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. 2023 Feb 13;8(7):6720–6728. doi: 10.1021/acsomega.2c07313

Fabricating Heterostructures for Boosting the Structure Stability of Li-Rich Cathodes

Yao Li †, Qing Zhao †, Mengke Zhang †, Lang Qiu †, Zhuo Zheng ‡, Yang Liu §, Yan Sun ∥, Benhe Zhong †, Yang Song †,*, Xiaodong Guo †
PMCID: PMC9948178  PMID: 36844563

Abstract

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Li-rich Mn-based oxides are regarded as the most promising new-generation cathode materials, but their practical application is greatly hindered by structure collapse and capacity degradation. Herein, a rock salt phase is epitaxially constructed on the surface of Li-rich Mn-based cathodes through Mo doping to improve their structural stability. The heterogeneous structure composed of a rock salt phase and layered phase is induced by Mo6+ enriched on the particle surface, and the strong Mo–O bonding can enhance the TM–O covalence. Therefore, it can stabilize lattice oxygen and inhibit the side reaction of the interface and structural phase transition. The discharge capacity of 2% Mo-doped samples (Mo 2%) displays 279.67 mA h g–1 at 0.1 C (vs 254.39 mA h g–1 (pristine)), and the discharge capacity retention rate of Mo 2% is 79.4% after 300 cycles at 5 C (vs 47.6% (pristine)).

Introduction

Li-rich Mn-based cathode materials have great advantages over other cathode materials due to their high operating voltage and high energy density.1−8 The high capacity is mainly provided by the cation redox reaction and anion redox reaction.9−12 At 2–4.45 V, the transition metal ions undergo reversible redox processes (Ni2+/Ni4+ and Co3+/Co4+) which are the cation redox reaction.13−16 This process is often accompanied by transition metal migration, thus inducing phase transition.17−22 At 4.45–4.8 V, lattice O undergoes a redox reaction to provide charge compensation, which is the anion redox reaction.23−25 With irreversible O2 loss, it leads to structure collapse, capacity degradation, and poor cycling performance.16,26,27 The severe challenge of structural attenuation and capacity degradation in the electrochemical cycle also seriously hinders their practical application.4

Researchers have done a lot to address the above problems, such as surface coating28−33 and doping.34−37 Surface coating layers can effectively isolate electrolytes from the cathode surface and inhibit side reactions and HF corrosion, thus suppressing phase transition,29 but there is an interface gap between the surface coating layer and particle, which also increases the Li+ transmission resistance. Furthermore, it is unable to stabilize the O in a crystal lattice effectually.38 On the contrary, ionic doping can change the O network and Li+ diffusion route, which stabilizes the lattice O and enhances the rate performance of Li-rich Mn-based cathode materials.39 However, the surface cannot be protected from side reactions. Moreover, we find that the rock salt phase is electrochemically inert and possesses a high reaction energy barrier with the electrolyte and thus can effectively inhibit side reactions.40 The electrochemical performance of materials can be partly improved by the above modification measures. Considering comprehensively, we try to regulate the lattice and surface of materials at the same time, that is, to achieve the stability of lattice O and surface protection while avoiding the gap transmission resistance of the coating layer. For this purpose, we use Mo doping to situ induce the formation of heterostructures combined with a layered phase and rock salt on the surface. The surface-enriched Mo6+ converts part of Ni3+ into Ni2+ due to the charge conservation, and the increased Ni2+ occupies the Li site, thus forming a rock salt phase as a protective layer.41

For this subject, we chose (NH4)6Mo7O24·4H2O as the molybdenum source, calcined at 850 °C to realize Mo doping and construct a composite structure of a layered phase and rock salt phase on the surface in situ. Due to the charge conservation, the formation of the rock salt phase can be induced by Mo6+.41 Besides, a strong Mo–O bond can enhance the TM–O covalence and stabilize the O in the crystal lattice.39 Compared with the pristine (PLLR), the electrochemical performances of the modified samples are greatly improved, particularly for the Mo 2% sample. This work provides a new idea for consolidating the structure stability of Li-rich materials.

Experimental Section

Materials Preparation

The precursor (Mn0.75Ni0.25CO3) is prepared by the carbonate coprecipitation method. The mixed solution of 2 M NiSO4·6H2O and MnSO4·H2O (Ni/Mn molar ratio is 1:3), 2 M Na2CO3 (precipitant), and 0.2 M NH3·H2O (chelating agent) are added into a 500 mL continuous stirred tank reactor through three peristaltic pumps. During the coprecipitation process, the pH is always kept at 8.0; the temperature is maintained at 50 °C; and the carbonate precursor is obtained after continuous stirring for 8 h, filtration with pure water, washing, and drying in an oven at 120 °C for 12 h.

