Fluorinated carbonate electrolytes for use in a lithium secondary battery

Abstract: A lithium secondary battery comprising a negative electrode using as an active material at least one kind selected from the group of metallic lithium, lithium alloys or materials which are capable of electrochemically occluding and releasing lithium ions, a positive electrode using as an active material at least one kind of compounds which are capable of electrochemically occluding and releasing lithium ions, and an organic electrolyte. It is characterized in that a solvent for the organic electrolyte is composed mainly of a mixture solvent prepared by mixing an organic solvent indicated by structure expression I, i.e. 4-trifluoromethyl-1,3-dioxolane-2-one, with one kind selected from the group of organic solvent indicated by structure expression II, e.i. 1-trifluoroethylmethyl carbonate, and/or an organic solvent indicated by structure expression III, i.e. di-1-trifluoroethyl carbonate, as mentioned below. As a result of using the solvent which contains the above-mentioned mixture solvent as a main component for the electrolyte, a lithium secondary battery with improved high-rate discharge characteristics, cycle characteristics and safety characteristics is obtained.

Description: BACKGROUND OF THE INVENTION With recent rapid progress in the field of the electric and electronic industries, electric and electronic devices have been made higher in performance, smaller in size and portable. As such, the development of these devices has generated considerable demand for secondary batteries with high energy density which can be used for these electronic devices. As a secondary battery to be used for these electronic devices, lead acid batteries, Ni-Cd (nickel-cadmium) batteries or Ni-MH (nickel-hydrogen) batteries have been conventionally used. In addition to them, lithium secondary batteries which use metallic lithium or carbon material capable of electro-chemically occluding and releasing lithium ions as an active material of a negative electrode, and oxide materials containing lithium capable of electro-chemically occluding and releasing lithium ions as an active material of a positive electrode, have been put into practice and widely used recently. Lithium secondary batteries of this kind have, as compared with other conventional batteries, high discharge voltage and high energy density per unit volume or unit weight. Therefore it is said that they are the most promising secondary batteries today. Presently, as an electrolyte used in this kind of lithium secondary batteries, there has been used such a one that lithium salt such as LiPF 6 , etc. is dissolved in a non-aqueous mixture solvent wherein the main component is a mixture solvent of an organic solvent such as propylene carbonate, etc. , which has dielectric constant, and an organic solvent such as methyl ethyl carbonate, etc., which has low viscosity. (see Tokkai-Hei 6-13109) However, this kind of lithium secondary battery has such a problem that the organic solvents in the electrolyte is decomposed during charging, and the battery capacity is decreased by repetition of charge-discharged cycles. Therefore, the improvement in the length of battery life has been desired. Also, since these conventional electrolytes are combustible, the lithium secondary battery using this kind of electrolyte has such dangers as firing or explosion when it is mishandled, as compared with such batteries using the aqueous electrolyte such as lead acid, Ni-Cd battery or Ni-MH battery. SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery which has equal or higher high-rate discharge characteristics and more excellent cycle characteristics than the conventional lithium secondary battery using the conventionally proposed electrolytes, by using an organic electrolyte including a mixture solvent which is more stable chemically and electrically as compared with the conventional electrolytes. To achieve this object of the present invention among the above-mentioned objects, according to the present invention, a lithium secondary battery is characterized in that a solvent for the organic electrolyte is composed mainly of a mixture solvent prepared by mixing an organic solvent indicated by structure expression I, i.e. 4-trifluoromethyl-1,3-dioxolane-2-one, and one kind or two selected from the group of organic solvents