WO1998059387A2

WO1998059387A2 - Lithium batteries

Info

Publication number: WO1998059387A2
Application number: PCT/EP1998/003997
Authority: WO
Inventors: Louis Merckaert
Original assignee: Solvay (Societe Anonyme)
Priority date: 1997-06-23
Filing date: 1998-06-16
Publication date: 1998-12-30
Also published as: AU8630198A; GB9713226D0; ZA985428B; GB2327293A; TW389001B; WO1998059387A3

Abstract

Electrochemical cells known as lithium batteries wherein at least one of the anodes, cathodes and/or separator elements thereof contains PVdF homopolymers and/or copolymers and in which calcium carbonate is also incorporated in said battery and, more particularly, in at least one of the anode, cathode and/or separator element thereof. The incorporation of such calcium carbonate acts to reduce or eliminate free acid available which may contribute to the occurrence and/or intensity of runaway exothermic reactions in said battery. Calcium carbonates having a mean particle size of at least about 10 νm are preferred.

Description

LITHIUM BATTERIES

The present invention relates to electrochemical lithium battery cells having a negative electrode, a positive electrode and a separator element therebetween wherein at least one of the electrodes or the separator contains a synthetic polymer material. Lithium batteries are batteries of the type having at least one electrochemical cell in which lithium ions move from anodes to cathodes during the discharge cycle (and, in the case of rechargeable lithium batteries, from cathodes to anodes during the charge cycle) of the cell. Such batteries can be in either single cell or plural cell arrangements in which each cell includes a negative electrode (an anode), a positive electrode (a cathode) and a separator element which is located between the cathode and the anode.

There are many different types of batteries based on lithium. In a first generation of lithium batteries, the anodes are a lithium metal and the cathodes include compounds that are capable of intercalating the lithium ions within their structure during the discharge of the cell. An example of such a lithium battery is disclosed by U. Von Sacken et al., in Solid State Ionics, 69 (1994) 284-290.

In a second generation of lithium batteries, it has been proposed to provide anodes wherein the lithium metal has been replaced by various forms of carbon, such as graphite, petroleum coke, etc., capable of intercalating lithium, so that the electrodes thereof are capable of reversibly intercalating the lithium ions within their structures. An example of such a lithium battery is disclosed by U. Von Sacken et al., in Solid State Ionics, 69 (1994) 284-290.

Lithium batteries further have separator elements (or "membranes") of various structures and constructions which contain an electrolyte through which the lithium ions move between the cell electrodes during the discharge cycle (and, in the case of rechargeable batteries, the charge cycle) of the cell. This electrolyte can be in the form of a liquid (for example, a solution of a lithium salt, such as LiPF6, L-BF4 or LiClO-}, in an organic solvent, such as linear or cyclic alkene carbonate, diethoxyethane or dimethylformanide), a gel (for example, PVdF and carbonates, such as ethylene-, propylene- and dimethyl carbonates) or a solid (for example, polyvinylidene fluoride ~ PVdF — and carbonates, such as ethylene-, propylene- and dimethyl carbonates, as well as specially-designed glassy material having high ionic conductivity).

Examples of such lithium batteries are disclosed in United States Patent Nos. 5,587,253 and 5,580,682. Other lithium batteries have been proposed in which the cathode also includes a material which is capable of intercalating lithium ions, so that both the cathode and the anode include intercalation materials

It has also been found to be useful, for various purposes, to employ polymeric materials in the construction and lamination of the electrodes (as a binder thereof) and/or the separator element of lithium batteries. Examples of such polymeric materials are polyvinylidene fluoride (PVdF) homo and copolymer resins, such as poly(vinylidene fluoride-chlorotrifluoroethylene), poly(vinylidene fluoride-tetrafluoroethylene), poly(vinylidene fluoride-hexafluoropropylene) (PVdF:HFP) and poly(vinylidene fluoride-trifluoroethylene). Identified as especially preferred in this regard is that PVdF:HFP copolymer, containing 8% HFP, which is commercially available from SOL V AY (Belgium) under the trademark SOLEF® 20810 and those SOLEF® 3XXXX-series poly(vinylidene fluoride-chlorotrifluoroethylene) (PVdF:CTFE) copolymers, containing about 8% to 20% CTFE, which are commercially available from SOL V AY (Belgium). An example of a lithium battery employing such polymeric material is given in United States Patent No. 5,571,634.

