Design Of CO₂ Capture Membrane Under Low Partial Pressure And Its Application In Lithium-Air Batteries

In recent years,with the widespread application of secondary batteries in portable electronic devices,new energy vehicles,energy storage stations,and other fields,higher performance requirements have been put forward.Lithium air batteries（LABs）have attracted widespread attention as the batteries with the highest energy density at present.Their theoretical energy density can reach up to 3500 Wh/kg,which is about ten times the energy density of commercial lithium-ion batteries.They are the most promising new generation of energy storage devices.However,the recent reports on lithium-air batteries mainly operate under pure oxygen conditions,which directly affects the full performance of the battery.To achieve the application of lithium-air batteries,it is necessary to solve the problems they face when working in real air:1.Positive electrode:As lithium-air batteries are a semi open system,carbon dioxide in the air reacts with lithium peroxide,a discharge product of the positive electrode,to generate more thermodynamic stable lithium carbonate.Lithium carbonate is difficult to decompose even at high charging voltages,which leads to low charging and discharging efficiency and battery cycle life;2.Oxygen,carbon dioxide,water vapor,etc.in the air can corrode the lithium metal in the negative electrode,leading to lithium metal pulverization and a rapid decrease in battery cycle life.In response to the above issues,this article proposes the design of a carbon dioxide capture membrane under low partial pressure to suppress the entry of carbon dioxide into the battery system,reduce the generation of discharge product lithium carbonate,and reduce charging overpotential;At the same time,it can reduce the decomposition of organic electrolytes to a certain extent,thereby improving the cycle life of lithium-air batteries.The specific research content includes:Firstly,in response to the current situation of low CO₂ content in the air,a composite membrane based on loaded lithium hydroxide activated carbon was designed to achieve carbon dioxide capture at low partial pressure.The lithium hydroxide is filled in the activated carbon with high specific surface area and high pore volume to realize the adsorption and fixation of CO₂ in both physical and chemical ways,improving the active site of the reaction between lithium hydroxide and carbon dioxide,the reaction rate and the adsorption efficiency compared with the pure lithium hydroxide.The activated carbon loaded with lithium hydroxide is deposited into activated carbon fiber felt with high specific surface area and rich microporous structure through adhesive.Activated carbon fibers can provide sufficient space to accommodate the activated carbon loaded with lithium hydroxide.The rich pore structure of activated carbon and activated carbon fiber felt,as well as the gaps between activated carbon fibers,provide good conditions for gas diffusion.Breakthrough experiments have shown that the composite membrane has excellent carbon dioxide capture performance and oxygen permeability.At the same time,ultra-high carbon dioxide capture can be achieved at a partial pressure of 15%CO₂.The capture capacity of carbon dioxide can reach up to115 cm³/g（4.69 mmol/g）.Secondly,regarding the reported work on grafting amines onto porous carriers,most of them are mesoporous materials,and there is no detailed exploration of the adsorption performance of carbon dioxide at ultra-low pressure.Based on this,we designed a polyethylene imine grafted carbon dioxide capture membrane and explored the relationship between carbon dioxide adsorption capacity and the amount of amine grafting at ultra-low pressure.Polyethyleneimine was grafted onto the surface of activated carbon fiber felt using chemical methods.The surface of activated carbon fiber felt is sequentially modified by carboxylation and acyl chlorination.The acyl chloride functional group is prone to react with the amine group in polyethylene imine,thereby grafting polyethylene imine onto the surface of activated carbon fiber.Through XPS analysis,polyethylene imine was successfully grafted onto the surface of activated carbon fibers.Using physical methods,strictly control the amount of polyethylene imine and evenly load it onto the surface of activated carbon fibers.The results showed that after grafting polyethylene imine onto the surface of activated carbon fibers,the adsorption capacity of carbon dioxide was 16 cm³/g at 5%CO₂ partial pressure,which was three times that of activated carbon fibers without polyethylene imine loading.Thirdly,the prepared composite film loaded with lithium hydroxide activated carbon is attached to the outside of the lithium-air battery,and the lithium-air battery can operate stably in air containing 4%CO₂.The charging and discharging cycles were carried out at a current density of 250 mA/g and a limited capacity of 500 mAh/g.The lithium-air battery without a carbon dioxide capture membrane can operate for 220hours,while a lithium-air battery with a carbon dioxide capture membrane can stably charge and discharge for over 310 hours.At a current density of 250 mA/g and a cut-off capacity of 500 mAh/g,a lithium-air battery without a carbon dioxide capture film can operate for 220 hours,while a lithium-air battery with a carbon dioxide capture film can stably charge and discharge for over 310 hours.At the same time,the charging overpotential of a lithium-air battery with a carbon dioxide capture membrane is reduced from 4.25V to 4.15V,significantly improving efficiency.In addition,the full discharge capacity of lithium-air batteries with carbon dioxide capture membranes has increased from 27948mAh/g to 36252 mAh/g.The relevant results indicate that the prepared carbon dioxide capture membrane has achieved good results in the practical application of lithium-air batteries,providing a new solution for achieving the application of lithium-air batteries in high carbon dioxide concentrations.