The Review of Hybridization of Transition Metal-Based Chalcogenides for Lithium-Ion Battery Anodes

Transition metal chalcogenides as potential anodes for lithium-ion batteries have been widely investigated. For practical application, the drawbacks of low conductivity and volume expansion should be further overcome. Besides the two conventional methods of nanostructure design and the doping of carbon-based materials, the component hybridization of transition metal-based chalcogenides can effectively enhance the electrochemical performance owing to the synergetic effect. Hybridization could promote the advantages of each chalcogenide and suppress the disadvantages of each chalcogenide to some extent. In this review, we focus on the four different types of component hybridization and the excellent electrochemical performance that originated from hybridization. The exciting problems of hybridization and the possibility of studying structural hybridization were also discussed. The binary and ternary transition metal-based chalcogenides are more promising to be used as future anodes of lithium-ion batteries for their excellent electrochemical performance originating from the synergetic effect.


Introduction
With the increasingly serious energy crisis, more and more attention has been paid to green energies, such as wind energy and solar energy [1,2]. To resolve the disadvantages of the intermittency and randomness of these green energies, the development of energy conversion and storage systems is becoming increasingly significant. Lithium-ion batteries (LIBs) have been widely explored and applied in portable electronics, electric vehicles, and high-power energy storage systems, on account of their long cycle life, and high specific capacity [3,4]. However, traditional carbon-based anodes are facing the challenges of improving the energy density and rate performance raised by high-power energy storage systems and artificial intelligence vehicles [5,6]. Despite the advantages of abundance, relatively low lithiation voltage, and high lithium storage for the alloying anodes, such as Si-based, Ge-based, and Sn-based materials, the large destructive effects of volumetric expansion during cycling and low intrinsic electronic conductivity prohibit the further application of these alloying anodes [7][8][9].
As a representative of the conversion reaction, transition metal chalcogenides, as one of the most promising alternatives to carbon-based anodes, have been extensively explored, exhibiting high specific capacity and outstanding rate performance [10][11][12]. For practical application, the drawbacks of volume expansion and low conductivity for these anode materials during the cycling process should be further resolved. Therefore, diverse nanostructures have been designed and prepared using many preparation methods. The nanostructures can increase the specific surface and decrease the intercalation path, and therefore increase the specific capacity of the transition metal chalcogenide anodes [13][14][15][16][17]. Cubic hollow CuS nano-boxes anodes exhibited superior cycle stability and high specific capacity at 20 C [13]. Synthesized by a sol-gel method, CuS nanorods achieved excellent

Hybridization of X and Y-Chalcogenide
Compared to homoatomic metal doping, heteroatom hybridization could be easily accomplished by controlling the kinds and ratios of the chemical precursors. The hybrid of transition metal chalcogenide and heteroatoms tended to exhibit more remarkable electrochemical performance than pure transition metal chalcogenide.
Wang et al. found that the concentration of the doped Cu could regularly affect the morphologies and structures of the CoO/Cu nanocomposites. At a certain concentration, the CoO/Cu nanocomposites showed remarkable cycling stability to 1000 cycles and a high specific capacity of 580 mAh/g at 10 C, which are much better than pure CoO nanomaterials [33]. The outstanding electrochemical performance of the CoO/Cu anodes originated from the improved electron conductivity, enhanced catalytic function, and significant structure stability of CoO/Cu induced by Cu doping. The structure, morphologies, and electrochemical performance of a series of Ce-doped Mn 3 O 4 nanocrystals with different Ce concentrations were thoroughly investigated. Mn 2.98 Ce 0.02 O 4 anodes exhibited the highest capacity of 754.2 mAh/g up to 100 cycles, which was attributed to the bigger diffusion channels, more robust structure, and lower charge transfer impedance induced by the doping of Ce [34]. Krishnan et al. proposed a simple, environmentally friendly, and low-cost method to synthesize Mo-doped Cu 2 O:Mo with a porous microsphere structure. The remarkable electrochemical performance of the Cu 2 O:Mo anodes originated from the improvement of the specific surface area, electrical conductivity, Li-ion transports, and electrochemical kinetics induced by Mo doping [35]. The Zn-doped Co 3 O 4 nanocomposites with a hollow concave structure displayed excellent rate capability and high specific capacities with stable cyclability. The outstanding electrochemical performance originated from the unique hollow concave structure and the defects induced by Zn doping, which could effectively improve the electronic structure of Co 3 O 4 composites and then enhance the electrochemical reaction kinetics [36]. Wu [26]. (b) Comparison of Cu/Cu2O@Ppy, Cu2O@Ppy, and pure Ppy at 100 mA/g [30].

