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Nature Communications volume 15, Article number: 9995 (2024 ) Cite this article Sodium Sulphide Nonahydrate
Sodium sulfide (Na2S) as an initial cathode material in room-temperature sodium-sulfur batteries is conducive to get rid of the dependence on Na-metal anode. However, the micron-sized Na2S that accords with the practical requirements is obstructed due to poor kinetics and severe shuttle effect. Herein, a subtle strategy is proposed via regulating Na2S redeposition behaviours. By the synergistic effect from both conductive structure and cuprous sulfide (Cu2S) catalysis, the micron-sized Na2S particles are broken down and redeposited to nano-size during the initial cycle which can be fully utilized in subsequent cycles. Consequently, the Na2S/CPVP@Cu2S||Na cell delivers excellent cyclability (670 mAh gS−1 after 500 cycles) with a remarkable average Coulombic efficiency over 99.7% and rate capability (480 mAh gS−1 at 4 A gS−1). Besides, the Na-free anodes are used to prove the application prospects. This work provides an innovative idea for utilizing micron-sized Na2S and offers insights into its conversion pathway.
In the past decades, global energy consumption has shown an overall growth trend, with renewable energy generation rapidly gaining a larger proportion among all energy sources, resulting in a swift surge in demand for energy storage1. Lithium-ion batteries as state-of-the-art technology may not be able to meet this rising requirement subject to the price fluctuations and disequilibrium of mineral resources2,3. Room-temperature sodium–sulfur (RT Na–S) batteries, which use low-cost, non-toxicity and rich-reserve sodium (Na) and sulfur (S) as active materials, exhibit promising future in energy storage system due to exceptionally high energy density (1274 Wh kg−1)4,5,6,7. Conventional RT Na–S batteries often use Na-metal as anode, which faces great challenges in safety and interfacial stability, severely restrict its commercialization prospects8,9,10,11. Therefore, the key to its practical application is to promote the development of Na-free anode battery system.
Sodium sulfide (Na2S) emerges as the most promising initial cathode material in RT Na–S batteries subject to Na-free anode systems, it can be originated from industrial waste which follows the principle of environmental and resource sustainability5,12,13,14. Moreover, Na2S in a full discharged state attains its maximum volume, which tends to contract during initial disintegration, providing interspace for subsequent volume expansion 15. Unfortunately, it is known that Na2S suffer from the poor electronic conductivity, slow transfer kinetics and severe shuttle effects of sodium polysulfides (NaPSs) intermediates6,16,17. Up to now, there are only few reports on Na2S cathode through novel structural design to create unique Na2S@C nanostructures which serve to enhance electrical conductivity and fasten transfer kinetics12,18,19,20. For instance, Li et al18. developed a frogspawn-coral-like hollow Na2S nanostructure, in which Na2S nanospheres were embedded within a highly layered and sponge-like carbon matrix. The shortened Na+ diffusion pathway and rapid electron transfer lead to a satisfying electrochemical performance. Meng et al20. effectively immobilized Na2SO4 within nano-scale threads through electrospinning followed by in-situ reduction to obtain Na2S@C nanofibers, achieving a capacity retention of 174 mAh gS−1 after 500 cycles under anode protection. Despite the accomplishments achieved on the enhanced electrochemical performance, the nano-structural Na2S@C materials require complex fabricating process and high cost, posing difficulties for large-scale production10,21. Therefore, addressing the use of easily sourced Na2S with micrometer size is of important in the practical application of RT Na–S batteries. Currently, there remains absent in long-cycled micron-sized Na2S, which necessitates an urgent breakthrough.
Considering the inherent electronic insulation of Na2S, the solid-liquid-solid conversion pathway is considered as an effective approach for achieving the activation of micron-sized Na2S22,23. However, the use of electrolytes with high polysulfide solubility can cause severe shuttle effect, while the lengthy activation process exacerbated this phenomenon4,12. Therefore, it is imperative to enhance the kinetics of polysulfide conversion on the cathode side in order to facilitate its continuous conversion rather than diffusion towards the anode24,25. Moreover, special attention should be paid to the accumulation and deactivation of Na2S on the cathode due to its low electronic conductivity5,26. With the accelerated conversion rates, it is essential to fabricate the cathode with significantly enlarged specific surface area and abundant nucleation sites, thereby ensuring homogeneous redeposition of Na2S.
