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Article

Controllable Synthesis and Surface Modifications of a Metastable O2-Type Li-Rich Cathode Material

by
Yiming Sun
1,
Junjie Huang
2,
Hongzhou Zhang
2,*,
Lianqi Zhang
2,* and
Defa Wang
1,*
1
School of Materials Science and Engineering, Tianjin University, Tianjin 300350, China
2
School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(8), 1154; https://doi.org/10.3390/cryst13081154
Submission received: 30 June 2023 / Revised: 15 July 2023 / Accepted: 18 July 2023 / Published: 25 July 2023
(This article belongs to the Special Issue Advanced Energy Storage and Conversion Nanomaterials)
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Abstract

:
Li-rich materials have become one of the most promising cathode candidates for next-generation lithium-ion battery systems due to their high capacity and operating voltage. Conventional O3-type Li-rich materials undergo a structural transition from a layered to a spinel phase during cycling, leading to the degradation in their electrochemical performance, especially in terms of their voltage decay. The oxygen atoms comprising the structure of O2-type Li-rich materials are stacked in the ABAC configuration, which can effectively suppress these harmful phase transitions. However, O2-type Li-rich materials are metastable structures and can only be synthesized via the means of complex ion exchange methods. In addition, the surface of the material is susceptible to side reactions with the electrolyte when charged to high voltages. Here, we explored the optimal conditions for the synthesis of O2-type Li[Li0.25Ni0.1Co0.05Mn0.6]O2 (LLNCM) in more detail by preparing the precursors using the sol-gel method. Meanwhile, the modification of the material’s surface via low-temperature hydrolysis of aluminum isopropoxide has been proposed for the first time in this study to avoid the damage of metastable materials by the high-temperature coating process. The surface-modified materials prepared under optimal conditions exhibited an excellent electrochemical performance, indicating that a highly stable O2-type bulk phase structure with effective surface modification is a potential way to promote the commercial applications of Li-rich cathode materials.

1. Introduction

The capacity of the traditional ternary cathode materials (NCM, NCA etc.) for electric vehicles is approaching its theoretical limit, thereby making it difficult to meet the demand for a higher range in the future [1,2,3,4,5]. Li-rich materials have a much higher capacity than traditional ternary materials due to their special crystal structure and anionic redox properties [6,7,8,9,10]. The Li-rich cathode material xLi2MnO3-(1 − x)LiMO2 (M = transition metal) consists of LiMO2 with a R 3 ¯ m space group and Li2MnO3 with a C2/m space group, respectively. Due to the partial substitution of the Mn elements with Li in the Li2MnO3 transition metal layer, Li2MnO3 can also be written as Li[Li1/3Mn2/3]O2 [11,12,13]. Since the (001) crystal plane spacing in Li2MnO3 is similar to that of (003) in LiMO2 (~0.47 nm), the Li2MnO3 and LiMO2 phases can achieve a mutual compounding at the atomic level [14,15].
When the Li-rich material is charged to below 4.4 V, the lithium ions are taken off from the LiMO2 phase, accompanied with the occurrence of the transition metal oxidation process [16,17,18,19]. Subsequently, upon charging up to 4.5 V, the Li2MnO3 phase starts to be activated and an anion oxidation process takes place, resulting in the generation of Li2O along with the removal of lithium ions [20,21,22,23]. However, the redox of anions in the bulk phase is coupled with the migration of the transition metals. The arrangement of the oxygen atoms in this structure is cubic dense stacking (ABCABC), which is the same as the arrangement of oxygen in the spinel structure [24]. Following the removal of the lithium ions, the transition metal ions are subsequently prone to migrate to the lithium layer and form vacancies in the original position after occupying the lithium vacancies, irreversibly forming a spinel phase and causing voltage decay as a result. In recent years, a more stable O2-type structure different from the conventional O3 structure mentioned above has been proposed. In this nomenclature, O represents the chemical environment of the Li-O octahedra where the oxygen atoms are located, while 2 represents the minimum number of transition metal layers that can be contained in one cell [25,26]. In the O2-type structure, the stacking of oxygen is ABAC instead of ABCABC, and the Li-O octahedra are in face-to-face contact with the Mn-O octahedra, while the Li-O octahedra are in edge-to-edge contact with the Mn-O octahedra in the O3 structure, respectively [27,28,29]. The face-to-face contact raises the energy barrier for transition metal migration during the anion redox process, thereby making it difficult for Mn atoms to migrate to the Li vacancies. This effectively avoids the phase transition due to the transition metal migration, and thus suppresses the voltage decay. However, since the structure of O2 is metastable in nature, the current mainstream method is obtained by first preparing Na-based P2-type precursors followed by ion exchange [30]. This process is highly tedious and requires careful control of the raw material ratios and preparation conditions. There are only a few reports that are dedicated to the preparation process, making the repeated preparation of O2-type Li-rich materials difficult and is a major constraint to the development of such materials [31,32]. Meanwhile, as Li-rich materials operate at high voltages, the surface of the material inevitably reacts with the electrolyte, causing a phase change on the surface of the material while causing the decomposition of the electrolyte. Since the O2 structure is metastable and prepared after quenching, conventional high-temperature coating methods could readily affect the bulk phase after annealing. Past coating methods typically required high temperatures of 500–700 °C [33]. Furthermore, conventional Al2O3 cladding also typically involves aluminum nitrate as a precursor, which normally requires at least 500 °C to decompose it completely [34].
In this study, an O2-type Li-rich material with low cobalt content Li[Li0.25Ni0.1Co0.05Mn0.6]O2 (LLNCM) was designed and the preparation process was assessed in further detail. The calcination temperature and Li/Na ratio control of the precursor preparation process was analyzed with emphasis. Subsequently, the hydrolytic properties of aluminum isopropoxide were utilized for the low-temperature coating of the material, and the optimal operating temperature was then discussed.

