Electrochemical Migration Behavior of Ag Electrodes in Negative Temperature Coefficient (NTC) Sensors Under Complex Environmental Conditions

To investigate electrochemical migration (ECM) and related failure of Ag electrodes in negative temperature coefficient (NTC) sensors under complex environments, screen-printed coplanar Ag electrodes were tested by means of real-time resistance monitoring and in situ CCD (Charge-Coupled Device) observation. The effects of water films, applied bias, NaCl concentration, and solution pH were examined. No observable migration occurred at 80 °C and 3 V without liquid water. In pure water, Ag dendrites grew from the cathode toward the anode and formed conductive bridges; increasing the bias from 3 to 4 V shortened the time to short circuit. In neutral NaCl media, 0.1 mmol L−1 NaCl allowed Ag-containing species to migrate across the electrode gap and form dendrites, whereas 1 and 5 mmol L−1 NaCl promoted AgCl-related deposits that suppressed sustained Ag dissolution, with the extent of inhibition depending on dissolution flux and surface coverage. In HCl media, surface deposits provided insufficient long-term protection, leading to continued Ag dissolution and short-circuit failure. In Na2CO3 media at pH = 9 and pH = 12, insoluble Ag-containing deposits formed on the anode and no dendritic bridging was observed. Overall, Ag ECM is governed by the competition among anodic dissolution, surface deposition/passivation, and the transport of Ag-containing species across the electrode gap. The results provide a basis for material selection, package design, and reliability assessment of NTC sensors for power batteries.