Non-aqueous electrolyte secondary battery

Abstract: Disclosed is a negative electrode active material offering a long life non-aqueous electrolyte secondary battery with high energy density that shows excellent cycle life characteristics. The negative electrode active material comprises a compound represented by the formula Li κ Z ε X γ wherein Z represents at least two elements selected from the group consisting of metals and semi-metals at least one of which is selected from group D consisting of Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Ag, Zn, Cd and Pd, and X represents at least one element selected from the group consisting of O, S, Se and Te and wherein 0<κ+γ+ε≤25 , 0≤κ<10, 0<ε<10 and 0<γ≤8.

Description: BACKGROUND OF THE INVENTION The present invention relates to a non-aqueous electrolyte secondary battery, particularly an improvement of a negative electrode used therefor. There have been various vigorous studies on a non-aqueous electrolyte secondary battery including lithium or a lithium compound as a negative electrode, because it is to be expected to offer a high voltage as well as a high energy density. To date, oxides and chalcogens of transition metals like LiMn 2 O 4 , LiCoO 2 , LiNiO 2 , V 2 O 5 , Cr 2 O 5 , MnO 2 , TiS 2 , MoS 2 and the like are known positive electrode active materials for non-aqueous electrolyte secondary batteries. Those compounds have a layered or tunneled crystal structure that allows free intercalation and deintercalation of lithium ions. On the other hand, there are many previous studies on metallic lithium as the negative electrode active material. However, metallic lithium has a drawback that when used as the negative electrode active material, a deposition of lithium dendrites occurs on the surface of lithium during charging, which reduces charge/discharge efficiency or causes internal short-circuiting due to contact between formed lithium dendrites and the positive electrode. As one measure for solving such drawback, the use of a lithium alloy such as lithium-aluminum alloy which not only suppresses the growth of lithium dendrites but also can absorb therein and desorb therefrom lithium as the negative electrode has been under investigation. However, the use of such lithium alloy has a drawback that repeated charge/discharge operation causes pulverization of the alloy as the electrode, which in turn deteriorates the cycle life characteristics of a battery. Therefore, there are proposals to suppress pulverization of the electrode by using a lithium-aluminum alloy including additional elements as electrode (e.g., Japanese Laid-Open Patent Publications Sho 62-119856 and Hei 4-109562). Under the circumstance, however, the improvement remains unsatisfactory. At present, lithium ion batteries have been put into practical use that include as the negative electrode a highly safe carbon material capable of reversibly absorbing and desorbing lithium and having exceptional cycle life characteristics although smaller in capacity than the above-mentioned negative electrode active materials. In an attempt to realize a higher capacity, various proposals have been made of an application of oxides to negative electrodes. For example, it is suggested in Japanese Laid-Open Patent Publications Hei 7-122274 and Hei 7-235293 that crystalline oxides such as SnO and SnO 2 may serve as negative electrode active materials with higher capacities than the conventional oxide WO 2 . There is another proposal in Japanese Laid-Open Patent Publication Hei 7-288123 to use non-crystalline oxides such as SnSiO 3 or SnSi 1-x P x O 3 for the negative electrode in order to improve the cycle life characteristics. But, the improvement is still unsatisfactory. BRIEF SUMMARY OF THE INVENTION The primary object of the present invention is to provide a negative electrode for non-aqueous electrolyte secondary batteries having excellent charge/discharge cycle life characteristics. Another object of the present invention is to provide a negative electrode affording a high electric capacity and an exceptional cycle life by absorbing lithium upon charging without growing lithium dendrites The present invention provides a non-aqueous electrolyte secondary battery comprising a chargeable and dischargeable positive electrode, a non-aqueous