Mechanical collapse as primary degradation mode in mandrel-free 18650 Li-ion cells operated at 0 °C

Low temperature charging of Li-ion batteries threatens undesired deposition of lithium dendrites which are often blamed for catastrophic, thermal runaway failures. This work identifies three distinct degradation modes experienced by 0 °C operated 18650 Li-ion batteries, revealing safety and performance risks beyond lithium plating. Two equivalently rated (2.6Ah) commercial cells from different vendors, one possessing a mandrel and one lacking, were cycled at 0 °C. Through a combination of techniques, it is identified that the first degradation mode is dominated by cell polarization at low temperature, causing lower coulombic efficiency and undesired side reactions. Next, mechanical deformation or jellyroll collapse occurs, followed by accelerated metal plating. However, in the mandrel-containing cell, physical constraint of the electrodes prevents both the deformation and metal plating modes. These degradation modes, difficult to isolate with electrochemistry alone, were diagnosed with non-destructive micro X-ray computed tomography and destructive physical analysis and confirmed with differential capacity assessment. The implication of the physical conditions induced by 0 °C operation on cell safety was assessed with accelerated rate calorimetry. Our observations emphasize the presence of thermo-mechano-electrochemical interplay in commercial cells operated in extreme conditions and assesses the safety implications of their coupling.