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Ionic conductivity, lithium insertion and extraction of lanthanum lithium titanate
Solid State Ionics 144 Ž2001. 51–57 www.elsevier.comrlocaterssi
Ionic conductivity, lithium insertion and extraction of lanthanum lithium titanate C.H. Chen, K. Amine ) Chemical Technology DiÕision, Electrochemical Technology Program, Argonne National Laboratory, 9700 South Cass AÕenue, Bldg. 203-C110, Argonne, IL 60439, USA Received 6 November 2000; received in revised form 8 May 2001; accepted 18 May 2001
Abstract The lithium ionic conductivity and electrochemical stability of perovskite La 2r3yx Li 3 xTiO 3 ŽLLTO. have been determined with AC impedance spectroscopy, cyclic voltammetry and galvanostatic cycling. Ionic conductivity of La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 pellets sintered from four different powders was measured in the temperature range from 30 to 110 8C. Bulk conductivity was found to be closely related to the calcination temperature of the powders. Pellets from 1100 8C-calcined powders had higher bulk conductivity than from 1200 8C-calcined powders. The grain-boundary conductivity was mainly determined by the sample composition. The activation energies were 0.14–0.18 eV for bulk conduction and 0.41–0.43 eV for grain-boundary conduction. Lithium was intercalated into LLTO below about 1.8 V vs. Li. With addition of acetylene black, about 0.48 Li was reversibly inserted into and extracted out of La 0.55 Li 0.35TiO 3. A phase transition is proposed to take place during the lithium insertion. Published by Elsevier Science B.V. Keywords: Ionic conductivity; Lithium insertion; Lanthanum lithium titanate
1. Introduction The development of all-solid-state lithium-ion batteries has received considerable attention because of their possible application to the new generation of energy sources in microelectronic and information industry w1–4x. Some obvious advantages over the current lithium-ion batteries can be expected with the liquid-free batteries. These include thermal stability, absence of leaks and pollution, resistance to shocks and vibrations, and a possible large electrochemical window allowing the use of 5-V cathodes. However, the main impediment is finding a sound solid elec) Corresponding author. Tel.: q1-630-2523838; fax: q1-6302524176. E-mail address: [email protected]. ŽK. Amine..
0167-2738r01r$ - see front matter. Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 8 8 4 - 0
trolyte that has a reasonably high lithium ionic conductivity and good stability. To date, the fastest lithium-ion-conducting electrolytes are the perovskite-type ŽABO 3 . lanthanum lithium titanates La 2r3yx Li 3 xTiO 3 ŽLLTO. and their variants w5–9x. In the structure of these materials, there are a substantial number of A-site vacancies through which lithium can transport. At room temperature, they possess a bulk conductivity of 10y3 Srcm and a grain-boundary conductivity of 10y4 –10y5 Srcm. These conductivities are comparable with those of the commonly used liquid electrolyte. However, there are reports that La 0.56 Li 0.33TiO 3 w10x and La 0.57 Li 0.29 TiO 3 w11x can intercalate lithium in the structure and introduce electronic conductivity at a potential below about 1.7 V vs. Li. The maximum lithium uptake was found to be equal to, or smaller than, the
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C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
number of A-sites available in the perovskite structure. This means that the use of these materials as electrolyte is probably unsuitable while metallic lithium or lithiated carbon is employed simultaneously as an anode. As an alternative, a less energetic anode such as the 1.5-V electrode Li 4Ti 5 O12 may be used with some sacrifice of battery energy w12,13x. In our study, the ionic conductivity of four LLTO samples with either different compositions or different synthesis conditions was compared in order to find an optimum for different application. Furthermore, the electrochemical stability of LLTO was checked with button-cell technology, and the maximum lithium uptake was found to be more than the number of available A-site vacancies in the perovskite.
2. Experimental A solid-state reaction procedure was adopted to prepare the perovskite lanthanum lithium titanate with two target compositions: La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 . The compounds La 2 O 3 , Li 2 CO 3 , and TiO 2 were mixed together with ethanol and ball-milled for 15 h. The mixtures were calcined at either 1100 or 1200 8C, followed by drying of ethanol. After grinding, four powders were obtained: La 0.55 Li 0.35TiO 3 calcined at 1100 8C, La 0.55 Li 0.35 TiO 3 calcined at 1200 8C for 12 h, La 0.57 Li 0.29TiO 3 calcined at 1100 8C, and La 0.57 Li 0.29TiO 3 calcined at 1200 8C. X-ray diffraction analysis confirmed that these powders were pure perovskite phase. The powders were pressed into pellets Ž15 mm in diameter. and sintered at 1200 8C for 12 h in air. The sintering temperature of 1200 8C was selected because a thermal analysis of these powders indicated that they melted at temperatures above 1250 8C, although sintering at 1350 8C has been reported in the literature w5x. The density of the pellets was measured by the Archimedes method. Two sides of the pellets were sputtered with a thin gold layer for conductivity measurement. A CHI660 model Electrochemical Workstation ŽCH Instruments. was used to acquire the AC impedance spectra of these LLTO samples in the frequency range from 1 mHz to 100 kHz, and the temperature range from room temperature to 110 8C.
The La 0.55 Li 0.35TiO 3 powder calcined at 1100 8C was made into electrode laminates to check the electrochemical stability. Two laminates with and without carbon addition were prepared on 15-mmthick copper foil by the tape-casting technique. The carbon-free laminate was composed of 92 wt.% LLTO and 8 wt.% polyvinylidene ŽPVDF., while the carbon-containing laminate was composed of 84 wt.% LLTO, 8 wt.% acetylene black, and 8 wt.% PVDF. Button cells Žsize 2032. were made using the punched laminates as cathode, lithium foil as anode, and 1 M LiPF6 in ethylene carbonate ŽEC.rdiethyl carbonate ŽDEC. as electrolyte. The cycling of these cells was performed on a Maccor cycler. Cyclic voltammetry and AC impedance spectroscopy were also used to characterize the cells on the CHI660 model Electrochemical Workstation. For comparison, a button cell using a laminate composed of 80 wt.% acetylene black and 20 wt.% PVDF as electrode was also tested.