For the preparation of the original sample (Li1.2Mn0.6Ni0.2CO3), the precursor is ground and mixed with an excess of 5% Li2CO3 in a mortar, and the mixture is put in a tubular furnace and calcined at 500 °C for 5 h at a heating rate of 5 °C min–1 and then calcined at 850 °C for 12 h. Then the mixture is cooled naturally to room temperature (25 °C).

For the preparation of the modified sample, the precursor is ground, and the precursor and ammonium molybdate are mixed in different molar proportions (1%, 2%, and 3%) with an excess of 5% Li2CO3 in a mortar. The the mixture is put in a tubular furnace and calcined at 500 °C for 5 h at a heating rate of 5 °C min–1 and then calcined at 850 °C for 12 h. The the mixture is cool naturally to room temperature. Corresponding to doping content, the modified sample is referred to as Mo x% (x = 1, 2, and 3).

Physical Characterization

The details of the materials’ crystal structures are tested by powder X-ray diffraction (XRD), at a scan speed of 0.03367° s–1 with Cu Kα radiation. The Fullprof software is used to perform the Refine treatment. The surface composition and elemental valence state are obtained by X-ray photoelectron spectroscopy (XPS, Al Kα radiation) and calibrated by C 1s (284.8 mV). and in situ XPS is obtained to observe the change of O 1s during the first cycle of charging and discharging. The XPS Peak-Fit software is used to fit spectrum data. The morphology of all materials is evaluated by scanning electron microscopy (SEM). High-resolution transmission electron microscopy (HRTEM) with FEI Talos F200x is used to explore the selected area electron diffraction (SAED) patterns and elemental distribution mapping (EDS mapping). The Digital-Micrograph software is used for obtaining the Fourier transform (FFT) images.

Electrochemical Measurements

The electrochemical test is conducted in a CR2025 coin cell battery. The cathode material of CR2025, the coin cell battery is: 80% active material, 10% conductive agent (acetylene black), 10% poly(vinylidene fluoride) adhesive (PVDF), and n-methyl-2-pyrrolidine (NMP) as the solvent. It is mixed and ground and then evenly coated on an aluminum foil surface, dried at 120 °C for 12 h in a vacuum, and cut it into a circular electrode with a 10 mm diameter. The battery assembly process is completed in a glovebox filled with Ar. The cathode material of the battery is lithium metal, and the PP diaphragm is used as a separator and pressed under the pressure of 0.65 kPa with electrolytes. The electrochemical test is finished at 25 °C at 2.0–4.8 V through the Sunway channel. Electrochemical impedance spectroscopy (EIS) is obtained on the Zennium IM6 workstation at 2.0–4.8 V and a frequency range from 100 kHz to 10 mHz. For galvanostatic intermittent titration technique measurement (GITT), we discharge/charge the battery with a constant current for 10 min and then place it on the open-circuit table for 1 h.

Results and Discussion

Mo doping and heterostructure construction are realized after sintering (Figure 1a). All samples show a typical spherical porous secondary particle composed of uniform and dense primary particles (Figure 1b–e). The secondary particles of PLLR, Mo 1%, Mo 2%, and Mo 3% have similar diameters and display particle sizes of 11.48, 11.61, 11.97, and 12.16 μm, respectively, but the primary particles become bigger after modification (Figure 1f). Mo6+ doping can decrease the surface formation energy of the primary particles and increase their particle size, thereby changing the particle morphology.42 EDS mapping images (Figure S1) further show that Mo can be uniformly distributed.

Figure 1.

Figure 1

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(a) Experimental flowchart of Mo6+ doping. (b) SEM images of pristine. (c) Mo 1%, (d) Mo 2%, and (e) Mo 3% at different magnifications. (f) Statistical diagram of particle size. (g) X-ray patterns of all samples.

