Studies on Nano-CaO·SnO2 and Nano-CaSnO3 as Anodes for Li-Ion

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Studies on Nano-CaO · SnO2 and Nano-CaSnO3 as Anodes for Li-Ion Batteries Yogesh Sharma, N. Sharma, G. V. Subba Rao, and B. V. R. Chowdari* Department of Physics, National UniVersity of Singapore, Singapore 117542 ReceiVed March 11, 2008. ReVised Manuscript ReceiVed August 30, 2008

The nanocomposite “CaO · SnO2” and nano-CaSnO3 are prepared by the thermal decomposition of CaSn(OH)6 precursor and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HR-TEM) along with selected area electron diffraction (SAED) and density measurements. Nanosize (3-6 nm) grains of CaO and SnO2 in the X-ray amorphous CaO · SnO2 and particles of ∼ 60 nm size in nano-CaSnO3 are obtained. Galvanostatic cycling of both the phases vs Li metal is performed in the voltage ranges 0.005-1.0 V and 0.005-1.3 V at the current rate, 60 mA g-1 (0.12 C). Stable and reversible capacities of 490 ((5) and 550 ((5) mA h g-1 are observed for nano-CaO · SnO2 respectively up to 50 cycles in the above voltage windows. These values correspond to 3.8 and 4.2 mol of cyclable Li per mole of CaO · SnO2 in comparison to the theoretical value of 4.4 mol of Li. A capacity of 420 ((5) mA h g-1 is observed at a rate of 0.4 C. Nano-CaSnO3 showed a stable capacity of 445 ((5) mA h g-1 (3.4 moles of Li) up to 50 cycles when cycled in the voltage window, 0.005-1.0 V. The average discharge and charge potentials are 0.2 V and 0.5 V, respectively, for both the phases. The reasons for the superior Li-cycling performance of nano-CaO · SnO2 in comparison to nano-CaSnO3 are discussed. Ex situ XRD, TEM, and SAED studies are carried out to support the reaction mechanism. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) data as a function of voltage are presented and discussed to complement the galvanostatic results. The “apparent” Li-ion diffusion coefficient (DLi+) estimated from EIS is ∼1.0 × 10-14 cm2 s-1 at V e 1.0 V during the first cycle and 11th discharge cycle.

1. Introduction Since 1990 when Sony Company introduced the rechargeable Li-ion batteries (LIBs) as a dc-power source for portable appliances, worldwide, extensive research has been in progress on LIB cathodes1-4 and anodes.5-12 Major research accomplishments are the proposal of Sn-based 1,5,12-23 and * Corresponding author. Tel.: (65) 6516 2531. Fax: (65) 6777 6126. E-mail: [email protected].

(1) Nazri, G. A., Pistoia G. Eds. Lithium Batteries: Science and Technology; Kluwer Academic: New York, 2003. (2) Thackeray, M. Nat. Mater. 2002, 1, 81. (3) Chung, S.-Y.; Bloking, J. T.; Chiang, Y.-M. Nat. Mater. 2002, 1, 123. (4) Morcrette, M.; Rozier, P.; Dupont, L.; Mugnier, E.; Sannier, L.; Galy, J.; Tarascon, J.-M. Nat. Mater. 2003, 2, 755. (5) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395. (6) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nature 2000, 407, 496. (7) Sharma, Y.; Sharma, N.; Subba Rao, G. V.; Chowdari, B. V. R. AdV. Funct. Mater. 2007, 17, 2855. (8) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366. (9) Tirado, J. L. Mater. Sci. Eng. R 2003, 40, 103. (10) Yu, Y.; Chen, C.-H.; Shui, J.-L.; Xie, S. Angew. Chem., Int. Ed. 2005, 44, 7085. (11) Sharma, Y.; Sharma, N.; Subba Rao, G. V.; Chowdari, B. V. R. J. Power Sources 2007, 173, 495. (12) Ying, Z.; Wan, Q.; Cao, H.; Song, Z. T.; Feng, S. L. Appl. Phys. Lett. 2005, 87, 113108. (13) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (14) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2943. (15) Sharma, N.; Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R. Electrochem. Commun. 2002, 4, 947. (16) Sharma, N.; Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R. J. Power Sources 2005, 139, 250.

Co- based oxides 1,6,7,10,11 as the possible alternatives to the graphite anode presently used in LIBs. Their potential lies in the high reversible capacities achievable with the above oxides in contrast to the graphite. Transition metal oxides react with Li through the “conversion reaction” in which the metal oxides are reduced to the metal nanoparticles embedded in the amorphous Li2O matrix.1,6-11 The reverse reaction occurs on charging and contributes to the capacity of the electrode. The conversion reactions (oxidation/reduction), however, occur at relatively higher potential versus Li metal, which reduces the overall output voltage of LIBs incorporating such anodes.6-11 The Sn-based binary and ternary oxides with various morphologies have shown the ability to deliver high reversible capacity at lower potential versus Li.12-16 But, still, nonretention of high capacity on long-term cycling has been one of the major drawbacks of Sn-based oxides.12-16,20-23 Composites of Sn-metal particles dispersed in electrochemically active/inactive matrices like carbon (C) and cobalt (Co) have received considerable attention recently as a possible (17) (18) (19) (20)

Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045. Li, N.; Martin, C. R. J. Electrochem. Soc. 2001, 148, A164. Behm, M.; Irvine, J. T. S. Electrochim. Acta 2002, 47, 1727. Sandu, I.; Brousse, T.; Schleich, D. M.; Danot, M. J. Solid State Chem. 2004, 177, 4332. (21) Yuan, Z.; Huang, F.; Sun, J.; Zhou, Y. J. Mater. Sci. Lett. 2003, 22, 143. (22) Huang, F.; Yuan, Z.; Zhan, H.; Zhou, Y.; Sun, J. Mater. Chem. Phys. 2004, 83, 16. (23) Huang, F.; Yuan, Z.; Zhan, H.; Zhou, Y.; Sun, J. Mater. Lett. 2003, 57, 3341.

10.1021/cm8020098 CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

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anode for LIBs. Derrien et al.24 found a stable capacity of ∼500 mA h g-1 for at least 200 cycles in the voltage window, 0.005-1.5 V, at a rate of 0.8 C for nano-Sn-C composite. For mechanically alloyed (CoSn2 + C + Co) and (CoSn + C) nanocomposites, Ferguson et al.25 observed capacities of ∼450 and ∼280 mA h g-1 stable in the range of 10-100 cycles when cycled in the voltage window 0.005-1.2 V at a rate of 0.2 C. The reversible capacity of tin oxide-based electrodes arises due to the alloying-dealloying reaction of Sn metal with Li, after the formation of Sn metal by the reduction of Sn oxide.1,5,9,14-16 The volume variation in unit cell during charge and discharge reaction leads to cracking and crumbling of electrode causing extensive capacity- fading up on electrochemical cycling.1,5,9,14-20 Thus, realization of a stable cycling response of Sn-oxides over a large number of cycles is one of the major goals, and this issue is addressed to some extent by using nanoparticles of starting materials and/or by using some matrix elements which could buffer the volume variation during the alloying-dealloying reaction1,14-18 and by choosing appropriate voltage range of cycling.14-17,19 Among various matrix elements studied in conjunction with tin oxides, we have shown that the sol-gel derived nanoCaSnO3 which contains SnO6 octahedra and adopts a perovskite structure has shown good cycling performance with a reversible capacity of 380 mA h g-1 stable up to 100 cycles in the voltage range 0.005-1.0 V.15,16 This capacity value corresponds to only 2.9 mol of Li per mole of CaSnO3 as compared to the theoretically obtainable value, 4.4 mol of Li. Therefore, scope exists to enhance the capacity of CaSnO3 by tailoring its particle size as well as morphology by employing different synthesis routes. In the past few years, various tin-composite oxides obtained by the decomposition of ASn(OH)6, A ) Mn, Mg, and Co,21-23 have been investigated for their Li-cyclability. It has been found that the amorphous composite (e.g., MgO · SnO2 22) shows a higher reversible capacity in comparison to the crystalline compounds but exhibits poor capacity retention on long-term cycling. By keeping in mind the beneficial effect of Ca counterion in the cycling performance of crystalline-CaSnO3, presently we prepared X-ray amorphous CaO · SnO2 nanocomposite and X-ray crystalline nanophase CaSnO3 by the thermal decomposition of CaSn(OH)6 and tested them for Li-cyclability at room temperature in two different voltage windows, that is, 0.005-1.0 V and 0.005-1.3 V versus Li. Results show the improved and stable reversible capacity values of 490 ((5) mA h g-1 and 550 ((5) mA h g-1, respectively, for nanoCaO · SnO2 in the afore-stated voltage ranges at least up to 50 cycles. 2. Experimental Section The metal salts, CaCl2 (0. 1M, Fluka; >97%) and SnCl4 (0.1 M, Merck; >99%) were dissolved separately in deionized water (20 mL) and then mixed together. Drop by drop, 0.65 M NaOH (24) Derrein, G.; Hassoun, J.; Panero, S.; Scrosati, B. AdV. Mater. 2007, 19, 2336. (25) Ferguson, P. P.; Todd, A. D. W.; Dahn, J. R. Electrochem. Commun. 2008, 10, 25.

