Abstract:
The instant application relates to an X-ray sensitive battery separator for a secondary lithium battery and a method for detecting the position of a separator in a secondary lithium battery. The X-ray sensitive battery separator includes a microporous membrane having an X-ray detectable element. The X-ray detectable element constitutes less than 0.1% by weight of the microporous membrane. The method for detecting the position of a separator in a battery includes the following steps: (1) providing a battery including an X-ray sensitive battery separator; (2) subjecting the battery to X-ray radiation; and (3) thereby detecting the position of said separator in said battery.

Description:
FIELD OF INVENTION 
   The instant application relates to an X-ray sensitive battery separator and a method for detecting the position of a separator in a battery. 
   BACKGROUND OF THE INVENTION 
   A battery separator is used to separate the positive and negative electrodes of a battery, for example, a secondary lithium battery. A battery separator is typically microporous to allow ionic current with least possible resistance. 
   In general, a battery separator is sandwiched between the positive electrode and the negative electrode of a secondary lithium battery. It is important for a battery separator to remain in its proper position because even a minute displacement may cause a short in the battery. Currently there are no prevailing techniques to determine the position of a separator in a battery to prevent the introduction of flawed batteries, i.e. those batteries in which the battery separator was displaced during the manufacturing process, into the consumer market. 
   U.S. Pat. No. 3,861,963 discloses a porous separator, which is coated or impregnated with an inorganic material. The inorganic coating may be an insoluble metal oxide and insoluble hydrous metal oxides such as the oxides and hydrous oxides of Zr, Ti, Sb, W, Si, Sc, Bi, V, Al, and Ce. The coating, however, constitutes about 15-32 percent by weight of the separator. 
   U.S. Pat. No. 5,154,988 discloses a battery separator comprising of three layers, which include a base web, a glass mat, and a natural rubber. The base web includes a filler such as silica and small amounts of alum and other retention aids such as cationic or anionic copolymers, including cationically or anionically modified high molecular weight polyacrylamide. 
   U.S. Pat. No. 5,336,573 discloses a separator comprising a microporous membrane sheet composed of substantially uniform mixture of a polymer and a filler present in a weight ratio of about 1:2.5 to about 1:30 which has a fibrous sheet encapsulated therein. 
   U.S. Pat. No. 5,654,115 discloses a hydrogen-absorbing alloy for a battery capable of satisfying leading characteristics of high electrode capacity, and long life. An X-ray microanalyzer is employed to determine the weight percent of the hydrogen-absorbing particles. 
   U.S. Pat. No. 6,348,286 discloses an alkaline battery separator comprising a fiber sheet containing, on the outer surface of fiber sheet, a substance having a peak of bond energy at 503.5 to 5.31.5 eV. The peak of the bond energy is measured via an X-ray photoelectron spectrometer. 
   U.S. Pat. No. 6,420,070 discloses an electrode made of a graphite material capable of intercalating and de-intercalating lithium ions. CuK α as the X-ray source is utilized to determine the wide angle X-ray diffraction of the electrode. 
   U.S. Pat. No. 6,423,445 discloses a battery separator comprising a gas-permeable sheet that contains a hydrophilic portion carrying a methacrylic/ethylene copolymer component having a crystalinity of 25% or more on at least a part of surface of the hydrophobic portion. Fluorescent X-ray method is utilized to measure X-ray intensity thereby determining the amount of sulfur atoms per unit of area. 
   U.S. Pat. No. 6,432,586 discloses a separator for a high-energy rechargeable lithium battery. The separator comprises a ceramic composite layer and a polymeric microporous layer. The ceramic composite layer comprises a matrix material having inorganic particles dispersed therethrough. The inorganic particles may be selected from SiO 2 , Al 2 O 3 , CaCO 3 , TiO 2 , SiS 2 , SiPO 4 , and the like, or mixtures thereof. The inorganic particles form approximately 20-95% by weight of the ceramic composite layer. 
   U.S. Pat. No. 6,713,217 discloses a hybrid separator. 
   Despite the research efforts in developing battery separators, there is a still a need for a battery separator, which is readily detectable when embedded in a battery to determine its position within the battery, and it is relatively easy to manufacture at a low cost. Furthermore, there is still a need for a method for detecting the position of a separator in a battery, which is relatively easy and cost effective. 
   SUMMARY OF THE INVENTION 
   The instant application relates to an X-ray sensitive battery separator for a secondary lithium battery and a method for detecting the position of a separator in a secondary lithium battery. The X-ray sensitive battery separator includes a microporous membrane having an X-ray detectable element. The X-ray detectable element constitutes less than 0.1% by weight of the microporous membrane. The method for detecting the position of a separator in a battery includes the following steps: (1) providing a battery including an X-ray sensitive battery separator; (2) subjecting the battery to X-ray radiation; and (3) thereby detecting the position of said separator in said battery. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
       FIG. 1  is a first embodiment of an X-ray sensitive battery separator according to the instant invention; 
       FIG. 2  is a second embodiment of an X-ray sensitive battery separator according to the instant invention; and 
       FIG. 3  is an exploded view of a battery including the X-ray sensitive battery separator of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings wherein like numerals indicate like elements, there is shown, in  FIG. 1 , a first embodiment of an X-ray sensitive battery separator  10 . The X-ray sensitive battery separator  10  includes a microporous membrane  12 , which contains an X-ray detectable element  14  dispersed therethrough. 