2. Experimental Section

2.1. Material Preparation

2.1.1. Preparation of the Precursors Nax[Li0.25Ni0.1Co0.05Mn0.6]O2 (NLNCM) via the Sol-Gel Method

NLNCM was prepared using the sol-gel method. Stoichiometric Ni(CH3COO)2 (0.024 mol), Mn(CH3COO)2 (0.144 mol), Co(CH3COO)2 (0.012 mol), Na2CO3 (0.1 mol), and citric acid (0.1 mol) were dissolved in 50 mL of deionized water with continuous stirring until they were completely dissolved. The mixed solution was transferred to a muffle furnace and heated at 120 °C for 10 h to obtain a fluffy dry gel and was then continued to be heated to 480 °C for 6 h to fully remove the organic matter. The product obtained after pretreatment was mixed with Li2CO3 powder and ground, and then calcined again in the muffle furnace at 700–800 °C for 6 h to obtain the P2-type Na-based precursor NLNCM.

2.1.2. Preparation of the O2-Type Li-Rich Cathode Material LLNCM via the Molten Salt Ion Exchange Method

The above obtained precursor NLNCM was transferred into a mortar and the mixed lithium salt of LiNO3 and LiCl was added in a 1:1 molar ratio, where LiNO3:LiCl = 88:12 (w/w). The precursor and lithium salt were fully ground, mixed, and then transferred into a muffle furnace for the molten salt-ion exchange process. The temperature was increased to 280 °C at a heating rate of 2 °C/min and held for 4 h. Immediately after the ion exchange process, the material was removed and quenched in deionized water. After that, the material was filtered and washed with deionized water and alcohol, respectively, in turn. Finally, the products were transferred to a drying oven at 100 °C for 12 h to finally obtain the O2-type Li-rich cathode material LLNCM.

