Structural Predication And First-Principles Studies Of Two-Dimensional Vanadium-Based Materials For Electrochemical Energy Storage Applications

Batteries are the most widely used energy storage devices,and the lithium-ion battery（LIB）is the most heavily commercialized and the most widely used rechargeable battery in the industry.However,conventional LIBs could no longer satisfy the current soaring energy demands.Exploring the high capacity,long life cycle,low cost,and reliable safety are the desperate requires.Therefore,many studies have been focused on exploration and design the battery materials,in which first principles caculations based on quantum mechanics have played an important role.In this paper,the energy storage mechanism for alkali ion battery systems through the prediction of new two-dimensional（2D）vanadium-based materials has been studied based on the first principles calculations method,and looked for the alternative solutions to the problems of limited lithium storage,high cost and safety in current LIBs,for achieving high specific capacity and fast ion transmission of rechargeable battery systems.This work have given full play to the advantages of theoretical calculation,and investigated the crystal configurations,stabilities,physical intrinsic natures,as well as the ion storage mechanism and transmission mechanism,electrochemical properties of the predicted 2D vanadium-based materials used as rechargeable battery electrode materials.Besides,the relationship between crystal microstructures and intrinsic natures and macrostructures of electrode materials has been discussed,which provided a reliable theoretical basis for exploring and designing the new electrodes.The main research contents are as follows:（1）VC_2,a new 2D transition metal carbide containing C₂ dimers,was predicted by the swarm-intelligent global-structure search method.The structural properties and Li⁺storage ability of VC₂ monolayers and stacked VC₂ multilayers were systematically investigated by first-principles calculations,and the high structural stability and electronic conductivity of the materials suggested promising Li⁺storage properties.VC₂ monolayers showed a theoretical capacity of 1073 mA?h?g^-1 based on multilayer Li⁺adsorption,while stacked VC₂ showed an even larger theoretical capacity of 1430 mA?h?g^-1.Intercalated Li⁺formed ordered arrangements between VC₂ layers,retaining a well-ordered layered structure.Li⁺near the VC₂ layer formed ionic bonds with the host material of stacked VC₂ multilayers,while Li in middle layers formed metallic Li-Li bonds.All Li⁺was stored in the interlayer space with low diffusion barriers,which demonstrated high rate capability of the material for lithium ion batteries.Remarkably,the predicted VC₂ carbide achieved more than 1000mA?h?g^-1 capacity and excellent rate capability,as well as low work voltage irrespective being in monolayer or stacked layer structures,which indicating that VC₂carbide could be used as a new potential anode material for LIBs.（2）Identifying high performance electrode materials particularly with a large capacity and appropriate working voltage is one of the most promising approaches for improving the energy density of rechargeable batteries.Herein,a 2D tetra-VN₂ with intrinsic thermal/dynamical stability and excellent electronic conductivity is described that was identified using energy and stability directed screening as a potential electrode material for rechargeable alkali ion batteries.The maximum alkali ion storage was found to be Li₂VN₂,Na₄VN₂ and K₄VN₂,which corresponded to specific capacities of 679,1358 and 1358 mA h g^-1.The average working voltages of tetra-VN₂ in Li-,Na-,and K-ion batteries were 2.59,1.59,and 1.62 V,which produced a theoretical specific energy of 1761,2162,and 2206 Wh kg^-1,which were much larger than most well-known cathode materials.This suggested that the tetra-VN₂ could be a promising cathode material for alkali ion batteries with high energy density.Interestingly,different from the intercalation-type cathode materials,alkali ions were stored in the tetra-VN₂ via an adsorption/desorption process.With this surface storage mechanism,Li⁺,Na⁺and K⁺ions could migrate in the electrode with low energy barriers 0.237,0.018 and 0.075 eV,respectively,which indicating the excellent rate capability of the tetra-VN₂ in rechargeable batteries.（3）Exploring high-performance electrode materials is one of the most effective ways to improve the specific capacity of rechargeable batteries to satisfy soaring energy demands.Using the particle swarm optimization-based global structure search method combined with first principles calculations,we reveal that 2D VB₂ diboride could serve as a compelling anode material for rechargeable batteries.VB₂ is not only dynamically and thermally stable,but also intrinsic metallic nature before and after ionization,showing excellent electrical conductivity.The fully ionized phases of VB₂are Li₂VB₂ and Na₃VB₂,corresponding to specific capacity of 739 and 1108 mA h g^-1,respectively,superior to the capacity of traditional electrode materials and many of the previously reported 2D candidates.Meanwhile,the diffusion barrier of VB₂ in sodium-ion batteries（SIBs）（0.28 eV）is lower than that of LIBs（0.57 eV）.In addition,the average open-circuit voltage is 0.83 V in SIBs,which lower than that of 1.48 V in LIBs.These findings suggest that VB₂ could be used as a potential anode material of rechargeable batteries,particularly,which for SIBs rather than LIBs.（4）Finally,the novel 2D V₂B₂ boride has been successfully predicted by using CALYPSO code.Subsequently,the stabilities,electronic structure and electrochemical properties of V₂B₂ as anode materials for rechargeable batteries have been investigated systematically based on first principles calculation.The calculated phonon spectra and ab initio molecular dynamics simulations indicated that V₂B₂ was thermodynamically stable,moreover,which can withstand a temperature as high as1500 K.Li⁺/Na⁺could be absorbed stably on the surface of V₂B₂ to obtain the relative high theoretical specific capacity of 434 and 543 mA h g^-1,respectively.The average open circuit voltage is⁰.7 V for LIBs/NIBs.Especially,the diffusion energy barriers of Li⁺and Na⁺migrated on the surface of V₂B₂ are only⁷.5 meV and¹0 meV,indicating their ultra-fast ion diffusion capability.This study indicates that V₂B₂ could be used as a promising anode material with the excellent fast charge/discharge capability for LIBs/SIBs.In a word,we have studied the structures and properties of four kinds of 2D vanadium-based materials by means of particle swarm optimization combined with first-principles calculation,and further explore their potential applications in the field of rechargeable batteries,which provided a reliable theoretical reference for the design of new 2D electrode materials.