Preparation Of CuS And NiCo₂S₄ Nanomaterials And Their Application In Lithium-Air Batteries

In recent years,the problems of environmental pollution and energy shortage caused by traditional fossil energy which promotes the rapid development of economy are becoming more and more serious.Therefore,the development of clean energy and its corresponding storage and transformation technology to meet social needs is an important development direction at present.Li-Air batteries（LABs）,which have theoretical energy densities of 5 to 10 times that of existing lithium-ion batteries,have received widespread attention from researchers around the world and are considered to be the next-generation alternatives to lithium-ion batteries.However,there are still some problems in lithium-air batteries,such as unstable electrolyte,low energy efficiency,poor rate performance,short cycle life and high charge/discharge overpotential,which make the actual capacity difficult to reach the theoretical value.It is found that most of these problems depend on the interfacial reaction of the cathode catalyst.Therefore,the study of cathode catalysts is of great importance for the lithium-air batteries.Transition metal chalcogenides play a special role in the adsorption of the intermediate products of electrochemical reactions during the charge and discharge of lithium-air batteries because of their special band structure and outer electronic structure.What’s more,due to their low price,rich sources,easy preparation and excellent performance,transition metal chalcogenides have become the research focus of cathode catalysts.In this paper,CuS nanomaterials were synthesized by a chemical method and coated with graphene to prepare CuS@rGO nanocomposites.In addition,bimetallic sulfide NiCo₂S₄ nanomaterials were also prepared.The electrochemical performance of these catalyst materials as cathodes for lithium-air batteries was systematically studied.The main results of this work are as follows:1.Preparation of CuS nanomaterials with different morphologies and their electrochemical properties.In this paper,a simple,template free and mild solution chemistry method was used to synthesize copper sulphide（CuS）nanomaterials with different morphologies.In the electrochemical performance test of lithium-air battery for CuS nanotubes and CuS nanoflakes,CuS nanoflakes have a lower first charge voltage platform and better cycling stability.At a current density of 200 mA g-1 with a limited capacity of 500 mAh g-1 CuS nano flakes can circulate 73 cycles and CuS nanotubes 50 cycles.Because CuS nanoflakes provide larger specific surface area,better electrolyte penetration,and have more catalytic active sites and storage space for discharge products,they show superior electrochemical performance over CuS nanotubes.2.Preparation and electrochemical performance of CuS@rGO nanocompositesThe CuS@GO precursor was synthesized in water bath heating condition by using graphene aqueous solution as a component of the reaction solvent.After reduction by hydrothermal treatment,CuS@rGO nanocomposites were obtained.The high conductivity of graphene can facilitate the transportation of electrons between catalysts,discharge products and electrolyte.At the same time,the large specific surface area of graphene nanosheets can provide high density deposition active sites for insoluble discharge product Li2O2 and avoid cathode clogging.Therefore,the electrochemical properties of CuS@rGO nanocomposites have been greatly improved after graphene coating.3.Preparation and electrochemical performance of NiCo₂S₄ nanomaterialsBy using PEGylated DES as a solvent,NiCo₂S₄ nanomaterials were synthesized in a one step solventthermal method.The electrochemical performance test showed that NiCo₂S₄ nanomaterials have a lower charge and discharge overpotential of the first cycle and better cycle stability as a cathode catalyst for lithium air batteries.This is attributed to the relatively high specific surface area of NiCo₂S₄ nanomaterials,which can provide sufficient active sites and promote the diffusion of electrolytes and oxygen,thereby improving OER catalytic activity and cycle stability.