Preparation And Electrochemical Properties Of (Mg,Ni,Zn,Cu,Co)O-based High Entropy Oxides

High-entropy oxides are a new type of single-phase solid solution material,which shows excellent reversible lithium storage properties due to its unique entropy stability effect,and has received extensive attention in recent years.However,as anode materials for Li-ion batteries,High-entropy oxides still suffer from low reversible specific capacity,low Initial Coulomb efficiency（<60%）,and large voltage hysteresis.To address these issues,based on the review of the research development of high-entropy oxides,the modification of（Mg,Ni,Zn,Cu,Co）O based high-entropy oxides anode materials was studied by means of synthetic atmosphere control,chemical prelithiation and cationic regulation strategies.The structural characteristics,electrochemical performance and mechanisms related to the improved performances were elucidated.First,the effect of synthesis atmosphere（air and argon）on the phase composition and electrochemical performance of（Mg,Ni,Zn,Cu,Co）O high-entropy oxides was investigated.The（Mg,Ni,Zn,Cu,Co）O synthesized in the air environment had a single-phase structure,while in the argon environment,the raw material Cu O is decomposed to form stable Cu₂O,resulted in the presence of Cu₂O impurities in the obtained oxides.（Mg,Ni,Zn,Cu,Co）O synthesized in air environment exhibited a reversible capacity of 501 m Ah g^-1 at the current density of 100 m A g^-1,while high-entropy oxides synthesis lacks sufficient oxygen sources in an argon environment,resulted in a relatively low content of high valence Cu²⁺and Co³⁺in the synthesized oxides mixture,resulted in a lower reversible capacity and a reversible capacity of 465 m Ah g^-1 at a current density of 100 m A g^-1.Therefore,an oxygen rich air environment is an essential condition for synthesizing single-phase high entropy oxides.In order to improve the reversible capacity and initial Coulomb efficiency of（Mg,Ni,Zn,Cu,Co）O,a series of Li_x（Mg,Ni,Zn,Cu,Co）_1-xO high-entropy oxides were synthesized by pre-lithiation of Li H as a lithium source.Low valence Li⁺was successfully introduced into the lattice of（Mg,Ni,Zn,Cu,Co）O,and due to the charge compensation effect,Co³⁺,Ni³⁺,and oxygen vacancies were formed in the high-entropy oxides.The presence of high valence Co³⁺and Ni³⁺increases the electron transfer number of the conversion reaction,and increases the reversible specific capacity of Li_x（Mg,Ni,Zn,Cu,Co）_1-xO high-entropy oxides.Meanwhile,the formation of oxygen vacancy improves the electron and ion transport capacity of the high-entropy oxides,and further improves the kinetics of the conversion reaction.In addition,the inactive Mg²⁺makes the high entropy oxides maintain the stable rock salt structure upon lithiation/delithiation.Therefore,the Li_0.16（Mg,Ni,Zn,Cu,Co）_0.84O high-entropy oxides anode material with good structural stability and kinetic properties exhibits a high reversible specific capacity of 679 m Ah g^-1,initial Coulomb efficiency of 63.7%,and a small electrode polarization（charge and discharge platform voltage difference of 0.9V）at a current density of 100 m A g^-1.At a high current density of 1A g^-1,the reversible specific capacity remains at 651 m Ah g^-1 after 800 cycles,showing excellent cycle stability and rate characteristics.Based on the above research,cation regulation（removal of Cu element）was used to further improve the electrochemical performance of the high-entropy oxides anode.The removal of Cu element from Li_x（Mg,Ni,Zn,Co）_1-xO results in single-phase high entropy oxides with rock salt structure at a Li doping amount of x=0.2.The removal of Cu elements with weak electrochemical activity increased the reversible specific capacity of Li_x（Mg,Ni,Zn,Co）_1-xO.At the same time,the formation of Co³⁺,Ni³⁺and oxygen vacancies in the high-entropy oxides due to charge compensation improves the conversion reaction electron transfer number and reaction kinetics.In addition,the high-entropy oxides have good structural stability during the cycle.Therefore,the cation regulated Li_0.2（Mg,Ni,Zn,Co）_0.8O high-entropy oxides exhibits improved reversible specific capacity and initial Coulombic efficiency,as well as good cycling stability and rate characteristics.At a current density of 100 m A g^-1,the reversible capacity is 760 m Ah g^-1 and the initial Coulombic efficiency is 66.4%,there was no decline in specific capacity after 100 cycles.After 350 cycles at a current density of 1 A g^-1,the reversible specific capacity still remains at483 m Ah g^-1.