Nonaqueous electrolyte secondary battery

Abstract: A nonaqueous electrolyte secondary battery compri­ sing a positive electrode (4) housed in a case (1) a negative electrode (6) arranged in the case (1) so that a separator (5) is sandwiched between the positive and negative electrodes (4, 6) consisting of lithium or lithium-containing material, and a nonaqueous electro­ lyte contained in the case (1) and prepared by dissolving an electrolytic salt consisting of lithium phosphate hexafluoride (LiPF₆) or lithium borofluoride (LiBF₄) in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and at least one ester-based nonaqueous solvent selected from the group consisting of butylene carbonate, dimethyl carbonate, γ-butyrolactone, and sulfolane.

Description: The present invention relates to a nonaqueous electrolyte secondary battery and, more particularly, to a nonaqueous electrolyte secondary battery containing an improved electrolyte. In recent years, a nonaqueous electrolyte battery has attracted attention as a high energy density battery. Of such nonaqueous electrolyte batteries, a primary battery using a light metal such as lithium, sodium, or aluminum as a negative electrode active material and manganese dioxide (MnO₂), carbon fluoride [(CF) n ], thionyl chloride (SOCl₂), or the like as a positive electrode active material is already widely used as a power source of a timepiece or a backup battery of a memory. In addition, as the sizes and weights of various types of electronic equipment such as communication equipment have been decreased, a demand for a secondary battery having a high energy density which can be suitably used as a power source of such equipment has been increased, and a nonaqueous electrolyte secondary battery has been actively studied. For example, a nonaqueous electrolyte secondary battery using lithium as a negative electrode and an electrolyte prepared by dissolving an electrolytic salt such as LiCℓO₄, LiBF₄, LiAsF₆, or LiPF₆ in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-BL), or tetrahydrofuran (THF) has been studied. In addition, a compound which topochemically reacts with lithium such as TiS₂, MoS₂, V₂O₅, or V₆O₁₃ has been studied as a positive electrode material. The above secondary battery, however, has not been put into practical use yet. This is mainly because a charge/discharge efficiency of the battery is low and its number of charge/discharge times or cycle life is short. The reason for this is assumed that lithium as a negative electrode is degraded due to a reaction with an electrolyte. That is, lithium dissolved in an electrolyte as lithium ions upon discharge reacts with a solvent and its surface is partially deactivated when it precipitates upon charge. Therefore, when charge/discharge is repeated, lithium is precipitated in the form of dendrites or small spheres, or is separated from a collector. Since a combination of an electrolyte constituting an electrolytic salt and a nonaqueous solvent has a large effect on a degree of such degradation in lithium, an optimal combination has been studied. For example, Japanese Patent Disclosure (Kokai) No. 62-105375 discloses a method of using an electrolyte containing a solvent mixture composed of 10 to 50 vol% of ethylene carbonate and 50 to 90 vol% of 2-methyltetrahydrofuran. In addition, it is known to use electrolytes containing, as a nonaqueous solvent, solvent mixtures of sulfolane and 1,2-dimethoxyethane, sulfolane and tetrahydrofuran, ethylene carbonate and tetrahydrofuran, and ethylene carbonate and propylene carbonate. None of nonaqueous electrolyte secondary batteries using the above nonaqueous electrolytes, however, has achieved a sufficient charge/discharge efficiency. Elecrtrochem, Acta, 30, 1715 (1985) reports that high lithium charge/discharge can be obtained by using a nonaqueous electrolyte prepared by dissolving 1.5 mol/ℓ of LiAsF₆ as an electrolytic salt in a solvent mixture of ethylene carbonate and 2-methyltetrahydrofuran. LiAsF₆, however, has a problem of toxicity. Therefore, the use of LiPF₆ or LiBF₄ having substantially the same molar conductivity as that of LiAsF6 has been studied. Since, however, both LiPF₆ and LiBF₄ have problems of poor chemical stability and the like, it is difficult to obtain a sufficient lithium charge/discharge efficiency by using a nonaqueous electrolyte prepared by dissolving an electrolyte of this type. Japanese Patent Disclosure (Kokai) No. 59-134568 discloses a nonaqueous electrolyte secondary battery using an electrolyte containing a solvent mixture of ethylene carbonate, propylene carbonate, and at least one member selected from the group consisting of 2-methyltetrahydrofuran, 1,2-dimethoxyethane, and 1,3-dioxolane as the third solvent. Japanese Patent Disclosure (Kokai) No. 63-292578 discloses a technique of improving an electrolyte. This invention describes the use of an electrolyte prepared by dissolving LiPF₆ in 2-methyltetrahydrofuran and electrolyzing the resultant solution by using lithium as an anode. Another known electrolyte improving technique is "THE EFFECT OF DESICCANTS ON THE CYCLING EFFICIENCY OF THE LITHIUM ELECTRODE IN PROPYLENE CARBONATE-BASED ELECTROLYTES" presented by V.R. Koch, et al. in Electrochimice. Acta., Vol. 23, PP. 55 - 62. This reference discloses an effect of performing a neutral alumina treatment and pre-electrolysis for an electrolyte prepared by dissolving LiAsF₆ in polypropylene carbonate in relation to a lithium cycling efficiency. French patent specification No. FR 2 589 631 discloses a rechargeable battery having a lithium anode and an electrolyte solution consisting of a mixture of organic solvents in which is disolved a lithium salt. The organic solvent is constituted by ethylene carbonate mixed with at least one other solvent chosen from a group which includes 2-methyltetrahydrofuran. It is an object of the present invention to provide a nonaqueous electrolyte secondary battery which suppresses degradation in a negative electrode consisting of lithium or lithiuim-containing material caused by a nonaqueous electrolyte, and suppresses precipitation of lithium or lithium-containing material in the form of a dendrite or a small sphere caused by repetitive charge/discharge, thereby improving a