NaCl interphase enables stable Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1-based all-solid-state sodium batteries

Abstract

All-solid-state sodium (Na) batteries (ASSSBs) have attracted considerable attention due to their inherent safety and low cost. Developing sodium sulfide electrolytes with high Na⁺ ionic conductivity and excellent stability in contact with sodium metal anodes is crucial to realize high-performance ASSSBs. Herein, a tungsten (W) and chlorine (Cl) co-doped Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 solid electrolyte is effectively prepared through melt-quenching combined with a subsequent annealing process. Through W–Cl co-doping, Na vacancies are introduced into the Na₃SbS₄ electrolyte, thereby enhancing the ionic conductivity from 0.92 mS cm⁻¹ to 12.66 mS cm⁻¹. Meanwhile, the in situ formed NaCl-based electronically insulating interphase layer between Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 and the sodium metal effectively suppresses interfacial side reactions and improves interfacial stability. The obtained Na/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na symmetric cell demonstrates stable cycling over 800 h at 0.05 mA cm⁻². Moreover, the TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na ASSSB realizes an initial charge capacity of 142.2 mAh g⁻¹ at 0.1C, maintaining a capacity retention of 81.6% after 100 cycles. This work presents a viable approach for designing sodium sulfide electrolytes that combine high ionic conductivity with superior stability with a sodium anode.

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1. Introduction

Sodium-ion batteries have attracted significant attention due to the abundance of sodium resources and their low cost.^1–3 However, conventional sodium-ion batteries that rely on organic liquid electrolytes raise safety concerns, including the risk of electrolyte leakage and flammability.^4–6 All-solid-state sodium batteries (ASSSBs) exhibit enhanced safety because of the replacement of liquid electrolytes with solid electrolytes. Moreover, the use of high specific capacity sodium metal as an anode holds promise for significantly improving a battery's energy density.^7–9

Sodium sulfide electrolytes exhibit excellent mechanical flexibility, which facilitates robust interfacial adhesion with active materials via cold-pressing techniques, thereby demonstrating great potential for application in ASSSBs.^10,11 As a representative sodium sulfide electrolyte, Na₃SbS₄ exhibits excellent air stability along with a relatively high ionic conductivity reaching 10⁻³ S cm⁻¹.¹² Zhang et al. synthesized a tetragonal-phase Na₃SbS₄ solid electrolyte and found that only 80% of the Na2 sites were occupied by Na⁺ ions, with the remaining 20% being vacancies. The as-prepared Na₃SbS₄ demonstrated a room-temperature ionic conductivity of 3 mS cm⁻¹, indicating that sodium vacancies facilitate Na⁺ transport within the solid electrolyte.¹³ Tsuji et al. synthesized a Mo⁶⁺-doped Na₃SbS₄ solid electrolyte by substituting Sb⁵⁺ with Mo⁶⁺. Based on the principle of charge compensation, this aliovalent substitution introduced Na⁺ vacancies and replaced SbS₄³⁻ structural units with MoS₄²⁻, resulting in an improved ionic conductivity of 3.9 mS cm⁻¹ for Na_2.88Sb_0.88Mo_0.12S₄.¹⁴ Similarly, Hayashi et al. reported that replacing Sb⁵⁺ with W⁶⁺ resulted in the formation of Na⁺ vacancies and the substitution of SbS₄³⁻ units with WS₄²⁻, leading to a significantly enhanced ionic conductivity of 32 mS cm⁻¹ for ball-milled Na_2.88Sb_0.88W_0.12S₄.¹⁵

