Patent Publication Number: US-2023141498-A1

Title: Lithium ion battery

Description:
TECHNICAL FIELD 
     The present disclosure relates to a lithium-ion battery including a positive electrode with a positive electrode mixture layer which contains a positive electrode active material, and a negative electrode with a negative electrode mixture layer which contains a negative electrode active material. Charging and discharging are caused by lithium ions moving between the positive electrode and the negative electrode. 
     BACKGROUND ART 
     Lithium (Li) ion batteries, in which charging and discharging are caused by lithium ions moving between a positive electrode and a negative electrode are widely used. For a negative electrode active material of a negative electrode mixture layer in lithium ion batteries, graphite-based materials are often used. The graphite-based negative electrode active material may sometimes be used with Si. However, in this case, a significant volume change may occur during charging or discharging, capacity retention properties is likely to be deteriorated, and a relatively high cost may be required. 
     In light of above, negative electrode active materials other than the graphite-based materials are proposed. For example, Patent Literature 1 discloses using an alloy having a crystal structure of La 3 Co 2 Sn 7  used for the negative electrode active material. 
     A binder may be used in the negative electrode mixture layer to reduce peeling and cracking. However, because too much binder would lower the battery reaction efficiency of the negative electrode active material, a reduction in the amount of the binder is desirable. Patent Literature 2 discloses a binder in the amount of 0.5 weight% to 5.0 weight%. 
     Citation List 
     Patent Literature 
     
