Patent Publication Number: US-2015086707-A1

Title: Method for manufacturing a battery

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a battery or cell provided with an electrode sheet having an active material layer on which a separator layer is integrally formed. 
     BACKGROUND ART 
     Conventionally, there has been known a battery including an electrode sheet (a positive electrode sheet or a negative electrode sheet) having an active material layer (a positive active material layer or a negative active material layer) on which a separator layer is integrally formed. The separator layer is constituted of for example resin particles and a binding agent interposed between the resin particles to bond the particles. This separator layer is formed in such a manner that a dispersion liquid containing resin particles, a binding agent, and others in a dispersed state is applied to an active material layer to form an undried separator layer, which is then dried by heating. In this separator layer, the binding agent also coats the surfaces of active material particles (positive active material particles or negative active material particles) constituting the active material layer. As a conventional art of the battery including an electrode sheet having an active material layer integrally formed thereon with a separator layer, Patent Document 1 is provided. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: JP-A-2000-149906 
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     However, many of binding agents do not have ion conductivity (e.g., ion conductivity of lithium ions in a lithium ion secondary battery). Thus, a portion of each active material particle, coated with a binding agent, is less likely to cause battery reaction. Thus, a reaction area of each active material particle is decreased and the internal resistance of the battery is increased. On the other hand, to avoid this problem, it is conceivable to eliminate the binding agent contained in a separator layer or reduce the content of the binding agent. However, this results in weakened binding strength between the resin particles and causes the resin particles forming the separator layer to likely drop off at the time of manufacturing or using the battery. 
     The present invention has been made in view of the circumstances and has a purpose to provide a method for manufacturing a battery to strongly bond resin particles to each other, whereas to eliminate or reduce binding agent to reduce internal resistance of the battery. 
     Means of Solving the Problems 
     To achieve the above purpose, one aspect of the invention provides a method for manufacturing a battery provided with an electrode sheet including: an active material layer made of active material particles; and a separator layer integrally formed on the active material layer and made of thermoplastic resin particles, wherein the method includes: a coating step of applying a dispersion liquid in which the resin particles are dispersed to the active material layer to form an undried separator layer; and a heat-drying step of drying the undried separator layer by heating to form the separator layer, wherein the heat-drying step includes drying the undried separator layer by heating at a temperature within a surface melting temperature range in which each of the resin particles does not melt at a center portion but melts at a surface portion. 
     According to this battery manufacturing method, it is possible to manufacture a battery in which resin particles are strongly bonded to each other, whereas a binding agent is eliminated or reduced, so that the battery has reduced internal resistance. 
     In the aforementioned method for manufacturing a battery, further preferably, the resin particles have a thermal property that offers, in differential scanning calorimetry, a peak of total melting heat appearing when each resin particle wholly melts and a peak of surface melting heat appearing when each resin particle does not melt at the center portion but melts at the surface portion, and the surface melting temperature range is a range of ±10.0° C. with respect to the peak temperature of the surface melting heat. 
     In the aforementioned method for manufacturing a battery, further preferably, the temperature at which the undried separator layer is to be dried by heating in the heat-drying step is a temperature in a range of ±7.0° C. with respect to the peak temperature. 
     In the aforementioned method for manufacturing a battery, further preferably, the dispersion liquid is a non-binder dispersion liquid containing no binding agent to be interposed between the resin particles to bond the resin particles. 
