Lithium cell

Abstract: There is provided a lithium cell comprising a negative electrode made of lithium or a lithium containing material and an electrolytic solution based on an organic solvent wherein a positive electrode active material comprises a copper compound oxide and an atomic ratio of a number of lithium atoms (Li) which react with one molecule of the copper compound oxide during discharge of the lithium cell to a number of oxygen atoms (O) contained in one molecule of the copper compound oxide, which ratio is expressed as Li/O, is at least 1.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a lithium cell. Particularly, the present invention relates to improvement of a positive electrode active material of the lithium cell. Description of the Related Art A lithium cell in which copper oxide is used as a positive electrode active material (hereinafter, sometimes referred to as "Li/CuO" for abbreviation) has been hitherto proposed as a large capacity density lithium cell. For example, reference can be made to Broussely M., Jumel Y. and Gabano J. P. (1977) 152nd Electrochemical Society Meeting, Atlanta (Abstract No. 9 (p. 33)). However, a closed circuit voltage of the above Li/CuO is as low as about 1.5 V. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a lithium cell which has a high voltage and a large capacity density. According to the present invention, the above object has been achieved by using, as a main component of a positive electrode active material in a lithium cell, a copper compound oxide such as a compound comprising copper, tellurium and oxygen as main component elements. Namely, there is provided a lithium cell comprising a negative electrode made of lithium or a lithium containing material and an electrolytic solution based on an organic solvent wherein a positive electrode active material comprises a copper compound oxide. The copper compound oxide may comprise, in place of or in addition to tellurium, other element such as Ti, W, Mo, Mg and Ge. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows a cross sectional view of an embodiment of a coin-shaped lithium cell according to the present invention; Fig. 2 shows X-ray diffraction patterns of Cu₃TeO₆ separately produced in a different manner which is used in the positive electrode active material of the present invention; Fig. 3 shows discharge patterns of cells produced in Examples 1 to 3; Fig. 4 shows a discharge pattern of a cell produced in Comparative Example 1; Fig. 5 shows a discharge pattern of a cell produced in Comparative Example 2; and Fig. 6 shows a graph in which an initial portion of the patterns of Fig. 3 is enlarged. DETAILED DESCRIPTION OF THE INVENTION Preferable examples of the compound comprising copper, tellurium and oxygen as the main component elements are Cu₃TeO₆, CuTeO₃, CuTeO₅, CuTeO₄ and so on. In a preferred embodiment of the present invention, Cu₃TeO₆ is used as a main component of the positive electrode active material of the lithium cell. Reasons why the lithium cell of the preferred embodiment according to the present invention has a large density capacity are not necessarily clarified. However, not bound by any theory, it is possible to explain as below using, as an example, Cu₃TeO₆ selected from the above compounds. A discharge reaction in a lithium cell in which lithium is used as a negative electrode material and Cu₃TeO₆ is used as a positive electrode active material (hereinafter, sometimes referred to as "Li/Cu₃TeO₆" for abbreviation) proceeds according to a following reaction equation: Cu₃TeO₆ + 6Li → 3Cu + Li₆TeO₆ According to the above discharge equation, a number of lithium atoms (Li) to be reacted is six per six oxygen atoms. The volume of the oxide material which is used as a positive electrode active material depends. mainly on the volume occupied by oxygen atoms, and thus a number of oxygen atoms (O). A discharge capacity is determined mainly by the number of lithium atoms (Li) which are to be reacted. Therefore, the ratio of the number of lithium atoms to the number of oxygen atoms, namely Li/O can be used as a measure which predicts whether or not the capacity density of the lithium cell is large. Hitherto, Cu₄O(PO₄)₂ has been proposed for the use as the positive electrode active material so as to produce a lithium cell which can provide so high a voltage as about 2.0 - 2.5 V. In this case using Cu₄O(PO₄)₂, the ratio of Li/O is 8/9. To the contrary, Cu₃TeO₆ which is used according to the present invention has the Li/O ratio of 6/6, namely Li/O = 1, which is larger than the ratio in the case of Cu₄O(PO₄)₂. Therefore, it can be qualitatively understood that the Cu₃TeO₆ using cell according to the present invention has a larger capacity than the conventional Cu₄O(PO₄)₂ using cell. Although tellurium belongs to the VI b group of the Periodic Table and is positioned below selenium, its toxicity is less than that of selenium and thus hexavalent tellurium (Te⁶⁺) is safe to use. Since tellurium has a large atomic weight, Cu₃TeO₆ has so very large a density as 6.34 g/cc among oxides. Thus, when Cu₃TeO₆ is compared with Cu₄O(PO₄)₂, while there is no large difference in a capacity density per unit weight (mAh/g) between them, Cu₃TeO₆ can provide a larger capacity density per unit volume (mAh/cc). Therefore, the use of Cu₃TeO₆ is very advantageous so as to make a lithium cell having a large capacity of which the inner volume is limited as in the case of a coin-shaped cell or a cylinder-shaped cell. Especially, for the use in the cell of which the inner volume is not larger than 5 cc, it is preferred to use the compound having a large density above 6 g/cc such as Cu₃TeO₆. Thus, in a preferred embodiment of the present invention, the ratio of Li/O of the copper oxide compound is equal to or larger than 1 on the basis of the above consideration on Cu₃TeO₆ which provides the larger capacity density per unit volume (mAh/cc). The compound which is used in the positive electrode active material according to the present invention preferably comprises copper, tellurium and oxygen as the main component elements. Namely, the compound may consist essentially of copper, tellurium and oxygen or may include, in addition to copper, tellurium and oxygen, any additional element in a small amount. Among the compounds comprising copper, tellurium and oxygen as the main component elements, the compound having an atomic ratio (copper : tellurium : oxygen) of 3 : 1 : 6, namely Cu₃TeO₆ is especially preferred since its Li/O ratio is larger than any other copper-tellurium compound oxide and its safety to the human organism is high. In the lithium cell using the copper compound oxide, a discharge voltage (closed circuit voltage) is lowered during the initial period of the discharge. After about 10 % of its discharge capacity has been discharged, the cell recovers its inherent discharge voltage. Reasons why the discharge voltage is lowered during the initial period of the discharge are not clarified. Not bound by any theory, it may be contemplated that almost no electron conductivity of the copper compound oxide such as Cu₃TeO₆ may play a role. It has been found that the discharge voltage lowering is overcome by adding, to the positive electrode active material comprising the copper compound oxide, a positive electrode active material for a lithium secondary cell having an electric potential higher by at least 2 V above that of lithium. Thus, in another aspect of the present invention, there is provided a lithium cell comprising the copper compound oxide as a positive electrode active material characterized in that the positive electrode active material for the lithium secondary cell having an electric potential higher by at least 2 V than that of lithium. The addition of the positive electrode active material for the lithium secondary cell having an electric potential higher by at least 2 V than that of lithium is applicable not only to the lithium cell comprising the copper compound oxide having the ratio of Li/O of at least 1, but also to the lithium cell comprising the copper oxide compound having the ratio of Li/O of less than 1. When lithium is used as the negative electrode material and Cu₃TeO₆ is used as the positive electrode active material so that the lithium cell (Cu₃TeO₆/Li) is formed and then the cell is discharged, the discharge voltage is about 2 V in a flat discharge voltage region. However, the discharge voltage is lowered to near 1.7 V during the initial discharge period. Such discharge voltage lowering occurs again once the discharge is stopped and the discharge is re-started, even when the flat discharge voltage region has already been reached before the discharge stop. A compound which has an electric potential higher by 2.5 - 1.8 V above lithium such as TiS₂ (titanium disulfate) and which can be charged and discharged is added to Cu₃TeO 6. The resultant mixture is used as the positive electrode active material so that the lithium cell is formed, the other conditions being the same as the above lithium cell which has the discharge voltage lowering. When the cell thus formed is discharged, the discharge voltage lowering is suppressed since TiS₂ discharges during the initial discharge voltage lowering period of the Cu₃TeO₆-cell (Cu₃TeO₆/Li). When the inherent discharge voltage of the