charge/discharge cycle life. It is another object of the present invention to provide a nonaqueous electrolyte secondary battery which improves stability of a nonaqueous electrolyte to improve storage properties. It is still another object of the present invention to provide a nonaqueous electrolyte secondary battery which improves operation characteristics at low temperatures. According to an aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery comprising a positive electrode housed in a case, a negative electrode housed in said case so that a separator is sandwiched between the positive and negative electrodes consisting of lithium or a lithium-containing material, and a nonaqueous electrolyte contained in said case and having a composition prepared by dissolving a lithium phosphate hexafluoride or lithium borofluoride in a solvent mixture consisting of at least ethylene carbonate and 2-methyltetrahydrofuran, and containing not more than 100 ppm of an organic peroxide as an impurity and not more than 20 ppm of water. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a partial sectional view showing a nonaqueous electrolyte secondary battery according to the present invention; Fig. 2 is a sectional view showing a conduction treatment apparatus; Fig. 3 is a graph showing a change in discharge capacity as a function of a cycle number in a nonaqueous electrolyte secondary battery of each of Examples 1 and 2 and Controls 2, 7, and 8; Fig. 4 is a graph showing a change in discharge capacity as a function of a cycle number in a nonaqueous electrolyte secondary battery of each of Examples 3 and 4 and Control 1; Fig. 5 is a graph showing a change in discharge capacity as a function of a cycle number in a nonaqueous electrolyte secondary battery of each of Examples 5 to 9 and Controls 1 and 2; Fig. 6 is a graph showing a change in discharge capacity as a function of a cycle number in a nonaqueous electrolyte secondary battery of each of Examples 10 to 14 and Controls 1 and 2; Fig. 7 is a graph showing a cycle number obtained when a discharge capacity reaches 350 mAh in a nonaqueous electrolyte secondary battery of each of Examples 15-1 to 15-4 and Controls 9-1 and 9-2; and Fig. 8 is a graph showing a change in discharge capacity as a function of a cycle number in a nonaqueous electrolyte secondary battery of each of Examples 16 to 18 and Control 1. A nonaqueous electrolyte secondary battery according to the present invention will be described below with reference to Fig. 1. Referring to Fig. 1, a cylindrical case 1 having a bottom houses an insulator 2 arranged on its bottom and electrodes 3. The electrodes 3 have a structure in which a band-like member obtained by stacking a positive electrode 4, a separator 5, and a negative electrode 6 in the order named is spirally wound so that the negative electrode 6 is located outside. The case 1 contains a nonaqueous electrolyte prepared by dissolving an electrolytic salt in a solvent mixture. Insulating paper 7 having an opening formed in its central portion is placed above the electrodes 3 housed in the case 1. An insulating opening sealing plate 8 is arranged at an upper opening portion of the case 1 and liquid-tightly fixed to the case 1 by calking the upper opening portion inwardly. A positive terminal 9 is fitted in the center of the plate 8. One end of a positive lead 10 is connected to the positive electrode 4 and the other end to the positive terminal 9. The negative electrode 6 is connected to the case 1 as a negative terminal via a negative lead (not shown). The case 1 consists of, e.g., stainless steel. The positive electrode 4 consists of a material containing an active material such as amorphous vanadium pentoxide, a manganese oxide, e.g., manganese dioxide and a lithium-manganese composite oxide, titanium disulfide, molybdenum disulfide, and molybdenum selenide. The negative electrode 6 consists of lithium or lithium-containing material. The lithium-containing material is one element selected group consisting of a lithium alloy such as a lithium-aluminum alloy, carbon-containing lithium ions, conductive polymer-containing lithium ions and a lithiuted transion-metal chalcogene. The nonaqueous electrolyte has any of the following compositions. Nonaqueous Electrolyte (a) This nonaqueous electrolyte has a composition prepared by dissolving an electrolytic salt in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and at least one ester-based nonaqueous solvent selected from the group consisting of butylene carbonate, dimethyl carbonate, γ-butyrolactone, and sulfolane. Ethylene carbonate is not easily oxidized and has an effect of increasing the conductivity of the nonaqueous electrolyte due to dissolution of the electrolyte. When the content of ethylene carbonate in the solvent mixture is increased, however, ethylene carbonate precipitates as a solid in the nonaqueous electrolyte to reduce a charge/discharge cycle life of the battery. 2-methyltetrahydrofuran has an effect of reducing the viscosity of the nonaqueous electrolyte and is stable with respect to lithium. 2-methyltetrahydrofuran is easily oxidized to produce an impurity which degrades the negative in the nonaqueous electrolyte. The ester-based nonaqueous solvent has high stability against oxidation and does not precipitate unlike ethylene carbonate even if it is contained in a large amount. When the content of the ester-based nonaqueous solvent in the solvent mixture is increased, however, the conductivity of the nonaqueous electrolyte may be reduced in relation to ethylene carbonate and 2-methyltetrahydrofuran, or the stability with respect to lithium may be reduced. A mixing ratio of these solvents constituting the above solvent mixture is preferably 20 to 60 vol% of ethylene carbonate, 10 to 50 vol% of 2-methyltetrahydrofuran, and 10 to 50 vol% of the ester-based nonaqueous solvent in order to cancel the drawbacks of the solvents and effectively utilize their advantages. Examples of the electrolytic salt are one or two types of lithium salts selected from the group consisting of LiPF₆ and LiBF₄. These lithium salts are particularly effective since they have no toxicity and can increase the conductivity of the nonaqueous electrolyte to be higher than those obtained by other