In this work, a promising Na2S/CPVP@Cu2S cathode is subtly designed via regulating Na2S redeposition behaviors. By thermal treating the micron-sized Na2S cathode, Polyvinylpyrrolidone (PVP) binder is carbonized to a continuous electron-conductive matrix encapsulating Na2S particles, which offers abundant nucleation sites as well as refinement effect during Na2S-S-Na2S transformation process. The oxide layer on the surface of copper (Cu) current collector in-situ reacts with Na2S to form cuprous sulfide (Cu2S) catalysts during the high-temperature process. Comprehensive mechanism studies further indicate that the micron-sized Na2S particles quickly self-refine into nanometer Na2S particles during the initial cycle through a solid-liquid-solid conversion, and the size remain stable in subsequent cycles benefiting from the synergistic effect of Cu2S catalyst and excellent conductive network with abundant nucleation sites. Owing to the fast transfer kinetics that effectively restrains the shuttling of NaPSs, the obtained Na2S/CPVP@Cu2S cathode exhibits remarkable cycling stability and rate capability in an ether-based electrolyte. After 500 cycles at a specific current of 0.4 A gS−1, the Na2S/CPVP@Cu2S||Na remains a highly reversible capacity of 670 mAh gS−1, with an impressive average Coulombic efficiency (CE) over 99.7%. To showcase its practical application, the Na2S/CPVP@Cu2S cathode is successfully integrated with HC anode in a full-cell configuration, maintaining a capacity of 518 mAh gS−1 over 200 cycles. This work realizes the stable circulation of Na2S originated from industrial waste, which offers significant reference for research and practical application of Na-free anode system in RT Na–S batteries.
The raw Na2S was extracted from barium sulfate industry, as illustrated in Fig. S1a, b. To veritably reproduce the Na2S waste from barium sulfate industry, the barium sulfide with purity of 85% was selected. The as-prepared Na2S was denoted as purified Na2S with the size range of 2 μm to 5 μm (Fig. S2), in which only small amount of impurity sodium sulfate (Na2SO4) was detected by X-ray diffraction (XRD) (Fig. S3). The cathode composed of purified Na2S is rationally designed, following the schematic illustration of the fabrication process shown in Fig. 1a. The Na2S/PVP cathode was obtained by mixing purified Na2S powder, PVP binder, conductive agents and then blade-coated onto Cu foil. Afterwards, the Na2S/PVP cathode underwent heat treatment to carbonize PVP as well as simultaneously render the Cu2O/CuO layer on the surface of Cu foil react with Na2S to in-situ generate Cu2S catalysts (denoted as Na2S/CPVP@Cu2S). With the removal of non-conductive PVP binder, the carbonized PVP forms conductive network and greatly improves the electrical conductivity as well as provides abundant nucleation sites for Na2S and Cu2S.
a Schematic illustration of Na2S/PVP and Na2S/CPVP@Cu2S. b TGA curve for Na2S/PVP and PVP from 30–500 °C. c XRD patterns of Na2S/PVP and Na2S/CPVP@Cu2S. d High-resolution TEM image of pristine Na2S/CPVP@Cu2S. e, f SEM image of pristine e Na2S/PVP and f Na2S/CPVP@Cu2S (Inset is the corresponding optical photograph). g–j Corresponding EDS mapping of Na, S, C and Cu in (f). Source data are provided as a Source Data file.
To investigate the transformation during carbonization process, XRD, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the component and morphology of cathode. From the TGA curves in Fig. 1b, it can be observed that pure PVP and Na2S/PVP started to carbonize at around 400 °C, reaching a minimum weight at 450 °C, suggesting complete carbonization. The weight reduction in pure PVP closely matches the expected weight reduction in Na2S/PVP, indicating that the change in weight during carbonization is totally contributed by the carbonization of PVP rather than other reactions. Furthermore, Raman spectra in Fig. S4 show characteristic peaks ascribed to graphitic carbon at 1585 cm−1 and 1360 cm−1 in carbonized PVP (CPVP). The XRD patterns of the Na2S/PVP and Na2S/CPVP@Cu2S (Fig. 1c) do not exhibit significant changes except for some new peaks on 37.2°, 45.7° and 48.1° from Cu2S (PDF #84-0207). The morphology and element distribution of Na2S/CPVP@Cu2S were tested by TEM and energy-dispersive spectroscopy (EDS), as illustrated in Figs. S5 and S6, proving Na, S and Cu are uniformly distributed in the Na2S/CPVP@Cu2S cathode. The lattice fringes of 0.231 nm in the high-resolution TEM image match well with the spacing of the (220) plane of Na2S (PDF #77-2149). The lattice fringes of 0.198 nm, indexed to the (110) plane of Cu2S (PDF #84-0206), can also be found around the Na2S particles, confirming the intimate contact between them (Fig. 1d). The SEM image of Na2S/CPVP@Cu2S exhibits a more loosely arranged carbon-conductive network while the Na2S/PVP possesses a pasty surface (Fig. 1e, f). Besides, both samples display Na2S particles with sizes approximately from 2 μm to 5 μm, which are identified to be similar to purified Na2S particles in size (Fig. S2). More importantly, some new particles observed around Na2S are identified to be Cu2S nanoparticles, which is supported by EDS mapping in Figs. 1g–j and S6 b–e that S and Cu are overlapped in certain area, while Na and S exhibit similar spatial distributions. The combined evidence confirms that PVP underwent carbonization, and Cu2S was simultaneously in situ formed during the heating-treatment process. Approximately 6.4% Na2S participated in the reaction with Cu2O during the high-temperature process which is proved by the ICP-MS tests (Table S1).
