expected to be a promising candidate for emerging smart grid technology in the near future. Nevertheless, the scarcity and uneven distribution of lithium resources hamper its further development. In pursuit of alternatives to LIBs for large-scale applications, sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have great potential, owing to the low cost and high abundance of Na (2.36 wt%) and K (2.09 wt%) in the Earth's crust, as well as its similar chemical properties to those of lithium. [1][2][3][4][5] Much work so far has focused on SIBs, and significant progress has been achieved in the past few years. [6][7][8] On the contrary, the development of PIBs is still in its infancy, probably due to the larger ionic radius of K + (1.38 Å) than those of Na + (1.02 Å) and Li + (0.76 Å). [9] However, PIBs possess several advantages compared with SIBs, such as the more negative standard potential of K + /K (−2.93 V vs SHE, compared with −2.71 V for Na + / Na), reversible intercalation/deintercalation of K + in graphite (theoretical capacity of 279 mA h g −1 ), and fast ionic conductivity of K + in liquid electrolyte. [10][11][12] These properties of PIBs offer exciting opportunities to achieve low-cost batteries with high energy density and good rate performance. Nevertheless, it remains challenging to fabricate suitable electrode materials, The potassium-ion battery (PIB) represents a promising alternative to the lithium-ion battery for large-scale energy storage owing to the abundance and low cost of potassium. The lack of high performance anode materials is one of the bottlenecks for its success. The main challenge is the structural degradation caused by the huge volume expansion from insertion/extraction of potassium ions which are much larger than their lithium counterparts.Here, this challenge is tackled by in situ engineering of a yolk-shell FeS 2 @C structure on a graphene matrix. The yolk-shell structure provides interior void space for volume expansion and prevents the aggregation of FeS 2 . The conductive graphene matrix further enhances the charge transport within the composite. The PIB fabricated using this anode delivers high capacity, good rate capability (203 mA h g −1 at 10 A g −1 ), and remarkable long-term stability up to 1500 cycles at high rates. The performance is superior to most anode materials reported to date for PIBs. Further in-depth characterizations and density functional theory calculations reveal that the material displays reversible intercalation/deintercalation and conversion reactions during cycles, as well as the low diffusion energy barriers for the intercalation process. This work provides a new avenue to allow the proliferation of PIB anodes.

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Ti 3 C 2 T x (MXene) exhibits attractive properties in different applications. However, traditional synthesis leads to unsatisfactory yield of two-dimensional (2D) Ti 3 C 2 T x , e.g., lower than 20%, which stems from the strong interactions of potential Ti−Ti bonds and residual Ti−Al bonds between the adjacent Ti 3 C 2 layers, hindering the effective intercalation and delamination. Herein, we propose a facile hydrothermalassisted intercalation (HAI) strategy to boost the yield of 2D sheets, achieving a record high value of 74%. This HAI assists the diffusion and intercalation of reagent effectively, promoting the subsequent delamination; meanwhile, an antioxidant is applied to protect these Ti 3 C 2 T x from oxidation during the HAI process. Therefore, massive Ti 3 C 2 T x 2D sheets can be easily synthesized. Thanks to the synergistic effect of high conductivity and substantial terminated functionalities, these Ti 3 C 2 T x 2D sheets show promising application in supercapacitor, providing a high capacitance of 482 F g −1 . Besides, the ultrafast carrier dynamics results of Ti 3 C 2 T x 2D sheets clearly imply the promising application in photocatalysis due to the relatively long bleaching relaxation time. Our work not only paves the way for the mass production of Ti 3 C 2 T x 2D sheets but also provides insights into their electronic and optical properties. KEYWORDS: hydrothermal-assisted intercalation (HAI), Ti 3 C 2 T x , MXene, high yield, facile

We have systematically changed the number of atomic layers stacked in 2D SnO nanosheet anodes and studied their sodium ion battery (SIB) performance. The results indicate that as the number of atomic SnO layers in a sheet decreases, both the capacity and cycling stability of the Na ion battery improve. The thinnest SnO nanosheet anodes (two to six SnO monolayers) exhibited the best performance. Specifically, an initial discharge and charge capacity of 1072 and 848 mAh g were observed, respectively, at 0.1 A g. In addition, an impressive reversible capacity of 665 mAh g after 100 cycles at 0.1 A g and 452 mAh g after 1000 cycles at a high current density of 1.0 A g was observed, with excellent rate performance. As the average number of atomic layers in the anode sheets increased, the battery performance degraded significantly. For example, for the anode sheets with 10-20 atomic layers, only a reversible capacity of 389 mAh g could be obtained after 100 cycles at 0.1 A g. Density functional theory calculations coupled with experimental results were used to elucidate the sodiation mechanism of the SnO nanosheets. This systematic study of monolayer-dependent physical and electrochemical properties of 2D anodes shows a promising pathway to engineering and mitigating volume changes in 2D anode materials for sodium ion batteries. It also demonstrates that ultrathin SnO nanosheets are promising SIB anode materials with high specific capacity, stable cyclability, and excellent rate performance.

An effective multifaceted strategy is demonstrated to increase active edge site concentration in Ni0.33Co0.67Se2 solid solutions prepared by in situ selenization process of nickel cobalt precursor. The simultaneous control of surface, phase, and morphology result in as‐prepared ternary solid solution with extremely high electrochemically active surface area (Cdl = 197 mF cm−2), suggesting significant exposure of active sites in this ternary compound. Coupled with metallic‐like electrical conductivity and lower free energy for atomic hydrogen adsorption in Ni0.33Co0.67Se2, identified by temperature‐dependent conductivities and density functional theory calculations, the authors have achieved unprecedented fast hydrogen evolution kinetics, approaching that of Pt. Specifically, the Ni0.33Co0.67Se2 solid solutions show a low overpotential of 65 mV at −10 mV cm−2, with onset potential of mere 18 mV, an impressive small Tafel slope of 35 mV dec−1, and a large exchange current density of 184 µA cm−2 in acidic electrolyte. Further, it is shown that the as‐prepared Ni0.33Co0.67Se2 solid solution not only works very well in acidic electrolyte but also delivers exceptional hydrogen evolution reaction (HER) performance in alkaline media. The outstanding HER performance makes this solid solution a promising candidate for mass hydrogen production.

Silicene is a monolayer of Si atoms in a two-dimensional honeycomb lattice, being expected to be compatible with current Si-based nanoelectronics. The behavior of silicene is strongly influenced by the substrate. In this context, its structural and electronic properties on MgX2 (X = Cl, Br, and I) have been investigated using first-principles calculations. Different locations of the Si atoms are found to be energetically degenerate because of the weak van der Waals interaction with the substrates. The Si buckling height is below 0.55 Å, which is close to the value of free-standing silicene (0.49 Å). Importantly, the Dirac cone of silicene is well preserved on MgX2 (located slightly above the Fermi level), and the band gaps induced by the substrate are less than 0.1 eV. Application of an external electric field and stacking can be used to increase the band gap.

The Dirac physics of silicene is preserved on the WSe2 substrate with a sufficiently large band gap to withstand thermal fluctuations.

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