Ultrafast Na storage enabled by in-situ formed metal nanoparticles in a self-assembled 3D Na2S framework

Abstract

Most alloying-type anodes for Na-ion batteries are fundamentally limited in ultrafast-charging applications due to the formation of Zintl phases: intermetallic compounds with intrinsically high electrical resistivity. This work demonstrates how to overcome this fundamental limitation by employing metal sulfide-based conversion anodes (NiS, CuS, and MnS), which follow a distinct electrochemical pathway that avoids Zintl-phase formation. During cycling in ether-based electrolytes, these sulfides undergo conversion reactions that spontaneously generate highly conductive, in-situ formed metal nanoparticles embedded within a self-assembled three-dimensional nanoporous Na2S matrix. This unique composite structure forms a dual-function architecture that enables both efficient ion diffusion and long-range electron transport, even when using inexpensive microsized sulfide particles. Among the tested materials, NiS exhibits the best performance, delivering a reversible capacity of 600 mAh g–1 at 1C, exceptional cycling stability over 3800 cycles at 10C, and a high-rate capacity of 358 mAh g–1 at 30C. Density functional theory and machine-learning-based molecular dynamics simulations reveal that the strong Ni–S bonding in the intermediate phases suppresses nanoparticle coarsening, resulting in uniformly distributed, nanoscale Ni particles that form an efficient percolation network. These findings establish a new design paradigm for Zintl-free conversion anodes, offering a practical and scalable route toward high-performance, fast-charging Na-ion batteries.