The Impact of Thermal Gradients on the Aging Behavior of Lithium-Ion Batteries

Summary:

Temperature is a dominant parameter in lithium-ion batteries’ aging behavior. Aging mechanisms in lithium-ion batteries correspond to unwanted side reactions, often associated with film formation on the electrodes. This work shows measurement results illustrating the effect of thermal gradients on the aging behavior of a commercial nickel-rich pouch lithium-ion battery. Using a carefully designed test bench, we aged multiple battery cells under different homogeneous and inhomogeneous thermal conditions.

The test bench consists of two copper plates that clamp the battery cell with a constant pressure of 0.5 bar in a spring setup. Six Peltier elements on each cell side impose the thermal boundary conditions using the copper plates to ensure a homogeneous in-plane temperature distribution. The aging test procedure consisted of repeating check-ups and cycling sections. During cycling, 7 Ah, which correspond to a nominal depth of discharge of 60 %, were charged and discharged with constant 1C between 4.2 V and a varying lower voltage, but not lower than 2.7 V. The cells’ datasheet covers all electrical and thermal conditions. Every 150 cycles, the capacity was measured in a check-up at homogeneous 25 °C.

The capacity plot shows results at both homogeneous (15, 25, 35 °C) and symmetrical inhomogeneous (25 °C average and 5, 20, or 30 K maximum difference) thermal conditions. While the battery at 15 °C didn’t survive the first set of cycles due to plating, the cell at 25 °C reached close to 500 equivalent full cycles (EFC). At 540 EFC, the aging test at 35 °C is still running. Compared to the homogeneous 25 °C, the aging results of the inhomogeneous thermal conditions show the possible negative impact of thermal through-plane gradients. Compared to homogeneous 25 °C, the cells aged with a thermal gradient von 20 K and 30 K don’t reach half of the EVC. Overall, however, most cells didn’t meet the cycle life of 3,000 cycles specified by the manufacturer.

Images from the post-mortem analysis of the fully discharged cell aged at 25 °C with a through-plane gradient of 30 K illustrate why the cells reached their end of life rather quickly. The depicted three anodes located at the warm and cold end and in the middle of the cell, respectively, show that all anodes are covered with clustered depositions, albeit with varying size distributions. These depositions consist of lithium compounds that permanently bind lithium. The SEM image from anode 17 displays the size of the large deposition clusters and shows that smaller depositions also cover the area in between. These large clusters can reach multiples of 100 μm in size and can get as high as 50 μm, as the laser microscopy image illustrates. The nickel-rich cathode didn’t show any signs of aging.