Characterizing the Mechanical and Electrochemical Behavior of Preloaded Electrodes in Lithium Ion Pouch Cells

Mechanical stress on electrodes in pouch cells can be generated internally during the intercalation of lithium ions into the host lattice of electrode materials or imposed externally during the manufacturing process. Since a long time the former inducement is a field of active research. The influence of the later inducement is less concerned, although the external stress is preloaded inevitably.     

To understand the effect of the mechanical preloading in form of bending on the electrode performance four pouch cells consisting of single cathode (65x45mm) and single anode (70x50mm) using NMC (nickel manganese cobalt oxide) and MCMB (Meso Carbon Micro Beads) as electrode materials for cathode and anode respectively were bent into U-profile with different bending diameters ranging from 1.5mm to 12.5mm before the electrochemical formation. An aging test according to a CC scheme with 1C for 800 cycles is conducted prior to a cycling test using CC scheme with different current between 0.2C and 5C for 3 cycles at each current. The mechanical behavior of the bending electrodes during the cycling is characterized by monitoring the electrode displacement in situ applying DIC (digital image correlation) technique. To compare with the performance of the bending samples a flat pouch cell with the identical material system and in the same size cycled parallel using the same test procedure as above is considered as a reference sample.

Variation in bending diameter results in diverse cycle stability characteristics over the cycling. In comparison to the reference sample with 60% remaining capacity, the samples with the bending diameter of 3mm and 10mm show improved capacity retention with the value of 72% and 66% after 800 cycles. By contrast, the cycle stability of the cells with a smaller bending (1.5mm) and a larger bending (12.5mm) is not upgraded. Their remaining capacities are comparable to the reference sample, at 60% and 59%, respectively. The cells with enhanced cycle stability are also found to exhibit a stable performance at higher charge rate, while the reference sample displays a slight capacity fading at the same charge rate. The mechanical behavior of the flat electrode in the reference cell is inhomogeneous and elusive. In the central area, the displacement behaves reversibly in accordance to the volumetric expansion and contraction due to the insertion and extraction of the lithium ions indicating the reversible electrochemical process inside the electrodes. In contrary, the behavior in the edge areas is anomalous. The displacement at the electrode edge exhibits a persistent increase which is indicative of an irreversible electrochemical process in these regions leading to gradually capacity fading. This accumulated displacement can reach a high value (0,03mm) after a long operation time. In comparison to the displacement behavior on unloaded electrodes, the bended samples show no obvious displacement during charge and discharge at different current. In the bending area, the displacement intensity is below 0.003mm. However, the displacement evolution plot demonstrates a reversible wavelike performance corresponding to the repeated electrochemical reaction within the electrode, which indicates that the active volumetric change due to the intercalation and deintercalation is effectively suppressed by the bending; the reaction, nevertheless, is less affected by this kind of preloading and under these circumstances the mechanical behavior can no more reflect the electrochemical process proceeding in the electrode quantitatively.         

Taken together our findings conclude that all the improvements can be attributed to the positive effect due to pre-bending of the electrode, which may stabilize the electrode morphology against the negative influence of repeatable volume change and improve therefore the capacity retention and cycle stability during high rate charge and discharge to prolong the life of the cells. The improvement is only effective in an optimal range of the bending diameter (3-10mm). Larger diameter causes a mild mechanical loading that is insufficient to protect the electrode. A smaller bending diameter is harmful due to the severe stress it brings to the electrode.