3. Results and discussion 3.1. Ionic conductiÕity and actiÕation energy of LLTO As a representative of the AC impedance spectra of the sintered pellets, Fig. 1 shows the results for La 0.55 Li 0.35TiO 3 calcined at 1100 8C and sintered at 1200 8C. Each spectrum consists of a semicircle in the high-frequency range and a straight line in the low-frequency range ŽFig. 1a.. In addition, there is an intercept at the high-frequency end of the semicircle ŽFig. 1b.. This impedance spectrum is typical for a pure ionic conductor with blocking electrodes w14x and consistent with that observed by Fragnaud and Schleich w1x. In fact, the electronic conductivity of LLTO estimated from a DC measurement is in the order of 10y7 Srcm at room temperature. In principle, a semicircle between the origin and the intercept would be observed if much higher frequencies are used in the experiment. The high-frequency limit should be as high as 0.1–1 GHz because the time constant of this bulk process Ž RCgeo . is 1 ns with assuming a geometric capacitance Ž Cgeo . of 10–1 pF. The high-frequency limits used in this study Ž100 kHz. and in Inaguma et al.’s Ž13 MHz. are obviously not sufficient. The intercept represents the total ionic
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
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tra can be fit with such a simple equivalent circuit satisfactorily Ž x 2 around 10y3 –10y4 .. The obtained grain-boundary capacitance is in the order of 10 nF, which is typical for polycrystalline ionic conductors. Fig. 2 presents Arrhenius plots of the bulk conductivity and grain-boundary conductivity of these pellets at different temperatures. The room temperature Ž30 8C. conductivity and activation energy are also given in Table 1. It can be seen that, consistent with the literature data w5,6x, the bulk conductivity of LLTO at 30 8C is around 10y3 Srcm and the grain-boundary conductivity at 30 8C is above 10y5 Srcm. By comparing the results from the four different pellets, one can see that the bulk conductivi-
Fig. 1. AC impedance spectra of a La 0.55 Li 0.35TiO 3 pellet at different temperatures after it had been calcined at 1100 8C and sintered at 1200 8C: Ža. full spectra and Žb. high-frequency part.
resistance of LLTO grains, while the semicircle is usually assigned to the relaxation process in LLTO grain boundaries. The straight line is related to the lithium-ion diffusion occurring at the interface between LLTO and the sputtered gold layers. With increasing temperature and thus causing a faster diffusion, the interface tends to shift from an infinite space to a finite space. Therefore, more charge accumulation process, which corresponds to a capacitive behavior, is involved at higher temperatures. Hence, the angle of the straight line in Fig. 1a increases with temperature. Under AC measurement conditions, no detectable lithium deposition on gold due to this diffusion process is observed. These impedance spec-
Fig. 2. Arrhenius plots of the lithium ionic conductivity: Ža. bulk conductivity and Žb. grain-boundary conductivity. Four pellets: ŽA. La 0.55 Li 0.35TiO 3 calcined at 1100 8C Ždiamonds with solid line., ŽB. La 0.55 Li 0.35TiO 3 calcined at 1200 8C Žtriangles with solid line., ŽC. La 0.57 Li 0.29TiO 3 calcined at 1100 8C Žsquares with dashed line., and ŽD. La 0.57 Li 0.29TiO 3 calcined at 1200 8C Žcrosses with dashed line..
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
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Table 1 Density, ionic conductivity and activation energy of La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 Samples
Composition
Tcalcination Ž8C.
d Žg cmy3 .
s bulk,30 Ž10y3 Srcm.
Ea,bulk ŽeV.
sgb,30 Ž10y5 Srcm.
Ea,gb ŽeV.
A B C D
La 0.55 Li 0.35TiO 3 La 0.55 Li 0.35TiO 3 La 0.57 Li 0.29TiO 3 La 0.57 Li 0.29TiO 3
1100 1200 1100 1200
4.50 4.37 4.80 4.48
1.19 0.89 1.17 0.66
0.14 0.16 0.15 0.18
2.64 2.71 4.68 3.71
0.43 0.41 0.43 0.42
ties of two pellets ŽA and C. from the 1100 8Ccalcined powders are very close, yet higher than those of the other two pellets ŽB and D. from 1200 8C-calcined powders ŽFig. 2a.. It appears that the pellet density has an important effect on the bulk conductivity. This condition is usually true for grain-boundary conductivity because a high density suggests the presence of relatively thin grainboundaries. However, the real reason might be related to the composition change of the samples. It is known that at temperatures above 900 8C lithium oxide ŽLi 2 O. can be evaporated from lithium-containing solid solutions such as lithium zirconates w15x and Li x Ni 1yxO w16x. More lithium loss could be expected to take place in the samples B and D than A and C, leading to the difference in bulk conductivity. The activation energy for the bulk conduction is 0.14–0.18 eV, which is considerably smaller than Inaguma et al.’s w5x result of 0.40 eV for this low temperature range. However, Inaguma et al. also obtained another activation energy, 0.15 eV, for the bulk conduction in the temperature range from 100 to 400 8C. The activation energy for the grainboundary conduction is 0.41–0.43 eV, which agrees well with Inaguma et al.’s results. The results of grain-boundary conductivity ŽFig. 2b. show that the lithium ions diffuse faster in the grain boundaries of La 0.57 Li 0.29TiO 3 pellets than in those of La 0.55 Li 0.35TiO 3 pellets. This may be attributed to the compositional difference of the grain boundaries because different bulk compositions may lead to different segregation kinetics and, therefore, a different composition at the grain boundaries. For the La 0.57 Li 0.29TiO 3 , the pellet from 1100 8C-calcined powder has higher conductivity than that from 1200 8C-calcined powder because the density of former sample is greater. Nevertheless, very little difference is observed for the composition La 0.55 Li 0.35TiO 3 .
