Studies On Advanced Lithium-rich Cathode Materials For Lithium-ion Batteries

Rechargeable lithium-ion batteries have made great success in the past 20 years and are extensively used for consumer electronic devices due to their high energy density and long cycle life. Moreover, they are considered as crucial power sources for electric vehicles (EVs) and green grid. However, the performance of current lithium-ion batteries, which is linked intimately to the properties of electrode and electrolyte materials, falls short of meeting the requirements for EVs in prospect. Therefore, tremendous efforts have been dedicated to exploring advanced electrode materials that deliver more energy at rapid charge and discharge rates, which are essential for further enhancement of the energy and power density of rechargeable lithium-ion batteries (LIBs).Throughout the search for alternative advanced cathode materials to current LiCo02 that shows practical capacities of only about 150 mA h g"1 at high cost, the lithium-rich cathode materials have been recognized as one of the most promising cathode materials for high-energy-density LIBs due to their high reversible capacities of～250 mA h g-1 at low cost recently. However, the microstructure and mechanism of such materials are quite complex as there is an ongoing debate in prior structural studies on whether they are in solid solution notation as Li[NixLi(1-2x)/3Mn(2-x)/3]O2 (0 < x< 0.5) or represented as composite with the general formula xLi2MnO3·(1-x)LiMO2 (M=Mn, Co, Ni, etc.). Considerable research has revealed that the Li2MnO3 component plays a crucial role in the electrochemical performance of xLi2MnO3·(1-x)LiMO2 (0<x<1; M=Mn, Co, Ni, etc.) electrodes, which not only serves as the electrochemical active phase providing large reversible capacities, but also improves the structural stability of layered LiMO2 at high potentials giving rise to excellent cyclic performance and thermal stability. Despite their outstanding advantages, these materials are plagued with problems that have hindered their widespread practical applications. They show the large irreversible capacity loss associated with lithium loss and electrolyte oxidation during the initial charge, poor rate capability caused by the slow kinetics of oxygen diffusion or electron/lithium-ion transport at the plateau region, and severe voltage decay upon cycling.To circumvent these challenges, this thesis developed a facile molten-salt method to prepare the high performance lithium-rich cathode materials, provided new insights into the correlation between the electrochemical properties and the microstructure e.g. stacking faults, and studied the electrochemical degradation and structural evolution of xLi2MnO3·(1-x)LiMO2 materials and the role of Li2MnO3 component during cycling from a material synthesis and structural point of view through careful structural analysis methods. The detailed content is summarized as follows:In Chapter 3, high-performance xLi2MnO3·(1-x)LiMn1/3Ni1/3Co1/33O2 (x=0.3,0.5, and 0.7) structurally integrated nanomaterials towards high capacity and high power cathode for advanced lithium-ion batteries are developed by a facile KCl molten-salt strategy. The effects of heat-treatment temperature, time, and the molar ratio of KCl flux to reaction precursor on the particle size as well as the electrochemical properties are extensively explored. In this progress, molten-salt of KCl flux not only acts as a mineralizing agent for highly crystalline, but also serves as an inactive liquid reaction media that enables molecular level mixing of the reactants and restricts the growth of the particles. Our results demonstrated that 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 material consists of mono-dispersed irregular nano-particles with diameters on the order of 100-200 nm, delivers high reversible capacities of ～310 mA hg-1 with significantly enhancement in initial coulombic efficiency (87%) at room temperature, exhibits superior rate capability with 197 mA h g"1 at a rate of 6 C and shows improved electrochemical properties over wide range of operation temperature, in particular at low temperatures addressing capacities of～180 mAhg-1 even at -20℃.In Chapter 4, layered lithium-rich transition-metal oxides with various degree of stacking faults have been successfully prepared via a molten-salt method using KCl, Li2CO3, and LiNO3 fluxes. The frequency of the stacking faults is highly dependent on the temperature and molten salt type. In an inactive KCl flux, layered Li1.18Mn0.54Ni0.13Co0.13O2 nanoparticles with larger amount of stacking faults and slightly lithium deficient were obtained at 800℃ and delivered a high reversible capacity of～310 mA h g-1. Although well-crystallized Li1.39Mn0.54Ni0.13Co0.13O2 sample with smaller amount of stacking faults and an excess of lithium could be prepared at 800℃ using an oxidizing LiNO3 flux, the electrode exhibited the worst electrochemical performance among the samples synthesized using the different fluxes and showed an insignificant voltage plateau at ～4.5 V when initially charged to 4.8 V. The results reported herein demonstrate the dependence of the electrochemical properties of layered lithium-rich oxides on their microstructures and provide new insights for materials design and synthesis of Li-rich cathode materials.In Chapter 5, the electrochemical degradation and structural evolution of xLi2MnO3·(1-x)LiMn1/3Ni1/3Co1/3O2 (x=0.3,0.5, and 0.7) materials and the role of Li2MnO3 component during electrochemical cycling are systematically studied through careful analysis of electrochemical data, ex-situ XRD, and HR-TEM observations. The materials consisting of higher Li2MnO3 content show better cyclic performance with more significant voltage decay compared to that of xLi2MnO3·(1-x)LiMn1/3Ni1/3Co1/3O2 electrodes with low Li2MnO3 content. The electrochemical degradation of xLi2MnO3·(1-x)LiMn1/3Ni1/3Co1/3O2 electrodes upon cycling not only results from the remarkably increase in impedance caused by the damage of the electrode surface, in particular for low Li2MnO3 content; but also arises from structural rearrangement, especially for high Li2MnO3 content. Upon cycling, high Li2MnO3 content in the crystal structure of lithium-rich transition metal oxides can stabilize the electrode\electrolyte interface at high potentials, facilitates the rapid formation of cracks and porosity in the cycled electrodes, and promotes the distortions and breakdown of the original well-layered lattice.In Chapter 6, the electrochemical properties and structural evolution of layered-layered-spinel composite are extensively explored to counter the voltage decay upon cycling. The results have determined that the composite consisting of LiNi0.5Mn1.5O4 phase shows excellent cyclic performance and good rate capability with～130 mA h g-1 at a rate of 10 C. Besides, the composite exhibits slow voltage decay upon cycling, indicating that the voltage degradation and the formation of cracks and cores upon cycling can be suppressed to some extent owing to the introduction of LiNi0.5Mn1.5O4 spinel phase.In Chapter 7, a new type of rechargeable lithium-ion battery consisting of a structurally integrated 0.4Li2MnO3·0.6LiMn0.4Ni0.4Co0.2O2 cathode and a hard carbon anode have been developed. The battery shows good reversibility with a sloping voltage from 1.5 V to 4.5 V, and delivers a capacity of 105 mA h g-1 and a specific energy of 315 Wh kg-1 based on the total weight of the both active electrode materials. The drawback of the high irreversible capacities loss encountered at the positive or negative electrode materials, occurring at the first charge/discharge process, can be counterbalanced each other during operation by the electrode materials combination.