Improved Electrode Reversibility of Nanosized Li_1.15Nb_0.15Mn_0.7O₂ through Li₃PO₄ Integration

Abstract

A lithium-excess cation-disordered rocksalt oxide, Li_1.15Nb_0.15Mn_0.7O₂, is synthesized and tested as positive electrode materials for battery applications. Although nanosized Li_1.15Nb_0.15Mn_0.7O₂ delivers a large reversible capacity using cationic/anionic redox reaction, the inferior capacity retention hinders its use for practical applications. Such degradation of electrode reversibility, including electrochemical and structural reversibility, is anticipated to originate from the gradual oxygen loss for the electrode materials with anionic redox. Herein, Li₃PO₄ is integrated into Li_1.15Nb_0.15Mn_0.7O₂ by high-energy mechanical milling, and 7 mol% Li₃PO₄ integrated Li_1.15Nb_0.15Mn_0.7O₂, Li_1.2P_0.06Nb_0.13Mn_0.61O₂, shows much improved cyclability when compared with the sample without Li₃PO₄. Approximately 80 % of reversible capacity is retained after 100-cycle test at a rate of 200 mA g⁻¹. Moreover, electrode kinetics are significantly improved by Li₃PO₄ integration, and Li_1.2P_0.06Nb_0.13Mn_0.61O₂ delivers a discharge capacity of 200 mA h g⁻¹ at a rate of 640 mA g⁻¹. Li_1.2P_0.06Nb_0.13Mn_0.61O₂ also shows improved thermal stability at elevated temperatures. From these results, the effectiveness of Li₃PO₄ integration into nanosized disordered rocksalt oxides with anionic redox is discussed, and this finding leads to the development of metastable high-capacity positive electrode materials for advanced Li-ion batteries.

1. Introduction

To satisfy the rapidly growing demand for lithium-ion batteries (LIBs) in the global market, the scientific community is focused on developing high-performance electrode materials.^1–7 Among several candidates, Li-excess oxides with a cation-disordered rocksalt (DRS) structure are promising positive electrode materials due to their high theoretical capacities (>350 mA h g⁻¹).^8–12 DRS oxides used to be disregarded as a positive electrode material because of the absence of a lithium migration path in DRS structure. However, recently, it is found that facile Li ion conduction is possible by forming percolative networks for Li sites in oxides with DRS structure, and the enrichment of Li ions in host structures is an effective approach to increase the probability for percolative pathways. When compared with layered structures, Li enrichment is, therefore, essential to improve electrode performance of electrode materials with DRS structure, and better electrode kinetics are observed for Li-excess DRS oxides.¹³ In addition, nanosizing is proved as an efficient strategy to overcome inferior kinetics in DRS structure.^8,10,14 The optimization of Li contents in host structures and particle size/morphology are necessary to effectively improve the electrode performance of oxides with DRS structure for battery applications.

Another important strategy to increase reversible capacity is the activation of anionic redox, coupled with classical and conventional cationic redox. Although anionic redox reaction is activated and stabilized in Li-excess DRS oxides with Mn ions through π-type interaction with Mn t_2g orbitals,¹⁵ electrode reversibility, coupled with electrochemical and structural reversibility, is not high enough for practical battery applications associated with oxygen dimerization and irreversible oxygen loss.^8,10,16 Recently, anionic redox with σ-type interaction of Ni e_g orbitals in DRS structure, which is not activated in the ordered layered structure, has been also reported.¹⁵ An effective way to suppress the oxygen release is the optimization of chemical compositions, and Mn enrichment results in better reversibility for Li₂TiO₃–LiMnO₂ binary system.¹⁷ However, the improvement mainly relies on the enrichment of use for cationic redox, and the use of anionic redox is partly restricted.¹⁰ Another methodology to suppress the oxygen release is found in the substitution of anionic species, for instance, fluorine substitution. Oxidation states of transition metal ions are also decreased by the substitution of O²⁻ by F⁻. Therefore, the partial substitution by fluoride ions effectively modifies the balance between cationic and anionic redox. In addition, because fluoride ions have the highest electronegativity, electrochemical oxidation is hindered for fluoride ions, and thus oxygen dimerization is partially suppressed.^18,19 A similar phenomenon is found in the oxide after the integration of phosphate anions. Phosphorus ions are successfully integrated into nanosized LiMnO₂ by high-energy mechanical milling.²⁰ Thanks to the more covalent nature of P-O bonds compared with Mn-O and Li-O bonds, phosphorus ion integration can effectively suppress the excessive oxidation of oxygen ions and oxygen dimerization. Improved structural and thermal stability are also anticipated. However, factors of integrated Li₃PO₄ affecting electrochemical properties for oxides with DRS structure are not fully understood.

