Abstract:
An electrochemical energy store comprising a separator ( 40, 40   a,    40   b ) is described, wherein said electrochemical energy store has a positively charged electrode ( 20 ), a negatively charged electrode ( 30 ), an electrolyte, and a porous separator ( 40, 40   a,    40   b ) which separates the positively charged electrode ( 20 ) and the negatively charged electrode ( 30 ) from each other. The separator ( 40, 4   a,    40   b ) includes at least one microporous foil which is produced using ion irradiation, among other things. The separator ( 40, 40   a,    40   b ) farther includes ion ducts ( 43 ) extending at different angles from one another.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an electrochemical energy store having a positively charged electrode, a negatively charged electrode and a porous separator. The porously designed separator is used to isolate the positively charged electrode and the negatively charged electrode from one another. 
       PRIOR ART 
       [0002]    The prior art discloses various types of electrochemical energy stores which are used to supply electrically operated appliances with power. Such energy stores are usually called batteries or accumulators. When the battery or accumulator is discharged, chemical energy is converted to electrical power by an electrochemical redox reaction. Said electrical power can be used in a wide variety of ways by an electrical load connected to the electrochemical energy store. 
         [0003]    Electrochemical energy stores can generally be classified into a first group of nonchargeable primary batteries and a second group of rechargeable secondary batteries. In this case, secondary batteries can be returned, following discharge, to a charge state which largely corresponds to the original charge state prior to discharge, which means that it is possible to repeatedly convert chemical energy to electrical power and back. 
         [0004]    Essential quality criteria of primary and secondary batteries are high energy density, good thermal stability and the delivery of a constant voltage over the discharge period. In addition, preferred batteries have no “memory effect”, which means that they do not suffer any loss of capacity even with multiple charging/discharge operations. Furthermore, the raw materials used in the batteries should be sufficiently present in nature, as a result of which these battery types can be produced inexpensively even in the long term. 
         [0005]    The way in which batteries work is based on an electrochemical redox reaction which is known to a person skilled in the art, wherein the battery discharge involves the occurrence of reducing processes at a positively charged electrode (cathode) and oxidizing processes at a negatively charged electrode (anode). There is thus ion transportation, which takes place within an electrolyte, wherein the process can be reversed in the case of a rechargeable secondary battery in order to recharge the battery. In order to isolate the anode and the cathode from one another physically and electrically, a separator is used in the battery. Said separator is wetted with the electrolyte and has the particular task of preventing electrical shorts within the battery, but at the same time needs to be permeable to ions in order to be able to guarantee the electrochemical reactions. 
         [0006]    The separator is therefore an important element which concurrently influences the properties of the battery to a significant degree. The internal resistance, the charge capacity, the charging/discharge current and further electrical properties of the battery are concurrently determined by the separator to a definitive degree. The separator should be mechanically robust and have good ion permeability. The demands on batteries include not only high energy density but also, in particular, high power density in order to be able to provide a large volume of power within a short time. However, the power density is influenced particularly by the permeability of the separator. The separator should accordingly be designed such that it transmits as large a volume of ions as possible per unit time. Inter alia, the thickness of the separator should therefore be as small as possible. Furthermore, the separator should be easily wettable, have long-term robustness toward the chemicals and solutions which occur in the battery, and react insensitively to temperature fluctuations as may occur in batteries. 
         [0007]    The prior art primarily uses separators which are based on polyolefins. However, these have the disadvantage that they react sensitively to increased temperatures and particularly to temperatures of above 150° C. Thus, the melting temperature of polyolefins is relatively low, and a separator designed in this manner has low dimensional stability in respect of heating. This can cause shorts inside the battery, which in turn result in a rise in temperature. The battery is permanently damaged as a result. Specifically in the field of batteries of high-power design or when external shorts occur, however, very severe internal heating may arise which the separator should withstand so as not to irreversibly damage the battery. 
