| Next-generation lithium-ion batteries (LIBs) require high capacity cathode materials to meet the ever-growing energy demand for futuristic applications such as electric vehicles with the same performance as with the internal combustion engines. The conventional approach to developing high energy cathode relies on the lithium-excess materials. Yet, they fail to satisfy the requirements because of their unsolved issues upon cycling, such as voltage fading and structural transition. Moreover, the sources of lithium are limited, and their stockpile is depleting fast. Hence, a lithium-deficient ratio is adopted as a new approach to attain high capacity at high voltage for layered oxide cathodes. Multiphase LixNiyMnzCo1-y-zO 2 (x <1) (NMC) cathode materials with different nickel contents -- LixNi1/3Mn1/3Co1/3O2 (NMC111), LixNi0.6Mn0.2Co0.2O 2 (NMC622), and LixNi0.8Mn0.1Co 0.1O2 (NMC811) -- have been synthesized by sintering of metal hydroxide precursors with lithium source. The metal hydroxide precursors were synthesized by a commercially feasible spray drying method. During the solid-state sintering of spray dried metal hydroxide precursors, the change in the structure of the cathode materials was observed by in-situ XRD. All three variations have appeared to have the layered structure (space group: R-3m) in the final cathode material. Raman study has suggested NMC111 contains a higher amount of secondary spinel phase (space group: Fd-3m) than NMC622 and NMC811. Synchrotron x-ray diffraction (SR-XRD) of NMC 111 revealed the formation of a layered R-3m structure as well as a spinel Fd-3m structure. In addition to that, SR-XRD also reveals the lattice parameter expansion of the R-3m phase which is likely due to the transition metal ion migration to the spinel phase during synthesis. High-resolution transmission electron microscopy (HR-TEM) and selective area electron diffraction (SAED) data, from the cross-section of the particle after focused ion beam (FIB) milling, also confirmed the formation of spinel phase near the high cobalt areas, approving the cation migration in spinel phase. XPS study of the cathode materials also identified that the most responsible entity to lead this phase transition is Co3+ which is analogous to the findings from the SR-XRD and SAED. With the structurally stable spinel phase in the material, NMC111 showed an outstanding result of high cycling stability at high voltage (up to 4.5V vs. Li+) and high temperature (60 °C). The findings from this study provide a new perspective compared to the conventional layered cathode materials offering high-energy and high-power applications and denying the focus for lithium excess material. At the same time, it provides insights to the fundamental understanding of LixNiyMnzCo1-y-zO 2 (x <1) cathode materials suggesting that the role of cobalt is dominant than that of nickel while forming the spinel phase, as opposed to the general belief of nickel taking the leading role. |
