Abstract:
A method for fabricating an electrode includes: determining a thickness of an active layer; selecting lithium (Li) foil having a specified thickness; determining widths of one or more Li strips based on an active layer to Li layer weight ratio or volume ratio; laminating the active layer onto a conductive substrate; forming one or more grooves in the active layer exposing a bare surface of the conductive substrate; and pressing the one or more Li strips into the one or more grooves, wherein widths of the one or more grooves are slightly larger than the widths of the Li strips.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/359,031, filed Jul. 6, 2016, the disclosure of which is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND 
     1. Technical Field 
       [0002]    Apparatuses and methods consistent with the present inventive concept relate to energy storage devices, and more particularly to electrodes for energy storage devices. 
       2. Related Art 
       [0003]    Lithium (Li) doped negative electrodes are widely used in energy storage devices, for example, lithium-ion (Li-ion) batteries and Li-ion capacitors. 
         [0004]    In the external Li attachment and pre-dope method, an electrode pack is constructed with at least one positive electrode, at least one separator, at least one Li film laminated on a current collector, and at least one negative electrode. The negative electrode is connected to the Li film electrode through the current collector tab. The electrode pack is immersed in electrolyte that contains Li ions. Through the pre-dope process, the Li film is converted into Li ions through the electrode and the Li ions migrate and are doped into the negative electrode. 
         [0005]    Production of Li doped negative electrodes requires precise control of the amount of Li attaching to the negative electrode. A low amount of attached Li results in incomplete doping of the negative electrode causing sub-optimal electrode performance. On the other hand, over attaching Li to the negative electrode causes metal Li residue on the negative electrode after the pre-dope process that may cause safety issues for energy storage devices that include the electrodes. 
         [0006]    In the external Li attachment and pre-dope method, thin Li metal films are normally provided only on the uppermost and lowermost layers of an electrode package. During the Li pre-dope process, the Li ions may be non-uniformly doped into the stacked negative electrode, and the Li metal films may remain on the electrode package after completion of the pre-doping process. More than twenty days are typically required to uniformly dope lithium to the negative electrode inside the electrode laminates. 
         [0007]    In order to improve upon the long manufacture time necessary for the external Li attachment and pre-dope method, direct contact methods were proposed by different inventors. In the direct contact methods, Li powder or Li film were pressed directly onto the electrode surface layer. The direct contact methods shortened the Li pre-dope time. However, instantaneous electrical shorting between the Li metal and the negative electrode active layer materials (i.e., the surface of the electrode) induced by immersing the electrode pack into electrolyte caused severe reactions. These severe reactions resulted in damage to the electrode and separator. 
       SUMMARY 
       [0008]    Various embodiments provide Li attached electrodes and methods for fabricating internal Li attached electrodes are provided. 
         [0009]    According to various embodiments there is provided a method for fabricating an electrode. In some embodiments, the method may include: determining an electrode active layer thickness; selecting lithium (Li) pieces or strips having a specified thickness equal or slightly larger than the electrode active layer thickness; determining Li piece sizes or Li strip widths based on the active layer to Li layer weight or volume ratio requirements; coat or laminate the active layer onto the conductive substrate which may or may not contain a conductive binder interlayer, the electrode surface contains at least one of the following or both: grooves and holes, where bare conductive substrate is exposed and no active layer materials or conductive binder interlayer in it, and the hole sizes are slightly larger than the sizes of the Li pieces or the groove widths are slightly larger than the widths of the Li strips, and press the Li pieces or strips into the holes or grooves of the electrode. The grooves may be located anywhere on the electrode, including at the end of the active layer. 
         [0010]    According to various embodiments there is provided an electrode. In some embodiments, the electrode may include: a conductive substrate which may or may not contain a conductive binder interlayer; an active layer adhered to the conductive substrate, the electrode surface contains at least one of the following or both: grooves and holes, where bare conductive substrate is exposed and no active layer materials or conductive binder interlayer in it; and lithium (Li) pieces or strips disposed in the holes or the grooves of the active layer. The grooves may be located anywhere on the electrode, including at the end of the active layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is a diagram illustrating a view of a structure of an electrode according to various example embodiments; 
           [0012]      FIG. 1B  is a diagram illustrating a view of the layers of an electrode according to various example embodiments; 
           [0013]      FIG. 2A  is a diagram illustrating one Li strip disposed in one groove in the active layer on one of the two sides of an electrode  100  according to various example embodiments; 
           [0014]      FIG. 2B  is a diagram illustrating a plurality of lithium pieces disposed in a plurality of holes in the active layer on one of the two sides of an electrode according to various example embodiments; and 
           [0015]      FIG. 3  is a flowchart illustrating a method for fabricating an electrode according to various example embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    While certain embodiments are described, these embodiments are presented by way of example only, and are not intended to limit the scope of protection. The apparatuses, methods, and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions, and changes in the form of the example methods and systems described herein may be made without departing from the scope of protection. 