3.2. Lithium insertion and extraction in LLTO Fig. 3 shows the cyclic voltammogram of a freshly made 2032 button cell using the laminate consisting of 92 wt.% La 0.55 Li 0.35TiO 3 and 8 wt.% PVDF as cathode. During the initial lithiation half-cycle, lithium can be intercalated into LLTO below the potential 1.8 V. This finding is in agreement with the literature reports, although there is a small discrepancy about the intercalation onset potential w10,11x. However, no extraction peak is observed during the first charge shown in Fig. 3. Furthermore, no significant active signals appear in the subsequent cycles. This means that the lithium insertion process seems irreversible when no conducting additive is used in the sample. This irreversibility is likely caused by the very high over-potential due to the very poor electronic conductivity of the sample. Similar phenomena were observed in the lithiation of hollandite-type TiO 2 w17x. Nevertheless, this process is reversible after adding acetylene black in the laminate to enhance the electronic conductivity Žsee below..
Fig. 3. Cyclic voltammogram of a cell using a LLTO laminate consisting of 92 wt.% La 0.55 Li 0.35TiO 3 and 8 wt.% PVDF vs. Li. The scan rate was 1 mVrs. Cycle numbers are indicated.
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
Fig. 4. Cyclic voltammograms of Ža. cell using a LLTO laminate consisting of 84 wt.% La 0.55 Li 0.35TiO 3 , 8 wt.% acetylene black, and 8 wt.% PVDF vs. Li and Žb. cell using a laminate consisting of 80 wt.% acetylene black and 20 wt.% PVDF vs. Li. The scan rate was 1 mVrs. The first cycle is indicated.
Fig. 4a shows the cyclic voltammograms of the cell using a cathode consisting of 84 wt.% La 0.55 Li 0.35TiO 3 , 8 wt.% acetylene black, and 8 wt.% PVDF and Li anode. The result is quite different from the voltammogram for carbon-free laminate ŽFig. 3.. In the first lithiation half-cycle, two significant intercalation steps, from 1.8 to 1.1 V and from 0.6 to 0 V, and a small peak around 0.9 V are observed. On the first delithiation half-cycle and the subsequent cycles, four well-resolved peaks appear quite reversibly in each charge–discharge cycle. The peak potentials are approximately 1.5, 1.04, 0.55 and 0 V for lithium insertion during the anodic scans, and, correspondingly, approximately 1.67, 1.2, 0.77, and 0.55 V for lithium extraction during the cathodic scans. Therefore, the averages of the potentials for insertion and corresponding extraction peaks are ap-
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proximately 1.6, 1.1, 0.66, and 0.27 V, respectively. On the other hand, the cyclic voltammogram of a cell using an acetylene black laminate as cathode ŽFig. 4b. has peaks at 1.1 and 0.27 V, which are regarded as the contribution from insertion and extraction of lithium in and out of acetylene black. Therefore, the 1.6 and 0.66 V steps in Fig. 4a can be ascribed to the lithium insertionrextraction in LLTO. In addition to the reversible lithium insertion and extraction steps shown in Fig. 4a, the charge quantities involved in the two lithiation steps, from 1.8 to 1.1 V and from 0.6 to 0 V, on the first anodic scan are obviously substantially more than those in the subsequent cycles. Similar to usual carbon anodes in electrochemical lithium cells, the step from 0.6 to 0 V probably includes the formation of thin solid electrolyte interface ŽSEI. layers on the acetylene black particles due to decomposition of electrolyte at low potential. A similar big step is also observed without mixing LLTO in the laminate ŽFig. 4b.. The possibility of forming SEI layers on LLTO particles is very small because of their poor electronic conductivity. The step from 1.8 to 1.1 V is apparently only related to the lithium insertion in LLTO. More charge quantity involved in this suggests that only part of the lithium intercalated in the first lithiation half-cycle can be reversibly cycled afterwards. Figs. 5 and 6 show the galvanostatic cycling results from cells without and with acetylene-blackcontaining laminates, respectively. In general, they are very much consistent with the cyclic voltammetry results discussed before. As shown in Fig. 5,
Fig. 5. Cycling curve of a cell using a LLTO laminate consisting of 92 wt.% La 0.55 Li 0.35TiO 3 and 8 wt.% PVDF vs. Li. Current density is 0.0625 mArcm2 . The spikes on the curve resulted from 30-s interruption.
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C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
Fig. 6. Cycling curve of a cell using an LLTO laminate consisting of 84 wt.% La 0.55 Li 0.35TiO 3 , 8 wt.% acetylene black, and 8 wt.% PVDF vs. Li. Current density is 0.0625 mArcm2 : Ža. first 19 cycles and Žb. first 4 cycles.
