Patent Publication Number: US-2016233485-A1

Title: Lithium ion secondary cell

Description:
TECHNICAL FIELD 
     The present invention relates to a lithium ion secondary cell. 
     BACKGROUND ART 
     Conventionally, lithium ion secondary cells have been employed in hybrid automobiles, electric automobiles and the like. From such lithium ion secondary cells for automobiles, rapid discharge characteristics are particularly required. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 4219391 
     Patent Literature 2: Japanese Unexamined Patent Publication No. 2009-252705 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, with conventional lithium ion secondary cells, rapid discharge characteristics are not sufficient. 
     The present invention has been made in consideration of the problem described above, and has an object to provide a method for producing a lithium ion secondary cell excellent in rapid discharge characteristics. 
     Solution to Problem 
     The lithium ion secondary cell according to the present invention comprises a negative electrode mixture layer that satisfies the following formula and a positive electrode mixture layer that contains a lithium-containing transition metal oxide. 
       (Initial charge capacity per unit mass of the negative electrode mixture layer)/(Initial discharge capacity per unit mass of the negative electrode mixture layer)≧1.3
 
     Then, the negative electrode mixture layer contains 5 to 45% by mass of at least one negative electrode active material selected from the group consisting of hard carbon, soft carbon, Sn, Sn alloys, Si, Si alloys, SiO x  (0&lt;x&lt;2), Ge, Ge alloys, carbon nanotubes, and carbon nanofibers. 
     Also, it is preferable that the negative electrode mixture layer further contain 105 to 500 parts by mass of graphite based on 100 parts by weight of the negative electrode active material. 
     Also, it is preferable that a negative electrode active material other than the graphite be SiO x  (0&lt;x&lt;2). 
     Advantageous Effects of Invention 
     According to the present invention, a lithium ion secondary cell excellent in rapid discharge characteristics is provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a lithium ion secondary cell according to one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one example of an embodiment according to the present invention will be described with referring to the drawing.  FIG. 1  is a schematic cross-sectional view of a lithium ion secondary cell  100  according to the present embodiment. As shown in  FIG. 1 , the lithium ion secondary cell  100  mainly comprises a positive electrode  10 , a separator  20 , a negative electrode  30  and a case  70 , and a liquid electrolyte. 
     (Positive Electrode) 
     The positive electrode  10  includes a positive electrode collector  12 , and a positive electrode mixture layer  14  provided on the positive electrode collector  12 . The positive electrode mixture layer  14  may be provided on only one side of the positive electrode collector  12 , as shown in  FIG. 1 , or may be provided on each side of the positive electrode collector  12 . 
     The positive electrode collector  12  is made of an electrically conductive material. Examples of the material of the positive electrode collector  12  include metal materials such as stainless steel, titanium, nickel, and aluminum or electrically conductive resins. Particularly, as the material of the positive electrode collector  12 , aluminum is suitable. The thickness of the positive electrode collector  12  is not particularly limited, and for example, can be a foil form (15 to 20 μm). 
     The positive electrode mixture layer  14  contains a positive electrode active material and a binder. The positive electrode active material is a lithium-containing transition metal oxide. Examples of the lithium-containing transition metal oxide include mixed oxides containing at least one element selected from the group consisting of Ni, Mn, and Co; and Li. Examples of the mixed oxide include a lithium-cobalt mixed oxide LiCoO 2 , a lithium-nickel mixed oxide LiNiO 2 , lithium-manganese mixed oxides LiMnO 2  and LiMn 2 O 4 , lithium-nickel-cobalt mixed oxides LiNi a Co b O 2  (a+b=1, 0&lt;a&lt;1, 0&lt;b&lt;1), lithium-manganese-cobalt mixed oxides LiMn a Co b O 2  (a+b=1, 0&lt;a&lt;1, 0&lt;b&lt;1), and lithium-cobalt-nickel-manganese mixed oxides LiCo p Ni q Mn r O 2  (p+q+r=1, 0&lt;p&lt;1, 0&lt;q&lt;1, 0&lt;r&lt;1). Examples of the mixed oxide include LiCo 1/3 Ni 1/3 Mn 1/3 O 2 , LiNi 0.6 Co 0.2 Mn 0.2 O 2 , LiNi 0.5 CO 0.2 Mn 0.3 O 2 , LiCoO 2 , LiNi 0.8 Co 0.2 O 2 , and LiCoMnO 2 . 
     Alternatively, the positive electrode active material can also be a lithium-containing transition metal oxide represented by the formula: LiCo p Ni q Mn r D s O 2  (p+q+r+s=1, 0&lt;p≦1, 0≦q&lt;1, 0≦r&lt;1, 0&lt;s&lt;1). D is at least one element selected from the group consisting of Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, and Na. 
     Alternatively, as the positive electrode active material, it is possible to use oxide solid solutions that contain any of Li 2 MnO 3 , LiFePO 4 , LiMnPO 4 , Li 2 FeP 2 O 7 , Li 2 FeSiO 4 , Li 2 MnSiO 4 , LiNi 0.5 Mn 1.5 O 4 , and the aforementioned oxides. 