         
         PATENT LITERATURE 1: JP 4127692 B 
         PATENT LITERATURE 2: JP 2007-258127 A 
       
    
     SUMMARY 
     Patent Literature 1 describes a use of polyvinylidene fluoride (PVDF) as a binder. However, it was found as a test result that when PVDF was used as the binder with La 3 Ni 2 Sn 7  used as an active material, these materials reacted each other to cause gelation of the mixture slurry used to form a negative electrode mixture layer, making a coating process difficult. In order to enable the coating by lowering the reaction between the La 3 Ni 2 Sn 7  and PVDF, the particle sizes of the negative electrode active material should be increased. However, as the negative electrode active material of larger particle sizes reduce reaction between the negative electrode active material and Li, the capacity is likely to be declined. 
     A lithium-ion battery according to an embodiment of the present disclosure comprises a positive electrode with a positive electrode mixture layer containing a positive electrode active material, and a negative electrode with a negative electrode mixture layer containing a negative electrode active material. Charging and discharging are caused by lithium-ions moving between the positive electrode and the negative electrode. The negative electrode mixture contains the negative electrode active material represented by a general formula M 3 Me 2 X 7  (where M includes at least one of La and Ca; Me includes at least one of Mn, Ni, Fe, and Co; and X includes at least one of Ge, Si, Sn, and Al). The negative electrode mixture also contains a binder containing a cyano group. The ratio of the binder in the negative electrode mixture layer is 0.5 weight% to 7.0 weight%. 
     In an embodiment according to the present disclosure, a material represented by a general formula M 3 Me 2 X 7  is used as the negative electrode active material, and a binder containing a cyano group is added in the mount of 0.5 weight% to 7.0 weight%. This enables coating application of the negative electrode layer. Further, because a relatively small amount of the binder is applied, the decline in the capacity can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
         FIG.  1    is a longitudinal cross sectional view of a cylindrical secondary battery  10  according to an embodiment of the present disclosure. 
         FIG.  2    is a graph showing initial efficiencies of Examples 1 to 5 and Comparative Example 1. 
         FIG.  3    is a graph showing discharge capacities of Examples 1 to 5 in the first cycle. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments according to the present disclosure are described below with reference to the attached drawings. It should be noted that the present disclosure is not limited to the embodiments described below. 
     Negative Electrode Material 
     Negative electrode materials used for lithium-ion batteries are desired to have a high energy density and a low expansion. Various types of research and development have been performed. For the negative electrode active material, a use of an intermetallic compound represented by M 3 Me 2 X 7  (M = La or Ca; Me = Mn, Ni, Fe, or Co; and X = Ge, Si, Sn, or Al), for example, La 3 Ni 2 Sn 7 , is proposed. As such an intermetallic compound performs Li absorption and release through intercalation reactions, expansion can be low and a long life can be achieved. 
     However, it has been found that further improvement is required to actually use the material. First of all, as described above, use of PVDF as the binder may cause gelation of the negative electrode mixture slurry, making it difficult to apply the negative electrode mixture layer. When the gelation is reduced by using the binder of larger particle diameters, the battery reaction may be impeded. 
     In the present disclosure, the gelation of the negative electrode mixture slurry is impeded by using a binder which contains a cyano group, such as polyacrylonitrile (PAN). This can reduce the amount of the binder required to be applied to the negative electrode mixture layer to a range from 2.0 weight% to 5.0 weight%. 
     As described above, the coating application of the negative electrode mixture layer is enabled and a battery having a high energy density can be provided by using the negative electrode active material having a crystal structure of M 3 Me 2 X 7  and the binder containing a cyano group. 
     Structural Embodiments of Present Disclosure 
       FIG.  1    shows a longitudinal cross section of a cylindrical secondary battery  10  according to an embodiment of the present disclosure. In the secondary battery  10  shown in  FIG.  1   , an outer housing body  15  houses an electrode assembly  14  and non-aqueous electrolyte. The electrode assembly  14  has a rolled structure in which a positive electrode  11  and a negative electrode  12  are rolled via a separator  13 . As the non-aqueous solvent (an organic solvent) of the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, and esters may be used. These solvents may be used alone or in mixture of two or more solvents. For a mixture of two or more solvents, a mixture solvent containing a cyclic carbonate and a chain carbonate may be used. For the cyclic carbonate, for example, ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC) may be used. For the chain carbonate, for example, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC) may be used. For the electrolyte salt of the non-aqueous electrolyte, LiPF 6 , LiBF 4 , LiCF 3 SO 3  may be used alone or in mixture. The amount of the electrolyte salt to be dissolved in the non-aqueous solvent may be, for example, 0.5 to 2.0 mol/L. In the description below, in order to facilitate positional description, the side on which a sealing assembly  16  is located is referred to as “top”, and other side on which the bottom of the outer housing body  15  is located as “bottom”. 
