Lithium ion secondary battery

Abstract: The invention provides a lithium ion secondary battery with high capacity and excellent current characteristics. The lithium battery of this invention mainly comprises a positive electrode, a negative electrode and an electrolyte; negative electrode contains ceramics particles such as Al 2 O 3 not responsible for charging and discharging reaction of the battery. The presence of the ceramics particles in the negative electrode leads to a decrease in the internal resistance of the battery because of the enhancement of ion conductivity in the negative electrode, resulting in higher capacity at high rate discharge of the lithium ion secondary battery.

Description: Background of the Invention The present invention relates to a lithium ion secondary battery, and more particularly to its electrode. The lithium battery, in particular, the rechargeable lithium secondary battery is intensively researched and developed recently as a new secondary battery having high voltage and high energy density. In earlier studies, the lithium secondary battery using lithium metal in the negative electrode has been highly expected as the battery of high energy density. However, when lithium metal is used in the negative electrode, dendric lithium formed at the time of charging is grown in the course of charging and discharging of battery, which leads to internal short circuit of battery or abnormal temperature rise of battery. Such safety problems have not been solved yet. To solve these problems, instead of using lithium metal alone, it has been attempted to use an alloy of lithium metal with low melting metal such as aluminum, lead, indium, bismuth or cadmium, as the negative electrode. In these cases, however, as charging and discharging are repeated, pulverized alloy penetrates through the separator to cause internal short circuit, and it is far from practical and the problems are not solved. Recently, to solve the problems, using carbon in the negative electrode, the lithium secondary battery using transition metal compound containing lithium in the positive electrode is coming into the mainstream. In this battery system, since charging and discharging are done by occlusion and release of lithium ions into carbon in the negative electrode, dendrite is not formed by charging. Hence, the battery has an excellent cycle characteristic and is excellent in safety. In the existing lithium secondary battery, as mentioned above, carbon is used as active substance in the negative electrode, and charging and discharging are effected by occlusion and release of lithium ions into carbon. When powder is used as active substance, required conditions of the negative electrode of the battery include, aside from the lithium ion occluding and releasing capacity of the carbon itself, the filling performance of filling the limited volume of the battery with how much carbon. In the lithium secondary battery, usually, a mixed paste of carbon and adhesive is applied on both sides or one side of thin metal films used as current collectors, and the obtained plates are dried and rolled to form electrodes. In the plates of high capacity type having such high filling performance, it is a technical problem to accelerate ion conduction in the limited gap existing in the active substance grain boundary. That is, by obtaining a smoother ion diffusion in the negative electrode, the internal resistance of the electrode is curtailed, and the lithium secondary battery of high capacity is realized also in high rate discharge. Further researches are also made into the lithium secondary battery using polymer electrolyte, instead of organic electrolyte solution, as the electrolyte, and it is expected as a lithium secondary battery of next generation featuring small size, light weight, and freedom of shape. However, the ion conductivity of polymer electrolyte is about 10 -4 S/cm at most, which is about two digits lower as compared with the organic electrolyte solution. Hence, to obtain a conductivity similar to that of the organic electrolyte solution, gel electrolyte impregnating organic electrolyte solution in polymer is used. The gel electrolyte is manufactured, for example, in the following method as disclosed in Japanese Laid-open Patent No. 5-109310. A mixed solvent composed of an optical crosslinking polymer of polyethylene glycol diacrylate, an optical crosslinking monomer of trimethylol propane ethoxylated triacylate, an electrolyte solution solvent of propylene carbonate or polyethylene oxide, an electrolyte salt of LiCF 3 SO 3 , and others is applied on a flat plate, and it is irradiated with electron beams to polymerize and cure the monomer, so that a transparent and soft film of gel electrolyte is obtained. In the gel electrolyte, since ion conduction is mostly conducted through electrolyte solution phase, a high ion conductivity of about 3 × 10 -3 S/cm can be obtained at room temperature. In the lithium polymer secondary battery, as the substitute for separator used in the ordinary battery system, the polymer electrolyte is bonded with positive electrode and negative electrode, and the battery is composed. In the case of an ordinary secondary lithium battery using organic electrolyte solution, the positive electrode is composed of active substance, conductive agent and bonding agent, and electrolyte solution is impregnated, so that a favorable electrochemical interface with the active substance is obtained. However, the gel electrolyte