Hexagonal CoSe₂ nanosheets: rational design towards stable sodium-ion storage

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries (LIBs) due to sodium's abundance and low cost, but the larger ionic size of Na⁺ causes slower ion diffusion, structural degradation, and limited material compatibility, leading to poor cycling stability and low rate performance. In this work, two-dimensional hexagonal CoSe₂ nanosheets embedded in N-doped carbon (2DH-CoSe₂@NC) were fabricated using a solvothermal process followed by gas-phase selenation. The unique architecture, combining a N-doped carbon matrix with an ultrathin 2D hexagonal morphology, enables strong synergistic interactions that enhance electrochemical behavior. As a result, the derived 2DH-CoSe₂@NC anode demonstrates remarkable rate performance and good cycling durability when applied in SIBs. Even after 2600 cycles at 5 A g^-1, the electrode delivers a stable reversible capacity of 516.2 mAh g^-1 and sustains coulombic efficiency close to 100% throughout prolonged cycling. Furthermore, the constructed full cells based on 2DH-CoSe₂@NC∥NVP exhibit a significant reversible capacity of 364.8 mAh g^-1 when operated at a current density of 1 A g^-1, highlighting their promising applicability in high power-density SIBs.