Understanding the Effect of Low Temperature Degradation on Lithium-Ion Battery Performance and Safety

As efforts to decarbonise the world economy ramp up, lithium-ion batteries have become a crucial component in facilitating the transition of key industries to clean energy. Successfully tackling this challenge requires continuous improvements in energy-density, longer cell lifetimes and better assurances in safe cell operation. In recent years, numerous high-profile failures of Li-ion batteries have been reported¹, contributing to wider concerns about the safety of high energy density cells. Inadequate management of such systems, in applications such as electric vehicles, aerospace, and portable commercial devices, can be costly in both human health and financial damages. Therefore, it is important to study battery safety in order to provide a reliable alternative to fossil fuels. Apprehensions in cell safety can mainly be attributed to a cell’s ability to enter thermal runaway, a process that is most often caused by an internal short circuit (ISC). Such an event can be triggered by three different modes of abuse: thermal, mechanical, and electrical.<br/>Conditions of thermal and mechanical abuse are mostly because of a cells external environment and can be controlled more easily; however, electrical abuse often originates from internal conditions that accelerate the growth of lithium dendrites that can pierce the separator – a mechanism that is more difficult to monitor or predict. At the anode, metallic lithium can precipitate onto the surface via three main conditions: overcharge, high charging currents, and low charging temperatures. Each of which create a saturation of intercalated lithium ions in the crystallographic active sites near the anode surface. This lowers the anodes surface potential until it is sufficiently low enough for lithium plating to occur. To better understand this process and how it affects battery safety, our work investigates the degradation of commercial cylindrical cells (with NCA cathodes) at low temperatures and how it changes their response to abusive conditions. Here we show that as the temperature is lowered, plating becomes more aggressive and pseudo-homogenous due to the reduced rate of lithium-ion diffusion into the bulk graphite.<br/>By utilizing various <i>in-situ</i> lithium detection techniques, such as post-charge voltage relaxation, Electrochemical Impedance Spectroscopy, and Differential Capacity Analysis, the extent of a cell’s degradation and its state of safety can be estimated by monitoring real-time electrochemical data. Further optimization of these techniques to generate quantitative measurements on the aforementioned parameters would greatly improve battery management systems and their predictive capabilities of cell failure. Moreover, subsequent Accelerated Rate Calorimetry (ARC) studies highlight that a cells thermal stability diminishes as degradation becomes more advanced. These findings demonstrate how lithium plating has an impact on battery safety and why more work is needed on developing real-time assessments of a battery’s state of degradation. Insights into the mechanisms at play in such failures have been achieved through post-mortem X-ray computed tomography (CT) and <i>operando</i> ultra-high-speed tomography and radiography. Together, these techniques act as a valuable tool in elucidating failure behaviours.<br/><br/><i>References:</i><br/>Bisschop, R., Willstrand, O., Amon, F. & Rosengren, M. SAFETY & TRANSPORT FIRE RESEARCH Fire Safety of Lithium-Ion Batteries in Road Vehicles.