Therefore, research on the degradation mechanism of the bimodal NCM cathode at a local scale should be conducted. The Raman spectra of various layered cathode materials. a) SEM image of the NCM 523 electrode in cross-section after 400 cycles. SEM images of the (a) SNCM cathode with an electrode density of 3.5 g cm-3 and the (b) LNCM cathode with an electrode density of 3.1 g cm-3.
Raman spectra of the NCM 622 unimodal cathode, (a) the SNCM electrode, and (b) the LNCM uni-modal cathode. Plot of (c) R597/547 value of each uni-modal cathode in clean state (green bar) and cycle efficiency reached about 80% (red bar). a) Raman spectra of the NCM 622 bi-modal cathode after formation (blue line) compared to the three spots, the SNCM region (red line), the LNCM region (yellow line) and the mixed region (green line) after cycling reached about 80%. Pellet density of uni-modal LCO, bimodal NCM and single crystal cathode materials.
Introduction
Fundamental degradation factors of the Li[NixCoyMnz] O2 cathodes
The phenomenon leads to great shrinkage of the a-axis of unit cells, and H3 phase is formed. Consequently, repeated expansion and contraction causes each of the primary particles to move apart, causing secondary particles to collapse. Phase transition of the NCM cathodes can be demonstrated by the differential capacity analysis (dQ/dV).
However, when more lithium ions are de-intercalated from the lattice structure, the Li-deficient H2 phase disappears at x<0.73, leading to significant cell volume change of at least 2% due to the rapid decrease in cell parameters. When the Li/Ni cation disorder is too high after the synthesis process, the lithium diffusivity is lower due to hindrance of the transition metals in the Li plates. Consequently, the transformed layer induces surface overpotential and accelerates capacity loss a) HADDF-STEM image of the pristine NCM622 primary particles.
Therefore, the transition metals are quickly dissolved in the electrolyte, and the interfacial properties of the cathode materials become significantly unstable. Therefore, open-chain EC radicals (ROCO2-Li+) can be generated, which can initiate the anionic polymerization of the EC molecules. Therefore, the organic byproducts with different chain lengths form major components of the outer SEI layer during cycling.
In Figure 13-(b), at the cutoff potential above 4.5 V, defects in the layered structure are continuously generated due to oxygen evolution, which are points to initiate premature crack formation. Meanwhile, as lithium-deficient cells form over the cycles, the variation of the c4-axis lattice parameter also increases.
Surface degradation analysis through Raman Spectroscopy
Therefore, the Raman peak of the TMO6 bonds can be predicted through the single-layer cathode materials. In the works, the deterioration of the specific area in the bimodal cathodes can be measured through the ratio. Through the co-precipitation process, different size ranges of the NCM 622 cathode materials can be synthesized.
Therefore, to understand the degradation mechanism of the bi-modal NCM cathode, investigate the degradation. Uni-modal NCM cathode with high electrode density = 3.5g cm-3 has similar condition with the SNCM region and LNCM region of the bi-modal NCM cathode. Indeed, SEM image of the SNCM electrodes shows extreme particle fractions due to the calendaring process (Figure 22-a).
If each particle in the bimodal cathode undergoes different degradation according to the particle size, the cycle retention of the BNCM cathode may be an intermediate level between the SNCM and LNCM cathodes. A1g peaks of the MnO6 and CoO6 bonds in the NCM 622 cathode are also shown by ex-situ Raman spectra. Likewise, in the case of point 3, all LNCM particles are surrounded under similar conditions.
SEM images (3-a) and time-of-flight secondary ion mass spectra (TOF-SIMS, 3-b) of the spherical SEI layer. In the research, point 3 (LNCM region) of the bi-modal NCM cathode has the same degree of degradation as the uni-modal LNCM cathode.
Objective in the works
Experimental
Preparation of the electrodes: The positive electrode was prepared by a suspension casting process using 3 types of cathode materials (SNCM, LNCM and BNCM), super P (TIMCAL) and 6wt%. The suspension was mixed by a mixer at 1500 rpm for 20 min and cast onto the aluminum foil with the loading level of ~11 mg cm−2. The negative electrode was prepared by a suspension casting process using artificial graphite(BTR), super P(TIMCAL), and 1.5wt% Styrene Butadiene(SBR) solution with a mass ratio of 96:2:2.
The slurry was mixed with a mixer at 2000 rpm for 15 minutes and cast onto the copper foil at a loading level of 6.6 mg/cm 2 . Electrochemical characterization: All half-cell tests were performed with the 2032R type coin cell with the lithium metal as the counter electrode in the Ar-filled glove box. 1M LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (3:4:3, v/v/v) was used as an electrolyte.
