Design of Positive Electrodes for Li-Ion Full Cells with Silicon

Wound pouch cells with a Li(Ni0.6Mn0.2Co0.2)O2 (NMC622) positive electrode and graphite/Si alloy negative electrode are shown to have significantly better performance than equivalent cells with a LiCoO2 (LCO) positive electrode. Silicon containing NMC622 full cells have better performance with a 4.35 V upper cutoff voltage (UCV) than with a 4.25 V UCV. The addition of small amounts (≤8 wt%) of high surface area NMC622 to LCO-based positive electrodes in single layer pouch cells with graphite/Si alloy negative electrodes improves cycling performance and coulombic efficiency. Adding fluoroethylene carbonate (FEC) further improves the performance of cells showing the benefits of NMC622 and FEC can be combined. High precision calorimetry and cycling, Archimedes volume measurements, and cross-section scanning electron microscopy support the hypothesis of interplay between the parasitic reactions at the positive and negative electrodes. The NMC622 positive electrode is proposed to stabilize the Si alloy in the negative electrode via a CO2 release mechanism, though further confirmation of the mechanism is needed.