lithium may be inserted in the LLTO electrode in the initial lithiation, but is difficult to extract from the acetylene-black-free cell. As shown in Fig. 6, lithium can be inserted as well as extracted in the acetyleneblack-containing cell. Nevertheless, the initial lithiation capacity obtained is considerably higher than the subsequent cycle capacity. As discussed before, the cell capacity in the acetylene-black-containing cell is partially contributed by acetylene black. From the half-cells using a cathode consisting of 80 wt.% acetylene black and 20 wt.% PVDF, we obtained an initial intercalation capacity ranging from 445 to 676 mA hrg, and a reversible capacity 160 mA hrg in subsequent cycles for acetylene black in the voltage window between 2.5 and 0 V. After subtracting the contribution from acetylene black, we determined the cell capacity and its converted equivalent number of insertedrextracted
lithium per formula of La 0.55 Li 0.35TiO 3 . Fig. 7 shows these results, together with the cycling results of the acetylene-black-free cell. It can be seen that about 0.48 Li may be inserted into one formula of La 0.55 Li 0.35TiO 3 in the initial lithiation half-cycle in the acetylene-black-free cell. For the acetyleneblack-containing cell, about 0.75–0.9 Li can be initially inserted into LLTO; however, only 0.48 Li can be reversibly inserted into and extracted out of one formula of La 0.55 Li 0.35TiO 3 in subsequent cycles. These values Ž0.75–0.9 and 0.48. are well above the number of available vacant perovskite A-sites Ž0.1. in La 0.55 Li 0.35TiO 3 . Other researchers w6,7x did not observe this effect, partly because they did not extend the voltage window to 0 V. We propose a possible phase transition mechanism to explain this phenomenon. In the initial lithiation half-cycle, the perovskite structure ŽABO 3 . might be transformed to A 2 BO 3 monoclinic phase after filling all the vacant perovskite A-sites. This A 2 BO 3 phase could be similar to rechargeable Li 2y x RuO 3 w18,19x. A point of inflection at potential of about 0.68 V is noticed on the lithiation curve of the LLTO laminate in Fig. 5 and indicated by an arrow. This point of inflection could be caused by the proposed phase transition at the intercalation of about 0.24 Li. Further compositional and structural analyses are needed to confirm the occurrence of this phase transformation and to explain why it occurs at the intercalation of 0.24 Li
Fig. 7. Capacity and corresponding number of Li ions inserted into or extracted out of LLTO: charge Žtriangles. and discharge Žcrosses. of the acetylene-black-free cell, and charge Žsquares. and discharge Ždiamonds. of the acetylene-black-containing cell.
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
Žnot 0.1 Li as predicted by La 0.55 Li 0.35TiO 3 composition..
4. Conclusions Pellets of La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 sintered from four different powders were studied with AC impedance spectroscopy. Bulk conductivity was found to be closely related to the calcination temperature of the powders. Pellets from 1100 8Ccalcined powders had higher bulk conductivity than those from 1200 8C-calcined powders. The grainboundary conductivity was mainly determined by the sample composition. The activation energies were 0.14–0.18 eV for bulk conduction and 0.41–0.43 eV for grain-boundary conduction. Lithium was intercalated into LLTO below about 1.8 V vs. Li. With addition of acetylene black, about 0.48 Li was reversibly inserted into and extracted out of La 0.55 Li 0.35TiO 3 . A phase transition is proposed to take place during the first lithium insertion step. Because of the lithium insertion into LLTO at the potential below 1.8 V vs. Li, and thus the introduction of electronic conductivity, LLTO is obviously not a suitable electrolyte material in lithium ion batteries that use either metallic lithium or lithiated carbon as anode material. It can probably be used as electrolyte materials when some high potential oxide materials, for instance, Li 4Ti 5 O12 are used as anode.
Acknowledgements This study was supported by the Laboratory Director R & D ŽLDRD. program of Argonne National Laboratory.
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References w1x P. Fragnaud, D.M. Schleich, Sens. Actuators, A 51 Ž1995. 21. w2x P. Birke, W.F. Chu, W. Weppner, Solid State Ionics 93 Ž1997. 1. w3x P. Birke, W. Weppner, Electrochim. Acta 42 Ž1997. 3375. w4x C.H. Chen, Thin-film components for lithium-ion batteries, PhD thesis, Delft University of Technology, 1998. w5x Y. Inaguma, L. Chen, M. Itoh, T. Makamura, T. Uchida, M. Ikuta, M. Wakihara, Solid State Commun. 86 Ž1993. 689. w6x Y. Inaguma, L. Chen, M. Itoh, T. Nakamura, Solid State Ionics 70r71 Ž1994. 196. w7x M. Itoh, Y. Inaguma, W.H. Jung, L. Chen, T. Nakamura, Solid State Ionics 70r71 Ž1994. 203. w8x Y. Harada, T. Ishigaki, H. Kawai, J. Kuwano, Solid State Ionics 108 Ž1998. 407. w9x H. Kawai, J. Kuwano, in: P. Vincenzini ŽEd.., Advances in Science and Technology 3D, Ceramics: Charting the Future. Techna, 1995, p. 2641. w10x O. Bohnke, C. Bohnke, J.L. Fourquet, Solid State Ionics 91 Ž1996. 21. w11x P. Birke, S. Scharner, R.A. Huggins, W. Weppner, J. Electrochem. Soc. 144 Ž1997. L167. w12x D.M. Scheich, T. Brousse, P. Fragnaud, R. Marchand, Abs. No. 609, Meeting of Electrochemical Society, San Antonio, TX. 1996, p. 745. w13x T. Brousse, R. Marchand, P. Fragnaud, D.M. Schleich, Abs. No. 841, Meeting of Electrochemical Society, San Antonio, TX. 1996, p. 1037. w14x J.R. Macdonald, Impedance Spectroscopy. Wiley-Interscience, New York, 1987. w15x Y. Zou, A. Petric, J. Phys. Chem. Solids 55 Ž1994. 493. w16x E. Antolini, J. Mater. Chem. 8 Ž1998. 2783. w17x L.D. Noailles, C.S. Johnson, J.T. Vaughey, M.M. Thackeray, J. Power Sources 81–82 Ž1999. 259. w18x H. Kobayashi, R. Kanno, Y. Kawamoto, M. Tabuchi, O. Nakamura, M. Takano, Solid State Ionics 82 Ž1995. 25. w19x H. Kobayashi, R. Kanno, M. Tabuchi, H. Kageyama, O. Nakamura, M. Takano, J. Power Sources 68 Ž1997. 686.
