Uptake of Lithium by Layered Molybdenum Oxide and Its Tin Exchanged Derivatives: High Volumetric Capacity Materials

Solid State Ionics 133 (2000) 37±50 www.elsevier.com/locate/ssi

Uptake of lithium by layered molybdenum oxide and its tin exchanged derivatives: high volumetric capacity materials

Fabrice Leroux1 , Linda F. Nazar* Department of Chemistry, University of Waterloo, Ontario, Canada N2L 3G1 Received 20 July 1999; received in revised form 26 April 2000; accepted 5 May 2000

Abstract

The materials A0.25 MoO 3 (A 5 Na, Li, Sn), prepared by a `chimie douce' route, are a promising alternative as anode materials in Li ion batteries. These materials present large reversible charge capacities, greater than 900 mAh/g, with a good capacity retention on cycling. At least 65% of the charge capacity (600 mAh/g) is maintained under 1.5 V vs. Li. The gravimetric capacities, on the order of 4000 mAh/cm3 , are three to four times greater than for high capacity carbon materials and twice that of Sn oxide-based glasses. A mild heat treatment and an appropriate discharge cut-off potential stabilizes the cycling behavior. A discharge cut-off of 5 mV is associated with a large polarization, and fading charge retention, probably related to the oxygen diffusion process into the highly sub-stoichiometric oxide during the charge sweep. Conversely, raising the charge potential to 200 mV may conserve the oxygen environment surrounding the Mo centre to some degree, thus facilitating oxygen migration during charge. The irreversible capacity and the high average potential in charge are the major drawbacks in these systems. By utilizing the exchange capability of the interlayers ions, Sn can be incorporated into the material, thus lowering the average charge potential but at the expense of capacity fading. Finally, a catalytic effect of the carbon black in these composite electrodes via an interface effect is present, which must be accounted for by methods other than simple subtraction of the carbon contribution to the total capacity by mass fraction.  2000 Elsevier Science B.V. All rights reserved.

Keywords: Lithium; Molybdenum oxide; Tin exchanged derivatives; Lithium-tin-molybdenum oxides; Negative electrodes

1. Introduction ple is based on the simultaneous insertion and deinsertion of Li ions from negative and positive An area of current and intense research activity in electrode host materials [5,6]. This concept has the Li ion batteries is the development of new negative advantage of avoiding metallic lithium in the battery, electrode materials [1±4]. The Li ion battery princi- which may result in a short circuit in the cell as a result of dendritic growth. A good negative electrode candidate must ful®ll *Corresponding author. the following expectations: E-mail address: [email protected] (L.F. Nazar). 1Current address: UniversiteÂ Blaise-Pascal, Laboratoire des Mat- eriauxÂÁÂ Inorganiques, ESA no. 6002, 63177 Aubiere Cedex, • a low as possible average intercalation potential France; ¯[email protected] vs. Li,

• a high as possible reversibility towards Li de- als in lithium ion batteries. To overcome these insertion, drawbacks, Si±C±O based polymers (polysiloxanes), • a low as possible polarization between these two Si or Sn pitches dispersed in carbon, and more electrochemical processes, recently, active/inactive nanocomposites have been • other practical considerations such as low cost investigated [25,26]. Such an approach may stabilize and low toxicity. the capacity by isolating the Li alloy nanoparticles within an active matrix to prevent bulk alloy forma- Amongst the compounds which possess the property tion. Following the same principle, FujiFilm Celltec to intercalate Li at low potential, graphite and more announced the development of a Li ion battery, using generally carbonaceous materials have been and still an amorphous tin-based oxide as a negative elec- are the most frequently studied materials [7±13]. trode, although this has yet to appear on the market Now, however, an anode material of greater capacity [27]. is required to match the new emerging cathode In this paper, our leitmotif is based on two materials which have been developed with increased arguments, related to our study of the reaction of Li capacity. Other materials aside from graphite also with Na0.25 MoO 3 at low potential, which have been have the ability to intercalate Li at low potential. The communicated in preliminary form [24]. The capaci- concept of using an oxide as the negative electrode, ty fade experienced by metallic alloys such as Sn for example was introduced 10 years ago by Auborn may be partly avoided by surrounding the Sn par- and Barberio based on a MoO222 or WO /LiCoO ticles with a reduced matrix MoOx (where x , 1at based `rocking-chair' battery [1]. The characteristics low potential). From the viewpoint of MoOx addition of these materials and especially the poor stability of of Sn may decrease the average charge potential. their electrolyte at low potential inhibited further Here, we report the electrochemical characteristics of development. The renewed interest in negative elec- the series A0.25 MoO 3 (A 5 Li, Na, Sn) with consid- trode materials, with rapid growth in portable elec- eration of the effects of thermal treatment, carbon tronic device production, has resulted in the discov- black contribution and discharge cut-off voltage. ery of unexpected properties in some crystalline metal oxides, such as LiMVO4 (M 5 Zn, Cd, Ni) [18], Lixz MO and Li xy M V12yz O (M 5 Ti, V, Mn, Co, 2. Experimental Fe, Ni, Cr, Nb and Mo) [19±21] and in amorphous compounds such as InVO426 [22] and MV O1d (M 5 2.1. Synthesis of the sodium molybdenum bronze Fe, Mn, Co) [23]. Recently, we reported a gravimet- and its hLi, Naj derivatives ric capacity of 950 mAh/g, and volumetric capacity 3 1 z2 of over 4000 mAh/cm for a partially reduced [Na (H2 O)nz ] [MoO3 ] (denoted Naz MoO3 ); molybdenum oxide [24]. where z represents the degree of reduction of the

