Abstract:
The invention concerns an electrochemical generator element ( 102 ) comprising successively a first electrode layer having a polarity ( 114 ), a first electrolyte layer ( 110 ), a second electrode layer with reverse polarity ( 108 ), a second electrolyte layer ( 112 ), a second electrode layer with said polarity ( 116 ). The electrode layers of said polarity ( 114, 116 ) are connected by a parallel connection. It further comprises current collectors ( 118, 120 ) connected to the electrode layers with said polarity ( 114, 116 ). The thickness of the first electrode layer with said polarity ( 114 ) is different from the thickness of said second electrode layer with said polarity ( 116 ). The invention is applicable to lithium-polymer storage batteries.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an electrochemical generator element successively comprising a first electrode layer of one polarity, a first electrolyte layer, an electrode layer of a reverse polarity, a second electrolyte layer and a second electrode layer of said polarity, said electrode layers of said polarity being connected by a connection in parallel, the element further comprising current collectors, connected to the electrode layers of said polarity. 
   The invention is applicable, for example, to lithium-polymer electrochemical batteries for electric vehicles or steady-state applications. 
   Electrochemical generators with lithium having a polymer electrolyte are known. Such generators generally comprise elements consisting of two half-elements electrically connected in parallel. 
   Each half-element consists of a cathode layer which is applied via an electrolyte layer onto one of the two faces of a lithium layer. The current is drawn by current collectors arranged on the free surface of the cathodes and of the collecting leads connected to the lithium layer. 
   The cathode layers on the two sides of the lithium layer have the same thickness ( FIG. 1 ). To increase the specific energy (energy per unit mass) of such an electrochemical generator, the thickness, which has the same value for both cathode layers, is increased. Correspondingly, the thickness of the lithium layer is also increased. This has the effect that the maximum specific power (power per unit mass) decreases as the resistance of the element increases. 
   SUMMARY OF THE INVENTION 
   The aim of the invention is to alleviate this drawback and to provide an electrochemical element in which the specific energy is substantially independent of the maximum specific power. 
   To this end, the subject of the invention is an electrochemical generator element of the aforementioned type, characterized in that the thickness of said first electrode layer of said polarity is different from the thickness of said second electrode layer of said polarity. 
   The subject of the invention is also an electrochemical battery comprising at least one electrochemical generator element of the type defined above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood on reading the following description, given solely by way of example and made with reference to the appended drawings in which: 
       FIG. 1  is a cross section through a known lithium-polymer electrochemical generator element; 
       FIG. 2  shows the specific energy and the specific power of an electrochemical element of the prior art as the function of the cathode thickness; 
       FIG. 3  is a cross section through a lithium-polymer electrochemical generator element according to the invention; and 
       FIG. 4  shows the specific energy and the specific power of the electrochemical elements according to the invention and indicates the ratio of the two elements as a function of the cathode thickness. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For the effect of the invention to be better understood, the problem posed by the prior art will first of all be described with reference to  FIGS. 1 and 2 . 
     FIG. 1  shows an electrochemical generator element  2  of the prior art in cross section. The element comprises two half-elements  4 ,  6 , each of which consists of a common lithium layer  8 , an electrolyte layer  10 ,  12 , a positive electrode layer  14 ,  16  and a current collector  18 ,  20 . 
   Electrical conductors  22 ,  24  are connected to the collectors  18 ,  20  and an electrical conductor  26  is connected to the lithium layer  8 . 
   Depending on the accumulator type, the conductors  24 ,  22  may be connected directly to the positive electrode layers  14 ,  16 , such that the current is collected by the conductors  24 ,  26 . 
   The two electrical conductors  22 ,  24  are connected to one side of a user (not shown), while the electrical conductor  26  is connected to the other side of the user such that a parallel connection of the two half-elements  4 ,  6  is formed. 
   The electrolyte layers  10 ,  12  each have a thickness of 30 μm and separate the positive electrode layers  14 ,  16  from the common lithium layer  8  which acts as a negative electrode. 
   The lithium layer has a thickness between 10 μm and 150 μm, preferably between 30 μm and 70 μm. 
   Each of the positive electrode layers  14 ,  16  has (in this example) a thickness of 90 μm and are manufactured from a material which contains a certain proportion Y of V 2 O 5 , generally more than 50%. This proportion of V 2 O 5  determines the specific capacity which is, if Y=54%, 153 Ah/kg. 
   As a variant, nickel, cobalt, manganese oxide or a mixture of these oxides can be used instead of V 2 O 5 . 
   The internal surface resistance of a half-element is calculated by the following formula:
 
