Abstract:
A battery cell includes a negative electrode and a positive electrode. The battery cell also contains a thermally expandable graphite intercalation compound.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to patent application number DE 10 2014 207 678.1, filed on Apr. 24, 2014 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates to a battery cell and to a method of operating the same. 
         [0003]    Lithium ion batteries have now become widespread in products such as cellular phones and electric vehicles. A lithium ion battery usually comprises a cathode and an anode between which a separator comprising a carbonate-based electrolyte is located. In a first charging of the battery, lithium ions are liberated by the cathode, migrate through the separator and are stored in the material of the anode. During discharge, a reverse flow of the lithium ions is established. 
         [0004]    Over a prolonged period of operation with numerous charging and discharging cycles of the lithium ion battery, short circuits attributable to the formation of lithium dendrites or metallic impurities which affect both the anode and the cathode can occur in the battery. Such short circuits lead to a rapid increase in the internal battery temperature to temperatures greater than 200° C.-250° C. If the thermal energy liberated here exceeds a particular limit, the carbonate-based electrolyte decomposes and produces large volumes of predominantly toxic gases. These large amounts of toxic gases are given off into the surroundings of the battery in a relatively short time or the battery explodes as a result of the excessive internal pressure. 
         [0005]    To avoid such short circuits, lithium ion batteries of this type normally comprise at least one layer composed of a metal oxide such as aluminum oxide or titanium dioxide which is positioned on the large area of the anode or the cathode. This layer is referred to as safety function layer (SFL) or as heat retardant layer (HRL). It is electrically nonconductive and thus increases the internal, electrical resistance of the battery and adversely affects the electrochemistry within the lithium ion cell. Furthermore, it increases the weight of the battery. 
         [0006]    Thermally expandable graphite intercalation compounds are known from a different context, for example as fire protection materials. Thus, such compounds are disclosed, for example, in EP 1489136 A1, WO 2011/039301 A1 and EP 1323670 A2. The production of expanded graphite is disclosed, for example, in DE 102007053652 A1 and the production of expandable graphite intercalation compounds is described, for example, by the book N. N Queenwood and A. Earlshore, “Chemistry of the elements” Second Edition, Elsevier Butterworf-Heinemann 2005, p. 293 to p. 296. 
       SUMMARY 
       [0007]    Compared to the above, the disclosure provides a battery cell and a method of operating the same having the characterizing features described herein, by means of which effective protection against overheating of battery cells can be realized in a simple way. 
         [0008]    This is based, in particular, on the battery cell of the disclosure having at least one electrode which comprises a thermally expandable graphite intercalation compound. The particular advantage of thermally expandable graphite intercalation compounds is that when high temperatures, for example above 150° C., occur or when an electric short circuit occurs, they display a structural transformation which results in a substantial increase in volume and a significantly decreasing electrical conductivity of the graphite intercalation compound. This effect is based, in particular, on thermally expandable graphite intercalation compounds displaying a change in the layer structure of the graphite forming the basis of the graphite intercalation compound at elevated temperatures. The layers of SP2-hybridized carbon which are usually arranged parallel to one another in the graphite structure are broken up to form an essentially no longer parallel arrangement of carbon layers accompanied by an increase in volume by, for example, a factor of from 300 to 500. For example, if the thermally expandable graphite intercalation compound is applied in the form of a layer to the large area of one of the two electrodes of the battery cell, in particular to the large area of the respective electrode facing the other electrode, the formation of thermally expanded graphite leads, owing to the increasing volume of the thermally expandable graphite intercalation compound, to an increase in the distance between the two electrodes and thus possibly to elimination of an electrical short circuit which already exists between the electrodes. Furthermore, the increase in volume of the thermally expandable graphite intercalation compound is associated with consumption of energy. 
         [0009]    A further advantage of the use of thermally expandable graphite intercalation compounds is that they display a sufficiently good electrical conductivity for battery applications but when thermal expansion of the graphite intercalation compounds occurs, a layer of expanded graphite which displays only a low residual electrical conductivity is formed on thermal expansion. This too, may lead to existing electric short circuits within the battery cell being eliminated. 
         [0010]    It is particularly advantageous for metal oxide layers in the form of safety function layers (SFL) or heat retardant layers (HRL) present until now in the battery cell to be replaced by a layer of thermally expandable graphite intercalation compounds (GIC). 