     The secondary battery  10  is sealed with the sealing assembly  16  sealed around the opening edge of the outer housing body  15 . Insulating plates  17 ,  18  are respectively provided on the top and at the bottom of the electrode assembly  14 . A positive electrode lead  19  extends upward through a through hole of the insulating plate  17  and welded on a bottom surface of a filter  22  which is a bottom plate of the sealing assembly  16 . In the secondary battery  10 , a cap  26 , which is the top plate of the sealing assembly  16  electrically connected to the filter  22  serves as a positive electrode terminal. A negative electrode lead  20  extends downward through a through hole of the insulating plate  18  to the bottom of the outer housing body  15  and is welded on the inner bottom surface of the outer housing body  15 . In the secondary battery  10 , the outer housing body  15  serves as a negative electrode terminal. When the negative electrode lead  20  is positioned around the rolling end, the negative electrode lead  20  may extend on the outer side of the insulating plate  18  to the bottom of the outer housing body  15  and is welded on the inner bottom surface of the outer housing body  15 . 
      The outer housing body  15  may be, for example, a cylindrical metal can housing with a bottom. A gasket  27  may be provided between the outer housing body  15  and the sealing assembly  16  to ensure sealing of the secondary battery  10 . The outer housing body  15  may include a grooved portion  21  for supporting the sealing assembly  16 . The grooved portion  21  may be formed by, for example, pressing the side surface from the outer side. The grooved portion  21  may be formed to extend annularly along the outer circumference of the outer housing body  15 . The upper surface of the grooved portion  21  supports the sealing assembly  16  via the gasket  27 . 
     The sealing assembly  16  may include the filter  22 , a lower vent member  23 , an insulating member  24 , an upper vent member  25 , and the cap  26  positioned in this order from the electrode assembly  14  side. Each of these components of the sealing assembly  16  has, for example, a disk or ring shape, and all the components except for the insulating member  24  are electrically connected to each other. The lower vent member  23  and the upper vent member  25  may be connected to each other at the center of these components with the insulating member  24  sandwiched therebetween around the circumference edges of these components. When the internal pressure of the battery increases due to overheat, the lower vent member  23  may be broken, and the upper vent member  25  may swell towards the cap  26 , moving away from the lower vent member  23 . This disconnects the electrical connection between the upper vent member  25  and the lower vent member  23 . If the internal pressure increases further, the upper vent member  25  may rupture, and gas may be exhausted from an opening  26   a  of the cap  26 . 
     The positive electrode  11 , the negative electrode  12 , and the separator  13  of the electrode assembly  14  are described below. In particular, the negative electrode active material of the negative electrode  12  is described in detail below. 
     Positive Electrode 
     The positive electrode  11  includes a positive electrode core and a positive electrode mixture layer disposed on a surface of the positive electrode core. The positive electrode core may be made from a foil of metal such as aluminum that is stable within a potential range of the positive electrode  11 , or a film which includes the metal at the outermost layer. The positive electrode core may have a thickness of, for example, 10 µm to 30 µm. The positive electrode mixture layer may include a positive electrode active material, a binder, and a conductive agent. The positive electrode mixture layer may be provided on each side of the positive electrode core except for the area to which the positive electrode lead  19  is connected. The positive electrode  11  may be manufactured by forming the positive electrode mixture layer on each side of the positive electrode core by, for example, coating the surfaces of the positive electrode core with the positive electrode mixture slurry containing the positive electrode active material, the binder, and the conductive agent, and drying and compressing the coated film. 
     The positive electrode active material contains a lithium-transition metal oxide as the main substance. The positive electrode active material may be made from a lithium-transition metal oxide substantially alone, or a lithium-transition metal oxide with inorganic compound particles, such as the ones containing an oxide aluminum or a lanthanoid attached to particle surfaces of the lithium-transition metal oxide. A single type or two or more types of the lithium-transition metal oxide may be used. 
