JP5741936B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  In this specification, “secondary battery” refers to a general power storage device that can be repeatedly charged. In the present specification, the “lithium ion secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by the movement of electrons accompanying the lithium ions between the positive and negative electrodes.

  Regarding such a lithium ion secondary battery, for example, JP 2009-32677 A (Patent Document 1) discloses that boehmite is used as a separator. This publication discloses that when boehmite is used for a separator, the amount of Na contained in a porous film (heat-resistant layer) in which boehmite is bound with a binder is set to 1000 ppm or less. The point that the capacity | capacitance fall of a battery can be suppressed by making the quantity of Na contained in the porous film which bound boehmite with the binder into 1000 ppm or less is indicated.

JP 2009-32677 A

By the way, Na contained in the porous film (heat-resistant layer) takes in moisture (H ₂ O) in the electrolytic solution. H ₂ O reacts with the electrolytic solution (LiPF ₆ ) to generate HF (hydrofluoric acid). When such a side reaction occurs, the battery capacity decreases and the battery resistance increases. In order to obtain a high-performance lithium ion secondary battery, we want to suppress such side reactions of the electrolyte.

  A lithium ion secondary battery according to an embodiment of the present invention includes a positive electrode current collector, a positive electrode active material layer containing a positive electrode active material, held by the positive electrode current collector, a negative electrode current collector, and a negative electrode current collector A negative electrode active material layer containing a negative electrode active material, and a separator interposed between the positive electrode active material layer and the negative electrode active material layer. The separator includes a resin-made base material and a heat-resistant layer that is held on the base material and includes a filler and a binder. The heat-resistant layer contains four elements of Na, S, Ca and Si at a ratio of 1000 ppm or less.

  According to such a lithium ion secondary battery, since the four elements Na, S, Ca, and Si are included in the heat-resistant layer at a ratio of 1000 ppm or less, the heat-resistant layer includes the heat-resistant layer including the filler and the binder. When the separator is used, it contributes to improving the output characteristics of the lithium ion secondary battery.

  In this case, the heat-resistant layer may contain four elements of Na, S, Ca, and Si at a ratio of 5 ppm or more. The heat-resistant layer may contain three elements of Na, S, and Si at a ratio of 200 ppm to 800 ppm, respectively, and Ca may be included at a ratio of 100 ppm to 350 ppm.

  Further, the relationship between the number of moles of Na (Mna) in the heat-resistant layer, the number of moles of S (Ms), and the number of moles of Ca (Mca) may be Mna <(Ms + Mca) / 2. Here, the filler may be, for example, alumina. In this case, the alumina is preferably boehmite, for example.

  A separator according to an embodiment of the present invention includes a resin base material and a heat resistant layer that is held on the base material and includes a filler and a binder. The heat resistant layer includes Na, S, and Ca. And 4 elements of Si are preferably contained at a ratio of 1000 ppm or less.

FIG. 1 is a diagram illustrating an example of the structure of a lithium ion secondary battery. FIG. 2 is a view showing a wound electrode body of a lithium ion secondary battery. 3 is a cross-sectional view showing a III-III cross section in FIG. 2. FIG. 4 is a cross-sectional view showing the structure of the positive electrode active material layer. FIG. 5 is a cross-sectional view showing the structure of the negative electrode active material layer. FIG. 6 is a side view showing a welding location between an uncoated portion of the wound electrode body and the electrode terminal. FIG. 7 is a diagram schematically illustrating a state of the lithium ion secondary battery during charging. FIG. 8 is a diagram schematically showing a state of the lithium ion secondary battery during discharge. FIG. 9 is a diagram showing a lithium ion secondary battery according to an embodiment of the present invention. FIG. 10 is a cross-sectional view of a separator of a lithium ion secondary battery according to an embodiment of the present invention. FIG. 11 is a diagram illustrating a vehicle equipped with a secondary battery.

