Metastable and Nanosized Li_1.2Nb_0.2V_0.6O₂ for High-Energy Li-ion Batteries

Abstract

A Li-excess cation-disordered rocksalt oxide, Li_1.2Nb_0.2V_0.6O₂, with higher theoretical capacity than traditional stoichiometric and layered oxides, is synthesized and tested as positive electrode materials for battery applications. Although Li_1.2Nb_0.2V_0.6O₂ cannot be synthesized by conventional calcination method, a single phase and metastable oxide is successfully synthesized by high-energy mechanical milling. Electrode performance of metastable and nanosized Li_1.2Nb_0.2V_0.6O₂ is significantly improved by heat treatment at 600 °C. Heat treated Li_1.2Nb_0.2V_0.6O₂ with a partial cation ordered layered structure delivers a high reversible specific capacity of 320 mAh g⁻¹ on the basis of highly reversible two-electron redox of V ions. Moreover, inferior cyclability originating from the dissolution of V ions is successfully improved by using concentrate electrolyte solution, and over 90 % capacity retention is achieved after 50 cycles. This finding opens a new way to design high-capacity metastable Li-excess oxides for advanced Li-ion batteries with higher energy density.

1. Introduction

To realize a fossil fuel-free society, progress on Li-ion battery technology is indispensable. The development of high-capacity positive electrode materials is necessary to further increase the energy density of Li-ion batteries. Li-excess metal oxides have been extensively studied for potential advanced positive electrode materials in the past decade due to their potential higher reversible capacity than traditional stoichiometric oxides with the layered structure and phosphates.¹ By considering solely one-electron cationic redox for transition metal ions, all Li ions in the host structure of Li-excess metal oxides cannot be used. Therefore, the introduction of anionic redox chemistry is promising to realize the full use of Li ions in Li-excess metal oxides. Among them, Li-excess manganese-based oxides, Li₂MnO₃, have been extensively studied for potential high-capacity positive electrode materials.^2–6 However, the researches on the Li₂MnO₃-based systems have revealed that anionic redox triggers partial oxygen loss and subsequent irreversible phase transition, resulting in voltage decay on continuous cycles and restricting its use for practical battery applications.^7,8

Recently, our group has reported that the xLi₃NbO₄–(1 − x)LiVO₂ binary system can be used for high-capacity positive electrode materials without anionic redox involvement.^9,10 Unlike Li₂MnO₃-based systems, better capacity retention is achieved in Li-excess vanadium-based oxides, Li_1.25Nb_0.25V_0.5O₂ (x = 0.33 in xLi₃NbO₄–(1 − x)LiVO₂), with two-electron cationic redox of V³⁺/V⁵⁺. The incorporated d⁰ transition metal ion, Nb⁵⁺, is electrochemically inactive but plays an important role in stabilizing the structure.^11,12 Nb⁵⁺ without d electrons has no particular octahedral shape preference and therefore can accommodate octahedral distortions with the lower energy cost, leading to the stabilization of cation-disordered rocksalt (DRS) structure.¹³ As for Li-excess positive electrode materials without anionic redox, the theoretical capacity is jointly determined by Li and V contents in the host structure.¹⁴ Therefore, compared to reported chemistry composition Li_1.25Nb_0.25V_0.5O₂ (x = 0.33) with a suboptimal theoretical specific capacity of 300 mAh g⁻¹, the optimum chemistry composition is found in Li_1.2Nb_0.2V_0.6O₂ (x = 0.25) with a higher theoretical specific capacity of 360 mAh g⁻¹, which is essential to achieve high-energy-density. However, the thermodynamic metastability of Li_1.2Nb_0.2V_0.6O₂ makes it sensitive to phase segregation during the solid-state calcination reaction process. Single phase Li_1.2Nb_0.2V_0.6O₂ cannot be directly synthesized by the conventional calcination method, inevitably resulting in the reduction of experimentally observed reversible capacity.¹⁰

In this study, to overcome the problem of metastability and phase segregation, high-energy mechanical milling (MM) is adopted to synthesize single phase Li_1.2Nb_0.2V_0.6O₂. Mechanical energy, including shear stress and friction, often makes it possible to synthesize metastable DRS materials.^15–17 Especially, high-energy MM can produce a synthesis condition corresponding to heating at ≈1750 °C on the basis of empirical evidence,¹⁸ which would be expected to be higher than the melting point of the majority of Li-excess metal oxides. The electrochemical performance of metastable and nanosized Li_1.2Nb_0.2V_0.6O₂ with single phase is systematically evaluated in detail.

