Nanoscale Morphological and Chemical Changes of High
Voltage Lithium–Manganese Rich NMC Composite Cathodes with
Cycling
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Abstract
Understanding the evolution of chemical
composition and morphology
of battery materials during electrochemical cycling is fundamental
to extending battery cycle life and ensuring safety. This is particularly
true for the much debated high energy density (high voltage) lithium–manganese
rich cathode material of composition Li<sub>1 + <i>x</i></sub>M<sub>1 – <i>x</i></sub>O<sub>2</sub> (M =
Mn, Co, Ni). In this study we combine full-field transmission X-ray
microscopy (TXM) with X-ray absorption near edge structure (XANES)
to spatially resolve changes in chemical phase, oxidation state, and
morphology within a high voltage cathode having nominal composition
Li<sub>1.2</sub>Mn<sub>0.525</sub>Ni<sub>0.175</sub>Co<sub>0.1</sub>O<sub>2</sub>. Nanoscale microscopy with chemical/elemental sensitivity
provides direct quantitative visualization of the cathode, and insights
into failure. Single-pixel (∼30 nm) TXM XANES revealed changes
in Mn chemistry with cycling, possibly to a spinel conformation and
likely including some Mn(II), starting at the particle surface and
proceeding inward. Morphological analysis of the particles revealed,
with high resolution and statistical sampling, that the majority of
particles adopted nonspherical shapes after 200 cycles. Multiple-energy
tomography showed a more homogeneous association of transition metals
in the pristine particle, which segregate significantly with cycling.
Depletion of transition metals at the cathode surface occurs after
just one cycle, likely driven by electrochemical reactions at the
surface