The cycling curves for all transition metal oxides, MoO3 layers, was synthesized using a redox reaction however, exhibit large polarization associated with in aqueous media according to Thomas and McCar- high charge potential (| 1.5 V vs. Li). Other possible ron's method [28]. This preparation prevents the negative electrode materials include main-group co-intercalation of protons and gives reproducible metals, amongst the ®rst studied for their capacity to results, with z 5 0.25. Two grams (13.9 mmol) of react with Li at low potential [14±17]. Al, Si, Ge, MoO3 (Johnson Matthey) were suspended in 100 3 Sn, Pb, Sb and Bi are all known to form Li alloys cm of Millipore water and N2 was bubbled through with a stoichiometry as high as 4.4 Li/M (e.g. the suspension for 2 h. Sodium dithionite, Na22 S O 4 , Li22 Sn 5 ). Unfortunately, formation of the alloy is (0.8 g, 4.6 mmol-Aldrich) and Na242 MoO ? 2H O(24 associated with a one to two-fold volume expansion, g, 100 mmol-Johnson Matthey) were simultaneously thus pulverizing the electrode material and resulting added to the MoO3 suspension. The reduction re- in loss of electrical contact between the material action was carried out for 3 h at room temperature grains on cycling. For both oxides and metals, these under nitrogen atmosphere. The resulting powder problems hinder their practical use as anode materi- was collected by suction-®ltration and washed with F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50 39 water until the ®ltrate was colourless. The ®nal system. A heating rate of 10 K/min was used from product was a dark blue crystalline powder with a room temperature up to 8008C. X-ray powder dif- metallic lustre. The composition was con®rmed by fraction (XRD) patterns were obtained on a Siemens elemental analysis (vide infra). D500 diffractometer equipped with a diffracted beam Lithium molybdenum bronze was synthesized via monochromator, using Cu Ka radiation. two routes. One involved the same protocol as A Varian Liberty 100-ICP-AES was used for above: formation of the lithium molybdenum bronze chemical analysis. X-ray photoelectron spectroscopy 1 z2 was identical to that of [Na (H2 O)nz ] [MoO3 ] (XPS) measurements were performed at the Western except Li242 MoO ? 2H O was used as the buffer Science Centre (London, Ontario, Canada). The Sn± instead of sodium molybdate. This material is de- Mo oxide sample was mounted by pressing it onto noted as Liz MoO3 . The other route proceeded via a indium foil. cation exchange, accomplished by suspending fresh, Charge compensation was achieved using a low wet sodium molybdenum bronze (2 g) with LiI (4 g) energy ¯ood gun and by covering the sample with a in 50 ml of water. The exchange was repeated twice nickel grid. Due to the extensive charging, an under nitrogen atmosphere. Residual LiI was re- internal calibration standard was needed. Oxygen moved from the yielding material by washing re- (1s), as a metal oxide (529.9 eV), was chosen since peatedly with 1-butanol to yield `(Li,Na)z MoO3 '. the carbon signal from adventitious carbon was ambiguous.

2.2. Synthesis of the tin (SnII ) exchanged molybdenum oxides 2.5. Electrochemical measurements

The tin exchange with A0.25 MoO 3 (A 5 Na, Li) The composite electrodes were prepared from was carried out overnight at room temperature under active material, carbon black (Super S, Chemetals a nitrogen atmosphere using a ratio A:Sn of 1:10. An Inc.) and Kynar FlexE 2820-00 as an organic binder. aqueous suspension of 350 mg of NaMoO3 in 200 The powders in the weight proportion 85, 12, 3, ml of water was sonicated for 20 min, and 1.24 g of respectively, were mixed in cyclopentanone and the

SnCl22? 2H O (BDH) was added. The pH dropped slurry was spread onto a nickel disk. Electrodes were from 4.3 to 1.8 during the reaction. The same treated at 1008C for 2 h before their assembly in the procedure was used for (Li,Na)0.25 MoO 3 , adjusting glove box. A mixture of 1 M LiPF6 in a 2:1 solution the amounts to re¯ect the change in stoichiometry. of vacuum-distilled dimethyl carbonate (DMC- The ®nal dark blue-gray powders were collected by Aldrich) and ethylene carbonate (BC-Aldrich), was  suction-®ltration and washed with water until the used as the electrolyte. The Swagelock cells were ®ltrate was colourless. assembled in an argon ®lled glove box, containing less than 2 ppm of water and oxygen. Composite 2.3. Synthesis of amorphous molybdenum-tin oxide electrodes had a surface area of 1 cm2 and contained 1±2 mg of active material. Lithium metal was used

Aqueous solutions of SnCl22? 2H O and Na 2 MoO 4 as anode. A multichannel galvanostatic/potentio- were mixed and stirred together at room temperature static system (Mac-PileE) was used for the electro- under a nitrogen atmosphere. A dark green powder chemical study. rapidly precipitated, and was collected as described The electronic density (per mole V21 ) vs. voltage above. The absence of Na or Cl in the product was curves were calculated by numerical differentiation con®rmed by ICP, yielding a general composition, of the voltage composition dependence curves. Fol-

