Synthesis of γ-LiV₂O₅/VO₂ mixture by thermal lithiation of vanadium (+4, +5) oxides

LI Zhi-you(李志友), CAO Du-meng(曹笃盟), ZHOU Ke-chao(周科朝)

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Received 13 October 2006; accepted 28 December 2006

Abstract:

γ-LiV₂O₅/VO₂ composites were synthesized through thermal lithiation reaction of mixed valence (+4, +5) vanadium oxides by lithium bromide. The phase evolution, morphology and discharge behavior at 500 ℃ were investigated by thermal gravimeter/differential thermal analysis (TG/DTA), X-ray diffraction(XRD), scanning electron microscopy(SEM) and specific surface analysis(BET). The mixed vanadium oxides are obtained from the pyrolytic decomposition of ammonium metavanadate, with V₆O₁₃ as main phase. Results show that the lithiation reaction begins at about 258 ℃, with γ-LiV₂O₅ and VO₂(B) as the product. VO₂(B) can transit to VO₂(R) in the range of 400-500 ℃, following by grain growth and crystalline development with the increase of temperature and roasting time. The ratio of γ-LiV₂O₅ to VO₂ can be modified by the additive content of lithium bromide. A lattice shearing model about the nucleation and growth of Li_xV₂O₅ and VO₂(B) inside mixed valence (+4, +5) vanadium oxides (e.g. V₆O₁₃, V₃O₇) is speculated, which is relative to oxygen-/vacancy-diffusion and structural evolution inspired by lithium-insertion. The open-circuit voltage of 2.6 V is observed in the single cell of Li-B/LiCl-KCl/(γ-LiV₂O₅/VO₂) at 500 ℃, and the specific capacities of 146 and 167 mA?h/g (cut-off voltage 1.4 V) are measured for the positive material at 100 mA/cm² and 200 mA/cm², respectively.

Key words:

thermal cell; γ-LiV2O5/VO2 composite material; lithiation;

1 Introduction

Transition-metal oxides were investigated as the potential cathode materials of thermally activated molten salts battery. Attention was focused on the oxides based on vanadium, such as V₂O₅ and LiV₃O₈. These materials are more thermally stable and have higher open-circuit voltages than widely used disulfides (FeS₂ and CoS₂ mainly), but show lower usable capacities and exhibit sloping discharge curves due to frequent structure changes or insertion reactions[1-2]. The resultant product obtained by in situ thermally reacting V₆O_13+x with LiBr is usually entitled as LVO (lithiated vanadium oxide) that consists of γ-LiV₂O₅ and VO₂ mainly. LVO was introduced as the cathode material of thermal cell with lithium alloys anode firstly in 1986[3]. It has more flat discharge profiles than γ-LiV₂O₅, VO₂, their machine-made mixtures and other vanadium oxides, and is compatible with many molten electrolytes such as LiCl-KCl, LiCl-LiBr-LiF. Another material named vanadium-oxide-carbon(VOC), produced by baking the mixture of V₂O₅, LiCl-KCl eutectic salt and acetylene black in inert atmosphere, has the same discharge active compounds of γ-LiV₂O₅ and VO₂ that are also from the lithiation and reduction reaction of V₂O₅[4-5]. Although the effects of the addition of LiBr on thermal stability and electrochemical characteristics of LVO were investigated later[6-7], any detail on the synthesis and physical performances of LVO and VOC has not been reported up to now.

The crystallographic structure of LiV₂O₅ and VO₂ are both variable and temperature-dependent. It’s reported that VO₂ has at least six polymorphs. Rutile VO₂(R) (above 68 ℃), monoclinic VO₂(M) (below 68 ℃) and triclinic VO₂(T) phase are similar in structure and convertible at 52-67 ℃[8]. There are other three VO₂ phases designated as tetragonal VO₂(A)[9], monoclinic VO₂(B)[10] and layered VO₂(C)[11], respectively. There are also a series of compounds Li_xV₂O₅ based on α-V₂O₅ in the range of 0≤x≤1. The one with 0.88≤x≤1 is generally recorded as LiV₂O₅ that has at least three polymorphs denoted δ, ε and γ, respectively. γ-LiV₂O₅ is the high temperature phase,while δ phase appears only at low temperature and can transform spontaneously to another form ε at 110-130 ℃[12-13]. But the temperature range relating to the ε→γ transition is still not well defined. Several values were observed, such as 175-220 ℃[12], above 300 ℃[13] or 350 ℃[14]. It’s not known whether any structural transformations about LiV₂O₅ and VO₂ run in the preparation of LVO.

