Non-aqueous electrolyte solvent and battery containing this electrolyte

Abstract: A solvent for cell electrolyte comprising a carbamate of formula (I): wherein R¹, R² and R³ independently represent linear or branched alkyl groups may be used in batteries showing high withstand voltage and excellent charge/discharge efficiency.

Description: The present invention relates to a non-aqueous electrolyte for electric cells and a solvent for the electrolyte and particularly it relates to an electrolyte and a solvent for the electrolyte suitable for batteries, e.g. a secondary cell or rechargeable cell. The present invention further relates to a non-aqueous electrolyte battery utilizing lithium, more specifically a non-aqueous electrolyte battery comprising a specific non-aqueous solvent which prevents rapid heat generation and breakdown of the cell which may be caused by overcharge of the cell. Recently, various types of portable electronic equipment such as camera-integrated VTR's, portable phones and lap-top computers have been developed. Effort to make such equipment smaller and lighter is still being continued. For this reason, much attention has been paid to batteries as a portable electric source for the portable electronic equipment and research for providing batteries capable of realizing a high energy density is being conducted. In particular, lithium batteries are actively being investigated because lithium batteries can provide a higher energy density as compared with batteries using an aqueous solution electrolyte such as lead batteries and nickel/cadmium cells. Electric cells utilizing a non-aqueous electrolyte have been conventionally and widely used as electric sources in various types of consumer electronic equipment because of their high voltage, high energy density and excellent properties relating to reliability of the cells such as storage characteristics and anti-leak property. Conventional non-aqueous electrolytes usually comprise propylene carbonate. Such electrolytes have been widely used for various primary cells including lithium cells because propylene carbonate is a good solvent for supporting salts, is stable to alkali metals, and can show excellent discharging characteristics. While batteries are of course required to have a high energy density, excellent discharge characteristics and low self-discharge rate like primary cells, the batteries, in addition to the properties mentioned above, must satisfy the requirements that they have a high energy efficiency upon charging and discharging (charge/discharge efficiency) and that the reversibilities of the chemical and physical properties of the active materials are maintained upon repetition of the charge/discharge cycles. The charge/discharge efficiency and the internal resistance of batteries are greatly influenced by the type of electrolyte. For example, if the electroconductivity of the electrolyte is high, the charge/discharge efficiency is improved and the internal resistance is lowered. Therefore, electrolytes for batteries are required to have, for example, a high electroconductivity, anti-redox properties and high withstand voltages. However, non-aqueous electrolytes generally have an electroconductivity which is tens to hundreds times lower than that of aqueous solution electrolytes. In particular, non-aqueous electrolytes having a high withstand voltage generally have poor charge/discharge characteristics and insufficient internal resistance. For example, when propylene carbonate is used as the non-aqueous electrolyte for batteries, the charge/discharge efficiency (discharge capacity/charge capacity) is lowered to about 50 to 60%. Further, propylene carbonate cannot provide a sufficient electroconductivity because it has a high viscosity and hence a low ion migration rate. Although attempts to improve the electric double layer conductivity of the cells by adding, to the propylene carbonate, cyclic ethers such as 1,3-dioxolanes and tetrahydrofurans or linear chain ethers such as 1,2-dimethoxyethane (DME) and diethyl ether, have been disclosed (e.g., DENKI KAGAKU (Electrochemistry) 53, No. 3, 173 (1985)), the addition of these ether compounds impairs the oxidation resistance of the solvent and then lowers the withstand voltage of the electrolyte. Further, because of the high reactivity of metallic lithium, metallic lithium deposited in lithium cells during the charge/discharge cycles may react with the solvent. For instance, N-methyl-2-oxazolidinone reacts with metallic lithium and hence is yellowed. Such a reaction may badly affect the charge/discharge efficiency and the lifetime of the cells. Further, electric cells are generally manufactured in a sealed structure, but when the internal pressure of the cells is increased for some reason, the cells may be broken and hence dysfunction. For instance, when non-aqueous electrolyte cells are overcharged by an electric current of a larger amount than