METHOD OF APPLYING AN ELECTRICALLY CONDUCTING CURRENT COLLECTOR ON A SELF-SUPPORTING ELECTRODE AND ASSEMBLY OF ELECTRODE AND CURRENT COLLECTOR

Abstract: The invention relates to physical or chemical vapor deposition, such as evaporation, of a metal current collector on a self-supporting electrode. Such an electrode can be used in Li ion rechargeable batteries. The invention is characterized in that the electrically conducting layer forming the current collector is provided such that the electrically conducting layer follows the surface of the electrode on account of the fact that said layer is formed on said surface in situ. Electrically conducting current collectors may thus be provided an anode and cathode materials by various techniques, such as physical or chemical vapor deposition, sputtering or electroplating, the physical vapor deposition being preferred on account of the good surface contact between the electrode and the current collector. The invention further relates to a battery built up from, in that order, a current collector, an anode, a separator, a cathode, a current collector, and an elelctrolyte, which battery according to the present invention is characterized in that at least of the component parts: current collector/anode and current collector/cathode is an assembly according to the invention.

Description: METHOD OF APPLYING AN ELECTRICALLY CONDUCTING CURRENT COLLECTOR ON A SELF-SUPPORTING ELECTRODE AND ASSEMBLY OF ELECTRODE AND CURRENT COLLECTOR The invention relates to a method of applying an electrically conducting current collector on a self-supporting electrode. Furthermore, the invention relates to an assembly of an electrode and an electrically conducting current collector obtained by such a method and to a battery built up from, in that order, one or several layers of a current collector, an anode, a separator, a cathode, a current collector, and an electrolyte. Such a method of applying an electrically conducting current collector on a self-supporting electrode is known per se from US patent 5,470,357. For the manufacture of a lithium ion battery, a positive electrode film or membrane is separately manufactured as a coated layer of a dispersion of an intercalation electrode assembly, for example an LiMn 2 O 4 powder in a copolymer matrix solution, which is subsequently dried so as to form the membrane. A positive current collector layer of aluminum foil is pretreated with a polymer- type material which is compatible with the matrix copolymer for the purpose of improving the adhesion to the positive electrode which is subsequently laid on the collector. An electrolyte/separator membrane, which is formed as a dried coating layer from a composition comprising a solution of NdF: HFP copolymer and a plasticizer is subsequently laid on the positive electrode film. A negative electrode membrane, which is formed as a dried coating layer of a powdery carbon dispersion in a copolymer matrix solution, is laid on the separator membrane layer in a corresponding manner, and a negative copper collector foil pretreated in a manner corresponding to that used for the positive collector is laid on the negative electrode layer, thus completing the cell construction. This construction is subsequently heated under pressure for achieving the bond between the plasticized copolymer matrix components and the collector foils, so that a lamination of the cell elements into a coherent flexible battery cell structure is achieved. A disadvantage of such self-supporting electrodes is that the applied current collector does not have a sufficiently good contact with the subjacent electrode, in particular cathode or anode. In addition, a good contact must be maintained between the particles of the electrode and the current collectors throughout battery life. In the present state of the art, therefore, the battery is enclosed in canisters in which the pressure can be maintained throughout battery life. The contact between the current collector and the electrode is of a physical nature only in such an embodiment and is thus defined by the contact surface area at the boundary between the electrode and the current collector. It is an object of the present invention, accordingly, to increase the contact surface area between the current collector and the electrode so as to achieve a substantial improvement in battery efficiency. Another object of the present invention is to provide a method of applying an electrically conducting current collector on a self-supporting electrode in which any irregularities in the surface of the electrode do not substantially lead to a decrease in the contact surface area between the current collector provided and the self-supporting electrode. The method as mentioned in the opening paragraph of the present document is characterized in that the electrically conducting layer forming the current collector is provided such that the electrically conducting layer follows the surface of the electrode on account of the fact that said layer is formed on said surface in situ. Electrically conducting current collectors may thus be provided on anode and cathode materials by various techniques, such as physical or chemical vapor deposition, sputtering, or electroplating, the physical vapor deposition being preferred on account of the good surface contact between the electrode and the current collector. It will be obvious that the term "cathode" is used here to indicate a positive electrode and the term "anode" to indicate a negative electrode. To ensure that a fast penetration of the electrolyte remains safeguarded during the manufacture of the battery, it is preferred that the electrically conducting layer is provided in dots on the self-supporting electrode. The expression "in dots" is understood to mean here that not the entire surface area of the electrode is provided with the electrically conducting layer. In the method according to the invention, it is possible to provide both an electrically conducting layer on the positive self-supporting electrode and an electrically conducting layer on the negative self-supporting electrode. If an electrically conducting layer, preferably of aluminum, is applied on the cathode, an assembly of a positive current collector and cathode will be obtained; if an electrically conducting layer, preferably made of copper, is provided on the anode, an assembly of a negative current collector and anode will be obtained. The invention further relates to a battery built up from, in that order, a current collector, an anode, a separator, a cathode, a current collector, and an electrolyte, which battery according to the present invention is characterized in that at least one of the component parts: current collector/anode and current collector/cathode is an assembly according to the invention. The invention will now be explained with reference to an embodiment to which, however, the invention is by no means limited. Manufacture of an anode A mixture of 5.3 g meso-carbon micrograins, 5 g decaline, and 0.05 g UHMWPE with a molecular weight of more than one million was prepared. The mixture was heated to 160 °C and a film was cast. The excess quantity of solvent was removed and the film was densified by rolling. Manufacture of a cathode The cathode was manufactured in the same way as the anode, except that first a mixture was prepared of 5 g LiCoO , 5 g decaline, 0.05 g UHMWPE, and 0.3 g soot. The mixture was then heated and treated in the same manner as for the negative electrode. Dry plates were obtained after removal of the solvent. Manufacture of an assembly of current collector and anode and current collector and cathode The cathode and anode were obtained as described above, whereupon layers of copper for the negative electrode and aluminum for the positive electrode, each having a thickness of 1 μm, were provided by vapor deposition. An assembly of current collector and cathode provided with an aluminum layer and current collector and anode provided with a copper layer was thus obtained. Example 1 To measure the contact resistance between the cathode material and a current collector, a cathode material with a thickness of 100 μm manufactured by the method described above was used, from which material a disc with a diameter of 1.5 cm was cut. A comparison sample was manufactured, using two 10 μm thick perforated aluminum films with the same surface area as the cathode material which were provided between three cathode discs in an alternating arrangement and laminated under the influence of heat and pressure in the presence of a small quantity of solvent. The sample according to the present invention was prepared by direct evaporation of aluminum electrodes onto the surfaces of the cathode materials. The resistance values of the samples as a function of time were measured after the samples had been moistened with acetone. The results are shown in the attached Figs. 1 and 2. It is apparent from these Figs, that the resistance is high already in the beginning in the embodiment of the laminated electrode (Fig. 2). This value increases by a factor of more than 10 the moment the sample is moistened. Since acetone evaporates from the system, the resistance value decreases, but it does not reach its initial value. In the embodiment with the electrode obtained by physical vapor deposition (Fig. 1), there is a slight increase in resistance, and the initial value is restored after the acetone has evaporated from the system. Example 2 The electrodes obtained by physical vapor deposition as manufactured in example 1 were used with a porous separator of polythene being interposed between the anode and cathode materials. The battery construction was subsequently provided with an electrolyte composition comprising 1 mole LiPF 6 in EC:DEC of 1 :1. The battery gave no problems when subjected to a charge/discharge cycle.

