Transition Metal Chalcogenide Cathode Materials Applied in Aluminum-Ion Batteries

Xiaoqiong Feng; Yunlong Ma; Hong Ning; Shiying Zhang; Changsheng An; Jinfeng Li

doi:10.7536/PC210111

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (2) : 319-327 DOI: 10.7536/PC210111

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Transition Metal Chalcogenide Cathode Materials Applied in Aluminum-Ion Batteries

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Abstract

As a promising cathode of aluminum ion batteries (AIBs), transition metal chalcogenides (MX₂ (X=S, Se, Te)) have excellent theoretical specific capacity and lower electronegativity, which endow it with great potential in commercial AIBs. Here, the relationship between the aluminum storage mechanism and the electrochemical properties of the transition metal sulfide compound (MX₂ (X = S, Se, Te)) is summarized. According to the current problems of transition metal chalcogenides, we propose the corresponding solutions proposed by researchers and summarize the main material modification techniques. Finally, the development direction of transition metal chalcogenide cathode materials is prospected, and feasible strategies to improve its overall electrochemical performance are discussed.

Contents

1 Introduction

2 Aluminum storage of transition metal chalcogenides

2.1 Intercalation mechanism

2.2 Conversion mechanism

3 Transition metal chalcogenide based on conversion mechanism

3.1 Metal sulfide

3.2 Metal selenide

3.3 Metal telluride

3.4 Other chalcogenide

4 Conclusion and outlook

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Keywords

aluminum-ion battery / cathode material / transition metal chalcogenide / aluminum storage performance / material modification

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Xiaoqiong Feng, Yunlong Ma, Hong Ning, Shiying Zhang, Changsheng An, Jinfeng Li. Transition Metal Chalcogenide Cathode Materials Applied in Aluminum-Ion Batteries. Progress in Chemistry, 2022, 34(2): 319-327 DOI:10.7536/PC210111

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1 Introduction

At present, lithium ion batteries (LIBs) have problems such as scarcity of lithium resources and potential safety hazards, which seriously limit their large-scale application in the field of energy storage^[1⇓~3]. Therefore, the development of secondary batteries with low cost and high safety has been widely concerned by the society^[4⇓~6]. Aluminum ion batteries (AIBs) have the advantages of abundant aluminum resources, high safety and high theoretical volume capacity (8040 mAh/cm³). Compared with traditional LIBs, AIBs have high theoretical specific capacity due to the participation of many electrons in the electrode reaction during the charge-discharge process, which makes AIBs gradually become a research hotspot in the field of energy storage. Since Holleck et al. Used aluminum as the negative electrode and metal chloride as the positive electrode to form a molten salt electrolyte composed of AlCl₃, KCl and NaCl, the study of non-aqueous AIBs has attracted the attention of researchers^[7]. However, due to the high electrode potential of aluminum (-1.676 V vs standard hydrogen electrode) and the large radius of ions participating in the electrode reaction, the energy density of AIBs is low, and the electrochemical performance is not enough to meet the actual needs. It was not until 2015 that Lin et al. Built a model of AIBs with new ionic liquids (AlCl₃/EMImCl) as electrolytes that the deadlock in the development of aluminum-ion batteries was broken^[8]. In recent years, different types of materials have been applied to the cathode materials of non-aqueous AIBs, such as metal oxides, transition metal chalcogenides, carbon-based materials and other materials^[9⇓~11]^[12⇓~14]^[15⇓~17]^[18,19]. Among them, carbon-based materials (such as graphite, graphene and carbon nanotubes, etc.) Are the most widely used in AIBs, showing higher discharge voltage and better cycle stability, but the guest ion in the aluminum storage mechanism of carbon-based materials is AlC