indicated by structure expression II, e.i. 1-trifluoroethylmethyl carbonate, and an organic solvent indicated by structure expression III, i.e. di-1-trifluoroethyl carbonate. Further, the lithium secondary battery according to the present invention is characterized in that the volume of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of organic solvents indicated by structure expression II and structure expression III occupied in the whole solvent for the organic electrolyte is 65% or more when measured under environmental temperature of 25°C, and that the volume of the organic solvent indicated by structure expression I occupied in the mixture solvent is 35 % or more when measured under environmental temperature of 25°C. Thereby, a lithium secondary battery is provided which has, in addition to the equal or higher high-rate discharge characteristics and the excellent cycle life characteristics, a self-extinguishing characteristics of the organic electrolyte which is excellent in safety. Also, a lithium secondary battery, according to the present invention is further characterized in that the volume of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of organic solvents indicated by structure expression II and structure expression III which are occupied in the whole solvent for the organic electrolyte is 80% or more when measured under environmental temperature of 25°C, and thereby there is offered the battery which is particularly good in cycle characteristics. And also, a lithium secondary battery, according to the present invention is further characterized in that the volume of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of organic solvents indicated by structure expression II and structure expression III occupied in the whole solvent for the organic electrolyte is 80% or more when measured under environmental temperature of 25°C, and that the volume of the organic solvent indicated by structure expression I occupied in the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of organic solvents indicated by structure expression II and structure expression III is 35-65 % when measured under environmental temperature of 25°C, and thereby there is offered one of the most preferable batteries which is particularly good in high-rate discharge characteristics and cycle characteristics and is excellent in safety. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be described in details as follows. A lithium secondary battery according to the present invention makes no differences from conventional lithium secondary batteries in constructional parts except for using an organic electrolyte prepared by dissolving at least one kind of lithium salts in a whole solvent has higher chemical and electrical stability than the conventional electrolytes. Here, the whole solvent which has higher stability chemically and electrically than the conventional electrolytes is mainly composed of the above-mentioned mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III. As for a solvent with high dielectric constant to be selectively used here, it is important that the solvent itself is high in chemically and electrically stable, but it is more important that the resultant mixture solvent prepared by mixing it with any other kind of solvent is also high in the above-mentioned stability. As a result of various and many investigations, it has been found that there is brought about excellent cycle characteristics when the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III is used. Specific actions for offering the foregoing excellent cycle characteristics are unknown, but it is presumed that it is due to the fact that the organic solvent itself indicated by structure expression I has the high chemical and electrical stability and also due to the interactions between the organic solvent indicated by structure expression I and one kind or two selected from the group of the organic solvent indicated by structure expression II and the organic solvent indicated by structure expression III as resulted from the mixture thereof. Thus, if the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III is present as a main component in the whole organic solvent for the electrolyte, there is brought about the excellent cycle characteristics. In that case, the inventors have found that if the volume of the mixture solvent occupied in the whole solvent