Sharma et al. aqueous solution was added to the mixed chloride solution. This has resulted in a white precipitate of CaSn(OH)6. Warming at 50 °C and stirring of the solution along with the precipitate was carried out for 2 h using stirrer-cum-hot plate, after which it was filtered, washed several times with deionized water, and then dried at 80 °C for 12 h in an air oven. The white powder was calcined in air at 400, 500, 600, and 900 °C for 6 h each separately and another batch at 600 °C for 24 h and cooled to room temperature. Structural characterization and thermogravimetric analysis were carried out by an X-ray diffractometer (XRD) (Philips X’PERT MPD instrument, equipped with Cu KR radiation) and a TA Instrument (SDT2960 Simultaneous DTA-TGA) at a heating rate of 5 °C min-1, in air, respectively. Density measurements were carried out on the compounds prepared at 600 °C for 24 h and at 900 °C for 6 h, using AccuPyc 1330 pycnometer (Micromeritics, U.S.A.). The compounds were pressed into pellets and vaccuum-dried for density determination. Microstructural characterizations were carried out by a scanning electron microscope (SEM) (JEOL JSM-6700F, Field Emission Electron Microscope) and high resolution transmission electron microscope (HR-TEM) (JEOL JEM 3010 operating at 300 kV). For TEM and selected area electron diffraction (SAED) measurements, the powder was dispersed in ethanol using the ultrasonic miller (Transsoinc 660/H Elma), and after obtaining a homogeneous dispersion, a drop of it was deposited on a holey carbon-coated Cu-grid, dried, and then transferred to the TEM chamber. The doctor blade technique was used to prepare the electrode for electrochemical studies. The active material, Super P carbon powder (MMM Ensaco), and binder (Kynar 2801) in the weight ratio 70:15:15 were mixed thoroughly using N-methyl-pyrrolidinone (NMP) as the solvent for the binder. Etched copper foil (∼10 µm thick) was the current collector. The thick film (20-30 µm) electrode was dried at 80 °C in an air oven for 12 h to evaporate the NMP and then pressed between twin rollers to ensure intimate contact with the current collector and cut into circular disks (16 mm diameter and containing 3-4 mg active material’s mass) followed by vacuum drying at 70 °C for 12 h. These electrodes were transferred to an Ar-filled glovebox (MBraun, Germany) which maintains the H2O and O2 content less than 1 ppm. Coin cells (size 2016) were assembled in the glovebox with Li metal (Kyukoto Metal Co., Japan) as the counter electrode, glass microfiber filter (Whatmann) as the separator, and 1 M LiPF6 in ethylene carbonate (EC) + diethyl carbonate (DEC) (1:1, v/v, Merck; Selectipur LP 40) as the electrolyte. The cyclic voltammetry and galvanostatic charge-discharge cycling of the cells were carried out at room temperature after aging the cells for 24 h (to ensure the percolation of the electrolyte in to the electrode) by Macpile II (Biologic, France) and computer controlled Bitrode multiple battery tester (model SCN, Bitrode, U.S.A.), respectively. Electrochemical impedance spectroscopy was performed using the Solartron impedance/phase-gain analyzer (SI 1255) coupled with a battery testing unit (1470). The measurements were carried out at room temperature in the frequency range 5 mHz to 0.10 MHz, with an ac signal of amplitude 5 mV. Data acquisition and analysis were done, respectively, using the electrochemical impedance software, Zplot and Zview (version 2.2, Scribner Associates Inc., U.S.A.).

3. Results and Discussion 3.1. Structural and Morphological Characterization. The XRD pattern along with the Miller indices of synthesizedCaSn(OH)6 is shown in Figure 1. It crystallizes in a cubic structure. The calculated lattice parameter a ) 8.14(2) Å matches well with the 8.15 Å reported in JCPDS card no.

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Figure 1. X-ray diffraction patterns of as prepared cubic-CaSn(OH)6 and of the products calcined in air at 400, 500, and 600 °C for 6 h at each temperature and at 600 °C for 24 h. Formation of X-ray crystalline CaSnO3 is seen in the latter case. Miller indices (hkl) are shown.

Figure 2. TGA and derivative weight loss curves of CaSn(OH)6 at a heating rate of 5 °C per min, in air.

74-1823. It is known that CaSn(OH)6 decomposes on heating at temperature T > 250 °C, to form the composite CaO · SnO2, and at T > 500 °C forms the compound CaSnO3.26,27 The TGA and derivative of weight loss (DWL) curves of CaSn(OH)6 are shown in Figure 2. An intense peak in the DWL curve is observed at ∼290 °C which can be attributed to the decomposition of CaSn(OH)6 to form the composite CaO · SnO2. The decomposition equation can be written as CaSn(OH)6 f CaO·SnO2 + 3H2O

(1)

The observed weight loss, ∼18% up to 600 °C, is almost consistent with the theoretically estimated loss, ∼20% according to eq 1. The small peaks observed in the DWL curves at 50-70 °C is due to desorption of the adsorbed water. To ascertain the temperature at which the composite CaO · SnO2 transforms to the well defined CaSnO3, the CaSn(OH)6 was calcined in air at various temperatures as mentioned in Experimental Section. The XRD patterns indicate that the CaSn(OH)6 heated at 400, 500, and 600 °C for 6 h are X-ray amorphous in nature, and only broad peaks with 2θ at ∼32° and ∼52° are observed (Figure 1). The well-defined peaks corresponding to orthorhombic CaSnO3 are observed only when CaSn(OH)6 is heated at 600 °C for 24 h, and as expected, at 900 °C for 6 h (pattern not shown). The derived lattice parameters, a ) (26) Inagaki, M.; Kuroishi, T.; Yamashita, Y.; Urata, M. Z. Anorg. Allg. Chem. 1985, 527, 193. (27) Jena, H.; Kutty, K. V. G.; Kutty, T. R. N. Mater. Chem. Phys. 2004, 88, 167.