2. Experimental

2.1 Material preparation and characterization

As-prepared Li_1.2Nb_0.2V_0.6O₂ (LNVO-SS) was synthesized by a solid-state (SS) reaction, primary precursors, Li₂CO₃ (98.5 %, Kanto Kagaku), Nb₂O₅ (99.9 %, Wako Pure Chemical Industries), and V₂O₃ (98 %, Sigma-Aldrich Japan) were mixed by wet ball-milling at 300 rpm with ethanol for 5 h and the mixtures were pressed into pellets after drying in air. Excess amount (3 mol%) of Li₂CO₃ was utilized to compensate for the vaporization of Li source on heating. The pellets were heated at 950 °C for 12 h in an argon atmosphere. For the material synthesis with mechanical milling (MM) method, LiVO₂ and Li₃NbO₄ were prepared by SS calcination and used as precursors. From the mixture of precursors, LNVO-MM300, and LNVO-MM600 samples were prepared by MM method at 300 and 600 rpm, respectively, with a container and balls made by ZrO₂. After milling for 12 h, the mixture was taken out from the container and mixed with a mortar and pestle to ensure sample uniformity for following milling treatment. The total ball milling time was 48 h for LNVO-MM300 and 24 h for LNVO-MM600. The as-prepared samples (LNVO-SS, LNVO-MM300 and LNVO-MM600) were mixed with acetylene black (HS-100, Denka, sample : AB = 90 : 10 in wt%) using a planetary ball mill at 300 rpm for 5 h. The carbon composited LNVO-MM600 was further heat-treated at 400–600 °C for 4 h in Ar and the sample heated at 600 °C is denoted as LNVO-MM600-HT and further characterized. Finally, all prepared samples were stored in an Ar-filled glove box until use to avoid contact with moist air.

Particle morphology of the samples was observed using a scanning electron microscope (SEM, JCM-6000, JEOL) with an acceleration voltage of 15 kV and transmission electron microscopy (TEM) (JEM-2100F) operated under 200 kV. The structure characterizations of the samples were collected by X-ray diffraction (XRD) patterns using an X-ray diffractometer (D2 PHASER, Bruker Corp. Ltd.) and electron diffraction (ED). The surface chemistry was analyzed by X-ray photoelectron spectroscopy (XPS) (QuanteraSXM). The synchrotron XRD measurement is conducted at BL19B2¹⁹ in the SPring-8 synchrotron facility in Japan. The wavelength of X-rays was calibrated to be 0.500 Å. Structural refinement analysis was carried out using GSAS software with EXPGUI interface.^20,21 Hard X-ray absorption spectroscopy (XAS) was performed at beamline BL-12C of the Photon Factory Synchrotron Source in Japan. The hard XAS spectra were obtained with a silicon monochromator in a transmission mode. The intensity of the incident and transmitted X-ray was measured using an ionization chamber at room temperature. The composite electrodes were rinsed with dimethyl carbonate and sealed in a water-resistant polymer film in the Ar-filled glovebox. XAS spectra was normalized using the program, IFEFFIT.²² A cubic spline procedure was utilized for the estimation of post-edge background.

2.2 Electrochemical measurements

Electrode performance of carbon composite samples was examined. The slurry consisted of 76.5 wt% active material, 13.5 wt% acetylene black, and 10 wt% poly(vinylidene fluoride), was mixed and pasted on an aluminum foil. The electrodes were dried under vacuum and further heated at 120 °C in vacuum. Metallic lithium (Honjo Metal) was used as the negative electrode. The electrolyte solution used was 1.0 mol dm⁻³ LiPF₆ dissolved in ethylene carbonate : dimethyl carbonate (1 : 1 by volume) (Kishida Chemical) and lithium bis(fluorosulfonyl)amide (LiFSA) and DMC (LiFSA : DMC = 1 : 1.1 in molar ratio). A polyolefin microporous membrane was used as a separator. Two-electrode cells (TJ-AC, Tomcell Japan) were assembled in an Ar-filled glovebox. The cells were cycled at a current density ranging from 10 mA g⁻¹ to 2560 mA g⁻¹ at room temperature and 50 °C.

