Application Of Crystallographic Structure Tuning Of Transition Metal Oxide In Energy Storage

With the rapid development and popularization of miniaturized electronic devices and electric vehicles in recent years,the energy storage devices have become the research hotspot in the corresponding field.In order to meet the growing demand for energy,it is the top priority to develop energy storage devices with high energy density,high power density and good cycle performance.Among all kinds of energy storage devices,supercapacitors and lithium/potassium ion batteries draw the most attention and are widely used.Supercapacitors and lithium/potassium ion batteries have different electrochemical characteristics with unique advantages,due to their distrinct energy storage mechanisms.They can be utilized individually or combined according to the practical application.Carbon materials and transition metal oxides are the most commonly explored active materials applies in supercapacitors and lithium/potassium ion batteries.High conductive carbon materials exhibit superb power capability,yet their theoretical capacity is low due to the limitation of energy storage mechanism,which can hardly meet the energy needs in the practical application.Transition metal oxides have high theoretical capacity,but their intrinsic poor ion/electron conductivities and the volume changes during charging and discharging in lithium/potassium ion batteries will undermine their advantages.In order to obtain high energy density and power density simultaneously in the energy storage devices,researchers devote themselves to synthesize nanostructured composite materials.Traditional planar electrode assembly usually requires to introduce insulating polymers to bind active materials to current collector,which will unavoidably result in the increment of contact resistance between the active materials as well as between the active materials and current collector,it will also give rise to some dead volume electrolytes cannot entered,which reduces the utilization rate of the active materials.Thus,in this dissertation,transition metal oxides are epitaxially integrated into a three-dimensional conductive metal skeleton with nanoporous structure,which can not only improve the electronic conductivity of the transition metal oxides,but also promote the ion transport within the electrode.Moreover,the crystallographic structure of transition metal oxide is regulated by crystal design and cation preintercalation to accelerate the ion/electron transport inside the active materials.These measures can improve the utilization rate of active materials,obtain a higher capacity,energy/power density and good cycle stability,and achieve efficient and stable energy storage.The research contents include the following three parts:1.Remarkable improvements in volumetric energy and power of 3D MnO₂microsupercapacitors by tuning crystallographic structuresWe take MnO₂ nanocrystals as a model system to demonstrate that the micro supercapacitors based on pseudo-capacitive transition metal oxides can minimize the internal resistance by tuning the crystallographic structures of transition metal oxide components to achieve large volume capacitance,high energy/power density and superb rate capability.We mainly study on two typical structures of MnO₂,?-MnO₂ with tunnel structure and?-MnO₂with layered structure,which are incorporated onto Au current collectors with a 3D bicontinuous nanoporous architecture in microsupercapacitor.In this unique nanostructure,MnO₂ component lies between the highly efficient electron channel and the ion channel.The tuning crystallographic structures from tunneled?-MnO₂ to layered?-MnO₂ can not only enhance the accessibility and diffusion of Na⁺,but also dramatically reduce the charge transfer resistance at the interface of NP Au/MnO₂/electrolyte.As a result of 3D hybrid nanostructure and greatly enhanced ion/electron conductivities,NP Au/?-MnO₂microelectrode can achieve?922 F cm^-3 volumetric capacitance(?1.5-fold higher than?-MnO₂,?617 F cm^-3)and excellent rate performance.This enlists?-MnO₂microsupercapacitor to deliver ultrahigh stack electrical powers(up to?295 W cm^-3)while maintaining volumetric energy density much higher than that of thin-film lithium battery.The high performance enlists them to be promising candidates as next-generation micropower sources in miniaturized portable electronics and MEMS.2.Lithium ion breathable electrodes with 3D hierarchical architecture for ultrastable and high-capacity lithium storageConversion-type transition metal oxides usually suffer from large volume change during repeated charging and discharging process,which leads to numerous problems in both material and electrode level,such as material pulverization,instability of solid-electrolyte interphase?SEI?and electrode failure.We design and fabricate lithium-ion breathable hybrid electrodes with 3D architecture to tackle all these problems.The hybrid electrode uses a typical conversion-type transition-metal oxide,Fe₃O₄,of which nanoparticles are anchored onto 3D current collectors of Ni nanotube arrays?NTAs?and encapsulated by?-MnO₂ layers?Ni/Fe₃O₄@MnO₂?.The?-MnO₂ layers reversibly switch lithium insertion/extraction of internal Fe₃O₄ nanoparticles and protect them against pulverizing and detaching from NTA current collectors,securing exceptional integrity retention and efficient ion/electron transport.In terms of capacity contribution,?-MnO₂ coating layers can not only accommodate additional lithium ions but also effectively capture the high-capacity LiF produced by the decomposition of LiPF₆ electrolyte during the discharge of first cycle and provide additional capacity for the system.Benefiting from these advantages,the Ni/Fe₃O₄@MnO₂ electrodes exhibit high-capacity lithium storage and superior cyclability(retaining?1450 mAh g^-1,?96%of initial value at 1 C rate after 1000 cycles).3.High-energy and high-power flexible aqueous potassium-ion microbattery based on layered metal oxides with dual-phase nanostructuresIn virtue of its advantages of low cost and safety,aqueous potassium-ion microbattery is expected to be a potential substitute for lithium-ion battery.However,the narrow potential window of aqueous electrolyte less than 1.23 V limits the realization of high energy density.In order to expand the potential window of the aqueous microbattery,we adopt two transition metal oxides with large work function difference and incorporate them onto Au current collectors with a 3D bicontinuous nanoporous architecture in aqueous potassium-ion microbattery,in which the layered?-K_0.23MnO₂ as cathode and the layered amorphous/crystalline dual-phase structured K_0.25V₂O₅ as anode.As a result of the enhanced electronic/ionic conductivities and enlarged potential window,the NP Au/K_0.25V₂O₅//NP Au/?-K_0.23MnO₂ aqueous potassium-ion microbattery exhibits ultrahigh energy density of0.1 Wh cm^-3 with electrical powers²00-fold higher than those of lithium thin-film batteries,in addition to excellent rate capability(⁵1.6%capacity retention with the scan rate ranging from 10 to 1000 mV s^-1)and cycling stability(retains⁸0%of the initial value after 10000cycles at 500 m V s^-1).