Hybridization of X and Y-Chalcogenide
Compared to homoatomic metal doping, heteroatom hybridization could be easily accomplished by controlling the kinds and ratios of the chemical precursors. The hybrid of transition metal chalcogenide and heteroatoms tended to exhibit more remarkable electrochemical performance than pure transition metal chalcogenide.
Wang et al. found that the concentration of the doped Cu could regularly affect the morphologies and structures of the CoO/Cu nanocomposites. At a certain concentration, the CoO/Cu nanocomposites showed remarkable cycling stability to 1000 cycles and a high specific capacity of 580 mAh/g at 10 C, which are much better than pure CoO nanomaterials [33]. The outstanding electrochemical performance of the CoO/Cu anodes originated from the improved electron conductivity, enhanced catalytic function, and significant structure stability of CoO/Cu induced by Cu doping. The structure, morphologies, and electrochemical performance of a series of Ce-doped Mn3O4 nanocrystals with differ-  [26]. (b) Comparison of Cu/Cu 2 O@Ppy, Cu 2 O@Ppy, and pure Ppy at 100 mA/g [30].
Other metal-doped compound anodes for lithium-ion batteries were also reported. The physical properties and electrochemical performance of Mn-doped Co 2 (OH) 3 Cl xerogels depending on the Mn concentration were systematically investigated. Compared to pure Co 2 (OH) 3 Cl, the 4% Mn-doped Co 2 (OH) 3 Cl xerogels displayed a superior capacity and an outstanding rate capability, which could be mainly ascribed to the increased electric conductivity and charge transfer ability induced by Mn doping [38]. Through a ball milling activating technique, Li 4 Ti 5−x Y x O 12 anode materials were obtained by Y doping to Li 4 Ti 5 O 12 and exhibited fast diffusion of lithium ions, outstanding rate capability, and excellent long-cycle stability. The introduction of hetero valence Y can establish a new charge balance with the spinel structure unchanged, by which the Li diffusion coefficient, electrical conductivity, and electrochemical performances were improved [39]. The better electrochemical performance of the hybrid of X and Y-chalcogenide than pure transition metal chalcogenide was summarized in Table 2.

Hybridization between Different Transition Metal Chalcogenides
Recently, many binary and ternary transition metal chalcogenides have been prepared and exhibited outstanding electrochemical performance. The hybridization of transition metal chalcogenides could promote the advantages of each chalcogenide and suppress the disadvantages of each chalcogenide. Therefore, the excellent electrochemical performance of a hybrid of transition metal chalcogenides could be obtained on account of the synergetic effect.

Hybridization of X-Chalcogenide and X-Chalcogenide
Due to the multivalent state of the transition metals, the two phrases of X-chalcogenide in the above title represent two different X-based chalcogenides, such as CoO and Co 3 O 4 , MnO and Mn 3 O 4 , CuO and Cu 2 O. Therefore, the hybridization of the same transition metal-based but different chalcogenides is discussed in this part.