lithium salts. The lithium salt is preferably dissolved in the solvent mixture in an amount of 0.2 mol/ℓ to 1.5 mol/ℓ. This is because if the content falls outside the above range, the conductivity and the lithium charge/discharge efficiency may be reduced. Nonaqueous Electrolyte (b) This nonaqueous electrolyte has a composition prepared by dissolving the electrolytic salt described above in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and at least one ether-based nonaqueous solvent selected from the group consisting of tetrahydrofuran, 2-methylfuran, 1,2-dimethoxyethane, diethoxyethane, 1,3-dioxolane, and 1,3-dimethoxypropane. The ether-based nonaqueous solvent has an effect of forming a stable protection film having a conductivity on the surface of lithium to prevent degradation in negative electrode caused by an impurity contained in the nonaqueous electrolyte. The ether-based nonaqueous solvent, however, is easily oxidized like 2-methyltetrahydrofuran and easily reacts with lithium. Therefore, if the content of the ether-based nonaqueous solvent in the solvent mixture is increased, the nonaqueous electrolyte and the negative electrode are easily degraded upon repetitive charge/discharge in relation to ethylene carbonate and 2-methyltetrahydrofuran. A mixing ratio of these solvents constituting the above solvent mixture is preferably 30 to 70 vol% of ethylene carbonate, 10 to 60 vol% of 2-methyltetrahydrofuran, and 0.5 to 30 vol% of the ether-based nonaqueous solvent in order to cancel the drawbacks of the solvents and effectively utilize their advantages. The nonaqueous electrolyte (b) may consist of a solvent mixture prepared by mixing an ester-based solvent described in the nonaqueous electrolyte (a) in addition to ethylene carbonate, 2-methyltetrahydrofuran, and the ether-based nonaqueous solvent. A ratio of the components constituting such a solvent mixture is preferably 20 to 60 vol% of ethylene carbonate, 10 to 50 vol% of 2-methyltetrahydrofuran, 0.5 to 30 vol% of the ether-based nonaqueous solvent, and 10 to 50 vol% of the ester-based nonaqueous solvent. Nonaqueous Electrolyte (c) This nonaqueous electrolyte has a composition prepared by dissolving 0.1 mol/ℓ (inclusive) to 1 mol/ℓ (exclusive) of lithium phosphate hexafluoride (LiPF₆) in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran or a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and the ether-based nonaqueous solvent described above. A mixing ratio of these solvents constituting the solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran is preferably 40 to 80 vol% of ethylene carbonate and 20 to 60 vol% of 2-methyltetrahydrofuran for the following reason. That is, if the mixing ratio of ethylene carbonate is less than 40 vol%, the conductivity of the nonaqueous electrolyte is reduced. In addition, since the amount of 2-methyltetrahydrofuran as the other solvent is relatively increased, 2-methyltetrahydrofuran is oxidized by an oxidizing decomposition product of LiPF₆ to produce an impurity which degrades the negative electrode. As a result, polarization of the negative electrode may be increased, or a charge/discharge efficiency may be reduced. If the mixing ratio of ethylene carbonate exceeds 80 vol%, the amount of 2-methyltetrahydrofuran as the other solvent is relatively decreased. Therefore, ethylene carbonate easily reacts with lithium constituting the negative electrode, thereby reducing the stability in the nonaqueous electrolyte. A more preferable ratio of the solvents constituting the above solvent mixture is 50 to 60 vol% of ethylene carbonate and 40 to 50 vol% of 2-methyltetrahydrofuran. A mixing ratio of the solvents constituting the solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and the ester-based nonaqueous solvent is preferably the same as the ratio described in the nonaqueous electrolyte (a). This is because precipitation of ethylene carbonate at a low temperature (room temperature or lower) can be suppressed when the LiPF₆ concentration is less than 1 mol/ℓ. The dissolution amount of LiPF₆ in the above solvent mixture is limited for the following reason. That is, if the dissolution amount of LiPF₆ is less than 0.1 mol/ℓ, the conductivity of the nonaqueous electrolyte cannot be increased, and polarization of the negative electrode and that of the positive electrode upon discharge are increased. If the dissolution amount of LiPF₆ is 1 mol/ℓ or more, 2-methyltetrahydrofuran is oxidized by an oxidizing decomposition product of LiPF₆ to produce an impurity which degrades the negative electrode. As a result, the negative electrode is degraded to reduce its charge/discharge efficiency. Nonaqueous Electrolyte (d) This nonaqueous electrolyte has a composition prepared by dissolving lithium borofluoride (LiBF₄) in a solvent mixture consisting of 50 vol% (exclusive) to 80 vol% (inclusive) of ethylene carbonate and 20 vol% (inclusive) to 50 vol% (exclusive) of 2-methyltetrahydrofuran. The mixing ratio of these solvents constituting the above solvent mixture is limited for the following reason. If the mixing ratio of ethylene carbonate is 50 vol% or less, the conductivity of the nonaqueous electrolyte is reduced. In addition, since the amount of 2-methyltetrahydrofuran as the other solvent is relatively increased, 2-methyltetrahydrofuran is oxidized by an oxidizing decomposition product of LiBF₄ to produce an impurity which degrades the negative electrode. As a result, polarization of the negative electrode is increased, or a charge/discharge efficiency is decreased. If the mixing ratio of ethylene carbonate exceeds 80 vol%, the amount of 2-methyltetrahydrofuran as the other solvent is reduced. Therefore, ethylene carbonate easily reacts with lithium constituting the negative electrode, thereby reducing the stability in the nonaqueous electrolyte. A more preferable ratio of the solvents constituting the above solvent mixture is 55 to 75 vol% of ethylene carbonate and 25 to 45 vol% of 2-methyltetrahydrofuran. The dissolution amount of LiBF₄ in the above solvent mixture is preferably 0.2 mol/ℓ to 1.5 mol/ℓ for the following reason. That is, if the dissolution amount of LiBF₄ is less than 0.2 mol/ℓ, the conductivity of the nonaqueous electrolyte cannot be increased, and polarization