         [0008]    EP 0 851 523 discloses a separator which comprises a membrane based on a polyethylene terephthalate (PET) nonwoven. The thermal stability of this membrane is significantly increased in comparison with the separators which are based on polyolefins. Further such purely PET-based separators are likewise described in US 2003/0190499 and US 2006/0019164. However, a drawback of such separators is the effect of the relatively large pores, which have an average diameter of between 5 μm and 15 μm. Furthermore, the variance in the pore diameter is large, which means that short-circuit currents may be produced particularly in the region of relatively large pores. Furthermore, the nonwoven-type structure of the separator means that it does not have well-defined ion channels, but rather has a spongy quality. The path of the ions from one to the other side of the separator membrane acting as a depth filter is significantly extended thereby, and the pore size varies to an accordingly great extent both in the direction through the separator and over the surface area of the separator. A further known problem of such separators is what is known as dendritic growth. This involves the formation, starting from the electrodes, of a type of enlarging “stalactites”, which sometimes pass through the separator and can therefore form an internal short. Separators which have a spongy structure are susceptible to this dendritic growth particularly because, firstly, sometimes excessively large pores, which cause high local current density, are already present, and, secondly, the thinly produced sponge structures are easily perforated. 
         [0009]    Further PET-based separators are specified in JP 2005/293891 and CN  2009 / 69179 . 
         [0010]    In order to improve the properties of a lithium ion battery and to reduce the pore size of the separator, EP 2 077 594 and US 2003/0190499 specify separators in which a respective PET-based nonwoven is coated with an organic polymer such as polyvinylidene fluoride (PVdF). US 2006/0019164 describes a PET separator with a ceramic coating. A drawback of these separators, however, is the effect of the depth filter structure, in particular, and in the case of ceramic also of the fragility and complicated production. 
       PRESENTATION OF THE INVENTION 
       [0011]    It is an object of the present invention to specify an electrochemical energy store which has a separator which eliminates the aforementioned drawbacks. 
         [0012]    This object is achieved by an electrochemical energy store having the features of claim  1 . Further embodiments are specified in the dependent claims. 
         [0013]    The present invention thus provides an electrochemical energy store having a separator which has the following features: 
         [0014]    a positively charged electrode, 
         [0015]    a negatively charged electrode, and 
         [0016]    an electrolyte. 
         [0017]    The separator isolates the positively charged electrode and the negatively charged electrode from one another and is of porous design. Furthermore, the separator has at least one microporous membrane which has ion channels formed in it which are produced by means of exposure to radiation from ions, inter alia. 
         [0018]    The ion channels in this arrangement are each at different angles to one another. 
         [0019]    The electrochemical energy store may be a primary battery or a secondary battery. This may involve any battery type within these two groups, wherein particularly the positively charged electrode and the negatively charged electrode and also the electrolyte are then designed from an appropriate material. In the group of primary batteries, for example, a lithium battery would be conceivable. In the case of a secondary battery, the electrochemical energy store may relate to battery types such as a lead acid battery, a lead gel battery, a sodium sulfur battery, a nickel lithium battery, a lithium iron phosphate battery, a lithium titanate battery or a lithium air battery. With particular preference, the electrochemical energy store is a lithium ion battery, however, in which the positively charged electrode has a lithium-containing metal oxide and the negatively charged electrode is suitable for receiving and emitting lithium ions. 
         [0020]    Producing the microporous membrane by means of exposure to ion radiation is advantageous particularly because it allows the formation of well-defined ion channels. Exposure to ion radiation therefore prompts the formation of the ion channels. The microporous membrane may thus be produced not only by the exposure to radiation from ions but also by further method steps which can be seen in the finished membrane under a microscope, such as particularly by subsequent chemical etching. Such etching allows the removal of molecule chains which have been split up during the exposure to ion radiation, in order to form pores completely. Further and alternative further treatment steps are possible. This exposure to radiation from ions in combination with possible further method steps such as the etching described thus prompts formation of ion channels which can be seen under a microscope. In contrast to separators from the prior art which have the spongy structure of a depth filter, such a separator according to the invention allows the passage of ions on a direct, zero-resistance path. Such a separator may thus simultaneously have relatively low porosity and nevertheless very good ion permeability. It is therefore also mechanically relatively robust. The good ion permeability of the separator improves the electrical properties of the battery to a substantial degree, and the mechanical robustness of the separator facilitates production of the battery, in particular. 
         [0021]    The separator may have a single microporous membrane, in particular. Furthermore, it may be formed solely therefrom. 
         [0022]    Preferably, the microporous membrane is produced at least partly from polyethylene terephthalate (PET) and in particular exclusively from polyethylene terephthalate (PET). As a result, the separator is stable over a very wide temperature range. The melting point of such a PET separator is 220° C., and the separator can be operated in a range from −40° C. to 180° C. without altering its structure. By way of example, this allows the battery to be operated at high power too. In addition, PET is easily wetted with an electrolyte and has good properties in respect of processing. 