l 4 -

, which produces only one third of the theoretical specific capacity of Al³⁺ under the condition of embedding the same number of ions, so its specific discharge capacity is lower. And transition metal oxide are concerned because of their high specific discharge capacity, but at present, that report on transition metal oxides are mainly focused on vanadium-based oxides, and the report on other transition metal oxides are less.This is due to the high surface charge density of Al³⁺ and the high energy barrier of multi-valent ions in the oxide crystal structure, which makes the strong Coulomb interaction between Al³⁺ and material lattice and hinders its interlattice diffusion. Compared with other cathode materials, transition metal chalcogenides can exchange three electrons in the process of aluminum storage, which can show higher theoretical specific capacity^[20,21]. In addition, transition metal chalcogenides are considered to be promising cathode materials for aluminum ion batteries because of their wide interlayer spacing, low electronegativity and abundant resources, and the Coulomb interaction between transition metal chalcogenides and Al³⁺ is weaker than metal oxides. However, due to the large radius of the ions participating in the electrode reaction during the charge-discharge process, transition metal chalcogenides have some problems in the charge-discharge process, such as low specific discharge capacity, poor cycle stability, complex deintercalation/intercalation reactions and unclear reaction mechanism^[22,23]. At present, researchers have designed and optimized the structure of transition metal chalcogenides to improve their inherent defects and electrochemical properties, and analyzed their aluminum storage mechanism by modern characterization methods.

In order to improve the aluminum storage performance of transition metal chalcogenides, this paper reviews the aluminum storage mechanism of transition metal chalcogenides (MX₂(X=S, Se, Te)) cathode materials in non-aqueous AIBs, the latest research progress and the technical means to improve their aluminum storage performance. At the same time, the application prospect of transition metal chalcogenides in non-aqueous AIBs was prospected, and the solutions to optimize their electrochemical performance were proposed.

2 Mechanism of Aluminum Storage in Transition Metal Chalcogenides

Non-aqueous AIBs are battery systems designed based on the chloroaluminate ion electrolyte system (IL) as the electrolyte^[24]. Among them, transition metal chalcogenides as AIBs cathode materials have two reversible energy storage mechanisms, namely intercalation reaction and conversion reaction.

2.1 Intercalation reaction

At present, there are individual studies that confirm the reversible intercalation and deintercalation of chloroaluminate anions (

A l C l 4 -

) in transition metal chalcogenide cathode materials. During charging, the Al atom in the metal Al is converted into a

A l C l 4 -

on the negative electrode side in an electrochemical process. The

A l C l 4 -

ions are intercalated into the layers of the cathode material upon charging, and are correspondingly extracted from the layers upon discharging, while the metal Al and

A l C l 4 -

are converted into Al₂C

l 7 -

[25]. The specific electrochemical equation is as follows.

During charging and discharging

Positive electrode: M + nAlC

l 4 -

-ne^-↔M[AlCl₄]_n.

Negative :4Al₂C

l 7 -

+3e^-↔Al+7AlC

l 4 -

Due to the large ionic radius of

A l C l 4 -

(0. 528 nm) and less charge than Al³⁺, the large lattice resistance of most metal chalcogenides leads to the difficulty of reversible intercalation and deintercalation, the limited electrochemical reaction rate and the low specific discharge capacity, so the research progress of this kind of AIBs cathode materials based on intercalation mechanism is limited. To solve the above problems, researchers have focused on the construction of electrode materials with large interlayer size to withstand the reversible intercalation and deintercalation of

A l C l 4 -

during charge and discharge. On this basis, Hu et al. prepared a graphene-supported SnS₂(G-SnS₂) layered material with a large interlayer spacing (0.589 nm) as the cathode of AIBs, and demonstrated that

A l C l 4 -

ions can be reversibly intercalated and deintercalated between SnS₂ lattice sites by means of ex situ characterization^[26]. In addition, the existence of graphene (rGO) matrix also provides abundant active sites for

A l C l 4 -

deintercalation. Zhao et al. Prepared a star-shaped two-dimensional WS₂ microsheet and applied it to AIBs for the first time^[27]. The intercalation mechanism of

AlCl 4 -

in WS₂ electrode was deeply revealed by density functional theory (DFT) calculations, XPS and XRD, and the DFT calculation results verified that the formation energy of

AlCl 4 -

intercalation in WS₂ electrode was − 1.17 eV, indicating the feasibility and stability of

