LITHIUM BATTERY COMPRISING A GEL-ELETROLYTE

Abstract: A lithium battery, comprising a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte solution between the negative and the positive electrode. At least the negative electrode material and the positive electrode material are provided with a pattern of holes which accommodate a polymeric material which sticks and presses the negative electrode, the positive electrode and the separator together. In order to decrease the tendency of the electrolyte solution to sublime, thereby expanding the packaging material around the battery, the non-aqueous electrolyte solution comprises a solution of lithium salts, solvent molecules and a polymer, forming a gel-electrolyte.

Description: Lithium battery comprising a gel-electrolyte. The invention relates to a lithium battery, at least comprising: a negative electrode, comprising negative electrode material and a negative current collector; a positive electrode, comprising positive electrode material and a positive current collector; a separator, separating the negative electrode and the positive electrode; and a non-aqueous electrolyte solution between the negative and the positive electrode, wherein at least the negative electrode material and the positive electrode material are provided with a pattern of holes which accommodate a polymeric material which sticks and presses the negative electrode, the positive electrode and the separator together. Due to a growing market for lightweight, portable cordless consumer products, such as CD-players, mobile telephones, laptop computers and video cameras, there is an increased need for high-density batteries. Specifically, very thin and flexible batteries are required. In order to provide for an acceptable portability, the batteries contained in said consumer products should provide the necessary amount of energy at the smallest possible weight and volume. However, the thinner the battery gets, the more difficult the application of pressure, which is needed to maintain sufficient contact between the respective components ofthe battery, becomes. A battery according to the preamble is known from the International patent application with publication number 00/04601. Said battery has thin and flexible form and provides at the same time a very high energy density. Moreover, the contact between the electrodes and the separator is obtained and maintained in a very efficient way. The battery can be packed in a thin walled can, as the walls of said can are not needed to maintain sufficient pressure on the respective components of the battery. At higher temperatures the liquid electrolyte made up of low molecular weight organic molecules, sometimes tends to sublime, giving rise to the expansion of the packaging material around the battery. It is an object ofthe invention to provide a battery according to the preamble, in which the above problem is avoided. This object is achieved in a battery as specified in the preamble, characterized in that the non-aqueous electrolyte solution comprises a solution of lithium salts, solvent molecules and a polymer, forming a gel-electrolyte. As a result of its lower vapour pressure, such gel-electrolyte will show a considerably reduced sublimation at higher temperatures, compared to the electrolyte used in the prior art. Therefore, by providing a gel-electrolyte between the positive and the negative electrode, it is prevented that the packaging material around the battery components expands at higher temperatures. In order to decrease the tendency of the liquid electrolytes to sublime preferably polymeric gelling agents are used. Such polymeric gelling agents can be brought into the battery stack, anode and cathode materials and/or into the separator separately. Various types of polymeric materials which can advantageously be used are described below in the exemplary embodiment In a preferred embodiment, the separator comprises the gel-electrolyte and more preferably consists ofthe gel-electrolyte. In this latter embodiment, the gel-electrolyte has to have sufficient mechanical strength to act as a separator between the positive electrode and the negative electrode. The 'usual' separator can then be omitted and can be replaced by the gel-electrolyte which then acts as a separator. The negative electrode (also called anode) in the lithium battery according to the invention basically comprises negative electrode active material for the battery reaction and a negative current collector serving to transmit electrons upon charging and discharging. The negative electrode material preferably comprises an intercalatable material, preferably carbon, such as amorphous carbon or graphite, dispersed in a polymeric binder matrix. Lithium can be intercalated between crystal layers of carbon, for example in a quantity of 1 lithium atom