Electrochemical Lithium Insertion/Extraction in ...

Z. Phys. Chem. 218 (2004) 1259–1272  by Oldenbourg Wissenschaftsverlag, München

Electrochemical Lithium Insertion/Extraction in Partially Reduced α-MoO3 By A. M. Hashem ∗, H. Abbas, and A. E. Abdel-Ghany National Research Centre, Inorganic Chemistry Department, Behoes Street, Dokki, Cairo, Egypt (Received April 15, 2004; accepted in revised form July 12, 2004)

Insertion / Lithium Batteries / Cathodes and MoO 3 This work concerns with synthesis, structural, thermal and electrochemical characterization of molybdenum oxides (stoichiometric MoO3 , sub-stoichiometric oxide MoO3−x (where x is a small fraction) with layered type structure. XRD investigations of the samples proved that the crystal structure of the layered α-MoO3 has been maintained after the reduction process. The reduced samples exhibited a drastically improved charge/discharge cycling stability and capacity retention on cycling in 1 M LiClO4 /propylene carbonate. At higher cycle numbers (approx. cycle 50) the discharge capacity of the reduced molybdenum oxides stabilizes at a level of approx. 50 mAh g −1 , whereas the non-reduced MoO3 has retained only about 45 mAh g −1 after 20 cycles. This significant improvement of the rechargeability may be related to improvement in the electronic conductivity after reduction process.

1. Introduction During the past two decades much interest has been given to the study of transition metal oxides, as these oxides exhibit interesting optical and electrical properties and especially the electrochromic effect that is of particular interest in “smart windows” [1]. They may have also wide application in optical data storage and electrochromic display devices [2], in micro batteries [3] and in switching devices [4]. Transition metal oxides that can undergo reversible lithium intercalation/ extraction at ambient temperature are of great technical interest as cathode materials for secondary lithium batteries [5]. Within this group of oxides, the molybdenum oxides have received special attention due to their chemical and electrochemical properties [6]. The interest in these materials arises * Corresponding author. E-mail: [email protected]

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from i) the orthorhombic phase (α-MoO3 ) has the unique two-dimensional layered structure [7] ii) oxide compounds always exhibit higher electrochemical activity than chalcogenides for instance; iii) thin film of MoO3 can be easily prepared using numerous evaporation techniques; iv) MoO3 is one of the oxide compounds with highest stability and insoluble in most common electrolytes [8], also shows excellent discharge capabilities when coupled with lithium [5, 8, 9]. The ability of ionic insertion in MoO3 leads to their use in micro batteries [3]. The crystal lattice of MoO3 mono crystal has orthorhombic symmetry with the space group Pbnm and the cell parameters defined as a = 3.963 Å, b = 13.855 Å, C = 3.696 Å [10]. Besenhard et al. [11] observed that slight oxygen deficiencies of MoO3 did result in an improved electrode performance, which was also found to be true for addition of inert electronic conductors such as graphite. The colour of sub-stoichiometric molybdenum oxide contain a number of oxygen vacancies varied from light blue to very deep blue. This attributed to the increase of the lack of oxygen atoms [12]. Sub-stoichiometric MoO3−x films offer possibilities for alkali intercalation and hence superionic conduction with application in solid state micro battery technology [13]. When a layer of MoO3 is sandwiched between two metal electrodes, Schottky batteries are formed at the metal-insulator interface, giving interesting properties which find applications in switching devices, dividers and so on. Different workers [14, 15] have observed by X-ray photoelectron spectroscopy the formation of the Mo5+ oxidation state in the blue samples of MoO3 films. This was attributed to an internal electron transfer from oxygen to metallic orbitals by thermal ionisation creating an Mo 5+ oxidation state. Molybdenum oxides with smaller O/Mo ratio have higher conductivity. The decrease of the conductivity with the heat treatment of sub-stoichiometric samples in ambient atmosphere is attributed to the partial filling of oxygen vacancies. It is well known that the conduction is mainly due to the presence of the low oxidation state of Mo which results in indirect conduction. The annealing treatment in O2 atmosphere decreases the number of vacancies, i.e., the molybdenum oxide becomes more stoichiometric. The increasing O/Mo ratio induces a decrease of the conductivity. On the other hand, the change in coloration of the blue sample over the transparent one is due to the mobility of electrons in the defect band as reported by Rabalais et al. [16]. The effect of an annealing treatment in vacuum is an enhancement of the conductivity. This result comes from an opposite mechanism that has been described in ambient atmosphere annealing treatment. The vacuum treated samples exhibit a smaller O/Mo ratio inducing an increasing conductivity and a decreasing activation energy. These samples are sub-stoichiometric and posses more oxygen vacancies in their structure [17]. In this contribution we study the effect of a slight reduction process of α-MoO3 on their electrochemical performance.