electrolyte and a chargeable and dischargeable negative electrode, wherein the negative electrode comprises a compound represented by the formula as claimed in claim 1. Further, preferred embodiements are described in dependent claims 2-5. BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING FIG. 1 is a brief longitudinal cross-sectional view of a test cell for evaluating the characteristics of an electrode of an active material in accordance with the present invention. FIG. 2 is a longitudinal cross-sectional view of a cylindrical battery used for embodying the present invention. FIG. 3 is an X-ray diffraction pattern obtained from a negative electrode active material of a test cell upon cathode polarization at the 10th cycle. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, a highly reliable non-aqueous electrolyte secondary battery with a high energy density as well as an exceptional cycle life which is free from development of dendrite-induced short-circuiting. In the following, the present invention will be described referring to specific examples, although the present invention is not limited thereto. Example 1 (* according to the invention) In the present example, compounds or oxides represented by the formula (1) and whose X is oxygen were evaluated. First, for evaluation of the electrode characteristics of those oxides as negative electrode active materials, test cells as shown in FIG. 1 were fabricated. A mixture was prepared by mixing 3 g of a graphite powder as a conductive agent and 1 g of a polyethylene powder as a binder with 6 g of each active material powder. Then, 0.1 g of the mixture was pressure-molded to a disc of 17.5 mm in diameter. An electrode 1 thus formed was placed in the center of a case 2 and disposed thereon with a separator 3 of a microporous polypropylene film. A non-aqueous electrolyte prepared by dissolving 1 mol/l lithium perchlorate (LiClO 4 ) in a mixed solvent of ethylene carbonate and dimethoxyethane in a volumetric ratio of 1:1 was injected over the separator 3. Next, the case 2 was combined with a sealing plate 6 having a polypropylene gasket 5 on the periphery thereof and attached with a metallic lithium disc 4 of 17.5 mm in diameter to the inner surface thereof, then was sealed to complete a test cell. Each of the test cells thus produced was subjected to cathode polarization (corresponding to charging when the active material electrode is taken as the negative electrode) at a constant current of 2 mA until the electrode potential became 0 V vs. lithium counter electrode. Then, the test cell was subjected to anode polarization (corresponding to discharging) until the electrode potential dropped to 1.5 V vs. lithium counter electrode. Cathode and anode polarizations were repeated and the electrode characteristics were evaluated in all the test cells. For comparison, conventional oxides and sulfides of metals listed in Table 1 were used. The present example used those oxides listed in Table 2. The discharge capacities per gram of the active material at the 1st cycle in the test cells are summarized in Tables 1 and 2, respectively. All of the test cells including oxides of Example 1 in accordance with the present invention were found chargeable and dischargeable. Upon completion of cathode polarization at the 10th cycle, the test cells were disassembled and found to have no deposits of metallic lithium. The above results indicated that the electrodes including the active materials in accordance with the present invention absorb therein lithium upon cathode polarization and desorb therefrom absorbed lithium upon anode polarization without growing dendrites of metallic lithium. Next, for evaluation of the cycle life characteristics of the batteries applied with the negative electrodes of the active materials in accordance with the present invention, cylindrical batteries as shown in FIG. 2 were produced. The batteries were produced as follows: First, a positive electrode active material LiMn 1.8 Co 0.2 O 4 was synthesized by mixing Li 2 CO 3 , Mn 3 O 4 and CoCO 3 in a predetermined molar ratio, followed by heating at 900 °C. The resultant was further filtered through 100 mesh or less before used as the positive electrode active material of Example 1. Then, to 100 g of the positive electrode active material, 