After gaining the Na2S/CPVP@Cu2S cathode, Na metal was used as the standard anode to evaluate its performance, and Na2S/PVP||Na coin-cell was assembled for comparison. Unless otherwise specified, potentials are referenced to Na metal in the following study. To activate the micron-sized Na2S, an ether-based electrolyte with high solubility to NaPSs, 1 M NaOTf in diethylene glycol dimethyl ether (DIGLYME) was used to facilitate the solid-liquid-solid transformation reaction, which easily leads to serious shuttle effect when the dissolved NaPSs cannot be swiftly converted.
As discussed above, the key issue affecting the utilization of Na2S refers to its redeposition behaviors. Therefore, SEM characterization of Na2S/PVP and Na2S/CPVP@Cu2S cathode were conducted to identify the redeposition morphology at the end of the 1st charge, 1st discharge, and 100th discharge. For Na2S/PVP cathode, after the first charge, the micron-sized Na2S particles disappears and transforms into sulfur, which accumulates on the cathode surface and presents a clustered and agglomerated morphology (Fig. 2a). Since the active substance needs to be in proximity to the conductive matrix and electrolyte to participate in electrochemical reactions6, the accumulation of non-conductive sulfur results in “dead S” and further decreases electronic conductivity. As a result, after the first discharge, redeposited Na2Sx (1 ≤ x < 4) exists in the form of large agglomerates with sizes is even larger than the initial Na2S (Figs. 2b and S7a). After 100 cycles, the particle size further increases, even exceeding 10 mm (Fig. 2c). The large agglomerated Na2Sx formed on the cathode loses electrochemical activity, causing a significant decrease in the utilization efficiency of active material, which leads to poor reversibility. This situation is improved a lot in Na2S/CPVP@Cu2S cathode. As shown in Fig. 2d, the micron-sized Na2S particles transformed into S nanoparticles with sizes smaller than 200 nm during the first charge, which was uniformly distributed on the conductive matrix without surface accumulation. At the end of the first discharge, Na2S were observed with the redeposition of smaller particle size (<600 nm) (Figs. 2e and S7b). This phenomenon can be attributed to the abundant nucleation sites facilitate the deposition of S species. Besides, Cu2S nanoclusters founded in pristine Na2S/CPVP@Cu2S are also observed at different cycles (Fig. 2d–f). The catalysis of Cu2S significantly accelerates the multiple-step conversion of Na2S-NaPSs-S, leading to a higher utilization rate of active materials. Therefore, the smaller Na2S particles can be reutilized in the following charging-discharging process. Consequently, after 100 cycles, Na2S particles can still maintain small sizes and exhibit fine adhesion with the conductive matrix, demonstrating an outstanding reversibility (Fig. 2f). Besides, this self-refinement process also reduces the cell impedance (Fig. S8a, b), which further improves the sulfur conversion kinetics.
a–f SEM images of Na2S/PVP after a 1st charge, b 1st discharge, c 100th discharge, and Na2S/CPVP@Cu2S after d 1st charge, e 1st discharge, f 100th discharge.