SnMoxy O ? nH2 O for the resultant material. lowing convention, the charge is denoted by the positive values. The irreversible capacity, Qirr,is 2.4. Instrumentation taken as the difference in capacity between the ®rst discharge and the ®rst charge. The reversible capaci-

Thermogravimetric analysis was carried out on a ty, Qrev, is taken as the ®rst charge capacity and the PL Thermal Sciences STA-1500 thermal analysis subsequent cycles. The average voltages for the 40 F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50

insertion process, Vav.D (second discharge), and the Sn obtained by XPS; Mo and Sn 3d energy levels deinsertion process, Vav.C (®rst charge), are reported; (not shown) were investigated. The Mo 3d popula- the difference DV5Vav.C2V av.D, which is twice the tion was resolved into two components, attributed to IV VI cell polarization provides an estimate of the Mo (E(3d5/2 )5229.3 eV) and Mo (E(3d 5/2 )5 `feasibility' of the process. The charge capacity at 230.5 eV) [30]. The energy splitting of the two 3d the nth cycle divided by the ®rst charge capacity is doublets was slightly greater than 3 eV. The same the retention of capacity (Rnth/1) at the nth cycle. Gaussian ®t procedure was carried out for the Sn 3d Special attention was also taken to evaluate the levels that were also distributed into two compo- II carbon black contribution to the total electrochemical nents, attributed to Sn (E(3d5/2 )5484.5 eV) and IV response of the composite (black carbon1active Sn (E(3d5/2 )5486.1 eV), with an energy splitting material) electrodes (vide infra). of the 3d level of |8.4 eV. The chemical formula can hence be written as

II IV IV VI (Sn0.88 Sn 0.12 )(Mo 0.58 Mo 0.42 ) 0.92 O 3.35? nHO 2 3. Results and discussion (n 5 1.44 by TGA)

3.1. Characterization of the materials The water content was determined by TGA, where the weight loss ascribed to H2 O was readily discern- ible as a discrete step. The presence of reduced MoIV The uptake of the solvated Na ions in MoO3 was correlated with an increase in the interlayer distance. may be the result of the redox reaction: The interlayer spacing of MoO increased from 6.92 22 211→ 41 3 2MoO421 Sn 1 8H 2MoO 1 Sn 1 to 11.18 AÊ for the fully hydrated materials. After 8H2 O (1) several days in a dry N2 atmosphere, the interlayer distance equilibrated to a value of 9.63 A,Ê in The excess of reduced MoIV could be explained by a agreement with results obtained by Sotani et al., for partial dissolution of Sn21 ions. To our knowledge, the partially hydrated bronze [29]. The combination except for a eutectic mixture SnO23 ±MoO formed at of ICP and TG analysis con®rmed the composi- 7508C, which dissociates into SnO23 and MoO at tion [Na(H2 O) 2 ] 0.26 MoO 3 , henceforth denoted room temperature [31], and SnMo28 O [32], no other Na0.25 MoO 3 or Naz MoO3 . Similar analytical experi- molybdenum tin oxide has been reported in the ments gave rise to a composition of literature.

[Li0.20 Na 0.06 (H 2 O) 0.52 ]MoO 3 for Liz MoO3 (d 0015 Ê 9.45 A) and Li0.07 Na 0.13 (H 2 O) 0.40 MoO 3 for 3.2. Electrochemical study Ê (Li,Na)z MoO3 (d 00159.53 A). Ion-exchange of the II alkali ions in Naz MoO3 for Sn yielded materials 3.2.1. Voltage pro®le for Na0.25 MoO 3 vs. discharge with a composition of Na0.252xx Sn MoO3 (where x5 cut-off potential 0.1). For simpli®cation, in the following, SnII ex- Numerous authors who have studied the insertion changed materials from (Li,Na)0.25 MoO 3 and of Li ions into MoO 3 and Mo bronzes in the Na0.25 MoO 3 are denoted Sn Li,Na MoO 3 and potential window 4.0±1.0 V vs. Li [33±36] have SnNa MoO 3 , respectively. The tin-exchanged materi- reported a topotactic reaction inherent to the lamellar als exhibit a broad diffraction peak centered at 9.12 nature of these oxides in this voltage range. They A,Ê slightly smaller than the pristine materials, con- also drew attention to the poor ionic and electronic sistent with the difference in ionic radius (102 ppm conductivity of MoO3 in its discharged state [37± for Na1 and 93 for Sn21 ). 39]. Li insertion into Mo bronzes has not been

