VOLUME 17, NUMBER 23
NOVEMBER 15, 2005
© Copyright 2005 by the American Chemical Society
Communications Fabrication of All-Solid-State Lithium Polymer Secondary Batteries Using Al2O3-Coated LiCoO2 Hajime Miyashiro,*,† Yo Kobayashi,† Shiro Seki,† Yuichi Mita,† Akira Usami,† Masanobu Nakayama,‡ and Masataka Wakihara‡ Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japan, and Department of Applied Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed August 2, 2005 ReVised Manuscript ReceiVed September 23, 2005
Lithium secondary batteries are being researched and developed worldwide to realize ubiquitous energy storage devices from mobile phones to electric vehicles.1 For cathode active materials, one of the most important components in such batteries, a high capacity and a high operation voltage are required for a high energy density. However, current commercially available batteries using a layered LiCoO2 cathode material suffer from a relatively small capacity, because only half of the Li ions in LiCoO2 can be extracted reversibly.2 It was previously observed that, once more than half of the lithium ions were removed from a LiCoO2 structure in a high-voltage region (>4.2 V), the cycle performance severely degenerated in a liquid organic system. Recently, Dahn et al. and Cho et al. reported an inorganic oxide (e.g., Al2O3, ZrO2) coating of a cathode with a view
to improve its reversibility at a high cathode potential for lithium secondary batteries using an organic electrolyte solution.3-8 The charge-discharge performance in a highvoltage region can be improved by stabilizing the cathode surface. Moreover, for instance, in the case of using a LiCoO2 cathode, because the number of mobile lithium ions increases with an increase in upper cutoff voltage, high-capacity batteries can be realized. However, to our knowledge, the mechanism of improvement still requires further investigation. For example, Cho et al. claimed that coating oxides serve as a mechanical constraint that prevents the LixCoO2 lattice from a large volume change during a chargedischarge reaction in a high-voltage region. On the other hand, Chen and Dahn suggested that the poor cycle performance of noncoated LiCoO2 is caused by impedance growth due to some side reactions and that an inorganic oxide coating that modifies surface properties of cathode materials suppresses impedance growth. An all-solid-state lithium polymer secondary battery (LPB) that uses a polyether-based solid polymer electrolyte (SPE) as a fundamentally safe battery system has been investigated by many researchers.9-14 As SPEs do not contain a volatile organic electrolyte solution, the fabrication of large-scaled (3) (4) (5) (6) (7) (8)
* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-3-3480-3401. Tel.: +81-3-3480-2111. † Central Research Institute of Electric Power Industry. ‡ Tokyo Institute of Technology.
(1) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359. (2) Wang, H.; Jang, Y.; Huang, B.; Sadoway, D. R.; Chang, Y. J. Electrochem. Soc. 1999, 146, 473.
(9) (10) (11)
Chen, Z.; Dahn, J. R. Electrochem. Solid-State Lett. 2002, 5, A213. Chen, Z.; Dahn, J. R. Electrochem. Solid-State Lett. 2003, 6, A221. Chen, Z.; Dahn, J. R. Electrochim. Acta 2004, 49, 1079. Cho, J.; Kim, G. B.; Lim, H. S.; Kim, C. S.; Yoo, S.-I. Electrochem. Solid-State Lett. 1999, 2, 607. Cho, J.; Kim, Y. J.; Kim, T.-J.; Park, B. Angew. Chem., Int. Ed. 2001, 40, 3367. Miyashiro, H.; Yamanaka, A.; Tabuchi, M.; Seki, S.; Nakayama, M.; Ohno, Y.; Kobayashi, Y.; Mita, Y.; Usami, A.; Wakihara, M. J. Electrochem. Soc., submitted. Matsui, S.; Muranaga, T.; Higobashi, H.; Inoue, S.; Sakai, T. J. Power Sources 2001, 97-98, 772. Noda, K.; Yasuda, T.; Nishi, Y. Electrochim. Acta 2004, 50, 243. Seki, S.; Kobayashi, Y.; Miyashiro, H.; Yamanaka, A.; Mita, Y.; Iwahori, T. J. Power Sources 2005, 146, 741.
