Exploring hybrid hard carbon/Bi2S3-based negative electrodes for Na-ion batteries

Abstract

The study presents a hybrid hard-carbon/nanocrystalline-Bi₂S₃ material applicable for negative electrodes in sodium-ion batteries. Through a series of comprehensive analyzes, including electrochemical measurements, operando XRD, ex situ solid-state NMR, and high-resolution STEM imaging, the effectiveness of the HC/Bi₂S₃ hybrid configuration in the negative electrode function is elucidated with a focus on the underlying charge storage mechanism. Electrochemical analysis demonstrates the improved performance of the hybrid materials over the pristine HC negative electrode and highlights the robustness and stability of the HC/Bi₂S₃ hybrids over prolonged cycling even under high current densities. Here, the final capacities observed after 100 cycles reached a value of 252 mA h g⁻¹, compared to 216 mA h g⁻¹ of pristine HC. Cyclic voltammetry measurements demonstrate a complex charge storage behavior that integrates both surface and diffusion-driven processes at different potentials during reduction and oxidation. A series of phase transformations during cycling observed in operando XRD expose irreversible reactions during the initial cycle between Bi₂S₃ and sodium ions, such as the breakdown of the Bi₂S₃ nanocrystal structure. This phenomenon is further confirmed by the detection of Na₂S species using ex situ solid-state NMR. High-resolution STEM imaging reveals morphological changes in Bi₂S₃ nanocrystals and highlights their resistance to pulverization due to their nanoscale dimensions. This work provides comprehensive insights into the electrochemical performance of the HC/Bi₂S₃ and sheds light on specific mechanisms and reactions occurring during cycling.