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Scientific Reports volume 14, Article number: 28468 (2024 ) Cite this article indium oxide 9999 99999
Rechargeable aluminum-ion batteries (AIBs) stand out as a potential cornerstone for future battery technology, thanks to the widespread availability, affordability, and high charge capacity of aluminum. However, the efficacy of current AIBs on the market is significantly limited by the charge storage process within their graphite cathodes. To fully realize the capabilities of AIBs, the discovery of a new cathode material is essential. Transition metal sulfides present an attractive option for cathode materials, although there has been a variety of conflicting reports regarding the exact nature of their charge storage mechanisms. This paper investigates cobalt sulfide (CoSx) cathodes in AIBs, with a particular focus on deciphering the mechanisms of charge storage. Through synthesis, electrochemical testing, and post-cycling characterization, we illuminate the roles of AlCl4− intercalation, cobalt sulfide to Al2S3 conversion, and sulfur to Al2S3 conversion in charge storage. As cycling progresses, Al2S3 synthesis from segregated sulfur segments emerged as the predominant mechanism, showcasing its potential to fully leverage the high capacity of aluminum metal and propel AIBs towards higher energy densities. Despite these promising findings, the study also uncovered significant challenges, notably material loss, intra-cathode diffusion limitations, and irreversible reactions that precipitously diminish charge capacity over time. These issues highlight the critical need for enhanced electrode stability, improved electrolyte compatibility, and accelerated aluminum diffusion. The research paves the way for further exploration of transition metal sulfides as cathode materials in AIBs, highlighting the imperative for innovations that bolster mechanical and chemical stability while optimizing ion transport. This work not only contributes to the fundamental understanding of charge storage in AIBs but also charts a course for the development of more durable and efficient battery systems.
Since their inception, lithium-ion batteries (LIBs) have revolutionized electrical energy storage, paving the way for the widespread adoption of electric vehicles and the enhancement of personal electronics, feats that were once unattainable1. However, the surge in demand for electrical energy storage is outpacing the production capabilities of LIBs, primarily due to the constraints in lithium metal extraction2,3. Consequently, there is a pressing need for the development of a new battery chemistry that does not rely on lithium. Among various metals under consideration, aluminum stands out due to its abundance, availability, and cost-effectiveness4. Additionally, aluminum exhibits superior electrochemical properties, with the trivalent aluminum ion offering the highest theoretical charge capacity among all potential lithium alternatives, boasting a volumetric charge density of 8056 mAh cm− 3 and a gravimetric charge density of 2596 mAh g− 15. Despite these advantages, the overall performance of aluminum-ion batteries (AIBs) is currently hampered by the limitations of their cathode materials. The majority of research on AIB cathodes has concentrated on graphite-based materials that accommodate aluminum tetrachloride (\(\:{AlCl}_{4}^{-}\) ) ions during the charging process. Yet, these graphite cathodes offer a modest theoretical charge capacity of 128 mAh g–16,7, lower than that of LIB cathodes employing nickel-manganese-cobalt oxide (NMC)8. Furthermore, charging AIBs with graphite electrodes consists of the insertion of \(\:{AlCl}_{4}^{-}\) into the cathode (as described in reaction 1), which is not symmetrical to the charging reaction occurring at the anode (outlined in reaction 2). As a result, the overall charging reaction (shown in reaction 3) reduces the Al2Cl7− concentration of the electrolyte. This asymmetry means that in any AIB employing graphite cathodes, the maximal charge density of the anode, and thus the cell, is not determined by the charge-dense aluminum metal, but by the availability of Al2Cl7− ions9. A mixture of the commonly used \(\:\left[EMIMCl\right]\left[AlC{l}_{3}\right]\) electrolyte has a maximum charge capacity of only 49 mAh g− 19. Therefore, even in optimal conditions, an AIB with graphite cathodes has a maximum capacity of only 36 mAh per gram of active material (Qam). At approximately 25% of the Qam of current LIBs, this system is clearly insufficient9.
To circumvent this limitation, it is crucial to develop a new cathode for AIBs capable of storing charge symmetrically to the anode’s plating reaction (as described in a generalized reaction 4), resulting in a full cell reaction which does not alter the overall electrolyte composition (as recorded in reaction 5). To achieve this symmetry, the most promising charge storage mechanisms are Al3+ intercalation reactions, where the aluminum ions are inserted in the cathode’s existing lattice structure, and conversion reactions, where the aluminum interacts with the cathode’s active material to form a new chemical compound. By storing charge utilizing one of these mechanisms, a cathode with a charge capacity of only 100 mAh g− 1 would enable an overall cell charge capacity of Qam = 97 mAh g− 1. This advancement would represent a significant step towards enhancing the energy density and overall efficiency of AIBs, moving them closer to or even surpassing the performance benchmarks set by current LIBs.
For the implementation of these charge storage mechanisms, transition metal sulfides (TMS) have emerged as a highly promising category of materials. Both the intercalation of Al3+ ions and conversion reactions have been observed in various TMS cathodes, as reported by Leisegang et al. in 2019 10. Among these materials, cobalt sulfide stands out due to its impressive conductivity and stability within aggressive electrolytes, a finding supported by Li et al. in 2014 11. This stability and conductivity have facilitated the analysis of charge storage mechanisms in other battery systems, as noted by Grindal and Azimi in 2024 12. Additionally, cobalt sulfide has shown remarkable stability and longevity in previous AIB studies, with its performance metrics documented in Table 1. This combination of properties makes cobalt sulfide an attractive candidate for addressing the current limitations in AIB cathode materials, potentially leading to the development of more efficient and durable batteries.