     In the aforementioned method for manufacturing a battery, further preferably, the resin particles are polyethylene particles. 
     In the aforementioned method for manufacturing a battery, further preferably, the resin particles have an average particle diameter of 1.0 to 5.0 μm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a lithium ion secondary battery in an embodiment; 
         FIG. 2  is a longitudinal cross sectional view of the lithium ion secondary battery in the embodiment; 
         FIG. 3  is an exploded view of an electrode body, showing a state where a positive electrode sheet and a negative electrode sheet are overlapped one on the other in the embodiment; 
         FIG. 4  is a perspective view of the negative electrode sheet in the embodiment; 
         FIG. 5  is a magnified sectional photograph of a separator layer in the embodiment; 
         FIG. 6  is a partial magnified sectional view of the separator layer, drawn by an outline of the photograph of  FIG. 5  in the embodiment; 
         FIG. 7  is a DSC curve of resin particles in the embodiment; 
         FIG. 8  is an explanatory view showing a process of manufacturing the negative electrode sheet in the embodiment; 
         FIG. 9  is an explanatory view showing how to form the separator layer on a negative active material layer in the embodiment; 
         FIG. 10  is a magnified sectional photograph of a separator layer in a comparative example 4; and 
         FIG. 11  is a partial magnified sectional view of the separator layer, drawn by an outline of the photograph of  FIG. 10  in the comparative example 4. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A detailed description of a preferred embodiment of the present invention will now be given referring to the accompanying drawings.  FIGS. 1 and 2  show a lithium ion secondary battery  100  (hereinafter, also simply referred to as a battery  100 ).  FIG. 3  shows an exploded state of a wound electrode body  120  of the battery  100 .  FIG. 4  shows a negative electrode sheet  131 . The following explanation is given on the assumption that a thickness direction BH, a width direction CH, and a height direction DH of the battery  100  are defined as shown in  FIGS. 1 and 2 . Further, in the following explanation, a top side of each of  FIGS. 1 and 2  is regarded as an upper side of the battery  100  and a bottom side of each figure is regarded as a lower side of the battery  100 . 
     This battery  100  is a rectangular sealed battery to be mounted in a vehicle such as a hybrid vehicle and an electric vehicle or in a battery using device such as a hammer drill. This battery  100  includes a rectangular parallelepiped battery case  110 , the wound electrode body  120  of a flattened shape accommodated in this battery case  110 , a positive terminal  150  and a negative terminal  160  each supported in the battery case  110 , and others (see  FIGS. 1 and 2 ). In the battery case  110 , a non-aqueous electrolyte  117  is also stored. 
     The battery case  110  is made of metal (concretely, aluminum). This battery case  110  is constituted of a box-shaped case body member  111  opening only at an upper side, and a case lid member  113  welded to the case body member  11  to close an opening  111   h  thereof (see  FIGS. 1 and 2 ). The case lid member  113  is provided, near a center in a longitudinal direction (the width direction CH of the battery  100 ), with a non-return safety valve  113   v . In the vicinity of this safety valve  113 , a liquid inlet  113   h  is provided for injection of the electrolyte  117  into the battery case  110 . This liquid inlet  113   h  is hermetically sealed with a seal member  115 . 
     Furthermore, in the case lid member  113 , near both ends in the longitudinal direction (the width direction CH of the battery  100 ), the positive terminal (a positive terminal member)  150  and the negative terminal (a negative terminal member)  160  are separately fixed to extend out of the battery case  110 . To be concrete, those terminals  150  and  160  are fixed to the case lid member  113  respectively with bolts  153  and  163  for fastening external connection terminals such as a bus bar or a crimp-type terminal to the terminals  150  and  160 , and further through insulating members  155  and  165  made of resin. 