While being useful for their purposes, a considerable problem encountered with lithium batteries has been the occurrence of internal short-circuiting and over-charging which can cause runaway exothermic reactions. Such short- circuits and over-charging can result from problems inherent with the design of the battery (for example, as a result of the formation of lithium dendrites) or they can be the consequence of external factors such as, for example, improper use or accidents. These runaway exothermic reactions, the exact origin of which is not yet well-known, are particularly dangerous in that they can result in, for example, sometimes intense explosions.

Suggestions for eliminating and/or controlling such runaway reactions have principally centred on changing/replacing the electrolyte used in such batteries (see, for example, United States Patent No. 4,830,939 and European Patent Application No. 0 766 329). While PVdF is not regarded as the source for such short-circuits and/or over-charging, it is thought that the runaway exothermic reaction resulting therefrom may lead to dehydrofluorination of the PVdF homo and/or copolymers, thereby liberating acids, such as HF and/or HCl, which are known to autocatalyze the degradation of PVdF homo and/or copolymers. We believe that this can contribute to the intensity of the phenomenon. Consequently, it has been proposed to replace the PVdF homo and/or copolymers used therein with other polymers, such as polyethylene oxide (PEO), polyacrylonitrile (PAN) and polytetrafluoroethylene (PTFE). Unfortunately, these other polymers do not provide the advantageous properties, such as ionic conductivity over a broad temperature range and gel formation, which are provided by PVdF homo and/or copolymers.

Accordingly, it can be seen that there remains a need for other solutions to resolve problems associated with runaway exothermic reactions and to provide a lithium battery which has PVdF homo and/or copolymers incorporated into at least one of the electrodes and/or the separator element thereof and in which the runaway exothermic reactions are eliminated or reduced in intensity.

It is a primary object of the present invention to provide anodes, cathodes and/or separator elements for lithium batteries which have PVdF homo and/or copolymers incorporated therein and in which the danger of runaway exothermic reactions is eliminated or reduced in intensity. It is another primary object of the present invention to provide lithium batteries which have PVdF homo and/or copolymers incorporated into at least one of the electrodes and/or the separator element thereof and in which the danger of runaway exothermic reactions is eliminated or reduced in intensity. It is still another object of the present invention to identify what, if any, role the presence of PVdF homo and/or copolymers has in runaway exothermic reactions resulting from short-circuits/over-charging in lithium batteries and to eliminate and/or reduce said role.

It is still yet another object of the present invention to provide a method for eliminating and/or reducing in intensity the danger of runaway exothermic reactions which can occur in lithium batteries of the type which have PVdF homo and/or copolymers incorporated in at least one of the anode, cathode and/or separator element thereof.

In accordance with the teachings of the present invention, disclosed herein are lithium batteries of the type having at least one of the anode, cathode and/or separator element having polyvinylidene fluoride (PVdF) homo and/or copolymers incorporated therein and further wherein said at least one anode, cathode and/or separator element further includes calcium carbonate (CaCO3). In this manner, the calcium carbonate (CaCO3) may act as an acid scavenger (for example, for HF and/or HCl), thereby reducing or eliminating the free acid (HF and/or HCl) available which may contribute to the occurrence and/or intensity of runaway exothermic reactions therein.

In a preferred embodiment, the calcium carbonate is included in at least the anode of such lithium batteries.

In another preferred embodiment, the carbon is in the form of graphite.

In a further preferred embodiment, the calcium carbonate has a mean particle diameter size of at least about 0.10 μm.

In another preferred embodiment, said calcium carbonate has a mean particle diameter size of at least about 0.18 μm.

