High-sulfur coal-based carbon dots as an electrolyte additive to enhance the performance of aqueous zinc-ion batteries

Abstract

Aqueous zinc-ion batteries (AZIBs) are promising for large-scale energy storage but suffer from zinc anode issues like dendrite growth and side reactions. This study synthesized sulfur-containing carbon dots (S-CDs) from high-sulfur coal via hydrodynamic cavitation. These quasi-spherical S-CDs have abundant surface groups (O–H, C C, C–S) and good aqueous compatibility. As a 0.5 mg mL⁻¹ additive in 2 mol per L ZnSO₄ electrolyte, S-CDs weakened Zn²⁺–water interaction, bound Zn²⁺via negative groups, inhibited hydrogen/oxygen evolution and corrosion (corrosion current down to 0.168 mA from 2.3 mA), restricted Zn²⁺ diffusion, and induced dense (002) plane zinc deposition to suppress dendrites. Electrochemical tests showed Zn‖Zn symmetric batteries with S-CDs had R_ct reduced to 200 Ω (from 610 Ω), cycling stably over 1120 h (1 mA cm⁻²) and 2000 h (5 mA cm⁻²). Zn‖NH₄V₄O₁₀ full cells delivered 310.1 mAh g⁻¹ at 3 A g⁻¹ (vs. 250.5 mAh g⁻¹ for pristine electrolyte) and retained 90.3% capacity after 500 cycles (vs. 72.1%). This work enables high-value use of high-sulfur coal and provides a scalable additive to boost AZIBs' practicality in large-scale energy storage.

To highlight the significance of coal-based carbon dots, this figure can well illustrate the relevant mechanism and phenomenon, making it highly appropriate for inclusion in the paper.

1. Introduction

With the global energy structure transitioning toward clean energy, the large-scale application of renewable energies such as wind and solar power urgently requires support from efficient and stable energy storage systems. Aqueous zinc-ion batteries (AZIBs) avoid the flammability issues of organic electrolytes by using aqueous electrolytes. Meanwhile, zinc resources are abundant and low-cost, making AZIBs a research hotspot in the field of large-scale energy storage.^1–5 The zinc anode possesses an ultra-high volumetric capacity (5822 mAh cm⁻³) and a low redox potential of −0.76 V (vs. SHE, 25 °C, 1 mol per L Zn²⁺), granting it inherent advantages in aqueous batteries.⁶ However, the zinc anode faces two core challenges during cycling: first, zinc ions tend to form dendrites during deposition, leading to increased electrode surface area, intensified side reactions, and even battery short circuits; second, active water molecules in the aqueous electrolyte easily react with zinc, generating by-products such as Zn(OH)₂ and Zn₄(OH)₆SO₄·xH₂O, which cause electrode passivation and capacity fading.⁷

The key to addressing the stability issues of the zinc anode lies in constructing a stable electrode–electrolyte interface. In existing research, artificial coating technology protects the zinc surface by forming a protective layer to isolate electrolyte erosion, but the coating is prone to cracking and failure during cycling. Electrode structure modification, such as porous zinc and alloyed zinc, can improve deposition uniformity but involves complex preparation processes and high costs.⁸ In contrast, electrolyte additive technology enables molecular-level regulation of zinc ion deposition behavior and interface reactions, offering advantages of simple operation and broad applicability.⁹ Previous studies have shown that organic additives like disodium ethylenediaminetetraacetate can regulate the deposition process by complexing with Zn²⁺, but they require high addition amounts and may reduce electrolyte conductivity. Inorganic additives such as In³⁺ and Bi³⁺ can inhibit dendrite growth, yet they are costly and may cause environmental pollution.¹⁰

Carbon-based nanomaterials exhibit excellent performance in the electrochemical field due to their unique electronic structure and surface chemical properties. As a new type of carbon-based material, carbon dots (CDs) feature small size (2–10 nm), abundant surface functional groups, and good biocompatibility, and have been applied in batteries, catalysis, and other fields.¹¹ In previous studies, single-doped carbon dots (e.g., nitrogen-doped or sulfur-doped carbon dots) have been proven to regulate the Zn²⁺ solvation structure or adsorb on the electrode surface to induce uniform deposition.¹² However, single-doped carbon dots usually have a single function and cannot simultaneously solve the problems of dendrite growth and side reactions. Nitrogen–sulfur co-doping can enhance the coordination ability of carbon dots with Zn²⁺ through the synergistic effect of the two heteroatoms, while introducing more active sites to regulate crystal plane growth, holding promise for achieving dual-functional synergy.¹³