for the organic electrolyte is 65% or more when measured under environmental temperature of 25°C and the volume of the organic solvent indicated by structure expression I occupied in the mixture solvent is 35% or more when measured under environmental temperature of 25°C, there can be provided a lithium secondary battery which has self-extinguishing characteristics of the electrolyte and is excellent in safety, in addition to the excellent cycle characteristics. These characteristic features are presumed to be due to the interactions between the respective solvents resulted from mixing the organic solvent indicated by structure expression I and one kind or two selected form the group of the organic solvents indicated by structure expression II and structure expression III. Thus, if the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III is present as a main component for the whole solvent, there can be an excellent cycle characteristics. Furthermore, the inventors also have found that if the volume of the mixture solvent occupied in the whole solvent for the organic electrolyte is 80 % or more when measured under environmental temperature of 25°C, there can be obtained a lithium secondary battery which is especially excellent in cycle characteristics. The inventors assume that the reason for this is because the actions (influence(s)) of other solvent(s) than the solvents indicated by structure expressions I, II and III are hardly presented under environmental temperature of 25°C if the volume rate of the mixture solvent in the whole solvent is 80 % or more. Here, solvents used in the electrolyte other than the solvents indicated by structure expressions I, II and III can be, for example, cyclic esters such as propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-methyl-γ-butyrolactone, acetyl-γ-butyrolactone and γ-valerolactone, chain esters such as dimethyl carbonate, methyl ethyl carbonate, propyl methyl carbonate, methyl butyl carbonate, diethyl carbonate, propyl ethyl carbonate, ethyl butyl carbonate, dipropyl carbonate, butyl propyl carbonate, dibutyl carbonate, alkyl propionates ester, dialkyl malonates ester and alkyl acetate ester, cyclic ethers such as alkyl acetates, tetrahydrofuran, alkyl tetrahydrofuran, dialkyl tetrahydrofuran, alkoxy tetrahydrofuran, dialkoxy tetrahydrofuran, 1,3-dioxolane, alkyl-1,3-dioxolane, 1,4-dioxolane, etc. and chain ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, ethylene glycol dialkyl ether, diethylene glycol dialkyl ether, triethylene glycol dialkyl ether and tetraethylene glycol dialkyl ether. However, it is not necessary to be limited to the solvents as listed above. Thus, if the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III is present in the whole solvent as main component, there is brought about an excellent characteristics. Further, the inventors have also found that it is preferable that the volume of the mixture solvent occupied in the whole solvent for the organic electrolyte is 80 % or more when measured under environmental temperature of 25°C, and the volume of the organic solvent indicated by structure expression I occupied in the mixture solvent is 35-65 % when measured under environmental temperature condition of 25°C. More in detail, the inventors have found that especially when the volume of the organic solvent indicated by structure expression I is limited in the above range, there can be obtained a lithium secondary battery which has an especially high specific electric conductivity, and thus there can be obtained a large discharge capacity even if it is discharged with a current in a wide range of 0.2CA - 2.5CA, in addition to the especially excellent cycle characteristics and a high safety resulted from the self-extinguishing characteristics of the electrolyte. And, if the foregoing volume rate of the organic solvent indicated by structure expression I is less than 35%, the lithium salt can not be dissociated sufficiently, resulting in decrease in the specific electric conductivity and increase in the internal resistance of the battery, so that it becomes difficult to take out a sufficient capacity. On the other hand, if the volume thereof is more than 65%, the viscosity of the electrolyte is increased, resulting in decrease in mobility of lithium ions and decrease in the specific conductivity and increase in the internal resistance. As to lithium