Recently, Weng et al. synthesized W⁶⁺-doped Na_2.95Sb_0.95W_0.05S₄ solid electrolytes via a melt-quenching method, which achieved an impressive ionic conductivity of 10.3 mS cm⁻¹.¹⁶ However, despite the improved conductivity, W⁶⁺ doping was found to compromise interfacial stability due to the easy reduction of W⁶⁺ to metallic W⁰ upon contact with sodium metal, thus leading to increased interfacial resistance and rapid battery degradation.¹⁷ To address this issue, a common strategy involves constructing an interphase layer that is electronically insulating but ionically conductive through elemental doping. Such interphases effectively prevent direct contact between the solid electrolyte and sodium metal, thereby suppressing undesirable side reactions and promoting long-term cycling stability.^18–21 For instance, Wang et al. synthesized a W–B co-doped Na₃SbS₄ solid electrolyte where a Na–B compound interfacial layer was formed upon reaction with sodium metal. This interfacial layer allowed Na⁺ transport while blocking electron transfer, effectively suppressing W⁶⁺ reduction and mitigating interfacial degradation. Consequently, the Na/Na₃Sb_0.95W_0.05S_3.95B_0.05/Na symmetric cell achieved durable operation over 500 h under 0.05 mA cm⁻².²² Similarly, Meng et al. systematically investigated the properties of the interface between metallic sodium and a Cl-doped Na₃PS₄ solid electrolyte through X-ray photoelectron spectroscopy (XPS), and identified NaCl and S–P–Cl compounds within the solid electrolyte interphase (SEI) layer formed at the electrolyte/sodium interface. This SEI layer effectively reduces the electronic conductivity at the interface, thereby inhibiting electrolyte decomposition and improving interfacial stability. The assembled TiS₂/Na_2.9375PS_3.9375Cl_0.0625/Na cell maintained a discharge capacity of 80 mAh g⁻¹ after 10 cycles at a current density of 0.149 mA cm⁻².²³ Inspired by these doping strategies, a W and Cl co-doping approach was proposed to optimize Na₃SbS₄ due to the similar ionic radii of W⁶⁺ (60 pm) and Sb⁵⁺ (60 pm), and Cl⁻ (181 pm) and S²⁻ (184 pm),²⁴ thereby ensuring minimal strain and preserving a tetragonal framework after structural substitution. Furthermore, the lower charge density of Cl⁻ compared to S²⁻ weakens the electrostatic interaction with mobile Na⁺ ions, thereby lowering the migration energy barrier. Therefore, it is anticipated that aliovalent co-doping of W⁶⁺ and Cl⁻ into Na₃SbS₄ could yield a solid electrolyte exhibiting both enhanced ionic conductivity and improved interfacial stability toward sodium metal.

In this work, Na_2.95−xSb_0.95W_0.05S_4−xCl_x (x = 0, 0.05, 0.1, 0.15, 0.2) was prepared through melt-quenching followed by an annealing process. Based on the charge compensation mechanism, Na vacancies can be introduced by the substitution of Sb⁵⁺ with W⁶⁺ and S²⁻ with Cl⁻. The optimized Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 exhibits a room-temperature ionic conductivity of 12.66 mS cm⁻¹, significantly surpassing that of Na₃SbS₄ of 0.92 mS cm⁻¹. The incorporation of Cl leads to the in situ formation of an electronically insulating NaCl interfacial layer between the solid electrolyte and sodium metal, which effectively suppresses interfacial side reactions and enhances interfacial stability. The Na/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na symmetric cell demonstrates stable cycling over 800 h. Furthermore, the TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na ASSSB exhibits an initial charge capacity of 142.2 mAh g⁻¹ at 0.1C, maintaining 81.6% of this capacity after 100 cycles.

2. Results and discussion

The Na₃SbS₄ and Na_3−xSb_1−xW_xS₄ (x = 0, 0.025, 0.05, 0.075, and 0.1) solid electrolytes were synthesized via a melt-quenching and annealing method. The optimized Na_2.95Sb_0.95W_0.05S₄ electrolyte demonstrates an ionic conductivity of 10.02 mS cm⁻¹, which represents a tenfold increase compared to the 0.92 mS cm⁻¹ observed in pristine Na₃SbS₄ (Fig. S1 and Table S1).^15,25 Based on the Na_2.95Sb_0.95W_0.05S₄ solid electrolyte, the W–Cl co-doped Na_2.95−xSb_0.95W_0.05S_4−xCl_x (x = 0.05, 0.1, 0.15, and 0.2) solid electrolyte was synthesized. The X-ray diffraction (XRD) patterns of Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄, and Na_2.95−xSb_0.95W_0.05S_4−xCl_x (x = 0.05, 0.1, 0.15, and 0.2) are shown in Fig. 1a. The peak splitting at 30.2° and 35.1° in Na₃SbS₄ confirms its tetragonal structure,²⁶ and all doped solid electrolytes maintain the same structure. Owing to the identical ionic radii of W⁶⁺ and Sb⁵⁺, and similar ionic radii of Cl⁻ and S²⁻, the W–Cl co-doped Na₃SbS₄ exhibits no discernible shift in peak position.¹⁶ Notably, the absence of NaCl peaks in Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 confirms successful Cl⁻ doping.