Ionic conductivity, lithium insertion and extraction of lanthanum lithium titanate C.H. Chen, K. Amine ) Chemical Technology DiÕision, Electrochemical Technology Program, Argonne National Laboratory, 9700 South Cass AÕenue, Bldg. 203-C110, Argonne, IL 60439, USA Received 6 November 2000; received in revised form 8 May 2001; accepted 18 May 2001
Abstract The lithium ionic conductivity and electrochemical stability of perovskite La 2r3yx Li 3 xTiO 3 ŽLLTO. have been determined with AC impedance spectroscopy, cyclic voltammetry and galvanostatic cycling. Ionic conductivity of La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 pellets sintered from four different powders was measured in the temperature range from 30 to 110 8C. Bulk conductivity was found to be closely related to the calcination temperature of the powders. Pellets from 1100 8C-calcined powders had higher bulk conductivity than from 1200 8C-calcined powders. The grain-boundary conductivity was mainly determined by the sample composition. The activation energies were 0.14–0.18 eV for bulk conduction and 0.41–0.43 eV for grain-boundary conduction. Lithium was intercalated into LLTO below about 1.8 V vs. Li. With addition of acetylene black, about 0.48 Li was reversibly inserted into and extracted out of La 0.55 Li 0.35TiO 3. A phase transition is proposed to take place during the lithium insertion. Published by Elsevier Science B.V. Keywords: Ionic conductivity; Lithium insertion; Lanthanum lithium titanate
1. Introduction The development of all-solid-state lithium-ion batteries has received considerable attention because of their possible application to the new generation of energy sources in microelectronic and information industry w1–4x. Some obvious advantages over the current lithium-ion batteries can be expected with the liquid-free batteries. These include thermal stability, absence of leaks and pollution, resistance to shocks and vibrations, and a possible large electrochemical window allowing the use of 5-V cathodes. However, the main impediment is finding a sound solid elec) Corresponding author. Tel.: q1-630-2523838; fax: q1-6302524176. E-mail address: [email protected]. ŽK. Amine..
0167-2738r01r$ - see front matter. Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 8 8 4 - 0
trolyte that has a reasonably high lithium ionic conductivity and good stability. To date, the fastest lithium-ion-conducting electrolytes are the perovskite-type ŽABO 3 . lanthanum lithium titanates La 2r3yx Li 3 xTiO 3 ŽLLTO. and their variants w5–9x. In the structure of these materials, there are a substantial number of A-site vacancies through which lithium can transport. At room temperature, they possess a bulk conductivity of 10y3 Srcm and a grain-boundary conductivity of 10y4 –10y5 Srcm. These conductivities are comparable with those of the commonly used liquid electrolyte. However, there are reports that La 0.56 Li 0.33TiO 3 w10x and La 0.57 Li 0.29 TiO 3 w11x can intercalate lithium in the structure and introduce electronic conductivity at a potential below about 1.7 V vs. Li. The maximum lithium uptake was found to be equal to, or smaller than, the
52
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
number of A-sites available in the perovskite structure. This means that the use of these materials as electrolyte is probably unsuitable while metallic lithium or lithiated carbon is employed simultaneously as an anode. As an alternative, a less energetic anode such as the 1.5-V electrode Li 4Ti 5 O12 may be used with some sacrifice of battery energy w12,13x. In our study, the ionic conductivity of four LLTO samples with either different compositions or different synthesis conditions was compared in order to find an optimum for different application. Furthermore, the electrochemical stability of LLTO was checked with button-cell technology, and the maximum lithium uptake was found to be more than the number of available A-site vacancies in the perovskite.
2. Experimental A solid-state reaction procedure was adopted to prepare the perovskite lanthanum lithium titanate with two target compositions: La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 . The compounds La 2 O 3 , Li 2 CO 3 , and TiO 2 were mixed together with ethanol and ball-milled for 15 h. The mixtures were calcined at either 1100 or 1200 8C, followed by drying of ethanol. After grinding, four powders were obtained: La 0.55 Li 0.35TiO 3 calcined at 1100 8C, La 0.55 Li 0.35 TiO 3 calcined at 1200 8C for 12 h, La 0.57 Li 0.29TiO 3 calcined at 1100 8C, and La 0.57 Li 0.29TiO 3 calcined at 1200 8C. X-ray diffraction analysis confirmed that these powders were pure perovskite phase. The powders were pressed into pellets Ž15 mm in diameter. and sintered at 1200 8C for 12 h in air. The sintering temperature of 1200 8C was selected because a thermal analysis of these powders indicated that they melted at temperatures above 1250 8C, although sintering at 1350 8C has been reported in the literature w5x. The density of the pellets was measured by the Archimedes method. Two sides of the pellets were sputtered with a thin gold layer for conductivity measurement. A CHI660 model Electrochemical Workstation ŽCH Instruments. was used to acquire the AC impedance spectra of these LLTO samples in the frequency range from 1 mHz to 100 kHz, and the temperature range from room temperature to 110 8C.
The La 0.55 Li 0.35TiO 3 powder calcined at 1100 8C was made into electrode laminates to check the electrochemical stability. Two laminates with and without carbon addition were prepared on 15-mmthick copper foil by the tape-casting technique. The carbon-free laminate was composed of 92 wt.% LLTO and 8 wt.% polyvinylidene ŽPVDF., while the carbon-containing laminate was composed of 84 wt.% LLTO, 8 wt.% acetylene black, and 8 wt.% PVDF. Button cells Žsize 2032. were made using the punched laminates as cathode, lithium foil as anode, and 1 M LiPF6 in ethylene carbonate ŽEC.rdiethyl carbonate ŽDEC. as electrolyte. The cycling of these cells was performed on a Maccor cycler. Cyclic voltammetry and AC impedance spectroscopy were also used to characterize the cells on the CHI660 model Electrochemical Workstation. For comparison, a button cell using a laminate composed of 80 wt.% acetylene black and 20 wt.% PVDF as electrode was also tested.