salts to be dissolved in the whole solvent for the electrolyte, any kind which is capable of dissociating in the organic solvent and releasing lithium ions can be used. For example, there are inorganic lithium salt such as LiClO 4 , LiBF 4 , LiPF 6 , LiAsF 6 , LiCl, LiBr etc., and organic lithium salt such as LiB(C 6 H 5 ) 4 , LiN(SO 4 CF 3 ) 2 , LiC(SO 2 CF 3 ) 3 , LiOSO 2 CF 3 etc.. Among the above lithium salts, fluorine-containing lithium salts are preferable in terms of safety, and especially LiPF 6 alone or a mixture composed mainly of LiPF 6 mixed with any other lithium salt(s) are preferable because of its high electro-conductivity. The active materials to be used for the positive electrode are, for example, lithium-containing complex oxides such as LiCoO 2 , LiNiO 2 , LiMnO 2 or LiMn 2 O 4 , etc., or chalcogen compounds such as TiO 2 , MnO 2 , MoO 3 , V 2 O 5 , TiS 2 , MoS 2, etc.. Specially, a lithium compound having a structure of α-NaCrO 2 such as LiCoO 2 , LiNiO 2 , LiMnO 2 etc. or LiMn 2 O 4 etc. are more preferable because of its high discharge voltage and electro-chemical stability. EMBODIMENT EXAMPLES The embodiment examples of the present invention will be explained concretely as follows, but the present invention is not to limit to them. LiCoO 2 powder as an active material for a positive electrode, graphite powder as an electro-conductive agent, polyfluoro vinylidene resin as a binder, and N-methyl-2-pyrrolidone as a solvent for the binder were stirred to be mixed by a homogenizer to obtain a slurry active material mixture for a positive electrode. One side of an electric collector made of aluminum foil was coated with this slurry mixture using a slot die coater, and thereafter dried at 130°C in an oven to remove the solvent. The other side of the electric collector was coated therewith, and thereafter the solvent was removed by the same manner as above in order to coat both sides of the electric collector with active material mixture. Then, it was pressed with a roller press and heated in a vacuum oven to remove the moisture to obtain a positive electrode. On the other hand, a negative electrode was made in such a manner that carbon powder capable of electro-chemically occluding and releasing lithium ions, styrenebutadiene rubber type resin and ethyl acetate was stirred to be mixed by a homogenizer to obtain a slurry active material mixture. This slurry active material mixture coated on one side of an electric collector made of copper foil using a slot die coater, and thereafter it was dried in an oven at 130°C to remove the solvent. The other side of the electric collector was also coated therewith and the solvent was removed in the same manner as above. The electric collector with the active material mixture on both sides thereof thus obtained was then subjected to a heat treatment so as to cure styrene butadiene rubber type resin, and then pressed with a heated roller press and was then dried to remove the moisture to obtain a negative electrode. 1. Evaluation of cycle characteristics The positive electrode and the negative electrode thus obtained were stacked one upon another through a separator made of micro porous resin film which has a threedimensional (sponge-like) structured polyolefine resins ( polyethylene, polypropylene or a copolymer thereof). And it was wound to form a spiral electrode assembly. The spiral electrode assembly thus obtained was put in a tubular container made of stainless steel. The opening of the container was closed with a cover, and after an electrolyte was poured, to obtain a sealed lithium secondary battery of AA-size with 500 mAh rated capacity. In the course of manufacturing a lot of the foregoing batteries, the batteries were poured with respective electrolytes prepared by dissolving LiPF 6 as a solute in the respective whole solvents for the electrolytes corresponding to examples A - AG, comparative example A and conventional examples A - C having the respective compositions in the volume ratios of component solvents, when measured at 25°C as shown in Table 1, so as to become the concentration of the solute 1 mol/l. In Table 1, [A] represents the volume % of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expressions II and III occupied in the whole solvent at temperature of 25°C. [B] represents the volume % of other solvents than the organic solvents indicated by structure