Cu₃TeO₆-cell is recovered, TiS₂ which has discharged is charged to about 2.0 V. As a result, even when the discharge is re-started after the discharge has been stopped, the discharge voltage lowering at the beginning is suppressed since TiS₂ which has been previously charged discharges again when the discharge voltage of Cu₃TeO₆ is lowered. When the truly initial discharge voltage lowering is only suppressed, the positive electrode active material to be added is not necessarily required to be chargeable. Considering prevention the discharge voltage lowering after the discharge is once stopped and the discharge is restarted, the positive electrode active material to be added is required to be the chargeable material for the positive electrode active material of the secondary cell. In the present invention, as the positive electrode active material having the electric potential higher by at least 2 V above lithium for the lithium secondary cell which is added to the copper compound oxide such as Cu₃TeO₆, in addition to or instead of TiS₂, chalcogenides such as MoS₂, VSe, MoS₃, Fe0.25V0.75S2 and NbSe₃, lithium compound oxides such as Li₂MoO₃ and LiMn₃O₆, V₆O₁₃, amorphus V₂O₅ and any combination thereof may be used depending on the positive electrode active material used in the cell and an object of the cell. Particularly, the chalcogenides are preferred since each has a higher electron conductive ability. Reasons why the positive electrode active material for the lithium secondary cell which is to be added to the copper compound oxide such as Cu₃TeO₆ is required to have the electric potential larger by at least 2 V above that of lithium is that the discharge voltage during the initial period of the discharge is lowered to about 1.7 V which value may be slightly changed depending on the kind of the copper compound oxide and the discharge conditions, and such voltage lowering has to be compensated until the flat voltage (of about 2 V) period is reached. It can be clearly seen also from such reasons that the positive electrode active materials for the lithium secondary cell to be added are preferably those having a large capacity in a range of the electric potential of 1.7 - 2.0 V above lithium. An amount of the positive electrode active material for the secondary lithium cell having the electric potential larger by at least 2 V above lithium which is to be added to the copper compound oxide such as Cu₃TeO₆ is not specifically limited. However, it is preferably that a content of the positive electrode active material for the secondary lithium cell having the electric potential larger by at least 2 V above lithium is 2 to 30 % by weight based on the total weight of the positive electrode active material comprising the copper compound oxide such as Cu₃TeO₆. and the positive electrode active material for the secondary lithium cell; namely, (Positive electrode active material for secondary lithium cell) (Cu 3 TeO 6 + Positive electrode active material for secondary lithium cell) = 2-30 % by weight As to the discharge capacity, Cu₃TeO₆ has a theoretical discharge capacity of 388 mAh/g while TiS₂ has a theoretical discharge capacity of 240 mAh/g. Thus, based on a unit weight of the active material, Cu₃TeO₆ has about 1.6 times as large as that of TiS₂. By mixing the positive electrode active material for the lithium secondary cell having the electric potential larger by at least 2 V above lithium, the above effect is provided. Examples of the copper compound oxide used as the positive electrode active material of the lithium cell according to the present invention in which the effect of the addition of the positive electrode active material for the secondary lithium cell is obtained are, in addition to Cu-Te compound oxides such as Cu₃TeO₆ described above, CuGeO₃, Cu₂GeO₄, Cu₃B₂O₆, Cu₂P₂O₇, Cu₃P₂O₈, Cu₄P₂O₉, Cu₅P₂O₁₀, Cu₂V₂O₇, Cu₃WO₆, Cu₃TiO₅ and Cu₂MgO₃. Each oxide has a different electric potential and a different discharge capacity, and selection of the oxide depends on the purpose of the cell. A positive electrode according to the present invention is produced by mixing the copper compound oxide as the above and the positive electrode active material for the secondary lithium cell having an electric potential larger by at least 2 V above lithium, and optionally an electron conductive aid such as acetylene black and a binder such as a polytetrafluoroethylene to have a resultant mixture as a positive electrode composition and forming the mixture into the positive electrode using a conventional means. When a material having electron conductivity, for example TiS2 and MoS2, is added, the addition of the electron