Transition metal dichalcogenide-based materials for rechargeable aluminum-ion batteries: A mini-review

Transition metal dichalcogenide-based materials for rechargeable aluminum-ion batteries: A mini-review Sunny Nandi[a] and Martin Pumera*[a, b, c, d] Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 6/20] 1 ChemSusChem 2024,17, e202301434 (1 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem www.chemsuschem.org Review doi.org/10.1002/cssc.202301434 Rechargeable aluminum-ion batteries (AIBs) have emerged as a promising candidate for energy storage applications and have been extensively investigated over the past few years. Due to their high theoretical capacity, nature of abundance, and high safety, AIBs can be considered an alternative to lithium-ion batteries. However, the electrochemical performance of AIBs for large-scale applications is still limited due to the poor selection of cathode materials. Transition metal dichalcogenides (TMDs) have been regarded as appropriate cathode materials for AIBs due to their wide layer spacing, large surface area, and distinct physiochemical characteristics. This mini-review provides a succinct summary of recent research progress on TMD-based cathode materials in non-aqueous AIBs. The latest developments in the benefits of utilizing 3D-printed electrodes for AIBs are also explored. 1. Introduction In recent times, there has been intense research interest in the development of high-performance electrode materials for rechargeable multivalent-ion batteries.[1–4] Although lithium-ion batteries (LIBs) have emerged as one of the most promising storage systems in portable electronic gadgets, they are still unfit to meet the growing demand for sustainable energy storage in the near future due to their high cost and safety concerns.[5] Consequently, the investigation of new materials with high energy and power densities has highlighted the need to fulfil the fast-growing demand in the energy sector. To guarantee the dependency of future clean energy-storage devices, huge endeavors have been expended into various rechargeable univalent (Na+or K+) or multivalent (Mg2+, Zn2+, Al3+) ion batteries.[6–43] Among the latter, rechargeable aluminum-ion batteries (AIBs) have drawn significant attention, as Al features the highest theoretical volumetric and gravimetric capacity, i.e., 8040 mAhcm3and 2980 mAhg1, respectively.[4] In addition, the low cost of aluminum, derived from its high abundance, safety, and environmental friendliness, makes it a promising alternative storage system for large-scale applications.[4,29] However, a primary obstacle in the advancement of AIBs is the limited availability of cathode materials with the ability to efficiently incorporate aluminum ions. This challenge stems from the complexity of developing cathode materials that can reliably and reversibly intercalate aluminum ions, thereby impacting the energy density and overall performance of AIBs.[4] Until date, metal oxides/sulfides, carbon-based materials, and Prussian Blue analogs (PBAs), have been investigated as cathode materials for non-aqueous AIBs.[4,30–46] However, significant challenges still exist in addressing the poor cycling stability and several capacity drops during battery cycling.[47–48] Ever since the rediscovery of graphene, the quest for 2D and layered materials with semiconducting properties has been one of the hottest research topics among the scientific community.[49–52] Transition metal dichalcogenide (TMD), which are semiconductors of the type MX2, where X is a chalcogen atom such as sulfur, selenium, or tellurium and M is a transition metal atom, have gained a lot of attention in the areas of optoelectronics devices, photocatalysis, sensors, batteries and supercapacitors, etc.[53–62] Unlike some 2D materials like graphene, which possess intrinsic metallic properties and require further modification to function as electrodes in AIBs, TMDs on the other hand offer intriguing potential for use as cathode materials for non-aqueous AIBs owing to the following reasons: i) The layered TMD nanomaterials possess a large specific surface area, which enables a substantial contact interface between the electrode and the electrolyte. This characteristic facilitate rapid ion diffusion and electron transport leading to enhanced charge-discharge rates and improved electrochemical performance. ii) The vacant edge sites present in these materials might function as potential sites for the adsorption of metal ions, hence enhancing the overall capacity for AIBs. iii) The weak van der Waals forces between adjacent XM-X layers in layered TMD nanomaterials facilitates the formation of interlayer gaps. These gaps play a crucial role in enabling rapid ion diffusion, insertion, and extraction, as well as enhancing the overall utilization of materials during the insertion/extraction process.