     The electrode body  120  will be explained below (see  FIGS. 2 and 3 ). This electrode body  120  is accommodated sideways in the battery case  110  so that an axis of the electrode body  120  is parallel to the width direction CH (see  FIG. 2 ). This electrode body  120  is formed from a strip-shaped positive electrode sheet  121  and a strip-shaped negative electrode sheet  131  that are overlapped each other (see  FIG. 3 ), wound about an axis, and compressed into a flattened shape. Part of an exposed portion  122   m  mentioned later of the positive electrode sheet  121  protrudes in a spiral form on one side AC in the axial direction (an upper side in  FIG. 3  and a left side in  FIG. 2 ), and is connected (welded) to the aforementioned positive terminal (the positive terminal member)  150 . An exposed portion  132   m  mentioned later of the negative electrode sheet  131  protrudes in a spiral form on the other side AD in the axial direction (a lower side in  FIG. 3  and a right side in  FIG. 2 ), and is connected (welded) to the aforementioned negative terminal (the negative terminal member)  160 . 
     The positive electrode sheet  121  includes, as a core member, a strip-shaped positive electrode foil  122  made of aluminum. Part of the positive electrode foil  122  in the width direction (the upper side in  FIG. 3 ) forms the exposed portion  122   m  extending in a strip shape in the longitudinal direction (right-left direction in  FIG. 3 ). On the other hand, a remaining part (the lower side in  FIG. 3 ) other than the exposed portion  122   m  is formed, on both main surfaces, with positive active material layers (positive mixture layers)  123  each extending in a strip shape in the longitudinal direction. These positive active material layers  123  are made of positive active material particles, conducting material, and binding agent. The present embodiment uses lithium-cobalt-nickel-manganese composite oxide (concretely, LiCo 1/3 Ni 1/3 Mn 1/3 O 2 ) particles as the positive active material particles and carbon black (concretely, acetylene black) as the conducting material, and polyvinylidene fluoride (PVDF) as the binding agent. 
     The negative electrode sheet  131  (see  FIGS. 3 and 4 ) includes, as a core member, a strip-shaped negative electrode foil  132  made of copper. Part of the negative electrode foil  132  in the width direction FH (the lower side in  FIG. 3 ) forms the exposed portion  132   m  extending in a strip shape in the longitudinal direction EH (the right-left direction in  FIG. 3 ). On the other hand, a remaining part (the upper side in  FIG. 3 ) other than the exposed portion  132   m  is formed, on both main surfaces, with negative active material layers (negative mixture layers, active material layers)  133  each extending in a strip shape in the longitudinal direction EH. On these negative active material layers  133 , the separator layers  141  each extending in a strip shape in the longitudinal direction EH are respectively integrally formed. The negative active material layers  133  are made of negative active material particles  135 , a binding agent, and a thickening agent. The present embodiment uses graphite particles as the negative active material particles  135 , styrene-butadiene rubber (SBR) as the binding agent, and carboxymethyl cellulose (CMC) as the thickening agent. 
     Each separator layer  141  is a porous film having a thickness of 30 μm and made of thermoplastic resin particles  143  and a thickening agent  144 , without containing a binding agent. Specifically, as shown in a photograph of  FIG. 5  and  FIG. 6 , the adjacent resin particles  143  melt and three-dimensionally bonded to each other through respective surface portions  143   h , and gaps (voids) KG are formed to three-dimensionally continuously extend among the resin particles  143 . A porosity Ca of the separator layer  141  is 35.4%. The present embodiment uses polyethylene (PE) particles as the thermoplastic resin particles  143  and carboxymethyl cellulose (CMC) as the thickening agent  144 . A ratio of the resin particles  143  to the thickening agent  144  is 99.7:0.3 at weight ratio. The thickening agent (CMC)  144  is considered to stick to the resin particles  143 . 
     The porosity Ca (%) of the separator layer  141  is measured by the following method. Specifically, coating weight per unit area (g/cm 2 ) and the thickness of the separator layer  141  are actually measured, and density ρ1 (g/cm 3 ) of the separator layer  141  is calculated. Then, the porosity Ca (%) is calculated by the following formula from this density ρ1 and real density ρ2 (at a porosity of 0%) as the material of the resin particles  143 . 
         Ca={ 1−(ρ1/ρ2)}×100
 
