Regulating The Electrolyte Environment And Electrochemical Performance Of Lithium Metal Batteries By Using Phthalocyanine Complexes

The depletion of non-renewable fossil energy sources such as oil,coal and natural gas has spurred the development of secondary battery systems for storage devices with high energy density and long cycle life.The Li metal anode is a competitive candidate for next-generation secondary battery systems for its low redox potential and ultra-high theoretical specific capacity.Nevertheless,obstacles regarding dendrite growth and poor coulombic efficiency limit the practical application of Li metal batteries.Metal phthalocyanine complexes(MPcs)are macromolecular structures with 18π electron metal-N4 structure.It has significant interaction with the solvent and solute of the electrolyte,which can effectively regulate the mass transfer process,suppress Li dendrites and improve cycling stability.In this paper,by adjusting the structure of MPcs,we research two kinds of complexes based on iron phthalocyanine(FePc)and dilithium phthalocyanine(Li2Pc)on regulating the electrolyte environment.Meanwhile,theoretical calculation and electrochemical experiments are adopted to clarify the dendrite suppression effect and structure-activity relationship of MPcs on the Li anode.The main research contents are as follows:Li2Pc additive was added into the ether-based electrolyte,which is highly effective for regulating of ion solvation and transport towards the dendrite-free Li metal anode.Due to a high charge density,the Li ions in the center cavity of Li2Pc created strong interactions with the solute and solvent of the electrolyte and effectively regulated the solvation structure of anions and migration process of Li+,which leads to suppress the dendrite growth of Li anode.At the same time,Li2Pc greatly promoted the solute dissociation to increase the number of free cations in the electrolyte.As a result,the ionic conductivity of the electrolyte was significantly improved.Therefore,the Li-S batteries with N/P=2 in the Li2Pc modified electrolyte achieve a high initial capacity of 966 mAh g-1 at 0.5 C,and can steadily operate for 500 cycles(calculated based on the theoretical specific capacity of sulfur).Tetraaminodilithium phthalocyanine(Li2TaPc)with strong sulfiphilic/lithiophilic properties was prepared by introducing amino functional groups on the periphery of Li2Pc.Herein,we demonstrate a multifunctional separator based on dilithium tetraaminophthalocyanine(Li2TaPc)self-assembled on reduced graphene oxides(rGO)by strong π-π interactions.Electrochemical tests show that the modified separator exhibits a comprehensive regulation effect on important ion transport properties in one configuration.Firstly,Li2TaPc presents strong sulfiphilic/lithiophilic binding sites for confining polysulfides within the cathode region to improve sulfur utilization.Secondly,charge transfers from the central cavity of the TaPc2-ligand to the rGO promotes the dissociation of Li+ cations to produce additional charge carriers.Meanwhile,the negatively-charged TaPc2-ligand exhibits strong electrostatic repulsions to the salt anions,hindering migration across the separator.Finally,the strong coordination effects of Li+with Li2TaPc enable high retention of Li+conductivity.The apparently increased Li+transference number suppresses dendrite formation and improves the reversibility of the Li anode as well as electrochemical performance of Li-S cells.Pouch cells with a high areal sulfur loading and limited electrolyte addition can steadily operate for 80 cycles at 0.5 C.This study demonstrates a modified separator with a high catalytic activity that is developed from a self-assembled composite of Fe tetraaminophthalocyanine(FeTaPc)physically adsorbed on rGO.In contrast with dilithium cations of Li2TaPc,the Fe2+cation at FeTaPc significantly improved the slow redox kinetics and the utilization rate of S.Therefore,this modified separator provides effective trapping of LiPSs,catalyzed sulfur redox,as well as suppressed formation of Li dendrites in one configuration.The theoretical calculation reveals that FeTaPc@rGO modified separator enables catalyzed sulfur redox via electron superexchange between FeTaPc@rGO and LiPSs.In addition,the strong coordination effect of Li+with amino groups and ligand reduces the diffusion barrier of Li+,thereby promoting the uniform diffusion of Li+.As expected,the Li-S cells with areal sulfur loadings up to 5 mg cm-2 based on FeTaPc@rGO modified separator deliver an areal capacity of 5.0 mAh cm-2 after the initial cell activation(calculated based on the theoretical specific capacity of sulfur).The capacity remained above 4.0 mAh cm-2 after 200 cycles of charge/discharge tests.The above study found a significant crystallographic preferred orientation(i.e.,textures)in the Li metal electrodeposition process under a high Li+transference number environment.The electrochemical deposition of Li+is an electrocrystallization process where preferred crystallographic growth can be formed.The anisotropy of chemical and physical properties associated with the resulting textures can then be utilized to strengthen the Li metal anode.Therefore,on the basis of the above work,texture formation and evolution during homoepitaxial growth were preliminarily studied.Based on pole figures,inverse pole figures and orientation distribution functions from X-ray diffraction,compression is found to be a critical factor for forming {110} textures during deposition on a Li substrate.Without compression and under a high Li+ transference number environment,a sharp {100}<110>texture is formed in the epitaxial layer up to the high deposition capacity of 20 mAh cm-2,inheriting and intensifying the pre-existing texture of the Li foil substrate.Compression alters the homoepitaxial texture by deformation and dynamic recrystallization to produce a peak-type{110} texture component close to {110}<111>.Due to the low diffusion barrier of Li adatoms and rapid redox process of Li/Li+,the {110} texture is confirmed to be a benefiting factor that enables reduced overpotentials,improved stability and suppressed moss-like dendrites in Li metal anodes.This study adds crystallographic factors to the electrochemo-mechanical correlations of Li metal anodes and provides new opportunities for further optimizing the electrochemical performance of Li metal anodes.