[63–66] In recent times, several literature reviews have provided comprehensive summaries of the advancements and development of cathode materials pertaining to carbon-based, oxides/sulfides, and PBAs for AIBs.[4,47–48,65–68] However, the electrochemical performance of transitional metal-based dichalcogenides (TMDs) for non-aqueous AIBs and methods to improve the storage capacity are rarely reported.[4,47–48,65–68] In this mini-review, we will highlight the recent progress in TMDbased electroactive materials for non-aqueous rechargeable AIBs. The structural design and battery fabrication for better space utilization of AIBs have been emphasized. [a] Dr. S. Nandi, Prof. M. Pumera New Technologies – Research Centre, University of West Bohemia, Univerzitní 8, Plzeň 30614, Czech Republic E-mail: [email protected] [b] Prof. M. Pumera Future Energy and Innovation Laboratory, Central European Institute of Technology, Brno University of Technology, Purkyňova 656/123, Brno, CZ-616 00, Czech Republic [c] Prof. M. Pumera Energy Research Institute @ NTU (ERI@N), Research Techno Plaza, X-Frontier Block, Nanyang Technological University, 50 Nanyang Drive, Singapore 03722, Singapore [d] Prof. M. Pumera Faculty of Electrical Engineering and Computer Science, VSB - Technical University of Ostrava, 17. listopadu 2172/15, 70800 Ostrava, Czech Republic © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 7/20] 1 ChemSusChem 2024,17, e202301434 (2 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 2. Transitional metal dichalcogenides-based cathode material for aluminum-ion batteries The potential of TMD materials as cathode electrodes for AIBs has been studied recently.[48,69–70] Owing to their unique layered structure, few TMD-based materials, such as MoS2, WS2, VS2, etc., provide a favorable pathway for achieving high storage capacity.[71–85] However, TMDs encounter few challenges that restrict their performance and potential in advanced AIBs. Several issues that arise include: i) The limited rate performance and energy density of AIBs can be attributed due to the poor conductivity exhibited by (TMDs). ii) The capacity fading and cycle stability of TMDs are compromised due to the structural instability caused by the repetitive insertion and extraction process. iii) TMDs undergo a significant volume change during the insertion and extraction process. As a result, it induces mechanical stress leading to pulverization of the active materials.[58–60] Nevertheless, several engineering approaches have been investigated in order to address the aforementioned challenges. These approaches include nanostructuring, the incorporation of protective layers to prevent undesirable reactions, and enhancing the electrode design to mitigate mechanical strain, etc. and such strategies have also been briefly discussed in other literature reviews, with the aim of improving the electrochemical performance of TMD materials.[58,60,63,85] In this section, we will mainly cover the recent progress of TMD materials and their storage mechanisms, such as MoS2, WS2, WSe2, VS2, and VSe2, for non-aqueous AIBs. 2.1. Molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2) Molybdenum disulfide material has drawn considerable research interest in energy storage applications over the past few years.[57–62] Due to its lamellar layered structure and large interlayer distance (0.62 nm), MoS2is the most widely studied electrode material among all other TMDs, and it has been demonstrated as one of the feasible electrodes for Li+, Na+, K+, and Zn2+ion storage.[86–94] Recently, MoS2was introduced as a suitable cathode material for Al-ion storage. It was demonstrated a rechargeable AIB with MoS2microsphere cathode and Al as anode in a mixture of 1 M AlCl3/1-ethyl-3ethylimidazolium chloride ([EMIm]Cl) ionic liquid electrolyte with a molar ratio of 1.3:1.[71] It was found that the assembled battery could deliver a stable discharge capacity of only 66.7 mAhg1over 100 cycles at a current rate of 40 mAg1. Based on spectroscopic investigations, it was proposed that during the electrochemical reaction, the SMo-S sites (abbreviated as A1) shown in Figure 1a tend to lose their capacity and go through a phase transition during the charge process, whereas the space created by the MoS2layers (abbreviated as A2) is relatively weak due to weak van der Waals forces, thereby providing more feasible sites for Al3+ion intercalation/de-intercalation. In other words, the reaction mechanisms during the discharge and charge processes can be written:[71] Cathode :MoS2þxAl3þþ3xe! AlxMoS2(1) Anode :Al þ7xAlCl4! 