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Li2MTi6O14 (M=Sr, Ba): new anodes for lithium-ion batteries
Electrochemistry Communications 5 (2003) 435–438 www.elsevier.com/locate/elecom
Li2MTi6O14 (M ¼ Sr, Ba): new anodes for lithium-ion batteries I. Belharouak *, K. Amine Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Received 1 April 2003; received in revised form 7 April 2003; accepted 7 April 2003
Abstract Isostructural Li2 MTi6 O14 (M ¼ Sr, Ba) materials, prepared by a solid state reaction method, have been investigated as insertion electrodes for lithium battery applications. These titanate compounds have a structure that consists of a three-dimensional network of corner- and edge-shared [TiO6 ] octahedra, 11-coordinate polyhedra for the alkali-earth ions, and [LiO4 ] tetrahedra in tunnels that also contain vacant tetrahedral and octahedral sites. Electrochemical data show that these compounds are capable of reversibly intercalating four lithium atoms in a three-stage process between 1.4 and 0.5 V vs. metallic lithium. The electrodes provide a practical capacity of approximately 140 mAh/g; they are, therefore, possible alternative anode materials to the lithium titanate spinel, Li4 Ti5 O12 . The lithium intercalation mechanism and crystal structure of Li2 MTi6 O14 (M ¼ Sr, Ba) electrodes are discussed and compared with the electrochemical and structural properties of Li4 Ti5 O12 . The area-specific impedance (ASI) of Li=Li2 SrTi6 O14 cells was found to be significantly lower than that of Li=Li4 Ti5 O12 cells. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Lithium-ion batteries; Anode; Titanate; Structure
1. Introduction Rechargeable lithium-ion batteries with their lightweight and high energy density have become an important source for powering many applications, particularly those in the electronics sector. These batteries are gaining more attention because of their possible application in high power devices such as hybrid electric vehicles [1]. Graphite/LiCoO2 is the most commonly used electrochemical couple in lithium-ion batteries, in which LiCoO2 plays the role of the positive electrode (cathode) and graphite acts as the negative electrode (anode). At full charge, the highly lithiated graphite (LiC6 ) electrode is highly reactive because it operates close to the potential of metallic lithium. In order to address the safety limitations of lithium-ion cells, particularly those containing nickel-based cathodes, alternative anodes to graphite have been suggested, such as the spinel Li4 Ti5 O12 that operates at approximately 1.5 V vs. Li0 [2–5]. Although Li4 Ti5 O12 is an insulator, doping the structure with small amounts of Mg2þ , Al3þ *
Corresponding author. Tel.: 1-630-252-4450; fax: 1-630-252-4176. E-mail address: [email protected] (I. Belharouak).
has been reported to improve the electronic conductivity of the spinel by many orders of magnitude [6]. In this paper, we introduce, as an alternative to Li4 Ti5 O12 , a new family of lithium-ion conducting titanate materials Li2 MTi6 O14 (M ¼ Sr, Ba) that can act as insertion electrodes in lithium cells.
2. Experimental Li2 MTi6 O14 (M ¼ Sr, Ba) compounds were prepared by heating stoichiometric quantities of Li2 CO3 , SrCO3 or BaCO3 , and TiO2 . The reagents were carefully mixed and progressively heated up to 600 °C in air to decompose the carbonate salts. After grinding, the powders were sintered at 1000 °C for 24 h. Chemical analyses of the resulting products were performed by the inductively coupled plasma technique (ICP) to check their composition. The analytical results were in excellent agreement with the expected theoretical values. For example, the (Sr + Ti)/Li ratio and the lithium content in the final Li2 SrTi6 O14 product were determined to be 27.05 and 2.2 wt%, in contrast to the theoretically expected values of 27.01 and 2.26 wt%, respectively.
