Impact of Graphite Materials on the Lifetime of NMC811/Graphite Pouch Cells: Part II. Long-Term Cycling, Stack Pressure Growth, Isothermal Microcalorimetry, and Lifetime Projection

Abstract

Part II of this 2-part series examines the impact of competitive graphite materials on NMC811 pouch cell performance using Ultra-High Precision Coulometry (UHPC), isothermal microcalorimetry, and in-situ stack growth. A simple lifetime projection of the best NMC811/graphite cells as a function of operating temperature is made. We show that graphite choice greatly impacts fractional fade, while fractional charge endpoint capacity slippage was largely unchanged due to identical cathodes. We show that an increase in graphite 1st cycle efficiency due to limited redox-active sites—favourable for minimizing Li inventory loss—is concomitant with an increase in negative electrode charge transfer resistance. Further, we demonstrate that cells with competitive artificial graphites (AG) have a lower parasitic heat flow (∼0.060 mW A−1 h−1 at 40 oC) compared to cells with natural graphites (NG), and that the cells with the AGs had minimal stack thickness change with cycling. Finally, we model SEI growth for NMC811 cells limited to 4.06 V with the square-root time model, and project that the best NMC811/graphite cells here can have decades-long lifetimes at 20 –30 °C when Li plating is avoided. Such cells are excellent candidates for grid storage applications where energy density is less important compared to long lifetime.