  Temperature is known to impact Li-ion battery performance and safety,
however, understanding its effect on Li-ion batteries has largely been limited
to uniform high or low temperatures. While the insights gathered from such
research are important, much less information is available on the effects of
non-uniform temperatures which more accurately reflect the environments that
Li-ion batteries are exposed to in real world applications. In this paper, we
characterize the impact of a microscale, temperature hotspot on a Li-ion
battery using a combination of in situ micro-Raman spectroscopy, in situ
optical microscopy and COMSOL Multiphysics thermal simulations. Our results
show that mild temperature heterogeneity induced by the micro-Raman laser can
cause lithium to locally leach out from different lithiated graphite phases
(LiC6 and LiC12) in the absence of an applied current. The Li metal is found to
be largely localized to the region heated by the micro-Raman laser and is not
observed upon uniform heating to comparable temperatures suggesting that
temperature heterogeneity is uniquely responsible for causing Li to leach out
from lithiated graphite phases. A mechanism whereby localized temperature
heterogeneity induced by the laser induces heterogeneity in the degree of
lithiation across the graphite anode is proposed to explain the localized Li
leaching. This study highlights the sensitivity of lithiated graphite phases to
minor temperature heterogeneity in the absence of an applied current.
