Abstract:
A vehicle traction battery includes a battery cell, the battery cell including a cathode, and anode, and a separator the anode and cathode. The separator includes at least one protective layer that is impermeable to polysulfides and at least one ion-conducting conductive layer whose composition is different than that of the protective layer and that is designed as a copolymer which includes a stabilizing phase and an ionically conductive phase, the protective layer including an inorganic substance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is the national stage of International Pat. App. No. PCT/EP2015/069931 filed Sep. 1, 2015, and claims priority under 35 U.S.C. §119 to DE 10 2014 218 803.2, filed in the Federal Republic of Germany on Sep. 18, 2014. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a separator for separating an anode and a cathode in a battery cell, a battery cell incorporating such a separator, and to a vehicle traction battery incorporating such a battery cell. 
       BACKGROUND 
       [0003]    Separators are used in batteries, in particular in lithium-ion batteries, to separate the anode from the cathode and, in particular, to prevent short circuits. Ions, in particular lithium ions, migrate from one electrode to the other during the charging and discharging of the battery cell. For this purpose, the separator is designed to be ionically conductive. Chemical reactions take place on the cathode and on the diode during the operation of the battery cell. In particular, lithium-ion storage batteries exist, which include graphite anodes, in which lithium ions migrate from the anode to the cathode through a separator during discharging and are intercalated into a transition metal oxide, for example Li 1−X Ni 0.33 Mn 0.33 O 2 . The lithium ions are intercalated into the anode during charging. 
         [0004]    Lithium storage batteries that include metal anodes are also known, which contain, in particular, lithium. The cathode contains an organic or inorganic sulfur compound or pure sulfur. During discharging, lithium ions are chemically combined with the sulfur or the sulfur compound in the cathode, whereby polysulfides, for example, are formed. An electrolyte is situated between the electrochemically active sulfur compounds. Polysulfides, among other things, may be released thereby. The released polysulfides may accumulate on the anode and contaminate it and thereby destroy the battery cell. 
         [0005]    It is known for separators made from organic materials, for example made from polyethylene or polypropylene, to be highly ionically conductive but also permeable to polysulfides. Separators made from inorganic materials are furthermore known, which are not permeable to polysulfides, but which have a relatively low conductivity for ions. 
         [0006]    In the battery cell, lithium ions which are deposited onto the anode during charging may form dendrites thereon, which grow toward the cathode through the separator and trigger a short circuit upon reaching the cathode. Dendrites of this type may penetrate, in particular, separators made from organic materials. 
         [0007]    A separator is known from U.S. Pat. No. 3,625,771, which includes a polymer mixture as well as particles of inorganic materials. Polysulfones, polyepoxides, and polyphenyl oxide, for example, are provided in the polymer mixture. 
         [0008]    WO 00/336671 A1 describes a separator, including a microporous pseudoboehmite layer, which is in contact with a protective layer. The protective layer includes a polymer made from multiple monomers, for example acrylates, styrene or epoxide. The protective layer can also include a mixture of organic and inorganic material. 
         [0009]    A separator for batteries is also described in WO 2006/088 959 A2. The separator contains a layer of additives, for example clay, clay minerals as well as metal sulfides. 
       SUMMARY 
       [0010]    According to an example embodiment of the present invention, a separator, for separating an anode and a cathode in a battery cell, includes at least one protective layer that is impermeable to polysulfides and at least one ion-conducting conductive layer whose composition is different than that of the protective layer. The conductive layer is designed as a copolymer, which includes a stabilizing phase and an ionically conductive phase. The protective layer includes an inorganic substance. 
         [0011]    According to an example embodiment of the present invention, the inorganic substance of the protective layer includes chemically reactive particles that react with polysulfides. The protective layer acts as a getter layer. The inorganic substance can also be made entirely from the aforementioned chemically reactive particles. 
         [0012]    The chemically reactive particles of the protective layer, for example, contain an alkali metal or an alkali metal compound or they are made from an alkali metal or an alkali metal compound. 