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2. Experimental 2.1 Samples preparation In this contribution, we compare the performance of three Mo-O samples. Sample 1 and 2 were prepared according to the method of Freedman [18]. Acidification of a 1 M Na2 MoO4 aqueous solution with 3 N HCl at 100 ◦ C leads to the yellow powder MoO3 · 2H2 O. Thereafter, a part of this dried powder was heat treated at 350 ◦ C for six hours in air (sample 1) and another part was heated under vacuum at 350 ◦ C for six hours (sample 2). Sample 1 and 2 exhibited a light yellow and blue colours, respectively. The partially reduced sample 3 has been prepared from sample 1 by reaction with a given amount of ascorbic acid (as a reducing agent) [the molar ratio of ascorbic acid/MoO3 was 0.11] in an organic solution as follows. Dispersions of ascorbic acid and MoO3 powders were stirred for 24 hours in dry n-hexane. After filtration the blue powder were heated at 350 ◦ C in vacuum for 6 hours.

2.2 Samples characterisation X-ray diffraction (XRD) patterns were measured on a Bruker D-5005 θ/θ powder diffractometer using Cu K α -radiation (40 KV, 30 mA), a position sensitive detector system (PSD; Braun) and a scintillation counter, respectively. X-ray diffraction diagrams were recorded with a constant stepwidth of 0.02◦ (in 2θ) and with 1 second/step in the range of 8–80◦ (in 2θ). Thermogravimetry (TGA) and differential scanning calorimetry (DSC) were applied simultaneously for the characterisation of the molybdenum oxide powders. The instrument was a Netzsch-STA 409 thermal analyser controlled via a TASC 414/2 interface. The measurements were performed in air with a heating rate of 3 ◦ C/min. within the temperature range of 25–700 ◦ C. The electronic conductivity was measured by the four probe Van der Pauw method [19]. About 1.5 g of the sample powder mixed with poly(vinylidene fluoride) (PVDF, Aldrich) as an organic binder in a weight ratio (95 w/o of the sample and 5 w/o binder) was pressed into disks of about 10 mm diameter and about 1.3 mm thickness. At four diagonal points on the side of the disk by using gold electrodes, the measurements were performed at room temperature. Each sample took one minute in measurement. At a constant current of 0.01 to 0.04 mA (power supply, Knick) applied to the sample, the potential difference in each value of current was measured by (voltammeter, 7060 systems, Shlumberger). Electronic conductivity was calculated by using a special version of Microsoft Origin 50 program, according to Van der Pauw method [19]. Electrochemical lithium insertion/extraction reactions on these samples were done in a three electrode configuration either with a constant current of 20 mA g −1 (with respect to electrode mass) or voltametrically with a scan rate of 0.05 mV s −1 . 1 M LiClO4 (battery grade, Merck) dissolved in anhydrous propylene carbonate (PC, battery grade, Merck) was used as electrolyte. The

