Elsevier

Journal of Power Sources

Volume 373, 1 January 2018, Pages 1-10
Journal of Power Sources

Electrochemical behavior of LiV3O8 positive electrode in hybrid Li,Na–ion batteries

https://doi.org/10.1016/j.jpowsour.2017.10.086Get rights and content

Highlights

  • LiV3O8 inserts 1Na per formula unit while preserving the crystal structure.

  • More than1Na intercalated into LiV3O8 leads to amorphisation of the material.

  • Chemical diffusion coefficients of Li and Na in LiV3O8 are comparable.

  • Co-insertion of Li and Na into LiV3O8 occurs in Li,Na-hybrid cells with Na anode.

Abstract

Vanadium(V)-containing oxides show superior intercalation properties for alkaline ions, although the performance of the material strongly depends on its surface morphology. In this work, intercalation activity of LiV3O8, prepared by a conventional solid state synthesis, is demonstrated for the first time in non-aqueous Li,Na-ion hybrid batteries with Na as negative electrode, and different Na/Li ratios in the electrolyte. In the pure Na-ion cell, one Na per formula unit of LiV3O8 can be reversibly inserted at room temperature via a two-step process, while further intercalation leads to gradual amorphisation of the material, with a specific capacity of 190 mAhg−1 after 10 cycles in the potential window of 0.8–3.4 V. Hybrid Li,Na-ion batteries feature simultaneous intercalation of Li+ and Na+ cations into LiV3O8, resulting in the formation of a second phase. Depending on the electrolyte composition, this second phase bears structural similarities either to Li0.7Na0.7V3O8 in Na-rich electrolytes, or to Li4V3O8 in Li-rich electrolytes. The chemical diffusion coefficients of Na+ and Li+ in crystalline LiV3O8 are very close, hence explaining the co-intercalation of these cations. As DFT calculations show, once formed, the Li0.7Na0.7V3O8-type structure favors intercalation of Na+, whereas the LiV3O8-type prefers to accommodate Li+ cations.

Graphical abstract

LiV3O8 positive electrode in Li,Na-hybrid cells. Co-insertion of Li+ and Na+ into LiV3O8 occurs because of similar diffusion coefficients.

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Introduction

Nowadays sodium-ion batteries are recognized as promising candidates for stationary energy storage applications due to the low cost and abundance of sodium [1], [2], [3]. Despite the larger ionic radius of Na+ in comparison to Li+ (1.02 Å vs. 0.76 Å for the octahedral oxygen surrounding [4]), fast kinetics with Na+ as charge carrier could nevertheless be expected, since i) weaker Lewis acidity of the Na+ cations reduces their solvation energy, hence facilitating their transfer at the electrolyte/electrode phase boundary [5], and ii) larger polarizability of Na+ can reduce the activation barrier for diffusion in the bulk [6].

Since the operation principle of sodium-ion cells is very similar to lithium-ion batteries, many of the lithium insertion compounds may be considered as sodium insertion hosts and thus as electrode materials for sodium-ion batteries. Polyanionic Li- and Na-insertion positive electrodes with three-dimensional structures or two-dimensional layered structures containing vanadium(V), are of special interest owing to the variable oxidation state of vanadium, which may facilitate multi-electron redox processes and high theoretical capacities. However, the preparation of Na-containing materials, which are isostructural to the corresponding Li-compounds, requires often a complicated synthesis procedure different from the one for the Li-analogue as in the case of olivine-like NaFePO4 [7]. Moreover, the larger size of Na+ puts additional restrictions on the host structures that should sustain larger lattice expansion. Many of the materials undergo amorphisation or show lower capacity values upon repeated cycling with Na+ intercalation.

In order to overcome this drawback, an innovative concept of hybrid non-aqueous Li+/Na+ rechargeable batteries can be proposed following the widely studied Li,Mg- hybrid cells [8], [9], [10]. These cells feature a Mg anode and an electrolyte composed of Mg- and Li-salts, thus using advantages of both ingredients. While Mg is abundant and evades dendrite formation, Li+ supports fast diffusion in the bulk and allows a large choice of suitable high capacity electrode materials. In the ideal case, the exchange of the smaller Li+ ions should occur between the electrolyte and the positive electrode (“cathode”), while the divalent Mg2+ ions are exchanged between the electrolyte and the negative electrode (“anode”). During cell charge and discharge, the total concentration of Li+ and Mg2+ ions in the electrolyte provides charge neutrality, but the Li+/Mg2+ ratio is changed [9]. The capacity, operating voltage in terms of cell polarisation, and the stability of such batteries depend on the electrolytes [9]. However, simultaneous intercalation of both metal ions into one electrode can also occur, provided that the insertion potentials and intercalation kinetics are similar for both ions [11].

Up to now, most studies were focused on the mixed-ion Li,Mg- [8], [9], [10], Na,Mg- [12], [13], and aqueous Li,Na- [14] hybrid batteries. To the best of our knowledge, the only implementation of the mixed Li,Na-battery reported so far involves a Li2RuO3 positive electrode, a Na negative electrode, and a purely Na-salt electrolyte [15]. However, the total amount of Li in the electrolyte did not exceed 4% after complete cell charge, and common principles of Li,Na-hybrid cells could not be studied in this system.