conductive aid may be omitted. As the negative electrode in the cell according to the present invention, lithium, a lithium alloy such as lithium aluminum alloy or a lithium containing material such as a lithium-carbon mixture may be used depending on the purpose of the cell. As an electrolytic solution of the cell according to the present invention, at least one of electrolytes such as LiCF₃SO₃, LiClO₄, LiPF₆, LiBF₄, and LiC₄F₉SO₃ dissolved in an organic solvent such as 1, 2-dimethoxyethane, 1, 2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 1, 3-dioxolan or diethyl carbonate or a mixture thereof may be used depending on the purpose of the cell. The present invention will be, hereinafter, explained in detail with reference to below Examples. However, the present invention is not limited to the Examples. EXAMPLES Example 1 Cu₃TeO₆ was synthesized in the following two different ways: Copper oxide (CuO) and telluric acid (H₆TeO₆) were mixed in a molar ratio of 3 : 1 (= CuO : H₆TeO₆) to have a resultant mixture, and the mixture was heated in a air to 700 °C for 36 hours to have a material colored pale yellow-green. The prepared material was in a form of particles having an average particle size of about 1 µm and identified as Cu₃TeO₆ with an X-ray powder diffractometer using Cu-Kα. A diffraction pattern was obtained using an X-ray source under conditions of an applied voltage 50 kV and a current of 150 mA and 2  was in a range of 10° to 100°. The diffraction curve thus obtained is shown in an upper side of Fig. 2. Copper carbonate (CuCO₃· Cu(OH)₂· H₂O) was used in place of copper oxide and reacted with telluric acid (H₆TeO₆) in the above synthesis, so that a material colored pale yellow-green as described above was produced. The material was identified as Cu₃TeO₆ by the powder X-ray diffractometer as described above. The diffraction curve of Cu₃TeO₆ originated from copper carbonate is also shown in a lower side of Fig. 2. It is clearly seen from Fig. 2 that the material produced from copper oxide (CuO) as the starting material and the material produced from copper carbonate (CuCO₃·Cu(OH)₂·H₂O) as the starting material have substantially the same peaks each other while intensity ratios of some peaks are different, and that both materials are Cu₃TeO₆. A positive electrode active main material was prepared by mixing 95 % by weight of Cu₃TeO₆ thus prepared as a main component and 3.5 % by weight of CuGeO₃, 0.5 % by weight of Cu₃WO₆, 0.5 % by weight of Cu₃TiO₅ and 0.5 % by weight of Cu₂MgO₃ as sub-components. An additive material was prepared by mixing 90 % by weight of TiS₂ as a main component and 5 % by weight of each MoS₂ and LiMn₃O₆. Then, a positive electrode active material was prepared by mixing 85 % by weight of the positive electrode active main material and 15 % by weight of the additive material to have a positive electrode active material according to the present invention. A mixture of flake graphite and acetylene black of which weight ratio of 1 : 1 as an electron conductive aid and a polytetrafluoroethylene as a binder (commercially available from Mitsui-Dupont under a trade name of Teflon-6J) was added to the positive electrode active material thus obtained (weight ratio of the positive electrode active material : the mixture of the flake graphite and the acetylene black : the polytetrafluoroethylene was 80 : 18 : 2) so that a positive electrode composition was prepared. The composition of the positive electrode active material is shown in below Table 1. Composition of Positive Electrode Active Material Preferable Mixing Ratio of Positive Active Main Material and Additive Material Component (% by weight in each material) Positive Active Main Material 85 wt % -100 wt % Cu₃TeO₆ (95) CuGeO₃ (3.5) Cu₃WO₆ (0.5) Cu₃TiO₅ (0.5) Cu₂MgO₃ (0.5) Additive Material 15 wt % - 0 wt % TiS₂ (90) MoS₂ (5) LiMn3O6 (5) 60 Grams of the positive electrode composition was charged in a mold, press-molded into a disc having a diameter of 10 mm and a density of about 4 g/cm³ under a pressure of 1 ton/cm² and then thermally treated at a temperature of 250 °C for 4 hours in an atmosphere of mixture gas of Ar/O₂ (having a volume ratio of 80/20) so that a positive electrode was formed. Using the positive electrode thus formed, a coin (or button) shaped lithium cell according to the present invention was produced as shown in Fig. 1. In Fig. 1, the numeral 1 indicates the positive electrode formed as the above, and 2 indicates a disc-shaped negative electrode made of lithium having a diameter of 14 mm. The numeral 3 indicates a separator made of a polypropylene non - woven fabric, 4 indicates a positive electrode can made of