3. Results and Discussion

As summarized in Fig. 1, Li_1.2Nb_0.2V_0.6O₂ was prepared by different methods, solid-state (SS) calcination reaction (LNVO-SS), low-energy MM at 300 rpm (LNVO-MM300), and high-energy MM at 600 rpm (LNVO-MM600). The detailed structural evolutions of LNVO-MM600 and LNVO-MM300 with increased milling time are shown in Fig. S1a. The total milling time was 48 h for LNVO-MM300 and 24 h for LNVO-MM600. The particle morphology of the three samples observed by SEM is also shown in Fig. 1. It is noted that non-uniform and micrometer-sized particles are observed for LNVO-SS because of the easier crystallization process on heating at high temperatures (Fig. 1). Similarly, excessive growth of particle size of LiVO₂, which is used for the precursor of MM, is noted (Fig. S1b). In contrast, nanosized and agglomerated particles are observed after MM, which is a common feature for the samples synthesized by MM.^23,24 Additionally, the uniform distribution of O, V, and Nb ions at the nanoscale is evidenced by using a transmission electron microscope coupled with energy-dispersive X-ray spectroscopy (Fig. S2).

Figure 1.

A scheme of the synthesis procedure of Li_1.2Nb_0.2V_0.6O₂ by different methods. Particle morphology observed by SEM is also shown for comparison.

Figure 2a shows X-ray diffraction (XRD) patterns of the binary system of x = 0.25 in xLi₃NbO₄–(1 − x)LiVO₂ with both end members (x = 0 and 1.0). Li₃NbO₄ (x = 0) and LiVO₂ (x = 1) crystallize into a cation-ordered structure with distinct octahedral site locations for Li, Nb, and V ions (Fig. 2b). As for LNVO-SS sample, a major phase can be assigned as partially cation-ordered layered structure because of the presence of a diffraction line at 18°.^9,10 The enlarged XRD patterns in Fig. 2a reveal that clear phase segregation exists in LNVO-SS sample. A shoulder peak is observed at 44°, which suggests the presence of a minor second phase with a different cation distribution from the major phase. Although the shoulder peak observed for LNVO-SS is less obvious in the XRD pattern for LNVO-MM300, the segregation accompanied with a low crystallinity phase can be identified by the electron diffraction (ED) study as shown in Fig. 2c. After high-energy milling (LNVO-MM600), a single phase sample with a low crystallinity DRS structure is successfully obtained, in which Li, Nb, and V ions occupy the same octahedral 4a site (Fig. 2b). No phase segregation and smaller particle sizes (10–200 nm), when compared with LNVO-MM300, are also evidenced by ED study for LNVO-MM600 (Fig. 2c). Galvanostatic charge/discharge evaluation of thus prepared samples was conducted and is reported in Fig. 3a. As expected, LNVO-MM600 without phase segregation delivers a larger reversible specific capacity of 320 mAh g⁻¹. The initial charge capacity is clearly smaller than that of a second charge capacity. This fact suggests partial oxidation of V ions caused by high-energy milling, and this point is further discussed in the later section. Note that LNVO-MM300 also displays 290 mAh g⁻¹ even though the sample contains an impurity phase. In addition, for as-prepared LNVO-SS, its electrochemical performance is mainly restricted by its large particle size. However, as shown in Fig. S3, ball milled LNVO-SS delivers an improved specific capacity of 290 mAh g⁻¹, which is almost as same as the capacity of LNVO-MM300. However, as shown in Fig. 3b, a rapid decrease in discharge capacity is observed for all samples. The cyclability can be partially improved with lower cut-off voltage (Fig. S4a), which partly originates from higher solubility of V ions at high voltage for nanosized oxides with larger surface area. Although LNVO-MM600 delivers a high reversible capacity at a slow rate, inferior rate capability as electrode materials is observed (Fig. S4b).

Figure 2.

(a) Structural characterization by XRD of Li_1.2Nb_0.2V_0.6O₂ synthesized by different methods. The data of Li₃NbO₄ and LiVO₂ are also shown. Schematic illustrations of the crystal structures of the samples, which were drawn using the VESTA program,²⁵ are also shown in (b). TEM images and electron diffraction patterns of the samples are also shown in (c).