4xAl2Cl7þ3xe (2) In order to improve the electrochemical performance of MoS2, Yang et al. fabricated a self-standing flexible MoS2on carbon nanofibers, abbreviated as MoS2/CNFs composite cathode, by a simple electrospinning technique (Figure 1b), followed by thermal annealing.[72] These composites showed a discharge capacity of 126.6 mAhg1even after 200 cycles at a current rate of 100 mAg1(Figure 1c). Such excellent cycling stability may be attributed to the uniform distribution of MoS2 on CNF (Figures 1d, e), providing a large surface area and more active sites for Al3+ion intercalation/de-intercalation during the discharge/charge processes. The authors further revealed the aluminum storage mechanism based on the spectroscopic measurements, which state that during the discharge state, the diffraction peak at 2θ=13.8°(marked as * in Figure 1f) disappears; then, during the charge state, it emerges again. This suggests that during the discharge state, Al3+ions could Sunny Nandi received his M.Sc. and Ph.D. in Physics from National Institute of Technology, Meghalaya, India and Tezpur University, Assam, India, respectively. During his Ph.D. period, he was awarded with Fulbright-Nehru Doctoral Research Fellowship 2021–22 to carry out research work for nine months at Department of CBE, University of California, Los Angeles, USA under mentorship of Prof. Yuzhang Li. In 2023, he joined the research group of Prof. Martin Pumera as a postdoctoral researcher at NTC, University of West Bohemia, Pilsen, Czech Republic. His research interest mainly focuses on the development of novel materials using 3D printing technique for electrochemical conversion and storage devices. Martin Pumera is a group leader of Materials Chemistry Lab at NTC and Advanced Nanorobots & Multiscale Robotics Lab at TUO. He became a tenured group leader at National Institute for Materials Science (NIMS), Japan, in 2006 and joined Nanyang Technological University, Singapore, as a professor in 2010. His research interests include 2D nanomaterials, 3D printing, electrochemistry, and micro/ nanomachines. Martin is “2017, 2018, 2019, 2020 and 2021 Highly Cited Researcher” by Clarivate Analytics. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 8/20] 1 ChemSusChem 2024,17, e202301434 (3 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License intercalate into the interlayer of MoS2layers and de-intercalate during the charge state, which was further supported by the exsitu XPS results.[72] Further, Tan et al. demonstrated a novel phase engineering approach to prepare MoS2-RGO composites via the hydrothermal method.[73] Owing to the synergistic effect of MoS2and reduced graphene oxide (RGO), as indicated by the FESEM and TEM images (Figures 1g, h), the MoS2-RGO composite facilitates faster Al3+ion diffusion and leading to improved electrochemical performance for AIB when compared to pristine MoS2. As a result, the as-prepared MoS2-RGO composites delivered a high specific discharge capacity of 150.2 mAhg1 after 100 cycles at a current rate of 1000 mAg1(Figure 1j), whereas pristine MoS2nanoflowers displayed a discharge capacity of only 80.9 mAhg1after 100 cycles at the same current rate (Figure 1i), which is substantially lower than that of MoS2-RGO composites. Similarly, the same group also demonstrated the combination of MoS2with MXene composite as a stable cathode material for AIBs.[74] The MoS2-MXene composite exhibited an initial discharge capacity as high as 224 mAhg1. The Ti3C2TXMXene function not only acts as a strong and conductive framework, but it also prevents pulverization, resulting in rapid ion and electron movement. Similar to MoS2, MoSe2is also a TMD layered material that has been widely used in rechargeable Li+, Na+, K+and Zn2+ion batteries.[95–98] Due to its wide interlayer spacing (0.64 nm), which is larger than graphite (0.32 nm), it can facilitate an easy pathway for ion intercalation/de-intercalation. MoSe2has recently been investigated as a promising electrode material for AIBs.[75–78] For example, Zhao and co-workers assembled an AIB using an N-MoSe2@C cathode electrode, which was obtained by an ionic complexation approach.[75] The Al//N-MoSe2@C cell could deliver an initial discharge capacity of 267 mAhg1at a current density of 0.1 Ag1(Figure 2a). It is important to note here that the battery could still demonstrate an outstanding reversible discharge capacity of 117 mAhg1with a columbic efficiency of 100% over 5000 cycles at a current density of 1 Ag1(Figure 2b).