Fig. 1 (a) XRD patterns and (b) Raman spectra of Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄ and Na_2.95−xSb_0.95W_0.05S_4−xCl_x (x = 0.05, 0.1, 0.15, and 0.2) solid electrolytes; (c) W 4f and (d) Cl 2p XPS spectra of Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄, and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 solid electrolytes.

Raman spectroscopy was further employed to analyze the structures of the were electrolytes (Fig. 1b). Characteristic peaks at approximately 360, 380, and 410 cm⁻¹ are observed in all the solid electrolytes, corresponding to the SbS₄³⁻ units. In Na_2.95Sb_0.95W_0.05S₄ and Na_2.95−xSb_0.95W_0.05S_4−xCl_x (x = 0.05, 0.1, 0.15, and 0.2), an additional peak appeared at 470 cm⁻¹, corresponding to the formation of WS₄²⁻ units, indicating that W⁶⁺ successfully substituted Sb⁵⁺ in these solid electrolytes.^15,27 XPS was employed to conduct a detailed analysis of the chemical bonding and valence states of the elements in Na_2.95Sb_0.95W_0.05S₄ and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1. In both solid electrolytes, W⁶⁺ is identified as the dominant valence state (Fig. 1c), which is consistent with the formation of WS₄²⁻ units, indicating successful W doping in the solid electrolytes.^22,28 The Cl 2p XPS spectrum of Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 (Fig. 1d) exhibits two peaks at 198.2 eV and 199.1 eV, corresponding to Sb–Cl and W–Cl bonds, respectively.^29,30 No characteristic peak of NaCl was observed, indicating that Cl had successfully substituted for S in the solid electrolyte. Collectively, the XRD, Raman and XPS results demonstrate the successful synthesis of the W–Cl co-doped tetragonal Na₃SbS₄ solid electrolyte by the melt-quenching approach.

For the W–Cl co-doped Na_2.95−xSb_0.95W_0.05S_4−xCl_x (x = 0, 0.05, 0.1, 0.15, and 0.2) solid electrolytes, the incorporation of Cl⁻ leads to a further enhancement in ionic conductivity compared to the solely W-doped composition. When x = 0.1, Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 exhibits an ionic conductivity of 12.66 mS cm⁻¹ (Fig. 2a and Table S2). The effects of W doping and W–Cl co-doping on the activation energy were further compared (Fig. 2b and S2). It can be observed that Na₃SbS₄ exhibits an activation energy of 0.26 eV. After W doping, the activation energy decreases to 0.21 eV for Na_2.95Sb_0.95W_0.05S₄, and is further reduced to 0.18 eV for the W–Cl co-doped Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1. This progressive reduction in the activation energy is consistent with the introduction of additional Na⁺ vacancies induced by aliovalent W and Cl substitution, which facilitates Na⁺ transport. In addition, as shown in Fig. S3, the W-doped sample exhibits a noticeably higher electronic conductivity of 3.56 × 10⁻⁸ S cm⁻¹ than the Na₃SbS₄ with 4.64 × 10⁻⁹ S cm⁻¹, likely due to the variable valence of W, which introduces additional electronic states. Nevertheless, the incorporation of Cl effectively reduces the electronic conductivity to 2.16 × 10⁻⁸ S cm⁻¹ due to the insulating nature of Cl, which could mitigate the risk of internal dendrite nucleation.³¹

Fig. 2 (a) Ionic conductivities of Na_2.95−xSb_0.95W_0.05S_4−xCl_x (x = 0, 0.05, 0.1, 0.15, and 0.2) solid electrolytes; (b) Arrhenius plots of Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄, and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 solid electrolytes; Rietveld refinements of XRD patterns for (c) Na₃SbS₄, (d) Na_2.95Sb_0.95W_0.05S₄ and (e) Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1; and (f) relative migration energy profiles of Na⁺ in W–Cl co-doped Na₃SbS₄ along three typical migration pathways: Na1–Na1, Na2–Na2 and Vac–Vac.

