Application Of Low-dimensional Germanium-based Nanomaterials For Metal-ion Battery Anodes

With the depletion of fossil fuels and the development of clean energy like wind and solar energy,the global energy is shifting from fossil fuels to clean energy.The transformation of energy depends on advanced large-scale energy storage technologies.Among various energy storage technologies,lithium-ion batteries（LIBs）are a mature energy storage technology with advantages of high energy density,low self-discharge rate,and long battery life.However,the development of LIBs is hampered by the limited and expensive lithium resources.Therefore,low-cost and higher-capacity non-lithium-ion batteries,such as sodium-ion batteries（SIBs）and magnesium-ion batteries（MIBs）,have attracted much attention.Since graphene was discovered in 2004,two-dimensional（2D）materials have attracted much interest because of their unique properties.Compared to conventional materials,2D materials possess excellent properties.In the research of anode materials for metal-ion batteries,2D materials stand out among many studies because large specific surface area is more suitable for the adsorption and diffusion of metal ions.2D germanium-based materials have attracted a lot of attention for their easily tunable energy bands and high carrier mobility.After fully investigating the development status of metal-ion batteries and the research status of 2D materials as anodes,we explore the electrochemical properties of three 2D germanium-based materials as the anodes of metal-ion batteries using the first-principles calculations.The main studies in this thesis are as follows:（1）2D BGe possesses graphene-like structure,and the surface electrons exhibit localized characteristics which is favorable for metal adsorption.We conduct a comprehensive evaluation of its potential as an anode material for MIBs.The results show that Mg can be stably adsorbed on the BGe surface with an adsorption energy of-1.34 e V,revealing that the adsorption process is a spontaneous exothermic reaction,which is favorable to the stable adsorption of metals.A low diffusion barrier（0.14～0.76 e V）on the BGe ensures a good charge/discharge rate of the MIBs.The open-circuit voltage（0.264 V）and theoretical capacity(3856 m Ah·g^-1)indicate the superior electrochemical performance of 2D BGe.The small volume expansion（5.63%）of BGe during charging and discharging processes is beneficial to maintain the stability of the anode.After that,we further investigate the effects of solvent on the performance of 2D BGe anode,and the results show that the adsorption energy increases with the increasing DC,while the diffusion barrier decreases.So 2D BGe demonstrates excellent performance in the application of MIBs anodes.（2）The electron mobility of germanether is much higher than that of phosphorene,while the special groove’s structure may improve the diffusion rate of metal ions.Therefore,we systematically investigate the performance of germanether,a new 2D material,as an anode material for SIBs.Na can be stably adsorbed on the surface of germanether with an adsorption energy of-1.32 e V,and the system shows metallic properties.Even at low Na concentrations of Na_0.031Ge₂O,the system still demonstrates metallic characteristics,ensuring good electrical conductivity.The diffusion barrier of 0.73 e V could deliver the high Na-ion mobility.Germanether has a suitable open-circuit voltage（1.12 V）,and theoretical capacity(167.1 m Ah·g^-1).The volume expansion of germanether is 10.8%under full adsorption,and after sodium removal,the germanether is still able to return to its original structure,which is beneficial to extend the battery life and increase the number of battery cycles.All the results of this work indicate that 2D germanether monolayer has potential as an anode material for SIBs.（3）2D tetrahex-Ge C₂ possesses a very high electron mobility,demonstrating its great potential for nanoelectronics.Therefore,we want to know whether 2D tetrahex-Ge C₂ has the potential as an anode material for SIBs.The results show that Na atoms can be adsorbed on 2D tetrahex-Ge C₂with the adsorption energy of-0.87e V,and the system exhibits metallic,which ensure the electrical conductivity of the system.The diffusion barrier is 0.61 e V,which is beneficial for improving the multiplicative performance of the battery.Tetrahex-Ge C₂ exhibits an open-circuit voltage（1.10 V）comparable to that of germanether,but its theoretical capacity(552.6 m Ah·g^-1)is higher than that of germanether.The volume expansion rate of tetrahex-Ge C₂ is 7%,which is smaller than that of graphite,ensuring the structural integrity of the battery and then extending the battery life.These results indicate that monolayer tetrahex-Ge C₂ can be used as an anode material for SIBs,and the performance of tetrahex-Ge C₂ is superior to that of germanether.