In still another preferred embodiment, said calcium carbonate has a mean particle diameter size of at least about 0.2 μm. In still yet another preferred embodiment, said calcium carbonate has a mean particle diameter size of at least about 0.36 μm.

In another aspect of the present invention, and in further accordance with the teachings of the present invention, disclosed herein are anodes for use in lithium batteries, said anode having polyvinylidene fluoride (PVdF) homo and/or copolymers incorporated therein and further wherein said anode further includes calcium carbonate (CaCO3). In this manner, the calcium carbonate (CaCO3) may act as an acid scavenger (for example, for HF and/or HCl), thereby reducing or eliminating the free acid (i.e., HF and/or HCl) available which may contribute to the occurrence and/or intensity of runaway exothermic reactions therein. It is further preferred that the carbon is in the form of graphite.

In a preferred embodiment, the calcium carbonate (CaCO3) is a calcium carbonate which is commercially-available under the trademark SOCAL®.

In a further preferred embodiment, the calcium carbonate has a mean particle diameter size of at least about 0.10 μm. In another preferred embodiment, said calcium carbonate has a mean particle diameter size of at least about 0.18 μm.

In still another preferred embodiment, said calcium carbonate has a mean particle diameter size of at least about 0.2 μm.

In still yet another preferred embodiment, said calcium carbonate has a mean particle diameter size of at least about 0.36 μm. In yet another aspect of the present invention, and in yet further accordance with the teachings of the present invention, disclosed herein is a cathode for use in lithium batteries of the type having polyvinylidene fluoride homo and/or copolymers and further wherein said cathode further includes calcium carbonate. In this manner, the calcium carbonate acts as an acid scavenger (for, for example, HF and/or HCl), thereby reducing or eliminating the free acid (i.e., HF and/or HCl) available which may contribute to the occurrence and/or intensity of runaway exothermic reactions in said battery.

In a preferred embodiment, the calcium carbonate is a calcium carbonate which is commercially-available under the trademark SOCAL®.

In another preferred embodiment, the calcium carbonate has a mean particle diameter size of at least about 0.10 μm.

In a further preferred embodiment, the calcium carbonate has a mean particle diameter size of at least about 0.18 μm. In a still further preferred embodiment, the calcium carbonate has a mean particle diameter size of at least about 0.2 μm.

In a yet further preferred embodiment, the calcium carbonate has a mean particle diameter size of at least about 0.36 μm.

Preferrably, the cathode is further comprised of carbon. In a further preferred embodiment, the carbon is in the form of graphite.

In still another aspect of the present invention, disclosed herein is a method for eliminating and/or reducing in intensity the danger of runaway exothermic reactions which can occur in lithium batteries of the type which have PVdF homo and/or copolymers incorporated in at least one of the anode, cathode and/or separator element thereof. This method is characterised by the incorporation of calcium carbonate in at least one of said anode, cathode and/or separator element of the lithium battery.

In a preferred embodiment of this method, the calcium carbonate is a SOCAL®. In another preferred embodiment of this method, the calcium carbonate has a mean particle diameter size of at least about 0.10 μm.

The present invention involves our belief that runaway exothermic reactions occurring in lithium cells during improper use or accidents may possibly involve acid (such as HF and/or HCl) autocatalyzed dehydrofluorination of PVdF homo and/or copolymers. In this regard, the use (by its incorporation into the anode and/or cathode and/or separator element of the battery) of calcium carbonate (CaCO3), which acts as an acid scavenger, reduces or eliminates the available free acid (such as HF and HCl) which may contribute to the occurrence and/or intensity of the runaway exothermic reaction. Moreover, the introduction of some CaCO3 within the anode and/or cathode and/or separator element does not modify the electrochemical properties of the lithium ion cell.

The present invention further involves our additional, and quite surprising, finding that calcium carbonates having a mean particle diameter size of at least about 0.10 μm, and preferably at least about 0.18 μm, display markedly greater efficiency as an "acid scavenger" (and, more particularly, as an "HF scavenger") than calcium carbonates having a mean particle diameter size of less than about 0.10 μm.

