Research on the Electrochemical Performance of Single Crystal and Polycrystalline Ultra High Nickel!

Ultra high nickel layered oxide (NCM) is considered one of the most promising cathode materials for the new generation of high-energy lithium-ion batteries due to its high discharge specific capacity. However, the NCM cathode undergoes adverse structural changes, severe surface side reactions, and lattice oxygen loss during electrochemical cycling, resulting in rapid capacity decay after high-voltage cycling. The irreversible migration of nickel ions leads to the transformation of layered structures into spinel and rock salt structures. Secondly, the strong oxidizing substances on the surface of high nickel materials (such as generated reactive oxygen species, oxygen vacancies, and high valent Ni ions) promote side reactions during the cycling process, leading to an increase in interface resistance and internal capacity decay. Meanwhile, O ions may escape from within the particles, leading to oxygen loss; Cation mixing (Li+/Ni2+mixing) has been a concern of researchers for many years. Due to the similar ionic radii of Ni2+and Li+, Ni2+ions are prone to occupying Li+sites during synthesis and electrochemical processes, which seriously hinder the commercialization of NCM.
In recent years, single crystal NCM materials (S-NCM) have been introduced as an alternative strategy to improve cycling stability. Compared with polycrystalline NCM (P-NCM) particles, single crystal particles have unidirectional shrinkage/expansion properties, which is the main advantage in reducing crack formation. S-NCM particles can be co precipitated with polycrystalline Ni0.9Co0.05Mn0.05 (OH) 2 hydroxide precursor particles, and then calcined with LiOH · H2O at a sintering temperature slightly higher than P-NCM. To avoid excessive particle aggregation, S-NCM particles are usually smaller in size than PNCM particles. Due to limited/free grain boundaries, smaller particles are less likely to form cracks.
Thanks to its unique morphology, S-NCM outperforms polycrystalline materials with the same composition in several aspects. In terms of long-term cycling stability, there are no microcracks on the surface or inside of S-NCM particles. In terms of high voltage performance, there are few side reactions at the electrode/electrolyte interface, high reversibility of reactions, and good capacity maintenance. In addition, SNCM has shown good compaction density and high volume energy density in practical applications. The role of particle size in electrochemical reactions is crucial, as the smaller the particle size, the better the mechanical integrity and kinetics during the charge discharge cycle. However, due to harmful phase transitions, the impact of surface degradation will also increase. The above introduction does not demonstrate a systematic study of the degradation/improvement mechanism of S-NCM, nor does it determine the mechanism of S-NCM crack generation.
To address the aforementioned issues, this study synthesized single crystal and polycrystalline cathode materials, and systematically investigated the effects of single crystal and polycrystalline materials on the structure, morphology, and electrochemical performance of cathode materials.

1 Experiment
1.1 Sample preparation
The preparation process of single crystal and polycrystalline cathode materials is as follows. Single crystal and polycrystalline precursors were synthesized using hydroxide co precipitation method. The obtained precursor was thoroughly mixed with an excess of 6% LiOH · H2O. Under an oxygen atmosphere, the single crystal precursor was calcined at 500 ℃ for 10 hours, followed by calcination at 830 ℃ for 15 hours to obtain the single crystal cathode material S-NCM. The production process of polycrystalline materials is basically the same as that of single crystals, with only slight differences in sintering temperature. The first sintering temperature is 400 ℃ and calcined for 10 hours, while the second sintering temperature is 780 ℃ and calcined for 15 hours to obtain polycrystalline cathode material P-NCM.
1.2 Sample characterization
Study the morphology of single crystal and polycrystalline cathode materials using field emission scanning electron microscopy and transmission electron microscopy. Analyze the crystal structure of single crystal and polycrystalline materials using an X-ray diffractometer.
1.3 Electrochemical performance testing
A button type battery was prepared using lithium metal as the negative electrode, single crystal and polycrystalline as the positive electrode, separator, and electrolyte, respectively. The battery is activated by charging and discharging at 0.1C, and the current is increased to 0.5C to measure the cycling performance. Characterize the impedance and diffusion coefficient of single crystal and polycrystalline cathodes using electrochemical impedance spectroscopy (EIS).
2 Results and Analysis
2.1 Material morphology and phase characterization
Figure 1 (a) shows the scanning electron microscopy image of polycrystalline P-NCM particles with a particle size of approximately 10 μ m. The SEM images of the material confirm that it is in a spherical particle shape with no obvious pores. The primary particles are arranged uniformly, with uniform size and morphology. The relatively wide particle size distribution of the secondary particles can maximize the particle density in the electrode. Figure 1 (b) shows the SEM image of S-NCM particles, which have a polyhedral morphology and a relatively narrow particle size distribution. The overall structure is a 2-4 μ m single crystal with no particle aggregation.