         [0023]    Preferably, the pores of the microporous membrane are each in the form of essentially cylindrical ion channels. By “essentially”, it is meant that the diameter of the ion channels may alter slightly along the longitudinal extent thereof. The cylindrical shape of the ion channels may be hose-like or, in particular, tubular in this case. Various ion channels may also intersect. In the case of a significant majority of the pores, however, it is possible to see a clearly defined, hose-like ion channel which has at least one considerable longitudinal section which is unbranched and is not intersected by another ion channel. Such a pore structure is optimum, since the cross-sectional area of the pores can be determined very precisely, and the path for the ions through the separator is direct and without resistance. 
         [0024]    In particular, the ion channels are each at different angles to one another. This means that the ion channels extend in different spatial directions randomly in each case. Preferably, the ion channels are each at different angles to one another not only along one dimension but also along two dimensions which each extend parallel to the membrane surface. The different ion channels are thus advantageously each askew with respect to one another in space. The mean pore diameter of the separator therefore has much lower variance particularly in the case of a high pore density. The probability of occurrence of parallel ion channels which have partially overlapping cross-sectional areas and therefore together form an excessively wide pore is substantially reduced. 
         [0025]    Of particular advantage is an embodiment in which the angle between the surface of the separator membrane and the ion channels is at least 45° in each case. This limits the length of the ion channels. Preferably, however, at least 50% of the ion channels are at an angle of less than 70° to the surface of the separator membrane. This ensures that the angles of the ion channels to the membrane surface each differ to a sufficiently high degree from ion channel to ion channel. 
         [0026]    The ion channels may each have an opening which widens toward the outside, as can be seen under a microscope, on both sides of the separator. Preferably, the openings in this case each widen conically toward the outside, as a result of which a single ion channel can be called double conical, and as a whole it has a kind of “hourglass shape”. This facilitates the entry of the ions into the ion channel, which benefits both the properties of the charging operation and those of the discharge operation. 
         [0027]    In order to achieve good ion permeability for low internal resistance, on the one hand, and to ensure the mechanical robustness of the separator, on the other hand, the separator preferably has a thickness of between 12 μm and 36 μm. In this case, particularly a thickness of the separator of between 20 μm and 28 μm, preferably of approximately 23 μm, is advantageous. 
         [0028]    In order to improve the wettability of the separator with the electrolyte, and hence to facilitate the passage of ions through the separator, the separator may have a modification to the surface which improves the wettability with liquids. This may be a chemical or a physical modification. In particular, it may also be a coating of the surface with another material, which has improved properties in terms of wettability. 
         [0029]    In one preferred embodiment, the porosity of the separator is less than 30%. This improves the mechanical and chemical robustness. Even more advantageous in this case is an embodiment in which the porosity of the separator is less than 20%, in particular even less than 15%. 
         [0030]    The present invention furthermore specifies a separator for use in an electrochemical energy store, wherein the separator is designed as described above, in particular is of porous design. In addition, the invention claims the use of a microporous membrane as a separator for an electrochemical energy store. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Preferred embodiments of the invention are described below with reference to the drawings, which are used merely for explanation and should not be interpreted as restrictive. In the drawings: 
           [0032]      FIG. 1  shows a perspective view of an inventive battery according to a first embodiment, cut open for illustrative purposes; 
           [0033]      FIG. 2  shows a schematic illustration of the polymer structure of a separator as can be found in the battery in  FIG. 1  prior to exposure to ion radiation; 
           [0034]      FIG. 3  shows a schematic illustration of the polymer structure of a separator as can be found in the battery in  FIG. 1  following exposure to ion radiation; 
           [0035]      FIG. 4  shows a schematic illustration of the polymer structure of a separator as can be found in the battery in  FIG. 1  following exposure to ion radiation and during the etching operation; 
           [0036]      FIG. 5  shows a microscopic view of the surface of a separator as can be found in the battery in  FIG. 1 ; 
           [0037]      FIG. 6  shows a microscopic sectional view at right angles to the surface of a separator as can be found in the battery in  FIG. 1 ; 
           [0038]      FIG. 7  shows a microscopic view of the surface of a separator based on the prior art; 
           [0039]      FIG. 8  shows a microscopic sectional view at right angles to the surface of a separator based on the prior art; 
           [0040]      FIG. 9  shows an apparatus for producing a separator as can be found in the battery in  FIG. 1 ; and 
