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

## TL;DR

This study explores how a cobalt-free lithium-rich oxide cathode changes during initial charging, leading to improved battery performance.

## Contribution

The paper reveals the structural and redox mechanisms in Co-free, Ni-poor LRLOs during formation, which were previously unknown.

## Key findings

- Activation compresses layer spacing and increases structural breathing in Li1.28Ni0.15Mn0.57O2.
- Oxygen redox is delocalized and reversible without oxygen evolution, preventing degradation.
- Structural changes in MnO6 octahedra enable superior electrochemical performance.

## Abstract

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.