To address the aforementioned challenges of dendrite growth and severe side reactions on zinc anodes that hinder the practical application of AZIBs, this study proposes a novel and sustainable solution by utilizing high-sulfur coal, an abundant and low-value industrial byproduct, as the raw material to synthesize S-CDs via a facile hydrodynamic cavitation method. The as-prepared S-CDs exhibit a quasi-spherical morphology with abundant surface functional groups, and excellent aqueous compatibility, endowing them with unique advantages as electrolyte additives. When introduced into the ZnSO₄ electrolyte, the additive effectively weakens the Zn²⁺–water interaction and binds tightly with Zn²⁺ through negatively charged surface groups. This dual effect suppresses the hydrogen/oxygen evolution reactions and corrosion, restricts the two-dimensional diffusion of Zn²⁺, and induces dense and stable Zn deposition to inhibit dendrite growth. Electrochemical tests confirm that batteries with S-CDs show significantly reduced charge transfer resistance, enhanced cycling stability, and improved specific capacity compared to those with pristine electrolyte. This work not only realizes the high-value utilization of high-sulfur coal but also provides a scalable, cost-effective additive to simultaneously mitigate the key issues of zinc anodes, thereby promoting the practicality of AZIBs in large-scale energy storage.

2. Experimental section

2.1. Materials

High-sulfur coal was obtained from Bijie (Guizhou, China), the industrial and elemental analysis of high-sulfur coal are shown in Tables 1 and 2.

Table 1. Industrial analysis and elemental compositions^a.

	Proximate analysis			Element analysis
	Ad (wt%)	Vdaf (wt%)	FCd (wt%)	C (wt%)	S (wt%)	O (wt%)	H (wt%)	N (wt%)
High-sulfur coal	8.05	7.56	66.17	69.23	5.16	6.19	1.85	0.94

^aA_ad: ash content; V_daf: volatile matter; F_Cd: fixed carbon.

Table 2. Sulfur forms of high-sulfur coal^a.

	St,ad	So,ad	Ss,ad	Sp,ad
High-sulfur coal	5.16	3.38	0.06	1.72

^aS_t,ad: total sulfur; S_o,ad: organic sulfur; S_s,ad: sulfate sulfur; S_p,ad: pyritic sulfur.

2.2. Preparation of S-CDs

The S-CDs was synthesized by hydrodynamic cavitation method. The experimental setup consists of two closed-loop circuits including a feed tank, a pump of power rating 1.2 kW, a venturi tube as cavitation device, a flowmeter and three control valves. In a typical synthesis procedure, 50 g of coal powder were added into 1000 mL DI-water and stirred to obtain a suspension. Then, the suspension was transferred and circulated in the setup at a flow rate of 200 mL min⁻¹. As the pump working, the cavitation phenomena continuously occurred and produced various effects. These comprehensive effects disrupted the crosslinking net between crystallites and fragmentize the high-sulfur coal powders into small nanocrystallites, acting as sulfur-containing CDs (named S-CDs). After fragmentizing 12 h, the suspension containing S-CDs was firstly centrifuged at 6000 rpm for 5 min and then was filtered with slow-speed quantitative filter paper to remove big fragments. The finally obtained supernatant was the original aqueous solution of S-CDs. The solid sample can be further obtained by freeze drying technique.

2.3. Preparation of cathode materials

1.82 g of NH₄VO₃ was dissolved in 60 mL of deionized water. The mixture was stirred at 80 °C for 30 minutes, and 2 mol per L H₂C₂O₄·2H₂O was slowly added, followed by continued stirring for 1 hour. The solution was transferred to a 100 mL autoclave and reacted at 180 °C for 8 hours. After the reaction, the precipitate was collected by centrifugation, washed alternately with deionized water, and then dried in a vacuum oven at 60 °C for 12 hours to obtain NH₄V₄O₁₀.¹⁴