The X-ray diffraction spectra and their refinement results of S-NCM and P-NCM are shown in Figure 2 and Table 1. It can be seen from the reflection intensity ratios of (003) and (104) both exceeding 1.5 that the Li/Ni mixing degree of the materials is low, the internal structure is complete, and both materials exhibit ordered layered structures. In addition, the clear splitting of reflections from (006) and (012), as well as reflections from (018) and (110), further confirms that the layered structure of α - NaFeO2 is ordered [Figure 2 (b-c)].




2.2 Electrochemical performance testing
During high voltage cycling, transition metal dissolution and cycling degradation may occur in ultra-high nickel cathode materials. A 2032 type button cell was used to measure the functionality of the single crystal structure at a high cut-off voltage of 4.6V.
After activation at 0.1C (1C set to 200mA/g), P-NCM had a specific capacity of only 55mAh/g and a capacity retention rate of only 28.4% of its initial value of 193.5mAh/g after 500 cycles. The polycrystalline cathode suffered from severe capacity decay and structural evolution; Under the same testing conditions, the reversible specific capacity of the S-NCM positive electrode is 107.5mAh/g, and the capacity retention and discharge specific capacity are about twice that of PNCM [Figure 3 (a)]. At the same time, the corresponding discharge medium voltage shows that the S-NCM positive electrode maintains well above 3.4V, significantly higher than the 3.0V of P-NCM [Figure 3 (b)]; After 500 cycles, the energy density of the S-NCM positive electrode is about twice that of P-NCM, indicating that the material has a higher capacity retention efficiency [Figure 3 (c)].

                                                Figure 3 Performance curves of S-NCM and P-NCM at 2.8-4.6V
In order to explore the reasons for improving electrochemical performance, in-depth exploration was conducted on the electrochemical data, and the corresponding charge discharge curves are shown in Figure 4 (a) and (b). It is evident that the low polarization and increased overpotential of the S-NCM positive electrode are the reasons for maintaining long-term stability. At the same time, the coincidence of the charge discharge curves is good, and the corresponding Coulombic efficiency is relatively high. During long-term cycling, the discharge capacity and retention rate are significantly higher than those of P-NCM. The corresponding dQ/dV curve clearly indicates that the first phase transition (H1 → M) has a stronger and clearer peak, indicating that there are more Li ions in the S-NCM cathode during the H1 → M phase transition stage. At the same time, the second phase transition of M → H2 and the final phase transition of H2 → H3 transfer to a slightly higher voltage, with lower peak intensity. The phase transition of H2 → H3 is always suppressed, and under high charge conditions, the H2 − H3 phase transition causes anisotropic volume changes, which will lead to the formation of microcracks inside the particles. The inhibition of the H2 − H3 phase transition by S-NCM is beneficial for improving electrochemical performance. Meanwhile, as the cycle progresses, the oxidation-reduction peak of the P-NCM positive electrode gradually disappears, the layered structure is destroyed, and the corresponding Li ion transport rate deteriorates.

