Fabrication of High-Voltage, High-Capacity All-Solid-State Lithium

Chem. Mater. 2005, 17, 2041-2045

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Fabrication of High-Voltage, High-Capacity All-Solid-State Lithium Polymer Secondary Batteries by Application of the Polymer Electrolyte/Inorganic Electrolyte Composite Concept Shiro Seki,* Yo Kobayashi, Hajime Miyashiro, Yuichi Mita, and Toru Iwahori Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japan ReceiVed December 10, 2004. ReVised Manuscript ReceiVed February 14, 2005

For the purpose of oxidation decomposition control of the polyether-based solid polymer electrolyte (SPE) in the high-voltage state, lithium phosphate (Li3PO4)-coated cathode (LiCoO2) powder was prepared via a mechanical coating technique. An all-solid-state lithium polymer secondary battery that used Li3PO4-coated LiCoO2 was prepared. By the oxidation suppression enabled by the Li3PO4 that was intervened between SPE and LiCoO2, charging up to a high voltage (4.6 V vs Li/Li+) was made possible and the discharge capacity was observed to be over 200 mAh g-1. Detailed electrochemical analysis was enabled by a combination of constant current-constant voltage charging measurements and electrochemical impedance spectroscopy (EIS). As a result, it became clear that the oxidation decomposition that takes place at the SPE/cathode interface can be controlled by a coating of Li3PO4 on LiCoO2.

1. Introduction Research and development of all-solid-state lithium batteries has been carried out with the goal of realizing an electric power load leveling system for customer use.1 In particular, because all-solid-state lithium batteries do not contain organic electrolyte solution, their safety will be markedly improved. Currently, both inorganic materials and organic materials are being studied as a solid electrolyte for the all-solid-state lithium batteries. With respect to the former, Thio-LISICON,2 Li2S-P2S5 glass,3 and LiPON4 are mentioned as typical examples. In particular, ref 2 indicates the high ionic conductivity for Thio-LISICON of 2.2 × 10-3 S cm-1 at 298 K. However, inorganic electrolytes do not accept the volume change of electrodes and have poor processing workability and formability, and thus they are unsuitable for enlargement of battery systems. On the other hand, as for the latter, solid polymer electrolytes (SPEs) are mentioned as a representative example. Conventional SPEs used in the all-solid-state lithium polymer secondary batteries (LPBs) are poly(ethylene)oxide (PEO) and its derivatives.5-12 Many polyether-based polymer electrolytes have a low glass transition temperature (170* To whom correspondence should be addressed. Tel.: +81-3-3480-2111. Fax: +81-3-3480-3401. E-mail: [email protected].

(1) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359. (2) Kanno, R.; Murayama, M. J. Electrochem. Soc. 2001, 148, 742. (3) Hayashi, A.; Hama, S.; Minami, T.; Tatsumisago, M. Electrochem. Commun. 2003, 5, 111. (4) Bates, J. B.; Gruzalski, G. R.; Dudney, N. J.; Luck, C. F.; Yu, X. Solid State Ionics 1994, 70-71, 619. (5) Nishimoto, A.; Agehara, K.; Furuya, N.; Watanabe, T.; Watanabe, M. Macromolecules 1999, 32, 1541. (6) Noda, K.; Yasuda, T.; Nishi, Y. Electrochim. Acta 2004, 50, 243. (7) Spindler, R.; Shriver, D. F. J. Am. Chem. Soc. 1988, 110, 3036. (8) Rawsky, G. C.; Fujinami, T.; Shriver, D. F. Chem. Mater. 1994, 6, 2208. (9) Blonsky, P. M.; Shriver, D. F.; Austin, P.; Allcock, H. R. J. Am. Chem. Soc. 1984, 106, 6854. (10) Allcock, H. R.; Kuharcik, S. E.; Reed, C. S.; Napierala, M. E. Macromolecules 1996, 29, 3384.