Such a finding is quite surprising in that the ability for CaCO3 to act as a scavenger for acids (such as, for example, HF and/or HCl) is generally regarded as being directly proportional to the total surface area which is available for contacting and "scavenging" the acid (that is to say, chemically reacting with the liberated acid in such a manner as to chemically bind the acid therein). In this regard, it is generally considered that CaCO3 having smaller mean particle diameters would, due to the presence of a larger overall surface area, be more efficient in acting as an "acid scavenger" than the same quantity of CaCO3 having the larger mean particle diameters disclosed and employed herein. In this regard, the particularly high efficiency as an "acid scavenger" observed for calcium carbonates having a mean particle diameter size of at least about 0.18 μm are particularly surprising and noteworthy.

As used herein "mean particle diameter" and "mean particle diameter size" are used to refer to those size measurements calculated by the formula of Carman and Malherbe (J. Appl. Chem., I, March 1951 at 105-108) starting from permeability measurements obtained by the method described by Blaine (American Society for Testing and Materials (ASTM) norm C 204-81).

The types of CaCO3 which may be employed in the lithium batteries and the anodes of the present invention include, but are not limited to, those sold under the trademark SOCAL®, (commercially available from SOL V AY) and Hydrocarb 95T (natural calcium carbonate).

Preferred types of SOCAL® useful herein include, but are not limited to, SOCAL®E2, SOCAL®Np, SOCAL®90A, SOCAL®92E, SOCAL®N2, SOCAL® N2R and SOCAL® 91 C V. Particularly preferred in this regard is SOCAL®91CV. The quantity of CaCO3 to be employed in the lithium batteries (and, in particular, in the anodes, cathodes and/or separator elements) of the present invention must be sufficient as to provide for the global efficacy thereof but not so much that one loses space for reactive material, such as carbon or graphite, in the component (such as an electrode and/or the separator element).

In this regard, it is contemplated that, within the teachings of the present invention, the quantity of CaCO3 to be employed in the lithium batteries and in the anodes of the present invention may be as little as about 0.5% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated. More preferred is that a quantity of at least about 1% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed. More preferred is that a quantity of at least about 5% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed. Still more preferred is that a quantity of at least about 7% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed. Still yet more preferred is that a quantity of at least about 8% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed. Still yet more preferred is that a quantity of at least about 10% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed. Preferably, a quantity of no more than about 20% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed. More preferred is that a quantity of no more than about 11% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed. Most preferred is that a quantity of no more than about 10% (w/w) of the total weight of the coating of the electrode (i.e., the anode) in which it is incoφorated be employed.

The cathode of the lithium battery of the present invention may have a coating comprised of a Hthiated transition metal oxide, carbon, polyvinylidene fluoride (PVdF) homo and/or copolymer resins and CaCO3.

The Hthiated transition metal oxides useful in the coating of the cathodes of the present invention include LiχCoθ2, Li Mn2θ4, LiχNiθ2, LiχV2θ5 and mixed oxides, such as LiχNi_aCo_DMn_cθ2, wherein a + b + c = 1. Examples of the polyvinylidene fluoride (PVdF) homo and/or copolymer resins useful in the coating of the cathodes of the present invention include the poly(vinylidene fluoride-hexafluoropropylene) (PVdF:HFP) copolymer, containing 8% (w/w) HFP, which is commercially available from SOL V AY (Belgium) under the trademark SOLEF® 20810 and those SOLEF® 3XXXX-series poly(vinylidene fluoride-chlorotrifluoroethylene) (PVdF: CTFE) copolymers, containing about 8% (w/w) to 20% (w/w) CTFE, which are commercially available from SOL V AY (Belgium). A particularly preferred PVdF useful in the cathodes of the present invention is that PVdF homopolymer commercially available from SOL V AY (Belgium) under the trademark SOLEF® 1008.

Examples of cathodes into which CaCO3 may be incoφorated into the coatings thereof according to the present invention are those cathodes described in European Patent Appln. No. 0 492 586 and Japanese Patent Applications Nos. 63/121262 and 04/095363 and European Patent Appln. No. 0 205 856.

