Organic electrolyte solution type cell

Abstract: An organic electrolytic solution type cell using an electrolytic solution of a lithium salt in a polar solvent which comprises a compound having an organophobic group and an organophilic group, which cell has improved storage stability and good charge-discharge characteristics.

Description: ORGANIC ELECTROLYTE SOLUTION TYPE CELL The present invention relates to an organic electrolytic solution type lithium cell comprising a non-aqueous electrolytic solution. As the non-aqueous electrolytic solution for the organic electrolytic solution type cell, a solution of a lithium salt in a polar solvent is used. Examples of the lithium salt are LiClO₄, LiAsF₆ and LiPF₆, and examples of the polar solvent are propylene carbonate, γ-butyrolactone, dimethoxyethane and dioxolane. In the cell comprising the non-aqueous electrolytic solution, a negative electrode Li reacts with a harmful material such as water, oxygen gas, nitrogen gas and impurities which are contained in the electrolytic solution or, in some case, components of the electrolytic solution, so that the surface of lithium electrode is deactivated. Thereby, cell performances such as closed circuit voltage are deteriorated during storage. When the organic electrolytic solution type cell is used as a secondary cell, the reaction between the harmful materials in the electrolytic solution and the lithium electrode deteriorates the charge-discharge cycle characteristics. LiPF₆ is practically preferable as an electrolyte since it is safe unlike LiClO₄ or it is atoxic unlike LiAsF₆. However, it is thermally unstable and decomposed during storage at a high temperature. Therefore, the cell performances are further deteriorated. To improve the thermal stability of LiPF₆, it is proposed to add hexamethylphosphoric triamide (hereinafter referred to as "HMPA") or tetramethylethylenediamine (hereinafter referred to as "TMEDA"). Since such additives do not achieve sufficient effects and HMPA may react with active lithium metal of the negative electrode, improvement of storage stability cannot be expected. JP-A-58 106 771 discloses an organic electrolyte cell with an electrolytic solution of a lithium salt dissolved in a polar solvent mixture of propylene carbonate and N,N-dimethylacetamide. JP-A-62 217 578 discloses a cell incorporating a polar organic solvent, a lithium salt and a dicyclic tertiary amine which may have a range of molecular weights based on increases in the - (CH₂) - function. FR-A-21 24 388 discloses an organic electrolyte cell containing lithium chlorate/dioxolane and propylene carbonate/ether and methyl acetate. One object of the present invention is to provide an organic electrolytic solution type cell having improved storage stability and charge-discharge cycle characteristics. Another object of the present invention is to provide an organic electrolytic solution type cell having thermal stability even when LiPF₆ is used as the electrolyte. These and other objects of the present invention are achieved by an organic electrolytic solution type cell as described in claim 1. Further embodiments of the invention can be found in the dependent claims. With the compound having the organophobic group and the organophilic group, the surface of the lithium electrode may be protected so that the reaction of lithium metal with the electrolytic solution, impurities or water may be prevented. In a preferred embodiment of the present invention, a trialkylamine of the formula: wherein R₁, R₂ and R₃ are the same or different and each an alkyl group having at least 3 carbon atoms at least one of hydrogen atoms of which may be substituted with a fluorine atom is used in combination with LiPF₆ as the electrolyte. In such combination, the trialkylamine (I) not only protects the lithium electrode but also stabilizes LiPF₆, the cell has good storage stability. In addition, when the trialkylamine (I) is used together with other stabilizer for LiPF₆, the storage stability of the cell can be improved. Fig. 1 shows a model of the function of the compound having the organophobic group and the organophilic group in the organic electrolytic solution type cell. Since the organophobic group of the compound is very slightly soluble in the electrolytic solution, it faces the lithium electrode surface, while since the organophilic group of the compound