Rietveld refinements were further performed on the XRD patterns of the solid electrolytes, i.e. Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄ and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 (Fig. 2c–e). All three electrolytes have tetragonal crystal structures with the space group P4̄2₁c. The refined lattice parameters and site occupancies of each element are listed in Tables S3–S5. In Na₃SbS₄, both the Na1 and Na2 sites are fully occupied with no Na⁺ vacancies observed. In contrast, Na_2.95Sb_0.95W_0.05S₄ shows 95% occupancy at the Na2 sites, while Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 exhibits only 85% occupancy, indicating that W–Cl co-doping effectively increases the number of Na⁺ vacancies within the Na₃SbS₄ framework.

To elucidate the correlation between vacancy concentration and enhanced ionic conductivity, density functional theory (DFT) calculations were performed to determine the energy profiles of three typical migration pathways in W–Cl co-doped Na₃SbS₄: Na1–Na1, Na2–Na2, and Vac–Vac (Fig. 2f). The calculated migration energy barriers are 0.68 eV (Na1–Na1), 0.32 eV (Na2–Na2), and 0.08 eV (Vac–Vac). These results indicate that the vacancy-mediated pathway requires the least energy, enabling more efficient Na⁺ transport. Clearly, these findings demonstrate that W–Cl co-doping enhances ionic conductivity by increasing Na⁺ vacancy concentration and reducing the diffusion energy barrier in Na₃SbS₄.¹³

Scanning electron microscopy (SEM) reveals similar microstructures among the Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄, and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 solid electrolytes, indicating that the Cl doping does not induce significant morphological changes (Fig. 3 and S4). Energy-dispersive X-ray spectroscopy (EDS) reveals a homogeneous dispersion of the primary elements Na, Sb, and S as well as dopant elements W and Cl in the solid electrolytes, confirming the successful synthesis of the W–Cl co-doped Na₃SbS₄ solid electrolyte.

Fig. 3 SEM images and corresponding elemental mapping images of Na_2.95Sb_0.95W_0.05S₄ and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 powders.

The chemical stability of the solid electrolyte in humid air was evaluated by measuring the H₂S released from the solid electrolyte at 45% humidity at 25 °C over a period of 120 min (Fig. 4). Na₃SbS₄ exhibited minimal H₂S release of only 0.07 cm³ g⁻¹, whereas W doping the solid electrolyte significantly increased the release to 0.555 cm³ g⁻¹, indicating greater susceptibility to moisture. In contrast, the W–Cl co-doped Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 exhibited a much lower H₂S release of 0.119 cm³ g⁻¹, demonstrating that Cl doping effectively improves moisture resistance and mitigates humidity-induced degradation.

Fig. 4 Amount of H₂S gas generated in Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄ and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 solid electrolytes at 45% air humidity at 25 °C.

In Na₃SbS₄ solid electrolytes, the SbS₄ units react with moisture in the air, resulting in the substitution of S by O and the release of H₂S gas.³² Upon W doping, WS₄ units are formed in the Na_2.95Sb_0.95W_0.05S₄ solid electrolyte. With W–Cl co-doping, additional SbS₃Cl units are introduced in the Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 solid electrolyte (Fig. S5). To further elucidate the effect of W–Cl co-doping on the humid-air stability and structural integrity of Na₃SbS₄ electrolytes, the hydrolysis energies (ΔE_hy) of the main structural units (SbS₄, WS₄, and SbS₃Cl) in the three electrolytes were evaluated using DFT calculations. The calculated ΔE_hy values of the SbS₄, WS₄, and SbS₃Cl units are as follows (Table S6):

SbS₄ + H₂O → SbS₃O + H₂S, ΔE_hy (SbS₄) = 0.61 eV

WS₄ + H₂O → WS₃O + H₂S, ΔE_hy (WS₄) = −0.07 eV

SbS₃Cl + H₂O → SbS₂OCl + H₂S, ΔE_hy (SbS₃Cl) = 0.74 eV

Compared with the SbS₄ units, the negative ΔE_hy value of WS₄ indicates that W doping facilitates the reaction between the solid electrolyte and moisture, leading to structural degradation and reduced humid-air stability. However, after Cl doping, the SbS₃Cl units exhibit a higher ΔE_hy than SbS₄, suggesting that Cl doping effectively suppresses hydrolysis and enhances the humid-air stability of the solid electrolyte.

