Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the si Contact online >>
Thank you for visiting nature . You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.
Chalcogenide-based (S, Se) chemistries offer the potential for higher ionic conductivities than oxides11,12,13,14,15,16,17,18,19,20. Though it is likely that sulfide and selenide-based solid electrolytes may exhibit lower intrinsic electrochemical stability, the formation of passivating phases at the electrode-solid electrolyte interface can potentially mitigate further reactions21,22,23. Sulfide electrolytes, particularly sodium sulfides, also tend to be softer than oxides24, which allows intimate contact between electrode and solid electrolyte to be achieved via cold pressing instead of high-temperature sintering.
Using DFT calculations, we first performed a systematic investigation of the dopability of potential halide X− (X = F, Cl, Br and I) into both the tetragonal and cubic phases of Na3PS4. As the DFT predictions for both phases are extremely similar, we will henceforth present only the results of the more stable tetragonal phase for brevity, and interested readers are referred to the Supplemental Information for the results on the cubic phase.
Figure 1 shows the crystal structure of t-Na3PS4. There are two symmetrically distinct Na sites in t-Na3PS4, Na1 (4d) and Na2 (2a), and the PS4 tetrahedra are centered at the 2b positions27. The symmetrically distinct Na sites in c-Na3PS4 are labeled similarly as cubic Na1 (6b) and Na2 (12d). The Na1 (4d) and Na2 (2a) sites in t-Na3PS4 essentially occupy the same positions as the Na1 (6b) sites in c-Na3PS431.
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.
A single halide dopant was introduced into a 2 × 2 × 2 supercell of t-Na3PS4 by replacing one of the S atoms with X, and simultaneously a Na vacancy was introduced to form Na47P16S63X, or t-Na3−xPS4−xXx with x = 0.0625. All symmetrically distinct configurations (Kröger–Vink notation is adopted) were evaluated, and the lowest energy configuration was used for subsequent analyses.
Despite the fact that F− doping is predicted to be the most favorable, we have selected Cl− doping for further investigation due to several considerations. First, NaCl, aka table salt, is by far a more commonly available precursor than NaF. Second, fluoride chemistry inherently comes the possibility of HF exposure. Finally, the Cl− anion has an ionic radius that is closest to S2−, which would minimize the local structural distortion introduced at the substituted site. The doping concentration of Cl is fixed at x = 0.0625, given that previous work in doping I− (which has a similar dopant formation energy at the same concentration) into c-Na3PS4 at x > 0.1 resulted in formation of unknown phases32.
As demonstrated in recent work by some of the co-authors30, pristine c-Na3PS4, i.e., without interstitial or vacancy defects, is predicted to be an extremely poor ionic conductor in AIMD simulations. Using AIMD simulations (see Methods section), a similar result is obtained with t-Na3PS4 in this work, which is not surprising given that the small differences in lattice parameters and atomic positions between the cubic and tetragonal polymorphs.
Figure 2a shows the Arrhenius plot of the log of the conductivity-temperature product () versus 1/T for t-Na2.9375PS3.9375Cl0.0625 obtained from AIMD simulations. With the introduction of a defect pair, the Na+ conductivity at 300 K is predicted to be 1.38 mS cm−1 with an activation barrier of 232 meV (see Table 2). During the preparation of this article, it has come to our attention that Klerk et al. has also performed AIMD simulations on halide doping in t- and c-Na3PS431. Though the qualitative conclusions of vacancy-induced conductivity are similar, we note that Klerk et al. only performed relatively short AIMD simulations at a single temperature of 525 K; room-temperature Na+ conductivities and activation energies were therefore not obtained.
(a) Arrhenius plots of the t-Na3−xPS4−xClx 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-Na3−xPS4−xClx (x = 6.25%) at 800 K, with P = 0.0001 a0−3 (a0 is the Bohr radius).
From the Na+ probability density distribution (Fig. 2b), we may observe that t-Na2.9375PS3.9375Cl0.0625 is predicted to be a 3D diffuser comprising of chains of Na1 sites along the c direction interconnected via the Na2 sites, which form a body-centered cubic sublattice. Such a 3D diffusion network is expected to be highly robust against the potential introduction of blocking defects33.
