Electrochemical Preparation And Lithium Storage Properties Of 3D Self-supported Transition Metal Oxides

As the anode materials of lithium ion batteries,conversion-type transformed transition metal oxides have become one of the potential candidates for commercial graphite anode material because of its high theoretical specific capacity and high safety.However,there appears a second phase Li₂ O in the discharge?lithium storage?of conversion-type transition-metal oxides.In the charge?removal of lithium?process,the second phase Li₂ O cannot release of all the lithium,resulting in irreversible capacity.Meanwhile,the large volume changes of conversion-type transformed transition metal oxide during the elctrochemical cycling leading to craking and exofiation of active materials,and also leading to the solid electrolyte interphase?SEI?film formed on the electrode surface continuously grow thicken due to its repeated cracking and repairing.Besides,conversion-type transformed transition metal oxides have low intrinsic electrical conductivity and lithium ion diffusion coefficient,causing their high-rate capacities are lowIn this thesis,three dimensional porous and self-supported structures were introduced into the Mn₃O₄ and NiOx to improve the reversible capacity,cycle life and first coulomb efficiency through accommodating the volume changes and relieving the enormous stress during the electrochemical cycling.Three dimensiaonal porous self-supported Mn₃O₄ naosheet arrays were produced by high temperature annealing the as-electrodeposited MnHxOy precursors under Ar atmosphere using the three dimensiaonal porous Cu prepared by electroless as the substrate.Three dimensional porous self-supported nanosheets array structure can reduce "dead lithium" in Mn₃O₄ active particles during charging/discharging and achieve excellent cycle life by effectively alleviating the volume changes,The electrochemical test indicate the three dimensiaonal porous self-supported Mn₃O₄ naosheet arrays deliver high specific capacity of 667.9 mAh g^-1 after 1000 cycles at 1000 mA g^-1.The self-supported structure can reduce the electrons transfer distance since the electrons can direcly transfer between the Mn₃O₄ nanosheets array units and the three dimensional porous Cu substrate.Meanwile,the small thickness of nanosheets reduce the lithium ion transfer distance and the three dimensiaonl porous structure provides rapid transfer channels.Therefore,excellent high rate capacities are achieved with 216.7 mAh g^-1 remained at high current density of 20000 mA g^-1.Three dimensiaonal porous self-supported NiOx was produced by high temperature annealing the as-electrodeposited NiHxOy precursors under Ar atmosphere using the three dimensiaonal porous Cu prepared by electroless as the substrate.Three dimensional porous self-supported nanosheets array structure is beneficial to achieve excellent cycle life because it can reduce "dead lithium" in NiOx active particles during charging/discharging by effectively alleviating the volume changes.The three dimensiaonal porous self-supported NiOx demonstrates high specific capacity of 473.7 mAh g^-1 after 150 cycles at 1000 mA g^-1.Meanwhile,three dimensiaonal porous self-supported structure increase the contact area between the NiOx and Cu substrate,reducing “dead lithium” and electrochemical polarization,resulting in high first coulombic efficiency of up to 86.76 %.Besides,excellent high rate capacity with high specific capacity of 489 mAh g^-1 are achieved at 20000 mA g^-1 owing to the three dimensional porous structure can provide rapid transfer channels for lithium ions.