expected, the electrolyte, for instance, decomposes and generates gas. The gas may elevate the internal pressure. Further, if such an overcharge state lasts long, abnormal reactions such as rapid decomposition of the electrolyte and the active materials and rapid elevation of the cell temperature may be caused. These may invite breakdown of the cells. To solve this problem, sealed cells of an explosion proof type have been proposed. The cells of this type are provided with a current cut-off device or a pressure release device, which operates in response to an increase of the internal pressure of the cells. For example, if the internal pressure of the cells is elevated as a result of accumulation of gases generated by chemical reactions caused by a long lasting overcharge state inside of the cells, the current cut-off device operates to cut the charging current, or the pressure release device operates to release the internal pressure. However, if the overcharge state is continued for a long time in the conventional sealed cells of the explosion proof type, the abnormal exothermic reactions often proceed even after the current cut-off device or the pressure release device has operated, and thus the cell temperature may elevate from 50 or 60°C to 300 or 400 °C. This rapid temperature elevation may cause rapid elevation of the internal pressure, which may lead to breakdown of the cells, and is considered to be a serious problem. In view of the above-described problems, the present invention seeks to provide an electrolyte and solvent for the electrolyte which have excellent electroconductivity, no(or less)reactivity with alkali metals and which are able to withstand voltage. The present invention also seeks to provide an electrolyte and a solvent for the electrolyte capable of providing batteries with excellent charge/discharge characteristics and stability. Further, the present invention seeks to prevent the abnormal exothermic reactions which may be observed even after the current cut-off device or the pressure release device has operated, to prevent the rapid elevation of the cell temperature and the cell internal pressure, and thereby to avoid breakdown of the cells. US-A-3,686,038 discloses a battery comprising a solvent consisting essentially of from 100 to 60 % by weight of a carbamate. According to the first aspect of the present invention, there is provided a solvent for cell electrolyte comprising: at least one carbamate of formula [I]: wherein R 1 , R 2 and R 3 are the same or different and each is a linear or branched alkyl group, in an amount of from more than 0.02% to less than 30% by volume based on the total volume of the solvent; and at least one cyclic carbonate of formula (II); wherein R 4 and R 5 are the same or different and each is hydrogen, methyl or ethyl. R 1 , R 2 and R 3 are preferably alkyl groups having 1 to 4 carbon atoms. According to the second aspect of the invention, there is provided a cell electrolyte comprising an electrolyte solvent as defined above. The electrolyte may comprise at least one solute electrolyte selected from LiPF 6 , LiBF 4 and LiClO 4 . According to the third aspect of the invention, there is provided a non-aqueous electrolyte battery comprising a negative electrode capable of being reversibly doped with lithium, a positive electrode and a non-aqueous electrolyte comprising a solute electrolyte in solution in a non-aqueous solvent as defined above or an electrolyte as defined above. The carbamate of formula [I] is preferably present in an amount of from 0.05 to 20% by volume of the total volume of the non-aqueous solvent. The positive electrode of the cell may comprise Li x MO 2 wherein M is at least one transition metal and 0.05 ≦ x ≦ 1.10. Fig. 1 shows an embodiment of the present invention, a coin-like shape cell. Fig. 3 is a sectional view of the cells manufactured in Examples 1 to 9 and Comparative Examples 1 to 4. The cells comprise a negative electrode 1, negative current collector la, positive electrode 2, positive current collector 2a, separator 3, insulator 4, cell housing can 5, sealing gasket 6, cell lid 7, current cut-off plate 8, negative electrode lead 9 and positive electrode lead 10. The solvent for cell electrolyte of the present invention will now be explained in more detail. Examples of the carbamate of formula (I), namely the alkyl N,N-dialkylcarbamate, include methyl N,N-dimethylcarbamate, methyl N,N-diethylcarbamate, ethyl N,N-dimethylcarbamate, ethyl N,N-diethylcarbamate, propyl N,N-dimethylcarbamate, propyl N,N-diethylcarbamate, propyl N,N-dipropylcarbamate, isopropyl N,N-dimethylcarbamate, isopropyl N,N-diethylcarbamate, iosopropyl N,N-diisopropylcarbamate, butyl N,N-dimethylcarbamate, butyl N,N-diethylcarbamate, butyl N,N-dipropylcarbamate and butyl N,N-diisopropylcarbamate. These carbamates may be used alone or in any combination. Particularly preferred are methyl N,N-dimethylcarbamate, ethyl N,N-dimethylcarbamate, ethyl