         [0013]    Alternatively, the chemically reactive particles of the protective layer contain an alkaline earth metal or an alkaline earth metal compound, or they are made from an alkaline earth metal or an alkaline earth metal compound. 
         [0014]    The chemically reactive particles of the protective layer are preferably situated a distance from each other, clearances remaining between the chemically reactive particles. 
         [0015]    According to an example embodiment of the present invention, the inorganic substance of the protective layer includes composite particles that are situated close together, multiple of the composite participles being in contact with each other. This results in a tight bond which is impermeable to liquids and chemicals, in particular to polysulfides. The protective layer thus acts as a barrier layer. The inorganic substance of the protective layer is ion-conducting and can be introduced into the separator on both sides. 
         [0016]    The composite particles of the protective layer contain, for example, ion-conducting ceramic or are made from ion-conducting ceramic. 
         [0017]    According to a example embodiment a getter layer is provided in combination with a barrier layer. 
         [0018]    Alternatively, the composite particles of the protective layer contain argyrodite or are made from argyrodite. 
         [0019]    The ionically conductive phase in the copolymer of the conductive layer preferably contains acrylate or polyethylene oxide. 
         [0020]    According to an example embodiment of the present invention, a battery cell includes at least one separator as described herein. 
         [0021]    According to an example embodiment of the present invention, a battery cell including a separator as described herein is advantageously used in a traction battery of a vehicle, in particular a hybrid vehicle (HEV), a plug-in hybrid vehicle (PHEV), or an electric vehicle (EV). 
         [0022]    The barrier layer as well as the getter layer as the protective layer of the separator reliably prevent polysulfides from migrating from the cathode to the anode and accumulating thereon. The solid ceramic, inorganic barrier layer of the separator furthermore prevents dendrites growing on the anode from reaching the cathode and thereby triggering a short circuit. However, the separator nevertheless has a relatively high conductivity for ions. In addition, the protective layer of the separator is also designed to be hermetically impermeable to gases, liquids and chemicals. This is particularly advantageous for lithium-sulfur cells or lithium-air cells, in which decomposition products, such as carbon dioxide, are released during the electrochemical reactions which occur. The separator has a mechanically strong yet flexible design and can thus be rolled or folded between the cathode and the anode during assembly. 
         [0023]    Specific example embodiments of the present invention are explained in greater detail on the basis of the drawings and the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a sectional view of a separator, including a getter layer, according to an example embodiment of the present invention. 
           [0025]      FIG. 2  is a sectional view of a separator, including a barrier layer, according to an example embodiment of the present invention. 
           [0026]      FIG. 3  is a sectional view of a separator according to another example embodiment of the present invention. 
           [0027]      FIG. 4  is a sectional view of a separator with a three-part design, including a getter layer, according to an example embodiment of the present invention. 
           [0028]      FIG. 5  is a sectional view of a separator having a three-part design, including a getter layer, according to another example embodiment of the present invention. 
           [0029]      FIG. 6  is a sectional view of a separator having a three-part design, including a barrier layer, according to an example embodiment of the present invention. 
           [0030]      FIG. 7  is a sectional view of a part of a battery cell including a two-part separator having a getter layer, according to an example embodiment of the present invention. 
           [0031]      FIG. 8  is sectional view of a part of a modification of the battery cell of  FIG. 7 , including a two-part separator having a barrier layer, according to an example embodiment of the present invention. 
           [0032]      FIG. 9  is a sectional view of a part of another modification of the battery cell from  FIG. 7 , according to another example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    According to example embodiments of the present invention, a separator  1  is situated between an anode and a cathode in a battery cell. Separator  1  is designed to be ionically conductive, on the one hand, and prevents a through-flow of contaminating substances, in particular polysulfides and sulfur, from the cathode to the anode, on the other hand. Separator  1  is furthermore used mechanically as a spacer between the anode and the cathode and prevents a short circuit between the anode and the cathode. 
         [0034]    A separator  1  according to a first exemplary embodiment, which is illustrated in  FIG. 1 , includes a protective layer  40  acting as a getter layer and a conductive layer  50 , which abut each other. Protective layer  40  is delimited by an underside  6  on its side facing away from conductive layer  50 , and conductive layer  50  is delimited by an upper side  5  on its side facing away from protective layer  40 . 
