Self-standing Three-dimensional Electrodes For Thin Film Lithium-Ion Batteries

All-solid-state thin film lithium-ion batteries(TFBs)have been widely used in smart cards,electronic tags,integrated circuits,and implantable medical devices due to their high specific energy density,extremely long cycle life,low self-discharge rate,flexible size and compatible thin film fabrication processes.Besides,TFBs have been emerged as the most attractive power solution for micro-/nanoelectronics and wearable electronics.However,as more energy is required in microelectronic devices,two-dimensional(2D)TFBs are no longer the ideal battery design because the limited footprint area can result in a compromise between energy density and power density for the built-in power source.A move to three-dimensional(3D)architectures has been proposed as a promising approach to tackle this challenge and can achieve both high energy and power densities within the footprint area by increasing the electrode/electrolyte contact area and shortening the ions diffusion length.Thus,it is meaningful to fabricate high-performance self-standing 3D electrodes to construct 3D TFBs.In this dissertation,self-standing 3D anodes based on titanium-based oxides and self-standing 3D cathodes based on manganese-based oxides were designed,prepared and characterized for 3D TFBs.TFBs with 3D structure were further constructed by using the as-prepared 3D electrodes.As a result,the 3D TFBs show improved electrochemical performances than corresponding 2D TFBs,revealing the great advantages of 3D structure design.The main contents are as follows:1.Oxygen-deficient black 3D mesoporous Li₄Ti₅O_12-?nanowall arrays have been prepared by a controllable chemical solution method and post heat treatment.The abundant pore structure in the 3D Li₄Ti₅O_12-?anode can greatly shorten the ions diffusion length.While the electrical conductivity and electrode kinetics of the electrode are significantly improved by introducing oxygen vacancies in the Li₄Ti₅O_12-?structure.Consequently,the3D Li₄Ti₅O_12-?electrode exhibited greatly improved rate performance compared to the stoichiometric Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/TiO₂ dual phase electrodes.In specific,the 3D Li₄Ti₅O_12-?electrode can deliver a high reversible capacity of 115 m Ah g^-1 at 20 C(1C=175 m A g^-1)as well as excellent cycling stability of 93%capacity retention after 500cycles.2.A novel one-step“hydrothermal lithiation”method was developed to directly prepare self-standing 3D porous Li Mn₂O₄ nanowall arrays on conductive substrates at low temperature by using Mn₃O₄nanowall arrays as the template.The transformation mechanism from Mn₃O₄ to Li Mn₂O₄was well characterized and studied.The 3D Li Mn₂O₄electrode,featuring porous structure and highly crystallized nanoparticles,exhibit large specific capacity(131.8 m Ah g^-1at 20 C,1 C=148 m A g^-1),excellent rate capability(97.1m Ah g^-1 at 20 C),and outstanding cycling stability(96%capacity retention after 200cycles).For the first time,a flexible 3D Li Mn₂O₄/Li₄Ti₅O₁₂full cell was constructed by using 3D Li Mn₂O₄ nanowall arrays as cathode and 3D L_i4Ti₅O₁₂ nanowall arrays mentioned in part 1 as anode in organic liquid electrolyte.The excellent electrochemical performance of the 3D Li Mn₂O₄/Li₄Ti₅O₁₂full cell demonstrates the structural advantages of the 3D electrode design of both the thin-film anode and cathode.3.Self-standing 3D Li Mn₂O₄ nanowall arrays are directly deposited on conductive substrates by magnetron sputtering via controlling the thin film growth mode.The 3D LMO nanowall arrays and 2D LMO thin film were first characterized and tested in half cells using liquid electrolyte to demonstrate the structural advantages of the 3D architecture.Followed by Li PON electrolyte film(by magnetron sputtering)and Li anode film(by thermal evaporation)deposition,3D and 2D all-solid-state LMO/Li PON/Li TFBs have been successfully constructed by using the 3D LMO nanowall arrays and 2D LMO thin films as cathodes,respectively.The 3D architecture of the 3D TFB can not only greatly increase the cathode/electrolyte contact area and shorten the Li⁺diffusion length,but also effectively enhance the structural stability.More importantly,as compared to the2D planar thin film,the vertically aligned nanowall array architecture of the 3D cathode can signifcantly mitigate disordered LMO formation at the cathode surface during the Li PON depositon,resulting in a greatly reduced interface resistance and improved rate performance.As a result,the 3D TFB exhibited higher specific capacity,greatly improved rate capability and cycling performance than the 2D TFB.4.3D MnO_x nanosheet arrays and 2D MnO_x thin film are directly deposited on conductive substrates by magnetron sputtering via controlling the thin film growth mode.These thin films are composited by the major amorphous MnO₂ and some crystalline Mn₃O₄ phase.Followed by Li PON electrolyte film(by magnetron sputtering)and Li anode film(by thermal evaporation)deposition,3D and 2D all-solid-state MnO_x/Li PON/Li TFBs have been successfully constructed by using the 3D MnO_x nanosheet arrays and 2D MnO_xthin films as cathodes,respectively.As found in last chapter,the Li PON depsotion will cause the surface change of the cathode material.In this chapter,a simple heat treatment was carried out on the MnO_x thin film after the Li PON deposition,and the Li ions from Li PON may pre-insert into the MnO_x thin film and transform the MnO_x into tunneled Li_xMnO₂ with intergrowth 1×3 and 1×2 tunnels.The tunneled Li_xMnO₂ with excellent structure stability during charge and discharge enables the long cycle life of the TFBs.Compared with the 2D TFB,the 3D structure design in the 3D TFB can significantly increase the contact and reaction area between the electrode and electrolyte,leading to more Li ions insertion and more efficient phase transformation,providing more active sites for redox reaction and more paths for rapid ion transport.Besides,the 3D architecture of the cathode can provide more space for volume change to enhance the mechanical stability of the TFB during repeated charge and discharge.As a consequence,the 3D Li_xMnO₂/Li PON/Li TFB exhibited high specific capacity(185 m Ah g^-1at 50 m A g^-1)and excellent cycle stability(81.3%capacity after 1000 cycles),which outperformed the 2D TFB,demonstrating the great advantages of the 3D electrode design for 3D TFBs.More importantly,the all-solid-state design of the 3D TFBs can efficiently solve the Mn dissolution issue in liquid electrolyte for the manganese-based cathodes.Consequently,the3D TFBs shown apparently longer cycle life than the 3D MnO_x cathode tested in half cells using liquid electrolyte.