of the negative electrode and that of the positive electrode upon discharge may be increased. If the dissolution amount of LiBF₄ exceeds 1.5 mol/ℓ, 2-methyltetrahydrofuran is oxidized by an oxidizing decomposition product of LiBF₄ to produce an impurity which degrades the negative electrode. As a result, the negative electrode may be degraded to reduce its charge/discharge efficiency. Nonaqueous Electrolyte (e) This nonaqueous electrolyte has a composition prepared by dissolving the electrolytic salt described in the nonaqueous electrolyte (a) in a solvent mixture consisting of at least ethylene carbonate and 2-methyltetrahydrofuran and containing 100 ppm or less of an organic peroxide as an impurity and 20 ppm or less of water. The solution obtained by dissolving the electrolytic salt in the solvent mixture may have a composition prepared by dissolving an electrolytic salt such as LiPF₆ or LiBF₄ in a solvent mixture consisting of 20 to 80 vol% (more preferably, 40 to 60 vol%) of ethylene carbonate and 20 to 80 vol% (more preferably, 40 to 60 vol%) of 2-methyltetrahydrofuran or the composition described in any one of the nonaqueous electrolytes (a) to (d). The content of the impurity (organic peroxide) in the nonaqueous electrolyte is limited for the following reason. That is, if the content of the organic peroxide exceeds 100 ppm, the negative electrode is degraded from the beginning of a cycle, and the battery capacity and the cycle life are reduced. More preferably, the content of the organic peroxide is 30 ppm or less. The content of water in the nonaqueous electrolyte is limited since the negative electrode and the electrolyte are degraded if the content exceeds 20 ppm, thereby degrading long-time storage properties and cycle life performance. The above nonaqueous electrolyte is prepared by dissolving the above electrolytic salt in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran to prepare a solution and performing a treatment of bringing the resultant solution into contact with an insoluble absorbent and a conduction treatment for the resultant solution. The treatment using the insoluble absorbent is performed by, e.g., the following methods. That is, an insoluble absorbent such as active alumina or a molecular sieve of an inorganic material which does not react with the above solution is added to the solution and separated from the resultant solution by filtering or the like after the solution is stirred. Alternatively, the above solution is flowed through a column containing the above insoluble absorbent. The conduction treatment is performed by, e.g., the following method. That is, an electrode consisting of lithium is dipped as an anode in the above solution, and an electrode consisting of lithium or another metal is dipped as a cathode therein. Subsequently, a current is continuously or intermittently applied between the anode and cathode to precipitate lithium on the cathode. In this case, a constant voltage may be used instead of the constant current. Alternatively, lithium precipitated on the cathode may be repeatedly dissolved and precipitated. Although each of the treatment of bringing the solution into contact with the insoluble absorbent and the conduction treatment need only be performed once, it is more effective to perform the treatment of bringing the solution into contact with the insoluble absorbent before and after the conduction treatment. The treatment of bringing the solution into contact with the insoluble absorbent and the conduction treatment can be simultaneously performed by using a conduction treatment apparatus shown in Fig. 2. This conduction treatment apparatus will be described below. Referring to Fig. 2, the interior of a conduction cell 21 consisting of pyrex glass is divided into three chambers 23a to 23c by two porous glass mats 22a and 22b. Plug receiving holes 24a to 24c are formed in upper walls of the conduction cell 21 corresponding to the chambers 23a to 23c, respectively. A granular insoluble absorbent 25 is filled in the chamber 23b through the plug receiving hole 24b at the center of the conduction cell 21. A plug 26 is inserted in the hole 24b after the insoluble absorbent 25 is filled. An anode 27 is arranged in the chamber 23c at the right side of the cell 21. A lead wire 28 extending from the upper end of the anode 27 is connected to a power source (not shown) through a plug 29 inserted in the hole 24c. A cathode 30 is arranged in the chamber 23a at the left side of the cell 21. A lead wire 31 extending from the upper end of the cathode 30 is connected to the power source (not shown) through a plug 32 inserted in the hole 24a. The solution described above is contained in each of the chambers 23a and 23c of the conduction treatment apparatus shown in Fig. 2, and a constant current or voltage is applied from the power source (not shown) to the anode 27 and the cathode 30. Therefore, since the conduction treatment can be performed while the solution is in contact with the insoluble absorbent 25, a treatment time can be reduced. Note that the electrodes of the nonaqueous electrolyte secondary battery shown in Fig. 1 have a spirally wound structure. The electrodes, however, may have a multilayered structure obtained by stacking a plurality of positive electrodes and negative electrodes at both sides of a separator. According to the present invention, the positive and negative electrodes are housed in the case so as to sandwich the separator therebetween, and one of the nonaqueous electrolytes (a) to (e) is contained in the case. Since the nonaqueous electrolytes (a) to (e) have high conductivities and can prevent degradation in the negative electrode to suppress dendrite precipitation of lithium, a nonaqueous electrolyte secondary battery having a long charge/discharge cycle life can be obtained. The effects obtained by the nonaqueous electrolytes (a) to (e) will be described in detail below. (1) Nonaqueous electrolyte secondary battery using nonaqueous electrolyte (a) Of the electrolytes consisting of various lithium salts, LiPF₆ and LiBF₄ are most effective since they have no toxicity and can increase the conductivity of a nonaqueous electrolyte to be higher than those obtained by the other lithium salts. Since, however, LiPF₆ or LiBF₄ is chemically unstable, it produces a Lewis acid such as PF₅ or BF₃ upon decomposition. Note that LiBF₄ is more stable than LiPF₆ and therefore does not easily produce BF₃. The Lewis acid reacts with 2-methyltetrahydrofuran mixed in the nonaqueous electrolyte (a) and is weak against oxidation to produce an impurity which degrades lithium constituting the negative electrode. As in the system of the nonaqueous electrolyte (a), however, the reaction between 2-methyltetrahydrofuran and the Lewis acid can be suppressed by mixing, in addition to ethylene carbonate and 2-methyltetrahydrofuran, at least one ester-based nonaqueous solvent selected from the group consisting of butylene carbonate, dimethyl carbonate, γ-butyrolactone, and sulfolane. As a result, since the impurity which degrades the negative electrode can be prevented from being produced in the nonaqueous electrolyte, a nonaqueous electrolyte secondary battery having an improved charge/discharge cycle life and good storage properties can be obtained. In the nonaqueous electrolyte consisting of the two components, i.e., ethylene carbonate and 2-methyltetrahydrofuran, if the mixing ratio of ethylene carbonate is increased, ethylene carbonate precipitates at a low temperature (below an ice point) to interfere with a low-temperature operation of the secondary battery. As in the system of the nonaqueous electrolyte (a), however, precipitation of ethylene carbonate at a low temperature can be suppressed by mixing the ester-based nonaqueous solvent as the third component. As a result, a nonaqueous electrolyte secondary battery having improved low-temperature operation characteristics can be obtained. (2) Nonaqueous electrolyte secondary battery using nonaqueous electrolyte (b) As described above, an impurity which degrades lithium constituting the negative electrode is produced in a nonaqueous electrolyte using LiPF₆ or LiBF₄ as an electrolytic salt and 2-methyltetrahydrofuran as one component of a solvent. As in the system of the nonaqueous electrolyte (b), however, by mixing, in addition to ethylene carbonate and 2-methyltetrahydrofuran, at least one ether-based nonaqueous solvent selected from the group consisting of tetrahydrofuran, 2-methylfuran, 1,2-dimethoxyethane, diethoxyethane, 1,3-dioxolane, and 1,3-dimethoxypropane, a conductive protection film capable of preventing a reaction with the impurity can be formed on the surface of the negative electrode. As a result, since degradation in the negative electrode caused by the nonaqueous electrolyte can be suppressed, a nonaqueous electrolyte secondary battery having an improved charge/discharge cycle life can be obtained. (3) Nonaqueous electrolyte secondary battery using nonaqueous electrolyte (c) As described above, an impurity which degrades lithium constituting the negative electrode is produced in a nonaqueous electrolyte using LiPF₆ as an electrolytic salt and 2- methyltetrahydrofuran as one component of a solvent. As in the system of the nonaqueous electrolyte (c), however, by setting the dissolution concentration of LiPF₆ to be 0.1 mol/ℓ (inclusive) to 1 mol/ℓ (exclusive), decomposition of LiPF₆ can be remarkably suppressed, and therefore the Lewis acid which oxidizes 2-methyltetrahydrofuran can be prevented from being produced. As a result, since an impurity which degrades the negative electrode can be prevented from being produced in the nonaqueous electrolyte, a nonaqueous electrolyte secondary battery having an improved charge/discharge cycle life and good storage properties can be obtained. (4) Nonaqueous electrolyte secondary battery using nonaqueous electrolyte (d) As in the system of the nonaqueous electrolyte (d), by using LiBF₄ as an electrolytic salt, an impurity which degrades lithium constituting the negative electrode can be prevented from being produced more effectively than in a nonaqueous electrolyte using LiPF₆ as an electrolytic salt. In addition, as in the system of the nonaqueous electrolyte (d), by setting an ethylene carbonate amount in a solvent mixture to be 50 vol% (exclusive) to 80 vol% (inclusive), the conductivity can be significantly increased in relation to LiBF₄. As a result, a nonaqueous electrolyte secondary battery having an improved charge/discharge cycle life can be obtained. (5) Nonaqueous electrolyte secondary battery using nonaqueous electrolyte (e) As in the nonaqueous electrolyte (e), degradation in lithium constituting the negative electrode can be remarkably suppressed by a composition prepared by dissolving an electrolytic salt in a solvent mixture consisting of at least ethylene carbonate and 2-methyltetrahydrofuran and containing 100 ppm or less of an organic peroxide as an impurity and 20 ppm or less of water. This effect most conspicuously appears when LiPF₆ or LiBF₄ is used as the electrolytic salt. The nonaqueous electrolyte (e) is prepared by dissolving an electrolytic salt in a solvent mixture consisting of at least ethylene carbonate and 2-methyltetrahydrofuran to prepare a solution and performing a treatment of bringing the resultant solution into contact with an insoluble absorbent and a conduction treatment for the resultant solution. LiPF₆ and LiBF₄ are effective since they are stable with respect to the conduction treatment. As a result, a nonaqueous electrolyte secondary battery having a long charge/discharge cycle life and good storage properties can be obtained. The present invention will be described in detail below by way of its examples. Control 1 1.0 mol/ℓ of LiBF₄ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50) to prepare a nonaqueous electrolyte. The resultant nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Control 2 1.0 mol/ℓ of LiPF₆ was dissolved in a solvent mixture consisting of ethylene-carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50) to prepare a nonaqueous electrolyte. The resultant nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Control 3-1 1.0 mol/ℓ of LiPF₆ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50) to prepare a nonaqueous electrolyte. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Control 3-2 1.5 mol/ℓ of LiPF₆ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50) to prepare a nonaqueous electrolyte. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Controls 4-1 & 4-2 1.0 mol/ℓ and 1.5 mol/ℓ of LiPF₆ were dissolved in solvent mixtures each consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50) to prepare two types of nonaqueous electrolytes. Each of the prepared nonaqueous electrolytes and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom, thereby assembling two types of nonaqueous electrolyte secondary batteries shown in Fig. 1. The obtained batteries were left to stand at 30°C for one year. Control 5 1.0 mol/ℓ of LiBF₄ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 40 : 60) to prepare a nonaqueous electrolyte. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Control 6 1.0 mol/ℓ of LiBF₄ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 30 : 70) to prepare a nonaqueous electrolyte. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 1 80 wt% of an amorphous vanadium pentoxide powder, 15 wt% of acetylene black, and 5 wt% of a polytetrafluoroethylene powder were mixed to form a sheet, and the sheet was crimped on an expanded metal collector to manufacture a sheet-like positive electrode. Subsequently, the positive electrode, a separator formed of a polypropylene porous film, and a negative electrode formed of a sheet-like lithium foil were stacked in the order named and spirally wound so that the negative electrode was located outside, thereby manufacturing electrodes. 1.0 mol/ℓ of LiPF₆ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50), and 10 g of active alumina were added to 100 mℓ of the resultant solution mixture. This solution was intermittently stirred for 12 hours or more and filtered to remove the active alumina. Subsequently, an anode and a cathode each formed of an 8-cm² are lithium plate were arranged in the solution, and a current having a current density of 1 mA/cm² flowed for 10 hours or more to perform a conduction treatment. A contact treatment of the active alumina and the conduction treatment were performed again to prepare a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The electrodes and the nonaqueous electrolyte were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 2 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiPF₆ in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 1 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Control 7 1.0 mol/ℓ of LiPF₆ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50), and 10 g of active alumina were added to 100 mℓ of the resultant solution. This solution was intermittently stirred for 12 hours or more, and only an active alumina contact treatment for filtering active alumina was performed, thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain water, but in small amounts of 20 ppm or less. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 1 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Control 8 1.0 mol/ℓ of LiPF₆ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50), and an anode and a cathode each formed of a lithium plate having an area of about 8 cm² were arranged in the resultant solution. Thereafter, a current having a current density of 1 mA/cm² was flowed for ten hours or more to perform only a conduction treatment, thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide, but in small amounts of 100 ppm or less. The prepared nonaqueous electrolyte and electrode similar to those used in Example 1 were housed in a cylindrical stainless steel case having a bottom to assemble a nonaqueous electrolyte secondary battery shown in Fig. 1. Charge/discharge was repeatedly performed for each of the nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Controls 7 and 8 with a charge current of 100 mA and a discharge current of 100 mA, and a discharge capacity and a cycle life of each battery were measured. The measurement results are shown in Fig. 3. Fig. 3 also shows the measurement results of the nonaqueous electrolyte secondary battery of Control 2. As is apparent from Fig. 3, although initial discharge capacities of the nonaqueous electrolyte secondary batteries of Examples 1 and 2 are substantially similar to those of the batteries of Controls 2, 7, and 8, their cycle lives are much longer than those obtained by Controls 2, 7, and 8. In addition, the nonaqueous electrolyte secondary batteries of Examples 1 and 2 had better storage properties than those of the batteries of Controls 2, 7, and 8. Example 3 80 wt% of a spinell lithium-manganese composite oxide (LiMn₂O₄) powder, 15 wt% of acetylene black, and 5 wt% of a polytetrafluoroethylene powder were mixed to form a sheet, and the sheet was crimped on an expanded metal collector to form a sheet-like positive electrode. Subsequently, the positive electrode, a separator formed of a polypropylene porous film, and a negative electrode formed of a sheet-like lithium foil were stacked in the order named and spirally wound so that the negative electrode was located outside, thereby manufacturing electrodes. 1.0 mol/ℓ of LiBF₄ was dissolved in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50), and 10 g of active alumina were added to 100 mℓ of the resultant solution. This solution was intermittently stirred for 12 hours or more and filtered to remove the active alumina. Subsequently, an anode and a cathode each formed of a lithium plate having an area of about 8 cm² were arranged in the solution, and a current having a current density of 1 mA/cm² was flowed for 10 hours or more, thereby performing a conduction treatment. The active alumina contact treatment and the conduction treatment were performed again to prepare a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 4 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiBF₄ in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Charge/discharge was repeatedly performed for each of the nonaqueous electrolyte secondary batteries of Examples 20 and 21 with a charge current of 100 mA and a discharge current of 100 mA, and a discharge capacity and a cycle life of each battery were measured. The measurement results are shown in Fig. 4. Fig. 4 also shows the measurement results of the nonaqueous electrolyte secondary battery of Control 1. As is apparent from Fig. 4, although initial discharge capacities of the nonaqueous electrolyte secondary batteries of Examples 3 and 