AlCl 4 -

intercalation. The assembled WS₂∥Al cell exhibited excellent electrochemical performance, the specific capacity was stable at 119 mAh·g^-1 after 500 cycles at a large current density of 1 A·g^-1, and the aluminum ion cell could work stably in a wide temperature range of -20 to 70 ° C. Divya et al. studied a series of two-dimensional chalcogenides with large interlayer spacing (MoS₂, MoSe₂ and MoSSe) as the cathode of AIBs, and their DFT calculation results showed that the interlayer spacing of these materials would be significantly reduced when they were supposed to be intercalated due to the high charge density of Al³⁺. Therefore, they set

A l C l 4 -

to be intercalated between the layers of the cathode material.The theoretical calculation results are quite different from the reaction mechanism of most two-dimensional chalcogenides, as shown in Figure 1A. This kind of material has two vacancies, M1 (X-Mo-X interatomic) and M2(MoX₂ interlayer), for intercalation. Due to the large radius of

A l C l 4 -

ions, they will be properly deformed in the intercalation of M2 sites to adapt to the interlayer sites, which is consistent with the conclusion reported by Takahashi et al^[28]^[29].

Different from the interlayer insertion site mentioned above, Jiang et al. studied the aluminum storage mechanism of Cu_2-_xSe nanorods with large unit cell volume based on ex-situ XPS and XRD, and the results showed that the insertion point of

A l C l 4 -

in Cu_2-_xSe was its (200) crystal plane in electrochemical reaction^[30]. So far, when

A l C l 4 -

is intercalated into guest ions, its univalent electrochemical reaction can not reflect the multi-electron transfer advantage of AIBs, and its large size will lead to the collapse of the structure of electrode materials, resulting in the limited actual capacity of electrode materials based on this reaction mechanism, which is difficult to meet the practical needs^[31]. Therefore, in addition to designing transition metal chalcogenide cathode materials from the aspects of material scale, layer spacing and structural stability, it is also one of the feasible strategies to shift the focus to the study of electrode materials based on Al³⁺ conversion mechanism.

2.2 Conversion Reaction

At present, in most reports of metal chalcogenides (MX₂(X=S, Se, Te)) as AIBs cathode materials, the conversion mechanism mostly involves the reversible conversion of substances containing multivalent elements, which is mainly based on the intercalation and deintercalation reaction of Al³⁺. Al³⁺ is intercalated into the lattice of the host material during discharge, otherwise, it is deintercalated during charge. The specific reaction mechanism is as follows:^[32]^[33,34]

mnAl³⁺+M_nX_m+3mne^-↔mA_nX+nM⁰

Due to the occurrence of multi-electron redox reaction during the charge-discharge process, AIBs based on the conversion reaction can theoretically achieve higher capacity output, and the migration of multi-ions and the interaction between cathode materials and intercalation ions are the key to improve the electrochemical performance and fast charge-discharge of AIBs to some extent. Therefore, it is absolutely necessary to comprehensively and systematically expound the related research on the metal chalcogenides (MX₂(X=S, Se, Te)) based on the conversion reaction mechanism.

3 Transition Metal Chalcogenides Based on Conversion Reaction

3.1 Metal sulfides

3.1.1 Molybdenum Sulfide

The MoS₂ has a hexagonal layered structure with weak van der Waals force between the layers, and its large interlayer spacing (0. 65 nm) is beneficial to the intercalation of Al³⁺^[35]. Li et al. firstly applied the petal-shaped MoS₂ microspheres to AIBs (Figure 2 (a)), and the electrochemical test results showed that the initial specific discharge capacity was 253.6 mAh·g^-1 at a current density of 20 mA·g^-1, and the specific discharge capacity was 66.7 mAh·g^-1 after 100 cycles at a current density of 40 mA·g^-1^[36]. As the cathode material of AIBs, the MoS₂ electrode material shows a high specific capacity in the first cycle, but in the subsequent charge-discharge process, the low intrinsic conductivity of MoS₂ and the electrostatic effect with Al³⁺ lead to its poor cycle stability. Therefore, improving the structural stability and electronic conductivity of two-dimensional layered MoS₂ and making the effective contact between electrolyte and active material is one of the key problems to be solved urgently. Subsequently, Lu et al. prepared the MoS₂/CNFs composite (Figure 2(b)),MoS₂ nanosheets were uniformly dispersed on the surface of carbon nanotubes (Figure 2 (C)) using a self-supporting structure design^[37]. Due to the introduction of the CNFs, the conductivity of the MoS₂/CNFs composite material is effectively improved, the contact area of an electrode material and an electrolyte is increased, and the electrochemical performance of the MoS₂/CNFs composite material is effectively improved. At a current density of 100 mA·g^-1, the initial discharge specific capacity is as high as 293.2 mAh·g^-1, and the reversible specific capacity of 126.6 mAh·g^-1 is maintained after 200 cycles, with a Coulombic efficiency higher than 95%. In addition, ex-situ X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to confirm the aluminum storage mechanism of MoS₂/CNFs, and Al³⁺ could be reversibly deintercalated into the (002) plane of the MoS₂ layer during the charge-discharge process. The results show that the coating of carbon materials can effectively improve the structural stability of electrode materials in the process of aluminum storage, improve the electronic conductivity of transition metal sulfides, and thus significantly improve their electrochemical performance.