         [0011]    The increase in volume during the expansion of thermally expandable graphite intercalation compounds is based, in particular, on the intercalated molecules within the graphite layers decomposing at elevated temperatures and thus leading to formation of expanded graphite. The respective temperature at which expansion of the thermally expandable graphite intercalation compound occurs is dependent on the type of the intercalated compound. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The figures show advantageous embodiments of the present disclosure and these are explained further in the following description of the figures. The figures show: 
           [0013]      FIG. 1 : the schematic cross section of an electrode of a battery cell according to the disclosure as per a first embodiment of the present disclosure and 
           [0014]      FIG. 2 : a schematic depiction of a plan view onto an electrode of a battery cell according to the disclosure as per a second embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows a battery cell  10  according to the disclosure as per a first embodiment of the present disclosure. This comprises, for example, a housing  12  which is shown in sections and is made, for example, of a polymer or a metal. The battery cell  10  comprises a first electrode  14  which represents, for example, an anode of the battery cell  10 . The first electrode  14  is, for example, positioned on a copper foil as power outlet lead  16 . A protective layer  18  containing a thermally expandable graphite intercalation compound has, for example, been applied to the large area of the first electrode  14  facing away from the power outlet lead  16 . The general method of preparing thermally expandable graphite intercalation compounds is described, for example, in W. Zheng, S. C. Wong “Electrical conductivity and dielectric properties of PMMA/expanded graphite composites” Composites Science and Technology 63 (2003), pp. 225-235. As intercalates for thermally expandable graphite intercalation compounds, it is possible to use, in particular, cations such as alkali metal cations of sodium, potassium, cesium or barium and also cations of the alkaline earth metals strontium, barium and calcium and also cations of the rare earth metals ytterbium and europium. Graphite intercalates of lithium ions, which are usually formed in the region of the anodes of lithium ion cells and can also be additionally formed here are not encompassed by this definition. 
         [0016]    Further suitable intercalates for thermally expandable graphite intercalation compounds are halides, with metal halides such as iron chloride or copper chloride being particularly suitable. Further possible anions as intercalates are hexafluorophosphates, hexafluoroarsenates, perchlorates or hydrogensulfates. Sulfur trioxide as gas is also a suitable possible intercalate. 
         [0017]    In addition, organometallic compounds such as Cs(C 2 H 4 )—C 24 , Ba(NH 3 ) 25 ,—C 10.9 , K(NH 3 ) 4.3 —C 24 , RbN 2 —C 24 , KN 2 —C 24  and/or C x FeCl 3 —CH 3 NO 2  are suitable as possible thermally expandable graphite intercalation compounds. 
         [0018]    The thermally expandable graphite intercalation compound used according to the disclosure displays decomposition of the corresponding intercalate at elevated temperature, whereupon an increase in volume of the thermally expandable graphite intercalation compound occurs so that this is present as thermally insulating and electrically insulating thermally expanded graphite. In this way, electric short circuits and a thermal runaway reaction of battery cells can be effectively prevented. The respective temperature at which thermal expansion of the thermally expandable graphite intercalation compound occurs can be controlled via the type of intercalate. Thus, for example, graphite oxidized by means of sulfuric acid in the form of a graphite hydrogensulfate decomposes above 150° C. and graphite oxidized by means of nitric acid in the form of graphite nitrate decomposes above 210° C. 
         [0019]    In an alternative embodiment, the protective layer  18  can, in addition or as an alternative, be applied to a second electrode (not shown in  FIG. 1 ) of the battery cell  10 . 
         [0020]      FIG. 2  shows a battery cell  10  as per a second embodiment of the present disclosure. This comprises, within the housing  12 , a first electrode  14  in the form of an anode which has on its surface a protective layer  18   a  in the form of individual areal segments, for example areal segments which are not joined to one another, containing a thermally expandable graphite intercalation compound. The particular advantage of this embodiment is that a protective layer  18   a  consisting merely of areal segments results in a lower electrical resistance against charge transfer between the first electrode  14  and a further electrode (not shown) of the battery cell  10 . If a heat-related expansion of the protective layer  18   a  made up of areal segments occurs, an essentially full-area covering of the large area of the electrode  14  with expanded graphite is nevertheless achieved purely because of the increasing volume of the expanded graphite. 
         [0021]    During operation of the battery cell  10 , the thermally expandable graphite intercalation compound present in the protective layer  18  is initially present in an electrically conductive form. If a short circuit or excessive overheating occurs during operation of the battery cell  10 , decomposition of the intercalates present in the thermally expandable graphite intercalation compound occurs as a result of thermal activation and the formation of expanded graphite on the surface of the first electrode  14  follows. Since expanded graphite is both thermally and electrically insulating, subsequent damage caused by the electric short circuit or a thermal runaway reaction in the battery cell  10  is avoided. 
         [0022]    The battery cell  10  according to the disclosure can be used, for example, in the form of battery modules in mobile and stationary applications such as hybrid or electric vehicles and for the storage of electric energy in stationary applications.