Calendering, one of the crucial steps in the manufacturing of lithium-ion cells, is much like rolling a steamroller over asphalt. It compresses the battery's electrode into a thin, dense sheet that improves the electrical contact between active particles & the conductive network, enhancing the battery's energy density & overall performance. The process is not just about making things compact; it's about creating paths for lithium ions, ensuring they don't encounter hurdles on their way to generate power. But there's a delicate balance to strike. Compress too little, and the pathways are too broad, too resistive; compress too much, and you choke the lanes, obstructing the vital flow of energy.
The study by Lu X. et al. explores the impact of calendaring on battery performance. In-situ X-ray nano-computed tomography shows the state of lithiation (SoL) at 60% depth of discharge (DoD) in uncalendered & calendered electrodes (Figures A to F). It shows how densely packed particles in a less porous structure (resulting from calendering) exhibit differing lithiation profiles, with warmer colors indicating higher SoL. In A, B, & C, the uncalendered & 22.5% calendered large active material particles (AM_L) at discharge rates of 3C & 5C show a notable difference in lithiation, especially at the higher discharge rate. Figures D, E, & F show the smaller active material particles (AM_S), which maintain a more consistent lithiation even after calendering & at higher discharge rates.
Figures G to L show the spatial distribution of activation overpotential (ηact) at the active particle/electrolyte interface. It captures the voltage loss required to drive the electrochemical reactions. We see increased voltage losses in calendered electrodes, particularly at the 5C discharge rate, implying that calendering can affect the efficiency of electron transfer, especially under more demanding conditions.
In Figures M through R, the concentration of the electrolyte (Cey) is evaluated. It shows how the electrolyte concentration within the electrode changes as a result of both calendering & increased discharge rates, with the uncalendered structures maintaining a more uniform distribution compared to their calendered counterparts.
The flux of lithium ions (Jp) across the electrodes is mapped from S to X showing pathways taken by li-ions during discharge. The calendering alters these pathways, especially at 5C, where the paths become more restricted & less uniform in calendered samples, potentially leading to inefficiencies in the battery's operation.
The visual data points to the importance of optimizing electrode calendering to achieve desired battery performances.