Topotactic Transformation Synthesis of 2D Ultrathin GeS2 Nanosheets toward High-Rate and High-Energy-Density Sodium-Ion Half/Full Batteries

Currently, development of metal sulfide anodes for sodium-ion batteries (SIBs) with high capacity, fast charging/discharging, and good cycling performance continues to present a great challenge. Hence, a topochemical conversion strategy is reported to fabricate 2D ultrathin GeS₂ nanosheets (thickness: ∼1.2 nm) as the potential anodes for sodium storage. The 2D ultrathin nanostructure can mitigate the electrode-electrolyte contact issue faced by bulk material and provide shorter transport/diffusion pathways for Na ions and electrons, resulting in excellent rate performance. Impressively, ultrathin GeS₂ nanosheets can bring a large capacity of 515 mAh g^-1 even after 2000 cycles under 10 A g^-1. Additionally, as revealed by calculations and in situ/ex situ technique analysis, a favorable mechanism of Na⁺ intercalation/deintercalation into/from the GeS₂ interlayer region (GeS₂ ↔ Na_xGeS₂) is demonstrated. Furthermore, when coupled with the advanced cathode of Na₃V₂(PO₄)₂O₂F, the sodium-ion full cell shows a stable high energy density (213 Wh kg^-1), which makes our ultrathin GeS₂ nanosheets a promising candidate for SIBs.