In this study, phosphorus integration into Li_1.15Nb_0.15Mn_0.7O₂ with DRS structure is targeted, and 7 mol% of Li₃PO₄ is integrated into Li_1.15Nb_0.15Mn_0.7O₂ by high-energy mechanical milling. Metastable and nanosized Li_1.2P_0.06Nb_0.13Mn_0.61O₂ with DRS structure shows much improved electrode kinetics and capacity retention when compared with nanosized Li_1.15Nb_0.15Mn_0.7O₂ without phosphorus integration. From these results, the possibility of a metastable high-capacity electrode material, Li_1.2P_0.06Nb_0.13Mn_0.61O₂, is discussed for lithium storage applications.

2. Experimental

2.1 Material preparation and characterization

Li_1.15Nb_0.15Mn_0.7O₂ was synthesized by a solid-state reaction, calcinating a mixture of Li₂CO₃ (98.5 %; Kanto Kagaku), Mn₂O₃ and Nb₂O₅ (99.9 %, Wako Pure Chemical Industries) at 1050 °C for 12 h in Ar. Mn₂O₃ can be produced by heating MnCO₃ at 850 °C for 12 h in air. All the compounds were pressed into pellets before heating. Li₃PO₄ (7 mol%) was further integrated into Li_1.15Nb_0.15Mn_0.7O₂ by mechanical milling (PULVERISETTE 7; FRITSCH) in a zirconia pot with zirconia balls at 600 rpm for 24 h. The mixture of Li₃PO₄ and Li_1.15Nb_0.15Mn_0.7O₂ was taken out from the zirconia pot after 12 h, and mixed by hand grind using an alumina mortar and pestle to ensure powder uniformity. To study the influence of Li₃PO₄ integration, a nanosized sample without Li₃PO₄ (nanosized Li_1.15Nb_0.15Mn_0.7O₂) was also synthesized by high-energy mechanical milling at 600 rpm for 24 h. The obtained samples were kept in a glovebox before use.

X-ray diffraction (XRD) patterns of the samples were collected using an X-ray diffractometer (D2 PHASER; Bruker Corp., Ltd.) equipped with a one-dimensional X-ray detector using Cu Kα radiation generated at 300 W (30 kV and 10 mA) with a Ni filter. An airtight sample holder was used for the XRD measurement. In-situ XRD data were collected from an X-ray diffractometer (D8 ADVANCE, Bruker) using Cu Kα radiation generated at 1600 W (40 kV and 40 mA) with a Ni filter. In-situ XRD patterns were collected by a battery cell equipped with a Be window. Morphological features of samples were observed using a scanning electron microscope (JCM-6000, JEOL) with acceleration voltage of 15 kV and scanning transmission electron microscopy (STEM) (ARM200F, JEOL) at 200 kV. Elemental distributions are measured using STEM with an energy dispersive X-ray spectrometer (EDX, JED-2300T, JEOL). Raman spectra of the samples were collected by using a Raman microscope (inVia reflex) with a 532 nm laser. The measurement was conducted with a sample holder (LIBcell, Nanophoton) without exposure to air.

