Preparation And Electrolyte Improvement Of Mn-Based Cathode Materials For Li-CO₂ Batteries

Li-CO₂ battery can realize CO₂ capture and energy storage at the same time,and has a high theoretical energy density.It is one of the most potential energy storage systems.It can be widely used in CO₂ rich environments,such as submarine operations,Mars exploration,etc.However,the high charging overvoltage and short service life of Li-CO₂ battery limit its large-scale application,mainly because the discharge product Li₂CO₃ is an insulating material,which is difficult to decompose during charging.Therefore,building an efficient positive catalyst is one of the key strategies to improve the performance of Li-CO₂ battery.In view of the above problems,this paper takes Mn based metal compounds as the research object to explore their impact on the performance of Li-CO₂ batteries as positive catalysts,and further optimize the battery performance through gel electrolyte modification.The main research contents of this paper are as follows:（1）The effects of different Ni doping ratios on the morphology,catalytic activity and battery performance of MnO₂ nanowires were investigated.MnO₂ with different Ni doping ratios was prepared by controlling the addition ratio of Ni（NO₃）₂·6H₂O.With the increase of Ni doping ratio,the crystallinity of MnO₂ gradually decreases,and 10%Ni doped MnO₂ shows the highest catalytic activity for the reduction and precipitation of CO₂,indicating that the synergetic catalysis of Ni and Mn is more conducive to the reduction and precipitation of CO₂.At the same time,the high conductivity brought by bimetals greatly improves the electron transport ability and promotes the electrochemical reaction process.In-situ/ex-situ characterization found that Li₂CO₃ was uniformly deposited on the positive electrode surface during the power generation process,and Li₂CO₃ completely disappeared after charging,which confirmed the reversible realization of the main product Li₂CO₃ in the charge discharge process.（2）Based on the high catalytic properties of transition metal materials with 3D frame structure,a 3D structural material with Mn₂Co₂C nanoparticles dispersed in a connected nitrogen doped carbon skeleton was prepared（Mn₂Co₂C@NC）.The cathode catalyst used for Li-CO₂ battery has high discharge capacity(18962 m Ah g^-1),low charge potential（4.2 V）and excellent cycle life（330 times,nearly 1800 h）.This is because the carbonized frame structure has a large surface area,which can expose more Mn/Co NC and nitrogen doped carbon active sites,which is beneficial to the enhancement of the catalytic process.At the same time,the deposition space of the discharge products is enlarged,and the discharge capacity is effectively improved.Density functional theory calculation shows that Mn/Co NC and nitrogen doped carbon active sites have strong Li⁺and CO₂ adsorption capacity,reduction catalytic performance and good conductivity.（3）A gel electrolyte based on polyvinylidene fluoride hexafluoropropylene（PVDF-HFP）filled with polypropylene carbonate（PPC）was prepared by phase transfer method,and a quasi-solid Li-CO₂ battery was assembled.The leakage and volatilization of liquid organic electrolyte are reduced,and the safety is improved.The electrolyte has high ionic conductivity(1.55×10^-3 S/cm)and wide voltage window（5 V（V vs.Li+/Li））.It has excellent flexibility and porous absorption,ensuring close contact with the electrode.The Li-CO₂ battery assembled with PVDF-HFP/PPC gel electrolyte has excellent electrochemical performance(high discharge specific capacity of 9390 m Ah g^-1,high cycle stability of 120 cycles,and long continuous working time of 750 h),which is superior to the same type of Li-CO₂ battery previously reported Mn₂Co₂C@NC cathode catalyst and PVDF-HFP/PPC gel electrolyte can be well applied to quasi-solid Li-CO₂ batteries,providing theoretical basis and experimental support for the manufacture of new batteries with high specific energy,long life,high safety and flexible wearability.