3.1.2 Iron Sulfide

As early as several decades ago, FeS₂ was used as the positive electrode of AIBs in molten system and ILs electrolytes. Uchimoto et al studied the FeS₂ as the cathode of AIBs in chloroaluminate ILs, and studied the reaction mechanism through a variety of modern characterization and testing methods, indicating that the original FeS₂ was transformed into FeS with low crystallinity and amorphous Al₂S₃ during charge and discharge^[38]. The results show that the conversion process of FeS₂ mainly depends on the surface-controlled reaction, and its energy density is also determined by the specific surface area of FeS₂^[39]. Subsequently, Li et al. Designed a new multilayer FeS₂@C composite and applied it to the anode of AIBs^[40]. The multilayer structure morphology can provide a high specific surface area to ensure that that electrolyte effectively permeates the surface of the electrode material. Benefiting from this unique structure, FeS₂@C maintains a capacity of about 120 mAh·g^-1 at 1 A·g^-1 with little attenuation over 1000 cycles.

3.1.3 Cobalt sulphide

Hu et al. Prepared free-standing cobalt sulfide/carbon nanotube/fiber composites (Co₉S₈@CNT-CNF)^[41]. As the cathode material of AIBs, the Co₉S₈@CNT-CNF composite exhibits high flexibility and excellent mechanical strength, and the self-supporting structure design reduces the possibility of active material pulverization and side reactions (Figure 3 (a)), showing excellent electrochemical performance. The discharge capacity is 315 mAh·g^-1 at 0.1 A·g^-1; At 1 A·g^-1, the reversible specific capacity of 87 mAh·g^-1 was maintained after 6000 cycles (Fig. 3 (B)). At the same time, the introduction of conductive materials can accelerate the charge transfer, so that the reaction kinetics can be optimized to a certain extent. In addition, ex-situ XRD, XPS and Raman spectroscopy were used to analyze the mechanism of aluminum storage in Co₉S₈@CNT-CNF electrode materials, and the results showed that the ions participating in the electrode reaction were Al³⁺.

3.1.4 Other metal sulfides

In addition, other metal sulfides based on the conversion reaction mechanism are summarized, including CuS, Ni₃

S 2 [43]

, and Sn

S 2 [44]

^[42]. Jiao et al. reported the use of Ni₃S₂/ graphene composite as a novel cathode material for AIBs^[43]. The Al³⁺ is mainly intercalated and deintercalated in the structure of the Ni₃S₂, and the conversion between the Ni₃S₂ and NiS is reversible. At a current density of 0.1 A·g^-1, the initial discharge capacity of Ni₃S₂/ graphene is 300 mAh·g^-1, and the discharge plateau is high (~ 1.0 V). However, after 100 cycles, the discharge capacity is still only 60 mAh·g^-1, and the rapid decrease of the capacity may be due to the destruction of the crystal structure by the intercalation/deintercalation of Al³⁺. To solve the problem of relatively low capacity in Ni₃S₂/ graphene AIBs, they fabricated a 3D layered nanostructured CuS @ C microsphere composed of nanosheets as the anode of AIBs^[42]. The assembled AIBs exhibited a high average discharge voltage of about 1.0 V. In addition, a discharge capacity of about 240 mAh·g^-1 was obtained at a current density of 0.02 A·g^-1 in the initial discharge cycle. Benefiting from the single crystal structure and uniform nanosheets, the specific capacity of about 90 mAh·g^-1 was stably maintained after 100 cycles. Liang et al. reported a self-supporting porous SnS/PF cathode material. The existence of the porous structure effectively increased the contact area with the electrolyte and avoided possible side reactions between the binder and the electrolyte. The SnS/PF electrode material showed high specific discharge capacity and excellent cycle stability^[44].

