Theoretical Design And Performance Study Of New Two-Dimensional Group Ⅳ Sulfide Electrode Materials

Since graphene was successfully prepared in 2004,two-dimensional（2D）materials have become one of the most popular research fields in condensed matter physics.The thickness of 2D materials is usually at the nano level.The special structure makes 2D materials have many excellent electrical,optical,mechanical and other properties that are not found in traditional materials.On the other hand,traditional fossil energy is facing problems such as resource depletion and environmental pollution.Metal ion batteries are considered to be promising energy storage devices.2D materials are promising anode materials because of their high specific surface area,excellent electrical conductivity,rich electrochemical active sites,and high mechanical flexibility.In this paper,the research status and problems of 2D materials and metal ion batteries are introduced in the first chapter.Then,the basic theory and calculation details used in this paper are introduced in chapter 2.Finally,from chapter 3to chapter 6,the theoretical prediction of two 2D materials are studied.Their basic properties and the potential application of these 2D materials as anode materials are explored.The main research contents of this paper are as follows.A new 2D silicon based sulfide Si₂S is successfully designed by arranging the Si-S-Si skeleton of SiH₃SSiH₃.It has excellent thermal,mechanical,and dynamic stabilities.Si₂S is a semiconductor with an indirect band gap of 1.93 e V.The transition from indirect band gap to direct band gap occurs under strain.The calculated highest carrier mobility of Si₂S is 182.72 cm² V^-1s^-1.The band gap and carrier mobilities of Si₂S are comparable with 2D Mo S₂,which makes it promising to be used in the field of nanoelectronics.Si₂S has significant absorption of ultraviolet light.The calculation results of mechanical properties show that it is a soft material,which makes it promising to be applied in the field of flexible electronics.The properties of 2D Si₂S as anode material for Na-/K-ion batteries are studied.The larger adsorption energy shows that 2D Si₂S can closely adsorb Na/K atoms.The adsorbed systems show metallicity,which ensures the conductivity during the cycle.The lowest diffusion barrier of Na（K）atom on 2D Si₂S is only 0.04（0.53）e V,which makes rapid charge/discharge rate.The calculated maximum specific capacity of Na and K is 303.73 m A h g^-1.The stabilities of these adsorption systems are confirmed by AIMD simulations.The average electrode potentials of Na and K are 0.54and 0.66 V respectively,which are within the appropriate range of anode materials.The volume change of Si₂S during the sodiation/potassiation process is small,which is beneficial to prolong the cycle life.A novel 2D germanium based sulfide Ge₂S is predicted.Its stabilities,electrical and mechanical properties are studied.2D Ge₂S has excellent dynamic and thermal stabilities.It has an indirect band gap（1.59 e V）and high carrier mobility(800.5 cm²V^-1 s^-1),indicating its potential for application in nano optoelectronic devices.The results of light absorption show that 2D Ge₂S has good absorption of ultraviolet light.It is worth noting that 2D Ge₂S has a very special property of negative Poisson’s ratio（NPR）.The large in-plane NPR makes 2D Ge₂S a promising material for novel nanodevices.The properties of 2D Ge₂S as anode material for Na-/K-/Mg-ion batteries are studied.2D Ge₂S can stably adsorb Na/K/Mg atoms,and there is a significant charge transfer between the adsorbed ions and 2D Ge₂S.Na/K/Mg ions tend to diffuse along the x direction on 2D Ge₂S,and the calculated diffusion barriers are 0.12,0.36,and 0.76 e V,respectively.2D Ge₂S can adsorb Na/K/Mg atoms in a ratio of 1:1.The stabilities of these maximum concentration adsorption systems are verified by the convex hull of formation energies.The average electrode potential of Na,K,and Mg is in the range of 0.30-0.7 V,which can maximize the power density.During the Na/K/Mg adsorption process,the small volume change of Ge₂S suggests a good cycle stability.