*3.2. Li Ions Battery (LIB)*

MXenes also display great potential in LIB due to their large available surface areas for electrolyte ion adsorption, satisfactory electrical conductivity, desirable ion transfer within the inter-layers (up to 1.3 nm), and rapid surface redox reaction. Additionally, the MXenes (e.g., Ti3C2) can also be used as functional host matrix for integrating other materials for further enhancing the overall energy storage performance of LIB [91]. For instance, Junjie Niu's group synthesized a hybrid material comprising Ti3C2 MXene wrapped with germanium oxide layer (GeOx (x = 1.57) @MXene) through the wet-chemical strategy. The high electronic conductivity of MXene and germanium enable 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 cycle. Especially, the as-designed LIB also possesses temperature dependence properties [91]. Likun Pan's group prepared a SnS2/Sn3S4 modified multi-layered Ti3C2 MXene hybrid (denoted as S-TC) as anode material for LIB via the solvothermal and calcination strategy. The MXene substrate provides high electronic conductivity and suppresses the aggregation and volume change problems of active components while the nano-sized SnS2/Sn3S4 acts as a "spacer" to effectively inhibit restacking of the Ti3C2 layer. Due to these reasons, the S-TC anode delivers superb rate capability (216.5 mAh g−<sup>1</sup> at 5 A g−1) coupled with long cycling stability [92]. Wang et al. rationally designed an interconnected MXene hybrid aerogel composited with Fe2O3 nanospheres, which exhibits superb energy density of 216 Wh kg−<sup>1</sup> at 400 W kg−<sup>1</sup> for lithium-ion capacitors due to the synergistic effect between the two components [93]. Husam N. Alshareef et al. synthesized a HfO2 coated SnO2/MXene composite anode material via atomic layer deposition. The deposited SnO2 on the MXene can effectively suppress the degradation of MXene while the thin inactive layer of HfO2 would serve as an artificial solid-electrolyte

interphase (SEI) layer for further enhancing the cycling stability [94]. Wang et al. found that the integrating of nitrogen and vanadium in the forms of C–V–OH, C–V–O, V–O, and Ti–O–N species into the Ti-deficient Ti3C2Tx can further enhance the charge storage capability for approximately 40% [95]. Wang and co-workers modulated the interfacial properties by fabricating a crumpled S-functionalized Ti3C2Tx heterostructure embedded with Fe3O4/FeS. Due to the tuned electronic properties, the heterostructure displays improved kinetics and structural stability for LIB. The authors claimed that the S terminations boosted the extra (pseudo)capacitive ability of MXene for lithium storage. Due to these reasons, the optimized anode of the heterostructure material delivers a superb long-term stability (913.9 mAh g−<sup>1</sup> after 1000 cycles) with desirable rate performance. They also found that the heterostructure material exhibits an asymmetric conversion mechanism by experiencing stepwise phase transformations under discharge process coupled with a relatively uniform reconversion under the charge process. This work gives an in-depth understanding about MXene-based heterostructure for Li ion storage [96].

Yan and co-workers improved the properties of MXene by employing a liquid nitrogen quenching method to roll up the MXene sheets into Ti3C2Tx scrolls. They found that the prepared scrolls display unclosed topological structure and can effectively relieve the restacking effect. Meanwhile, they also reported that the produced scrolls possess superior electrical conductivity and can be used as buffer matrixes of Si nanoparticles for electrochemical energy storage. Due to these reasons, the as-designed Ti3C2Tx scroll anode material exhibits a reversible capacity of 226 mAh g−1, desirable rate performance, and excellent long-term cycling performance with 81.6% capacity retention after 500 cycles in LIB.

Additionally, MXene materials can be directly used as conductive and highly stable hosts by forming artificial solid electrolyte interface (SEI) film for greatly reducing the topical current density on the surface of the electrode, adjusting the electric field, and efficiently inducing the uniform growth of lithium dendrites. As exemplified, Shubin Yang's group demonstrated that the parallelly aligned MXene layers (denoted as PA-MXene-Li) can effectively regulate the uniform nucleation and growth of lithium, forming horizontal-growth of lithium on the surface of MXene. Moreover, the fluorine terminations of MXene make forming a durable and artificial solid electrolyte interface with lithium fluoride and homogenizing the electro-migration for lithium ions during the energy storage process possible [97].

Flexible MXene thin films or free-standing electrodes. The diversification development of LIBs (portable, flexible, wearable, stretchable, and foldable, etc.) is urgently required for ever-increasing demands for future energy-storage devices. In this respect, exploring conductive and flexible substrate with strong coupling of active materials is the key factor for the application of flexible LIBs technology. Given the self-assembly ability, 2D MXene are considered as ideal matrix to load active materials for forming composite paper electrodes in flexible LIBs. For example, Yitai Qian's group developed a MXene/liquid metal paper by confining the low-melting point GaInSnZn liquid metal in the MXene substrate. Due to the excellent conductivity and satisfactory wettability between GaInSnZn and MXene matrix, the obtained flexible anode for LIB exhibits a higher capacity of 638.79 mAh g−<sup>1</sup> at 20 mA g−<sup>1</sup> coupled with desirable rate performance (which is better than that of liquid metal coated Cu foil), indicating that the great potential of MXene matrix for compositing with various active materials in flexible LIB [98]. Feng et al. demonstrated that the construction of the Silicon/MXene composite papers via vacuum filtration to covalently anchor silicon nanospheres on MXene can accommodate the volumetric expansion of silicon and restrain the restacking of MXene sheets, which offers superior capacity of 2,118 mAh·g−<sup>1</sup> at a current density of 200 mA·g−<sup>1</sup> [18]. The electrochemical performances of different MXene-based materials for LIBs are compared and listed in Table 2.


**Table 2.** Comparison of the electrochemical properties of different MXene-based materials for LIBs.