[75] Additionally, the assembled Al//NMoSe2@C cell shows impressive performance in both low and high temperatures. Such outstanding electrochemical performance may be due to the homogeneous distribution of active materials on the highly conductive N-doped carbon matrix. Yang et al. prepared a free standing composite cathode material for AIBs by simple electrospinning and hydrothermal methods.[76] The as-prepared material consists of graphene-like MoSe2nanosheets anchored to N-doped carbon nanofibers abbreviated as MoSe2@NCNF. It was found that the prepared MoSe2@NCNF cathode could deliver a high initial discharge capacity of around 296.3 mAhg1with a columbic efficiency of Figure 1. (a) Schematic illustration of MoS2crystal structure with possible Al3+ion intercalation sites at A1and A2. Reprinted (adapted) with permission from ref.[71] Copyright 2018 American Chemical Society. (b) Illustration for the preparation of flexible free-standing composite and digital images of MoS2/CNFs before and after the long-term cycling test. (c) Variation of specific capacities with cycle numbers, and coulombic efficiency at a current rate of 100 mAg1. (d, e) FESEM and TEM images of MoS2/CNFs. (f) Ex-situ XRD patterns of MoS2/CNFs before and after fully discharge/charge states. Reprinted (adapted) with permission from ref.[72] Copyright 2019 American Chemical Society. (g, h) SEM and TEM images of MoS2-RGO composite, and variation of specific capacities with cycle number and coulombic efficiency for (i) MoS2and (j) MoS2-RGO at a current rate of 1000 mAg1. Reprinted (adapted) with permission from ref.[73] Copyright 2020 Elsevier. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 9/20] 1 ChemSusChem 2024,17, e202301434 (4 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 97% at a current density of 100 mAg1. Even after 200 cycles, it could still maintain a reversible discharge capacity of approximately 169.9 mAhg1at the same current rate, whereas pure MoSe2could maintain a reversible capacity of only 94.8 mAhg1.[76] Additionally, this as-prepared cathode electrode exhibits excellent rate capability and this may be attributed to the synergistic interactions between MoSe2nanosheets and the NCNF substrate. In contrast to bulk MoSe2, the graphene-like MoSe2nanosheets adhered uniformly to the surface of NCNFs to create a hierarchical core-shell nanostructure, which is beneficial for boosting the electron transport and ion diffusion. Later, Zhou et al. synthesized a sheet-like MoSe2@C nanocomposite by a typical hydrothermal method followed by thermal annealing.[77] The results show that MoSe2@C nanocomposite displayed an initial discharge capacity of 294.97 mAhg1at a current rate of 100 mAg1. In addition, the MoSe2@C nanocomposite electrode displayed excellent cycling performance. As shown in Figure 2c, the MoSe2@C nanocomposite exhibited a stable discharge capacity of approximately 151.6 mAhg1over 1000 cycles at the same current rate, with a columbic efficiency reaching 98%.[77] It is worth to mention here that during the first few cycles, there is rapid capacity decay and this may be attributed due to the formation of solid electrolyte interface.[77] From the ex-situ XPS results shown in Figures 2d–f, the authors further revealed the aluminum storage mechanism. During discharge/charge processes, Al3+ions could intercalate and de-intercalate reversibly into the MoSe2layered structure. Besides, the authors put forward that during the electrochemical discharge/charge process, neither Se nor C contribute to the electrochemical reaction (Figures 2e, f), suggesting the charge is balanced by the variations in Mo (Figure 2d), which is consistent with the reported literature.[72–73,75] Very recently, Ali and co-workers demonstrated promising binder-free three-dimensional MoSe2helical nanorod arrays (3D MoSe2HNRAs) as cathode material for AIBs.[78] This binder-free 3D MoSe2HRNs was grown on polyimide substrate using glancing angle deposition followed by a low-temperature plasma-assisted selenization process shown in Figure 3a. The binder-free 3D MoSe2HNRAs delivered a high initial discharge Figure 2. (a) Galvanostatic charge/discharge curves at different current rates, and (b) variation of specific capacities with cycle number and coulombic efficiency at a current rate of 1 A g1for the assembled Al//N-MoSe2@C cell. Reprinted (adapted) with permission from ref.[75] Copyright 2019 American Chemical Society. (c) Variation of specific capacities with cycle number and coulombic efficiency at a current rate of 100 mAg1, and ex-situ XPS spectra of (d) Mo 3d, (e) Se 3d, and (f) C 1s before and after fully discharge/charge states of the fabricated Al//MoSe2@C device. Reprinted (adapted) with permission from ref.[77] Copyright 2020 Elsevier. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 10/20] 1 ChemSusChem 2024,17, e202301434 (5 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License capacity of 753 mAhg1at a current density of 0.3 Ag1 (Figure 3b). The long-term stability of binder-free 3D MoSe2 HNRAs was also tested at a higher current density of 5 Ag1. As shown in Figure 3c, a specific capacity of 138 mAhg1could be obtained at an initial cycle and even after 10000 cycles it could still deliver a reversible specific capacity of 116 mAhg1. Such outstanding performance can be mainly attributed to the helical interconnected nanorod arrays; the absence of binder could also alleviate pulverization and provide more active sites for electron and ion transport. In addition, the authors thoroughly investigated the electrochemical behavior and storage mechanism of the 3D MoSe2HNRAs cathode and found that multistep reactions occurred during the discharge/charge processes. In addition, an interdigital AIB with a 3D MoSe2 HNRA cathode and an Al plate anode (Figure 3d) was also integrated, which demonstrated impressive performance (Figures 3e, f) in a variety of various bending and stretching states.[78] Such interesting results indicate a promising future for flexible batteries in wearable electronic devices. 2.2. Tungsten disulfide (WS2) and tungsten diselenide (WSe2) Apart from MoS2and MoSe2cathode materials, tungsten disulfide (WS2) and tungsten diselenide (WSe2) have also shown some promising results for AIBs.[79–81] For example, Yang et al. fabricated a flexible, free-standing cathode electrode for AIBs.[79] In this work, the authors prepared ultra-small few layered WS2 nanoplates by nitrogen-doping on the carbon nanofibers (WS2@NCNFs) through electrospinning and thermal annealing. As a result, the WS2@NCNFs cathode exhibits a discharge capacity of 195.81 mAhg1after 100 cycles at a current density of 100 mAg1, which is better than the bulk WS2cathode (Figure 4a). This can be attributed to the interconnected CNFs on WS2nanoplates, which provide an easy pathway for Al3+ion intercalation. Post-mortem analysis on WS2@NCNFs shows that even after 100 cycles, the morphology of WS2@NCNFs could still be maintained. The authors put forward a plausible storage mechanism based on the ex-situ XPS and TEM analyses. During discharge, Al3+can intercalate into two different sites in WS2: one of which is the interlayer between distinct WS2layers that is contacted by van der Waals forces and the other one is at the WS2unit, where strong SWS ion bonds are bonded tightly (Figure 4b). A similar mechanism was also proposed by Zhao and his co-workers for a MoSe2cathode for AIBs.[75] Hence, it can be commented that during the discharge/charge processes, Al3+ions find it far simpler for intercalation and de-intercalation in the first position, which accounts for the majority of the capacity. Later, however, intercalation and de-intercalation become difficult due to the strong electrostatic interaction of Al3+ions, which results in irreversible capacity as some Al3+ ions may get trapped as depicted in Figure 4b. Overall, the Figure 3. (a) Schematic representation of MoSe2helical nanorod arrays (HNRAs) on polyimide substrates via glancing angle deposition followed by the plasma-assisted process. (b) Galvanostatic charge/discharge curves at a current rate of 0.3 A g1. (c) variation of specific capacity with cycle number at a high current rate of 5 Ag1of MoSe2(HNRAs), (d) Digital photographs of the plane-interdigital AIBs. (e) The corresponding charge/discharge curves, and (f) the cycling performance of MoSe2(HNRAs) tested at different stretching stages ranging from 0 to 30% at a current rate of 20 μAcm2. Reprinted (adapted) with permission from ref.[78] Copyright 2020 American Chemical Society. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 11/20] 1 ChemSusChem 2024,17, e202301434 (6 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License proposed mechanism during the discharge/charge processes can be written:[79] Cathode :WS2þxAl3þþ3xe! AlxWS2, and (3) Anode :Al þ7xAlCl4! 4xAl2Cl7þ3xe, respectively (4) Zhao and co-workers developed a star-shaped 2D WS2 microsheet structure based on the polymerization method coupled with annealing strategies as shown in Figure 4c. The prepared 2D WS2microsheet was used as a cathode electrode for AIBs.[80] The 2D WS2microsheet displayed a high reversible capacity of 254 mAhg1at 0.1 Ag1, with exceptional rate performance from 0.1 to 5 Ag1, and also demonstrated excellent cycling stability of 119 mAhg1after 500 cycles at 1 Ag1(Figures 4d, e). The authors also delve into the storage mechanism in the 2D WS2cathode during the discharge/charge processes. It is to be noted here that the storage mechanism of metal chalcogenides in AIBs are numerous and include the intercalation of either AlCl4or Al3+due to the diversity of crystal structure and chemical composition. For instance, layerstructured metal chalcogenides, like SnS2, were able to intercalate AlCl4reversibly while exhibiting electrochemical properties akin to those of carbon-based materials.