Li₆PS₅X: A Class of Crystalline Li‐Rich Solids With an Unusually High Li⁺ Mobility

Abstract

The mineral argyrodite (Ag 8 GeS 6 ) was the first representative of a group of solids known as argyrodites [1,2] that are characterized, in general, by a high ionic conductivity and mobility of their Ag + ions. The structural and conductivity data of these compounds have been called on repeatedly to explain these physical properties in light of the complex argyrodite structure type. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] These studies have improved the general understanding of the diffusion paths of the mobile ions as a function of the temperature and structural properties. The Ag + ions in these compounds can be substituted by other cations, usually Cu + , and versatile substitution reactions of P and S whilst maintaining the topology of the argyrodite structure have also been reported.Considering the great interest in the mobility of Li + ions in solids it was surprising to us that there are only a few reports of experimental work with the corresponding crystalline Li analogues, and none of these appear to be aware of, or even mention, the argyrodite connection. One of these papers reports the synthesis of a black powder containing Li 7 PS 6 and Li 8 P 2 S 9 (minority phase), [19] and another gives detailed NMRspectroscopy-based information on the behavior of Li + ions in a series of pre-reacted amorphous and crystalline mixtures of Li 2 S and P 2 S 5 .[20] A few papers have been published on the application of glass ceramics obtained by high-energy milling of Li 2 S/P 2 S 5 mixtures and their application in secondary batteries in recent years [21][22][23] although, again the possible argyrodite connection, is not mentioned. Thus, well-established syntheses of crystalline single-phase Li argyrodites and reliable structural and physical data are not available.Although there is some controversy about the radii of the univalent cations of Group 11, [24] it can be assumed that the radii of Cu + and Li + are quite similar and different to Ag + (Cu + = 74 pm, Li + = 73 pm, Ag + = 114 pm; all coordination number(CN) = 4). This situation clearly favors a partial or full mutual substitution of Li + and Cu + ions in argyrodites and thus the existence of synthetic "Li-argyrodites". The enhanced reactivity of elemental Li and its compounds at higher temperatures compared to Ag and Cu, however, requires special precautions concerning appropriate container materials and may be the reason for this apparent knowledge gap.The crystal structures of the high-temperature phases of argyrodites are based on a tetrahedral close packing of the nonmetal atoms (chalcogen/halogen) following the topology of a cubic Laves phase (e.g., MgCu 2 ). The 24 chalcogen atoms in the unit cell of Ag 9 AlSe 6 (Z = 4[12] ) form 136 tetrahedral holes which are occupied by four Al 3+ (ordered) and 36 Ag + ions. These Ag + ions are dynamically and/or statically disordered, which explains the high Ag + (Cu + ) ionic conductivities of many argyrodites.Li 6 PS 5 X (X: Cl, Br, I) represents a series of argyrodites whose chemical formula i...

Preparation of high lithium-ion conducting Li6PS5Cl solid electrolyte from ethanol solution for all-solid-state lithium batteries

“…The electrochemical tests were performed at a constant current density of 0.13 mA cm À2 in the voltage range from 2.0 V to 3.6 V vs. LieIn (corresponding to the voltage range from 2.6 V to 4.2 V vs. Li). have reported that the argyrodite-type solid electrolyte Li 6 PS 5 X (X ¼ Cl, Br, I) was constructed of PS 4 3À units [22]. The powder prepared from EtOH solution thus retained the main structure unit of the original Li 6 PS 5 Cl.…”

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

“…[6][7][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.…”

Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction

“…7 While it is well-known that silver- 8,9 and sodium-ion 10,11 conductors can have ionic conductivities comparable to that of liquid electrolytes, 12 recent breakthroughs have led to marked increases in lithium-ion conductivity. Considerable research has focused on a number of crystal structures including LISICON-like (lithium superionic conductor), 13 argyrodites, 14 garnets, 15 NASICON-like (sodium superionic conductor), 16 lithium nitrides, 17,18 lithium hydrides, 19 perovskites, 20,21 and lithium halides, 22 where increasing conductivities can be achieved by structural and compositional tuning within a given family of structures. Lithium-ion conductivities in the argyrodites, 14 thio-LISICON, 23 diminished flammability.…”