  Here, a structural example of a lithium ion secondary battery will be described first. Thereafter, a lithium ion secondary battery according to an embodiment of the present invention will be described with reference to such structural examples as appropriate. In addition, the same code | symbol is attached | subjected suitably to the member and site | part which show | play the same effect | action. Each drawing is schematically drawn and does not necessarily reflect the real thing. Each drawing shows an example only and does not limit the present invention unless otherwise specified.

  FIG. 1 shows a lithium ion secondary battery 100. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 200 and a battery case 300. FIG. 2 is a view showing the wound electrode body 200. FIG. 3 shows a III-III cross section in FIG.

  As shown in FIG. 2, the wound electrode body 200 includes a positive electrode sheet 220, a negative electrode sheet 240, and separators 262 and 264. The positive electrode sheet 220, the negative electrode sheet 240, and the separators 262 and 264 are respectively strip-shaped sheet materials.

≪Positive electrode sheet 220≫
The positive electrode sheet 220 includes a strip-shaped positive electrode current collector 221 and a positive electrode active material layer 223. For the positive electrode current collector 221, a metal foil suitable for the positive electrode can be suitably used. As the positive electrode current collector 221, for example, a strip-shaped aluminum foil having a predetermined width and a thickness of about 15 μm can be used. An uncoated portion 222 is set along the edge on one side in the width direction of the positive electrode current collector 221. In the illustrated example, the positive electrode active material layer 223 is held on both surfaces of the positive electrode current collector 221 except for the uncoated portion 222 set on the positive electrode current collector 221 as shown in FIG. The positive electrode active material layer 223 contains a positive electrode active material. The positive electrode active material layer 223 is formed by applying a positive electrode mixture containing a positive electrode active material to the positive electrode current collector 221.

<< Positive Electrode Active Material Layer 223 and Positive Electrode Active Material Particles 610 >>
Here, FIG. 4 is a cross-sectional view of the positive electrode sheet 220. In FIG. 4, the positive electrode active material particles 610, the conductive material 620, and the binder 630 in the positive electrode active material layer 223 are schematically illustrated so that the structure of the positive electrode active material layer 223 becomes clear. As shown in FIG. 4, the positive electrode active material layer 223 includes positive electrode active material particles 610, a conductive material 620, and a binder 630.

As the positive electrode active material particles 610, a material that can be used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of the positive electrode active material include LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide), LiNiO ₂ (lithium nickelate), LiCoO ₂ (lithium cobaltate), LiMn ₂ O ₄ (lithium manganate), LiFePO ₄ ( Lithium transition metal oxides such as lithium iron phosphate). Here, LiMn ₂ O ₄ has, for example, a spinel structure. LiNiO ₂ or LiCoO ₂ has a layered rock salt structure. LiFePO ₄ has, for example, an olivine structure. LiFePO ₄ having an olivine structure includes, for example, nanometer order particles. Moreover, LiFePO _{4 having} an olivine structure can be further covered with a carbon film.

≪Conductive material 620≫
Examples of the conductive material 620 include carbon materials such as carbon powder and carbon fiber. One kind selected from such conductive materials may be used alone, or two or more kinds may be used in combination. As the carbon powder, various carbon blacks (for example, acetylene black, oil furnace black, graphitized carbon black, carbon black, graphite, ketjen black), graphite powder, and the like can be used.

≪Binder 630≫
Further, the binder 630 binds the positive electrode active material particles 610 and the conductive material 620 included in the positive electrode active material layer 223, or binds these particles and the positive electrode current collector 221. As the binder 630, a polymer that can be dissolved or dispersed in a solvent to be used can be used. For example, in a positive electrode mixture composition using an aqueous solvent, a cellulose polymer (carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), etc.), a fluorine resin (eg, polyvinyl alcohol (PVA), polytetrafluoroethylene, etc.) (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP, etc.), rubbers (vinyl acetate copolymer, styrene butadiene copolymer (SBR), acrylic acid-modified SBR resin (SBR latex), etc.) A water-soluble or water-dispersible polymer such as can be preferably used. In the positive electrode mixture composition using a non-aqueous solvent, a polymer (polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyacrylonitrile (PAN), etc.) can be preferably employed.

