Abstract:
A galvanic element includes the following elements in the order listed: a current collector associated with an anode; the anode; an ion-conducting separator in the form of a continuous layer; a cathode; and a current collector associated with the cathode. The anode encompasses an ion-conducting support structure, and both the ion-conducting support structure and the separator encompasses an ion-conducting material. The ion-conducing support structure is porous.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field Of The Invention 
         [0002]    The present invention relates to a galvanic element encompassing a current collector associated with the anode, an anode, a separator, a cathode, and a current collector associated with the cathode. The invention further relates to a battery cell encompassing a galvanic element of this kind, and to a battery encompassing multiple such battery cells. 
         [0003]    2. Description Of The Related Art 
         [0004]    Lithium ion batteries are notable inter alia for a very high specific energy and extremely low self-discharge. Lithium ion cells possess at least one positive and at least one negative electrode (cathode or anode); lithium ions migrate from one electrode to the other electrode as the battery charges and discharges. A so-called “lithium ion conductor” is necessary in order to transport the lithium ions. In lithium ion cells used at present, which are utilized e.g. in the consumer sector (mobile telephone, MP3 player, etc.) or as energy reservoirs in electric or hybrid vehicles, the lithium ion conductor is a liquid electrolyte that often contains the conductive lithium salt lithium hexafluorophosphate (LiPF 6 ) dissolved in organic solvents. A lithium ion cell encompasses the electrodes, the lithium ion conductor, and current collectors that represent the electrical terminals. 
         [0005]    The lithium ion cells can be enclosed in a package. Composite aluminum films, for example, can be used as a package. Cells packaged in this manner are also referred to, because of their soft packaging, as a “pouch” or “soft pack.” In addition to the soft pack package design, hard metal housings are also utilized as packages, for example in the form of deep-drawn or cold-extruded housing parts. The term “hard housing” or “hard case” is used in this instance. 
         [0006]    Lithium ion cells having a liquid electrolyte are disadvantageous in that under mechanical and thermal stress, the liquid electrolyte component can break down and an overpressure occurs in the cell. Without corresponding protective measures this can cause the cell to burst or even burn. 
         [0007]    It is possible to use a solid ceramic or inorganic lithium ion conductor instead of a liquid electrolyte. This concept avoids bursting of the battery cell or leakage of substances upon damage to the package. 
         [0008]    Published German patent application document DE 10 2012 205 931 A1 discloses an electrochemical energy reservoir as well as a method for manufacturing it. The electrochemical energy reservoir encompasses at least one electrode assembly in which an ion-conducting and electrically insulating separator layer is embodied on a coated surface. The ion-conducting layer is used as an electrolyte, so that a liquid electrolyte no longer needs to be used. For the embodiment as a lithium ion cell, the active materials proposed for the electrode assemblies are a lithium metal oxide, e.g. lithium cobalt oxide, for the cathode, and graphite for the anode. A ceramic powder having, for example, a particle size of 0.3 to 3 μm, for example lithium garnet, is proposed as a starting material for the ion conductor. The ceramic powder can be applied onto the surface to be coated, for example, in the form of an aerosol. 
         [0009]    The use of a graphite anode as proposed in the existing art is disadvantageous because it has only a low energy density compared with an anode based on lithium metal. With lithium metal-based anodes in turn, it is more difficult to implement manufacture of a galvanic element because the metallic lithium is highly reactive, and is stable only in completely dry environments. 
         [0010]    When electrodes based on lithium metal are used with the known solid lithium ion conductors, the problem furthermore occurs that a high contact resistance occurs between the metallic lithium and the ion conductor, and thus only small ionization currents can flow. This problem becomes worse once a few charge-discharge cycles have occurred, since lithium ions become dissolved out of the anode upon discharge and the volume of the anode changes as a result. 
         [0011]    A good contact that has been made upon manufacture, for example by press-joining, is then lost after a few charge-discharge cycles have occurred, since the lithium metal anode is no longer abutting tightly and with full coverage against the lithium ion conductor. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The invention proposes a galvanic element encompassing, in this order: a current collector associated with the anode, an anode, an ion-conducting separator in the form of a continuous layer, a cathode, and a current collector associated with the cathode, the anode encompassing an ion-conducting support structure, and both the ion-conducting support structure and the separator encompassing an ion-conducting material and the ion-conducing support structure being porous. 
