Synthetic Tailoring of Ionic Conductivity in Multicationic Substituted, High‐Entropy Lithium Argyrodite Solid Electrolytes

Abstract

Superionic conductors are key components of solid‐state batteries (SSBs). Multicomponent or high‐entropy materials, offering a vast compositional space for tailoring properties, have recently attracted attention as novel solid electrolytes (SEs). However, the influence of synthetic parameters on ionic conductivity in compositionally complex SEs has not yet been investigated. Herein, the effect of cooling rate after high‐temperature annealing on charge transport in the multicationic substituted lithium argyrodite Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I is reported. It is demonstrated that a room‐temperature ionic conductivity of ∼12 mS cm−1 can be achieved upon cooling at a moderate rate, superior to that of fast‐ and slow‐cooled samples. To rationalize the findings, the material is probed using powder diffraction, nuclear magnetic resonance and X‐ray photoelectron spectroscopy combined with electrochemical methods. In the case of moderate cooling rate, favorable structural (bulk) and compositional (surface) characteristics for lithium diffusion evolve. Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I is also electrochemically tested in pellet‐type SSBs with a layered Ni‐rich oxide cathode. Although delivering larger specific capacities than Li6PS5Cl‐based cells at high current rates, the lower (electro)chemical stability of the high‐entropy Li‐ion conductor led to pronounced capacity fading. The research data indicate that subtle changes in bulk structure and surface composition strongly affect the electrical conductivity of high‐entropy lithium argyrodites.

Tuning Ion Mobility in Lithium Argyrodite Solid Electrolytes via Entropy Engineering

“…The data for EQ in (d) and (e) was taken from the literature. [15,21] If error bars are not visible, the standard deviation is smaller than the data symbol.…”

“…This is consistent with previous observations on other lithium thiophosphate SEs. [5,14,15,21,23,31,32] The 31 P intensity decreased from P 0.75 to Sb 0.75 , with the peak gradually shifting from 94.05 to 91.3 ppm (Figure S4, Supporting Information). In the case of P 0.75 , the spectrum also revealed several contributions indicated by shoulder peaks at higher ppm values.…”

“…However, both the appearance of a single peak and the chemical shift confirm the [SiS 4 ] 4À tetrahedral environment and corroborate the absence of impurity phases. [15,21,[33][34][35] Additionally, 6 Li MAS NMR spectra were recorded, revealing a narrow peak located around 1.2 ppm, which is indicative of fast Li motion (Figure S5, Supporting Information). [15,21,23,29,31] Overall, similar spectra with no apparent signs of impurity contributions were obtained for the multication-substituted SEs, confirming the robustness of the argyrodite lattice.…”

“…With regards to compositionally complex SEs, only a few materials have been reported up until now, including garnet, perovskite, Na SuperIonic CONductor (NASICON), halide, argyrodite, and Li 10 GeP 2 S 12 (LGPS)-type samples. [15,16,[18][19][20][21][22][23][24][25][26] For high-entropy…”

“…LGPS-type (Li ), high ionic conductivities (> 10 mS cm À 1 at room temperature) could be realized in practice. [15,16,21,23] To unlock the potential of high-entropy ion conductors for SSB application, a detailed understanding of the role of configurational entropy on charge transport, as well as on the (electro)chemical and mechanical properties is required. However, direct correlation between the degree of disorder and the ionic conductivity remains elusive because of the challenge in separating configurational entropy from other effects.…”