10.1021/cm0517115 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/14/2005
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batteries (e.g., electric power storage systems) is expected from the aspect of safety. Such large-scaled batteries require a large cell capacity, because the improved capacity retention by coating is favorable for LPBs. In this research, for maximum use of the above-mentioned advantages, LPBs using the Al2O3-coated LiCoO2 were prepared and evaluated by various electrochemical methods. An Al2O3-coated LiCoO2 powder was prepared by a spray coating method. The details of this method were described elsewhere. An Al2O3 sol solution was sprayed and dried in a fluidized LiCoO2 powder (Powrex MP-01). The amount of Al2O3 used was 3 wt % versus LiCoO2. The obtained powder was annealed at 823 K for 15 h under an O2 flow condition. Surface morphologies of noncoated LiCoO2 and Al2O3-coated LiCoO2 were confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and BET surface area measurement. The cathode sheet used consisted of LiCoO2 (noncoated and Al2O3-coated, 82 wt %), acetylene black (5 wt %), and P(EO/MEEGE)-LiBETI complex (13 wt %) as a lithium ionic conductive binder. The matrix polymer used for the SPE sheet in this research was a P(EO/MEEGE/AGE)LiBF4 complex (Daiso Co., Ltd.), the ratio of lithium salt in the polymer was [Li+]/[O] ) 0.06, and the thickness of the SPE film was 55 µm. For various electrochemical measurements, 2032-type coin cells were prepared using a lithium metal anode. Charge-discharge tests for the prepared LPBs were performed at cutoff voltages of 3.0-4.4 V versus Li/Li+ (V) with a current density of 0.05 mA cm-2. To determine the degradation magnitude of a battery, combination measurements with constant current-constant voltage charging and alternating current impedance measurement were carried out as follows: once LPBs were charged to 4.4 V, the impedance measurements were performed at 1 h intervals, while the cell potential was maintained at 4.4 V (200 kHz-50 mHz; applied voltage, 10 mV).11 The obtained time dependences of the impedance spectra were analyzed using the fitting program ZSimpWin. All electrochemical measurements for LPBs were performed at 333 K. Liquid batteries were also prepared and measured to confirm the basic properties of Al2O3-coated LiCoO2, and the details are shown in Supporting Information. The surface morphologies of noncoated LiCoO2 and Al2O3-coated LiCoO2 were investigated. Figure 1 shows SEM images of noncoated LiCoO2 (a) and Al2O3-coated LiCoO2 (b). As seen in the figure, noncoated LiCoO2 has clear outlines and flat surfaces. On the other hand, plain outlines could not be confirmed for Al2O3-coated LiCoO2. It is considered that the LiCoO2 surface is covered with an Al2O3 layer. Although D50 of Al2O3-coated LiCoO2 (12.8 µm) was determined to be slightly larger than that of noncoated LiCoO2 (10.2 µm) by BET surface area measure(12) Kobayashi, Y.; Seki, S.; Yamanaka, A.; Miyashiro, H.; Mita, Y.; Iwahori, T. J. Power Sources 2005, 146, 719. (13) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Mita, Y.; Iwahori, T. Chem. Mater. 2005, 17, 2041. (14) Kobayashi, Y.; Seki, S.; Tabuchi, M.; Miyashiro, H.; Mita, Y.; Iwahori, T. J. Electrochem. Soc. 2005, 152, A1985.
Communications
Figure 1. SEM images of (a) noncoated LiCoO2 and (b) Al2O3-coated LiCoO2.
Figure 2. TEM cross-sectional images of (a) noncoated LiCoO2 and (b) Al2O3-coated LiCoO2.
ments, recohesion by Al2O3 did not progress and both morphologies were approximately equal. To observe the surface microstructure of the cathode particle in more detail, TEM studies were performed. Figure 2 shows TEM images of noncoated LiCoO2 (a) and Al2O3-coated LiCoO2 (b). Al2O3-coated LiCoO2 had a uniform Al2O3 layer with a thickness of approximately 100 nm. On the other hand, Chen and Dahn and Cho et al. reported the formation of oxidecoated cathode particles by an evaporation-drying process in a coating solution. They also reported a rough coating layer and a coating particle with a relatively large diameter. Spray coating might lead to a uniform Al2O3 coating layer. We also confirmed the possibility of the formation of a solid solution at the Al2O3/LiCoO2 interface by energydispersed X-ray (EDX) analysis. The EDX profile of each point of the Al2O3-coated LiCoO2 cathode is shown in Supporting Information. A small amount of Al was detected on the top surface of LiCoO2 (see b-2 in Figure 2). However, no Al signal was detected in the LiCoO2 bulk (see b-3 in Figure 2). On the other hand, a clear Co signal was confirmed at the Al2O3-coating bulk layer (see b-1 in Figure 2). This suggests the formation of the solid solution of Al and Co in the coated layer. Oh et al. also reported the depth profile of Al2O3-coated LiCoO2 obtained by Auger electron spectroscopy.15 They reported that the depth of the diffusion of Al3+ into LiCoO2 is approximately 15 nm, which is in good agreement with our results. The lattice parameters of Al2O3coated LiCoO2 [a ) 0.281 597(45) nm, c ) 1.405 29(2)] were nearly the same as those of noncoated LiCoO2 (a ) 0.281 536(5) nm, c ) 1.405 13(2)]. On the other hand, no impurity peaks were observed in Al2O3-coated LiCoO2 by XRD analysis. Thus, the Al2O3 layer was amorphous, the formation of a solid solution occurred mainly toward the Al2O3-coated layer, and the bulk properties of LiCoO2 did not change by Al2O3 coating. Next, Figure 3 shows the cycle performances of the LPBs composed of Al2O3-coated LiCoO2 and noncoated LiCoO2.