     The lithium-transition metal oxide may contain the following metal elements: nickel (Ni), cobalt (Co), magnum (Mn), aluminum (Al), boron (B), magnesium (Mg), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), gallium (Ga), strontium (Sr), zirconium (Zr), niobium (Nb), indium (In), tin (Sn), tantalum (Ta), or tungsten (W). A preferable example of the lithium-transition metal oxide is a composite oxide which is represented with a general formula Li a Ni x M (1-x) O 2  (0.1 ≤ α ≤ 1.2, 0.3 ≤ x &lt; 1, where M contains at least one of Co, Mn, and Al). For example, the positive electrode may be made from NCA in which nickel is partially replaced with cobalt, and to which aluminum is added. 
     The conductive agent in the positive electrode mixture layer may be made from, for example, a carbon material such as carbon black, acetylene black, Ketjenblack, carbon nanotube, carbon nanofiber, and graphite. The binder in the positive electrode mixture layer may be made from, for example, a fluororesin such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), a polyimide resin, an acrylic resin, and a polyolefin resin. Together with these resins, for example, a cellulose derivative such as carboxymethyl cellulose (CMC) or a salt thereof, and polyethylene oxide (PEO) may be used in combination. 
     Negative Electrode 
     The negative electrode  12  includes a negative electrode core and a negative electrode mixture layer disposed on a surface of the negative electrode core. The negative electrode core may be made from a foil of metal such as copper that is stable within a potential range of the negative electrode  12 , or a film which includes the metal at the outermost layer. The negative electrode core may have a thickness of, for example, 5 µm to 15 µm. The negative electrode mixture layer includes a negative electrode active material and a binder. The negative electrode mixture layer may be provided on each side of the negative electrode core except for the area to which the negative electrode lead  20  is connected. The negative electrode  12  may be manufactured by forming the negative electrode mixture layer on each side of the negative electrode core by, for example, coating the surfaces of the negative electrode core with a negative electrode mixture slurry containing the negative electrode active material and the binder, and drying and compressing the coated film. A conductive agent may be applied to the negative electrode mixture slurry. The conductive agent enables a uniform distribution of conductive paths. Similarly to the positive electrode mixture layer, the negative electrode mixture may contain the conductive agent, such as acetylene black 
     The negative electrode mixture layer may contain an intermetallic compound (an alloy of M 3 Me 2 X 7  type crystal) represented by a general formula M 3 Me 2 X 7  (where M includes at least one of La and Ca; Me includes at least one of Mn, Ni, Fe, and Co; and X includes at least one of Ge, Si, Sn, and Al). The negative electrode active material may be, for example, La 3 Co 2 Sn 7 , La 3 Mn 2 Sn 7 , or La 3 Ni 2 Sn 7 . Among them, in order to increase capacity, La 3 Co 2 Sn 7  and La 3 Mi 2 Sn 7  are preferable, and La 3 Ni 2 Sn 7  is particularly preferable. 
     The particle diameter of M 3 Me 2 X 7 , which is the negative electrode active material, may be 1 to 30 µm, more preferably, 2 to 20 µm, and most preferably, 2 to 10 µm. When M 3 Me 2 X 7  having too large particle diameters is used, reaction with Li is slowed, and the resistance between particles increases because the contact areas between particles decrease. In contrast, when M 3 Me 2 X 7  having too small particle diameters is used, the filling density of the negative electrode active material is reduced, and thereby the capacity is assumed to decline. The average particle diameter of M 3 Me 2 X 7  may be 3 to 15 µm, or 5 to 10 µm. The particle diameters of M 3 Me 2 X 7  may be obtained by measuring the diameters of circumscribed circles of M 3 Me 2 X 7  particles in a cross sectional view of the negative electrode mixture layer observed with a scanning electron microscope (SEM). The average particle diameter may be calculated by averaging the diameters of 100 arbitral particles. 
     The intermetallic compound represented by M 3 Me 2 X 7  may be obtained by arc melting, and annealing may be performed after arc melting. La for M may be substituted up to about 50%. For example, with about 40% of La substituted by Ca, a large charge/discharge capacity (initial charge capacity of 301 mAh/g, initial discharge capacity of 223 mAh/g (1,718 mAh/cc)) was obtained with a small volume change rate (0.5% or less). 
     The negative electrode active material may contain M 3 Me 2 X 7  as the main substance (the substance having the highest weight ratio), or M 3 Me 2 X 7  substantially alone. Alternatively, the negative electrode active material may contain other active materials, such as intermetallic compounds other than M 3 Me 2 X 7 , carbon-based active materials such as graphite, or Si-based active materials. When used with graphite, the graphite content may be 50 to 90 weight% with respect to the weight of the negative electrode active material. 