     A separator layer generally contains a binding agent selected for example from a fluorine resin binding agent such as polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and polytetrafluoroethylene (PTFE), and a rubber binding agent such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), and acrylic rubber (ACM). However, the separator layer  141  in the present embodiment does not contain any of those binding agents. 
     Next, the details of the resin particles  143  will be explained. The resin particles  143  are spherical particles made of PE and have an average particle diameter of 2.5 μm. As this average particle diameter, a value of D50 determined by a laser diffraction/scattering particle size distribution measuring method is used. For measuring the average particle diameter, Microtrac by Nikkiso Co., Ltd. is used. 
     The resin particles  143  have a thermal property that offers two endothermic peaks P 1  and P 2  as seen in curves of differential scanning calorimetry (DSC) shown in  FIG. 7 . Specifically, in DSC, there appear the second peak P 2  of total melting heat derived when each whole resin particle  143  melts and also the first peak P 1  of surface melting heat derived when each resin particle  143  does not melt at a center or core portion  143   g  but melts at the surface portion  143   h , the first peak P 1  appearing in a temperature region lower than the second peak P 2 . In the resin particles  143  in the present embodiment, a peak temperature Tb of the first peak P 1  is 62.7° C. and a peak temperature Tc of the second peak P 2  is 101.2° C. 
     A DSC measuring device used herein is Q100 by TA Instruments. For data processing, TRC-THADAP-DSC by Toray Research Center, Inc. was used. A measurement atmosphere was nitrogen fluid flow (50 ml/min), a temperature-heat amount calibration was high-purity indium (Tm=156.61° C., ΔHm=28.71 J/g), a temperature range was about −50 to 250° C., a temperature increasing speed was 10° C./min, an amount of sample was about 5 mg, and a sample vessel was an aluminum standard sample vessel. A region hatched in  FIG. 7  was obtained by curve fitting on the assumption of a distribution with two V-shaped protrusions in the DSC measurement result. 
     From those results, the following states are conceived. In every resin particle  143 , the center portion  143   g  and the surface portion  143   h  does not melt at a temperature less than 40.0° C. At a temperature from 40.0 to 52.7° C., only a small amount of the resin particles  143  fuse or melt at their surface portions  143   h . On the other hand, at a temperature from 52.7 to 72.7° C., it is expected that much resin particles  143  melt at their surface portions  143   h , but do not melt at their center portions  143   g . Therefore, the temperature range from 52.7 to 72.7° C. is regarded as a “surface melting temperature range TD”. This temperature range TD corresponds to Tb ±10.0° C. with respect to the aforementioned peak temperature Tb (concretely, 62.7° C.). At a temperature exceeding 72.7° C., each resin particle  143  wholly melts. 
     A method for manufacturing the battery  100  mentioned above will be explained. Manufacturing of the negative electrode sheet  131  is first explained (see  FIG. 8 ). Specifically, the negative active material particles  135 , the binding agent, and the thickening agent are dispersed in a solvent to prepare negative electrode paste. To be concrete, natural graphite (negative active material), SBR (binding agent), and CMC (thickening agent) are mixed and adjusted in viscosity with a solvent (e.g., water) to obtain slurry (negative electrode paste). Separately, the strip-shaped negative electrode foil  132  made of copper is prepared. 
