A Study On Rechargeable Magnesium Batteries Based On Cu-based Conversion Cathodes

The urgent attention to energy crisis and environmental issues has led to the urgent need for low-cost,sustainable,safe and efficient energy storage batteries.Among many energy storage battery technologies,rechargeable magnesium battery has attracted more and more attention due to the fact that metal magnesium is not easy to form dendrites in the process of reversible deposition and has high natural abundance.However,searching for available cathode materials remains a significant challenge blocking its development.Mg²⁺has two positive charges,resulting in large charge density,strong polarization.Thus embedded cathode materials show low reversible capacity and dynamic performance.The conversion-type material,which is not limited by lattice host,has high theoretical capacity and selection diversity.On the other hand,at present,the main electrolyte for rechargeable magnesium battery is flammable and volatile organic liquid,which brings great security risks to the practical application of magnesium battery,and all solid polymer electrolyte has attracted much attention due to its unique high security.Given the hard issues of development of rechargeable magnesium battery,this thesis will choose CuS and Cu_2-xSe as high-capacity cathode materials.The conversion-type materials including CuS and Cu_2-xSe are systematically studied in the electrochemical storage of magnesium and reaction mechanism in this paper.At the same time,a polymer electrolyte based on PVDF-HFP is developed,and a rechargeable magnesium battery based on CuS and the polymer electrolyte is constructed.This work provides a theoretical basis and technical support for development of rechargeable magnesium batteries with low cost,high specific energy and high safety.The main research contents are as follows:1.Three kinds of hollow CuS nanocubes are synthesized and their magnesium storage properties and reaction mechanism are studied.The results show that the concentration of reactant will affect the size of the hollow structure——the reactant with lower concentration will produce the smaller size of hollow CuS nanocube.From the relationship between structure and performance,we can know that smaller particle size and thinner wall can help to shorten the solid-phase diffusion path,so as to improve the magnesium storage performance and kinetics.Reaction mechanism proves that CuS is a conversion-type material.Among the CuS hollow nanocubes,the smallest one（side length about 200 nm,wall thickness about 20 nm）shows the highest reversible capacity(200 m Ah g^–1 at the current density of 100 m A g^–1),the best rate performance and excellent cycle stability,which indicates that the morphology control is of great significance for the performance optimization of CuS materials.2.Two kinds of Cu_2-xSe materials are synthesized——hollow Cu_2-xSe nanocube and Cu_2-xSe nanoparticle.The magnesium storage properties and reaction mechanism are studied.The results show that Cu_2-xSe nanoparticle displays a reversible capacity of 200 m Ah g^–1 at the current density of 100 m A g^–1 while hollow Cu_2-xSe nanocube has higher reversible capacity(250 m Ah g^–1 at the current density of 100 m A g^–1),better rate performance and more excellent cycle stability.The above results show that the rechargeable magnesium storage performance of cathode materials depends on both the structure and morphology.The ex-situ reaction mechanism confirms that Cu_2-xSe is a conversion-type material.3.A series of polymer electrolytes are prepared based on PVDF-HFP and Mg（HMDS）₂ using solvent casting method.The results of physical and chemical characterization show when the mass ratios of Mg（HMDS）₂and PVDF-HFP are from1:2 to 1:10,the polymer electrolytes are amorphous and have good mechanical strength and toughness and the melting temperature is about 150°C.With the increase of magnesium salt content,the ionic conductivity increases gradually and the corresponding polarization of reversible magnesium deposition is relatively small.Furthermore,the cycling performance of Mg/polymer electrolyte/CuS battery system shows that the electrolyte membranes with higher magnesium salt content have higher capacities.