2.1.3. Al2O3 Coating of the O2-Type Li-Rich Cathode Material LLNCM

The Al2O3 coating on the as-synthesized LLNCM was carried out using a wet chemical method with aluminum isopropoxide. The mixture of LLNCM powder and aluminum isopropoxide (with the molar ratio of aluminum in the mixture being 1%) was stirred in absolute ethyl alcohol until the solution transformed into a milky white suspension. Then the suspension was then heated and stirred at 90 °C until all the ethanol completely evaporated. The obtained product was calcined at 250–350 °C for 4 h under oxygen conditions to obtain the Al2O3 coating layer. The obtained product Li[Li0.25Ni0.1Co0.05Mn0.6]O2@Al2O3 was subsequently recorded as LLNCM@ Al2O3.

2.2. Materials Characterization

The crystal structure of the as-prepared product was examined using X-ray diffraction with Cu Kα radiation (XRD, D/MAX-2500 Rigaku, Tokyo, Japan) between 2θ = 10–80° at a scan rate of 1° min−1. Micromorphological characterizations of the product were observed on a scanning electron microscope (SEM, MERLIN Compact, ZEISS, Oberkochen, Germany). TEM images were acquired on a transmission electron microscope (TEM, Tecnai G2, FEI, Hillsboro, OR, USA). The XPS test was used to test and analyze the valence state of the elements present on the surface of the sample (ESCALAB250Xi, THERMO SCIENTIFIC, Waltham, MA, USA).

2.3. Electrochemical Measurements

The LLNCM and LLNCM@Al2O3 electrochemical performance were assessed using 2032-type coin cells with metallic lithium foil as the anode. The cathode electrode was fabricated with 80 wt.% cathode material, 10 wt.% acetylene black, and 10 wt.% polyvinylidene difluoride (PVDF) binder on an Al foil. The assembled battery was then assessed for its electrochemical performance in the voltage range of 2.0–4.8 V at 25 °C.

