Designing a binary sulfide/carbon polyhedron for secondary batteries with high electrochemical and thermal performances

Abstract

Sodium-ion (Na-ion) batteries are attractive for large-scale energy storage owing to the abundance of Na, its ionization energy comparable to Li, and the low Na⁺/Na redox potential. However, currently available anode materials remain suboptimal, limited by sluggish ion/electron transport and large volume changes during cycling. Here, we report a heterostructured binary sulfide/carbon (Cu₇S₄/Co₉S₈/C) polyhedron for a Na-ion battery anode, which exhibits high performance across diverse cycling rates and temperatures. In situ X-ray diffraction and Raman spectroscopy demonstrate reversible structural evolution during cycling. The Cu₇S₄/Co₉S₈/C anode achieves a high capacity of 556 mAh g⁻¹ after 300 cycles at 0.5 A g⁻¹ with a coulombic efficiency >99% and maintains 508 mAh g⁻¹ after 1300 cycles at 3.0 A g⁻¹. It also exhibits strong thermal tolerance, retaining 486 mAh g⁻¹ after 500 cycles at 50 °C. Moreover, pairing the Cu₇S₄/Co₉S₈/C anode with a NaVPO₄ cathode yields excellent full-cell performance, underscoring practical potential. To further evaluate the thermal properties, the 3ω method is employed to quantify the effective thermal conductivity of the composite. The Cu₇S₄/Co₉S₈/C architecture delivers a thermal conductivity of 0.30 W m⁻¹ K⁻¹, improving by ∼25% and ∼20% over Cu₇S₄/C and Co₉S₈/C, respectively. These findings highlight a generalizable heterostructure design strategy for high-performance anodes and provide guidance for engineering energy-storage materials and safe secondary batteries.