Construction,Physicochemical Properties And Li-Storage Mechanisms Of Sphalerite-Structured Cation-Disordered Phosphides

Lithium-ion batteries（LIBs）have attracted much attention in the field of energy storage systems due to the merits of small size,high energy conversion efficiency,environmental benign.However,the conventional graphite anodes of LIBs only have the theoretical specific capacity of 372 m A h g^-1,and thus can’t meet the current demands of the high-energy-density LIBs for the next-generation vehicles.Phosphorus with a high specific capacity of 2596 m A h g^-1,environmental benign and high abundance,is expected to be a strong anode candidate of high-energy-density LIBs.Herein,we develop a series of equimolar cation-disordered Sphalerite-structured（Zn S）phosphides and non-equimolar cation-disordered phosphides respectively by means of the composition regulation approach in this thesis.The structural characteristics,physicochemical properties,and lithium storage mechanisms were analyzed by virtue of a combined method of the experimental measurements and theoretical calculations.And the Li-storage performances were further improved by using the graphite-modified phosphides.The main results and innovations of this thesis are summarized as follows:（1）Construction of equimolar Sphalerite-structured cation-disordered phosphidesConsidering the all-lithium-reactive components and the intriguing physicochemical properties of multinary compounds,we for the first time prepared the equimolar Sphalerite-structure cation-disordered compound of GaAlP₂,Ga Si P₂,Ga Ge P₂ and In Ge Si P₃ on the basis of the similar crystalline structure（Sphalerite）of Al P,Ga P,In P,Ge P and Si P utilizing a facile mechanical ball milling method.We performed the characterizations on the as-prepared compounds,the cation-disordered phase with Zn S structure were confirmed by virtue of X-ray diffraction（XRD）refinement,high-resolution transmission electron microscopy（TEM）,Raman,and X-ray photoelectron spectroscopy（XPS）analyses.For those compounds,the cations（Zn）are evenly distributed by Ga and Al;Ga and Si（Ge）;In,Ge and Si,respectively,while the anions（S）are completely replaced by phosphorus.（2）Electrochemical properties of equimolar Sphalerite-structured cation-disordered phosphidesThe electrochemical properties of the equimolar cation-disordered phosphides of GaAlP₂,Ga Si P₂,Ga Ge P₂,and In Ge Si P₃ were investigated.The results show that the as-prepared cation-disordered phosphides have smoother discharge and charge profiles,higher initial Coulombic efficiency（ICE）,and better cycle stability than its corresponding ball-milled intermediates.Moreover,the Ga Si P₂,Ga Ge P₂ compounds present better lithium storage performances compared with the Ga P,Si,and Ge electrodes.Based on the analysis of experiments and theoretical calculations,the excellent lithium storage performances of these phosphides can be attributed to the fast Li-ion diffusion capability,fast electronic conductivity and strong resistance to volume expansion endowed by the cation-disordered lattice.The electrochemical performances of the GaAlP₂@C,Ga Si P₂@C,and Ga Ge P₂@C composites prepared after further ball milling with carbon at a mass ratio of 6:3 were further improved,high specific capacities of 342,800,and 525 m A h g^-1 can be provided at current densities of 20.0,10.0,and 20.0 A g^-1,respectively.At the same time,our research also shows that,compared with low-entropy In Ge P₂ and In Si P₂,due to the increase of structural entropy,the medium entropy In Si Ge P₃ compound has been improved in terms of thermal stability,electron and lithium ion diffusion rate,and volume expansion resistance.（3）Lithium storage mechanisms of equimolar Sphalerite-structured cation-disordered phosphidesLithium storage mechanisms of the as-prepared equimolar cation-disordered phosphides of GaAlP₂,Ga Si P₂,Ga Ge P₂,and In Ge Si P₃ were investigated by means of various characterization techniques including in-situ XRD,Raman and XPS.We unveiled the fundamental reasons why these multinary phosphides present excellent lithium storage properties via a comprehensive analysis on each electrochemical process.After the first cycle,these multinary phosphides become amorphous state,which can alleviate the volume expansion during the subsequent cycles,thereby presenting the electrochemical cycling stability.Moreover,the electrochemical intermediates of Li Ga,Li In,Li₂₁Si₈,Li₁₅Ge₄ and Li₃P during cycling facilitate to improve the lithium ion and electron diffusion capability,thus improving the electrochemical reaction kinetics.Finally,we take Ga Ge P₂ as an example to confirm the gradually enhanced reaction kinetics after intercalation of lithium ions using in-situ electrochemical impedance spectroscopy,which demonstrate the lithium storage advantages of these multinary phosphides.（4）Extension of the equimolar Sphalerite-structured cation-disordered phosphidesConsidering that the cationic ratios of the phosphides of GaAlP₂,Ga Si P₂,and Ga Ge P₂compounds are equimolar,the cationic ratios of Ga:Al,Ga:Si,and Ga:Ge were further tuned and a series of non-equimolar phosphides of Ga-Al-P,Ga-Si-P,and Ga-Ge-P were obtained.Then,based on the triple cation-disordered In Ge Si P₃,we further introduce two cations of Cu,Zn and Cu,Ga to synthesize high-entropy phosphides of Cu Zn In Ge Si P₅ and Cu Ga In Ge Si P₅,respectively.Furthermore,these phosphides exhibit high ICE,suitable working potential,and high reversible specific capacity,which provide guidance for the design of the next-generation of LIBs anode materials.