3. Results and Discussion

Metastable O2-type materials are obtained via the ion exchange process inheriting the structure of the precursor, meaning that synthesis of the P2-type sodium-based precursors possessing a high purity is a key step. Stable Nax[Li0.25Ni0.1Co0.05Mn0.6]O2 structures are usually sodium-deficient, and the number of sodium defects is more often an empirical value; according to reports, x may be 2/3, 5/6, or others [25]. Here, the exploration began with x = 5/6, which is equivalent to a 10:3 ratio of the Na source to the Li source. In addition, the sintering temperature is usually the primary consideration. The XRD patterns of the precursors obtained by calcination at 700, 750, and 800 °C are shown in Figure 1a.
When sintered at 800 °C, the material displayed a crystal structure that is not identical to that of the reported P2-type sodium precursor [26], demonstrating that an excessive sintering temperature causes the material to undergo a phase transition. Furthermore, at 700 °C calcination, the material exhibited a crystal structure that is closer to the P2-type precursor with the P63/mmc space group. However, the main peak of the material was not obvious and clear, proving that the lower sintering temperature leads to an insufficient crystallinity of the material. At 750 °C, the material exhibited a typical P2-type precursor crystal structure and has a sharp and well-defined peak. However, even at the optimized sintering temperature, impurities that can be attributed to the Li2MnO3 phase were still visible. This may be due to the Li exceeding the limit of the accommodation of the P2-type precursor lattice, leading to the formation of a separate phase. Thus, the effect of different Li/Na dosing ratios on the crystal structure of these materials during the sol-gel process was continued to be explored. Figure 1b shows the XRD patterns of the three materials with different dosage ratios. Compared to the initial Na:Li = 10:3, when the Na:Li was elevated to 10.8:3, the XRD pattern obtained did not contain any spurious peaks, and the positions of each peak corresponded to the structure of the P2-type precursor. When the ratio was elevated to 11.6:3, the peak of Na2CO3 appeared, suggesting an excess of the Na source. Figure 1c–h displays the SEM images of the precursors prepared with the different Li/Na ratios. All the P2-type precursors assessed in this study were thin flake materials of 3–5 µm. At Na:Li = 10:3, a few fine particles were observed in addition to the main body of the flake material, which may prove that the raw material was not fully reacted. When the ratio was increased to 10.8:3, the unreacted fine particles were significantly reduced, and when the ratio continued to increase further, the fine particles increased more again in terms of their quantity. This also indicates to some extent in that the reaction was mostly complete and that the product had the highest purity when Na:Li = 10.8:3.
Following the ion exchange process, the transition metal layer will slip along the (2/3, 1/3, z) or (1/3, 2/3, z) direction and the layer spacing changes, and Li+ occupies the Li-O octahedral position, completing the transformation of the material from the P2 phase to the O2 phase, respectively [14,35]. Figure 2a shows the XRD patterns of the cathode materials obtained following the ion exchange process of the precursors prepared with different Li/Na ratios. The peak positions of each material can be indexed to the O2-type layered structure with the P63/mmc space group. The material with Na:Li = 10.8:3 was found to have the sharpest peak intensity, indicating the highest crystallinity. For the remaining two materials, the impurity-related peaks disappeared due to the ion exchange and water washing processes. As shown in Figure 2b–g, all the assessed materials displayed smoother surfaces compared to their corresponding precursors, and only trace amounts of fine particles remained.
The electrochemical properties of the materials prepared with different Li/Na ratios are shown in Figure 3. Figure 3a shows the initial charge/discharge curves. When Na:Li = 10:3, the material exhibited a higher discharge specific capacity, which may be due to the small amount of independent Li2MnO3 impurities that were still present in the material, which provide part of the capacity during the first cycle of charging and discharging. Meanwhile, when Na:Li = 11.6:3, the O2 phase was not highly crystalline, and the Li2MnO3 component in the bulk phase was determined to be less, resulting in a lower discharge capacity. Figure 3b,c demonstrates the cycling performance and the average discharge voltage for a current density of 0.5 C and a voltage range of 2.0–4.8 V. Although the capacity of the material was high when Na:Li = 10:3, the capacity then decayed rapidly due to the poor reversibility of the Li2MnO3 structure; the capacity retention rate was calculated to be 67.4% after 30 cycles. When Na:Li = 10.8:3, the capacity retention rate reached 75.8% at a high capacity due to the proper ratio. When Na:Li = 11.6:3, the capacity retention, although also higher, accompanied a lower capacity. As shown in Figure 3c, compared with the conventional O3-type material, the voltage decay of the O2-type material prepared under all three conditions was found to be insignificant, and the voltage decay of the material with Na:Li = 10.8:3 was only 0.0233 V [36,37], with the voltage decay of the material with Na:Li = 10:3 only being 0.0439 V in comparison. The dQ/dV curves of the first three cycles of the materials prepared with the different Na/Li ratios are shown in Figure S1. A clear oxidation peak appeared for the three materials during the first cycle of charging to 4.6 V, with the peak then disappearing in both the second and third cycles, indicating that these materials experience an irreversible oxygen loss during the first charge and discharge. Meanwhile, when Na:Li = 10:3, the oxidation peak of the second cycle was significantly shifted compared with the first cycle, indicating its inferior structural reversibility.
