First-principles Study On Charge And Discharge Mechanism Of δ-MnO₂ Cathode For Zinc Ion Batteries

Zinc is a kind of metal with abundant reserves,with the advantages of low cost and environment friendly.In addition,zinc can be used as the anode material of aqueous battery,which ensures the safety of the battery.Aqueous zinc ion battery attracted a lot attention of researchers with all these reasons.However,due to the lack of suitable cathode materials with high energy density and excellent sustainability,further development on zinc ion batteries is hindered.δ-MnO₂has a unique layered structure for ion insertion/extraction.Due to the existence of interlayer water molecules,δ-MnO₂has larger interlayer spacing than other MnO₂structures.Therefore,it is conducive to the rapid insertion/extraction of ions and shows excellent rapid charge discharge performance.These makeδ-MnO₂be a potential cathode material.First principles method based on density functional theory is used in this study.The electronic structure ofδ-MnO₂and the adsorption and diffusion mechanics were further studied.The reaction voltage and theoretical capacity after adsorption,the structure after water absorption,the diffusion properties of water molecules were calculated.Firstly,properties were calculated in non-aqueous system with Zn atom absorption.Bulkδ-MnO₂exhibits the semiconductor characteristics with band gap of1.2 e V.The calculation results about the electronic structure shows that the band gaps after Zn absorption are 0.2 e V,respectively.The diffusion energy barrier of zinc atom in bulk is 0.44 e V.,The theoretical storage of zinc atom is about 616 m Ah/g,and the ratio of zinc to manganese in the adsorption system is 1:1 at the highest storage.The initial discharge voltage is 1.29 V,and the final discharge voltage is 0.99 V.Then,properties were calculated with H atom asorption.Band gap is 0.6 e V after H atom absorption.So the adsorption can improve conductivity.The diffusion energy barrier is 3.05 e V.This means the hydrogen atom is difficult to diffuse.The theoretical storage of hydrogen atom in the system is 308 m Ah/g.The voltage of conversion reaction is 1.42 V,which is higher than that of zinc atom adsorption,and therefore the chemical conversion precedes the embedded discharge reaction.Finally,properties in aqueous system were calculated.When the water molecules are added into the system,the interlayer spacing ofδ-MnO₂increases rapidly from5.37（?）to about 7（?）.This shows that the water molecule is the decisive factor affecting the interlayer spacing.The diffusion barrier is 0.053 e V in the system with 7（?）interlayer spacing.After the increasing of the layer spacing,the band gap ofδ-MnO₂system is 1.5 e V.The results show that the band gap after Zn adsorbed in the system is 1.1 e V.The conductivity of the system is increased.The adsorption energy of zinc atoms is 0.488 e V.In system of which c lattice is 13.5（?）,the diffusion barrier less than 10%of that in non-aqueous system.The system with larger interlayer spacing can not increase the theoretical zinc capacity,which is still 616 m Ah/g.The study shows thatδ-MnO₂has a broad application prospect as the cathode material of aqeuous battery.It has fast charging and discharging performance.In the process of discharge,hydrogen first reacts withδ-MnO₂.After the chemical conversion reaction,zinc ions intercalated rapidly in the interlayer,and water molecules were further de intercalated to complete the discharge reaction.