230 K). Moreover, SPEs are also flexible at room temperature and can produce a thin film. However, the oxidation decomposition of the SPE takes place at over 4 V vs Li/ Li+.13 Therefore, the most commonly reported cathode materials for the LPB have been 3V-class ones (e.g., V2O514-16 and LiFePO417,18), and the research and development (laboratory scale) of 4V-class (e.g., LiCoO219 and LiMn2O420) LPB systems is now in progress. We have so far proposed the concept of a “polymer electrolyte/inorganic electrolyte composite” as a guiding principle in the design of a battery with the capacity for high voltage operation. By placing an oxidation-resistant inorganic electrolyte such that it intervenes between the cathode active material and the SPE, we hope to achieve battery operation in a voltage region higher than the potential window of SPEs.21 To date, we have conducted experiments using thin film cathodes, to examine the principle described above.22 However, when the utilization of LPBs is considered, it is essential to examine the powder cathode sheet of the cathode (11) Allcock, H. R.; Laredo, W. R.; Kellam, E. C. V.; Morford, R. V. Macromolecules 2001, 34, 787. (12) Allcock, H. R.; Prange, R.; Hartle, T. J. Macromolecules 2001, 34, 5463. (13) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Yamanaka, A.; Mita, Y.; Iwahori, T. J. Power Sources, in press. (14) Prosini, P. P.; Capiglia, C.; Saito, Y.; Fujieda, T.; Vellone, R.; Shikano, M.; Sakai, T. J. Power Sources 2001, 99, 1. (15) Andre, P.; Deniard, P.; Brec, R.; Lascaud, S. J. Power Sources 2002, 105, 66. (16) Zaghib, K.; Gauthier, M.; Armand, M. J. Power Sources 2003, 119121, 76. (17) Appetecchi, G. B.; Hassoun, J.; Scrosati, B.; Croce, F.; Cassel, F.; Salomon, M. J. Power Sources 2003, 124, 246. (18) Wittingham, M. S. Chem. ReV. 2004, 104, 4271. (19) Matoba, S.; Matsui, S.; Tabuchi, M.; Sakai, T. J. Power Sources 2004, 137, 284. (20) Prosini, P. P.; Mancini, R.; Petrucci, L.; Contini, V.; Villano, P. Solid State Ionics 2001, 144, 185. (21) Kobayashi, Y.; Miyashiro, H.; Takeuchi, T.; Shigemura, H.; Balakrishnan, N.; Tabuchi, M.; Kageyama, H.; Iwahori, T. Solid State Ionics 2002, 152-153, 137.

10.1021/cm047846c CCC: $30.25 © 2005 American Chemical Society Published on Web 03/19/2005

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active material, the SPE binder, and the electronically conductive additive.23,24 In this research, the realization of a polymer-inorganic composite electrolyte was achieved by coating lithium phosphate (Li3PO4) onto the cathode powder (LiCoO2) surface. Direct solid (LiCoO2)-solid (Li3PO4) coating was proposed as the method for coating the Li3PO4 onto the LiCoO2 surface. By using the Li3PO4-coated LiCoO2 cathode powder, high-voltage and high-capacity LPBs were demonstrated. 2. Experimental Section 2.1. Preparation of Li3PO4-Coated LiCoO2 Cathode Powder. The Li3PO4-coated LiCoO2 powder was prepared using the mechanical powder-powder coating method.25 Stoichiometric LiCoO2 powder and Li3PO4 powder were filled into the hybridization system (NHS-0, Nara Machinery Co., Ltd.). The hybridization of particles was performed at a high rotation speed of 9700 rpm for 3 min. The surface morphologies of noncoated LiCoO2 and Li3PO4-coated LiCoO2 were observed by means of SEM measurement (E-SEM 2700L, Nikon) and BET surface area measurement (Micromeritics FlowSorb II 2000, Shimadzu). The crystalline structure or, to be more precise, the presence of impurities, the variation of the lattice parameters, and the mass fraction of the Li3PO4-coated LiCoO2 were measured by means of XRD analysis (Mac Science) and were determined using RIETAN-2000. 2.2. Preparations of Powder Cathode Sheet and All-SolidState Lithium Polymer Batteries (LPBs). The matrix polymer used for the SPE sheet in this research was P(EO/MEEGE/AGE) ) 82/18/1.7 (Daiso Co., Ltd.), which is the copolymer of ethylene oxide, 2-(2-methoxyethoxy) ethyl glycidyl ether, and allyl glycidyl ether.23,24 Lithium (tetrafluoro) borate (LiBF4, Kishida Chemical) was used as the electrolyte salt. The ratio of lithium salt in the polymer was [lithium cation]/[ether oxygen] ) 0.06, and the thickness of the SPE film was approximately 55 µm. The powder cathode sheet consisted of LiCoO2 (noncoated and Li3PO4-coated, 82 wt %) as a cathode active material, acetylene black (5 wt %, Denka) as an electrically conductive additive, and P(EO/MEEGE)lithium bis(pentafluoroethylsufonyl)amide (LiBETI: LiN(SO2CF2CF3)2, Kishida Chemical) complex (13 wt %) as an ionically conductive binder. These constitutive materials were thoroughly agitated together with acetonitrile in a homogenizer. The obtained paste was applied onto an aluminum current collector using an automatic applicator. After the cathode paste was dried, the cathode sheet was compressed to increase the packing density and to improve the electrical conductivity. The thickness of the powder cathode layer was approximately 20 µm. Dried SPE sheet samples for the electrochemical measurements were cut into disks and sandwiched between lithium metal anode foil (Honjo Metal: 0.3 mm thickness) and two types of powder cathode sheets and were encapsulated in 2032-type coin-type cells in a dry-argon-filled glovebox ([O2] < 0.4 ppm, [H2O] < 0.1 ppm, Miwa MFG Co., Ltd.) 2.3. Electrochemical Measurements. Charge-discharge tests of the cells were performed at 3.0-4.4 and 3.0-4.6 V vs Li/Li+ with the current density of 0.05 mA cm-2 (constant current chargeconstant current discharge), in the noncoated LiCoO2 and the (22) Kobayashi, Y.; Miyashiro, H.; Takei, K.; Shigemura, H.; Tabuchi, M.; Kageyama, H.; Iwahori, T. J. Electrochem. Soc. 2003, 150, 1586. (23) Matsui, S.; Muranaga, T.; Higobashi, H.; Inoue, S.; Sakai, T. J. Power Sources 2001, 97-98, 772. (24) Seki, S.; Tabata, S.; Matsui, S.; Watanabe, M. Electrochim. Acta 2004, 50, 379. (25) Home page of Nara Machinery Co., Ltd. (http://www.nara-m.co.jp/).

