Growth of idiomorphic LiMnPO₄ crystals in molten NaCl–KCl and LiF–NaCl–KCl fluxes

Abstract

Olivine-type idiomorphic polyhedral LiMnPO₄ crystals grown from a molten LiF–NaCl–KCl ternary flux at 700 °C are found to be surrounded with well-defined facets of major {100}, {101}, and {210} faces with minor {011} faces. The average size of these crystals of ∼20.7 μm was around ten times larger than those grown from a molten NaCl–KCl binary flux. In situ XRD and TG-DTA measurements revealed that LiMnPO₄ crystals grew from the melt during heating and holding at 700 °C with little assistance from the supersaturation produced by flux evaporation. Time-dependent scanning electron microscopy observation found that Ostwald ripening caused by repeated partial dissolution and reprecipitation of the LiMnPO₄ crystals is the dominant factor in both the development of crystal facets and the mean diameter of individual crystals. The addition of LiF is therefore considered to promote the development of well-defined polyhedral crystals by increasing the solubility of LiMnPO₄ in the molten flux.

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Introduction

High quality olivine-based silicates with a formula of (Mg²⁺, Fe²⁺)₂SiO₄ have a bright pale-green color that has made them a popular jewel known as “peridot” since the B.C. era. A large variety of other olivine-type materials with different cations are also known to exist, including tephroite (Mn₂SiO₄), fayalite (Fe₂SiO₄), monticellite (CaMgSiO₄), and kirschsteinite (CaFeSiO₄). In recent years, these olivine-type compounds have attracted increased attention for their potential use in various applications due to their unique physicochemical properties that are derived from their electric structure and phase stability. For example, Fe₂GeS₄ is known to exhibit photovoltaic,¹ optoelectric,² and thermoelectric properties.³ Olivine-type lithium phosphates with a formula of LiMPO₄ (where M = Fe, Co, Mn or Ni) are also currently being studied as promising candidates for active materials in durable, long-life, high-energy lithium ion batteries (LIBs). This is made possible by the strong covalent bonding of P–O that is present in the olivine lattice, which gives it significant structural stability at high temperature and in a highly-delithiated state.^4–6 In addition, LiMPO₄ responds to a wide range of redox voltages from 3.4 to 5.1 V vs. Li⁺/Li through a simple change in the active metal cation.^7–10

The electrochemically active olivine-type lithium phosphate LiMnPO₄ has a three-dimensional framework made up of alternating corner-sharing PO₄ tetrahedra and MnO₆ octahedra, which places it in the orthorhombic system with a space group of Pnma.¹¹ The voids in the octahedral sites of the framework are occupied by Li ions stacked in a one-dimensional direction along the b-axis, giving it lattice parameters of a = 10.4367 Å, b = 6.0959 Å, and c = 4.7417 Å. Of the various methods that have been reported for the synthesis of LiMnPO₄^12,13 (e.g., solid-state reaction,^14–16 ultrasonic spray pyrolysis,¹⁷ and hydro- and solvothermal methods^18–20), most have had little success in producing individually monodispersed single crystals with well-defined shapes needed to further understand the intrinsic properties of this material. Although sol–gel methods have yielded primary LiMnPO₄ particles, additional post-heating was required to promote crystallization¹⁴ that in most cases caused the particles to aggregate into irregular secondary particles.

Flux growth from a hot melt is a well-known liquid-phase method that can produce high-quality crystals with well-developed surface facets^21,22 in an unconstrained fashion. The advantages of this method are that crystals can grow free from mechanical or thermal constraints, though their growth in a low-temperature solution has no specific benefit in terms of crystal quality. The most difficult issue faced with this approach is selecting a suitable flux, as previous studies have reported that using LiCl and NaCl fluxes for LiFePO₄ crystal growth results in side reactions that form Li₃PO₄ and Na₃Fe₃(PO₄)₄, respectively.²³ With LiMnPO₄, flux selection is made even more difficult by its instability in air and at high temperatures; yet we are not aware of any systematic studies of the flux growth of LiMnPO₄ crystals. Kang et al. have reported that producing LiMnPO₄ under air results in lithium phosphates such as Li₃PO₄ and Li₄P₂O₇ being formed as by-products,²⁴ while Shiratsuchi et al. have demonstrated that carbothermal reduction occurring on carbon-coated LiMnPO₄ forms MnP₂ at 900 °C in an Ar atmosphere.²⁵ In light of this work, we herein investigate the low-temperature growth of idiomorphic LiMnPO₄ crystals in binary NaCl–KCl and ternary LiF–NaCl–KCl molten halide fluxes under an Ar atmosphere.