Despite the potential of cobalt sulfide in AIBs, the research on this system is still in its infancy, with ample room for advancement. A notable issue is that the charge capacity of these cells has yet to achieve and maintain the computationally-derived theoretical maximum of 294 mAh g− 1 for the most charge dense cobalt sulfide, CoS213. More critical is the lack of a systematic approach in studying the charge storage mechanism within these batteries. Various studies have employed a wide array of techniques to discern the underlying reactions, including X-ray photoelectron spectroscopy14,15,16,17, density functional theory simulations18, both in situ and post-situ Raman spectroscopy14,17, ex-situ energy dispersive X-ray spectroscopy14,17, and in situ and post-situ X-ray diffraction14,16,17. This diversity in methodology has led to inconsistent and sometimes contradictory findings regarding the charge storage mechanisms in different cobalt sulfides, with some studies suggesting conversion reactions and others pointing to Al3+ intercalation.
Attempts to diagnose the reaction by electrochemical measurement have likewise produced contradictory results17,19. Cyclic voltammetry in the previous cobalt sulfide studies produced midpoint potentials from 0.45 to 1.2 V, a wide range indicative of multiple different reactions. Possible reactions in this voltage window include Al2S3 formation from Co9S8 (0.56 V), Co3S4 (0.58 V), CoS2 (0.76 V), and pure sulfur (1.03 V), as well as Al3+ intercalation in CoS2 (0.79 V), as computationally-derived from the Materials Project database13.
The disparities in these findings underscore a critical need for a more coordinated and comprehensive investigation into the charge storage mechanisms of cobalt sulfide cathodes in AIBs. Resolving these uncertainties is essential for advancing our understanding and optimization of AIB chemistry. This study aims to systematically explore the system and mechanisms involved, pinpointing the limitations that have led to the observed discrepancies. By establishing a clear understanding of the charge storage processes, this work seeks to pave the way for the realization of AIBs with enhanced performance and reliability, moving closer to exploiting their full potential in practical applications.
In this study, a conclusive investigation into the charge storage mechanisms of cobalt sulfide cathodes in AIBs was conducted. Cobalt sulfide crystals were synthesized within a carbon nanotube matrix to create a conducive and stable environment for electrochemical reactions and were subjected to a thorough examination through both electrochemical and physical characterization techniques. Extensive characterization throughout the cell’s cycling lifespan was conducted, including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), ex-situ energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), and electrical impedance spectroscopy (EIS).
These techniques determined that the conversion to Al2S3 plays the key role in charge storage within these systems. Additionally, it was observed that AlCl4− intercalation in the carbon matrix occurred at the upper voltage limit of the stable window. The findings highlight the importance of addressing certain key areas to enhance the overall performance and longevity of AIBs. Future research and development efforts should focus on optimizing cobalt and sulfur retention within the electrode, a critical factor in extending cell life. Furthermore, improving the rate of aluminum diffusion within the cathode is essential for enhancing the system’s capacity and current density. Such advancements are necessary to overcome current limitations and unlock the full potential of cobalt sulfide-based AIBs, paving the way for their practical application in high-demand energy storage solutions.
This work goes beyond previous investigations of cobalt sulfide, aiming to systematically determine the charge storage mechanism. This is the first work to characterize cycled cobalt sulfide cathodes in both charged and discharged states using a combination of HRTEM, SAED, EDX, and XPS. In doing so, this work aims to improve the state-of-the-art understanding of the cobalt sulfide cathodes in AIBs.
The synthesis of the precursor ZIF-67 was confirmed by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), as illustrated in Fig. 1. The XRD results, depicted in Fig. 1a, displayed peaks that aligned with both the angle and relative intensity of those expected for ZIF-67, similar to its ZIF-8 counterpart, as noted in the powder diffraction file [PDF 00–062–1030]24. Notably, peaks were observed at 2θ angles of 10.4°, 12.7°, 14.7°, 16.4°, 18°, 24.5°, 26.5°, and 29.6°, corresponding to the (002), (112), (022), (013), (222), (233), (134), and (044) planes, respectively, confirming the expected crystalline structure.
The SEM imaging, showcased in Fig. 1b and c, revealed the ZIF-67 powder predominantly consisted of smooth, rhombohedral shapes ranging from 200 to 1000 nm in size, consistent with the anticipated morphology of ZIF-67 and indicative of a successful synthesis process. Additionally, a minority of particles exhibited diameters less than 100 nm; these are presumed to be smaller ZIF-67 particles that nucleated later in the process and did not undergo significant growth.
Characterization of the synthesized ZIF-67 particles by (a) XRD, as compared against the ZIF-67-equivelent ZIF-8 [PDF 00–062–1030], and by SEM at (b) low and (c) high magnification.
The CoSx-CNT composite was then synthesized by the two-stage pyrolysis process, as detailed in the “Materials and Methods” section, to create the cobalt sulfide crystal structure shown in Fig. 2a. XRD analysis of the resultant black powder is presented in Fig. 2b. The exhibited pattern most closely matches that of Co9S8 [DB 00–004–4525], with space group Fm3̅m. The crystalline peaks of this material can be observed at 2θ = 29.8°, 31.2°, 47.6°, and 52.1°, corresponding to the (311), (222), (511), and (440) planes of Co9S8, respectively. Additionally, the XRD pattern showed distinct smaller and broader peaks indicative of amorphous carbon [DB 00–023–0064] at 2θ = 26.5°, 44.6°, and 54.6°, associated with the (002), (101), and (004) planes, respectively. It is possible that the Co9S8 diffraction pattern concealed the diffraction patterns of other cobalt sulfides such as Co3S4 [DB 00-047-1738] and CoS2 [DB 00-041-1471]. However, Co9S8 is the primary cobalt sulfide.
