Materials Design And Mechanism Exploration For High Energy Density Batteries

The fast development of the markets of the portable electronic devices and electric vehicles requires secondary batteries with higher energy density,longer cycling life and lower cost.The most popular cathode materials for the current commercial Li-ion batteries include layer-structured LiCoO₂ and LiNi_xCo_yMn_1-x-yO₂,olivine LiFePO₄ and spinel LiMn₂O₄.The charge compensation in these materials upon Li ion extraction/insertion is realized by the redox reaction of the transition metals?TM?.Therefore,the specific capacity of these materials is closely related to the electrons that the TM can donate or accept.In order to increase the energy density of the cathode material,more charge transfer is needed,by increasing the charge potential and/or by activating the inert anions.At present,the anion redox mainly occurs in some lithium-rich layer oxides when they are charged to high potentials.However,the mechanism behind the anion oxidation and its impacts on the structural stability and the reversibility of the structural transformation as well as the electrochemical performances of the cathode material is still unclear.Boosting the specific capacity of the anode is also critical in developing new generation of secondary batteries.As the common high-capacity anode material for the new Li-metal batteries such as Li-S,Li-O₂ and the solid-state lithium batteries,the importance of addressing issues such as the dendritic growth of the Li-metal anode cannot be overestimated.This thesis will focuses on the charge compensation by oxygen redox and its impacts on the structural transformation and electrochemical performances of NaFeO₂and Na_4/7[Mn_6/7(V_Mn)_1/7]O₂ as cathode materials for the Na-ion batteries and the controlled deposition of Li metal.The iron?Fe?-based electrode materials have obvious resource advantages for the Na-ion batteries and will promote their applications in large-scale energy storage as an alternative to the Li-ion batteries.In this study,the structural transition of NaFeO₂ was characterized and the driving force for the irreversible Fe migration and O oxidation were explored during Na extraction.The Fe migration from the Fe layer to the Na layer was observed for the first time at the atomic scale by scanning transmission electron microscopy?STEM?.The chemical states and coordination environment of the Fe ions within the Na layer were determined by synchrotron X-ray absorption near edge structure?XANES?.The density functional theory?DFT?calculations were carried out to understand the Fe migration and O oxidation.No TM vacancies have been observed in the pristine Li-rich oxide cathode materials to date.Anion redox could increase the specific capacity of the current Li-rich cathode materials,but that also results in severe problems such as the TM migration and the resultant drop of the discharge potential.Herein,we introduce Na_4/7[Mn_6/7(V_Mn)_1/7]O₂(V_Mnn for vacancies in the Mn-O layer)as a state-of-the-art vacancy-containing cathode material for the Na-ion batteries.The existence of these intrinsic vacancies offers this material with attractive properties including high structural flexibility and stability in a wide range of Na-ion extraction and insertion and high reversibility of the oxygen redox reaction.XANES and X-ray photoelectron spectroscopy studies demonstrate that the charge compensation is dominated by the oxygen redox reaction between 2.3 and 4.4 V vs.Na⁺/Na,and by the Mn³⁺/Mn⁴⁺redox reaction between 1.5 and 2.3 V.The STEM and extended X-ray absorption fine structure demonstrate the high structural flexibility and stability of the[Mn_6/7(V_Mn)_1/7]O₂ slabs.DFT calculations further deepen our understanding of the charge compensation by oxygen and manganese redox reactions and the immobility of the Mn ions in the material.Grooves with different shapes were prepared on the titanium?Ti?foil by microfabrication.It was found that metallic lithium prefer to be deposited in the grooves.In light of this,micro-/nano-architectured porous silicon?Si?and ZnO nanowire array were prepared to guide the deposition of Li metal and were proved equally effective as the grooves on the Ti foil.A qualitative explanation was also proposed for the controlled/preferential deposition of metallic lithium in the micro-grooves.