  A major challenge in the development of new battery materials is
understanding their fundamental mechanisms of operation and degradation. Their
microscopically inhomogeneous nature calls for characterization tools that
provide operando and localized information from individual grains and
particles. Here we describe an approach that images the nanoscale distribution
of ions during electrochemical charging of a battery in a transmission electron
microscope liquid flow cell. We use valence energy-loss spectroscopy to track
both solvated and intercalated ions, with electronic structure fingerprints of
the solvated ions identified using an ab initio non-linear response theory.
Equipped with the new electrochemical cell holder, nanoscale spectroscopy and
theory, we have been able to determine the lithiation state of a LiFePO4
electrode and surrounding aqueous electrolyte in real time with nanoscale
resolution during electrochemical charge and discharge. We follow lithium
transfer between electrode and electrolyte and observe charging dynamics in the
cathode that differ among individual particles. This technique represents a
general approach for the operando nanoscale imaging of electrochemically active
ions in a wide range of electrical energy storage systems.
