Amidst the accelerating global carbon neutrality strategy, electrochemical energy storage technologies have emerged as pivotal solutions to reconcile the intermittency of renewable energy sources with grid stability, owing to their high energy conversion efficiency and versatile application scenarios. Current mainstream electrochemical storage systems include lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), zinc-ion batteries (ZIBs), and supercapacitors (Cao et al., 2025; Xie et al., 2025; Wei and Chen, 2025; Maurya et al., 2025; Jing et al., 2025; Tao et al., 2025; Ma J. et al., 2024). Among these, LIBs dominate the market due to their high energy density and mature commercialization. However, the scarcity of lithium resources (0.0065% crustal abundance), uneven geographical distribution, and environmental concerns have driven urgent demands for alternative energy storage systems (Gao et al., 2024; Wang et al., 2024). Aluminum-ion batteries (AIBs) have garnered significant attention as compelling candidates for large-scale energy storage, capitalizing on aluminum’s exceptional crustal abundance (8%), exceptional theoretical capacity (2,980 mAh g⁻¹), and cost-effectiveness. Notably, AIBs exhibit intrinsic safety (dendrite-free characteristics) and environmental compatibility, positioning them as promising solutions for grid-scale storage and electric vehicles (Gupta et al., 2024; Yang et al., 2024; Yang and Huang, 2024).

Despite their potential, practical AIBs applications face multifaceted challenges. In aqueous systems, issues such as aluminum corrosion, hydrogen evolution side reactions, and passivation layer formation severely degrade cycling stability (Yu et al., 2024; Stephanie et al., 2024). More critically, cathode material limitations remain the primary bottleneck for non-aqueous AIBs. Since the pioneering work by Lin et al. (2015), who established the AlCl3/EMImCl ionic liquid electrolyte system, research on non-aqueous AIBs has expanded rapidly. However, non-aqueous AIBs practical performance are hindered by sluggish electrochemical kinetics attributed to the low electrode potential of aluminum and the large ionic radius involved in electrode reactions. Recent efforts have explored diverse cathode materials, including metal oxides, transition metal chalcogenides (TMCs), carbon-based materials, and others (Gupta et al., 2024; Yang et al., 2024; Yang and Huang, 2024). Carbon-based materials (graphite, graphene) demonstrate high discharge voltages and cycling stability but suffer from limited capacity due to AlCl₄ ^– intercalation mechanisms, yielding only one-third of the theoretical capacity achievable via Al³⁺ intercalation. Transition metal oxides offer high specific capacity but face severe Coulombic interactions and lattice diffusion barriers caused by high charge density of Al³⁺. In contrast, TMCs exhibit superior promise for AIBs cathodes owing to their multi-electron redox activity, wide interlayer spacing, and low electronegativity, enabling high theoretical capacities and improved Al³⁺ diffusion kinetics (Ma D. et al., 2024; Wu et al., 2019; Huang et al., 2024; Liu et al., 2021).

Among TMCs, cobalt sulfide (Co₉S₈) has attracted particular interest due to its unique crystal structure and electrochemical properties. Recent studies highlight its versatility in energy storage: Bai et al. engineered core-shell Co₉S₈ nanospheres via hydrothermal and annealing methods, demonstrating exceptional sodium-ion storage performance (Zhang et al., 2018). Feng et al. developed Co₉S₈ hollow boxes with dual-open ends via solvothermal synthesis, achieving remarkable rate capability and cycling stability in SIBs through pseudocapacitive effects (Yin et al., 2020). Zhang et al. developed an innovative hierarchical architecture of Co₉S₈-MoB MBene heterostructures. The precisely embedded Co₉S₈ nanoparticles within the interlayer spacing of MoB MBene frameworks endowed the composite with exceptional lithium storage properties, demonstrating superior rate capability and cycle stability as a high-performance cathode material for LIBs, outperforming state-of-the-art transition metal sulfide-based cathodes (Wang et al., 2025). Additionally, Zhang et al. fabricated a three-dimensional porous N-doped carbon composite embedded with MOF-derived Co₉S₈ nanoparticles (Co₉S₈@NPC) through a templating strategy coupled with in situ high-temperature pyrolysis. When evaluated as an anode material for potassium-ion batteries (KIBs), the Co₉S₈@NPC composite demonstrated superior electrochemical performance compared to previously reported metal sulfide-based anode materials for KIBs (Ma et al., 2022). Furthermore, reports exist on the application of Co₉S₈ in AIBs; however, a specific investigation into its electrode energy storage mechanism is lacking (Grindal and Azimi, 2024).

