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All issues Volume 3 / No 6 (2024) Natl Sci Open, 3 6 (2024) 20240010 Full HTML
Issue
Natl Sci Open
Volume 3, Number 6, 2024
Special Topic: Key Materials for Carbon Neutrality
Article Number 20240010
Number of page(s) 16
Section Materials Science
DOI https://doi.org/10.1360/nso/20240010
Published online 24 May 2024

© The Author(s) 2024. Published by Science Press and EDP Sciences.

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

INTRODUCTION

Ternary layered LiNixCoyMn1–x–zO2 (x + y + z = 1) compounds alleviate the ever-growing range anxiety for electric vehicles (EVs) because of their high energy densities; these materials have driven the rapid development of EVs worldwide [1,2]. Now, LiNi0.6Co0.2Mn0.2O2 (denoted as NCM622) has become one of the most mainstream cathode materials for EV and consumer electronics [3,4]. Unfortunately, NCM622 still contains substantial Co, which has a fragile supply chain and raises a series of political and ethical issues, resulting in a high cost for battery utilization [5–7]. In addition, considerable lattice parameter changes and severe phase transitions cause particle fragmentation in the form of intragranular and intergranular cracking during long cycling (Figure 1a) [8,9], which are exacerbated at high voltages accompanied by oxygen evolution [10,11].

thumbnail Figure 1

Schematic illustration of the dynamics and stability evolution mechanisms for NCM622 (a), NM64 (b) and Co-NM64 (c).

Recent researches have shown that LiNi0.6Mn0.4O2 (denoted as NM64) not only saves raw material costs significantly over conventional NCM622 cathodes but also achieves excellent cycling and thermal stability at high voltages (Figure 1b), because the abundant Mn stabilizes the lattice oxygen and provides the Li/Ni disorder as pillars [8,12–15]. However, the tremendous Li/Ni disorder is also a two-edged sword because the impediment to Li+ transport makes it difficult to boost kinetics, resulting in its low reversible capacity and poor rate performance [16–18]. Also, the presence of residual Li compounds in nickel-rich cathodes can act as electronic and ion insulator and degrade electrochemical performance [19]. Therefore, NM64 should be further modified to achieve comparable kinetic performance to that of commercial NCM622, and maintain low cost, outstanding cycling and thermal stability.

It is accepted that the interface between the electrolyte and electrode plays a significant role in dominating the electrode reaction process. Thus, sluggish interfacial reaction kinetics can be effectively improved by controlling the surface chemistry [20]. Cai et al. designed a lanthurized surface architecture to effectively inhibit oxygen evolution kinetics and remove electron transport bottlenecks [21]. Yoon et al. employed cobalt boride coating to improve kinetics and stabilize the structure of Ni-rich cathodes [22]. In this work, we design a rational structure composed of a robust conductive protective layer, gradient Li+ ions conductive layer and stable bulk phase by reconstructing the surface of NM64, which successfully enables boosted kinetics and excellent stability. With scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) mapping, a Co3O4 and LixCoO2 mixed spinel phase (~2 nm, region I in Figure 1c) was found at the surface, and the Co element diffused in the cathode gradually to form a near-perfect layered solid solution (~200 nm, region Ⅱ in Figure 1c) to eliminate the Li/Ni disorder. The cobaltized surface (region I + II in Figure 1c) can boost the reaction kinetics by accelerating ionic and electronic transport. In addition, the mixed spinel phase (region I in Figure 1c) serving as a robust protective layer mitigates structure degradation and parasitic side reactions on the surface. Meanwhile, the modified NM64 also has excellent structural stability, far better than that of commercial NCM622. As a result, it notably improves rate capability compared to that of NM64 (118.7 vs 53.5 mAh g−1 at 5 C), and provides excellent electrochemical performance with an impressive 92.0% (2.7–4.5 V, 100 cycles) and 95.0% (2.7–4.3 V, 300 cycles) capacity retention in the half cell (better than 70.2% and 14.3% of commercial NCM622), indicating great potential applications in the field of long-cycle and high-energy-density lithium ion batteries.