The utilization of active substance can be supported by cyclic voltammetry (CV) tests and the first charging-discharging curves. Figure 3a shows the CV curves of Na2S/PVP||Na cell, with the voltage window from 0.5 V to 3.0 V, exhibiting a high oxidation peak at around 2.3 V during the first charge, which corresponds to the activation of Na2S. Besides, the high oxidation current above 2.3 V was observed, indicating severe shuttle effect within the cell due to the high solubility of NaPSs in ether-based electrolyte. The reaction between Na anode and shuttling NaPSs caused “dead S” deposited on Na anode, thus, active sulfur is greatly lost and only a small amount of sulfur can participate in the subsequent electrochemical reaction, which can be proved by the much weaker oxidation/reduction peaks in the 2nd and 3rd CV curves. Interestingly, a slightly lower activation voltage with a conspicuous smaller oxidation peak and a descent oxidation current above 2.3 V was observed in Na2S/CPVP@Cu2S||Na that indicates the significantly weakening of shuttle effect (Fig. 3b). Notably, the reduction current during the first discharge is conspicuously higher for Na2S/CPVP@Cu2S than that for Na2S/PVP, manifesting a great improvement in the utilization of active materials. In the 2nd and 3rd CV curves, oxidation peaks are observed at 1.5, and 2.1 V, and reduction peaks at 1.6, 1.2 and 0.8 V, suggesting the presence of multiple intermediate products in solid–liquid–solid reactions. Besides, the Na+ storage behaviors of Cu2S also occur at this stage, corresponding to the similar oxidation/reduction peaks27. Compared with Na2S/PVP, Na2S/CPVP@Cu2S displays the much higher oxidation-reduction current as well as highly overlapped oxidation-reduction peaks, manifesting more Na2S are activated and can be recycled owing to the shuttle effect is effectively restrained. The first galvanostatic charge-discharge curves of two cells also reveal similar results. From Fig. 3c, the Na2S/PVP||Na displays severe overcharge capacity (2582 mAh gS−1) and low discharge capacity (only 136 mAh gS−1). It is worth noticing that the sluggish electrochemical activation of micron-sized Na2S particles increases the duration of NaPSs, which amplifies the shuttle effect. Impressively, the shuttle effect is effectively restrained in Na2S/CPVP@Cu2S, processing an initial charge capacity of 967 mAh gS−1 and a discharge capacity of 790 mAh gS−1. The voltage polarization is also greatly reduced in Na2S/CPVP@Cu2S, indicating the effective synergy of Cu2S catalyst and continuously conductive carbon network.
a, b The CV curves at the scan rate of 0.1 mV s−1 of a Na2S/PVP||Na and b Na2S/CPVP@Cu2S||Na. c The galvanostatic capacity to voltage curves at 1st cycle of Na2S/PVP and Na2S/CPVP@Cu2S. d, e Depth profiles and distributions for NaS- secondary ions of d Na2S/CPVP@Cu2S and e Na2S/PVP before and after 10 cycles. f S 2p spectra of corresponding cathodes. Source data are provided as a Source Data file.
To deeply investigate the spatial distribution of redeposited Na2S, time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) tests were performed on Na2S/PVP and Na2S/CPVP@Cu2S cathodes before and after 10 cycles. As illustrated in Fig. 3d, the content of NaS- secondary ion (corresponding to Na2S) in both pristine and cycled Na2S/CPVP@Cu2S cathode are barely reduced, which demonstrates that active material encounters minimal loss and can be evenly distributed at different depths after cycles. However, the content of NaS- is significantly reduced in cycled Na2S/PVP, and the concentration on the surface is much higher than that inside the cathode, indicating there is a great loss of active substances after 10 cycles, and the redeposited Na2S tends to accumulate on the surface rather than distribute uniformly throughout the cathode (Fig. 3e). XPS spectra of Na2S/PVP and Na2S/CPVP@Cu2S cathodes present similar results. From Fig. 3f, the peak at 161.4 eV in the S 2p spectra corresponding to Na2S/Cu2S is remained in the cycled Na2S/CPVP@Cu2S cathode while insoluble Na2Sx (1 < x < 4) is observed in the cycled Na2S/PVP cathode20, indicating the “dead S species” is formed in cycled Na2S/PVP. The SOx2- signals which are detected after cycles are attributed to the reduction production of Na+OTf−. Besides, the distribution of CuS- secondary ion was investigated, which is representative of Cu2S. In Na2S/CPVP@Cu2S, it could be observed that Cu2S is massively distributed in pristine cathode and the concentration tends to increase upon etching (Fig. S9a). This is attributed to the fact that the reaction between Na2S and Cu2O/CuO layer occurs in solid phase, resulting in the inhomogeneously diffused Cu2S. In the subsequent 10 cycles, owing to the unique sulfurophilic property of Cu28,29, Cu2S is capable of being evenly distributed throughout the cathode. However, for Na2S/PVP cathodes, only a small amount of Cu2S can be observed in deep layers after 10 cycles (Fig. S9b). The XPS spectra of Cu 2p also confirms this result (Fig. S9c). Apparently, Cu 2p3/2 at 932.8 eV and Cu 2p1/2 at 952.7 eV, corresponding to Cu+30, is detected in the pristine and cycled Na2S/CPVP@Cu2S cathodes, which cannot be detected in Na2S/PVP cathodes.