         [0035]    Conductive layer  50  of separator  1  is designed as a copolymer and has a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . Stabilizing phase  2  effectuates the mechanical strength of separator  1  and contains, for example, polystyrene, epoxides and/or siloxanes. 
         [0036]    Ionically conductive phase  3  of separator  1  is used to conduct ions through separator  1  and contains, for example, polyethylene oxide (polyethylene glycol) and/or acrylates. To increase the ion conductivity, a conductive salt, which is not illustrated here, can be optionally embedded in ionically conductive phase  3 . 
         [0037]    In the present case, conductive layer  50  transitions smoothly into protective layer  40 . Protective layer  40  is also designed as a copolymer and has a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . Chemically reactive particles  7  are embedded in ionically conductive phase  3 . Chemically reactive particles  7  are distributed at a distance from each other in ionically conductive phase  3 , so that clearances  31  remain between chemically reactive particles  7 . Aforementioned clearances  31  ensure a relatively good ion conduction through ionically conductive phase  3  of protective layer  40  and through separator  1 . 
         [0038]    Chemically reactive particles  7  contain an inorganic substance and react, in particular, with polysulfides, which come from the cathode. Chemically reactive particles  7  thus prevent polysulfides and other substances that may contaminate the anode from penetrating protective layer  40  and thus separator  1 . 
         [0039]    Aforementioned clearances  31  are configured in a size which ensures that a sufficiently large space is possible for conducting the ions through ionically conductive phase  3  of protective layer  40  even after a reaction of chemically reactive particles  7  with polysulfides. 
         [0040]    For example, alkali metals, alkali metal compounds, alkaline earth metals, alkaline earth metal compounds as well as other metals and chemical compounds can be considered as materials for chemically reactive particles  7 . 
         [0041]    A separator  1  according to a second exemplary embodiment, which is illustrated in  FIG. 2 , has a similar structure to the first exemplary embodiment and includes a protective layer  40  and a conductive layer  50 , which abut each other. Conductive layer  50  has the same structure as the one in the first exemplary embodiment. 
         [0042]    In contrast to the first exemplary embodiment, composite particles  8  are embedded in ionically conductive phase  3  of protective layer  40  acting as a barrier layer. Composite particles  8  are arranged close together in ionically conductive layer  3  and are in contact with each other. Composite particles  8  thus prevent polysulfides and other substances that may contaminate the anode from penetrating protective layer  40  and thus separator  1 . Composite particles  8  are used as a protective layer for thermal events. Composite particles  8  conduct lithium ions sufficiently well. 
         [0043]    Composite particles  8  contain an inorganic substance, for example, an oxide, a ceramic, a garnet, garnet-like Li 7 La 3 Zr 2 O 12  (LLZO), a glass-ceramic membrane referred to as LISICON, a non-oxidic ion conductor such as sulfur or phosphorus, an argyrodite, or a lithium argyrodite. 
         [0044]    Composite particles  8  are ionically conductive but have a lower ion conductivity than ionically conductive phase  3  of the copolymer. Protective layer  40  has a relatively thin design. The mechanical stability of separator  1  is ensured by stabilizing phase  2  in the copolymer of conductive layer  50 . 
         [0045]    A separator  1  according to a third exemplary embodiment, which is illustrated in  FIG. 3 , has a similar structure to the second exemplary embodiment and includes a protective layer  40  acting as a barrier layer and a conductive layer  50 , which abut each other. Conductive layer  40  has the same structure as the one in the second exemplary embodiment. 
         [0046]    In contrast to the second exemplary embodiment, chemically reactive particles  7  are embedded in ionically conductive phase  3  of conductive layer  50 , similarly to the protective layer according to the first exemplary embodiment. 
         [0047]    Separator  1  according to the third exemplary embodiment thus includes a conductive layer  50 , which has chemically reactive particles  7 , and a protective layer  40  acting as a barrier layer, which includes composite particles  8 . A separator  1  of this type is preferably situated in a battery cell in such a way that conductive layer  50  faces the anode and protective layer  40  faces the cathode. 