Subsequent chemical analysis of the bulk material was conducted using Raman spectroscopy. The Raman spectra, depicted in Fig. 2c, featured sharp peaks at 667, 604, 515, 470, and 186 cm− 1, characteristic of Co9S821,25, alongside broad peaks around 1370 and 1580 cm− 1, representing the D and G bands of graphite26. Given these XRD and Raman results, it can be asserted that Co9S8 and graphite were the primary phases of the bulk synthesized material.
The X-ray photoelectron spectroscopy (XPS) spectra of the material, shown in Fig. 2d, was also collected to determine the surface composition of the synthesized material. The presence of Co 2p peaks in Fig. 2e at 783.3/798.3 eV and 778.1/793.2 eV corresponded to the Co2+ and Co0 states, respectively27. The detection of this Co0 peak specifically suggests the formation of Co9S8, as detailed in previous studies28. Conversely, the most prominent features in the S 2p spectrum, illustrated in Fig. 2f, were peaks at 170.0 eV and 171.2 eV, which denoted the oxidation of surface sulfur to the S6+ sulfate form. Additional peaks at 161.4/162.6 eV and 163.8/165.0 eV were indicative of the presence of sulfide species and residual sulfur, respectively28. These observations indicated that the surface of the synthesized powder was primarily CoSO4 and graphitic carbon, with an additional presence of cobalt sulfides and cobalt oxides.
Characterization of the synthesized CoSx-CNT material, showing (a) the predicted crystal structures of Co9S8, (b) the XRD pattern of the synthesized material, with reference XRD patterns of Co9S8 [DB 00-004-4525] and graphite [DB 00-023-0064], (c) the recorded Raman spectra, with characteristic Co9S8 peaks indicated in blue and graphite peaks indicated in black, and the XPS spectra of (d) the entire energy range, (e) the Co 2p signal, and (f) the S 2p signals.
The morphology of synthesized material was characterized using SEM and transmission electron microscopy (TEM), revealing three distinct phases, as depicted in Fig. 3. Figure 3a showcases previously polyhedral architecture of ZIF-67 amalgamated into a substantial hierarchical microporous structure primarily composed of carbon. Carbon nanotubes (CNTs) formed during pyrolysis of the ZIF-67 are present on the particle surface. The CNTs, visible in the secondary electron image displayed in Fig. 3b, directly connect to the graphite matrix, ensuring electrical conductivity through the interconnected particles. Finally, the brightfield TEM image shown in Fig. 3c indicated the presence of nanocrystals containing high atomic number (high-Z) elements, both on the surface and within the carbon matrix. This multi-phase composition underscores the complex structural integration achieved through the synthesis process.
Electron microscope images of the synthesized material, by (a) SEM secondary electrons, (b) TEM secondary electrons, and (c) TEM brightfield imaging.
The elemental composition of these phases was further analyzed using energy dispersive X-ray spectroscopy (EDX) in conjunction with the TEM. In Fig. 4a-c, bright areas in the secondary electron image strongly correlated with the presence of cobalt and sulfur, suggesting that these regions were composed of cobalt sulfides. Figure 4d confirmed the primary composition of the matrix as carbon, which was uniformly distributed throughout the structure. The distribution of oxygen, shown in Fig. 4e, appeared to be loosely associated with sulfur, consistent with the sulfate presence observed in Fig. 2f. Oxygen was also present to a lesser extent over the entire surface, indicating a degree of surface oxidation.
TEM characterization of a single Co9S8-CNT particle by (a) secondary electrons, and (b–e) EDX elemental mapping.
Energy dispersive X-ray spectroscopy (EDX) was utilized to ascertain the elemental composition of both the carbon matrix and the cobalt sulfide phases within the synthesized material. The findings, summarized in Table 2, detail the elemental percentages, including the proportions of cobalt and sulfur in the bulk material as determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). According to the ICP-OES analysis, the bulk composition of the material consisted of 32.6 wt% cobalt and 14.6 wt% sulfur, with the remainder presumably being carbon. In contrast, the nanoparticles exhibited a composition of 48.9 wt% cobalt and 47.0 wt% sulfur, translating to atomic percentages of 32.4 at% cobalt and 57.3 at% sulfur. This ratio suggests that the nanocrystals were composed of various cobalt sulfide forms, ranging from CoS2 to Co9S8. Some excess sulfur may also have remained in an unreacted or sulfate state. Meanwhile, the carbon matrix was found to be predominantly carbon, demonstrating a distinct separation of elements into their respective phases.
TEM crystallographic measurements of the CoSx-CNT powder by (a) SAED and HRTEM (b–f). Crystals of (b) Co9S8 and metallic cobalt, (c) graphic carbon, and CNTs were observed, and their d-spacing was confirmed by FFT of the highlighted area (d–f).
The electrochemical behavior of the CoSx-CNT cathodes in AIBs was assessed by galvanic charge-discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The outcomes of these tests are depicted in Figs. 6 and 7.
From the results of the CV, it is apparent that charge storage occurred both by faradaic and capacitive methods. Faradaic reactions were clearly visible in Fig. 6a at all scan rates, with anodic peaks at 1.4 V and above 1.7 V, and cathodic peaks at 1.0 V and 0.7 V. Significant capacitive charge storage was also observable, likely resulting from the high surface area of the carbon component.
The faradaic and capacitive fractions of charge storage can be deconvoluted to achieve a better understanding of the mechanisms of charge storage. In a cyclic voltammogram, an imperfect capacitor is known to produce linear cathodic and anodic scans with constant, non-zero slopes. Thus, the capacitive charge storage can be estimated by extrapolating the linear regions of the cyclic voltammogram, as shown in Fig. 6b. By dividing the overall area under the curve by the area under this estimated capacitive region, the fraction of charge was stored by electric double layer capacitance can be determined. Using this methodology, it was determined that at high scan rates as much as 44% of charge storage resulted form capacitive charge storage.