Particularly preferred for use in the lithium batteries of the present invention are cathodes which include, on aluminium foil, a coating of about 60- 94% (w/w) of a lithiated transition metal oxide, about 3-10% (w/w) of carbon (graphite and/or carbon black), about 3-10% (w/w) of a PVdF homo and/or copolymer and about 0.5-20% (w/w) of CaCO3, as was noted above.

The cathodes of the lithium batteries of the present invention may contain further elements and components, such as additives, stabilisers, etc., as desired. The anode of the lithium battery of the present invention may have a coating comprised of carbon, especially graphite, polyvinylidene fluoride (PVdF) homo and/or copolymer resins and CaCO3.

The preferred carbon useful in the coating of the anodes of the present invention is graphite. The polyvinylidene fluoride (PVdF) homo and/or copolymer resins useful in the coating of the anodes of the present invention include PVdF homopolymers and copolymers. Examples of such PVdF copolymers are that poly(vinylidene fluoride-hexafluoropropylene) (PVdF:HFP) copolymer, containing 8% (w/w) HFP, which is commercially available from SOL V AY (Belgium) under the trademark SOLEF® 20810 and those SOLEF® 3XXXX-series poly(vinylidene fluoride-chlorotrifluoroethylene) (PVdF: CTFE) copolymers, containing about 8% (w/w) to about 20% (w/w) CTFE, which are commercially available from SOL V AY (Belgium). A particularly preferred PVdF useful in the anodes of the present invention is that PVdF homopolymer commercially available from SOLVAY (Belgium) sold under the trademark SOLEF® 1008. Examples of anodes into which CaCO3 may be incoφorated into the coatings thereof according to the present invention are those anodes described in European Patent Appln. No. 0 492 586 and Japanese Patent Applications Nos. 63/121262 and 04/095363 and European Patent Appln. No. 0 205 856.

Particularly preferred for use in the the lithium batteries of the present invention are anodes which include, on copper foil, a coating of about 60-99% (w/w) of carbon (graphite and/or carbon black), about 1-20% (w/w) PVdF homo and/or copolymer and about 0.5-20% of CaCO3, as was noted above.

If desired, the coatings of the anodes of the lithium batteries of the present invention may contain further additional elements and components, such as additives, stabilisers, vulcanisation agents, etc. An example of such additional elements and components are bismuth succinate, aluminium hydroxide, calcium hydroxide and antimony hydroxide.

Such additives may be included in concentrations as low as 0% (w/w) in relation to the PVdF homo/copolymer content of the coating. Further, such additives may be included in concentrations as high as about 5% (w/w) of the PVdF homo/copolymer content [0.05 to 1% (w/w) of the total content of the coating of the electrode].

The electrolyte of the lithium battery of the present invention may be any suitable solid, gel or liquid electrolyte. Examples of such solid electrolytes useful in the lithium battery of the present invention are PVdF homo and/or copolymers and carbonates, such as ethylene-, propylene- and dimethyl carbonates, as well as specially-designed glassy materials having high ionic conductivity. Examples of such gel electrolytes useful in the lithium battery of the present invention are PVdF homo and/or copolymers and carbonates, such as ethylene-, propylene-, dimethyl- and diethyl-carbonates. Examples of such liquid electrolytes useful in the lithium battery of the present invention are LiPFg, L-BF4 and LiClθ4, in a mixture of ethylene-, propylene-, dimethyl and diethyl-carbonates.