Layered lithium vanadium(V) oxide, LiV3O8, is a promising positive electrode material for such hybrid Li,Na-cells. This compound has been studied since the 1980s as positive electrode in non-aqueous Li-ion batteries [16], [17], [18] and as negative electrode in aqueous rechargeable Li-ion batteries [19]. Although the use of vanadium implies similar concerns in terms of environmental compatibility and sustainability as commercially applied Co-, Ni- and Mn-containing materials, LiV3O8 is considered superior to common positive electrode materials due to its outstanding electrochemical performance. During Li-insertion, more than 3 Li+ ions per formula unit can be reversibly inserted into the structure between vanadium-oxygen layers, resulting in the valence state of vanadium below 4+. As the pristine compound exists in charged state, a lithium-containing negative electrode is needed. The crystal structure does not undergo significant transformations upon Li-insertion [20], see Fig. 1a. The most important changes include some shifts of metal and oxygen atoms resulting in the transformation of VO5 pyramids into distorted VO6 octahedra. The Na-insertion into this compound was not studied so far, although a mixed Li,Na vanadium oxide with a related structure is known, see Fig. 1b [21]. In a Li,Na-hybrid cell, one could expect mostly Li intercalation because of its smaller ionic radius in comparison to Na+ as well as a larger thermodynamic stability of the lithiated phase compared to the sodiated one, because of the larger repulsion between the Na and Li ions in the NaLiV3O8 structure than in Li2V3O8.

In the present work, we study LiV3O8 as a host material in hybrid Li,Na-cells with a Na-anode and different Li and Na cation compositions of the electrolyte. We show that 1 Na per LiV3O8 formula unit can be reversibly intercalated into pure Na-cells via a two-phase process without any structural decomposition. In Li,Na-hybrid cells, electrochemical behavior of LiV3O8 was studied in dependence on the Li/Na cation ratio in the electrolyte, also from the structural point of view with in situ experiments. Simultaneous intercalation of the Li+ and Na+ cations into LiV3O8 is established in hybrid cells with equal availability of both kind of cations in the electrolyte because of kinetic and thermodynamic considerations. At the same time, Li-deposition on metallic Na was observed upon the first cell charge, thus leading to an “inverse” hybrid cell, in which Li+ ions are mostly exchanged between the negative electrode and electrolyte, while Na+ ions shuffle between the electrolyte and the positive electrode. Possible reasons behind this behavior are discussed.

Section snippets

Synthesis and characterization

LiV3O8 samples were prepared by solid-state reaction from stoichiometric amounts of Li2CO3 (Fluka, 98,0%) and NH4VO3 (Merck, 99,0%) placed in Al2O3 crucibles at 500 °C for 40 h and cooled down to room temperature. Since particle size of the active material affects its electrochemical performance, one LiV3O8 sample was ball-milled (Fritsch Pulverisette 7) in an Ar-filled glove-box at room temperature for 2.5 h at 300 rpm using a grinding bowl and grinding balls from tungsten carbide, with 10 wt.

Material characterization

According to laboratory XRD analysis (Fig. S1, Supporting Information), LiV3O8 samples prepared via solid-state synthesis were single-phase crystallizing in the monoclinic P21/m space group with lattice parameters a = 6.6319(1) Å, b =  3.58651(4) Å, c = 11.9714(1) Å, β = 107.8127(8)° in good agreement with the literature data [20]. The cation ratio Li:V = 1:3 in the material was confirmed by ICP-OES analysis after dissolving the sample in HNO3 solution. In order to reduce the particle size, the

Discussion

Successful operation of a hybrid-ion battery requires that two ions are present in the electrolyte and exhibit different behavior with respect to their redox potentials and diffusion in electrode materials. In the present work, the comparison of Li- and Na-ion batteries assembled with a LiV3O8 positive electrode displayed very similar chemical diffusion coefficients for Li and Na at room temperature as well as similar activation energies for the cation transport in the material. Therefore,

Conclusions

Lithium trivanadium (V) oxide, LiV3O8, prepared via a conventional solid-state route, was tested for the first time as positive electrode in Na- and hybrid Li,Na-ion batteries. In a pure Na-cell, it revealed a reversible electrochemical insertion of about 1 Na per formula unit without any deterioration of the material, thus yielding a reversible capacity of 60 mAh/g. Further Na intercalation leads to a gradual amorphisation with each cycle, reflected in XRD and XPS studies, and in a less stable

Acknowledgment

This work has benefited from financial support within the BMBF project “DESIREE”, grant number 03SF0477B. The authors are indebted to Dr. T. Jaumann (IFW Dresden, Germany) for help in performing the SEM studies, and A. Voss (IFW Dresden, Germany) for performing the ICP-OES analyses. AT was supported by Federal Ministry of Education and Research through the Sofja Kovalevskaya award of the Alexander von Humboldt Foundation.

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