A new crystalline LiPON electrolyte: Synthesis, properties, and electronic structure

doi:10.1016/j.ssi.2012.12.013

Solid State Ionics

Volume 233, 21 February 2013, Pages 95-101

https://doi.org/10.1016/j.ssi.2012.12.013 Get rights and content

Abstract

The new crystalline compound, Li₂PO₂N, was synthesized using high temperature solid state methods starting with a stoichiometric mixture of Li₂O, P₂O₅, and P₃N₅. Its crystal structure was determined ab initio from powder X-ray diffraction. The compound crystallizes in the orthorhombic space group Cmc2₁ (# 36) with lattice constants a = 9.0692(4) Å, b = 5.3999(2) Å, and c = 4.6856(2) Å. The crystal structure of SD-Li₂PO₂N consists of parallel arrangements of anionic chains formed of corner sharing (PO₂N₂) tetrahedra. The chains are held together by Li⁺ cations. The structure of the synthesized material is similar to that predicted by Du and Holzwarth on the basis of first principles calculations (Phys. Rev. B 81, 184106 (2010)). The compound is chemically and structurally stable in air up to 600 °C and in vacuum up to 1050 °C. The Arrhenius activation energy of SD-Li₂PO₂N in pressed pellet form was determined from electrochemical impedance spectroscopy measurements to be 0.6 eV, comparable to that of the glassy electrolyte LiPON developed at Oak Ridge National Laboratory. The minimum activation energies for Li ion vacancy and interstitial migrations are computed to be 0.4 eV and 0.8 eV, respectively. First principles calculations estimate the band gap of SD-Li₂PO₂N to be larger than 6 eV.

Highlights

► High temperature solid state methods were used to synthesize the new crystalline compound Li₂PO₂N. ► X-ray analysis shows the synthesized compound to have a structure similar to first-principles predictions. ► The structure is characterized by parallel chains of corner sharing PO₂N₂ tetrahedra with planar P single bond NPN backbones. ► Li₂PO₂N is chemically and structurally stable in air up to 600 °C and in vacuum up to 1050 °C. ► The measured Arrhenius activation energy for ionic conductivity of Li₂PO₂N in pressed pellet form is 0.6 eV.

Introduction

Lithium phosphorous oxy-nitride “LiPON” electrolytes with the composition Li_xPO_yN_z, where x = 2y + 3z − 5, were pioneered at Oak Ridge National laboratory [1], [2], [3], [4], [5], [6], [7], [8], [9]. These compounds were physically deposited as amorphous thin film electrolytes for use in all solid state micro-batteries. In the course of a computational study of the broad class of crystalline lithium phosphorus oxy-nitride materials, Du and Holzwarth [10] recently predicted a stable crystalline material with the stoichiometry Li₂PO₂N. The predicted material was computationally derived from the known crystal structure [11] of LiPO₃ by using the following modification of the phosphate chains. Each oxygen in a bridging site between two phosphorus ions was replaced with nitrogen and one lithium was added to the chemical composition to maintain electroneutrality. The computational study showed that placing N rather than O at the bridging sites leads to the stabilization of a planar structure of the P single bond NPN backbone along the chains, consistent with an electronic configuration on the N site with sp² hybridization, compared with the twisted POPO backbone along the phosphate chains found in LiPO₃.

Various strategies have been explored to achieve stable LiPON materials, with reasonably high ionic conductivity and crystallinity, by employing diverse synthetic methods. However, most of the synthesis methods, with a few exceptions, have not produced crystalline phases of LiPON. The Oak Ridge group showed that a solid state reaction using stoichiometric amounts of Li₃N and LiPO₃ under N₂ atmosphere produces microcrystalline Li_2.88PO_3.73N_0.17 at relatively low temperature [4]. The ionic conductivity of this material was found to be 1 × 10^− 13 S/cm, which is significantly larger than the structurally similar pure lithium phosphate (γ-Li₃PO₄) but too small for electrolyte applications. By contrast, commercial LiPON thin films with ionic conductivities of typically 2 × 10^− 6 S/cm at 25 °C are prepared by deposition of material from radio frequency magnetron sputtering of ceramic Li₃PO₄ targets using nitrogen in the process gas [1], [9].