 R   s     —     de   =R   s     —     Li/el   +r   s     —     el   ×e   el   +r   s     —     cath   ×e   cath   (1.1)
         where R s     —     Li/el  is the surface resistance of the interface between the electrolyte  10 ,  12  and the lithium anode layer  8 . Its value is 10 Ωcm 2 . r s     —     el  is the specific resistance of the electrolyte  10 ,  12  (r s     —     el =0.2 Ωcm 2 /μm). r s     —     cath  is the specific resistance of the cathodes  14 ,  16  at 80% discharge (r s     —     cath =3,5 Ωcm 2 /μm).       

   e el  and e cath  are the thickness of the electrolyte layer  10 ,  12  (e el =30 μm) and the thickness of the cathode layer  14 ,  16  (e cath =90 μm). 
   The surface resistances indicated are valid for a temperature of 90° C. 
   The electrical resistances of the current collectors  18 ,  20 , and of the lithium  8  are considered to be negligible compared to the surface resistances of the interface, electrolyte and cathode. 
   For the given electrochemical element  2 , a half-element  4 ,  6  therefore has the surface resistance:
 
 R   s     —     de =10 Ωcm 2 +0.2 Ωcm 2 /μm×30 μm+3.5 Ωcm 2 /μm×90 μm=331 Ωcm 2 .
 
   The internal surface resistance of such an element is calculated from the following formula (parallel connection): 
   
     
       
         
           
             R 
             s_e 
           
           = 
           
             
               
                 
                   R 
                   s_de 
                 
                 ⁢ 
                 
                   R 
                   s_de 
                 
               
               
                 
                   R 
                   s_de 
                 
                 + 
                 
                   R 
                   s_de 
                 
               
             
             = 
             
               
                 R 
                 s_de 
               
               2 
             
           
         
       
     
   
   For the given example, the surface resistance is 
   
     
       
         
           
             R 
             s_e 
           
           = 
           
             
               
                 331 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Ω 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   cm 
                   2 
                 
               
               2 
             
             = 
             
               165.5 
               ⁢ 
               
                   
               
               ⁢ 
               Ω 
               ⁢ 
               
                   
               
               ⁢ 
               
                 cm 
                 2 
               
             
           
         
       
     
   
   The maximum surface power produced by such a lithium-polymer element  2  is given by the following equation: 
                   P   s_max     =         U   0   2       4   ⁢           ⁢     R   s_e         .             (     1.2   .     )               
where U 0  is the voltage in vacuo of the element  2  (2.2 V at 80% discharge).
 
   The energy per unit area is given by the following formula:
 
 E   s     —     e   =E   s     —     de1   +E   s     —     de2 =( e   cath1   +e   cath2 )×ρ cath   ×Y×E   s   ×U   mean   (1.2.)
 
where E s     —     deX  is the surface energy of the half-element  1  or  2 ; e cathX  is the thickness of the corresponding cathode, ρ is the density of the cathode (ρ=2.1 g/cm 3 ), Y is the V 2 O 5  content of the cathode in % weight (54%), E s  is the specific capacity of V 2 O 5  (153 Ah/kg) and U mean  is the mean voltage of 2.55 V.
 
 E   s     —     e =7.98 mWh/cm 2  for the example given.
 
   In order to increase the specific surface energy of a lithium-polymer element  2 , it has already been proposed to increase the thickness of the two cathode layers  14 ,  16 . This has the result that the surface resistance of the two half-elements  4 ,  6  increases while the voltage in vacuo of the elements  2  remains constant and thus the surface power and, consequently, the specific power of the element  2  decreases. 
   The thickness of the lithium layer remains constant. 
   The ratio of the maximum power of the element  2  to the specific energy of the element  2  decreases as the thickness of the cathode layers increases. 
   Table 1 shows, as a function of the cathode thickness, the values of surface resistance, of surface power at 80% discharge, of surface energy, the ratio of the surface power to the surface energy, the surface mass, the specific power and the specific energy. The values were measured at a temperature of 90° C. 
   
     
       
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Cathode 
                 
               Max. 
                 
                 
                 
               Max. 
                 