is soluble in the electrolytic solution, it faces the electrolytic solution. Due to such properties of the organophobic and organophilic groups of the compound, the molecules of the compound are arranged and orientated as shown in Fig. 1. As the result, the reaction between the lithium electrode surface and the harmful materials is prevented. The molecules of the compound do not greatly suppress liberation of the lithium ions (Li⁺) during discharge of the cell, since the molecules are present on the lithium electrode surface in the form of a single molecule layer and the lithium ions can be liberated into the electrolytic solution via the organophilic groups. In the present specification, the organophobic group is intended to mean a part of an organic molecule which part has low solubility in the polar solvent. The organophobic group of the compound has at least 3 carbon atoms, preferably at least 4 carbon atoms to impart organophobicity to the group, wherein the alkyl group is a straight chain one. Preferably, the number of the carbon atoms in the alkyl group does not exceed 10. The organophilic group is intended to mean a part of an organic molecule which part has high solubility in the polar solvent and polarity to some extent. Selected examples of the organophilic group are an amino group, a ketone group, an ether group and an ester group. The amino group has large affinity with the lithium ions and large solubility in the solvent. The ketone group and the ester group have relatively large organophilicity and less reactivity with the lithium metal. The ether group has adequate organophilicity and is most stable against the lithium metal. One organophilic group is bonded with at least two organophobic groups. The combination of the organophobic group and the organophilic group and the amount of the compound to be added to the electrolytic solution are determined according to the functions of those groups. The present invention is now explained in detail by using the trialkylamine as an example of the compound having the organophobic group and the organophilic group. The trialkylamine is represented by the formula (I). The number of the carbon atoms in R₁, R₂ or R₃ is at least 3, preferably at least 4. Examples of the trialkylamine are tributylamine, trihexylamine and tridecylamine. The content of the trialkylamine in the electrolytic solution is generally from 0.05 to 5 % by volume, preferably from 0.1 to 1.5 % by volume. When the content of the alkylamine is too small, the intended effects are not achieved. When the content is too large, the cell performances such as the closed circuit voltage (CCV) of the cell during discharge are decreased. The trialkylamine not only protects the lithium electrode surface but also stabilizes LiPF₆. Therefore, the trialkylamine is preferably used when the electrolytic solution contains LiPF₆. Together with the trialkylamine, other stabilizer for LiPF₆ may be used. In this case, the lithium electrode surface is protected and the stabilizing effect of the trialkylamine is strengthened. Examples of the other stabilizer are a compound having a bond of the formula: >N-P(=O) (e.g. HMPA), tetraalkyldiamines (e.g. TMEDA) and pyridines. Preferably, an N-dialkylamide of the formula: wherein R₄ and R₅ are the same and different and each a saturated hydrocarbon group having 1 to 10 carbon atoms, preferably 1 to 3 carbon atoms and optionally an oxygen atom or a nitrogen atom in a carbon chain, and R₆ is a hydrogen atom or the same hydrocarbon group as above, provided that two of R₄, R₅ and R₆ may together form a ring. The N-dialkylamide (II) strongly stabilizes LiPF₆ and hardly reacts with the lithium negative electrode. Specific examples of the N-dialkylamide (II) are 1-methyl-2-piperidone, 1-methyl-2-pyrrolidinone, 1-ethyl-2-pyrrolidinone, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide, N,N-dimethylpropionamide, 1,5-dimethyl-2-pyrrolidinone,, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 4-formylmorpholine, 1-formylpiperidine, 1-(3-methylbutyryl)pyrrolidine, N-methylcaprolactam, bis-pentamethyleneurea, 1-cyclohexyl-2-pyrrolidinone, N,N-dimethyldodecaneamide, N,N-diethylformamide, N,N-diethylpropioneamide, 1,3-dimethyl-2-imidazolidinone and the like. The amount of the stabilizer for LiPF₆ in the electrolytic solution may be from 0.1 to 5 % by volume, preferably from 0.2 to 1.5 % by