The specific surface capacity of the electrodes (cathodes and anodes) of the lithium batteries of the present invention may range from 0.5 mAh/cm^ (as determined by the electrode coating thickness of 0.025 mm after drying and calendering) to 10 mAh/cm^ (as determined by the electrode coating thickness of 0.5 mm after drying and calendering). Preferred is a specific surface capacity of about 3 mAh/cm2 (as determined by the electrode coating thickness of 0.15 mm after drying and calendering) to 6 mAh/cm^ (as determined by the electrode corresponding to a thickness of 0.3 mm after drying and calendering). The lithium-ion batteries of the present invention may be constructed in any conventional manner well-known to those skilled in the art. For example, thin active materials may be coated on an even thinner metal foil and/or grid. The carbon or graphite anode is coated, using a polymer binder, onto a copper foil and/or grid and the lithium transition metal oxide cathode onto aluminium. These electrodes are then either stacked on one another or wound into a round or elliptical "jelly roll" with a microporous polymer separator element. The stacked or wound "jelly roll" electrodes are then placed in a metal container, electrolyte added and given a formation cycle. As first produced, the cell is in the discharged state with all of the lithium ions in the cathode.

The lithium batteries disclosed herein are useful for a variety of puφoses including, but not limited to, batteries for electric vehicles.

Having thus described the lithium batteries and the anodes, cathodes and separator elements of the present invention, as well as the methods used to produce and obtain the same, the following examples are now presented for the puφoses of illustration only and are neither meant to be, nor should they be, read as being restrictive. Example 1

Construction of Negative Electrode C Anode) Fourteen negative electrodes (anodes) were constructed as follows : Respective 98.8 gram samples of N-Methyl-2-Pyrrolidone (NMP) were placed in respective stainless steel beakers and maintained therein under an argon flow. To each respective sample of NMP, 4.39 grams of a particular CaCO3,, as set forth in Table 1 , were added under gentle agitation:

Table 1

Crystal Forms : 1 = Calcite trigonal scalenohedral

2 = Aragonite orthorhombic

3 = Calcite trigonal rhombohedral

Each of the samples were then treated with ultrasound (Braun Labsonic 2000 B, high level) for two (2) minutes to form a dispersion. The dispersions were then placed under vacuum (1 mm Hg) for one (1) hour. The samples were then once again treated with ultrasound (Braun Labsonic 2000 B; high level) for two (2) minutes.

7.65 grams of PVdF was then progressively added to each respective sample under agitation (1000 RPM) which was then maintained until dissolution was complete as evidenced by the solution being clear (10-20 minutes).

Respective 43.3 gram samples of carbon, in the form of graphite, were then slowly added to each sample under agitation (1000 RPM). The agitation was then increased to 4000 RPM for 1 minute with the agitator disc being moved up and down in the paste in order to ensure the homogeneity of the mixture.

The respective stainless steel beakers containing the respective samples were then progressively placed under vacuum for degassing the sample (paste) while taking care not to let the product spill over. The samples were maintained for 10 minutes under a maximum vacuum (5-10 mm Hg) to complete degassing.

Fourteen copper foils (200 x 60 mm), which had been contemporaneously degreased by submersion in a bath of CFC-113 for 30 minutes were placed in an air-tight container with paper separators between them to conserve them.

The foils were then individually labelled and weighed (to within about 0. Img). They were then made perfectly flat by placing on a rigid perforated metal plate and applying a vacuum to the underside of the latter.

The fourteen foils were then coated with a respective one of the paste samples so as to have a thickness (of paste) of about 300 μm. The respective foils were then placed on stainless steel supports which had been previously coated with polytetrafluoroethylene (PTFE) and the supports with the coated foils thereon were placed in a ventilated oven at 150°C for 30 minutes

After drying, the electrodes were allowed to cool to ambient temperature. The thickness of the coated surface and the weight of t^'.ie electrodes were then measured.

The electrodes were then passed one-by-one through a BRABENDER laboratory calender. The space between the rolls of the calender was adjusted in such a way as to have, at the first passage, a maximal reduction of thickness which is equivalent to 10%. Subsequent passages were performed as needed until the thickness no longer varied (meaning a thickness in the order of about 55% of that of the wet paste had been obtained).

In this fashion, fourteen electrodes containing different types of CaCO3 were obtained.

Example 2

Construction of Positive Electrode (Cathode)

Preparation of positive electrodes (cathodes) may be constructed as follows :

67.5 grams of N-Methyl-2-Pyrrolidone (NMP), previously dried over molecular sieves, will be weighed into a stainless steel beaker. 2.5 grams of calcium carbonate, such as SOCAL®91CV, will then be added to the NMP under gentle agitation.