         [0013]    The anode of the galvanic element encompasses a porous ion-conducting support structure. A structure of this kind can in principle be generated using any method known to one skilled in the art. The starting material used for the support structure contains an ion-conducting material, in particular a ceramic material. Depending on the manufacturing method the starting material can be present, for example, in the form of a powder—therefore in the form of a ceramic powder in the case of a ceramic starting material. 
         [0014]    When the galvanic element is embodied as a lithium ion battery cell, a material that is lithium ion-conducting is preferred. Suitable materials are, for example, lithium ion-conducting ceramics. Lithium garnet is particularly suitable. Alternatively, the material can be selected from perovskites (LLTO) Li 3x La 2/3x -TiO 3 , phosphates (LATP) Li 1+x Ti 2−x M x (PO 4 ) 3  (where M =Al, Ga, In, Sc), sulfide glasses containing Li 2 S and P 2 S 5  as well as doping elements such as Ge and Sn, and argyrodites Li 6 PS 5 X (where X =I, Cl, or Br). 
         [0015]    If a ceramic powder is used as a starting material, an aerosol coating, a sol-gel synthesis, or a solid state ceramic synthesis is particularly suitable. A pore-forming agent can be added, for example, to the starting material in order to generate the pores of the ion-conducting support structure. A suitable pore-forming material is, for example, cellulose, carbon fibers, or potato starch. Alternatively, a polymer that is later burned out can also be used. With polymers of high hardness, co-deposition by aerosol coating is also possible. 
         [0016]    The proportion of pores in the ion-conducting support structure is, for example, between 20 vol % and 90 vol %. The proportion of pores is preferably between 50 vol % and 80 vol %. The porosity is selected so that it is as high as possible but mechanical stability still exists. 
         [0017]    Depending on the embodiment, the ion-conducting support structure can firstly be generated on a substrate and later detached therefrom and introduced into the galvanic element. In other embodiments the ion-conducting support structure can be deposited with the aid of the coating method directly onto a constituent of the galvanic element. In order to increase electrical conductivity the support structure can be provided with a carbon-containing layer, e.g. by way of a chemical vapor deposition (CVD) process. 
         [0018]    The separator of the galvanic element likewise encompasses an ion-conducting material. In particular, the ion-conducting materials suitable for the separator are the same ones as for the ion-conducting support structure. The separator is embodied, however, in such a way that it forms a continuous layer. The separator is moreover embodied in such a way that it is not electrically conductive. 
         [0019]    Substantially the same coating methods, i.e. for example solid state ceramic synthesis, sol-gel synthesis, or aerosol coating, can be used to manufacture the separator. Aerosol coating is preferably used, although no pore-forming agent is added to the starting material. The separator manufactured in this manner has a residual porosity of less than 5 vol %; no open porosity is present, and the separator is therefore completely sealed. 
         [0020]    The current collectors of the galvanic element are usually embodied as metal films. For the current collector associated with the anode, for example, a copper film having a thickness of between 6 μm and 12 μm is used. For the current collector associated with the cathode, for example, an aluminum film having a thickness of between 13 μm and 15 μm is used. 
         [0021]    In further variant embodiments it is conceivable to use, instead of a metal film, a carrier material coated respectively with copper and aluminum. It is likewise conceivable to subject the current collector to a surface treatment in order to prevent a reaction with metallic lithium or with other constituents of the galvanic element. 
         [0022]    The cathode preferably encompasses a mixture of an optionally pre-lithiated cathode active material, an electrically conductive material, and an ion conductor (catholyte). 
         [0023]    In an embodiment of the invention, the conductive material is selected from carbon nanotubes, a conductive carbon black, graphene, graphite, or a combination of at least two of these materials. 
         [0024]    In a preferred embodiment, in order to increase the electrical conductivity the material of the cathode can be present as a composite material having carbon. In an embodiment of the invention the composite material encompasses a mixture of sulfur particles as cathode active material, graphite and conductive carbon black in order to increase the electrical conductivity, and optionally a binder such as PVdF (polyvinylidene fluoride). In a further embodiment of the invention the material of the cathode encompasses a mixture of SPAN (sulfur polyacrylonitrile), graphite and/or conductive carbon black, and a lithium ion-conducting polymer. In a further embodiment the composite material encompasses a mixture of optionally carbon as well as nanoparticles of LiF and a metal, for example Fe, Cu, Ni. In a further embodiment the composite material encompasses a mixture of optionally carbon as well as nanoparticles of Li 2 S and a metal, for example Fe, Cu, Ni. In another embodiment the pre-lithiation of the material has already occurred, and the composite material is made up of carbon and a lithium-containing metal hydride, metal sulfide, metal fluoride, or metal nitride. 
