^aNational Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

^bHonda Engineering Co., Ltd., 6-1 Hagadai, Haga-Machi, Haga-Gun, Tochigi 321-3395, Japan

^cARC Centre for Functional Nanomaterials, School of Engineering, The University of Queensland, St. Lucia, Brisbane, Qld 4072, Australia

^dInstitute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia

^eDepartment of Biomolecular Functional Engineering, Faculty of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan

Abstract

Two types of layered compounds, Li_0.96VO₂ and Li_0.86V_0.8O₂ , were synthesized with a solid-state and a hydrothermal reaction, respectively, and their structural modifications caused by electrochemical lithium extraction were investigated. The Li_0.96VO₂ and Li_0.86V_0.8O₂ compounds are characterized by the ordered and disordered arrangement of vanadium ions in the alternate layers of octahedral 3a (0 0 0) and 3b (0 0 1/2) sites, respectively. According to the electrochemical lithium extraction, the Li_0.96VO₂ changed to a disordered arrangement; such a structural modification is irreversible and takes place in the initial stage of the lithium extraction. For the Li_0.86V_0.8O₂, no significant structural modification was detected.

Keywords: LiVO₂ ; Structural modifications; Electrochemical lithium extraction

Article Outline

1. Introduction

2. Experimental

3. Results and discussion

4. Conclusions

Acknowledgements

References

1. Introduction

LiVO₂ has a structure that is similar to that of LiNiO₂; it is characterized as a layered rhombohedral structure related to α-NaFeO₂, corresponding to the space group symmetry of [7] and [8]. The structure of LiVO₂ takes successive layers of vanadium, oxygen, and lithium ions. The vanadium and lithium ions occupy the octahedral sites of a cubic close packing of oxygen ions, where the vanadium, lithium, and oxygen ions are positioned at the 3a (0 0 0), 3b (0 0 1/2), and 6c (0 0 x) sites, respectively. It was also reported that a similar structural destabilization occurs in the LiVO₂.

Recently, we synthesized two types of LiVO₂, namely, Li_0.96VO₂ and Li_0.86V_0.8O₂, and found that these compounds are characterized by the ordered and disordered arrangements of vanadium ions in alternate layers of octahedral 3a and 3b sites. We also found significant differences in the electrochemical properties of both materials.

In this paper, we discuss the structural modifications due to electrochemical lithium extraction and insertion. Secondly, the electrochemical properties for both materials are reported in connection with their structural modifications.

2. Experimental

The materials of Li_0.96VO₂ and Li_0.86V_0.8O₂ were synthesized through a solid-state and a hydrothermal reaction, respectively. In short, for Li_0.96VO₂, equal molar quantities of Li₂CO₃ and V₂O₅ powders were reacted under flowing hydrogen, initially at 500 °C for 3 h and then at 700 °C for 10 h [7]. For Li_0.86V_0.8O₂, V₂O₃ and LiOH·H₂O for the molar ratio of LiOH·H₂O/V₂O₃ = 20 were hydrothermally reacted at 180 °C for 4 days. After filtration with suction and washing with water, the product was dried at 70 °C to give a black powder of Li_0.86V_0.8O₂.

The lithium and vanadium contents of the resulting materials were determined by inductively coupled plasma (ICP) atomic emission spectrometry. For the structural investigations, powder X-ray diffraction (XRD) experiments were performed using a rotating cathode X-ray diffractometer (JEOL JDX3500) at 35 kV and 300 mA with a 2θ–θ step-scanning mode and graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å). Rietveld refinements based on the XRD data were also conducted using the RIETAN computational program system [9].

Electrochemical lithium extraction and insertion were conducted using a coin-type cell with Li metal as an anode and a 1 mol dm⁻³ LiClO₄ solution in mixed ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 in volume) as an electrode. The cathodes consisted of a titanium grid on which the dried materials (ca. 14 mg), mixed with 20 wt% carbon black and 10 wt% polytetrafluoroethylene (PTFE) powders, were pressed. The electrochemical lithium extraction and insertion were performed at current densities of ±7.14 mA g⁻¹ in the cell voltage range of 1.5–4.5 V.