The resulting mixture will then be treated with ultrasound (Braun Labsonic 2000 B, high level) for two (2) minutes to form a dispersion. The dispersion will then be placed under vacuum (1 mm Hg) for one (1) hour and subsequently treated again with ultrasound (Braun Labsonic 2000 B; high level) for two (2) minutes.

4.95 grams of PVdF will then be progressively added to the sample under agitation (1000 RPM) which then maintained until dissolution is complete as evidenced by the solution being clear (5-10 minutes).

3.3 grams of carbon black, followed by 3.3 grams of graphite, will then be progressively added under agitation (1000 RPM). 70.95 grams of LiNiθ2 (previously dried in a vacuum oven at 150°C) will then be added and the stirring speed increased to 4000 RPM for 1 minute with the agitator disc being moved up and down in the paste in order to ensure the homogeneity of the mixture.

The beaker and its contents will then be placed under vacuum for degassing the sample (paste) while taking care not to let the product spill over. The sample (paste) will be maintained for 5 minutes under a maximum vacuum (5-10 mm Hg) to complete degassing.

An aluminium foil (200 x 60 mm), degreased by submersion in CFC-113 for 30 minutes and weighed to within 0.1 mg will then be made perfectly flat by placing on a rigid perforated metal plate and applying a vacuum to the underside of the latter. The foil will then be coated with the paste sample (prepared as described above) to a thickness (of paste) of about 300 μm using a doctor blade. The coated foil will then be placed on a stainless steel support (previously coated with polytetrafluoroethylene (PTFE)) and the support with the coated foil thereon will be placed in a ventilated oven at 150°C for 30 minutes.

After drying, the cathode will be allowed to cool to ambient temperature. The thickness of the coated surface and the weight of the cathode will then be measured.

The cathode will then be passed through a BRABENDER laboratory calender. The space between the rolls of the calender being adjusted in such a way as to have, at first passage, a maximal reduction of thickness of 10%. Subsequent passages will be performed as needed with a reduced space between the rolls, so that the final thickness of the coating is about 45% of the thickness of the wet paste applied to the aluminium foil.

In this fashion, cathodes containing different types of CaCO3, may be obtained.

Example 3

Measuring efficiency of various types of CaCO3 as acid scavengers

Using the electrodes obtained as described in example 1, the metallic supports were removed therefrom and the remainder of each of the electrodes was placed and weighed in a respective "boat" comprised of a material which is inert to the acid (HF). These "boats" were then individually introduced into a tube made up of the same material as the container. The tubes were then individually placed in a tubular oven and heated at a constant temperature (500°C) for 30 minutes during which time a pre-heated nitrogen gas flow was passed through the tubes which gas flow carried off the acid (HF) which had been liberated due to the heating but not scavenged by the CaCO3. The acid (HF) carried off by the gas flow was then absorbed in a known volume of an acid buffer solution (pH of between 5 and 6) of acetic acid/sodium acetate.

The measurement of the acid (HF) absorbed in the buffer solution was then carried out by direct potentiometry with the use of an electrode (an indicator electrode) specific to F^" ions and a reference electrode. The potential difference measured between this indicator electrode and the reference electrode was continuously recorded. Following recording, the measured potential difference was compared to a preestablished reference curve. This comparison permitted the determination of the fluoride content at any particular moment and, therefore, the quantity of acid (HF) liberated at that moment.

The "Acid Scavenged" and the "Acid Scavenging Efficiency" were then calculated, as follows : - From a reference experiment with the coating containing no added CaCO3, it was determined that 31.9 grams of acid (HF)/kg of coating was liberated.

- For the samples containing CaCO3 (79.2 grams/kg coating), the maximum amount of acid (HF) which can be formed by dehydrofluorination of the PVdF was calculated to be 29.4 grams/kg coating (due to the "dilution" effect of the CaCO₃).

- The maximum amount of acid (HF) which can theoretically be neutralised by 79.2 grams CaCU3 is 31.7 grams acid (HF).