3. Results and discussion 3.1. Ionic conductiÕity and actiÕation energy of LLTO As a representative of the AC impedance spectra of the sintered pellets, Fig. 1 shows the results for La 0.55 Li 0.35TiO 3 calcined at 1100 8C and sintered at 1200 8C. Each spectrum consists of a semicircle in the high-frequency range and a straight line in the low-frequency range ŽFig. 1a.. In addition, there is an intercept at the high-frequency end of the semicircle ŽFig. 1b.. This impedance spectrum is typical for a pure ionic conductor with blocking electrodes w14x and consistent with that observed by Fragnaud and Schleich w1x. In fact, the electronic conductivity of LLTO estimated from a DC measurement is in the order of 10y7 Srcm at room temperature. In principle, a semicircle between the origin and the intercept would be observed if much higher frequencies are used in the experiment. The high-frequency limit should be as high as 0.1–1 GHz because the time constant of this bulk process Ž RCgeo . is 1 ns with assuming a geometric capacitance Ž Cgeo . of 10–1 pF. The high-frequency limits used in this study Ž100 kHz. and in Inaguma et al.’s Ž13 MHz. are obviously not sufficient. The intercept represents the total ionic
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
53
tra can be fit with such a simple equivalent circuit satisfactorily Ž x 2 around 10y3 –10y4 .. The obtained grain-boundary capacitance is in the order of 10 nF, which is typical for polycrystalline ionic conductors. Fig. 2 presents Arrhenius plots of the bulk conductivity and grain-boundary conductivity of these pellets at different temperatures. The room temperature Ž30 8C. conductivity and activation energy are also given in Table 1. It can be seen that, consistent with the literature data w5,6x, the bulk conductivity of LLTO at 30 8C is around 10y3 Srcm and the grain-boundary conductivity at 30 8C is above 10y5 Srcm. By comparing the results from the four different pellets, one can see that the bulk conductivi-
Fig. 1. AC impedance spectra of a La 0.55 Li 0.35TiO 3 pellet at different temperatures after it had been calcined at 1100 8C and sintered at 1200 8C: Ža. full spectra and Žb. high-frequency part.
resistance of LLTO grains, while the semicircle is usually assigned to the relaxation process in LLTO grain boundaries. The straight line is related to the lithium-ion diffusion occurring at the interface between LLTO and the sputtered gold layers. With increasing temperature and thus causing a faster diffusion, the interface tends to shift from an infinite space to a finite space. Therefore, more charge accumulation process, which corresponds to a capacitive behavior, is involved at higher temperatures. Hence, the angle of the straight line in Fig. 1a increases with temperature. Under AC measurement conditions, no detectable lithium deposition on gold due to this diffusion process is observed. These impedance spec-
Fig. 2. Arrhenius plots of the lithium ionic conductivity: Ža. bulk conductivity and Žb. grain-boundary conductivity. Four pellets: ŽA. La 0.55 Li 0.35TiO 3 calcined at 1100 8C Ždiamonds with solid line., ŽB. La 0.55 Li 0.35TiO 3 calcined at 1200 8C Žtriangles with solid line., ŽC. La 0.57 Li 0.29TiO 3 calcined at 1100 8C Žsquares with dashed line., and ŽD. La 0.57 Li 0.29TiO 3 calcined at 1200 8C Žcrosses with dashed line..
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
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Table 1 Density, ionic conductivity and activation energy of La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 Samples
Composition
Tcalcination Ž8C.
d Žg cmy3 .
s bulk,30 Ž10y3 Srcm.
Ea,bulk ŽeV.
sgb,30 Ž10y5 Srcm.
Ea,gb ŽeV.
A B C D
La 0.55 Li 0.35TiO 3 La 0.55 Li 0.35TiO 3 La 0.57 Li 0.29TiO 3 La 0.57 Li 0.29TiO 3
1100 1200 1100 1200
4.50 4.37 4.80 4.48
1.19 0.89 1.17 0.66
0.14 0.16 0.15 0.18
2.64 2.71 4.68 3.71
0.43 0.41 0.43 0.42
ties of two pellets ŽA and C. from the 1100 8Ccalcined powders are very close, yet higher than those of the other two pellets ŽB and D. from 1200 8C-calcined powders ŽFig. 2a.. It appears that the pellet density has an important effect on the bulk conductivity. This condition is usually true for grain-boundary conductivity because a high density suggests the presence of relatively thin grainboundaries. However, the real reason might be related to the composition change of the samples. It is known that at temperatures above 900 8C lithium oxide ŽLi 2 O. can be evaporated from lithium-containing solid solutions such as lithium zirconates w15x and Li x Ni 1yxO w16x. More lithium loss could be expected to take place in the samples B and D than A and C, leading to the difference in bulk conductivity. The activation energy for the bulk conduction is 0.14–0.18 eV, which is considerably smaller than Inaguma et al.’s w5x result of 0.40 eV for this low temperature range. However, Inaguma et al. also obtained another activation energy, 0.15 eV, for the bulk conduction in the temperature range from 100 to 400 8C. The activation energy for the grainboundary conduction is 0.41–0.43 eV, which agrees well with Inaguma et al.’s results. The results of grain-boundary conductivity ŽFig. 2b. show that the lithium ions diffuse faster in the grain boundaries of La 0.57 Li 0.29TiO 3 pellets than in those of La 0.55 Li 0.35TiO 3 pellets. This may be attributed to the compositional difference of the grain boundaries because different bulk compositions may lead to different segregation kinetics and, therefore, a different composition at the grain boundaries. For the La 0.57 Li 0.29TiO 3 , the pellet from 1100 8C-calcined powder has higher conductivity than that from 1200 8C-calcined powder because the density of former sample is greater. Nevertheless, very little difference is observed for the composition La 0.55 Li 0.35TiO 3 .