5.52(2) Å, b ) 5.67(2) Å, and c ) 7.89(2) Å match well with the reported values, in the JCPDS card no. 77-1797 (a ) 5.532(2) Å, b ) 5.681(2) Å, and c ) 7.906(2) Å). The crystallite size of crystalline-CaSnO3 was calculated using the Scherrer’s formula, P ) Kλ/(β1/2 cos θ).28 Here, K is a constant, λ is the wavelength of Cu KR radiation in Å, β1/2 is the full width at half-maximum (fwhm) in radians, and θ is the scattering angle. The instrumental resolution of 0.15° is determined using the standard compounds, LiNbO3 and LiCoO2. This value is subtracted from the measured fwhm’s, and then P was calculated using the fwhm of the maximum intensity peak (112), λ ) 1.54059 Å, and K ) 0.9. The estimated crystallite size for the CaSnO3 prepared by heating CaSn(OH)6 at 600 °C for 24 h is 60 ((5) nm, thereby establishing it as a nanophase. The crystallite size of the CaSnO3 prepared at 900 °C for 6 h is 120 ((5) nm, meaning it is submicrocrystalline. The XRD data are in tune with the TGA curve (Figure 2) where 600 °C was found to be the appropriate temperature to obtain crystalline CaSnO3 from CaSn(OH)6 and agree with the findings in the literature reports.26,27 The experimentally observed density of X-ray crystalline-CaSnO3 (prepared at 600 °C, 24 h) is 5.25 ((0.05) g/cm3 and is slightly smaller than theoretical X-ray density of 5.52 g/cm3, which could be attributed to the porosity present in the compound. The value determined for CaSnO3 prepared by heating CaSn(OH)6 at 900 °C for 6 h is 5.58((0.05) g/cm3 and matches well with the X-ray density. Morphological characterization was carried out on X-ray amorphous (CaSn(OH)6, heated at 600 °C; 6 h) and X-ray crystalline (CaSn(OH)6, heated at 600 °C; 24 h) CaSnO3 by SEM. Agglomeration of fine particles and their inhomogeneous distribution is observed in both the cases. However, the X-ray crystalline CaSnO3 showed a denser packing of the agglomerates of fine particles (Figure S1, Supporting Information). The high resolution (HR)-TEM lattice image of the X-ray amorphous CaSnO3 shown in Figure 3a clearly indicates nano-crystalline regions (size, 3-5 nm) dispersed in an amorphous phase. Some of the regions are marked as 1, 2, and 3. Careful measurements of the interplanar d-spacings of the nanosize regions, that is, 1 and 2, give the values of 3.35 ((0.02) Å and 2.78 ((0.02) Å, respectively. These values can be assigned to the Miller indices, (110) of tetragonal SnO2 (JCPDS no. 72-1147) and (111) of cubic CaO (JCPDS no. 82-1691), respectively. However, the d ) 2.78 ((0.02) Å can also be indexed as the (112) plane of orthorhombic-CaSnO3 (JCPDS no. 77-1797). The regions marked as 3 are comprised of various sets of lattice planes attributable to CaO, SnO2, and CaSnO3 which, however, cannot be indexed unambiguously to any one of them. The selected area electron diffraction (SAED) pattern of the X-ray amorphous CaSnO3, shown in Figure 3 b, exhibits concentric diffuse rings without specific spots indicative of an amorphous nature of the compound as a whole. The d values corresponding to these concentric rings were evaluated by measuring the radius of the ring from the center, and they match well with the values derived from the nanophase regions in the HR-TEM lattice image (Figure 3a). (28) Jha, P.; Arya, P. R.; Ganguli, A. K. Mater. Chem. Phys. 2003, 82, 355.

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Figure 3. (a) HR-TEM lattice image of X-ray amorphous nano-CaO · SnO2 (The precursor CaSn(OH)6 heated at 600 °C for 6 h.) showing the presence of nanocrystalline regions (size, 3-6 nm) dispersed in the amorphous region, marked as 1, 2, and 3. The regions marked 1 and 2 correspond to SnO2 and CaO, respectively. (b) The SAED pattern. The assignment of the diffuse rings to the phases CaO, SnO2, and CaSnO3 are shown along with the Miller indices. (c) The HR-TEM lattice image of X-ray crystalline nano-CaSnO3 (the precursor CaSn(OH)6 heated at 600 °C for 24 h). The d-values estimated from the lattice fringes are assigned to CaSnO3. (d) The corresponding SAED pattern of nano-CaSnO3. The assigned Miller indices of CaSnO3 to the spots are shown.

The HR-TEM lattice image along with SAED pattern of X-ray crystalline CaSnO3 are shown in Figure 3c,d, respectively. The lattice fringes of the nanosize grains are clearly seen in Figure 3c. The d-values corresponding to some of the planes are 2.78 ((0.02) Å and 2.30 ((0.02) Å, which correspond to the Miller indices (112) and (022), respectively, of the orthorhombic-CaSnO3. The SAED pattern comprises of well-defined but diffuse spots, and some of the spots are indexed by comparing the d-values of crystalline CaSnO3 (Figure 3d). Therefore, on the basis of XRD, HR-TEM, and SAED measurements, we can conclude that the X-ray amorphous phase is comprised of nanocomposite CaO · SnO2 (and possibly a small amount of nano-CaSnO3), whereas the CaSn(OH)6 calcined at 600 °C for 24 h in air is comprised of nanophase crystalline CaSnO3. Henceforth, the nanocomposite is designated as nano-CaO · SnO2 and X-ray crystalline nanophase as nano-CaSnO3. 3.2. Electrochemical Studies. 3.2.1. GalVanostatic Cycling. Figure 4a shows the capacity-voltage profiles of the first discharge and charge cycle of nano-CaO · SnO2 and nano-CaSnO3 at a current density of 60 mA g-1 in the voltage window 0.005-1.0 V. The profiles of the former in the range 0.005-1.3 V are also shown. The discharge profile commenced from the OCV (∼2.3-2.5 V) to the lower cutoff voltage, 0.005 V. The first discharge profile of nano-CaSnO3 shows a small plateau at ∼ 0.8 V with an onset at ∼1.0 V followed by a sloping profile until a capacity of ∼500 mA h g-1 is reached. Thereafter, a large plateau region sets in