The interfacial stability between the solid electrolytes and sodium metal was assessed by examining the polarization voltage of symmetric cells with Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄ and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 (Fig. 5). A notable increase in the polarization voltage is observed for the Na/Na₃SbS₄/Na cell after 400 h of operation, primarily due to the solid electrolyte decomposition that generates Sb⁰ and Na₂S.³³ These decomposition products reduce ion transportation at the interface, thereby leading to a progressive rise in polarization voltage until cell failure occurs. Upon W doping, the electrolyte undergoes accelerated decomposition accompanied by the formation of metallic W⁰, resulting in severe instability at the interface. Consequently, the polarization voltage of the Na/Na_2.95Sb_0.95W_0.05S₄/Na cell rapidly increases after 280 h. In contrast, the Na/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na cell maintains stable cycling over 800 h with a polarization voltage of ±0.79 V. These results demonstrate that Cl doping significantly improves the interfacial compatibility between the solid electrolyte and sodium metal.

Fig. 5 Cyclic performances of Na//Na symmetrical cells using Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄ and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 at 0.05 mA cm⁻² and 0.05 mAh cm⁻² at 25 °C.

To elucidate the mechanism underlying the enhanced interfacial stability, XPS analysis was performed to examine the elemental valence state and chemical bonding at the interface between Na and the solid electrolyte after cycling in symmetric cells. At the Na/Na₃SbS₄ interface following cycling, the Sb 3d and S 2p spectra reveal the presence of SbS₄³⁻ units. However, partial reduction of Sb⁵⁺ to Sb⁰ along with decomposition products, such as Sb₂S₃ and Na₂S, are also detected (Fig. 6a and b),^33,34 which indicates structural degradation of the Na₃SbS₄ solid electrolyte, thereby leading to increased interfacial resistance. At the Na/Na_2.95Sb_0.95W_0.05S₄ interface, the W 4f spectrum reveals only the W⁰ signal, indicating complete reduction of W⁶⁺ to W⁰ after cycling (Fig. 6c). This reduction is accompanied by continuous solid electrolyte decomposition and a corresponding increase in polarization voltage until cell failure. Simultaneously, XPS signals for Sb⁰, Sb₂S₃, and Na₂S are also detected in the Sb 3d and S 2p spectra (Fig. 6a and b), confirming that both Na₃SbS₄ and Na_2.95Sb_0.95W_0.05S₄ undergo severe interfacial reactions with the sodium metal.

Fig. 6 XPS spectra of interfacial phases formed at Na metal and Na₃SbS₄, Na_2.95Sb_0.95W_0.05S₄ and Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 solid electrolytes after symmetric cell cycling: (a) Sb 3d; (b) S 2p; (c) W 4f; and (d) Cl 2p.

