MXPA00003507A - Lithium ion polymer cell separator - Google Patents

Abstract

A lithium ion cell with a polymeric anode and cathode and a separator therebetween. The separator, such as of non-woven polymeric fibers is provided with its own discrete structure, without carrier substrate, and with structural integrity, apart from being laminated/compressed between the polymeric anode and cathode elements. Operable cells are made thinner despite the self supporting discrete structure, with obtained improved rate capacity and high temperature performance.

Description

SEPARATOR FOR ION LITIO PILE OF POLYMER TYPE The present invention relates to separators used in thin laminated electrochemical cells and particularly to separators used in polymer-type lithium ion cells.

BACKGROUND OF THE INVENTION In polymer-type cells individual cells are typically constructed of cathode and anode materials laminated on opposite faces of a thin solid electrolyte polymer separating membrane, with respective electrically conductive current collectors located adjacent to or within of the anode and cathode, respectively. The anode, cathode, current collectors and separator, together with the electrolyte, make up the individual pile assembly, which is typically placed inside a metallized plastic laminate, which is thermally closed under pressure to form a completely sealed stack, being the respective electrically accessible current collectors from the outside. The respective layers of the electrodes and the electrolyte separator are very thin, generally in the order of 5-6 mils for the anode, 9-12 mils for the cathode and 3-9 mils for the polymer separator. electrolyte, with a total pile thickness of about 18-25 thousandths of an inch. In the past, in such polymeric stacks, the polymer materials of the electrodes and the separator have been of the same material, or of very similar material. The piles were made by compression by rolling the thin separator between the electrodes to increase the interface contact by means of a common material interface. This improved interfacial contact was considered necessary to provide sufficient ion transport capacity through the separator to the electrodes, said ion transport having been minimized due to the low amounts of liquid electrolyte contained in the polymer type cells. Polymer-type batteries contain all, or almost all, of their electrolyte absorbed within the electrodes and the separator, leaving little, if any, free liquid electrolyte. Accordingly, it was deemed necessary to provide a pressed sheet structure with all its component parts consisting of common polymer components to provide the required interface for acceptable performance of the battery and the battery. Consequently, in polymer-type piles the separator had the conformation of a thin membrane constituted of the common polymeric material, with little structural integrity of its own. Consequently, it was necessary, during the construction of the battery and the battery, to manipulate the separator on a base substrate as a carrier until the separator was laminated together with the electrodes, the carrier substrate being removed at that time. However, despite the integration of the anode, cathode and separator into polymer-type piles, as has been described, fluctuations in battery performance often occurred in terms of power, capacity and behavior under high battery temperatures. . All this, in addition to the complications in the manufacture produced by the construction of said batteries with carrier substrates as components and the sensitivity of the material to mechanical handling. As a specific example, in a typical polymer stack, the anode, the cathode and the stack separator are each constituted by a combination of a copolymer matrix of polyvinylidene fluoride (PVdF) and a compatible organic plasticizer, which maintains a homogeneous composition in film form. In commercial embodiments, the copolymer composition of the separator includes about 75 to about 92% by weight of polyvinylidene fluoride and about 8 to about 25% by weight of hexafluoropropylene (HFP), (both commercially available from Elf AtoChem. North America under the name Kynar FLEX ™), and an organic plasticizer. The composition of the copolymer is also used as a binder material in the manufacture of the respective electrodes to ensure an inferible compatible with the separator. The most common organic plasticizing materials are high-boiling plasticizers such as dibutyl phthalate, dimethyl phthalate, diethyl phthalate, and tris-butoxyethyl phosphate. Additionally, inorganic fillers such as volatized alumina and volatized silanized silica were often added to improve the physical strength and melting viscosity of the separating membrane, and to increase the absorption level of the electrolyte solution. Due to the thin film nature required in the separators, they were usually made by processes that included molding or forming of films, in addition to the described use of membranes with substrates carrying polymer compounds. In a typical polymer-type lithium-ion battery, the cathode is generally composed of the polymer used at the anode and the separator, with an amount of manganese oxide and lithium added to the mixture instead of graphite (used at the anode ) as host intercalation material for lithium ions. The current collectors, assembled to be in direct electrical contact with the cathode and the anode, are usually made, respectively, of aluminum and copper and a sheet or configuration similar to a grid.

To facilitate conductivity and ion transport, the anode, cathode and separator (usually as a unit, after lamination and pressing) are made porous by solvent extraction of the plasticizer material such as DBP, which, after extraction , leaves matrices or pores in the electrodes and in the separator. However, it should be understood that this is only an illustrative embodiment and that other extraction methods are possible, as well as other methods to provide the required porosity. The porous electrodes and separator are immersed in the electrolyte, before the assembly of the battery, to charge the electrolyte in the cell. The free liquid electrolyte is minimized or eliminated in this way.

The electrolyte, which is added to the electrodes and the separator, is composed of a lithium salt solution soluble in one or more organic solvents such as ethylene carbonate and dimethyl carbonate (EC-DMC). Other non-aqueous solvents commonly used include (-butyrolactone ((-BL), tetrahydrofuran (THF), 1,2-dimethoxyethane (1,2-EMD), propylene carbonate (PC), diethyl carbonate (DEC), carbonate methyl ethyl (MEC), diethoxyethane (DEE), dioxolane (DOL) and methyl formate (MF) Generally the soluble electrolyte is present in molar solutions of about 1 to 2, with soluble common lithium salts being preferable. electrolyte such as LiPF6, L? AsF4, L1BF4, L1CIO4, L1CF3SO3, L? N (CF2S02) 3, and LiN (C2F5S02) 3, with L1PF4 being particularly preferred.The types of electrodes, anodes and cathodes, as well as the structure and method of Construction of polymer batteries is known in the technical field (for example as described in U.S. Patent No. 5,296,318) with carbon anodes and spinel cathodes such as those of L? Mn204, and separators containing electrolytes , as described.