2.4. Battery assembly

S-CDs were added to 2 mol per L ZnSO₄ aqueous solution at concentrations of 0.5 mg mL⁻¹ and were then ultrasonically dispersed for 30 minutes to prepare electrolytes (named as ZnS-CDs). The zinc electrode was made of 0.1 mm thick zinc foil (purity 99.99%), cut into 14 mm diameter discs, ultrasonically cleaned with ethanol for 10 minutes, and dried for later use. NH₄V₄O₁₀, conductive carbon black, and PVDF were mixed at a mass ratio of 7 : 2 : 1. An appropriate amount of NMP was added, and the mixture was stirred for 6 hours to form a uniform slurry. The slurry was coated onto a titanium foil (thickness 0.02 mm), dried in a vacuum oven at 60 °C for 12 hours, and cut into 12 mm diameter discs. Battery assembly: CR2016 coin cell casings were used, with Whatman GF/F glass fiber as the separator, and 100 µL of electrolyte was injected. After assembly, the cells were allowed to stand at room temperature for 6 hours before electrochemical testing to ensure sufficient wetting of the electrodes and separator by the electrolyte.

2.5. Characterization and electrochemical testing

Morphological and structural characterization of materials: transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100) were used to observe the morphology and microstructure of the materials. X-ray diffraction (XRD, Bruker D8 Advance) was employed to analyze the crystal structure, with a scanning range of 2θ = 5–80°. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) was used to determine the elemental composition and chemical valence states. Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50) was utilized to characterize surface functional groups, with a scanning range of 400–4000 cm⁻¹.

Electrochemical performance testing: cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV) tests were performed on a CHI760E electrochemical workstation. The CV tests were conducted within a voltage range of 0–1.8 V (vs. Zn/Zn²⁺) at scan rates of 0.1–1.0 mV s⁻¹. The EIS tests were carried out over a frequency range of 10⁻²–10⁵ Hz with an applied AC signal of 5 mV amplitude. The LSV tests were performed within a voltage range of −1.0–3.0 V (vs. Zn/Zn²⁺) at a scan rate of 1 mV s⁻¹. For Zn‖Zn symmetric cells, the test current densities ranged from 1 to 10 mA cm⁻² with a capacity of 0.5 mAh cm⁻². For full cells, the test voltage ranges were 0.2–1.8 V (Zn‖NH₄V₄O₁₀), with current densities ranging from 0.1 to 5 A g⁻¹. All tests were performed at room temperature.

3. Results and discussion

The S-CDs prepared via hydrodynamic cavitation exhibit a quasi-spherical shape and are evenly dispersed in aqueous solution without any agglomeration, indicating its excellent water solubility, as shown in Fig. 1a. The high-resolution TEM of S-CDs, as inset in Fig. 1a, displays clear lattice fringes with spacing distance about 0.21 nm, which corresponds to the (100) diffraction facet of graphite structure. After counting more than 150 dots, S-CDs have small sizes with an average of 3.6 nm. These morphological and structural results have demonstrated that the S-CDs have been successfully fragmentized from high-sulfur coal. To explore the surface state of the S-CDs, FT-IR was used to analyze their surface functional groups. As shown in Fig. 1b, the broad absorption peak at 3449 cm⁻¹ corresponds to the stretching vibration of hydroxyl groups (O–H); the peaks at 1638, 1388, and 1129 cm⁻¹ correspond to the stretching vibrations of carbon–carbon double bonds (C C), hydroxyl groups (C–OH), and ether bonds (C–O–C) respectively; the absorption peak at 623 cm⁻¹ is attributed to the stretching vibration of carbon–sulfur bonds (C–S) formed by residual sulfur element in the coal.^15,16 XPS spectra further confirm that the S-CDs are mainly composed of carbon (C), oxygen (O), and sulfur (S) (Fig. 1e): the high-resolution spectrum of C 1s consists of three peaks at 288.9 eV (C–S), 287.1 eV, 285.8 eV (C–O), and 284.8 eV (C–C); the high-resolution spectrum of O 1s can be decomposed into two peaks at 531.6 eV (C O) and 533.1 eV (C–O);¹⁴ the high-resolution spectrum of S 2p consists of two peaks at 169.2 eV (–C–SO₄⁻) and 168.0 eV (–C–SO₃⁻).^15–18 The results demonstrate the S-CDs contain abundant sulfur element and surface functional groups. The results show a strong signal of sulfur element, indicating a high sulfur content in the CDs, and the mapping images confirm the uniform doping of sulfur. These test results demonstrate the successful synthesis of sulfur-doped carbon quantum dots with abundant surface functional groups.

Fig. 1. Morphological and element characterization of the synthesized CDs. (a) TEM image. (b) FT-IR spectra of CDs. (c) XPS survey spectra, and (d–f) high-resolution C 1s, O 1s, S 2p spectra of CDs.