         [0025]    In order to prevent migration of fluorine and thus a reaction with the ion conductor, a reaction with the current collector, or reactions with other battery components, in a preferred embodiment the composite material is equipped with a coating made, for example, of carbon or of an oxide (e.g. Al 2 O 3 ) or a fluoride (e.g. AlF 3 ) or an oxyfluoride. In the sulfur-containing embodiment, a coating can also prevent the diffusion of polysulfides. 
         [0026]    In a further embodiment of the invention the cathode active material is selected from a lithiated transition metal oxide, for example Li(NiCoMn) 0   2 , LiMn 2 O 4  (or a higher Li content), Li 2 MO 3 -LiMO 3  (where M is, for example, Ni, Co, Mn, Mo, Cr, Fe, Ru, or V), LiMPO 4  (where M is, for example, Fe, Ni, Co, or Mn), Li(Ni 0.5 Mn 1.5 )O 4  (or a higher Li content), Li x V 2 O 5 , Li x V 3 O 8 , or further cathode materials known to one skilled in the art, such as borates, phosphates, fluorophosphates, silicates. 
         [0027]    In a further embodiment of the invention the cathode active material is selected from a lithiated sulfur, for example Li 2 S, the material preferably being encapsulated in a carbon composite matrix, for example in the form of small spherules, in order to prevent dissolution or secondary reactions with the catholyte. 
         [0028]    In an embodiment of the invention the ion conductor is a solid electrolyte based on polyethylene oxide (PEO) or on soy. In this embodiment the cathode active material and the conductive material are embedded in the solid electrolyte. 
         [0029]    In a further embodiment of the invention the ion conductor in the cathode (catholyte) is a further porously configured support structure having an ion-conducting material. The same materials as those already used for the ion-conducting support structure of the anode and for the separator can be used as an ion-conducting material. In contrast to the separator, the material of the ion conductor can additionally have an even higher conductivity, although this need not necessarily be the case. In order to increase the electrical conductivity the support structure can be equipped with a carbon-containing layer, for example by way of a CVD process. 
         [0030]    In a variant embodiment, an electrolyte layer encompassing a polymer electrolyte is disposed between the separator and the cathode. A polyethylene oxide-based electrolyte is, for example, suitable. 
         [0031]    In another embodiment of the invention a liquid electrolyte is used as an ion conductor. 
         [0032]    In a variant embodiment of the invention, a further separator, impregnated with a liquid electrolyte, is disposed between the separator and the cathode. The material of the further separator is preferably selected from glass fibers, polyethylene (PE), or polypropylene (PP) with or without ceramic filling. Suitable electrolytes are, for example, carbonate-containing electrolytes such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), optionally with additives such as vinylene carbonate (VC) or monofluoroethylene carbonate (FEC). 
         [0033]    Metallic lithium is preferably used as an active material for the anode. In an embodiment of the invention, the anode active material is placed in the form of a film of metallic lithium onto the porous ion-conducting support structure, and press-joined to it. In another embodiment the anode active material is introduced as a lithium melt into the porous ion-conducting support structure. 
         [0034]    In an embodiment of the invention a cathode that encompasses a lithiated active material is used, and the anode is generated by electrochemical deposition when the galvanic element is first charged. When the galvanic element is first charged, lithium ions migrate out of the lithiated active material of the cathode through the separator, and become deposited on the current collector associated with the anode, and optionally in the pores of the ion-conducting support structure, in the form of a layer of metallic lithium. 
         [0035]    By way of example, the following process occurs (based here on a sulfur-containing conversion cathode material): 
         [0000]        2 Li 2 S +Fe 0 &lt;-&gt;FeS 2  + 4 Li + + 4 e −   
         [0000]    In this case the cathode encompasses an active material that can be reversibly lithiated again upon discharge of the galvanic element. 
         [0036]    A battery cell encompassing a cell package and a galvanic element of this kind is furthermore proposed. The cell package can be a soft pack package design or a hard housing. 
         [0037]    Also proposed is a battery encompassing one or more such battery cells. 
         [0038]    In the context of this description the term “battery” or “battery cell” is used in the manner usual in everyday speech, i.e. the term “battery” encompasses both a primary battery as well as a secondary (rechargeable) battery. The term “battery cell” similarly encompasses both a primary and a secondary cell. 
