Failure Analysis of Li-Cu Reference Electrode in NCM811||Gr Pouch Battery

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.