expressions I, II and III occupied in the whole solvent at a temperature of 25°C. [A1] ] represents the volume % of the organic solvent indicated by structure expression I occupied in the mixture solvent of the organic solvents indicated by structure expressions I, II and III at a temperature of 25°C. [A2] represents the volume % of one kind or two selected from the group of the organic solvents indicated by structure expressions II and III occupied in the mixture solvent of the organic solvents indicated by structure expressions I, II and III at a temperature of 25°C. [II] represents the volume % of the organic solvent indicated by structure expression II occupied in the mixture solvent indicated by structure expressions II and III at a temperature of 25°C. [III] represents the volume % of the organic solvent indicated by structure expression III occupied in the mixture solvent indicated by structure expressions II and III at a temperature of 25°C. Further, PC stands for propylene carbonate and MEC stands for methyl ethyl carbonate. Composition of solvents for electrolytes (volume %) [A] [B] Composition of [A] [A1] [A2] Composition of [A2] Composition of [B] [II] [III] PC MEC Example A 50 50 50 100 0 50 100 0 Example B 60 50 50 100 0 40 100 0 Example C 70 50 50 100 0 30 100 0 Example D 80 50 50 100 0 20 100 0 Example E 90 50 50 100 0 10 100 0 Example F 100 50 50 100 0 0 - - Example G 50 50 50 0 100 50 0 100 Example H 60 50 50 0 100 40 0 100 Example I 70 50 50 0 100 30 0 100 Example J 80 50 50 0 100 20 0 100 Example K 90 50 50 0 100 10 0 100 Example L 100 50 50 0 100 0 - - Example M 50 50 50 50 50 50 50 50 Example N 60 50 50 50 50 40 50 50 Example O 70 50 50 50 50 30 50 50 Example P 80 50 50 50 50 20 50 50 Example Q 90 50 50 50 50 10 50 50 Example R - 100 50 50 50 50 0 - - Example S 100 30 70 100 0 0 - - Example T 100 35 65 100 0 0 - - Example U 100 55 45 100 0 0 - - Example V 100 65 35 100 0 0 - - Example W 100 70 30 100 0 0 - - Example X 80 30 70 0 100 20 50 50 Example Y 80 35 65 0 100 20 50 50 Example Z 80 55 45 0 100 20 50 50 Example AA 80 65 35 0 100 20 50 50 Example AB 80 70 30 0 100 20 50 50 Example AC 70 30 70 50 50 30 50 50 Example AD 70 35 65 50 50 30 50 50 Example AE 70 55 45 50 50 30 50 50 Example AF 70 65 35 50 50 30 50 50 Example AG 70 70 30 50 50 30 50 50 Comparative example A 50 100 0 - - 50 0 100 Conventional example A 0 - - - - 100 50 50 Conventional example B 50 0 100 100 0 50 100 0 Conventional example C 50 0 100 0 100 50 100 0 Each of the batteries thus manufactured was charged and discharged repeatedly 10 cycles for the initial activation in such a manner that it was charged with a current density of 0.2CmA, at a temperature of 25°C until the battery voltage reached 4.2V, and after allowed to stand for ten minutes, it was discharged with the same current as above until the battery voltage became 2.75V, and it was then allowed to stand for ten minutes, and the same charging as above was carried out again. Thereafter, for measuring the cycle characteristics, a life test was carried out for each of the batteries in such a manner that it was charged with a current of 0.2CmA, at a temperature of 25°C, until the battery voltage reached 4.2V, and after it was allowed to stand for ten minutes, it was discharged with a current of 0.7CmA until the battery voltage became 2.75V, and after, it was allowed to stand for ten minutes, the same charging was carried out again. Here, the battery life is determined by the number of charge / discharge cycles counted until a discharge capacity of the each battery has been reduced to 70 % of the discharge capacity thereof at the first cycle in the life test. The result thereof is shown Table 2 below. Examples A - AG which are the batteries made according to the present invention exhibit longer in life as compared with conventional examples A - C which are conventionally made batteries and with comparative example A which is a battery made for comparison. Accordingly, it has been found therefrom that the electrolytes according to the present invention are effective in improving the cycle characteristics of the lithium secondary batteries. Furthermore, especially, examples D - F, examples J - L and examples P - R, among examples A - R, in which the volume of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III are 80% or more of the whole solvent under environmental condition of 25°C, exhibit more than 500 cycles which prove longer life than the other examples each in which the volume rate thereof is less than 80%. Also, it has been found therefrom that if the volume of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III is 80% or more of the whole solvent under environmental condition of 25°C, it is much effective in improvement in the cycle characteristics of the lithium secondary batteries. 