MATERIALS AND METHODS

Material synthesis

The Ni0.6Mn0.4(OH)2 precursor was synthesized by a typical coprecipitation method in CNGR Advanced Material Co., Ltd. A mixed aqueous solution A contained 1.2 M NiSO4·6H2O and 0.8 M MnSO4·5H2O. The solution A, 10.8 M NaOH and 5.3 M NH4OH aqueous solutions were continuously pumped into a stirred tank reactor (capacity of 100 L) under feeding rates of 5000, 1900 and 120 mL h−1, respectively. In the reaction process, the vessel (100 L) was filled with the reaction solution, with feeding and overflowing occurring simultaneously. The pH was maintained at 11.6 at 60°C. After reacting for 90 h, the resulting hydroxide precipitate was filtered and washed with deionized water until the pH of the filtrate dropped below 7. It was then collected and dried at 100°C for 15 h. The precursor mixed with battery-grade Li2CO3 (the molar ratio of Li:[Ni + Mn] = 1.05:1) was sintered at 500°C for 5 h and 850°C for 15 h to obtain LiNi0.6Mn0.4O2 (NM64) in an air atmosphere with a heating rate of 1°C min−1. In order to obtain Co-NM64, 0.1 g polyvinylpyrrolidone (PVP, M=40000, Sinopharm) was firstly added to 10 mL ethanol, and stirred at 45°C to obtain a solution. Then, 5 g NM64 was added in the above solution, and stirred for 0.5 h. In the following, Co(NO3)2·6H2O (98%, Sinopharm) was added with the molar ratio of Co:NM64 = 0.005:1, with a continuous stir for another 0.5 h. Then, the above solution was dried overnight at 80°C in a vacuum oven, and the obtained powder was annealed at 600°C for 5 h in air atmosphere. The modifications using other elements were similar to Co-NM64. The Ni0.6Mn0.4(OH)2 precursor, Li2CO3 and Co(NO3)2·6H2O (the molar ratio of Li:[Ni + Mn]:Co = 1.05:1:0.005) were thoroughly mixed in an agate mortar and ground for 0.5 h. The mixtures were calcined under the same conditions with the NM64 to create cobalt-doped NM64. NCM622 commercial cathode material was purchased from Ningbo Shanshan Co., Ltd.

Electrochemical measurement

The cathode electrodes were obtained by uniformly stirring 80% cathode material, 10% superconducting carbon (Super P Li, TIMCAL) and 10% polyvinylidene fluoride (PVDF, Solvay 5130) binder using N-methyl-1,2-pyrrolidone solvent (99.9%, NMP, Sinopharm) as the solvent, followed by coating and drying. The loading of each cathode electrode (12 mm diameter, single side) is ~5 mg cm−2. A Celgard 2400 type polypropylene (PP) separator (19 mm diameter), lithium metal foil (400 μm thick, 15.6 mm diameter, China Energy Lithium Co., Ltd) and 1.2 M LiPF6 (VEC:VEMC=3:7) electrolyte (Nanjing Mojiesi Energy Tech., Co., Ltd., MJS-G01) were used to prepare half cells. The coin cell was assembled in an argon-filled glove box (H2O < 0.5 ppm, O2 < 0.5 ppm) using a 2032-coin cell (all metal parts are 304 stainless steel, Guangdong Canrd New Energy Technology Co.,Ltd.). The 25 μL volume of electrolyte was used for the coin cells. The assembled cells were left to stand in a temperature-controlled room for 12 h to ensure sufficient wetting of the separator and electrodes before cycling was started. The constant current-constant voltage (CC-CV, CC with each C rate and CV with a cut-off current of 0.05 C) cycling mode was used in charging Li-ion cells. After cells were charged to 4.0 V (versus Li/Li+), the electrochemical impedance measurements were performed using an electrochemical workstation (Ivium, Vertex.One.EIS) with an amplitude of 10 mV and a frequency range of 0.01 Hz–100 kHz. The galvanostatic intermittent titration test (GITT) consisted of 30 min of titration at 0.1 C discharge current and 180 min of relaxation step. For full-cell tests, the anode electrodes were obtained by uniformly stirring 80% graphite (BTR New Material Group Co., Ltd.) anode material, 10% superconducting carbon and 10% carboxymethyl cellulose (CMC, Daicel 2200) binder using water and ethanol as the solvent to coat onto a Cu foil, and dried at 80°C for 12 h. The loading of each anode electrode (12.5 mm diameter, single side) was ~2.6 mg cm−2. The cathode loading was ~5 mg cm−2 (12 mm diameter, single side, ~0.9 mAh g−1). The ratio of negative to positive electrode capacity (N/P ratio) was about 1.1. The formation cycle at 0.1 C was conducted before 1 C cycling within the range of 2.7–4.4 V.