                                             Figure 4: Charge discharge curves and dQ/dV curves of P-NCM and S-NCM
In order to better understand electrochemical behavior, electrochemical impedance spectroscopy (EIS) was used to compare the electrochemical kinetics at the interface between the positive electrode and electrolyte after 500 cycles in the voltage range of 2.8~4.6V. All curves are composed of a semicircle and a diagonal line. Due to its superior rate performance (short diffusion path) or low resistance of lithium ions through the surface layer, the composite resistance of the semicircle after cycling, including charge transfer resistance (Rct and RSEI film resistance), is related. The tilted line in the low-frequency region represents the Warburg impedance (W), corresponding to the equivalent circuit of the lithium ion dynamics EIS results in the NCM structure. The composite resistance Rer values of P-NCM and S-NCM after 500 cycles are 110 and 55 Ω, respectively. Correspondingly, the internal resistance of P-NCM increases, which is significantly unfavorable for the rapid transmission of lithium ions (see Figure 5).


At the same time, this result was validated from the corresponding Li+ion diffusion coefficients. The diffusion coefficients of P-NCM and S-NCM after cycling are shown in Table 2. This result indicates that the single crystal structure can suppress unnecessary side reactions at the interface, thereby ensuring the long-term cycling stability of the battery. The layered structure is also maintained during the cycling process.


2.3 Material characterization after cycling
In order to further understand the relationship between crack development and surface degradation, SEM and TEM were used to analyze the differences in morphology and structural evolution of single crystal and polycrystalline cathode materials after cycling under high pressure. From the SEM image in Figure 6 (a), it can be seen that there are deep cracks on the surface of the P-NCM material, and the primary particles are broken into many small particles, which will directly cause the electrolyte to penetrate into the interior of the primary particles and damage the layered structure of the material. However, the S-NCM electrode in Figure 6 (b) has almost no such structural deformation, and the intergranular nano/micro cracks of the electrode are difficult to detect on the outer surface. The micron level single crystal structure has been fully preserved.


Meanwhile, the rock salt NiO phase can be clearly detected on the surface of P-NCM material, which diffuses along the particle surface towards the interior of the material with a thickness of 50nm, as shown in the HRTEM image in Figure 7 (a). This phenomenon should be attributed to the adverse electrode/electrolyte side reaction caused by the infiltration of the electrolyte, resulting in an irreversible phase transition from R3m layered structure to Fm3 rock salt structure, as shown in Figure 7 (b~c). On the contrary, HRTEM results showed that only a thin layer of disordered phase appeared on the surface of S-NCM material, as shown in Figure 7 (d). The internal structure basically maintained an R3m layered structure, with a corresponding lattice spacing of 0.206nm, corresponding to the (104) crystal plane of S-NCM. This phenomenon indicates that the single crystal structure has a unique advantage in stabilizing the layered structure of the material, as shown in Figure 7 (e~f).


3 Conclusion
In summary, single crystal and polycrystalline precursors were calcined to obtain positive electrode materials. Compared with P-NCM, S-NCM exhibits excellent cycling stability. The initial discharge specific capacity of P-NCM at 0.1C in the voltage range of 2.8~4.6V is 223.2mAh/g. After 500 cycles at 0.5C, the capacity retention rate is about twice that of S-NCM. For P-NCM, due to the volume anisotropy change accompanied by local stress generation, micro scale intergranular cracks are prone to form within the secondary particles. In addition, the appearance of nanoscale cracks will exacerbate the infiltration of electrolyte within a single particle, especially under high pressure. Intense electrode/electrolyte side reactions lead to irreversible transition from layered structure to rock salt structure, exacerbating structural collapse. The rock salt phase hinders the diffusion of lithium ions, ultimately accelerating the degradation of lithium ion capacity. On the contrary, the micrometer scale single crystal structure strengthens mechanical connections, effectively preventing the formation of intergranular nano/micro cracks and reducing the formation of disordered rock salt phases. Therefore, the quasi single crystal structure strengthens the inherent microstructure of the material, significantly enhances cycling stability, and prevents unnecessary phase transitions. This work provides ideas for the development of single crystal cathode materials for lithium-ion batteries.

Created on:2024-06-05 14:19
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