In addition to the CV faradaic reactions at 1.4/0.7 V and > 1.7/1.0 V, the GCD profiles in Fig. 6c also revealed an additional anodic reaction at 0.85 V and a cathodic reaction at 0.52 V, which only occurred during the first two cycles. There are multiple potential sources for each observed faradic reaction. Regarding the reaction at 0.85 V and 0.52 V, previous investigations have claimed reactions in this range resulted from Al3+ intercalation into different cobalt sulfide13,14,18. Alternatively, the theoretical voltage of Co9S8, Co3S4, and CoS2 to Al2S3 conversion all lie in this range13. Finally, this reaction may have resulted from a side reaction with some contaminant, such as Al3+ intercalation in the surface cobalt oxide29.
In any case, the reaction is definitively irreversible. After the initial discharge, the charge capacity halved by the second cycle and was entirely depleted by the third. This irreversibility, observed in both conversion and Al3+ intercalation, might stem from the saturation of the cobalt sulfide surface layer with aluminum. Once aluminum storage occurs, the slow diffusion rate of aluminum through the lattice could effectively passivate it9. This passivation might prevent access to the internal structure of cobalt sulfide, thereby explaining the reduced charge capacity observed, which is lower than theoretical expectations. Additionally, material loss could account for these deficiencies and contribute to the limited lifespan of any reaction caused by a surface contaminant.
Beyond the initial cycles, the predominant charge transfer observed in the CV was associated with the reactions at 1.4 V and 0.7 V. An electrochemical reaction at this potentials has previously been attributed to the intercalation of Al3+ into vacancies within a cobalt sulfide lattice17. Alternatively, the formation of Al2S3 from sulfur is expected at E° = 1.03 V, in line with previous investigations of aluminum-sulfur batteries30. Regardless of the precise mechanism, the substantial peak separation indicates significant kinetic limitations, likely due to the high migration energy required for aluminum atoms within the lattice9.
The final observed faradaic reaction involved the charging and discharging peaks above 1.7 V and at 1.0 V, respectively. While these reactions have been observed in other studies involving cobalt sulfide, their occurrence at the edge of the voltage window is infrequently discussed14,16,19. Possible explanations for these reactions include the intercalation of \(\:{AlCl}_{4}^{-}\) ions into the carbon structure of the cathode23. Another theory posits electrolyte degradation, though this seems less likely given the reversibility of the reaction, and that the operational voltages are below the threshold for electrolyte breakdown31.
Quantitative charge capacity measurements were taken by GCD to evaluate the performance of the system under galvanic conditions. Figure 6c displays the GCD profile of a cell which was discharged and charged for 20 cycles at a low charge density of 25 mAh g− 1.
An initial charge capacity of 28 mAh g− 1 was observed, which then declined to just 15 mAh g− 1 after 20 cycles. This charge capacity was significantly lower than previous investigations, primarily due to the smaller extent of the faradaic reaction at 1.4/0.7 V. As this reaction is typically associated with the aluminum conversion or intercalation reaction with the sulfide, it can be asserted that only a small fraction of the initial 47.2 wt% cobalt and sulfur was both accessible to the electrolyte and in electrical contact with the current collector. Given the larger particle size of the active material synthesized here, the slow diffusion of the aluminum atom through the cobalt sulfide matrix was one possible cause of the low capacity. Immediate active material loss is another potential cause of the lower-than-expected charge capacity and is evidenced by the immediate decline in the cobalt and sulfide atomic fraction following the initial discharge (see Table 3).
Immediately following the initial charge-discharge, the 0.85/0.52 V reaction declined significantly, ceasing entirely by the third cycle. Afterwards, further declines in charge storage were caused by a decrease in the occurrence of the reaction observed at 1.4/0.7 V. As this reaction resulted from the incorporation of aluminum atoms in the cobalt sulfide matrix, possible causes for the decline include the irreversible filling of cobalt sulfide vacancies, irreversible conversion of the material to Al2S3, passivation of active material, or further loss of the active material into the electrolyte. The mechanism is investigated in more depth by direct characterization after cycling in Sect. 2.3. Given the poor quantitative performance of the material, long-term cycling performance was not quantified in depth.
Electrochemical characterization of the CoSx-CNT cathodes in AIBs, by (a) cyclic voltammetry at multiple sweep rates, (b) cyclic voltammogram showing the projected capacitive current, and (c) galvanic charge discharge of selected cycles at 25 mA g−1.
Electrochemical impedance spectroscopy (EIS) was utilized to analyze the internal structure of the cells and to understand how the structure evolved throughout cycling, with measurements taken before and after 100 cycles. The resulting Nyquist plots, illustrated in Fig. 7, revealed two distinct segments in both spectra. The first segment, a semi-circle observed at high frequencies, suggested the presence of a capacitive-resistive barrier that dominated the high-frequency behavior of the cell. The second segment, appearing as a straight line in the low-frequency region, indicated diffusion-controlled processes, consistent with Warburg impedance behavior. These elements together typify the structure of a conventional Randles cell, depicted in Fig. 7a.
There were three significant changes in the cell’s parameters after cycling. The resistance at the highest frequency, typically considered the electrolyte resistance, dropped significantly from 143.1 Ω to 21.3 Ω after 100 cycles. This change suggests that an insulating layer was initially present, possibly an oxide contaminant, but was removed during cycling.