         [0017]    Various embodiments provide a new non-direct contact method for producing Li attached electrodes. The new non-direct contact method may reduce the long manufacture time used in the conventional external Li attachment and pre-dope method, and may prevent electrode and separator damage caused during the pre-dope process in the direct contact methods. 
         [0018]    Various embodiments provide electrodes having controlled electrode potentials for energy storage devices. In various embodiments, the electrodes may be configured with a controlled amount of attached lithium. The electrodes may be incorporated into energy storage devices, for example, but not limited to, lithium-ion batteries, lithium-ion capacitors, etc.  FIG. 1A  is a diagram illustrating a view of a structure of an electrode  100  according to various example embodiments. Referring to  FIG. 1A , the electrode  100  may include a substrate  110  which may or may not contain include a conductive binder interlayer  140 , an active layer  120  having a groove  150 , and a lithium strip  130  disposed in the groove  150  of the active layer  120 . In various example embodiments the substrate  110  may not include the conductive binder interlayer  140 . 
         [0019]    The substrate  110  may be an innermost layer of the electrode  100  and may be a conductive substrate formed from, for example, but not limited to, copper or other conductive material. A conductive binder interlayer  140  may or may not be added to the conductive substrate  110 . The active layer  120  may be adhered to the substrate  110  or may be adhered to the conductive binder interlayer  140  when the conductive binder interlayer  140  is provided. A thickness t of the active layer  120  may be determined based on energy density and power density specifications for an energy storage device. The Li strip  130  may be formed from a Li sheet or other Li products, such as, but not limited to, foils, wires or melted powders. 
         [0020]      FIG. 1B  is a diagram illustrating a view of the layers of an electrode  100  according to various example embodiments. Referring to  FIGS. 1A and 1B , the Li strip  130  may be adhered directly onto surface of the substrate  110  in the groove  150  formed by a top surface  122  of active layer  120  and a bottom surface  124  of active layer  120 . As illustrated in  FIGS. 1A and 1B , a similar groove  150  may be formed in the active layer  120  disposed on both sides of the substrate  110 . 
         [0021]    The Li strip  130  may not cover up to one hundred percent of the substrate  110  within the groove  150 . The Li strip  130  may be constrained within the groove  150  on the top surface  122  of active layer  120  and the bottom surface  124  of active layer  120 . The Li strip  130  may be equal to or thicker than the active layer  120 . The groove  150  and the Li strip  130  may be located anywhere along the substrate  110  of the electrode  100 , including at either or both ends of the active layer  120 . Length, width, and thickness reference directions for the various layers are illustrated in  FIG. 1B . 
         [0022]    While the groove  150  illustrated in  FIG. 1B  is shown as a rectangular groove, one of ordinary skill in the art will appreciate that the groove may have other shapes enabling the Li strip  130  to be adhered directly onto the surface of the substrate  110  without departing from the scope of the present inventive concept. Alternatively or additionally, one or more discrete holes may be formed in the active layer to accommodate discrete Li pieces such that the Li pieces may be adhered directly onto surface of the substrate  110 . Similar holes may be formed in the active layer  120  disposed on both sides of the substrate  110 . 
         [0023]      FIG. 2A  is a diagram illustrating one Li strip  130  disposed in one groove  150  in the active layer  120  on one of the two sides of an electrode  100  according to various example embodiments.  FIG. 2B  is a diagram illustrating a plurality of lithium pieces  135  disposed in a plurality of holes  155  in the active layer  120  on one of the two sides of an electrode  100  according to various example embodiments. One of ordinary skill in the art will appreciate that the Li strip  130  and pieces  135  as well as the groove  150  and holes  155  in the active layer  120  illustrated in  FIGS. 2A and 2B  are merely examples and that other shapes of Li strips, pieces, grooves, and holes may be used without departing from the scope of the present inventive concept. In various embodiments, shapes of the Li metal may be, for example, but not limited to, discrete Li dots, squares, or circles, Li strips, Li wires, etc. The shapes and number of Li pieces or strips forming the Li metal patterns are determined by minimizing the Li diffusion time from the Li strips  130  or pieces  135  to the furthest area of the active layer  120  balanced by manufacturing considerations, for example, but not limited to, fabrication and placement of the Li strips  130  or pieces  135 . 
         [0024]    When fabricating an electrode  100 , the electrode parameters, such as the thickness t of the active layer  120  and the weight or volume ratio of the active layer  120  to the Li metal, may be specified based on energy density and power density requirements of the electrode  100 . Knowing the density of the active layer  120  in the electrode  100  and the density of the Li metal, by selecting a commercially available Li foil with specific thickness, one can calculate the size of the Li pieces  135  and thus the hole  155  pattern and sizes in the active layer  120 , or the Li strip  130  widths and thus the groove  150  widths in the active layer  120 . 
         [0025]    Table 1 lists an example of a width calculation for a Li strip  130 . The width calculation assumes that there is one Li strip  130  and one groove  150  in each active layer  120  and that the length of the Li strip  130  is equal to the length of the active layer  120 . Length, width, and thickness reference directions for the various layers are illustrated in  FIG. 1B . 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Requirement 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 Weight ratio of active layer to Li layer (WR) 
                 10:1 
               