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Figure 3. Relationships between cycle number and discharge capacity for noncoated LiCoO2 and Al2O3-coated LiCoO2 LPBs.
The discharge capacities in the first cycle were 169 mA h g-1 (noncoated LiCoO2) and 172 mA h g-1 (Al2O3-coated LiCoO2), which are close to the theoretical capacity of LiCoO2 (voltage region: 4.4-3.0 V). However, in the 10th cycle, the noncoated LiCoO2 LPB had a discharge capacity of 115 mA h g-1, and the Al2O3-coated LPB had a discharge capacity of 158 mA h g-1. Although in this research we used a polyether-based polymer electrolyte as the electrolyte layer, it is interesting to note that we improved the cycle performance in a high-voltage region as mentioned above. We examined the “constant voltage impedance measuring method” as a method of determining the degradation magnitude of batteries at a high voltage (>4 V), where the cell maintains the required voltage, and impedance measurements were performed at certain intervals. This method enabled interfacial resistance separation in the LPBs. The LPBs have mainly three types of resistance, namely, SPE bulk resistance (Rb), lithium anode/electrolyte interfacial resistance (Rlithium), and cathode/electrolyte interfacial resistance (Rcathode). The relationships between voltage applied (4.4 V) time and Rb, Rlithium, and Rcathode are indicated in Figure 4 [(a) noncoated LiCoO2 and (b) Al2O3-coated LiCoO2]. Although the increase in Rcathode was predominant in both systems, the Al2O3 coating markedly reduced it. The intervening Al2O3 might have prevented interfacial degradation, resulting in the stability of both the electrolyte and cathode active material. We have reported the coating of Li3PO412,13 and LAGP [Li1.5Al0.5Ge1.5(PO4)3]14 on the LiCoO2 powder thus far and that all the coating materials contain lithium. In this report, the Al2O3 coating that does not contain lithium has been selected. Moreover, a uniform Al2O3 coating was observed by TEM measurement (Figure 2). To realize the oxidation(15) Oh, S.; Lee, J. K.; Byun, D.; Cho, W. I.; Cho, B. W. J. Power Sources 2004, 132, 249.
Figure 4. Applying voltage time dependence of SPE bulk resistance (Rb), SPE/Li anode interfacial resistance (Rlithium), and SPE/cathode interfacial resistance (Rcathode) for all-solid-state LPBs [(a) noncoated LiCoO2 and (b) Al2O3-coated LiCoO2].
reduction reaction (charge-discharge reaction) between the cathode and the electrolyte, the interchange of lithium ions at the LiCoO2/Al2O3/SPE interface is required. Maybe, as a consequence, the Al2O3 coating containing lithium temporarily (for instance, LiAlO2) may assist the oxidationreduction reaction. On the other hand, although a small portion of Co diffuses to the Al2O3 layer, the activity of diffused Co is sufficiently low compared with that of bare LiCoO2. As a result of the contact fraction of highly oxidized Co4+ and O2- with SPE being smaller than that of noncoated LiCoO2, the side reaction at the cathode/SPE interface is controlled. There is a possibility that mechanisms can be investigated by applying a composite technique with the defect coating of Al2O3 (e.g., mechanical coating and simple mixing). Acknowledgment. The authors are grateful to Daiso Co., Ltd., for supplying us with valuable chemicals [P(EO/MEEGE/ AGE), P(EO/MEEGE)] for our research. Supporting Information Available: Relationships between cycle number and relative discharge capacity for noncoated LiCoO2 and Al2O3-coated LiCoO2 liquid batteries (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0517115