The FT-IR spectra of 2 mol per L ZnSO₄ aqueous solution and a mixed solution with 0.5 mg per mL S-CDs are shown in Fig. 2a, respectively. It is observed that the stretching vibration peak of hydroxyl groups (O–H) gradually shifts from 3216 cm⁻¹ to 3222 cm⁻¹, indicating a weakening of the interaction between Zn²⁺ and water molecules. Besides, the zeta potential of S-CDs aqueous solution was tested and shown in Fig. 2a (inset). The obtained potential value of approximately −26 mV is mainly due to the negatively charged groups on the surface of the S-CDs, facilitating their tight binding with Zn²⁺.¹⁹ Zn‖Zn symmetric batteries were assembled and a series of electrochemical tests were conducted. From the LSVcurve of the battery (Fig. 2b), it can be found that the onset potentials of the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) of the symmetric battery using the ZnS-CDs electrolyte are higher than those of the battery using the ZnS electrolyte, indicating that the S-CDs can inhibit the kinetic processes of HER and OER, thereby expanding the electrochemical stability window.²⁰ In addition, Tafel curve of the symmetric battery was tested at a scanning rate of 1 mV s⁻¹ (Fig. 2c). The measured corrosion current of the symmetric battery using the ZnS electrolyte is 2.3 mA, while the corrosion current of the battery using the ZnS-CDs electrolyte is significantly reduced to 0.168 mA, indicating that the added S-CDs can reduce the occurrence of corrosion side reactions, improve the cycling stability of the electrode, and reduce the probability of corrosion and by-product formation.²¹ The fluctuation of the current–time curve can reflect the changes in the electrode surface morphology during the deposition process. The results of the chronoamperometry (CA) test (Fig. 2d) show that after applying an over potential of −150 mV, the current of the symmetric battery using the ZnSO₄ electrolyte increases continuously in the 200-second test period, indicating that a continuous and unconstrained two-dimensional diffusion occurs, and the deposition growth is rough. Zn²⁺ diffuses laterally along the electrode surface to find the charge transfer site with the optimal energy. To reduce the surface energy and exposed area, Zn²⁺ tends to aggregate and form dendrites.²¹ In contrast, the symmetric battery using the ZnS-CDs electrolyte quickly transitions from the initial disordered two-dimensional diffusion (lasting about 70 seconds) to a uniform and stable three-dimensional diffusion, indicating that Zn²⁺ is locally reduced to zinc under the constrained two-dimensional surface diffusion.²² This change is due to the coordination interaction between the oxygen-containing functional groups on the surface of S-CDs and Zn²⁺. This interaction limits the two-dimensional diffusion of Zn²⁺ on the electrode surface, forcing Zn²⁺ to deposit near the initial adsorption site, avoiding concentrated deposition at a few low surface energy sites, and thus forming more small-sized nucleation sites. Combined with the low nucleation energy barrier brought by the S-CDs-regulated solvation shell, the uniformity of subsequent deposition can be effectively improved.²³ In summary, these tests show that the S-CDs additive can inhibit side reactions and promote the uniform deposition of zinc, which helps maintain the long-term cycling stability of the battery and extend its service life.

Fig. 2. Various spectra and electrochemical tests for the ZnS-CDs electrolyte system. (a) FTIR spectra for ZnS and ZnS-CDs electrolytes. (b) LSV curves at scan rate of 1 mV s⁻¹ reflecting HER and OER performance of the system; (c) Tafel plots representing corrosion behaviors; (d) CA curves indicating Zn²⁺ diffusion process of Zn electrode in ZnS and ZnS-CDs electrolytes.