which is a solid form lacks fluidity, and hardly permeates into the inside of the electrode. Accordingly, a compound electrode containing polymer electrolyte preliminarily in the electrode is formed, and it is bonded with the polymer electrolyte to fabricate the battery. However, the conventional gel polymer electrolyte described above is characterized by containing organic electrolyte solution, and although the polymer electrolyte shows a high ion conductivity, it has not reached the level of organic electrolyte solution yet in terms of characteristics. When the battery is composed by introducing this polymer electrolyte into the electrode, owing to the low ion conductivity of the electrolyte itself, the internal resistance of the electrode is increased, and the charging and discharging capacity of the battery is extremely spoiled. Hence, to fabricate the battery of high capacity type, it is required to enhance the ion conductivity of polymer electrolyte in the electrode, and compose a battery of low internal resistance. The invention is to solve such problems, and to propose a new lithium ion secondary battery of high capacity. Summary of the Invention It is an object of the invention to lower the internal resistance of the electrode in the lithium ion secondary battery. It is other object of the invention to achieve a high capacity in high rate discharge. To achieve the objects, the electrode and electrolyte have been studied. As a result, the lithium ion secondary battery of the invention comprises a positive electrode using lithium transition metal compound oxide as active substance, a negative electrode containing at least one active substance selected from the group consisting of lithium occluding and releasing carbon, metal oxide and polymer, and an organic electrolyte solution containing LiPF 6 as a solute, wherein the negative electrode contains Al 2 O 3 particles of not larger than 10 µm diameter and 0,01 to 20 parts by weight in 100 parts by weight of active substance. According to the invention, by mixing ceramic particles, i.e. Al 2 O 3 particles, in the negative electrode, the ion conductivity is enhanced, and the internal resistance of the electrode is lowered. By combining such positive and negative electrode for lithium battery and the electrolyte, a lithium ion secondary battery of high capacity particularly in high rate discharge is obtained. At the same time, by adding ceramic particles in the plates, the plate strength is increased, and the lithium secondary battery excellent in cycle life characteristic can be manufactured. As disclosed in Japanese Laid-open Patent No. 8-509100, an inorganic filler such as alumina or silanated fumed silica is added in the electrolyte, and the mechanical strength of the separator film is increased, and the electrolyte absorption level is heightened, but its constitution is different from the present invention in which ceramic particles are contained in the electrode, and it is also different in purpose, action and effect from the present invention which is intended to enhance the ion conductivity and lower the internal resistance of the battery. In Japanese Laid-open Patent No. 7-235293, it is claimed that the electron conductivity is enhanced by using the active substance, having a compound mainly composed of semimetal belonging to group IV-B or V-B of periodic table as the dopant. It is the reformation of the active substance itself of the negative electrode, and is different from the present invention in its purpose and means. Besides, by adding Al 2 O 3 , In 2 O 3 , SnO 2 , ZnO in the positive electrode in Japanese Laid-open Patent No. 7-153495, or mixing BaO, MgO, CaO in the positive electrode in Japanese Laid-open Patent No. 7-153496, it is reported that the stability of the positive electrode active substance having lithium ions de-doped in charged state is enhanced, thereby improving the capacity deterioration in the course of charging and discharging cycles. It is only intended to stabilize the positive electrode active substance by adding to the electrode, which is radically different from the constitution, action and purpose of the present invention in which additives are added to the negative electrode of the lithium ion battery. Brief Description of the Drawings Fig. 1 is a longitudinal sectional view of a lithium ion secondary battery using electrodes of the invention. Fig. 2 is a diagram showing a discharge curve of lithium secondary battery. Fig. 3 is a diagram showing rate characteristic of lithium secondary battery. Detailed Description of the Preferred Embodiments The invention relates to a lithium secondary battery using electrodes containing ceramic particles in at least negative electrode. By containing ceramics in the negative electrode, the ion conductivity is enhanced, so that a lithium secondary battery of low internal resistance is obtained. As a result, the lithium secondary battery capable of charging and discharging at high rate and higher in capacity is obtained. The ceramic is Al 2 O 3 used in particles, and the particle size is 10 microns or less. For transmission of lithium ions, the surface porous portion of ceramic particles, more specifically the porous volume is responsible, and by using ceramic particles of small particle size and wide surface area, more effective ion diffusion capacity is obtained. The ceramic content is 0.01 to 20 parts by weight in 100 parts by weight of active substance. By the presence of ceramic particles, the ion conductivity of electrode is enhanced, but its effect appears at 0.01 parts by weight or more, and if too much ceramic is present in the electrode, the electrolyte volume as the ion transmission route decreases suddenly, and therefore the ceramic to be mixed in the electrode is appropriately 20 wt.