Materials characterization

The morphologies were characterized using a Zeiss ΣIGMA field emission scanning electron microscope (SEM) (operating at 10 kV). The cathode powder (D50, ~10 μm) was placed in a silicon zero-background sample holder and then flattened using a glass slide. XRD measurement was conducted within the 2 theta range of 10–80° at 0.02° intervals using a RIGAKU RINT-2000 (Cu Kα). For in-situ XRD, a slurry composed of 80 wt% active material, 10 wt% superconductive carbon and 10 wt% PVDF was dropped onto an ultra-thin Al foil with NMP as the solvent. Subsequently, it underwent overnight drying at 80°C. The in-situ XRD cell included components such as the base, Be window, cathode material coated on an ultra-thin Al foil, separator, Teflon guide bushing, separator, lithium foil, compression ring, spring, spacer and upper cover with 1.2 M LiPF6 (VEC:VEMC=3:7) as electrolyte. The in-situ cell was assembled (bottom to top) similar to a coin cell as depicted in the Figure S27. In-situ XRD was conducted using the Bruker D8 ADVANCE (Cu Kα) X-ray diffractometer in the range of 10° to 38° (2θ) at 0.02° intervals. The cells were cycled at 0.1 C with a voltage range of 2.7–4.5 V for two cycles using the Neware BTS-53 system. XRD results were analyzed using the GSAS package [23]. Cross-sections of the three cathode materials were characterized by dual-beam focused ion electron microscope (FIB-SEM, Helios 5 CX) and ion milling (Hitachi IM4000ii). Before TEM tests, the specimens were thinned to less than 200 nanometers using FIB-SEM. High-resolution atomic images of all cathode materials were conducted by the spherical aberration-corrected electron transmission microscope (TEM, Themis Z 3.2). The elemental and spectrum analysis was obtained using EDS and EELS detector equipped with a TEM. Small-angle X-ray scattering (SAXS) experiments were performed using the SAXS beamline at the Australian Synchrotron with camera parameters of 7 m and 0.9 m. The differential scanning calorimeter (DSC) and thermogravimetry analysis (TG) were conducted within the range of room temperature to 400°C at a heating rate of 10°C min−1. Before tests, the cycled cathodes were washed with DMC to remove residual electrolytes.

Detailed analysis of GITT

Assuming that the solid phase comprises spherical particles, the DLi+ values of three cathodes could be calculated using Equation (1):

D s = ( π / 4 τ ) ( R s / 3 ) 2 ( ∆ V s / ∆ V t ) 2 , (1)

in which τ is the time duration of the pulse, and RS is the radius of spherical particles, which was obtained from particle size analysis. The voltage drops ( and ) are achieved from discharge curves of the GITT measurements (Figure 2c and Figure S16) [24].

thumbnail Figure 2

Structural comparison of NCM622, NM64 and Co-NM64. (a) SEM images of Co-NM64. (b) EDS line profile along the arrow. (c) EDS mapping of Co in the cross section. (d–f) XRD diffraction and refinement of NCM622 (d), NM64 (e) and Co-NM64 (f). (g1–i1) Surface scanning transmission electron microscopy under high-angle annular dark-field mode (HAADF-STEM) image of NCM622 (g1), NM64 (h1) and Co-NM64 (i1). (g2–i2) Bulk HAADF-STEM image of NCM622 (g2), NM64 (h2) and Co-NM64 (i2). (j) HAADF-STEM image of Co-NM64 marked by yellow and green boxes. (k–l) Corresponding HAADF signal profiles of the yellow and green boxes in (j). Scale bars, 200 nm (a), 2 μm (an inset), 50 nm (b,c), 2 nm (g–j).

Detailed analysis of SAXS

The inverse proportional relationship between the scattering factor Q and the physical scale d can be derived from the following formula:

d = 2 π / Q. (2)

Under cycling at high voltage (≥4.5 V), transition metals in layered cathode materials catalyze the oxidation of O. With increasing number of cycles, the oxidation of the accumulated O will lead to an increase in the density of nanoholes in the cathode material; this provides the possibility for the characterization of SAXS. Therefore, the increase in intensity in the range of 0.4 Å−1 < Q < 0.9 Å−1 is attributed to the nanovoids created by the oxidation of O [25,26].

RESULTS

Design strategy based on NM64

The modification strategy for NM64 cathode materials should meet the following conditions: (1) The stable structure of the original bulk phase should be maintained, so the heat-treatment temperature and long sinter times should be avoided in resintering. (2) It should avoid the formation of passivation coating layer on the surface, ensuring rapid Li+ and electron transport. (3) The surface phase should be coherent with the bulk phase in the lattice. (4) It should effectively improve the overall reaction kinetics of the cathode material.