In this study, γ-LiV₂O₅/VO₂ composites were in situ synthesized via thermal reaction of LiBr·H₂O and vanadium oxides obtained by heating NH₄VO₃ in argon atmosphere. Attention was focused on the effect of synthesis condition on phase composition and resultant morphology in order to show the lithiation process. The positive discharge performances of the resultant at 500 ℃ were also investigated.

2 Experimental

2.1 Synthesis of γ-LiV₂O₅/VO₂ mixed oxides

The mixed valence (+4, +5) vanadium oxides were firstly produced by calcining NH₄VO₃(A.R) in argon atmosphere at 450 ℃ for 4 h. It was then ball-milled with appropriate LiBr·H₂O (A.R) in an anhydrated ethanol using a resin bottle and thpolytetrafluoroethylene balls. After dried at 120 ℃, the mixtures were heated in vacuum at 320-600 ℃ for several hours to produce lithiated vanadium oxide.

2.2 Product characterizations

The phase compositions were evidenced by X-ray powder diffraction using a Rigaku D/max2550VB⁺ diffractometer with Cu K_α radiation (40 kV, 300 mA, λ= 0.154 06 nm, 10?≤2θ≤80? or 5?≤2θ≤85?). Thermogravimetric analysis(TGA) and differential thermal analysis(DTA) were performed in a TAS100 analyzer. Morphology of the powders was observed with a JSM-5600LV scanning electron microscope. The specific surface area of BET was measured using an ASAP2010 type instrument.

2.3 Electrochemical test of γ-LiV₂O₅/VO₂ composite at 500 ℃

The discharge process of Li-B/LiCl-KCl/(γ-LiV₂O₅/ VO₂) pellet single cell was simulated at 500 ℃, and curves of voltage (vs time) were monitored by an x-y function recorder. The Li-B alloy negative with 67% Li (mass fraction, the same below) is a composite material with Li₇B₆ and pure Li (self-made, thickness of 0.6-0.65 mm). The electrolyte separator (mass of 0.4 g) is the powder pressed pellet of LiCl-KCl eutectic salt (with 45% LiCl, melting point 352℃) and 30% porous powdered MgO binder. 30% powdered LiCl-KCl was added into γ-LiV₂O₅/VO₂ powder (obtained at 600 ℃ for 4 h, the excess of LiBr·H₂O was 20%), then kept at 450℃ for 1 h to make LiCl-KCl be adsorbed homogeneously. After cooled and crushed, the mixture was pressed as the positive pellet (mass of 0.4 g). The diameter of electrode was 17.5 mm, and two pieces of molybdenum (thickness of 1.0 mm) were used as the collector. The preparation of electrodes and cells was performed in a glove box (relative humidity lower than 2%) to prevent the electrodes from being wetted or oxidized. The cells were loaded at several current densities to cut-off voltage of 1.4 V after being heated between two isothermal metallic tables (at 500 ℃) for 1 min.

3 Results and discussion

3.1 Thermal decomposition of NH₄VO₃

X-ray diffraction pattern of the product from thermal decomposition of NH₄VO₃ at 450 ℃ for 4 h is shown in Fig.1. The product consists of several vanadium oxides. They are V₆O₁₃, V₃O₇, V₂O₅ and VO₂(B). Similar result was found in the Ref.[3], in which V₆O₁₃, VO₂ and V₃O₇ still co-existed even further being heated at 550 ℃ for 2 h. Since the thermal decomposition reaction of NH₄VO₃ is very complex, the decomposition process is usually described by the following two steps[1]:

2NH₄VO₃→V₂O₅+2NH₃+H₂O                  (1)

3V₂O₅+6NH₃→V₆O_13+x+oxidation outcome of NH₃  (2)

Fig.1 XRD pattern of NH₄VO₃ decomposition

Reactions (1) and (2) represent the decomposition of NH₄VO₃ and the reduction of V₂O₅, respectively. But the reduction reaction does not end at V₆O_13+x; it can react further with NH₃ and the pyrolytic productions of NH₃. As a result, a serial of nonstoichiometric vanadium-oxygen compounds are formed unavoidably.

