The Research On High Energy-Desnity Cathodes For Rechargeable Mg Batteries

With the increasing demand for green energy,lithium ion batteries?LIBs?a re becoming the dominant batteries for mobile electronic devices and electric vehicles.With the decade's development,the energy densities of LIBs are approaching the limitation of intercalation chemistry.Its capacities and energy density are hardly to b e greatly improved.Rechargeable metal batteries,which pair metal anodes with various cathode materials,offer great promise for further improving battery energy density,because metal anodes provide much higher capacity and lower reduction potential than intercalation anodes.Among all metal anodes,the high capacity?especially volumetric capacity of 3833 mA h/cm³ for Mg vs.2046 mA h/cm³ for Li?,low reduction potential?-2.4 V vs.SHE?and most importantly,dendrite-free deposition with 100%coulombic efficiency in some electrolytes make Mg metal an ideal anode.For this reason,there is a growing interest to develop rechargeable Mg batteries during the past decade.However,due to its bivalent nature,the diffusion of Mg²⁺in solid state cathode materials is much sluggish than monovalent cations like Li⁺etc.,which leads to large voltage hysteresis and low magnesiation degree for most materials.For this reason,finding cathode materials with acceptable kinetics and high energy density has become the major challenges for the development of rechargeable Mg batteries,which is also the emphasis of this dissertation:?1?Oxides are the most common cathode materials for LIBs.However,due to the high electronegativity and strong interaction between Mg²⁺and oxide,oxides suffer from sluggish kinetics in rechargeable Mg batteries.Therefore,applying anions with larger ion radius and less electronegativity,like sulfur and selenium,will lower the interaction and enlarge the diffusion channel of Mg²⁺,thus improving the kinetics of cathodes.As a cathode for LIBs,layered NbSe₃ has achieved fast kinetics,which inspires us to apply it in rechargeable Mg batteries.We prepared layered NbSe ₃ by solid state reaction,and used it as the cathode for rechargeable Mg ba tteries.NbSe₃can achieve high magnesiation level at room temperature,good kinetics with stable cycling performance and high coulombic efficiency.The weak interaction between layers of NbSe₃ contributes to the the diffusion of Mg²⁺while metallicity of Nb and electronegativity of Se is weak,which is benefit for the charge redistribution during diffusion of Mg²⁺.However,the energy density of NbSe₃?85 Wh/kg?is lower than Mo₆S₈?140 Wh/kg?,hindering its application in rechargeable Mg batteries.?2?To improve the energy density of cathode materials,we used NbS ₃ with higher capacities and voltage as the cathode for rechargeable Mg batteries.However,limited by sluggish kinetics,NbS₃ hardly intercalate any magnesium.Therefore,we investigated how the anionic chemistry of cathodes affects Mg²⁺diffusion kinetics.For the first time,we systematically investigated how different anionic chemistry?O,S,and Se?affect Mg²⁺migration in layered MX₂?M=Ti,V;X=O,S,Se?,by combining theoretical calculations with electrochemical characterizations.Selenides can achieve high electrochemical activation with high capacity retention after 40cycles.In contrast,sulfides can hardly intercalate any magnesium with few capacities after 20 cycles.Besides,the diffusion coefficient of Mg²⁺was calculated by GITT and EIS,in which selenides are three orders of magenitude higher than sulfides.The migration barriers for Mg²⁺in oxides,sulfides,and selenides are decreasing in this series,which is due to the difference in electronegativity and ionic radius.In addition,we proposed three criteria to explain why selenides have better kinetics than sulfides and oxides:larger diffusion channel due to larger interlayer spacing,weaker interaction between Mg²⁺and anions?Se?,and high electronic conductivity of selenide.Through extensive discussions,these three criteria not only can be applied to layered compounds,but also Chevrel phase,spinel,and olivine compounds.The method and disscussions in this chapter will be benefit for the research on the similar topic,and give a direction to design and modify cathode materials for rechargeable Mg batteries.?3?To further improve the energy density of rechargeable Mg batteries,we turn to CuS with higher theoretical capacity?560 mA h/g?.In view of the good electrochemical performance in LIBs,we hope CuS can further improve the energy density of rechargeable Mg batteries.When used as cathodes for rechargeable Mg batteries,CuS achieve a capacity of 430 mA h/g in the first discharge process at room temperature.Between 0.5 and 2.2 V,CuS can achieve the reversible capacity of 400mA h/g,corresponding to the energy density of 598 Wh/kg and 948 Wh/L,which are the highest among all the reported cathodes for rechargeable Mg batt eries.In addition,ex situ XRD demonstrate that CuS undergoes a displacement reaction process during magnesiation,resulting in good kinetics and cycling performance.From GITT,the calculated diffusion coefficient of Mg²⁺in CuS is 1.97×10^-14 cm²/s,higher than that of Li⁺in LiFePO₄.In view of the high capacities and energy density of CuS,it offer s great promising as the cathode for rechargeable Mg batteries.?4?In spite of high capacities and energy density of CuS,the kinetics is still less satisfied.While organic moleculars have the weak interaction with intercalated ions,and the excellent kinetics have been demonstrated in LIBs.HATN is an organic materials with high theoretical capacity,whose redox sites are based on the carbon and nitrogen double bonds.Based on that,we design poly-hexaazatrinaphthalene?PHATN?,in which HATN are linked via a carbon-carbon bond between two phenyl rings,utilizing the good electrochemical performance of HATN and low solubility in electrolyte.In sodium ion batteries,PHATN will achieve high capacity and excellent kinetics.At the current density of 5C even 25 C,it still delivers high capacities,which inspires us to use it in rechargeable Mg batteries.When used as cathode for rechargeable Mg batteries,at the current density of 20mA/g,PHATN can achieve the energy density of 219 Wh/kg.When the current densities are 20,50,100,and200mA/g,the capacities are 125,100,80,and 65 mAh/g,showing good rate performance.Therefore,PHATN has great potential for environmentally-friendly,fast,and stable organic cathode materials for rechargeable Mg batteries.