MXenes exhibit high redox capacitance and high specific surface area, making them highly attractive for energy storage devices that require high volumetric energy density. Despite the excellent conductivity and layered structure of MXenes, the performance of pure MXene materials in electrochemical energy storage devices is still limited by some inherent drawbacks, such as poor structural stability and limited ion storage capacity. To overcome these issues, researchers have been exploring the combination of MXenes with other conductive materials, metal materials, or transition metal compounds to create composite materials that take advantage of synergistic effects, thereby enhancing energy density, cycling stability, and power density of the electrodes. For example, Gogotsi et al. [93] employed atomic layer deposition (ALD) to grow oxides on 2D Ti-MXene sheets to prepare SnO₂/MXene composite anodes (Figure 10a). This composite electrode not only utilizes the high lithium-ion capacity of SnO₂ but also maintains the structural and mechanical integrity of the MXene platform. Therefore, as a lithium-ion battery anode, the conductive MXene/SnO₂ sheets can buffer the volume changes that occur during lithiation and delithiation processes, exhibiting excellent electrochemical performance. Based on this, the HfO₂-passivated SnO₂/MXene electrodes further improve the structural integrity of the SnO₂/MXene anode during cycling. As a result, the composite anode demonstrates a stable specific capacity of up to 843 mAh g⁻¹. Wang et al. [94] utilized an H⁺-induced electrostatic self-assembly method to construct a hollow Fe₃O₄/C microsphere with inherent positive charges and few-layer MXene with inherent negative charges, thereby forming the Fe₃O₄/C@MXene composite structure (Figure 10b). XPS characterization results indicated that the spinel-structured Fe₃O₄ exhibited a mixed Fe²⁺/Fe³⁺ ratio, confirming its mixed valence state. Furthermore, Fe₃O₄ was successfully in situ encapsulated within an amorphous carbon matrix (Figure 10c). This encapsulation effectively protected Fe₃O₄ from degradation, thereby enhancing the stability and electrochemical performance of the composite material. The uniform and interconnected Fe₃O₄/C@MXene composite structure significantly promotes the rapid diffusion of Li⁺, smooth electron/charge transfer, and suppresses the pulverization of the material. As a result, the composite anode maintains an undegraded specific capacity of 907 mAh g⁻¹ over long-term cycling and demonstrates high energy density and cycling stability when applied in a 4.0 V lithium-ion capacitor (Figure 10d).

Compared to TMOs, many transition metal sulfides (TMSs), such as NiS_x and CoS_x, exhibit higher electronic conductivity due to the lower electronegativity of sulfur compared to oxygen. This gives TMSs a higher specific capacity than their oxide counterparts (e.g., NiO_x, CoO_x). However, TMSs often suffer from volume expansion during redox reactions, leading to rapid capacity decay and poor cycling stability in electrochemical energy storage applications. Hybridization and composite strategies with MXenes offer a potential solution to these issues. For instance, to improve the performance of nickel sulfide, Luo et al. [95] synthesized Ni-S/d-Ti₃C₂ nanohybrids via a simple solvothermal reaction (Figure 11a). The composite of d-Ti₃C₂ nanosheets with NiS not only prevented the aggregation of Ni-S nanostructures and alleviated their pulverization during cycling but also formed a synergistic interaction that enhanced the electron transport at the interface of the nanohybrid materials during charge/discharge processes. The XRD pattern demonstrated that the primary diffraction peaks of Ni-S/d-Ti₃C₂ corresponded with the typical peaks of the Ni-S phase. The (002) peak of d- Ti₃C₂ exhibited an increased interlayer spacing, which is beneficial for facilitating electrolyte ion diffusion (Figure 11b). Further confirmation came from XPS spectra, which indicated the interface interaction between Ni-S and d-Ti₃C₂ (Figure 11c). High-resolution Ni 2p spectra showed an increased Ni²⁺/Ni³⁺ratio (22.72% vs. 10.42%), promoting redox reactions (Figure 11d). Additionally, the slight left shift of the Ni 2p peak suggested the presence of chemical bonding between Ni-S and d-Ti₃C₂, optimizing the interface electronic structure and thereby enhancing the electrochemical activity of the composite. Furthermore, the re-stacking of d-Ti₃C₂ nanosheets was suppressed by the inserted Ni-S nanostructures, maintaining a larger active surface area. Consequently, the Ni-S/d-Ti₃C₂ nanohybrid exhibited significantly improved specific capacity and cycling stability compared to pure Ni-S materials. Similarly, Wang et al. [96] synthesized a highly stable 1T-MoS₂ nanosheet and Ti₃C₂ MXene composite with a three-dimensional interconnected microstructure. This 2D/2D heterostructure combined the collective advantages of 1T-MoS₂ nanosheets and Ti₃C₂ MXene, forming a robust synergistic interaction that effectively expanded the ion storage space, thereby enhancing capacitance. Additionally, due to the ultrafast electron transport from Ti₃C₂ MXene, the 1T-MoS₂/Ti₃C₂-based all-solid-state flexible symmetric supercapacitors exhibited outstanding rate performance. In a similar vein, MXene’s excellent electronic and ionic conductivity, anisotropic charge mobility, and rich tunable surface functional groups make it an attractive candidate for composites with small molecules and organic materials. This approach has been widely explored to address the solubility and low conductivity issues of organic materials in organic electrolytes. The functional group interactions between Ti₃C₂T_x MXene and organic materials can induce the polymerization and assembly of organic materials on the surface of the 2D MXene, resulting in uniform organic–inorganic hybrids, which is one of the common composite strategies. Gao et al. [97] successfully synthesized PAQS@Ti₃C₂T_x MXene hybrids through hydrogen bonding and S-Ti interactions (Figure 11e). This hybrid exhibits a typical intercalated structure, with Ti₃C₂T_x serving as an electronic and ionic transport channel, facilitating rapid charge and ion transfer. DFT calculations indicated that the introduction of MXene effectively reduced the adsorption energy of Na⁺, promoting the reaction kinetics. As a result, the PAQS@MXene hybrid material demonstrated excellent sodium storage performance when used as a cathode in SIBs, exhibiting exceptional rate capability. It is noteworthy that the composite of MXene with organic materials has also enhanced its application in lithium–sulfur (Li-S), sodium–sulfur, and even, aluminum–sulfur batteries. This is because functional organic/MXene composite materials not only inhibit the shuttle effect but also improve conductivity/ionic conductivity and provide a high sulfur load due to the metallic conductivity and rich MXene terminal groups. For example, various organic materials, such as CTF, CTF/TNS, and Ti₃C₂@iCON, have been combined with MXene to form composites that serve as multifunctional sulfur host materials to enhance the performance of Li-S batteries [98,99].