To sum up, the ion diffusion kinetics and specific discharge capacity of sulfide cathode materials can be improved to a certain extent by adjusting the structure of sulfide cathode materials, such as constructing nano-scale or porous structure, increasing the specific surface area of materials, shortening the ion diffusion path, and compounding highly conductive carbon-based materials to improve the electronic conductivity. On the other hand, the self-supporting structure design without adhesive can effectively increase the contact area between the active material and the electrolyte, improve its electrochemical performance and better match with the electrolyte, and provide effective guidance for optimizing its aluminum storage performance.

3.2 Metal selenide

3.2.1 Cobalt selenide

Selenium and sulfur are located in the same main group.Compared with metal sulfides, metal selenides have higher conductivity, and selenium has lower electronegativity and more easily polarized anion skeleton, so cobalt selenide may be more conducive to the intercalation/deintercalation of Al³⁺ than cobalt sulfide. Cai et al. prepare a porous CoSe₂@rGO composite material with a hollow structure through a hydrothermal method, as shown in fig. 4 (a-c), and use that composite material as a cathode material of AIBs^[45]. Thanks to its special hollow porous structure, it increases the contact area between the electrode material and the electrolyte, ensures the effective diffusion of the electrolyte in the electrode material, and provides more channels for ion transmission. In addition, the introduction of graphene can improve the conductivity of the composite, alleviate the pulverization of the electrode material during charging and discharging, and effectively improve the cycle stability of the CoSe₂@rGO composite. As shown in Figure 4D, at a current density of 1 A·g^-1, the specific capacity is 143 mAh·g^-1 after 500 cycles. In addition, the researchers further discussed the aluminum storage performance and capacity fading mechanism of the material through ex situ characterization and DFT calculation. The results show that the initial lattice spacing of the CoSe₂ is 0. 587 nm, but it changes to 0. 582 nm after the first discharge, which can be attributed to the partial replacement of Co²⁺(0.069 nm) by the smaller Al³⁺(0.053 nm), which is also consistent with the DFT calculation data. This work provides a good idea to solve the problem of structural instability of conversion-type electrode materials during charge-discharge process, and lays a theoretical foundation for the application of transition metal selenides in AIBs.

3.2.2 Antimony selenide

The Sb₂Se₃ crystal is composed of band-shaped (Sb₄Se₆)_n, the interior of the (Sb₄Se₆)_n is combined by Sb-Se covalent bonds, and the bands are combined by weak van der Waals forces, which is beneficial to the reversible intercalation and deintercalation of Al³⁺^[46]. Wei et al. firstly prepared a composite material of single crystal Sb₂Se₃ nanorods and nitrogen-doped graphene (SNG) as the cathode material of AIBs. Compared with the pure Sb₂Se₃ electrode material, the introduction of hetero-element doped graphene further effectively improved the conductivity of the Sb₂Se₃@SNG composite material, thus showing high specific discharge capacity and excellent cycle stability^[47]. At a current density of 0.5 A·g^-1, the first discharge specific capacity of the Sb₂Se₃@SNG composite is as high as 343 mAh·g^-1; The specific discharge capacity is 125 mAh·g^-1, 106 mAh·g^-1 after 1000 long cycles at a higher current density of 0.5 and 1 A·g^-1, respectively, and the electrochemical performance is effectively improved.