     A binder is a resin that is blended to fix the active material to the collector. Examples of the binder include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluorine rubber; thermoplastic resins such as polypropylenes and polyethylenes; imide-based resins such as polyimides and polyamideimides; and alkoxysilyl group-containing resins. The amount of the binder can be 1 to 30 parts by mass based on 100 parts by mass of the active material. 
     The positive electrode mixture layer  14  can further contain a conductive assistant as required. Examples of the conductive assistant include carbon-based particles such as carbon black, graphite, acetylene black (AB), Ketjenblack (registered trademark) (KB), and vapor grown carbon fibers (VGCFs). These can be added singly, or two or more of these can be added in combination. The amount of the conductive assistant used is not particularly limited, and for example, can be 1 to 30 parts by mass based on 100 parts by mass of the active material. 
     The positive electrode collector  12  has a tab portion  12   t  on an end of which the positive electrode mixture layer  14  is not formed. To the tab portion  12   t , a lead  16  described below is electrically connected. 
     (Negative Electrode) 
     The negative electrode  30  comprises a negative electrode collector  32 , and a negative electrode mixture layer  34  provided on the negative electrode collector  32 . The negative electrode collector  32  is made of an electrically conductive material. As the material of the negative electrode collector  32 , metals that are not alloyed with lithium can be used, and particularly, copper is preferable. The negative electrode collector  32  can be a foil form as the positive electrode collector  12 . 
     The negative electrode mixture layer  34  contains a negative electrode active material and a binder. The negative electrode mixture layer  34  may contain a conductive assistant as required. Examples of the binder and the conductive assistant can be similar to those described in the positive electrode  10 . The amount of the binder can be 1 to 30 parts by mass based on 100 parts by mass of the negative electrode active material. The amount of the conductive assistant can be 1 to 30 parts by mass based on 100 parts by mass of negative electrode active material. 
     In the present embodiment, the negative electrode mixture layer  34  contains 5 to 45% by mass of at least one negative electrode active material selected from the group consisting of hardly-graphitizable carbon (hard carbon), easily-graphitizable carbon (soft carbon), Sn, Sn alloys, Si, Si alloys, SiO x  (0&lt;x&lt;2), Ge, Ge alloys, carbon nanotubes, and carbon nanofibers. A combination of a plurality among these negative electrode active materials also can be used. The initial charge capacity/initial discharge capacity of these negative electrode active materials can be 1.3 or more. 
     The hardly-graphitizable carbon (hard carbon) is the generic name of carbons that form a crystalline structure in which the average surface interval d 002  of the surface (002) exceeds 3.40 Å when thermally treated at 2500° C. in an inert atmosphere. The hard carbon can be obtained by calcining, for example, a thermosetting resin such as a phenolic resin, a melamine resin, a urea resin, a furane resin, an epoxy resin, an alkyd resin, an unsaturated polyester resin, a diallyl phthalate resin, a furfural resin, a resorcinol resin, a silicone resin, a xylene resin, and a urethane resin, and hardly-graphitizable coke. 
     The easily-graphitizable carbon (soft carbon) is a generic name of carbons that form a crystalline structure in which the average surface interval d 002  of the surface (002) is 3.40 Å or less, preferably from 3.35 to 3.40 Å, when thermally treated at 2000 to 3000° C. in an inert atmosphere. The soft carbon is a carbon material obtained by calcining a polymer from which a graphite crystalline structure is likely to develop by a high temperature treatment, for example, a curable resin, a thermoplastic resin, petroleum-based or coal-based tar or pitch, and furthermore, a compound prepared by crosslinking the tar, pitch or the like. The soft carbon can be obtained by calcining, for example, pitch such as petroleum-based pitch, coal-based pitch, and mesophase-based pitch; and easily-graphitizable coke such as petroleum-based needle coke, coal-based needle coke, anthracene, polyvinyl chloride, and polyacrylonitrile. 
     Examples of the Sn alloy include Sn—Ni alloys, Sn—Zn alloys, P—Sn alloys, Sn—Cu alloys, and Sn—Ag alloys. 
     Examples of the Si alloy include Si—Cu alloys, Si—Co alloys, and Si—Cr alloys. 
     SiO x  is a silicon oxide represented by the composition SiO x  (0&lt;x&lt;2). If x is less than 0.5, volume changes on charging and discharging become too large because the ratio of the Si phase becomes higher, and the cycling characteristics tend to hardly increase. Alternatively, if x exceeds 1.5, the ratio of the Si phase decreases, and there may be a case where the energy density decreases. Accordingly, it is preferable that x be 0.5 to 1.5, and it is more preferable that x be 0.7 to 1.2. 
     Examples of the Ge alloy include Si—Ge alloys, Si—Ge—Ti alloys, and Ge—Cr alloys. 
     The carbon nanotube is tubular carbon which is formed from a monolayer or multilayer graphene sheet and of which diameter is about 100 nm or less. 
     The carbon nanofiber is fibrous carbon fiber which is formed by laminating graphene sheet and of which diameter is about 100 nm or less. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Initial 
                   