Hard X-ray absorption spectroscopy (XAS) at Mn K-edge was performed at beamline BL-12C of the Photon Factory synchrotron source in Japan. Hard XAS spectra were collected with a silicon monochromator in the transmission mode. The intensities of the incident and transmitted X-rays were measured using an ionization chamber at room temperature. Samples for XAS measurements were prepared using the two-electrode cells at a rate of 10 mA g⁻¹. The composite electrodes were rinsed with dimethyl carbonate and sealed in a water-resistant polymer film in the Ar-filled glovebox. Normalization of the XAS spectra was carried out using the program code IFEFFIT.²¹ The post-edge background was determined using a cubic spline procedure. Schematic illustrations of crystal structures of samples were drawn using the program VESTA.²² Thermal stability of electrode materials was tested by differential scanning calorimetry (DSC, DSC-60 Plus, SHIMADZU). For the sample preparation, the cell was charged to 4.8 V at a rate of 10 mA g⁻¹ to obtain fully oxidized sample, then the cell was disassembled in the glovebox. The positive electrode with Al foil was soaked in dimethyl carbonate to remove residual electrolyte, and then dried in vacuum. The powder of charged samples was peeled off from Al foil, and then used for DSC measurement. Electrolyte solution (0.75 µL) was added to 2.5 mg of fully oxidized samples, and the mixture was sealed in a high pressure-resistant container made of stainless steel. Containers were heated at 5 °C min⁻¹ in a nitrogen atmosphere from 50 to 400 °C.

2.2 Electrochemical measurements

Before preparing composite electrode slurry, the samples were further mixed with 10 wt% acetylene black using planetary ball milling at 300 rpm for 12 h to obtain uniform mixtures of oxides and acetylene black. Composite electrodes consisted of 76.5 wt% active materials, 13.5 wt% acetylene black, and 10 wt% poly(vinylidene fluoride), pasted on aluminum foil as a current collector. The electrodes were dried at room temperature for 2 h in vacuum and then heat at 120 °C in a vacuum for 2 h. The electrolyte solution used was 1.0 M LiPF₆ dissolved in ethylene carbonate and dimethyl carbonate (3 : 7 by volume) (battery grade, Kishida Chemical). All the cells are assembled with the electrolyte amount of 300 µL. Polyolefin microporous membrane was used as a separator. Metallic lithium (Honjo Metal) was used as a negative electrode. Two-electrode cells (TJ-AC, Tomcell Japan) were assembled in the Ar-filled glovebox. Electrochemical impedance measurement was conducted by using a potentiostat equipped with a frequency response analyzer (SP-200, Bio-Logic). All the electrochemical measurements were carried out at room temperature (25 °C on average).

3. Results and Discussion

Results of structural and morphological characterizations of nanosized Li_1.15Nb_0.15Mn_0.7O₂ with or without the integration of Li₃PO₄ are summarized in Fig. 1. X-ray diffraction (XRD) patterns of starting materials, as-prepared Li_1.15Nb_0.15Mn_0.7O₂ and Li₃PO₄ are shown in Fig. 1a. Li_1.15Nb_0.15Mn_0.7O₂ is crystallized into a single cubic phase with a cation-disordered rocksalt (DRS) structure (space group; Fm-3m). Nanosized Li_1.15Nb_0.15Mn_0.7O₂ is obtained after mechanical milling at 600 rpm for 24 h (Fig. 1a). Similar crystallinity and diffraction pattern are also obtained for the sample mixed with Li₃PO₄. Note that diffraction lines of Li₃PO₄ were completely lost by milling. Lattice parameters are also changed from 416.1 to 417.3 pm (4.161 to 4.173 Å) before and after milling with Li₃PO₄. Similar particle morphology is found in nanosized Li_1.15Nb_0.15Mn_0.7O₂ with or without Li₃PO₄ as shown in Fig. 1b, but less agglomeration is evidenced for the sample with Li₃PO₄. This fact will be beneficial for the electrode materials with DRS structure to mitigate inferior electrode kinetics.²³ Detailed nanostructures and atomic distributions, including P ions, were analyzed by STEM/EDX analysis (Fig. 1c and Supporting Figs. S1 and S2). Nanosized crystalline particles (∼5 nm) are observed, and uniform distributions for P ions are noted on elemental maps. No phase segregation of P ions in nanoscale is clearly found from EDX mapping.

Figure 1.