Experimental

LiMnPO₄ crystals were grown from molten NaCl–KCl and LiF–NaCl–KCl salts using Li₂CO₃, NH₄H₂PO₄ and MnCO₃ as the starting materials. All reagents were purchased from Wako Pure Chemical Industries Ltd. and were used without any further purification. Stoichiometric amounts of each starting material were mixed and added to an alumina crucible, which was then loosely sealed with an alumina lid and placed in an electric furnace. Crystal growth was achieved through a two-step process of precursor formation by solid-state reaction (heating to 400 °C for 3 h under Ar),¹³ followed by heating of the pulverized precursor with a solid-state NaCl–KCl or LiF–NaCl–KCl flux in a furnace at a rate of 300 °C h⁻¹ under an Ar atmosphere till molten. After growth, the powders were cooled naturally to room temperature in the furnace, with the LiMnPO₄ crystals being isolated from the remaining flux in warm water by washing several times and drying at 100 °C under air. The flux growth conditions are summarized in Table 1.

Table 1 Growth conditions for LiMnPO₄ crystals

Run no.	Solute concentration/mol%	Starting materials			Fluxes			Holding temperature/°C	Holding time/h	Cooling rate/°C h^−1
		Li2CO3/g	MnCO3/g	NH4H2PO4/g	NaCl/g	KCl/g	LiF/g
1	100	0.165	0.514	0.515	0	0	0	700	3	Natural
2	50	0.165	0.514	0.515	0.123	0.168	0	700	3	Natural
3	10	0.049	0.152	0.152	0.348	0.444	0	700	3	Natural
4	5	0.026	0.081	0.081	0.394	0.495	0	700	3	Natural
5	50	0.169	0.526	0.526	0.131	0.14	0.013	700	3	Natural
6	50	0.169	0.526	0.526	0.131	0.14	0.013	700	3	5
7	50	0.169	0.526	0.526	0.131	0.14	0.013	700	0	Natural
8	50	0.169	0.526	0.526	0.131	0.14	0.013	700	1	Natural

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The crystal phases present in the pulverized samples were identified using X-ray diffraction (XRD, RIGAKU, MiniflexII) with Cu Kα radiation (λ = 0.154 nm) at 30 kV and 20 mA, and a scan speed of 1° min⁻¹ across a 2θ range of 10° to 80°. The morphologies of the obtained powders were characterized using scanning electron microscopy (SEM, JEOL, JCM-5700) at an acceleration voltage of 10 kV. Particle size distributions were evaluated using a nanoparticle size analyzer (SHIMAZU, SALD-7100). Chemical elements were identified by inductively coupled plasma optical emission spectrometry (ICP-OES, Hitachi High-Tech, SPS5510) and X-ray photoelectron spectroscopy (XPS, JEOL, JPS-9010MX). The reaction mechanism during growth using a heating rate of 300 °C h⁻¹ was studied by thermogravimetric analysis coupled with differential thermal analysis (TG-DTA, Rigaku, Thermo plus EVOII) and in situ XRD measurement with an infrared furnace system (Rigaku, SmartLab, Cu Kα radiation (λ = 0.154 nm)) and a quartz-based sample holder covered with Au foil. Individual XRD profiles were continuously measured at a scan speed of 40° min⁻¹ across a 2θ range of 10° to 80°. The temperature difference between the beginning (2θ = 10°) and the end (2θ = 80°) of measurement was less than 30 °C.