The deoxidization of V₂O₅ to form V₆O₁₃ or VO₂(B) can be described by a model of structure shear[15], in which V₆O₁₃ and VO₂(B) structures are obtained from shearing the idealized V₂O₅ lattice by removing every third or every second (020) plane of oxygen atoms and closing the oxygen-lost polyhedra along the shear vector 1/2[110], respectively, as shown in Fig.2. V₃O₇ is an intermediate structure between V₆O₁₃ and V₂O₅, whose structure contains both single and double chains of VO₆ octahedra, like V₆O₁₃, as well as some edge-shared zig-zag chains of VO₅ trigonal bipyramids, as those in V₂O₅. By viewing of the structural intrinsic relationship, the mixture of V₆O₁₃, VO₂(B) and V₃O₇ is produced inevitably if V₂O₅ is deoxidized asymmetrically. To obtain single V₆O₁₃, the molar ratio of vanadium to oxygen is needed to be controlled strictly, and the solid state reaction of stoichiometric V₂O₅ and vanadium is usually chosen[3,15]. Based on the structural relationship of these vanadium oxides, the product obtained at 450 ℃ is used directly as the vanadium oxide source to get LVO with high specific surface area.

Fig.2 Sketch map of shearing ideal V₂O₅ crystal lattice to form VO₂(B) and V₆O₁₃ lattice (Blank circles represent vacancies by removing oxygen atoms)

3.2 In situ lithiation synthesis of γ-LiV₂O₅/VO₂ oxides

TGA and DTA curves of the mixture powder with an excess of 20% LiBr·H₂O under flowing argon atmosphere are shown in Fig.3. There are two temperature ranges, in which the mass loses sharply. The first is from 80 ℃ to 150 ℃, with the mass loss of 1.2 mg (11%), corresponding to two endothermic peaks at 118 ℃ and 148 ℃ on the DTA curve. The mass loss may be due to the evaporation of the residual balling medium, free water and bond water. The second is 150-260 ℃, with 9.2% mass loss, corresponding to an exothermic peak at 258 ℃. The mass loss and the heat release in this range may result from the reactions between the vanadium oxides and LiBr. It is awaited to be further confirmed whether the exothermic peak at 377 ℃ is related to VO₂(B)→VO₂(R) phase transition. This transition was observed at (440±20) ℃[15], or began at 320 ℃ and finishes at 500 ℃ for nanometer VO₂(B)[16]. But in our experiment the transformation isn’t completed even after being kept at 500 ℃ for 4 h. There is the difference of the synthesis technique and initial morphology of VO₂(B) affecting the VO₂(B)→VO₂(R) transition. The great and broad endothermic peak at 630 ℃ is related to the melting of γ-LiV₂O₅. Because it is an incongruent melting compound, two melting-related endothermic peaks at 572 and 628 ℃ are once observed in a DTA study[17].

Fig.3 TG/DTA curves for vanadium oxides with LiBr·H₂O (Initial mass of 11.0 mg, 10 ℃/min)

The lithium-inserted structures of V₂O₅, V₆O₁₃ and V₃O₇ are changeable at high temperature. Li_xV₆O₁₃ is disassembled at 230-265 ℃[15]. Lithium-intercalation makes V₃O₇ transit to lithium-rich V₆O_13+x even at room temperature[15], also makes V₂O₅ structure transform frequently. Based on their inherent stacking relationship and lithium-insertion properties, in the similar way do V₆O₁₃ and V₃O₇ form γ-LiV₂O₅/VO₂(B) as V₂O₅. Namely, the lithiation reactions of V₃O₇ and V₆O₁₃ may contain two steps including the shearing breakup of the matrix resulting from the diffusion of oxygen vacancy or anion, and the nucleation and growth of new structures, as illustrated in Fig.4.

The structural micro-region of α-V₂O₅ in V₆O₁₃ and V₃O₇ crystal is more thermodynamically advantageous to lithium-ion insertion (viz. lithiation) than that of VO₂(B), which can be substantiated by the fact that V₂O₅ has higher electrochemical intercalation voltage than VO₂(B) [18]. As lithium ions move into V₂O₅ micro-regions, a new structure is formed. But its frontier keeps the same structure as α-V₂O₅, and shares the plane made up of double octahedral layers as the structural boundary with matrix crystal. The shear-coherence bond raises the lattice distortion near the interface, which is beneficial to breaking the double octahedral sheets. When a (020) oxygen plane in the double layers shears along  direction, a stride taking toward V₆O₁₃ or V₃O₇ is realized. The shear reaction is the reverse process of the transformation of V₂O₅ to V₆O₁₃ shown in Fig.2. The cleavage and bonding on one hand leads to the shift of the interfaces, on the other hand increases the concentration of oxygen vacancy near the interface. High oxygen vacancy concentration induces oxygen anions in V₆O₁₃ or V₃O₇ to configurationally transfer to the interfaces (or oxygen vacancies near the interface to the matrix phase), and in turn increases the concentration of oxygen vacancy in the matrix phase. Since a great deal of oxygen vacancy appears at VO₅ micro-region ahead VO₂(B) matrix, it’s helpful for the edge-shared zig-zag layers of VO₅ trigonal bipyramids to undergo the shearing along 1/2[110], as shown in Fig.2. As a result, the shear drives the interfaces to move along and VO₂(B) structure region spread. Vice versa, the shear that makes VO₂(B) structure region spread is helpful for the extending of V₂O₅ structure. So the lithiation of V₆O₁₃ and V₃O₇ is perhaps the phenomenon that the structural decomposition is firstly induced by the diffusion of vacancy or anion, which produces the structure regions of nonstoichiometric V₂O₅, Li_xV₂O₅ and VO₂(B), and that γ-LiV₂O₅ is obtained from further lithiation of Li_xV₂O₅.