at ∼0.20 V, whereas the nano-CaO · SnO2 exhibits a small shoulder plateau at ∼0.8 V and then continuous a sloping profile from ∼0.6 V to the lower cutoff voltage, 0.005 V. This difference in the first-discharge profiles can be attributed to the presence of electrochemically inactive-CaO in nanoCaO · SnO2 as compared to the nano-CaSnO3. The electrochemical process occurring during the first discharge is the structure destruction (amorphization of crystal lattice), that is, the formation of electrochemically inactive CaO matrix and active Sn-metal (eq 2) in the case of nano-CaSnO3. For nano-CaO · SnO2, the reaction of eq 3 occurs. The freshly formed Sn-metal reacts with Li to form the intermetallic alloy, Li4.4Sn at ∼0.2 V according to the forward reaction of eq 4. It has been observed that CaO will not be reduced to the Ca-metal by Li because of strong bonding between Ca and O.15,16 CaSnO3 + 4Li++4e- f CaO + 2Li2O + Sn

(2)

CaO·SnO2 + 4Li+ + 4e- f CaO + 2Li2O + Sn

(3)

4.4Li+ + 4.4e- + Sn T Li4.4Sn

(4)

The above eqs show the overall consumption of 8.4 mol of Li per mole of nano-CaSnO3 or CaO · SnO2 and constitute 1089 mA h g-1 as the overall first-discharge capacity. However, the experimentally observed capacity in both nanoCaO · SnO2 and nano-CaSnO3 is higher than the theoretically envisaged values, namely, 1310 ((30) mA h g-1 (∼10.1

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Figure 5. Capacity vs cycle number plots at the current of 60 mA g-1 (0.12C), (a) for nano-CaO · SnO2, and (b) for nano-CaSnO3. Voltage windows are indicated. (c) The capacity vs cycle number plot at various current rates of nano-CaO · SnO2 in the voltage window, 0.005-1.0 V. The filled and open symbols represent discharge and charge capacities, respectively. The C values corresponding to different current rates are shown, assuming 1 C ) 500 mA g-1.

Figure 4. Voltage vs capacity profiles at the current rate of 60 mA g-1 (0.12 C) at room temperature. (a) The first cycle for nano-CaO · SnO2 and nano-CaSnO3 in the two voltage windows, namely, 0.005-1.0 V and 0.005-1.3 V vs Li. (b) The profiles for nano-CaO · SnO2 during the 2-65 cycles in the voltage window, 0.005-1.3 V, and (c) during the 2-50 cycles in the voltage window 0.005-1.0 V. Only selected cycles are shown for clarity. The numbers refer to the cycle number.

mol of Li per mole of nano-CaO · SnO2) and 1165 ((20) mA h g-1 (∼9.0 moles of Li per mole of nano-CaSnO3), respectively. These extra capacity values, over and above the theoretical ones, can be ascribed to the solid-electrolyte interphase (SEI) formation due to the reaction of Li with the solvents of the electrolyte.13-23 The difference between the observed first-discharge capacities in nano-CaSnO3 versus nano-CaO · SnO2 is possibly due to the difference in the well crystalline nanophase versus nanophase X-ray amorphous nature and a thicker SEI formation in the case of nanoCaO · SnO2. Analogous results were noted with the compounds MnO · SnO2, MgO · SnO2, and CoO · SnO2 reported in the literature.21-23 The voltage-capacity profiles of nanoCaSnO3 show clear and relatively flat voltage plateau regions in comparison to nano-CaO · SnO2 indicative of well defined reaction potentials in the case of the former as compared to latter and can be attributed to the coexistence of two-phase regions. The energy required for Li to react with the nanoCaO · SnO2 is lesser, and therefore, a sloping profile rather than clearly defined flat plateau potential is seen. The first charge profiles of nano-CaO · SnO2 in two different voltage windows (0.005-1.0 V and 0.005-1.3 V) and of nano-CaSnO3 in the range 0.005-1.0 V show similar behavior, and the first charge capacity is 420 ((5) mA h