In contrast, at the Na/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 interface, only weak signals of Sb⁰ and Sb₂S₃ are observed in the Sb 3d and S 2p spectra, indicating that Cl doping effectively suppresses solid electrolyte decomposition induced by metallic sodium (Fig. 6a and b). Moreover, the W 4f spectrum can be deconvoluted into three oxidation states of W⁰, W⁵⁺ and W⁶⁺, with significantly reduced W⁰ intensity (Fig. 6c),^35–38 which suggests that Cl doping also inhibits the reduction reactions of sodium metal with W species. The Cl 2p XPS spectrum of Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 after cycling shows peaks at 198.2 and 199.1 eV, corresponding to Sb–Cl and W–Cl bonds, respectively. In contrast to the results shown in Fig. 1d, a new peak at 199.7 eV corresponding to NaCl is detected,³⁹ indicating the generation of NaCl at the interface between sodium and the solid electrolyte (Fig. 6d). As an electronic insulator, NaCl can effectively suppress side reactions caused by electron leakage, thereby preventing the development of layers with mixed ionic/electronic conductivities.¹⁸ The reduced W⁰ and other decomposition products demonstrate that the NaCl layer mitigates both the reduction of W and the degradation of the solid electrolyte, ultimately enhancing the interfacial stability between the solid electrolyte and sodium metal. To explicitly demonstrate the superior comprehensive performance of the as-prepared electrolyte, a comparative analysis was conducted on the key metrics among recently reported high-performance sulfide electrolytes, as summarized in Table S7. Clearly, the Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 not only exhibits high ionic conductivity, but also maintains outstanding interfacial durability and robust air stability, thereby validating the effectiveness of the dual-doping and interface engineering strategy.

All-solid-state TiS₂/solid electrolyte/Na batteries were constructed and tested within a voltage range of 1.0–2.5 V (Fig. 7a, b, S6a, and c). The discharge/charge specific capacities of the TiS₂/Na₃SbS₄/Na, TiS₂/Na_2.95Sb_0.95W_0.05S₄/Na, and TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na batteries are 145.3/97.2, 226.4/141.2, and 250.3/142.2 mAh g⁻¹ at 0.1C, respectively. The low ionic conductivity of Na₃SbS₄ limits the initial capacity of the all-solid-state TiS₂/Na₃SbS₄/Na battery. Meanwhile, the formation of W⁰ at the Na/Na_2.95Sb_0.95W_0.05S₄ interface results in a continuous increase in interfacial impedance, leading to significant polarization in the TiS₂/Na_2.95Sb_0.95W_0.05S₄/Na battery and a capacity drop to only 5.1 mAh g⁻¹ after 60 cycles. In contrast, benefiting from the enhanced ionic conductivity of the Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 electrolyte and the formation of a protective NaCl interfacial layer at the Na/solid electrolyte interface, the TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na battery demonstrates an improved initial charge capacity and significantly enhanced cycling stability with 81.6% of its initial capacity (116.1 mAh g⁻¹) after 100 cycles.

Fig. 7 (a) Cyclic performances of TiS₂/Na₃SbS₄/Na, TiS₂/Na_2.95Sb_0.95W_0.05S₄/Na and TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na at 0.1C between 1.0 and 2.5 V at 25 °C. (b) Charge/discharge profiles of the battery using Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 at 0.1C; Nyquist plots and equivalent circuit of the TiS₂/Na₃SbS₄/Na, TiS₂/Na_2.95Sb_0.95W_0.05S₄/Na, and TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na batteries (c) before and (d) after cycling at 0.1C.

Electrochemical impedance spectroscopy (EIS) results for TiS₂/solid electrolyte/Na batteries before and after cycling at 0.1C are shown in Fig. 7c and d. In the equivalent circuit model, R_e represents the bulk resistance of both the electrodes and solid electrolyte, R_ct corresponds to the interfacial charge transfer resistance, the constant phase element (CPE) corresponds to the non-ideal double-layer capacitive behavior at the electrode–electrolyte interface, and Z_w represents the Warburg impedance, which is related to sodium ion diffusion into the bulk electrode. To accurately determine the individual resistance components, the EIS data were fitted using the equivalent circuit shown in the figures, and the corresponding fitted Nyquist plots are presented in Fig. S7, while the specific extracted values are listed in Table S8. For the TiS₂/Na₃SbS₄/Na battery, interfacial reactions with sodium metal induce the solid electrolyte decomposition, leading to a substantial increase in R_e from 41.3 to 2786.6 Ω. Meanwhile, the decomposition products hinder ion transport across the interface, increasing R_ct from 441.5 to 615.4 Ω. In comparison, the Na_2.95Sb_0.95W_0.05S₄-based battery shows the most severe interfacial degradation, with R_e increasing from 69.1 Ω to 2872.2 Ω and R_ct increasing from 297.3 Ω to 2704.3 Ω. Furthermore, with Cl doping, the interfacial stability between Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 and the sodium metal is significantly improved, effectively mitigating both electrolyte decomposition and interface side reactions. Consequently, R_e only moderately increases from 76.8 Ω to 795.4 Ω, and the R_ct increases from 44.9 Ω to 316.2 Ω.