2. Evaluation of discharge capacity The above-mentioned positive electrode and negative electrode were stacked one upon another through a separator made of a microporous resin film having a three dimensional sponge-like structure of such a polyolefine resin as polyethylene, polypropylene or a copolymer thereof, and it was wound to form a spiral electrode assembly. The spiral electrode assembly thus obtained was put in a tubular container made of stainless steel. The opening of the container was closed with a cover, after an electrolyte was poured, to obtain a lithium secondary battery of AA-size. In the course of manufacturing a lot of the foregoing batteries, the batteries were poured with respective electrolytes prepared by dissolving LiPF 6 as a solute in the respective whole solvent for the electrolyte corresponding to examples F, L and R-AG and conventional example A having the respective compositions in the volume ratios of the component solvents when measured at 25°C as shown in Table 1, so as to become the concentration of the solute 1 mol/l. Each of the batteries thus manufactured was charged and discharged repeatedly 10 cycles for the initial activation in such a manner that it was charged with a current of 0.2CmA, at a temperature of 25°C until the battery voltage reached 4.2V, and after allowed to stand for ten minutes, it was discharged with the same current as above until the battery voltage became 2.75V, and after it was allowed to stand for ten minutes, the same charging as above was carried out again. Thereafter, a discharge capacity test was carried out for each battery in such a manner that it was charged with a current of 0.2CmA , at a temperature of 25°C, until the battery voltage reached 4.2V, and after it was allowed to stand for ten minutes, it was discharged with a current of 1CmA until the battery voltage became 2.75V. The result of measuring the discharge capacity of each battery is shown in Table 3 below. Those lithium secondary batteries were so designed as to obtain the same capacity by including the same amounts of the active materials in the positive and negative electrodes. It has been found therefrom that examples F, L and R-AG, which are the batteries made according to the present invention, are all provided with equal or more discharge capacity to or than the conventional batteries and enough to be acceptable as a practical use, in spite of such a large discharge current that is 1CA, and especially, examples F, L, R, T - V, Y-AA and AD - AF, in which the volume of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expressions II and III is more 80% or more of the whole solvent, when measured under environmental temperature of 25°C, and the volume of the organic solvent indicated by structure expression I occupied in the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expressions II and III is in the range of 35 - 65 % when measured under environmental temperature of 25°C, exhibit such a large discharge capacity that is more than 500mAh, and thus there can be obtained lithium secondary batteries which are excellent also in the high-rate discharge characteristics, in addition to the above-mentioned much excellent cycle characteristics. 3. Evaluation of self-extinguishing characteristics The following test was carried out for confirming self-extinguish characteristics of the electrolytes used for the batteries of the present invention. Various electrolytes were prepared by dissolving LiPF 6 as a solute in the respective whole solvent corresponding to the example AH - ER having the respective composition in the volume ratios of the component solvents at 25°C as shown in Table 4, so as to become the concentration of the solute 1 mol/l. Here, in Table 4, [A], [B], [A1], [A2], [II] and [III] respectively represent the same as in Table 1. PC stands for propylene carbonate and MEC stands for methyl ethyl carbonate. Composition of solvents for electrolytes (volume %) [A] [B] Composition