Preparation Of Mixed Metal Sulfide Composites And Its Application In Sodium-ion Capacitors

Sodium-ion capacitors(SICs)combined the high energy density of sodium-ion batteries with the high power density of supercapacitors and had received widespread attention.At present,the main bottlenecks faced by SICs were the low specific capacity,poor cycle performance,and sluggish reaction kinetics of the negative electrode materials,which were caused by the large radius of sodium ions.Among many negative electrode materials,metal sulfides based on conversion or alloying reaction mechanisms owned higher theoretical specific capacities and were very suitable as negative electrode materials for SICs.However,intrinsic metal sulfides suffered from poor cycle stability and sluggish kinetics during sodium storage.Therefore,in order to further improve the electrochemical performance of metal sulfides,we prepared the mixed metal sulfides and reduced graphene oxide(RGO)composites.Among them,mixed metal sulfides with mixed phases of different metal sulfides had higher specific capacity,which was mainly due to the richer redox reactions of mixed metal sulfides.In addition,RGO provided a good carbon substrate for mixed metal sulfide,so that the mixed metal sulfide nanoparticles grew uniformly on the RGO surface,and produced a pseudocapacitive effect.The pseudocapacitive effect can speed up the reaction kinetics of mixed metal sulfides and improve their rate performance.In order to better study the relationship between material structure and performance,this paper divided mixed metal sulfides into three categories according to the energy storage mechanism.The first was the mixing of metal sulfides based on conversion and conversion energy storage mechanisms(conversion-conversion),and its representative materials were CoS₂/NiS₂-RGO,MoS₂/CoS₂-RGO,and Mo₂S₃/NiS₂-RGO.The second was the mixing of metal sulfides based on conversion and alloying energy storage mechanisms(conversion-alloying),and its representative materials were Sn₂S₃/CoS₂-RGO and MoS₂/SnS₂-RGO.The third was metal sulfide mixing based on alloying and alloying energy storage mechanism(alloying-alloying),and its representative substance was SnS₂/ZnS-RGO.The six mixed metal sulfides-RGO composite materials were prepared by a one-step hydrothermal method.The main research contents of this paper were:CoS₂/NiS₂-RGO,MoS₂/CoS₂-RGO,Mo₂S₃/NiS₂-RGO,Sn₂S₃/CoS₂-RGO,MoS₂/SnS₂-RGO,and SnS₂/ZnS-RGO in 1 M Na Cl O₄(EC+DEC+5 wt%FEC)specific capacity,rate performance,cycle stability,sodium storage mechanism,pseudocapacitive performance and diffusion coefficient in electrolyte.The experimental results showed that the electrochemical performance of all mixed metal sulfide-RGO composites was better than that of single metal sulfide-RGO composites in their respective systems.More importantly,by comparing the six composite materials,it can be seen that MoS₂/SnS₂-RGO had the highest specific capacity(818.9 m A h g^-1at 0.1 A g^-1)due to its unique three-dimensional structure and the high theoretical specific capacity of SnS₂.Mo₂S₃/NiS₂-RGO had the highest rate performance,which was mainly due to its high pseudocapacitive effect(The contribution rate of the pseudocapacitive of Mo₂S₃/NiS₂-RGO can reach 99%at1.0 m V s^-1).SnS₂/ZnS-RGO did not show the highest specific capacity,which was due to the fact that the metal produced by the conversion reaction had not all undergone alloying reaction,resulting in a decrease in specific capacity.Finally,in order to verify the practical application of mixed metal sulfide-RGO composite materials in SICs,each composite materials and carbon materials constitute a sodium-ion capacitor:CoS₂/NiS₂-RGO//commercial carbon(CC),MoS₂/CoS₂-RGO//CC,Mo₂S₃/NiS₂-RGO//CC,Sn₂S₃/CoS₂-RGO//MBSC(porous carbon derived from mung bean skin),MoS₂/SnS₂-RGO//CC,and SnS₂/ZnS-RGO//PCPSK(porous carbon derived from peanut shell).The six SICs all showed good specific energy,specific power and cycle stability.The above results indicated that the mixed metal sulfide-RGO composite materials had great application potential in SICs negative materials.