Further Evidence for Energy Landscape Flattening in the Superionic Argyrodites Li_6+xP_1–xM_xS₅I (M = Si, Ge, Sn)

Abstract

All-solid-state batteries are promising candidates for next-generation energy-storage devices. Although the list of candidate materials for solid electrolytes has grown in the past decade, there are still many open questions concerning the mechanisms behind ionic migration in materials. In particular, the lithium thiophosphate family of materials has shown very promising properties for solid-state battery applications. Recently, the Ge-substituted Li6PS5I argyrodite was shown to be a very fast Li-ion conductor, despite the poor ionic conductivity of the unsubstituted Li6PS5I. Therein, the conductivity was enhanced by more than 3 orders of magnitude due to the emergence of I–/S2– exchange, i.e., site disorder, which led to a sudden decrease of the activation barrier with a concurrent flattening of the energy landscapes. Inspired by this work, two series of elemental substitutions in Li6+x P1–x M x S5I (M = Si and Sn) were investigated in this study and compared to the Ge analogue. A sharp reduction in the activation energy was observed at the same M4+/P5+ composition as previously found in the Ge analogue, suggesting a more general mechanism at play. Furthermore, structural analyses with X-ray and neutron diffraction indicate that similar changes in the Li sublattice occur despite a significant variation in the size of the substituents, suggesting that in the argyrodites the lithium substructure is most likely influenced by the occurring Li+–Li+ interactions. This work provides further evidence that the energy landscape of ionic conductors can be tailored by inducing local disorder.

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

“…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.…”

How Certain Are the Reported Ionic Conductivities of Thiophosphate-Based Solid Electrolytes? an Interlaboratory Study

“…Fast ionic conductors such as lithium and sodium thiophosphates are currently being investigated for their possible application in all-solid-state batteries. 1,2 Recent research efforts have found a variety of Li + and Na + based materials such as the thiophosphates Li11-xM1-xP2+xS12 (M = Si, Ge, Sn), [3][4][5][6][7][8] Li2S-P2S5 glasses, [9][10][11][12][13][14][15] Li6PS5X (X = Cl, Br, I), [16][17][18][19][20][21][22] Na3PS4 [23][24][25][26][27] and Na11Sn2PS12. 11,28,29 These materials can exhibit ionic conductivities > 1 mScm -1 , making them viable for solid-state battery applications.…”

Changing the Static and Dynamic Lattice Effects for the Improvement of the Ionic Transport Properties Within the Argyrodite Li6PS5-xSexI

“…[12][13][14][15][16] , and the lithium thiophosphates such as Li10GeP2S12, [17][18][19][20][21] Li2S-P2S5 glasses, [22][23][24][25][26] the thio-LISICONS, [27][28][29][30][31] and the Li6PS5X argyrodites. [32][33][34][35][36][37][38][39][40][41][42] While the thiophosphates are sensitive to ambient atmosphere and may not have the largest potential window, they demonstrate high ionic conductivity and high ductility, improving the processability and resulting contact to the active materials. 17,43,44 Standing out within this family of materials, the halide-containing argyrodites Li6PS5X (X = Cl, Br, I) have been explored with increasing interest.…”

“…38 A variety of different substitution series have already been performed on the argyrodites to probe underlying influences on the conductivity. 32,[36][37][38][39][45][46][47] Substitution of the halides to form solid solutions of Cl/Br or Br/I has demonstrated clear effects on alteration of the anion sitedisorder, with associated changes in ionic conductivity, but the disorder effects are coupled with compositional differences. 38 Aliovalent substitution has also led to conductivity enhancements in the iodide structures, by substituting P with Si, Ge, and Sn, with large reductions in activation energies; effects in this case are also coupled with increases in Li content.…”

“…38 Aliovalent substitution has also led to conductivity enhancements in the iodide structures, by substituting P with Si, Ge, and Sn, with large reductions in activation energies; effects in this case are also coupled with increases in Li content. 36,37 Introduction of Se in place of S in Li6PS5Br was explored as a method to increase the polarizability of the lattice, but induced competing effects by reduction of the beneficial S 2-/Brdisorder. 46 Li6PS5I has the lowest ionic conductivity of the halide argyrodites (also having the lowest anion site disorder), but the capacity to improve its conductivity based on these substitutions justifies more exploration of the composition.…”