         [0048]    Polysulfides, which can still penetrate through protective layer  40  to a limited degree between composite particles  8 , react in conductive layer  50  with chemically reactive particles  7  present therein. Composite particles  8  increase the strength of separator  1 . Composite particles  8  which are situated close together, furthermore prevent dendrites that spread from the anode and partially penetrate separator  1  from forcing chemically reactive particles  7  out of separator  1 . 
         [0049]    A separator  1  according to a fourth exemplary embodiment is illustrated in  FIG. 4 . Separator  1  includes two conductive layers  50 , which are also referred to as upper layer  10  and lower layer  11 , as well as a protective layer  40  acting as a getter layer, which is also referred to as inner layer  9 . Inner layer  9  is situated between upper layer  10  and lower layer  11 . 
         [0050]    Upper layer  10  and lower layer  11  are each designed as a copolymer and each have a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . Inner layer  9  has only one ionically conductive phase  3 , in which chemically reactive particles  7  are embedded. 
         [0051]    Clearances  31  remain between chemically reactive particles  7 . The materials specified in relation to the first exemplary embodiment can also be used as materials for chemically reactive particles  7 . 
         [0052]    The three-layer structure of separator  1 , in which protective layer  40  is surrounded on both sides by a conductive layer  50  made from a copolymer, prevents protective layer  40  from coming into direct contact with the anode and with the cathode. As a result, undesirable chemical reactions of the anode material and the cathode material with chemically reactive particles  7  in protective layer  40  are avoided. 
         [0053]    A separator  1  according to a fifth exemplary embodiment is illustrated in  FIG. 5  and also includes two conductive layers  50 , which are also referred to as upper layer  10  and lower layer  11 , as well as a protective layer  40  acting as a getter layer, which is also referred to as inner layer  9 . As in the fourth exemplary embodiment, inner layer  9  is situated between upper layer  10  and lower layer  11 . Upper layer  10  and lower layer  11  are each designed as a copolymer and, as in the fourth exemplary embodiment, each has a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . 
         [0054]    Inner layer  9  is also designed as a copolymer and has a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . Stabilizing phase  2  increases the adhesive forces between upper layer  10  as well as lower layer  11  and inner layer  9  and thereby increases the strength of separator  1 . 
         [0055]    Chemically reactive particles  7  are embedded in ionically conductive phase  3  of inner layer  9 . Clearances  31  remain between chemically reactive particles  7  in inner layer  9 . The materials specified in relation to the first exemplary embodiment can also be used as materials for chemically reactive particles  7 . 
         [0056]    The three-layer structure of separator  1 , in which protective layer  40  is surrounded on both sides by a conductive layer  50  made from a copolymer, prevents protective layer  40  from coming into direct contact with the anode and with the cathode. As a result, undesirable chemical reactions of the anode material and the cathode material with chemically reactive particles  7  in protective layer  40  are avoided. 
         [0057]    A separator  1  according to a sixth exemplary embodiment, which is illustrated in  FIG. 6 , has a similar structure to the fourth exemplary embodiment. Separator  1  includes two conductive layers  50 , which are also referred to as upper layer  10  and lower layer  11 , as well as a protective layer  40  acting as a barrier layer, which is also referred to as inner layer  9 . As in the fourth exemplary embodiment, inner layer  9  is situated between upper layer  10  and lower layer  11 . 
         [0058]    Upper layer  10  and lower layer  11  are each designed as a copolymer and each has a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . Inner layer  9  has only one ionically conductive phase  3 , in which composite particles  8  are embedded. Composite particles  8  are arranged close together in ionically conductive phase  3  and are in contact with each other. The materials specified in relation to the second exemplary embodiment can also be used as materials for composite particles  8 . 
         [0059]    The three-layer structure of separator  1 , in which protective layer  40  is surrounded on both sides by a conductive layer  50  made from a copolymer, prevents protective layer  40  from coming into direct contact with the anode and with the cathode. As a result, undesirable chemical reactions of the anode material and the cathode material with composite particles  8  in protective layer  40  are avoided. 
