It is critical for large-scale energy storage to develop sodium-ion batteries (SIBs) due to their cost-effectiveness and abundant sodium resources. However, some challenges like sluggish kinetics from large Na⁺ radii and poor structural stability of electrode materials hinder their practical application. Transition metal sulfides (TMS) like SnS, NiS, and FeS exhibit high theoretical capacities, but suffer from some intrinsic drawbacks (i.e., low conductivity and severe volume expansion during cycling). To address these limitations, this study was to propose a novel strategy, i.e., designing a mixed-metal sulfide composite of (NiS/Cu₂(Fe,Co,Ni)SnS₄/CNT) with heterointerfaces and multi-metal synergy. The integration of Fe, Co, Ni, Cu, and Sn could enhance electronic/ionic transport, mitigate volume changes, and leverage catalytic effects of transition metals. Carbon nanotube (CNT) were introduced to further improve conductivity and structural integrity. This mixed-metal sulfide composite exhibited the excellent performance at a ultra-high current density.

The composite was synthesized via a co-precipitation method and a subsequent high-temperature sulfidation. For NiS/Cu₂(Fe,Co,Ni)SnS₄/CNT, stoichiometric amounts of FeSO₄, CoSO₄, NiSO₄, CuSO₄, and SnSO₄ were dissolved and co-precipitated with NaOH. The CNT slurry was ultrasonically dispersed and incorporated into the precursor. After sulfidation with thioacetamide in N₂ at 500 ℃, the final product was obtained. NiS/CNT (control sample) followed the similar process using only NiSO₄. The crystallinity, morphology, and elemental states of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). To evaluate their kinetics, rate capability and cycling stability, CR2032 half-cells assembled with Na metal counter electrodes were tested by cyclic voltammetry (CV), constant-current charge/discharge test, galvanostatic intermittent titration (GITT) and electrochemical impedance spectroscopy (EIS), respectively.

The results show that the complex consists of Cu₂(Fe,Co,Ni)SnS₄ and NiS biphasic phases, and the heterogeneous interfaces exist, which are confirmed by high-resolution transmission electron microscopy (HRTEM). The built-in electric field at the heterogeneous interface accelerates the charge transfer, and the multimetal synergy (Fe/Co/Ni/Cu/Sn) provides abundant catalytic sites and reduces the Na⁺ diffusion barrier (i.e., 32% reduction in the diffusion barrier according to density-functional theory calculations). The three-dimensional conductive network constructed by the CNTs inhibits the volume expansion efficiently (i.e., the thickness of the electrodes increases by only 37.9% after cycling, which is significantly lower than that of the NiS/CNT). For the electrochemical performance, NiS/Cu₂(Fe, Co, Ni)SnS₄/CNT exhibits an excellent high rate charge/discharge performance and a long cycle stability, and still maintains a discharge specific capacity of 626 mA·h·g^-1 after 1000 cycles at a current density of 5 A·g^-1, and 399 mA·h·g^-1 after 8000 cycles at an ultra-high current density of 20 A·g^-1. Capacitive-dominated storage and enhanced Na⁺ diffusivity outperform monometallic NiS/CNT. The TEM images and CV indicate a reversible phase transition between sulfide and Na₂S, thus validating the reaction mechanism.

The built-in electric field formed by the heterogeneous interface and the synergistic effect of polymetallic cations could enhance the ion and charge transfer rate, and the reduction of polymetallic ions to metal monomers facilitated the ion and electron transport during the charging and discharging process, effectively suppressing the polysulfide shuttling phenomenon, thus decreasing the polarization of the battery. The introduction of carbon nanotubes provided electron transport paths, which further enhanced the structural stability of the material. This study could have a great potential of mixed metal sulfides for application in anode materials for sodium-ion batteries.