Furthermore, the real impedance of the high frequency semi-circle halved during cycling. Prior to cycling, this semi-circle is typically considered the charge transfer resistance of the system. Often, following cycling a second distinct semi-circle would become visible, resulting from emergence of a capacitive-resistive solid electrolyte interface. In this case, however, a distinct semi-circle was not visible in the Nyquist plot; hence, the effect of the SEI cannot be deconvoluted from the effect of the charge transfer barrier. Regardless, after 100 cycles, the combined resistance of charge transfer and SEI was half that of the initial charge transfer resistance. These changes in the high frequency region were indicative of the replacement of an initial surface oxide on the cathode, likely with a small SEI, resulting in eased ion transport and improved electrode stability.
Finally, the phase angle in the low-frequency region of the Nyquist plot was initially 45°, indicative of semi-infinite Warburg diffusion, suggesting that the cell’s primary limitation at low frequencies was the diffusion of charge carriers through the electrolyte. At frequencies below 0.5 Hz, however, the phase angle increased, shifting towards finite space Warburg diffusion. This change indicated that at these lower frequencies, the electrochemical reaction rate was constrained by the rate at which charge carriers were consumed at the electrode surface, rather than by their diffusion through the electrolyte32.
After 100 cycles, the behavior in the low-frequency region exhibited a notable change; the phase angle remained above 45° across the entire linear region for frequencies below 10 Hz. This observation suggests that the cathode surface had reached a saturation point, affecting the cell’s performance even at medium frequencies. Therefore, the limiting factors in the aged cell were not just the diffusion of charge carriers through the electrolyte but also the rate of reaction and intra-cathode diffusion. The increased phase angle across a wider range of frequencies highlights a shift towards limitations imposed by the electrode surface’s ability to accommodate and react with incoming charge carriers, reflecting changes in the cell’s internal dynamics and efficiency over time.
EIS spectra of an AIB cell using a CoSx-CNT cathode prior to and after 100 cycles, showing (a) the full spectrum and (b) the high frequency spectrum.
To directly analyze the charge transfer mechanisms within the coin cells, three such cells were assembled, subjected to cycling, and subsequently dismantled for in-depth analysis of the cathodes’ compositional and structural evolution. The first cathode was recovered after a single discharge cycle, while the second and third cathodes were analyzed after enduring 100 cycles at 50 mAh g− 1, with one in the discharged state and the other in the charged state. These cathodes were characterized through TEM imaging, SAED, and XPS.
The SAED and TEM imagery for the cathode that underwent a single discharge are depicted in Fig. 8. The SAED pattern, as shown in Fig. 8a, unveiled a variety of crystalline structures, consistent with metallic cobalt, cobalt sulfides, and graphite—components initially identified in the material depicted in Fig. 5. Notably, new distinct d-spacing signals were observed at 2.4 Å and an additional ring corresponding to a d-spacing of 1.8 Å. These measurements are characteristic of Co3O4 (311) and Al(CoS2)2 (333) crystals, respectively, highlighting the initial presence of an oxide, as well as the intercalation of aluminum within the cobalt sulfide lattice13. The detection of Al(CoS2)2 was further corroborated by high-resolution TEM (HRTEM) in Fig. 8c and d, where a d-spacing of 5.6 Å pointed to the (011) plane’s presence. Collectively, the TEM analyses lend substantial support to the hypotheses that the initial reactions observed at 0.85/0.52 V in Fig. 6a can be attributed to the aluminum intercalation in the cobalt sulfide.
Characterization of an AIB CoSx-CNT cathode after a single discharge, by (a) SAED, (b) brightfield TEM, and (c, d) HRTEM, with lattice spacing evaluated by FFT analysis.
After undergoing 100 cycles, the SAED pattern for the discharged electrode demonstrated a notable transformation, illustrated in Fig. 9a. This pattern lacked any distinct lattice fringes or rings with d-spacings characteristic of cobalt sulfides or aluminum-intercalated cobalt sulfide, signaling a significant shift in the electrode’s composition. The diffraction pattern revealed only the presence of metallic cobalt and graphite, as well as Al2S3. This observation was echoed in the HRTEM lattice fringe measurements depicted in Fig. 9c-f, which confirmed these findings. Additionally, the cobalt oxide signal was no longer present, indicating the surface layer was removed, as indicated by the EIS results in Fig. 7.
These results collectively suggest that, following 100 cycles, the predominant chemical charge storage mechanism within the electrode transitioned to a conversion reaction producing aluminum sulfide. Given that the midpoint potential of this reaction by the hundredth cycle is approximately 1 V, this Al2S3 formation is likely the result of a reaction with pure sulfur in the electrode, as described in reaction 6. This volume of pure sulfur appears to be the result of substantial segregation of cobalt from sulfur during cycling, which also explains the increase in metallic cobalt signals. This alteration underscores a shift from mixed charge storage mechanisms to a reliance on aluminum sulfide conversion for electrochemical energy storage in the evaluated system.
Characterization of a discharged AIB CoSx-CNT cathode after 100 cycles, by (a) SAED, (b) brightfield TEM, and (c–f) HRTEM, with lattice spacing evaluated by FFT analysis.
In the analysis of the charged cathode after 100 cycles, the SAED pattern, depicted in Fig. 10a, showcased a stark contrast with earlier observations. This pattern lacked clearly distinguishable individual crystals, excluding four faint signals corresponding to the Co9S8 (311) plane. The remaining detectable features were polycrystalline rings with d-spacings indicative of metallic cobalt and graphite, suggesting the development of a simplified phase composition after extensive cycling, and confirming the segregation of the most cobalt from sulfur, as observed from Fig. 9.