EIS tests were conducted on the symmetric batteries and the curves are shown in Fig. 3a. The results of equivalent circuit fitting show that the initial charge transfer resistance (R_ct) of the symmetric battery with ZS electrolyte is as high as 610 Ω. After adding S-CDs into electrolyte, the R_ct of the Zn‖Zn symmetric battery is significantly reduced to 200 Ω. The reduced impedance indicates that the S-CDs is beneficial for desolvation and efficient charge transfer, enabling faster deposition kinetics.²⁴ At a current density of 1 mA cm⁻², the cycling performance of symmetric batteries was tested and shown in Fig. 3b. The symmetric battery using the ZnS-CDs electrolyte has the best performance, with a cycle life of 1120 hours and a low polarization voltage, indicating that the reversibility of zinc deposition/stripping is better in the presence of S-CDs. As a control, the symmetric battery using the ZnSO₄ electrolyte failed after only 100 hours of cycling; while all batteries with S-CDs added showed a significant improvement in cycle life. When the cycling behavior of the batteries was tested with current density of 5 mA cm⁻², the symmetric battery using the ZnS-CDs electrolyte still maintains a small polarization voltage. Subsequently, long-term cycling tests were also conducted at high current density of 5 mA cm⁻²: the symmetric battery using the ZnS-CDs electrolyte can cycle stably for more than 2000 hours under the conditions of 5 mA cm⁻² and 0.5 mAh cm⁻² (Fig. 3c). From the enlarged detail diagrams, it can be seen that during the early 1000.1–1000.6 h stages of cycling, the battery shows a stable voltage curve, indicating a stable internal working state, proving that the added S-CDs can effectively improve the cycling ability of the battery under high current densities.

Fig. 3. Cyclic performance of Zn‖Zn symmetric cells using ZnS and ZnS-CDs electrolytes (a) EIS profiles and (b–d) long-term galvanostatic charging/discharging under different current densities.

The symmetric batteries cycled in ZnS and ZnS-CDs electrolytes for 50 and 100 cycles were disassembled, respectively, and the surface of the zinc electrode was analyzed using XRD technology, as shown in Fig. 4a. For the symmetric battery using the ZnS electrolyte, the signal of ZnSO₄(OH)₆·4H₂O on the electrode surface increases with the increase of cycling times; while for the symmetric battery using the ZnS-CDs electrolyte, no relevant ZnSO₄(OH)₆·4H₂O signals are detected on the electrode surface, confirming the ZnSO₄(OH)₆·4H₂O inhibition ability of this additive. In addition, it was found that in the symmetric battery using the ZnS electrolyte, the zinc metal is mainly dominated by the (101) crystal plane; while in the symmetric battery using the ZnS-CDs electrolyte, the (002) crystal plane of zinc metal accounts for a high proportion and increases with the increase of cycling times. The (002) crystal plane zinc is arranged at a small angle to the substrate, with dense and uniform deposition, high stability in the electrolyte, and strong corrosion resistance as shown in Fig. 4b and c. This indicates that the CD additive can also induce the formation of specific crystal planes on the electrode. The zinc on the surface is mainly in the (101) crystal plane, which is prone to corrosion and dendrite growth, and is not conducive to the long-term cycling stability of the battery. This also explains why the symmetric battery is more stable under high current densities: during cycling at high current densities, S-CDs help the electrode quickly construct and update the (002) crystal plane, maintaining the electrode stability; under low current densities, due to the slow deposition rate, the electrode is more significantly affected by side reactions, resulting in poor cycling performance.

Fig. 4. Morphology characterization of Zn electrode (a) XRD patterns of Zn electrode after 50 and 100 cycles in ZnSO₄ and ZnS-CD0.4 electrolyte, respectively. Electrochemical performances of Zn‖NVO full cells with different electrolytes. (b) CV curves, (c) rate capability under different current densities and (d) long-term cycling performance and corresponding coulombic efficiency.

Zn‖ZnS‖NH₄V₄O₁₀ (NVO) and Zn‖ZnS-CDs‖NVO full cells were assembled using NH₄V₄O₁₀ as cathode material. The Cyclic Voltammetry (CV) curves of the full cells using ZnS and ZnS-CDs electrolytes are similar (Fig. 4d), indicating that the introduction of the S-CDs additive does not change the type of redox reaction of the full cells. The rate performance of the full cells was tested at different discharge rates of 0.2, 0.5, 1, 2, 3, and 5 A g⁻¹ (Fig. 4e): under all current conditions, the specific capacity of the full cell using the ZnS-CDs electrolyte is higher than that of the full cell using the ZnS electrolyte; and when the current density is restored to 0.2 A g⁻¹, the specific capacity of the full cell using the ZnS-CDs electrolyte basically returns to the initial state. This shows that the S-CDs additive performs excellently under different current conditions, and can achieve rapid redox kinetics and stable interfacial charge transfer even under high currents. The long-term working ability of the full cell was tested under the fast charge–discharge condition of 3 A g⁻¹ (Fig. 4f): after 5 activation cycles, the specific capacity of the full cell using the ZnS-CDs electrolyte reaches 310.1 mAh g⁻¹, which is significantly higher than that of the full cell using the ZS electrolyte (250.5 mAh g⁻¹); in the subsequent cycles, due to the inhibition of side reactions and the improvement of electrode stability by the S-CDs additive, the full cell using the ZnS-CDs electrolyte shows excellent capacity retention, which is 90.3% after 500 cycles, while that of the full cell using the ZnS electrolyte is only 72.1%. The charge–discharge curves of the full cell during cycling also confirm the full cell using the ZnS-CDs electrolyte has better cycling reversibility and lower polarization.

