Inducing High Ionic Conductivity in the Lithium Superionic Argyrodites Li_6+xP_1–xGe_xS₅I for All-Solid-State Batteries

Abstract

Solid-state batteries with inorganic solid electrolytes are currently being discussed as a more reliable and safer future alternative to the current lithium-ion battery technology. To compete with state-of-theart lithium-ion batteries, solid electrolytes with higher ionic conductivities are needed, especially if thick electrode configurations are to be used. In the search for optimized ionic conductors, the lithium argyrodites have attracted a lot of interest. Here, we systematically explore the influence of aliovalent substitution in Li 6+x P 1−x Ge x S 5 I using a combination of X-ray and neutron diffraction, as well as impedance spectroscopy and nuclear magnetic resonance. With increasing Ge content, an anion site disorder is induced and the activation barrier for ionic motion drops significantly, leading to the fastest lithium argyrodite so far with 5.4 ± 0.8 mS cm −1 in a cold-pressed state and 18.4 ± 2.7 mS cm −1 upon sintering. These high ionic conductivities allow for successful implementation within a thick-electrode solid-state battery that shows negligible capacity fade over 150 cycles. The observed changes in the activation barrier and changing site disorder provide an additional approach toward designing better performing solid electrolytes.

A Versatile Sn‐Substituted Argyrodite Sulfide Electrolyte for All‐Solid‐State Li Metal Batteries

“…The enlarged crystal unit cell is derived from the replacement of P with Sn, which has a larger atomic radius ( R <Sn> 1.40 Å > R <P> 1.10 Å), and the increased Li + ion concentration in the specific unit cell. The characteristic unit cell is expected to benefit the Li + ion conduction and to increase the ionic conductivity . A similar phenomenon is observed in the LPSBr‐ x Sn ( x = 0, 1, 5, 10, 12.5, 15, 20, 30) system (Figure S1, Supporting Information), but cannot occur in the LPSCl‐ x Sn ( x = 0 and 30) system (Figure S2, Supporting Information).…”

“…A similar phenomenon is observed in the LPSBr‐ x Sn ( x = 0, 1, 5, 10, 12.5, 15, 20, 30) system (Figure S1, Supporting Information), but cannot occur in the LPSCl‐ x Sn ( x = 0 and 30) system (Figure S2, Supporting Information). The reason is related to the atomic radius of X in the argyrodite structure (X = Cl, Br, and I), where the larger size of the I atom endows the LPSX structure with more possibilities of various aliovalent elemental substitutions 16a…”

“…Nevertheless, the air stability of the sulfide electrolytes is predicted to be improved after Sn substitution in the PS 4 3− structure. Furthermore, the aliovalent element (Sn) substitution with larger atomic radius and lower valence compared with phosphorus (P) can expand the cell volume and increase the Li solubility in the unit cells, which can synergistically increase the corresponding ionic conductivity …”

“…However, the high I concentration in the electrolyte itself is highly anticipated to stabilize the Li metal/LPSI‐based electrolyte interface in ASSLMBs if decent ionic conductivity can be obtained. Recent studies show that the ionic conductivity of LPSI SSE can be significantly improved by aliovalent element substitutions 16a,20. Different from previous work that is fundamentally dedicated to improve the ionic conductivity fundamentally, we aim in using a versatile strategy to achieve an excellent sulfide SSE with good Li metal compatibility, improved air stability, and decent ionic conductivity, simultaneously.…”

On the Functionality of Coatings for Cathode Active Materials in Thiophosphate‐Based All‐Solid‐State Batteries

“…Meanwhile, polymer-, oxide-, and sulfide-based ionic conductors are being heavily investigated as the solid electrolyte (SE) separator. [19,20] Unfortunately, thiophosphates also have a rather narrow electrochemical stability window, i.e., the onset of oxidative decomposition begins even before 3 V versus Li + /Li due to S(0)/S(−2) redox reactions, while reductive decomposition is theoretically expected at potentials of about 1.7 V versus Li + /Li due to P(+5)/P(−3) redox reactions. [2,3,18] Currently, only thiophosphates allow for the preparation of thick cathode composites with sufficient rate capability.…”