEM and XRD analyses of precursors, EDS mapping, Rietveld-refined EDS data, SEM images before and after annealing, NCM from fluxes of Li2SO4 and Na2CO3, NCM after recrystallization (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research is partially supported by the Super Cluster Program from the Japan Science and Technology Agency and JSPS KAKENHI Grant Number 25249089.

■

REFERENCES

(1) Wang, F.; Li, C.; Sun, L.-D.; Wu, H.; Ming, T.; Wang, J.; Yu, J. C.; Yan, C.-H. J. Am. Chem. Soc. 2011, 133, 1106−1111. (2) Wu, Y.; Cai, S.; Wang, D.; He, W.; Li, Y. J. Am. Chem. Soc. 2012, 134, 8975−8981. (3) Kimijima, T.; Kanie, K.; Nakaya, M.; Muramatsu, A. Appl. Catal., B 2014, 144, 462−467. (4) Yu, Z.; Guo, R.; Bhalla, A. S. Appl. Phys. Lett. 2000, 77, 1535− 1537. (5) Park, M. H.; Kim, H. J.; Kim, Y. J.; Moon, T.; Hwang, C. S.; Lee, W.; Kim, K. D. Appl. Phys. Lett. 2014, 105, 072902−4. (6) Kim, Y. ACS Appl. Mater. Interfaces 2012, 4, 2329−2333.

Figure 6. Voltage-capacity profiles of the NCM crystal grown at 1000 °C, 80 mol % (a) before and (b) after heat treatment. 2622

DOI: 10.1021/acs.cgd.5b01723 Cryst. Growth Des. 2016, 16, 2618−2623

Crystal Growth & Design

Article

(41) Kabalnov, A. S.; Shchukin, E. D. Adv. Colloid Interface Sci. 1992, 38, 69−97.