Herein, we present the comprehensive study of Co₉S₈ as an AIBs cathode, elucidating its electrochemical dynamics and atomic-scale Al³⁺ storage mechanisms. Nanocrystalline Co₉S₈ particles were synthesized via a one-step hydrothermal method, with structural and morphological properties characterized by XRD and SEM. Electrochemical performance was evaluated through CV, galvanostatic charge-discharge testing, and EIS. The Co₉S₈ cathode demonstrated notable cycling stability, retaining ∼48 mAh g⁻¹ after 500 cycles at 100 mA g⁻¹, with a minimal capacity decay rate of 0.04% per cycle. First-principles calculations revealed a distinctive “pseudo-isomorphic substitution” mechanism, where Al³⁺ preferentially occupies octahedral Co. sites in the Co₉S₈ lattice (binding energy: 0.92 eV). By elucidating structure-property relationships and Al³⁺ storage dynamics, this study provides critical insights for designing high-performance AIB cathodes and advances the development of cost-effective, durable energy storage systems.

In order to study the phase structure of the synthesized materials, X-ray diffraction (XRD) testing was performed on the sample. The results of XRD phase analysis on the sample are shown in Figure 2A. The analysis shows that the characteristic peaks located at 2θ = 15.4°, 25°, 4°29.8°, 31.2°, 36.2°, 39.5°, 44.7°, 47.6°, 52.1°, 61.2° and 62.1° correspond to the (111), (220), (311), (222), (400), (331), (420), (511), (440), (533) and (622) crystal planes of cubic Co₉S₈, respectively. All characteristic peak positions are highly consistent with the diffraction data of the JCPDS standard card (No.86-2273), and no diffraction signals of other impurities are detected (Tian et al., 2022; Feng et al., 2018). It is noteworthy that the full width at half maximum (FWHM) of each diffraction peak is less than 0.5, indicating that the prepared material has high crystallinity. The above structural characterization results fully demonstrate that the experiment successfully obtained Co₉S₈ nanomaterials with single phase and high crystallinity by optimizing the hydrothermal synthesis conditions. Figure 2B shows the scanning electron microscopy (SEM) image of Co₉S₈. It can be seen from the figure that the synthesized sample has an irregular shape, consisting of smaller nanoparticles clustered into larger structures. The exposed active sites of this large particle structure are reduced, leading to a decrease in the effective contact area between the electrode and the electrolyte, and a reduction in the interfacial reaction activity (Wang et al., 2015).

The surface chemical composition and elemental electronic states of the material surface are systematically analyzed by X-ray photoelectron spectroscopy (XPS). By deeply analyzing the characteristics of each peak in the XPS spectrum, the chemical valence states and chemical bond properties of elements such as cobalt and sulfur in the sample can be inferred. The high-resolution spectrum of S 2p (Figure 3A) was deconvoluted into six distinct peaks after peak fitting. The double peaks at 161.5 eV and 162.6 eV correspond to the S 2p3/2 and S 2p1/2 orbitals of sulfide species (S2-), respectively (Feng et al., 2016; Feng et al., 2015). The characteristic peaks at 163.8 eV and 165.1 eV are attributed to disulfide (S2- 2) and polysulfide (S2- n) species, consistent with the crystal structure characteristics of Co₉S₈ (Tian et al., 2022; Zhou et al., 2015; Lv et al., 2024). Additional peaks at 166.6 eV and 168.6 eV indicate the presence of surface-oxidized sulfur species (SOx), likely due to partial oxidation of surface sulfur species during prolonged air exposure (Wang et al., 2025; Lv et al., 2024). The Co 2p high-resolution spectrum (Figure 3B) displays typical multiple splitting characteristics, mainly including a pair of Co 2p3/2, a pair of Co 2p1/2, and a pair of satellite peaks. Among them, the main peaks at 780.1 eV (Co 2p3/2) and 795.9 eV (Co 2p1/2) are assigned to Co³⁺ species, while the secondary peaks at 781.7 eV (Co 2p3/2) and 797.3 eV (Co 2p1/2) originate from Co²⁺ species (Tian et al., 2022; Wang et al., 2025; Zhou et al., 2015; Lv et al., 2024). In addition, satellite peaks observed at 786.7 eV and 802.2 eV further confirm the multivalent nature of cobalt, which is closely related to the complex electronic structure of Co₉S₈.