Claims: CLAIMS: 1. A method of applying an electrically conducting current collector on a self- supporting electrode, characterized in that the electrically conducting layer forming the current collector is provided such that the electrically conducting layer follows the surface of the electrode on account of the fact that said layer is formed on said surface in situ. 2. A method as claimed in claim 1 , characterized in that the layer is obtained through physical or chemical vapor deposition, sputtering, or electroplating. 3. A method as claimed in claim 1, characterized in that physical vapor deposition is used. 4. A method as claimed in claims 1 to 3, characterized in that the electrically conducting layer is provided in dots. 5. A method as claimed in claims 1 to 4, characterized in that a corrosion-resistant layer is provided on the electrically conducting layer. 6. An assembly of an electrode and an electrically conducting current collector obtained by a method as defined in claims 1 to 5. 7. An assembly as claimed in claim 6, characterized in that an electrically conducting layer is provided on the positive electrode. 8. An assembly as claimed in claim 7, characterized in that aluminum is used. 9. An assembly as claimed in claim 6, characterized in that an electrically conducting layer is provided on the negative electrode. 10. An assembly as claimed in claim 9, characterized in that copper is used. 11. A battery built up from, in that order, one or several layers of a current collector, an anode, a separator, a cathode and current collector, and an electrolyte, characterized in that at least one of the assemblies of current collector and anode, and current collector and cathode is an assembly as claimed in claim 7 or an assembly as claimed in claim 9.

Inventors: HIKMET RIFAT A M, HIKMET, RIFAT, A., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H01M4
IPC codes: H01M4, H01M10