Although the voltage decay of the synthesized O2-type material was significantly suppressed, its cycling performance was still unsatisfactory, which may be related to the strong side reaction between the surface of the material and the electrolyte. In order to obtain a better overall performance and thus advance the practical use of this material, modification of the surface is therefore necessary. Since the O2-type material is metastable, the annealing temperature needs to be as low as possible when generating the coating layer using a liquid phase coating technology combined with a heat treatment. Aluminum isopropoxide can easily be hydrolyzed in the air to form aluminum hydroxide, which can then be decomposed into a stable Al2O3 coating layer by annealing at a lower temperature. However, the annealing temperature still needs to be carefully controlled in order to enhance the coating effect as much as possible without destroying the bulk phase of the material. Figure 4a shows the XRD patterns of the LLNCM@ Al2O3 materials annealed at 250, 300, and 350 °C, respectively. Following calcination, the materials were all found to have maintained the O2 phase structure without experiencing any significant phase transitions. However, the peaks associated with Al2O3 were not able to be detected due to the low amount of coating. The initial charge/discharge curves of the LLNCM@ Al2O3 materials prepared under different temperatures in the voltage interval 2.0–4.8 V are shown in Figure 4b. These materials were found to have discharge specific capacities of 226.4 mAh g−1, 245.7 mAh g−1, and 258.9 mAh g−1 when annealed at 250 °C, 300 °C, and 350 °C, respectively. The capacity of the material was similar to that of the original material when annealed at 300 °C, while the specific capacity decreased significantly when annealed at 250 °C, respectively. This may be due to the fact that the isopropanolic aluminum failed to completely decompose under the lower annealing temperature and instead adhered to the LLNCM surface, which hindered the Li+ transport as a result and led to the degradation of the electrochemical properties of the material. At higher temperatures (350 °C), the structure of the material may become partially damaged, also resulting in a lower capacity. Based on these findings, a temperature of 300 °C was considered to be the optimum temperature for annealing treatment, and subsequently the LLNCM@ Al2O3 mentioned below specifically refers to the material prepared at 300 °C. Figure S2a,b shows the low and high magnification SEM images of the LLNCM material obtained following the coating treatment, respectively. The morphology did not change significantly when compared to the bare LLNCM material, proving that the coating technique exhibits a small effect on the material’s morphology. Figure S3a–c show the XPS spectra of Ni, Co, and Mn for the LLNCM and LLNCM@ Al2O3. The peak positions of all the samples did not shift following the application of the coating, thereby proving that the coating process does not trigger a phase transition on the surface of the materials. Ni 2p displayed two spin orbital lines, wherein the 2p1/2 binding energy was 872.4 eV, with its satellite peak located at 878.8 eV, while the 2p3/2 binding energy was 854.5 eV, with its satellite peak located at 860.9 eV, respectively, showing that the main valence of the Ni element in both materials was a +2 valence. Co 2p showed a 2p1/2 binding energy of 794.8 eV and a 2p3/2 binding energy of 780.0 eV, and its two satellite peaks were located at 803.3 eV and 788.5 eV, respectively, revealing that the Co element had a +3 valence. The 2p1/2 binding energy of Mn 2p was 653.1 eV, while the 2p3/2 binding energy was 642.1 eV, meaning it can be determined that the main valence states of the Mn elements in both materials were of a +3 and +4 valence, respectively.
The TEM images of LLNCM@ Al2O3 are shown in Figure 5a,b. The coating layer on the surface of the material was about 18 nm. Further EDS-mapping analysis was performed, and from Figure 5d–g, there is a clear enrichment of Al in the outermost layer of the material.
Figure 6a shows the initial charge/discharge curves of the bare material and LLNCM@ Al2O3 at 0.1 C in the voltage range of 2.0–4.8 V. The charge/discharge voltage plateaus of the two materials nearly overlap and their capacities were found to be similar, which proves that the coating layer does not change the material’s bulk phase structure. Figure 6b shows the long cycle performance of the material at a current density of 0.5 C. Since Al2O3 protects the surface structure of the material and reduces the surface phase change, the coated material exhibits a higher capacity at the higher current densities. After 100 cycles, the capacity retention rate of the coated material reached 72.9%, while the capacity retention rate of the bare material was only 50.5%, respectively. This proves that this material is effectively isolated from the corrosive effects of the electrolytes on the electrode after coating, which stabilizes the material’s surface structure as a result and improves the electrochemical performance.
The dQ/dV curves of LLNCM@Al2O3 and LLNCM are shown in Figure S4. The LLNCM@Al2O3 material displays a small shift in the position of the oxidation and reduction peaks after 100 cycles. In contrast, the reduction peak of the LLNCM material significantly shifted to the left, indicating that this material underwent a severe phase transition. Overall, these results reveal that the coating layer was able to effectively prevent the occurrence of side reactions between itself and the electrolyte and was also able to suppress the surface phase transition.
Figure 7 shows the impedance spectra of LLNCM and LLNCM@ Al2O3 that were formed at different numbers of cycles. The data were fitted by the equivalent circuit diagram in Figure 7a, where Rs represents the solution impedance and Rct is related to the surface film and the surface charge transfer.
The fitted data are shown in Table 1. The Rct of LLNCM@ Al2O3 was found to be much lower than that of the LLNCM material at different cycles, indicating that LLNCM@ Al2O3 encompasses a more stable interface during periods of long cycling.
In order to directly observe the protective effects of the coating layer, Figure 8 shows the SEM images of the LLNCM and LLNCM@ Al2O3 materials after cycling. From Figure 8b, it is shown how the LLNCM material was significantly corroded by the electrolytes that were present on the surface after 100 cycles. In contrast, the surface of the LLNCM@ Al2O3 material still appeared to be smooth, and there was no obvious difference observed with the surface of the material prior to the cycling.