of [A] [A1] [A2] Composition of [A2] [II] [III] kind of solvent Example AH 65 35 65 100 0 35 PC Example Al 70 35 65 100 0 30 PC Example AJ 80 35 65 100 0 20 PC Example AK 90 35 65 100 0 10 PC Example AL 65 35 65 0 100 35 PC Example AM 70 35 65 0 100 30 PC Example AN 80 35 65 0 100 20 PC Example AO 90 35 65 0 100 10 PC Example AP 100 35 65 0 100 0 - Example AQ 65 35 65 50 50 35 PC Example AR 70 35 65 50 50 30 PC Example AS 80 35 65 50 50 20 PC Example AT 90 35 65 50 50 10 PC Example AU 100 35 65 50 50 0 - Example AV 65 35 65 100 0 35 MEC Example AW 70 35 65 100 0 30 MEC Example AX 80 35 65 100 0 20 MEC Example AY 90 35 65 100 0 10 MEC Example AZ 65 35 65 0 100 35 MEC Example BA 70 35 65 0 100 30 MEC Example BB 80 35 65 0 100 20 MEC Example BC 65 35 65 50 50 35 MEC Example BD 70 35 65 50 50 30 MEC Example BE 80 35 65 50 50 20 MEC Example BF 100 35 65 50 50 0 - Example BG 65 50 50 100 0 35 PC Example BH 65 50 50 0 100 35 PC Example BI 70 50 50 0 100 30 PC Example BJ 80 50 50 0 100 20 PC Example BK 90 50 50 0 100 10 PC Example BL 65 50 50 50 50 35 PC Example BM 70 50 50 50 50 30 PC Example BN 80 50 50 50 50 20 PC Example BO 90 50 50 50 50 10 PC Example BP 65 50 50 100 0 35 MEC Example BQ 70 50 50 100 0 30 MEC Example BR 80 50 50 100 0 20 MEC Example BS 90 50 50 100 0 10 MEC Example BT 65 50 50 0 100 35 MEC Example BU 65 50 50 50 50 35 MEC Example BV 70 50 50 50 50 30 MEC Example BW 80 50 50 50 50 20 MEC Example BX 90 50 50 50 50 10 MEC Example BY 65 60 40 100 0 35 PC Example BZ 70 60 40 100 0 30 PC Example CA 80 60 40 100 0 20 PC Example CB 90 60 40 100 0 10 PC Example CC 100 60 40 100 0 0 - Example CD 65 60 40 0 100 35 PC Example CE 70 60 40 0 100 30 PC Example CF 80 60 40 0 100 20 PC Example CG 90 60 40 0 100 10 PC Example CH 100 60 40 0 100 0 - Example CI 65 60 40 50 50 35 PC Example CJ 70 60 40 50 50 30 PC Example CK 80 60 40 50 50 20 PC Example CL 90 60 40 50 50 10 PC Example CM 100 60 40 50 50 0 - Example CN 65 60 40 100 0 35 MEC Example CO 70 60 40 100 0 30 MEC Example CP 80 60 40 100 0 20 MEC Example CQ 90 60 40 100 0 10 MEC Example CR 65 60 40 0 100 35 MEC Example CS 70 60 40 0 100 30 MEC Example CT 80 60 40 0 100 20 MEC Example CU 90 60 40 0 100 10 MEC Example CV 65 60 40 50 50 35 MEC Example CW 70 60 40 50 50 30 MEC Example CX 80 60 40 50 50 20 MEC Example CY 90 60 40 50 50 10 MEC Example CZ 65 80 20 100 0 35 PC Example DA 70 80 20 100 0 30 PC Example DB 80 80 20 100 0 20 PC Example DC 90 80 20 100 0 10 PC Example DD 100 80 20 100 0 0 - Example DE 65 80 20 0 100 35 PC Example DF 70 80 20 0 100 30 PC Example DG 80 80 20 0 100 20 PC Example DH 90 80 20 0 100 10 PC Example Dl 100 80 20 0 100 0 - Example DJ 65 80 20 50 50 35 PC Example DK 70 80 20 50 50 30 PC Example DL 80 80 20 50 50 20 PC Example DM 90 80 20 50 50 10 PC Example DN 100 80 20 50 50 0 - Example DO 65 80 20 100 0 35 MEC Example DP 70 80 20 100 0 30 MEC Example DQ 80 80 20 100 0 20 MEC Example DR 90 80 20 100 0 10 MEC Example DS 65 80 20 0 100 35 MEC Example DT 70 80 20 0 100 30 MEC Example DU 80 80 20 0 100 20 MEC Example DV 90 80 20 0 100 10 MEC Example DW 65 80 20 50 50 35 MEC Example DX 70 80 20 50 50 30 MEC Example DY 80 80 20 50 50 20 MEC Example DZ 90 80 20 50 50 10 MEC Example EA 60 35 65 100 0 40 PC Example EB 60 35 65 0 100 40 PC Example EC 60 35 65 50 50 40 PC Example ED 60 35 65 100 0 40 MEC Example EE 60 35 65 0 100 40 MEC Example EF 60 35 65 50 50 40 MEC Example EG 65 30 70 100 0 35 PC Example EH 65 30 70 0 100 35 PC Example EI 65 30 70 50 50 35 PC Example EJ 65 30 70 100 0 35 MEC Example EK 65 30 70 0 100 35 MEC Example EL 65 30 70 50 50 35 MEC Example EM 60 30 70 100 0 40 PC Example EN 60 30 70 0 100 40 PC Example EO 60 30 70 50 50 40 PC Example EP 60 30 70 100 0 40 MEC Example EQ 60 30 70 0 100 40 MEC Example ER 60 30 70 50 50 40 MEC Also, various mixture electrolytes were prepared by dissolving LiPF 6 as a solute in the respective whole solvents for the electrolytes corresponding to examples C, D, E, F, L, R and T, comparative example A, and conventional examples A -C having the respective compositions in the volume ratios of the component solvents as shown in Table 1, so as to adjust the solute concentration to 1 mol/l. Self-extinguishing characteristics of each of the electrolytes prepared as above was examined by a test where sheets of paper were immersed in the respective electrolytes to be tested, and were then set fire by a flame of a burner