10 g of a carbon powder as a conductive agent, 8 g (in solids) of an aqueous dispersion of polytetrafluoroethylene as a binder and pure water were added to form a paste. The paste was applied on a titanium core material, dried and rolled. In this way, a positive electrode plate was produced. Separately, a negative electrode plate was prepared as follows: Each of various active materials, a graphite powder as a conductive agent and polytetrafluoroethylene as a binder were mixed in a weight ratio of 60:30:10 and the mixture was made into a paste using a petroleum solvent. The paste was applied on a copper core material, followed by drying at 100 °C to form a negative electrode plate. A porous polypropylene film was used as the separator. Then, a positive electrode plate 11 having a spot-welded positive electrode lead 14 made of the same material as that of the core material was combined with a negative electrode plate 12 similarly having a spot-welded negative electrode lead 15 made of the same material as that of the core material together with a band-like separator 13 of a porous polypropylene film interposed therebetween and the combination was spirally rolled up to make an electrode group. The electrode group was placed in a battery case 18 after adhering polypropylene insulating plates 16 and 17 to the top and the bottom of the electrode group. A step was formed at the upper part of the battery case 18 and a non-aqueous electrolyte prepared by dissolving 1 mol/l lithium perchlorate in a mixed solvent of ethylene carbonate and dimethoxyethane in an equivolumetric ratio was injected into the battery case 18. Then, the case was sealed using a sealing plate 19 provided with a positive terminal 20 to form a battery. Each of the batteries thus formed was subjected to a charge/discharge cycle test under conditions of a temperature of 30 °C, a charge/discharge current of 1 mA/cm 2 and a charge/discharge voltage in a range of 4.3 to 2.6 V. Tables 1 and 2 summarize the discharge capacity maintenance ratios after 100 cycles in the batteries including negative electrodes of the oxides of the comparative example and Example 1 according to the invention (*), using their discharge capacities at the 2nd cycle as preference. Oxide or sulfide Capacity (mAh/g) Capacity maintenance rate (%) WO 2 190 9 Fe 2 O 3 185 10 SnO 522 5 SnSiO 3 453 20 PbO 453 2 SnS 498 6 PbS 436 3 SnSi 0.8 P 0.2 O 3.1 406 25 Oxides Capacity (mAh/g) Capacity maintenance rate (%) Al 2 BaO 4 390 85 BaSnO 3 400 95 BaSiO 3 400 85 BaPbO 3 580 90 BaCdO 2 390 80 BaBi 2 O 6 410 80 BaIn 2 O 4 530 85 BaZnO 2 400 85 BaGa 2 O 4 400 90 Ba 2 GeO 4 500 90 Ba 0.5 Sr 0.5 SnO 3 600 95 (*) Ba 0.7 Sr 0.3 SnO 3 620 95 (*) Ba 0.9 Sr 0.1 SnO 3 630 95 Ba 0.5 Ca 0.5 SnO 3 600 90 Ba 0.5 Mg 0.5 SnO 3 580 90 Ba 0.5 Sr 0.5 SiO 3 500 90 Ba 0.5 Sr 0.5 PbO 3 620 90 Al 2 Na 2 O 4 420 90 Na 2 SnO 3 400 90 Na 2 SiO 3 400 85 Na 2 PbO 3 600 80 Na 2 CdO 2 400 85 Na 2 Bi 2 O 6 380 80 Na 2 In 2 O 4 550 85 Na 2 ZnO 2 400 85 Na 2 Ga 2 O 4 400 90 Na 4 GeO 4 500 85 Al 2 K 2 O 4 430 90 K 2 SnO 3 450 90 K 2 SiO 3 420 90 As is evident from the tables, the batteries using the negative electrodes of the oxide active materials in accordance with the present invention are improved drastically in the cycle life characteristics as compared with those using the negative electrodes of the conventional oxides. Next, the factor contributing to the improved cycle life characteristics of the above-mentioned active materials of the present invention was analyzed. FIG. 3 shows an X-ray diffraction pattern obtained from the test cell using MgSnO 2 as the negative electrode active material upon completion of cathode polarization (charged state of the negative electrode active material) at the 10th cycle. The figure also lists the result in the test cell using the comparative example active material SnO 2 . Noting the peak around 2=38 °, a sharp peak clearly indicating the presence of an Li-Sn alloy was observed in the comparative example oxide. On the other hand, a very broad peak with a low peak intensity was observed in the oxide of Example 1. The above findings suggested that the charge/discharge reaction in the comparative example oxide SnO 2 develops basically by the alloying reaction between Sn and Li. In the oxide MgSnO 3 of Example 1, although it was