The crystal structures of the PLLR, Mo 1%, Mo 2%, and Mo 3% are explored by powder X-ray diffraction (XRD) (Figure 1g). All samples have characteristic diffraction peaks corresponding to the R3̅m space group of the α-NaFeO2 structure. In addition, typical characteristic peaks of the Li2MnO3 phase (C2/m structure group) are shown between 20° and 25°.43 The clear splits of the (006)/(102) and (108)/(110) peaks mean all samples show a well-layered crystal structure, indicating that Mo doping does not damage the structure of the material.44 The (003) peak offsets to a low angle after modification, indicating that Mo6+ is successfully doped into the particles.45 As shown in Figure S2a–d and Table S1, the lattice parameters are analyzed by the Rietveld refinement method. With the increase of doping amount, both c and v increase in varying degrees. This is because the radius of Mo6+ (0.59 Å), which is bigger than Ni3+and Mn4+ (0.56 and 0.53 Å), makes the layer space enlarged. Besides, part of Ni3+ (0.56 Å) is turned into larger Ni2+ (0.69 Å) due to the charge conservation.46 However, when Mo doping is excessive, c shrinks a little due to the strong Mo–O bond energy. I(003)/I(104) before and after modification is greater than 1.2, suggesting that all samples have a well-order layered structure.3

Moreover, we study the surface composition and elemental valence state of the samples before and after modification by X-ray photoelectron spectroscopy (XPS). In the full XPS spectrum, the characteristic peak of Mo 3d can be seen clearly, proving the existence of Mo (Figure 2a). In the Mo 3d spectrum, the characteristic peaks of Mo 3d5/2 (235 eV) and Mo 3d3/2 (232 eV) exist, which can prove the valence state of Mo in the particles (Mo6+).41 For the Ni 2p spectra of PLLR and Mo 2%, both Ni2+ (854.4 eV) and Ni3+ (856.0 eV) can be observed in Ni 2p3/2, and the relative intensity of the Ni2+ peak increases from 65.9% to 75.3% after modification, indicating that the content of Ni2+ increases.47,48 The radius of Ni2+ is similar to that of Li+, making it more inclined to occupy the Li site, thus forming a rock salt phase as a protective layer, protecting the crystal structure and inhibiting phase transition.49 It can be known that the ΔE3s of PLLR and Mo 2% are similar, at 4.50 and 4.65 eV, respectively (Figure 2d). In addition, Mn 3s ΔE3s and the valence states of Mn have the following linear relationship:3,50

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Figure 2.

Figure 2

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X-ray photoelectron spectroscopy (XPS) of different elements in PLLR and Mo 2%. (a) Full spectrum of XPS. (b) Mo 3d, (c) Ni 2p, and (d) Mn 3s.

According to this formula, the valence states of Mn in the PLLR and Mo 2% are +3.96 and +3.76, respectively. It is speculated that the valence state change of Mn is related to the charge compensation caused by Mo6+ doping because the low oxidation valence state of Mn supports its further oxidation and provides capacity.46 In the O 1s spectrum (Figure S3a), the characteristic peak at 529.5 eV represents oxygen in the form of lattice oxygen (TM–O), and the characteristic peak at 532.1 eV represents weak carbonate species on the surface (CO32–).51,52 The analysis results show that the proportion of lattice oxygen increases slightly after modification (66.2% → 66.8%), reducing residual Li on the surface. In addition, the chemical characteristics of the outer surface area of the PLLR sample and Mo 2% particles are further investigated by XPS within the etching depth of 60 nm (0.33 nm/s), as shown in Figure S3b–h. The results show that the concentrations of all elements hardly changed near the particle surface, except Mo. However, the concentration of Mo distributes a gradient trend, which decreases from outside to inside, indicating that Mo6+ enters the material and is enriched on the surface after calcination. The quantitative analysis results (Table S2) show that the Mo content gradually decreases with the depth. At the same time, the peak value of Mo6+ does not shift significantly with the deepening of the etching depth, indicating that the valence state of Mo does not change with the change of etching depth. Combined with XRD analysis, it can be inferred that as the concentration of Mo6+ decreases with depth more Mo6+ near the surface leads to the increase of Ni2+ and the aggravation of cation mixing. Some Ni2+ migrates from the transition metal layer to the Li layer, resulting in the formation of the rock salt phase near the surface.49

To study the effect of Mo doping on the redox reaction of lattice oxygen, in situ XPS was carried out on the O 1s region of the PLLR and Mo 2% when charging to 4.0, 4.4, and 4.8 V and then discharging to 2.0 V during the first cycle (Figure 3). During charging to 4.8 V, the anion redox reaction results in the oxidation of the lattice oxygen (O2–) to form peroxo-like oxygen ((O2)n−). Therefore, the intensity of the characteristic peaks of peroxo-like oxygen (530.7 eV) in both samples increases significantly, and the intensity of the characteristic peaks of lattice oxygen (529.8 eV) decreases. When discharging to 2 V, the intensity of the characteristic peaks of peroxo-like oxygen decreases with various degrees. It was found that the characteristic peaks of peroxides in the modified sample basically disappeared, and the lattice oxygen peak increased compared with the PLLR sample, indicating that the anion redox reaction in the Mo 2% sample was more reversible.53,54

Figure 3.

