Exploring the Role of Spin Polarization in Enhancing Sodium Storage Capabilities of Two-Dimensional Transition Metal Thiophosphites

Two-dimensional ternary transition metal thiophosphites (MPS₃), as an emerging class of anode candidates for sodium ion batteries (SIBs), exhibit tremendous application potential. This study focuses on a series of 2D MPS₃ materials containing different transition metal elements (M = Mn, Fe, Co, and Ni) synthesized by the chemical vapor transport method. In-depth theoretical analysis employing density functional theory combined with crystal orbital Hamilton population reveals that NiPS₃ exhibits the lowest Na⁺ diffusion barrier and the strongest Na⁺ adsorption capability among the four materials. And these exceptional characteristics originate from the distinctive low-spin electronic configuration inherent in NiPS₃, triggering a pronounced electronic repulsion between bridging Ni and S atoms, consequently diminishing the strength of Ni-S bonds and substantially enhancing the electrochemical reactivity. Furthermore, the S element in NiPS₃ exhibits a highest p-band center energy level in the four samples, resulting in a corresponding decrease in the occupancy of the Na-S antibonding sigma orbital, which in turn strengthens the interaction between S and Na⁺. This enhanced interaction significantly improves the adsorption capability of NiPS₃ towards Na⁺, effectively facilitating the charge transfer at the interface. Electrochemical test results indicate that NiPS₃ exhibits the most superior electrochemical performance among the four materials, maintaining a specific capacity of 500.7 mA h g⁻¹ after 1100 cycles at 5 A g⁻¹. This study, through the profound integration of theory and experiments, fundamentally elucidates and verifies the underlying reasons for the superior properties of NiPS₃ in MPS₃ anodes.