Figure 3.

(a) Galvanostatic charge/discharge curves at a rate of 10 mA g⁻¹ and (b) cyclability of Li_1.2Nb_0.2V_0.6O₂ prepared by different methods, (c) changes in ex situ XAS spectra on electrochemical cycles of LNVO-MM600.

Reaction mechanisms on lithiation/delithiation in Li-excess DRS oxide LNVO-MM600 were examined by XAS. The changes in absorption energies of V and Nb spectra prove the charge compensation is only achieved by V ion redox on the lithiation/delithiation processes (Fig. 3c), and Nb ions serve as a stabilizer for the host structure (Fig. S5). Additionally, the increase of pre-edge peak intensity of V K-edge XAS spectra at 5469 eV on charge is noticed. This fact suggests that V³⁺ ions, originally located at the octahedral site, are oxidized to V⁵⁺ ions and then migrate to neighboring and face-sharing tetrahedral sites.²⁶

Insufficient reversibility for as-prepared LNVO-MM600 shown in Fig. 3 would also be expected to originate from a highly reactive surface with structural defects induced by high-energy mechanical milling. Therefore, to eliminate these defects, LNVO-MM600 was further heat treated. As shown in Fig. S6a, the crystallinity is improved after heat-treatment for 4 h at 400–600 °C. In addition, a new peak appeared at 18°, which is indicative of the presence of partial cation ordering. However, sample heating at 700 °C results in phase segregation into Li₃NbO₄ and LiVO₂. Therefore, the sample heated at 600 °C is used for further characterization, and hereafter denoted as LNVO-MM600-HT. As shown in Fig. 4a, structural analysis of LNVO-MM600-HT was further conducted by synchrotron XRD measurement. A layered structure with a $R\bar{3}m$ space group is adopted as a model and all diffraction lines were well fitted. Refined structural parameters are also summarized in Table 1. The cation mixing (antisite defects) between Li and transition metal ions is 22 %. Besides, the study by X-ray photoelectron spectroscopy (XPS) (Figs. 4b and S7) reveals that the presence of oxidized V ions and by-product Li₂O is caused by high-energy milling. Similar to the V system, molybdenum oxidation induced by high-energy milling has been also reported.²⁷ XPS study also indicates that V ions are partly reduced by acetylene black upon heat-treatment coupled with the removal of Li₂O at the surface and incorporation of Li ions into the crystal lattice. The loss of carbon and oxygen is evidenced by the decreased weight after heating, and the same amount of acetylene black was added for the preparation of composite electrode, leading to a different composition from the sample without heat treatment. The electrode composition used was 78 wt% LNVO-MM600-HT, 12 wt% acetylene black, and 10 wt% poly(vinylidene fluoride). As shown in Figs. 4c and S6b, the initial charging capacity increases from 215 for LNVO-MM600 to 305 mAh g⁻¹ when the heat-treating temperature reaches up to 600 °C, which potentially makes full cell assembly with graphite possible. Gradual increase in the initial charge capacity as a function of heating temperatures also supports that V ions reduction by acetylene black on heating (Fig. S6b). Although LNVO-MM600 shows almost the same capacity before and after heat treatment, differential capacity curves (Fig. 4d) show that LNVO-MM600-HT has a higher operating voltage than LNVO-MM600, which is essential for higher energy density. Additionally, improved rate capability is also achieved after heat treatment as shown in Figs. 4e and S6c. Discharging capacity increases to 170 mAh g⁻¹ at over 2500 mA g⁻¹ for LNVO-MM600-HT, which is more than twice as much as LNVO-MM600 without heat-treatment. Moreover, superior rate capability can be further obtained by lower cut-off voltage and higher working temperature as shown in Fig. 4e. By decreasing cut-off voltage from 4.8 to 4.3 V, severe side reactions, including electrolyte decomposition, can be effectively suppressed. At elevated working temperatures, faster Li transportation can be achieved in both electrode materials and electrolyte thanks to lower migration barriers. And thus, a superior discharging specific capacity of over 200 mAh g⁻¹ for LNVO-MM600-HT can be delivered at the high specific current density of over 2500 mA g⁻¹. Furthermore, LNVO-MM600-HT with better crystallinity results in an improvement in cyclability, as shown in Fig. 4f. Indeed, the capacity retention is significantly increased from 22 % for LNVO-MM600 to 65 % for HT-600 after 50 cycles at a rate of 50 mA g⁻¹. Nevertheless, continuous V ion dissolution cannot be avoided as shown in Fig. S8 and results in the loss of reversible capacity, similar to Mo-based oxides reported in literature.²⁸ Therefore, substituting conventional electrolyte solution (1 M LiPF₆ in EC/DMC) with concentrated electrolyte solution (LiFSA : DMC = 1 : 1.1 in molar ratio)²⁹ improves the cyclability over 90 % after 50 cycles (Fig. 4f) because of the suppression of dissolution of V ions without free solvent for concentrated electrolyte solution.^28,30

