polycrystalline vs monocrystalline(1)

High nickel material polycrystalline vs monocrystalline - why monocrystalline better

Based on ternary lithium battery cathode material and top 5 cathode ternary material companies technology are continuously upgraded, monocrystalline LiNi1-x-yCoxMnyO2 (NCM, 1-x-y ≥ 0.6) cathode materials are gaining more and more attention. Because of their better structural stability compared to polycrystalline vs monocrystalline based cells, they have a higher cycle life. However, in-depth understanding of the less obvious degradation mechanisms of single-crystal NCMs is still lacking.

However, in-depth understanding of the less obvious degradation mechanisms of single-crystal NCMs is still lacking. Pouch cells with single- and polycrystalline LiNi0.60Co0.20Mn0.20O2 (NCM622) cathodes were compared after 1375 discharge/charge cycles on the graphite anode.

The specific introduction of polycrystalline vs monocrystalline

The thickness of the cationic disordered layer formed in the near-surface region of cathode particles did not differ significantly between polycrystalline vs monocrystalline particles, while cracks were evident in polycrystalline particles but almost absent in monocrystalline particles.

Quantification of transition metal dissolution on the surface of the cycled graphite anode by time-of-flight mass spectrometry showed that the amount of dissolution of single-crystal NCM622 was greatly reduced. Likewise, quantified by electrochemical mass spectrometry, the carbon dioxide gas evolution of single-crystal NCM622 is also greatly reduced in the first two cycles.

Benefiting from these advantages, the graphite/single crystal NMC622 pouch cell has a cathode areal capacity of 6 mAh cm-2 and a capacity retention rate of 83% after 3000 cycles to 4.2 V. The comparison of polycrystalline vs monocrystalline underscores the potential of monocrystalline NCM622 as a cathode material for next-generation Li-ion batteries.

Study on PC-NCM622 and SC-NCM622 electrodes

Morphological and structural characteristics of PC-NCM622 and SC-NCM622

Figure 1 – Morphological and structural characteristics of PC-NCM622 and SC-NCM622.

Galvanostatic cycling of 1C1D pouch cells

Figure 2 – Galvanostatic cycling of 1C/1D pouch cells at 25°C.

SC-NCM622 electrodes

Figure 3-a,b) Ion-milled cross-sectional SEM images of SC-NCM622 and e,f) PC-NCM622 electrodes. HAADF-STEM images from the cycled c, d) PC-NCM622 and g, h) SC-NCM622 electrodes. i) Schematic illustration of the degradation of PC-NCM622 and j) SC-NCM622 particles after long-term cycling.

Figure 3a,b shows the ion-milled cross-sectional SEM images of the PC-NCM622 electrode at two different magnifications, showing severe crack formation, resulting in several fully collapsed secondary particles. Therefore, the formation of intergranular cracks is the main reason for the observed capacity fading and voltage polarization. The new surface is exposed to the electrolyte along the crack, exacerbating and accelerating the degradation.

In addition to the cracks, holes can be seen on flakes extracted with a focused ion beam from the cycled PC-NCM622 electrode in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image in Fig. 3c.

Interestingly, these cavities are located inside the primary particles, possibly created by the dissolution of transition metals. Particle crushing dominates the voltage polarization in PC-NCM622.

Figure 3 i,j show the differences in the degradation mechanisms of PC-NCM622 and SC-NCM622 particles. After prolonged cycling, severe crack formation resulted in pulverization of PC-NCM622 particles, while only some microcracks appeared in SC-NCM622 particles.

Furthermore, the cationic disordered layer (indicated in purple) is only formed on the surface of the SC-NCM622 particles, but penetrates into the interior of the PC-NCM622 particles along the intergranular cracks entering the electrolyte.

TOF-SIMS chemical map of surface cycling of graphite anodes

Figure 4 – TOF-SIMS chemical map of surface cycling of graphite anodes.

The spatially integrated TOF-SIMS signal intensities extracted from the TOF-SIMS chemical map indicated that SC-NCM622 had significantly reduced transition metal dissolution (Fig. 4c,d). The dissolution of transition metals is responsible for the formation of cationic disordered surface layers on NCM particles.

Furthermore, the addition of Ni, Co, and Mn species to the SEI grown on the surface of the graphitic anode increases the electronic conductivity of the SEI, triggering the continuous growth of the SEI and the conversion of active Li to dead Li, thereby reducing the reversible capacity of the battery.

On-line electrochemical mass spectrometer tracking CO2 and O2 gas evolution of PC-NCM622 and SC-NCM622 during the first two cycles

Figure 5—On-line electrochemical mass spectrometer (OEMS) tracking CO2 and O2 gas evolution of PC-NCM622 and SC-NCM622 during the first two cycles.

On-line electrochemical mass spectrometry (OEMS) to quantify the gas evolution of carbon dioxide and oxygen in PC-NCM622 and SC-NCM622 during the first two cycles. The peak instantaneous carbon dioxide gas release rate of the PC-NCM622 electrode is 1.6 times that of the SC-NCM622 electrode, and the combined amount is about 3 times that of the SC-NCM622 electrode. Interestingly, in the second cycle, SC-NCM622 did not detect CO2 gas release, while PC-NCM622 still released transient CO2 gas in the second cycle. This difference can be partly attributed to the higher Li2CO3 and LiOH concentrations on the surface of PC-NCM622.

Li2CO3 may be both electrochemically oxidized and chemically decomposed, while LiOH may catalyze the decomposition of ethylene carbonate through a ring-opening process, both of which trigger CO2 release. Compared with the measured amount of SC-NCM622, the total amount of Li2CO3 and LiOH of PC-NCM622 is about 2 times higher.

By analyzing the cross-sectional SEM images in Fig. 5e,f, it can be confirmed that more NCM surfaces are exposed to the electrolyte. In the second cycle, more cracks formed, exposing more of the NCM surface, consistent with the observed CO2 evolution in the second cycle (Fig. 5g,h). In contrast, SC-NCM622 did not release any carbon dioxide in the second cycle because no additional NCM surface was exposed to the electrolyte (Fig. 5i,j).

Density versus applied pressure

Figure 6-a) Density versus applied pressure, and b-e) cross-sectional SEM images of PC-NCM622 and f-i) SC-NCM622 at different pressures.

Tests show that the density of the SC-NCM622 electrode is slightly higher than that of the PC-NCM622, which is beneficial to the volumetric energy density of the battery. The structure of SC-NCM622 particles can withstand higher pressures than PC-NCM622 particles, and also achieve a higher tap density (3.92 vs 3.68 g cm-3).

Conclusion

1. The 2-4 micron SC-NCM622 particles are more robust to crack formation during cycling than the 10 micron PC-NCM622 particles, making the pouch battery with excellent long-term cycling stability.
2.The thickness of the cationic disordered surface layer of SC-NCM622 and PC-NCM622 is comparable, but for PC-NCM622, the layer also forms along the intergranular cracks.
3.When PC-NCM622 was cycled, the transition metal dissolution detected on the surface of the SEI formed on the graphite was much more pronounced than when SC-NCM622 was cycled.
4.In addition, the voltage polarization of PC-NCM622 increases more rapidly, which is mainly due to particle crushing caused by cracking of PC-NCM622 particles, which is also consistent with the CO evolution observed in the first two cycles.
5.The NCM622 material presented here does not use any dopants or protective layers, which shows that the single crystal method is very effective in mitigating degradation at the cathode level compared to polycrystalline vs monocrystalline.

It is not only the process application of monocrystalline that drives the continuous optimization of ternary cathode materials, but also high nickel and high voltage battery. For more information, please refer to the following related articles.

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