  Li-ion battery degradation processes are multi-scale, heterogeneous, dynamic
and involve multiple cell components through cross talk mechanisms. Correlated
operando characterization capable of measuring several key parameters are
needed to accelerate understanding on these complex degradation processes. In
particular, degradation mechanisms during overcharge of LiNiO2/Graphite-Silicon
is well known at the material level featuring O2 gas release and concomitant
surface reconstruction of LiNiO2. However, there are still debates regarding
the role of high voltage O1 phase formation on gas production and no
information on the effect of produced gases on the cell components (anode or
sensors), or effect of overcharge on electrode level behavior. In this work, we
simultaneously measured the gas produced using operando mass spectrometry while
spatially resolving nanostructure and lattice changes using operando micro
SAXS/WAXS mapping during the formation and over charge of a LiNiO2/Gr-Si pouch
cell. This new correlated operando characterization experiment allowed to (1)
confirm the absence of O1 phase even with substantial gas produced at end of
charge, (2) unveil the effect of gases on reference and negative electrodes,
(3) show that overcharge increases in-plane reaction heterogeneities by
creating local degraded spots lagging behind the ensemble electrochemistry.
These findings will be important to optimize ageing of devices based on similar
chemistries, in particular Ni-rich NMC, while showing the strength of
correlated characterization leading to more efficient and robust information on
complex mechanisms.