4 are substantially similar to that of the battery of Control 1, their cycle lives are much longer than that obtained by Control 1. In addition, the nonaqueous electrolyte secondary batteries of Examples 3 and 4 had better storage properties than those of the battery of Control 1. Example 5 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiPF₆ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran and sulfolane (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary batteries shown in Fig. 1. Example 6 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiBF₄ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and sulfolane (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 7 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiPF₆ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and γ-butyrolactone (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 8 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiBF₄ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and γ-butyrolactone (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 9 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiPF₆ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and butylene carbonate (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Charge/discharge was repeatedly performed for each of the nonaqueous electrolyte secondary batteries of Examples 5 to 9 with a charge current of 100 mA and a discharge current of 100 mA, and a discharge capacity and a cycle life of each battery were measured. The measurement results are shown in Fig. 5. Fig. 5 also shows the measurement results of the nonaqueous electrolyte secondary batteries of Controls 1 and 2. As is apparent from Fig. 5, although initial discharge capacities of the nonaqueous electrolyte secondary batteries of Examples 5 to 9 are substantially similar to those of the batteries of Controls 1 and 2, their cycle lives are much longer than those obtained by Controls 1 and 2. In addition, the nonaqueous electrolyte secondary batteries of Examples 5 to g had better storage properties than those of the batteries of Controls 1 and 2. Example 10 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiPF₆ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and 2-methylfuran (mixing volume ratio = 50 : 49 : 1) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 11 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiBF₄ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and 2-methylfuran (mixing volume ratio = 50 : 49 : 1) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 12 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiPF₆ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and tetrahydrofuran (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 13 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiBF₄ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and tetrahydrofuran (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 14 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiPF₆ in a solvent mixture consisting of ethylene carbonate, 2-methyltetrahydrofuran, and 1,3-dimethoxypropane (mixing volume ratio = 50 : 25 : 25) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm of less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Charge/discharge was repeatedly performed for each of the nonaquenous electrolyte secondary batteries of Examples 10 to 14 with a charge current of 100 mA and a discharge current of 100 mA, and a discharge capacity and a cycle life of each battery were measured. The measurement results are shown in Fig. 6. Fig. 6 also shows the measurement results of the nonaqueous electrolyte secondary batteries of Controls 1 and 2. As is apparent from Fig. 6, although initial discharge capacities of the nonaqueous electrolyte secondary batteries of Examples 10 to 14 are substantially similar to those of the batteries of Controls 1 and 2, their cycle lives are much longer than those obtained by Controls 1 and 2. In particular, the battery of each of Examples 12 and 14 has a very long service life. Examples 15-1 - 15-4 0.5 mol/ℓ, 0.2 mol/ℓ, 0.1 mol/ℓ, and 0.8 mol/ℓ of LiPF₆ were dissolved in solvent mixtures each consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50), and 10 g of active alumina was added to 100 mℓ of each of four types of the resultant solutions. Each solution was intermittently stirred for 12 hours or more and filtered to remove the active alumina. Subsequently, an anode and a cathode each formed of a lithium plate having an area of about 8 cm² were arranged in each solution, and a current having a current density of 1 mA/cm² or more was flowed for 10 hours or more, thereby performing a conduction treatment. The active alumina contact treatment and the conduction treatment were performed again to prepare four types of nonaqueous electrolytes. Each nonaqueous electrolytes, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. Each of the prepared nonaqueous electrolytes and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom, thereby assembling four types of nonaqueous electrolyte secondary batteries shown in Fig. 1. Controls 9-1 & 9-2 1.0 mol/ℓ and 1.5 mol/ℓ of LiPF₆ were dissolved in solvent mixtures each consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 50 : 50), and 10 g of active alumina were added to 100 mℓ of each of two types of the resultant solutions. Each solution was intermittently stirred for 12 hours or more and filtered to remove the active alumina. Subsequently, an anode and a cathode each formed of a lithium plate having an area of about 8 cm² were arranged in each solution, and a current having a current density of 1 mA/cm² of more was flowed for 10 hours or more, thereby performing a conduction treatment. The active alumina contact treatment and the conduction treatment were performed again to prepare two types of nonaqueous electrolytes. Each nonaqueous electrolytes, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. Each of the prepared nonaqueous electrolytes and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom, thereby assembling two types of nonaqueous electrolyte secondary batteries shown in Fig. 1. Charge/discharge was repeatedly performed for each of the nonaqueous electrolyte secondary batteries of Examples 15-1 to 15-4 and Controls 