     In a coating step S 1  of negative active material layer, on one of main surfaces of the negative electrode foil  132 , the aforementioned negative electrode paste is applied by a die coating method to form a strip-shaped negative electrode paste layer. Thereafter, in a heat-drying step S 2  of negative active material layer, the negative electrode paste layer is dried by heating using hot air when passing through a drying furnace, thereby forming a negative active material layer  133 . Similarly, on the other main surface of the negative electrode foil  132 , the negative electrode paste is applied to form a negative electrode paste layer (the coating step S 1  of negative active material layer), dried by heating to form a negative active material layer  133  (the heat-drying step  2  of negative active material layer). In a press step S 3 , thereafter, the negative active material layers  133  are pressed by pressure rolls, so that the negative active material layers  133  are compressed in their thickness direction to increase the density (into a predetermined thickness). 
     A dispersion liquid BS for forming a separator layer  141  (see  FIG. 9 ) is separately prepared. Specifically, the thermoplastic resin particles  143  and the thickening agent  144  are dispersed in the solvent, thus producing the dispersion liquid BS. To be concrete, the dispersion liquid BS is obtained by mixing PE particles (resin particles)  143  and CMC (thickening agent)  144  at a weight ratio of 99.7:0.3 and dispersing this mixture in a solvent (e.g., water). 
     In a coating step S 4  of separator layer, the dispersion liquid BS is applied onto one of the negative active material layers  133  to form an undried separator layer  141   p  (see  FIG. 9 ). In the present embodiment, a gravure coating device  200  is used to apply the dispersion liquid BS onto the negative active material layer  133 . This gravure coating device  200  includes a gravure roll  201 , a dispersion liquid bath or vessel  203 , and a doctor blade  205 . In this coating step S 4 , the gravure roll  201  is rotated at a predetermined speed to sequentially transfer the dispersion liquid BS stored in the bath  203  to a surface  201   h  of the gravure roll  201 . On the other hand, the dispersion liquid BS excessively transferred to the surface  201   h  of the roll  201  is scraped by the doctor blade  205 . Thus, an appropriate amount of dispersion liquid BS remaining on the surface  201   h  of the roll  201  is sequentially applied to the negative active material layer  133  on the one main surface of the negative electrode foil  132  unreeled from a wind-off roll  211 , thereby forming the undried separator layer  141   p.    
     Successively, in a heat-drying step S 5  of separator layer (see  FIG. 8 ), the undried separator layer  141   p  is dried by heating to form the separator layer  141  (see  FIG. 9 ). In this heat-drying step S 5 , the negative electrode foil  132  on which the undried separator layer  141   p  and the negative active material layer  133  are formed is fed forward by rotation of feed rollers  213 ,  215 , and  217  to pass through the drying furnace  230 . In the drying furnace  230 , a plurality of hot air dryers  231  are installed from each of which hot air NF of a temperature Ta (concretely, 62.0° C.) is blown against the undried separator layer  141   p  to heat and dry the undried separator layer  141   p , thereby forming the separator layer  141 . Then, the negative electrode foil  132  formed with the separator layer  141  and the negative active material layer  133  is wound up by a wind-up roll  219 . 
     The above temperature Ta (concretely, 62.0° C.) is a temperature falling within the surface melting temperature range TD (concretely, 52.7 to 72.7° C.) in which each resin particle  143  melts at its surface portion  143   h  but does not melt at its center portion  143   g . This surface melting temperature range TD is a range of Tb±10.0° C. with respect to the peak temperature Tb (concretely, 62.7° C.) of surface melting heat as mentioned above. 