Figure 3

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In situ XPS of O 1s measured at C-4.0, 4.4, and 4.8 V and D-2.0 V for (a) PLLR and (b) Mo 2%.

TEM analyses are carried out to study the surface microstructure, and it can be observed that the microstructure near the particle surface of PLLR is a complete and uniform layered structure (Figure 4a). The local HRTEM images and the corresponding FFT diagrams can confirm that, with point O as the starting point of the vector, three planes are (110), (−112), and (−202) planes of the C2/m phase structure, respectively. The analysis of Mo 2% shows that the material structure is mainly divided into two areas, site 1 and site 2, which correspond to the C2/m phase and Fm3̅m phase, respectively (Figure 4b–d). The formation of the rock salt phase on the surface is mainly attributed to the doping of some Mo6+, which leads to the migration of some Ni2+ from the TM layer to the Li layer.49 The epitaxial rock salt phase growth is helpful to maintain the surface and structure stability. Combined with the above XPS and XRD analyses, it can be concluded that Mo doping can construct a rock salt phase on the surface.

Figure 4.

Figure 4

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Sample HRTEM images before the cycles: (a) PLLR and (b–d) Mo 2%.

The electrochemical properties of all samples are characterized, as shown in Figure 5. The specific discharge capacities (0.1 C) of PLLR, Mo 1%, Mo 2%, and Mo 3% are 254.39, 277.27, 279.67, and 268.75 mA h g–1 corresponding to their first-Coulomb efficiencies of 74.3%, 86.5%, 85.4%, and 84.9% and the specific discharge capacities of 1 C of 193.59, 197.69, 212.98, and 199.85 mA h g–1, respectively. In addition, as for the rate performance, the improved electrodes show better ability than the PLLR electrode at different rates. At 5 C, the discharge capacity of the Mo 2% sample reaches 138.60 mA h g–1, much higher than the original 62.17 mA h g–1 (Figure 5b). Besides, the cycle performance of the modified samples is also improved. After 200 cycles at 1 C, the capacity is 148.23, 160.82, 178.39, and 173.73 mA h g–1 with a capacity retention of 75.6%, 81.3%, 83.8%, and 86.9%, respectively (Figure 5c). The PLLR and Mo 2% electrodes are cycled for 300 cycles at 5 C, and the Mo 2% sample still has a specific discharge capacity of 118.03 mA h g–1 and a capacity retention rate of 79.4%, which is much better than 65.33 mA h g–1 and 47.6% of the PLLR (Figure 5d). Comparing the discharge curves of PLLR and Mo 2% samples at the 1st, 25th, 50th, 100th, and 200th cycles, it can be found that the voltage attenuation is suppressed after Mo doping (Figure S4). The above results show that this modification measure improves the electrochemical performance effectively, and the cycle stability of Mo 2% is optimal. It also further proves that a rock salt phase can be epitaxially constructed on the surface by Mo doping. The formation of heterostructures can effectively slow down the reaction of electrolyte, inhibit HF attack, stabilize lattice O, and improve material structure stability.36,41 Subsequent impedance calculations further support this view (Figure 6). The first charging process of the material contains two oxidation reactions: (1) Li+ is extracted from the LiMO2 component (R3̅m phase) at 4.0 V (O2), and a Ni2+/Ni4+ oxidation reaction occurs. (2) Partially irreversible Li+, at 4.5 V (O1), O2–/O2n– (1 < n < 3), is extracted from the Li2MnO3 component (C2/m phase) through the anion oxidation reaction of O2. The corresponding peaks of O1 and O2 are reduction reactions of R1 (4.3 V) and R2 (3.7 V), respectively. The R3 peak is related to the Mn4+/Mn3+ reaction at 3.4 V.7 Mo 2% shows a larger R3 peak than PLLR, representing more reduction reactions and discharge capacity, and the degradation and polarization of the battery potential of Mo-doped samples during the cycle do not significantly decrease, which is speculated to be related to the increase of Ni2+ leading to the intensification of cation mixing.

Figure 5.

Figure 5

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(a) First-Coulomb efficiency and first cycle discharge capacity at 0.1 and 1 C of all samples. (b) The discharge capacity of all samples at different rates (0.2 C, 0.5 C, 1 C, 3 C, 5 C). The cycling performance at (c) 1 C and (d) 5 C.

Figure 6.

Figure 6

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dQ/dV curves of (a) the PLLR and (b) Mo 2%.

