Metal Coordinated Polymer As Three-Dimensional Network Binder For High Sulfur Loading Cathode Of Lithium Sulfur Battery

Lithium-sulfur（Li/S）batteries have the advantages of environmental friendliness and high theoretical energy density(2500 Wh kg^-1),which are promising energy storage devices.However,in the case of high sulfur loading on the cathode,Li/S batteries face the technical bottleneck of poor cycle stability and low actual energy density.The above phenomenon is mainly due to the loose cathodic structure and slow reaction kinetics under high sulfur loading condition.Specifically,the poor mechanical stability of traditional two-dimensional linear polymer binders cannot ensure the interaction between the components of cathode with high loading,resulting in the loss of sulfur species during battery cycling.what is worse,with the increase of sulfur loading,the electrode thickness gradually increases,leading to slow electron transfer rate between sulfur（S）and lithium sulfide（Li₂S）.In order to solve above problems,a new three-dimensional polymer binder is designed,which improves the structural stability of high loading cathode and improves the cathodic reaction kinetics,so that Li/S batteries show better cycling performance and higher discharge capacity.The main research ideas and contents of this paper are as follows:（1）Aiming at the problems of low mechanical strength and poor adhesion of traditional two-dimensional linear polymer binder,we synthesized a three-dimensional polymer binder coordinated by metal ions,which significantly improved the structural stability and redox activity of the cathode.The polar functional groups（-NH₂）in the binder significantly enhance the interaction between the components of cathode.Under high loading condition,the cathodic surface is complete and smooth with the use of three-dimensional binder.In contrast,the use of two-dimensional binders caused cracking and collapse of the cathode.Furthermore,metal ions are coordinated with the amino groups（-NH₂）on binder,and the optimal content of metal ions is explored.The introduction of metal ions significantly reduces the activation energy at each stage of the cathodic reactions,so that the batteries exhibit smaller polarization voltage,which proves the improvement of reaction kinetics.（2）In view of the poor cycle stability and low capacity of Li/S batteries under high sulfur loading,the three-dimensional polymer binder coordinated by metal ions are used to significantly extend the cycle life and improve the capacity of batteries.The construction of three-dimensional network alleviates the severe volume change of cathode,which effectively improves the stability during battery cycling.The introduction of metal ions regulates the binding capacity between-NH₂ and polysulfides（Li₂S_n）in the polymer substrate.Therefore,the metal coordinated three-dimensional polymer binder effectively adsorb Li₂S_n while accelerating redox conversion,contributing to high battery capacity.At the sulfur loading of 4.81 mg cm^-2,batteries with the new three-dimensional binder shows a high discharge platform and the oxidation potential of Li₂S is decreased,which proved that the cathodic reaction kinetics was improved.Furthermore,under the low electrolyte volume condition with higher requirements for cathodic redox kinetics,the two-dimensional linear polymer binder fail to prevent the large increase of polarization voltage,and the battery capacity is only 346.6 m Ah g^-1,while the metal ions coordinated three-dimensional polymer binder significantly increase the discharge capacity(938.0 m Ah g^-1)and improve the cycle stability of the battery.In summary,due to the low strength and weak adhesion of traditional two-dimensional linear polymer binders,high loading cathodes often have structural instability and slow reaction kinetics,resulting in low actual energy density of Li/S batteries.In order to solve the above problems,a novel three-dimensional polymer binder with metal ion coordination was constructed.The three-dimensional network ensures the overall structural stability of the cathode and provides support for high sulfur loading.More importantly,metal ions accelerate the oxidation and reduction of Li₂S_n and improve the redox reaction kinetics of high-loading cathodes.Therefore,Li/S batteries with metal ions coordinated three-dimensional polymer binders exhibit better cycle stability and higher discharge capacity under high sulfur loading,which help the batteries achieve higher energy density.