The amorphous molybdenum tin oxide, SnMoxy O ? undertaken at low potential (,1 V), however. Fig. 1 nH2 O, was characterised by a combination of tech- shows the ®rst cycling curve for Na0.25 MoO 3 as a niques, as follows. The atomic ratio of Mo/Sn was function of the discharge cut-off potential, Vd. For found to be 0.92 by ICP, and the oxygen content yVd .500 mV vs. Li, the voltage pro®le maintains the was calculated from the oxidation states of Mo and feature of a single phase transformation (Fig. 1a). F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50 41

mV) and the material undergoes a major transformation during the ®rst discharge. Ex-situ X-ray diffraction experiments performed after one cycle at each

step of Vd show that complete amorphisation of the material was achieved after a discharge to Vd ,500 mV. For Vd 5500 mV, extremely weak diffraction peaks appear at 11.92 and 8.81 AÊ that can be attributed to the presence of residual lamellar 1 z2 [Na (H2 O)nz ] [MoO3 ] where n55 and 2, respectively. The Li reaction within the entire voltage range is described by two consecutive processes: → For Vd0.253. 0.5V, [Na MoO ] (t)Lia [Na0.25 MoO 3 ]

1 (12 t)Lib2a MoOy (2)

For Vd , 0.5 V,(t)Lia [Na0,25 MoO 3 ] → 1 (12 t) Lib2acy MoOy Li MoO9

1 d(Li,Na)2 O (3) The parameter t denotes the degradation of the lamellar structure that occurs during discharge; a represents the Li contribution from the topotactic reaction; (b2a) and c are the Li content in the

amorphous materials for Vd .0.5 V and ,0.5 V, respectively; and d represents the formation of

lithium oxide (irreversible capacity, or Qirr). Such a description, involving the creation of a Mo-suboxide, has been con®rmed by NMR and EXAFS studies at

Fig. 1. Voltage±capacity curve (C/3) for Na0.25 MoO 3 in the different depths of discharge of NaMoO3 [24]. In 3.5-Vddvoltage range, with V 5500 (a), 300 (b), 200 (c) and 0.005 terms of structural integrity, Eq. (2) is partly revers- (d). First and subsequent three sweeps are shown. ible whereas Eq. (3) is completely irreversible. However, `a loss of structural integrity during cy- After the ®rst discharge, the Li insertion process is cling does not necessarily translate into a loss of highly reversible as shown in Fig. 1a (inset) and rechargeability' [35] as exempli®ed by comparison more than 100 cycles were performed without sub- of the reversible component in Fig. 1. stantial capacity fading. The deinsertion process is To estimate the carbon black response component centered at 2.4 V vs. Li as observed previously for of the total coulometric titration, the carbon additive

MoO3 . was studied as an active material (Fig. 2). Unlike The cutoff potential does not only in¯uence the graphite, the discharge curve does not show the capacity, but it also dramatically affects the voltage presence of any staged phases (Fig. 2a) since the pro®le charge process (Fig. 1b, c and d). For Vd 5 acetylene-based carbon black does not have crys- 300 mV, another charge process is observed at 1.3 V. talline order. The ®rst discharge capacity is not

Lower discharge limits (Vd 5200 and 5 mV vs. Li) completely recovered in the charge process, giving result in complete removal of these features in rise to a large irreversible loss (223 mAh/g). This charge, i.e. no Li insertion is observed above 2 V. lost capacity is commonly attributed to the formation This shows that the single phase reaction is not of a passivation layer (solid electrolyte interface conserved after discharge at low potential (Vd ,500 (SEI)) on the surface of the carbon black arising 42 F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50

Fig. 3. Voltage pro®les for Na0.25 MoO 3 in the voltage windows: (a) 3.5±0.005 V; (b) 3.5±0.2 V vs. Li (b). The current used corresponds to the insertion of 1 Li in 3 h.

ished compared to Fig. 1c and d, not only for the ®rst Fig. 2. (a) Voltage±capacity curve for carbon black in the 3.5± 0.005 V vs. Li voltage window, at a current density of 106 mA/g cycle but also for subsequent cycles. This difference in discharge and 73 mA/g in charge (corresponding to C/4 and which may be due to the poor conductivity of C/3, respectively); (b) voltammogram for carbon black obtained NaMoO3 indicates that relation (4) is not appro- from potentiodynamic cycling at 10 mV/0.2 h. The SEI peak is priate, since a term which includes the interfacial evident at 750 mV vs. Li. interaction between the carbon black and the active material is needed. Hence, the carbon contribution from its reaction with the electrolyte. The following was ®rst determined by means of a potentiodynamic cycles show excellent reversibility (242 mAh of Li/g experiment (Fig. 2b). The sum of the insertion carbon). To take the carbon black contribution in the current for the carbon black and NaMoO3 (alone) is composite into account, a typical method is to compared to the current of the composite electrode subtract its capacity from the total reversible capacity via a potentiodynamic study in Fig. 4. The poten- according to its relative weight percentage in the tiodynamic experiments were carried out within the composition (i.e. X512%): same voltage range (3.5±0.010 V vs. Li) and a voltage step of 10 mV/0.2 h was applied in each Qrev(composite) 5 (12 X%) Q rev(active material) case. The current is normalized per gram of material and then summed according to the relative mass 1 X% Qrev(carbon) (4) fraction of each material. As is evident in Fig. 4, the This is not strictly accurate, as demonstrated by Fig. sum of the two components, taken separately, within

3 which shows the voltage pro®le of NaMoO3 the composite electrode is smaller than the response without any carbon additive for two values of Vd: of the composite electrode itself. note the discharge capacity is substantially dimin- These results highlight the role of the interfacial F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50 43