         [0039]    The galvanic element according to the present invention has a large capacity and a high energy density. 
         [0040]    The separator is embodied in the form of a continuous layer of an ion-conducting material, with which the anode and cathode are reliably electrically insulated from one another. In addition, dendrites that can form upon deposition of the lithium ions onto the anode cannot penetrate through the continuous layer of the separator and thus cannot short-circuit the galvanic element. At the same time, the anode of the galvanic element has a porously configured ion-conducting support structure that makes possible an intimate contact between the anode and the separator that also serves as an ion conductor. When contact between the anode and the separator serving as an ion conductor is poor, large contact resistance values occur and this limits the currents in the galvanic element. 
         [0041]    To further reduce the contact resistance, an additional (gel) layer can optionally be used. The proposed improvements permit the use of metallic lithium as an anode material, which enables an increase in the energy density on the anode side by an order of 10 as compared with the graphite anodes usual in the existing art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIG. 1  shows a separator and an ion-conducting support structure for the anode. 
           [0043]      FIG. 2   a  shows the separator and the ion-conducting support structure in a charged state of the galvanic element. 
           [0044]      FIG. 2   b  shows the separator and the ion-conducting support structure in a partly discharged state of the galvanic element. 
           [0045]      FIG. 3  shows a first embodiment of a galvanic element. 
           [0046]      FIG. 4  shows a second embodiment of a galvanic element. 
           [0047]      FIG. 5  shows a third embodiment of a galvanic element. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0048]    In the description below of exemplifying embodiments of the invention, identical or similar components and elements are labeled with identical or similar reference characters, repeated description of said components or elements in individual cases being omitted. The Figures depict the subject matter of the invention merely schematically. 
         [0049]      FIG. 1  depicts a separator  14  and an ion-conducting support structure  16 , which ion-conducting support structure  16  encompassing an ion-conducting material is disposed on separator  14 . Ion-conducting support structure  16  has pores  15 , the proportion of pores being between 20 vol % and 90 vol %, preferably between 60 vol % and 80 vol %. A first boundary layer  41  forms between ion-conducting support structure  16  and separator  14 . Ion-conducting support structure  16  has been manufactured using a coating method. Suitable coating methods encompass an electrode coating method or compression with subsequent solid ceramic synthesis, sol-gel synthesis, or aerosol coating. The starting material is usually present in the form of a powder. Lithium ion-conducting garnets, in particular lithium garnets, are particularly suitable as a starting material. 
         [0050]    Pore-forming agents, for example cellulose, can be added to the starting material in order to form the pores. 
         [0051]      FIG. 2   a  depicts separator  14  and ion-conducting support structure  16  in a charged state of galvanic element  10 . In a variant of the invention, after separator  14  is manufactured, a film having metallic lithium is placed onto ion-conducting support structure  16  and is press-fitted thereonto, with the result that metallic lithium  30  penetrates into pores  15 . 
         [0052]    In another variant, only the current collector associated with the anode is applied onto ion-conducting support structure  16 , and the active material for the cathode is applied onto that side of separator  14  which faces away from ion-conducting support structure  16 . When the galvanic element is first charged, lithium ions then move out of the active material of the cathode through separator  14 , and then become deposited partly in pores  15  and partly on the current collector associated with the anode. 
         [0053]      FIG. 2   b  depicts separator  14  and ion-conducting support structure  16  in a (partly) discharged state of galvanic element  10 . As is evident from the depiction in  FIG. 2 , lithium ions have been dissolved out of the metallic lithium  30 , have migrated through separator  14 , and have become deposited again in the cathode material. Lithium  30  therefore no longer completely fills up pores  15 . If applicable, in the context of a complete discharge all the lithium can even diffuse back into the cathode. 
         [0054]      FIG. 3  depicts a first embodiment of a galvanic element  10  according to the present invention. 
         [0055]    Galvanic element  10  encompasses a current collector  12  associated with the anode, an anode  13 , a separator  14 , as well as a cathode  24  and a current collector  28  associated with the cathode, in that order. A second boundary layer  42  therefore forms between current collector  12  associated with the anode and anode  13 , a third boundary layer  43  between separator  14  and cathode  24 , and a fourth boundary layer  44  between cathode  24  and current collector  28  associated with the cathode. First boundary layer  41  is located between anode  13  and separator  14 . 