     Similarly, the aforementioned dispersion liquid BS is applied onto the negative active material layer  133  on the other side to form the undried separator layer  141   p  (the coating step S 4  of separator layer). Then, this undried separator layer  141   p  is dried by heating to form the separator layer  141  (heat-drying step S 5  of separator layer). In this way, the negative electrode sheet  131  is produced (see  FIG. 4 ). 
     Furthermore, the positive electrode sheet  121  is separately produced. Specifically, a strip-shaped positive electrode foil  122  made of aluminum is prepared. On one of main surfaces of this positive electrode foil  122 , positive electrode paste containing lithium-cobalt-nickel-manganese composite oxide (positive active material particles), acetylene black (conducing material), and PVDF (binding agent) is applied and dried. Thus, the positive active material layer  123  is formed. Similarly, on the other main surface of the positive electrode foil  122 , the aforementioned positive paste is applied and dried to form a positive active material layer  123 . Then, the positive active material layers  123  are compressed by pressure rolls to increase their density. In this way, the positive electrode sheet  121  is produced (see  FIG. 3 ). 
     The positive electrode sheet  121  and the negative electrode sheet  131  are overlapped each other (see  FIG. 3 ) and wound about an axis by use of a winding core. Thereafter, this winding is compressed into a flattened shape to form the electrode body  120 . Separately, the case lid member  113 , the positive terminal member  150 , the negative terminal member  160 , and bolts  153  and  163  are prepared and set in a die for injection molding. Then, insulating members  155  and  165  are integrally molded to fix the positive terminal member (positive terminal)  150  and the negative terminal member (negative terminal)  160  respectively to the case lid member  113 . 
     Successively, the positive terminal  150  and the negative terminal  160  are each connected (welded) to the electrode body  120 . Then, the electrode body  120  is put in the case body member  111  prepared, and the opening  111   h  of the case body member  111  is closed with the case lid member  113 . The case body member  111  and the case lid member  113  are welded by laser to each other (see  FIGS. 1 and 2 ). After that, the electrolyte  117  is poured in the battery case  110  through the liquid inlet  113   h , and then the liquid inlet  113   h  is hermetically sealed with the seal member  115 . This battery  100  is then subjected to initial charge, aging, and various tests. Thus, the battery  100  is completed. 
     Examples and Comparative Examples 
     Next, results of tests conducted to verify the effects of the method for manufacturing the battery  100  in the present embodiment will be explained. In Example 1, the negative electrode sheet  131  was manufactured by the manufacturing method of the present embodiment. In this manufacturing method, as mentioned above, the temperature Ta in the heat-drying step S 5  of the separator layer  41  is 62.0° C. In Example 2, as shown in Table 1, the temperature Ta in the heat-drying step S 5  was set to 57.0° C. lower by 5.0 degrees than in Example 1, and remaining conditions were the same as those in Example 1. In Example 3, the temperature Ta in the heat-drying step S 5  was set to 67.0° C. higher by 5.0 degrees than in Example 1, and remaining conditions were the same as those in Example 1. 
     On the other hand, in Comparative example 1, the temperature Ta in the heat-drying step S 5  was set to 40.0° C. In Comparative example 2, the temperature Ta was set to 50.0° C. In Comparative example 3, the temperature Ta was set to 75.0° C. In Comparative example 4, the temperature Ta was set to 85.0° C. Remaining conditions of the comparative examples are the same as those in Example 1. Each temperature Ta is a value measured with a K-type thermocouple attached near a hot-air outlet of each hot-air dryer  231 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Temp. Ta 
                 Crack Test 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 (difference from 
                   