Zhu et al. prepared dandelion-like CoO/Co 3 O 4 /C nanocomposites by a hydrothermal method and the following unique combustion method. The CoO/Co 3 O 4 /C anodes exhibited stable and high capacity at different current densities [40]. Prepared by a hydrothermal reaction and a subsequent calcination and reduction process, porous Co 3 O 4 @CoO nanosheet anodes showed an outstanding rate capability even at 5 A/g and an excellent specific capacity [41]. Li et al. synthesized Co(OH) 2 /Co 3 O 4 /Co nanoparticles derived from the EDTA-Co(II) sodium complex. The anode materials exhibited a high reversible capacity, good cycling stability, and high-rate performance, which originated from the porous structure, the presence of conductive Co, and especially the synergetic effect of Co(OH) 2 and Co 3 O 4 [42]. The MnO/Mn 3 O 4 @NC composites with a porous structure were prepared and used as anodes of LIBs. The relatively high electrochemical performance resulted from the special porous structure and the co-doping of N and Mn [43]. After 630 cycles, the MnO/Mn 3 O 4 /SeO x (x = 0, 2) hybrid anodes exhibited a reversible specific capacity of up to 1007 mAh/g at 3 A/g, resulting from the synergetic effect between MnO and Mn 3 O 4 phases [44]. Grown on conductive collectors of β-NiS@Ni 3 S 2 , the bind-free anodes of NiO nanosheet arrays exhibited outstanding lithium storage properties owing to the special heterostructures and the synergetic effect of the Ni 3 S 2 , β-NiS, and NiO components [45]. The CuS/Cu 1.8 S nanocomposite anodes showed a better electrochemical performance than that of most of the previous reports, owing to the nano-size structure and the assistance of the Cu 1.8 S component [46].
Yolk-shell structured Cu 2 O@CuO nanocomposites were prepared by a multistep chemical method at room temperature. The anode nanocomposites exhibited superior cycling stability and rate capability with a reversible capacity of up to 854 mAh/g at 0.1 C after 200 cycles, which could be mostly attributed to the improved conductivity induced by the spur-CuO bridge and CuO shell [47]. Quaternary Cu 2 O/CuO/Cu/Carbon-polymer composite fibers were obtained and exhibited good rate performance, excellent cycle stabil-ity, and high capacity due to the fiber structure and the combination of metal and metal oxides [48]. Compared to pure CuO nanowires, the CuO/Cu 2 O/Cu exhibited much better electrochemical performance, which was ascribed to the synergetic effect of the ternary composites [49]. The outstanding electrochemical performance of the CuO/Cu 2 O/C anodes with uniform spherical morphology was attributed to the synergetic effect of the two-component CuO/Cu 2 O, porous structure, and conducting carbon coating [50]. Ternary CuO/Cu 2 O/Cu composites were designed and prepared by a method of electrodeposition and following calcination in air. The obtained bind-free CuO/Cu 2 O/Cu anodes displayed a high capacity, which originated from the cypress-like structure and the synergetic effect between the conductive Cu and the active Cu oxides [51]. Used in lithium/sodium-ion batteries, the CuO/Cu 2 O anodes anchored in graphite matrix exhibited high capacity and cycling stability, mainly due to the synergetic effect of the CuO and Cu 2 O nanoparticles [52]. Other CuO/Cu 2 O nanocomposite anodes with various structures were prepared by different methods and exhibited relatively outstanding electrochemical performance. The synergetic effect of the components of CuO and Cu 2 O plays an important role in enhancing the rate capability, specific capacity, and cycling stability of these nanocomposite anodes [53][54][55]. From Figure 2 and Table 3, we could draw the conclusion that the hybridization of the homoatomic transition metal chalcogenides can effectively enhance the electrochemical performance of a single pure transition metal chalcogenide [45,49].
Yolk-shell structured Cu2O@CuO nanocomposites were prepared by a multistep chemical method at room temperature. The anode nanocomposites exhibited superior cycling stability and rate capability with a reversible capacity of up to 854 mAh/g at 0.1 C after 200 cycles, which could be mostly attributed to the improved conductivity induced by the spur-CuO bridge and CuO shell [47]. Quaternary Cu2O/CuO/Cu/Carbon-polymer composite fibers were obtained and exhibited good rate performance, excellent cycle stability, and high capacity due to the fiber structure and the combination of metal and metal oxides [48]. Compared to pure CuO nanowires, the CuO/Cu2O/Cu exhibited much better electrochemical performance, which was ascribed to the synergetic effect of the ternary composites [49]. The outstanding electrochemical performance of the CuO/Cu2O/C anodes with uniform spherical morphology was attributed to the synergetic effect of the twocomponent CuO/Cu2O, porous structure, and conducting carbon coating [50]. Ternary CuO/Cu2O/Cu composites were designed and prepared by a method of electrodeposition and following calcination in air. The obtained bind-free CuO/Cu2O/Cu anodes displayed a high capacity, which originated from the cypress-like structure and the synergetic effect between the conductive Cu and the active Cu oxides [51]. Used in lithium/sodium-ion batteries, the CuO/Cu2O anodes anchored in graphite matrix exhibited high capacity and cycling stability, mainly due to the synergetic effect of the CuO and Cu2O nanoparticles [52]. Other CuO/Cu2O nanocomposite anodes with various structures were prepared by different methods and exhibited relatively outstanding electrochemical performance. The synergetic effect of the components of CuO and Cu2O plays an important role in enhancing the rate capability, specific capacity, and cycling stability of these nanocomposite anodes [53][54][55]. From Figure 2 and Table 3, we could draw the conclusion that the hybridization of the homoatomic transition metal chalcogenides can effectively enhance the electrochemical performance of a single pure transition metal chalcogenide [45,49].