% or less. More preferably, it may be 10 wt.% or less, so that a lithium secondary battery of high capacity is obtained. The invention relates to a lithium secondary battery containing organic electrolyte solution containing LiPF 6 as a solute. Example 1 A cylindrical lithium ion secondary battery of the invention was composed by using a negative electrode mixing alumina (Al 2 O 3 ) particles. Fig. 1 shows a longitudinal sectional view of a cylindrical lithium ion secondary battery of the invention. In the diagram, reference numeral 11 is a negative electrode mixing Al 2 O 3 particles, and it was manufactured in the following manner. First, as negative electrode active substance, to 90 parts by weight of graphite powder, 10 parts by weight of polyvinylidene fluoride as binder and N-methyl-2-pyrrolidone as solvent were mixed, and a specified amount of Al 2 O 3 particles with specified particle size were mixed and kneaded, and a paste was prepared. A specified amount of this compound was applied on both sides of a copper foil as negative electrode current collector, and dried, rolled, and cut to a specified size, and a negative electrode 11 for lithium secondary battery was fabricated. Reference numeral 13 is a positive electrode, and it was manufactured in the following manner. To 100 parts by weight of LiCoO 2 positive electrode active substance, 3 parts by weight of acetylene black and 9 parts by weight of aqueous dispersion of polytetrafluoroethylene were added and kneaded to prepare a paste. It was applied on both sides of an aluminum foil as positive electrode current collector, and dried and rolled, and cut to a specified size, and a positive electrode 13 for lithium ion secondary battery was obtained. Reference numeral 15 is a separator composed of a polyethylene microporous film, and it is interposed between the positive electrode 13 and negative electrode 11, and a spiral plate group is composed on the whole. At upper and lower ends of the plate group, an upper insulator 16 and a lower insulator 17 of polypropylene are disposed, and the assembly is inserted into a nickel plated iron case 18. A positive electrode lead plate 14 is spot-welded to a titanium sealing plate 20 and a negative electrode lead plate 12, to the bottom of the case 18, and a specified amount of electrolyte solution is injected into the case, and the battery is sealed with the sealing plate 20 through a gasket 19, and a cylindrical lithium secondary battery of the invention is obtained. The battery .measures 14 mm in diameter and 50 mm in height. Reference numeral 21 is a positive electrode terminal of the battery, and the battery case 18 serves as the negative electrode terminal. Herein, the electrolyte solution is a nonaqueous electrolyte solution dissolving 1.5 mol/liter of LiPF 6 in a mixed solvent of ethylene carbonate and ethyl methyl carbonate at 25:75 by volume. Example 2 The cylindrical lithium ion secondary battery of the invention was composed in the same manner as in example 1, except that the particle size of Al 2 O 3 particles to be mixed in the negative electrode was fixed at 0.5 micron, and that the additive rate in 90 parts by weight of graphite and 10 parts by weight of binder was 0.01, 5, 10, 20 and 30 parts by weight. Example 3 The cylindrical lithium ion secondary battery of the invention was composed in the same manner as in example 1, except that the additive rate of Al 2 O 3 particles mixed in the negative electrode was fixed at 5 parts by weight in 90 parts by weight of graphite and 10 parts by weight of binder, and that the particle size of Al 2 O 3 particles was 0.5, 1.0, and 10 and further 20 microns not forming part of the invention. Comparative example 1 The cylindrical lithium ion secondary battery of the invention was composed in the same manner as in example 1, except that Al 2 O 3 particles were not contained in the negative electrode. The characteristics of these batteries in examples 1 to 3 and comparative example 1 were evaluated. The results are shown in Fig. 2, and Fig. 3. Fig. 2 shows the discharge curve at the tenth cycle of the lithium secondary batteries obtained in example 1 and comparative example 1. In the test, the battery was charged at a constant current of 100 mA and discharged at 500 mA, the charge end voltage was 4.2 V and discharge end voltage was 3.0 V, and the cycle test was conducted in the environments at 20°C. As a result, the battery of comparative example 1 had a capacity of 405 mAh, and the battery of the invention in example 1 mixing Al 2 O 3 particles in the negative electrode was increased in capacity by 6% to 430 mAh. This is because the internal resistance of the battery is low, the voltage drop in discharge is small, and a high voltage is maintained. In Fig. 3, the current-capacity characteristics of the lithium secondary batteries obtained in example 1 and comparative example 1 are expressed as the discharge capacity to the discharge current. In the test, the battery was charged at a constant current of 100 mA, and discharged at 100, 250, 500, and 1000 mA. In low rate discharge at 100 mA, in the battery of example 1 mixing Al 2 O 3 particles in the negative electrode, the capacity was decreased by 4% as compared with the battery of comparative example 1 due to decline of quantity of active substance, but in high rate discharge at 250 mA and higher, the capacity was improved. Besides, the battery of comparative example 1, in discharge at 1000 mA, the capacity was about 83% that in discharge at 100 mA, while a high capacity maintenance rate of 90% was observed in the battery of example 1. In the test, the battery of example 2 was charged at 100 mA and discharged at 500 mA at 20°C. As a result, the discharge capacity increased together with the addition of Al 2 O 3 particles, showing the maximum value of 430 mAh at 5 wt.