Li-ion secondary batteries are used in many applications and technologies as energy storage devices because of their relatively high Coulombic efficiency and energy and/or power density. However, Li-ion batteries (LIBs) are prone to safety concerns (e.g., thermal runaway), which limit their application [1]. Thermal conditions are one of the major parameters that influence the safety and performance of LIBs due to temperature-dependent electrochemistry. Low temperatures [2] and high temperatures [3] have distinct effects on LIB electrochemistry and are well-reported in the literature. Although non-uniform thermal conditions are more practical during use, the effect of various non-uniform thermal conditions is not fully understood. A non-uniform temperature distribution can easily develop inside Li-ion cells by surface cooling, and the resulting thermal gradient can have adverse effects. Moreover, the direction of the thermal gradient can dictate local transport mechanisms and can cause different degradation modes [4]. Thus, further assessment of the impact of thermal gradients is needed to fully understand the fundamental electrochemical mechanisms, performance, and safety implications. In this work, we quantify the thermo-electrochemical sensitivity of LIBs to modulating interelectrode thermal gradients in synchronization with electrochemical cycling using a custom test facility. Instrumented single-layer pouch cells are fabricated using NMC cathodes and graphite anodes. The internal temperature of each electrode is obtained in real-time using a thin thermistor. Electrochemical impedance spectroscopy is performed before and after galvanostatic cycling at C/5, and the magnitude and the direction of the thermal gradient in synchronization with cell cycling is varied.

Acknowledgments
The authors thank Dr. Michele Anderson (Office of Naval Research, grant N00014-21-1-2307) for financial support of this work. The authors also acknowledge Dr. Corey Love and Dr. Rachel Carter (U.S. Naval Research Laboratory) for technical discussion of this work.

References
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