Thus, after screening many suitable elements including Co, Zn, Al, Mg, Zr and Cd, we found that Co and Zn whose atomic numbers were close to those of Ni and Mn, could uniformly diffuse into the NM64 lattice after resintering at a low temperature, with no evident coating layer being appeared on the surface (Figures S2–S8). Specially, only Co formed the interdiffusion layer with high electron and ionic conductivities (Figure S9). The Co-rich surface composed of Co3O4 and LixCoO2 mixed spinel phase (region I in Figure 1c) and the gradient distribution within the range of ~200 nm (region II in Figure 1c) ensures the high electrochemical activity of interfacial reactions, thus boosting the electrode reaction kinetics. Meanwhile, the bulk phase with moderate lattice change suppresses the formation of particle cracking.

Reconstructed surface and stable bulk

To obtain surface-reconstructed NM64 (denoted as Co-NM64), NM64 was added to an ethanol solution of polyvinylpyrrolidone (PVP) and Co(NO3)2, followed by drying and sintering in air. The amount of Co loading in the materials is 0.5 wt% obtained from the inductively coupled plasma-atomic emission spectroscopy (ICP-AES). A schematic diagram of the synthesis is shown in Figure S1. Commercial NCM622 was used as a control sample.

Both pristine NM64 and Co-NM64 (Figure S11) have a typical polycrystalline microstructure with spherical secondary particles, which consist of fine-grained primary particles. The surface of the primary grains of Co-NM64 is similar to that of NM64, and there is no rough coating on the surface (Figure 2a). To gain a deeper understanding of the difference between the Co-NM64 and NM64 surfaces, STEM with EDS was used to characterize the elemental composition of the surface. Figure 2b, c, and Figures S12, S13 show the EDS mapping of the cross section of Co-NM64, and it can be observed that Co is distributed on the surface and inside of the grains. TEM EDS line scans along the black arrows (from the surface to ~200 nm into the bulk) indicate that Co exhibits a gradient distribution. The Li-residue compounds such as LiOH and Li2CO3 at the cathode surface could react with Co(NO3)2 to form LiCoO2 (LCO) during the annealing process [27–29]. LCO can easily diffuse into NM64 through the interdiffusion phenomenon because of the similar crystal structure to NM64 [30], and the NM64 surface eventually becomes LiNi1-x-yCoxMnyO2, where x gradually decreases from the surface to the interior of the particle.

thumbnail Figure 3

Electrochemical performance of NCM622, NM64 and Co-NM64. (a) Rate capability of NCM622, NM64 and Co-NM64 within a voltage range of 2.7–4.5 V versus Li/Li+ (1 C = 180 mA g–1). (b) Discharge curves of NM64 and Co-NM64 at different current densities. (c) Discharge curves of the GITT measurements at a current density of 0.1 C. (d) 1 C cycling of NCM622, NM64 and Co-NM64 in the range of 2.7–4.5 V versus Li/Li+. (e) 1 C cycling of NCM622, NM64 and Co-NM64 in the range of 2.7–4.3 V versus Li/Li+. (f) Initial dQ/dV curves of NCM622, NM64 and Co-NM64 at a current density of 0.1 C. (g) Comparison of the specific capacity and capacity retention between the Co-NM64 and typical cathodes previously reported for LIBs in the half cell.

The refined XRD results are shown in Figure 2d–f and TableS3. Among these three cathodes, NCM622 exhibits the smallest a and c lattice unit cell parameters because the incorporation of trivalent Co in lattice reportedly leads to a reduction of the a and c lattice unit cell parameters [31,32]. Co-NM64 shows a slight reduction in lattice parameters compared to NM64, suggesting that a small amount of Co elements diffuse into NM64 lattice, which is consistent with TEM-EDS results. NCM622, NM64, and Co-NM64 all belong to the R–3m space group, where Li occupies the 3a sites, while Ni and Mn (Co) occupy the 3b sites, and O occupies the 6c sites. Due to the presence of abundant Co, the Li/Ni disorder of NCM622 is only 4.7%, while the Li/Ni disorder of NM64 increases rapidly to 9.0% after all Co was replaced by Mn [33]. The Li/Ni disorder of Co-NM64 is consistent with NM64, which suggests that the high structural stability of Co-NM64 that resulted from a moderate amount of Li/Ni disorder was not changed compared to that of NM64.

HAADF-STEM was performed to investigate the atomic structure of both the particle surface and bulk region. As shown in Figure 2g1 and g2, the atomic arrangement inside the NCM622 particle is consistent with the surface. When a similar inspection was applied to NM64 as shown in Figure 2h1 and h2, we found substantial Li/Ni disorder on the surface and in the bulk phase [8]. Unlike NM64 and NCM622 that have a consistent atomic arrangement on the surface and interior of the particle, the Co-NM64 surface exhibits a near perfect layered structure and no bright spots are captured in the Li layer, which indicates that the Li/Ni disorder could be negligible, while an appropriate amount of Li/Ni disorder acts as pillars in bulk phase (Figure 2i1 and i2). Further investigations in Figure 2j–l and Figure S14 indicate that the spinel structure in the range of ~2 nm on the surface is probably a stable Co3O4 and middle-temperature LixCoO2 (x<1) mixed spinel phase [28,34,35].

