<p>The
mitigation of decomposition reactions of lithium-ion battery electrolyte
solutions is of critical importance in controlling device lifetime and
performance. However, due to the
complexity of the system, exacerbated by the diverse set of electrolyte
compositions, electrode materials, and operating parameters, a clear
understanding of the key chemical mechanisms remains elusive. In this work, operando
pressure measurements, solution NMR, and electrochemical methods were combined
to study electrolyte oxidation and reduction at multiple cell voltages. Two-compartment
LiCoO<sub>2</sub>/Li cells were cycled with a lithium-ion conducting glass-ceramic
separator so that the species formed at each electrode could be
identified separately and further reactions of these species at the opposite
electrode prevented. One principal finding is that chemical oxidation (with an
onset voltage of ~4.7 V vs Li/Li<sup>+</sup> for LiCoO<sub>2</sub>), rather
than electrochemical reaction, is the dominant decomposition process at the
positive electrode surface in this system. This is ascribed to the well-known
release of reactive oxygen at higher states-of-charge, indicating that reactions
of the electrolyte at the positive electrode are intrinsically linked to
surface reactivity of the active material. Soluble electrolyte decomposition products formed
at both electrodes are characterised, and a detailed reaction scheme is
constructed to rationalise the formation of the observed species. The insights
on electrolyte decomposition through reactions with reactive oxygen species identified
through this work have direct impact on understanding and mitigating
degradation in high voltage/higher energy density LiCoO<sub>2</sub>-based
cells,<sub> </sub>and more generally for cells containing nickel-containing
cathode materials (e.g. LiNi<sub>x</sub>Mn<sub>y</sub>Co<sub>z</sub>O<sub>2</sub>;
NMCs), as they lose oxygen at lower operating voltages.</p>