                 Winding core 
                 Winding core 
                 Winding core 
               
               
                   
                 1 st  peak temp. Tb) 
                 Porosity 
                 diameter 
                 diameter 
                 diameter 
               
               
                   
                 (° C.) 
                 (%) 
                 1 mm 
                 2 mm 
                 3 mm 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 62.0 (−0.7) 
                 35.4 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Example 2 
                 57.0 (−5.7) 
                 35.2 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Example 3 
                 67.0 (+4.3) 
                 35.1 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Comparative 
                 40.0 (−22.7) 
                 35.5 
                 X 
                 X 
                 X 
               
               
                 Example 1 
               
               
                 Comparative 
                 50.0 (−12.7) 
                 35.8 
                 X 
                 X 
                 ◯ 
               
               
                 Example 2 
               
               
                 Comparative 
                 75.0 (+12.3) 
                 30.2 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Example 3 
               
               
                 Comparative 
                 85.0 (+22.3) 
                 27.7 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Example 4 
               
               
                   
               
            
           
         
       
     
     A porosity Ca (%) of the separator layer of each negative electrode sheet in Examples 1 to 3 and Comparative examples 1 to 4 was determined by the aforementioned method. Each negative electrode sheet was subjected to a “Crack Test” of a separator layer and the bonding property of the resin particles  143  was evaluated. To be concrete, each negative electrode sheet was wound on the winding core having a 1-mm diameter, 2-mm diameter, or a 3-mm diameter. The surface states of the separator layers were visually observed. Samples in which no crack was generated in the separator layers are determined to be good (indicated with “0” in Table) and samples in which a crack(s) was generated are determined to be defective (“x” in Table). These results are shown in Table 1. 
     Referring to the porosity Ca of separator layers, firstly, as is seen in Table 1, the separator layers in Comparative examples 3 and 4 have a small porosity Ca (27.7 to 30.2%).  FIGS. 10 and 11  show the separator layer of Comparative example 4. The reason why the porosity Ca in the separator layer is small in Comparative examples 3 and 4 is conceived as that since the heat-drying step S 5  was conducted at a high temperature Ta (75.0° C. or 85.0° C.) exceeding the surface melting temperature range TD (52.7 to 72.7° C.), the resin particles  143  in the undried separator layer  141   p  melted not only at the surface portions  143   h  but also at the center portions  143   g . Accordingly, some adjacent resin particles  143  fused with each other (see  FIGS. 10 and 11 ). It is thus conceivable that gaps (voids) KG between the resin particles  143  are made small, resulting in a small porosity Ca. 
     In contrast, the separator layers in Examples 1 to 3 and Comparative examples 1 and 2 have a large porosity Ca (35.1 to 35.8%) as compared with the separator layers in Comparative examples 3 and 4. The reason why the porosity Ca in the separator layer is large in Examples 1 to 3 is conceived as that since the heat-drying step S 5  was conducted at a temperature Ta (57.0° C., 62.0° C., or 67.0° C.) falling within the surface melting temperature range TD (52.7 to 72.7° C.), the resin particles  143  in the undried separator layer  141   p  melted only at the surface portions  143   h  without melting at the center portions  143   g . Accordingly, some adjacent resin particles  143  fused with each other only in the surface portions  143   h . It is thus conceivable that gaps (voids) KG between the resin particles  143  are made large, resulting in a large porosity Ca. 
     Also, the reason why the porosity Ca of the separator layers in Comparative examples 1 and 2 is large is conceived as that the heat-drying step S 5  was conducted at a temperature Ta (40.0° C. or 50.0° C.) lower than the surface melting temperature range TD (52.7 to 72.7° C.) and thus the resin particles  143  in the undried separator layers  141   p  little melted even at the surface portions  143   h  and the resin particles  143  did not fuse with each other. Accordingly, gaps (voids) KG between the resin particles  143  are made large, resulting in a large porosity Ca. 
     Referring to the result of “Crack Test”, as is seen in Table 1, the negative electrode sheets in Comparative examples 1 and 2 were cracked in the separator layers. The reason why the cracks were caused is conceived as that the heat-drying step S 5  was conducted at a temperature Ta (40.0° C. or 50.0° C.) lower than the surface melting temperature range TD (52.7 to 72.7° C.) and thus the resin particles  143  in the undried separator layer  141   p  little melted at the surface portions  143   h  and the resin particles  143  did not sufficiently fuse with each other. Therefore, bonding strength between the resin particles  143  was weak, causing the resin particles  143  to be separated due to stress applied on the negative electrode sheet when wound on a winding core, and the separator layers were cracked. 
     In contrast, the negative electrode sheets in the Examples 1 to 3 and Comparative examples 3 and 4, the separator layers were not cracked. The reason why no crack was generated is conceived as that the heat-drying step S 5  was conducted at the temperature Ta (Examples 1 to 3) within the surface melting temperature range TD or at the higher temperature Ta (Comparative examples 3 and 4) and thus the resin particles  143  in each undried separator layer  141   p  melted in at least the surface portions  143   h  and thereby the adjacent resin particles  143  fused with each other. Therefore, the bonding strength between the resin particles  143  was strong. The resin particles  143  were not separated even if stress was exerted on the negative electrode sheet when wound on a winding core, and the separator layers were not cracked. 