%. However, at 20 wt.% or more, to the contrary, the discharge capacity decreased due to addition. This seems because quantities of ceramic fine particles are present in the electrode and the electrolyte volume as the ion transmission route is decreased suddenly. Hence, ceramic fine particles to be mixed in the electrode should be preferably 20 wt.% or less, and a lithium secondary battery of high capacity is obtained at this time. The discharge capacity in terms of particle size of Al 2 O 3 particles mixed in the negative electrode of the lithium secondary battery obtained in example 3 was improved as the particle size of mixed Al 2 O 2 particles decreased, and a high capacity of over 420 mAh was obtained at particle size of 10 microns or less. This is considered because the surface porous portion of ceramic particles, more specifically, the porous volume is closely related to transmission of lithium ions, and an effective ion diffusion is achieved by using ceramic particles of small particle size and wide surface area. In both charge and discharge states, the X-ray diffraction analysis of the negative electrode after charging and discharging disclosed no difference in Al 2 O 3 peaks. Hence, Al 2 O 3 is not considered to be related to occlusion and release of lithium. Moreover, the cycle characteristics of the lithium secondary batteries of example 1 and comparative example 1 were studied. The battery test was conducted at the discharge current of 500 mA, upper limit end voltage of 4.2 V, lower limit end voltage of 3.0 V, and in the environment of 20°C. Results of discharge capacity at 0, 100, and 500 cycles are shown in Table 1. Initial capacity (mAh) Capacity after 100 cycles (mAh) Capacity after 500 cycles (mAh) Example 1 430 414 404 Comparative example 1 405 376 365 As clear from Table 1, in the battery of comparative example 1, the capacity decrease to 93% of initial capacity in 100 cycles, and 90% in 500 cycles, but the battery of example 1 of the invention maintained 94% of initial capacity after 500 cycles. The battery was decomposed after 500 cycles, and the negative electrode was observed, and the plate of example 1 containing Al 2 O 3 was less in dropout of negative electrode compound and higher in strength as compared with the plate of comparative example 1 without additive. In the embodiments, LiPFs was used as the solute of nonaqueous electrolyte solution. Also in the embodiments, as the solvent of the nonaqueous electrolyte solution, a mixed solvent of propylene carbonate and ethylene carbonate, and a mixed solvent of ethylene carbonate and ethyl methyl carbonate were used, but other organic solvents or their mixed solvents may be used. As the positive electrode active substance of the embodiments, V 6 O 13 and LiCoO 2 were used, but they may be also replaced by MnO 2 , Li 1+x Mn 2 O 4 (0≦ X≦ 0.1), LiNiO 2 , LixMnO 2 (0<X ≦0.5), and other lithium transition metal composite compound. Lithium metal and graphite powder were used in the negative electrode of the embodiments, but other carbon material, metal oxide, metal nitride, and other metal compounds may be also used. In the embodiments, as ceramic fine particles, Al 2 O 3 were used. Ceramic particles were used in the embodiments, but the shape of particles is not limited to granular form, but same effects were obtained in fibrous ceramic particles having a wide specific surface area. Thus, according to the invention, by mixing ceramic particles in the negative electrode, the electrode capacity was enhanced particularly in high rate discharge. By combining the positive and negative electrodes for lithium battery and the electrolyte, the lithium secondary battery of high capacity was obtained.

Claims: A lithium ion secondary battery mainly comprising: a positive electrode using lithium transition metal compound oxide as active substance, a negative electrode containing at least one active substance selected from the group consisting of lithium occluding and releasing carbon, metal oxide and polymer, and an organic electrolyte solution containing LiPF 6 as a solute, wherein the negative electrode contains Al 2 O 3 particles of not larger than 10 pm diameter and 0,01 to 20 parts by weight in 100 parts by weight of active substance. A lithium ion secondary battery of claim 1, wherein the negative electrode is a graphite and the organic electrolyte solution contains ethylene carbonate as a solvent. A lithium ion secondary battery of claim 2, wherein the solvent of the organic electrolyte solution is a mixed solvent of ethylene carbonate and ethyl methyl carbonate.

Inventors: EDA NOBUO, ISHIDA AKIKO, KITAGAWA MASAKI, NISHIMURA KEN, OGAWA MASAHIKO, SAKAI TETSUHISA, EDA, NOBUO, ISHIDA, AKIKO, KITAGAWA, MASAKI, NISHIMURA, KEN, OGAWA, MASAHIKO, SAKAI, TETSUHISA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H01M4
IPC codes: H01M4