             
          
           
               
                   
                 Active layer thickness (t a ) 
                 100 
                 μm 
               
               
                   
                 Density of active material (D a ) 
                 1.08 
                 g/cm 
               
               
                   
                 Density of Li (D Li ) 
                 0.543 
                 g/cm 3   
               
               
                   
                 Li strip thickness (t Li ) 
                 100 
                 μm 
               
               
                   
                 Li strip width (G) 
                 16.59 
                 mm 
               
               
                   
                   
               
             
          
         
       
     
         [0026]    In Table 1, the thickness t of the active layer  120  and the weight ratio of active layer  120  to the Li strip  130  (or lithium pieces  135 ) may be specified design parameters, a commercially available Li foil having a specified thickness may be selected, the densities of the active material and Li are known based on the materials, and the weight of the active layer  120  and Li weight may be calculated based on the densities and thus, the width of the Li strip  130  may be calculated. An electrode  100  having a groove  150  in the active layer  120  may be fabricated with the width of the groove  150  being slightly larger than the width of the Li strip  130 . 
         [0027]    The width of the groove  150  should be made minimal to reduce the material cost and to improve the electrode efficiency but large enough to prevent direct contact of the Li metal to the electrode active layer materials. For example, the width of the groove  150  may be about 0.5 mm larger than the width of the Li strip  130  creating a small gap between the Li strip  130  and the active layer  120 . Similarly, the sizes of the holes may be about 0.5 mm larger than the sizes of the Li pieces  135  creating a small gap between the Li pieces and the active layer  120 . Control of the gap may be achieved by calculations of the Li metal extension during the Li press process (i.e., final Li width) vs. groove width or hole sizes. 
         [0028]    Based on the weight ratio (WR) of active layer to Li strip, and the densities and weights of the active layer  120  and Li strip  130 , and the thicknesses of both the Li strip  130  and the active layer  120 , the width G of the Li strip  130  may be calculated by Equation (1): 
         [0000]    
       
         
           
             
               
                 
                   G 
                   = 
                   
                     
                       
                         100 
                          
                         
                           ( 
                           
                             t 
                             a 
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             D 
                             a 
                           
                           ) 
                         
                       
                       
                         
                           
                             
                               t 
                               Li 
                             
                              
                             
                               ( 
                               
                                 D 
                                 Li 
                               
                               ) 
                             
                           
                            
                           
                             ( 
                             WR 
                             ) 
                           
                         
                         + 
                         
                           
                             t 
                             a 
                           
                            
                           
                             ( 
                             
                               D 
                               a 
                             
                             ) 
                           
                         
                       
                     
                     = 
                     
                       
                         
                           100 
                            
                           
                             ( 
                             
                               100 
                                
                               
                                   
                               
                                
                               μm 
                             
                             ) 
                           
                            
                           
                             ( 
                             
                               1.08 
                                
                               
                                   
                               
                                
                               
                                 g 
                                 
                                   cm 
                                   3 
                                 
                               
                             
                             ) 
                           
                         
                         
                           
                             100 
                              
                             
                                 
                             
                              
                             
                               μm 
                                
                               
                                 ( 
                                 
                                   .543 
                                    
                                   
                                     g 
                                     
                                       cm 
                                       3 
                                     
                                   
                                 
                                 ) 
                               
                             
                              
                             
                               ( 
                               10 
                               ) 
                             
                           
                           + 
                           
                             100 
                              
                             
                                 
                             
                              
                             
                               μm 
                                
                               
                                 ( 
                                 
                                   1.08 
                                    
                                   
                                       
                                   
                                    
                                   
                                     g 
                                     
                                       cm 
                                       3 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                       = 
                       
                         16.59 
                          
                         
                             
                         
                          