These excellent performance originate from the enhancement of S-CDs on the solvation of ZnSO₄ electrolyte. In pristine electrolyte, Zn²⁺ primarily exists as [Zn(H₂O)₆]²⁺ with a solvation shell composed of 6 tightly coordinated water molecules. S-CDs, featuring abundant surface functional groups (e.g., –OH, C–S, –SO₃⁻) and negative zeta potential (−26–−28 mV), exhibit stronger binding affinity with Zn²⁺ (binding energy: −11.74–−12.38 eV) compared to water molecules (−4.87 eV).¹ This enables S-CDs to preferentially displace partial coordinated water molecules, forming a hybrid solvation structure of “Zn²⁺–S-CDs-residual water molecules” and breaking the original uniform hydration shell. FT-IR results (both documents) confirm that the O–H stretching vibration peak of water shifted from 3216 to 3222 cm⁻¹, reflecting weakened constraints of Zn²⁺ on water molecules and a decrease in the number of coordinated water molecules (less than 6 in the hybrid solvation shell). The blue shift of the O–H stretching vibration peak directly indicates reduced electrostatic interaction between Zn²⁺ and the oxygen atoms of water molecules. This attenuation reduces the activation of coordinated water molecules, thereby inhibiting side reactions (e.g., hydrogen evolution, corrosion) induced by active water at the electrode–electrolyte interface. Tafel tests verify that the corrosion current decreases sharply from 2.3–2.4 mA to 0.158–0.168 mA, which is attributed to the reduced reactivity of water molecules caused by weakened Zn²⁺–O bonds. The reconstruction of the Zn²⁺ solvation shell by S-CDs not only reduces the content of active water and weakens Zn²⁺–O bond strength but also lowers the Zn²⁺ nucleation energy barrier. Combined with the restriction of Zn²⁺ two-dimensional diffusion by S-CDs, this promotes uniform deposition of Zn with a dense (002) crystal plane, collectively suppressing dendrite growth and side reactions. This specific solvation regulation mechanism provides clear chemical insights into the role of S-CDs as bifunctional electrolyte additives for aqueous zinc-ion batteries.

4. Conclusions

This study successfully synthesizes S-CDs from high-sulfur coal via a hydrodynamic cavitation method. The S-CDs exhibit a quasi-spherical morphology with an average size of 3.6 nm, abundant surface functional groups (e.g., O–H, C C, C–S), and ensuring excellent water solubility and compatibility with aqueous electrolytes. When used as an electrolyte additive for aqueous AZIBs, the S-CDs demonstrate that they weaken the interaction between Zn²⁺ and water molecules and bind tightly with Zn²⁺ through negatively charged surface groups. This inhibits hydrogen/oxygen evolution reactions and corrosion side reactions, reducing the corrosion current from 2.3 mA to 0.168 mA and expanding the electrochemical stability window. The S-CDs also induce uniform Zn deposition by restricting two-dimensional diffusion of Zn²⁺ and promoting the formation of dense (002) crystal plane zinc, which suppresses dendrite growth. For Zn‖Zn symmetric batteries, the additive reduces charge transfer resistance from 610 Ω to 200 Ω, enabling stable cycling for over 1120 hours at 1 mA cm⁻² and 2000 hours at 5 mA cm⁻² with low polarization. In Zn‖NH₄V₄O₁₀ full cells, the S-CDs do not alter the redox reaction mechanism but significantly enhance performance. This work provides a cost-effective and sustainable strategy for valorizing high-sulfur coal into high-performance battery materials. The S-CDs additive offers a simple, scalable solution to improve the cycling stability and electrochemical performance of AZIBs, laying a foundation for their large-scale energy storage applications.

Conflicts of interest

All authors declare that there are no known financial interests or personal relationships that could affect the research reported in this paper.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.