Hybridization of X-Chalcogenide and Y-Chalcogenide
The hybridization of different transition metal-based chalcogenides, i.e., X-metal chalcogenide and Y-metal chalcogenide, was widely investigated and could be easily achieved by many different preparation methods. The electrochemical performance of the composites of binary or ternary transition metal-based chalcogenides was almost better than that of each transition metal-based chalcogenide because of the synergetic effect of the components. The hybridization of transition metal chalcogenides could effectively enhance the advantages of a single component at certain concentrations.
A hierarchical Fe 3 O 4 /CuO hybrid wire showed high capacities, excellent rate capabilities, and ultrafast diffusion of Li ions, which was much better than the pure Fe 3 O 4 or CuO nanomaterial, respectively, originating from the intelligent integration of the Fe 3 O 4 and CuO and the unique nanowires structure [56]. Due to the synergetic effect between Mn 3 O 4 and Fe 3 O 4 and the unique flower-like structure, the Mn 3 O 4 /Fe 3 O 4 nanocomposites showed superior lithium ion storage ability than pure Fe 3 O 4 or Mn 3 O 4, respectively [57].
Porous Co 3 O 4 /CuO composites also displayed better cycling stability and higher capacities than pure Co 3 O 4 or CuO electrodes because of the synergetic lithium storage effect of both Co 3 O 4 and CuO, as well as the porous hierarchical structure [58]. The ratios of Co and Cu could affect the morphologies, structures, and further electrochemical performance of the porous Co 3 O 4 /CuO composites, which indicated the vital role of hybridization to adjust electrochemical properties [58]. Directly grown on Cu foam, the heterostructured CoO/SiO 2 exhibited high reversible capacity and good rate capability mainly because of the excellent synergetic effect between the two active materials of CoO and SiO 2 , and the enhanced conductivity of the 3D copper foam [59]. The binary ZnO/CoO mesoporous microspheres with carbon shells were prepared and delivered a good rate capability and high reversible capacity of 1457 mAh/g after 500 cycles, which was ascribed to the synergetic effect of the two components [60]. Composed of cubic Co 3 O 4 and rhombus NiO, the flowerlike NiO/Co 3 O 4 composites exhibited a low charge transfer resistance and a high initial discharge capacity, indicating the improved conductivity and electrochemical reaction originated from the two metal cations [61]. For the hybrid nanofibrous Co 3 O 4 /TiO 2 , the initial discharge capacity was higher than the theoretical capacity of pure Co 3 O 4 at 50.6% Co 3 O 4 mass content [62]. The reversible capacity of amorphous hybridized SnO 2 /Co 3 O 4 nanoflakes with a certain ratio of Sn and Co was much better than that of pure Co 3 O 4 and SnO 2 electrodes, respectively, which was attributed to the different potentials and working mechanisms of the SnO 2 and Co 3 O 4 [63]. Prepared by facile solvothermal and freeze-dring methods, the 3D structured CoO/ZnO nanoclusters anodes delivered a much higher specific capacity, which indicated an improved electrochemical performance by the synergetic effect of CoO and ZnO components [64]. The Co 3 O 4 /CeO 2 heterostructure nanocomposites with different molar ratios of Co/Ce were prepared by a facile microwave assistance method. The excellent cycling stability and high reversible capacity of the 5Co 3 O 4 /CeO 2 anodes resulted from the appropriate hybridization of Co 3 O 4 and CeO 2 [65]. Synthesized by a series of chemical reaction processes, the hierarchical mesoporous structured CoO@TiO2@C anodes exhibited much better electrochemical performance than that of pure CoO anodes, which indicated the enhanced effect of the robust TiO 2 and amorphous carbon shells [66]. For the NiO/CoO nanoneedles, the enhancement of the electrochemical performance mainly resulted from the structural stability and the volume accommodation because of the interspace between NiO-CoO nanoneedles [67].
