Effect of Zinc Incorporation on the Structure, Conductivity and (Electro)chemical Stability of Li6–2 x Zn x PS5X (X = Br, Cl, I) Argyrodite Solid Electrolytes

Abstract Solid-state batteries (SSBs) are currently regarded as a potential alternative to dominant liquid-electrolyte Li-ion batteries, possibly offering higher energy and power densities. In this regard, lithium argyrodite solid electrolytes have received considerable attention due to their high ionic conductivity and mechanical softness allowing straightforward integration into SSB cells. However, they face issues with (electro)chemical stability leading to an accelerated capacity fade. Thus, mitigation strategies have been pursued via chemical substitution. Here we investigate the effects of Zn2+ incorporation into the argyrodite system Li6–2xZnxPS5X (X = Br, Cl, I). We show that there is limited solubility of Zn2+ reaching maximum values of x = 0.2 for Li6–2xZnxPS5Cl and Li6–2xZnxPS5Br while incorporation is not possible for Li6PS5I. Using complementary techniques of X-ray-, neutron powder diffraction and 31P magic-angle spinning nuclear magnetic resonance spectroscopy in combination with machine-learning based molecular dynamics (MD) we show that Zn2+ incorporation increases the S2–/X– site inversion for X = Br– but has virtually no effect for X = Cl–. Interestingly, the shift of certain vibrational bands observed via Raman spectroscopy measurements correlates well with an increased site disorder. Maximum ionic conductivities are detected for a Zn2+ content of x = 0.1. However, no improvement in cycle life was observed when the material was evaluated in solid-state batteries. Nevertheless, a slight enhancement in stability under ambient conditions was indicated by time-dependent H2S evolution measurements.