To systematically evaluate the energy storage performance of Co₉S₈ electrode in AIBs, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and constant current charge-discharge tests were conducted to comprehensively characterize its electrochemical behavior (Figure 4). The CV test was conducted at a scan rate of 0.5 mV s⁻¹ within a voltage window of 0.1–1.8 V to elucidate the electrochemical activity of the electrode, with the corresponding curve shown in Figure 4A. It can be observed from the figure that all CV curves exhibit a pair of positive-negative peaks at approximately 1.41 V and 0.72 V, respectively. It is noteworthy that CV curves shows a significant polarization gap (ΔE = 0.69 V) in the electrode. This significant polarization is attributed to sluggish ion diffusion kinetics caused by the large particle size, which prolongs the Al³⁺ transport pathway from the surface to the bulk. Additionally, CV curves of the Co₉S₈ electrode shows broad and weak characteristic peaks, indicating that its electrochemical reaction kinetics are significantly inhibited, which may be related to insufficient active site exposure and structural constraints (Lou et al., 2020; Luo et al., 2023; Kope, 2011). Figure 4B shows the EIS spectrum of the Co₉S₈ electrode, and the fitted curve clearly demonstrates a high degree of agreement with the experimental data. As can be seen from the figure, the EIS spectrum consists of a semicircle and a diagonal line. Specifically, the slanted line in the low-frequency range reflects the ion diffusion capability in the electrolyte, while the semicircle in the high-frequency range corresponds to the electrode/electrolyte interface charge transfer resistance (Rct) (Panigrahi et al., 2024). For the EIS spectrum of the Co₉S₈ electrode, the Rct value is as high as 186 Ω, which is directly related to the reduction in the electrode/electrolyte contact area caused by its large particle morphology. The large angle between the straight line in the low-frequency region and the real axis in the EIS spectrum indicates that the ion diffusion process is relatively slow (Singh et al., 2017; Zeng et al., 2024). Figure 4C shows the long-term cycling performance curve of AIBs using Co₉S₈ electrodes under the working condition of 100 mA g⁻¹. It should be noted that the Co₉S₈ electrode undergoes significant activation effects, which is similar to previous reports (Zhuang et al., 2021; Yang et al., 2019). The phenomenon of longer activation time for Co₉S₈ electrode may be attributed to the delayed accessibility of aluminum ions due to the maximum particle size of the surface-active substance, resulting in slower battery reaction kinetics (Zhaung et al., 2021; Elia et al., 2016). After activation, the electrode delivers a discharge capacity of about 65 mAh g⁻¹. Then, it undergoes a slow decay process and eventually stabilizes after 350 cycles (∼48 mAh g⁻¹). After that, the charge-discharge curve tends to a horizontal straight line, indicating that the electrode has good cycle stability. It is noteworthy that the Coulombic efficiency is stable at over 93%, indicating that the side reactions at the electrode interface are effectively suppressed.

Although the electrode exhibits good cycle stability, its discharge capacity is still significantly lower than the practical application requirements of AIBs. To elucidate the intrinsic capacity limitations, this study finally combined density functional theory (DFT) calculations with multiscale simulation methods to systematically explore the crystal structure characteristics and Al³⁺ storage behavior of Co₉S₈. Figure 5 shows the crystal structure model of Co₉S₈ and the energetics analysis results of different Al³⁺ storage mechanisms. Figure 5A illustrates the structural configuration of the Co₉S₈ primitive unit cell, which consists of 17 atoms in total. Specifically, this configuration comprises one octahedrally coordinated Co metal atom (denoted as M(O)), eight tetrahedrally coordinated Co metal atoms (denoted as M(T)), two bridging sulfur atoms (S(l)), and six face-capping sulfur atoms (S(f)). These structural characteristics reveal that sulfur atoms engage in distinct coordination modes (bridging and face-capping) to construct a three-dimensional metal sulfide framework. This structural arrangement provides critical insights into understanding the physicochemical properties of the compound, establishing essential correlations between atomic coordination environments and macroscopic material behavior. Based on the VASP software package, the PAW-PBE functional was used to perform geometric optimization on the cubic phase of Co₉S₈ supercell (space group Fm3m, No. 225), and a 3 × 3 × 3 supercell model was constructed as shown in Figure 5B. After optimization, the lattice constant a = 9.842 Å, with a deviation of less than 0.3% from the benchmark value in the Materials Project database, verifying the reliability of the computational method. Structural analysis showed that the optimized Co₉S₈ supercell exhibited a typical cobalt sulfide structure, with 3D ion channels composed of octahedral interstices and tetrahedral vacancies providing a topologically adapted transport pathway for Al³⁺ diffusion. There was a significant mismatch between this size and the diameter of Al³⁺ ions, and the spatial confinement effect of this structure may be a key factor contributing to the kinetic retardation of ion diffusion.