The core-shell CuO@ZnO composites with different contents of ZnO were synthesized by a chemical process of depositing ZnO on the CuO surface. Compared to pure CuO materials, the specific CuO@ZnO-6.5% anodes showed much superior electrochemical performance [68]. Cu 7.2 S 4 /C was introduced to form the core-shell Cu 7.2 S 4 /C@MoS 2 nanocomposites, which exhibited long-cycle stability and high specific discharge capacity owing to the special oxidation states of Cu and the improved conductivity by the Cu-rich Cu 7.2 S 4 component [69]. ZnO was introduced to suppress the agglomeration and migration of Cu 2 O or Cu particles during the carbonization process, and the molar ratios of Zn to Cu can directly affect the size of the bimetallic oxides. At a certain concentration, the carbon-confined Cu 2 O/ZnO showed a reversible capacity of up to 476 mAh/g [70]. Porous Cu 2 O/Mn 3 O 4 hetero-nanosheets exhibited good cycling stability, high reversible capacity, and competitive rate capability because of the synergetic effect of the two components [71]. The flower-like C@SnO 2 /Cu 2 O nanosheet cluster anodes showed much better storage capacity, rate performance, and cycle stability than pure SnO 2 anodes because of the carbon layer, the novel structure, and the synergetic effect between the nanosized SnO 2 and Cu 2 O [72]. Prepared by a facile dealloying process of MnCoAl, MnCoO x microspheres anodes exhibited better rate capability, superior cycling stability, and higher capacities than those of pure Mn 3 O 4 anodes [73]. Figure 3 exhibited the better electrochemical performance of the hybrid of Mn 3 O 4 than that of pure Mn 3 O 4 [57,63], while Figure 4 showed the better electrochemical performance of the hybrid of SnO 2 than that of each pure component [72,73], indicating the enhanced electrochemical performance by the hybridization of different transition metal-based chalcogenides.
can directly affect the size of the bimetallic oxides. At a certain concentration, the carbonconfined Cu2O/ZnO showed a reversible capacity of up to 476 mAh/g [70]. Porous Cu2O/Mn3O4 hetero-nanosheets exhibited good cycling stability, high reversible capacity, and competitive rate capability because of the synergetic effect of the two components [71]. The flower-like C@SnO2/Cu2O nanosheet cluster anodes showed much better storage capacity, rate performance, and cycle stability than pure SnO2 anodes because of the carbon layer, the novel structure, and the synergetic effect between the nanosized SnO2 and Cu2O [72]. Prepared by a facile dealloying process of MnCoAl, MnCoOx microspheres anodes exhibited better rate capability, superior cycling stability, and higher capacities than those of pure Mn3O4 anodes [73]. Figure 3 exhibited the better electrochemical performance of the hybrid of Mn3O4 than that of pure Mn3O4 [57,63], while Figure 4 showed the better electrochemical performance of the hybrid of SnO2 than that of each pure component [72,73], indicating the enhanced electrochemical performance by the hybridization of different transition metal-based chalcogenides.  [57]. Comparison of the MnCoOx anode and pure Mn3O4 material at 300 mA/g (c) and 1000 mA/g (d) [63].  [57]. Comparison of the MnCoO x anode and pure Mn 3 O 4 material at 300 mA/g (c) and 1000 mA/g (d) [63].  Due to the unique architecture and the synergetic effect between the two composites, the bind-free Co(OH)2@MnO2 nanosheets grown on Ni foam showed outstanding electrochemical performance [74]. Multilayer amorphous SnO2 and TiO2 thin films displayed good electrochemical performance due to the heterogeneous structure and the two components. The SnO2/TiO2 nanocomposite integrated the advantages of the long cycle life of Due to the unique architecture and the synergetic effect between the two composites, the bind-free Co(OH) 2 @MnO 2 nanosheets grown on Ni foam showed outstanding electrochemical performance [74]. Multilayer amorphous SnO 2 and TiO 2 thin films displayed good electrochemical performance due to the heterogeneous structure and the two components. The SnO 2 /TiO 2 nanocomposite integrated the advantages of the long cycle life of TiO 2 and the high specific capacity of SnO 2 , indicating the enhanced effect of hybridization [75]. The hybrid of transition metal selenides was also synthesized by different methods and exhibited superior electrochemical performance. The CoFeSe/NC anodes exhibited better electrochemical performances (775 mAh g −1 after 50 cycles at 0.2 A g −1 and 423 mAh g −1 at 3 A g −1 up to 1000 cycles) and dynamic kinetics than the single-metal selenides, profiting from the synergistic effect of the multi-metal components, and well-retained integrated architecture [76]. Zhang and Pang demonstrated the excellent electrochemical performance of the hybrid of transition metal selenides in energy storage systems, such as lithium-ion batteries, sodium-ion batteries, and supercapacitors, respectively [77,78]. The better electrochemical performance of the hybrid of X-chalcogenide and Y-chalcogenide than pure transition metal chalcogenide was summarized in Table 4.

