# Construction of Highly Active Co3S4/Fe7S8 Heterostructures Derived from Sodium Alginate for Enhanced Sodium Storage Performance

## TL;DR

This paper presents a new method to create Co3S4/Fe7S8 heterostructures from sodium alginate for better sodium storage in batteries.

## Contribution

The novel contribution is the fabrication of a Co3S4/Fe7S8 heterostructure using sodium alginate and a one-step sulfurization strategy.

## Key findings

- The SA-CoFe(1:4)-S composite shows high initial discharge/charge capacities of 723/1010 mAh·g−1 at 1 A·g−1.
- The material retains 806 mAh·g−1 after 800 cycles at 1 A·g−1 and 258 mAh·g−1 after 500 cycles at 3 A·g−1.
- Theoretical calculations confirm enhanced sodium ion adsorption and electrical conductivity due to the heterointerfaces.

## Abstract

Heterointerface engineering, especially the construction of heterointerfaces based on two highly active components, is an effective strategy to enhance the sodium storage capacity and accelerate the reaction kinetics of transition metal chalcogenide anodes. Herein, a series of SA-CoFe-S composites composed of two highly active metal sulfides, Co3S4 and Fe7S8, were fabricated through in situ chelation effects coupled with a one-step sulfurization strategy. The optimized SA-CoFe(1:4)-S is composed of fine nanoparticles encapsulated by uniformly distributed S-doped carbon. This unique carbon confinement effect and nano-sized active particles can alleviate volume expansion, shorten the ion diffusion distance, and accelerate electron transfer. In addition, the strong electric-field effect and rich heterointerfaces generated by the heterostructure provide more active sites for sodium storage and accelerate the sodium storage kinetics. The relevant theoretical calculation outcomes further confirm that the heterointerfaces formed between Co3S4 and Fe7S8 can enhance the adsorption energy toward sodium ions and boost the electrical conductivity of the composite material. As an anode material for sodium-ion batteries, the initial discharge/charge capacities were 723/1010 mAh·g−1, exhibited at 1 A·g−1, and the coulombic efficiency (CE) corresponding to this current density was measured to be 71.6%. Even after 800 cycles, the reversible discharge specific capacity of the electrode can still reach 806 mAh·g−1 at 1 A·g−1. Additionally, at an elevated current density of 3 A·g−1, the electrode sustains stable cycling over 500 cycles, with its discharge capacity kept at 258 mAh·g−1 after the long-term cycling test.