N,N-diethylcarbamate and propyl N,N-dimethylcarbamate. The carbamates of formula [I] have various advantages, for example 1) they are solvents having a low viscosity and high dielectric constant, 2) they exhibit a high withstand voltage, 3) they have relatively high electroconductivity when they dissolve solute electrolytes and 4) they may be used in a wide temperature range. Examples of the cyclic carbonate of formula (II) are ethylene carbonate, propylene carbonate, 2,3-butylene carbonate, 1,2-butylene carbonate, 2,3-amylene carbonate and 3,4-hexylene carbonate. Among these cyclic carbonates, ethylene carbonate, propylene carbonate and 2,3-butylene carbonate are particularly preferred. These carbonates may be used alone or in any combination by mixing with the carbamate compounds. Other non-aqueous solvents such as lactones and nitriles may also be used in the solvent of the present invention. The non-aqueous solvents are not particularly limited. Examples of the non-aqueous solvent which may be used with the carbamates and cyclic carbonates are vinylene carbonate, γ-butyrolactone, sulfolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolan, methyl propionate, methyl butyrate, dimethyl carbonate, diethyl carbonate and dipropyl carbonate. From the view point of stability to voltage, the carbamates and cyclic carbonates may be preferably used in combination with other cyclic carbonates such as vinylene carbonate, or linear carbonates such as dimethyl carbonate, diethyl carbonate and dipropyl carbonate. By adding the cyclic carbonates of formula (II) to the solvent, it becomes possible to further improve the solubility of the solute electrolyte in the solvent. Therefore, a sufficiently high concentration of solute electrolyte can be obtained, and thereby the electroconductivity and the charge/discharge efficiency can be much more improved. When the carbamate of formula (I) and the cyclic carbonate of formula (II) are mixed, they are mixed usually in a volume ratio, though it is not particularly limited of from 1:9 to 8:2, preferably of from 2:8 to 6:4. However, the amount of the carbamate in the solvent differs depending upon the type of cell or conditions where the cell is used. It may, for example, be less than 10 % by volume. As described above, the solvent for cell electrolyte of the present invention may further contain, in addition to the above-described carbamates and cyclic carbonates, any other non-aqueous solvents conventionally used as a solvent for cell electrolyte such as ethers and linear carbonates, so long as the characteristics of the electrolyte are not deteriorated. The cell electrolyte of the present invention is formed by dissolving a solute electrolyte in the electrolyte solvent as defined above. Preferred examples of the solute electrolyte are alkali metal salts including lithium salts such as LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , LiAlCl 3 , LiSiF 6 and LiN(CF 3 SO 2 ) 2 . Among these solute electrolytes, particularly preferred are LiPF 6 , LiBF 4 and LiClO 4 . The solute electrolyte is generally dissolved in the solvent in a concentration of from 0.1 to 3.0 mole/liter, preferably of from 0.5 to 2.0 mole/liter. The electrolyte solvent and the electrolyte of the present invention may be used for both primary cells and batteries. For example, they may be used for the batteries shown in Figs. 1 and 3. The electrolyte solvent and electrolyte of the present invention are particularly suitable for batteries having a negative electrode of carbon or a metal comprising an alkali metal and an active material of a positive electrode capable of reversibly and electrochemically reacting with the alkali metal. For example, the electrolyte of the present invention may be used for lithium cells utilizing metallic lithium as the negative electrode and a mixture of a lithium salt and graphite as the positive electrode. As described above, the carbamates of formula [I] are solvents having a low viscosity and a high dielectric constant and, when they dissolve an electrolyte, they can give a relatively high electroconductivity and a high withstand voltage to the electrolyte. Therefore, batteries having excellent electroconductivity and withstand voltage can be obtained by utilizing the organic solvent of the invention as an organic electrolyte solvent for an electrolyte containing an alkali metal salt dissolved in an organic electrolyte solvent. Further, by adding the cyclic carbonates to the carbamates of formula [I], the property of the solvent for dissolving the solute electrolyte is greatly improved, and accordingly a sufficient concentration of the solute electrolyte can be obtained. As a result, the electroconductivity of the electrolyte and the charge/discharge efficiency of the cell can be improved. The non-aqueous electrolyte batteries of the present invention will now be further explained. In the non-aqueous electrolyte battery of the