Investigation Of Micro Lithium Storage Mechanism Of High Entropy Oxides Via Room-temperature Synthesis

High entropy oxides（HEOs）,also known as entropy stabilized oxides,are a new class of stabilized single-phase solid solution formed from five or more metal cations in equimolar or near-equimolar ratios.Metal ions in HEO crystal structures occupy cations sites randomly and enhance configurational entropy greater than 1.5 R.HEOs have been used in the application fields of electrode materials of lithium-ion batteries,dielectric materials,magnetic materials and catalytic material for their excellent thermal,magnetic and electrical properties.Especially,their high specific capacity and excellent cycle stability as the electrode materials of lithium-ion batteries（LIBs）have been widely concerned.The traditional preparation methods of HEOs mainly include solid-state sintering and spray pyrolysis which need high-temperature environment,the metastable state at high temperature is reserved through rapid annealing.However,there is a risk of precipitation of the second phase after long-term storage at room temperature,which is not conducive to the stable use of LIBs.Meanwhile,the micro lithium storage mechanism of HEOs and the influence of functional units on electrochemical performance are not clear.In this paper,(Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)O high entropy oxide（CCMNZ-HEO）with good crystallinity was synthesized in one step by high-energy ball milling at room temperature.Its single-phase rock salt structure and random distribution of cations were proved by X-ray diffraction（XRD）,transmission electron microscopy（TEM）,scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）.Micro electrochemical charge/discharge experiment on CCMNZ-HEO based on in situ TEM identified the conversion-alloying mechanism during lithiation.The results of electrochemical analysis showed that the relatively high discharge capacity of the first cycle is 1119.2 m A h g^-1.After 50 cycles,the coulomb efficiency is above 98%,but rapidly capacity decrease appeared,discharge capacity after 725cycles is 270.9 m A h g^-1.Therefore,it is necessary to prepare high entropy oxides with better electrochemical performance by regulating functional units.(Co_0.2Ni_0.2Mn_0.2Zn_0.2Fe_0.2)O（CNMZF-HEO）with single-phase rock salt structure was synthesized by replacing Cu O and Mg O with Mn O and Fe₃O₄ which have larger theoretical discharge specific capacity.Reversible capacity of LIBs improved significantly by using CNMZF-HEO as negative electrode material,the specific discharge capacity is 646 m A h g^-1after 150 cycles and the capacity retention is more than 90%compared with the initial several cycles,this reversible capacity is higher than commercial carbon anode material.Meanwhile,the coulomb efficiency remains above 97%after initial 4 cycles.In situ TEM charge/discharge experiment combined with selected area electron diffraction（SAED）,high resolution transmission electron microscopy（HRTEM）and electron energy loss spectroscopy（EELS）showed that Co,Mn and Zn precipitated first in the process of lithium intercalation,marking the beginning of conversion reaction.Then Ni and Fe precipitated,finally Zn reacted with excess Li to form Li_xZn alloy.The understanding of the micro lithium storage mechanism of HEOs will provide a direct basis for the component design of new high-performance HEO electrode materials.