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00090-0
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I. Belharouak, K. Amine / Electrochemistry Communications 5 (2003) 435–438
Electrodes were made by coating a paste containing the Li2 MTi6 O14 active material (M ¼ Sr or Ba), a superP carbon conducting additive, and a polyvinylidene fluoride binder in a 80:10:10 wt% ratio on a copper foil collector. The loading amount of the active material was 4–5 mg=cm2 . The electrolyte was 1 M LiPF6 in a (1:1 wt%) mixture of ethylene carbonate and diethyl carbonate. The cells were assembled inside a helium-filled dry-box and were evaluated using coin-type cells (CR2032). The cells were discharged and charged between 2.0 and 0.5 V vs. the Li counter electrode at a constant current density of 0:1 mA=cm2 .
3. Results and discussion Both Li2 SrTi6 O14 and Li2 BaTi6 O14 are white materials, indicating that they are electronic insulators like the spinel, Li4 Ti5 O12 . Fig. 1 shows the X-ray diffraction (XRD) patterns of the two compounds; the close similarity of the patterns shows that they are isostructural compounds. No detectable amount of impurity phases was observed in either of these patterns. The X-ray patterns were indexed according to an orthorhombic Ccentered unit cell by a least squares refinement of the peak positions following the space group assignment
Fig. 1. X-ray diffraction patterns of (a) Li2 BaTi6 O14 , and (b) Li2 SrTi6 O14 . The pattern for Li2 SrTi6 O14 has been indexed to a Ccentered cell (Cmca).
(Cmca) for Li2 SrTi6 O14 by Koseva et al. [7] from a single-crystal structure refinement of this compound. The lattice parameters of Li2 SrTi6 O14 : a ¼ 16:566, are in excellent agreement b ¼ 11:148, and c ¼ 11:468 A with those reported by Koseva et al. (a ¼ 16:570, ). The refined lattice pab ¼ 11:150, and c ¼ 11:458 A rameters of Li2 BaTi6 O14 : a ¼ 16:575, b ¼ 11:268, and are similar to those of Li2 SrTi6 O14 . The c ¼ 11:579 A slight increase of the unit cell parameters and volume of Li2 BaTi6 O14 was expected because the ionic radius of Ba2þ ions is larger than that of Sr2þ ions. The Li2 MTi6 O14 structure can be described, in general, as consisting of a three-dimensional network of cornerand edge-shared [TiO6 ] octahedra, 11-coordinate polyhedra for the alkali-earth ions, and [LiO4 ] tetrahedra in tunnels that also contain vacant tetrahedral and octahedral sites. A [0 0 1] projection of the structure is shown in Fig. 2. A close examination of this structure type led us to predict that these compounds have an interstitial space that might be favorable for accommodating additional lithium and thus act as an electrode for lithium batteries. For example, Fig. 3(a) clearly shows that the [LiO4 ] tetrahedra share faces with neighboring vacant, tetrahedra and octahedra in the tunnel that would allow lithium-ion insertion and diffusion. Indeed, it has already been demonstrated, in the absence of any structural information, that a ‘‘Li2 O–BaO–TiO2 ’’ solid electrolyte with a sample composition 20 mol% Li2 O, 7 mol% BaO, and 73 mol% TiO2 exhibits a high Liþ -ion conductivity at 300 °C (4 103 Scm1 ) [8,9]. The Li2 MTi6 O14 structure also contains vacant octahedral sites that bridge the [MO11 ] polyhedra (Fig. 3(b)). A more detailed description of the Li2 SrTi6 O14 crystal structure is provided in [7]. Figs. 4(a) and (b) show typical voltage profiles of lithium cells with Li2 BaTi6 O14 (Fig. 4(a)) and Li2 SrTi6 O14 (Fig. 4(b)) electrodes, respectively; the dis-
Fig. 2. A [0 0 1] projection of the Li2 MTi6 O14 structure showing the [TiO6 ] octahedral framework, the positions of the M ions, and the tunnels that contain the [LiO4 ] tetrahedra.
