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.

A new class of 2D materials named “MXene” has recently received significant research interest as they have demonstrated great potential for the applications in batteries, supercapacitors, and electronic devices. However, the research on their thermal properties is still very limited. In this work, Ti3C2Tx films were prepared by the vacuum-assisted filtration of delaminated nano-flake Ti3C2Tx MXenes. The thermal and electrical conductivity of the Ti3C2Tx films were measured by the state-of-the-art T-type method. The results showed that the effective thermal conductivity of the films increased from 1.26 W·m−1·K−1 at 80 K to 2.84 W·m−1·K−1 at 290 K, while the electrical conductivity remained at 12,800 Ω−1·m−1 for the same temperature range. Thermal resistance model was applied to evaluate the inherent thermal conductivity of the Ti3C2Tx flakes, which was estimated to be in the range of tens to hundreds W·m−1·K−1.

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The microstructure of large-capacity anodes is of great importance in determining the performance of sodium- and potassium-ion batteries. Yolk–shell nanostructures promise excellent structural stability but suffer from insufficient charge transfer rate during cycles. Herein, we tackle this challenge by constructing a single-walled carbon nanotube (SWNT) internally bridged yolk–shell structure, inside which SWNTs cover the surface of the yolk and connect the yolk and shell, for better electron/ion transportation. Combining the merits of both yolk–shell structure and conductive SWNT channels, the as-prepared Fe1–x S/SWNT@C composite manifests high reversible capacity and ultralong cycling stability up to 8700 cycles. Moreover, it displays the best rate capability (317 mA h g–1 at 20 A g–1 for Na+ and 236 mA h g–1 at 10 A g–1 for K+) among the reported yolk–shell structures and iron-sulfide-based anodes thus far. The kinetic analysis and density functional theory calculations further reveal that the Fe1–x S/SWNT heterointerface can effectively enhance the reversibility of K+ storage and decrease the K+ diffusion energy barrier, leading to excellent pseudocapacitive behavior and fast ion transportation for outstanding rate capability.