         [0056]    Anode  13  encompasses an ion-conducting support structure  16  and metallic lithium  30  as an anode active material. The porously configured ion-conducting support structure  16  of anode  13  guarantees that the change in volume is less as compared with the use of a pure lithium film, and that even with a small change in the volume of anode  13 , good electrical contact is still ensured between separator  14 , which of course also serves as an ion conductor, and anode  13 . Loss of contact between metallic lithium  30  and separator  14  is prevented by the porously configured ion-conducting support structure  16 . 
         [0057]    The material of cathode  24  also encompasses, besides a cathode active material  26 , conductivity additives such as carbon nanotubes or a conductive carbon black. Cathode  24  furthermore encompasses an ion conductor (catholyte) in order to improve conductivity inside cathode  24 . In the embodiment depicted in  FIG. 3 , the ion conductor is embodied as a polymer electrolyte  34 , based e.g. on polyethylene oxide (PEO). 
         [0058]    Cathode active material  26  contains lithium, which upon charging of galvanic element  10  becomes dissolved out of cathode active material  26  in the form of lithium ions and migrates through separator  14  toward current collector  12  associated with the anode. The lithium ions then become deposited on anode  13  in the form of metallic lithium. Upon discharge, the lithium ions then in turn dissolve out of anode  13  and migrate through separator  14  back into cathode  24 , where they re-lithiate cathode active material  26 . 
         [0059]      FIG. 4  depicts a further embodiment of galvanic element  10 . Galvanic element  10  once again encompasses current collector  12  associated with the anode, anode  13 , separator  14 , cathode  24 , and current collector  28  associated with the cathode. 
         [0060]    In the embodiment depicted in  FIG. 4 , besides a cathode active material cathode  24  also encompasses conductivity additives, binders, and a liquid electrolyte as ion conductor. 
         [0061]    In contrast to  FIG. 3 , in the embodiment depicted in  FIG. 4  a further separator  20  is disposed between separator  14  and cathode  24 . A fifth boundary layer  45  thus forms between separator  14  and further separator  20 , and a sixth boundary layer  46  forms, instead of third boundary layer  43  described in  FIG. 3 , between further separator  20  and cathode  24 . Further separator  20  likewise acts as an electrical insulator but cannot itself conduct ions. For ion conduction, further separator  20  is impregnated with an electrolyte that is present, for example, in liquid form. 
         [0062]    Electrical contact between separator  14  and the cathode material is improved by the liquid electrolyte in further separator  20 . 
         [0063]    In a further variant of the invention it is conceivable to omit further separator  20 . 
         [0064]      FIG. 5  shows a third embodiment of galvanic element  10 . Once again galvanic element  10  encompasses current collector  12  associated with the anode, anode  13 , separator  14 , cathode  24 , and current collector  28  associated with the cathode. 
         [0065]    In the embodiment depicted in  FIG. 5 , cathode  24  has a further ion-conducting support structure  32  as an ion conductor. This further ion-conducting support structure  32  is embodied here as a garnet infiltrated with cathode active material  26 , the garnet serving as an ion conductor. It is conceivable for separator  14  also to be embodied as a garnet, the garnets of separator  14  and of further ion-conducting support structure  32  optionally being capable of having different chemical compositions. For example, Li 7 La 3 Zr 2 O 12  is suitable as a material for the separator; and Fe-doped and Li-enriched Li 4+x Ti 5 O 12  is suitable, thanks to its high ion conductivity, for further ion-conducting support structure  32 . 
         [0066]    The cathode can furthermore encompass additives to improve conductivity, for example carbon nanotubes or a conductive carbon black. 
         [0067]    In the embodiment depicted in  FIG. 5  an additional electrolyte layer  22  is disposed between separator  14  and cathode  24 . Electrolyte layer  22  is preferably embodied as a polymer electrolyte, for example based on polyethylene oxide. A seventh boundary layer  47  thus forms between separator  14  and electrolyte layer  22 , and an eighth boundary layer  48 , instead of third boundary layer  43  described in  FIG. 3 , forms between electrolyte layer  22  and cathode  24 . Electrolyte layer  22  serves to improve ion conduction between cathode  24  and separator  14 . 
         [0068]    The invention is not limited to the exemplifying embodiments described here, and to the aspects emphasized therein. A plurality of variants that are within the competence of one skilled in the art are instead possible within the scope indicated by the claims.