         [0060]    A separator  1  according to a seventh exemplary embodiment in a battery cell is illustrated in  FIG. 7 . The battery cell includes an anode arrangement having a current collector  13 , on which an anode material  16  is applied. In the present case, current collector  13  is a copper film. Anode material  16  is an electrochemically active material, such as lithium, magnesium or sodium. 
         [0061]    Carrier platelets  14  are also applied on current collector  13 , which are made from a polymer or a metal polymer composite material, for example a copper polymer composite material. Insulators  15  are situated on carrier platelets  14  on the side facing away from current collector  13  in each case. 
         [0062]    Separator  1  includes a protective layer  40  acting as a getter layer and a conductive layer  50 , which abut each other. Protective layer  40  abuts insulators  15 . Insulators  15  project farther away from current collector  13  than anode material  16 . A direct contact between protective layer  40  of separator  1  and anode material  16  is thus avoided. 
         [0063]    Conductive layer  50  of separator  1  is designed as a copolymer and has a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . Protective layer  40  is also designed as a copolymer and has a mechanically stabilizing phase  2  as well as an ionically conductive phase  3 . Chemically reactive particles  7  are embedded in ionically conductive phase  3  of protective layer  40 . Clearances  31  remain between chemically reactive particles  7 . The materials specified in relation to the first exemplary embodiment can also be used as materials for chemically reactive particles  7 . 
         [0064]      FIG. 8  shows a modification of the battery cell from  FIG. 7 . The battery cell includes an anode arrangement having a current collector  13 , on which an anode material  16  is applied. Carrier particles  17  are furthermore applied on current collector  13  for supporting separator  1 . Carrier particles  17  project farther away from current collector  13  than anode material  16 . A direct contact between separator  1  and anode material  16  is thus avoided. 
         [0065]    In the present case, separator  1  is structured as described in the second exemplary embodiment. Protective layer  40  of separator  1 , which acts as a barrier layer, faces current collector  13  of anode arrangement and rests on carrier particles  17 . However, a separator according to another exemplary embodiment can also be used. 
         [0066]      FIG. 9  also shows a modification of the battery cell from  FIG. 7 . The battery cell includes an anode arrangement having a current collector  13 , on which an anode material  16  is applied. Carrier particles  17 , which are used to support separator  1 , are furthermore applied on current collector  13 . 
         [0067]    Clearances  21  remaining between separator  1  and anode material  16  are filled with ionically conductive material. For example, the materials of the ionically conductive phase of the copolymer of separator  1  or a mixture of an electrolyte and the ionically conductive phase of the copolymer as well as a liquid or a gel-like electrolyte are suitable for this purpose. 
         [0068]    In the present case, separator  1  is structured as described in the second exemplary embodiment. 
         [0069]    Conductive layer  50  of separator  1  faces current collector  13  of anode arrangement and rests on carrier particles  17 . However, a separator according to another exemplary embodiment can also be used. 
         [0070]    The battery cell furthermore includes a cathode arrangement having a cathode layer  20 , which is essentially made from compressed, electrochemically active material. Cathode layer  20  can also contain material of the ionically conductive phase of the copolymer of separator  1  as well as an electrolyte. The cathode arrangement furthermore includes a mixed layer  19 , which includes more porous electrochemically active material having a larger surface, compared to cathode layer  20 . 
         [0071]    An intermediate layer  18  is situated between protective layer  40  of separator  1 , which acts as a barrier layer, and mixed layer  19 . Mixed layer  19  contains a copolymer having a stabilizing phase  2  and an ionically conductive phase  3 . A greater proportion of ionically conductive phase  3  and a smaller proportion of stabilizing phase  2  is included, compared to the copolymer of separator  1 . 
         [0072]    The present invention is not limited to the exemplary embodiments described here and to the aspects highlighted therein. Instead, a large number of modifications are possible within the range specified by the claims, which lie within the purview of those skilled in the art.