   Microporous membrane  12  may be any conventional microporous membrane. Microporous membranes are generally known in the art. Microporous membrane  12  may be made from any material, for example a polymer. A polymer, for example, may be any synthetic polymer, cellulose, or synthetically modified cellulose. The preferred synthetic polymers are polyolefins, e.g., polyethylene, polyproplyene, polymethylpentene, polybutylene, ultra high molecular weight polyethylene, copolymers thereof, and mixtures thereof. Microporous membrane  12  may have any porosity; for example, microporous membrane  12  may have a porosity in the range of about 20% to about 80%. Microporous membrane  12  may have any average pore size; for example, microporous membrane  12  may have an average pore size in the range of about 0.1 micron to about 5 microns. Microporous membrane  12  may have any thickness; for example, microporous membrane  12  may have a thickness in the range of about 10 microns to about 75 microns. 
   X-ray detectable element  14  may be any X-ray detectable material. For example, X-ray material may be a material selected from the group consisting of a metal oxide, a metal phosphate, a metal carbonate, an X-ray fluorescent material, and combinations thereof. The listed X-ray materials are not limiting. Exemplary metal oxides include, but are not limited to, metal oxides having a metal selected from the group consisting of Zn, Ti, Mn, Ba, Ni, W, Hg, Si, Cs, Sr, Ca, Rb, Ta, Zr, Al, Pb, Sn, Sb, Cu, Ni, and Fe. The listed metal oxides are not limiting. Exemplary metal phosphates include, but are not limited to, phosphate oxides having a metal selected from the group consisting of Zn, Ti, Mn, Ba, Ni, W, Hg, Si, Cs, Sr, Ca, Rb, Ta, Zr, Al, Pb, Sn, Sb, Cu, Ni, and Fe. The listed metal phosphates are not limiting. Exemplary metal carbonates include, but are not limited to, metal carbonates having a metal selected from the group consisting of Zn, Ti, Mn, Ba, Ni, W, Hg, Si, Cs, Sr, Ca, Rb, Ta, Zr, Al, Pb, Sn, Sb, Cu, Ni, and Fe. The listed metal carbonates are not limiting. Exemplary X-ray fluorescent materials include, but are not limited to, organic materials, inorganic materials, and combinations thereof. A fluorescent material, as used herein, refers to a material having electrons capable of becoming exited by X-ray radiation thereby providing detection signals. The listed X-ray fluorescent materials are not limiting. X-ray detectable element  14  may constitute any percentage of the weight of membrane  12 . For example, the X-ray detectable element may constitute in the range of 0.01 to 0.1 percent by weight of the membrane  12 . 
   In the alternative, referring to  FIG. 2 , the X-ray sensitive battery separator  10 ′ may be a multi-layer battery separator. Multi-layer, as used herein, refers to two or more layers. The X-ray battery separator  10 ′ preferably includes a microporous membrane  12 ′, which contains an X-ray detectable element  14 ′, and at least one other layer  16 . Preferably, the X-ray battery separator  10 ′ includes a plurality of layers  16 . Additionally, at least one layer may be a shutdown layer, i.e., one adapted to shut down ionic flow between the electrodes in the event of thermal runaway or internal short circuiting caused by internal or external circumstances. 
   Microporous membrane  12 ′ may be any conventional microporous membrane. Microporous membranes are generally known in the art. Microporous membrane  12 ′ may be made from any material, for example a polymer. A polymer, for example, may be any synthetic polymer, cellulose, or synthetically modified cellulose. The preferred synthetic polymers are polyolefins, e.g., polyethylene, polyproplyene, polymethylpentene, polybutylene, ultra high molecular weight polyethylene, copolymers thereof, and mixtures thereof. Microporous membrane  12 ′ may have any porosity; for example, microporous membrane  12 ′ may have a porosity in the range of about 20% to about 80%. Microporous membrane  12 ′ may have any average pore size; for example, microporous membrane  12 ′ may have an average pore size in the range of about 0.1 micron to about 5 microns. Microporous membrane  12 ′ may have any thickness; for example, microporous membrane  12 ′ may have a thickness in the range of about 10 microns to about 75 microns. 
