Preparation Of Manganese Dioxide With Optimized Electronic Structure By Atomic Engineering For Energy Storage

Manganese dioxide（Mn O₂）is a promising electrode material for SCs due to its high theoretical capacitance,low cost and high safety.However,the practical performance of Mn O₂ is seriously restricted by its low intrinsic electronic conductivity.Therefore,it is remaining a challenge to prepare high performance Mn O₂ electrode materials for SCs applications.The capacitive performance of Mn O₂ largely depends on the internal electron structure,such as the spin state and electric conductivity.Herein,we regulated the electronic structure of Mn O₂ by using an atomic engineering strategy.The electrochemical properties of the as-prepared Mn O₂ electrode material and the performance of assembled supercapacitor were tested,respectively.The influence of the electronic structure of Mn O₂ on the capacitance performance was deeply discussed.（1）Ni-doping Mn O₂（Ni-Mn O₂）was successfully prepared by oxidation precipitation method.Structure distortion was introduced in Mn O₂ to improve the storage performance of Na⁺.Density-functional theory（DFT）calculations verify that structure distortion can effectively adjust the electron configuration of Mn O₂,change the electron distribution,reduce the band gap,and then promote electron transfer.Meanwhile,the structurally distorted Mn O₂can efficiently suppress and accommodate the cooperative Jahn-Teller distortion,thus improving the cycling capability.The resultant Ni-Mn O₂ exhibits a desired cycle life of 85.0%retention over 10000 cycles.An asymmetric supercapacitor（ASC）fabricated with Ni-Mn O₂ cathode and AC anode delivers a maximum energy density of 114.6 Wh kg^-1 at a power density of 3600 W kg^-1,superior to the most Mn O₂-based supercapacitor currently reported.（2）Ni-dopingδ-Mn O₂（Ni-Mn O₂）was successfully prepared by chemical coprecipitation method.The delocalized spin of d-electron at Mn site ofδ-Mn O₂was induced by atomic engineering to improve the storage performance of Na⁺.Combined with the characterization techniques such as electron microscopy and structural analysis,the spin state transition of Mn O₂ electrode materials was investigated.Bader charge analysis and DFT calculation results show that Mn O₂has more d-electron delocalized spin states and better electrochemical energy storage performance.The obtained Ni-Mn O₂possesses a capacity of 327 F g^-1at 1 A g^-1,240 F g^-1 even at 20 A g^-1.The resultant ASC can deliver a high energy density of 70.8 Wh kg^-1 at a power density of 3600 W kg^-1.In summary,we have synthesized Ni-Mn O₂ electrode materials by using atomic engineering strategy for high performance SCs applications.Our results denmonstrate that the atomic engineering strategy can effectively improve Na ion storage performance by optimizing the Mn O₂ electron configuration.This thesis provides useful information for fabricating high performance Mn O₂electrode materials for SCs,which can be extended for the design of other electrode materials applied in various energy storage devices.