present invention, the non-aqueous electrolyte comprises more than 0.02 and less than 30 %, preferably 0.05 to 20 %, by volume of the carbamates based on the total volume of the carbamates and the other non-aqueous solvents. The inventors achieved the present invention on the finding that the abnormal exothermic reactions observed in the conventional cells even after the current cut-off device or the pressure release device has operated can be prevented by adding the alkyl N,N-dialkyl carbamates to the non-aqueous electrolyte. In this way the rapid temperature elevation and hence the breakdown of cells can be avoided. As described above, the alkyl N,N-dialkylcarbamate(s) is added to the electrolyte in an amount of more than 0.02 and less than 30 %, preferably from 0.05 to 20%, by volume of the total volume of the alkyl N,N-dialkylcarbamate and other non-aqueous electrolyte solvents. In this range of the amount of the carbamate, the advantages of adding the carbamate are sufficiently effected and the cell has long cell life time, that is it stands practical application under severe conditions where the cell is continuously charged at a high temperature such as at 60°C. Under such conditions, the speed of increasing the internal pressure of a conventional sealed cell becomes too high and hence the current cut-off device would operate within a short period of time. In the cell of the present invention, the solute electrolyte to be dissolved in the non-aqueous solvent is not particularly limited. A conventional electrolyte may be utilized. However, LiClO 4 , LiAsF 6 , LiPF 6 , LiBF 4 , LiCF 3 SO 3 and LiN(CF 3 SO 2 ) 2 can be mentioned as examples of the solute electrolyte. Among these, LiPF 6 and LiBF 4 are particularly preferred. The positive electrode of the cell is also not particularly limited so long as it is such that the cell can be charged and discharged. For example, a positive electrode comprising an active material mainly containing Li x MO 2 , wherein M represents one or more transition metals and 0.05 ≦ x ≦ 1.10, may be preferably used. In this case, it is preferred that at least one of Co, Ni and Mn is used as the transition metal M to improve the energy density of the cell. As the negative electrode, a material capable of being reversibly doped with lithium is preferably selected. Examples of the material for the negative electrode are pyrolyzed carbonaceous substances, cokes such as pitch coke, needle coke and petroleum coke, graphite, glassy carbons, sintered materials formed by sintering polymer compounds such as phenol resins and furan resins at a suitable temperature, carbon fibers, and other carbonaceous materials such as active carbon. Other materials usable for the negative electrode are metallic lithium, lithium alloys such as lithium/aluminium alloys and polymers such as polyacetylene and polypyrrole. Among these materials, carbonaceous materials capable of being reversibly doped with lithium are particularly preferred because such materials can improve the charge/discharge cycle efficiency of the cell. The shape of the cell is also not particularly limited. The cell may be formed in various shapes such as a cylindrical shape, square pillar shape, coin-like shape and button-like shape. Further, the cell is preferably provided with a current cut-off device or a pressure releasing device which can operate depending on the internal pressure of the cell. The current cut-off device and the pressure releasing device may be the same as the devices provided in conventional cells. Figure 1 shows a non-aqueous electrolyte cell of a coin-like shape having a diameter of 20mm and a height of 2.5mm. The cell has a negative electrode 1 of metallic lithium and a positive electrode formed by pressure-molding a mixture comprising 85 parts by weight of LiCoO 2 , 12 parts by weight of graphite as a conductor and 3 parts by weight of a fluorocarbon resin as a binder. The materials of the negative electrode 1 and positive electrode 2 are bonded to the negative electrode can 4 and positive electrode can 5 via a porous separator 3. The electrolyte is introduced into the cell from the sealing gasket 6 and the cell is then sealed. In the non-aqueous electrolyte battery of the present invention, the abnormal exothermic reaction, which is observed in the conventional batteries, even after the current cut-off device or the pressure releasing device has operated, is prevented by the addition of the alkyl N,N-dialkylcarbamate(s) to the non-aqueous electrolyte. Therefore, the rapid elevation of the internal pressure of the cell after the current cut-off device or the pressure releasing device has started to operate is prevented, and hence breakdown of the cell is avoided. It is not clear how the alkyl N,N-dialkylcarbamate restricts the abnormal exothermic reaction, but the mechanism is presumed to be as follows. It is considered that the abnormal exothermic reaction observed in the conventional