Room temperature synthesis and phase transformation of lithium phosphate Li3PO4 as solid electrolyte

ABSTRACT We successfully prepared single-phase sample of the low-temperature β-type form of lithium phosphate Li3PO4 by a stoichiometric mixing of LiOH solution and NH4H2PO4 powder at room temperature without heating. The orthorhombic lattice parameters with the space group of Pmn21 for β-Li3PO4 were refined to be a = 6.1281(6) Å, b = 5.2674(5) Å, c = 4.8923(2) Å, and V = 157.92(2) Å3. We also prepared the single-phase sample of high-temperature γ-Li3PO4 by heating the β-Li3PO4 at 650°C. The orthorhombic lattice parameters with the space group of Pmnb for γ-Li3PO4 were refined to be a = 6.12569(5) Å, b = 10.48730(9) Å, c = 4.92957(4) Å, and V = 316.685(4) Å3. The structure transformation from β- to γ-type phase was examined utilizing the XRD measurements. The XRD data confirmed the continuous phase change in the temperature range between 25°C and 650°C. The lattice contraction for the β-Li3PO4 phase was observed between 150°C and 350°C. The phase transformation from β- to γ-Li3PO4 was observed around 450°C. The spherical primary particle size was ca. 20–30 nm order for both β- and γ-Li3PO4 samples. Accordingly, the nano-sized particle Li3PO4 sample can be easily prepared using the present synthetic technique at room temperature.


Introduction
Lithium phosphate Li 3 PO 4 is one of the attractive solid electrolyte materials due to its wide electrochemical potential, strong glass-forming capability, and simple composition for the next generation all-solid-state lithium battery (LIB) applications [1][2][3][4]. Especially, an amorphous form of Li 3 PO 4 exhibits low electrode/solid electrolyte interfacial resistance in use as solid electrolyte for all-solid-state thin-film LIB [5][6][7].
There are three crystalline forms of Li 3 PO 4 ; α-, β-, and γ-type structures depending on the stable temperature region. The low-temperature β-form of Li 3 PO 4 is stable below 400°C and undergoes a phase transition to γ-type structure irreversibly [8]. Furthermore, γ-Li 3 PO 4 reversibly undergoes a phase transition to α-Li 3 PO 4 above 1167°C [8]. A phase relationship between βand γ-Li 3 PO 4 phases has been reported in the literatures. Zemann first determined the crystal structure of γ-Li 3 PO 4 , as shown in Figure 1(b) [9]. Tatre reported that the sample prepared by hydrothermal treatment at 245°C for 4 days crystallized in the low-temperature β-form of Li 3 PO 4 for the first time [10]. Keffer et al. described the crystal structure of β-Li 3 PO 4 (Figure 1 (a)) using the single-crystal sample prepared by neutralizing a slurry of lithium carbonate by slowly adding phosphoric acid [11]. They also reported the phase transformation from β-to γ-Li 3 PO 4 at 502°C irreversibly by differential thermal analysis [11]. On the other hand, a continuous and martensitic transformation phenomenon between 340°C and 410°C was investigated by ex-situ X-ray diffraction technique using the commercial grade of β-Li 3 PO 4 sample having the particle grain size approximately between 0.1 μm and 5 μm [12]. The relative intensities of the 002 reflection peaks for these phases were used as a measurement of the degree of the β/γ transformation. Moreover, Popović et al. reported the β/γ transition temperature above 580°C [13]. From these situations, the reported transition temperatures vary in a wide range of temperatures from 340°C to 580°C. This may be affected by the different synthetic procedures of β-Li 3 PO 4 at low temperatures, however, unfortunately, chemical information for these samples could not be reported in the literatures [12,13]. Recently, Ayu et al. re-investigated the crystal structure of β-Li 3 PO 4 using the sample prepared by reaction of LiOH and H 3 PO 4 at 40°C [14]. However, they have never revealed the structure transformation from β-to γ-Li 3 PO 4 using the as-prepared β-Li 3 PO 4 sample.
The β-Li 3 PO 4 has an ordered wurtzite structure in which lithium and phosphorus are ordered over one set of tetrahedral sites, as shown in Figure 1(a) [11]. The γ-Li 3 PO 4 has a closely related structure with LiO 4 and PO 4 tetrahedra (Figure 1(b)) [9]. The structural difference between β-and γ-Li 3 PO 4 can be explained by the arrangement of both the PO 4 and LiO 4 tetrahedra to the c-axis direction. In the β-Li 3 PO 4 structure, all the tetrahedra are arranged in the same direction, while half of the PO 4 and LiO 4 tetrahedra are arranged in opposite directions in the γ-Li 3 PO 4 structure [12].
In the present study, we successfully prepared the β-Li 3 PO 4 sample utilizing the room temperature reaction between LiOH solution and NH 4 H 2 PO 4 . The chemical composition and the lattice parameters of the β-Li 3 PO 4 sample were revealed by inductively coupled plasmaoptical emission spectroscopy (ICP-OES) analysis and X-ray diffraction (XRD) measurements. The nano-sized particle morphology of the present Li 3 PO 4 sample was observed by a field emission-scanning electron microscope (FE-SEM). The structural transformation upon heating of the present β-Li 3 PO 4 sample was demonstrated by ex-situ XRD measurements using the heated samples in the temperature range from 25°C to 650°C.

