Structural Rearrangements of a Cobalt-Free Lithium-Rich Layered Oxide Cathode during Formation.

Formation during the first cycles of Li-rich layered oxide (LRLO) cathode materials consolidates the interphase and leads to structural changes that are decisive for long-term cyclability. However, the nature and effect of the changes are material-dependent and unknown for the important class of Co-free, Ni-poor LRLOs. Here, we analyze the processes during the tailored formation procedure of a typical class member, Li1.28Ni0.15Mn0.57O2, and demonstrate that it remarkably changes lattice composition and structure as a prerequisite for stable cycling. We combine electrochemistry, operando mass spectrometry, X-ray diffraction, and X-ray absorption spectroscopy with density functional theory simulations. Activation most prominently compresses the layer spacing along the c-axis and increases reversible structural breathing. The large capacity of ∼250 mAh g-1 originates from the Ni2+/Ni4+ and O2-/O- redox couples. Electron exchange during O-redox is smeared over the entire anionic sublattice rather than localized on specific oxygen atomic sites. This redox mechanism is reversible without detrimental oxygen evolution, avoiding continued degradation common in conventional LRLOs. Sequential Ni- and O-redox during activation irreversibly distorts the coordination of the redox-inactive Mn4+ centers. This structural evolution of the MnO6 octahedra appears to enable the superior electrochemical performance of this LRLO phase. These findings define an activation pathway for the important class of Co-free, Ni-poor LRLOs, offering potential guidance for the rational design of high-performance, more sustainable cathode materials.