             
             
               thick- 
               Surface 
               surface 
               Surface 
                 
               Surface 
               specific 
               Specific 
             
             
               ness 
               resistance 
               power 
               energy 
               P max / 
               mass 
               power 
               energy 
             
             
               μm 
               Ohm · cm 2   
               mW/cm 2   
               mWh/cm 2   
               energy 
               Mg/cm 2   
               W/kg 
               Wh/kg 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               10 
               25.50 
               47.45 
               0.89 
               53.53 
               29.7 
               1597.68 
               29.85 
             
             
               15 
               34.25 
               35.33 
               1.33 
               26.57 
               31.8 
               1110.96 
               41.82 
             
             
               20 
               43.00 
               28.14 
               1.77 
               15.87 
               33.9 
               830.07 
               52.30 
             
             
               25 
               51.75 
               23.38 
               2.22 
               10.55 
               36 
               649.49 
               61.56 
             
             
               30 
               60.50 
               20.00 
               2.66 
               7.52 
               38.1 
               524.93 
               69.80 
             
             
               40 
               78.00 
               15.51 
               3.55 
               4.37 
               42.3 
               366.73 
               83.83 
             
             
               50 
               95.50 
               12.67 
               4.43 
               2.86 
               46.5 
               272.48 
               95.32 
             
             
               60 
               113.00 
               10.71 
               5.32 
               2.01 
               50.7 
               211.20 
               104.91 
             
             
               70 
               130.50 
               9.27 
               6.21 
               1.49 
               54.9 
               168.89 
               113.03 
             
             
               80 
               148.00 
               8.18 
               7.09 
               1.15 
               59.1 
               138.34 
               120.00 
             
             
               90 
               165.50 
               7.31 
               7.98 
               0.92 
               63.3 
               115.50 
               126.05 
             
             
               100 
               183.00 
               6.61 
               8.87 
               0.75 
               67.5 
               97.96 
               131.34 
             
             
               110 
               200.50 
               6.03 
               9.75 
               0.62 
               71.7 
               84.17 
               136.01 
             
             
               120 
               218.00 
               5.55 
               10.64 
               0.52 
               75.9 
               73.13 
               140.16 
             
             
               130 
               235.00 
               5.14 
               11.52 
               0.45 
               80.1 
               64.14 
               143.88 
             
             
               140 
               253.00 
               4.78 
               12.41 
               0.39 
               84.3 
               56.73 
               147.23 
             
             
               150 
               270.50 
               4.47 
               13.30 
               0.34 
               88.5 
               50.54 
               150.26 
             
             
                 
             
           
        
       
     
   
     FIG. 2  is a graph showing the specific energy and the specific power as a function of the cathode thickness of a known electrochemical element. 
   It is observed that an increase in the specific energy of a lithium-polymer cell by increasing the thicknesses of the two cathode layers by the same value is not possible without reducing the specific power. 
   Now the invention will be described with reference to  FIGS. 3 and 4 . 
     FIG. 3  shows an electrochemical generator element  102  with lithium having a polymer electrolyte according to the invention, in cross section. 
   In  FIG. 3 , elements similar to those in  FIG. 1  are denoted by reference numbers increased by 100. 
   The electrochemical generator element  102  comprises, like the element of  FIG. 1 , two half-elements  104 ,  106  each one comprising part of a common lithium layer  108 , an electrolyte layer  110 ,  112  and a cathode layer  114 ,  116  together with a layer forming a collector  118 ,  120 . 
   The thicknesses of the lithium layer  108  and of the electrolyte layers  110 ,  112  are the same as those of the layers  8 ,  10  and  12  of the element  2  of  FIG. 1 . 
   However, the thicknesses of the two cathode layers  114 ,  116  are different from each other while their sum is equal to that of the two cathode layers  14 ,  16  of the element  2  of  FIG. 1 . The thickness of the first layer  114  is, for example, 150 μm. For this example, it may be between 130 μm and 170 μm, preferably between 140 μm and 160 μm. The thickness of the second layer  116  is 30 μm, and may be between 10 μm and 50 μm, preferably between 20 μm and 40 μm. 
   In general, the thicker layer may have a thickness between 80 μm and 200 μm, preferably between 100 and 160 μm. 
   In order to be able to compare the power and the energy of the two elements  2  and  102 , the surface resistance of the element  102  will be calculated according to the invention as follows. 
   The electrical surface resistance of each of the two half-elements is calculated using formula (1.1.) for respective thickness values of the cathode layers  114 ,  116 . 
   Here again, the electrical surface resistances of the current collectors  118 ,  120  and of the lithium layer  108  are considered to be negligible. 
   The values of the specific resistances at an operating temperature of 90° C. remain the same. 
   For an element consisting of an assembly of two half-elements comprising cathode layers of 30 μm and 150 μm respectively, the calculation of the internal surface resistance of the two half-elements  104 ,  106  gives the following values:
 
 R   s     —     de1 =60.5 Ωcm 2 
 
 R   s     —     de2 =270.5 Ωcm 2 .
 
