Operando Temperature Characterization Inside Lithium Batteries via Upconverting Nanoparticle Thermometry

Charging lithium-ion batteries at high rates requires reliable, accurate temperature characterization to ensure safety. Conventional thermometers for batteries typically monitor external packaging or cell cases, which fails to capture internal thermal processes, particularly during high-rate charging when significant temperature gradients may develop. These gradients are difficult to predict due to limited understanding of the dynamic thermal properties of cell materials and interfaces. While advanced thermometry methods, such as X-ray techniques, Raman spectroscopy, optical fibers, and inserted thermistors have been used for internal battery thermometry, these approaches can be invasive or destructive, lack sufficient spatial resolution, or are unable to measure key internal components. In this work, we present a less-invasive, optical method for internal temperature monitoring of Li batteries, using the well-studied and temperature-dependent luminescence of upconverting nanoparticles (UCNPs) embedded in the coin cells [1].
We deposited UCNPs on various components before they were sealed off in the coin cell. Small openings are created in the cell case or components to enable optical measurements. The influence of UCNP implementation and cell structure modification was analyzed. Voltage profiles and cycling data showed negligible differences in the electrochemical performance between UCNP-integrated cells and pristine cells. Additionally, structural modifications made for optical measurements of the coin cell were found to have negligible impact on the temperature profile during discharge.
Next, after dissembling a commercial coin cell (CR2032), a high concentration of UCNPs was embedded into each component within the same cell. By measuring the spectra of UCNPs on the MnO₂ cathode, Li metal anode, and glass fiber separator, internal temperature measurements of each component were achieved. Under low current, consistent temperature readings from both an external thermocouple and internal UCNP thermometry validated the accuracy of our methodology. Focusing on the cathode temperature as the discharge current increased, a temperature difference of 2.3 °C was observed between the internal and the external temperatures. Furthermore, upon replacing the glass fiber separator with a polyolefin (Celgard®) separator, a maximum temperature difference of 7.9 °C was observed when the current was raised to 65 mA. This approach provides a novel solution for identifying critical thermal processes within batteries, which in the future can be extended to larger format lithium-ion battery cells that are expected to experience steeper internal temperature gradients. These measurements can reveal non-uniform internal temperature distributions and offer valuable insights for battery thermal management and safety improvements.

[1] Fei Hu^†, Ziyang Ye^†, Andrea D. Pickel^*, and Wyatt E. Tenhaeff^*, “Operando Temperature Characterization inside Lithium Batteries via Upconverting Nanoparticle Thermometry,” under review. (^† denotes equal contributions)