[69] Therefore, for the purpose of clarifying the atomic-level interactions in WS2 layers, the authors put forth a theoretical DFT model based on first-principles calculations. Based on this model, the formation energy was found to be negative, i.e., 1.17 eV, which indicates that AlCl4insertion in WS2layers is theoretically possible.[80] This was further supported by the ex-situ XRD and XPS measurements. Hence, the schematic illustration of the proposed mechanism for Al//2D WS2during the discharge/charge processes is shown in Figure 4f, where nrepresents the ratio of AlCl4anions to WS2. This work hints at a new approach to understanding the storage mechanism in AIBs. Tungsten dichalcogenide (WSe2), another favorable TMD material, has shown promising prospects as a cathode material for AIBs due to its high theoretical capacity, large interlayer distance, and good electrical conductivity.[81] However, due to the strong columbic electrostatic interaction and high charge density of Al3+ions, most of the cathode materials suffer irreversible structural deterioration and poor cycle life. In order Figure 4. (a) Comparison of specific capacities with cycle number for WS2@NCNFs and bulk WS2. (b) Schematic illustration of the proposed mechanism of WS2 with possible Al3+ion intercalation/deintercalation. Reprinted (adapted) with permission from ref.[79] Copyright 2019 Elsevier. (c) Illustration for the preparation of 2D WS2microsheet for AIBs. (d) Rate capabilities test with various current rates. Inset Figure 4d shows the charge/discharge curves of star-shaped 2D WS2at 0.1 Ag1. (e) Variation of specific capacities with cycle number and coulombic efficiency of star-shaped 2D WS2at a current rate of 1 Ag1. (f) Schematic illustration of the proposed mechanism of 2D WS2for AIBs. Reprinted (adapted) with permission from ref.[80] Copyright 2020 Elsevier. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 12/20] 1 ChemSusChem 2024,17, e202301434 (7 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License to mitigate these issues, a new family of superlattice-type WSe2 sodium dodecylbenzene sulfonate (SDBS) was proposed by Cui et al.[81] This new superlattice type, S-WSe2was prepared by a simple bottom-up solvothermal method. When assembled, this new superlattice S-WSe2cathode with Al as anode demonstrated a high initial discharge capacity of 354 mAhg1at 100 mAg1, whereas in the case of pure WSe2it could deliver only 282 mAhg1at the same current rate (Figure 5a).[81] Moreover, a reversible specific capacity of 110 mAhg1was found at a high current rate of 2 Ag1with no capacity decline even after 1500 cycles. Such remarkable performance may be attributed to the introduction of organic molecules (SDBS) that considerably enlarge the interlayer spacing and facilitate easy pathways for Al3+ion intercalation. To understand the storage mechanism, ex-situ/in-situ XRD measurements were performed, as shown in (Figures 5b, c). Based on these results, the authors revealed an intriguing anomaly in the diffraction peak for (002) at around 2θ=8.5°. During the first discharge state, this (002) peak shifted to a lower angle value and then went back to its initial value after the first charge stage. Accompanying with the XPS results, the intensity of Al 2ppeak is significantly higher at fully discharged state than in a fully charged state (Figure 5d). This may be due to Al3+ion intercalation into the S-WSe2during discharge and de-intercalation during charge processes.[81] Additionally, ex-situ TEM images further reveal that even after 1000 cycles the superlattice-like S-WSe2electrode could still retain its crystal morphology, whereas the WSe2has changed into an amorphous structure after 200 cycles. This again may be ascribed due to the impact of SDBS, which makes it favorable to stabilize the high charge density of Al3+during the intercalation/de-intercalation processes.[81] Therefore, the electrochemical mechanism of S-WSe2during the discharge process can be formulated as: S-WSe2þxAl3þþ3xe! AlxðS-WSe2Þ(5) 2.3. Vanadium sulfide (VS2) and vanadium selenide (VSe2) VS2and VSe2are a class of TMD materials that has received considerable attention as cathode materials for rechargeable aluminum-ion batteries.