Fig.4 Schematic drawing of lithiation evolution of V₃O₇ and V₆O₁₃

3.3 Effect of additive content of lithiated reagent

The additive content of LiBr·H₂O is calculated based on the following reaction for the preparation of lithiated vanadium oxides. The XRD patterns of composite materials with 0.8≤x≤1.3 in Li_xV₂O₅ are shown in Fig.5.

2LiBr·H₂O+2V₆O₁₃→2LiV₂O₅+8VO₂+Br₂↑+2H₂O↑    (3)

Fig.5 XRD patterns of LVO with different ratios (600 ℃, 4 h) of practical mass to calculated value

The ratio of LiV₂O₅ to VO₂ in the resultant products is reported according to the calculated value from Eqn.(3), and the optimum discharge property and the highest thermal stability can not be obtained until the additive content of LiBr is excessive[3,6-7]. To eliminate β-Li_0.13V₂O₅, α-Li_0.04V₂O₅ and V₂O₅ phases, 20%-30% excess of LiBr is needed[7]. However, LiV₂O₅/VO₂ composite is gained in this experiment after 600 ℃, 4 h treatment with only 80% of LiBr·H₂O calculated value, in which γ-LiV₂O₅ content is just relatively lower and VO₂ content is higher. γ-LiV₂O₅ content rises up and VO₂ drops off with the increase of LiBr·H₂O content from 0.8 to 1.3, as seen in Fig.6. Since the third intensity peak (103) of γ-LiV₂O₅ superposes the highest (011) peak of VO₂(R), the relative contents of both are characterized by the intensity of the highest (002) peak of γ-LiV₂O₅ and the second high (211) peak of VO₂(R), respectively. Based on the fact that γ-LiV₂O₅ and VO₂(R) are both nonstoichiometric[15], it’s thought that γ-LiV₂O₅ obtained at low LiBr·H₂O may be a lithium-poor phase and VO₂ a oxygen-rich phase, and that the oxygen-rich VO₂ can be further lithiated to form γ-LiV₂O₅ and oxygen-poor VO₂ phase, as well as increasing lithium content in γ-LiV₂O₅. What is more, the lower thermal stability (decomposition temperature) of LVO obtained at low LiBr content[7] perhaps results from the transformations that lithium-poor γ-LiV₂O₅ separates out β′ phase and oxide-rich VO₂ separates out V₂O₅ phase.

Fig.6 XRD peak intensities of γ-LiV₂O₅ and VO₂(R) versus LiBr·H₂O

3.4 Effects of experimental parameters

XRD patterns of the resultant products obtained at 320-600 ℃ for 4 h are shown in Fig.7. γ-LiV₂O₅ and VO₂(B) phase are gained at 320 ℃, with two unknown weak peaks at 2θ=(8-9)?, which disappear above 400 ℃. The resultants consist of γ-LiV₂O₅, VO₂(B) and distinguishable VO₂(R) after being heated at 400 ℃ and 500 ℃, while γ-LiV₂O₅ and VO₂(R) appear at 550 ℃ and 600 ℃. It’s shown that γ-LiV₂O₅ and VO₂(B) phases are firstly obtained from the lithiation reaction, and the metastable VO₂(B) will transform gradually to the more stable rutile VO₂(R) in the range of 400-550 ℃.

