The Liquid-phase Reaction Path Regulated By Fluorenone And Its Derivatives Increases The Cathode Reaction Rate Of Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries have become one of the most promising energy storage devices due to their low price,environmental friendliness and high energy density(2600 Wh kg^-1).However,under the conditions of low electrolyte volume and high sulfur loading（low E/S ratios）,the actual capacity of Li-S batteries is much lower than the theoretical value.So that,the actual energy density of Li-S batteries is limited.This phenomenon is mainly attributed to the slow cathodic reaction rate at low E/S ratios.Specifically,the content of liquid polysulfides（Li₂S_n,intrinsic redox mediators）is significantly decreased,which leads to difficult electron transfer between the sulfur（S₈）and the discharge product lithium sulfide（Li₂S）.As a result,the cathodic reactions of Li-S batteries are seriously hindered,and most of the sulfur species could not be utilized.In order to solve above problems,exogenous redox mediators（RM）are introduced as liquid phase catalysts into the electrolyte of Li-S batteries.The RM can provide new liquid-phase reaction pathways for the cathode redox reaction by interacting with Li₂S_n,which significantly increases the cathodic reaction rate and the actual capacity of Li-S batteries.On this basis,substituents are introduced to regulate the molecular structure.Then,exploring the effects of its redox properties and cathodic reaction kinetics of Li-S batteries caused by the changes of molecular structure.The main contents of this work can be summarized as follows:1.In view of the slow cathodic reaction rate of Li-S batteries,we propose an in situ radical supplementation strategy mediated by RM,which significantly accelerates the cathodic reaction rate.Compared with the conventional reaction pathway of S₈（?）Li₂S_n（?）Li₂S,RM can generate a new liquid electron transport pathway between the electrode and Li₂S_n by its redox and stabilizing of trisulfur radicals(S₃^·-)in the electrolyte.This new liquid-phase reaction pathway significantly improves the redox kinetics of Li₂S_n and reduces the activation energy at all stages of the cathodic reactions.At high current density of 2 C,the polarization voltage of Li-S batteries is reduced by 100 m V.Furthermore,under the low electrolyte content of E/S=5μL mg_s^-1,the limited cathodic reaction rate makes a large polarization voltage,with a capacity of only 218.7 m Ah g^-1,while the regulation of the liquid-phase reaction pathway significantly reduces the polarization voltage and increase the capacity to 1099.7 m Ah g^-1.2.Considering that substituents can regulate the redox properties of RM,we change the structure of the original RM,discuss the redox properties of different RM,and further explore their role in the cathodic reaction of Li-S batteries.Specifically,the introduction of electron-absorbing groups enhances the interaction between RM and Li₂S_n by regulating the obtain electrons ability of RM,resulting in the formation of new intermediates.Those new intermediates can optimize the original liquid-phase reaction pathway,which is mediated by Li₂S_n,significantly reducing the energy barrier at each reaction stage of the battery.The cyclic voltammetry test shows that the transient generation of Li₂S_n is not enough for effectively mediating the cathodic reactions,so that the large polarization voltage is occurred and the charging or discharging processes are limited.Nevertheless,RM makes Li-S batteries show higher electrochemical activity under the same conditions,and the whole charging and discharging process can be completely realized.Under the conditions of high sulfur loading(5.4 mg_s cm^-2)and low E/S ratio(5.7μL mg_s^-1),which require high cathodic reaction rate,the capacity of Li-S batteries is increased by nearly 2.5 times.In summary,in order to achieve high energy density,Li-S batteries must overcome the limited sulfur species utilization and actual capacity at low E/S ratios caused by slower cathodic reaction rate.In this paper,exogenous RM are introduced as functional additives to regulate the liquid-phase reaction pathway of cathode.As a result,electron transfer in the redox reactions is accelerated,which significantly increases the cathodic reaction rate,resulting in a smaller polarization voltage and higher discharge capacity at a low E/S ratio.This work provides a basic understanding of the redox reaction of Li-S batteries,and also shows new ideas and methods to solve the problem of slow kinetics of cathodic reactions at low E/S ratio.