     The binder in the negative electrode mixture layer may be made from a compound which contains a cyano group. When a widely used polyvinylidene fluoride (PVDF) is used as the binder with the above described M 3 Me 2 X 7  used as the negative electrode active material, gelation of the negative electrode mixture slurry occurs, making the application of the slurry difficult. However, as the use of the binder containing a cyano group improves dispersibility of the negative electrode active material, the gelation of the slurry is impeded. Because the binder containing a cyano group has a high affinity with M 3 Me 2 X 7 , even a small amount of the binder can sufficiently work. 
     The binder containing a cyano group may be, for example, polyacrylonitrile (PAN), polymethaclonitrile, poly-α-chloroacrylonitrile, and poly-α-etylacrylonitrile. Among them, PAN and polymethaclonitrile are preferable, and PAN is particularly preferable. The binder containing a cyano group may be, for example, synthesized by polymerizing a monomer containing a cyano group of a carbon number 5 or less. Alternatively, the binder may contain a copolymerization component containing no cyano group in a range not impeding the achievement of the purpose of the present disclosure. The binder containing the cyano group may be used alone or in combination of two or more. 
     The weight ratio of the binder containing a cyano group in the negative electrode mixture layer may be 0.5 weight% to 7.0 weight%. When the binder in the amount over 7.0 weight% is applied, the initial charge/discharge efficiency is significantly reduced, while when the binder in the amount below 0.5 weight% is applied, it becomes impossible to ensure the binding force between active material particles and between the active material particles and the core. A preferable binder content may be, for example, 1.0 weight% to 5.0 weights%, or 2.0 weight% to 3.0 weight%. The negative electrode mixture layer may include a binder containing no cyano group in a range not impeding the achievement of the purpose of the present disclosure. 
     Separator 
     The separator  13  may be a porous sheet having ion permeation and insulation properties. The porous sheet may be, for example, a fine porous film, a woven fabric, or a nonwoven fabric. As a material for the separator  13 , an olefin resin such as a polyethylene and polypropylene, and cellulose may be used. The separator  13  may have a single-layered structure or a multilayered structure. A heat-resistant layer containing a heat-resistant material may be formed on a surface of the separator  13 . The heat-resistant material may be, for example, a polyamide resin such as aliphatic polyamide and an aromatic polyamide (aramid), and a polyimide resin such as polyamide-imide and polyimide. 
     EXAMPLES 
     Although the present disclosure is described in more detail below with reference to examples, the present disclosure is not limited to any of these examples. 
     Example 1 
     Manufacturing Negative Electrode 
     La 3 Ni 2 Sn 7  having particle diameters of 2 to 20 µm was used as the negative electrode active material, and polyacrylonitrile (PAN) as the binder, and acetylene black as the conductive agent. A negative electrode mixture slurry was prepared by mixing the negative electrode active material, the binder, and the conductive agent at the weight ratio of 96:3:1, and adding N-methyl-2-pyrrolidone (NMP) as a dispersion medium. A negative electrode was obtained by coating a negative electrode core made from a copper foil with the negative electrode mixture slurry, drying and compressing the coated film, and then cutting to a predetermined size of the electrode. 
     Preparing Test Cell 
     An electrode assembly was obtained by disposing the negative electrode to oppose the positive electrode made from a lithium metal foil via the separator, and inserting the electrode assembly into a coin-shaped can housing. A coin-shaped test cell (non-aqueous electrolyte secondary battery) was obtained by sealing the can housing after injecting a predetermined non-aqueous electrolyte solution into the can housing. 
     Charging/Discharging Test (Capacity Assessment) 
     After CC charging the above prepared test cell with the constant current of 0.15 C to the battery voltage of 4.5 V in a room temperature environment, the test cell was CC discharged with the constant current of 0.15 C down to the battery voltage of 2.5 V. This charging and discharging process was repeated three times, while measuring a charging capacity and a discharging capacity in each cycle. The assessment results are shown in Table 1 along with particle diameters of the negative electrode active material, whether the coating with the slurry was possible or impossible, what was used as the binder, and the applied amount of the binder. 
     Examples 2 to 5 and Comparative Examples 1 to 4 
     The test cells were prepared and the charging and discharging tests were performed in the manner same as in Example 1 except that the applied amount of the binder, what was used as the binder, and the particle diameters of the negative electrode active material were changed as shown in Table 1. In Comparative Example 2, polyimide (PI) is used as the binder in place of PAN, while in Comparative Example 3, polyvinylidene fluoride (PVDF) is used in place of PAN. 
     Results 
     Table 1 shows the results of the charging and discharging tests of Examples 1 to 5 and Comparative Examples 1 to 4.  
     