I. Belharouak, K. Amine / Electrochemistry Communications 5 (2003) 435–438
Fig. 3. Slices of the Li2 MTi6 O14 structure showing (a) the tunnels in which the [LiO4 ] tetrahedra are linked by vacant face-shared octahedra and tetrahedra, and (b) a vacant octahedral site that bridges the 11coordinated M polyhedra.
charge profiles are essentially identical. The profiles show that lithium insertion occurs in three distinct steps, (1) at approximately 1.4 V (a two-phase reaction), (2) between 1.2 and 1.0 V, and (3) between 1.0 and 0.5 V. The reactions are reversible, although the reversibility of the individual processes appears to be better defined on charge for the Li2 SrTi6 O14 electrodes. On the initial discharge of the cells to 0.5 V, the Li2 BaTi6 O14 and Li2 SrTi6 O14 deliver capacities of 180 and 185 mAh/g; on the second discharge, the electrodes lose 28 and 30 mAh/ g, respectively. Thereafter, the cells perform with remarkable cycling stability for 40 cycles, as shown in Fig. 5. If all six of the Ti4þ ions could be reduced to Ti3þ , then the Li2 MTi6 O14 electrodes would provide theoretical capacities of 242 mAh/g (for Ba) and 262 mAh/g (for Sr). However, experiment shows that approximately 70% of the theoretical capacity is delivered on the initial discharge at a very low current rate (C/40), which corresponds to the uptake of approximately four Liþ ions per formula unit and the reduction of two-thirds of the available six Ti4þ ions. Cyclic voltammetry data of Li2 BaTi6 O14 and Li2 SrTi6 O14 electrodes are shown in Figs. 6(a) and (b), respectively. The data demonstrate that the reduction process that occurs at the electrodes during the initial sweep is different from subsequent cycles. The weak reduction peak that is observed at 0.65 V is only visible during the initial reduction process. Furthermore, the
437
Fig. 4. Voltage profile vs. the capacity of (a) Li2 BaTi6 O14 and (b) Li2 SrTi6 O14 . The current density was 0:025 mA=cm2 (C/40).
Fig. 5. Cycling performance of Li2 BaTi6 O14 () and Li2 SrTi6 O14 (). The current density was 0:1 mA=cm2 (C/7).
main reduction peak of the Li2 BaTi6 O14 electrode that is visible at 1.18 V shifts to 1.35 V, and the corresponding reduction peak of the Li2 SrTi6 O14 electrode at 1.24 V shifts to 1.32 V; these peaks remain more or less constant at these voltages during the following cycles. Of particular significance is that the second and third reduction processes, which are clearly visible in the voltage profiles in Figs. 4(a) and (b), are not as prominent in the CV data. The reason for this discrepancy is not yet clearly understood. Fig. 7 shows the area-specific impedance (ASI) of the Li=Li2 MTi6 O14 cells as a function of the state-of-charge of the cells, and compared with ASI values of a cell with a Mg-doped Li4 Ti5 O12 spinel electrode [6]. The ASI of cells with Li2 SrTi6 O14 and Li2 BaTi6 O14 electrodes are
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provement, it is possible that the three-step reduction process that occurs in Li2 MTi6 O14 electrodes provides a mixed-valence character to the electrodes that significantly improves the electronic conductivity of Li2 MTi6 O14 electrodes throughout charge and discharge. A more detailed account of the structure–electrochemical properties of these electrodes will be reported in a subsequent full-length paper.
4. Conclusions A new class of insertion compounds based on the Li2 MTi6 O14 (M ¼ Ba, Sr) structure type has been identified as electrodes for lithium batteries. Four lithium ions can be accommodated per Li2 MTi6 O14 formula unit in a three-step process between 1.5 and 0.5 V vs. metallic lithium. It has been demonstrated that these electrodes can deliver a stable capacity of approximately 140 mAh/g for at least 40 cycles. Lithium cells with these electrode materials exhibit a low ASI compared to Li4 Ti5 O12 spinel electrodes, making them attractive alternative anodes to Li4 Ti5 O12 in lithium-ion cells.