To clearly understand the Na2S evolution mechanism during initial cycle, in situ Raman combined with galvanostatic charge-discharge measurements was carried out to a Na2S/CPVP@Cu2S||Na cell. From Fig. 4a, during the first charge, the peak at 376 cm−1 representing S2− gradually weakened along with the formation of Na2S2 and Na2S4 at 434 cm−1 and 489 cm−1 7. When charged to 2.3 V, S62− appeared at 355 cm−1, S2− and S22− did not completely disappear, indicating the lengthy activation process of micron-sized Na2S particles. At the end of the first charge (2.8 V), all above peaks disappeared, and S stretching vibration band was founded at 472 cm−1 25, indicating the complete conversion from Na2S to S. Afterwards, during the discharging process, S was successively reduced to Na2S6, Na2S4, Na2S2 and Na2S. Notably, when soluble S62- appears, NaOTf can also be continuously detected accompanying with soluble long-chain NaPSs (Na2Sx, 4 ≤ x < 8), which can be attributed to the change from solid phase to liquid phase on the focusing points of Raman beams. This phenomenon was observed during solid-liquid-solid transformation, which is speculated to experience breaking, dissolution and redeposition of Na2S particles.
a In-situ Raman spectra of Na2S/CPVP@Cu2S||Na cell measured by galvanostatic charge-discharge measurements. b Schematic illustrations showing Na2S transformation of Na2S/PVP and Na2S/CPVP@Cu2S electrode at 1st cycle. c–g The Na2S/CPVP@Cu2S||Na cells c CV curves at different scan rates; d Peak current versus the square root of the scan rates of peak 1; e rate performance; f galvanostatic capacity to voltage curves at different cycles from 0.5 V to 2.8 V; g Cycling performance. Source data are provided as a Source Data file.
Based above results, two schemes are proposed to elaborate the different transformation mechanism of Na2S/PVP and Na2S/CPVP@Cu2S cathodes during the 1st cycle (Fig. 4b). For Na2S/PVP cathode, due to poor electron-conductive network and sluggish conversion kinetics, when micron-sized Na2S transform to S in the first charging process, large number of long-chain NaPSs intermedia prefer to dissolve in electrolytes rather than convert to S which causes a serious shuttle effect. Meanwhile, the generated S tends to accumulate on the cathode surface due to obstruct electron transport, eventually loses its electrochemical activity. When S transforms to Na2S in the subsequently discharging process, only a small portion of S can be conversed to Na2S which also accumulated on the cathode surface. Hence, Na2S/PVP cathode displays poor specific capacity and initial CE (ICE) (Fig. 3c). In contrast, for Na2S/CPVP@Cu2S cathode, owing to the fast transformation kinetics brought by the synergistic effect of catalysis of Cu2S, excellent electron-conductive network and abundant nucleation sides, Na2S can be smoothly converted to NaPSs and S without serious shuttle effect. Simultaneously, generated S species are evenly distributed onto conductive matrix to form smaller particles, which can be efficiently utilized in subsequently discharging process. After initial cycle, Na2S particles with a size of less than 600 nm are uniformly redeposited onto the conductive matrix. Consequently, the refined Na2S particles further enhance the transformation kinetics, which leads to the remarkable electrochemical performance.
To characterize the reinforced transfer reaction rate, the CV curves were recorded at different scanning rates (Figs. 4c and S10), in which the Na2S/CPVP@Cu2S||Na cell exhibited higher redox peak current and lower polarization at all scanning rates. According to the Randles–Sevcik equation (see methods section for details), the linear relationship between the peak current and the scanning rate can be used to represent the Na+ diffusion coefficient which is positively correlated with the slope of the line31. It can be noticed that Na2S/CPVP@Cu2S||Na cell possesses significantly increased slope in each peak, demonstrating the diffusion process is strengthened through every steps of sulfides conversion (Figs. 4d and S11a–d). Benefiting from both timely diffusion and low impedance, the Na2S/CPVP@Cu2S||Na exhibits an excellent rate performance from 0.1 A gS−1 to 4 A gS−1, and remains a capacity of 480 mAh gS−1 at 4 A gS−1 (Fig. 4e). In addition, a specific capacity of 670 mAh gS−1 (normalized to the total mass of Cu2S and Na2S is 262 mAh g−1) after 500 cycles at 0.4 A gS−1 (normalized to the total mass of Cu2S and Na2S is 156 mA g−1) was provided, with an average CE over 99.7% (Fig. 4f, g). In comparison, the Na2S/PVP||Na only retains a capacity of 143 mAh gS−1 after 200 cycles at 0.1 A gS−1, with an average CE of 92.7% (Fig. S12).