                         mm 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0029]    Thus, in the example above, assuming the Li strip  130  and the active layer  120  both have a thickness of 100 microns (μm), an electrode  100  with a groove  150  in the active layer  120  with a groove width slightly larger than 16.59 mm may be fabricated. The width of the groove  150  should be made minimal to reduce the material cost and to improve the efficiency of the electrode  100  but large enough to prevent the direct contact of the Li metal to the electrode active layer materials. 
         [0030]    Embodiments of the present inventive concept provide methods for fabricating electrodes using various commercially available freestanding Li foils or wires. In the various example embodiments, by specifying the Li foil thickness and applying the Li foil pieces or strips  130  to the conductive substrate  110  inside the groove  150  in the active layer  120 , an electrode  100  having specified energy density and power density requirements may be fabricated. 
         [0031]    Referring again to Table 1, by selecting a Li foil with a commercially available thickness (e.g., 150 μm), the width of the Li strip  130  may be recalculated as necessary, as shown in Equation (2): 
         [0000]    
       
         
           
             
               
                 
                   G 
                   = 
                   
                     
                       
                         100 
                          
                         
                           ( 
                           
                             100 
                              
                             
                                 
                             
                              
                             μm 
                           
                           ) 
                         
                          
                         
                           ( 
                           
                             1.08 
                              
                             
                                 
                             
                              
                             
                               g 
                               
                                 cm 
                                 3 
                               
                             
                           
                           ) 
                         
                       
                       
                         
                           150 
                            
                           
                               
                           
                            
                           
                             μm 
                              
                             
                               ( 
                               
                                 .543 
                                  
                                 
                                   g 
                                   
                                     cm 
                                     3 
                                   
                                 
                               
                               ) 
                             
                           
                            
                           
                             ( 
                             10 
                             ) 
                           
                         
                         + 
                         
                           100 
                            
                           
                               
                           
                            
                           
                             μm 
                              
                             
                               ( 
                               
                                 1.08 
                                  
                                 
                                     
                                 
                                  
                                 
                                   g 
                                   
                                     cm 
                                     3 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                     = 
                     
                       11.71 
                        
                       
                           
                       
                        
