Preparation And Electrochemical Performance Of Catalysts For Cathode Of Li-Air Battery

As a new generation of high energy density,safe and environmentally friendly electrochemical energy storage devices,Li-air batteries have attracted widespread attention from academia and industry.However,the development of Li-air batteries is still in its infancy,and many problems remain to be solved,including poor cycling performance,insufficient rate performance and many challenges in operating in ambient air.Although many researchers have been worked on the above problems and achieved many results,Li-air batteries are still far from meeting the requirements of practical applications.In view of the current problems of Li-air batteries,this paper has carried out relevant research work on the design and structural optimization of the cathode catalyst.The electrochemical performance of the catalysts was studied through electrochemical testing,material characterization and model simulation.In addition,we make the superhydrophobic modified catalysts based on the superhydrophobic interface principle to reduce the passivation of the cathodes and anodes of the Li-air batteries,so as to achieve high discharge capacity and long cycle performance of Li-air batteries in open ambient air.The details of the study are as follows:（1）At first,N-doped porous carbon was synthesized using SiO₂ as the occupied template,sucrose and urea as the carbon and nitrogen sources,and then RuO₂/N-C composite was synthesized with RuCl₃·H₂O by coprecipitation method and dehydration by high-temperature calcination.The microstructure and elemental composition of the composite were observed by SEM and XPS characterization.Finally,it was used as catalysts to assemble the Li-air（O₂）batteries,and the constant current charge and discharge tests and deep charge and discharge tests,and the obtained test curves were collected as comparison data.（2）The RuO₂/GPE/N-C catalyst was synthesized by a two-step templated method and coprecipitation method.The combination of RuO₂ particles,graphene and N-C can increase the catalytic active sites and give the composite a larger specific surface area.Li-O₂batteries equipped with RuO₂/GPE/N-C cathodes exhibits better electrochemical performance compared with Li-O₂ batteries equipped with RuO₂/N-C cathodes.Under the conditions of current density of 100 m A g^-1 and the cut-off capacity of 1000 m Ah g^-1,the Li-O₂ batteries with RuO₂/GPE/N-C cathodes cycled stably for 95 times,and the discharge capacity was 13122.1 m Ah g^-1.It also showed good cycling stability in humid oxygen and ambient air.SEM and XPS were used to study the morphology and composition of the cathode of the Li-O₂ battery after discharge.The discharge products showed a flake structure,and the main component was Li₂O₂.After charging,the discharge products decomposed and the impedance value of the battery returned to the initial value.The changes of the internal parameters of the cathode of Li-O₂ battery containing RuO₂/GPE/N-C cathode at the discharge current density of 500 m A g^-1 were also studied through the COMSOL Multiphysics finite element simulation platform.（3）Based on Cassie-Baxter model,low surface energy substances were grafted on the surface of RuO₂ nanoparticles,and then combined with rough and uneven nanostructure of RuO₂/N-C composite to form a superhydrophobic modified RuO₂/N-C composite.The grafting modification of the composite was verified using SEM,XPS and contact angle tests.After the superhydrophobic modification,the contact angle of the air cathode increased from 133°to 151.9°.By comparing the cycle performance and discharge capacity with Li-air（O₂）batteries containing RuO₂/N-C cathodes in different environments,it can be proved that Li-air（O₂）batteries containing superhydrophobic modified RuO₂/N-C cathode has better electrochemical performance.The two groups of batteries were placed in the ambient air for 10 cycles,and the SEM,XPS and XRD characterization results of the cathodes and anodes were compared and analyzed.It was found that the surface discharge products of the cathode and anode of Li-air battery with superhydrophobic modified RuO₂/N-C cathode protection were mainly Li₂O₂,with fewer by-products,makes the battery safer;while the surfaces of electrodes of Li-air battery containing RuO₂/N-C cathode were filled with by-products.At the same time,the EIS spectra of the two groups of batteries before and after discharge also showed that the superhydrophobic modified RuO₂/N-C electrode material had excellent cycling reversibility performance.