The Design,Preparation And Performance Optimization Of MnO_x Cathode Catalyst For Li-CO₂ Battery

With the rapid development of human society and the increasing demand for energy,the energy exhaustion and the global greenhouse effect caused by combustion of fossil fuel have drawn increasing concerns.It is urgent and efficient to find a technique that can capture and convert CO₂ into a fuel.The Li-CO₂ battery,which works with a process of 4Li+3CO₂?2Li₂CO₃+C,gives a new choice for utilization of CO₂.However,there are still several challenges during its development.1.One of the main discharge product is lithium carbonate,which can only be decomposed at a higher potential.Meanwhile,carbon based material,which demonstrated a higher charging overpotential,is still the mainstream of cathode catalyst.2.The high cost Ru based catalysts display no bright future for application.3.There is rarely reports on lithium anode protection in Li-CO₂ batteries.By addressing these challenges,investigation of cathode catalyst and in-situ Li anode protections have been conducted.Firstly,the porous Mn O_X were designed and prepared,and their electrochemical properties were systematically studied in Li-CO₂ batteries.In addition,the Li anode protection by in-situ nitridation for Li-CO₂ battery has been explored.The study mainly includes the following aspects:Firstly,a simple method was designed to prepare porous MnO_x,oxidation states of Mn have been tuned by optimized the sintering conditions.And the powders Mn₂O₃ with pure crystalline phase and three-dimensional continuous porous structure was obtained in the air atmosphere.We have found that Mn₂O₃ has a higher catalytic activity and stability through optimizing the composition and structure.The Li-CO₂battery with Mn₂O₃/KB cathode can be stabilized for 30 cycles,and the charging terminal voltage remains 4.42 V with a current density of 50 mA g^-1 and limited capacity of 1000 mAh g^-1.However,the charging voltage quickly reached 4.5 V only in the 4^th cycle with a pure KB cathode.Meanwhile,the Mn₂O₃ electrode reduced the charging overpotential（0.16 V lower than pure KB）.The morphology and composition of the discharge products were systematically analyzed by XRD,SEM,FTIR and Raman spectra.The results demonstrate that flake discharge product（Li₂CO₃）reversibly formed and decomposed during the repeated discharging and charging.In addition,the generation of carbon during discharging have been confirmed by using a pure Mn₂O₃ cathode.We can conclude that lithium carbonate and carbon formed in the discharging process,and decomposed in the charging process.Secondly,we use a facile method to protect the Li metal anode by in-situ nitridation.The Li₃N/Li anode was successfully prepared by using the self-made air battery test device.The SEM and EIS test of Li₃N/Li with different nitridation time confirmed the thin film growth mechanism.The Li₃N layer formed initially and then exhibited as separated island.According to the EIS results,the Li₃N/Li after 1 h treatment displayed a small charge transfer resistance.When applied in Li-CO₂battery as negative electrode,it still present a certain improvement and the charging and discharging voltage platform becomes more stable in the first 10 cycles.It provides a new choice for the research on negative electrode protection of Li-CO₂battery.