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Fig. 1. X-ray powder diffraction patterns of the investigated samples a) 1, b) 2 and c) 3. * are theoretical peak positions of orthorhombic α-MoO 3 (JCPDS card no. 35-0609).

experiments were carried out in glass cells with an excess of electrolyte. The working electrode was a composite electrode fabricated as follows: the above MoO3 samples were mixed with (44 w/o) of carbon black (Printex L6) as inert conductive additive and 6 w/o polyvinylidene fluoride (PVDF, Aldrich) as binder. 2 mg of the mixture were rolled and pressed onto a titanium mesh used as a current collector. The geometric electrode area was approx. 0.5 cm 2 . Lithium metal in excess was used as counter and reference electrode. All electrochemical measurements were carried out at room temperature in an argon filled glove box.

3. Results and discussion 3.1 Characterization by X-ray powder diffraction Fig. 1 shows the X-ray diffraction patterns of the investigated samples. The figure basically shows that all the samples have the structure of orthorhombic MoO3 . All the diffraction peaks for the samples are characteristic of the orthorhombic structure of α-MoO3 (JCPDS card no. 35-0609), JCPDS “Joint Commite on Powder Diffraction Standards” without discernable impurities ex-


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Fig. 2. Thermogravimetry (TGA) and differential scanning calorimetry (DSC) analysis of sample 1 in air, heating rate 3 ◦ C/min.

cept one peak related to Mo4 O11 (JCPDS card no. 05-0337) or MoO2 (JCPDS card no. 32-0671) was observed at 2θ = 37◦ in the case of sample 2, but the major phase is orthorhombic α-MoO3 . This corresponds well to structure of orthorhombic α-MoO3 reported by L. Khilborg [10]. Sample 3 maintained the same structure of sample 1 (starting material) in spite of the reduction by ascorbic acid, which caused a slight reduction for the sample (this reduction observed from the blue colour of the prepared sample). The only difference between reduced and non-reduced samples is that the XRD profiles of the reduced samples exhibit broader peaks with lower relative intensities, indicating a relatively poorer crystallinity.

3.2 Characterization by thermal analysis Thermogravimetry (TGA) and differential scanning calorimetry (DSC) curves for sample 1 are shown in Fig. 2. No change in the weight is observed during heating up to 700 ◦ C, which attributed to the thermal stability of α-MoO3 until its melting point. Fig. 3 shows the TGA and DSC analysis of sample 2 we observed an increasing in the weight (about 1%) started at about 300 ◦ C when the sample was heated in ambient atmosphere up to 700 ◦ C. This gain in the weight can be attributed to the partial oxidation process (= take up of oxygen), which causes partial filling of the oxygen vacancies in sub-stoichiometric molybdenum oxide. The orthorhombic sub-stoichiometric MoO3−x converts to stoichio-

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metric orthorhombic MoO3 . This observation is confirmed by the change of the colour from blue (MoO3−x , small O/Mo ratio) to yellow (MoO3 with normal O/Mo ratio) [17, 20] after heating in air. Fig. 4 shows the thermal analysis of ascorbic acid which used as a reducing agent for preparation of sample 3. The figure shows that the acid melts and decomposes at about 190 ◦ C (a corresponding endothermic peak related to phase change was observed at this temperature). Three successive exothermic peaks observed at 200, 300 and 465 ◦ C, these peaks may be related to the oxidation of ascorbic acid. At about 500 ◦ C, 100% weight loss was observed (completely sublimation of ascorbic acid). TGA and DSC analysis for sample 3 is shown in Fig. 5. There are two successive endothermic peaks at 200 and 320 ◦ C. These peaks may be related to removal of water molecules from the hydrated molybdenum oxides bronze H x MoO3 · yH2 O. Hydrated molybdenum oxides bronze H x MoO3 · yH2 O may be formed through the slight reduction of MoO3 by reaction with ascorbic acid by intercalation of hydrogen ions and/or water molecules (from ascorbic acid decomposition) between the layers of MoO3 . An exothermic peak observed at about 370 ◦ C with a total weight loss 4.5% could result from the oxidation of eventually remaining traces of ascorbic acid presented on the surface of the sample because complete sublimation of ascorbic acid occurs only at 500 ◦ C (Fig. 4). After 400 ◦ C there is no weight change due to formation of anhydrous thermodynamically stable orthorhombic α-MoO3 until its melting point above 720 ◦ C. It is predicted that water molecules in