Experimental procedure
In the present study, the sample preparation was performed at room temperature without heating. The starting materials were LiOH·H 2 O (Kojundo Chemical Lab. Co., Ltd.) and NH 4 H 2 PO 4 (FUJIFILM Wako Pure Chemical Corp.). First, LiOH·H 2 O was dissolved in deionized water. Then, NH 4 H 2 PO 4 powder was put into the LiOH solution with magnetic stirring for 10 min. The resulting precipitate was collected by filtration and dried in a vacuum at room temperature overnight. To reveal the structure transformation upon heating, the obtained Li 3 PO 4 sample was heated in alumina crucibles at 50°C, 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, and 650°C for 1 h using an oven or electric furnace in air.
The phase identification of the obtained samples was examined using the powder X-ray diffraction (XRD, Rigaku SmartLab) at room temperature with Cu Kα 1 radiation (λ = 1.54056 Å; operating conditions: 40 kV, 30 mA) equipped with Johansson-type X-ray mirror over a 2θ range from 5° to 100°. The lattice parameters were determined by Rietveld analysis using the reported structural data for β-Li 3 PO 4 [11] and γ-Li 3 PO 4 [15]. The refinement software Jana2006 [16] was used for the calculations. The particle size and morphology of the samples were observed by field emission-scanning electron microscope (FE-SEM, Hitachi S-4300) at an accelerating voltage of 5 kV. Chemical composition was analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Agilent 720-ES). Figure 2 shows powder XRD pattern of the Li 3 PO 4 sample after vacuum-drying at room temperature. All the diffraction peaks were attributed to orthorhombic β-Li 3 PO 4 structure having the space group of Pmn2 1 , and impurities were not detected in the XRD pattern. The lattice parameters were determined to be a = 6.1281(6) Å, b = 5.2674(5) Å, c = 4.8923(2) Å, and V = 157.92(2) Å 3 . These values were in good agreement with the previous results, e.g. a = 6.1295(2) Å, b = 5.2546(2) Å, and c = 4.8705(1) Å [14]. Chemical composition of the Li 3 PO 4 sample was analyzed by ICP-OES. The analytical ratio of Li:P was determined to be 3.11:1 suggesting the stoichiometric chemical composition of Li 3 PO 4 for the present sample. These results indicated that the single-phase sample of β-Li 3 PO 4 can be easily prepared at room temperature without heating in this study.  Figure 3 shows the particle morphology of Li 3 PO 4 prepared at room temperature, which is observed using a field emission scanning electron microscope (FE-SEM). The secondary particles were spheres of submicron size (0.2-0.3 μm). In addition, very small primary particles (spherical particle size: 20-30 nm) gather on the surface of the secondary particles were observed. Considering the effects of particle size and electrostatic force on the aggregation process, it is presumed that these small primary particles aggregate to form larger secondary particles in the vacuum drying process. Accordingly, the present synthetic procedure enables to produce nano-sized particles of the β-Li 3 PO 4 phase at room temperature.  . Figure 5 shows the FE-SEM image for the Li 3 PO 4 sample after heating at 650°C. The spherical secondary particle morphology having the submicron order of 0.-2-0.3 μm was maintained unchanged from the asprepared sample, as shown in Figure 3. Moreover, very small spherical primary particles with the size of ca. 30 nm on the surface of the secondary particles were also observed even after 650°C heating. Consequently, the nano-sized particles of the γ-type Li 3 PO 4 can be prepared utilizing the present synthetic route.

Phase change upon heating in the temperature range from 25°C to 650°C
The structural transformation upon heating of the present β-Li 3 PO 4 sample was demonstrated by exsitu XRD measurements using the heated samples in the temperature range from 25°C to 650°C. Figure  6 shows the powder XRD patterns of the present Li 3 PO 4 samples at elevated temperatures. As elevating the temperature, the peak width and intensity changed sharply. No impurity phases were observed in these patterns. Figure 7 shows the magnified XRD patterns for these samples. Additional peaks, e.g. 2θ = 19.9° (indicated by an arrow) suggesting the γtype structure can be observed above 450°C. This  fact indicated that the phase transformation temperature from β-to γ-Li 3 PO 4 was around 450°C for the present Li 3 PO 4 sample. Figure 8 and Table 1 summarize the lattice parameters for the present Li 3 PO 4 samples. It should be noted the refined lattice parameters confirmed the continuous phase change in the temperature range between 25°C and 650°C. Especially, a continuous change in the c-axis length upon heating is characteristics. The contraction for the c-axis length for the β-Li 3 PO 4 phase was observed between 150°C and 350°C. In fact, the peak position of 002 reflections drastically changed at these temperatures, as shown in Figure 7. These facts may be explained the continuous local structural change in both βand γ-type Li 3 PO 4 . We are now examining the precise structure determination upon heating using synchrotron XRD data and will publish in another paper.

Conclusion
In summary, we successfully prepared the β-Li 3 PO 4 sample utilizing the room temperature reaction without heating. The obtained sample showed nano-sized particle morphology of 20-30 nm that is suitable for the fabrication of dense solid electrolyte in all-solidstate lithium battery application. transformation temperature from β-to γ-Li 3 PO 4 was confirmed around 450°C for the present Li 3 PO 4 sample. In addition, the present β-Li 3 PO 4 sample exhibited a continuous structural change upon heating in the temperature range from 25°C to 650°C. Even for the β-Li 3 PO 4 phase, the lattice volume contraction was observed between 150°C and 350°C. We are now attempting to determine the precise crystal structure so as to reveal the mechanism of the continuous lattice parameter change upon heating. mental help and discussions regarding the structure change upon heating.

Disclosure statement
No potential conflict of interest was reported by the authors.