| The work of the thesis can be divided into two major parts:1. Effects of oxygen vacancies on the phase transitions and electrochemical performance of lithium-rich oxygen-deficient Li1.07Mn1.93O4-δ cathode materials. Oxygen nonstoichiometric Li1.07Mn1.93O4-δ with different oxygen deficiency values8were made by heating the starting material Li1.07Mn1.93O4in an Ar atmosphere at an annealing temperature of660℃for different length of time. The samples were studied at room temperature by transmission electron microscopy methods, including Selected Area Electron Diffraction (SAED), High Resolution Transmission Electron Microscopy (HRTEM), Bright and Dark Field image (BF and DF), together with the X-Ray powder Diffraction (XRD). It was found that,(1) The cubic spinel phase with space group F d3m is still the dominant component in the annealed samples at room temperature;(2) The number and contents of the new phases increase with increasing oxygen deficiency value8in the annealed samples(3) Monoclinic Li2MnO3phase with C2/m space group was identified in the annealed samples and the presence of lamellar rotational twinning domains in Li2MnO3with120°rotation angles, stacked along the [103]m/[111]c ("m" and "c" represent the monoclinic and cubic descriptions, respectively) axis was also confirmed.(4) The tetragonal phases with distortion level cp/ap ratio (ap and cp are the pseudo cubic unit cell parameters of the tetragonal phase) ranging from1.00to1.16were identified in the annealed samples. The diversity in values for the tetragonal phases is due to the difference in oxygen vacancy value among particles after annealing, which is related to the differences in their initial physical characteristics. (5) The electrochemical test showed that the electrochemical performance of annealed samples is dramatically deteriorated with increasing amount of oxygen vacancy.2. Effects of precursor, synthesis time and synthesis temperature on the physical and electrochemical properties of Li(Ni1-x-yCoxMny)O2(NCM) cathode materials. Single-phase Li(Ni1/3Co1/3Mn1/3)O2(NCM111) and Li(Ni0.4Co0.2Mn0.4)O2(NCM424) compounds with a layered structure and spheroidal morphology were synthesized by solid-state reactions using the corresponding commercially supplied precursors NCMOH111-α, NCMOH111-β, NCMOH111-γ, NCMOH424-a and NCMOH424-b. The effects of the synthesis temperature, the synthesis time and the nature of the transition-metal hydroxide precursors on the physical and electrochemical properties of Li(Ni1-x-yCoxMny)O2are studied. It was found that the physical properties and electrochemical performances of the samples depend highly on the choices of precursors, synthesis temperature and reaction time:(1) Larger primary particle size (PPS) and secondary particle size (SPS) of the precursors made a smaller PPS of NCM. NCM with smaller PPS has a higher rate capability but a worse cyclability;(2) Extending the reaction time led to slightly larger PPS and SPS, a better crystallinity and layered structure of NCM, resulting in a slightly lower tap density and substantially improved cyclability of NCM;(3) Higher synthesis temperatures resulted in larger PPS and SPS, broader SPS distributions and lower tap densities of NCM;(4) NCM with better crystallinity, a well-defined layered structure and better cation ordering has a higher specific capacity, better cycling performance and better rate capability; (5) The level of Ni2+/Li+disordering was higher with higher Ni2+content. And when the Ni content was decreased, an elevated synthesis temperature was required to prepare an NCM compound with appropriate physical properties and high electrochemical performance.The optimized synthesis conditions for precursors NCMOH111-α and NCMOH424-a under the current experimental conditions were950℃for12h and950℃for9h, respectively. The NCM111-α-950-12h sample delivered a discharge capacity of165.5mAhg-1during the initial cycle at0.1C with a columbic efficiency of87%, a3C rate capability of91.25%and a1C capacity retention rate of98.25%after40cycles. |
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