The HRTEM lattice fringe measurements, shown in Fig. 10c-e, also confirmed the presence of metallic cobalt, and indicated the presence of isolated CoS2. Nonetheless, the comprehensive TEM assessment of the cycled charged cathode revealed a substantial depletion of crystalline structures including sulfur. This erosion of material, likely resulting from dissolution into the electrolyte, provides another plausible explanation for the observed decline in charge capacity over the course of 100 cycles. This phenomenon underscores the challenge of maintaining the integrity and electrochemical performance of cathode materials in the face of prolonged cycling, particularly in the context of complex multi-phase systems such as those involving cobalt sulfide-carbon composites in aluminum-ion batteries.
Characterization of a charged AIB CoSx-CNT cathode after 100 cycles, by (a) SAED, (b) brightfield TEM, and (c–e) HRTEM, with lattice spacing evaluated by FFT analysis.
Energy dispersive X-ray spectroscopy (EDX) analyses were also carried out on the three cycled cathodes using the TEM. The determined elemental compositions, alongside the initial elemental composition of the CoSx-CNT powder as determined by EDX, are summarized in Table 3. The spatial distributions of cobalt and sulfur are shown in Fig. 11. The data indicates three discernible trends.
First, there was a marked reduction in the concentrations of both cobalt and sulfur in the cathodes, especially noticeable after the first discharge. Sulfur experienced a more pronounced reduction compared to cobalt; a phenomenon likely due the ease of polysulfide formation33. Interestingly, the sulfur content in the discharged cathode increased towards the end of the cell’s life, suggesting cyclic shuttling of sulfur to the cathode during the reduction phase later in the cell’s life33. During all three EDX measurements, it can be seen from the elemental spatial mapping that high sulfur densities were only present in regions which also contained a high density of cobalt. In contrast, several regions with a high cobalt concentration displayed little sulfur presence, particularly after long term cycling. This elemental distribution is further evidence of the presence of cobalt sulfide and metallic cobalt.
Second, there was a gradual increase in the aluminum content within the cathode over the life of the cell, with the discharged cathode exhibiting 1.1 at% aluminum by the 100th cycle. The absence of a chlorine signal in the EDX data indicated that the observed aluminum was incorporated by Al3+ intercalation or via aluminum sulfide conversion reactions. In contrast, the once-discharged cathode and the aged charged cathode were measured to contain a small fraction of chlorine, implying infiltration of electrolyte into the cathode, as well as potentially AlCl4− interaction in the charged cell.
Lastly, the notable aluminum signal in the aged charged cathode indicated that a significant fraction of the aluminum was stored irreversibly, and could not be expelled during the charging process. This irreversible storage mechanism likely contributed to the observed capacity fading over time.
EDX elemental separation of cobalt and sulfur after (a, b) a single discharge, (c, d) 100 cycles ending in the discharged stage, and (e, f) 100 cycles ending in the charged stage.
XPS was also employed to probe the surface chemistry of each cathode, offering a complementary perspective to the bulk analysis provided by EDX. The survey scan spectra, presented in Fig. 12, together with the quantitative elemental compositions outlined in Table 4, revealed trends in surface composition that largely mirrored those observed in the bulk material. Specifically, both cobalt and sulfur concentrations exhibited a decline through the cycling process, with a notable decrease in sulfur. The aluminum content on the cathode surface was found to be higher in the discharged state and increased as the cell aged.
Two distinct discrepancies between the XPS and EDX findings were identified. First, the sulfur content on the cathode surface exceeded the bulk sulfur concentration. This discrepancy supports the hypothesis of sulfur dissolution into the electrolyte and the cyclic shuttling of sulfur between electrodes. Second, the chlorine concentration was observed to be higher in the cycled discharged cathode compared with the cycled charged cathode. This variation could likely be attributed to alterations in the solid-electrolyte interphase (SEI) during the discharge process, which may increase the chlorine content detected on the cathode surface. Alternatively, this may result from increased electrolyte infiltration in the discharged cathode’s surface.
These observations underscore the complex interplay between surface and bulk chemistry in influencing the electrochemical performance and stability of battery cathodes, highlighting the critical role of surface processes such as electrolyte interaction and SEI dynamics in the overall behavior of aluminum-ion batteries.
XPS survey spectra of AIB CoSx-CNT electrodes dissembled after (a) a single discharge, (b) 100 cycles ending in the discharged stage, and (c) 100 cycles ending in the charged stage.
The regional XPS scans shown in Fig. 13 offered additional insight into the evolving surface chemistry of the cathodes throughout cycling. Aluminum 2p signals visible in Fig. 13a indicated that the concentration of aluminum was highest in the discharged cycled cathode. The 2p peak also shifted to a lower binding energy compared with the single-discharged electrode, from 75.4 eV to 74.8 eV during cycling, suggesting a change in aluminum bonding’s character. These peak energies suggest the initially observed aluminum on the surface resulted from an aluminum chloride, but after cycling the aluminum signal was observed from less polarized Al2S3 instead34,35.
As shown in Fig. 13b, the Cl 2p peaks at 198.5/199.6 eV confirmed that the detected chlorine was bonded with aluminum, reflecting electrolyte remnants on the cathode surface36. The slight initial shift of the peak to a higher binding energy may also indicate some chlorine-carbon bonding on the cathode surface37.
The cobalt 2p scan in Fig. 13c indicated most of the transition metal was initially present in the Co2+ state, which suggested the presence of cobalt sulfides or oxides. However, as cycling proceeded, an increase in the relative Co0 signals was observed, suggesting a decrease in the presence of surface cobalt sulfides or oxides. This increased presence of metallic cobalt is in line with the SAED observations in Figs. 9 and 10. This shift could reflect the cycling-induced transformation of cobalt species, or the removal of the surface oxide exposing the underlying lattice.
Finally, in the sulfur 2p scan, the initial prominence of the sulfate S6+ signal indicated significant preliminary surface oxidation. Over time, this sulfate presence diminished, supplanted by signals corresponding to S0 and S2−, evidence of the removal of the oxide layer, the expose of underlaying cobalt sulfide, and the emergence of a distinct sulfur phase28.