Synthesis of Li₃PO₄ integrated Li_1.15Nb_0.15Mn_0.7O₂ by mechanical milling: (a) X-ray diffraction patterns of nanosized Li_1.15Nb_0.15Mn_0.7O₂ with/without Li₃PO₄, (b) particle morphology, (c) a high-resolution lattice image of Li_1.2P_0.06Nb_0.13Mn_0.61O₂ and elemental distributions observed by STEM/EDX spectroscopy, (d) Raman spectra. The data of reference samples are also shown.

To further analyze local structures of the sample, Raman spectra of different samples are compared in Fig. 1d. Nanosized Li_1.15Nb_0.15Mn_0.7O₂ presents two characteristic peaks at 599 and 789 cm⁻¹, the former one is also found in mechanically milled LiMnO₂ and is assigned to a vibration mode of Mn-O bonds in MnO₆ octahedra, while the latter one is found in mechanically milled Li₃NbO₄ and is assigned to a vibration mode of Nb-O bonds in NbO₆ octahedra. Furthermore, for the sample with Li₃PO₄, an additional peak at 925 cm⁻¹ is noted, which is also found in Li₃PO₄ and originates from a symmetric stretching mode of PO₄ tetrahedra. Nevertheless, a clear difference for wave numbers is noted. This shift suggests that the P-O bond distance is elongated for the integrated phase, which is indicative of the dissolution of P ions into nanosized Li_1.15Nb_0.15Mn_0.7O₂ with DRS structure, as similarly discussed in LiMnO₂–Li₃PO₄ binary system.²⁰ These experimental observations suggest the formation of solid solution between Li_1.15Nb_0.15Mn_0.7O₂ and Li₃PO₄, and hereafter, a chemical formulation of Li_1.2P_0.06Nb_0.13Mn_0.61O₂ is used for the P integrated sample. The metastable character of Li_1.2P_0.06Nb_0.13Mn_0.61O₂ is confirmed by heat treatment at elevated temperatures (Supporting Fig. S3), as different phases are crystallized, especially at 700 °C.

Electrochemical properties of Li_1.15Nb_0.15Mn_0.7O₂ and Li_1.2P_0.06Nb_0.13Mn_0.61O₂ were examined as positive electrode materials in Li cells. Galvanostatic charge/discharge curves at a rate of 10 mA g⁻¹ in the voltage range of 1.5–4.8 V are reported in Fig. 2a. Li_1.2P_0.06Nb_0.13Mn_0.61O₂ delivers an initial discharge capacity of 280 mAh g⁻¹, which is slightly larger than that of Li_1.15Nb_0.15Mn_0.7O₂. On consecutive cycles, shown in Fig. 2b, Li_1.15Nb_0.15Mn_0.7O₂ presents clear capacity fading. In contrast, Li_1.2P_0.06Nb_0.13Mn_0.61O₂ shows better capacity retention, indicating that the P ion integration is beneficial for the improvement of electrode reversibility, even though the capacity based on cationic Mn redox is reduced and anionic oxygen redox is more activated. Moreover, lowering the cut-off voltage results in better capacity retention, and 96 % capacity is retained after 20 cycles with 4.0 V cut-off as reported in Fig. 2c. Rate capability of the samples with or without Li₃PO₄ integration was evaluated from 10 to 640 mA g⁻¹ at the voltage range of 1.5–4.6 V, and results are shown in Fig. 2d. Rate capability is clearly improved for Li_1.2P_0.06Nb_0.13Mn_0.61O₂. As displayed in Fig. 2d, even at a rate of 640 mA g⁻¹ Li_1.2P_0.06Nb_0.13Mn_0.61O₂ delivers a discharge capacity of 200 mAh g⁻¹, which is significantly better than Li_1.15Nb_0.15Mn_0.7O₂, <100 mAh g⁻¹ at the same rate. Although Li_1.15Nb_0.15Mn_0.7O₂ shows almost the same capacity with Li_1.2P_0.06Nb_0.13Mn_0.61O₂ at a slow rate of 10 mA g⁻¹, the P integration is beneficial for the improvement of electrode kinetics. To further support this observation, impedance measurement was conducted after charge to 3.8 V, corresponding to approximately 50 % depth of charge. The results shown in Fig. 2e denote that Li_1.2P_0.06Nb_0.13Mn_0.61O₂ has a much lower impedance, which supports better kinetics as electrode materials by P integration. An extended cycle test for continuous 100 cycles was further conducted for Li_1.2P_0.06Nb_0.13Mn_0.61O₂ and Li_1.15Nb_0.15Mn_0.7O₂ at a rate of 200 mA g⁻¹ with a voltage range of 1.5–4.8 V (Fig. 2f). Excellent capacity retention is achieved for Li_1.2P_0.06Nb_0.13Mn_0.61O₂, and 80 % of initial discharge capacity is retained after 100 cycles, which demonstrates the efficient improvement of electrode reversibility and enhanced electrode kinetics through the P ion integration. When lower cut-off voltage to 4.3 V is used, both two electrodes show relatively good cycle stability due to the less utilization of anionic redox and the suppression of electrolyte decomposition. However, Li_1.2P_0.06Nb_0.13Mn_0.61O₂ with superior electrode kinetics delivers a larger reversible capacity as electrode materials (Supporting Fig. S4). Additionally, as expected from the structural changes after heat treatment of Li_1.2P_0.06Nb_0.13Mn_0.61O₂ with metastability, the degradation of electrochemical performance is observed for heat treated samples (Supporting Figs. S5 and S6).

