Patent Publication Number: US-2016226064-A1

Title: Lithium ion rechargeable battery

Description:
TECHNICAL FIELD 
     The present invention relates to a lithium ion rechargeable battery, and more particularly, to a lithium ion rechargeable battery in which discharge characteristics are improved. 
     BACKGROUND ART 
     In recent years, there has been a growing need for rechargeable batteries used for hybrid cars, electric cars, and accumulation of power which have a high capacity, a small size and a low weight. Among the rechargeable batteries, the current focus is on lithium ion rechargeable batteries as they are considered the most important rechargeable batteries since it has been possible to achieve a higher capacity and a higher output in the lithium ion rechargeable batteries. It has been demanded that the capacity and the output in the lithium ion rechargeable batteries be further increased. 
     One of the techniques to improve the electric capacity of the lithium ion rechargeable battery is to provide a thick-film electrode in which a positive electrode active material layer or a negative electrode active material layer is formed on a current collector in such a way that the layer has as great a thickness as possible. Related techniques to promote the reaction of the thick electrode on the upper layer (electrolyte side) include, for example, Patent Literature 1 and 2. 
     Patent Literature 1 discloses an electrode in which a solid concentration decreases from a side of a current collector to an upper layer (electrolyte side) in an active material layer of a thick electrode. Patent Literature 2 discloses an electrode in which an active material having a small particle diameter is arranged in an upper layer (electrolyte side) in an active material layer of a rechargeable battery and a part having void sizes different from one another is provided. 
     CITATION LIST 
     Non Patent List 
     [Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2005-050755 
     [Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2011-175739 
     SUMMARY OF INVENTION 
     Technical Problem 
     When the lithium ion rechargeable battery is discharged at a high rate, much Li ion is consumed in the positive electrode surface layer (electrolyte side), which causes a so-called “lack of electrolyte solution” and results in discharge defects. This is because Li ion concentration is intensively consumed on the surface layer of the electrode. Since the active material located around the surface of the electrode selectively reacts in the surface layer active material layer in the thick electrode, it is difficult to sufficiently promote the performance of the active material on the side of the current collector and improve output corresponding to the thickness of the active material layer. 
     According to the methods disclosed in Patent Literature 1 and 2, the reactivity of the upper layer (electrolyte side) of the active material layer of the thick electrode increases in a short time in the lithium ion rechargeable battery. However, Patent Literature 1 and 2 do not consider a way to mitigate the reaction to the current collector side. Therefore, when a constant power discharge is carried out for a certain period of time, the reaction of the lower layer part (current collector side) decreases, and the speed of the decrease in the voltage of the battery increases. 
     The present invention has been made in view of the aforementioned problem and provides a lithium ion rechargeable battery that promotes reactivity on a side of a current collector of an electrode and improves a constant output discharge performance. 
     Solution to Problem 
     A lithium ion rechargeable battery according to one aspect of the present invention includes an electrode, the electrode including: a lower layer formed of a first active material and a second active material having a conductivity different from that of the first active material; and an upper layer formed of the first active material and the second active material, in which the lower layer is formed by alternately applying a first lower layer-forming slurry that contains the first active material and a second lower layer-forming slurry that contains the second active material in a stripe shape on a current collector, and the upper layer is formed by a first upper layer-forming slurry that contains the first active material applied on the second lower layer-forming slurry in multiple layers and a second upper layer-forming slurry that contains the second active material applied on the first lower layer-forming slurry in multiple layers. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a lithium ion rechargeable battery that promotes reactivity on a side of a current collector of an electrode and improves a constant output discharge performance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an electrode  1  of a lithium ion rechargeable battery according to a first embodiment of the present invention; 
         FIG. 2  is a partial view of the cross section of the electrode  1  according to the first embodiment of the present invention; 
         FIG. 3  is a graph showing some reaction characteristics of the lithium ion rechargeable battery when a pattern of the electrode  1  is changed according to the first embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of the electrode  1  when a pattern of the electrode  1  is changed according to the first embodiment of the present invention; 
         FIG. 5  is a cross-sectional view of an electrode  2  when a pattern of the electrode  2  is changed according to a second embodiment of the present invention; 
         FIG. 6  is a cross-sectional view of an electrode  3  when a pattern of the electrode  3  is changed according to a third embodiment of the present invention; 
         FIG. 7  is a compounding ratio of paste of an active material when a first layer  5  and a second layer  6  are formed on a current collector  12  of the electrode  1  using gravure pattern printing according to the first embodiment of the present invention; 
         FIG. 8  is a schematic view showing a method of applying paste that contains the active material on the current collector  12  using gravure pattern printing according to the first embodiment of the present invention; and 
         FIG. 9  is a schematic view showing a lithium ion rechargeable battery  100  according to the first embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, with reference to the drawings, a first embodiment of the present invention will be described.  FIG. 9  is a schematic view showing a lithium ion rechargeable battery  100  according to the first embodiment of the present invention. The lithium ion rechargeable battery  100  includes an electrode  1  (anode), an electrode  40  (cathode), and an electrolyte  50 . 
