Title Page
Abstract
Contents
Ⅰ. Introduction 12
1.1. Co-free Li-, Mn- rich oxide (LMR) as a promising cathode materials 13
1.2. Intrinsic problems of LMRs 15
1.3. Fundamental challenges of LMRs in practical perspectives 22
1.3.1. Low electrode density with small-sized primary particles 22
1.3.2. Side reactions with electrolyte at high operating voltage 24
1.4. Previous researches to improve the performance of LMRs 28
1.4.1. Morphology control during synthesis 28
1.4.2. Surface coating 30
1.5. Objective in the works 32
Ⅱ. Experimental section 33
Ⅲ. Result and Discussion 35
3.1. Flake-type Co-free LMR cathode materials for high-energy density 35
3.2. AlF₃ surface coating for high-performance LMR cathode materials 48
Ⅳ. Conclusion 69
References 70
Table 1. Lattice constants for pristine MN7525 and MNC622. 40
Table 2. Structure parameters for pristine MN7525 and MNC622. 41
Table 3. Initial charge/discharge capacity, Coulombic efficiency of MN7525 and MNC622 42
Table 4. Structure Parameters for 1, 3, 5wt% AlF₃-coated LMR samples. 50
Table 5. Initial charge/discharge capacity and Coulombic efficiency of pristine and 1, 3, 5wt% AlF₃-coated MN7525 samples. 53
Table 6. Impedance parameters obtained from electrochemical impedance spectroscopy (EIS). 59
Figure 1. Applications of LIBs in the three main fields including consumer electronics and devices, transportation, as well as grid energy and industry. 12
Figure 2. Comparison of theoretical and practical specific gravimetric capacities of representative cathode materials (Theoretical value is calculated based on the total amount of Li ions in the formula units). 13
Figure 3. Structural representation of (a) O3-type layered oxides (b) the overall cell of Li-rich layered oxides described as monoclinic and (c) M / Li ordering within LiM2 layer leading to a honey-... 14
Figure 4. Compositional phase diagram showing the electrochemical reaction pathways for a xLi₂MnO₃·(1-x)LiMO₂ electrode. 14
Figure 5. Model highlighting the depletion of global cobalt reserves specifically available for battery industries, leading to the shortage of cobalt.[내용없음] 8
Figure 6. Initial charge/discharge profiles of a Li / 0.3Li₂MnO₃·0.7LiMn₀.₅Ni₀.₅O₂ cell (5.0-2.0 V). 15
Figure 7. Schematic representation of the energy level versus density of states N(ε), showing the respective motion of the metal d band with respect to the oxygen p band in going from cationic to... 17
Figure 8. Schematic illustration of the layered-to-spinel phase transformation in Li₂MnO₃ during the initial cycle and after the multiple cycles. 18
Figure 9. Atomic models explaining the structural evolution pathway based on the close observation from the structural changes in cycled materials. 19
Figure 10. Mechanistic diagram of Mn and Ni ion migration differences upon cycling and its relation to phase transformation. 20
Figure 11. Electrochemical characterization of Li₁.₂Ni₀.₁₅Co₀.₁Mn₀.₅₅O₂. (a) Charge-discharge curves for Li₁.₂Ni₀.₁₅Co₀.₁Mn₀.₅₅O₂ for the 1st, 2nd, 25th, 50th and 75th cycles. (b) dQ/dV plot of... 21
Figure 12. Electrochemical performance of LMR material with different electrode density in the voltage range of 2.0-4.6 V. (a) Initial charge/discharge profiles at C/10. (b) Cycling performance... 23
Figure 13. Comparison of microstrain changes for different states of charge of (a) LMR and (b) NCA, which is determined from the Williamson-Hall analysis of the Bragg peak widths from... 25
Figure 14. TEM and STEM images of (a) non-cycled and (c) cycled LMR cathode. Cross-sectional images of (b) non-cycled and (d) cycled LMR cathode. 26
Figure 15. Scheme of the proposed mechanism of the successive reactions inside the closed system originated from the oxygen evolution out of the layered Li-excess metal oxides. 27
Figure 16. Schematic diagram showing electron migration through secondary particles composed of primary particles with different morphologies, either flake-shaped (left) or nanoparticle-shaped (right). 29
Figure 17. Schematic illustrations of the change of particle structure with and without AlF₃ coating during cycling. 31