The present paper reports the experimental preparation of the computationally predicted compound, Li₂PO₂N. Furthermore, its crystal structure determined ab initio from powder X-ray diffraction data was found to be similar to the two low-energy structures s₁ and s₂ found in the original optimization studies [10]. For ease of comparison with these previously predicted structures, we refer to the experimental obtained compound as SD-Li₂PO₂N. Computer optimization studies of the SD structure find it to stabilize at an energy lower by 0.1 eV/Li₂PO₂N compared to the energies of the s₁ and s₂ structures. The combination of experimental and computational studies of Li₂PO₂N reveal an interesting material composed of anionic flat chains of phosphorus oxy-nitride corner-shared tetrahedra, held together by Li⁺ cations. The structure of SD-Li₂PO₂N is similar to that of lithium metasilicate Li₂SiO₃ [12].

Section snippets

Synthesis methods

SD-Li₂PO₂N was synthesized using the chemical reaction ${Li}_{2} O (s) + \frac{1}{5} P_{2} O_{5} (s) + \frac{1}{5} P_{3} N_{5} (s) \to {Li}_{2} {PO}_{2} N (s) .$

The precursor materials, lithium oxide (Li₂O, 99.5% purity, Alfa Aesar), phosphorous pentoxide (P₂O₅, 99.99% purity, Sigma-Aldrich), and triphosphorous pentanitride (P₃N₅), received as a gift from Tianyu Chemical Co., Ltd. Zhejiang, China (> 99% purity with Mn ≤ 0.001%, Mg ≤ 0.005%, Cu ≤ 0.0001%, Fe ≤ 0.001%, and Si ≤ 0.01%), were weighed in an argon filled glove box in the molar ratio of 1:0.2:0.3 and ground for

Synthesis details and material properties

A high temperature solid state reaction method was employed to make SD-Li₂PO₂N. All manipulations were done inside an argon filled glove box. Although the precursors are not air-sensitive, it is important to handle them, especially the highly hygroscopic P₂O₅, under moisture free conditions. Approximately 90% yield relative to initial weight of the precursor mixture was recovered after the completion of the reaction. It was our experience that a slight excess of the P₃N₅ precursor was needed to

Summary and conclusions

LiPON electrolytes developed and used for applications in solid state microbatteries [1] have a disordered glassy form. By contrast the new SD-Li₂PO₂N has one of the simplest structures of the LiPON family of materials. The planar P single bond NPN backbone is stabilized by the N 2p π states. The strong bonding structure undoubtedly contributes to the chemical and structural stability of the material up to 1050 °C in vacuum and up to 600 °C in air. The first-principles calculation results are in excellent

Acknowledgments

The work was supported by the Wake Forest University Center for Energy, Environment, and Sustainability and by NSF grants DMR-1105485 and MRI-1040264. Computations were performed on the Wake Forest University DEAC cluster, a centrally managed resource with support provided in part by the University. Helpful discussions with R. T. Williams are gratefully acknowledged. Additional experimental help from David Hobart and Brian Hanson from Virginia Tech and Baxter McGuirt from Wake Forest University

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A new crystalline LiPON electrolyte: Synthesis, properties, and electronic structure

Abstract

Highlights

Introduction

Section snippets

Synthesis methods

Synthesis details and material properties

Summary and conclusions

Acknowledgments

Solid State Ionics

J. Solid State Chem.

J. Non-Cryst. Solids

J. Power Sources

Solid State Ionics

J. Power Sources

Solid State Ionics

Comput. Phys. Commun.

Comput. Phys. Commun.

Comput. Phys. Commun.

Chem. Phys. Lett.

J. Nucl. Mater.

Comput. Mater. Sci.

Interface

J. Electrochem. Soc.

Phys. Rev. B

Crystallogr. Rep.

Acta Crystallogr. B

DIFFRAC.EVA (Version 2.0)

DIFFRAC.TOPAS (Version 4.2)