S-Decorated Porous Ti₃C₂ MXene Combined with In Situ Forming Cu₂Se 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 Ti₃C₂ exhibits increased binding energy to sodium polyselenides, suggesting a better capture and restriction on intermediates. The obtained Se@S-decorated porous Ti₃C₂ (Se@S-P-Ti₃C₂) 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 Ti₃C₂ (Se@P-Ti₃C₂), Se@Ti₃C₂, 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 Ti₃C₂, which provides effective immobilization toward polyselenides, makes full use of nanosized Se, and alleviates volume expansion during sodiation/desodiation. Additionally, in situ forming Cu₂Se can generate Cu nanoparticles through discharge process and then transform polyselenides into solid-phase Cu₂Se, 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.