  Lithium-ion secondary batteries have been used in a wide variety of purposes,
such as for powering mobile devices and electric vehicles, but their
performance should be improved. One of the factors that limits their
performance is the non-uniformity of the chemical reaction in the process of
charging and discharging. Many attempts have been made to elucidate the
mechanism behind this reaction non-uniformity. In this paper, to detect
non-uniformity in various physical properties from Co K-edge two-dimensional
X-ray absorption spectroscopy (2D-XAS) images of lithium ion batteries, we
propose a method that consists of one-sided orthogonal non-negative matrix
factorization in combination with removal of the reference signal. The
difference between X-ray absorption spectra acquired at different positions in
the battery is very small. However, even in such a situation, our method can
decompose the 2D-XAS data into different spatial domains and their
corresponding absorption spectra. From the spectral decomposition of the
obtained absorption spectra, we confirmed a transition-energy shift of the main
peak as evidence for a change in the state of charge and also found spectral
changes due to orbital hybridization in the decomposed spectral components.