       FIG. 1  is a schematic view showing the electrode  1  of the lithium ion rechargeable battery according to the first embodiment of the present invention. 
     The electrode  1  includes a current collector  12  formed of metallic foil, a first layer  5  (lower layer) having one surface side formed on the current collector, and a second layer  6  (upper layer) formed on the other surface side of the first layer.  FIG. 2  is a cross-sectional view of the electrode  1  showing a part surrounded by the circle shown in  FIG. 1 . As shown in  FIGS. 1 and 2 , the first layer  5  is formed of an A layer  10  (layer including a first active material) and a B layer  11  (layer including a second active material) having a conductivity different from that of the A layer  10 . 
     The first layer  5  includes a plurality of strip-shaped A layers  10  having a constant width and a plurality of strip-shaped B layers  11  having a constant width alternately arranged therein and has a stripe shape. 
     The second layer  6  has a configuration similar to that of the first layer  5  formed of the A layers  10  and the B layers  11 . The A layer  10  of the second layer  6  is formed on the other surface side of the B layer  11  of the first layer  5  and the B layer  11  of the second layer  6  is formed on the other surface side of the A layer  10   a  of the first layer  5 . That is, in the electrode  1 , the A layers  10  and the B layers  11  are alternately arranged with respect to the z-axis direction and the y-axis direction from the side of the current collector  12 . 
     The A layer  10  is an active material in which the reactivity is large and the capacity is small. The A layer  10  is formed to include the active material having a small particle diameter (2 to 5 μm). 
     The B layer  10  is an active material in which the reactivity is small and the capacity is large. The B layer  11  is formed to include the active material having a large particle diameter (7 to 12 μm). The active material may be, for example, LiNi 1/3 Mn 1/3 Co 1/3 O 2 . 
     Next, the reaction of the electrode surface layer when a high-rate discharge is performed in the lithium ion rechargeable battery  100  will be described.  FIG. 3  is a graph showing some reaction characteristics of the lithium ion rechargeable battery when each electrode shown in ( 1 ) to ( 3 ) described later is used. The graph in  FIG. 3  shows a variation of voltage with time when the lithium ion rechargeable battery is discharged at a constant power when each electrode shown in ( 1 ) to ( 3  ) described later is used. 
       FIG. 4  is a cross-sectional view of electrodes  1   a,    1   b,  and  1   c  when each electrode is formed to have the patterns of ( 1 ) to ( 3 ). The table shown in  FIG. 4  shows a reaction time in the case of ( 1 ) to ( 3 ) when the lithium ion rechargeable battery  100  of each electrode is discharged at a constant power and the voltage decreases from 4.1 V to 3.0 V. The unit of the set power value is obtained by dividing a set power value to be output by an effective area of each electrode (mW/cm 2 ). 
     First, as shown in Case  1 -( 2 ) in  FIG. 4 , a case in which the electrode  1   b  is formed of the second layer formed of only the A layers  10  and the first layer formed of only the B layers will be described. As shown in  FIG. 3 , when the electrode is as shown in (2) at the time of discharge, the average reaction voltage increases. However, the reaction at the time of discharge occurs in around the surface layer (side of the electrolyte  50 ) of the A layer  10 , which causes a so-called “lack of electrolyte solution”. Then as shown in  FIG. 3  and the table in  FIG. 4 , in the electrode lb formed by the pattern of ( 2 ), the reactivity of the whole electrode decreases and the discharge time becomes short. 
