Research On Cathode Material And Catalyst In Lithium-Air Batteries

Owing to their high theoretical energy densities,lithium-air batteries have received widespread attention,and have a good prospect of application in the field of green energy storage.However,lithium-air batteries are still in their early stages.Before commercialization,there are many problems need to be solved.Especially,lithium-air batteries have serious polarization phenomenon during charge and discharge processes,which will reduce the energy efficiency.Meanwhile,the high charge voltage plateau can easily cause the decomposition of the electrolyte,resulting in deteriorating cycling stability.To address the above problems,by summarizing the research status of cathode materials and catalysts in lithium-air batteries,we systematically investigate the optimization of substrate materials and efficient catalysts of air electrodes.We also deeply analyze the influence of the air electrodes on the discharge products during charge and discharge processes.The achievements and progress are shown below:Firstly,we investigate the catalytic performance of a series of Pt-based catalysts?different contents and compositions?in lithium-O₂ batteries,and their influence on the discharge products.The experimental results show that the content and composition of Pt catalysts can influence the morphology of the discharge products?diameter and thickness?.Compared with Pt catalyst,Pt-Ru catalyst contributes to the production of more poorly crystalline Li₂O₂,especially during the initial discharge stage.Because the poorly crystallized Li₂O₂ easily decomposes during charge process,Pt-Ru catalyst can effectively reduce the charge overpotential,demonstrating its excellent catalytic performance.Secondly,the hollow graphene nanocages?HGNs?material with three-dimensional structure has been successfully designed and synthesized.The HGNs can preserve the original porous structure during the well-designed synthetic process,and provide numerous nano-sized tri-phase regions for oxygen reduction reactions.And the nano-sized Pt particles have been homogeneously deposited onto the HNGs?Pt-HGNs?by physical vapor deposition.The lithium-O₂ battery with Pt-HGNs as cathode can effectively reduce the charge overpotential.The charge voltage plateau is below 3.2 V at the current density of 100 mA/g with the fixed capacity of 1000 mAh/g,which is close to the theoretical value?2.96 V?.Meanwhile,the lithium-O₂ battery with Pt-HGNs as cathode achieves an outstanding cycling performance and rate capability.The discharge capacity of the Pt-HGNs electrode is around5600 mAh/g at the current density of 100 mA/g,which is much higher than that of the HGNs electrode.Owing to their strong absorption for oxygen,Pt nanoparticles can serve as the active sites for nucleation and growth of Li₂O₂.Furthermore,the unique structure of the HGNs has a positive influence on the catalytic activity of noble metal Pt catalysts deposited on the HGNs matrix.Thirdly,the boron-doped reduction graphite oxide?B-rGO?material with three-dimensional porous structure has been successfully prepared by a facile freeze-drying method.Owing to the cross-linking function of boron oxide,the B-rGO material has a meso/marcopore hierarchical structure,facilitating the diffusion of electrolyte and oxygen,and accommodating enough discharge products.Meanwhile,the boron-oxygen groups linking to the carbon surface and edge could serve as additional reaction sites for oxygen reduction reactions.Furthermore,the boron atoms have been successfully incorporated into carbon lattices,leading to the electrons in the?system being greatly activated,so that the electrical conductivity of the material is improved.The theoretical calculation demonstrates that,compared with reduction graphite oxide?rGO?material,the B-rGO material exhibits much stronger interaction with Li₂O₂,so that B-rGO electrode could more effectively activate Li-O bonds to decompose Li₂O₂ during charge.The lithium-O₂ battery with B-rGO material as cathode exhibits a large discharge specific capacity and excellent rate capability.At the current density of 100 mA/g,B-rGO electrode achieves a discharge capacity of¹8000mAh/g.At the large current density of 2000 mA/g with the fixed capacity of 1000 mAh/g,the charge capacity of B-rGO electrode is 1000 mAh/g.The experimental results demonstrate that the B-rGO material is an ideal substrate for lithium-O₂ batteries.Finally,using the boron-doped reduction graphite oxide material as the substrate,the nano-sized and micro-sized RuO₂ catalysts have been deposited on the B-rGO matrix by chemical deposition method.The prepared composite materials as the cathodes in lithium-O₂ batteries can effectively improve the electrochemical performance,and reduce the discharge/charge overpotential.Attributing to their good electrical conductivity,and strong adsorption for oxygen,RuO₂ catalysts can facilitate the kinetic processes of oxygen reduction reaction and oxygen evolution reaction.Meanwhile,RuO₂ catalysts can reduce the occurrence of side reactions,and decompose the by-products formed during discharge process.The experimental results show that micro-sized RuO₂ catalysts are more suitable for the non-aqueous lithium-O₂ batteries.Because the nano-sized catalysts are easily covered by the by-products or the discharge products undecomposed to loss their activities.It is worth noting that the sandwich-type conductive electrode structure can further improve the battery performance.This is because the electrode with sandwich-type conductive structure can increase the contact area between the discharge products and the conductive materials,which is beneficial for the formation and decomposition of discharge products.