Structural and Chemical Evolution of Li- and Mn-Rich
Layered Cathode Material
- Publication date
- 2015
- Publisher
Abstract
Lithium
(Li)- and manganese-rich (LMR) layered-structure materials
are very promising cathodes for high energy density lithium-ion batteries.
However, the voltage fading mechanism in these materials as well as
its relationships to fundamental structural changes is far from being
sufficiently understood. Here we report the detailed phase transformation
pathway in the LMR cathode (Li[Li<sub>0.2</sub>Ni<sub>0.2</sub>Mn<sub>0.6</sub>]O<sub>2</sub>) during cycling for samples prepared by the
hydrothermal assisted (HA) method. It is found that the transformation
pathway of the LMR cathode is closely correlated to its initial structure
and preparation conditions. The results reveal that the LMR cathode
prepared by the HA approach experiences a phase transformation from
the layered structure (initial <i>C</i>2/<i>m</i> phase transforms to <i>R</i>3̅<i>m</i> phase after activation) to a LT-LiCoO<sub>2</sub> type defect spinel-like
structure (with the <i>Fd</i>3̅<i>m</i> space
group) and then to a disordered rock-salt structure (with the <i>Fm</i>3̅<i>m</i> space group). The voltage fade
can be well correlated with Li ion insertion into octahedral sites,
rather than tetrahedral sites, in both defect spinel-like and disordered
rock-salt structures. The reversible Li insertion/removal into/from
the disordered rock-salt structure is ascribed to the Li excess environment
that permits Li percolation in the disordered rock-salt structure
despite the increased kinetic barrier. Meanwhile, because of the presence
of a large quantity of oxygen vacancies, a significant decrease in
the Mn valence is detected in the cycled particle, which is below
that anticipated for a potentially damaging Jahn–Teller distortion
(+3.5). Clarification of the phase transformation pathway, cation
redistribution, oxygen vacancy and Mn valence change provides unique
understanding of the voltage fade and capacity degradation mechanisms
in the LMR cathode. The results also inspire us to further enhance
the reversibility of the LMR cathode via improved surface structural
stability