Figure 2.

Electrochemical properties of Li_1.15Nb_0.15Mn_0.7O₂ and Li_1.2P_0.06Nb_0.13Mn_0.61O₂ at room temperature: (a) Galvanostatic charge/discharge curves, (b) cyclability, (c) influence of cutoff voltages on capacity retention, (d) rate capability, (e) Nyquist plot, and (f) the extended cycle test at a rate of 200 mA g⁻¹ in the voltage range of 1.5–4.8 V.

The electrode reversibility of Li_1.2P_0.06Nb_0.13Mn_0.61O₂ was further examined in Li cells by combinational studies by in-situ XRD and ex-situ XAS spectroscopy at initial cycle. Results of in-situ XRD study are presented in Fig. 3a and Supporting Fig. S7. The DRS structure is retained after electrochemical cycle, which is found from the fact that reversible changes for 200 and 220 diffraction lines. However, asymmetric changes of lattice parameters are noted for charge/discharge. A relatively small change in lattice parameters is observed above 4.0 V on charge, but clear lattice shrinkage is observed during the start of discharge. Such asymmetric changes are expected to originate from the large voltage hysteresis related to O-O dimerization formed by anionic redox reaction as proposed in literature.²⁴ Relatively large voltage hysteresis is generally observed for Li-excess Mn-based oxides.²⁵ Recently, a small or negligible volume change was reported in V system with DRS structure,^23,26 but the lattice shrinkage is unavoidable in Mn system with DRS structure. Volume change on charge/discharge is calculated to be 6.2 % for Li_1.2−yP_0.06Nb_0.13Mn_0.61O₂. Similar to this trend observed in lattice parameters, a non-linear change is also found in Mn K-edge XAS spectra. For the as prepared sample, the oxidation state of Mn ions is trivalent state, and after charge to 4.0 V, the XAS spectrum at Mn K-edge shifts to higher energy, indicating Mn oxidation from trivalent to tetravalent state. Absorption energy values for both oxidation states are also consistent with literature.²⁷ However, when charged above 4.0 V, a small change is noted at Mn K-edge XAS spectra, which is indicative of activation of anionic redox at a higher voltage region. In addition, on discharge, a small change is observed <2.5 V for Mn K-edge XAS spectra, suggesting the large voltage hysteresis for anionic redox. The voltage hysteresis is expected to be larger for oxygen anionic redox when compared with Mn cationic redox.

Figure 3.

Reaction mechanisms of Li_1.2P_0.06Nb_0.13Mn_0.61O₂: (a) In-situ XRD profile and a corresponding charge/discharge curve, (b) XAS spectra on electrochemical cycles (1.5–4.8 V), and (c) DSC curves of the fully charged samples.