Acknowledgements Fig. 6. Cyclic voltammograms of (a) Li2 BaTi6 O14 and (b) Li2 SrTi6 O14 . The scan rate was 0.5 mV/s.
This work was supported by the US Department of Energy, The Office of FreedomCar and Vehicle Technologies, under contract No. W-31-10-ENG-38.
References
Fig. 7. Area-specific impedance (ASI) as function of state-of-charge (SOC) of cells with Li2 BaTi6 O14 electrodes () and Li2 SrTi6 O14 electrodes (). An ASI plot of Mg-doped Li4 Ti5 O12 spinel () is shown for comparison.
significantly lower than that of the Li/spinel cell. This finding clearly attests that Li2 MTi6 O14 (M ¼ Ba, Sr) electrodes may have superior ionic and/or electronic properties. Although it is premature to explain this im-
[1] K. Amine, J. Liu, ITE Lett. 1 (2000) 59. [2] K.M. Colbow, J.R. Dahn, R.R. Haering, J. Power Sources 26 (1989) 397. [3] M.M. Thackeray, J. Am. Ceram. Soc. 82 (12) (1999) 3347. [4] E. Ferg, R.J. Gummow, A. de Kock, M.M. Thackeray, J. Electrochem. Soc. 141 (11) (1994) L147. [5] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (5) (1995) 1431. [6] C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, T. Goacher, M.M. Thackeray, J. Electrochem. Soc. 148 (1) (2001) A102. [7] I. Koseva, P. Pechev, S. Pechev, P. Gravereau, J.P. Chaminade, Z. Naturforsch. B: Chem. Sci. 57 (5) (2002) 512. [8] W.J. Zheng, R. Okuyama, T. Esaka, H. Iwahara, Solid State Ionics 35 (1989) 235. [9] L.M. Torres-Martinez, C. Suckut, R. Jimenez, A.R. West, J. Mater. Chem. 4 (1) (1994) 5.
Li2MTi6O14 (M ¼ Sr, Ba): new anodes for lithium-ion batteries I. Belharouak *, K. Amine Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Received 1 April 2003; received in revised form 7 April 2003; accepted 7 April 2003
Abstract Isostructural Li2 MTi6 O14 (M ¼ Sr, Ba) materials, prepared by a solid state reaction method, have been investigated as insertion electrodes for lithium battery applications. These titanate compounds have a structure that consists of a three-dimensional network of corner- and edge-shared [TiO6 ] octahedra, 11-coordinate polyhedra for the alkali-earth ions, and [LiO4 ] tetrahedra in tunnels that also contain vacant tetrahedral and octahedral sites. Electrochemical data show that these compounds are capable of reversibly intercalating four lithium atoms in a three-stage process between 1.4 and 0.5 V vs. metallic lithium. The electrodes provide a practical capacity of approximately 140 mAh/g; they are, therefore, possible alternative anode materials to the lithium titanate spinel, Li4 Ti5 O12 . The lithium intercalation mechanism and crystal structure of Li2 MTi6 O14 (M ¼ Sr, Ba) electrodes are discussed and compared with the electrochemical and structural properties of Li4 Ti5 O12 . The area-specific impedance (ASI) of Li=Li2 SrTi6 O14 cells was found to be significantly lower than that of Li=Li4 Ti5 O12 cells. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Lithium-ion batteries; Anode; Titanate; Structure
1. Introduction Rechargeable lithium-ion batteries with their lightweight and high energy density have become an important source for powering many applications, particularly those in the electronics sector. These batteries are gaining more attention because of their possible application in high power devices such as hybrid electric vehicles [1]. Graphite/LiCoO2 is the most commonly used electrochemical couple in lithium-ion batteries, in which LiCoO2 plays the role of the positive electrode (cathode) and graphite acts as the negative electrode (anode). At full charge, the highly lithiated graphite (LiC6 ) electrode is highly reactive because it operates close to the potential of metallic lithium. In order to address the safety limitations of lithium-ion cells, particularly those containing nickel-based cathodes, alternative anodes to graphite have been suggested, such as the spinel Li4 Ti5 O12 that operates at approximately 1.5 V vs. Li0 [2–5]. Although Li4 Ti5 O12 is an insulator, doping the structure with small amounts of Mg2þ , Al3þ *
Corresponding author. Tel.: 1-630-252-4450; fax: 1-630-252-4176. E-mail address: [email protected] (I. Belharouak).