a stainless steel, 5 indicates a positive electrode collector made of a stainless steel mesh, and 6 indicates a negative electrode can made of a stainless steel having a surface plating of nickel. The numeral 7 indicates a negative electrode collector made of a stainless steel mesh which is spot-welded to an inner surface of the negative electrode can 6. The negative electrode 2 is press-bonded to the negative electrode collector 7. The numeral 8 indicates a gasket in an annular form made of a polypropylene. The cell was filled with an electrolytic solution consisting essentially of a mixture solvent of ethylene carbonate and 1, 2-dimethoxyethane having a volume ratio of 1 : 1 to which LiCF₃SO₃ and LiClO₄ were added in ratios of 0.5 mol/l and 0.1 mol/l, respectively. An amount of the electrolytic solution filled in the cell was so selected that a ratio of a volume of the electrolytic solution (µl) to the discharge capacity of the positive electrode active material (mAh) corresponds to 0.9. Example 2 A lithium cell was prepared in the same manner as in Example 1 except that the positive electrode active material consisted of 95 % by weight of the positive electrode active main material of Table 1 and 5 % by weight of the additive material of Table 1, in place of 85 % by weight and 15 % by weight in Example 1, respectively. Example 3 A lithium cell was prepared in the same manner as in Example 1 except that no additive material as shown in Table 1 was added to the positive electrode active main material of Table 1. Comparative Example 1 A lithium cell was prepared in the same manner as in Example 1 except that copper oxide (CuO) was used as the positive electrode active material. Comparative Example 2 A lithium cell was prepared in the same manner as in Example 1 except that Cu₄O(PO₄)₂ was used as the positive electrode active material. Discharge properties of the cells produced in Example 1 to 3 and Comparative Examples 1 and 2 were measured by discharging the cells at a discharge current of 0.85 mA, respectively. Discharge curves of the cells are shown in Figs. 3, 4, 5 and 6. In the graphs shown in Figs. 3 to 6, the ordinate indicates a voltage (V) and the abscissa indicates a capacity density per unit weight (mAh/g). When the final discharge voltage of each cell is regarded to be 1.5 V, the capacity density per unit weight of each cell can be determined from the curves as follows: Examples 1 to 3: 370 to 390 mAh/g (seen from Fig. 3) Comparative Example 1: 110 mAh/g (seem from Fig. 4) Comparative Example 2: 400 mAh/g (seem from Fig. 5) When the above results are interpreted in view of the positive electrode active material, the density capacity is 390 mAh/g in the case of Cu₃TeO₆ (Example 3), 110 mAh in the case of CuO and 400 mAh/g in the ease of Cu₄O(PO₄)₂ at the final discharge voltage of 1.5 V. CuO which provides the low voltage has a small capacity density when the final discharge voltage is set to 1.5 V. However, the cell of the present invention using Cu₃TeO₆ has almost the same capacity density per unit weight as that of the cell using Cu₄O(PO₄)₂. The specific gravity of Cu₃TeO₆ is 6.34 g/cm³ and a specific gravity of Cu₄O(PO₄)₂ is 4.54 g/cm³. Thus, the capacity density per unit volume is 2400 mAh/cm³ in the case of Cu₃TeO₆ but 1900 mAh/cm³ in the case of Cu₄O(PO₄)₂. On the basis of the capacity density per unit volume, Cu₃TeO₆ can provide a larger density capacity than Cu₄O(PO₄)₂. Practically, a commercially available cell is required to have a standard size, and thus, an inner volume of the cell which is filled with an active material and so on is limited. Therefore, in order to make the cell having a large capacity, it is required for the active material to have a large capacity density per unit volume, and thus, it is contemplated that the copper compound oxide such as Cu₃TeO₆ is an extremely effective material. Especially, in a cell of which inner volume is limited to less than 5 cm³, it is preferable that a material having a density capacity not less than 2000 mAh/cm³ such as Cu₃TeO₆ is used in order to have an overall capacity density of the cell not less than 250 mAh/cm³. As clearly seen from Fig. 6 which shows an enlarged curves of Fig. 3 during the initial discharge period, the cell produced in Example 3 by using solely the positive electrode active main material comprising Cu₃TeO₆ as the main component showed the discharge voltage lowering during the initial period from the beginning in which period about 10 % of the total discharge capacity was discharged. In particular, the voltage was lowered to near 1.7 V at the beginning of the discharge. To the contrary, the cell produced in