Figure 1. SEM image of (a) noncoated LiCoO2 and (b) Li3PO4-coated LiCoO2 powder cathode.

Figure 2. Observed, calculated, and difference plots of X-ray diffraction patterns for Li3PO4-coated LiCoO2. Solid line indicates calculated intensities, dots indicate observed intensities, and ∆y is the difference between observed and calculated intensities.

Li3PO4-coated LiCoO2 systems. For analysis of the battery degradation without destruction, the combination measurement with constant current-constant voltage charging (constant currentconstant voltage charge) and the AC impedance measurement were carried out (the constant voltage impedance measurement method).13 The cells were charged to 4.4 V vs Li/Li+. The impedance measurements were then performed at 1 h intervals, while the cell potential was maintained at 4.4 V vs Li/Li+ (200 kHz to 50 mHz; impressed voltage: 10 mV, Princeton Applied Research VMP2/ Z). The time dependences of the impedance spectra, which were obtained by using the fitting program ZSimpWin, were discussed. All measurements were performed at 333 K.

3. Results and Discussion 3.1. Surface Morphology of Noncoated LiCoO2 and Li3PO4-Coated LiCoO2. Figure 1 shows the SEM images of noncoated LiCoO2 (a) and Li3PO4-coated (b) LiCoO2 powder. On the basis of the fact that there were no changes in particle form or size, it is considered that the crash of LiCoO2 particles has not occurred during the coating process. In addition, the noncoated LiCoO2 particles are clearly delineated in the SEM, whereas the outline of the coated particles is ill-defined. This difference in appearance presumably results from the presence of a very thin dispersed cloudlike Li3PO4 coating on the LiCoO2 surface. The Rietveld fitting results for Li3PO4-coated LiCoO2 are shown in Figure 2 and Table 1. The Li3PO4-coated LiCoO2 contained a negligible amount of impurities as a result of the solid-state reaction of hybridization between them. The weight ratio of the Li3PO4 was 5 wt %, which is in agreement with the EDX result (5.1 wt %). The lattice parameter values of the Li3PO4 and the base LiCoO2 in the coated sample were similar to those of the as-prepared one, indicating that there

Fabrication of All-Solid-State Lithium Batteries

Chem. Mater., Vol. 17, No. 8, 2005 2043

Table 1. X-ray Diffraction Rietveld Refinement Results of Li3PO4-Coated LiCoO2 LiCoO2 (R3hm), a ) 0.281505(4) nm, c ) 1.40482(2) nm, RI ) 2.14 atom

position

x

y

z

occupancy

B/Å2

Li Co O

3a 3b 6c

0.0000 0.0000 0.0000

0.0000 0.0000 0.0000

0.0000 0.5000 0.2396(1)

1.0000 1.0000 1.0000

1.2(1) 0.22(2) 0.55(4)

Li3PO4 (Pmn21), a ) 0.6113(2) nm, b ) 0.5234(1) nm, c ) 0.4855(2) nm, RI ) 5.92 atom

position

x

y

z

occupancy

B/Å2

Li1 Li2 P O1 O2 O3

4b 2a 2a 4b 2a 2a

0.23(1) 0.50 0.000 0.215(4) 0.000 0.500

0.33(2) 0.84(2) 0.791(3) 0.695(6) 0.121(9) 0.105(8)