volume, and a total amount of the trialkylamine (I) and the stabilizer is generally from 0.1 to 10 % by volume, preferably from 0.2 to 5 % by volume. Examples of the polar solvent are propylene carbonate, γ-butyrolactone, dimethylsulfoxide, ethylene carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 2-methyltetrahydrofuran, other aliphatic monoethers and polyethers and the like. Examples of the electrolyte are LiPF₆, LiCl0₄, LiCF₃SO₃, LiBF₄, LiCF₃CO₂, LiAsF₆, LiB(C₆H₅)₄, LiSbF₆ and the like. They may be used independently or as a mixture of two or more of them. The amount of the electrolyte is selected from the range of 0.2 to 1.5 mole/liter so that conductivity of the electrolytic solution is at least 3 mS/cm at 25°C according to the kind of electrolyte. The organic electrolytic solution type cell of the present invention uses the non-aqueous electrolytic solution of the electrolyte in the polar solvent to which the compound having the organophobic group and the organophilic group such as the trialkylamine (I) is added, and includes various primary and secondary cells. Examples of active materials for the positive electrode are metal oxides such as MnO₂, V₂O₅, MoO₃, Pb₃O₄, Bi₃O₄, Co₃O₄, TiO₂, Cr₃O₈, Cr₂O₅ and LiCoO₂, mixed oxides thereof, metal sulfides such as TiS₂, CuS and FeS, and mixtures thereof. Among them, MnO₂ is preferred since it has a high single-electrode potential and generates a high voltage of about 3 V in the cell comprising lithium as the negative electrode. In addition, complex MnO₂ or modified MnO₂ which are recently developed can achieve good charge-discharge cycle characteristics. Examples of active materials for the negative electrode are lithium and lithium alloys such as LiAl, LiIn, LiCd, LiSi, LiGa, etc. Among them, lithium metal is preferred. Fig. 2 shows a cross sectional view of a spiral wound cell according to the present invention. The cell comprises a positive electrode 1, a negative electrode 2, a bag shape separator 3 which wraps the positive electrode 1 and a non-aqueous electrolytic solution 4. The electrodes 1 and 2 are laminated and spirally wound and then installed in a cylindrical stainless steel cell case which acts as the negative electrode can. The whole electrodes are immersed in the electrolytic solution 4. In addition to the spiral wound cell of Fig. 2, the cell may be in any form such as a can-type cell, a button shape cell, a coin shape cell or other thin cells. Fig.3 is a graph showing the results of stability test of the cells in Examples and Comparative Examples. Example 1 A band shape lithium negative electrode having a thickness of 0.17 mm and a width of 30 mm and a band shape MnO₂ positive electrode having a thickness of 0.4 mm and a width of 30 mm which was wrapped in a bag shape separator made of microporous polypropylene were laminated and wound. To each electrode, a lead member was attached. Then, the wound electrodes were installed in a stainless steel cell case having an outer diameter of 15 mm. Separately, an electrolytic solution was prepared by dissolving 0.5 mole/liter of LiCl0₄ and 0.1 mole/liter of LiPF₆ in a mixed solvent of propylene carbonate, tetrahydrofuran and 1,2-dimethoxyethane in a volume ratio of 1:1:1, removing water from the solution and then adding 0.5 % by volume of tributylamine. The solution contained less than 50 ppm of water. The electrolytic solution was poured in the cell case which contained the electrodes. The opening of the cell case was closed, and the cell was stabilized and aged to obtain a cylindrical spiral wound cell having an outer diameter of 15 mm, a height of 40 mm and a structure shown in Fig. 2. Example 2 In the same manner as in Example 1 but using 0.5 % by volume of N,N-dimethylacetamide in addition to tributylamine, the cell was produced. Comparative Example 1 In the same manner as in Example 1 but using no tributylamine, the cell was produced. Comparative Example 2 In the same manner as in Example 1 but using 5 % by volume of N,N-dimethylacetamide in place of tributylamine, the cell was produced. Each of the cells produced in Examples 1 and 2 and Comparative Examples 1 and 2 was stored at 60°C for 100 days. Every 20 days, the closed circuit voltage after 0.5 second at 3 A was measured. The results are shown in Fig. 3, in which the curves 1a, 1b, 1c and 1d represent the results for the cells produced in Examples 