Example 1 by using the positive electrode active main; material comprising Cu₃TeO₆ as the main component mixed with 15 % by weight of the additive material did not show the voltage lowering as seen in the cell of Example 3, but showed an almost flat discharge curve. Also the cell produced in Example 2 by using the positive electrode active main material comprising Cu₃TeO₆ as the main component mixed with 15 % by weight of the additive material did not show the voltage lowering as seen in the cell of Example 3, but showed an almost flat discharge curve. For an application where an discharge current above 1 mA/cm² is used or an application at a temperature of below 0 °C, since the voltage lowering is likely to occur, the additive material is preferably added in an amount of 10 to 30 % by weight, as in the case of Example 1, on the basis of the mixture of the positive electrode active main material and the additive material. For the other applications, the amount of the additive material to be added of 2 to 10 % by weight is sufficient. An amount of the electrolytic solution provided in the cell is preferably so controlled that the ratio of the volume of the electrolytic solution (µl) to the discharge capacity of the positive electrode active material (mAh), namely (µl/mAh), is in a range of 0.8 to 1.5. In order to produce a pellet having a large density using a material of which density is as large as above 6 g/cm³, the electron conductive aid to be added in the positive electrode composition is preferably in the range of 2 to 25 % by weight of the composition. The amount of the binder is preferably in the range of 0.2 to 5 % by weight based on the composition. The amount of the electron conductive aid is determined by electron conductive properties and surface conditions of the positive electrode active main material which comprises Cu₃TeO₆ as the main component according to the present invention. The amount of the binder is optimally selected depending on the specific gravity and the particle size of the positive electrode active main material. In a practical cell, it- is preferable that the amount of Li of the negative electrode is so limited that it is less than the amount which corresponds to the discharge capacity of the positive electrode, namely, a negative electrode limited cell. It is because hexavalent tellurium may be converted to a smaller valent tellurium due to a reaction with Li, and such a smaller valent tellurium may adversely affect storage properties due to, for example, a reaction with an electrolytic solution. Cu₃TeO₆ has a flat voltage region of about 2 V above that of Li which voltage is gradually lowered to near 1 V thereafter. Concretely, it is preferable that the amount of Li of the negative electrode is limited to an amount which corresponds to the capacity of only the flat region and that Li does not react with the positive electrode thereafter. As explained above, according to the present invention, a lithium cell is provided which provides a high voltage and a large capacity density.

Claims: A lithium cell comprising a negative electrode made of lithium or a lithium containing material and an electrolytic solution based on an organic solvent wherein a positive electrode active material comprises a copper compound oxide and an atomic ratio of a number of lithium atoms (Li) which react with one molecule of the copper compound oxide during discharge of the lithium cell to a number of oxygen atoms (O) contained in one molecule of the copper compound oxide, which ratio is expressed as Li/O, is at least 1. The lithium cell according to claim 1 wherein a compound comprising copper, tellurium and oxygen as main component elements is used as the copper compound oxide. The lithium cell according to claim 2 wherein the copper compound is Cu₃TeO₆. The lithium cell according to claim 1 wherein the copper compound oxide comprises at least one selected from Cu₃WO₆, Cu₃TiO₅, and Cu₂MgO₃. The lithium cell according to claim 1 wherein a positive electrode active material for a secondary lithium cell having an electric potential higher by at east 2 V above lithium is added to the copper compound oxide. The lithium cell according to claim 5 wherein the positive electrode active material for a secondary lithium cell is at least one chalcogenide selected from TiS₂ and MoS₂ and added in an amount of 2 to 30 % by weight based on a total amount of the positive electrode active material.

Inventors: KAWAKAMI AKIRA, MATSUMOTO KAZUNOBU, YUMIBA HIDEAKI, KAWAKAMI, AKIRA, MATSUMOTO, KAZUNOBU, YUMIBA, HIDEAKI
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1993
Main classification: H01M4
IPC codes: H01M4