Tuning the anionic redox chemistry (O²⁻ → O₂ ⁿ⁻) activity and reversibility by crystal and/or electronic modulation is essential for Li-rich oxide cathode materials. Herein, we report a facile strategy to improve the activity and reversibility of both anionic and cationic redox by integrating oxygen vacancies and the spinel phase. The initial specific capacity (216.1 mA h g⁻¹vs. 316.3 mA h g⁻¹), coulombic efficiency (80% vs. 94.8%), long-term cycling stability (1000 cycles at 5C) and voltage decay have all been greatly improved due to the largely suppressed irreversible oxygen release. The underlying modulation mechanism has been unraveled. Firstly, the introduction of oxygen vacancies decreases the covalency of TM–O and the density of states of the O 2p band, which mitigates the irreversible oxygen release during oxygen redox. Secondly, the spinel phase integration induced by oxygen vacancies not only improves the Li-ion conductivity and the rate capability due to its 3D Li⁺ channel and the expanded Li layer but also enhances the structural stability. Thirdly, the first-principles calculations indicate that the increase of delocalized electrons around the transition metal also intensifies the MnO₆ octahedral distortion and the inactive Mn-ions are partially activated during the first cycle and participate in the charge compensation. This study sheds some new light on designing high-performance Li-rich layered oxide cathode materials by regulating the anionic and cationic redox with the incorporation of oxygen vacancies and the spinel phase.

Single crystals of ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ have been grown at a pressure of 30 kbar using the cubic anvil technique. Precipitation free crystals with $x<0.1$ were obtained as a result of the optimization of an already developed ${\mathrm{MgB}}_2$ crystal growth procedure. A systematic decrease of the $c$-axis lattice constant with increasing Al content, when the $a$-axis lattice constant is practically unchanged, was observed. Variation of the critical temperature on Al content in ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ crystals was found to be slightly different than that one observed for polycrystalline samples, since even a very small substitution of 1%–2% of Al leads to the decrease of $T_c$ by about 2–3 K. X-ray and high resolution transmission electron microscopy investigations indicate the appearance of the second precipitation phase in the crystals with $x>0.1$. This is in a form of nonsuperconducting ${\mathrm{MgAlB}}_4$ domains in the structure of a superconducting ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ matrix. Resistivity and magnetic investigations show the slight increase of the upper critical field, $H_{c2}$, for $H\Vert c$ for the samples with small $x$, a significant reduction of the $H_{c2}$ anisotropy at lower temperatures, and a decrease of the residual resistance ratio value for Al substituted samples as compared to those of unsubstituted crystals. Superconducting gaps variation as a function of Al content, investigated with point contact spectroscopy for the series of the crystals with $T_c$ in the range from 20 to 37 K, does not indicate on the merging of the gaps with decreasing $T_c$ down to 20 K. It may be related to an appearance of the precipitation phase in the ${\mathrm{Mg}}_{1-x}{\mathrm{Al}}_x{\mathrm{B}}_2$ structure.

Cation doping is an effective strategy for improving the cyclability of layered oxide cathode materials through suppression of phase transitions in the high voltage region. In this study, Mg and Sc are chosen as dopants in P2‐Na_0.67Ni_0.33Mn_0.67O₂, and both have found to positively impact the cycling stability, but influence the high voltage regime in different ways. Through a combination of synchrotron‐based methods and theoretical calculations it is shown that it is more than just suppression of the P2 to O2 phase transition that is critical for promoting the favorable properties, and that the interplay between Ni and O activity is also a critical aspect that dictates the performance. With Mg doping, the Ni activity can be enhanced while simultaneously suppressing the O activity. This is surprising because it is in contrast to what has been reported in other Mn‐based layered oxides where Mg is known to trigger oxygen redox. This contradiction is addressed by proposing a competing mechanism between Ni and Mg that impacts differences in O activity in Na_0.67Mg_xNi_0.33‐xMn_0.67O₂ (x < 0 < 0.33). These findings provide a new direction in understanding the effects of cation doping on the electrochemical behavior of layered oxides.