It should be emphasized here that the electrolyte with high NaPSs solubility is important for activating micron-sized Na2S particles through solid–liquid–solid conversion pathway. Fig. S13 shows the Na2S/CPVP@Cu2S||Na cell using a localized high-concentration electrolyte (LHCE, NaFSI:DME:TTE = 1:1.2:1)11, which has been reported to transfer S through a solid-solid process. The cell shows a low reversible capacity owing to un-activated Na2S particles. Besides, to prove the catalyzing effect of Cu2S, a Na2S/PVP cathode coated on Al foil was prepared and then underwent carbonization to obtain Na2S/CPVP. The remaining capacity of Na2S/CPVP||Na cell after 100 cycles is only 60 mAh gS−1 at 0.1 A gS−1 (Fig. S14), which confirms that the excellent performance of Na2S/CPVP@Cu2S comes from the combination of conductive structure and catalysis of Cu2S.
The Na2S/CPVP@Cu2S cathode was further assembled with a HC anode to expand its application in Na-free anode system, as illustrated in Fig. 5a. Before assembling Na2S/CPVP@Cu2S||HC full-cell, the HC electrode was firstly pre-activated to compensate its inherently irreversible capacities in the initial cycle, which was prepared by immersing the HC electrode into a sodium biphenyl solution32. The as-treated HC anode exhibits a high ICE of 114.89% in HC||Na half-cell, indicating the successful pre-activation (Fig. S15). The typical charge–discharge curves of the assembled Na2S/CPVP@Cu2S||HC full-cell present similar characteristics with Na2S/CPVP@Cu2S||Na (Fig. 5b). The full-cell also reveals stable cycling performance (Fig. 5c), with the ICE of 93.79%, maintaining a capacity of over 518 mAh gS−1 after 200 cycles.
a Schematic illustration of Na2S/CPVP@Cu2S||HC cell. b,c Typical galvanostatic capacity to b voltage curves (10th) and c cycling performance and CE of Na2S/CPVP@Cu2S||HC at 0.1 A gS−1. d Cycling performance of Na2S/CPVP@Cu2S||HC pouch cell. The inset shows a pouch cell lighting up a small LED light. e Comparison of this work with previously reported Na2S cathode (The electrochemical performance is evaluated in half-cell and the comparison of rate capability is based on the maximum current rate that is stably rechargeable for each rate test). f Cost analysis of battery systems (the theoretical energy densities are calculated based on the total mass of anode and cathode active materials). The material prices are adapted from ref. 36 of Aug. 31, 2024. Source data are provided as a Source Data file.
Furthermore, a monolayered Na2S/CPVP@Cu2S||HC pouch cell with designed capacity of 20 mAh was assembled as a validation for practical application (Fig. S16). As shown in Fig. 5d, the pouch cell maintains a capacity of 14.3 mAh after 25 cycles and demonstrates the ability to power a small LED light bulb. The results convincingly substantiate the feasibility of using Na2S originated from industrial waste to energy devices, which offers the incomparable electrochemical properties to those of other Na2S cathode proposed so far (Fig. 5e). Besides, a comparison in material costs to the state-of-art energy storage system is illustrated in Fig. 5f.33 Subject to the high activity of alkali metals, the Li–S and Na–S battery systems are still not commercially available. The Na2S paired with Na-free anode system shows the extreme low cost and its anode is selective (P/Si/Sn or anode free) which may realize the lower cost and wider application prospects33,34. Fig. S17 offers the Na2S/CPVP@Cu2S||anode-free concept cell, which successfully verified its feasibility, showcasing the possibility of further cost reduction.
Comparing with existing energy storage systems, RT Na–S batteries are very competitive on the premise of overcoming the safety and interfacial stability of Na–metal anode. Despite recent progress attempting to apply Na2S in Na-free anode systems, the complicated manufacturing process and serious capacity attenuation limit its further application. In this study, a rational design is proposed to utilize the Na2S sourced from industrial waste via solid-liquid-solid conversion pathway. The Na2S/CPVP@Cu2S cathode shows the remarkable cyclability (670 mAh gS−1 after 500 cycles at a specific current of 0.4 A gS−1) in half-cell and proves its capability of using in Na-free anode system. Unlike previous work trying to produce S/Na2S nanoparticles or using electrolytes with low NaPSs solubility, this work chooses the low-cost ether-based electrolyte and recyclable Na2S, realized the stable and long cycle of micron-sized Na2S by designing its redeposition behaviors, promoting the development of Na-free anode system in RT Na–S batteries.
