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Abstract2D transition‐metal carbides and nitrides, named MXenes, are promising materials for energy storage, but suffer from aggregation and restacking of the 2D nanosheets, which limits their electrochemical performance. In order to overcome this problem and realize the full potential of MXene nanosheets, a 3D MXene foam with developed porous structure is established via a simple sulfur‐template method, which is freestanding, flexible, and highly conductive, and can be directly used as the electrode in lithium‐ion batteries. The 3D porous architecture of the MXene foam offers massive active sites to enhance the lithium storage capacity. Moreover, its foam structure facilitates electrolyte infiltration for fast Li+ transfer. As a result, this flexible 3D porous MXene foam exhibits significantly enhanced capacity of 455.5 mAh g−1 at 50 mA g−1, excellent rate performance (101 mAh g−1 at 18 A g−1), and superior ultralong‐term cycle stability (220 mAh g−1 at 1 A g−1 after 3500 cycles). This work not only demonstrates the great superiority of the 3D porous MXene foam but also proposes the sulfur‐template method for controllable constructing of the 3D foam from 2D nanosheets at a relatively low temperature.

2D titanium carbide (Ti 3 C 2 T x MXene) has potential application in flexible/ transparent conductors because of its metallic conductivity and solution processability. However, solution-processed Ti 3 C 2 T x films suffer from poor hydration stability and mechanical performance that stem from the presence of intercalants, which are unavoidably introduced during the preparation of Ti 3 C 2 T x suspension. A proton acid colloidal processing approach is developed to remove the extrinsic intercalants in Ti 3 C 2 T x film materials, producing pristine Ti 3 C 2 T x films with significantly enhanced conductivity, mechanical strength, and environmental stability. Typically, pristine Ti 3 C 2 T x films show more than twofold higher conductivity (10 400 S cm −1 vs 4620 S cm −1 ) and up to 11-and 32-times higher strength and strain energy at failure (112 MPa, 1,480 kJ m −3 , vs 10 MPa, 45 kJ m −3 ) than films prepared without proton acid processing. Simultaneously, the conductivity and mechanical integrity of pristine films are also largely retained during the long-term storage in H 2 O/O 2 environment. The improvement in mechanical performance and conductivity is originated from the intrinsic strong interaction between Ti 3 C 2 T x layers, and the absence of extrinsic intercalants makes pristine Ti 3 C 2 T x films stable in humidity by blocking the intercalation of H 2 O/O 2 . This method makes the material more competitive for real-world applications such as electromagnetic interference shielding.

Isotropic negative permeability resulting from Mie resonance is demonstrated in a three-dimensional (3D) dielectric composite consisting of an array of dielectric cubes. A strong subwavelength magnetic resonance, corresponding to the first Mie resonance, was excited in dielectric cubes by electromagnetic wave. Negative permeability is verified in the magnetic resonance area via microwave measurement and the dispersion properties. The resonance relies on the size and permittivity of the cubes. It is promising for construction of novel isotropic 3D left-handed materials with a simple structure.

An electrically tunable negative permeability metamaterial consisting of a periodic array of split ring resonators infiltrated with nematic liquid crystals is demonstrated. It shows that the transmitted resonance dip of the metamaterial can be continuously and reversibly adjusted by an applied electric field, and the maximum shift is about 210MHz with respect to the resonance frequency around 11.08GHz. Numerical simulation shows that the permeability is negative near the resonance frequency, and the frequency range with negative permeability can be dynamically adjusted and widened by about 200MHz by the electric field. It provides a convenient means to design adaptive metamaterials.