3.2.3 Molybdenum selenide

The MoSe₂ has a graphene-like two-dimensional layered structure, which is beneficial to the rapid intercalation and desorption of Al³⁺ between the MoSe₂ lamellae, thereby obtaining high aluminum storage capacity. Recently, Nann et al. used pure MoSe₂ as the cathode of AIBs, which provided the first discharge capacity up to 110 mAh·g^-1 at a smaller current density of 40 mA·g^-1, indicating that MoSe₂ has great potential in AIBs^[28]. However, when the pure MoSe₂ is directly used as the positive electrode in AIBs, due to the strong Coulomb interaction with the Al³⁺, it will be limited by the severe volume expansion and material pulverization in the electrochemical charge/discharge process, resulting in rapid capacity decay. A large number of studies have shown that the electrochemical performance of MoSe₂ can be effectively improved by carbon coating modification. For example, Zhou et al. Prepared modified MoSe₂@C composites as the anode of AIBs based on the surface carbon coating strategy^[48]. The excellent first cycle discharge capacity of 295 mAh·g^-1 is provided at 0.1 A·g^-1, and the reversible specific capacity of 110 mA·g^-1 is maintained after a long cycle, showing good cycle stability, which confirms that the carbon coating strategy is an effective method to inhibit the volume expansion during charge/discharge. Recently, Feng et al. Prepared a molybdenum selenide/nitrogen-doped graphene (MoSe₂/N-G) composite material with heterojunction structure and large interlayer spacing as the anode of AIBs^[49]. Experimental verification and theoretical calculations reveal that the unique heterojunction structure and the conductive network of graphene doped with heterogeneous elements are conducive to improving the electrochemical reaction kinetics and providing more reaction sites for the rapid transport of electrolyte. It has an initial high specific discharge capacity of 167 mAh·g^-1 at 0.2 A·g^-1 and retains a capacity of 140 mAh·g^-1 after 1000 cycles. The DFT calculation results show that the potential barrier of Al³⁺ intercalation and deintercalation is -0. 19 eV, which indicates that the heterojunction structure design accelerates the electrolyte electron/ion (de) intercalation process, provides a large number of reactive active sites, and effectively provides more migration channels for Al³⁺ ions, thus obtaining high reaction kinetics and reversible capacity. Therefore, it is of great significance to design electrode materials based on the heterojunction structure of MoSe₂-based materials and large interlayer spacing to improve the performance of aluminum storage.

3.2.4 Manganese selenide

MnSe is expected to be a very promising cathode material for AIBs due to its special three-dimensional porous rock salt structure, high electronic conductivity and theoretical specific capacity^[50]. Du et al. reported a three-dimensional nanostructured α-MnSe microsphere. The special nano-microporous structure can provide more ion transmission paths, increase the specific surface area of the electrode material, improve the utilization rate of the active component, and effectively improve the electrochemical performance^[51]. When the current density is 0.2 A·g^-1, a high specific discharge capacity (408 mAh·g^-1) is exhibited; Even at higher current density (1 A·g^-1), the specific discharge capacity is still 131 mAh·g^-1 after 150 cycles, and the coulombic efficiency is 97%.

To sum up, metal selenides exhibit better aluminum storage properties than metal sulfides due to their high conductivity, low electronegativity and soft anion skeleton. In addition, the material scale design, such as the construction of a special nano-micro porous structure and the broadening of the selenide layer spacing, can also improve the diffusion rate of Al³⁺ in the electrode materials and optimize the electrode reaction kinetics, which provides a feasible reference direction for improving the electrochemical performance of AIBs through the structural design of electrode materials.

3.3 Metal telluride

3.3.1 Nickel telluride

NiTe has a typical nickel-arsenide crystal structure with high electrical conductivity than NiT

e 2 [52]

, Ni₃T

e 2 [53]

and Te, and has unique electrical, crystallographic and thermodynamic properties, which has attract extensive attention of many researchers^[54]. Yu et al. First prepared NiTe nanorods as the cathode material of AIBs by hydrothermal method^[55]. The reaction mechanism of aluminum storage was analyzed by means of modern testing and characterization methods. The results show that Te^2- are oxidized on the positive electrode and aluminum ions are deposited on the negative electrode during the charging process. In addition, the separator was also modified by single-walled carbon nanotubes (SWCNTs), which showed good cycle stability after the soluble intermediate was confined by the SWCNT layer. At a current density of 200 mA·g^-1, its initial specific discharge capacity is as high as 570 mAh·g^-1; At 500 mA·g^-1, the discharge capacity remained at 307 mAh·g^-1 after 100 cycles.