4. Conclusions

To effectively improve the electrochemical performance of Li-rich, Mn-based cathode materials and suppress the voltage decay during periods of long cycling, the key conditions for the preparation of the O2-type Li-rich materials were explored. The P2-type sodium-based precursors were prepared using the sol-gel method, and the effects of the calcination temperature and the Li/Na ratio on the precursors and cathode materials were investigated. When the calcination temperature was controlled at 750 °C and Na:Li = 10.8:3, the P2-type Na-based precursors were synthesized possessing a high crystallinity and no impurities. Furthermore, the O2-type Li-rich materials after ion exchange also exhibited excellent electrochemical properties. In order to suppress the phase change on the material surface and further optimize the cyclic stability of the material, Al2O3 coating of the material at a low annealing temperature was proposed for the first time in this study. The material annealed after coating at 300 °C exhibited a stable interface as well as demonstrating excellent electrochemical properties.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst13081154/s1. Figure S1: dQ/dV curves of LLNCM cathodes synthesized with Li/Na = (a) 10:3, (b) 10.8:3 and (c) 11.6:3; Figure S2: SEM image of LLNCM@Al2O3 cathode material; Figure S3: XPS (a) Ni 2p (b) Co 2p (c) Mn 2p spectra of the LLNCM and LLNCM@Al2O3 cathode material; Figure S4: dQ/dV curves of (a) LLNCM and (b) LLNCM@Al2O3 cathode materials.

Author Contributions

Conceptualization, Y.S. and H.Z.; Writing—original draft, Y.S.; Formal analysis, Y.S. and J.H.; Data curation, J.H.; Writing—review & editing, H.Z.; Supervision, L.Z.; Funding acquisition, L.Z.; Investigation, D.W.; Resources, D.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Key Research and Development Program of China (2018YFE0111600) and the National Nature Science Foundation of China (22005217, 52172224 and 22279090).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare that they have no competing financial interest.