Fig.7 XRD patterns of LVO obtained at different temperatures (for 4 h)

As reported, γ-LiV₂O₅ can be obtained from its low-temperature counterparts, even though the required temperature varies with the lithium content, and varies greatly with measurement methods. For example, for the precursor δ-LiV₂O₅ prepared in an organic solution, the ε→γ phase transition begins at 175-220 ℃ and completely finishes at 235 ℃ with a heating rate of 0.5 ℃/min from the synchrotron X-ray powder diffraction analysis[12]. The ε phase transits quickly at 280 ℃ from the DSC result[19], or at 300-320 ℃ or 350-370 ℃ on the well-accepted presumable phase diagram[12, 14]. It can also be directly gained though chemical reactions, such as, hydrothermal method at 160 ℃[20], organic solution processing at 120 ℃ using vanadium alkoxides and lithium[21], solid-state reaction of NH₄V₃O₈ with CH₃COOLi at 285.7-312 ℃[22]. This observation is similar to that of the Ref.[22]. The reactions progress quickly at 258 ℃ and end before 280 ℃ (shown in Fig.3). Although it’s not quite clear whether γ-LiV₂O₅ nucleates and grows directly inside the vanadium oxides or comes from the transformations of its low temperature polymorphs or other low lithium phases (e.g. β phase), it is reasonable that γ-LiV₂O₅ is formed though the structural transition induced by the intercalation of lithium step by step, as presumed in our model.

The lithiation reactions make the loose agglomerated particles be rock salts. The grain growth and the crystallinity characteristics are reinforced with the increase of calcination temperature and time (shown in Fig.8). The strip crystals with the radial length of less than 1 μm and the longitudinal length of above 10 μm come forth as bundles, and are sintered ultimately to be blocks. This indicates that the calcination, after the complete lithiation reaction, mainly enforces the growth of grain and the increasing of crystallinity except the transformation of VO₂(B) to VO₂(R).

Fig.8 SEM images of products obtained at different calcinated temperatures and time: (a) Vanadium oxides; (b) 500 ℃, 4 h; (c) 600℃, 4 h; (d) 600 ℃, 2 h

The specific surface area analysis results give another evidence for the gain growth (shown in Fig.9). The specific surface area of the vanadium oxides directly from NH₄VO₃ is 11.26 m²/g. The values of LVO are 5.12 m²/g and 0.44 m²/g at 400 ℃ and 600 ℃ for 4 h, respectively. It also declines continuously with the prolonging of exposure time.

Fig.9 Specific surface area of LVO versus time (at 600 ℃) (a) and temperature (for 4 h) (b)

3.5 Electrochemical test of γ-LiV₂O₅/VO₂ composite

The discharge plots of γ-LiV₂O₅/VO₂ material (with Li-B alloy anode at 500 ℃) are shown in Fig.10. The open-circuit voltage of single thermal cell of Li-B/LiCl- KCl/(γ-LiV₂O₅/VO₂) is 2.6 V. The discharge voltage and time drop off with increasing current density. The discharge can last for 650 s till 1.4 V at the current density of 100 mA/cm², and the specific capacity of the mixed oxides reaches 146 mA?h/g, while 167 mA?h/g at 200 mA/cm². The discharge property is comparable to that previously reported.

Fig.10 Electrochemical characteristics of γ-LiV₂O₅/VO₂(R) composite

4 Conclusions

1) The product from the decomposition of ammonium metavanadate is mainly made up of V₆O₁₃, and contains minor amount of impurities such as VO₂, V₃O₇ and V₂O₅. The multiphase coexistence results from the asymmetric oxygen-loss of V₂O₅, and does not seriously affect the phase composition of lithiated vanadium oxide.

2) The reactions between the mixed vanadium oxides and lithium bromide take place near 258 ℃ with γ-LiV₂O₅ and VO₂(B) as-obtained, and VO₂(B) turns to VO₂(R) in the range of 400-550 ℃. After the lithiation reaction, the particles grow and the crystalline morphology increases with the increased temperature and the prolonged time till the melting of γ-LiV₂O₅ near 630 ℃. The relative content of γ-LiV₂O₅ and VO₂ in LVO can be modified by the additive content of lithium bromide.

3) A nucleation-growth model is speculated to describe the lithiation process of mixed valence (+4, +5) vanadium oxides (e.g. V₆O₁₃, V₃O₇) to form Li_xV₂O₅ and VO₂(B), including deoxidized shearing and oxygen- assembled shearing both inspired by lithium-insertion.

4) The open-circuit voltage of single cell of Li-B/ LiCl-KCl/(γ-LiV₂O₅/VO₂) is 2.6 V. The specific capacity of LVO reaches 146 mA?h/g and 167 mA?h/g at 100 mA/cm² and 200 mA/cm² till 1.4 V, respectively.

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Foundation item: Project(2003AA05510) supported by the National High-Tech Research and Development Program of China

Corresponding author: LI Zhi-you; Tel: +86-731-8830464; Fax: +86-731-8710855; E-mail: lzpm@mail.csu.edu.cn

(Edited by YANG Bing)