and thereafter the flame was put out. Thus the self-extinguishing characteristics was confirmed by observing as to whether the firing was still continued or discontinued when the flame was put out. The results of self-extinguishing characteristics are shown in Table 5 below. As it is obvious from Table 5, while the electrolytes corresponding to conventional examples A - C and comparative example A continued firing even after putting out the burner's flame, the electrolytes corresponding to examples AH - DZ and examples and C, D, E, F, L, R and T used for the battery according to the present invention self-extinguished after putting out the burner's flame, and thus their self-extinguishing characteristics have been confirmed. However, it has been also confirmed that the electrolytes corresponding to examples EA-ER have no self-extinguish characteristics, because the firing thereof continued even after putting out the burner's flame. Therefore, it has been concluded that the electrolytes wherein the volume of the mixture solvent prepared by mixing the organic solvent indicated by structure expression I with one kind or two selected from the group of the organic solvents indicated by structure expression II and structure expression III is 65% or more of the whole solvent when measured under environmental temperature of 25°C, and the volume of the organic solvent indicated by structure expression I is 35 % or more of the mixture solvent, when measured under environmental temperature of 25°C, have more improved in self-extinguishing characteristics. Using such a specific range of electrolyte, a lithium secondary battery with high safety characteristics can be supplied. Thus, according to the present invention, a lithium battery with excellent charge / discharge cycle characteristics can be obtained. Furthermore, by limiting the mixing ratios of the mixture solvent in the electrolyte in the foregoing range, a lithium secondary battery which is provided with high safety and self-extinguishing characteristics in addition to the excellent cycle characteristics can be obtained. In addition, a lithium secondary battery provided also with high-rate discharge characteristics can be obtained.

Claims: A lithium secondary battery comprising a negative electrode using as an active material at least one kind selected from the group of metallic lithium, lithium alloys or materials which are capable of electrochemically occluding and releasing lithium ions, a positive electrode using as an active material at least one kind of compounds which are capable of electrochemically occluding and releasing lithium ions, and an organic electrolyte an improvement being that a solvent for the electrolyte is composed mainly of a mixture solvent prepared by mixing an organic solvent indicated by structure expression I, i.e. 4-tnfluoromethyf-1,3-dioxolane-2-one, and one kind or two selected from the group of an organic solvent indicated by structure expression II, e.i. 1-trifluoroethylmethyl carbonate, and an organic solvent indicated by structure expression III, i.e. di-1-trifluoroethyl carbonate, as mentioned below. A lithium secondary battery according to claim 1, wherein the volume of the mixture solvent occupied in the whole solvent for the organic electrolyte is 65% or more under environmental temperature of 25°C, and the volume of the organic solvent indicated by the structure expression I occupied in the mixture solvent is 35% or more under environmental temperature of 25°C. A lithium secondary battery according to claim 1 or claim 2, wherein the volume of the mixture solvent occupied in the whole solvent for the organic electrolyte is 80% or more under environmental temperature of 25°C. A lithium secondary battery according to claim 1, claim 2, or claim 3, wherein the volume of the mixture solvent occupied in the whole solvent for the organic electrolyte is 80% or more under environmental temperature of 25°C, and the volume of the organic solvent indicated by structure expression I occupied in the mixture solvent is 35 - 65% under environmental temperature of 25°C.

Inventors: SAKAI SHIGERU, YAMAMOTO MASAHIRO, SAKAI, SHIGERU, YAMAMOTO, MASAHIRO, Sakai, Shigeru c/o Iwaki Jigyosho, Yamamoto, Masahiro c/o Iwaki Jigyosho
Applicants: FURUKAWA BATTERY CO LTD, FURUKAWA DENCHI KABUSHIKI KAISHA
Application year: 1997
Main classification: H01M10
IPC codes: H01M6, H01M10