Potassium-ion battery anode materials with high capacity always hold one or more K ions and are companied by large volume swelling, which threatens the stability of solid-electrolyte-interface (SEI) layers, and results in low coulombic efficiency as well as inferior cycling stability. Herein, an avenue that induces the rapid formation of continuous SEI layers by the confinement effect to boost K ions storage property is proposed. CuS nanoplates are dispersed on the core layer of carbon nanofibers and further confined by the Nb 2 O 5-C shell layer, constructing core-shell structure CuS-C@Nb 2 O 5-C nanofibers (NFs). The shell layer protects the CuS nanoplates from immediate contact with the electrolyte and brings about volume expansion, assisting the rapid formation of continuous SEI layers. As a result, the capacity retention of the CuS-C@Nb 2 O 5-C NFs electrode remains at 93.1% after 100 cycles, much larger than that of the CuS-C NF electrode (74.6%); the process that coulombic efficiency stabilized above 99.0% shortens to 5 cycles from 30 cycles. This progress is also found in the CoS 2-C@ Nb 2 O 5-C and NiS 2-C@Nb 2 O 5-C NFs electrodes. The improved coulombic efficiency and cycling stability brought about by the confinement effect offer a facile approach to boost the K ion storage property of conversion reaction anodes.

working mechanism of LIBs, K ions in the potassium-ion batteries (KIBs) also shuttle between the cathode and anode through the separator. Luckily, the potassium resources are much sufficient in the crust (2.09 wt%), [1] which may endow the KIBs low cost. Moreover, merits such as low standard reduction potential of K + (−2.93 V vs SHE), fast K ions transport kinetics in electrolyte, and "internal fuse" effect brought by the low melting point of K metal (63.4 °C), [2] make KIBs potential alternatives to LIBs in some scenarios. Therefore, it is meaningful and urgent to develop KIBs electrode materials with excellent K ions storage properties. Among the candidates for KIBs anode, conversion-alloy reaction materials have attracted considerable attention recently. [3-7] These materials are always have high theoretical specific capacity and suitable working potentials. For example, antimony tri-sulfide (Sb 2 S 3) proceeds a conversion reaction for the formation of Sb and subsequent alloy reaction for the formation of K 3 Sb, delivering a theoretical capacity of 974 mAh g −1. [6] However, an inevitable volume expansion is companied and that destroy the microstructure of the active materials, leading the capacity degradation. What is worse, sulfur species in the intermediate products are ambiguous and readily dissolved in the electrolyte, resulting in the loss of sulfur in the sulfides. Liu et al. reported the by-product sulfur and poly-sulfides in the intermediates of Sb 2 S 3 anode, and the irreversible conversion of Sb 2 S 3 could be the main reason for the failure of Sb 2 S 3 anode in KIBs. [8] Wang et al. observed the formation of poly-sulfides (K 2 S 4) when adopting Bi 1.11 Sb 0.89 S 3 nanotube as KIBs anode. [7] Recently, Zhang and coauthors found that the potassiated products (K 3 Sb and K 2 S) can transform into the original Sb 2 S 3 when they evaluated Sb 2 S 3 @C nanowire for KIBs anode using in situ transmission electron microscopy (TEM), though the electrode also suffered a capacity degeneration. [3] These works suggest that the total electrochemical reaction reversibility of Sb 2 S 3 anode for KIBs is viable in thermodynamics, while the reaction route and the intermediate products may be mainly controlled by the kinetics. However, few works on Sb 2 S 3 anode achieved the full reversibility and long cycling stability up to now. Therefore, it is meaningful to stimulate reversibility of Sb 2 S 3 anode via constructing neat Conversion-alloy sulfide materials for potassium-ion batteries (KIBs) have attracted considerable attention because of their high capacities and suitable working potentials. However, the sluggish kinetics and sulfur loss result in their rapid capacity degeneration as well as inferior rate capability. Herein, a strategy that uses the confinement and catalyzed effect of Nb 2 O 5 layers to restrict the sulfur species and facilitate them to form sulfides reversibly is proposed. Taking Sb 2 S 3 anode as an example, Sb 2 S 3 and Nb 2 O 5 are dispersed in the core and shell layers of carbon nanofib...

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