Results and discussion

The precursor powders were synthesized by solid-state reaction as per a previous report,²⁴ which resulted in them forming irregular shapes (Fig. S1(a)†) that were found through XRD (Fig. S1(b)†) to contain Mn₂P₂O₇ and Li₂CO₃. The XRD patterns of the pulverized crystals obtained from the molten NaCl–KCl salt shown in Fig. 1 reveal that diffraction lines, consistent with those of LiMnPO₄, were observed regardless of the solute concentration. A NaMnPO₄ sub-phase²⁶ was formed with a 5 mol% solute concentration, which implies that an excess amount of NaCl–KCl flux promotes solid solution substitution of Li⁺ in the LiMnPO₄ lattice and Na⁺ in molten NaCl. Conversely, flux growth under higher solute concentrations of 10 and 50 mol% did not produce detectable diffraction lines in the XRD profile for NaMnPO₄, and so it is believed that NaMnPO₄ is either formed at the surface or not at all. Ions in hot solution can be exchanged immediately with ions in an organic crystal if they are at the surface, but for this exchange to proceed, the ions in solution have to move to ion exchange sites in the solid phase via solid-phase diffusion. Because both the size and position of these ion exchange sites are predetermined in an inorganic crystal, the exchange reaction is limited by solid-phase diffusion. This is particularly true in the case of one-dimensional ionic conductors, such as the olivine system. A similar result has been observed in the NaCl–KCl flux growth of LiFePO₄ crystals under high NaCl concentrations.²³ The lattice parameters of the LiMnPO₄ prepared at a solute concentration of 50 mol% were estimated to be a = 10.4467(13) Å, b = 6.1038(8) Å, and c = 4.7446(6) Å, which are still in the orthorhombic system and in agreement with reference data (a = 10. 4367 Å, b = 6.0959 Å, c = 4.7417 Å).¹¹ During synthesis without the flux, irregularly aggregated secondary particles were formed (Fig. 2(a)), whereas growth in the molten NaCl–KCl flux resulted in the formation of highly dispersed primary crystals (Fig. 2(b)–(d)). The average size of these crystals increased with flux concentration from 2.3 to 8.5, and 14.3 μm (see Fig. S2†). As the primary particles were larger than all of the starting materials, this suggests that the NaCl–KCl flux plays a key role in preventing aggregation.

Fig. 1 Powder XRD patterns of crystals prepared at 700 °C for 3 h under solute conditions of 100% (solid-state reaction, run no. 1), 50 mol% (run no. 2), 10 mol% (run no. 3), and 5 mol% (run no. 4). LiMnPO₄ (ICDD PDF 078-5417) is also shown for reference.

Fig. 2 SEM images of crystals grown from the NaCl–KCl flux at 700 °C for 3 h with various solute concentrations: (a) 100% (solid-state reaction, run no. 1), (b) 50 mol% (run no. 2), (c) 10 mol% (run no. 3), (d) 5 mol% (run no. 4).

In order to promote further growth of the LiMnPO₄ crystal, a LiF–NaCl–KCl ternary flux system with a eutectic composition of LiF : NaCl : KCl = 1 : 4.5 : 3.8 as the molar ratio, which is expected to give a lower melting point, was used instead of NaCl–KCl.²⁷ Furthermore, the addition of fluoride is known to promote crystal growth,²⁸ as the high electronegativity of F⁻ ions enhances coulomb interactions and makes it chemically stable with many ionic crystals, thus leading to an enhanced solubility of oxides. In the XRD patterns of the pulverized crystals prepared from the LiF–NaCl–KCl flux with a solute concentration of 50 mol% (run no. 5) at 700 °C for 3 h (Fig. 3), all the diffraction peaks were assigned to LiMnPO₄ without any by-products after several washing steps in warm water. The estimated lattice parameters of a = 10.4483(11) Å, b = 6.1053(6) Å, and c = 4.7467(5) Å are in accordance with those previously reported,¹¹ indicating that no external cations or anions were included in the crystals during growth in the molten LiF–NaCl–KCl flux. In the corresponding SEM images of the LiMnPO₄ crystals in Fig. 4, it can be seen that the crystals were individually dispersed and formed a well-defined polyhedral shape. The average size of these crystals was 20.7 μm (Fig. S3†), which is approximately ten times larger than that of the LiMnPO₄ crystals grown from the NaCl–KCl flux with the same concentration (see Fig. 2(b)). The crystal facets were identified through XRD measurement of the as-obtained LiMnPO₄ crystals without pulverizing (Fig. S4†), which revealed that the 200, 101 and 210 faces of LiMnPO₄ were more highly defined than those of the pulverized sample shown in Fig. 3. This suggests that the polyhedral LiMnPO₄ crystal surface is dominantly covered with {100}, {101}, and {210} faces. The crystallographic features of randomly picked-up LiMnPO₄ crystals were computationally replicated by these faces. An example is shown in Fig. 4(c). Note that the addition of minor {011} faces has a beneficial effect in terms of replicating the SEM image of the crystal shown in Fig. 4(b). As the SEM and XRD results differed from the equilibrium Wulff shapes calculated by ab initio DFT calculation,²⁹ the LiF–NaCl–KCl flux is considered to have an impact on the growth of crystals with well-defined polyhedral shapes surrounding the major {100}, {101} and {210} faces with minor {011}. This crystallographic difference between the experimental result and theoretical expectation is likely the result of kinetic factors and chemical interactions with the flux.

