1 research outputs found
Imaging Phase Segregation in Nanoscale Li<sub><i>x</i></sub>CoO<sub>2</sub> Single Particles
LixCoO2 (LCO)
is a common
battery cathode material that has recently emerged as a promising
material for other applications including electrocatalysis and as
electrochemical random access memory (ECRAM). During charge–discharge
cycling LCO exhibits phase transformations that are significantly
complicated by electron correlation. While the bulk phase diagram
for an ensemble of battery particles has been studied extensively,
it remains unclear how these phases scale to nanometer dimensions
and the effects of strain and diffusional anisotropy at the single-particle
scale. Understanding these effects is critical to modeling battery
performance and for predicting the scalability and performance of
electrocatalysts and ECRAM. Here we investigate isolated, epitaxial
LiCoO2 islands grown by pulsed laser deposition. After
electrochemical cycling of the islands, conductive atomic force microscopy
(c-AFM) is used to image the spatial distribution of conductive and
insulating phases. Above 20 nm island thicknesses, we observe a kinetically
arrested state in which the phase boundary is perpendicular to the
Li-planes; we propose a model and present image analysis results that
show smaller LCO islands have a higher conductive fraction than larger
area islands, and the overall conductive fraction is consistent with
the lithiation state. Thinner islands (14 nm), with a larger surface
to volume ratio, are found to exhibit a striping pattern, which suggests
surface energy can dominate below a critical dimension. When increasing
force is applied through the AFM tip to strain the LCO islands, significant
shifts in current flow are observed, and underlying mechanisms for
this behavior are discussed. The c-AFM images are compared with photoemission
electron microscopy images, which are used to acquire statistics across
hundreds of particles. The results indicate that strain and morphology
become more critical to electrochemical performance as particles approach
nanometer dimensions