Transition Metal Molybdenum Tungsten Oxide:Structure And Energy Storage Performance

Transition metal oxide（TMO）based on conversion reaction has the advantages of low cost and high weight specific capacity,which is a promising cathode material for secondary battery.Molybdenum oxide,including MoO₂,MoO₃,MoO_3-x（0<x<1）,is a promising candidate for high-efficiency electrochemical energy storage system because of its unique physical and chemical properties,such as Layered/tunnel structure,mechanical and thermal stability.At present,researchers’research on molybdenum based oxide energy storage mainly focuses on lithium-ion and sodium ion battery anodes and supercapacitors.In this paper,molybdenum tungsten mixed oxide,spherical molybdenum oxide and molybdenum tungsten binary oxide materials were obtained by precipitation method,solvothermal method and subsequent high-temperature calcination,and their structures and electrochemical energy storage properties were studied.The effects of molybdenum tungsten ratio and calcination temperature on the phase composition and microstructure of a series of molybdenum tungsten composite oxides prepared by precipitation method were studied.Symmetrical supercapacitors were assembled with a series of samples as working electrodes.We studied the capacitance properties of different samples in 1M H₂SO₄and 1M Na₂SO₄electrolytes.The sample calcined in air at 550℃with molybdenum tungsten ratio of1:3 showed the best capacitance performance in 1M H₂SO₄electrolyte(93.3 F·g^-1specific capacitance at 0.1 A·g^-1current density),and the sample calcined in air at 550℃with molybdenum tungsten ratio of 1:1had the best capacitance performance in 1M Na₂SO₄electrolyte(54.8F·g^-1specific capacitance at 0.1 A·g^-1current density).We studied the effects of different volume ratios of ethanol,ethylene glycol and adding citric acid on the structure and morphology of spherical molybdenum dioxide synthesized by solvothermal synthesis.And then,the energy storage properties of samples obtained under different solvothermal conditions were compared,and the lithium/sodium storage properties of molybdenum dioxide samples with uniform spherical morphology and under the solvothermal condition with ethanol glycol volume ratio of 1:1 and citric acid were studied deeply.The spherical molybdenum dioxide anode has a high specific charge capacity of 1126.8mAh·g^-1at low current density(0.05 A·g^-1),the highest charging specific capacity of 694.2 mAh·g^-1at 1 A·g^-1current density,（the capacity retention rate is 61.6%%）,which shows the great rate performance of this sample.In the sodium-ion half cells,the spherical molybdenum dioxide anode has the highest reversible specific capacity and good rate performance(219.7 mAh·g^-1charging specific capacity at0.05A·g^-1current density,147.7 mAh·g^-1charging specific capacity at 1A·g^-1,and 77.7 mAh·g^-1charging specific capacity at 5 A·g^-1).Finally,the different lithium/sodium storage mechanisms of the spherical molybdenum dioxide anode in lithium-ion battery and sodium ion battery were studied.In lithium-ion batteries,the spherical molybdenum dioxide anode stores lithium through coupled conversion reaction,while in sodium ion batteries,pseudocapacitive sodium ion storage is dominant in the spherical molybdenum dioxide anode.Molybdenum tungsten composite oxide with different molybdenum tungsten ratios were synthesized by solvothermal method and high temperature calcination,and their structures and energy storage properties were studied.The anode of samples calcined under nitrogen at 600℃with molybdenum tungsten ratio of 8:1 shows the best performance in lithium-ion half cells,and The anode of samples calcined under nitrogen at 600℃with molybdenum tungsten ratio of 1:2 has the best performance in sodium-ion half cells.The anode of samples with molybdenum tungsten ratio of 8:1 anode of lithium-ion battery has a specific charge capacity of 719.1 mAh·g^-1in the first cycle of charging and discharging cycle at 1 A·g^-1,and the initial coulomb efficiency is94.4%.After 1000 cycles,the specific charge capacity remains 420.1mAh·g^-1,and the capacity retention rate is 58.4%.The lithium storage mechanism of coupled insertion–conversion reaction and the associated electrode activation during the cycle in the anode of samples with molybdenum tungsten ratio of 8:1 are the reasons of higher reversible specific capacity and batter capacity retention compared with spherical molybdenum dioxide samples.The first charge and discharge specific capacity of the anode of samples with molybdenum tungsten ratio of 1:2of sodium-ion battery under 1 A·g^-1are 86.6 mAh·g^-1and 206.2 mAh·g^-1,and the coulomb efficiency is 41.99%.After 1000 cycles of charge and discharge cycle,the charge specific capacity is 74.1 mAh·g^-1,and the specific capacity retention rate relative to the first cycle is 85.6%,which shows that the anode of samples with molybdenum tungsten ratio of 1:2also has good reversibility under high current density.Electrochemical characterization shows that the anode of samples with molybdenum tungsten ratio of 1:2 has the above excellent properties because of the sodium storage mechanism of its“conversion+pseudocapacitance”reaction.