≪Thickener, solvent≫
The positive electrode active material layer 223 is prepared, for example, by preparing a positive electrode mixture in which the above-described positive electrode active material particles 610 and the conductive material 620 are mixed in a paste (slurry) with a solvent, applied to the positive electrode current collector 221, and dried. And is formed by rolling. At this time, as the solvent for the positive electrode mixture, either an aqueous solvent or a non-aqueous solvent can be used. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). The polymer material exemplified as the binder 630 may be used for the purpose of exhibiting a function as a thickener or other additive of the positive electrode mixture in addition to the function as a binder.

  The mass ratio of the positive electrode active material in the total positive electrode mixture is preferably about 50 wt% or more (typically 50 to 95 wt%), and usually about 70 to 95 wt% (for example, 75 to 90 wt%). It is more preferable. Moreover, the ratio of the electrically conductive material to the whole positive electrode mixture can be, for example, about 2 to 20 wt%, and is usually preferably about 2 to 15 wt%. The ratio of the binder to the whole positive electrode mixture can be, for example, about 1 to 10 wt%, and is usually preferably about 2 to 5 wt%.

<< Negative Electrode Sheet 240 >>
As illustrated in FIG. 2, the negative electrode sheet 240 includes a strip-shaped negative electrode current collector 241 and a negative electrode active material layer 243. For the negative electrode current collector 241, a metal foil suitable for the negative electrode can be suitably used. The negative electrode current collector 241 is made of a strip-shaped copper foil having a predetermined width and a thickness of about 10 μm. On one side in the width direction of the negative electrode current collector 241, an uncoated part 242 is set along the edge. The negative electrode active material layer 243 is formed on both surfaces of the negative electrode current collector 241 except for the uncoated portion 242 set on the negative electrode current collector 241. The negative electrode active material layer 243 is held by the negative electrode current collector 241 and contains at least a negative electrode active material. In the negative electrode active material layer 243, a negative electrode mixture containing a negative electrode active material is applied to the negative electrode current collector 241.

<< Negative Electrode Active Material Layer 243 >>
FIG. 5 is a cross-sectional view of the negative electrode sheet 240 of the lithium ion secondary battery 100. As shown in FIG. 5, the negative electrode active material layer 243 includes a negative electrode active material 710, a thickener (not shown), a binder 730, and the like. In FIG. 5, the negative electrode active material 710 and the binder 730 in the negative electrode active material layer 243 are schematically illustrated so that the structure of the negative electrode active material layer 243 becomes clear.

≪Negative electrode active material≫
As the negative electrode active material 710, one kind or two or more kinds of materials conventionally used for lithium ion secondary batteries can be used without any particular limitation. For example, a particulate carbon material (carbon particles) including a graphite structure (layered structure) at least in part. More specifically, the negative electrode active material is, for example, natural graphite, natural graphite coated with an amorphous carbon material, graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon ( Soft carbon) or a carbon material combining these may be used. Note that here, the negative electrode active material 710 is illustrated using so-called flake graphite, but the negative electrode active material 710 is not limited to the illustrated example.

≪Thickener, solvent≫
The negative electrode active material layer 243 is prepared, for example, by preparing a negative electrode mixture in which the negative electrode active material 710 and the binder 730 described above are mixed in a paste (slurry) with a solvent, applied to the negative electrode current collector 241, and dried. It is formed by rolling. At this time, any of an aqueous solvent and a non-aqueous solvent can be used as the solvent for the negative electrode mixture. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). For the binder 730, the polymer material exemplified as the binder 630 of the positive electrode active material layer 223 (see FIG. 4) can be used. Further, the polymer material exemplified as the binder 630 of the positive electrode active material layer 223 may be used for the purpose of exhibiting a function as a thickener or other additive of the positive electrode mixture in addition to the function as a binder. possible.