[82–84] The crystal structure of VS2consists of a vanadium layer sandwiched between two sulfur atoms held together by a weak van der Waals force. Due to its large interlayer spacing of 0.59 Å, VS2can be regarded as a promising candidate for the cathode material in AIBs.[82] For instance, Wu et al. demonstrated VS2/graphene (G-VS2) nanosheets as a novel cathode material for AIBs.[82] The modified G-VS2nanosheets had a synergistic impact that significantly improved the electrochemical performance of AIB when compared to pristine VS2. As shown in Figure 6a, G-VS2exhibits an initial discharge capacity Figure 5. (a) Galvanostatic charge/discharge curves at a current density of 100 mAg1. (b-c) ex-situ/in-situ XRD and (d) ex-situ XPS spectra of Al 2pbefore and after 1st discharge/1st charge states of S-WSe2for AIBs. Reprinted (adapted) with permission from ref.[81] Copyright 2022 Wiley and Sons. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 13/20] 1 ChemSusChem 2024,17, e202301434 (8 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License of 186 mAhg1at the current density of 100 mAg1; even after the 50th cycle, the discharge capacity remains 50 mAhg1with an outstanding coulombic efficiency of 100%. In comparison to G-VS2, the pristine VS2could deliver an initial discharge capacity of only 145.4 mAhg1and the capacity decreased to 22 mAhg1 after the 50th cycle at the same current rate. This abrupt decrease in capacity for VS2may originate from the destruction of the crystal structure with the intercalation/deintercalation of Al3+ions. Moreover, the authors also evaluated the CV performance of the AIBs within the voltage window of 0.1 V to 1.8 V and they were able to observe the anodic and cathodic peaks that correspond to the insertion and extraction of Al3+ions (Figure 6b). Based on the in-situ/ex-situ XRD measurements, the possible storage mechanism of G-VS2is illustrated in Figure 6c. During discharge, Al3+ions intercalate into the layers of G-VS2 cathode while reversible deposition and dissolution occur on the anode side.[82] Very recently, Wang et al. designed a hierarchical VS2@VS4composite as cathode material for AIBs.[83] The VS2@VS4composite was prepared using a VS4nanorod array grown on VS2nanosheets by the in-situ solvothermal method.[83] This VS2@VS4composite cathode material has a specific capacity of 116.5 mAhg1after 500 cycles at a current density of 0.3 Ag1(Figure 6d). In addition, the VS2@VS4composite shows impressive performance at low temperatures when compared to VS2and VS4(Figure 6e), indicating VS2@VS4has good low temperature endurance.[83] Similar to VS2, VSe2was also considered a promising electrode material for AIBs due to its unique layered structure and high theoretical capacity.[84] For example, Lei et al. prepared a brick-like VSe2cathode material for AIBs through hydrothermal method (Figure 7a).[84] It is intriguing that the capacity of the battery is provided by the change in valence states of Se and V throughout repeated charge/discharge processes as revealed by the XPS measurement. As a result, this material displayed a high initial discharge capacity of 650 mAhg1at 100 mAg1with a discharge voltage plateau of 1.3 V, and could retain a reversible capacity of 50 mAhg1after 250 cycles (Figures 7b, c). In addition, the CV profiles of VSe2shown in Figure 7d are well consistent with the charge/discharge voltage plateaus. However, there are considerable challenges with TMDbased materials for AIBs in terms of their practical applications. One main drawback is the poor structural stability during battery cycling, which causes rapid capacity decay. Therefore, future efforts should be given to the material design and structural engineering of other TMD cathode materials to improve the storage capacity as well as the rate capability for Figure 6. (a) Variation of specific capacities with cycle number at a current rate of 100 mAg1, (b) CV curves at a scan rates of 0.2 mVs1, and (c) ex-situ/in-situ XRD patterns of G-VS2at various discharge/charge states. Reprinted (adapted) with permission from ref.[82] Copyright 2018 Royal Chemical Society. Variation of charge/discharge capacities with cycle number and coulombic efficiency for (d) VS2@VS4at a current rate of 0.3 Ag1and (e) VS2, VS4, and VS2@VS4at 10°C. Reprinted (adapted) with permission from ref.[83] Copyright 2020 Royal Chemical Society. Wiley VCH Dienstag, 14.05.2024 2409 - closed* / 337100 [S. 14/20] 1 ChemSusChem 2024,17, e202301434 (9 of 15) © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH ChemSusChem Review doi.org/10.1002/cssc.202301434 1864564x, 2024, 9, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cssc.202301434 by Technical University Ostrava, Wiley Online Library on [14/10/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License