                       mm 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0032]    Thus, using an available Li foil thickness of 150 μm, an electrode  100  meeting specified energy density and power density requirements with an 11.71 mm wide Li strip  130  disposed in a groove  150  in the active layer  120  on top of the conductive substrate  110  may be fabricated. 
         [0033]    One of ordinary skill in the art will appreciate that more than one Li strip  130  or more than one groove  150  in the electrode active layer  120 , or other shapes of Li products, such as, for example, but not limited to, discrete Li dots, squares, or circles, Li wires, etc., may be used without departing from the scope of the present inventive concept. One of ordinary skill in the art will also understand that similar calculations may be performed for different shapes of Li products. 
         [0034]    Increasing the number of Li strips  130  or grooves  150  in the electrode active layers  120 , or using other shapes of the Li format, may reduce the Li diffusion time from the Li strips  130  or pieces  135  during Li doping process, however, it may increase the manufacture difficulties. Therefore, the shapes and number of Li strips  130  or pieces  135  forming the Li layer patterns should be determined by minimizing the Li diffusion time from the Li strips  130  or pieces  135  to the furthest area of the active layer  120  but be balanced by the ease of manufacturing considerations, for example, but not limited to, fabrication and placement of the Li strips  130  or pieces  135 . 
         [0035]    In various embodiments, the Li foil thickness may be equal to or less than double (i.e., two times) the thickness of the active layer  120 . A Li foil thickness greater than double the thickness of the active layer  120  may result in a long Li pre-dope process due to a smaller contact area between the Li strips  130  and the conductive substrate  110 , and/or portions of the Li foil forming the Li layer pattern may protrude from the surface of the active layer  120  and pierce the separator causing shorts between the electrodes. An Li foil thickness less than the thickness of the active layer  120  may result in loose contact between the Li layer and the conductive substrate  110 , or the width of the groove  150  or sizes of the holes  155  are too large such that the electrode pack made by the negative electrode may have low efficiency. 
         [0036]      FIGS. 2A and 2B  are diagrams illustrating Li strips and pieces, respectively, placed inside the active layer groove or holes, respectively, according to various example embodiments.  FIG. 2A  illustrates one surface (e.g., the top surface  122  or the bottom surface  124 ) of an active layer (e.g., the active layer  120 ) of an electrode (e.g., the electrode  100 ). Referring to  FIGS. 1A-2B , an electrode  100  may include an active layer  120  including a groove  150  disposed on a substrate (e.g., substrate  110 ) or conductive binder layer (e.g., interlayer  140 ). Li strips  130  or pieces  135  may be disposed on the substrate  110  and in the groove  150  in the active layer  120 . The width of the Li strip  130  or piece  135  sizes may be smaller than the width of the groove  150 . The thickness of the Li strip  130  or pieces  135  may be equal to or may exceed the thickness of the active layer  120 , but should not be more than double (i.e., two times) the thickness of the active layer  120 . The thickness of the Li strips  130  or pieces  135  may not be thinner than the thickness of the active layer  120 . 
         [0037]      FIG. 3  is a flowchart for a method  300  for fabricating Li electrodes according to various example embodiments. Referring to  FIG. 3 , at block  305 , a thickness of an electrode active layer  120  may be determined based on energy density and power density specifications of an energy storage device. The electrode active layer  120  may be a film layer formed with a combination of, for example, but not limited to, active materials (e.g., graphite, hard carbon, soft carbon, activated carbon, Lithium salts, Li oxides, silicon, etc.), conductive carbon, and binder. 
         [0038]    At block  310 , a thickness of the Li pieces or strips  130  or pieces  135  may be selected. For example, the Li thickness may be selected based on, for example, but not limited to, commercial availability, ability to handle the Li without damage, etc. The Li thickness may be at least equal to or greater than the thickness of the active layer  120 . At block  315 , a weight ratio of the active layer  120  to the Li layer may be determined. An electrode potential after pre-doping may be made as close as possible to a Li metal potential by maximizing the Li effects by use maximum amount of Li strips  130  or pieces  135 . However, the amount of Li strips  130  or pieces  135  may be optimized to minimize Li metal residue upon completion of a pre-doping process. For example, the weight ratio of the active layer  120  to the Li layer may be in a range of 5:1 to 15:1. 
         [0039]    At block  320 , the pattern for the groove or grooves  150  or holes  155  in the active layer  120  may be determined. The selection of the pattern for the groove or grooves  150  or holes  155  will be considered with both Li strips  130  or pieces  135  placement uniformity and dispersion, and ease of manufacture conditions, for example, but not limited to, fabrication and placement of the Li strips  130  or pieces  135 . The widths of the groove or grooves  150  or sizes of the holes  155  may also be determined. For example, width of the groove  150  in the active layer  120  may be determined by the width of the Li strip  130  which may be calculated by Equations (1) and (2). The width of the groove  150  should be larger than the width of the Li strip  130 . Similarly, the sizes of the holes  155  should be made larger than the sizes of the Li pieces  135 . 
         [0040]    The width of the groove  150  or sizes of the holes  155  should be made minimal to reduce the material cost and to improve the efficiency of the electrode  100  but large enough to prevent the direct contact of the Li metal to the electrode active layer materials. For example, the width of the groove  150  may be about 0.5 mm larger than the width of the Li strip  130  creating a small gap between the Li strip  130  and the active layer  120 . Similarly, the sizes of the holes may be about 0.5 mm larger than the sizes of the Li pieces  135  creating a small gap between the Li pieces and the active layer  120 . Control of the gap may be achieved by calculations of the Li metal extension during the Li press process (i.e., final Li width) vs. groove width or hole size. 
         [0041]    At block  325 , the electrode  100  with the grooves  150  or holes  155  in the active layer  120  may be fabricated. The electrode  100  may include the conductive substrate  110 , may or may not include the conductive binder interlayer  140 , and may include the active layer  120  with the grooves  150  or holes  155 . 
         [0042]    At block  330 , the Li strips  130  or pieces  135  may be arranged in groove or grooves  150  or holes  155 , respectively, in the active layer  120  on top of the conductive substrate  110  such that the diffusion time between the Li strips  130  or pieces  135  and the active layer  120  is minimized. Further, the Li strips  130  or pieces  135  (e.g., Li dots, Li squares, Li stripes, etc.) and the groove or grooves  150  or holes  155  in the active layer  120  that minimizes the Li diffusion distance from the Li layer (i.e., the Li strips  130  or pieces  135  and the active layer  120  is minimized. Further, the Li strips  130  or pieces  135 ) to the active layer  120  may be determined at least in part based on manufacturing considerations, for example, but not limited to, fabrication and placement of the Li strips  130  or pieces  135 . The grooves  150  or holes  155  may be located anywhere on the electrode  100 , including at the either or both ends of the active layer  120 . 
         [0043]    Although the present disclosure provides certain example embodiments and applications, other embodiments that are apparent to those of ordinary skill in the art, including embodiments which do not provide all of the features and advantages set forth herein, are also within the scope of this disclosure. Accordingly, the scope of the present disclosure is intended to be defined only by reference to the appended claims.