Abstract
Li-rich layered oxides (LLOs) are considered as one of the most promising cathode candidates for next-generation lithium-ion batteries. Unfortunately, their development is challenging, due to the detrimental structure changes and voltage decay that resulted from irreversible oxygen redox and transition metal (TM) migration. This thesis focuses on studying the structural evolution of Co-free LLO cathodes and improving their electrochemical performance. The mechanistic behaviour of Li<sub>1.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>O2 (LNMO) was comprehensively studied using a series of synchrotron-based characterizations. An intrinsic mechanistic behaviour transition from the monoclinic (C2/c) to the hexagonal (R3 ̅m) was demonstrated for the first time. Based on this understanding, electrochemically active 4d metal Ruthenium (Ru) was introduced into both R3 ̅m and C2/c components in LNMO to improve the stability of LNMO. With 3 w.t. % Ru doping, the oxygen lattice is strengthened and the best voltage retention (< 0.45 mV per cycle) is achieved for Co-free LNMO with a high reversible capacity (215 mAh g<sup>-1</sup> at 1C). To improve the cycling stability of LLOs, the inhibition of the irreversible TM migration needs also to be considered. A two-phase reaction was revealed in the heavily cycled LNMO, resulting from the accumulative irreversible transition-metal migration and loss. Chromium (Cr) doping was proposed to inhibit the formation of undesirable structural changes, by achieving reversible tetrahedral-octahedral TM migration. It was demonstrated that the oxidated Cr occupies the tetrahedral sites in the Li layer at the high delithiated state, which prevents TM ions from being trapped in the Li layers at the highly delithiated state and stabilizes the structure against cycling. It migrates back to the octahedral sites after lithiation due to energetical benefits. Compared with pristine LNMO, Cr-doped LNMO shows significantly enhanced structural stability with capacity retention up to 99% after 200 cycles at 1C and 71% after 500 long cycles, far surpassing pristine LNMO.