3.3.2 Cobalt telluride

Zhang et al. prepared CoTe₂@N-PC composite material as AIBs cathode material by mixing nanocrystals of ZIFs (ZIF-67) with tellurium powder and then pyrolyzing in an inert atmosphere so that metal cobalt ions grow in situ into CoTe₂ nanoparticles on a porous carbon matrix^[56]. The composite containing carbon skeleton can bear the mechanical strain generated by ion intercalation and deintercalation in the charging/discharging process; At that same time, the special nano-micro structure can reduce the generation of soluble tellurium aluminate compound and improve the problem of capacity loss cause by chemical dissolution and electrochemical reaction, thereby improving the cycle stability and showing excellent electrochemical performance. At a current density of 200 mA·g^-1, an ultrahigh reversible initial capacity of 635.8 mAh·g^-1 is exhibited; After 200 cycles, the specific discharge capacity is still as high as 168.6 mAh·g^-1, and the coulombic efficiency exceeds 90%.

To sum up, there are few reports on metal tellurides in AIBs, but because of their special physical and chemical properties, they show excellent specific discharge capacity in the charge-discharge process. Therefore, it is necessary to further develop more advanced AIBs metal telluride electrode materials by combining the above material construction and modification strategies.

3.4 Other sulfur elements

Sulfur-based cathode has high specific capacity (1675 mAh·g^-1) and gravimetric energy density (1340 Wh·kg^-1), so Al-S batteries have the advantages of low cost and high energy density, and have great potential in the field of large-scale energy storage^[57,58]. Cohn et Al. reported a non-aqueous Al-S battery system based on chloroaluminate ILs, which can achieve a specific capacity as high as the theoretical value. However, the dissolution of sulfur-containing intermediate discharge materials into the IL electrolyte will lead to a shuttle effect, resulting in a rapid capacity decay. Therefore, it is necessary to optimize the design of sulfur cathode^[59]. Gao et Al. prepared an optimally designed Al-S battery by dispersing sulfur /Al[EMIm]Cl₄ slurry on free-standing carbon nanofiber paper (CNF) (Fig. 5 (a, B))^[60]. A piece of GF coated with a thin layer of carbon nanotubes (SWCNTs) was used as a separator, with the coated surface facing the sulfur cathode (Fig. 5 (C)). This separator optimization strategy effectively limited the dissolution of polysulfides in the electrolyte and greatly improved its electrochemical performance. As shown in Fig. 5 (d, e), compared with the battery without coating GF separator, the capacity fading of the battery with modified separator is slowed down, and the cycle performance of Al-S battery is greatly improved. Although Al-S batteries are still in their infancy, they are an ideal alternative to high-capacity energy storage. Therefore, it is urgent to study the electrochemical reaction mechanism to avoid the dissolution of polysulfides in the electrolyte. The electronic conductivity of Te is higher than that of elemental sulfur, so Al-Te battery is also a promising energy storage system. Zhang et Al. Used Te nanowires to construct Al-Te batteries, and modified the separator by reducing graphene oxide and carbon nanotubes to solve the problem of capacity fading caused by the shuttle effect of Te during charging and discharging^[61]. The cycle performance test results showed that the specific capacity of the battery system could be stably maintained at about 400 mAh·g^-1 after 100 cycles, showing excellent discharge specific capacity and cycle stability.

In addition to the Al-S and Al-Te batteries with specific capacities reported so far, Se should also have a higher theoretical specific capacity because of its electrochemical properties similar to those of S and Te. In addition, the molecular weight of Se is smaller than that of Te, and it has higher volume capacity (3253 mAh/cm³) and conductivity (~1×10^-3S/m) compared with S, so it has great potential in the field of electrochemical energy storage of AIBs. In the AIBs with common separator, the selenium-aluminum battery shows a high discharge voltage of about 1. 52 V relative to Al/Al³⁺, but the capacity decays rapidly within 20 cycles, which is due to the dissolution of the oxidation products of Se cathode in the chloroaluminate electrolyte, resulting in a significant capacity decay. In order to solve the above problems, Huang et al. modified the separator with CMK-3 porous carbon coating to greatly improve the rechargeable capacity and cycle stability of selenium-aluminum battery (SAB)^[62]. The test results show that the initial specific discharge capacity can be as high as 1009 mAh·g^-1 at a current density of 1 A·g^-1. Nitrogen adsorption-desorption tests showed that the use of CMK-3 modified separator in AIBs could achieve more effective active material utilization. XRD, XPS and Raman spectroscopy were used to study the reaction mechanism during the charge-discharge process, and the results showed that the CMK-3 modified separator could well inhibit the shuttle effect caused by the soluble selenium chloroaluminate compound.

