First-principles Study On Magnesium Storage Performance Of Cathode Material δ-MnO₂ For Magnesium Ion Batteries

Lithium ion batteries（LIBs）have gradually become the main mobile power source for various applications due to their advantages of high energy density,long cycle life,and good power performance.However,the abundance of lithium in the earth’s crust limits its prospects.Therefore,the development of alternative batteries is essential.Among all metal anodes,the capacity is the largest,the reduction potential is low,the safety performance is good,the reserves are rich and the price is low,the most important is that there is no dendrite deposition in some electrolytes,and the Coulomb efficiency is 100%,making Mg metal an ideal anode.In view of these advantages,rechargeable magnesium ion batteries（r MIBs）,as a promising alternative to current lithium-ion battery technology,have received increasing attention.However,the lack of a suitable cathode provides high energy density and good sustainability,which greatly hinders the development of practical rechargeable magnesium ion batteries.Theδ-MnO₂ is a layered structure,there can be water,ions such as hydrogen ions,potassium ions,etc.between the layers,and the layer spacing can be changed according to the amount of water between the layers and the size of the ion between the layers.The greater the layer spacing.Therefore,it is very conducive to the insertion and extraction of ions,thereby improving the ion mobility and increasing the storage capacity.It is considered to be a very promising cathode material.In this paper,the first-principles method based on density functional theory is used to study the electronic structure and related electrochemical properties of bulk structure,monolayer and bilayer nanosheet structureδ-MnO₂ as cathode materials for Mg ion batteries,such as magnesium storage capacity,voltage and diffusion performance etc.First,three stableδ-MnO₂ structural models were obtained;then the adsorption stability,diffusion performance and rate performance of Mg among the three structuralδ-MnO₂were studied;finally,the magnesium insertion voltage was calculated And theoretical capacity,the actual performance of three different structures ofδ-MnO₂ as cathode materials for magnesium ion batteries is predicted.The conclusions we finally got are as follows:Theδ-MnO₂ systems of the three structures all exhibit semiconductor properties.The most stable adsorption sites for magnesium atoms are located directly above the hollow sites of three oxygen atoms on the surface of the inner layer and point to the outer oxygen atoms,regardless of whether it is bulk,monolayer or bilayer,After embedding Mg,the system exhibits semiconductor,metal and semiconductor properties respectively.Among the three structures ofδ-MnO₂ system,the MgMnO₂ system has the lowest formation energy after magnesium atoms are inserted.Theδ-MnO₂ system with bulk phase and bilayer nanosheet structure shows excellent performance as magnesium ions The potential of battery cathode materials:When the bulk phase and the bilayerδ-MnO₂ system form the MgMnO₂ system,the system can still exist stably,the structure deformation is small,and the corresponding magnesium storage capacity is about 617 mA h g^-1,the corresponding voltage It is 2.10 V and 1.54 V,and the corresponding diffusion energy barriers are 0.70 eV and 0.045 eV,respectively.The monolayer nanosheet structure,due to the strong bonding between Mg and O,makes the diffusion barrier of Mg as high as 1.21 eV.When the MgMnO₂ system is formed,the system structure changes from a layer to a tube-like structure,resulting in its poor cycle stability.