Article pubs.acs.org/cm
Local Structural Investigations, Defect Formation, and Ionic Conductivity of the Lithium Ionic Conductor Li4P2S6 Christian Dietrich,† Marcel Sadowski,‡ Sabrina Sicolo,‡ Dominik A. Weber,† Stefan J. Sedlmaier,§ Kai S. Weldert,† Sylvio Indris,∥ Karsten Albe,‡ Jürgen Janek,*,†,§ and Wolfgang G. Zeier*,† †
Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany Institute of Materials Science, Technische Universität Darmstadt, Jovanka-Bontschits-Strasse 2, D-64287 Darmstadt, Germany § BELLA-Batteries and Electrochemistry Laboratory, Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ∥ Institute for Applied Materials, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ‡
S Supporting Information *
ABSTRACT: Glassy, glass−ceramic, and crystalline lithium thiophosphates have attracted interest in their use as solid electrolytes in all-solid-state batteries. Despite similar structural motifs, including PS43−, P2S64−, and P2S74− polyhedra, these materials exhibit a wide range of possible compositions, crystal structures, and ionic conductivities. Here, we present a combined approach of Bragg diffraction, pair distribution function analysis, Raman spectroscopy, and 31P magic angle spinning nuclear magnetic resonance spectroscopy to study the underlying crystal structure of Li4P2S6. In this work, we show that the material crystallizes in a planar structural arrangement as a glass ceramic composite, explaining the observed relatively low ionic conductivity, depending on the fraction of glass content. Calculations based on density functional theory provide an understanding of occurring diffusion pathways and ionic conductivity of this Li+ ionic conductor.
S cm−1.22 In contrast, Li4P2S6, which can be crystallized from the glass under loss of elemental sulfur or via typical solid-state reaction, has been reported to have a variety of measured ionic conductivities ranging from 10−7 to 10−6 S cm−1.22,23 The glass itself consists of P2S74− ditetrahedra with P−S−P bonds, while the main building unit in Li4P2S6 is P2S64− with a characteristic P−P bond.24,25 In addition to the differences in the local bonding environment and oxidation states (+5/+4), the underlying structure of Li4P2S6 itself has not been determined unambiguously.23 According to the initial report by Mercier et al.,25 Li4P2S6 crystallizes in space group P63/mcm with one crystallographic phosphorus position for the P2S64− structural unit. Figure 1a shows a representation of the average structure as reported in literature. The Wyckoff position for P (4e) is 50% occupied, resulting in two average P−P distances of 1.066 and 2.233 Å. While the latter seems reasonable for a P4+−P4+ bond, the short bond is physically unlikely because of the strong Coulombic repulsion. Therefore, different structures of Li4P2S6 have recently been proposed, including the characteristic P2S64− structural units, but do not exhibit the very short P4+−P4+ distances.23 One of these structures is characterized by an
1. INTRODUCTION Lithium conducting thiophosphates have emerged as promising solid electrolytes for use in solid-state batteries because of their inherent high ionic conductivities.1,2 This class of compounds spans a wide variety of compositions and structures such as Li3PS4 (thio-LISICON),3,4 Li10MP2S12 (LGPS, where M = Si, Ge, or Sn),5−11 and the Li2S−P2S5 glasses, glass ceramics, and crystalline phases (e.g., Li7P3S11),12−18 respectively. While for most of these phases, isolated PS43− tetrahedra and cornersharing P2S74− ditetrahedra building units are the few underlying structural motifs, a wide range of conductivities spanning multiple orders of magnitude are found, and outstanding values on the order of 10−3 S cm−1 have been reported for Li10MP2S12 or Li7P3S11.7,13 The fact that large differences in ionic conductivity can be observed despite similar polyhedral arrangements demonstrates the need to study the influence of the local crystal structure of these compounds in more detail.10,19,20 Once a better understanding of the local structure and ion dynamics has been obtained, it may be possible to apply chemical concepts like doping to achieve, e.g., local “chemical pressure”21 for a widening of diffusion pathways and optimization of ionic transport. One prominent example of a thiophosphate with a comparably low conductivity can be found in the 67:33 Li2S:P2S5 system (corresponding to Li4P2S7). The glass with this nominal composition exhibits conductivities of up to 10−5 © 2016 American Chemical Society
Received: September 30, 2016 Revised: October 30, 2016 Published: November 8, 2016 8764
DOI: 10.1021/acs.chemmater.6b04175 Chem. Mater. 2016, 28, 8764−8773
Article
Chemistry of Materials
Figure 1. Structural representations of Li4P2S6 in its three possible space group configurations. All three structures exhibit a hexagonal pattern of Li and the P−S bonds, with a view along the P−P bond. Whereas in the literature structure the P position is 50% occupied, resulting in unphysical short (1 Å) P−P bonds, the local arrangement in the other two structures prevents this short P−P bond; P2S64− building units can be arranged in a zigzag manner (e) or as planes (f). P is colored purple, S orange, and Li yellow.
alternating zigzag arrangement of P2S64− building units in space group Pnnm as seen in Figure 1b, with two different crystallographic sites for P (Wyckoff position 4g). A second structure represents a layered arrangement of P2S64− planes (Figure 1c) with two crystallographic sites for P (Wyckoff position 2e) in space group P3̅1m. There are suggestions from theoretical calculations23 that both structures represent lowenergy configurations. It has not been possible, however, to obtain experimental evidence of the existence of either of these structures. Inspired by the conflicting reports about the measured ionic conductivity of Li4P2S6 as well as the unsolved questions about the underlying crystal structure, here we employ a combination of synchrotron Bragg diffraction, pair distribution function analysis, and Raman and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy to confirm the existence of the planar structure in Li4P2S6. More importantly from a practical point of view, we propose an explanation for the observed variation in experimentally determined ionic conductivities. As mentioned above, crystalline Li4P2S6 can be formed from Li4P2S7 glass by entropy-driven loss of sulfur at elevated temperatures and a corresponding reduction of phosphorus, thus forming dianions with a P−P bond. Even the preparation by solid-state reaction appears to lead to a glass−ceramic composite of the poorly conducting crystalline phase and the better conducting Li2S:P2S5 glass, in which the fraction of glass determines the observable ionic conductivity. Furthermore, from calculations within electronic density functional theory, we obtain an understanding of the prevalent diffusion pathways and use the calculated defect formation enthalpies to explain the measured ionic conductivities in Li4P2S6.
argon-filled glovebox (MBraun, O2 and H2O content of