   X-ray detectable element  14 ′ may be any X-ray detectable material. For example X-ray material  14 ′ may be a material selected from the group consisting of a metal oxide, a metal phosphate, a metal carbonate, and an X-ray fluorescent material. The listed X-ray materials are not limiting. Exemplary metal oxides include, but are not limited to, metal oxides having a metal selected from the group consisting of Zn, Ti, Mn, Ba, Ni, W, Hg, Si, Cs, Sr, Ca, Rb, Ta, Zr, Al, Pb, Sn, Sb, Cu, Ni, and Fe. The listed metal oxides are not limiting. Exemplary metal phosphates include, but are not limited to, phosphate oxides having a metal selected from the group consisting of Zn, Ti, Mn, Ba, Ni, W, Hg, Si, Cs, Sr, Ca, Rb, Ta, Zr, Al, Pb, Sn, Sb, Cu, Ni, and Fe. The listed metal phosphates are not limiting. Exemplary metal carbonates include, but are not limited to, metal carbonates having a metal selected from the group consisting of Zn, Ti, Mn, Ba, Ni, W, Hg, Si, Cs, Sr, Ca, Rb, Ta, Zr, Al, Pb, Sn, Sb, Cu, Ni, and Fe. The listed metal carbonates are not limiting. Exemplary X-ray fluorescent materials include, but are not limited to, organic materials, inorganic materials, and combinations thereof. A fluorescent material as used herein refers to a material having electrons capable of becoming exited by X-ray radiation thereby providing detection signals. The listed X-ray fluorescent materials are not limiting. X-ray detectable element  14 ′ may constitute any percentage of the weight of membrane  12 ′. For example, the X-ray detectable element may constitute in the range of 0.01 to 0.1 percent by weight of the membrane  12 ′. 
   Layer  16  may be any conventional microporous membrane. Microporous membranes are generally known in the art. Layer  16  may be made from any material, for example a polymer. A polymer, for example, may be any synthetic polymer, cellulose, or synthetically modified cellulose. The preferred synthetic polymers are polyolefins, e.g., polyethylene, polyproplyene, polymethylpentene, polybutylene, ultra high molecular weight polyethylene, copolymers thereof, and mixtures thereof. Layer  16  may have any porosity; for example, layer  16  may have a porosity in the range of about 20% to about 80%. Layer  16  may have any average pore size; for example, layer  16  may have an average pore size in the range of about 0.1 micron to about 5 microns. Layer  16  may have any thickness; for example, layer  16  may have a thickness in the range of about 10 microns to about 40 microns. 
   In manufacturing, referring to  FIG. 3 , the X-ray sensitive battery separator  10 , is sandwiched between a positive electrode  18  and a negative electrode  20 , and may be subsequently rolled into a jellyroll  15  (prismatic constructions are also possible). The jellyroll  15  may further include negative tab  24 , and positive tab  22 . Positive electrode  18  may include a metal sheet, e.g., aluminum foil, i.e. the current collector, upon which the positive electrode material or electrode active mix (not shown but conventional) has been spread in conventional manner. Negative electrode  20  may include a metal sheet, e.g., copper foil, i.e. the current collector, upon which the negative electrode material or electrode active mix (not shown but conventional) has been spread in conventional manner. Subsequently, jellyroll  15  is inserted into can  26 , which is filled with an electrolyte (not shown), and then, can  26  is sealed with cap  28 . can  26  may be a metallic (e.g., steel, stainless steel, aluminum) cylindrical can, a plastic box, of a foil (e.g., metallized foil) pouch. Electrolyte may be any substance capable of providing ionic conductivity. Electrolyte may, for example, be a liquid electrolyte, a solid electrolyte, or a gel electrolyte. A liquid electrolyte generally includes an electrolytic salt dissolved in a solvent, i.e. an inorganic solvent or an organic solvent. A gel electrolyte generally includes an electrolytic salt dissolved in non-aqueous solvent, and gelated with a polymer matrix. 
   In operation, a battery containing an X-ray sensitive battery separator  10  is subjected to X-ray radiation thereby facilitating the detection of the position of the X-ray sensitive battery separator  10  within the battery. For example, the separator is usually wider than the electrodes, so that the separator extends beyond the lateral edges of the electrodes. The separator extends beyond the lateral edges of the electrodes to prevent the electrodes from coming into physical contact and thereby creating the potential for short-circuiting. It is possible that during winding or in the battery assembly that the separator portion that extends beyond the lateral edges of the electrodes is removed or pushed back or otherwise misplaced thereby allowing the possibility of physical contact of the electrodes. An X-ray examination of the assembled battery allows a check to determine that the separator remains in position throughout manufacture. The X-ray visible separator can be observed, via X-ray examination, to ensure that it has maintained its position (i.e., a portion extending beyond the lateral edges of the electrodes). Further, it is possible that the inspection process could be automated, via computer, to increase the speed of inspection. 
   The present invention may be embodied in other forms without departing from the spirit and the essential attributes thereof, and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicated the scope of the invention.