           [0041]      FIG. 10  shows an illustration of the ion bombardment of a membrane in the apparatus in  FIG. 9 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0042]      FIG. 1  shows a perspective illustration of a preferred exemplary embodiment of an electrochemical energy store according to the invention. This electrochemical energy store, which is described below, is a secondary battery in the form of a lithium ion battery. However, this embodiment is only one possible example of an electrochemical energy store according to the invention. Self-evidently, the separator according to the invention can also be used in other electrochemical energy stores. 
         [0043]    In this embodiment, the battery has an essentially cylindrical housing  10  having a circumferential side wall which contains, as the most important parts of the battery, a positively charged electrode  20  and a negatively charged electrode  30  isolated by porous separators  40   a  and  40   b . In addition, the housing  10  contains an electrolyte which is in chemical contact with the two electrodes  20 ,  30  and which surrounds the two separators  40   a ,  40   b , wetting them in the process. In this case, the negative electrode  30  has a material which is active in the chemical reaction of the charging or discharge operation and which contains graphite. In the present exemplary embodiment, the positive electrode  20  contains particularly lithium metal oxides. The positively and negatively charged electrodes  20  and  30  are each in the form of a long, ribbon-like microporous sheet  21  or  31  in this case. Similarly, the separators  40   a  and  40   b  in the present exemplary embodiment are each as a whole in the form of a membrane. The battery has two separators  40   a  and  40   b  of the same type in this case. In order to produce the battery, these cited microporous membranes are each placed congruently above one another in the order positive electrode  20 —separator  40   a —negative electrode  30 —separator  40   b  and are then rolled up around a connecting pin  50  (possibly a plurality of times), wherein the positive electrode  20  comes to rest radially innermost. Even in the wound up state, the sheet  21  of the positive electrode  20  and the sheet  31  of the negative electrode  30  are thus isolated from one another at every location by respect of one of the two separators  40   a  and  40   b . The design of the separators  40   a  and  40   b  is described in detail further below. 
         [0044]    The connecting pin  50  is arranged centrally along the longitudinal axis of the housing  10  and is connected along a predominant portion of its length to an electrode connection  22  of the positively charged electrode  20 . This electrode connection  22  is formed along that edge of the sheet  21  of the positive electrode  20  which is inside in the rolled up state and which runs parallel to the connecting pin  50 . In this case, it is arranged on that side of the sheet  21  which points radially inward. The electrode connection  22  is formed particularly such that it can be connected to the connecting pin  50  and thereby makes an electrically conductive connection between the sheet  21  of the positive electrode  20  and the connecting pin  50 . 
         [0045]    The connecting pin  50  in turn is connected by means of an electrically conductive connection to a positive pole  70 , which in this embodiment is formed by a top area which closes off one side of the cylindrical housing  10  to form a seal. To produce the seal, a seal  110 —for example in the form of a sealing ring—is arranged between the housing  10  and the outer edge of this top area. The outwardly pointing side of the top area which forms the positive pole  70  is suitable particularly for applying a first contact of an electrical load (not shown), which may take a variety of forms. 
         [0046]    That side of the roll formed by the electrodes  20 ,  30  and the separators  40   a ,  40   b  which points towards the pole  70  has an insulator  61  fitted. The insulator  61  prevents the negatively charged electrode  30  from being in electrical contact with the connecting pin  50 , the pole  70  or another electrically conductive element arranged between the pole  70  and the negative electrode  30 . In this case, the insulator  61 , which is made from an electrically insulating material, surrounds the connecting pin  50  and extends circumferentially therefrom radially outward up to the side wall of the housing  10 . In the present exemplary embodiment, this ensures that the pole  70  is electrically connected to the winding exclusively by means of the connecting pin  50 , and no short inside the battery can arise between the pole  70  and the negative electrode  30 . 
         [0047]    In order to upwardly limit the temperature inside the battery, for example in the case of an external short, a PCT thermistor  100  may be provided within the electrical connection between the connecting pin  50  and the pole  70 . The thermistor  100  is a temperature-dependent electrical resistor which substantially increases its resistance value in the event of an increase in the current and thereby upwardly limits the flow of current and hence also the temperature. This protects the battery against increased temperature on account of an excessive flow of current, which prevents related, irreversible damage to the battery. 
         [0048]    In addition, a safety valve  90  may be formed in the region between the electrodes  20 ,  30  and separators  40   a ,  40   b  rolled up inside one another and the pole  70 . This safety valve  90  allows an overpressure produced during battery charging, for example, to escape from the inside of the battery to the outside. 
