Synthesis of polyvalent ion reaction of MoS2/CoS2-RGO anode materials for high-performance sodium-ion batteries and sodium-ion capacitors.

Metal sulfide is the most promising anode material for sodium storage devices due to its high theoretical capacity and low cost. However, the practical application of metal sulfide is largely hindered by huge capacity fading during the sodiation/desodiation process. Here mixed bimetallic sulfides grown on reduced graphene oxide (MoS2/CoS2-RGO) are prepared via a facile hydrothermal method. MoS2/CoS2-RGO displays a unique 2D structure which provides large specific surface area for pseudocapacitive charge storage, polyvalent ion reaction for ultrahigh capacity, and a heterostructure to high Na-ion diffusion rate. The optimized MoS2/CoS2-RGO shows a considerable reversible capacity of 593.6 mA h g-1 at 100 mA g-1 over 50 cycles and a high rate capability of 215.8 mA h g-1 even at a high specific current of 5000 mA g-1. A reaction kinetics and galvanostatic intermittent titration technique analysis indicates that MoS2/CoS2-RGO possesses fast pseudocapacitive charge storage and high Na-ion diffusion rate, benefiting the kinetics balance between anode and cathode. With this special structure, SICs containing the anode deliver a high specific energy of 152.98 W h kg-1 at 562.5 W kg-1. Similarly, the SIB exhibits a good capacities of 64 mA h g-1 at the high rates of 5C over 100 cycles.