Figure 18. Cross-sectioned SEM images of (a) flake-type Mn₀.₇₅Ni₀.₂₅(OH)₂ precursor obtained from co-precipitation, and (b) oxide precursor after 1st calcination at 600℃.[이미지참조] 36
Figure 19. X-ray diffraction (XRD) patterns of (a) flake-type Mn₀.₇₅Ni₀.₂₅(OH)₂ precursor obtained from co-precipitation, and (b) oxide precursor after 1st calcination at 600℃.[이미지참조] 37
Figure 20. (a) Cross-sectioned SEM images of Li₁.₂Mn₀.₆Ni₀.₂O₂. (b) Overall view of TEM image showing pristine MN7525 particle and (c) higher magnification TEM image showing its surface. (d)... 38
Figure 21. Morphological information for MN7525 and MNC622. SEM images of (a) flake-type MN7525 and (b) spherical-type MNC622. 39
Figure 22. X-ray diffraction (XRD) pattern and Rietveld refinement of pristine (a) MN7525 and (b) MNC622 powder. 40
Figure 23. Voltage profiles of MN7525 and MNC622 in the range of 2.5-4.7V at 0.2C. 42
Figure 24. Discharge capacity retention and average voltage of MN7525 and MNC622 as a function of the number of cycles between 2.5 and 4.6V at 1C. 43
Figure 25. Voltage profiles of (a) MN7525 and (b) MNC622 at the 1st, 10th, 20th, 30th, and 40th cycle with 1C after two formation cycles.[이미지참조] 43
Figure 26. Rate capabilities of MN7525 and MNC622 in voltage range 2.5-4.6V. 44
Figure 27. Voltage profiles obtained from the galvanostatic intermittent titration technique (GITT) of MN7525 and MNC622 after (a) formation and (b) 40 cycles. 44
Figure 28. Overpotential, non-ohmic voltage loss, and IR drop calculated from the galvanostatic intermittent titration technique (GITT) (a),(b) after formation and (c),(d) after 40 cycles. 45
Figure 29. Particle size distribution analysis of MN7525 and MNC622. 47
Figure 30. X-ray diffraction (XRD) patterns of pristine and AlF₃-coated (1,3,5wt%) LMR samples with an enlargement of peaks at 2θ=20-25° on the right. 49
Figure 31. SEM-EDS analysis and mapping of 3wt% AlF₃-coated LMR samples. 51
Figure 32. (a) High-resolution TEM image showing the AlF₃ coating layer on LMR samples and TEM-EDS mapping. (b) TEM-EDS line scanning of 3wt% AlF₃-coated LMR samples. 52
Figure 33. Voltage profiles of pristine and 1,3,5wt% AlF₃-coated samples in the range of 2.5-4.7V at 0.2C. 53
Figure 34. dQ/dV profiles at 0.2C derived from the initial cycles. 54
Figure 35. Schematic illustrations of the structure of AlF₃-coated LMRs after the first cycle. 55
Figure 36. The change of discharge capacity and the retention of pristine and 3wt% AlF₃-coated sample as a function of the number of cycles. 56
Figure 37. Rate capabilities of the pristine and 3wt% AlF₃-coated MN7525 in the voltage range 2.5- 4.6V. 57
Figure 38. Nyquist plots from electrochemical impedance spectroscopy (EIS) of (a) pristine and (b) 3wt% AlF₃-coated samples comparing the impedance before and after cycling. 59
Figure 39. Microstructure of pristine samples after cycling. (a) A HAADF-STEM image of pristine sample at discharge states after cycling and higher magnification image showing its surface. Fast... 61
Figure 40. Microstructure of 3wt% AlF₃-coated samples after cycling. (a) A BF-STEM image of 3wt% AlF₃-coated samples at discharge states after cycling. Fast Fourier transform patterns signal... 62
Figure 41. dQ/dV plots of (a) uncoated and (b) 3wt% AlF₃-coated samples comparing the 2nd cycle at 0.2C and the 41st cycle at 1C.[이미지참조] 63
Figure 42. Electron energy loss spectroscopy (EELS) spectra for O-k edge of (a) uncoated and (b) 3wt% AlF₃-coated samples comparing pristine states and discharge (2.0V) states after 40 cycles. 64
Figure 43. Electron energy loss spectroscopy (EELS) spectra for Mn-L of (a) uncoated and (b) 3wt% AlF₃-coated samples comparing pristine states and discharge (2.0V) states after 40 cycles. 66
Figure 44. Electron energy loss spectroscopy (EELS) spectra for Ni-L edge of (a) uncoated and (b) 3wt% AlF₃-coated samples comparing pristine states and discharge (2.0V) states after 40 cycles. 67
Figure 45. L₃/L₂ ratios obtained from EELS of uncoated and 3wt% AlF₃-coated samples comparing pristine states and discharge (2.0V) states after 40 cycles. 68