per 6 carbon atoms, in other words LiC 6 . The positive electrode (also called cathode) comprises positive electrode active material and a positive current collector. The positive electrode material can comprise a lithium intercalation compound, such as LiMn 2 O 4 , LiCoO 2 or LiNiO 2 ,dispersed in a polymeric binder matrix. The mixture also comprises a powdery conductive material, such as carbon black (e.g. acetylene black, thermal black), graphite powder, metal powder or the like. The amount of conductive material is in the range of 2 to 15 % by weight. For the binder matrix polymers are used, which include polysaccharide, thermoplastic polymers and polymers having rubber-like elasticity. Examples include carboxymethyl cellulose, polytetrafluoroethylene, polyethylene, polypropylene and styrene- butadiene rubber. The polymers may be used singly or in combination. The polymeric binder serves to bind together active material powders to prevent cracks and fix such powdery materials to the surface of the current collector. The amount ofthe polymeric binder is in the range of 2 to 30 % by weight. For the current collector use can be made of any electronic conductor provided that it does not induce chemical reactions in the battery. Examples of materials for the positive current collector (cathode) include stainless steel, aluminum and nickel. Examples of materials for the negative current collector (anode) include stainless steel, copper and nickel. The collector may be in the shape of a foil, film or sheet, and may be porous, punched or mesh-like. The thickness ofthe collector generally is in the range of 1 to 500 μm. The separator provided between the positive and negative electrode can be an insulated film having both high ion permeability and desired mechanical strength. It prevents short-circuits between the negative and the positive electrode, and holds the electrolytic solution. A generally used separator is made of glass fibre, or a porous sheet or non- woven fabric made of an olefin polymer such as polyethylene or polypropylene. The diameter ofthe pores generally is in the range of 0.01 to 10 μm. The thickness ofthe separator generally is in the range of 5 to 300 μm. As mentioned in the above, the usual separator can also be replaced by the gel electrolyte itself. The battery structure ofthe present invention may be activated with any ofthe numerous compositions used as liquid electrolyte solutions. For the solvent an ester, such as ethylene carbonate, propylene carbonate, methylethylcarbonate; an ether, such as tetrahydrofurane; dimethyl sulfoxide, and mixtures thereof may be used. Examples ofthe solute include salts composed of lithium ions (Li + ) and Lewis acid ions (BF ~ , PF 6 " , AsF 6 " , ClO " , CF 3 SO 3 " ) and mixed salts thereof. The concentration ofthe salt is between 0.5 and 2 mol/1. The battery is enclosed in a battery case, which may be a plastic metallic or a plastic resin case. Examples of these materials include stainless steel and plastics like polypropylene. The sealing may be made by an adhesive, welding or soldering. A flat flexible battery may be enclosed in an air and moisture-proof bag of polymer-coated aluminum foil. Sheets of such foil are commercially available, and can be sealed together at their edges. The sheets typically comprise an outer 15 μm polyester or polyethylene film, a 50 μm aluminum foil, and an inner 15 μm polyester or polyethylene film, bearing a 90 μm layer of heat sealing adhesive. The invention will be elucidated in greater detail by means of an exemplary embodiment in which the various types of polymeric materials which can advantageously be used are described. Exemplary embodiment. A mixture for the negative electrode material is prepared by mixing 6 g graphite particles having a particle size of 10 μm as the active positive material, 4.5 g carboxymethyl cellulose (1% aqueous solution) and 0.5 g styrene butadiene rubber (60% dispersion in water) as a binder, and formed into a paste to be applied as a coating onto both surfaces of a copper foil current collector. The thickness of the coating is 200 μm. The thickness ofthe copper foil amounts to 14 μm. The pasted current collector is pre-dried at 85°C for 15 minutes, heat-treated at 110°C for 3 hours and then pressed until the thickness becomes 110 μm. The negative electrode is cut out so as to be a square of 2 x 2 cm 2 . A mixture for the positive electrode material is prepared by mixing 6 g LiCoO 2 as the active positive material, 0.18 g acetylene black as a conductive material, 5 g carboxymethyl cellulose (1% aqueous solution) and 0.7 g polytetrafluoroethylene (60%) dispersion in water) as a binder, and formed into a paste to be applied as a coating onto both surfaces of an aluminum foil current collector. The thickness ofthe coating is 420 μm. The thickness ofthe aluminum