3.2. Lithium insertion and extraction in LLTO Fig. 3 shows the cyclic voltammogram of a freshly made 2032 button cell using the laminate consisting of 92 wt.% La 0.55 Li 0.35TiO 3 and 8 wt.% PVDF as cathode. During the initial lithiation half-cycle, lithium can be intercalated into LLTO below the potential 1.8 V. This finding is in agreement with the literature reports, although there is a small discrepancy about the intercalation onset potential w10,11x. However, no extraction peak is observed during the first charge shown in Fig. 3. Furthermore, no significant active signals appear in the subsequent cycles. This means that the lithium insertion process seems irreversible when no conducting additive is used in the sample. This irreversibility is likely caused by the very high over-potential due to the very poor electronic conductivity of the sample. Similar phenomena were observed in the lithiation of hollandite-type TiO 2 w17x. Nevertheless, this process is reversible after adding acetylene black in the laminate to enhance the electronic conductivity Žsee below..
Fig. 3. Cyclic voltammogram of a cell using a LLTO laminate consisting of 92 wt.% La 0.55 Li 0.35TiO 3 and 8 wt.% PVDF vs. Li. The scan rate was 1 mVrs. Cycle numbers are indicated.
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
Fig. 4. Cyclic voltammograms of Ža. cell using a LLTO laminate consisting of 84 wt.% La 0.55 Li 0.35TiO 3 , 8 wt.% acetylene black, and 8 wt.% PVDF vs. Li and Žb. cell using a laminate consisting of 80 wt.% acetylene black and 20 wt.% PVDF vs. Li. The scan rate was 1 mVrs. The first cycle is indicated.
Fig. 4a shows the cyclic voltammograms of the cell using a cathode consisting of 84 wt.% La 0.55 Li 0.35TiO 3 , 8 wt.% acetylene black, and 8 wt.% PVDF and Li anode. The result is quite different from the voltammogram for carbon-free laminate ŽFig. 3.. In the first lithiation half-cycle, two significant intercalation steps, from 1.8 to 1.1 V and from 0.6 to 0 V, and a small peak around 0.9 V are observed. On the first delithiation half-cycle and the subsequent cycles, four well-resolved peaks appear quite reversibly in each charge–discharge cycle. The peak potentials are approximately 1.5, 1.04, 0.55 and 0 V for lithium insertion during the anodic scans, and, correspondingly, approximately 1.67, 1.2, 0.77, and 0.55 V for lithium extraction during the cathodic scans. Therefore, the averages of the potentials for insertion and corresponding extraction peaks are ap-
55
proximately 1.6, 1.1, 0.66, and 0.27 V, respectively. On the other hand, the cyclic voltammogram of a cell using an acetylene black laminate as cathode ŽFig. 4b. has peaks at 1.1 and 0.27 V, which are regarded as the contribution from insertion and extraction of lithium in and out of acetylene black. Therefore, the 1.6 and 0.66 V steps in Fig. 4a can be ascribed to the lithium insertionrextraction in LLTO. In addition to the reversible lithium insertion and extraction steps shown in Fig. 4a, the charge quantities involved in the two lithiation steps, from 1.8 to 1.1 V and from 0.6 to 0 V, on the first anodic scan are obviously substantially more than those in the subsequent cycles. Similar to usual carbon anodes in electrochemical lithium cells, the step from 0.6 to 0 V probably includes the formation of thin solid electrolyte interface ŽSEI. layers on the acetylene black particles due to decomposition of electrolyte at low potential. A similar big step is also observed without mixing LLTO in the laminate ŽFig. 4b.. The possibility of forming SEI layers on LLTO particles is very small because of their poor electronic conductivity. The step from 1.8 to 1.1 V is apparently only related to the lithium insertion in LLTO. More charge quantity involved in this suggests that only part of the lithium intercalated in the first lithiation half-cycle can be reversibly cycled afterwards. Figs. 5 and 6 show the galvanostatic cycling results from cells without and with acetylene-blackcontaining laminates, respectively. In general, they are very much consistent with the cyclic voltammetry results discussed before. As shown in Fig. 5,
Fig. 5. Cycling curve of a cell using a LLTO laminate consisting of 92 wt.% La 0.55 Li 0.35TiO 3 and 8 wt.% PVDF vs. Li. Current density is 0.0625 mArcm2 . The spikes on the curve resulted from 30-s interruption.
56
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
Fig. 6. Cycling curve of a cell using an LLTO laminate consisting of 84 wt.% La 0.55 Li 0.35TiO 3 , 8 wt.% acetylene black, and 8 wt.% PVDF vs. Li. Current density is 0.0625 mArcm2 : Ža. first 19 cycles and Žb. first 4 cycles.
