Reliable monitoring of the internal state of lithium-ion batteries (LIBs) is
crucial for mitigating potential safety hazards. The incorporation of a
reference electrode (RE) within the battery constitutes a vital approach for
achieving single-electrode monitoring and understanding changes in electrode
state during cycling. Among these, the lithium-copper reference electrode (Li-Cu
RE) is particularly cost-effective and straightforward to prepare, being
fabricated by depositing lithium onto a copper wire. However, Li-Cu RE exhibits
a relatively short effective lifespan during long-term cycling, thereby limiting
its practical application. In this work, based on a self-fabricated
three-electrode single-layer pouch cell, the microstructural changes before and
after failure of the Li-Cu RE were characterized and analyzed, revealing its
failure evolution process. Post-failure microstructures observations exhibit
marked porosity in the electrode, attributed to substantial depletion of surface
lithium metal. Concurrently, the copper wire's elevated potential dominantly
influences the overall Li-Cu RE potential, causing its potential to rise and
destabilize. This induces a sharp decline in the measured electrode's potential
curve. Furthermore, comparative analysis of key factors influencing Li-Cu RE
lifespan were investigated. In the static state, the theoretical failure time of
Li-Cu RE differed by only approximately 9 hours from that in the cycling state.
Crucially, isolating the test electrode from the Li-Cu RE nearly doubled its
lifespan, revealing that current generated by the potential difference between
the test electrode and Li-Cu RE is the primary cause of failure under low-rate
cycling. This paper systematically elucidates the observed failure behavior of
the Li-Cu RE and comprehensively analyzes the various factors, which aids in
further understanding the failure mechanism of the Li-Cu RE and identifying
targeted solutions.