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Fig. 4. Thermogravimetry (TGA) and differential scanning calorimetry (DSC) analysis of ascorbic acid in air, heating rate 3 ◦ C/min.


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Table 1. Electronic conductivity results for the investigated samples. Sample

Specific conductivity S cm−1

1 2 3

2.5 × 10−9 [22] 6.25 × 10−5 1.75 × 10−5

MoO3 · H2 O consist of interlayer and co-ordinated water molecules. The interlayer water molecules are extracted at 216 ◦ C and the co-ordinated water molecules at 345 ◦ C. The co-ordinated water molecules and the interlayer water molecule are removed completely above 345 ◦ C producing anhydrous MoO3 [21].

3.3 Characterisation by electronic conductivity Table 1 shows the electronic conductivities of the investigated samples. The electronic conductivities of the reduced samples (2 and 3) is higher than that of non-reduced sample 1, which have also the same orthorhombic structure. For reduced samples the electronic conductivities are in the range 10−5 S cm −1 while it is about 10−9 S cm −1 for the non-reduced sample (due to the high resistively of sample 1 the method used here can not estimate its electronic conductivity, for comparison the value was taken from Ref. [22]. MoO3 is an n-type semiconductor. The reduced forms exhibit pseudo metallic conductivity, it is insulating when oxidised and conducting when reduced. B. Wang et al. [23] reported that molybdenum oxide is transformed into a more open structure when reduced. This open structure benefits the transport of electrons and cations (e.g. Li+ and H + ) in the oxide. When reduced MoO3 was oxidised, it possesses high resistance which blocks transport of electrons and cations.

3.4 Electrochemical characterization Figs. 6–8 show the cyclic voltammograms for sample 1, 2 and 3, respectively. The cyclic voltammograms were carried out between 2.0 and 3.3 V vs. Li/Li + at a scan rate 0.05 mV s −1 to confirm the reversibility of these samples. The results obtained by cyclic voltammetry are similar (but not identical) with respect to the potential for reduction and oxidation (= intercalation and deintercalation) processes. An irreversible large peak was observed at 2.7 V during first intercalation reaction only, it disappeared in the subsequent cycles. During subsequent intercalation and de-intercalation reactions, peaks are observed at 2.3 and 2.55 V


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Fig. 6. Cyclic voltammogram (scan rate: 0.05 mV s−1 ) of sample 1 (1 M LiClO4 /PC, 40 w/o C and cut-off potential: 2.0–3.3 V vs. Li/Li+ ). Numbers in the figure indicate the cycle number.


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respectively. These peaks are weekend during subsequent cycling. In case of sample 2, there is a small de-intercalation peak at about 2.3 V, this peak may be related to presence a trace amount of further reduced molybdenum oxide (e.g. Mo4 O11 or MoO2 ) formed during the reduction process (a corresponding XRD peak for this oxide was observed for sample 2 in Fig. 2). The galvanostatic discharge – charge curves for the investigated samples are shown in Figs. 9–11. The cells were discharged and charged at current density 20 mA g −1 between 2.0 and 3.5 V vs. Li/Li+ . For the three samples, two plateaus were observed in the first discharge curve. The first plateau is an irreversible (disappeared after the first cycle) was observed at about 2.75 V vs. Li/Li+ , whereas the second plateau is reversible (can be seen in the subsequent cycles) and was observed at about 2.35 V. In the charge curve there is only one reversible plateau observed in the range 2.35–2.6 V vs. Li/Li + according to the sample. Big capacity loss was observed between first and second discharge for all the samples. During the subsequent cycles, the capacity loss observed was less. The initial discharge capacities of samples 1, 2 and 3 are 295, 220 and 225 mAh g −1 respectively, which decrease to 57, 96 and 72 mAh g −1 at the end of 20 cycles. Sample 2 delivers 60 mAh g −1 at 100 th cycle, while sample 3 retained only 43 mAh g −1 at the 50 th cycle. Iriyama [24] and Tsumura [25] reported that the first irreversible reaction step at around 2.75 V vs. Li/Li+ corresponds to the lithiation of α-MoO3 ,


