Tetragonal Li10GeP2S12 and Li7GePS8 – exploring the Li ion dynamics in LGPS Li electrolytes

Abstract

Tetragonal Li 10 GeP 2 S 12 (LGPS) is the best solid Li electrolyte reported in the literature. In this study we present the first in-depth study on the structure and Li ion dynamics of this structure type. We prepared two different tetragonal LGPS samples, Li 10 GeP 2 S 12 and the new compound Li 7 GePS 8 . The Li ion dynamics and the structure of these materials were characterized using a multitude of complementary techniques, including impedance spectroscopy, 7 Li PFG NMR, 7 Li NMR relaxometry, X-ray diffraction, electron diffraction, and 31 P MAS NMR. The exceptionally high ionic conductivity of tetragonal LGPS of $10 À2 S cm À1 is traced back to nearly isotropic Li hopping processes in the bulk lattice of LGPS with E A z 0.22 eV.Lithium-ion batteries are considered to play an important role in future energy storage, especially for mobile applications such as vehicle propulsion. 1 One approach to overcome both safety and durability problems of state-of-the-art Lithium-ion batteries is the use of solid electrolytes, which must satisfy the criteria of having high Li ion conductivity and a wide electrochemical window. The lack of suitable materials triggered intense research efforts in the eld of solid state ionics. During recent years signicant progress has been achieved, such that the materials available now are suitable for commercial applications.2 Hereby, two main classes of solid electrolytes have attracted the attention of both academia and industry, namely (i) oxide-based garnet-type electrolytes such as Li 7 La 3 Zr 2 O 12 , [3][4][5] and (ii) sulde-based electrolytes.6-8 The oxide-based materials generally show a wider electrochemical window, whereas the sulde-based electrolytes usually exhibit a higher Li-ion conductivity. In 2011, a new solid electrolyte Li 10 GeP 2 S 12 (LGPS), a metastable phase occurring in the system xLi 4 GeS 4 : yLi 3 PS 4 , was reported.9 Tetragonal LGPS combines the most important prerequisites for a high-performance Li electrolyte: a room-temperature conductivity of 12 mS cm À1, an activation energy of 0.24 eV, and an electrochemical window of up to 4 V vs. Li/Li + . 9 While tetragonal LGPS has only been studied by means of MD and ab initio calculations since the original publication, 10-13 a fundamental study on the Li ion dynamics occurring in tetragonal LGPS has not been reported to date. Additionally, the fact that tetragonal LGPS is a solid solution with a rather broad range of existence has not been considered in the literature so far.In this study, both Li 10 GeP 2 S 12 and Li 7 GePS 8 , a new member of the solid solution of tetragonal LGPS with a Ge : P ratio of 1 : 1, were prepared and structurally characterized. The Li ion dynamics occurring in the materials was studied by several complementary techniques sensitive to (i) long-range Li diffusion (PFG-NMR), (ii) atomic-scale jumps (NMR relaxometry), and (iii) long-range charge transport (impedance spectroscopy). This combination of techniques allows us to connect diffusion (at the macroscopic scale) to ionic ho...

In-Channel and In-Plane Li Ion Diffusions in the Superionic Conductor Li10GeP2S12 Probed by Solid-State NMR

“…2,3 The exceptionally high ionic conductivity of tetragonal LGPS has been attributed to either one dimensional (1D) ionic tunnel or 3D network. 1,4-7, 9 Kamaya et. al.…”

“…experimentally determined Li occupation of interstitial site Li(4) by single crystal XRD, 8 and obtained an averaged activation energy 0.22 eV for overall Li 3D diffusion by measurements of Li NMR diffusion and NMR relaxation but incapable of distinguishing 2D diffusion process from the 1D one. 9 So far, theoretically predicated 1D and 2D diffusion processes in LGPS have not been verified experimentally.…”

Fast Na ion transport triggered by rapid ion exchange on local length scales

“…It is beyond any doubt that reporting such high conductivities 4–8,10,25–28 , especially for sodium-based batteries 19–21,29,30 , is of enormous importance. Understanding their roots, preferably with the help of theory 31,32 , is also crucial and would enable us to safely control their dynamic properties.…”

Fast Li-Ion Transport in Amorphous Li2Si2O5: An Ab Initio Molecular Dynamics Simulation

“…[9][10][11][12][13][14][15][16][17][18] However, high conductivity electrolytes are often found with less electrochemical/chemical stability against operation. Therefore, searching for new high conductivity and stable solid-state Li + conductors has become an active area of research in recent years.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] As a key component of SSLIBs, the electrolyte should possess a high Li + conductivity (σ) at room temperature and low activation energy (E a ) for use over a broad range of operating temperatures. In the meanwhile, properties such as electrochemical stability over a wide working voltage window and chemical stability against electrodes (layered cathode and Li metal anode) should also be preferred.…”