Investigating the Energy Storage Mechanism of SnS2-rGO Composite Anode for Advanced Na-Ion Batteries

Tin sulfide-reduced graphene oxide (SnS2-rGO) composite material is investigated as an advanced anode material for Na-ion batteries. It can deliver a reversible capacity of 630 mAh g(-1) with negligible capacity loss and exhibits superb rate performance. Here, the energy storage mechanism of this SnS2-rGO anode and the critical mechanistic role of rGO will be revealed in detail. A synergistic mechanism involving conversion and alloying reactions is proposed based on our synchrotron X-ray diffraction (SXRD) and in situ X-ray absorption spectroscopy (XAS) results. Contrary to what has been proposed in the literature, we determined that Na2S2 forms instead of Na2S at the fully discharge state. The as-formed Na2S2 works as a matrix to relieve the strain from the huge volume expansion of the Na-Sn alloy reaction, shown in the high resolution transmission electron microscope (HRTEM). In addition, the Raman spectra results suggest that the rGO not only assists the material to have better electrochemical performance by preventing particle agglomeration of the active material but also coordinates with Na-ions through electrostatic interaction during the first cycle. The unique reaction mechanism in SnS2-rGO offers a well-balanced approach for sodium storage to deliver high capacity, long-cycle life, and superior rate capability.