Activating the paddle-wheel effect towards lower temperature in a new sodium-ion solid electrolyte, Na3.5Si0.5P0.5Se4

Abstract

Designing solid electrolytes with high room temperature (RT) conductivity is essential for the development of the next generation of solid-state batteries. Beyond the static structural framework, interaction between cation mobility and anion dynamics, i.e., the paddle-wheel mechanism, may be the principle for enhanced cation mobility in solid electrolytes. Herein, we use first-principles calculations to study a promising polyanion-based solid electrolyte, Na₄SiSe₄, which meets the requirements of high ionic conductivity, and thermodynamic and dynamic stability simultaneously. Ab initio molecular dynamics reveal the fast Na diffusion dominated by the paddle-wheel mechanism, which is rationalized by the coupling of the translational motion of Na with the rotational motion of SiSe₄ in terms of time-space, vibrational properties, and energetics. Furthermore, by substituting Si with P, we theoretically predict Na_3.5Si_0.5P_0.5Se₄ with the paddle-wheel effect activated at 600 K, while Na₄SiSe₄ is activated at 1000 K. The calculated RT ionic conductivities of Na_3.5Si_0.5P_0.5Se₄ is up to 16.94 mS cm⁻¹. Our findings highlight that high cation mobility can be achieved by exploiting the anion rotation to invoke the paddle-wheel effect, especially in the low temperature range.