The evolution of sulfur species is the key affecting its reversible capacity. Our initial findings suggest that Na2S deposition behaviors can be induced by cathode structural design and the shuttle effect is effectively inhibited without using anode protection. However, the challenges originated from high Na2S-loading, lean electrolyte and Na loss need to be overcome in future large-scale applications. Besides, understand the mechanism of sulfur migration on cathode surface is also critical to further realizing high-performance RT Na–S batteries.
The chemicals polyvinylpyrrolidone (PVP k88-96, Sigma-Aldrich, average Mw = 55000), biphenyl (Bp), Ketjen Black (KB, ECP 600JD) and multi-walled carbon nanotubes (MWCNT) were dried at 80 °C for 12 h to get rid of moisture. N-methyl-2-pyrrolidone (NMP, anhydrous, Sigma-Aldrich) was dried over molecular sieve (3 Å). 1 M NaOTf in DIGLYME solvent electrolyte was bought from DUODUO (H2O < 30 ppm. The Cu foils as well as all cell components were dried for 12 h at 70 °C under vacuum before use. All the chemicals were inserted into the Ar-filled glove box (Mbraun, H2O < 0.1 ppm and O2 < 1 ppm) after prepared.
In order to veritably reproduce the production process of barium sulfate in industry, the barium sulfide (BaS, 85%, Xiya) was selected, configured it as aqueous solution, and mixed with sodium sulfate solution in 1:1 mole ratio at room temperature. After full mixing and reaction, the solution is filtered and then evaporated to obtain crude Na2S. The crude Na2S was subjected to ball milling for 30 min and vacuum drying at 150 °C for 12 h to dehydrate35, and then heated to 800 °C for 2 h with a flowing mixture of 5% H2 in Ar to remove possible impurities (Na2SO4, Na2S2O3, Na2S2). The as-prepared Na2S was denoted as purified Na2S and the yield of purified Na2S is 86.0%.
The purified Na2S samples were mixed with Ketjen Black (KB, ECP 600JD), multi-walled carbon nanotubes (MWCNT) and polyvinylpyrrolidone (PVP k88-96, Sigma-Aldrich, average Mw = 55,000), following a weight ratio of 70:15:5:10 in N-methyl-2-pyrrolidone (NMP, anhydrous, Sigma-Aldrich) stirring 6 h to form a slurry in a suitable viscosity. The as-prepared slurry was then coated onto a Cu foil and punched into disk with a diameter of 10 mm after vacuum dried at 70 °C in the glove box to get Na2S/PVP. Na2S/CPVP@Cu2S was obtained by thermal heated Na2S/PVP to 500 °C for 1 h in a tube furnace under Ar flow.
The HC anode were fabricated by mixing 80 wt% HC, 12 wt% Super P, and 8 wt% poly(vinyl difluoride) (PVDF) in N-methyl-2-pyrrolidinone (NMP) to form a slurry, which was then coated on a Cu foil and punched into disk with diameter of 12 mm after vacuum dried. The as-prepared HC anode was immersed in sodium biphenyl complex solution (0.5 M Na and 0.5 M biphenyl dissolve in 1,2-Dimethoxyethane) for 2 minutes to get pretreated HC32.
XRD measurements were carried out by a D8 diffractometer (Bruker) with Cu Kα radiation in the 2θ range of 10-70° to obtain crystallinity identification. The morphologies and element distribution of Na2S/PVP and Na2S/CPVP@Cu2S was performed by Scanning Electron Microscope (Regulus 8230) and Transmission Electron Microscope (TEM) (Tecnai G2 F20 S-TWIN) equipped with an EDS detector after washing the samples by DIGLYME and drying in vacuum. The Na:Cu ratio was determined by ICP-MS tests (Thermo, ICP-MS, America; radio-frequency power: 1550 W). Raman spectroscopy (LABRAM, HR) was tested with 532 nm laser excitation. XPS analysis was tested by using an ESCALAB 250Xi spectrometer (VG, Altrincham, UK) with Al Kα radiation, the samples were firstly washed by DIGLYME and dried in vacuum, then transferred into the XPS analysis chamber by a vacuum transfer device. The TGA test (TG 209 F1, NETZSCH, Germany) was firstly heated to 500 °C with a heating rate of 10 °C min−1 and then preserve for 2 h under a nitrogen atmosphere. TOF-SIMS analysis (TOF.SIMS5-100) was obtained by a 10 kV Bi+ ions beam and 1 kV Cs+ sputtering.
