S‐Decorated Porous Ti 3 C 2 MXene Combined with In Situ Forming Cu 2 Se as Effective Shuttling Interrupter in Na–Se Batteries

Given natural abundance of Na and superior kinetics of Se, Na─Se batteries have attracted much attention but still face the problem of shuttling effect of soluble intermediates. The first-principle calculations reveal the S-decorated Ti3 C2 exhibits increased binding energy to sodium polyselenides, suggesting a better capture and restriction on intermediates. The obtained Se@S-decorated porous Ti3 C2 (Se@S─P─Ti3 C2 ) exhibits a high reversible capacity of 765 mAh g-1 at 0.1 A g-1 (calculated based on Se), ≈1.2, 1.3, and 1.7 times of Se@porous Ti3 C2 (Se@P─Ti3 C2 ), Se@Ti3 C2 , and Se, respectively. It gives considerable capacity of 664 mAh g-1 at 20 A g-1 and impressive cycling stability over 2300 cycles with an ultralow capacity decay of 0.003% per cycle. The excellent electrochemical performance can be ascribed to the S-modified porous Ti3 C2 , which provides effective immobilization toward polyselenides, makes full use of nanosized Se, and alleviates volume expansion during sodiation/desodiation. Additionally, in situ forming Cu2 Se can generate Cu nanoparticles through discharge process and then transform polyselenides into solid-phase Cu2 Se, further suppressing the shuttling effect. This work provides a practical strategy to immobilize and transform sodium polyselenides for high-capacity and long-life Na─Se batteries.