Fig. 3 XRD pattern of a crystal grown from the LiF–NaCl–KCl flux with a solute concentration of 50 mol% (run no. 5). LiMnPO4 (ICDD-PDF 078-5417) is shown for reference.

Fig. 4 Low (a) and high (b) magnitude SEM images of the LiMnPO₄ crystal grown from a LiF–NaCl–KCl mixed flux with a solute concentration of 50 mol% (run no. 5). (c) Schematic view of the computationally-replicated crystal by using {100), {011}, {101}, and {210} facets.

The chemical composition of the as-grown LiMnPO₄ crystals was evaluated through ICP-OES analysis, which revealed that the atomic ratio of Li, Mn, P, Na, and K was 0.996, 0.989, 1.00, 0.020, and 0 (lower detection limit) respectively, i.e., which are almost stoichiometric LiMnPO₄ with tiny negligible contamination of the Na source from the flux. The XPS-Na 1s core-level spectrum of the LiMnPO₄ crystals revealed no signal coming from the Na⁺ source at around 1075 eV (Fig. S5†). This fact also strongly supports our consideration on the stoichiometry.

To better understand the manner in which LiMnPO₄ crystals grow in molten LiF–NaCl–KCl, they were subjected to thermal analyses and any time-dependent morphologic change during growth was observed. DTA measurements taken during the heating (Fig. S6(a)†) and holding process at 700 °C (Fig. S6(b)†) revealed that the LiF–NaCl–KCl flux exhibited a single endothermic peak assigned to the melting point of the ternary flux of 612 °C, which is lower than that of the NaCl–KCl binary flux system (657 °C) of NaCl : KCl = 1 : 1 molar ratio.³⁰ In the case of a mixture of a calcined precursor made up of Mn₂P₂O₇, Li₂CO₃ and the ternary flux, two endothermic peaks were observed at around 460 °C and 645 °C that were attributed to the formation of LiMnPO₄ and melting of the flux, respectively. The increase of the melting point of the flux in the latter case should be due to a contribution from the solute. TG measurements taken at the heating process (Fig. S6(c)†) presented no significant weight loss of the flux, while the mixture of the calcined precursor and the flux decreased to ca. 90% of the initial state. It indicates that the weight loss in this process was likely due to the formation of LiMnPO₄, accompanied with generation of CO₂ and NH₃. Further weight loss down to 25% was continuously observed (Fig. S6(d)†). As no further change was observed in both cases in the DTA curve after holding for 3 h at 700 °C (Fig. S6(b)†), more than 20 wt% of the flux was evaporated in this process. This weight loss is considered to represent the evaporation of the flux coupled with the continuous formation of LiMnPO₄. Changes in the in situ XRD profiles during the formation and growth of LiMnPO₄ in Fig. 5 strongly support this, with all peaks constantly shifting to a lower 2θ value with increasing temperature due to thermal expansion of the lattice. Diffraction lines assigned to Mn₂P₂O₇, NaCl and KCl were detected up to 374 °C, but no LiF was detected at any point during the process, possibly due to the low amounts involved. The formation of LiMnPO₄ was initiated before the melting of the flux at 469 °C, which coincides with the endothermic peak in the TG-DTA data at around 460 °C. The reaction of Mn₂P₂O₇ and Li₂CO₃ progressed with increasing reaction temperature up to 613 °C, with the diffraction lines of LiMnPO₄ becoming much more intense upon the disappearance of Mn₂P₂O₇. No degradation in the intensity of these LiMnPO₄ diffraction lines was observed during holding at 700 °C for 3 h, based on which it is considered that the LiMnPO₄ crystals do not fully dissolve in molten LiF–NaCl–KCl. The addition of LiF clearly did not change the reaction route of LiMnPO₄ as no intermediate fluorides were formed, suggesting that the addition of primary LiF contributed to enhancing the solubility.

Fig. 5 Change in in situ XRD profiles during the formation of LiMnPO₄ from Mn₂P₂O₇, Li₂CO₃ and LiF–NaCl–KCl mixed at a solute concentration of 50 mol% and with a heating rate of 5 °C min⁻¹. Scanning speed was 40° min⁻¹ and all experimental conditions were the same as in run no. 5.