Superior electrochemical performances with boosted kinetics

As illustrated in Figure S15, the initial discharge capacities with a CCCV mode (0.1 C//0.05 C, 4.5 V, 1 C = 180 mA g−1) within a voltage window of 2.7–4.5 V versus Li/Li+ are recorded as 206.5, 180.2, and 189.1 mAh g−1 for NCM622, NM64, and Co-NM64, respectively. Co-NM64 presents a lower voltage gap between the charge and discharge platforms compared to that of NM64 [36]. As shown in Figure 3a and b, Co-NM64 shows a dramatically enhanced rate capability, especially at a high current density of 5 C (118.7 mAh g−1) compared to NM64 (53.5 mAh g−1), which indicates that the reaction kinetics are greatly improved.

As shown in Figure 3c, the quasi-equilibrium voltage profiles of Co-NM64 show a significantly reduced polarization voltage compared to NM64, and its IR drop is the smallest among the three materials, even better than NCM622. The DS for the initial discharge is plotted in Figure S16, which shows that the average Li+ diffusion coefficient of Co-NM64 is approximately twice time that of NM64. The enhanced Li kinetics of Co-NM64 can explain its better rate capability relative to NM64.

Electrochemical impedance spectroscopy (EIS) tested at the charged state (4.0 V) before and after the extended cycles (Figure S17) further prove the higher reaction kinetics of the Co-NM64 material compared to that of NM64 [37]. The fitted results by the equivalent circuit are shown in Table S2. Before cycling, the Rct value of Co-NM64 is 65 Ω, while that of NM64 cathode reaches 125 Ω. The Rf and Rct values of Co-NM64 are significantly reduced compared to those of NM64, indicating a superior surface charge transport capability. The improved kinetic performance should be attributed to the accelerated electron and ion transport by the surface-enriched Co, which is consistent with the GITT results. Regardless, Co-NM64 always outperforms NM64 in kinetics during cycling (Figures S16 and S17).

The ultra-high Co content in NCM622 leads to the largest ion mobility among NCM622, NM64 and Co-NM64, whereas even a small amount of Co (0.5 wt%) in Co-NM64 also enhances electronic conductivity and ion mobility significantly compared to NM64. Moreover, the voltage loss of Co-NM64 is even smaller than that of NCM622. The bulk doped NM64 material with 0.5 wt% Co was synthesized, and its poor electrochemical performance (Figure S18) indicates that surface plays a key role in improving the dynamics. Therefore, the cobaltized surface composed of a Co3O4 and LixCoO2 mixed spinel phase and near-perfect layered solid solution may be the reason for the superior kinetic performance of Co-NM64.

Figure 3d shows the capacity retentions over 100 cycles in the range of 2.7–4.5 V (versus Li/Li+), which are 70.2%, 95.0% and 92.0% for NCM622, NM64 and Co-NM64 at a current density of 1 C, respectively. Besides, in the range of 2.7–4.3 V (versus Li/Li+) at a current density of 1 C, the capacity retentions (Figure 3e) are 90.1% and 95.0% for NM64 and Co-NM64 after 300 cycles. These prove that Co-NM64 still exhibits good cycle stability when improving the reversible capacity after surface reconstruction, which is far superior to the commercial NCM622 cathode materials. The severe capacity degradation in cycled NCM622 can be attributed to the rapid H1 (1st hexagonal)–M (monoclinic)–H2 (2nd hexagonal) phase-transition processes of NCM622 (two sharp peaks near 3.7 V), while NM64 and Co-NM64 show only a smooth H1–H2 phase-transition process (one broad peak) during charging in the dQ/dV curves (Figure 3f) [8,38]. Co-NM64, whose specific capacity and cycling stability are far superior to those previously reported NM64 half cells [8,16], enables its comprehensive electrochemical performance to surpass that of commercialized LiNi0.5Co0.2Mn0.3O2 (NCM523) [39–41], NCM622 [42–44] and LiNi0.8Co0.1Mn0.1O2 (NCM811) [45–47], as shown in Figure 3g.