has been reported to improve the electronic conductivity of the spinel by many orders of magnitude [6]. In this paper, we introduce, as an alternative to Li4 Ti5 O12 , a new family of lithium-ion conducting titanate materials Li2 MTi6 O14 (M ¼ Sr, Ba) that can act as insertion electrodes in lithium cells.
2. Experimental Li2 MTi6 O14 (M ¼ Sr, Ba) compounds were prepared by heating stoichiometric quantities of Li2 CO3 , SrCO3 or BaCO3 , and TiO2 . The reagents were carefully mixed and progressively heated up to 600 °C in air to decompose the carbonate salts. After grinding, the powders were sintered at 1000 °C for 24 h. Chemical analyses of the resulting products were performed by the inductively coupled plasma technique (ICP) to check their composition. The analytical results were in excellent agreement with the expected theoretical values. For example, the (Sr + Ti)/Li ratio and the lithium content in the final Li2 SrTi6 O14 product were determined to be 27.05 and 2.2 wt%, in contrast to the theoretically expected values of 27.01 and 2.26 wt%, respectively.
1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00090-0
436
I. Belharouak, K. Amine / Electrochemistry Communications 5 (2003) 435–438
Electrodes were made by coating a paste containing the Li2 MTi6 O14 active material (M ¼ Sr or Ba), a superP carbon conducting additive, and a polyvinylidene fluoride binder in a 80:10:10 wt% ratio on a copper foil collector. The loading amount of the active material was 4–5 mg=cm2 . The electrolyte was 1 M LiPF6 in a (1:1 wt%) mixture of ethylene carbonate and diethyl carbonate. The cells were assembled inside a helium-filled dry-box and were evaluated using coin-type cells (CR2032). The cells were discharged and charged between 2.0 and 0.5 V vs. the Li counter electrode at a constant current density of 0:1 mA=cm2 .
3. Results and discussion Both Li2 SrTi6 O14 and Li2 BaTi6 O14 are white materials, indicating that they are electronic insulators like the spinel, Li4 Ti5 O12 . Fig. 1 shows the X-ray diffraction (XRD) patterns of the two compounds; the close similarity of the patterns shows that they are isostructural compounds. No detectable amount of impurity phases was observed in either of these patterns. The X-ray patterns were indexed according to an orthorhombic Ccentered unit cell by a least squares refinement of the peak positions following the space group assignment
Fig. 1. X-ray diffraction patterns of (a) Li2 BaTi6 O14 , and (b) Li2 SrTi6 O14 . The pattern for Li2 SrTi6 O14 has been indexed to a Ccentered cell (Cmca).