Fig. 4. Voltammograms obtained from potentiodynamic cycling at 10 mV/0.2 h for the composite electrode hC1Na0.25 MoO 3j (1) and the sum hCj1hNa0.25 MoO 3j (2) (see text). (a) First cycle 2.5±0.010 V vs. Li, the higher voltage portion 0.5±3 V is shown in the inset; (b) second discharge. interaction and the effect of the electronic con- mixture (macrocomposite) that displays an interfacial ductivity from the additive. It is most clearly ob- enhancement effect, rather than a simple sum of each served for the charge process: only a small anodic component. This presumably arises from a combina- current is recorded for the arithmetic sum, as one tion of the conductivity enhancement, and/or] inter-] would expect from Fig. 3; however, the phenomenon facial metal oxide/carbon grain boundary sites that ]]]]]]]]]]]]]]] also occurs in discharge. Two other differences are uptake Li. In our analysis, we therefore consider the ]]] also observed when introducing the additive: split- total mass of the electrode in our calculations, as the ting of the 2.05 V-Li potential site for NaMoO3 to interaction term cannot be evaluated. Eq. (4) is two potential sites (1.85 and 2.25 V vs. Li) in the reformulated as: composite electrode (inset Fig. 4a), and a shift in the Q (composite) 5 Q (experimental)/(1 1 X%) SEI peak potential compared to the additive alone. rev rev C The composite must be considered as an intimate (5) 44 F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50

Since the carbon black has a reversible capacity ®ve have the same shape as before (Fig. 1c and d), but times less than the composite, we note that the the electrochemical performance is improved, as

NaMoO3 capacity is hence underestimated in our exempli®ed by the decrease of the irreversible calculation. portion and the better charge capacity recovery. Both

Comparison of the voltage pro®les (Fig. 4a and b) effects give rise to a greater reversible capacity Q1C con®rms the poor electronic conductivity of of 938 mAh/g for Vdd55 mV (Table 1). For V 5200 Na0.25 MoO 3 which may preferentially affect the mV, this thermal treatment does not have an effect on higher voltage portion of the curve (Vd .200 mV). the reversible contribution; namely Q1C remains the The reversible capacity for Vd 55 mV (Fig. 3a) is same, whereas the irreversible component is strongly | greater than the sum of the reversible capacities diminished ( 110 mA/g). To a lesser extent, Q1C is | obtained from Vd 5200 mV (Fig. 3b) added to the also decreased for Vd 55 mV, but only by 30 mAh/ capacity from 200 to 5 mV (3.8e/Mo compared to g. A complicated behavior akin to the interfacial 1.6e2211.8e , respectively). This reinforces the con- reaction seems to occur below 200 mV, as both cept of a change in the nature of Li insertion below contributions (Qirrand Q rev) are increased: neverthe- 200 mV, consistent with our EXAFS results [24,40]. less the net balance is positive. Fig. 5 summarizes the evolution of the reversible Fig. 7 shows the charge capacity on cycling for and irreversible capacity with the discharge cut-off. Naz MoO3 in two different voltage windows as a The carbon black contribution (and interaction) was function of Vd between 100 and 1708C. A cutoff taken into account for Vd ,500 mV, according to relation (5). The irreversible capacity Qirr does not increase as much as the reversible capacity Qrev.To limit this loss, attributable to residual water within the Naz3 MoO , thermal treatment of the material was carried out at a temperature corresponding to the point just before the layered structure collapses.

3.2.2. Effect of a thermal treatment Fig. 6 shows the voltage pro®les of the ®ve ®rst cycles for Naz MoO3 at two different voltage cut- offs, after a treatment at 1708C. The cycling curves

Fig. 6. Voltage pro®les of the ®rst ®ve cycles of Na0.25 MoO 3 after Fig. 5. Variation of the reversible (square dots), and irreversible a heat treatment at 1708C, in the 3.5±0.005 V (a) and 3.5±0.2 V (triangular dots) of the composite electrode, with 12% carbon (b) voltage range, at a current density of 71 and 44 mA/g in black, as a function of the discharge cut-off potential. For Vd ,500 discharge and charge, respectively. The capacity is expressed per mV, the total weight of the composite electrode is taken into gram of composite electrode. The loading of the composite account in the calculation of the capacities. electrode is the same as used for Fig. 1. F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50 45

Table 1

The percentage of the irreversible capacity during the ®rst cycle (with reference to the total ®rst discharge capacity) Qirr, the ®rst charge capacity, Q1C, with respect to the total mass of the composite electrode (see text), and the charge capacity retention after 20 cycles, R20/1, for a Na0.25 MoO 3 cycled in the 3.5±0.2 and 3.5±0.005 V voltage range, are listed Material T51208C T51708C

QQirr 1C R 20/1 QQ irr 1C R 20/1 (%) (mAh/g) (%) (mAh/g)

Na0.25 MoO 3 35 882 0.73 33 938 0.77

(Vd 55mV) b b Na0.25 MoO 3 38 650 0.76 31 652 0.88

(Vd 5200 mV)

C(Vd 55 mV) 48 242 0.92 48 245 0.93

C(Vd 5200 mV) 71 88 0.95 69 90 0.96 a These characteristics are given for the electrode materials after a treatment at 120 and 1708C. Data for carbon black (C) are given. b Note that the reversible charge capacity of the carbon black is overestimated in the calculation in this voltage range, since the contribution of the carbon black is more than seven times less than the composite capacity.

