Shiyao Zheng

In this work, Li-rich Li1.2Mn0.43+Mnx4+Ti0.4-xO2 (LMMxTO, 0 ≤ x ≤ 0.4) oxides have been studied for the first time. X-ray diffraction (XRD) patterns show a cation-disordered rocksalt structure when x ranges from 0 to 0.2. After Mn4+ substitution, LMM0.2TO delivers a high specific capacity of 322 mAh g-1 at room temperature (30 °C, 30 mA g-1), and even 352 mAh g-1 (45 °C, 30 mA g-1) with the energy density of 1041 Wh kg-1. The reason for such a high capacity of LMM0.2TO is ascribed to the increase of both cationic (Mn) and anionic (O) redox after Mn4+ substitution, which is proved by dQ/dV curves, XANES, DFT calculations and in situ XRD results. In addition, the roles of Mn3+ and Ti4+ in LMM0.2TO are also discussed in detail. A ternary phase diagram is established to comprehend and further optimize the earth-abundant Mn3+-Mn4+-Ti4+ system. This work gives an innovative strategy to improve the energy density, broadening the ideas of designing Li-rich materials with better performance.

As a parent compound of Li-rich electrodes, Li2MnO3 exhibits high capacity during the initial charge, however, suffers notoriously low Coulombic efficiency due to oxygen and surface activities. Here, we successfully optimize the oxygen activities towards reversible oxygen redox reactions by intentionally introducing protons into lithium octahedral vacancies in Li2MnO3 system with its original structural integrity maintained. Combining structural probes, theoretical calculations and resonant inelastic X-ray scattering results, a moderate coupling between the introduced protons and lattice oxygen at the oxidized state is revealed, which stabilizes the oxygen activities during charging. Such a coupling leads to an unprecedented initial Coulombic efficiency (99.2%) with a greatly improved discharge capacity of 302 mAh g-1 in the protonated Li2MnO3 electrodes. These findings directly demonstrate an effective concept for controlling oxygen activities in Li-rich systems, which is critical for developing high-energy cathodes in batteries.

Cation-disordered rock-salt oxides with O2-/O2n- redox reaction, such as Li1.2Mn0.4Ti0.4O2 (LMTO), are critical Li-rich cathode materials for designing high-energy-density batteries. Understanding the cationic-anionic redox accompanying with the structural evolution process is really imperative to further improve the performance. In this work, the cationic-anionic redox and capacity degradation mechanism of carbon coated LMTO during (dis)charge processes are elucidated by combining in-situ X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES), differential electrochemical mass spectrometer (DEMS), transmission electron microscope (TEM) and electrochemical analyses. It is concluded that the redox reaction of the Mn2+/Mn4+ is quite stable, while the severe degradation is mainly caused by the O2-/O2n- redox process. Moreover, we clearly clarify how the cationic-anionic interplay governs sluggish kinetics, large polarization, and capacity fading in LMTO, and reveal for the first time that a certain amount of carbon coating is capable of suppressing the irreversible lattice oxygen loss and result in encouraging cyclic performance. In summary, we elucidate the degradation of cationic-anionic redox processes in cation disordered cathode materials and propose strategies of adjusting electronic/ionic conductivity of the electrodes to modulate the oxygen redox reactions, setting a new direction for the design of better cation-disordered oxides.