Discussion and Prospect
As is shown in Tables 1-4 and Figures 1-4, the electrochemical performance of hybrid anode materials was always better than that of pure anode materials. Hybridization is an effective method to improve the electrochemical performance of transition metal chalcogenides. Some researchers ascribe the enhanced mechanism of hybridization to the synergetic effect of the different components [64][65][66], while some researchers believe the interspace of the different phases plays an important role [67]. However, the details of the enhanced mechanism or the synergetic effect have not been further investigated. Based on the experimental results, a suitable theoretical calculation model should be established, and the enhanced mechanism should be deeply studied by using classical molecular dynamics and first-principle calculations in detail. If the interaction of different atoms or different phases is explored clearly, transition metal chalcogenides with excellent electrochemical performance will be prepared and used as anodes practically.
Till now, the study of hybridization has always focused on the hybridization of different crystal components. The study of amorphous/amorphous and amorphous/crystal hybridization has not been widely explored. The amorphous materials have unique features, such as an isotropic nature and more defects, which could provide more active sites and accommodate the volume expansion during the charge-discharge process [79]. Many molecular dynamics simulations and experimental results indicate that the electrochemical performance of amorphous anodes is better than that of crystal anodes [80,81]. The electrochemical performance of amorphous and crystalline Sn@C composites was compared, and the better electrochemical performance of amorphous anodes was ascribed to the unique features of amorphous structures [82]. A novel amorphous MoS 2 /MoO 3 anode exhibited high specific capacity and cycling stability [83]. Compared to the crystal CoS anode, amorphous CoS exhibited much higher specific capacity and rate capability [10]. The amorphous CoO buffer was introduced to accommodate the volume expansion of CoO nanosheets, by which the CoO nanosheets/CoO film exhibited better electrochemical performance than CoO nanosheets [12]. Based on the unique features of amorphous structures, it is necessary to study structural hybridization combined with component hybridization, such as amorphous/amorphous and amorphous/crystal, which could accommodate the volume expansion and then improve the electrochemical performance of transition metal chalcogenides.

Conclusions
Due to the synergetic effect, hybridization is an effective way to enhance the electrochemical performance of transition metal chalcogenides as anodes in lithium-ion batteries. The hybridization could be divided into the hybridization between the transition metal and transition metal-based chalcogenide, and the hybridization between different transition metal-based chalcogenides. The electrochemical performance of a hybrid of transition metal chalcogenides is always better than that of pure transition metal chalcogenides. Though excellent electrochemical performance was obtained through the hybridization by many different preparation methods, the enhanced mechanism should be deeply investigated using classical molecular dynamics and first-principle calculations in the future. According to the unique features of amorphous structures, structural hybridization combined with component hybridization, such as amorphous/amorphous and amorphous/crystal, should be further investigated to improve the electrochemical performance. After a thorough understanding of the enhanced mechanism of the hybridization, we believe that a welldesigned hybrid of transition metal-based chalcogenides is more promising to be used as future anodes of lithium-ion batteries.
Author Contributions: All authors listed have made a substantial contribution to this review and approved it for publication. All authors have read and agreed to the published version of the manuscript.