     Next, as shown in Case  1 -( 3 ) in  FIG. 4 , a case in which the electrode  1   c  is formed of the second layer formed of only the B layers  11  and the first layer formed of only the A layers  10  will be described. As shown in  FIG. 3  and the table in  FIG. 4 , when the electrode is as shown in ( 3 ) at the time of discharge, the voltage abruptly decreases and reaches the lower-limit voltage (stops when the voltage reaches 3 V). On the other hand, in the electrode formed by the pattern of ( 3 ), the reaction becomes dull in the electrode lower layer (current collector side) and the reaction time when the voltage is equal to or larger than 3 V increases. That is, while the reaction average voltage decreases in the electrode where the upper layer is formed of the B layers  11 , the reaction time in the electrode lower layer increases ( 3 ). In this way, the electrode formed of the A layers  10  and the B layers  11  have both advantages and disadvantages. 
     Next, discharge characteristics of a case in which the electrode  1  (electrode  1   a ) according to the first embodiment of the present invention is used will be described ( 1 ). When the A layers  10  and the B layers  11  are alternately arranged in two layers, as shown in Case  1 -( 1 ) in  FIG. 4 , the reaction indicating the characteristics intermediate between those of ( 2 ) and ( 3 ), as shown in  FIG. 3  and the table in  FIG. 4 , is obtained. That is, the electrode la exhibits the characteristics in which the average voltage is higher than that of ( 3 ) and the discharge time until when the voltage reaches the lower-limit voltage (3.0 V) is longer than that of ( 2 ). 
     As stated above, by using the electrode  1  according to this embodiment, the reactivity on the side of the electrolyte  50  and the side of the current collector is promoted, whereby the lithium ion rechargeable battery  100  in which the constant output discharge performance is improved is obtained. 
     Next, with reference to the drawings, a method of manufacturing the electrode  1  according to this embodiment will be described.  FIG. 7  shows a compounding ratio of the slurry (paste) of the active material when the first layer  5  and the second layer  6  are formed on the current collector  12  of the electrode  1  using gravure pattern printing according to this embodiment. LiNi 1/3 Mn 1/3 Co 1/3 O 2  is used as the active material. Acetylene black (HS- 100 ) is used as the conductive auxiliary agent. Polyvinylidene fluoride (PVdF) is used as the binder. N-methyl-2-pyrrolidone (NMP) is used as the solvent. 
     Next, a method of preparing the slurry (paste) including the active material will be described. A slurry producing device is used to produce the slurry. This device may be a typical planetary mixer. 
     First, the active material and the conductive auxiliary agent are mixed. Then binder is input to the mixed material and the mixture is kneaded. Further, NMP is injected into the kneaded material and the mixture is further mixed and kneaded. According to the above processes, the slurry that contains the active material is obtained. 
     Next, a method of applying the slurry  23  that contains the active material onto the current collector  12  (in this embodiment, aluminium foil  24 ) will be described.  FIG. 8  is a schematic view showing the method of applying the slurry that contains the active material onto the current collector  12  using the gravure pattern printing. 
     First, the slurry  23  is rotated in a clockwise direction about the x axis while the slurry  23  is being uniformly applied to the lower part of a gravure roll  21  (−z direction) in the x direction. The slurry  23  is then scraped at certain intervals by a doctor blade  22  having grooves at certain intervals while the gravure roll  21  is being rotated. The slurry  23  that has been scraped at certain intervals is transferred to a blanket roll  20 . The slurry  23  that has been transferred to the blanket roll  20  is then transferred and applied to the aluminium foil  24  in a stripe shape. The condition for applying the slurry is, for example, 0.8 m/min. The condition for dying the slurry  23  after it is applied is, for example, 180 degrees. 
     The first layer  5  (lower layer) is formed by alternately applying the A layers  10  and the B layers  11  twice to form the A layers  10  that contain the first active material and the B layers  11  that contain the second active material when the electrode lower layer-forming slurry is applied while the compounding ratio of the active material is being changed. That is, the first layer  5  (lower layer) is formed by alternately applying the first lower layer-forming slurry that contains the first active material and the second lower layer-forming slurry that contains the second active material in a strip shape on the current collector. 