XPS comparison of signals at different stages of a cell’s lifetime, measuring the (a) Al 2p, (b) Cl 2p, (c) Co 2p, and (d) S 2p signals.
This study explored cobalt sulfide as a cathode material for aluminum-ion batteries (AIBs), aiming to definitively confirm or disprove the charge storage mechanisms claimed by previous studies. Through synthesis, electrochemical evaluation and post-cycling analysis, this research uncovered that charge storage initially occurred by processes including Al3+ intercalation into the cobalt sulfide lattice, Al2S3 formation from both cobalt sulfide and sulfur, and AlCl4− intercalation in the accompanying carbon component. As cycling progressed, the formation of Al2S3 emerged as the main ongoing charge storage mechanism. In the future, this mechanism may be used to enable the production of highly energy dense AIBs utilizing the full potential capacity of aluminum metal.
However, the investigation revealed significant remaining challenges. The performance of the system was hampered by material loss, irreversible reactions, and diffusion through the solid electrode, which led to a precipitous decline in charge capacity over time. These phenomena underscore the necessity for targeted improvements in the electrode material.
To advance the field of AIBs, future research should focus on enhancing the mechanical and chemical robustness of the electrodes, optimizing their interaction with the electrolyte for improved stability, and engineering solutions to expedite the diffusion of aluminum ions through the active material. These strategies may include the development of novel material composites, surface coatings to prevent direct interaction of the active material with the electrolyte, and the exploration of electrolyte formulations that can better facilitate ion transport and reduce the formation of detrimental side products.
While cobalt sulfide cathodes exhibit promising characteristics for AIBs, realizing their potential demands a multifaceted approach to address the inherent limitations observed in this study. Continued investigation into transition metal sulfides as AIB cathodes is warranted, with a focus on achieving a balance between high energy density, high current density, and long-term stability. This pursuit is not only crucial for advancing aluminum-ion battery technology but also for meeting the growing demand for sustainable and high-performing energy storage solutions.
Materials used in this study were procured from various suppliers as follows: Aluminum foil (50 μm thickness, 99.999% purity), polypropylene-laminated aluminum film (113 μm thickness), heat melt polymer adhesive (0.1 mm thickness), and molybdenum sheets (130 μm thickness, 99.95% purity) were obtained from Beijing Loyaltarget Tech. Co., Ltd., China. UV-transparent quartz slides (25.4 × 25.4 × 1 mm) were sourced from Ted Pella, Inc. Stainless steel coin cell casings and springs were acquired from Xiamen TOB New Energy Technology Co., Ltd. Super P carbon black conductive agent (99% purity) was purchased from Alfa Aesar. Trimethylamine hydrochloride (TMAHCl, > 97% purity) was supplied by TCI Chemicals. Concentrated nitric acid (ACS reagent grade) and polytetrafluoroethylene were procured from Fischer Scientific. Glass microfiber separator (Whatman GF/A), polyvinylidene fluoride filter membranes (Durapore 0.45 μm), anhydrous aluminum chloride (AlCl3, > 99.99% purity), 1-methyl-2-pyrrolidinone (NMP, anhydrous, > 99% purity), cobalt (II) nitrate hexahydrate (≥ 98% purity), 2-methylimidazole (99% purity), and sulfur powder (≥ 99.0% purity) were all purchased from Sigma-Aldrich Co., USA.
The synthesis of the precursor ZIF-67 (Zeolite imidazole framework) followed protocols established in prior research20,21,22, involving the dissolution of cobalt (II) nitrate hexahydrate and 2-methylimidazole in methanol at a mass ratio of 0.557:1. The mixture was stirred for 30 min and left to stand for 24 h. Subsequently, ZIF-67 was isolated from the solution using vacuum filtration, washed three times with methanol, and then dried in an oven at 65 °C for 24 h to evaporate any residual solvent. The dried solid was pulverized into a fine powder with a mortar and pestle.
For the fabrication of the CoSx-CNT composite, the powdered ZIF-67 underwent thermal treatment at 900 °C for four hours in a nitrogen environment, degrading into cobalt nanocrystals on an interconnected carbon nanotube matrix. The resultant black material was combined with sulfur powder at a mass ratio of 5.6:1 and homogenized using a mortar and pestle for 20 min. This blend was then subjected to heat treatment at 400 °C for two hours under a nitrogen atmosphere. The final black powder was cleansed with ethanol three times to complete the synthesis process.
Crystallographic analysis was carried out through X-ray diffraction (XRD) using a Rigaku MiniFlex 600, with the XRD patterns obtained at a scanning speed of 0.01° per second and utilizing a Cu Kα X-ray source at 8.04 keV. The Rigaku PDXL software was employed for phase identification based on the collected data.
Surface morphology was examined by scanning electron microscopy (SEM) on a Hitachi ultra-high-resolution SEM SU7000, which features a ZrO/W Schottky electron emitter for the electron source. Additionally, transmission electron microscopy (TEM) imaging and selected area electron diffraction (SAED) analyses were performed with a Hitachi HF3300-environmental-CFE-TEM, equipped with a cold field emission electron source.
The compositional analysis of the synthesized material’s phases was conducted using energy dispersive X-ray spectroscopy (EDX) integrated into the TEM setup. For a comprehensive elemental characterization of the bulk material, inductively coupled plasma optical emission spectroscopy (ICP-OES) was utilized, employing an Optima 8000 ICP-OES Spectrometer (PerkinElmer). The sample preparation for ICP-OES involved dissolving the material in aqua regia and heating at 60 °C for 24 h, followed by filtration to remove any insoluble carbon content using a syringe filter. The samples were then prepared for ICP-OES analysis with an auto-diluter (Hamilton Microlab), maintaining a balance with 5 wt% nitric acid. The concentration of cobalt was determined by measuring the emission at a wavelength of 228.616 nm, while sulfur concentration was measured through optical emissions at a wavelength of 181.975 nm.