To sum up, the use of conductive carbon materials to modify the separator can improve the effective utilization of the active material and reduce the shuttle effect in the charge-discharge process, which provides an effective way to greatly improve the electrochemical performance of AIBs.

4 Conclusion and prospect

Transition-metal chalcogenides as cathode materials for non-aqueous AIBs have lower electronegativity and more easily polarized ionic skeleton, which is more conducive to the intercalation and deintercalation of Al³⁺. In addition, it has many advantages, such as abundant reserves, economic cost and high theoretical specific capacity, and has become one of the research hotspots of non-aqueous AIBs cathode materials. In this paper, the existing problems of metal chalcogenides in AIBs and the main modification technologies were introduced, and the application prospects of metal chalcogenides in non-aqueous AIBs were prospected.

Although metal chalcogenides have many advantages as cathode materials for non-aqueous AIBs, it is still necessary to know the existing problems of chalcogenide materials, such as rapid capacity fading, poor rate performance and low coulombic efficiency in the first cycle, which need to be further solved. From the current research progress, although we can learn from the experience of other metal ion batteries, it is speculated that metal chalcogenide materials with fast intercalation and deintercalation and high discharge plateau can be beneficial to Al³⁺. However, the reaction mechanism of metal chalcogenide materials used in non-aqueous AIBs is quite different from that of other metal ion batteries, so it is more necessary to combine the relevant work experience of chalcogenide materials used in non-aqueous AIBs.

Therefore, in order to make them better used in non-aqueous AIBs, it is suggested to carry out relevant research from the following aspects: (1) optimize the structure of chalcogenide cathode materials, design and prepare electrode materials with nano-micro structures, so that they have high specific surface area and abundant active sites; (2) The strategy of compounding chalcogenide with other highly conductive carbon-based materials is adopted to improve the electrochemical stability of the materials while improving their conductivity, thereby improving the first efficiency of the battery; (3) designing and optimizing the strategy of the self-supporting material to avoid the side reaction between the binder and the electrolyte and improve the conductivity of the cathode material; (4) In-depth exploration of Al-X (X = S, Se, Te, etc.) battery systems with high energy density and practical benefits for commercialization; (5) the optimized modification design of the separator, which reduces the loss of the cathode material in the reaction process and slows down the dissolution of the cathode material through a simple modification strategy of the separator; (6) The mechanism of the intercalation of multivalent aluminum cations or chloroaluminate anions into the crystal structure of transition metal chalcogenides may involve a series of structural and compositional changes, and a variety of complex processes are not yet fully understood, which requires advanced characterization methods to study the reaction kinetics and dynamic changes of active substances; (7) Among the transition metal sulfides described herein (MX₂(X=S, Se, Te)), S and Se, which have smaller atomic numbers among the chalcogens, are more economical than Te in terms of material costs and preparation processes; And (8) the discharge potential of the cathode material is properly widened, and the structural stability and the electrochemical reaction reversibility of the cathode material are ensured while a high aluminum storage capacity is obtained. In addition, optimizing and modifying the electrolyte is also a key link to promote the commercialization of AIBs. At present, the electrolyte of non-aqueous AIBs is mainly composed of ionic liquids containing AlCl₃, which has the disadvantages of high cost, sensitivity to water and air, and narrow voltage window, which limits the development of non-aqueous AIBs.Therefore, it is urgent to develop novel, safe and stable electrolytes (including molten salt electrolytes, GPEs and hybrid electrolytes) and other safer electrolytes, and the use of electrolyte additives and increasing the solubility of aluminum salts can effectively optimize the kinetics of aluminum deposition and improve the electrochemical performance of AIBs. With the increasing demand for large-scale energy storage devices such as electric vehicles, it is believed that with the maturity of research and development of electrolytes and such materials, the application of transition metal chalcogenides in non-aqueous AIBs is worth looking forward to.

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