  The recent discovery of superconductivity in La3Ni2O7-{\delta} under high
pressure with a transition temperature around 80 K has sparked extensive
experimental and theoretical efforts. Several key questions regarding the
pairing mechanism remain to be answered, such as the most relevant atomic
orbitals and the role of atomic deficiencies. Here, we develop a new
energy-filtered multislice electron ptychography technique, assisted with
electron energy loss spectroscopy, to address these critical issues. Oxygen
vacancies are directly visualized and are found to primarily occupy the inner
apical sites, which have been proposed to be crucial to superconductivity. We
precisely determine the nanoscale stoichiometry and its correlation to the
oxygen K edge spectra, which reveals a significant inhomogeneity in the oxygen
content and electronic structure within the sample. The spectroscopic results
also unveil that stoichiometric La3Ni2O7 is in the charge-transfer regime, with
holes that are self-doped from Ni sites into O sites. The outer apical oxygen
is found to be less relevant to the low-energy physics and can be safely
disregarded in theoretical models. These observations will assist in further
development and understanding of superconducting nickelate materials. Our
imaging technique for quantifying atomic deficiencies can also be widely
applied in materials science and condensed matter physics.