Excellent mechanical and structural stability

In-situ XRD measurements on NCM622, NM64 and Co-NM64 were performed to monitor the evolution of the lattice structure. Figure 4a1–c1 show the contour plots during charge and discharge of 0.1 C within the voltage range of 2.7–4.5 V. The (003) diffraction peak at ~18.5° clearly indicates the structural changes regarding the lattice c-axis. During the charging process, the (003) peak position shifts to the left due to the repulsive interlayer spacing of lattice oxygen with the extraction of Li+. The continued extraction of Li+ in the later stage causes phase transition, resulting in a rapid decrease in the interplanar spacing and a right shift of the peak position, and the structural evolution is reversed during the discharge process [48,49]. The structural evolution of NCM622 shows a drastic lattice shrinkage (Δθ = 0.389°). Severe lattice distortion will destroy the integrity of the grains, resulting in crack generation [11,50]. Compared with NCM622, the structural change of Co-NM64 during the charge-discharge process is similar to that of NM64, and the shift angle of the peak is only 0.184°, indicating that a smooth phase-transition process is maintained. The c-lattice evolution (Figure S19) was calculated from in-situ XRD, and the trends in the changes of the lattice c parameter for the three cathodes are consistent with the shift observed in the XRD peak positions. Ab initio calculations (Figure S20) indicate that the Jahn-Teller distortion around Ni3+ has minimal influence on volume changes in NM64 and Co-NM64 during delithiation, owing to smaller and less anharmonic distortions relative to Mn3+, the presence of inert Mn4+, the disorder of Ni/Mn ions, and partial Li/Ni disorder [51–53]. Therefore, the volume changes in NM64 and Co-NM64 may be primarily attributed to electrostatic repulsion between O–O due to the removal of positively charged Li+ ions [54], followed by the H2-H3 (3rd hexagonal) phase transition [8].

thumbnail Figure 4

Comparison of the structural evolution and mechanical stability of NCM622, NM64 and Co-NM64. (a1–c1) In-situ XRD characterizations of NCM622 (a1), NM64 (b1) and Co-NM64 (c1) before cycling. (a2–c2) Cross-sectional images of NCM622 (a2), NM64 (b2) and Co-NM64 (c2) charged to 4.5 V after 100 cycles in the 2.7–4.5 V (vs. Li/Li+) voltage range. (a3–c3) HAADF-STEM image of NCM622 (a3), NM64 (b3) and Co-NM64 (c3) at a secondary-particle surface discharged to 2.7 V (vs. Li/Li+) after 100 cycles. (a4–c4) EELS line scan of NCM622 (a4), NM64 (b4) and Co-NM64 (c4) at different depths after 100 cycles. Scale bars, 4 μm (a2–c2), 2 nm (a3–c3).

To visualize the changes inside the grains, the cross-section particles were obtained for the three cathode materials. Figure 4a2–c2 and Figure S21 are cross-sectional images of the charged state after 100 cycles of 1 C. A high density of cracks were observed from the core to the surface of NCM622 particles (Figure 4a2 and Figure S21a1–a4). The generated cracks worsen the connectivity between the primary grains, which increases the resistance and impairs the overall capacity. The electrolyte penetrates along the cracks and the resulting parasitic side reactions form new CEI films, which accelerate the degradation of the surface phase [10,55–57]. The generation of NCM622 microcracks can be attributed to the mechanical strain generated by the change in lattice parameter c during charging and discharging. In comparison, no obvious cracks are observed in NM64 (Figure 4b2 and Figure S21b1–b4) and Co-NM64 (Figure 4c2 and Figure S21c1–c4), and the particles remain intact, indicating that both of them have good mechanical stability.

The surface structures of NCM622, NM64 and Co-NM64 after cycling were characterized by combining STEM with electron energy loss spectroscopy (EELS). For cycled NCM622 (Figure 4a3 and a4), the surface shows an 11 nm thick rock salt phase, accompanied by ~30 nm of oxygen defects and reduction of Ni4+. The same phase transition is observed for NM64 (Figure 4b3 and b4), but oxygen defects are detected only at ~10 nm of the surface. In contrast, only ~2 nm layer with antisite appears on the surface of Co-NM64 (Figure 4c3), without rock salt phase. Furthermore, as shown in Figure 4c4, very few oxygen defects and the decreased peak intensity of Ni L3 edge indicate that the mixed spinel phase of Co-NM64 as surface protective layer effectively mitigates structural degradation and the generation of side reactions on the surface. Furthermore, ex-situ X-ray photoelectron spectroscopy (Figure S22) was used to monitor the Co valence on the surface, which shows that even at a high potential of 4.5 V (1st cycle), Co exhibits a higher valence without reduction, indicating the consistent stability of lattice oxygen.