cells even after the current cut-off device or the pressure releasing device has started to operate is caused by the overcharge state lasting for a certain period involving the cell active materials. However, if the alkyl N,N-dialkylcarbamate(s) is added to the non-aqueous electrolyte, the carbamate is preferentially decomposed electrochemically by the overcharge state. This hastens the start of operation of the current cut-off device or the pressure releasing device to eliminate the overcharge state. Thus the abnormal exothermic reaction observed in the conventional cells even after the current cut-off device or the pressure releasing device has started to operate is prevented. EXAMPLES The present invention will be further explained in the following Examples. All of the alkyl N,N-dialkylcarbmates of formula (I) used in the following Examples were synthesized by the reaction of the corresponding N,N-dialkylamines and chlorocarbonate esters and purified by distillation in the presence of calcium hydride. The prepared alkyl N,N-dialkylcarbamates and the physicochemical properties thereof are listed in Table 1 below. The physical properties of the carbamates compound boiling point (°C) relative dielectric constant viscosity (cP,25°C) methyl N,N-dimethyl carbamate 131-133 12.5 0.8 ethyl N,N-diethyl carbamate 145-146 11.0 0.9 methyl N,N-diethyl carbamate 157-158 10.3 1.2 As seen from Table 1, the carbamates used in the present invention as a solvent for electrolyte have a relatively high boiling point and therefore they can be used in a wide temperature range. Further, because the carbamates have a low viscosity and a high dielectric constant, the electrolyte comprising these carbamates dissolved therein has a high electroconductivity. Examples 1 to 9 and Comparative Examples 1 to 4 The non-aqueous electrolyte battery of the present invention will be explained more specifically hereinafter. Fig. 3 is a sectional view of the cell used in Examples 1 to 9 according to the present invention and Comparative Examples 1 to 4. The cell has a negative electrode strip 1 comprising a negative current collector la applied with a negative acting material and a positive electrode strip 2 formed by applying a positive acting material to a positive current collector 2a. The negative and positive electrode strips are stacked with a separator 3 interposed between the strips, and the electrode strips and the separator are rolled into a cylindrical shape. Insulators 40 are placed on the both ends of the rolled electrodes and the electrodes are accommodated in a cell housing can 50. A sealing gasket 6 is placed on the peripheral part of the open end of the cell housing can 5, a cell lid 7 is placed on the rolled electrodes at the opening of the can 50, the can 50 is caulked so that the lid 7 is secured to the can via the gasket and the cell is sealed. The negative electrode 1 is connected to the bottom of the cell via a negative electrode lead 9, while the positive electrode 2 is connected to the lid 7 via a positive electrode lead 10 and a current cut-off thin plate 8. When the internal pressure of the cell increases excessively, a current cut-off plate 8 is deformed as indicated by an arrow in Fig. 3 and it's connection to the positive electrode lead 10 is broken to cut off the current. The cells of the above-described structure were manufactured as follows. The negative electrode was prepared as follows. A non-graphitizable carbon, which was used as the active material of the negative electrode and has properties close to those of glassy carbon, was prepared by introducing 10 to 20% of functional groups containing oxygen into a starting material, petroleum pitch (oxygen crosslinking), and then sintering the pitch at 1000 °C in an inert gas flow. By X-ray diffraction analysis, it was found that the non-graphitizable carbon had a distance between the (0 0 2) crystal faces of 3.76 Å and a true specific gravity of 1.58 g/cm 3 . This non-graphtizable carbon was pulverized to obtain a carbonaceous powder having an average diameter of 10 µm. 90 parts by weight of the powder was mixed with 10 parts by weight of a binder, polyvinylidene fluoride (PVDF), to obtain a negative electrode mixture. Then the mixture was dispersed in N-methyl-2-pyrrolidone to give a slurry. The slurry was uniformly applied to both surfaces of the negative current collector, a copper foil strip having a thickness of 10µm, and compression molded after drying to give the negative electrode strip 1. The positive electrode was prepared as follows. As the positive acting material, LiCoO 2 was prepared by mixing lithium carbonate and cobalt carbonate in a molar ratio of 0.5 1 and sintering the resultant mixture in the ambient atmosphere at 900 °C for 5 hours. 