                 Initial 
               
               
                   
                   
                 discharge 
                   
                 charge 
               
               
                 Negative 
                 Initial charge 
                 capacity of 
                 Irreversible 
                 capacity/ 
               
               
                 electrode 
                 capacity of the 
                 the active 
                 capacity of the 
                 Initial 
               
               
                 active 
                 active material 
                 material 
                 active material 
                 discharge 
               
               
                 material 
                 (mAh/g) 
                 (mAh/g) 
                 (mAh/g) 
                 capacity 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Graphite 
                 370 
                 330 
                 40 
                 1.1 
               
               
                 Hard carbon 
                 500 
                 400 
                 100 
                 1.3 
               
               
                 Soft carbon 
                 400 
                 300 
                 70 
                 1.3 
               
               
                 Sn 
                 950 
                 600 
                 350 
                 1.6 
               
               
                 Si 
                 4000 
                 2500 
                 1500 
                 1.6 
               
               
                 SiO x   
                 2500 
                 1500 
                 1000 
                 1.7 
               
               
                 Ge 
                 1500 
                 900 
                 600 
                 1.7 
               
               
                   
               
            
           
         
       
     
     In Table 1, the approximate initial charge capacity (per unit mass), initial discharge capacity (per unit mass), irreversible capacity, and (initial charge capacity/initial discharge capacity) of each material are shown. It should be noted that the initial charge capacity and initial discharge capacity can be measured for each negative electrode active material using metal lithium as a counter electrode. In a cell in which the positive electrode that contains the lithium-containing transition metal oxide or the like is used as the counter electrode, the negative electrode containing a negative electrode active material stores lithium ions on charging and releases lithium ions on discharging. However, in the cell in which metal lithium is used as the counter electrode, since the standard electrode potential becomes lower in metal lithium than in the negative electrode, the negative electrode stores lithium ions on discharging and releases lithium ions on charging. Accordingly, in the case where metal lithium is used as the counter electrode, the capacity on the initial discharging (on storing lithium ions in the negative electrode) will be the initial charge capacity of the negative electrode, and the capacity on the initial charging (on releasing lithium ions from the negative electrode) will be the initial discharge capacity of the negative electrode. 
     The particle size of the negative electrode active material is not particularly limited, but the average particle size D50 can be 10 μm or less. The average particle size D50 may be 1 nm or more. The average particle size D50 is a median diameter and can be obtained based on a volume-basis particle size distribution by the laser diffraction method. 
     The negative electrode mixture layer can contain graphite in addition to the negative electrode active material. The graphite is a carbon material having a graphite structure and functions as a negative electrode active material. By adding graphite, (Initial charge capacity per unit mass of the negative electrode mixture layer)/(Initial discharge capacity per unit mass of the negative electrode mixture layer) described below can be adjusted. The average particle size D50 of the graphite particles is not particularly limited, but is preferably from 1 to 50 μm. The graphite may be artificial graphite or natural graphite. The amount of the graphite is not particularly limited, but, in the negative electrode mixture layer, it is preferable to contain from 105 to 500 parts by mass, it is more preferable to contain from 110 to 450 parts by mass, based on 100 parts by weight of the negative electrode active material. 
     In the present embodiment, the negative electrode mixture layer  34  satisfies the following formula. 
       (Initial charge capacity per unit mass of the negative electrode mixture layer)/(Initial discharge capacity per unit mass of the negative electrode mixture layer)≧1.3
 