1 and 2 and Comparative Examples 1 and 2, respectively. As is clear from these results, the addition of tributylamine or a combination of tributylamine and N,N-dimethylacetamide to the electrolytic solution greatly improved the storage stability of the cells. With the non-aqueous electrolytic solutions prepared in Examples 1 and 2 and Comparative Examples 1 and 2, stabilizing tests were carried out as follows: Ten milliliters of the non-aqueous electrolytic solution was charged in a 10 ml vial. In the vial, a lithium metal piece of 1 cm x 4 cm was added. The opening of the vial was closed with a polyethlene stopper and sealed with an aluminum cap. Then, the vial was stored at 80°C for 10 days and opened. The surface condition of the lithium piece and color of the electrolytic solution were observed. The results are shown in Table 1. Example No. Condition of Li piece surface Color of electrolytic solution 1 Gloss Slightly colored 2 Gloss Transparent Comp. 1 Black colored over whole surfaces Brown Comp. 2 Slightly colored over whole surfaces Substantially transparent As understood from the results of Table 1, the non-aqueous solution of the present invention can protect the surfaces of lithium piece and stabilize LiPF₆. Examples 3 to 6 In the same manner as in Example 1 but using an additive listed in Table 2, the cell was produced. The cell had the same storage stability as those produced in Examples 1 and 2. The stabilizing test was carried out in the same manner as in Example 1. The results are shown in Table 2. Example No. Additive (vol. %) Condition of Li piece surface Color of electrolytic solution 3 THA (1) Gloss Slightly colored 4 TDA (1) Gloss Slightly colored 5 THA (0.5) HMPA (1) Gloss Substantially transparent 6 TBA (0.5) DEAD (1) Gloss Transparent Note: Examples 7 to 12 and Comparative Examples 3 and 4 In the same manner as in Example 1 but using an electrolyte and an additive listed in Table 3, the cell was produced. The cells produced in Examples had better storage stability than those produced in Comparative Examples. The stabilizing test was carried out in the same manner as in Example 1. The results are shown in Table 3. Example No. Electrolyte (mole/l) Additive (vol. %) Condition of Li piece surfaces Color of electrolytic solution 7 LiPF₆ (0.5) TBA (5) Gloss Pale yellow 8 ↑ TBA (1) HMPA (3) Gloss Substantially transparent Com.3 ↑ None Black colored over whole surfaces Dark brown 9 LiClO₄ (0.5) TBA (0.5) Gloss Transparent 10 ↑ DHK (1.0) Gloss Transparent 11 ↑ DHE (1.0) Gloss Transparent 12 ↑ BB (1.0) Gloss Transparent Com.4 ↑ None Partly black colored Substantially transparent Note:

Claims: An organic electrolytic solution type cell comprising a positive electrode, a negative electrode which comprises lithium and an electrolytic solution, characterized in that the electrolytic solution comprises a polar solvent, a lithium salt dissolved therein and a compound having at least two organophobic groups and an organophilic group which groups are directly bonded to each other where said organophobic groups are straight alkyl groups having at least 3 carbon atoms and said organophilic group is a group selected from the group consisting of an amino group, a ketone group, an ether group and an ester group. The cell according to claim 1, wherein said alkyl group has at least 4 carbon atoms. The cell according to claim 1, wherein said compound is a trialkylamine. The cell according to claim 3, wherein said trialkylamine is tributylamine. The cell according to claim 1, wherein said electrolytic solution contains 0.05 to 5 % by volume of said compound. The cell according to claim 5, wherein said electrolytic solution contains 0.1 to 1.5 % by volume of said compound. The cell according to claim 1, wherein said polar solvent comprises propylene carbonate. The cell according to claim 1, wherein said lithium salt contains LiClO₄ and LiPF₆. The cell according to claim 1, wherein said electrolytic solution further contains a stabilizer for LiPF₆. The cell according to claim 9, wherein said stabilizer is at least one selected from the group consisting of N,N-dimethylacetamide, N,N-diethylacetamide and 1-methylpiperidone.

Inventors: KAJITA KOZO, KAWAKAMI AKIRA, KITA FUSAJI, KAJITA, KOZO, KAWAKAMI, AKIRA, KITA, FUSAJI
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1990
Main classification: H01M10
IPC codes: H01M6, H01M10