High-Pressure Synthesis, Crystal Chemistry, and Ionic Conductivity of

Publication Date (Web): December 6, 2018. Copyright © 2018 American ... impedance were 3.4 × 10–7 and 2.1 × 10–6 S/cm at 399 and 456 K, respect...
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High-Pressure Synthesis, Crystal Chemistry, and Ionic Conductivity of a Structural Polymorph of Li3BP2O8 Eiichi Hirose,*,† Kunimitsu Kataoka,‡ Hiroshi Nagata,‡ Junji Akimoto,‡ Takuya Sasaki,§ Ken Niwa,§ and Masashi Hasegawa*,§ Department of Crystalline Materials Science, Graduate School of Engineering, and §Department of Materials Physics, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya,Aichi 464-8603, Japan ‡ National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Downloaded via UNIV OF GOTHENBURG on December 8, 2018 at 01:22:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

Li3BP2O8 exhibits higher ionic conductivity than that of ambient-pressure-synthesized Li3BP2O8 (LP-Li3BP2O8).5 Our findings demonstrate the possible advantage of using a highpressure technique to develop new lithium-ion conductors. The details of the crystal structure and the nature of high ionic conductivity is reported in this Communication. Figure S1 shows the powder X-ray diffraction (XRD) profiles of the ambient recovered samples from the synthesis experiments at high pressures (2 GPa, 600 °C and 4 GPa, 600 °C). As seen in these profiles, LP-Li3BP2O8 was synthesized at the conditions of 2 GPa and 600 °C, although a small amount of Li3PO4 remained. On the other hand, a completely different XRD profile from the known materials was observed from the sample synthesized at 4 GPa and 600 °C, which indicates that a new phase was successfully synthesized. Both samples contain a small amount of the starting material Li3PO4 probably because of the absorption of moisture on the surface of the starting materials. The single phase of HP-Li3BP2O8 was synthesized by adjusting the ratio of the starting materials (Figure S2). All diffraction peaks of the new phase were successfully indexed by using DICVOL06.7 On the basis of the reflection conditions of the diffraction peaks with h0l (h = 2n), h00 (h = 2n), and 0k0 (k = 2n), the space group was determined to be P21/a (No. 14). The initial structure of the new phase was searched with SUPERFLIP, based on the charge-flipping algorithm, and the crystal structure was refined by Jana2006.8,9 Figure 1 shows the result of Rietveld refinement for the new phase (HP-Li3BP2O8). The results of structure refinement for HP-Li3BP2O8 are summarized in Table 1. The Rietveld analysis yielded reliability factors of Rp = 2.73%, wRp = 4.02%, Robs = 4.08%, wRobs = 4.64%, and goodness-of-fit S = 0.58. The atomic coordinates, occupancies, and isotropic atomic displacement parameters of HP-Li3BP2O8 are listed in Table 2. Table S1 shows the calculated bond lengths and bond valence sums (Vi) for HP-Li3BP2O8. To the best of our survey on the database, there is no model structure of HP-Li3BP2O8. Figure S3 shows the crystal structures of LP- and HPLi3BP2O8. In HP-Li3BP2O8, boron (denoted as B1) and phosphorus (denoted as P1 and P2) coordinate with four oxide ions to form boron- and phosphorus-centered tetrahedra,

ABSTRACT: A new structural polymorph of Li3BP2O8 has been successfully synthesized via a solid-state reaction between Li3PO4 and BPO4 at 4 GPa and 600 °C. The high-pressure phase of Li3BP2O8 (HP-Li3BP2O8) was found to crystallize in monoclinic symmetry with the cell parameters of a = 8.57010(4) Å, b = 11.11812(5) Å, c = 5.55380(3) Å, and β = 97.7269(3)° [space group P21/a (No. 14)]. HP-Li3BP2O8 has a new crystal structure that has not been reported so far. The total ionic conductivities measured for the polycrystalline sample by alternatingcurrrent impedance were 3.4 × 10−7 and 2.1 × 10−6 S/cm at 399 and 456 K, respectively. The lithium ionic conductivity of HP-Li3BP2O8 was higher than that of the low-pressure phase Li3BP2O8 in the temperature range of 375−456 K. This is caused by the difference in the dimensions of the lithium arrangements between LP- and HP-Li3BP2O8.