         [0049]    In the present exemplary embodiment, the sheet  31  of the negative electrode  30  has an electrode connection which is fitted along that edge of the sheet  31  which is outside in the wound up state and which runs parallel to the connecting pin  50 . This electrode connection  32  is formed on that side of the sheet  31  which points radially outward, and that end of said electrode connection which is remote from the pole  70  has a tab which extends from the radial outer side of the sheet  31 , beyond the edge thereof, radially inward. The tab on the electrode connection  32  is connected to a negative pole  80  which is formed by a closure area which closes the housing  10  on that side which is opposite the positive pole  70 . The outer side of this closure area is suitable for applying a second contact of an electrical load—which is not shown here. 
         [0050]    Fitted between this closure area which forms the negative pole  80  and the sheets  21 ,  31 ,  40   a ,  40   b  which are rolled up inside one another is a second insulator  62 , which electrically isolates the negative pole  80  from the positive electrode  20 . In the region of the tab of the electrode connection  32 , the second insulator  62  is arranged between this tab and the rolled up sheets  21 ,  31 ,  40   a ,  40   b  in this case. In contrast to the first insulator  61 , the connecting pin  50  does not project through the second insulator  62 . 
         [0051]    The text below describes the production of the separators  40   a  and  40   b . A separator  40 , which is suitable for use as a separator  40   a  or  40   b  in a battery, is of porous design and, when used in the battery, isolates the positively charged electrode  20  and the negatively charged electrode  30  from one another. In this case, in the present exemplary embodiment, it is particularly permeable to lithium ions. The starting material for the separator  40  comprises a uniform, homogeneous polyester and may comprise polycarbonate, polyamide or polyimide or in particular, as in the present case, polyethylene terephthalate (PET). As illustrated in  FIG. 2 , this starting material is constructed at a molecular level by a multiplicity of polymer chains  41 , these being able to form a crystalline (corresponding to region A in  FIG. 2 ) through to amorphous (region B in  FIG. 2 ) structure in different regions as the case may be. 
         [0052]    To produce the pores, the starting material of the separator  40 , having been processed to form a membrane, is exposed to radiation by means of ions during a particular time. In this case, this exposure to radiation is effected essentially from a direction which is at right angles to the membrane surface, as indicated in  FIG. 2  by an arrow which indicates the direction of exposure to radiation. In this case, the rear and front membrane surfaces are on the left-hand and the right-hand side, respectively, in  FIG. 2 . Depending on the intensity and duration of this exposure to radiation, a different pore density can be determined in this case. Although there are local variations in the pore density, they are relatively small. The exposure to radiation destroys or breaks the polymer chains  41  in the respective regions in which the ions pass through the membrane, as shown in  FIG. 3 . In this case, a passage of ions involves the formation of a respective path of destroyed polymer chains  41  which extends through the membrane. This path, which is marked by two horizontal solid lines in  FIG. 3 , has a diameter d (see  FIG. 3 ) of between approximately 5 nm and 7 nm. 
         [0053]    The membrane according to this embodiment is then dipped in a bath which contains etching materials and is drawn through it. The etching materials used for this purpose are highly alkaline solutions, such as potassium hydroxide solution and sodium hydroxide solution. The etching operation removes particularly the polymer chains broken by the exposure to ion radiation, which produces a pore running through the membrane. As  FIG. 4  shows, the etching liquid spreads out during the etching operation not only at right angles to the membrane surface along the path formed by the exposure to ion radiation, but also in all directions at right angles thereto. In this case, when it spreads out, the etching liquid forms an etching front in the separator membrane. The speed V t  at which this etching front spreads out in the direction of the path formed by the ion bombardment is substantially, that is to say a multiple, higher than the speed V b  at which the etching front spreads out at right angles to this path, however. The reason for this is that the destroyed polymer chains make it significantly easier for the etching front to spread out in the relevant direction of the path formed by the exposure to ion radiation. After a certain time, the etching front has passed through the membrane and the pores are formed. In order to obtain a wider and precisely predetermined pore diameter, however, the membrane can remain in the bath with the etching liquid for even longer, which causes the pores to widen in accordance with the already cited speed V b . 
         [0054]    The production process can be completed by further steps such as neutralization, rinsing and drying. To this end, the separator membrane is drawn through appropriate baths in succession. The process can also be extended and, by way of example, comprise a step to modify the surface, which involves the microporous membrane, in which pores are already formed, being altered such that its wettability with liquids is improved. This modification can be made by chemical or by physical means. Further production steps are possible. 
         [0055]    As shown in a microscopic illustration in  FIGS. 5 and 6 , the pores  43  of the separator  40  are of essentially cylindrical form and connect the top of the separator membrane to the bottom on an essentially straight path. The pores  43  have a solid  42  formed between them which is impenetratable to ions. The pores  43  have a well-defined structure, and an ion passes through the separator  40  through one of the pores  43  on a rectilinear, direct path which is free of resistances. The pores  43  are thus actual ion channels which are clearly visible in the separator under a microscope. 