g-1 for nano-CaSnO3 (Figure 4a). In the case of nanoCaO · SnO2, the first charge capacities of 480 ((5) mA h g-1 and 570 ((5) mA h g-1 were observed in the voltage windows of 0.005-1.0 V and 0.005-1.3 V, respectively. These capacity values correspond to 3.7 and 4.4 mol of Li per mole of nano-CaO · SnO2, respectively (Figure 4a). These findings are encouraging since the theoretical capacity is achieved with nano-CaO · SnO2 in the voltage window 0.005-1.3 V. The theoretical irreversible capacity loss (ICL) during the first cycle is ∼48% (from eqs 2-4), whereas the observed ICL is ∼56% for the nano-CaO · SnO2. The observed second discharge and charge capacities of 580 ((5) mA h g-1 and 550 ((5) mA h g-1, respectively, are very close to the first charge capacity which confirms the validity of eq 4 and good reversibility. In the range of 2-50 cycles, the charge-discharge profiles for nano-CaO · SnO2 are well overlapped and show good reversibility in both the voltage windows (Figure 4b,c). The capacity versus cycle number plots are shown in Figure 5a,b. Both nano-CaO · SnO2 and nano-CaSnO3 show a stable and high capacity in the range of 0.005-1.0 V up to 50 cycles. As is evident from Figures 4 and 5a,b, the discharge and charge capacities in both the cases show a noticeable increasing tendency on cycling when cycled in the voltage range, 0.005-1.0 V. The 30th cycle reversible capacities of nano-CaO · SnO2 and nano-CaSnO3 are 490 ((5) and 445 ((5) mA h g-1, respectively. The Coulombic efficiency, η, is in the range 96-98%. When cycled in the range of 0.005-1.3 V, nano-CaSnO3 showed an initial reversible capacity of 570 ((5) mA h g-1 (theoretical value) but capacity-fading occurred relatively slowly up to 40 cycles and fairly rapidly in the range 40-70 cycles, and only a capacity of 380((5) mA h g-1 is retained at the 70th cycle (Figure 5b). On the other hand, the capacity of nanoCaO · SnO2 remains stable at 550 ((5) mA h g-1 (4.2 mol of Li) up to 50 cycles when cycled in the range 0.005-1.3 V at 0.12 C (assuming 1 C ) 500 mA g-1). The η is ∼96-98% in the range of 5-50 cycles. We note, however,

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capacity fading sets in beyond 50 cycles, and in the range of 50-80 cycles, the rate of fading is ∼2 mA h g-1 per cycle. It has been noted in the literature that the operating voltage window is crucial for better reversibility of Sn-based oxide compounds.14-17,20 It is noted that beyond 1.0 V the consumption of Li2O and reformation of tin oxide (e.g., SnO) is responsible for capacity fading due to the loss of holding ability of keeping Sn-metal particles apart from each other and destroying the stable nanocomposite structure of Sn-Li2O matrix.14,17,20,29,30 Therefore, the observed capacity fading in nano-CaO · SnO2 in the voltage range 0.005-1.3 V on cycling beyond 50 cycles can be attributed to the partial oxidation of Sn metal (Li2O consumption) to form Sn oxides. Both nano-CaSnO3 and nano-CaO · SnO2 show higher and stable capacities of 445 ((5) mA h g-1 and 490 ((5) mA h g-1, respectively, than the 380 ((5) mA h g-1, reported by us earlier 15,16 for the nano-CaSnO3 prepared by sol-gel method when cycled in the range of 0.005-1.0 V at 60 mA g-1. The good cycling response of nano-CaO · SnO2 and nano-CaSnO3 is attributed to the excellent buffering ability of CaO against the unit cell volume variations occurring during alloying-dealloying of Sn metal (eq 4). The better cycling performance of the nano-CaO · SnO2 in comparison to nano-CaSnO3 up to 50 cycles in both the voltage ranges (upper cutoff, 1.0V and 1.3 V) is attributed to the nanophase nature of X-ray amorphous CaO · SnO2 which contains smaller nanosize regions (3-6 nm) against the relatively larger crystallite size (∼ 60 nm) in nano-CaSnO3 (Figures 1 and 3). The electro-inactive CaO matrix also serves as the barrier against the agglomeration of Sn nanoparticles during cycling. 3.2.2. Rate Capability. The encouraging galvanostatic cycling performance in the form of high and stable capacity, supported by the cyclic voltammetry studies to be discussed later, motivated us to check the rate capability of nanoCaO · SnO2 at various current densities in the voltage window 0.005-1.0 V at ambient temperature. The specific current was increased after 8-10 cycles in steps from 0.06 to 0.4 C up to 40 cycles assuming 1 C ) 500 mA g-1. The capacity vs cycle number plot is shown in the Figure 5c. The capacity, 520 ((5) mA h g-1, observed at 0.06 C decreases to 420 ((5) mA h g-1 upon increasing the current almost sevenfold, to 0.4 C. However, at each C-rate, the capacity remains fairly stable on cycling. We note that the capacity observed at 0.4 C is higher than the theoretical capacity of graphite (372 mA h g-1). 3.2.3. Cyclic Voltammetry. The galvanostatic cycling performance of nano-CaO · SnO2 has been complemented by cyclic voltammetry carried out in the potential range of 0.005-1.0 V and 0.005-1.3 V at the slow scan rate of 58 µV s-1. The cyclic voltammograms (CV) are shown in Figure 6. As can be seen in Figure 6a, the first discharge commenced from the open circuit voltage and no cathodic peaks are seen until 1.0 V. A broad shoulder peak at ∼0.75 V is observed which can be ascribed to the onset of reduction (29) Chang, S. T.; Leu, I. C.; Liao, C. L.; Yen, J. H.; Hon, M. H. J. Mater. Chem. 2004, 14, 1821. (30) Mohamedi, M.; Lee, S.-J.; Takahashi, D.; Nishizawa, M.; Itoh, T.; Uchida, I. Electrochim. Acta 2001, 46, 1161.

