Battery calorimetric analysis of aging effects on the safety and thermal behavior of cylindrical lithium-ion cells

With increasing energy density safety and thermal management of Li-ion cells is becoming more and more important because the thermal runaway can cause an ignition or even explosion with simultaneous release of toxic gases. In order to prevent such hazards, extensive research is undertaken to determine the causes of thermal runaway, as well as the influencing factors. In the last nine years, we have established battery calorimetry as a versatile characterization technique, which allows advancements for the thermal management and the safety of batteries. With six adiabatic Accelerating Rate Calorimeters (ARC) of different sizes and two sensitive Tian-Calvet calorimeters combined with cyclers we operate Europe’s largest battery calorimeter center. This enables the evaluation of thermodynamic, thermal and safety data on material, cell and pack level under quasiadiabatic and isoperibolic environments for both normal and abuse conditions (thermal, electrical, mechanical).
The primary purpose of this work was to determine the correlation between cell aging and the thermal runaway of commercial Sony VTC6 18650 cells with a nominal capacity of 3 Ah, a Li-containing graphite anode and a high-Ni LiNi0.9Co0.075Al0.025O2 (NCA) cathode as determined by inductive coupled plasma optical emission spectroscopy (ICP-OES). ... mehrTherefore, a comprehensive ageing study was performed with 116 cells.
First, fresh cells were stored in temperature chambers at different temperatures and states of charge and then characterized at fixed time intervals in order to study the influence of this storage (calendar aging) on cell performance. Second, cells of the same type were aged in temperature chambers at different charge/discharge rates (cyclic aging) and comprehensively characterized every 100 cycles. The ageing procedure was stopped at a state-of-health (SOH) of 80%. Information on the causes of the aging was obtained using electrochemical impedance spectrometry (EIS), X-ray diffraction and measurement of the entropy changes using the potentiometric method, where the change in the open circuit voltage is monitored against the temperature. The change in entropy showed a capacity-dependent behavior. It was found that the cathode has no significant influence on the entropy changes and that the anode represents the dominating factor. This allows the determination of the loss of lithium inventory (LLI) in the anode and to correlate the LLI to the total capacity loss. The cathode only showed changes in the charge transfer resistance that was extracted from the EIS spectra.
After these characterizations, the cells were thermally abused using the Heat-Wait-Seek (HWS) test in an ARC. The cyclic aged cells showed increased exothermic reactions in the low temperature range. It is unclear whether these indicate lithium plating or decomposition of the SEI. The absence of these increases with calendar aging, in which plating can be ruled out, means that a larger SEI layer has a small influence on an increased temperature rate. Accordingly, it is assumed that the increase in cyclic aging in the low temperature range is due to a small amount of plating. The aged cells show a vent opening at higher temperatures than the fresh cells and minimal deviations in the temperature rates, which can be attributed to the lower capacity of the aged cells. Thus, it can be concluded that for these cells the safety level is not negatively affected when they reach a SOH of 80%.