Small-angle X-ray scattering (SAXS) was used to determine the size and distribution of ultrafine particles and micropores (1–100 nm), and the intensity distribution is only related to the particle size and distribution of the scatterers (particles and micropores) [58]. Compared with TEM and SEM, synchrotron SAXS has the characteristics of no damage to the sample and high throughput, and the obtained statistical average information can better reflect the overall state of the cathode material.

After cycling, the overall intensity of NCM622 increases significantly in the range of 0.4–0.9 Å−1 (Figure S23), while the intensities of NM64 and Co-NM64 change little (Figure S23). This result indicates that the oxygen evolution reaction of NM64 and Co-NM64 during cycling is significantly less than that of NCM622 [25,26], which is of great significance for maintaining the cathode structure, reducing the consumption of electrolyte solvents, and increasing battery safety. The differential scanning calorimeter (DSC) curves (Figure S25) also demonstrate that the thermal stability of NM64 and Co-NM64 is significantly better than that of commercial NCM622. As shown in Figure S23, in the middle Q region (2×10−3–10−1), corresponding to the particle/structure size of 6.3 nm–0.31 μm, the scattering intensity of NCM622 increased significantly after cycling compared with NM64 and Co-NM64; this means more pores or particles in this range, which indicates that NM64 and Co-NM64 have better structural stability [59]. As shown in Figures S24 and S26, the weak change of (003) peak after cycling also indicates that Co-NM64 has excellent structural stability.

Electrochemical performances in full cells

Cycling performance in full cells was explored using graphite as an anode at a current density of 1 C within the voltage range of 2.7–4.4 V. As shown in Figure 5a, the discharge curve of Co-NM64 at high voltages has almost no change over repeated deep lithiation/delithiation, thus showing 90.4% capacity retention after 300 cycles (Figure 5b). In sharp contrast, NM64 exhibits a pronounced discharge curve and lower reversible capacity, and NCM622 exhibits rapid capacity degradation. Figure 5c and Table S4 illustrate cycle retention rate of Co-NM64 at high voltages, demonstrating its competitiveness among other typical ternary cathode materials [60–68]. To understand the intrinsic reason for the electrochemical behavior of the three materials in the full cell, HRTEM was performed to observe the cathode surface after 50 cycles. As shown in Figure 5d–f, a thick layer of CEI covers the surface of NM64 and NCM622, accompanied by an irreversible phase transition of the surface structure due to serious side reactions between the particle surface and the electrolyte. In comparison, Co-NM64 only has a thinner CEI layer and still maintains a good layered structure on the surface after cycling, which is crucial for improving the dynamics. The suppression of parasitic side reactions and structure degradation may result from ~2 nm stable Co3O4 and middle-temperature LixCoO2 (x < 1) mixed spinel phase on the surface [28,34,35]. This is different from NM64 and NCM622 whose surfaces are completely exposed to the electrolyte.

thumbnail Figure 5

Electrochemical performance and microstructure of NCM622, NM64 and Co-NM64 in the full cell. (a) Discharge profiles of NCM622, NM64 and Co-NM64 using graphite as an anode within 2.7–4.4 V at 50 cycles at a current of 1 C. (b) 1 C cycling of NCM622, NM64 and Co-NM64 in the range of 2.7–4.4 V. (c) Comparison of the capacity retention between the Co-NM64 and typical cathodes previously reported for LIBs in the full cell. (d–f) HRTEM images of NCM622 (d), NM64 (e) and Co-NM64 (f) after 50 cycles. Scale bars, 5 nm (c–e), 1 nm (enlarged Figures in (d–f)).

DISCUSSION

In summary, we successfully proposed a rational structure design strategy that combines surface cobaltization and stable bulk composition using a small amount of Co. The reconstructed surface composed of mixed spinel phase and LiNi1-x-yCoxMnyO2 conductive layer not only protects layer structure from parasitic side reactions but also facilitates Li+ and electron transport, while the bulk composition suppresses the formation of cracks attributed to moderate lattice change. As a result, Co-NM64 delivers dramatically improved rate capability compared to that of NM64 (118.7 vs 53.5 mAh g−1 at 5 C), excellent capacity retention with an impressive 92.0% (2.7–4.5 V, 100 cycles) and 95.0% (2.7–4.3 V, 300 cycles), high thermal stability and low raw material cost. For cathode materials with good structure stability but poor kinetic performance, such as low-/zero-cobalt and nickel-rich cathodes, our study of simultaneously designing surface crystal structure and bulk phase is an effective way to ensure great electrochemical performance with low cost.

Data availability

The original data are available from corresponding authors upon reasonable request.