Plots of Na uptake per formula unit (f.u.) of t-Na2.9375PS3.9375Cl0.0625 (red solid), c-Na3.0625Sn0.0625P0.9375S4 (blue dashed) and c-Na3.0625Si0.0625P0.9375S4 (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.
At voltages above 2.4 V versus Na/Na+, we find that the t-Na2.9375PS3.9375Cl0.0625 solid electrolyte is predicted to be unstable against Na extraction to form NaPS3 + S + NaCl. However, if the operating voltage is kept below 2.4 V, the predicted phase equilibria at the charged cathode-solid electrolyte interface retains t-Na3PS4 + NaCl as the primary component.
Pure t-Na3PS4 was synthesized from Na2S and P2S5 precursors. The Cl− dopant was introduced by adding NaCl following the chemical reaction (1.5 − x)Na2S + 0.5P2S5 + xNaCl → Na3−xPS4−xClx. The resulting pellets were then densified via spark plasma sintering (SPS) to minimize porosity of the solid electrolyte. The synthesis details are given in the Methods section, and the dimensions and density of the pristine and doped pellets are provided in Supplementary Information (see Table S1).
(a) XRD patterns for t-Na3−xPS4−xClx with x = 0% and 6.25%, and previous study in ref. 27. (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-Na3−xPS4−xClx (x = 6.25%). Scale bar is 10 μm.
Cross-sectional SEM images of the pristine t-Na3PS4 and doped t-Na2.9375PS3.9375Cl0.0625 SPS samples under identical processing conditions are shown in Fig. 4d,e, respectively. We note that the Cl− doping does not lead to any significant morphology changes. The images show that the local morphology of the pellets is densely formed due to SPS processing in both compounds. An EDX measurement was collected from the doped sample (see Figure S1 in Supplementary Information). From the EDX measurement, we determined that sulfur and chlorine are uniformly distributed throughout the sample, with no noticeable aggregation of element-rich domains. In conjunction with the refinement results, the majority of the chloride dopant is found to be integrated into the host crystal lattice.
Charge-discharge profile of TiS2/t-Na2.9375PS3.9375Cl0.0625/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.
The design of an all-solid-state rechargeable battery is a multi-component, multi-property optimization effort; it is therefore insufficient to merely focus on bulk ionic conductivity of the solid electrolyte as the only target parameter. In this work, we have demonstrated how an integrated computational and experimental effort can significantly accelerate such multi-component, multi-property optimization, resulting in a promising new t-Na2.9375PS3.9375Cl0.0625 solid electrolyte that has been demonstrated in a full ss-SIB cell with good cyclability and capacity.
Aliovalent doping is a common strategy to introduce defects (vacancies or interstitials) into a solid electrolyte candidate to further enhance its conductivity. Surprisingly, we find halide doping in Na3PS4 to be somewhat more favorable than cation (Si4+, Ge4+ and Sn4+) doping, with slightly lower dopant formation energies of 0.76–1.11 eV vs 1.04–1.32 eV for cation30. Though dopant formation energies of 0.76–1.11 eV might appear at first glance to be relatively high, it should be noted that these values depend strongly on the chemical potential references used. At the elevated temperatures during synthesis, Na2S loss is likely to lower the chemical potentials of Na and S, significantly promoting formation.
About Rechargeable sodium ion batteries
As the photovoltaic (PV) industry continues to evolve, advancements in Rechargeable sodium ion batteries have become critical to optimizing the utilization of renewable energy sources. From innovative battery technologies to intelligent energy management systems, these solutions are transforming the way we store and distribute solar-generated electricity.
When you're looking for the latest and most efficient Rechargeable sodium ion batteries for your PV project, our website offers a comprehensive selection of cutting-edge products designed to meet your specific requirements. Whether you're a renewable energy developer, utility company, or commercial enterprise looking to reduce your carbon footprint, we have the solutions to help you harness the full potential of solar energy.
By interacting with our online customer service, you'll gain a deep understanding of the various Rechargeable sodium ion batteries featured in our extensive catalog, such as high-efficiency storage batteries and intelligent energy management systems, and how they work together to provide a stable and reliable power supply for your PV projects.