         [0056]    As can clearly be seen in  FIG. 6 , the ion channels or pores  43  are each oblique to one another, that is to say at different angles to one another, in particular. Such an obliquely running form of the ion channels is achieved by virtue of the ions consciously being deflected into corresponding, different spatial directions relative to the surface of the membrane, when the separator membrane is exposed to radiation. A possible method for producing such obliquely running ion channels is described further below with reference to  FIGS. 9 and 10 . Advantageously, the angle α (see  FIG. 6 ) of an ion channel relative to the membrane surface is in each case at least 45° in all directions, however. Preferably, more than 50% of all the ion channels are at an angle of less than 70° to the membrane surface. In this case, the angle of the ion channels  43  relative to the membrane surface is respectively determined during the exposure to ion radiation by the direction of the ion passage through the membrane. The fact that the ion channels  43  each run askew relative to one another ensures that, particularly in the case of a separator  40  with a high pore density, the cross-sectional areas of two or more pores do not coincide and that a pore with an enlarged cross-sectional area is not formed as a result. This would be possible if the ion channels were to run parallel to one another. Although it is possible for the ion channels  43  running obliquely relative to one another to intersect at the surface, for example, as can be seen multiple times in  FIG. 5 , or at another level of the membrane, that is to say to have an at least partially overlapping-cross-sectional area at one location, the oblique, random arrangement means that the ion channels  43  then run independently of one another and in different directions outside of this common point of intersection. The definitive cross-sectional area for ion passage thus continues to be determined by the diameter of the individual ion channel rather than by the common cross-sectional area at a point of intersection with another ion channel. The respective differently oblique course of the ion channels  43  thus allows the cross-sectional area of the pores to be defined precisely, and allows the variance in this cross-sectional area of pores over the entire separator  40  to be kept substantially lower. 
         [0057]    The ion channels  43  may be in a form such that they are in funnel-shaped form in the region of their openings with which they open outward at the two membrane surfaces, in which case they widen conically toward the outside. In this case, the ion channels may have such funnel-shaped openings on both sides of the membrane, that is to say may be double conical and have a type of “hourglass shape”. This facilitates the entry of an ion into an ion channel  43 . Such a double conical shape of an ion channel  43  is produced during the etching operation, since the etching chemical requires a certain period in order to penetrate the ion channels and produce them. As a result, the etching chemical acts for longer at the surface of the membrane or in the entry region of the ion channels than inside the ion channels. This prompts the formation of ion channel openings which widen conically toward the outside, which is clearly visible under a microscope particularly in the case of relatively thicker separator membranes. 
         [0058]    The pores  43  advantageously, have a diameter of between 0.01 μm and 10 μm, the separator  40  preferably having a pore density of between 10E5 and 10E9 pores per cm 2 . 
         [0059]    In one specific, preferred exemplary embodiment, the separator  40  is produced from polyethylene terephthalate (PET), wherein its surface is modified such that it has properties which improve wettability with liquids. The thickness of the separator  40  is 23±2 μm, and the pore diameter is 0.2±0.02 μm. The density of the pores is 320±40*10E6 pores per cm 2 . As a characteristic value for its ion permeability, such a separator allows, per cm 2 , an air throughput of more than 2.5 liters per minute and per bar. The bursting pressure of the separator is then more than 0.95 bar, and the separator has a temperature stability of up to above 220° C. 
         [0060]    The separator  40  produced in this manner has a porosity of approximately 12%. In comparison with separators from the prior art, which are based on polyolefins or coated PET nonwovens, for example, this value is very low. Nevertheless, the ion permeability in the case of the present separator is significantly improved in comparison with the separators from the prior art, particularly in respect of the ions transmitted per unit time. This can be explained with the special, rectilinear and tubular pore structure of the described separator  40 , as shown in  FIGS. 5 and 6 , in comparison with the pore structure of conventional separators. Such a pore structure of a separator  40 ′ from the prior art is shown in plan view in  FIG. 7  and in cross section in  FIG. 8 . To produce the pores, the separator material, which is based on polyolefins in this case, is pulled apart in a stretching process, as a result of which a fibrillar spongy structure is formed. 
         [0061]    The solid  42 ′ thereby forms a multiplicity of islands which are connected to one another by means of a multiplicity of branches, as can be seen in  FIG. 7 . In the interspaces, the pores  43 ′ are formed. However, these pores  43 ′ do not have a cylindrical, rectilinear structure but rather are formed by highly contorted and random paths through the dendritic structure of the separator solid  42 ′. A passage path for an ion from one side to the other of the separator  40 ′ is extended significantly as a result, and the pore diameter is not clearly determined and has a correspondingly large variance. Furthermore, the relatively poor wettability of the polyolefin-based material in comparison with PET has an adverse effect on the properties of the separator in this case. 