9-1 and 9-2 with a charge current of 100 mA and a discharge current of 100 mA, and a cycle number obtained when a discharge capacity reached 350 mAh was checked. The results are shown in Fig. 7. As is apparent from Fig. 7, the nonaqueous electrolyte secondary batteries of Examples 15-1 to 15-4 have much longer cycle lives than those of the batteries of Controls 9-1 and 9-2. Example 16 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiBF₄ in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 60 : 40) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm2 for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 17 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiB₄ in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 70 : 30) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Example 18 Active alumina powder was filled as the insoluble absorbent 25 in the chamber 23b of the conduction cell 21 of the above-mentioned conduction treatment apparatus shown in Fig. 2, and a solution prepared by dissolving 1.0 mol/ℓ of LiBF₄ in a solvent mixture consisting of ethylene carbonate and 2-methyltetrahydrofuran (mixing volume ratio = 55 : 45) was injected in the left and right chambers 23a and 23c. Subsequently, a conduction treatment was performed between the anode 27 formed of a lithium plate having an area of about 8 cm² and the cathode 30 formed of a nickel plate having an area of about 8 cm² with a current density of 1 mA/cm² for 10 hours or more by a power source (not shown), thereby preparing a nonaqueous electrolyte. The nonaqueous electrolyte, thus prepared, was found to contain an organic peroxide and water, but in small amounts of 100 ppm or less and 20 ppm or less, respectively. The prepared nonaqueous electrolyte and electrodes similar to those used in Example 3 were housed in a cylindrical stainless steel Case having a bottom to assemble the nonaqueous electrolyte secondary battery shown in Fig. 1. Charge/discharge was repeatedly performed for each of the nonaqueous electrolyte secondary batteries of Examples 16 to 18 with a charge current of 100 mA and a discharge current of 100 mA, and a discharge capacity and a cycle life of each battery were measured. The measurement results are shown in Fig. 8. Fig. 8 also shows the measurement results of the nonaqueous electrolyte secondary battery of Control 1. As is apparent from Fig. 8, although initial discharge capacities of the nonaqueous electrolyte secondary batteries of Examples 16 to 18 are substantially similar to that of battery Control 1, their cycle lives are much longer than that obtained by Control 1. As has been described above, according to the present invention, a nonaqueous electrolyte secondary battery having a long charge/discharge cycle life and a large battery capacity can be provided.

Claims: A nonaqueous electrolyte secondary battery comprising: a positive electrode (4) housed in a case (1); a negative electrode (6) housed in said case (1) so that a separator (5) is sandwiched between said positive and negative electrodes (4,6) consisting of lithium or lithium-containing material; and a nonaqueous electrolyte contained in said case (1) and having a composition prepared by dissolving lithium phosphate hexafluoride (LiPF₆) or lithium borofluoride (LiBF₄) in a solvent mixture consisting of at least ethylene carbonate and 2-methyltetrahydrofuran and containing not more than 100 ppm of an organic peroxide as an impurity, and not more than 20 ppm of water. The secondary battery according to claim 1, characterized in that said lithium-containing material is one element selected from the group consisting of a lithium alloy, carbon-containing lithium ions, conductive polymer-containing lithium ions and a lithiuted transition metal chalcogene. The secondary battery according to claim 1, characterized in that said nonaqueous electrolyte has a composition prepared by dissolving an electrolytic salt in a solvent mixture consisting of at least ethylene carbonate and 2-methyltetrahydrofuran and subjected to a treatment of bringing said composition in contact with an insoluble absorbent and a conduction treatment. The secondary battery according to claim 1, characterized in that said solvent mixture consists of 40 to 80 vol% of ethylene carbonate and 20 to 60 vol% of 2-methyltetrahydrofuran. The secondary battery according to claim 1, characterized in that said solvent mixture consists of ethylene carbonate, 2-methyltetrahydrofuran, and at least one nonaqueous solvent selected from the group consisting of propylene carbonate, butylene carbonate, dimethyl carbonate, γ-butyrolactone, sulfolane, tetrahydrofuran, 2-methylfuran, 1,2-dimethoxyethane, diethoxyethane, 1,3-dioxolane, and 1,3-dimethoxypropane. The secondary battery according to claim 1, characterized in that said nonaqueous electrolyte has a composition prepared by dissolving 0.1 mol/ℓ (inclusive) to 1 mol/ℓ (exclusive) of lithium phosphate hexafluoride (LiPF₆) in a solvent mixture consisting of at least ethylene carbonate and 2-methyltetrahydrofuran and containing not more than 100 ppm of an organic peroxide as an impurity and not more than 20 ppm of water. The secondary battery according to claim 1, characterized in that said nonaqueous electrolyte has a composition prepared by dissolving lithium borofluoride (LiBF₄) in a solvent mixture consisting of 50 vol% (exclusive) to 80 vol% (inclusive) of ethylene carbonate and 20 vol% (inclusive) to 50 vol% (exclusive) of 2-methyltetrahydrofuran and containing not more than 100 ppm of an organic peroxide as an impurity and not more than 20 ppm of water.

Inventors: INADA KUNIAKI C O INTELLECTUAL, KURISU NORIHITO C O INTELLECTU, OHSAKI TAKAHISA C O INTELLECTU, TAKABAYASHI JUNICHI C O INTELL, TAKAMI NORIO C O INTELLECTUAL, YAMADA SHUJI C O INTELLECTUAL, INADA, KUNIAKI, C/O INTELLECTUAL PROPERTY DIV., KURISU, NORIHITO, C/O INTELLECTUAL PROPERTY DIV., OHSAKI, TAKAHISA, C/O INTELLECTUAL PROPERTY DIV., TAKABAYASHI, JUNICHI, C/O INTELLEC. PROPERTY DIV., TAKAMI, NORIO, C/O INTELLECTUAL PROPERTY DIV., YAMADA, SHUJI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA BATTERY, TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA, TOSHIBA BATTERY CO., LTD.
Application year: 1990
Main classification: H01M10
IPC codes: H01M10