Preparation And Performance Investigation On Carbon-based Composite Cathode Materials For Li-CO₂ Battery

Over the past century,the surge in CO₂ emission arising from excessive fossil energy consumption has led to global warming and other serious environmental issues.It is urgent to capture and utilize CO₂ from the atmosphere to effectively address the greenhouse effect except for the deduction of CO₂ emissions.Fortunately,Li-CO₂ battery,as an emerging energy storage device,possesses the advantages of environment friendliness,relatively high energy and power densities.This device can not only alleviate the greenhouse effect by capturing CO₂ and taking advantage of the corresponding redox reaction between carbon dioxide and lithium,but also performs a broad prospect in Mars exploration.However,the discharge product of Li₂CO₃ during the CO₂ reduction process is an insoluble and wide band-gap insulator,which is inconducive for its thermodynamically electrochemical decomposition,resulting in the sluggish kinetics for the CO₂evolution reaction in the charging process.Moreover,the accumulation of Li₂CO₃ on the cathode surface may further inhibit the diffusion of CO₂/Li⁺and reduce the effective reaction space of the cathode.Therefore,Li-CO₂ batteries usually exhibit high overpotential,low energy efficiency,poor rate performance,and short cycle life.Therefore,designing and developing a highly active catalyst is an effective method to promote the reversible conversion between CO₂ and Li₂CO₃,thus achieve Li-CO₂ battery with high-performance.Herein,we have explored the following three aspects:（1）Ruthenium nanoparticles loaded on porous hollow carbon nanospheres（Ru AHCNs）were successfully synthesized after one-step hydrothermal treatment and high-temperature calcination.As a cathode for Li-CO₂ battery,Ru AHCNs displays an excellent electrochemical performance.During the full discharge process,the Ru AHCNs cathode displays a high discharge specific capacity of 11194.4 m A h g^-1at the current density of 100 m A g^-1.In addition,the Ru AHCNs cathode can stably run over 50 cycles and maintain a small overpotential（<1.38 V）with a limited capacity of 1000 m A h g^-1 at 100 m A g^-1.The morphology and chemical composition of the Ru AHCNs cathodes after the first and tenth cycles were further characterized via SEM and XPS measurements.The results undoubtly prove the reversible formation and decomposition of the discharge products,indicating that Ru AHCNs cathode possesses excellent bifunctional catalytic activity.（2）In order to better improve the catalytic performance of the materials,we further modified the carbon substrate and loaded ruthenium nanoparticles with bifunctional catalytic activity,in order to design and synthesize nanocatalysts with synergistic effects.Ultrafine Ru nanoparticles anchored on N,S co-doped graphene as cathode for Li-CO₂ batteries were obtained via a simple hydrothermal reaction with subsequent high-temperature calcination treatment.Remarkably,graphene materials possess a large specific surface area and good electrical conductivity.In addition,the doping of N,S heteroatoms into graphene can not only alter the surface polarities and electronic structure of the carbon material,but also significantly boost the electrocatalytic activity and the adsorption capacity of CO₂/Li⁺.More importantly,the bifunctional ultrafine Ru nanocatalyst can significantly reduce the decomposition voltage of the discharge product Li₂CO₃.Therefore,the Ru/NS-G cathode achieves an impressive discharge specific capacity of 12448 m A h g^-1 and a high coulombic efficiency of 94.6%at a current density of 100 m A g^-1.Attractively,the Ru/NS-G cathode can smoothly run over 100 cycles with a low overpotential（<1.40 V）and perform a superior cycling stability when the specific capacity is limited to 1000 m A h g^-1.（3）Cross-linked ultrathin K-birnessite-type MnO₂ nanoflowers combined with CNTs composites were fabricated via a facile one-step hydrothermal method and were utilized as the highly efficient cathode for Li–CO₂ battery.The interconnected network structure of K-δ-MnO₂/CNTs composites maintains a porous structure of the electrode,which is beneficial for the electrolyte infiltration,CO₂ diffusion,and exposure of numerous active sites.Simultaneously,the K⁺located in the interlayer space of the K-birnessite-type MnO₂layers can stabilize the catalyst configuration,promote charge balance,and further improve the conductivity and Li⁺diffusion rate.Hence,the K-δ-MnO₂/CNTs cathode exhibits superb electrocatalytic activity for the formation/decomposition of Li₂CO₃.It is worth to mention,the Li–CO₂ battery based on K-δ-MnO₂/CNTs cathode achieves a low overpotential of 1.05 V and a high average energy efficiency of 87.95%at current density of 100 m A g^-1,which is better than most of the previous reported Li–CO₂ batteries.Additionally,the K-δ-MnO₂/CNTs cathode can steadily run over 100 cycles（overpotential<1.20 V）.Moreover,a low overpotential of 1.47 V can be obtained even at a high current density of 1000 m A g^-1,indicating the superior rate performance of K-δ-MnO₂/CNTs.