     When the slurry for forming the second layer  6  (upper layer) is applied onto the first layer  5  (lower layer) in multiple layers, the second layer  6  (upper layer) is formed by applying the A layers  10  and the B layers  11  twice, that is, by applying the A layer-forming slurry of the second layer  6  (upper layer) onto the slurry for forming the B layer  11  of the first layer  5  (lower layer) in multiple layers in a stripe shape and further applying the slurry for forming the B layer  11  of the second layer  6  (upper layer) onto the slurry for forming the A layer  10  of the first layer  5  (lower layer) in multiple layers in a stripe shape. 
     That is, the second layer  6  (upper layer) is formed by the first upper layer-forming slurry that contains the first active material applied on the second lower layer-forming slurry in multiple layers and the second upper layer-forming slurry that contains the second active material applied on the first lower layer-forming slurry in multiple layers. 
     As stated above, by performing the process of applying slurry four times in total, the electrode  1  according to this embodiment is obtained. 
     Second Embodiment 
     Next, characteristics of an electrode  2 , which is obtained by changing the active materials of the A layer  10  and the B layer  11  from those in the first embodiment, will be described. In this embodiment, a hollow active material is used as the A layer  10  and a solid active material is used as the B layer  11 .  FIG. 5  is a cross-sectional view of electrodes  2   a,    2   b,  and  2   c  when each electrode is formed to have the patterns of ( 1 ) to ( 3 ). The experimental method and the patterns ( 1 ) to ( 3 ) of each electrode are similar to those of the first embodiment, and the overlapping descriptions will be omitted. 
       FIG. 5  shows a table indicating the discharge time until when the voltage reaches the lower-limit voltage (3.0 V) when a constant power discharge is performed. As shown in the table of  FIG. 5 , the discharge time of the electrode  2   a  shown in the pattern of ( 1 ) is the longest. 
     As described above, by using the electrode  2  according to this embodiment, the reactivity on the side of the electrolyte  50  and the side of the current collector is promoted, whereby the lithium ion rechargeable battery in which the constant output discharge performance is improved is obtained. 
     Third Embodiment 
     Next, characteristics of an electrode  3 , which is obtained by changing the active materials of the A layer  10  and the B layer  11  from those in the first embodiment, will be described. In this embodiment, an active material that contains a large amount of carbon is used as the A layer  10  and an active material that contains a small amount of carbon is used as the B layer  11 . 
       FIG. 6  is a cross-sectional view of electrodes  3   a,    3   b,  and  3   c  when each electrode is formed to have the patterns of ( 1 ) to ( 3 ). The experimental method and the patterns ( 1 ) to ( 3 ) of each electrode are similar to those of the first embodiment, and the overlapping descriptions will be omitted. 
       FIG. 6  shows a table indicating the discharge time until when the voltage reaches the lower-limit voltage (3.0 V) when a constant power discharge is performed. As shown in the table of  FIG. 6 , the discharge time of the electrode  3   a  shown in the pattern of ( 1 ) is the longest. 
     As described above, by using the electrode  3  according to this embodiment, the reactivity on the side of the electrolyte  50  and the side of the current collector is promoted, whereby the lithium ion rechargeable battery in which the constant output discharge performance is improved is obtained. 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-206951, filed on Oct. 2, 2013, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
       1  ELECTRODE 
       1   a  ELECTRODE 
       1   b  ELECTRODE 
       1   c  ELECTRODE 
       2   a  ELECTRODE 
       2   b  ELECTRODE 
       2   c  ELECTRODE 
       3   a  ELECTRODE 
       3   b  ELECTRODE 
       3   c  ELECTRODE 
       5  FIRST LAYER (LOWER LAYER) 
       6  SECOND LAYER (UPPER LAYER) 
       10  A LAYER 
       11  B LAYER 
       12  CURRENT COLLECTOR 
       20  BLANKET ROLL 
       21  GRAVURE ROLL 
       22  DOCTOR BLADE 
       23  SLURRY 
       24  METALLIC FOIL 
       40  ELECTRODE (CATHODE) 
       50  ELECTROLYTE 
       100  LITHIUM ION RECHARGEABLE BATTERY