Raman spectroscopy was utilized to investigate the chemical bonding within the bulk material, employing a Bruker SENTERRA dispersive Raman microscope. This instrument was set up with a 2 mW 532 nm laser and utilized a 50 × 1000 μm2 aperture, performing measurements with a single coaddition over an accumulation time of 10 s.
For surface chemical analysis, X-ray photoelectron spectroscopy (XPS) was conducted. Initial XPS analysis of the synthesized powder was performed using a Kratos AXIS Supra X-ray photoelectron spectrometer, employing a monochromatic Al Kα source at 15 mA and 15 kV. The spectrometer’s work function was calibrated, yielding an 83.96 eV binding energy for the Au 4f7/2 line of metallic gold, while the spectrometer dispersion was adjusted to achieve a binding energy of 932.62 eV for the Cu 2p3/2 line of metallic copper. For the survey scan analyses, an analysis area of 300 μm × 700 μm and a pass energy of 160 eV were used. High-resolution analyses were conducted with an analysis area of 300 μm × 700 μm and a pass energy of 20 eV. Spectra were charge-corrected relative to the primary line of the carbon 1s spectrum (adventitious carbon) set at 284.8 eV.
Post-cycling XPS was performed using a ThermoFisher Scientific ESCALAB 250Xi system. Analysis was facilitated by a micro-focussed, monochromatized Al Kα X-ray source, and charge neutralization was achieved using a low-energy electron/ion argon flood source. Survey spectra were captured with a pass energy setting of 200 eV and a step size of 1 eV. Detailed regional scans were performed at a pass energy of 20 eV, with a finer step size of 0.1 eV, and a dwell time of 50 ms. These measurements were taken at a standard angle of 90° relative to the sample holder’s plane. ThermoFisher’s Avantage software was utilized for the XPS data analysis. During fitting the full width at half max of the peaks was kept below 2 eV, while the Smart background method was used to eliminate the background from composition calculations, ensuring accurate atomic composition calculations within 0.1%.
To analyze both charged and discharged states of the electrodes, cells were cycled and then maintained at 1.5 V and 0.5 V, respectively, for a duration of 24 h. The coin cells containing these electrodes were carefully disassembled in an argon-filled glovebox, using non-conductive tools to avoid electrical shorts. Following disassembly, the electrodes were washed with ethanol to remove electrolyte residues and then left to dry vertically within the glovebox for 24 h, ensuring the complete evaporation of any remaining ethanol. XPS was subsequently employed for detailed surface chemistry analysis of these electrodes. After XPS analysis, the cathode material was carefully transferred from the electrode to a TEM grid using a razor blade for further characterization under transmission electron microscopy (TEM).
To fabricate cathodes for electrochemical evaluation, the active substance was blended with conductive carbon black and a polytetrafluoroethylene (PTFE) binder in a weight ratio of 80:10:10, using 1-methyl-2-pyrrolidinone (NMP) as the solvent. This mixture was stirred magnetically at 300 rpm until it formed a uniform slurry. This slurry was then applied to molybdenum substrates, each covering an area of about 1 cm2, via drop casting and using a doctor blade. The coated substrates were subsequently dried at 70 °C, yielding cathodes with an active material mass loading of roughly 1 mg cm− 2.
The electrolyte, a mixture of AlCl3 and Trimethylamine hydrochloride (TMAHCl), was prepared in an anhydrous state at a 1.7:1 molar ratio under an argon atmosphere to prevent moisture absorption. The AlCl3 was gradually introduced to TMAHCl, resulting in the formation of a homogeneous ionic liquid, which was then stirred magnetically for an entire day to ensure thorough mixing23.
Coin cells were assembled within an argon-filled environment. The assembly included a cathode, a glass fiber separator soaked with 20 L of AlCl3-TMAHCl ionic liquid electrolyte, an aluminum foil anode, a spring, and two molybdenum current collectors, all encapsulated within a coin cell casing and sealed under a pressure of 8.3 MPa.
Galvanostatic charging and discharging tests were conducted with a Neware CT-4008-5V10mA-164-U galvanostat. Measurements for cyclic voltammetry (CV), electrical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV) utilized a VersaSTAT 3 potentiostat.
The data will be made available upon request. Please contact the corresponding author, Dr. Gisele Azimi (g.azimi@utoronto.ca).
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The authors acknowledge the financial support provided by the New Frontiers in Research Fund (NFRF) (Exploration) and University of Toronto XSeed grant. The authors acknowledge Dr. Jeffrey Henderson and Dr. Peter Brodersen for their help with XPS, Dr. Raiden Cobas Acosta for help with XRD, Mr. Salvatore Boccia for help with SEM, Dr. Jonthan Kong for help with TEM characterizations.
Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, ON, M5S3E4, Canada
Andrew Grindal & Gisele Azimi
Laboratory for Strategic Materials, Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON, M5S3E5, Canada
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AG performed investigation and developed the methodology. AG performed formal analysis of data and prepared the figures. AG wrote the main manuscript text. GA conceptualized the research, acquired funding, provided resources and supervision and validated the results. All authors reviewed the manuscript.
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Grindal, A., Azimi, G. Advancing aluminum-ion batteries: unraveling the charge storage mechanisms of cobalt sulfide cathodes. Sci Rep 14, 28468 (2024). https://doi.org/10.1038/s41598-024-78437-9
DOI: https://doi.org/10.1038/s41598-024-78437-9
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