Summary of the Invention Based on the aforementioned background, an object of the present invention is to improve the manufacture of polymer-type lithium-ion batteries, but without degradation in the capacity or performance of the battery. It is a further object of the present invention to provide greater uniformity in the performance of the cells. Still another object of the present invention is to provide specific components and a method for the construction of cells with said components, in polymer-type lithium-ion batteries, which provide an improved performance of the cell as well as a total uniformity in said performance. In general, the present invention comprises a polymer-type lithium ion battery in which the separating component is a self-supporting discrete element without carrier substrate. Said separator includes a nonwoven porous textile material made of polymeric fibers that do not need to be of the same polymer contained in the anode or the cathode. The separator is used in its self-supporting form, with a minimum thickness of approximately 1 mil (0.001") and preferably approximately 25 mils, with a specific gravity per unit area of at least about 20 g / m2 and more preferably about 25 to 28 g / m2.The discrete separator can be used without carrier substrate, the anode and cathode being directly laminated on the separator, as well as with the membrane laminate.Alternatively, the discrete separator is coated on both sides (or coated on one side so as to allow filtering on the other side) with a thin layer of the common polymer material of the anode and cathode, and can therefore effectively function as a separating element and a carrier substrate not removable in situ for a separator consisting of the common anode and cathode polymer material, a preferred material for the separator independent of the present invention is a nonwoven fabric of polypropylene fibers, spun and blown (melt-blo n), commercially available from Johns Manville as a PX0074 grade material, with a specific gravity of 28 g / m2 and a thickness of 0.0025"and high porosity, other non-woven or polymeric materials, inert to the components of the pile, such as PVC, polyethylene and the like, or non-polymeric materials such as glass fibers, glass wool mats, etc. In accordance with the present invention, the manufacture is improved because the separator can be treated and positioned without a carrier base with the consequent removal of the same, resulting in materials with greater resistance.Another stages of fabrication of the anode and cathode laminate placement are the same, surprisingly it has been proven that not only the performance it does not degrade with the use of a discrete separator of an uncommon material, but the total efficiency of the pile is really improved, presenting a uniformity improved both the characteristics of the pile and its structural integrity.

DETAILED DESCRIPTION OF THE INVENTION To test and demonstrate the effectiveness of the present invention, two groups of identical lithium-ion batteries of the polymer type were constructed and tested identically, except that one of the groups (Example 1, with 31 cells) It was made with membrane separators composed of a PVdF copolymer matrix, of the prior art, as specifically described above, on carrier substrates, which were removed during the construction thereof, with anode and cathode lamination thereof. The other group of piles (Example 2, with 16 piles) was constructed using nonwoven fabric of polypropylene fibers, spun and blown (melt-blown), grade PX0074 of the firm Johns Manville, described above, as stand-alone separators, instead of the membrane separator with carrier substrate, and coated on both sides with PVdF in sufficient quantity to increase the weight thereof by about 50%, after which the anode and cathode were laminated to the coated separator. Both groups of batteries were placed under a load regime of 4, 2v of constant voltage load limiting the current to 0, 2A (regime C) until the current dropped below C / 20 or 2.5 hours to provide a load profile; and a constant discharge current discharge rate of cut from 0.2A to 3.0 volts to provide a discharge rate. The test results for both battery groups are given in the following Tables 1 and 2.

The last column in each OCV Table after 48 hours is very indicative of the stability of the stack and the uniformity of the stack performance. The prior art stacks generally show declines or fluctuations in the OCV voltage, while the stacks using the discrete separators according to the present invention show a uniform increase in OCV. Consequently, the use of discrete separators provides the benefits of facilitated manufacturing, as well as greater consistency in performance. It should be understood that the description and examples given above are only exemplary of the present invention and that changes may be made in the types, amount, configuration and composition of the components of the batteries according to the present invention without departing from the scope of the invention. present invention as defined in the following claims.

Claims (6)

1. Lithium ion battery of polymer type with an anode and a cathode polymer, characterized in that said anode additionally contains an intercalation carbon material and in which said cathode contains a material capable of reversibly containing lithium ions, said anode and cathode being laminated on a self-supporting discrete separator element.

A battery according to claim 1, characterized in that said self-supporting discrete separator element is composed of any of the group comprising non-woven fabrics of polymer fibers and microporous polymer, melt-blown, with a greater thickness than 1 thousandth of an inch.

Stack according to claim 2, characterized in that said melt-blown non-woven polymer fabric of the spacer element is made of polypropylene with a specific gravity per unit area of at least 25 g / m2.

A battery according to claim 3, characterized in that the anode, cathode and separator are made of the same polymer and said same polymer in the separator includes a coating on the non-woven fabric of polypropylene fibers, melt-blown , the anode and the cathode being laminated on the coating.

5. Pile according to claim 1, characterized in that said self-supporting discrete separator is constituted of glass.

6. Pile according to claim 5, characterized in that said glass separator is constituted by glass fibers.