         [0062]      FIG. 9  schematically shows a possible apparatus for producing ion channels inclined obliquely with respect to one another in a membrane. The apparatus has an ion source  200  which emits ions. The ions are accelerated within a magnetic field, which is formed in the acceleration sections  220 ,  221 ,  222  and  223 , along a longitudinal axis in the direction of a target, which in this case is a membrane  260 , particularly a PET membrane. The magnetic field strengths of the acceleration sections  220  to  223  may each be different in this case and, in particular, may rise continuously from the acceleration section  220  to the acceleration section  223 . After passing through the acceleration sections  220  to  223 , however, the energy of the ions must at any rate be sufficiently high to penetrate the target or the membrane  260 . On account of the length of the acceleration sections  220  to  223 , there is the assurance that the ions hit the target within a particular angle range. Such ion accelerators have been known for a long time in the prior art. 
         [0063]    Arranged between the ion source  200  and the acceleration sections  220  to  223  is what is known as a wobbler  210 , which is used to fan out the ion beam. The wobbler  210  surrounds the ion beam and in so doing exposes it to an electromagnetic field which is variable over time. In this case, a power supply  250  supplies an AC voltage to the wobbler. Since the wobbler  210  fans out the ion beam, the ions do not hit the target at a pinpoint location, but rather are scattered over a certain width or area. 
         [0064]    The membrane  260  to be exposed to radiation is rolled up on one of the winding rollers  241  in the winding chamber  240  and, during the exposure to ion radiation, is continuously rewound from one winding roller  241  to the other winding roller  241  using a proven method. In the process, the membrane  260  runs over a deflection roller  242  arranged between the two winding rollers  241 . The deflection roller  242  is arranged precisely on the longitudinal axis of the ion beam. As a result, the membrane  260  has a radius corresponding to the radius of the deflection roller  242  in that region in which said membrane is bombarded by the ion beam, as shown in  FIG. 10  (arrows represent the fanned out ion beam). The effect of this, in particular, is that the ions penetrate the membrane  260  at different angles and thereby produce ion channels with different inclinations. In this case, the membrane is therefore deliberately arranged relative to the direction of exposure to the ion radiation such that it is penetrated by the ions in different spatial directions. Alternatively or in addition, it is naturally also possible for the ions to be deflected relative to the membrane surface. This can be done using a wobbler, in particular. In the present exemplary embodiment, the wobbler  210  is also actually used to amplify the effect shown in  FIG. 10  by virtue of the wobbler  210  fanning out the ion beam such that the individual ions move through the acceleration sections  220 - 223  at least slightly different angles relative to the longitudinal axis of the ion beam. 
         [0065]    During the ion bombardment, the membrane  260  is advantageously guided more than once, in particular at least twice, via the deflection roller  242  or rewound from one of the winding rollers  241  to the other winding roller  241 . As a result, the membrane  260  is exposed to the ion bombardment more than once. Advantageously, the membrane  260  is in this case exposed to the ion beam such that the ion channels produced do not just run obliquely with respect to one another along one dimension but rather each have different inclinations relative to one another along two dimensions. The probability of parallel ion channels with partially overlapping cross-sectional areas occurring can be reduced further as a result. In order to achieve this, the membrane  260  can be guided via the deflection roller  242  in a different orientation for fresh ion bombardment, for example. However, it is also possible for the ions to be deliberately deflected in spatial directions which are perpendicular to one another and hence to be fanned out in two dimensions, for example. Various options are conceivable in this regard. 
         [0066]    The invention is self-evidently not limited to the above exemplary embodiment, and a large number of modifications are possible. In particular, the battery does not have to be a lithium ion battery. It also does not necessarily have to be a secondary battery. The electrochemical energy store could equally well be in the form of a primary battery. In such a case, the positive or negative electrode would accordingly be produced from a different material that is known to a person skilled in the art from the prior art. Similarly, the electrolyte would then have a different chemical composition, and then accordingly not lithium ions but rather other ions would be involved in the ion transportation through the separator. In such a case, the separator would naturally be matched to the specific battery type and particularly to the properties of the ions to be transmitted. Furthermore, the battery may have a different physical shape than the cylindrical one described, for example, and may be in the form of a button cell, flat battery or in the form of a block, for example. In addition, the battery may have a separator which has further surface coatings to improve its physical and/or chemical properties. A large number of further modifications are possible. 
       LIST OF REFERENCE SYMBOLS 
       [0067]      
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 10 
                 Housing 
               