     The initial charge capacity per unit mass of the negative electrode mixture layer and the initial discharge capacity per unit mass of the negative electrode mixture layer can be determined by charging and discharging the negative electrode before charging using the lithium counter electrode. Alternatively, by calculating the total initial discharge capacity and the total initial charge capacity of the total active materials including graphite by use of Table 1 and dividing the capacities by the mass of the total negative electrode active materials including the conductive assistant and the binder, it is possible to determine (Initial charge capacity per unit mass of the negative electrode mixture layer)/(Initial discharge capacity per unit mass of the negative electrode mixture layer). 
     By using the negative electrode mixture layer  34  satisfying Initial charge capacity/Initial discharge capacity≧1.3, it is possible to provide a lithium ion secondary cell excellent in rapid discharge characteristics. Meanwhile, when the proportion of negative electrode active materials other than the graphite in the negative electrode mixture layer becomes too large, the discharge capacity of the lithium ion secondary cell tends to decrease, and the proportion of negative electrode active materials other than graphite in the negative electrode mixture layer is from 5 to 45% by mass, and preferably 10 to 40% by weight. 
     The negative electrode collector  32  has a tab portion  32   t  on an end of which the negative electrode mixture layer  34  is not formed. To the tab portion  32   t , a lead  36  described below is electrically connected. It is also possible for the negative electrode to contain a negative electrode active material other than the above. 
     (Separator) 
     The separator  20  separates the positive electrode  10  and the negative electrode  30  to prevent short-circuit of current by contact of both electrodes while allowing lithium ions to pass. As the separator  20 , for example, porous films made of a synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene or porous films made of ceramic can be used. Alternatively, non-woven fabrics made of polyethylene terephthalate, polyvinyl alcohol, polyacrylonitrile, and cellulose can be used. 
     (Liquid Electrolyte) 
     A liquid electrolyte contains an electrolyte and a solvent that dissolves the electrolyte. The positive electrode mixture layer  14 , the separator  20 , and the negative electrode mixture layer  34  are impregnated internally with the electrolyte. 
     As examples of the electrolyte, salts generally used in lithium ion cells can be used. The examples include lithium salts such as LiBF 4 , LiPF 6 , LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , and LiN(CF 3 SO 2 ) 2 . These lithium salts may be used singly, or two or more of the lithium salts may be used in combination. 
     Examples of the solvent include cyclic esters, chain esters, and ethers. Two or more of these solvents can be mixed. Examples of the cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma-butyrolactone, vinylene carbonate, 2-methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone, and gamma-valerolactone. Example of the chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, ethylmethyl carbonate, alkyl propionate esters, dialkyl malonate esters, and alkyl acetate esters. Examples of the ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. 
     The concentration of the electrolyte in the liquid electrolyte can be 0.5 to 1.7 mol/L, for example. The liquid electrolyte may contain a gelling agent. 
     (Case) 
     A case  70  accommodates the positive electrode  10 , the separator  20 , the negative electrode  30 , and the liquid electrolyte. The material and form of the case are not particularly limited, and known various substances such as resins and metals can be used. 
     To the tab portion  12   t  of the positive electrode collector  12  and the tab portion  32   t  of the negative electrode collector  32 , leads  16  and  36  are connected, respectively. One end of each of the leads  16  and  36  is external to the case  70 . 
     (Function and Effect) 
     The lithium ion secondary cell according to the present embodiment is excellent in rapid discharge characteristics from the state of charge (SOC) of 50% or less. The reason why such characteristics can be obtained is unclear, but it is believed as follows: in the cell according to the present embodiment, the irreversible capacity is large; and compared with conventional lithium ion secondary cells of which irreversible capacity is small, the amount of lithium ions that have returned into the positive electrode is small even in the region with the SOC of 50% or less, which results in a small positive electrode resistance. 
     EXAMPLES 
     Example 1 
     SiO x  (average particle size D50=5 μm, x=1.0):natural graphite (average particle size D50=20 μm):acetylene black:polyimide were mixed in a proportion of 32:50:8:10 and diluted with NMP to prepare paste. This paste was applied on Cu foil of 20 μm in thickness. After drying, the mixture layer was pressed to about 1.1 g/cm 3  (except the Al foil) and thermally treated at 200° C. for 2 hours to prepare a negative electrode. 
     Li(Ni 0.5 Co 0.2 Mn 0.3 )O 2  (average particle size D50=10 μm):acetylene black:PVdF were mixed in a proportion of 94:3:3 (wt %) and diluted with NMP to prepare paste. This paste was applied on 20 μm Al foil. After drying, the mixture layer was pressed to approximately 3.0 g/cm 3  (except the Al foil) and thermally treated at 120° C. for 6 hours to prepare a positive electrode. The coating weight of the positive electrode was made to be 6.0 mg/cm 2 , and the coating weight of the negative electrode was made to be 1.0 mg/cm 2 . A pair of the positive electrode and the negative electrode obtained was used to prepare a cell of which opposing area was 7.5 cm 2 . 
     The liquid electrolyte contains a solvent of a volume ratio of EC:DEC=3:7 and 1M LiPF 6 . The separator was a monolayer polyethylene porous film of 20 μm in thickness. The cell prepared was CCCV charged at 0.8 C to 4.2 V for two hours, degassed, and subjected to 30 cycles of CC charging and discharging at 1 C from 3.0 to 4.2 V to obtain a cell to be evaluated. Based on the initial charging and discharging, Initial charge capacity/Initial discharge capacity of the negative electrode mixture layer was determined. Additionally, the charging and discharging operation and the output evaluation described below were both performed at 25° C. 
     Example 2 
     A negative electrode was prepared as described in Example 1 except that SiO x  (average particle size D50=5 μm, x=1.0):natural graphite (average particle size D50=20 μm):acetylene black:polyimide were mixed in a proportion of 16:66:8:10 and that the coating weight of the negative electrode was 1.5 mg/cm 2 . 
     Comparative Example 1 
     A negative electrode was prepared as in Examples except that natural graphite:acetylene black:polyimide=82:8:10 (wt %) and that the coating weight of the negative electrode was 3.1 mg/cm 2 . 
     (Evaluation) 
     The open circuit potential (OCV) was measured by charging and discharging each cell in the range of 2.5 to 4.2 V to determine the potential at which the SOC reached 50%. Then, discharging was performed once at 1 C to 2.5 V, and subsequently, CCCV charging was performed at 1 C for two hours to the potential at which the SOC reached 50%. From the potential at which the SOC reached 50%, discharging was performed plural times with the output varied under constant output discharging to 2.5 V. From the relationship between the discharged electric power value then and the time, the 10-second output, that is, the electric power that can be outputted in 10 seconds was determined. The results are shown in Table 2. 
     The cells of Examples were larger in the 10-second output than the cell of Comparative Example. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Ex- 
                 Ex- 
                 Comparative 
               