(Cmca) for Li2 SrTi6 O14 by Koseva et al. [7] from a single-crystal structure refinement of this compound. The lattice parameters of Li2 SrTi6 O14 : a ¼ 16:566, are in excellent agreement b ¼ 11:148, and c ¼ 11:468 A with those reported by Koseva et al. (a ¼ 16:570, ). The refined lattice pab ¼ 11:150, and c ¼ 11:458 A rameters of Li2 BaTi6 O14 : a ¼ 16:575, b ¼ 11:268, and are similar to those of Li2 SrTi6 O14 . The c ¼ 11:579 A slight increase of the unit cell parameters and volume of Li2 BaTi6 O14 was expected because the ionic radius of Ba2þ ions is larger than that of Sr2þ ions. The Li2 MTi6 O14 structure can be described, in general, as consisting of a three-dimensional network of cornerand edge-shared [TiO6 ] octahedra, 11-coordinate polyhedra for the alkali-earth ions, and [LiO4 ] tetrahedra in tunnels that also contain vacant tetrahedral and octahedral sites. A [0 0 1] projection of the structure is shown in Fig. 2. A close examination of this structure type led us to predict that these compounds have an interstitial space that might be favorable for accommodating additional lithium and thus act as an electrode for lithium batteries. For example, Fig. 3(a) clearly shows that the [LiO4 ] tetrahedra share faces with neighboring vacant, tetrahedra and octahedra in the tunnel that would allow lithium-ion insertion and diffusion. Indeed, it has already been demonstrated, in the absence of any structural information, that a ‘‘Li2 O–BaO–TiO2 ’’ solid electrolyte with a sample composition 20 mol% Li2 O, 7 mol% BaO, and 73 mol% TiO2 exhibits a high Liþ -ion conductivity at 300 °C (4 103 Scm1 ) [8,9]. The Li2 MTi6 O14 structure also contains vacant octahedral sites that bridge the [MO11 ] polyhedra (Fig. 3(b)). A more detailed description of the Li2 SrTi6 O14 crystal structure is provided in [7]. Figs. 4(a) and (b) show typical voltage profiles of lithium cells with Li2 BaTi6 O14 (Fig. 4(a)) and Li2 SrTi6 O14 (Fig. 4(b)) electrodes, respectively; the dis-
Fig. 2. A [0 0 1] projection of the Li2 MTi6 O14 structure showing the [TiO6 ] octahedral framework, the positions of the M ions, and the tunnels that contain the [LiO4 ] tetrahedra.
I. Belharouak, K. Amine / Electrochemistry Communications 5 (2003) 435–438
Fig. 3. Slices of the Li2 MTi6 O14 structure showing (a) the tunnels in which the [LiO4 ] tetrahedra are linked by vacant face-shared octahedra and tetrahedra, and (b) a vacant octahedral site that bridges the 11coordinated M polyhedra.
charge profiles are essentially identical. The profiles show that lithium insertion occurs in three distinct steps, (1) at approximately 1.4 V (a two-phase reaction), (2) between 1.2 and 1.0 V, and (3) between 1.0 and 0.5 V. The reactions are reversible, although the reversibility of the individual processes appears to be better defined on charge for the Li2 SrTi6 O14 electrodes. On the initial discharge of the cells to 0.5 V, the Li2 BaTi6 O14 and Li2 SrTi6 O14 deliver capacities of 180 and 185 mAh/g; on the second discharge, the electrodes lose 28 and 30 mAh/ g, respectively. Thereafter, the cells perform with remarkable cycling stability for 40 cycles, as shown in Fig. 5. If all six of the Ti4þ ions could be reduced to Ti3þ , then the Li2 MTi6 O14 electrodes would provide theoretical capacities of 242 mAh/g (for Ba) and 262 mAh/g (for Sr). However, experiment shows that approximately 70% of the theoretical capacity is delivered on the initial discharge at a very low current rate (C/40), which corresponds to the uptake of approximately four Liþ ions per formula unit and the reduction of two-thirds of the available six Ti4þ ions. Cyclic voltammetry data of Li2 BaTi6 O14 and Li2 SrTi6 O14 electrodes are shown in Figs. 6(a) and (b), respectively. The data demonstrate that the reduction process that occurs at the electrodes during the initial sweep is different from subsequent cycles. The weak reduction peak that is observed at 0.65 V is only visible during the initial reduction process. Furthermore, the
437
Fig. 4. Voltage profile vs. the capacity of (a) Li2 BaTi6 O14 and (b) Li2 SrTi6 O14 . The current density was 0:025 mA=cm2 (C/40).