The pouch type full cells were designed with the N/P ratio of ∼1.1 using the prepared electrodes with a cathode size of 20 mm x 25 mm and an anode size of 22 mm x 27 mm. The electrochemical performance of the half-cell was performed in the voltage window between 3.0 V and 4.3 V using a battery cycler (WBVS-3000, WonAtech). The formation data was obtained at a charge and discharge rate of 0.1C and 0.1C, and the cycle data was obtained at a rate of 0.5C and 0.5C.
Regarding the full cell, the formation was performed in the voltage window between 3.0 V and 4.2 V at room temperature, and formation data was obtained at a rate of C/10. High-resolution transmission electron microscopy (HR-TEM, ARM300, JEOL) was used to investigate structural breakdown of the cathode materials.
Result and discussion
Physical / electrochemical properties of the bi-modal NCM 622 cathode
In particular, the capacity retention of the entire BNCM cell decreases dramatically during the first 100 cycles, and the capacity decreases stabilize after the 100 cycles. Cycling performance of the unimodal cathodes and bimodal cathode after 250 cycles under room temperature. a) Voltage profile and (b) cycling performance of the complete BNCM cathode/graphite cell at 45°C compared to room temperature. In the case of the SNCM cathode with an electrode density = 3.5 g cm-3, a large film resistance can be observed (Rfilm, 8.6/Ω) and a low charge transfer resistance (Rct, 6.0/Ω).
Corrosion of the cathode materials can increase the specific surface area of the particles, therefore the side reaction between the exposed surface and the electrolyte can intensify during cycling. Furthermore, the temperature at 45°C increases the lithium diffusivity, which accelerates severe cation mixing towards the bulk of the SNCM particles. The phenomena are caused by anisotropic expansion/extraction of the LNCM particles during cycles, which induces the separation of each electrode material.
Consequently, the degree of porosity of the electrode caused the mechanical strength of the LNCM cathode to decrease, which can determine the Rct. Consequently, particle fraction has little influence on electrode degradation in LNCM cathodes, while mechanical strength between electrode materials is highly related to LNCM cathode degradation. Therefore, the SNCM particles can play a buffer role against the external force, so the LNCM particles are rarely damaged, and the Rfilm is also reduced.
However, not only the particle size but also the particle arrangement in a thick electrode can influence the degradation mechanism of the BNCM cathode. Electrochemical impedance spectroscopy (EIS, c,d) of the unimodal cathode after the formation (green dot), and the unimodal cathode with the different cathode densities at 3.1 g cm-3 and 3.5 g cm-3, after the cycle retention achieved at 80% below the high temperature. a) SEM images of the BNCM cathode with an electrode density ≈ 3.5 g cm-3.
Investigation on the local degradation through Raman spectroscopy
Through the peak intensity of the Ni2+O6 and Ni3+O6 compounds, the degradation of the cathode can be demonstrated. As a result, the degradation rate of the cathode can be demonstrated by the Raman shift from 597cm-1 to 547cm-1. Likewise, the a/b axis parameter can also be changed by decreasing the TM ion radius during the charging process.
Moreover, E.g. the tip is insensitive to the variation of the lattice parameters due to the stiffness of the vibrational mode. As a result, measuring the degradation of the NCM cathode through Raman spectroscopy should proceed with the two A1g peaks of NiO6. The phenomena lead to the decrease of the polarization of the NCM cathode, so the 597 cm-1 peak also decreases.
Similarly, the NCM 622 cathodes undergo lithium insertion during the discharge process, which can compensate for the low polarizability of the oxidized Ni3+O6 bonds. As a result, the surface of the NCM cathode becomes a lithium-deficient state, causing the peak generation at 547 cm-1. Therefore, the excellent performance of the bimodal NCM cathode is a result of the suppressed local-scale degradation factor.
The SEM image of the ion-milled bimodal cathode consisted of the three points; point 1 (blue circle) is concentrated in SNCM particles, point 3 (green circle) is concentrated in LNCM particles, and point 2 is a well-mixed region with the SNCM and LNCM particles. The comparison of the R597/547 value can demonstrate the relative cathode degradation at local locations. After a forming stage, the R597/547 values are similar to 1.45 regardless of the points, which is the higher R597/547 value than the unimodal cathodes due to the lower irreversible capacitance.
Meanwhile, the mechanical strength of the LNCM particles in the bimodal cathode can be maintained by the movement of the SNCM particles because the SNCM particles act as a buffer against the external force.
Overall cell degradation mechanism derived from the cathode structural collapse
Conclusion
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