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Positive electrode for use in lithium cell and lithium cell using the sameRelated Patent Categories: Chemistry: Electrical Current Producing Apparatus, Product, And Process, Current Producing Cell, Elements, Subcombinations And Compositions For Use Therewith And Adjuncts, Electrode, Chemically Specified Inorganic Electrochemically Active Material Containing, Alkali Metal Component Is Active Material, The Alkali Metal Is LithiumPositive electrode for use in lithium cell and lithium cell using the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060222952, Positive electrode for use in lithium cell and lithium cell using the same. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] The present invention concerns a positive electrode for use in lithium cell and a lithium cell using the same. [0002] As electrode materials for lithium secondary cell, it is generally to use carbon compounds such as graphite for a negative electrode and a composite oxide as an electrode active material for a positive electrode. Such positive and negative electrodes are manufactured, for example, by slurryfying an electrode active material, a binder and a electrically conducting agent such as conductive carbon used optionally for improving the electron conductivity with a dispersion medium to prepare a paste or a coating solution, coating the same to an electrode collector by a coater and evaporating the dispersion medium. [0003] An organic dispersion medium such as N-methyl-2-pyrrolidone has been used as the dispersion medium for manufacturing the coating solution but it involves a problem of toxicity in the manufacturing premise, a problem in view of safety, effects of the dispersion medium wastes on environments, a problem of processing cost, etc. In view of the above, it has been demanded to use water which reduces manufacturing cost and does not cause such problems. [0004] In the manufacture of the negative electrode, use of water has been enabled recently as a dispersion medium by using a synthetic rubber latex type adhesive and a viscosity improver, and a manufacturing method capable of avoiding the problems described above has become popular. [0005] On the other hand, in the manufacture of the positive electrode, while LiCoO.sub.2 has been widely used generally as the positive electrode active material, when water is used as the dispersion medium upon preparing a positive electrode by a positive electrode paste using LiCoO.sub.2 as the positive electrode active material, this results in a problem that LiOR, etc. are formed by reaction of lithium ion in the active material and water to cause corrosion of a collector and deteriorate the cell performance. Accordingly, it is still customary at present to use a styrene butadiene rubber or polyvinylidene fluoride (PVDF) for the binder and a non-aqueous dispersion medium such as N-methyl-2-pyrrolidone for the dispersion medium. [0006] For solving the problem such as corrosion of the collector and enabling the use of water as the dispersion medium, JP-A No. 2000-294252 describes a positive electrode formed by coating the surface of a collector with a protective film of an oxalate salt or a compound of elements selected from silicon, chromiun, and phosphorus. Further, JP-A No. 2003-157836 proposes to coat the surface of the positive electrode active material with a protective film having electroconductivity. [0007] However, the positive electrodes described above are not still sufficient in view of the cell performance and since they require a step of covering the collector, etc., they involve a problem that they are disadvantageous also in view of the cost. [0008] On the other hand, the present inventors have noted on lithium phosphate materials such as LiFePO.sub.4 as the positive electrode active material use of lithium phosphate in the organic dispersion medium system as the positive electrode active material is shown, for example, in JP-T No. 2000-509193, JP-A Nos. 9-134724, 2004-55493, etc. [0009] When the present inventors have attempted to manufacture of a positive electrode by using lithium phosphates such as LiFePO.sub.4 in an aqueous system, it has been found that the problem of the corrosion for the collector less occurs. However, it has been found that the positive electrode active material agglomerates upon preparing the paste, tending to cause unevenness on the paste coated surface. [0010] The present invention has been achieved in view of the above and it intends to provide a positive electrode for use in a lithium cell excellent both in electric characteristics and physical properties and a lithium cell using the same, by manufacture from an aqueous coating solution using safe water as a dispersion medium instead of organic dispersion media used so far thereby solving the problem of toxicity in the manufacturing premise, the problem of safety, and the problem of the effects of dispersion medium wastes on environments and the problem of the processing cost, as well as solving the problem of degradation of the cell performance caused by corrosion etc. of collectors and the problem for the occurrence of unevenness on the coated surface, SUMMARY OF THE INVENTION [0011] In order to solve the foregoing subject, the positive electrode for use in the lithium cell according to the present invention is formed from a positive electrode paste containing a positive electrode active material represented by the following formula (I), a binder ingredient comprising a water dispersible elastomer and a water soluble polymer as a viscosity improver, water as a dispersion medium, and a dispersing agent: Li.sub.xMPO.sub.4 (I) (in the general formula (I) above, M represents a metal atom containing at least one of metal atoms selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B and Mo, and 0<x<2). [0012] The "dispersion medium" used herein is a material for dispersing the constituent ingredients of the positive electrode paste and it includes those dissolving a portion of the constituent ingredients, that is, functioning also as a solvent. [0013] A preferred example of the positive electrode active material includes LiFePO.sub.4. [0014] As the dispersing agent, one or more of materials selected from the group consisting of nonionic surfactants, polycarboxylic acid compounds, compounds having sulfonic acid groups, and compounds having vinyl pyrrolidone structure can be used suitably. [0015] The lithium secondary cell according to the invention has a positive electrode of the invention described above, a negative electrode using an active material capable of intercalating/deintercalating metal lithium or lithium ion, and an electrolyte layer. [0016] In the invention, since a lithium phosphate compound such as LiFePO.sub.4 is used as the positive electrode active material and a specified positive electrode material composition containing a dispersing agent is adopted, even in a case where water is used as a dispersion medium in the positive electrode preparing step, corrosion of a collector less occurs and the problem of degrading the cell performance can be overcome without covering the collector differently from the prior art, and the problem for the occurrence of unevenness during paste coating can also be overcome. Further, the lithium phosphate compound is an active material capable of solving the problems in view of the resource, cost and safety of existent electrode active materials, being inexpensive and having high safety. Accordingly, a positive electrode for use in a lithium cell having high thermal stability as the cell characteristics and also excellent in the rate characteristic and the cycle characteristic can be provided at a reduce cost. [0017] That is, according to the invention, a lithium cell of high performance can be provided, as well as an effect in view of the manufacture capable of using safe water instead of organic dispersion media having high risk and involving various problems in view of handling can be obtained and it is also advantageous in view of the raw material. BRIEF DESCRIPTION OF THE DRAWINGS [0018] FIG. 1(a) is a graph showing the result of a rate characteristic test at 10.2 C, 1 C, 3 C, and 5 C for beaker cells of Example 1; [0019] FIG. 1(b) is a graph showing the result of a rate characteristic test at 0.2 C, 1 C, 3 C, and 5 C for beaker cells of Comparative Example 1; [0020] FIG. 1(c) is a graph showing the result of a rate characteristic test at 0.2 C, 1C, 3 C, and 5 C for beaker cells of Comparative Example 2; [0021] FIG. 2(a) is a graph showing the result of a cycle characteristic test at IC charge/discharge for beaker cells of Example 1; Continue reading about Positive electrode for use in lithium cell and lithium cell using the same... 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