   The surface resistance of the complete element  102  is calculated by the following formula relating to the parallel connection of the two elements. 
             R   s_e     =         R   s_de1     ⁢     R   s_de2           R   s_de1     +     R   s_de2               
where R s     —     deX  is the surface resistance of the half-element X (Xε[1,2]).
 
   For the given assembly, the total surface resistance of the element is:
 
 R   s     —     e =121 Ωcm 2 .
 
   The maximum power (at 80% discharge) per unit area is then, using formula 1.2.: 
   
     
       
         
           
             P 
             s_max 
           
           = 
           
             10 
             ⁢ 
             
                 
             
             ⁢ 
             
               mW 
               
                 cm 
                 2 
               
             
           
         
       
     
   
   The energy contained per unit area of the element  102  is calculated by the equation of formula 1.3.:
 
 E   s     —     e   =E   s     —     de1   +E   s     —     de2 =( e   cath1   +e   cath2 )×ρ cath   ×Y×E   s   ×U   mean 
 
   The ratio of the surface power to the surface energy is then: 
                 P   max       E   s_e       =   1.52     ,         
which means an increase of 60% with respect to the element of the prior art which has a ratio
 
               P   max       E   s_e       =   0.92         
(see Table 1).
 
   It is understood that, for an equivalent total cathode thickness (180 μm), the increase in thickness of the first cathode  114  from 90 μm to 150 μm, on the one hand, and the decrease in thickness of the second cathode  116  from 90 μm to 30 μm, on the other hand, makes it possible to decrease the internal surface resistance of the element from 160.5 Ωcm 2  to 121 Ωcm 2 . For a constant specific energy density, since the mass and volume have not changed, the specific power is considerably increased. 
   Table 2 shows the surface resistances of two half-elements  114 ,  116  of different thicknesses for an element  102  with a total cathode thickness of 180 μm. 
   It furthermore shows the resulting surface resistance of the corresponding element, and the power gain with respect to an element  2  with cathode layers of  FIG. 1  each having a thickness equal to 90 μm. 
     FIG. 4  is a graph showing the specific power, the specific energy and the ratio between the two as a function of the thinnest cathode layer thickness for an element according to the invention, the cathode layers of which have a total thickness of 180 μm. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
                 
               Surface 
               Surface 
               Surface 
                 
             
             
                 
                 
               resistance 
               resistance 
               resistance 
             
             
               d cath1   
               d cath2   
               half- el. 1 
               half- el. 2 
               element 
               P/PO 
             
             
               μm 
               μm 
               Ohm · cm 2   
               Ohm · cm 2   
               Ohm · cm 2   
               Gain % 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               90 
               90 
               331 
               331 
               165.5 
               0.00 
             
             
               80 
               100 
               296 
               366 
               163.6 
               1.13 
             
             
               70 
               110 
               261 
               401 
               158.1 
               4.68 
             
             
               60 
               120 
               226 
               436 
               148.8 
               11.19 
             
             
               50 
               130 
               191 
               471 
               135.9 
               21.79 
             
             
               40 
               140 
               156 
               506 
               119.2 
               38.80 
             
             
               35 
               145 
               138.5 
               523.5 
               109.5 
               51.11 
             
             
               30 
               150 
               121 
               541 
               98.9 
               67.37 
             
             
               25 
               155 
               103.5 
               558.5 
               87.3 
               89.54 
             
             
               20 
               160 
               86 
               576 
               74.8 
               121.17 
             
             
               15 
               165 
               68.5 
               593.5 
               61.4 
               169.49 
             
             
               10 
               170 
               51 
               611 
               47.1 
               251.60 
             
             
                 
             
           
        
       
     
   
   It is observed that it is possible, by increasing the thickness of one layer to 170 μm and decreasing the thickness of the other layer to 10 μm, to increase the specific power by a factor of 2.5 with respect to the able power of an element, the positive electrodes of which each have a thickness of 90 μm. 
   The invention therefore makes it possible to increase the specific power and, consequently, the power density available in an electrochemical generator element with lithium having a polymer electrolyte at constant specific energy. 
   It makes it possible to change the power to energy ratio depending on the application envisaged for a given energy density. 
   It is clear that the invention is not limited to the example given. The thicknesses of the electrode layers may be changed over large ranges and the sum of the thicknesses of the cathode layers is not limited to 180 μm but may also be changed. 
   The invention may also be applied to electricity generating cells which use positive or negative electrode materials other than lithium and polymer. 
   The invention may also be applied to any type of electricity generating cell with a thin-layer assembly. 
   As a variant, the invention may also be applied to cells having a common positive electrode layer and two negative electrode layers having different thicknesses, one layer of which is placed on each of the two sides of the positive electrode layer.