The impact of P ions on thermal stability, which is an important factor to ensure safety for practical applications, is further studied by DSC study. DSC curves of fully charged Li_1.15−yNb_0.15Mn_0.7O₂ and Li_1.2−yP_0.06Nb_0.13Mn_0.61O₂ samples with electrolyte are compared in Fig. 3c, and better thermal stability at fully charged state is supported from the suppression of heat generation. Both samples show heat generation from 200 °C, but total heat generation is significantly suppressed for the sample with P ions. Note that these data are successfully reproduced from different measurements as shown in Supporting Fig. S8. The results prove the importance of P integration for Mn-based electrode materials with anionic redox, resulting in the development of high-energy, durable, and safe electrode materials in the future.

4. Conclusions

This study has described the synthesis of metastable and nanosized Mn-based oxides with Li₃PO₄ integration by high-energy mechanical milling for Li storage applications. P ions are uniformly dissolved into the crystal lattice of Li_1.15Nb_0.15Mn_0.7O₂ without P ion clustering, which has been successfully proved by XRD, STEM/EDX, and Raman spectroscopy. The P integration effectively improves electrode performance for Li-excess Mn-based oxides with DRS structure, including electrode kinetics, cyclability, and better thermal stability. Inferior electrode kinetics for Mn-based oxides with DRS structure are effectively mitigated through P ion integration. The reversibility of anionic redox is also significantly improved. The findings provided in this study contribute to further research and technological progress for electrode materials using anionic redox, leading to the future development of high-capacity and high-energy positive electrode materials with abundant Mn ions.

Acknowledgments

This work has been partially supported by JSPS, Grant-in-Aid for Scientific Research (Grant Numbers 19H05816 and 21H04698), and JST, CREST Grant Number JPMJCR21O6, Japan. The synchrotron X-ray absorption work was done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2021G039). NY also thanks CBMM for the partial financial support.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.22180366.

• 1) A lithium-excess cation-disordered rocksalt oxide, Li_1.15Nb_0.15Mn_0.7O₂, is synthesized and tested as positive electrode materials for battery applications. Although nanosized Li_1.15Nb_0.15Mn_0.7O₂ delivers a large reversible capacity using cationic/anionic redox reaction, the inferior capacity retention hinders its use for practical applications. Such degradation of electrode reversibility, including electrochemical and structural reversibility, is anticipated to originate from the gradual oxygen loss for the electrode materials with anionic redox. Herein, Li₃PO₄ is integrated into Li_1.15Nb_0.15Mn_0.7O₂ by high-energy mechanical milling, and 7 mol% Li₃PO₄ integrated Li_1.15Nb_0.15Mn_0.7O₂, Li_1.2P_0.06Nb_0.13Mn_0.61O₂, shows much improved cyclability when compared with the sample without Li₃PO₄. Approximately 80 % of reversible capacity is retained after 100-cycle test at a rate of 200 mA g⁻¹. Moreover, electrode kinetics are significantly improved by Li₃PO₄ integration, and Li_1.2P_0.06Nb_0.13Mn_0.61O₂ delivers a discharge capacity of 200 mA h g⁻¹ at a rate of 640 mA g⁻¹. Li_1.2P_0.06Nb_0.13Mn_0.61O₂ also shows improved thermal stability at elevated temperatures. From these results, the effectiveness of Li₃PO₄ integration into nanosized disordered rocksalt oxides with anionic redox is discussed, and this finding leads to the development of metastable high-capacity positive electrode materials for advanced Li-ion batteries.

CRediT Authorship Contribution Statement

Yanjia Zhang: Data curation (Lead), Formal analysis (Lead), Validation (Lead), Writing – original draft (Lead)

Benoît D. L. Campéon: Data curation (Equal), Formal analysis (Equal), Writing – review & editing (Supporting)

Naoaki Yabuuchi: Conceptualization (Lead), Funding acquisition (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: 19H05816

Japan Society for the Promotion of Science: 21H04698

Japan Science and Technology Agency: JPMJCR21O6

Companhia Brasileira de Metalurgia e Mineração

Footnotes

Y. Zhang: ECSJ Student Member

N. Yabuuchi: ECSJ Active Member

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