     The above results reveal that it is preferable to perform the heat-drying step S 5  in the surface melting temperature range TD (52.7 to 72.7° C.) and particularly in a range of Tb ±7.0° C. with respect to the peak temperature Tb (concretely, 62.7° C.) of surface melting heat. 
     As explained above, according to the method for manufacturing the battery  100 , the undried separator layer  141   p  is dried by heating at the temperature Ta in the surface melting temperature range TD (a range of Tb ±10.0° C. with respect to the peak temperature Tb of surface melting heat) in which the center portion  143   g  of each resin particle  143  does not melt but the surface portion  143   h  does melt in order to form the separator layer  141 . Accordingly, only the surface portion  143   h  of each resin particle  143  melts, so that the resin particles  143  fuse with each other only at their surface portions  143   h . This can form the separator layers  141  in which the resin particles  143  firmly bond to each other and have gaps (voids) three-dimensionally continuous to each other between the resin particles  143 . This makes it possible to eliminate the binding agent contained in the separator layers  141 , while allowing the resin particles  143  to firmly bond to each other. Thus, the battery  100  with small internal resistance can be manufactured. 
     Furthermore, the temperature Ta in the heat-drying step is set in a range of Tb±7.0° C. with respect to the peak temperature Tb of surface melting heat. Accordingly, it is possible to appropriately melt the surface portions  143   h  of the resin particles  143  and further effectively suppress excessive advancing of fusion of the resin particles  143  and decrease in the porosity Ca of the separator layer  141 . This can easily form the separator layer  141  in which the resin particles  143  are firmly bonded to each other and with a large porosity Ca. 
     Since the dispersion liquid BS does not contain the binding agent to be interposed between the resin particles  143  to bond the resin particles  143  to each other, the negative active material particles  135  are not covered with the bonding agent. Thus, and the battery  100  with particularly small internal resistance can be manufactured. Since the resin particles  143  are made from PE particles, furthermore, the separator layer  141  can be formed with good resistance to the electrolyte  117 . The PE particles are low in melting point and thus easy to handle. By using the resin particles  143  having an average particle diameter of 1.0 to 5.0 μm, the gaps (voids) between the resin particles  143  can be formed in appropriate sizes. 
     The present invention is explained along the above embodiment but is not limited thereto. The invention may be embodied in other specific forms without departing from the essential characteristics thereof. For instance, although the invention is applied to form the negative electrode sheet  131  in the above embodiment, the invention may be applied to form the positive electrode sheet  121 . 
     In the above embodiment, heat-drying of the undried separator layer  141   p  in the heat-drying step S 5  is performed by blowing hot air NF against the undried separator layer  141   p , but it is not limited thereto. For instance, the undried separator layer  141   p  may be dried by heating using an IR heater, IH heater, or the like. 
     The above embodiment exemplifies PE particles as the thermoplastic resin particles  143  to form the separator layer  141 , but the invention is not limited thereto. The material of the thermoplastic resin particles may be for example polyethylene copolymer, polypropylene (PP), polypropylene copolymer, polyvinyl chloride (PVC), polystyrene (PS), polyvinyl acetate (PVAc), polytetrafluoroethylene (PTFE), acrylonitrile butadiene styrene resin (ABS resin), acrylonitrile styrene resin (AS resin), acrylic resin (PMMA), polyamide (PA), polyacetal (POM), polycarbonate (PC), polyethylene terephthalate (PET), polyphenylene sulfide (PPS), thermoplastic polyimide (PI), polyamide-imide (PAI), and others. 
     A particularly preferable one is polyolefin resin such as polyethylene, polyethylene copolymer, polypropylene, and polypropylene copolymer. This is because the polyolefin resin has a suitable melting point for a material constituting the separator layer. In other words, when the temperature of a battery rises over an upper limit temperature (a temperature at which the electrolyte begins altering, e.g., 60° C.) of battery operation, the separator layer promptly melts to close the gaps (voids) in the separator layer itself, thereby appropriately shutting down the battery (stopping battery reaction). 
     The above embodiment exemplifies CMC as the thickening agent  144  to be added to the dispersion liquid BS in order to form the separator layer  141 . However, the invention is not limited thereto. The thickening agent may be water-soluble polymer such as methyl cellulose (MC) and polyvinyl alcohol (PVA). 
     REFERENCE SIGNS LIST 
     
         
           100  Lithium ion secondary battery (Battery) 
           120  Electrode body 
           121  Positive electrode sheet 
           131  Negative electrode sheet (Electrode sheet) 
           132  Negative electrode foil 
           133  Negative active material layer (Negative mixture layer, Active material layer) 
           141  Separator layer 
           141   p  Undried separator layer 
           143  Resin particle 
           143   h  Surface portion 
           143   g  Center portion 
           144  Thickening agent 
         BS Dispersion liquid 
         KG Gap (Void) 
         NF Hot air 
         P 1  First peak 
         P 2  Second peak