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Fig. 9. Constant current discharge/charge cycling (current density: 20 mA g−1 ) of sample 1 (1 M LiClO 4 /PC, 40 w/o C and cut-off potential: 2.0–3.5 V vs. Li/Li+ ). Numbers in the figure indicate the cycle number.

Fig. 10. Constant current discharge/charge cycling (current density: 20 mA g−1 ) of sample 2 (1 M LiClO4 /PC, 40 w/o C and cut-off potential: 2.0–3.5 V vs. Li/Li+ ). Numbers in the figure indicate the cycle number.

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Fig. 11. Constant current discharge/charge cycling (current density: 20 mA g−1 ) of sample 3 (1 M LiClO4 /PC, 40 w/o C and cut-off potential: 2.0–3.5 V vs. Li/Li+ ). Numbers in the figure indicate the cycle number.

which proceeds in the stoichiometric range of 0 < x < 0.25 of Li x MoO3 . This two phase reaction (= unlithiated and lithiated MoO3 coexist during the lithiation process) results in an irreversible structural change of the host MoO3 and yields a low crystalline Li0.25 MoO3 . When x in Lix MoO3 is greater than 0.25 the pristine MoO3 has completely disappeared and from the second cycle the reaction goes in one step, on the contrary to the first reduction which is a two step process. In general the improvement in the cycle performance of the reduced samples 2 and 3 compared with the non reduced sample 1 may be attributed to their higher electronic conductivities, but to some extent compared with sample 1 (Table 1). Because when molybdenum oxide is partially lithiated, the effect of mixed transition metal oxidation states should also considerably increase the conductivity of the material. An enhanced conductivity before the intercalation would then, in any case, be levelled off at the beginning of lithium insertion. Sample 2 looks better than sample 3, this may be related to the thermal stability of sample 2 which contains no water. On the contrary sample 3 contains some intercalated water (as confirmed before in TGA curves). These water molecules play a bad role in attacking the lithium anode and hence declines the battery service life. Also it can be concluded that reduction by creation of oxygen vacancies is better in electrochemical performance than reduction by intercalation of H + between the layers of α-MoO3 .


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4. Conclusion In general, one can assume that the dark colour witness that the lattice contains a number of oxygen vacancies (MoO3−x ) as in sample 2 and/or a number of doping ions e.g., protons (sample 3). The reduced samples exhibited a drastically improved cycling stability and capacity retention on cycling in 1 M LiClO4 /propylene carbonate. This significant improvement of the rechargeability may be related to improvement in the electronic conductivity after reduction process. In the electrochemical performance, the first voltammetric or constant current discharge/charge is different from the following cycles. At about 2.75 V vs. Li/Li+ an irreversible reduction can be observed. Reversible lithium intercalation/de-intercalation proceeds in the potential range of approx. 2.2 V–2.5 V vs. Li/Li+ not only in the first but also in the later cycles. In other words, the first reduction is a two step process, the following reduction reactions proceed in one step.

Acknowledgement The author (A. M. Hashem) would like to thank Prof. Dr. J. O. Besenhard and Prof. Dr. J. Albering (Institute for chemical technology of inorganic materials, Graz university of technology, Graz, Austria) for their co-operation and advise.

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