               
                   
                 ample 1 
                 ample 2 
                 Example 1 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Negative 
                 Graphite (% by mass) 
                 50 
                 66 
                 82 
               
               
                 electrode 
                 SiO x  (% by mass) 
                 32 
                 16 
                 0 
               
               
                 active 
                 AB (% by mass) 
                 8 
                 8 
                 8 
               
               
                 material 
                 Polyimide (% by mass) 
                 10 
                 10 
                 10 
               
               
                 layer 
               
            
           
           
               
               
               
               
            
               
                 Parts by mass of graphite based on 100 
                 156.3 
                 412.5 
                 — 
               
               
                 parts by mass of SiO x   
               
               
                 Coating weight of the negative 
                 1.0 
                 1.5 
                 3.1 
               
               
                 electrode active material layer 
               
               
                 (mg/cm 2 ) 
               
               
                 Initial charge capacity per unit mass of 
                 1006 
                 671 
                 337 
               
               
                 the negative electrode active material 
               
               
                 layer (mAh/g) 
               
               
                 Initial discharge capacity per unit mass 
                 665 
                 484 
                 303 
               
               
                 of the negative electrode active material 
               
               
                 layer (mAh/g) 
               
               
                 Irreversible capacity per unit mass of 
                 341 
                 187 
                 34 
               
               
                 the negative electrode active material 
               
               
                 layer (mAh/g) 
               
               
                 (Initial charge capacity per unit mass of 
                 1.51 
                 1.39 
                 1.11 
               
               
                 the negative electrode active material 
               
               
                 layer)/(Initial discharge capacity per 
               
               
                 unit mass of the negative electrode 
               
               
                 active material layer) [—] 
               
               
                 Cell capacity (mAh) 
                 4.4 
                 4.9 
                 6.2 
               
               
                 SOC 50% potential (V) 
                 3.58 
                 3.63 
                 3.67 
               
               
                 10-second output (mW) 
                 497 
                 465 
                 446 
               
               
                   
               
            
           
         
       
     
     REFERENCE SIGNS LIST 
     
         
           10  positive electrode 
           30  negative electrode 
           12  positive electrode collector 
           32  negative electrode collector 
           12   t ,  32   t  tab portion 
           14  positive electrode mixture layer 
           34  negative electrode mixture layer 
           20  separator 
           16 ,  36  lead 
           70  case 
           100  lithium ion secondary cell