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 various 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. We show that an increase in 1st cycle efficiency due to limited redox-active sites, which is 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/Ahr at 40oC) compared to cells with natural graphites (NG), and that the cells with the AG materials 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 can have a century of lifetime at 15 oC when Li plating during charge is avoided. Such cells are an excellent candidate for grid storage applications where energy density is less important compared to long lifetime.

Effect of Annealing on the Structure, Composition, and Electrochemistry of NMC811 Coated with Al2O3 Using an Alkoxide Precursor

“…Aluminum oxides have three possible coordination environments for the aluminum atom [4-, 5-, and 6-coordinate sites ( [4] Al, [5] Al, and [6] Al)] that depend on the number of oxygen atoms coordinating to the aluminum. These environments give rise to distinct resonances with isotropic chemical shifts of approximately δ iso ( [4] Al) = 60 ppm, δ iso ( [5] Al) = 30 ppm, and δ iso ( [6] Al) = 0 ppm, 57 allowing the relative concentration of each site to be determined. No difference in signal intensity in the diamagnetic region is seen on going from an annealing temperature of 200 to 400 °C, but changes in relative intensity at 600 °C are seen and very little signal from diamagnetic species remains by 800 °C, again confirming that the diffusion process starts at temperatures higher than 400 °C.…”

“…Figure 6 shows the central (isotropic) resonances of 27 Al SS-NMR spectra measured on a 1 GHz spectrometer, performed at the higher magnetic field (23.5 T) to reduce the secondorder quadrupolar interaction and increase the signal-to-noise ratio. The three peaks corresponding to the [4] Al, [5] Al, and [6] Al environments nearby diamagnetic cations can be identified. There is a significant amount of aluminum in a five-coordinate environment in the 200 and 400 °C samples, which proves that the coating is amorphous, since no crystalline phase of aluminum oxide contains this local environment.…”

“…The Czjzek model accounts for the distribution of quadrupolar interactions present in amorphous alumina due to its disordered structure; it describes the distribution of local environments and thus quadrupolar interactions, leading to characteristic NMR signals with tails toward lower ppm values. 47 The percentages and absolute integrated intensities of the observed signals assigned to the [4] Al, [5] Al, and [6] Al sites are derived directly from the fit and shown in Figure 7. An increase in [4] Al and [5] Al and a decrease in the [6] Al signals are observed going from 200 to 400 °C.…”

“…4,5 Accordingly, excellent lifetimes are achievable in NMC811/graphite cells operated under more moderate conditions (e.g., 3.00−4.06 V and 20−30 °C). 6 Improvements in battery lifetime can also be attained by modifying the NMCs through surface coatings 7−15 or by using electrolyte additives. 16−18 Even with these strategies, it is not yet clear whether it is possible to obtain satisfactory performance at temperatures above 25 °C or with increased voltage windows for practical applications.…”

Erdite NaFeS2 as a New Anode Material for Lithium-Ion Batteries

“…As one of the key components in lithium-ion batteries (LIBs), the anode greatly affects the performance of the whole battery. − At present, the anode materials used in most LIBs are graphite in various forms because of their good conductivity, high reversibility, low cost, and suitable working voltage. , However, their theoretical specific capacity is rather low (372 mAh·g –1 ), which drags down the energy density and power density of the batteries. Many new kinds of anode materials with high theoretical capacity have been developed, such as alloy type (Si, Sn, P, Ge, etc.…”