               
                 20 
                 Positively charged 
               
               
                   
                 electrode 
               
               
                 21 
                 Electrode sheet 
               
               
                 22 
                 Electrode connection 
               
               
                 30 
                 Negatively charged 
               
               
                   
                 electrode 
               
               
                 31 
                 Electrode sheet 
               
               
                 32 
                 Electrode connection 
               
               
                 40, 40′, 40a, 40b 
                 Separator 
               
               
                 41 
                 Polymer chain 
               
               
                 42, 42′ 
                 Solid 
               
               
                 43, 43′ 
                 Pore 
               
               
                 50 
                 Connecting Pin 
               
               
                 61 
                 First insulator 
               
               
                 62 
                 Second insulator 
               
               
                 70 
                 Positive pole 
               
               
                 80 
                 Negative pole 
               
               
                 90 
                 Safety valve 
               
               
                 100  
                 Thermistor 
               
               
                 110  
                 Seal 
               
               
                 200  
                 Ion source 
               
               
                 210  
                 Wobbler 
               
               
                 220, 221, 222, 223 
                 Acceleration section 
               
               
                 230  
                 Radiation chamber 
               
               
                 240  
                 Winding chamber 
               
               
                 241  
                 Winding rollers 
               
               
                 242  
                 Deflection roller 
               
               
                 250  
                 Power supply 
               
               
                 260  
                 Membrane