standing of the different cycling behavior as a

function of Vd (Fig. 8). The area under the curves is directly proportional to a coulometric titration. For

Vd 5200 mV (Fig. 8a), the difference between the 2nd and 100th cycles is insigni®cant, whereas for

Fig. 7. Variation of the speci®c capacity of Na0.25 MoO 3 composite electrode on cycling as a function of the discharge cut-off potential, at a current density of 69 and 45 mA/g in discharge and charge, respectively (corresponding to C/15 and C/20). The capacity is expressed per gram of composite electrode; (a) and (c) correspond to 1708C-treated material; (b) and (d) are for 1008C- treated material; (a) and (b) are at Vdd55 mV, (c) and (d) at V 5200 mV. voltage of 200 mV gives rise to a good stabilization of the charge capacity on cycling, with a capacity retention of 88% after the 20th cycle and 75% at the 50th cycle. With deeper discharge, a greater speci®c capacity is recovered, but at the expense of reduced cyclability due to an increase of the polarization. The charge capacity fading in the 3.5±0.005 V range is almost twice that for the 3.5±0.2 V window (4.3 and 2.2 mAh/g per cycle, respectively). On the other hand, heat treatment at 1008C provides a lower, and Fig. 8. Electronic density vs. voltage curves (from the experi- less stable capacity on cycling (Fig. 7b and d). ments presented in Fig. 9) of Na0.25 MoO 3 at (a) 3.5±0.005 V; (b) The dx/dV vs. V curves provide a further under- 3.5±0.2 V. The number of cycles is noted on the ®gure. 46 F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50

Vd 55 mV, the intensity and features of the derivative potential (1.2±1.4 V vs. Li). In order to minimize curves are different (Fig. 8b). Hence, after the ®rst this, tin, which is well-known to uptake large cycle, we conclude that Naz MoO3 undergoes revers- amounts of lithium at low potential, was introduced ible transformations at Vd 5200 mV; conversely for between the layers of the oxide. The reaction of Li Vd 55 mV, the change in the electronic density pro®le with SnO itself proceeds according to the following on cycling shows that irreversible transformations proposed mechanism [41]: occur below 200 mV which increase the polarization. 12→ The effect of the charge cut-off potential Vc4.42(2) SnO 1 6.4Li 1 6.4 Li Sn 1 Li O (6) was also been examined. A decrease of Vc to 2.5 V vs. Li does not have a signi®cant effect on the Our recent results suggest that the reaction is more cycling behavior when a moderate current density complex than originally thought [42], especially for (C/15 in charge) was used. At a higher current the charge process. Migration of oxygen atoms close density, a higher value of Vc (.2.8 V) is necessary to the Sn centers is observed even at low degrees of to reach an appreciable capacity due to the increase charge, showing that the alloying and de-alloying in polarization. Under rapid cycling conditions (104 process does not completely explain the electro- and 75 mA/g for the discharge and charge current chemical behavior. The Li uptake by SnO is shown density corresponding to an effective C/l.6 and C/ for comparison in Fig. 9a. In accord with our other 2.3 rate, respectively), a charge capacity of 170 results, we note the irreversible capacity for SnO is mAh/g (approximately 700 mAh/cm3 ) was main- lower than one would expect according to Eq. (6) tained after 100 cycles in the voltage range 2.5±0.2 (281 mAh/g compared to 398 mAh/g, 1 Li2 O per V vs. Li. SnO), and the uptake is less than the theoretical (4.4). Nonetheless, the reaction above is a useful 3.2.3. Effect of the interlayer cations simpli®cation. The effect of substitution of the interlayer cations The voltage pro®les of the tin-exchanged materi- on the electrochemical characteristics was studied. als, SnNa MoO 3 and Sn Li,Na MoO 3 in the voltage (Li,Na)z MoO3 and Liz MoO3 exhibit similar features range 3±0.005 V vs. Li are shown in Fig. 9b and c. and cycling properties to those displayed by As expected, a lower charge potential in the 5±1000

Naz MoO3 (Table 2). Their performance is slightly mV range is qualitatively observed for SnLi,Na MoO 3 inferior to Naz MoO3Na3 , especially with respect to and Sn MoO , corresponding to a decrease of 170 irreversible capacity and charge retention, although and 70 mV compared to the parent materials, as before, thermal (1708C) treatment improves the (Li,Na)z MoO3 and Naz MoO3 , respectively (Tables 1 characteristics. Liz MoO3 shows the largest reversible and 2). In order to assess each component, a capacity for the ®rst cycle, although unfortunately calculation was carried out on the basis of the this is associated with rapid fading (R20/150.59). relative contributions of SnO and MoOx within the All these materials, like other transition metal exchanged materials. The capacity function, Q(y), oxides, suffer from a relatively high average charge can be written as:

Table 2

The percentage of the irreversible capacity of the composite electrode in the ®rst cycle (with respect to the total ®rst discharge capacity) Qirr, the ®rst charge capacity, Q1C, referenced to the total mass of the composite electrode (see text), and the charge capacity retention after 20 a cycles, R20/1, for Li 0.25 MoO 3 , and (LiNa) 0.25 MoO 3 -based electrodes, cycled in the 3.5±0.005 V voltage range Material T51208C T51708C