1.0(1) 1.0(1) 0.000 1.08(2) 1.09(3) 0.52(7)

1.0000 1.0000 1.0000 1.0000 1.0000 1.0000

1.0 1.0 0.5 1.0 1.0 1.0

Rwp ) 12.38, Rp ) 8.31, S ) Rwp/Re ) 1.37; Fraction Ratio: LiCoO2/Li3PO4 ) 0.95/0.05 reference LiCoO2 Li3PO4

a ) 0.281536(5) nm a ) 0.6115(1) nm

c ) 1.40513(2) nm b ) 0.52394(11) nm

Rwp ) 9.98 c ) 0.48554(10) nm

RI ) 2.40 (PDF25-1030)

was no change in the structure of Li3PO4 and LiCoO2 during the coating process. 3.2. Charge-Discharge Performance of LPB Used by Noncoated LiCoO2 and Li3PO4-Coated LiCoO2. Figure 3 shows the charge-discharge curves of the LPBs, those for the noncoated and Li3PO4-coated LiCoO2, in the first charge-discharge cycle. Charging to 4.6 V vs Li/Li+ was impossible for the cell using the noncoated LiCoO2 cathode powder. On the other hand, reversible charge-discharging to 4.6 V vs Li/Li+ was made possible by a coating of Li3PO4, and the discharge capacity was approximately 209 mAh g-1. This value is a high value, about 1.4 times the capacity reported conventionally (3.0-4.2 V vs Li/Li+ charge-discharge; approximately 140 mAh g-1). Moreover, the energy density of the Li3PO4-coated LiCoO2 was a very high value (approximately 830 mWh g-1) with an average discharge voltage of 4.04 V vs Li/Li+. It is expected that the stable Li3PO4 interposed between the LiCoO2 and SPE controlled the oxidation decomposition of SPE, and, furthermore, it also functioned as a lithium ionic conductor, without significant degradation. Figure 4 shows the relationship between the cycle number and discharge capacity, and the charge-discharge voltage regions are 4.4-3.0 and 4.63.0 V vs Li/Li+. Regarding the cycle characteristic of the Li3PO4-coated LPB, for which 4.6 V vs Li/Li+ was set as the upper voltage limit, the discharge capacity was reduced by half within approximately 10 charge-discharge cycles.

In the case that 4.6 V vs Li/Li+ is the upper voltage limit, oxidation decomposition at the SPE/LiCoO2 interface cannot be prevented completely. Therefore, the upper cathode potential limit was lowered to 4.4 V vs Li/Li+, and the cycle performance of both LPBs was compared. Although the first discharge capacity of the noncoated LiCoO2 LPB was approximately 170 mAh g-1, the discharge capacity was reduced to less than 100 mAh g-1 in approximately 10 cycles. On the other hand, the first discharge capacity of the Li3PO4-coated LiCoO2 LPB was 174 mAh g-1, and the discharge capacity at 20 cycles was maintained at 134 mAh g-1, which is 76% of the first discharge capacity. In the case of 4.4 V vs Li/Li+ as the upper voltage limit, the clear effect of the Li3PO4 coating could be shown. 3.3. Constant Voltage Impedance Measuring Method. As mentioned above, it has been confirmed that the concept of a “polymer electrolyte/inorganic electrolyte composite” is effective in LPBs in high-voltage charge-discharge operation. To determine the resistance change inside the LPBs, the constant voltage impedance measurement method was employed. This measurement allowed the clear separation of the SPE bulk resistance (Rb), the SPE/lithium anode interfacial resistance (Rlithium), and the SPE/LiCoO2 cathode interfacial resistance (Rcathode) in the LiCoO2 cathode system.13 In this study, because the charge to 4.6 V vs Li/Li+ could not be applied to the noncoated LiCoO2 system, the imped-

Figure 3. Charge-discharge curves of the Li/SPE/LiCoO2 cathode cells, noncoated and Li3PO4-coated, at the first cycle.

Figure 4. Relationships between the cycle number and discharge capacity; the charge-discharge voltage regions are 4.4-3.0 and 4.6-3.0 V vs Li/ Li+, respectively.

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Figure 5. Measured and calculated results of impedance plots for (a) noncoated LiCoO2 LPB, and Li3PO4-coated LiCoO2 LPB at 4.4 V vs Li/ Li+ for 5 h duration, and the assumed equivalent circuit of the LPBs.

Figure 7. Time dependence of SPE bulk resistance (Rb), SPE/lithium anode interfacial resistance (Rlithium), and SPE/LiCoO2 cathode interfacial resistance (Rcathode) using noncoated and Li3PO4-coated LiCoO2, at 4.4 V vs Li/Li+.