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ithium conducting oxides as solid electrolytes are of particular interest, especially with the recent developments of electric vehicles, which require new battery materials having higher energy density and very long lifespans. The glass and crystalline phases of lithium borophosphate have been investigated for the solid electrolyte.1−6 The borophosphate glasses in the Li2O−B2O3−P2O5 system were synthesized by Tien and Hummel.2 The lithium ionic conductivity and thermal stability of glasses of the Li2O−B2O3−P2O5 system were investigated by Magistris et al.,6 which demonstrated that the lithium ionic conductivity changes as a function of the [B]/[P] atomic ratio. The lithium ionic conductivity of 0.45Li2O− 0.275B2O3−0.275P2O5 glass was measured to be 1.6 × 10−7 S/ cm at room temperature.4 On the other hand, the crystalline phases of Li22B11P13O60 and Li2B3PO8 were also investigated, while their crystal structures had not been solved.2 Recently, Hasegawa and Yamane5 succeeded in characterizing the crystal structure of Li3BP2O8, which had so far been reported as Li22B11P13O60.2 In addition to their crystal structure analysis, the lithium ionic conductivity of Li3BP2O8 was also measured. In this study, we have succeeded in the synthesis of a new structural polymorph of Li3BP2O8 (HP-Li3BP2O8) under high pressure and high temperature. The experimental details are shown in the Supporting Information. It was found that HP© XXXX American Chemical Society

Received: November 9, 2018

A

DOI: 10.1021/acs.inorgchem.8b03155 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Li3BP2O8, while they coordinate with four, six, and six oxide ions, respectively. This indicates that the coordination number of lithium is increased when LP-Li3BP2O8 transforms to HPLi3BP2O8. Borophosphates contain complex anionic structures built of BO4, BO3, and PO4 groups. The structural chemistry of borophosphate anions already extends from isolated species, oligomers, rings, and chains to layers and frameworks.10 In the crystal structure of LP-Li3BP2O8, all of the vertexes of BO4 are linked to those of neighboring PO4 and two vertexes of PO4 are linked to those of neighboring BO4, which leads to the formation of one-dimensional ∞1[BP2O8]3− chains along the c-axis direction (Figure S4).5 On the other hand, the zigzag layer of 3− 1 ∞ [BP2O8] with 4- and 12-membered rings of the tetrahedra is formed along the ab plane in HP-Li3BP2O8, although the numbers of vertexes of BO4 and PO4 that are linked to those of neighboring PO4 and BO4 are the same in LP- and HP-Li3BP2O8 (Figures S4 and S5). Considering the lithium arrangement in LP- and HP-Li3BP2O8, formation of the ∞1[BP2O8]3− zigzag layer is accompanied by an increase in the coordination number of lithium. The lithium ionic conductivities of HP-Li3BP2O8 were measured by the alternating-current impedance method at 375, 399, 415, 426, 432, 444, and 456 K. Figure S6 shows the typical Nyquist plots of HP-Li3BP2O8. The semicircles of the typical Nyquist plots could not be separated into the independent contributions from the intragrain- and grainboundary parts. Thus, the total resistances of the HP-Li3BP2O8 polycrystalline bulk sample measured at 375, 399, 415, 426, 432, 444, and 456 K were 2.4 × 106, 9.4 × 105, 6.9 × 105, 4.6 × 105, 3.8 × 105, 2.4 × 105, and 1.5 × 105 Ω, respectively, which were converted to the lithium ionic conductivities of 1.3 × 10−7, 3.4 × 10−7, 4.6 × 10−7, 6.8 × 10−7, 8.3 × 10−7, 1.3 × 10−6, and 2.1 × 10−6 S/cm, respectively. Figure 2 shows the Arrhenius plots of

Figure 1. Result of the Rietveld refinement of the synchrotron XRD profile. The red dots (Iobs) and black line (Ical) represent the data and calculated profile, respectively. Bragg reflections of HP-Li3BP2O8 are shown with the tips.

Table 1. Summarized Results of the Structure Refinement of HP-Li3BP2O8 chemical formula fw space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalc (g cm−3) GOF Rp (%) wRp (%) Robs (%) wRobs (%)

Li3BP2O8 221.6 P21/a 8.57010(4) 11.11812(5) 5.55380(3) 97.7269(3) 524.380(4) 4 2.8066 0.58 2.73 4.02 4.08 4.64

Table 2. Atomic Coordinates, Occupancies,a and Isotropic Atomic Displacement Parameters of HP-Li3BP2O8 Li1 Li2 Li3 P1 P2 B1 O1 O2 O3 O4 O5 O6 O7 O8

x

y

z

Uiso (Å2)

0.0197(7) 0.6844(8) 0.2988(7) 0.60845(12) 0.83673(12) 0.9238(5) 0.6396(2) 0.8951(3) 0.7804(2) 0.5155(2) 0.7741(3) 0.5214(2) 0.9867(2) 0.7117(2)

0.5894(5) 0.6693(6) 0.0595(6) 0.17636(9) −0.07608(9) 0.1472(3) 0.12744(17) 0.02076(18) −0.01860(18) 0.29869(17) 0.21304(18) 0.09334(17) −0.16038(17) −0.15125(18)

0.3605(11) 0.2641(11) 0.1188(11) 0.14627(17) 0.38339(17) 0.2803(7) −0.0996(3) 0.2111(4) 0.6009(4) 0.1031(3) 0.2984(4) 0.2924(3) 0.4670(4) 0.2268(4)

