Polycrystalline vs single crystal high nickel - comparison of rate



Lithium-ion batteries are the main energy storage devices for electric vehicles due to their versatility, scalability, and reliability. Among them, the high-nickel LixNi1-y-zCoyMnzO2 (NCM) cathode active material (CAM) has high specific energy, high power and long cycle life, and is becoming the first choice for next-generation lithium ion battery cathode material.


However, compared to polycrystalline vs single crystal, the fracture of polycrystalline (PC) NCM particles and electrolyte permeation lead to side reactions and rapid capacity fading, limiting their applications. On the other hand, single-crystal (SC) NCMs have slower kinetics, which hinder the lithiation process during discharge, thus reducing material utilization and capacity. Compare polycrystalline vs single crystal cathode rate performance, explore the source of performance difference and main influencing factors, and quantify.

1. Evolution of polycrystalline vs single crystal high nickel morphologies

Compared with polycrystalline vs single crystal, the PC particles with high nickel NCM will form microcracks inside during cycling. In contrast, the SC material did not show any large cracks. The original PC NCM is dense and does not have any obvious gaps. However, after charging up to 3.8 V, cracks appeared inside the PC particles, and at 4.2 V, the cracks were more serious. In contrast, no difference was seen before and after charging the SC cathode, but cracks were also observed when charging to higher potentials.

Evolution of PC vs SC high nickel morphologies


The specific surface area of the positive electrode charged to different potentials was determined by BET. No BET area change was observed after charging the SC cathode. Compared with polycrystalline vs single crystal, the adsorption amount of PC cathode increases obviously with SOC. During the first charge, the gap is penetrated by the electrolyte, resulting in a larger surface area.

2. Lithium diffusion coefficient in the NCM

The galvanostatic batch titration technique is commonly used to determine the lithium chemical diffusion coefficient. Both polycrystalline vs single crystal starting material surface areas were determined by BET. During the first charge, the Warburg coefficients of both NCM materials decreased, corresponding to a potential of 3.8 V. Subsequently, the Warburg coefficient continued to decrease in the PC cathode while increasing in the SC cathode. During discharge, the Warburg coefficient of single crystal strictly follows the trend of the first charge, while the Warburg coefficient of polycrystalline is still very low.

At the beginning of the first charge, the diffusion coefficients of the two CAMs behave similarly. The increased concentration of lithium vacancies during delithiation leads to higher diffusion coefficients. Comparing polycrystalline vs single crystal, in SC NCM, the apparent lithium diffusion coefficient peaks close to 0.5, and when charged to higher potentials, PC NCM increases by more than an order of magnitude. Both polycrystalline vs single crystal diffusion coefficients are significantly reduced when 70% Li is removed, as the transition to the H3 crystal structure reduces the interlayer spacing and narrows the Li+ diffusion channels.

Lithium diffusion coefficient in the NCM


Lithium migration is fastest when the concentrations of vacancies and occupied Li+ sites are approximately equal. The difference in apparent lithium diffusion coefficient of PC NCM during charge and discharge may be caused by two factors: crystal structure change or surface area increase. The two CAMs have the same composition and no two-phase region is observed, so their different behaviors cannot be caused by different transport mechanisms within the primary particles. Furthermore, the apparent lithium diffusion coefficients of polycrystalline vs single crystal at the onset of charging are very similar, confirming that the two morphologies share the same physicochemical properties.

3. Charge transfer resistance of NCM

The increased surface area of the PC CAM also leads to a decrease in the charge transfer resistance between the NCM and the electrolyte. Polycrystalline vs single crystal, NCM charge transfer resistance changes at different SOC. At the beginning of the first charge, the two NCM batteries have similar impedance characteristics. The increased resistance of single-crystalline NCM cells can be attributed to NCM surface degradation and electrolyte decomposition at high potentials, but for polycrystalline cathodes, because of the larger surface area, but also because of the shorter intra-particle diffusion length.

Charge transfer resistance of NCM


Therefore, comparing polycrystalline vs single crystal NCM, the lithium diffusion coefficient value of PC NCM is significantly higher than that of SC NCM due to the change of material morphology. The diffusion overpotential can be directly obtained from the lithium diffusion coefficient. Small particles with larger surface area can be obtained at the PC positive electrode after charging. In contrast, liquid electrolyte infiltration does not work in single-crystal NCMs because of their smaller morphological changes.

4. Kinetic limitations of NCM while cycling

① Limitations of crack-free secondary particles

Particle size and surface area directly affect the diffusion overpotential during cycling. For small, unbroken PC NCM particles, the overpotential is comparable to that of single crystals due to the similar surface area and effective particle size of the CAMs. However, the larger secondary particles of PC NCM should exhibit high overpotential and poor specific capacity and rate capability. Therefore, the effect of secondary particle breakage on battery performance was further investigated. PC NCM particles in cells charged to high voltage (HV) are affected by cracks. In contrast, no apparent morphological changes occurred in CAMs charged to low voltage (LV) cells.

Due to the higher overpotential of LV batteries, the capacity is lower than that of HV batteries. Even with the same cycling conditions, the LV battery capacity is increased compared to before. After continuous discharge at constant current, the Coulombic efficiency is 55%, and the HV battery is again better than the LV battery. This strong contrast between polycrystalline vs single crystal NCM reconfirms the positive effect of cracks on the overall performance of LIBs. Stronger kinetic confinement in LV cells reduces capacity, but when discharge ends, most of the lost capacity is recovered.

Compared with the HV battery, the higher overpotential of the LV battery limits the discharge capacity, indicating that the overpotential accumulation occurs when the two batteries are discharged at different SOCs, confirming that the morphology of the CAM has a greater impact on the cycling performance than its composition. High diffusion overpotential limits the galvanostatic cycling performance of NCMs. The low coulombic efficiencies of the two cells are directly related to the slow migration of lithium ions in the NCM.

② Limitations of SC and fractured secondary particles

Cycling to 4.2 V, cracks also formed in the LV cell CAM. Similar to HV batteries, a lower total CE is observed, since LV batteries experience a small irreversible reaction during the first charge to high voltage, which also shows lower total CE in its first cycle. When cycling LV cells in the low potential range, irreversible reactions may still occur, further limiting CE.

Kinetic limitations of NCM while cycling


Then, in laps 5 and 6, the previous tests were repeated for HV and LV cells, demonstrating that irreversible cracking at high voltage results in lower overpotentials. The penetration of the liquid electrolyte into the larger secondary particle gap results in lower overpotentials, which can only be observed on PC CAMs, as SC CAMs do not have obvious cracks. The degradation that occurs on the PC surface is masked by the positive effect of the increased surface area.

5. Conclusion

In this paper, the chemical diffusion of lithium in the high-nickel cathode material NCM811 with different morphologies was analyzed. In addition, our website also organizes top 10 lithium ion battery anode materials companies in China, which will help you learn more about lithium-ion battery materials. Comparing polycrystalline vs single crystal NCM, the lithium transport kinetics of PC NCM secondary particles improved at potentials above 3.8 V, while NCM SC did not. While SC materials may have longer cycling stability, contributing to extended battery life, their inherently low lithium chemical diffusion coefficient is the limiting factor for rate performance. Therefore, the morphology and size optimization of SC CAM will play a decisive role in realizing fast charging and discharging.

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