Synthesis Of Sn-based Catalysts And Their Fundamental Applications In Lithium-Oxygen/Nitrogen Batteries

Facing the severe energy and environmental crisis strikes,developing advanced environmental-friendly electrochemical energy storage devices with high energy density are considered as the vital working goal in the energy field.Lithium-oxygen（Li-O₂）battery possess the ability to deliver a high theoretical energy density of 11140 Wh kg^-1 and has been widely studied and reported in the past decades,which is considered as the one of promising devices for energy storage and conversion in the sustainable human society.Recently,the concept of advanced lithium-nitrogen（Li-N₂）battery began to be put forward and studied.Until now,rechargeable Li-O₂/N₂ batteries still suffer from several technical challenges,including low actual discharge capacity,severe electrochemical polarization,poor energy conversion efficiency,short cycle life,and bad rate capability,which due to their sluggish electrochemical kinetics during the discharge-charge processes.Suitable catalysts can reduce the high energy barrier required for the gas adsorption/release process,thereby accelerating the electrochemical kinetics during cycling and enhancing the electrochemical performance of Li-O₂/N₂ batteries.Therefore,discovering catalyst cathodes with low price and high catalytic property is an important task for promoting the further development of Li-O₂/N₂ batteries.In this paper,the catalytic mechanism and application basis of Sn-based catalysts in Li-O₂/N₂ batteries were detailly studied,respectively.The specific content is as follows:Rapid interfacial charge transfer and effective catalytic active site for the catalyst are the key to improve its catalystic performance,therefore,aiming on designing the morphology of the catalyst,a low-cost atomic thin SnS₂ layer synthesized via a modified hydrothermal method is applied as the cathode catalyst applied in the Li-O₂ battery.Compared with traditional nano-flower SnS₂,atomic thin SnS₂ layer exhibits an improved cyclic property and rate capability.The improved electrochemical performance can be mainly attributed to the following two aspects.On the one hand,the atomic level ultra-thin layer structure reduces the charge transfer distance and accelerates charge transfer rate for the catalyst.On the other hand,the higher specific surface area of atomic thin SnS₂ layer can provide more catalytic active sites,and more storage space for insoluble discharge products.Ulteriorly,the catalytic properties of atomic thin SnS₂ layer in the Li-O₂ batteries are discussed.It is found that the generation of Sn⁴⁺/Sn²⁺redox couple in the SnS₂ catalyst may act as the active catalytic sites on promoting formation/decomposition for the intermediate Li O₂ and discharge product Li₂O₂ throughout the cycling processes.Moreover,it is calculated that the（001）plane for the SnS₂ cathode can reduce the adsorption energy barrier and the catalytic reaction energy gaps for the interface electrochemical reaction in the Li-O₂ batteries.Based on the former discussion about dominant crystal plane of SnS₂,from the perspective of controlling the growth of the dominant crystal plane,a series of nano flower shaped SnS₂materials with different Ru contents are prepared by a typical solvothermal method and applied as the cathode catalyst in Li-O₂ batteries.The electrochemical results show that the Ru doped SnS₂ cathode exhibit significantly improved cyclic property and rate capability.When the doping amount is 2 wt.%,2Ru-SnS₂ cathode exhibits the best electrochemical performance.It is found that the improvement in the performance is mainly attributed to the following two aspects.Firstly,the introduction of Ru ions can limit the growth process of SnS₂,resulting in the smaller particle size for the doped materials with higher specific surface area and more active catalytic sites.Moreover,Ru doping leads to the lattice distortion of SnS₂,which induces the generation of more sulfur vacancies and more exposed Snion activation centers,resulting in better adsorption ability for the doped SnS₂.Secondly,Ru ion doping can limit the growth of the inferior（102）crystal plane and induce the few-layer structure for the dominant（001）crystal plane,fundamentally improving the ionic conductivity and charge transfer for the Ru doped SnS₂ materials,which can reduce the reactive energy barrier required for the ORR/OER process and improve their O₂ selectivity and catalytic performance.At present,the operation of Li-O₂ battery is still based on pure oxygen environments,which restricts its working environment.Considering the highest N₂ content of 78%in the air,the operation of Li-N₂ batteries may provide a new approach to address the former issues.In addition,due to the toxicity of most sulfur sources,SnO₂ is used as a catalyst for the further research.N-doped carbon nanosheets loaded with SnO₂ nanoparticles（SnO₂@NC）is successfully prepared by a simple wet chemical process and used as the cathode in the advanced Li-N₂ battery.Electrochemical results show that manufactured Li-N₂ battery with SnO₂@NC shows high discharge capacity and better cycle stability at a high current density of 1000 m A g^-1,which can stably cycle for 100 cycles with a cut-off capacity of 500 m Ah g^-1,indicating a good development prospect.Furthermore,based on series of systematic experiments including TOF-SIMS,it is demonstrated that SnO₂@NC cathode can participate in electrochemical reactions as a catalyst rather than an electrode material,and verified the electrochemical reversibility of Li-N₂ batteries.Similarly,the Sn⁴⁺/Sn²⁺redox pairs generated in SnO₂ act as the active catalytic sites for promoting the NRR/NER process.At last,combined with theoretical calculations,the synergistic effect of SnO₂ and nitrogen doped carbon nanosheets on promoting N₂ adsorption and NRR process are discussed.To further improve the catalytic performance of SnO₂-based materials in the Li-N₂batteries,an oxygen-vacancy rich Ru-doped SnO₂ catalyst（OR-SnO₂@NC）is prepared by a simple wet chemical method.It is found that the introduction of Ru ions can limit the growth of SnO₂ nanoparticles and the aggregation of carbon nanosheets,resulting in the higher specific surface areas and more active catalytic sites under low sintering temperature.Moreover,the formation of Ru-O bond induces the generation of oxygen vacancy in the OR-SnO₂@NC,which exposes more Snatom active centers,thereby improving its N₂ selectivity and adsorption ability.As a result,OR-SnO₂@NC cathode shows an improved cyclic property and electrochemical reversibility.In addition,referring to the controversial working mechanisms of Li-N₂ batteries,a detailed study about the N₂ conversion pathway in the Li-N₂ batteries is conducted via introducing different water contents into the electrolyte（16 ppm,30 ppm,50 ppm and 100 ppm）,and new working mechanism about a multi-step NRR conversion path is put forward.Combining a series of in situ/non in situ tests and theoretical calculations,it is found that trace H₂O in the electrolyte can participate during the discharge process,thereby enabling the operation of NRR process under a higher work voltage.This in-depth study on the charge and discharge process of the Li-N₂ battery may promote development of renewable N₂ conversion and provide new design ideas for development of other metal-N₂ batteries.