       
         
          TABLE 1
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 Charging and Discharging Test Results 
               
               
                   
                 Negative Electrode 
                 1st 
                 2nd 
                 3rd 
                 Charge/Discharge Efficiency 
               
               
                 Active Material Particle Diameter 
                 Coating Possible/ Impossible 
                 Binder 
                 Applied Amount 
                 Charge mAh/cc 
                 Discharge mAh/cc 
                 Charge mAh/cc 
                 Discharge mAh/cc 
                 Charge mAh/cc 
                 Discharge mAh/cc 
                 1st Cycle 
                 2nd Cycle 
                 3rd Cycle 
               
             
            
               
                 Example 1 
                 2∼20 µm 
                 Possible 
                 PAN 
                 1 wt% 
                 834 
                 629 
                 637 
                 593 
                 593 
                 592 
                 75% 
                 93% 
                 100% 
               
               
                 Example 2 
                 2∼20 µm 
                 Possible 
                 PAN 
                 2 wt% 
                 840 
                 653 
                 647 
                 608 
                 579 
                 500 
                 78% 
                 94% 
                 86% 
               
               
                 Example 3 
                 2∼20 µm 
                 Possible 
                 PAN 
                 3 wt% 
                 834 
                 654 
                 655 
                 603 
                 455 
                 584 
                 78% 
                 92% 
                 78% 
               
               
                 Example 4 
                 2∼20 µm 
                 Possible 
                 PAN 
                 5 wt% 
                 822 
                 602 
                 614 
                 550 
                 413 
                 546 
                 73% 
                 90% 
                 76% 
               
               
                 Example 5 
                 2∼20 µm 
                 Possible 
                 PAN 
                 7 wt% 
                 767 
                 578 
                 581 
                 460 
                 279 
                 451 
                 75% 
                 79% 
                 62% 
               
               
                 Comparative Example 1 
                 2∼20 µm 
                 Possible 
                 PAN 
                 10 wt% 
                 1316 
                 646 
                 464 
                 242 
                 100 
                 136 
                 49% 
                 52% 
                 74% 
               
               
                 Comparative Example 2 
                 2∼20 µm 
                 Possible 
                 PI 
                 15 wt% 
                 4 
                 0.5 
                 1 
                 0.5 
                 0.9 
                 0.5 
                 13% 
                 50% 
                 56% 
               
               
                 Comparative Example 3 
                 About 10∼200 µm 
                 Possible 
                 PVDF 
                 3 wt% 
                 379 
                 238 
                 255 
                 205 
                 200 
                 144 
                 63% 
                 80% 
                 72% 
               
               
                 Comparative Example 4 
                 2∼20 µm 
                 Impossible 
                 PVDF 
                 3 wt% 
                 - 
                 - 
                 - 
                 - 
                 - 
                 - 
                 - 
                 - 
                 - 
               
            
           
         
       
     
     In Examples 1 to 5, relatively high charge/discharge efficiencies were observed in any of the first to third cycles. In contrast, in Comparative Example 1, the charge/discharge efficiency in the first cycle is significantly lower.  FIG.  2    is a graph showing initial efficiencies obtained in Examples 1 to 5 and Comparative Example 1. As shown in the graph, in Comparative Example 1, the initial efficiency is significantly lower than those in Examples 1 to 5. The reason for this can be assumed that Li was unable to sufficiently move to the active material due to too much binder. 
       FIG.  3    shows a graph about the initial discharge capacities of Examples 1 to 5. As shown in the graph, the discharge capacity of Example 5 is lower than those in Examples 1 to 4. As a result, the amount of binder of 1 weight% to 5 weight% can be assumed to be more preferable than the amount of binder of 1 weight% to 7 weight%. 
     Further, in Comparative Example 4, coating was impossible because the negative electrode mixture slurry gelled due to reaction between La 3 Ni 2 Sn 7  and PVDF. 
     It was found that a preferable non-aqueous electrolyte secondary battery can be obtained with La 3 Ni 2 Sn 7  used as the negative electrode active material, and adding PAN as the binder in the amount of 1 weight% to 7 weight% (more preferably, 1 weight% to 5 weight%). 
     
       
         
           
               
               
             
               
                 REFERENCE SIGNS LIST 
               
             
            
               
                 
                   10 
                 
                 Secondary battery 
               
               
                 
                   11 
                 
                 Positive electrode 
               
               
                 
                   12 
                 
                 Negative electrode 
               
               
                 
                   13 
                 
                 Separator 
               
               
                 
                   14 
                 
                 Electrode assembly 
               
               
                 
                   15 
                 
                 Outer housing body 
               
               
                 
                   16 
                 
                 Sealing assembly 
               
               
                   17 ,  18   
                 Insulating plates 
               
               
                 
                   19 
                 
                 Positive electrode lead 
               
               
                 
                   20 
                 
                 Negative electrode lead 
               
               
                 
                   21 
                 
                 Grooved portion 
               
               
                 
                   22 
                 
                 Filter 
               
               
                 
                   23 
                 
                 Lower vent member 
               
               
                 
                   24 
                 
                 Insulating member 
               
               
                 
                   25 
                 
                 Upper vent member 
               
               
                 
                   26 
                 
                 Cap 
               
               
                 
                   26 
                   a 
                 
                 Opening 
               
               
                 
                   27 
                 
                 Gasket