Figure 6. Time dependence of the impedance spectrum for the Li/SPE/ LiCoO2 cathode cells, the noncoated LiCoO2 and Li3PO4-coated LiCoO2, at 4.4 V vs Li/Li+.

ance measurements were carried out at 4.4 V vs Li/Li+. Figure 5 shows the impedance plots for the LPBs at 4.4 V vs Li/Li+ for 5 h duration, as an example. Two semicircular arcs were observed in both systems. On the basis of the response frequency (time constant of the various reactions), it is concluded that the impedance components are Rb, Rlithium, and Rcathode from high frequency. Therefore, we assumed the equivalent circuit (series circuit of Rb, (RlithiumQ), and (RcathodeQ); Figure 5), and the fitting of the impedance spectrum was carried out; it is shown in Figure 5. Q is a constant phase element. In the case of the LPB using the

Li3PO4-coated LiCoO2, in addition to a noncoated LiCoO2 system, Rcathode includes the resistances of the Li3PO4/SPE and Li3PO4/LiCoO2 interfaces (except that the coating of Li3PO4 is not perfectly complete), and the Li3PO4 bulk resistance. However, we could not observe further semicircles, and therefore Rcathode was attributed as a single component in the case with a noncoated LiCoO2 system for the present study. Figure 6 shows the voltage applied time dependence of the impedance spectrum for the LPBs, using noncoated LiCoO2 and Li3PO4-coated LiCoO2. Although the ionic conductivity of Li3PO4 was relatively low (σ ) 10-8 S cm-1 at room temperature), the total resistance of the Li3PO4-coated LPB system is smaller than that of the noncoated LPB system in every case. A thin Li3PO4 layer and the increase in surface area due to fine Li3PO4 particle adhesion might control the increase of the resistances (BET surface area: noncoated LiCoO2, 0.39 m2 g-1; Li3PO4-coated LiCoO2, 2.12 m2 g-1). The voltage applied time dependences of Rb, Rlithium, and Rcathode are shown in Figure 7a, b, and c, respectively. In the noncoated LiCoO2 system and the Li3PO4-coated LiCoO2 system, no significant change of the Rb value is apparent from Figure 7a. It has been suggested that the decomposition

Fabrication of All-Solid-State Lithium Batteries

of SPE bulk does not take place even if the SPE is applied at the high voltage, under the dielectric breakdown voltage. In other words, it is clarified that the SPE maintains a high ionic conductivity without significant alteration of the SPE bulk. Figure 7b shows the time dependence of Rlithium, and, again, no large change was apparent, as for Rbulk. On the other hand, Rcathode increases linearly with time, as seen in Figure 7c. Moreover, a significant difference was observed between the gradients of the two straight lines (noncoated LiCoO2: Li3PO4-coated LiCoO2 ) 12:1). The SPE/LiCoO2 interfacial resistance Rcathode of Li3PO4 coated on a LiCoO2 system is stable under the oxidization condition, and it is considered that Li3PO4 plays an important role as the oxidation barrier at the LiCoO2/SPE interface. In this research, although Li3PO4 was used as an inorganic electrolyte layer, further improvement in the characteristics is expected through the adoption of a higher ionic conductive inorganic electrolyte.26 Our group will report on the research and development of higher performance LPBs soon. (26) Kobayashi, Y.; Seki, S.; Tabuchi, M.; Miyashiro, H.; Mita, Y.; Iwahori, T. J. Electrochem. Soc., submitted.

Chem. Mater., Vol. 17, No. 8, 2005 2045

4. Conclusions All-solid-state lithium polymer secondary batteries were fabricated using the Li3PO4-coated LiCoO2 cathode powder, and their cathode/polymer electrolyte interfaces were investigated. The results are summarized as follows: (1) Charge-discharge to 4.6 V vs Li/Li+ was made possible, the discharge capacity was found to be 209 mAh g-1 at the first cycle, and the cycle performance of the cell was also improved by the oxidation suppression achieved by the Li3PO4 interposed between the SPE and LiCoO2. (2) Significant improvement of the SPE/cathode interface was confirmed through the impedance measurements. Upon using the Li3PO4-coated LiCoO2, it became clear that the degradation of the cathode/SPE interface could be controlled to be 1/12 of that of the noncoated LiCoO2 system. Acknowledgment. We acknowledge Daiso Co., Ltd., for supplying us with valuable chemicals (P(EO/MEEGE/AGE), P(EO/MEEGE)) for our research and Mr. Yasutaka Ohno for his technical support of the experiments. CM047846C