foil amounts to 20 μm. The pasted current collector is pre-dried at 85°C for 15 minutes, heat-treated at 250°C for 4 hours and then pressed until the thickness becomes 100 μm. The positive electrode is cut out so as to be a square of 2 x 2 cm 2 A 25 μm thick porous polyethylene foil is used as a separator. The negative electrode and the positive electrode are each provided with a pattern of holes by mechanical punching. The diameter ofthe holes is generally about 1 mm. The holes are provided in a two-dimensional array with a mutual hole distance of 5 mm. In order to manufacture a lithium battery according to the invention, a stack is formed ofthe negative electrode, the separator and the positive electrode. Subsequently, a polymeric gelling agent is brought into the battery stack; into the anode and cathode materials and/or separately into the separator. Thereupon the battery is activated by bringing in electrolyte into the battery. Preferably, the following types of polymeric gelling agents are used: A) In order to produce a chemically cross-linked polymer, a precursor with reactive groups can be used. Examples of such molecules are molecules with the general structure: Y— B— X in which: X = Y = acrylate, epoxy, vinyl ether or thiolene X or Y = CH3 B = long chain molecule, such as: X^O— CH 2 — CH 2 — CH 2 -CH 2 ^ιτ— Y X-f-O— CH 2 -CH 2 -CH 2 ^n— Y Molecules with reactive groups are dissolved in a solvent and brought into the components ofthe battery. The reactive end groups can chosen to be acrylate, epoxy, vinyl ether or thiolene. The reactive molecules may be mono or multi functional. Mono functional molecules should always be used in combination with multifunctional molecules in order to obtain cross-linking. The polymerisation can be induced using high energy radiation such as electrons and gamma rays or thermal initiator in the presence ofthe solvent or after drying. In all cases samples are dried before activating the battery. In the activation process when electrolyte is brought into the battery the cross-linked polymer becomes swollen by the electrolyte. B) In order to produce physically cross-linked gels a semi-crystalline polymer or a copolymer is advantageously used. The polymer is dissolved in a solvent. The battery components are dipped into the solution. Subsequently the solvent is extracted or evaporated out ofthe system leaving a dry polymer behind. During the activation when the electrolyte is brought into the battery, partially swelling ofthe polymer occurs. This means that polymer chains remains physically connected to each other. The connection points are the crystallites or the parts ofthe copolymer which are not dissolved by the electrolyte. Examples of various polymers are the following: -O— CH 2 -CH 2 -CH 2 - — O— CH 2 — CH 2 — CH 2 -CH 2 - Examples of X and Y -O— CH 2 -CH 2 - C) As a third preferred possibility, very high molecular weight polymer can be used. The polymer is brought into the battery components as described above. However in the present case when the battery is activated the polymer is dissolved in the electrolyte. Nevertheless due to the high molecular weight ofthe polymer it is highly entangled and it is highly viscous. Examples of polymer which can be used for this purpose are polymethylmethacrylate and polystyrene, polyvinyl methyl keton. The polymers which are mentioned under A and B can also be used in producing separators. Such separators can also be used in combination with standard electrodes which do not contain gelling agents.

Claims: CLAIMS: 1. A lithium battery, at least comprising: a negative electrode, comprising negative electrode material and a negative current collector; a positive electrode, comprising positive electrode material and a positive current collector; a separator, separating the negative electrode and the positive electrode; and a non-aqueous electrolyte solution between the negative and the positive electrode, wherein at least the negative electrode material and the positive electrode material are provided with a pattern of holes which accommodate a polymeric material which sticks and presses the negative electrode, the positive electrode and the separator together, characterized in that the non-aqueous electrolyte solution comprises a solution of lithium salts, solvent molecules and a polymer, forming a gel-electrolyte. 2. A lithium battery according to claim 1 , characterized in that the polymer comprises a polymeric gelling agent. 3. A lithium battery according to claim 1, characterized in that the separator comprises the gel-electrolyte. 4. A lithium battery according to claim 3, characterized in that the separator consists ofthe gel-electrolyte.

Inventors: FEIL HANS, HIKMET RIFAT A M, FEIL, HANS, HIKMET, RIFAT, A., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H01M
IPC codes: H01M4, H01M6, H01M10