The EIS diagrams (Figure S5) are composed of two semicircles. The high-frequency semicircle reflects the surface SEI film resistance (Rsf), and the medium frequency semicircle is related to the charge transfer (Rct) process.55 The smaller the charge transfer resistance, the lower the potential polarization during charging. There is little difference between the surface facial mask impedance (Rf) and charge transfer resistance (Rct) values of PLLR and Mo 2% samples before cycling, but after 200 cycles, the difference between Rct values of PLLR and Mo 2% increases significantly after 200 cycles. The increase in Rf and Rct of PLLR can be attributed to the formation of additional SEI layers caused by side reactions during cycling.55 Therefore, it is proved that the in situ generated heterostructure combined with the layered phase and rock salt can effectively avoid the transmission resistance caused by surface gaps, and the rock salt phase can effectively restrain the surface side reaction.

The GITT test results (Figure S6) show that the Mo 2% electrode is significantly higher than the PLLR electrode. The result is consistent with the previous charge–discharge test results. Moreover, the Mo doping can stabilize the lattice oxygen, limit anions to participation in redox, and inhibit irreversible O loss, resulting in a slight decrease in the DLi+ value under high voltage compared with the PLLR. Therefore, it can be seen that the modification has a significant effect on stabilizing the crystal structure and improving the electrochemical performance.

In addition, TEM analysis is carried out on PLLR and Mo 2% samples after 200 cycles at 1 C, as shown in Figure 7. We can see three different regions from the inside to the outside of the PLLR samples after circulation, the typical layered lattice stripe C2/m phase, the intricate transition phase, and part of the rock salt phase (Figure 7a–d). It shows that the structure of PLLR produces a large number of phase transitions and serious particle breakage during the cycle process. The thickness of sites 1 and 2 is about 28–30 nm. However, Mo 2% samples still maintain the internal and external dual-phase structure (Figure 7e–g). Under the protection of the rock salt phase, the internal structure still maintains a very clear layered lattice stripe and a very complete particle. This further shows that the rock salt phase is helpful to reduce the phase transition during the cycle.

Figure 7.

Figure 7

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Sample HRTEM images after the cycles: (a–d) PLLR and (e–g) Mo 2%.

Conclusions

In summary, the heterostructure of the rock salt phase and layer phase can be induced by Mo doping. The induced rock salt phase is combined with the layered phase closely, such that the gap transmission resistance can be avoided, and the surface rock salt phase has a high reaction energy level with the electrolyte, which can effectively inhibit side reactions and phase transformation. In addition, the strong Mo–O bond energy can enhance lattice O and improve the structure stability. The Mo 2% electrode shows an excellent cycle ability and rate performance, and the capacity retention rate is 79.7% after 300 cycles at 5 C. This strategy combines epitaxial heterostructure construction with strong TM–O bond formation, which can greatly improve the structural stability and provide a modification idea for layered oxides.

Acknowledgments

The authors kindly acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. U20A200201, 21878195, and 22108183), the Distinguished Young Scholars of Sichuan Province (2020JDJQ0027), 2020 Strategic cooperation project between Sichuan University and Zigong Municipal People’s Government (No. 2020CDZG-09), 2020 Strategic cooperation project between Sichuan University and Luzhou Municipal People’s Government (No. 2020CDLZ-20), State Key Laboratory of Polymer Materials Engineering (No. sklpme2020-3-02), Sichuan Provincial Department of Science and Technology (No. 2020YFG0471 and 2020YFG0022), Sichuan Province Science and Technology Achievement Transfer and Transformation Project (No. 21ZHSF0111), and Sichuan University’s postdoctoral interdisciplinary Innovation Fund.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07313.

  • EDS mapping diagrams of all elements in PLLR and Mo 2%. XRD Rietveld refinements and results for all samples. X-ray photoelectron spectroscopy (XPS) of O 1s in PLLR and Mo 2%. The element’s concentration depth distribution based on XPS data. Mo 3d, Ni 2p, Mn 3s, and O 1s spectra for PLLR. Mo 3d, Ni 2p, Mn 3s, and O 1s spectra for Mo 2%. Quantification of the change of Mo concentration in Mo 2% with the depth of the particles by XPS. The discharge curves of the PLLR and Mo 2% sample at the 1st, 25th, 50th, 100th, and 200th cycle. The EIS diagram of the PLLR and Mo 2% samples. The Li+ diffusion coefficient of the PLLR and Mo 2% electrodes before the cycle (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c07313_si_001.pdf (621.4KB, pdf)

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