91 parts by weight of the resultant LiCoO 2 , 6 parts by weight of an electroconductive material, graphite, and 3 parts by weight of a binder, polyvinylidene fluoride (PVDF), were mixed to obtain a positive electrode mixture. Then the mixture was dispersed in N-methyl-2-pyrrolidone to give a slurry. The slurry was uniformly applied to both surfaces of the positive current collector, an aluminium foil strip having a thickness of 20 µm, and compression molded after drying to give the positive electrode strip 2. The obtained positive electrode 2 and negative electrode 1 were stacked with a separator 3 consisting of a microporous polypropylene film having a thickness of 25µm between the electrodes, and the stacked electrodes were wound spirally many times to give a rolled body. The insulator plate 40 was placed on the bottom of a steel can plated with nickel as the cell housing can 50. The rolled body was then placed into the can. One end of a nickel negative electrode lead 9 was connected to the negative electrode 1 by contact bonding and the other end was welded to the cell housing can 50 to ensure negative current collecting. Further, one end of an aluminium positive electrode lead 10 was connected to the positive electrode 2 and the other end of the lead was electrically connected to the cell lid 7 through a current cut-off plate 8, which can cut off the current depending on the internal pressure of the cell. Then, a non-aqueous electrolyte formed by dissolving LiPF 6 in one of the solvents listed in Table 3 below at a concentration of 1 mole/liter was poured into the cell housing can 50. The insulating sealing gasket 6 applied with asphalt and the cell lid 7 were placed at the opening of the can 50 and the can 50 was caulked over the gasket 6 to fix the lid 7. Thus obtained cylindrical non-aqueous electrolyte cell had a diameter of 20 mm and a height of 50 mm. Twenty cells for each of Examples 1 to 9 and Comparative Examples 1 to 4 were subjected to an overcharged state at 25°C using a current of 3.7 A and whether or not rapid temperature elevation or cell breakdown occurred was observed. The results are also shown in Table 2. As seen from the results shown in Table 2, 100% cell breakdown was observed in Comparative Examples 3 and 4 where the non-aqueous solvents did not contain the alkyl N,N-dialkylcarbamate. Further, in Comparative Example 1 where the non-aqueous solvent contained the alkyl N,N-dialkylcarbamate but only in an amount of 0.02% by volume, cell breakdown was observed at a high percentage of 50%. In contrast, cell breakdown was perfectly avoided in the working Examples of the present invention. In addition, the cells of Comparative Example 2, where the non-aqueous solvent contained the alkyl N,N-dialkylcarbamate in an amount of more than 20% by volume, showed a rapid increase of the internal pressure of the cells when those cells were subjected to continuous charging at a high temperature of 60 °C. Thus those cells were not preferred for practical applications. In all of the cells of Examples 1 to 9 according to the present invention where the solvent contained the alkyl N,N-dialkylcarbamates in an amount of from 0.05 to 20% by volume, cell breakdown was completely prevented and excellent results were obtained regardless of the kind of alkyl N,N-dialkylcarbamate used. No. composition (vol%) of solvent of non-aqueous electrolyte occurence of cell breakdown ( % ) PC DEC carbamate example 1 49.975 49.975 0.05 0 example 2 49.95 49.95 0.1 0 example 3 49.50 49.50 1.0 0 example 4 45.0 45.0 10.0 0 example 5 40.0 40.0 20.0 0 example 6 49.95 49.95 0.1 0 example 7 49.50 49.50 1.0 0 example 8 49.95 49.95 0.1 0 example 9 49.50 49.50 1.0 0 comp.exam.1 49.99 49.99 0.02 50 comp.exam.2 35 35 30 0 comp.exam.3 50 50 - 100 comp.exam.4 50 50 - 100 As seen from the above description, by using an organic solvent containing the specific carbamates as an electrolyte solvent for a non-aqueous electrolyte comprising an alkali metal dissolved in an electrolyte solvent according to the present invention, it becomes possible to improve both the electroconductivity and withstand voltage of the electrolyte. Therefore, by utilizing the electrolyte of the present invention in batteries, it is possible to obtain batteries having an excellent withstand voltage and excellent charge/discharge efficiency. Further, because the electrolyte solvents of the present invention have a relatively high boiling point and a low melting point, they may be used in a wide temperature range. In addition, when the electrolyte solvents of the present invention are used in lithium cells, lithium cells which are stable over time, that is lithium cells having excellent cycle characteristics, can be obtained, because the electrolytes are stable to metallic lithium which is possibly deposited during the charge/discharge cycles. According to the non-aqueous elecrolyte battery of the present invention, an abnormal exothermic reaction caused by overcharging and rapid elevation of the temperature or the inner pressure of the battery are effectively avoided and hence breakdown of the battery is prevented.