Fig. 5. Cycling performance of Li2 BaTi6 O14 () and Li2 SrTi6 O14 (). The current density was 0:1 mA=cm2 (C/7).
main reduction peak of the Li2 BaTi6 O14 electrode that is visible at 1.18 V shifts to 1.35 V, and the corresponding reduction peak of the Li2 SrTi6 O14 electrode at 1.24 V shifts to 1.32 V; these peaks remain more or less constant at these voltages during the following cycles. Of particular significance is that the second and third reduction processes, which are clearly visible in the voltage profiles in Figs. 4(a) and (b), are not as prominent in the CV data. The reason for this discrepancy is not yet clearly understood. Fig. 7 shows the area-specific impedance (ASI) of the Li=Li2 MTi6 O14 cells as a function of the state-of-charge of the cells, and compared with ASI values of a cell with a Mg-doped Li4 Ti5 O12 spinel electrode [6]. The ASI of cells with Li2 SrTi6 O14 and Li2 BaTi6 O14 electrodes are
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I. Belharouak, K. Amine / Electrochemistry Communications 5 (2003) 435–438
provement, it is possible that the three-step reduction process that occurs in Li2 MTi6 O14 electrodes provides a mixed-valence character to the electrodes that significantly improves the electronic conductivity of Li2 MTi6 O14 electrodes throughout charge and discharge. A more detailed account of the structure–electrochemical properties of these electrodes will be reported in a subsequent full-length paper.
4. Conclusions A new class of insertion compounds based on the Li2 MTi6 O14 (M ¼ Ba, Sr) structure type has been identified as electrodes for lithium batteries. Four lithium ions can be accommodated per Li2 MTi6 O14 formula unit in a three-step process between 1.5 and 0.5 V vs. metallic lithium. It has been demonstrated that these electrodes can deliver a stable capacity of approximately 140 mAh/g for at least 40 cycles. Lithium cells with these electrode materials exhibit a low ASI compared to Li4 Ti5 O12 spinel electrodes, making them attractive alternative anodes to Li4 Ti5 O12 in lithium-ion cells.
Acknowledgements Fig. 6. Cyclic voltammograms of (a) Li2 BaTi6 O14 and (b) Li2 SrTi6 O14 . The scan rate was 0.5 mV/s.
This work was supported by the US Department of Energy, The Office of FreedomCar and Vehicle Technologies, under contract No. W-31-10-ENG-38.
References
Fig. 7. Area-specific impedance (ASI) as function of state-of-charge (SOC) of cells with Li2 BaTi6 O14 electrodes () and Li2 SrTi6 O14 electrodes (). An ASI plot of Mg-doped Li4 Ti5 O12 spinel () is shown for comparison.
significantly lower than that of the Li/spinel cell. This finding clearly attests that Li2 MTi6 O14 (M ¼ Ba, Sr) electrodes may have superior ionic and/or electronic properties. Although it is premature to explain this im-
[1] K. Amine, J. Liu, ITE Lett. 1 (2000) 59. [2] K.M. Colbow, J.R. Dahn, R.R. Haering, J. Power Sources 26 (1989) 397. [3] M.M. Thackeray, J. Am. Ceram. Soc. 82 (12) (1999) 3347. [4] E. Ferg, R.J. Gummow, A. de Kock, M.M. Thackeray, J. Electrochem. Soc. 141 (11) (1994) L147. [5] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (5) (1995) 1431. [6] C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, T. Goacher, M.M. Thackeray, J. Electrochem. Soc. 148 (1) (2001) A102. [7] I. Koseva, P. Pechev, S. Pechev, P. Gravereau, J.P. Chaminade, Z. Naturforsch. B: Chem. Sci. 57 (5) (2002) 512. [8] W.J. Zheng, R. Okuyama, T. Esaka, H. Iwahara, Solid State Ionics 35 (1989) 235. [9] L.M. Torres-Martinez, C. Suckut, R. Jimenez, A.R. West, J. Mater. Chem. 4 (1) (1994) 5.