3. Results and discussion

The results of chemical analyses by ICP are summarized in Table 1. The measurements of thermogravimetry (TG), not shown here, indicate that adsorbed or structural water rarely exists in the resulting materials. Thus, the compositions can be represented to be Li_0.96VO₂ and Li_0.86V_0.8O₂, respectively, for materials prepared by the solid-state and hydrothermal reactions.

Fig. 1 shows the XRD profiles of the resulting materials of Li_0.96VO₂ and Li_0.86V_0.8O₂. Each profile shows differences, especially regarding the significantly reduced intensity of the 0 0 3 reflection at 2θ = 18° for the material of Li_0.86V_0.8O₂ [7]. By simulation, using model structures, it was confirmed that this behavior primarily takes place according to the disordered arrangement of vanadium ions into 3b sites. Fig. 2 shows the simulated XRD profiles of the LiVO₂ materials with various levels of such disordered arrangement, which were calculated using a computational program system (CaRIne crystallography). We found that the intensity of the 0 0 3 reflection decreases with an increase in the vanadium occupancy for the 3b site and is nearly zero for the value of 0.55 (Fig. 2(a)). The 0 0 3 reflection is due to the layered rock-salt structure of Rm, whereas the 1 0 4 reflection results from the layered and cubic rock-salt structures.

Referring to Fig. 1, the profile of Li_0.86V_0.8O₂ shows an extraordinary reduced intensity of the 0 0 3 reflection compared to that of Li_0.96VO₂. This fact suggests that the structure of Li_0.86V_0.8O₂ is comprised of the disordered arrangement of vanadium ions at the 3a and 3b sites. In fact, for the material of Li_0.86V_0.8O₂, we found that the Rietveld refinement based on the XRD data only converges for the disordered arrangement of the vanadium ions. The refined parameters for the Rietveld refinement are summarized in Table 2. Here, it is noted that the precise crystallographic information on lithium ions cannot be determined based on the XRD data alone; thus, we are conducting the Rietveld refinement on the basis of the neutron diffraction data.

To clarify the structural modifications caused by electrochemical lithium extraction for the materials, the XRD profiles of the materials at various stages of electrochemical lithium extraction and insertion are shown in Fig. 3 and Fig. 4. It is noteworthy that the XRD measurements were conducted using the cathode samples with a titanium grid, and, thus, reflections due to titanium are observed in almost every profile. For Li_0.96VO₂ (Fig. 3), except for the initial sample, a significantly reduced 0 0 3 reflection is observed in each profile, which indicates that vanadium disordering takes place at least when x ≤ 0.74 for the composition of Li_xVO₂. In addition, since the profile of Li_0.65VO₂ (Fig. 3(d)), which was obtained through the electrochemical insertion process followed by the lithium extraction process, also shows a reduced 0 0 3 reflection, such vanadium disordering is an irreversible modification. On the other hand, for Li_0.86V_0.8O₂ (Fig. 4), no apparent structural modification was detected, indicating structural stability.

It appears that such structural properties of Li_0.96VO₂ and Li_0.86V_0.8O₂ influence their electrochemical performance. Fig. 5 shows the variation in the cell voltage versus the amount of lithium for the Li/Li_0.96VO₂ and Li/Li_0.86V_0.8O₂ cells according to the lithium extraction and insertion. A significant hysteresis was detected for the Li/Li_0.96VO₂ cell, whereas almost symmetrical behavior was observed for the Li/Li_0.86V_0.8O₂ cell. These events could be primarily responsible for their structural behavior. Here, it is noteworthy that not every electrochemical performance can be explained by such structural modifications. In particular, the electrochemical capacities of the materials could be related to the presence of vanadium vacancies. In connection with this, a quantitative study using neutron measurements is underway.

4. Conclusions

We have demonstrated the preparation of two types of layered compounds (Li_0.96VO₂ and Li_0.86V_0.8O₂) and their structural modifications according to electrochemical lithium extraction or insertion. The material of Li_0.96VO₂ is transformed into the disordered structure of vanadium ions at the 3a and 3b sites, and this transformation is an irreversible change and occurs at the initial stage of lithium extraction. On the other hand, no apparent structural modification was observed for the material of Li_0.86V_0.8O₂. The results also show that such structural properties influence their electrochemical performance.

Acknowledgement

This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sport, Science, and Technology in Japan.