lithium may be inserted in the LLTO electrode in the initial lithiation, but is difficult to extract from the acetylene-black-free cell. As shown in Fig. 6, lithium can be inserted as well as extracted in the acetyleneblack-containing cell. Nevertheless, the initial lithiation capacity obtained is considerably higher than the subsequent cycle capacity. As discussed before, the cell capacity in the acetylene-black-containing cell is partially contributed by acetylene black. From the half-cells using a cathode consisting of 80 wt.% acetylene black and 20 wt.% PVDF, we obtained an initial intercalation capacity ranging from 445 to 676 mA hrg, and a reversible capacity 160 mA hrg in subsequent cycles for acetylene black in the voltage window between 2.5 and 0 V. After subtracting the contribution from acetylene black, we determined the cell capacity and its converted equivalent number of insertedrextracted
lithium per formula of La 0.55 Li 0.35TiO 3 . Fig. 7 shows these results, together with the cycling results of the acetylene-black-free cell. It can be seen that about 0.48 Li may be inserted into one formula of La 0.55 Li 0.35TiO 3 in the initial lithiation half-cycle in the acetylene-black-free cell. For the acetyleneblack-containing cell, about 0.75–0.9 Li can be initially inserted into LLTO; however, only 0.48 Li can be reversibly inserted into and extracted out of one formula of La 0.55 Li 0.35TiO 3 in subsequent cycles. These values Ž0.75–0.9 and 0.48. are well above the number of available vacant perovskite A-sites Ž0.1. in La 0.55 Li 0.35TiO 3 . Other researchers w6,7x did not observe this effect, partly because they did not extend the voltage window to 0 V. We propose a possible phase transition mechanism to explain this phenomenon. In the initial lithiation half-cycle, the perovskite structure ŽABO 3 . might be transformed to A 2 BO 3 monoclinic phase after filling all the vacant perovskite A-sites. This A 2 BO 3 phase could be similar to rechargeable Li 2y x RuO 3 w18,19x. A point of inflection at potential of about 0.68 V is noticed on the lithiation curve of the LLTO laminate in Fig. 5 and indicated by an arrow. This point of inflection could be caused by the proposed phase transition at the intercalation of about 0.24 Li. Further compositional and structural analyses are needed to confirm the occurrence of this phase transformation and to explain why it occurs at the intercalation of 0.24 Li
Fig. 7. Capacity and corresponding number of Li ions inserted into or extracted out of LLTO: charge Žtriangles. and discharge Žcrosses. of the acetylene-black-free cell, and charge Žsquares. and discharge Ždiamonds. of the acetylene-black-containing cell.
C.H. Chen, K. Aminer Solid State Ionics 144 (2001) 51–57
Žnot 0.1 Li as predicted by La 0.55 Li 0.35TiO 3 composition..
4. Conclusions Pellets of La 0.55 Li 0.35TiO 3 and La 0.57 Li 0.29TiO 3 sintered from four different powders were studied with AC impedance spectroscopy. Bulk conductivity was found to be closely related to the calcination temperature of the powders. Pellets from 1100 8Ccalcined powders had higher bulk conductivity than those from 1200 8C-calcined powders. The grainboundary conductivity was mainly determined by the sample composition. The activation energies were 0.14–0.18 eV for bulk conduction and 0.41–0.43 eV for grain-boundary conduction. Lithium was intercalated into LLTO below about 1.8 V vs. Li. With addition of acetylene black, about 0.48 Li was reversibly inserted into and extracted out of La 0.55 Li 0.35TiO 3 . A phase transition is proposed to take place during the first lithium insertion step. Because of the lithium insertion into LLTO at the potential below 1.8 V vs. Li, and thus the introduction of electronic conductivity, LLTO is obviously not a suitable electrolyte material in lithium ion batteries that use either metallic lithium or lithiated carbon as anode material. It can probably be used as electrolyte materials when some high potential oxide materials, for instance, Li 4Ti 5 O12 are used as anode.
Acknowledgements This study was supported by the Laboratory Director R & D ŽLDRD. program of Argonne National Laboratory.
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References w1x P. Fragnaud, D.M. Schleich, Sens. Actuators, A 51 Ž1995. 21. w2x P. Birke, W.F. Chu, W. Weppner, Solid State Ionics 93 Ž1997. 1. w3x P. Birke, W. Weppner, Electrochim. Acta 42 Ž1997. 3375. w4x C.H. Chen, Thin-film components for lithium-ion batteries, PhD thesis, Delft University of Technology, 1998. w5x Y. Inaguma, L. Chen, M. Itoh, T. Makamura, T. Uchida, M. Ikuta, M. Wakihara, Solid State Commun. 86 Ž1993. 689. w6x Y. Inaguma, L. Chen, M. Itoh, T. Nakamura, Solid State Ionics 70r71 Ž1994. 196. w7x M. Itoh, Y. Inaguma, W.H. Jung, L. Chen, T. Nakamura, Solid State Ionics 70r71 Ž1994. 203. w8x Y. Harada, T. Ishigaki, H. Kawai, J. Kuwano, Solid State Ionics 108 Ž1998. 407. w9x H. Kawai, J. Kuwano, in: P. Vincenzini ŽEd.., Advances in Science and Technology 3D, Ceramics: Charting the Future. Techna, 1995, p. 2641. w10x O. Bohnke, C. Bohnke, J.L. Fourquet, Solid State Ionics 91 Ž1996. 21. w11x P. Birke, S. Scharner, R.A. Huggins, W. Weppner, J. Electrochem. Soc. 144 Ž1997. L167. w12x D.M. Scheich, T. Brousse, P. Fragnaud, R. Marchand, Abs. No. 609, Meeting of Electrochemical Society, San Antonio, TX. 1996, p. 745. w13x T. Brousse, R. Marchand, P. Fragnaud, D.M. Schleich, Abs. No. 841, Meeting of Electrochemical Society, San Antonio, TX. 1996, p. 1037. w14x J.R. Macdonald, Impedance Spectroscopy. Wiley-Interscience, New York, 1987. w15x Y. Zou, A. Petric, J. Phys. Chem. Solids 55 Ž1994. 493. w16x E. Antolini, J. Mater. Chem. 8 Ž1998. 2783. w17x L.D. Noailles, C.S. Johnson, J.T. Vaughey, M.M. Thackeray, J. Power Sources 81–82 Ž1999. 259. w18x H. Kobayashi, R. Kanno, Y. Kawamoto, M. Tabuchi, O. Nakamura, M. Takano, Solid State Ionics 82 Ž1995. 25. w19x H. Kobayashi, R. Kanno, M. Tabuchi, H. Kageyama, O. Nakamura, M. Takano, J. Power Sources 68 Ž1997. 686.