(7) Kim, J.-S.; Kim, K.; Cho, W.; Shin, W. H.; Kanno, R.; Choi, J. W. Nano Lett. 2012, 12, 6358−6365. (8) Dixit, H.; Zhou, W.; Idrobo, J.-C.; Nanda, J.; Cooper, V. R. ACS Nano 2014, 8, 12710−12716. (9) Jiang, C.; Wei, M.; Qi, Z.; Kudo, T.; Honma, I.; Zhou, H. J. Power Sources 2007, 166, 239−243. (10) Fu, F.; Xu, G.-L.; Wang, Q.; Deng, Y.-P.; Li, X.; Li, J.-T.; Huang, L.; Sun, S.-G. J. Mater. Chem. A 2013, 1, 3860−3864. (11) Dixit, H.; Zhou, W.; Idrobo, J.-C.; Nanda, J.; Cooper, V. R. ACS Nano 2014, 8, 12710−12716. (12) Yang, Z.; Wang, X.; Yang, X.; Liu, L.; Liu, Z.; Bai, Y.; Wang, Y. J. Solid State Electrochem. 2012, 16, 1229−1237. (13) Bugaris, D. E.; zur Loye, H.-C. Angew. Chem., Int. Ed. 2012, 51, 3780−3811. (14) Shi, S. J.; Tu, J. P.; Tang, Y. Y.; Liu, X. Y.; Zhao, X. Y.; Wang, X. L.; Gu, C. D. J. Power Sources 2013, 241, 186−195. (15) Chang, Z.-R.; Yu, X.; Tang, H.-W.; Yuan, X.-Z.; Wang, H. Powder Technol. 2011, 207, 396−400. (16) Teshima, K.; Lee, S.; Mizuno, Y.; Inagaki, H.; Hozumi, M.; Kohama, K.; Yubuta, K.; Shishido, T.; Oishi, S. Cryst. Growth Des. 2010, 10, 4471−4475. (17) Zettsu, N.; Nishikawa, K.; Yubuta, K.; Sakurai, K.; Yamamoto, Y.; Mizuno, Y.; Oishi, S.; Teshima, K. J. Mater. Chem. A 2015, 3, 17016−17021. (18) Mizuno, Y.; Zettsu, N.; Inagaki, H.; Komine, S.; Kami, K.; Yubuta, K.; Wagata, H.; Oishi, S.; Teshima, K. CrystEngComm 2014, 16, 1157−1162. (19) Yamada, T.; Zettsu, N.; Handa, N.; Oishi, S.; Teshima, K. Cryst. Growth Des. 2015, 15, 3922−3928. (20) Kimijima, T., Zettsu, N., Yubuta, K., Hirata, K., Kami, K., Teshima, K. J. Mater. Chem. A., DOI: 10.1039/C6TA01593J (21) Mizuno, Y.; Wagata, H.; Onodera, H.; Yubuta, K.; Shishido, T.; Oishi, S.; Teshima, K. Cryst. Growth Des. 2013, 13, 479−484. (22) Kimijima, T.; Zettsu, N.; Onodera, H.; Yubuta, K.; Oishi, S.; Teshima, K. CrystEngComm 2015, 17, 3487−3492. (23) Xiao, X.; Wagata, H.; Hayashi, F.; Onodera, H.; Yubuta, K.; Zettsu, N.; Oishi, S.; Teshima, K. Cryst. Growth Des. 2015, 15, 4863− 4868. (24) Oishi, S.; Teshima, K.; Kondo, H. J. Am. Chem. Soc. 2004, 126, 4768−4769. (25) Park, J.-H.; Lee, D.-H.; Shin, H.-S.; Lee, B.-K. J. Am. Ceram. Soc. 1996, 79, 1130−1132. (26) Zhu, X.; Zhou, J.; Jiang, M.; Xie, J.; Liang, S.; Li, S.; Liu, Z.; Zhu, Y.; Zhu, J.; Liu, Z. J. Am. Ceram. Soc. 2014, 97, 2223−2232. (27) Hayashi, Y.; Kimura, T.; Yamaguchi, T. J. Mater. Sci. 1986, 21, 757−762. (28) Kimura, T.; Yamaguchi, T. J. Mater. Sci. 1982, 17, 1863−1870. (29) Nørby, P.; Johnsen, S.; Iversen, B. B. ACS Nano 2014, 8, 4295− 4303. (30) Cheong, S.; Watt, J.; Ingham, B.; Toney, M. F.; Tilley, R. D. J. Am. Chem. Soc. 2009, 131, 14590−14595. (31) Geselbracht, M. J.; Noailles, L. D.; Ngo, L. T.; Pikul, J. H.; Walton, R. I.; Cowell, E. S.; Millange, F.; O’Hare, D. Chem. Mater. 2004, 16, 1153−1159. (32) Hayashi, Y.; Kimura, T.; Yamaguchi, T. J. Mater. Sci. 1986, 21, 2876−2880. (33) Li, J.; Daniel, C.; Wood, D. J. Power Sources 2011, 196, 2452− 2460. (34) Manthiram, A. J. Phys. Chem. Lett. 2011, 2, 176−184. (35) Yabuuchi, N.; Ohzuku, T. J. Power Sources 2003, 119−121, 171−174. (36) Liu, S.; Xiong, L.; He, C. J. Power Sources 2014, 261, 285−291. (37) Lee, M.-H.; Kang, Y.-J.; Myung, S.-T.; Sun, Y.-K. Electrochim. Acta 2004, 50, 939−948. (38) Izumi, F.; Momma, K. Solid State Phenom. 2007, 130, 15−20. (39) Shinova, E.; Stoyanova, R.; Zhecheva, E.; Ortiz, G. F.; Lavela, P.; Tirado, J. L. Solid State Ionics 2008, 179, 2198−2208. (40) Yin, S.-C.; Rho, Y.-H.; Swainson, I.; Nazar, L. F. Chem. Mater. 2006, 18, 1901−1910. 2623

DOI: 10.1021/acs.cgd.5b01723 Cryst. Growth Des. 2016, 16, 2618−2623