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Figure 1. XRD patterns of the P2-type NLNCM precursor synthesized at different sintering temperatures (a) and with different Li/Na ratios (b). SEM image of the P2-type NLNCM precursor synthesized with Li/Na = (c,d) 10:3, (e,f) 10.8:3, and (g,h) 11.6:3, respectively.
Figure 1. XRD patterns of the P2-type NLNCM precursor synthesized at different sintering temperatures (a) and with different Li/Na ratios (b). SEM image of the P2-type NLNCM precursor synthesized with Li/Na = (c,d) 10:3, (e,f) 10.8:3, and (g,h) 11.6:3, respectively.
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Figure 2. (a) XRD patterns of the O2-type LLNCM cathode materials synthesized with different Li/Na ratios and SEM image of the O2-type LLNCM cathode material synthesized with Li/Na = (b,c) 10:3, (d,e) 10.8:3, and (f,g) 11.6:3, respectively.
Figure 2. (a) XRD patterns of the O2-type LLNCM cathode materials synthesized with different Li/Na ratios and SEM image of the O2-type LLNCM cathode material synthesized with Li/Na = (b,c) 10:3, (d,e) 10.8:3, and (f,g) 11.6:3, respectively.
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Figure 3. (a) Initial charge/discharge curves, (b) cycle performance, and (c) voltage decay of LLNCM synthesized with different Li/Na ratios.
Figure 3. (a) Initial charge/discharge curves, (b) cycle performance, and (c) voltage decay of LLNCM synthesized with different Li/Na ratios.
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Figure 4. (a) XRD patterns and the (b) initial charge/discharge curves of the LLNCM@Al2O3 cathode material’s handling under different annealing temperatures.
Figure 4. (a) XRD patterns and the (b) initial charge/discharge curves of the LLNCM@Al2O3 cathode material’s handling under different annealing temperatures.
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Figure 5. (a) TEM images and the (b) local magnification of the LLNCM@Al2O3 sample, (c) EDS-mapping, and (d–g) distributions of the Ni, Co, Mn, and Al elements in the sample.
Figure 5. (a) TEM images and the (b) local magnification of the LLNCM@Al2O3 sample, (c) EDS-mapping, and (d–g) distributions of the Ni, Co, Mn, and Al elements in the sample.
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Figure 6. (a) Initial charge/discharge curves and the (b) cycle performance of the LLNCM@Al2O3 and LLNCM cathode materials.
Figure 6. (a) Initial charge/discharge curves and the (b) cycle performance of the LLNCM@Al2O3 and LLNCM cathode materials.
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Figure 7. Electrochemical impedance spectra (EIS) of the (a) LLNCM and (b) LLNCM@Al2O3 electrodes at various cycles.
Figure 7. Electrochemical impedance spectra (EIS) of the (a) LLNCM and (b) LLNCM@Al2O3 electrodes at various cycles.
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Figure 8. SEM image of the (a,b) LLNCM and (c,d) LLNCM @Al2O3 materials after cycling.
Figure 8. SEM image of the (a,b) LLNCM and (c,d) LLNCM @Al2O3 materials after cycling.
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Table
Table 1. The simulated results for the EIS spectra of the LLNCM and the LLNCM@Al2O3 samples.
Table 1. The simulated results for the EIS spectra of the LLNCM and the LLNCM@Al2O3 samples.
SampleRs (Ω)Rct (Ω)
LLNCM-307.03330.03
LLNCM@Al2O3-309.21914.41
LLNCM-507.11936.38
LLNCM@Al2O3-506.3615.98
LLNCM-1007.65887.26
LLNCM@Al2O3-1006.85927.43
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Sun, Y.; Huang, J.; Zhang, H.; Zhang, L.; Wang, D. Controllable Synthesis and Surface Modifications of a Metastable O2-Type Li-Rich Cathode Material. Crystals 2023, 13, 1154. https://doi.org/10.3390/cryst13081154

AMA Style

Sun Y, Huang J, Zhang H, Zhang L, Wang D. Controllable Synthesis and Surface Modifications of a Metastable O2-Type Li-Rich Cathode Material. Crystals. 2023; 13(8):1154. https://doi.org/10.3390/cryst13081154

Chicago/Turabian Style

Sun, Yiming, Junjie Huang, Hongzhou Zhang, Lianqi Zhang, and Defa Wang. 2023. "Controllable Synthesis and Surface Modifications of a Metastable O2-Type Li-Rich Cathode Material" Crystals 13, no. 8: 1154. https://doi.org/10.3390/cryst13081154

APA Style

Sun, Y., Huang, J., Zhang, H., Zhang, L., & Wang, D. (2023). Controllable Synthesis and Surface Modifications of a Metastable O2-Type Li-Rich Cathode Material. Crystals, 13(8), 1154. https://doi.org/10.3390/cryst13081154

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