  LiMn2O4 (LMO), cathodes present large stability when cycled in aqueous
electrolytes, contrasting its behavior in conventional organic electrolytes in
Lithium-ion batteries (LIBs). To elucidate the mechanisms underlying this
distinctive behavior, we employ unconventional characterization techniques,
including Variable Energy Positron Annihilation Lifetime Spectroscopy (VEPALS),
Tip-Enhanced Raman Spectroscopy (TERS) and macro-Raman Spectroscopy (with
mm-size laser spot). These still rather unexplored techniques in the battery
field provide complementary information across different length scales,
revealing previously hidden features.
  VEPALS offers atomic-scale insights, uncovering cationic defects and
sub-nanometer pores that tend to collapse with cycling. TERS, operating at the
nanometric range at the surface, captured the presence of Mn3O4 and its
dissolution with cycling, elucidating dynamic changes during operation.
Additionally, TERS highlights SO42- accumulation at grain boundaries.
Macro-Raman Spectroscopy focuses on the micrometer scale, depicting small
changes in the cathode's long-range order, suggesting a slow but progressive
loss of crystalline quality under operation.
  Integrating these techniques provides a comprehensive assessment of LMO
cathode stability in aqueous electrolytes, offering multifaceted insights into
phase and defect evolution that can help to rationalize the origin of such
stability when compared to conventional organic electrolytes. Our findings
advance the understanding of LMO behavior in aqueous environments and provide
guidelines for its development for next-generation LIBs.
