Room-Temperature All-solid-state Rechargeable Sodium-ion Batteries with a Cl-doped Na3PS4 Superionic Conductor

Abstract

All-solid-state sodium-ion batteries are promising candidates for large-scale energy storage applications. The key enabler for an all-solid-state architecture is a sodium solid electrolyte that exhibits high Na(+) conductivity at ambient temperatures, as well as excellent phase and electrochemical stability. In this work, we present a first-principles-guided discovery and synthesis of a novel Cl-doped tetragonal Na3PS4 (t-Na3-xPS4-xClx) solid electrolyte with a room-temperature Na(+) conductivity exceeding 1 mS cm(-1). We demonstrate that an all-solid-state TiS2/t-Na3-xPS4-xClx/Na cell utilizing this solid electrolyte can be cycled at room-temperature at a rate of C/10 with a capacity of about 80 mAh g(-1) over 10 cycles. We provide evidence from density functional theory calculations that this excellent electrochemical performance is not only due to the high Na(+) conductivity of the solid electrolyte, but also due to the effect that "salting" Na3PS4 has on the formation of an electronically insulating, ionically conducting solid electrolyte interphase.

Figure 1. Crystal structure of pristine t-Na₃PS₄.

The tetragonal polymorph of the Na₃PS₄ crystal. There are symmetrically distinct Na sites in t-Na₃PS₄, Na1 (4d) and Na2 (2a), and the PS₄ tetrahedra are centered at the 2b positions.

Figure 2. Ionic conductivity and Li⁺ probability density distribution of t-Na_3−xPS_4−xCl_x.

(a) Arrhenius plots of the t-Na_3−xPS_4−xCl_x with x = 0 (blue) and 6.25% (red) obtained from SPS measurements, and x = 6.25% (green) from AIMD simulations. The filled green triangle indicates the extrapolated ionic conductivity at 300 K from AIMD simulations. (b) Isosurface of the Na⁺ probability density distribution (P, in green) in the t-Na_3−xPS_4−xCl_x (x = 6.25%) at 800 K, with P = 0.0001 a₀⁻³ (a₀ is the Bohr radius).

Figure 3. Electrochemical decomposition products of doped Na₃PS₄ compounds.

Plots of Na uptake per formula unit (f.u.) of t-Na_2.9375PS_3.9375Cl_0.0625 (red solid), c-Na_3.0625Sn_0.0625P_0.9375S₄ (blue dashed) and c-Na_3.0625Si_0.0625P_0.9375S₄ (green dashed) solid electrolytes against voltage vs Na/Na⁺. At low voltage (high Na chemical potential), each solid electrolyte undergoes reduction and uptakes Na, while at high voltage (low Na chemical potential), each solid electrolyte is oxidized and loses Na. Text indicates the predicted phase equilibria at corresponding regions of the profile. Only selected regions are annotated for brevity.

Figure 4. Characterization and morphology of t-Na_3−xPS_4−xCl_x.

(a) XRD patterns for t-Na_3−xPS_4−xCl_x with x = 0% and 6.25%, and previous study in ref. . (b) Refinement plot of the pristine t-Na₃PS₄. (c) Refinement plot of Cl-doped t-Na₃PS₄. Solid red and black lines denote the observed and calculated XRD patterns, while the green ticks mark the position of the reflections allowed by the space groups of t-Na₃PS₄ () and NaCl (). The difference between the observed and calculated patterns is signified by the blue line. (d) SEM image of pristine t-Na₃PS₄ SPS sample, and (e) SEM image of SPS sample of doped t-Na_3−xPS_4−xCl_x (x = 6.25%). Scale bar is 10 μm.

Figure 5. Electrochemical performance of TiS₂/t-Na_2.9375PS_3.9375Cl_0.0625/Na all-solid-state full cell.

Charge-discharge profile of TiS₂/t-Na_2.9375PS_3.9375Cl_0.0625/Na full cell at room-temperature. Cell was cycled under constant current conditions with a current density of 0.149 mA cm⁻² (C/10 rate) from 1.2 V to 2.4 V. The cell was able to routinely deliver 80 mAh g⁻¹ over 10 cycles. Red and black lines in the charge-discharge profile denote charging and discharging, respectively. Similarly, red and black markers signify charge and discharge capacities, while the green circles mark the coulombic efficiency by cycle.