Theoretical Study Of Two-dimensional Titanium-based Materials As Anodes For Metal-ion Batteries

With the increasing demand for clean,efficient and portable energy equipment,the development of energy storage technology has reached a new stage.In recent years,metal ion batteries,as an important energy storage device,have attracted much attention due to their long life,low cost and good safety.However,the lack of high-performance electrode materials greatly limits the development of the battery.Two-dimensional（2D）material has a unique layered structure and a large specific surface area,which can achieve rapid ion diffusion and provide more ion storage sites.It is suitable as an anode material for metal ion batteries.In addition,titanium is relatively high in the earth’s crust,and has the advantages of non-toxicity,light weight,and chemical stability.2D titanium-based materials are considered to be promising anode candidates for ion batteries.Here,we systematically explore the potential applications of three 2D titanium-based materials as anodes of ion batteries by the first-principle calculations based on density functional theory.The main research contents are as follows:（1）Bulk Ti Cl₂ has been successfully synthesized in 1948.It has van der Waals layered structure and metallic property.We predict a monolayer Ti Cl₂ which can be exfoliated from its bulk phase with small exfoliation energy（0.64 J/m²）.Monolayer Ti Cl₂ is an antiferromagnetic structure with metallic property.It shows good stability,as demonstrated by its high cohesive energy,positive phonon modes,and high thermal stability.Next,we systematically explore the potential performance of 2D Ti Cl₂ as an electrode material for sodium-ion batteries（NIBs）.Monolayer Ti Cl₂has high storage capacity(451 m A h g^-1),low diffusion energy barrier（0.02–0.14 e V）,moderate average open-circuit voltage（0.81 V）,and small lattice change（2.37%）.Moreover,bilayer Ti Cl₂can significantly enhance the Na adsorption strength but reduce the Na-ion diffusion ability.These results suggest that Ti Cl₂ is a promising anode candidate for NIBs.（2）We propose a novel 2D titanium material,namely titanene.Firstly,the stability of two-dimensional titanium is systematically analyzed.Titanene has good energy stability with large cohesive energy（5.90 e V/atom）.The phonon spectrum shows no imaginary phonon modes in the whole Brillouin zone,and the elastic constants and Young’s modulus are within a reasonable range,indicating that the structure is dynamic and mechanically stable.At the same time,ab initio molecular dynamics simulation shows that the structure of titanene can remain stable at 600 K,confirming that the structure is thermodynamically stable.Afterwards,we explore the performance of titanene monolayer as the anode material for lithium-ion batteries（LIBs）.The titanene monolayer exhibits an excellent,comprehensive performance with outstanding electronic conductivity,low diffusion barrier（0.07–0.11 e V）,ultra-high theoretical specific capacity(1679 m A h g^-1),relatively low average open circuit voltage（0.84 V）,and small volume change（5.74%）.Titanene maintains metal conductivity throughout the lithiation process,which is beneficial to the improvement of electrode conductivity.In addition,graphene can be added to the top of titanene as a protective cover to further explore the adsorption and diffusion properties of the graphene/titanene heterostructure surface.The introduction of graphene can well protect the metallicity of titanene,enhance the adsorption capacity of lithium ions on the side of titanene,and maintain fast ion diffusivity.These theoretical findings indicate that titanene is quite ideal as a high-performance anode material for LIBs.（3）The performance of 2D Ti OF as an anode material for potassium-ion batteries（KIBs）is studied.The structure of the Ti OF monolayer is optimized and found to have metal property,which is consistent with previous research results.The larger adsorption energy（1.52 e V）indicates that Ti OF has strong potassium ion adsorption.The lowest diffusion barrier of potassium ions in the Ti OF monolayer is only 0.20 e V,which makes rapid charge/discharge rate.In addition,by gradually increasing the adsorption concentration of potassium ions,the maximum theoretical capacity of 323m A h g^-1 and the open circuit voltage of 0.68 V are calculated.When the potassium ions are completely adsorbed,the volume change rate of the Ti OF monolayer is less than 5%,ensuring excellent cycle stability.The stability of the system after adsorption is confirmed by AIMD simulations.The system after adsorption still shows metal property,which ensures the conductivity during the battery cycle.