High Performance Transition Metal Oxides And Metal Sulfides Based Anode Material For Lithium-ion Batteries

Since the commercialization of lithium-ion batteries,they have been widely used in people’s lives.However,energy and power density of the existing batteries are difficult to satisfy the increasing demand.The exploration of anode materials with excellent rate performance and long cycle life has become the mainstream of current lithium-ion battery research.This article focuses on the shortcomings of transition metal oxides and transition metal sulfides such as low charge-discharge specific capacity,low cycle stability,insufficient conductivity,and slow electrochemical dynamics.We conduct a study of TiO₂,Ni Co₂O₄,Co₃O₄and Co₉S₈,using high conductivity high-rate carbon material composite,composite with high specific capacity metal oxide,reasonable nanocomposite structure design,integrated electrode structure design without binder and conductive agent,etc.to improve the reversible capacity,cycle performance and rate of the material and other lithium storage capacity.Contents of the research in this thesis are as follows:（1）The process of assembling commercial TiO₂ nanoparticles（P25）by polypyrrole and then carbonizing was designed to prepare a composite material（P25）similar to the watermelon-like amorphous carbon balls inlaid with melon flesh and melon seeds and assembled with TiO₂nanoparticles（P25@C）.The material structure characterization found that P25@C has a bonded small ball structure with a diameter of 150-200 nm.The P25@C sphere has abundant mesopores,and the specific surface area is as high as 196.5 m²g^-1.As a negative electrode material for lithium-ion batteries,P25@C has excellent rate performance,with an average discharge capacity of 496,416,297,240,180,99,49 and 25 m Ah g^-1at current densities of 0.5C,1C,5C,10C,20C,50C,100C,and 200C,exhibits excellent long cycle performance.After5000 cycles at 20C,the discharge specific capacity still reaches 106.9 m Ah g^-1,and the average capacity loss rate per cycle is only 0.008%.The outstanding lithium storage capacity comes from the unique watermelon-like composite structure,which not only significantly improves the conductivity of TiO₂,but also effectively disperses TiO₂ nanoparticles.Through the qualitative and quantitative analysis of the cyclic voltammetry curve,it is found that P25@C has a higher specific capacity and excellent rate performance.This is due to the surface pseudocapacitance lithium storage and bulk lithium storage of P25@C.Surface pseudocapacitive lithium storage enhanced by P25@C is because the entire surface of each TiO₂nanoparticle participates in the electrochemical reaction,so it produces a strong capacitive lithium storage effect.（2）Through the reaction of metal organic frame ZIF-67 nanosheets with Ni（NO₃）₂,cyclic voltammetry was used to coat polypyrrole and then carbonized to synthesize C@Ni Co₂O₄hollow nanosheets on the surface of foamed nickel.Ni Co₂O₄hollow nanosheets grow directly on the surface of nickel foam in an array.The height of the nanosheets is about 900 nm,and they are assembled from Ni Co₂O₄grains with a diameter of about 10 nm.There are abundant mesopores between the grains,and the surface is covered with Amorphous carbon,the thickness of carbon film is about 30-40nm.As an anode material,C@Ni Co₂O₄exhibits great charge and discharge performance,cycle performance and rate performance.After being cycled for 150times at a current density of 0.8C,C@Ni Co₂O₄can still provide a discharge capacity of 910 m Ah g^-1.It has a high capacity retention rate of 85.7%and an average discharge capacity of 961 m Ah g^-1.At current densities of 0.1C,0.2C,0.5C,1C,2C,the average discharge capacities of C@Ni Co₂O₄are 1096,1080,998,905,and 840 m Ah g^-1,respectively.Great storage capacity is benefit from the coordination of the material structure and integrated electrode structure.The carbon shell coating effectively reduces the volume of Ni Co₂O₄The change also makes up for the disadvantage of poor conductivity;hollow structure increases the contact area with the electrolyte,can promote the penetration of the electrolyte,and effectively alleviate the deformation stress of Ni Co₂O₄,and comprehensively improve the electrochemical performance of the material.（3）On the basic of cobalt carbonate nanowires,a capsule material TiO₂@Co₃O₄with Co₃O₄nanocrystals encapsulated in TiO₂nanotubes was prepared.The average diameter of Co₃O₄nanocrystalline grains is about 40 nm,and there are abundant mesopores between the nanocrystalline grains.The thickness of the TiO₂shell is about10 nm,and the tube diameter is 150-200 nm.The encapsulated TiO₂@Co₃O₄has high reversible capacity and cycle stability.At 0.5 A g^-1,the discharge is still 836 m Ah g^-1through 500 cycles.At 0.1,0.2,0.5,1 and 2 A g^-1,the reversible discharge capacity is836,828,760,660 and 540 m Ah g^-1,respectively.Significantly enhanced electrochemical performance benefits from the excellent nanocapsule structure of TiO₂@Co₃O₄.The encapsulation of the TiO₂shell effectively limits the large volume change of Co₃O₄.The abundant mesopores inside the capsule allow the electrolyte to fully contact the Co₃O₄nanocrystalline grains.At the same time,it can alleviate the expansion and contraction of Co₃O₄during charge and discharge,enhance the structural stability of the material,and improve its lithium ion storage performance.（4）On the basis of encapsulated TiO₂@Co₃O₄,Co TiO₃@Co₉S₈nanotubes were synthesized by gas-phase vulcanization method.The thickness of the Co TiO₃tube shell was about 20 nm,and a large number of Co₉S₈nanocrystalline grains were filled inside.About 40 nm,there are a large number of mesopores between the nanocrystalline grains,and the average pore diameter of the mesopores is about 5-6 nm.Electrochemical tests show that Co TiO₃@Co₉S₈nanotubes have excellent charge-discharge performance and cycle performance.At the end of 150 cycles,the discharge is still 951 m Ah g^-1,and the average discharge capacity was 785 m Ah g^-1.This is the result of the combined effect of the Co TiO₃nanotube shell and Co₉S₈nanoparticles.When the Co TiO₃shell is combined with Co₉S₈,it can not only play the role of supporting structure to limit the expansion and decomposition of Co₉S₈,but also provide the charge and discharge capacity equivalent to Co₉S₈;The extremely high conductivity of Co₉S₈also makes up for the poor conductivity of Co TiO₃.In addition,this nanotube shell/nanoparticle composite structure promote the transfer of electrons/charges.