*2.3. Other Synthesis Method*

Given that most of the commonly used synthetic methods have low exfoliation yields, some alternative and effective strategies were developed for boosting the yield of MXenes. Recently, Wu and co-workers reported a gentle water FAT method to prepare the MXene nanosheets by inserting water molecules into the multilayer of MXene and using the volume expansion process to raise the exfoliation efficiency. The yield of these FAT-MXene can reach to 39% and can further increase to 81.4% by the sonication treatment [51]. In addition, due to the water-freezing expansion force, the method can prevent the restacking and expand the space between the multilayer during the reaction process. The FAT-MXene also possesses larger flake size compared with MXenes obtained from other methods (Figure 2). Therefore, films or current-collector-free electrodes assembled from the FAT-MXene exhibit a high level of layer alignment and a low flake-to-flake contact resistance, which enables the FAT-MXene-based on-chip micro-supercapacitor to display a high areal capacitance of 23.6 mF cm−<sup>2</sup> and a high volumetric capacitance of 591 F cm<sup>−</sup>3.

**Figure 2.** The synthesis process of FAT assisted method [51].

#### **3. Electrochemical Energy Storage Applications**

The electrochemical energy storage systems implement energy conversion through electrophysical process (e.g., electrosorption of ions) or electrochemical redox reactions coupled with ions and electrons migration within electrolytes and electrodes. In these systems, 2D materials, such as graphene, black phosphorene, and MoS2 attract extensive concern due to their favorable morphological and electrical properties [52,53]. Additionally, recent advances in MXenes have enhanced the performance for storage devices (e.g., supercapacitors and metal-ion batteries) and exhibit extraordinary physical and chemical properties, such as mechanical flexibility, high tap density (4.0 g cm−3), high electrical conductance (3.1 × <sup>10</sup><sup>3</sup> S m−<sup>1</sup> for MXene and 2.0 × <sup>10</sup><sup>3</sup> S m−<sup>1</sup> for graphene) [42,54,55], outstanding volumetric capacity (up to 1500 F cm<sup>−</sup>3), and favorable hydrophilicity [56,57].