Figure 4.

(a) Structural refinement on the synchrotron XRD pattern of LNVO-MM600-HT, (b) XPS spectra, (c) galvanostatic charge/discharge curves at a rate of 10 mA g⁻¹, and (d) differential dQ/dE plots, obtained from (c), for LNVO-MM600 before and after heat treatment at 600 °C. (e) Rate capability of LNVO-MM600 before and after heat treatment with different temperatures. Discharge curves were obtained at different rates after charge at 100 mA g⁻¹ to 4.3 V and then held at 4.3 V for 1 h. (f) Cyclability of LNVO-MM600 before and after heat treatment with conventional and concentrated electrolytes.

Table 1. Crystallographic parameters obtained by Rietveld analysis on the synchrotron diffraction pattern of LNVO-MM600-HT.

Atom	Site	x	y	z	Occupancy	B†/Å2
Li1	3a	0	0	0	0.42(2)	0.5
V1	3a	0	0	0	0.43(1)	0.5
Nb1	3a	0	0	0	0.15(1)	0.5
Li2	3b	0	0	0.5	0.78(2)	0.7
V2	3b	0	0	0.5	0.17(1)	0.7
Nb2	3b	0	0	0.5	0.05(1)	0.7
O	3c	0	0	0.258(2)	1.0†	0.8

Space group, R-3m, a = 2.888(1) Å, c = 14.57(1) Å, and V = 105.24 ų, R_wp = 1.6 %, >20 % cation mixing between 3a and 3b sites is noted. ^†Not refined.

4. Conclusions

This study has described that high-energy mechanical milling enables the synthesis of metastable and nanosized Li_1.2Nb_0.2V_0.6O₂ without phase segregation. Heat treatment of Li_1.2Nb_0.2V_0.6O₂ effectively improves electrode performance, and the higher reversible capacity of >300 mAh g⁻¹ with good rate capability is achieved. Moreover, Li_1.2Nb_0.2V_0.6O₂ shows excellent cyclability with concentrated electrolyte solution associated with the suppression of V ion dissolution. The results provide some implications for the further increases in energy density of Li-ion batteries through the development of thermodynamically metastable Li-excess electrode materials with high-energy density positive electrode materials.

Acknowledgments

NY acknowledges the partial support from JSPS, Grant-in-Aid for Scientific Research (Grant Numbers 19H05816, 21H04698, and 21K18815), and MEXT program “Elements Strategy Initiative to Form Core Research Center (JPMXP0112101003)”, MEXT; Ministry of Education Culture, Sports, Science and Technology, Japan. NY also thanks CBMM for the financial support. The synchrotron X-ray absorption work was done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2021G039). The synchrotron radiation experiments were performed at the BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2021B1722).

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.19084178.

CRediT Authorship Contribution Statement

Ruijie Qi: Formal analysis (Lead), Investigation (Lead), Writing – original draft (Lead)

Benoît D. L. Campéon: Investigation (Supporting), Writing – review & editing (Supporting)

Itsuki Konuma: Investigation (Supporting)

Yoshihiko Sato: Investigation (Supporting)

Yuko Kaneda: Investigation (Supporting)

Masashi Kondo: Investigation (Supporting)

Naoaki Yabuuchi: Conceptualization (Lead), Funding acquisition (Lead), Validation (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 Society for the Promotion of Science: 21K18815

Ministry of Education, Culture, Sports, Science and Technology: JPMXP0112101003

Companhia Brasileira de Metalurgia e Mineração

Footnotes

R. Qi: ECSJ Student Member

N. Yabuuchi: ECSJ Active Member

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