Claims: A solvent for cell electrolyte comprising at least one carbamate of formula (I); wherein R 1 , R 2 and R 3 are the same or different and each is a linear or branched alkyl group, in an amount of from more than 0.02% to less than 30% by volume based on the total volume of the solvent; and at least one cyclic carbonate of formula (II); wherein R 4 and R 5 are the same or different and each is hydrogen, methyl or ethyl. A solvent according to claim 1, wherein R 1 , R 2 and R 3 are the same or different and each is alkyl of 1 to 4 carbon atoms. A solvent according to claim 2 comprising at least one carbamate of formula (I) selected from methyl N,N-dimethylcarbamate, ethyl N,N-dimethylcarbamate, ethyl N,N-diethylcarbamate and propyl N,N-dimethylcarbamate. A solvent according to any one of claims 1 to 3 comprising at least one cyclic carbonate of formula (II) selected from ethylene carbonate, propylene carbonate and 2,3-butylene carbonate. A solvent according to any one of claims 1 to 4 wherein the volume ratio of the carbamate of formula (I) to the cyclic carbonate of formula (II) is 1:9 to 8:2. A solvent according to any one of claims 1 to 5 which is non-aqueous. A non-aqueous solvent according to claim 6 which comprises the carbamates in an amount of from 0.05 to 20% by volume of the total volume of the non-aqueous solvent. A cell electrolyte comprising an electrolyte solvent according to any one of claims 1 to 7. A cell electrolyte according to claim 8 wherein the electrolyte comprises at least one solute electrolyte selected from LiPF 6 , LiBF 4 and LiClO 4 . A cell electrolyte according to claim 9 wherein the concentration of the solute electrolyte is from 0.1 mol/l to 3 mol/l. A non-aqueous electrolyte battery comprising a negative electrode capable of being reversibly doped with lithium, a positive electrode and a non-aqueous electrolyte comprising a solute electrolyte in solution in a non-aqueous solvent according to claim 6 or claim 7 or an electrolyte according to claim 8 or claim 9. A battery according to claim 11 wherein the non-aqueous solvent additionally comprises at least one carbonate solvent compound. A battery according to claim 11 or claim 12 wherein the positive electrode comprises Li x MO 2 wherein M represents at least one transition metal and x satisfies the relation: 0.05 ≤ x ≤ 1.10.

Inventors: FUJITA SHIGERU, HIWARA AKIO, OMARU ATSUO, YOKOYAMA KEIICHI, FUJITA, SHIGERU, HIWARA, AKIO, OMARU, ATSUO, YOKOYAMA, KEIICHI, FUJITA, SHIGERU, SONY CORPORATION, Hiwara, Akio, c/o Mitsui Petrochem. Ind. Ltd., OMARU, ATSUO, SONY ENERGYTEC INC. KORIYAMA PLANT, Yokoyama, Keiichi, c/o Mitsui Petrochem. Ind. Ltd.
Applicants: MITSUI PETROCHEMICAL IND, SONY CORP, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1993
Main classification: H01M10
IPC codes: H01M10