*3.3. Zn Ions Storage Devices*

Recent works suggest that the MXene can also be used as both anode and cathode materials for zinc ion hybrid supercapacitors (ZHSCs). As Qi Yang and co-workers demonstrated, the Zn−MXene capacitor fabricated by Ti3C2 cathode and the Zn@Ti3C2 anode displays 82.5% capacitance retention after 1000 cycles with excellent anti-self-discharge ability of 6.4 mV h−<sup>1</sup> [99]. The MXene (Ti3C2Tx) is directly used as anode coupled with manganese dioxide/carbon nanotubes (MnO2-CNTs) cathode material to fabricate a zincion capacitor (ZIC). The obtained ZIC in aqueous electrolyte possesses a high specific capacitance, energy density of 98.6 Wh kg−1, and high capacitance retention of ~83.6% after 15,000 cycles [100]. Wang et al. reported that when the δ-MnO2 cathode and MXene are grown on the cotton cloth, the assembled device would exhibit a high energy density of 90 Wh kg−<sup>1</sup> with capacitance retention of ~80.7% after 16,000 cycles [101]. Others would like to employ the MXene as a protective layer for suppressing the growth of Zn dendrites. Qian's group investigated the growth mechanism of metal zinc on the surface of Zn foil and binder-free Ti3C2Tx MXene@Zn hybrid firm and found that the Ti3C2Tx MXene@Zn film effectively restrains the growth of Zn dendrites with reversible plating/stripping of Zn. The developed MXene film would be a valid strategy for dendrite-free electrochemical storage devices [3]. Zeng et al. adopted the MXene with the reduced graphene oxide (rGO) to fabricate a 3D porous MXene-rGO aerogel for preventing the restacking of MXene flakes and enhancing the electrical conductivity and hydrophilicity of the final materials. The electrochemical results show that the ZHSC with the MXene-rGO aerogel cathode delivers a high specific capacitance of 128.6 F g−<sup>1</sup> coupled with high energy density of 34.9 Wh kg−<sup>1</sup> and long term stability for 75,000 cycles [2]. The comparison of the electrochemical performance of various MXene-based materials for Zn ions storage devices are listed in Table 3.

**Table 3.** Comparison of the electrochemical properties of different MXene-based materials for Zn ions storage devices.


#### **4. Conclusions**

In summary, this review was mainly focused on the synthetic method, the construction of MXene hybrids, and composite films or fibers in terms of their performance for various energy storage devices. The most recent advances of technologies (e.g., vacuum filtration, extrusion printing technique, and directly writing) for fabricating patterned MXene-based composite films or fibers with geometric flexibility in various energy storage devices were described. The multifunctional properties of MXenes enable them to be used as electrodes for SCs and anode materials for both lithium- and zinc-ion batteries. As we introduced

above, the electrochemical performance of MXene-based materials can be enhanced by tuning the chemical components, constructing the micro/nano structures, and enlarging the interlayer of MXene flakes. Advanced MXene-based macroscopically assembled films and fibers with dedicated design were also demonstrated in this work, which are expected to significantly enhance the geometric flexibility for portable energy storage devices. Thus, it is expected that the MXene-based nanostructures and advanced architectures for films and fibers will provide intriguing opportunities for next generation energy storage devices.

1. MXene offers attractive properties in SC applications, such as excellent conductivity, ultrahigh rate capability, adjustable composition, hydrophilic, and volumetric capacitance. Still, the stacking and aggregating problems reduce the interlayer spaces and lead to the sluggish kinetics for energy storage. Therefore, many MXene-based composites (e.g., 2D/2D 1T-MoS2/Ti3C2 heterostructures, Ti3C2/CuS, MXene/LDH, Ni–S/Ti3C2 nanohybrid, N-doped MXene, N, O co-doped carbon@MXene composite, dodecaborate/MXene) were designed for solving the problems via the surface modification, heteroatom doping, or crumpling process. Among the various materials, the Ni–S/Ti3C2 composite exhibits a superior capacity of 840.4 C g−<sup>1</sup> with a retention of 64.3% at 30 A g−<sup>1</sup> and a long cycle life. In addition, in order to settle the easy oxidation issue at positive potential (anodic oxidation process) of MXenes and most MXene-derived composites, some works focus on the enhancing of the WF to boost the antioxidant ability of MXene-based materials. PANI@MXene cathode material exhibits a larger WF due to the existence of the oxidation-resistive PANI layer on the surface of MXene compared with the pure metallic MXene, which enhance the electrochemical stability at a wider positive operating potential (0–0.6 V).

2. The special flexibility and self-assembly capability of MXenes enable them to be a versatile unit for constructing the macroscopic film and fiber electrodes. Several assembly strategies are developed, including spin-casting method, vacuum-assisted filtration method, and electrophoretic deposition/spin coating coupled method. Additionally, the energy storage performance can be further enhanced by optimizing the MXene synthesis conditions, tuning the MXene interlayer spacing, altering the types or amount of surface terminations, and compositing with other functional materials. Many recent works also revealed that the addition of reinforcement (e.g., carboxymethylated cellulose nanofibrils, bacterial cellulose) also acts as spacers for MXene to fabricate robust film electrodes with strong mechanical strength.

3. MXenes can be employed as the printing material for constructing the geometric flexible, printable, and free-standing electrochemical devices (e.g., MXene-based coplanar interdigital electrodes, printable or direct-write SC devices) due to their hydrophilicity, mechanical flexibility, and modifiability. In this regard, the design of certain formulations of MXene-based conductive inks with specific rheological properties for the compatibility with various patterning methods (e.g., screen printing, direct writing, and extrusion printing) was crucial for the fabrication of the printable energy storage devices.

4. MXenes can also be used for LIBs and Zn ions storage devices due to their layer structure for electrolyte ion adsorption, satisfactory electrical conductivity, and desirable ion transfer ability within the layers. Many recent works reveal that the compositing with other functional materials can further enhance the overall energy storage performance (e.g., GeOx (x = 1.57)@MXene, SnS2/Sn3S4 modified multi-layered Ti3C2 MXene hybrid, SnO2/MXene composite, MXene/liquid metal, silicon/MXene, and MXene-rGO aerogel). Among the various materials, the GeOx (x = 1.57)@MXene exhibits a high rate capability, superb capacity retention of ~929.6 mAh g−<sup>1</sup> at 1.0 C with high Coulombic efficiency of 99.6% after 1000 cycles.

With further substantial research effort on optimized methodologies, MXenes and their derivatives represent a promising platform in scalable and customizable manufacturing of multipurpose electrodes with wearable, flexible, and lightweight properties in the future. **Author Contributions:** Conceptualization, H.M. and S.Y.; investigation, C.J. and H.C.; data curation, H.C.; writing—original draft preparation, C.J. and H.C.; writing—review and editing, C.J. and H.C.; visualization, H.C.; supervision, S.Y.; project administration, S.Y.; funding acquisition, S.Y. and C.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number 21805237, U2003132. Fundamental Research Funds for the Central Universities and the Natural Science Foundation of Shaanxi Province, grant number 2021GXLH-Z-082 and 2020JZ-02 (S. Yang). Natural Science Foundation of Xinjiang Uygur Autonomous Region, grant number 2018D01C053. Tianshan Youth Planning Program of Science & Technology Department of Xinjiang Uygur Autonomous Region, grant number 2018Q013. Opening Foundation of the State Key Laboratory of Fine Chemicals, grant number KF2003.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No data support.

**Conflicts of Interest:** The authors declare no conflict of interest.