For the storage site mechanism of Al³⁺, the system considers two possible substitution mechanisms, namely, Al-Co substitution mechanism and Al-S substitution mechanism, and constructs two typical substitution models (Figures 5C,D). Al³⁺ occupies the octahedral Co. atom sites (Figure 5C), and the total energy of the crystal after substitution is −414.11 eV, with a calculated formation energy ΔEsub-Co of 0.92 eV. Figure 5D is a schematic diagram of another substitution structure of A³⁺, where Al³⁺ occupies the tetrahedral S atom sites, and the total energy of the crystal after substitution is −416.03 eV, with a formation energy ΔEsub-S of 1.98 eV. Although the formation energies obtained in both cases are within the thermodynamically acceptable range, they still belong to positive values, indicating the presence of resistance in the process. By comparing the formation energy, a lower formation energy indicates that the reaction of Al substituting for Co atoms is more thermodynamically favorable. Therefore, it can conclude that aluminum is more likely to replace Co in the Co₉S₈ electrode system, and the charge-discharge mechanism of the Co₉S₈ electrode system for AIBs is also more likely to be the mechanism of Al ion substitution reaction. To evaluate the structural stability, XRD analysis was performed on the cycled electrode (Figure 6A). Comparison reveals that the XRD pattern of the cycled electrode is largely unchanged from that of the pristine material (Figure 2A), with the exception of reduced intensity in certain diffraction peaks. This indicates excellent structural integrity of the electrode material after multiple cycles. The charge-discharge process of Co₉S₈ electrode system can be summarized as shown in Figure 6B.

This study reveals the capacity limiting factors and energy storage mechanism of Co₉S₈-based electrodes through a deep combination of experimental characterization and theoretical simulation, providing a clear direction for subsequent material design. Based on the above mechanism, in the future, it is possible to design gradient solid solutions, such as using transition metals such as Fe and Ni to partially replace Co. to form (Co.,M)9S8 solid solutions, and expand the size of ion channels through lattice regulation. This integrated experimental-theoretical approach provides critical insights into the performance limitations of Co₉S₈-based electrodes, guiding rational design strategies for advanced AIBs cathode materials.

This study successfully synthesized Co₉S₈ nanoparticles via a one-step hydrothermal method, and systematically investigated their energy storage mechanism and performance-limiting factors in AIBs. Structural characterization revealed that the synthesized material formed micron-sized agglomerates consisting of interconnected primary nanoparticles. XPS analysis demonstrated the coexistence of mixed Co²⁺/Co³⁺ valence states on the material surface, accompanied by significant charge delocalization in the S 2p orbital, which collectively established multiple electron transfer pathways at the electrode-electrolyte interface. Electrochemical tests showed that although the 3D ionic channels of the Co₉S₈ cubic phase are theoretically beneficial for the transport of Al³⁺, the material agglomeration morphology leads to significant interfacial charge transfer resistance (Rct = 128 Ω) and electrode polarization (ΔE = 0.69 V), ultimately limiting its discharge specific capacity to 48 mAh g⁻¹ at 100 mA g⁻¹ over 500 cycles. Density functional theory (DFT) calculations revealed a “pseudo-isomorphic substitution” mechanism, where Al³⁺ ions preferentially occupy cobalt lattice sites with a favorable substitution energy of 0.92 eV. The comprehensive investigation electrochemical behavior and pseudo-isomorphic substitution mechanism of Co₉S₈ based cathode establishes an atomic-scale theoretical framework for rational electrode design in AIBs. In the future, the capacity can be improved focus on synergistic engineering approaches. For example, optimizing crystallographic orientation to enhance the diffusion kinetics of Al³⁺ through low-energy migration and regulating electronic structure through anion/cation co doping to promote multi-electron redox reactions.