Unveiling the Synergy of Architecture and Anion Vacancy on Bi 2 Te 3- x @NPCNFs for Fast and Stable Potassium Ion Storage.

Large volume strain and slow kinetics are the main obstacles to the application of high-specific-capacity alloy-type metal tellurides in potassium-ion storage systems. Herein, Bi 2 Te 3- x nanocrystals with abundant Te-vacancies embedded in nitrogen-doped porous carbon nanofibers (Bi 2 Te 3- x @NPCNFs) are proposed to address these challenges. In particular, a hierarchical porous fiber structure can be achieved by the polyvinylpyrrolidone-etching method and is conducive to increasing the Te-vacancy concentration. The unique porous structure together with defect engineering modulates the potassium storage mechanism of Bi 2 Te 3 , suppresses structural distortion, and accelerates K + diffusion capacity. The meticulously designed Bi 2 Te 3- x @NPCNFs electrode exhibits ultrastable cycling stability (over 3500 stable cycles at 1.0 A g -1 with a capacity degradation of only 0.01% per cycle) and outstanding rate capability (109.5 mAh g -1 at 2.0 A g -1 ). Furthermore, the systematic ex situ characterization confirms that the Bi 2 Te 3- x @NPCNFs electrode undergoes an "intercalation-conversion-step alloying" mechanism for potassium storage. Kinetic analysis and density functional theory calculations reveal the excellent pseudocapacitive performance, attractive K + adsorption, and fast K + diffusion ability of the Bi 2 Te 3- x @NPCNFs electrode, which is essential for fast potassium-ion storage. Impressively, the assembled Bi 2 Te 3- x @NPCNFs//activated-carbon potassium-ion hybrid capacitors achieve considerable energy/power density (energy density up to 112 Wh kg -1 at a power density of 1000 W kg -1 ) and excellent cycling stability (1600 cycles at 10.0 A g -1 ), indicating their potential practical applications.