Time-dependent SEM observations were also carried out in individually prepared specimens to assess their growth behavior and the effect of the cooling rate. The SEM image in Fig. 6(a) shows a typical crystal prepared by slow cooling at a rate of 5 °C h⁻¹ (run no. 6), which retains the same polyhedral shape and shows that the cooling rate has little effect on growth. With natural cooling, crystals grown at 700 °C with a holding time of 0 and 1 h (run no. 7 and 8 in Table 1) formed secondary aggregates with slightly developed facets and an average particle size of 4.8 (Fig. 6(b)) and 6.1 μm (Fig. 6(c)), respectively (also see their particle distribution analyses in Fig. S7†). Note also that the grain boundaries produced in the aggregates during the early stage of reaction became unclear with increasing reaction time. Further drastic morphological changes continued to appear in the crystal until the holding time was increased beyond 3 h.

Fig. 6 SEM images of LiMnPO₄ crystals prepared from the molten LiF–NaCl–KCl flux with a solute concentration of 50 mol% by holding at 700 °C for: (a) 3 h (run no. 6), (b) 0 h (run no. 7), (c) 1 h (run no. 8).

The most plausible process for the growth of polyhedral LiMnPO₄ crystals from binary NaCl–KCl and ternary LiF–NaCl–KCl molten flux systems, based on all the results presented here, is shown schematically in Fig. 7. Note that the LiMnPO₄ phase first starts to form at 450 °C, which is below the melting point of the flux. Mn₂P₂O₇ also reacts with Li₂CO₃ to form LiMnPO₄ during the early stages of this solid-state reaction, and this process is accelerated in molten LiF–NaCl–KCl above 645 °C with the simultaneous flux growth of LiMnPO₄ crystals. As these LiMnPO₄ crystals were not completely dissolved in the LiF–NaCl–KCl flux at 700 °C, crystal growth is believed to be initiated by heterogeneous nucleation. Repeated dissolution and reprecipitation of the crystal creates bidirectional material transfer at the interface between it and the molten flux, which would promote the formation of well-defined crystal facets as a means to reduce the surface energy. Surface reconstruction of the aggregates was promoted to form a flat surface by increasing holding times at 700 °C, which suggests that the LiMnPO₄ crystals preferentially dissolve in the flux at their grain boundaries. Based on the time-dependent SEM observation, Ostwald ripening is considered to contribute to the increase in the average crystal diameter and narrowing of the size distribution as the holding time is increased. Furthermore, given that the evaporation ratio during holding was 25%, the resulting supersaturation during the temperature-holding process is the most likely candidate for the driving force of crystal growth; i.e., an increased rate of flux cooling has little effect. Thus, the addition of LiF to the NaCl–KCl flux helps stimulate the development of crystal facets and increase the diameter of crystals by enhancing their solubility.

Fig. 7 Schematic illustration showing the growth of LiMnPO₄ crystals in multi-halide NaCl–KCl and LiF–NaCl–KCl molten flux systems.

Conclusions

This study of the growth of LiMnPO₄ crystals from Mn₂P₂O₇ and Li₂CO₃ in binary NaCl–KCl and ternary LiF–NaCl–KCl fluxes has demonstrated that LiMnPO₄ is formed by solid-state reaction at temperatures below the melting point of the flux. Subsequent growth through heterogeneous nucleation produces a crystal surface primarily surrounded with well-defined facets of major {100}, {101}, and {210} faces with minor {011} faces. The formation of these well-defined crystal facets and the surface reconstruction of individual crystals are enhanced through repeated dissolution and reprecipitation of the crystal at its surface, and so the growth process is dominantly controlled by the evaporation of the flux and Ostwald ripening. The addition of LiF further promotes the development of well-defined polyhedral crystals through an increase in solubility. This growth process differs from the conventional flux growth, and so has the potential to be applied to other olivine-type crystals. We believe that the highly developed LiMnPO₄ single crystals produced in this manner may encourage further studies of their intrinsic shape-dependent physical properties.

Acknowledgements

This work was supported by JSPS Grant-in-Aid for Scientific Research (A) 25249089.

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Footnote

† Electronic supplementary information (ESI) available: SEM images and XRD profiles of the precursor powder calcined at 400 °C for 3 h, particle size distribution of LiMnPO₄ crystals grown from binary and ternary halide fluxes at various solute concentrations, XRD pattern of single-crystal LiMnPO₄ grown from the LiF–NaCl–KCl flux, XPS spectrum of LiMnPO₄ grown from the LiF–NaCl–KCl flux, TG-DTA plots of the precursor calcined with the LiF–NaCl–KCl flux, and time-dependent particle distribution of LiMnPO₄ crystals prepared at different holding times. See DOI: 10.1039/c6ce02114j

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