<i>Operando</i> characterization of lithium battery internal temperatures <i>via</i> upconverting nanoparticle thermometry

Charging lithium batteries at high rates requires reliable, accurate temperature characterization for battery safety. Monitoring the external temperature of battery packaging does not provide satisfactory insight into thermal processes within the cell, especially at high rates when significant temperature gradients can develop. For <i>operando</i> characterization of the internal temperatures of Li batteries, a novel thermometry technique based on the luminescence of upconverting nanoparticles (UCNPs) was demonstrated. NaYF₄:Yb³⁺,Er³⁺ UCNPs with an average diameter of 27 nm were introduced onto the surface of battery cell components (anode, cathode, and separator) and shown to have negligible adverse effects on the cell's performance, while enabling <i>operando</i> temperature measurements of all three components in a single cell. With application of a discharge current of 65 mA in a commercial primary coin cell (CR2032), a maximum temperature difference of 7.9 °C was measured between the cell separator and external packaging. It is envisioned that this technique can be extended to larger format lithium-ion battery cells, revealing non-uniform internal temperature distribution within the cells to better understand critical thermal processes.