Funding

This work was supported by the National Natural Science Foundation of China (52074113, 22005091 and 22005092), the Hunan University Outstanding Youth Science Foundation (531118040319), the Science and Technology Innovation Program of Hunan Province (2021RC3055), the Changsha Municipal Natural Science Foundation (kq2014037), the CITIC Metals Ningbo Energy Co. Ltd. (H202191380246), the Chongqing Talents: Exceptional Young Talents Project (CQYC202105015), the Shenzhen Virtual University Park Basic Research Project of Free Exploration (2021Szvup036), and the National Key Research and Development Program of China (2022YFB2402400).

Author contributions

Q.Z. and J.Z. conceived and directed the experiments. Q.Z. performed the experiments. C.C., H.C., J.Z. and B.L. co-wrote the manuscript. All authors discussed the results and helped with the preparation of the final manuscript.

Conflict of interest

The authors declare no conflict of interest.

Supplementary information

Supplementary file provided by the authors. Access here

The supporting information is available online at https://doi.org/10.1360/nso/20230033. The supporting materials are published as submitted, without typesetting or editing. The responsibility for scientific accuracy and content remains entirely with the authors.

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All Figures

thumbnail Figure 1

Schematic illustration of the dynamics and stability evolution mechanisms for NCM622 (a), NM64 (b) and Co-NM64 (c).
In the text
thumbnail Figure 2

Structural comparison of NCM622, NM64 and Co-NM64. (a) SEM images of Co-NM64. (b) EDS line profile along the arrow. (c) EDS mapping of Co in the cross section. (d–f) XRD diffraction and refinement of NCM622 (d), NM64 (e) and Co-NM64 (f). (g1–i1) Surface scanning transmission electron microscopy under high-angle annular dark-field mode (HAADF-STEM) image of NCM622 (g1), NM64 (h1) and Co-NM64 (i1). (g2–i2) Bulk HAADF-STEM image of NCM622 (g2), NM64 (h2) and Co-NM64 (i2). (j) HAADF-STEM image of Co-NM64 marked by yellow and green boxes. (k–l) Corresponding HAADF signal profiles of the yellow and green boxes in (j). Scale bars, 200 nm (a), 2 μm (an inset), 50 nm (b,c), 2 nm (g–j).
In the text
thumbnail Figure 3

Electrochemical performance of NCM622, NM64 and Co-NM64. (a) Rate capability of NCM622, NM64 and Co-NM64 within a voltage range of 2.7–4.5 V versus Li/Li+ (1 C = 180 mA g–1). (b) Discharge curves of NM64 and Co-NM64 at different current densities. (c) Discharge curves of the GITT measurements at a current density of 0.1 C. (d) 1 C cycling of NCM622, NM64 and Co-NM64 in the range of 2.7–4.5 V versus Li/Li+. (e) 1 C cycling of NCM622, NM64 and Co-NM64 in the range of 2.7–4.3 V versus Li/Li+. (f) Initial dQ/dV curves of NCM622, NM64 and Co-NM64 at a current density of 0.1 C. (g) Comparison of the specific capacity and capacity retention between the Co-NM64 and typical cathodes previously reported for LIBs in the half cell.
In the text
thumbnail Figure 4

Comparison of the structural evolution and mechanical stability of NCM622, NM64 and Co-NM64. (a1–c1) In-situ XRD characterizations of NCM622 (a1), NM64 (b1) and Co-NM64 (c1) before cycling. (a2–c2) Cross-sectional images of NCM622 (a2), NM64 (b2) and Co-NM64 (c2) charged to 4.5 V after 100 cycles in the 2.7–4.5 V (vs. Li/Li+) voltage range. (a3–c3) HAADF-STEM image of NCM622 (a3), NM64 (b3) and Co-NM64 (c3) at a secondary-particle surface discharged to 2.7 V (vs. Li/Li+) after 100 cycles. (a4–c4) EELS line scan of NCM622 (a4), NM64 (b4) and Co-NM64 (c4) at different depths after 100 cycles. Scale bars, 4 μm (a2–c2), 2 nm (a3–c3).
In the text
thumbnail Figure 5

Electrochemical performance and microstructure of NCM622, NM64 and Co-NM64 in the full cell. (a) Discharge profiles of NCM622, NM64 and Co-NM64 using graphite as an anode within 2.7–4.4 V at 50 cycles at a current of 1 C. (b) 1 C cycling of NCM622, NM64 and Co-NM64 in the range of 2.7–4.4 V. (c) Comparison of the capacity retention between the Co-NM64 and typical cathodes previously reported for LIBs in the full cell. (d–f) HRTEM images of NCM622 (d), NM64 (e) and Co-NM64 (f) after 50 cycles. Scale bars, 5 nm (c–e), 1 nm (enlarged Figures in (d–f)).
In the text

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