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Figure 6. Cyclic voltammograms of nano-CaO · SnO2 in the voltage window (a) 0.005-1.0 V and (b) 0.005-1.3 V vs Li at the slow scan rate of 58 µV s-1. Numbers indicate the cycle number.

of SnO2 to form the Sn metal (eq 3). This is followed by a smooth sloping curve and a broad peak at ∼0.1 V. This peak can be attributed to the formation of Li4.4Sn alloy (forward reaction of eq 4). The first charge (anodic) sweep up to the cutoff potential, 1.0 V, showed only one well- defined peak at ∼0.55 V. From the second cycle onward, the discharge and charge CV show peaks at 0.2V and 0.55 V, respectively (Figure 6a). The splitting of the cathodic peak into a doublet, discernible at 0.1-0.15 V during 2-25 cycles, indicates that the Li-Sn alloying reaction occurs in two stages instead of a single step, as shown in eq 4. Nevertheless, the overlapping of anodic and cathodic peaks during the 2 to 25 cycles indicates good Li cyclability and reveals the average discharge and charge potentials, 0.2 and 0.5 V, respectively, for alloying and dealloying reaction as per the reversible reactions of eq 4 (Figure 6a). Figure 6b shows the CV of nano-CaO · SnO2 in the voltage range 0.005-1.3 V. The first-cycle CV is identical to that of Figure 6a, as expected, indicating excellent reproducibility of a duplicate cell. During the 2-20 cycles, the CVs overlap well and qualitatively resemble those of Figure 6a. However, a cathodic peak at 0.55 V and a broad anodic peak at ∼1.2 V are clearly seen during the 2-20 cycles in Figure 6b. The latter peak may be associated with oxidation of Sn nanoparticles to form tin oxide, which means that the corresponding reduction of the tin oxide to Sn metal is occurring at ∼0.55 V. We note, however, that the formed tin oxide may not be the well-defined SnO, since studies by Courtney and Dahn17 have shown that SnO reduction by Li metal under electrochemical conditions occurs only at ∼1.1 V. Thus, the above cathodic and anodic peaks may represent reduction and oxidation of a suboxide, SnOx (x < 1), which may exist as a nanophase under room temperature electrochemical cycling. Indeed, 119Sn Mossbauer studies by Sandu et al.20 indicated the presence of unusual tin species (“exotic” forms of Sn(II) or Sn(0)) during the electrochemical reduction of SnO2 by Li metal. Therefore, we conclude that the CV results are in good agreement with the galvanostatic cycling data.

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Figure 7. Ex situ XRD patterns of the electrode of bare nano-CaSnO3 and those discharged to 0.5 V and 0.005 V and charged to 1.0 V after 30 cycles. Lines due to Cu substrate are indicated. Cu KR radiation.

3.3. Ex Situ XRD and TEM. To supplement the galvanostatic cycling and cyclic voltammetry data, ex situ XRD on nano-CaSnO3 and TEM studies on nano-CaO · SnO2 were performed, and the results are shown in Figures 7 and 8, respectively. For ex situ XRD, duplicate cells were discharged/ charged to different voltages and then equilibrated for 2-3 h. The cells were then dismantled inside the glovebox, and the composite electrodes were recovered, washed with solvent DEC to remove the electrolyte, and then covered by parafilm “M” (American National Can, U.S.A.) to ensure minimal contact with air. Figure 7 shows the XRD patterns of the electrodes recorded at various voltages, OCV, 0.5 V and 0.005 V, during first discharge, and at 1.0 V after 30 discharge-charge cycles. As can be seen, the characteristic lines of CaSnO3 were observed at OCV and at 0.5 V during first discharge, which reveals that the crystal structure is not destroyed at 0.5 V. In the XRD pattern taken at 0.005 V, the lines due to CaSnO3 completely disappeared (amorphization of lattice) and peaks corresponding to Li-Sn alloy and small intensity peaks due to Sn metal are seen. As per the forward reaction of eq 4, only alloy (Li-Sn) formation can be expected at 0.005 V. The observed lines due to Sn metal may be ascribed to the slight decomposition of alloy under ex situ conditions and exposure to X-rays. The XRD pattern, devoid of lines due to Sn metal, is seen at 1.0 V after 30 cycles (Figure 7). Since ex situ XRD was unable to detect the small crystalline regions that may be present in the charged state (1.0 V) after 30 cycles, ex situ TEM studies were carried out on the composite electrode of nano-CaO · SnO2. The lattice image shown in Figure 8a indicates the existence of nanocrystalline regions, size of 5-10 nm embedded in amorphous regions of mainly Li2O and CaO. The two regions marked 1 and 2 can be assigned to the nano-Sn-metal crystallites with interplanar distances (d-values) of 2.95 ((0.02) Å and 2.83 ((0.02) Å. These values correspond to the (200) and (101) planes of tetragonal Sn metal, respectively (JCPDS no. 02-0709). The SAED pattern was recorded to complement the lattice image data and is shown in Figure 8b. The diffuse spots superimposed by the diffuse rings are clearly seen due to the nanophase crystalline regions coexisting with the amorphous regions. The d-values corresponding to these rings and spots were derived by measuring the diameters of the rings. The d-values of 2.96 ((0.02) Å, 2.83 ((0.02) Å, and 2.09 ((0.02)Å can be assigned to the (200), (101), and (220) planes of Sn-metal, respectively. Therefore,