QQirr 1C R 20/1 QQ irr 1C R 20/1 (%) (mAh/g) (%) (mAh/g)

Li0.25 MoO 3 39 802 0.64 36 970 0.59

(Li,Na)0.25 MoO 3 49 840 0.39 35 910 0.73 a These characteristics are given for the electrode materials after treatment at 120 and 1708C. F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50 47

Table 3 The percentage of the irreversible capacity of the composite electrode in the ®rst cycle (with reference to the total ®rst

discharge capacity) Qirr, the ®rst charge capacity, Q 1C, with respect to the total mass of the composite electrode (see text), and

the charge capacity retention after ®ve cycles, R5/1, for MoO 3 ,

MoO22 , MoS , and SnO 2 , cycled in the 3.5±0.005 V voltage range are listed

Material Qirr(%) Q 1C(mAh/g) R 5/1

MoO3 29 773 0.77 a MoO2 17 674 0.83

MoS2 28 781 0.72

SnO2 35 818 0.69 a For MoO2 , the electrochemical response under low current density has been used (see text).

where y represents the degree of exchange, and Q represents the capacity for A MoO . AMoO3 0.25 3 Q and Q are taken from Tables 1, 2 and 3. AMoO3 SnO Since the relative increase of Li capacity with y is relatively greater than the capacity of the starting Mo bronzes:

Q QAMoO ]SnO] 2.]]3] O SDMM Sn AMoO3 Q(y) is a positive function. (8) The theoretical capacities are 922 and 953 mAh/g

for SnLi,Na MoO 3 and Sn Na MoO 3 , respectively. These values are close to the experimental capacities (Table 4); thus, we deduce that the behavior of the tin- exchanged phase is indeed the sum of the contribution of each component. Given this, and the fact that in principle a greater

proportion of Sn would provide a lower VC,we investigated the electrochemical behavior of the

amorphous mixed oxide, SnMo0.92 O 3.35 . The dis- Fig. 9. Voltage pro®les for (a) Sn(Li, Na); (b) Sn (Na); (c) charge-charge curve for this material is shown in amorphous SnMo; (d) SnO; at the same current densities as in Fig. Fig. 9c. Contrary to our expectations, the capacity of

6. The capacity is determined by taking into account the total mass SnMo0.92 O 3.35 is only 790 mAh/g, associated with of the electrode (and factoring out the carbon contribution, as in a large irreversibility (Table 4). There is a decrease Eq. (5) in the text). of the charge retention [along with a respective

decrease of Vav.C] with increasing Sn content: Mo0.92 SnO 3.35,Sn Li,Na MoO 3,Sn Na MoO 3 . The fading on cycling is mostly contained within the ®rst Y ? QSnO1 Q AMoO Q(y) 5 ]]]]]]]]]3 ]*26806 cycles for the Sn exchanged molybdenum oxide, in y ? M 1 (0.25 2 2y) M 1 M Sn A MoO3 contrast to SnO and the amorphous mixed oxide (7) which exhibit a continuous capacity decrease (Fig. 48 F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50

Table 4

The percentage of the irreversible capacity during the ®rst cycle (with reference to the total ®rst discharge capacity) Qirr, the reversible capacity, Qrev5Q 1C, (with respect to the total mass of the composite electrode), the average potentials, Vav.Cand V av.D, and their difference,

DV, and the charge capacity retention after ®ve cycles, R5/1, for SnMo 0.92 O 3.35 , (LiNa) 0.25 MoO 3 , SnLi,NaMoO 3 , Na 0.25 MoO 3 and a SnNaMoO3 cycled in the 3.0±0 V voltage range, are listed

Material VVav.D av.CDVQ 1C5QQR rev irr 5/1 (V vs. Li) (V vs. Li) (V vs. Li) (mAh/g) (%) (%)

(Li,Na)z MoO3 0.52 1.38 0.86 910 35 95

Naz MoO3 0.52 1.39 0.87 938 33 92 SnO 0.45 0.88 0.43 722 28 73

SnLi,Na MoO 3 0.52 1.21 0.69 919 36 79

SnNa MoO 3 0.60 1.32 0.74 965 30 87

Mo0.92 SnO 3.35 0.50 1.19 0.69 552 49 74 a These characteristics are given for the electrode materials after a treatment at 1708C.

10). It appears that the incorporation of Sn via 3.2.4. Understanding Li uptake in Na0.25 MoO 3 exchange helps to inhibit the growth of the Lix Sn Free energy calculations (obtained from a close to particles to some extent. equilibrium Li titration) have been previously used to The difference in capacity retention amongst the calculate the thermodynamical stability of the elec- hSn, Moj materials may be explained by the relative trolyte components. For instance, propylene carbon- Sn content but also in the way the LiSn particles are ate (PC) reacts with Li to form propene and lithium distributed within the lithium molybdenum oxide carbonate, according to matrix. In the amorphous oxide, the Sn alloying centers are randomly dispersed, whereas the collapse CH (CHOCH O)CO 1 2Li121 2e → Li CO z2 32 23 of the MoO3 layers upon discharge may preserve the initial Sn±Sn distance of the regularly interleaved 1 C36 H (9) Sn cations (as the collapse of the lamellar framework occurs before the LiSn alloying process). Seemingly, The free energy of 269 kcal/mol indicates that the the chance of Sn agglomerate formation is less in driving force is thermodynamically very favorable to this case. the breakdown of PC. Indeed, this occurs at |750 mV vs. Li, and hence PC cannot be used below this potential unless the kinetics of the breakdown process hinder the reaction. Such calculations are also pertinent to the understanding of the reaction of Li

with MoO3 and SnO. In the latter, the free energy of | Li22 Sn 5 formation was found to be 28 kcal/mol (|235 kJ/mol), according to:

