An Analysis of Artificial and Natural Graphite in Lithium Ion Pouch Cells Using Ultra-High Precision Coulometry, Isothermal Microcalorimetry, Gas Evolution, Long Term Cycling and Pressure Measurements

Abstract

Natural graphite (NG) negative electrode materials can perform poorly compared to synthetic, or artificial, graphite (AG) negative electrodes in certain lithium ion cells. LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532)/(AG or NG) pouch cells were tested with various loadings of an electrolyte additive blend to study the effect of the graphite type as well as the formed solid electrolyte interphase (SEI). Cells underwent testing using ultra-high precision coulometry, isothermal microcalorimetry, in-situ pressure measurements, long term cycling and in-situ gas measurements. In short term experiments NMC532/AG and NMC532/NG cells showed similar coulombic efficiencies, parasitic heat flows, and gas production with large electrolyte additive loadings, but NG cells showed worse capacity retention in long-term tests. With low additive loadings NMC532/NG cells showed lower coulombic efficiency, higher capacity fade, more parasitic heat flow, and more gas production. In-situ cell stack pressure measurements showed that NMC532/NG cells irreversibly expanded during cycling while NMC532/AG cells did not. Although these results lead one to propose a simple model for the poor performance of NMC532/NG cells, NMC622/NG and NMC622/AG cells showed very different behavior in long term tests suggesting that positive/negative interactions play a strong role in governing the behavior of graphites in Li-ion cells. Next generation lithium ion batteries require higher energy density, longer life, better safety, and lower cost to fulfill the ever-increasing demand for electric vehicles and renewable grid-level energy storage. By increasing the energy density of cells while keeping lifetime consistent, one can in turn decrease the cost of Li-ion cells. Much work has been focused on increasing the upper cutoff voltage of cells in order to achieve this increase in energy density. However, increasing the upper cutoff potential increases the rates of unwanted reactions in cells which can compromise lifetime.1-3 These unwanted reactions are commonly termed parasitic reactions.Another way of addressing the issue of cost is to use higher energy density materials, such as natural graphite (NG) as a negative electrode material instead of synthesized graphite, here called artificial graphite (AG). NG is known to perform poorly in some cells, which has in the past been attributed to surface exfoliation and cracking of particles.4-7 Park et al. found spherical natural graphite showed signs of particle swelling and cracking caused by mechanical strain during cycling, which could be suppressed using a carbon coating process. 5Carbon coatings on natural graphite negative electrodes have been studied in the past to avoid exfoliation from propylene carbonatecontaining electrolytes, but these coatings may decrease the energy density.4,6 AG performance reported in the literature appears to outperform natural graphite, however, few direct comparisons of artificial and natural graphite exist in the literature. Lee et al. 8 found that plasma treated AG performed better i...

High-Precision Coulometry Studies of the Impact of Temperature and Time on SEI Formation in Li-Ion Cells

“…The ranks obtained after the two formation protocols (by reading the graph left-right and down-up) are not identical, but they are very similar. The same well-established conclusions can be made 10 cycles after the preferred protocol (rank the red points bottom to top) and 20 cycles after the standard formation (rank the points left to right): i) cells with single crystal NMC532 positive electrodes and artificial graphite negative electrodes are better than all those with polycrystalline NMC622 positive electrodes and natural graphite negative electrodes; 35 ii) cells with the ternary additive blend PES211 outperform cells with 2% VC; 29 iii) within cells of the same chemistry, the CIE increases with increasing cycling voltage; 29 iv) in cells with PES211, the Al 2 O 3 -coated positive electrodes outperform those with the proprietary "high voltage" coating. 36 These results suggest that a UHPC experiment, to rank new cells with different chemistries, can be shortened to 10 cycles, if the preferred formation protocol were used.…”

Ultrasonic Scanning to Observe Wetting and “Unwetting” in Li-Ion Pouch Cells

“…Figure 7E shows that the volume change (before degassing), thickness change, and capacity loss (of the check-up cycles) for the 100% DOD cell are relatively large after 2.5 years testing at 40 C. Interestingly, the volume change of the cell tested under the 100% DOD condition is exactly equal to that predicted based on the thickness change and the area of the jelly roll ( Figure 7E). This suggests that the volume change comes about completely due to swelling of the electrode stack, most likely due to swelling of the natural graphite negative electrode as observed by Glazier et al 26 Figure 7E shows the volume of the electrode stack has increased by about 0.25 mL, and Figure 3 clearly shows that a shortage of 0.25 mL of electrolyte is sufficient to cause poor ultrasonic transmission. Therefore, we believe that Figures 7D and 7E indicate that the cell tested under 100% DOD conditions is ''unwetting,'' that is, there is no longer enough electrolyte to fill the increased pore space.…”

“…were examined. As shown by Glazier et al, 26 the natural graphite negative electrodes used in these cells show more swelling during cycling than the artificial graphite used in the cells described by Figures 5 and 6. Figure 7A shows the capacity versus time for the two cells tested at 40 C. One cell was tested at 100% depth of discharge (DOD) between 3.0 and 4.1 V at C/3, while the other was tested between 3.77 and 4.1 V (25% DOD) at C/10.…”

Comprehensive Aging Analysis of Volumetric Constrained Lithium-Ion Pouch Cells with High Concentration Silicon-Alloy Anodes

“…Over the past few years, only a handful of publications have dealt with the measurement and effect of stress induced by (de-)lithiation in various electrodes [3,[42][43][44][45][46][47][48]. A few researchers have successfully used stress measurements on pouch cells of various conventional chemistries for SoC [46,47] and SoH [45,46,48] estimations. However, to the author's best knowledge, only one publication briefly mentions the evolution of pressure over the lifetime of pouch cells with silicon-based anodes (SiO-graphite, Si-alloy and nano-Si:carbon) [3].…”