speculated that this oxide also experiences the same reaction, it shows a broad peak with very low peak intensity on the X-ray diffraction pattern as compared with the comparative example. This suggests very low crystallinity of the Li-Sn alloy synthesized during charge of MgSnO 3 as compared with the comparative example oxide SnO 2 . Although the details remain to be clarified more, the low crystallinity was considered to result from the prevention by the presence of the group D element Mg of a reduction of the reactive surface area or inactivation due to an aggregation of the group A element Sn. This seems to have led to improved cycle life characteristics. Although only MgSnO 3 was exemplified above, the same observations were obtained from the rest of the active materials. Example 2 (* according to the invention) In the present example, test cells were produced in the same manner as in Example 1 for evaluation of the electrode characteristics of the negative electrode active materials of various sulfides listed in Table 3. The evaluation was performed under the same conditions as applied in Example 1. The results are summarized in Table 3. All the test cells of Example 2 were found chargeable and dischargeable. Upon completion of cathode polarization at the 10th cycle, the test cells were disassembled and found to have no deposits of metallic lithium. From this, it was indicated that the electrodes including the active materials in accordance with the present invention absorb therein lithium upon cathode polarization and desorb therefrom absorbed lithium upon anode polarization without growing dendrites of metallic lithium. Next, for evaluation of the cycle life characteristics of the batteries applied with the negative electrodes of the various sulfides in accordance with the present invention, cylindrical batteries as used in Example 1 were produced and evaluated under the same conditions as in Example 1. The results according to the invention (*) are summarized in Table 3. Sulfides Capacity (mAh/g) Capacity maintenance rate (%) Al 2 BaS 4 370 85 BaSnS 3 410 95 BaSiS 3 440 85 BaPbS 3 450 90 BaCdS 2 390 80 BaBi 2 S 6 400 80 BaIn 2 S 4 490 85 BaZnS 2 400 85 BaGa 2 S 4 400 90 Ba 2 GeS 4 500 90 Ba 0.5 Sr 0.5 SnS 3 620 95 (*) Ba 0.1 Sr 0.3 SnS 3 630 95 (*) Ba 0.9 Sr 0.1 SnS 3 630 95 Ba 0.5 Ca 0.5 SnS 3 580 90 Ba 0.5 Mg 0.5 SnS 3 570 90 Ba 0.5 Sr 0.5 SiS 3 500 90 Ba 0.5 Sr 0.5 PbS 3 620 90 Al 2 Na 2 S 4 390 90 Na 2 SnS 3 470 90 Na 2 SiS 3 420 85 Na 2 PbS 3 580 80 Na 2 CdS 2 400 85 Na 2 Bi 2 S 6 380 80 Na 2 In 2 S 4 520 85 Na 2 ZnS 2 370 85 Na 2 Ga 2 S 4 400 90 Na 4 GeS 4 500 85 Al 2 K 2 S 4 400 90 K 2 SnS 3 450 90 K 2 SiS 3 420 90 As is clear from Table 3, the batteries using the various sulfides in accordance with the present invention as the negative electrode active materials are improved in the cycle life characteristics as compared with those using the conventional negative electrode active materials. Example 3 (* according to the invention) In Example 3, test cells were produced in the same manner as in Example 1 for evaluation of the electrode characteristics of the negative electrode active materials of various selenates listed in Table 4. The evaluation was performed under the same conditions as applied in Example 1. All the test cells of Example 3 were found chargeable and dischargeable. Upon completion of cathode polarization at the 10th cycle, the test cells were disassembled and found to have no deposits of metallic lithium. From this, it was indicated that the electrodes including the active materials in accordance with the present invention absorb therein lithium upon cathode polarization and desorb therefrom absorbed lithium upon anode polarization without growing dendrites of metallic lithium. Next, for evaluation of the cycle life characteristics of the batteries applied with the negative electrode active materials of various selenates in accordance with the present invention (*), cylindrical batteries as used in Example 1 were produced and evaluated under the same conditions as in Example 1. The results are summarized in Table 4. Selenides Capacity (mAh/g) Capacity maintenance rate (%) Al 2 BaSe 4 390 85 BaSnSe 3 400 95 BaSiSe 3 400 85 BaPbSe 3 580 90 BaCdSe 2 390 80 BaBi 2 Se 6 410 80 BaIn 2 Se 4 530 85 BaZnSe 2 400 85 BaGa 2 Se 