Na2S/PVP cathode contained 68 wt% sodium sulfide (Na2S), 15 wt% Ketjen Black (KB), 5 wt% multi-walled carbon nanotubes (MWCNT), and 12 wt% polyvinylpyrrolidone (PVP). In this work, to be consistent with the previous RT Na–S batteries works, the specific capacity values were calculated based on all the sulfur contained in Na2S and Cu2S. The actual mass of sulfur in Na2S active substance will be lower than the data used.
All electrochemical tests are based on three times measurements. Coin-type (CR2032) cells were assembled in an Ar-filled glove box with Na metal or HC as anode. The Na2S/CPVP@Cu2S cathode was cut into a disk with a diameter of 10 mm. Glass fiber GF-A (Shanghai Jinpan Biotechnology Co., Ltd.) was used as the separator. 1 M NaOTf in DIGLYME solvent was used as electrolyte in all cells. The Na2S loading of the coin-type cells is 1.2 mg cm-2, E/S ratio in the coin cells is 25 μL mg−1. The N/P ratio in Na2S/CPVP@Cu2S||HC full-cell is 1.2. For Na2S/CPVP@Cu2S||HC pouch cells, all electrode preparation steps were same to coin-type cells, while the electrode sizes are 67 mm*47 mm and the Na2S loading is 2.1 mg cm-2. The N/P ratio is 1.16. The electrolyte injection amount was controlled at 16 μL mg–1, and glass fiber (Whatman GF/B) was used as separator. All the Electrochemical tests were carried out using the Neware battery test system (CT-4000, Shenzhen Neware Technology Co. Ltd.) at the environmental chamber (25 °C). Electrochemical Impedance Spectroscopy (EIS) was tested by VPM300 analyzer (Bio-logic, France). The Na2S/CPVP@Cu2S||Na half-cells were tested by galvanostatic method in the voltage range from 0.5 V to 2.8 V, and the Na2S/CPVP@Cu2S||HC full-cells were tested from 0.1 V to 2.5 V. Cyclic voltammetry (CV) measurements were evaluated using a VPM300 analyzer (Bio-logic, France) with various scan rates from 0.1 mV s−1 to 0.5 mV s−1 each interval 0.1 mV s−1 in the potential range of 0.5 V and 3.0 V. Randles–Sevcik equation which is used to identify the Na+ diffusion coefficient31:
Where \({i}_{p}\) is current maximum in amps; \(n\) is number of electrons transferred in the redox event; \(A\) is electron area in cm2; F is Faraday constant in C mol−1; \(D\) is diffusion coefficient in cm2 s−1; \(C\) is concentration in mol cm−3; \(v\) is scan rate in V s−1; \({R}\) is gas constant in J k−1 mol−1; \(T\) is temperature in K. Since all other variables can be treated as constant under this circumstance, the square root of the scan rate is linearly related to peak current with a slope of square root of diffusion coefficient.
The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon request. Source data are provided with this paper.
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This work was supported by the National Key R&D Program of China (Grant No. 2022YFE0207300) and the National Natural Science Foundation of China (Grant Nos. 22075314 and 22179142). XPS and TOF-SIMS characterizations were supported by Nano-X (Vacuum Interconnected Nanotech Workstation, Chinese Academy of Sciences, Suzhou 215123, China).
School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, Hefei, China
Suwan Lu, Yang Liu, Jingjing Xu, Shixiao Weng, Lingwang Liu, Guochao Sun & Xiaodong Wu
i-lab, Vacuum Interconnected Nanotech Workstation (Nano-X), Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou, China
Suwan Lu, Yang Liu, Jingjing Xu, Shixiao Weng, Jiangyan Xue, Lingwang Liu, Guochao Sun, Yiwen Gao, Qingyu Dong & Xiaodong Wu
College of Material Science and Engineering, Hohai University, Nanjing, Jiangsu, China
Tianmu Lake Institute of Advanced Energy Storage Technologies Co., Ltd, Liyang, China
Zhicheng Wang, Can Qian, Hong Li & Xiaodong Wu
Beijing Advanced Innovation Center for Materials Genome Engineering Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing, China
Zhicheng Wang & Hong Li
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S.L. and Y.L. contributed equally to this article.
Correspondence to Jingjing Xu or Xiaodong Wu.
The authors declare no competing interest.
Nature Communications thanks Yuan Yang and the other, anonymous, reviewer for their contribution to the peer review of this work. A peer review file is available.
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Lu, S., Liu, Y., Xu, J. et al. Design towards recyclable micron-sized Na2S cathode with self-refinement mechanism. Nat Commun 15, 9995 (2024). https://doi.org/10.1038/s41467-024-54316-9
DOI: https://doi.org/10.1038/s41467-024-54316-9
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