- With the amount of acid (HF) eluted by the nitrogen carrier gas in the experiment described above being x grams acid (HF)/kg coating, then :

"Acid Scavenged" = 29.4 - x grams/kg coating; and

294 - x "Acid Scavenging Efficiency" = — ^: • 100%

[It is noted that, due to the use of a slight excess of CaCO3 relative to the HF liberated in the reference experiment, the maximum possible "acid scavenging efficiency", as defined herein, is 92.7 % (corresponding to x = 0)].

Table 2

Obviously, many modifications may be made without departing from the basic spirit of the invention. Accordingly, it will be appreciated by those skilled in the art that, within the scope of the appended claims, the invention may be practised other than has been specifically described herein.

Claims

C L A I M S

1 - A lithium battery of the type having at least one of the anode, cathode and/or separator element having polyvinylidene fluoride homo and/or copolymers incoφorated therein characterised in that said at least one anode, cathode and/or separator element further includes calcium carbonate, wherein the calcium carbonate acts as an acid scavenger, thereby reducing or eliminating the free acid available which may contribute to the occurrence and/or intensity of runaway exothermic reactions in said battery.

2 - The lithium battery of claim 1, further characterised in that the calcium carbonate is included in at least the anode and the cathode thereof.

3 - The lithium battery of claim 1, further characterised in that the calcium carbonate is included in at least the anode and the separator element thereof.

4 - The lithium battery of claim 1, further characterised in that the calcium carbonate is included in at least the cathode and the separator element thereof.

5 - The lithium battery of claim 1 , further characterised in that the calcium carbonate is included in at least the anode, the cathode and the separator element thereof.

6 - The lithium battery of any of the preceeding claims, further characterised in that the calcium carbonate is a SOCAL®.

7 - The lithium battery of any of the preceeding claims, further characterised in that the calcium carbonate has a mean particle diameter size of at least about 0.10 μm.

8 - The lithium battery of any of claims 1-6, wherein the calcium carbonate has a mean particle diameter size of at least about 0.18 μm.

9 - The lithium battery of any of claims 1-6, wherein the calcium carbonate has a mean particle diameter size of at least about 0.2 μm.

10 - The lithium battery of any of claims 1-6, wherein the calcium carbonate has a mean particle diameter size of at least about 0.36 μm. 1 1 - An anode for use in lithium batteries of the type having polyvinylidene fluoride homo and/or copolymers, said anode characterised by having calcium carbonate, wherein the calcium carbonate acts as an acid scavenger, thereby reducing or eliminating the free acid available which may contribute to the occurrence and/or intensity of runaway exothermic reactions in said battery.

12 - The anode according to claim 11, further characterised in that the calcium carbonate is a SOCAL®.

13 - The anode according to either of claims 1 1 and 12, further characterised in that the calcium carbonate has a mean particle diameter size of at least about 0.10 μm.

14 - A cathode for use in lithium batteries of the type having polyvinylidene fluoride homo and/or copolymers, said cathode characterised by having calcium carbonate, wherein the calcium carbonate acts as an acid scavenger, thereby reducing or eliminating the free acid available which may contribute to the occurrence and/or intensity of runaway exothermic reactions in said battery.

15 - The cathode according to claim 14, further characterised in that the calcium carbonate is a SOCAL®.

16 - The cathode according to either of claims 14 and 15, further characterised in that the calcium carbonate has a mean particle diameter size of at least about 0.10 μm.

17 - A method for eliminating and/or reducing in intensity the danger of runaway exothermic reactions in lithium batteries of the type which have PVdF homo and/or copolymers incoφorated in at least one of the anode, cathode and/or separator element thereof, said method characterised by the incoφoration of calcium carbonate in at least one of said anode, cathode and/or separator element of the lithium battery.

18 - The method according to claim 17, further characterised in that the calcium carbonate is a SOCAL®. 19 - The method according to either of claims 17 and 18, further characterised in that the calcium carbonate has a mean particle diameter size of at least about 0.10 μm.