0.0071(16) 0.0190(18) 0.0153(17) 0.0017(2) 0.0038(2) 0.0038(10) 0.0025(6) 0.0013(6) 0.0017(6) 0.0010(6) 0.0066(6) 0.0017(6) 0.0018(6) 0.0028(6)

Figure 2. Arrhenius plots of the lithium ionic conductivities of LPLi3BP2O85 and HP-Li3BP2O8.

the lithium ionic conductivities of LP-Li3BP2O85 and HPLi3BP2O8. The activation energy of HP-Li3BP2O8 (49 kJ/mol) was lower than that of LP-Li3BP2O8 (68 kJ/mol).5 The lithium ionic conductivity of HP-Li3BP2O8 was higher than that of LPLi3BP2O8 in the temperature range of 375−456 K (Figure 2). Figure S7 shows the projective views of lithium in HPLi3BP2O8. Table S2 shows the calculated Li−Li distances for HP-Li3BP2O8. Taking into account the Li−Li distances below 3 Å, there are independent units that consisted of Li1, Li2, and Li3

a

The site occupancies of all atoms are equal to 1.

BO4 and PO4, as well as LP-Li3BP2O8.5 For the lithium ions, three crystallographic sites denoted as Li1, Li2, and Li3 exist in the structure of LP-Li3BP2O8,5 and they coordinate with five, four, and four oxide ions, respectively. The three different sites of lithium ions denoted as Li1, Li2, and Li3 also exist in HPB

DOI: 10.1021/acs.inorgchem.8b03155 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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in HP-Li3BP2O8 (Figure S7a), while in the case of Li−Li distances below 3.2 Å, the two-dimensional Li−Li network is formed along the ab plane with Li−Li distances of 3.112(9) Å (Li1−Li2) and 3.108(8) Å (Li1−Li3) (Figure S7b1,b2 and Table S2). Further extending the Li−Li distance to 3.279(9) Å forms the three-dimensional Li−Li network (Figure S7c1,c2), which is almost the same as that of the two-dimensional one. Therefore, HP-Li3BP2O8 may have two- or three-dimensional ion diffusion pathways. In LP-Li3BP2O8, the Li2 and Li3 sites align on the ac plane, forming zigzag chains along the a + c direction (Figure S8). The distances of Li2−Li2, Li2−Li3, and Li3−Li3 are in the range 2.531(5)−2.624(7) Å, while the Li−Li distances between the zigzag chains are 3.688(7)−4.588(6) Å. The relatively short Li−Li interatomic distances in the chains may be suggested as a lithium-ion diffusion pathways of LPLi3BP2O8.5 The Li−Li distances of HP-Li3BP2O8 are in the range 2.567(9)−3.279(9) Å (Table S2), which are longer than those of LP-Li3BP2O8. However, the lithium ionic conductivity of HP-Li3BP2O8 is higher than that of LP-Li3BP2O8. It is conceivable that a three-dimensional ion diffusion pathway yields higher ionic conductivity than a one-dimensional one. For example, the lithium ionic conductivities of Li7La3Zr2O12 (σtotal = 7.74 × 10−4 S/cm at 291 K) and Li10GeP2S12 (σtotal = 5 mS/cm at room temperature) with three-dimensional ion diffusion pathways have higher ionic conductivities than LiAlSiO4 (σbulk = 2.41 × 10−7 S/cm at 500 K) with one-dimensional ion diffusion.11−13 However, the chemical compositions of these compounds are different from each other, which leads to the difficulty of a direct comparison of the lithium ionic conductivities with respect to the dimensions of the lithium arrangement. In this study, lithium-ion diffusion pathways are evaluated based on a comparison of the structural polymorph of Li3BP2O8. LP-Li3BP2O8 exhibits the alignment of lithium ions in zigzag chains,5 while in HP-Li3BP2O8, the lithium sites are twoor three-dimensionally arranged, which might lead to the high lithium ionic conductivity. In summary, we have succeeded in synthesizing a new structural polymorph of Li3BP2O8 under high pressure. HPLi3BP2O8 with a two- or three-dimensional Li−Li network has higher lithium ionic conductivity than LP-Li3BP2O8 with Li−Li zigzag chains. A state-of-the-art high-pressure technique allows one to stabilize unique high-pressure lithium-based phases without any change of the composition, which offers direct experimental evidence on the effect of the lithium arrangement on the ionic conductivity.

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Communication

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.H.). *E-mail: [email protected] (M.H.). ORCID

Eiichi Hirose: 0000-0003-0199-9859 Takuya Sasaki: 0000-0002-0241-4544 Ken Niwa: 0000-0003-1037-675X Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The XRD experiments were conducted at beamline BL5S2 of the Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (Proposals 201705094 and 201804090).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03155. Experimental details, XRD profiles, crystal structures, typical Nyquist plots, and tables of bond lengths and Li− Li distances (PDF) Accession Codes

CCDC 1865009 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. C

DOI: 10.1021/acs.inorgchem.8b03155 Inorg. Chem. XXXX, XXX, XXX−XXX