4 400 90 Ba 2 GeSe 4 500 90 Ba 0.5 Sr 0.5 SnSe 3 600 95 (*) Ba 0.7 Sr 0.3 SnSe 3 620 95 (*) Ba 0.9 Sr 0.1 SnSe 3 630 95 Ba 0.5 Ca 0.5 SnSe 3 600 90 Ba 0.5 Mg 0.5 SnSe 3 580 90 Ba 0.5 Sr 0.5 SiSe 3 500 90 Ba 0.5 Sr 0.5 PbSe 3 620 90 Al 2 Na 2 Se 4 420 90 Na 2 SnSe 3 400 90 Na 2 SiSe 3 4 00 85 Na 2 PbSe 3 600 80 Na 2 CdSe 2 400 85 Na 2 Bi 2 Se 6 380 80 Na 2 In 2 Se 4 550 85 Na 2 ZnSe 2 400 85 Na 2 Ga 2 Se 4 400 90 Na 4 GeSe 4 500 85 Al 2 K 2 Se 4 430 90 K 2 SnSe 3 450 90 K 2 SiSe 3 420 90 As is evident from Table 4, the batteries using the various selenates in accordance with the present invention as the negative electrode active materials are improved in the cycle life characteristics as compared with those using the conventional negative electrode active materials. Example 4 (* according to the invention) In this example, test cells were produced in the same manner as in Example 1 for evaluation of the electrode characteristics of the negative electrode active materials of various tellurides listed in Table 5. The evaluation was performed under the same conditions as applied in Example 1. All the test cells of Example 4 were found chargeable and dischargeable. Upon completion of cathode polarization at the 10th cycle, the test cells were disassembled and found to have no deposits of metallic lithium. From this, it was indicated that the electrodes including the active materials in accordance with the present invention absorb therein lithium upon cathode polarization and desorb therefrom absorbed lithium upon anode polarization without growing dendrites of metallic lithium. Next, for evaluation of the cycle life characteristics of the batteries applied with the negative electrodes of various tellurides in accordance with the present invention (*), cylindrical batteries as used in Example 1 were produced and evaluated under the same conditions as in Example 1. The results are summarized in Table 5. Tellurides Capacity (mAh/g) Capacity maintenance rate (%) Al 2 BaTe 4 390 85 BaSnTe 3 400 95 BaSiTe 3 400 85 BaPbTe 3 580 90 BaCdTe 2 390 80 BaBi 2 Te 6 410 80 BaIn 2 Te 4 530 85 BaZnTe 2 400 85 BaGa 2 Te 4 500 90 Ba 2 GeTe 4 400 90 Ba 0.5 Sr 0.5 SnTe 3 600 95 Ba 0.7 Sr 0.3 SnTe 3 620 95 Ba 0.9 Sr 0.1 SnTe 3 630 95 Ba 0.5 Ca 0.5 SnTe 3 600 90 Ba 0.5 Mg 0.5 SnTe 3 580 90 Ba 0.5 Sr 0.5 SiTe 3 500 90 Ba 0.5 Sr 0.5 PbTe 3 620 90 Al 2 Na 2 Te 4 420 90 Na 2 SnTe 3 400 90 Na 2 SiTe 3 400 85 Na 2 PbTe 3 600 80 Na 2 CdTe 2 400 85 Na 2 Bi2Te 6 380 80 Na 2 In 2 Te 4 550 85 Na 2 ZnTe 2 400 85 Na 2 Ga 2 Te 4 400 90 Na 4 GeTe 4 500 85 Al 2 K 2 Te 4 430 90 K 2 SnTe 3 450 90 K 2 SiTe 3 420 90 As shown in Table 5, the batteries using the various tellurides in accordance with the present invention as the negative electrode active materials are improved in the cycle life characteristics as compared with those using the conventional negative electrode active materials. Example 5

Claims: A non-aqueous electrolyte secondary battery comprising a chargeable and dischargeable positive electrode, a non-aqueous electrolyte and a chargeable and dischargeable negative electrode, said negative electrode comprising a compound represented by the formula Ba 1-j Sr j Sn X 3 wherein X represents one element selected from the group consisting of O, S, Se and Te and wherein 0.1 ≤j ≤0.5. The non-aqueous electrolyte secondary battery in accordance with claim 1 wherein said negative electrode compound is represented by the formula Ba 1-j Sr j SnO 3 wherein 0.1 ≤j ≤0.5. The non-aqueous electrolyte secondary battery in accordance with claim 1 wherein said negative electrode compound is represented by the formula Ba 1-j Sr j SnS 3 wherein 0.1 ≤j ≤0.5. The non-aqueous electrolyte secondary battery in accordance with claim 1 wherein said negative electrode compound is represented by the formula Ba 1-j Sr j SnSe 3 wherein 0.1 ≤j ≤0.5. The non-aqueous electrolyte secondary battery in accordance with claim 1 wherein said negative electrode compound is represented by the formula Ba 1-j Sr j SnTe 3 wherein 0.1 ≤j ≤0.5.

Inventors: BITO YASUHIKO, ITO SHUJI, MATSUDA HIROMU, MURATA TOSHIHIDE, SATO TOSHITADA, TOYOGUCHI YOSHINORI, BITO, YASUHIKO, ITO, SHUJI, MATSUDA, HIROMU, MURATA, TOSHIHIDE, SATO, TOSHITADA, TOYOGUCHI, YOSHINORI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H01M4
IPC codes: H01M10, H01M4