000 DG 5 (x 2 x9)/2DG f2(Li O) 1DG f(Lix9 Sn) 0 2DG f (SnO) 52xF´ (10)

0 where DG f , F and ´ represent the free energy of formation, the Faraday constant and the average Fig. 10. Variation of the speci®c capacity of exchanged tin- 0 potential at the equilibrium, respectively and DG f 2 molybdenum oxide composite electrodes on cycling at a current 21 (Li2 O)52134.13 kcal/mol (559 kJ mol ), the density of 69 and 45 mA/g in discharge and charge, respectively 0 (corresponding to C/15 and C/20); the capacity is expressed per DG f (SnO)5261.4 kcal/mol. Similarly, molyb- gram of composite electrode. denum trioxide has been studied at low voltage, and F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50 49

the Gibbs free energy formation was calculated Az MoO3 materials (Table 4). Az MoO3 -based elec- according to: trodes perform better than these materials (Table 3),

000 and present a better speci®c capacity and charge DG 5 (x 2 x9)/2DG f2(Li O) 1DG f(Lizy MoO ) capacity retention than other Mo-oxides. 0 2DG f3(MoO ) 52xF´ (11)

0 where DG f3(MoO )52159.66 kcal/mol. The 0 4. Conclusions DG f (Lizy MoO ) was found to be 2135 kcal/mol. The nature of whether or not the reduced form is still The materials A MoO , prepared by a `chimie an oxide is independent of this calculation. Im- 0.25 3 portantly, from a thermodynamic point of view, the douce' route, are a promising alternative as anode reduction of MoO is more favourable than Li Sn materials in Li ion batteries. These materials present 3 y . alloy formation, which is known to occur. Neverthe- large reversible charge capacities, 900 mAh/g of less, it is evident that the large polarization observed composite electrode, with a good capacity retention on cycling. At least 65% of the charge capacity (600 in charge (MoO3 and Liz MoO3 ) cannot be explained by the free energy of formation (or that needed for mAh/g of composite electrode) is maintained under 1.5 V vs. Li. The gravimetric capacities obtained the backreaction) but by the nature of the resulting 3 deeply-reduced material. correspond to more than 4000 mAh/cm (density for MoO is of 4.69). These capacities per unit volume Our previous explorations of this issue using XAS 3 with Li uptake by molybdenum oxides have sug- are three to four times greater than for high capacity gested that this is indeed the case, and our more carbon materials and twice that of Sn-based glasses. detailed experiments that have investigated the An interfacial effect of the carbon black in these charge process have con®rmed it [24,40]. Namely, a composite electrodes is present, that appears to result comparison of the pseudoradial distribution functions from enhanced conductivity in the composite and/or from the EXAFS showed that the reduction of interfacial sites for Li uptake that arise from intimate contact of the carbon and metal oxide particles. A Na0.25 MoO 3 is associated with a drastic change in the local environment. Nonetheless, at each step, a soft heat treatment and an appropriate discharge Mo-oxygen interaction is still present around the cut-off potential stabilize the charge capacity on | cycling (|600 mAh/g after 100 cycles). The ir- central atom, such that even at Vd 250 mV, the Mo±O bond is described by an interaction of Mox reversible capacity and the high average potential in and O22 . Diminution of the Mo±O interaction at charge are the major drawbacks in these systems. lower potential, and on extended cycling explains the The average charge potential can be lowered by the difference in the electrochemical behavior between incorporation of Sn but at the expense of capacity fading. Introduction of Sn into the metal oxide via an Vofd 5 and 250 mV. At the higher level of reduction, the Li reaction is cyclable, as shown on Fig. 8b, exchange process in the lamellar material gives rise whereas very deep reduction introduces irreversible to less fading, apparently as a result of a greater breakdown (Fig. 8a). It appears that the oxygen degree of dispersion of the Sn centers which may atoms are still maintained within the host matrix inhibit agglomerate formation. during the discharge process to very low potential, and their number per transition metal decreases only slightly. In contrast, tin (SnO or SnO2 ) undergo Acknowledgements phase separation on reaction with Li to a greater degree, to form bulk tin and lithium oxide [42]. Financial support for this research was provided by NSERC through the strategic program (LFN). 3.2.5. Comparison to other systems The authors thank Tracy Kerr for preparation of

The cycling curves of MoO20.253 (Johnson Matthey), Na MoO . LFN is also grateful to Professor MoO32 , MoS (Aldrich) and SnO 2 (Aldrich) (after a Raymond Brec (IMN, Nantes), for helpful discus- heat treatment at 1708C) have been compared to sions. 50 F. Leroux, L.F. Nazar / Solid State Ionics 133 (2000) 37 ±50

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