Physical Properties Needed to Enable Extreme Fast Charging of High Energy Density Graphite/NMC Cells
Authors: Colclasure Andrew M. , Dunlop Alison R , Trask Stephen E. , Polzin Bryant J. , Jansen Andrew N. , Smith Kandler 
Published: February 2020

The U.S. Department of Energy (DOE) has acknowledged extreme fast charging as one of the remaining challenges in enabling wide spread adoption of electric vehicles (EVs). The current DOE goal is to enable a 10-15 minute recharge time for high energy density cells with at least 200-250 Wh/kg. There are existing lithium-ion batteries that can be charged at very high rates (>5C), but these typically have prohibitively low energy density from use of thin electrodes and high cost. The charging time of EV type cells is limited by lithium plating, rapid temperature rise, and particle cracking. Through a combination of experimental data and Newman pseudo 2-D (P2D) modeling, this presentation will discuss physical properties needed to enable extreme fast charging of high energy density graphite/NMC 532 cells while avoiding lithium plating.

The ANL team fabricated graphite/NMC 532 coin cells with loadings of 1.5, 2.7, 4.3 and 5 mAh/cm2 with similar electrode porosities and N/P ratio. The cells used standard “Generation 2” electrolyte consisting of 1.2 M LiPF6 in EC/EMC (3:7 by weight). After formation cycling, cells where charged at 6C with CC-CV protocol up to 4.1 V with a total charge time of 10 minutes at a temperature of 30 °C. At the lowest loading of 1.5 mAh/cm2, the cell achieved 90% SOC during 6C charging and almost all the capacity occurred during the CC portion. For this loading, cells could be 6C charged/C/2 discharged for 800 cycles and retain 70-80% of the initial capacity. However, at the higher loadings the achieved SOC during 6C charging rapidly fell off to less than 60% SOC and most of the capacity occurred during the CV portion. Further, the capacity fade is dramatically higher for thicker cells during continued 6C/C/2 cycling. For instance, cells with a loading of 4.3 and 5 mAh/cm2 capacity faded by 20% in only 5 cycles.

A macro-homogeneous P2D model is used to investigate the poor 6C performance of thick-electrode cells and explore options from enabling 6C charging of high energy density cells. The model has been previously tuned to this chemistry under the DOE Computer-Aided Engineering for Electric-Drive Vehicle Batteries (CAEBAT) program. For loadings of 2.7, 4.3, and 5 mAh/cm2, the model predicts severe electrolyte depletion in the anode and saturation in the cathode are limiting high rate charging. The model and data seem to indicate solid-state diffusion and intercalation chemistry are sufficient to enable 6C charging. Based on this, the model is used to explore how the following might enable extreme fast charging of high energy density cells:

Reducing electrode tortuosity

Improving electrolyte properties

Increasing electrode porosity

Increasing charging temperature

Increasing N/P ratio

A combination of increased temperature, reduced electrode tortuosity, and improved electrolyte properties should enable 6C charging for cells with a loading of 4 mAh/cm2 and high energy density.

Figure 1