Figure 8. Ex situ TEM of nano-CaO · SnO2 charged to 1.0 V after 30 cycles. (a) Lattice image. The nanocrystalline regions marked 1 and 2 correspond to metallic tin. The other regions (circled) show the overlapping of various planes of crystalline metallic tin. (b) The SAED pattern. The Miller indices corresponding to the diffuse spots are assigned to the metallic tin. Scale bar is 5 nm.

in the nano-CaSnO3 and nano-CaO · SnO2-Li systems, on the basis of ex situ XRD of former and TEM and SAED of the latter, we conclude that the product formed upon the first discharge to 0.005 V consists of (Li-Sn) alloy. When charged to 1.0 V during the first cycle or after 30 cycles, the product formed is nanosize Sn-metal crystallites as indeed expected from the reverse reaction of eq 4. 3.4. Electrochemical Impedance Spectroscopy. Electrochemical Impedance spectroscopy (EIS) studies have been reported on other prospective anode materials like SnO,31 K2(LiSn)8O16,32 Co3O4,33,34 Fe2O3,35 and CaWO436 to understand the electrode reaction kinetics. Presently, impedance (31) Aurbach, D.; Nimberger, A.; Markovsky, B.; Levi, E.; Sominski, E.; Gedanken, A. Chem. Mater. 2002, 14, 4155. (32) Sharma, N.; Plevert, J.; Subba Rao, G. V.; Chowdari, B. V. R.; White, T. J. Chem. Mater. 2005, 17, 4700. (33) Kang, Y.-M.; Song, M.-S.; Kim, J.-H.; Kim, H.-S.; Park, M.-S.; Lee, J.-Y.; Liu, H. K.; Dou, S. X. Electrochim. Acta 2005, 50, 3667.

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Figure 10. (a) Equivalent circuit used for fitting the impedance spectra of Figure 9. Different resistances, Ri and/or Ri|CPEi components, and the Warburg element, W, are shown.

Figure 9. Family of Nyquist plots (Z′ vs -Z′′) for the cell with nanoCaO · SnO2 as cathode at different voltages vs Li. (a) During the first discharge reaction from open circuit voltage (OCV ∼ 2.6 V) to 0.005 V. (b) During the first-charge reaction from 0.005 to 1.0 V and (c) the 11th discharge cycle, after 10 charge-discharge cycles, from 1.0 to 0.005 V. Selected frequencies in the impedance spectra are shown. Symbols represent the experimental data points. Continuous lines show fitting with the equivalent circuit of Figure 10. Geometric area of the electrode is 2 cm2.

studies were carried out on the cell comprising nanoCaO · SnO2 as the working electrode versus Li during the first discharge and charge cycle and during the 11th discharge. The cell was discharged or charged to a specific voltage and relaxed at that voltage for 3 h, and the impedance data were collected. The results are presented as Nyquist plots (Z′ vs -Z′′), where Z′ and Z′′ refer to the real and imaginary parts of cell impedance, respectively (Figure 9 a-c). The impedance spectra have a qualitative resemblance to those observed with SnO31 and another Sn-containing oxide, K2(LiSn)8O16.32 The spectra were fitted with an equivalent electrical circuit using series and parallel combinations of resistances (R), CPEs, where CPE is the constant phase element and W the Warburg impedance. The experimental data points are represented as symbols, and fitting is represented as continuous lines in Figure 9a-c. The equivalent circuit is shown in Figure 10.32,35 The overall cell impedance is a summation of the resistances offered by the electrolyte (Re), surface film (sf) impedance components (Rsf and CPEsf), charge transfer (ct) resistance and the corre(34) Liu, Y.; Mi, C.; Su, L.; Zhang, X. Electrochim. Acta 2008, 53, 2507. (35) Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, Z. X.; Lim, C. T.; Subba Rao, G. V.; Chowdari, B. V. R. AdV. Funct. Mater. 2007, 17, 2792. (36) Sharma, N.; Subba Rao, G. V.; Chowdari, B. V. R. Electrochim. Acta 2005, 50, 5305.

sponding double layer (dl) CPE (Rct+ CPEdl), the bulk (b) impedance components (Rb and CPEb), and the Warburg impedance (W). Since the Nyquist plots show depressed semicircles, indicative of deviation from the ideal behavior, CPEs have been used instead of the pure capacitors. The impedance of constant phase element can be derived from the equation, ZCPE ) 1/[Ci(jω)n] where j ) -1, ω is the angular frequency, Ci is the capacitance, and n is a constant. The value of n (