Flexible K-type thin-film thermocouples enabling accurate in-situ temperature monitoring of lithium-ion batteries

• A flexible K-Type thermocouple was made on polyimide via optimized sputtering, achieving dense films. Calibration showed high linearity (R² ≥ 0.998), with low Seebeck deviation (1.13%) and batch CV (1.35%). • Multiphysics simulations revealed over 40% signal attenuation from "heat leakage" due to sensor-source mismatch, providing theoretical basis for precise battery sensor deployment. • The study revealed non-linear errors from the "micro-fin effect" at high convection, with peak temps dropping sharply (e.g., 215°C to 80°C). The sensor showed stable performance across -50 to 250°C. In addressing the pressing need for sensor miniaturisation, enhanced precision, and in-situ real-time response in the thermal safety monitoring of lithium-ion batteries, this study proposes a K-Type (NiCr-NiSi) thin film thermocouple (TFTC) based on a flexible polyimide (PI) substrate. A process known as 'optimised magnetron sputtering' was utilised in the fabrication of 'dense nanocrystalline functional thin films' with a controllable composition. This process enabled the integration of the device to be realised in a flexible and micro-invasive manner. Static calibration experiments demonstrate that the sensor exhibits exceptional linearity (R² ≥ 0.998) across a wide temperature range of 25–200°C. The deviation of the measured Seebeck coefficient from that of a standard K-type thermocouple is merely 1.13%, with a batch-to-batch coefficient of variation as low as 1.35%, indicating superior measurement accuracy and consistency. Moreover, when combined with multiphysics numerical simulations, this study provides a comprehensive insight into the mechanisms of heat transfer and the evolution of error laws under complex operating conditions. Specifically, the 'heat leakage' effect caused by the spatial mismatch of the heat source was quantified (resulting in 40% signal attenuation), and the non-linear measurement errors induced by the 'micro-fin effect' under high convective heat transfer boundaries were elucidated. Furthermore, linear superposition and transient response advantages were verified from -50 to 250 degrees Celsius. This work develops a high-performance device for internal battery temperature monitoring and provides a significant theoretical basis and technical support for sensor deployment strategies and precise thermal management in complex thermal environments.