The rate capability was further evaluated at various current densities. Due to the low ionic conductivity of Na₃SbS₄, the TiS₂/Na₃SbS₄/Na battery exhibits relatively low reversible capacities of 98.8, 95.5, 91.6 and 81.0 mAh g⁻¹ at 0.1, 0.2, 0.3 and 0.5C, respectively. After W doping, the TiS₂/Na_2.95Sb_0.95W_0.05S₄/Na battery shows reversible capacities of 139.4, 107.2, 91.9 and 68.3 mAh g⁻¹ under identical charge–discharge rate conditions due to severe interfacial degradation between the solid electrolyte and metallic sodium (Fig. 8a, S6b and d). In contrast, W–Cl co-doping effectively enhances both the ionic conductivity and interfacial stability, resulting in significantly enhanced rate performance. The TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na battery delivers higher reversible capacities of 144.2, 120.4, 107.5 and 86.2 mAh g⁻¹ at 0.1, 0.2, 0.3 and 0.5C, respectively (Fig. 8b). Upon returning to 0.1C, the capacity recovers to 135.8 mAh g⁻¹ (Fig. 8a). Furthermore, the TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na battery retains 70.2% of its initial charge capacity after 200 cycles at 0.2C (Fig. 8c) and achieves stable cycling over 250 cycles at 0.5C (Fig. 8d).

Fig. 8 (a) Rate capability of TiS₂/Na₃SbS₄/Na, TiS₂/Na_2.95Sb_0.95W_0.05S₄/Na and TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na batteries between 1.0 and 2.5 V at 25 °C. (b) Charge/discharge curves of TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na from 0.1 to 0.5C. Cyclic performances of TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na at (c) 0.2C and (d) 0.5C.

3. Conclusions

Based on the principle of charge compensation, aliovalent co-doping of W⁶⁺ and Cl⁻ introduces sodium vacancies into the crystal lattice, thereby significantly enhancing the ionic conductivity of the solid electrolyte. Consequently, the Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1 electrolyte demonstrates an ionic conductivity of 12.66 mS cm⁻¹, substantially higher than that of undoped Na₃SbS₄ of 0.92 mS cm⁻¹. The in situ formed electronically insulating NaCl interphase effectively suppresses the reduction of W and as well as the degradation of the solid electrolyte, thereby improving the interfacial stability between the solid electrolyte and sodium metal. As a result, the Na/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na symmetric cell exhibited stable cycling performance over 800 hours at 0.05 mA cm⁻². Furthermore, the TiS₂/Na_2.85Sb_0.95W_0.05S_3.9Cl_0.1/Na battery delivered a specific charge capacity of 142.2 mAh g⁻¹ at 0.1C and retained 81.6% of its initial charge capacity after 100 cycles. It also maintained 70.2% capacity retention after 200 cycles at 0.2C and demonstrated stable operation for over 250 cycles at 0.5C.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data needed to evaluate the conclusions in this paper are present in the main manuscript or the supplementary information (SI). the SI includes experimental procedures, EIS measurements, SEM characterization, DFT computational details, and supplementary figures and tables. See DOI: https://doi.org/10.1039/d5sc09154c.

Acknowledgements

The work was supported by the Commanding Heights of Science and Technology of Chinese Academy of Sciences (Grant No. LDES150000), National Natural Science Foundation of China (Grant No. U21A2075, 52172253, 22309194, 52372244), Ningbo S&T Innovation 2025 Major Special Programme (Grant No. 2023Z106), Zhejiang Provincial Key R&D Program of China (Grant No. 2022C01072, 2024C01095), Jiangsu Provincial S&T Innovation Special Programme for carbon peak and carbon neutrality (Grant No. BE2022007), Baima Lake Laboratory Joint Funds of the Zhejiang Provincial Natural Science Foundation of China (LBMHD24E020001) and Youth Innovation Promotion Association CAS (Y2021080).

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