<< Separators 262, 264 >>
The separators 262 and 264 are members that separate the positive electrode sheet 220 and the negative electrode sheet 240 as shown in FIG. 1 or FIG. In this example, the separators 262 and 264 are made of a strip-shaped sheet material having a predetermined width and having a plurality of minute holes. As the separators 262 and 264, for example, a single layer structure separator or a multilayer structure separator made of a porous polyolefin resin can be used. In this example, as shown in FIGS. 2 and 3, the width b1 of the negative electrode active material layer 243 is slightly wider than the width a1 of the positive electrode active material layer 223. Furthermore, the widths c1 and c2 of the separators 262 and 264 are slightly wider than the width b1 of the negative electrode active material layer 243 (c1, c2>b1> a1).

  In the example shown in FIGS. 1 and 2, the separators 262 and 264 are made of sheet-like members. The separators 262 and 264 may be members that insulate the positive electrode active material layer 223 and the negative electrode active material layer 243 and allow the electrolyte to move. Therefore, it is not limited to a sheet-like member. The separators 262 and 264 may be formed of a layer of insulating particles formed on the surface of the positive electrode active material layer 223 or the negative electrode active material layer 243, for example, instead of the sheet-like member. Here, as the particles having insulating properties, inorganic fillers having insulating properties (for example, fillers such as metal oxides and metal hydroxides) or resin particles having insulating properties (for example, particles such as polyethylene and polypropylene). ).

≪Battery case 300≫
In this example, as shown in FIG. 1, the battery case 300 is a so-called square battery case, and includes a container body 320 and a lid 340. The container main body 320 has a bottomed rectangular tube shape and is a flat box-shaped container having one side surface (upper surface) opened. The lid 340 is a member that is attached to the opening (opening on the upper surface) of the container body 320 and closes the opening.

  In an in-vehicle secondary battery, it is desired to improve the weight energy efficiency (battery capacity per unit weight) in order to improve the fuel efficiency of the vehicle. For this reason, in this embodiment, lightweight metals, such as aluminum and an aluminum alloy, are employ | adopted for the container main body 320 and the cover body 340 which comprise the battery case 300. FIG. Thereby, the weight energy efficiency can be improved.

  The battery case 300 has a flat rectangular internal space as a space for accommodating the wound electrode body 200. Further, as shown in FIG. 1, the flat internal space of the battery case 300 is slightly wider than the wound electrode body 200. In this embodiment, the battery case 300 includes a bottomed rectangular tubular container body 320 and a lid 340 that closes the opening of the container body 320. Electrode terminals 420 and 440 are attached to the lid 340 of the battery case 300. The electrode terminals 420 and 440 pass through the battery case 300 (lid 340) and come out of the battery case 300. The lid 340 is provided with a liquid injection hole 350 and a safety valve 360.

  As shown in FIG. 2, the wound electrode body 200 is flatly pushed and bent in one direction orthogonal to the winding axis WL. In the example shown in FIG. 2, the uncoated part 222 of the positive electrode current collector 221 and the uncoated part 242 of the negative electrode current collector 241 are spirally exposed on both sides of the separators 262 and 264, respectively. As shown in FIG. 6, in this embodiment, the intermediate portions 224 and 244 of the uncoated portions 222 and 242 are gathered together and welded to the tip portions 420 a and 440 a of the electrode terminals 420 and 440. At this time, for example, ultrasonic welding is used for welding the electrode terminal 420 and the positive electrode current collector 221 due to the difference in materials. Further, for example, resistance welding is used for welding the electrode terminal 440 and the negative electrode current collector 241. Here, FIG. 6 is a side view showing a welded portion between the intermediate portion 224 (244) of the uncoated portion 222 (242) of the wound electrode body 200 and the electrode terminal 420 (440), and VI in FIG. It is -VI sectional drawing.

  The wound electrode body 200 is attached to the electrode terminals 420 and 440 fixed to the lid body 340 in a state where the wound electrode body 200 is pressed and bent flat. The wound electrode body 200 is accommodated in a flat internal space of the container body 320 as shown in FIG. The container body 320 is closed by the lid 340 after the wound electrode body 200 is accommodated. The joint 322 (see FIG. 1) between the lid 340 and the container main body 320 is welded and sealed, for example, by laser welding. Thus, in this example, the wound electrode body 200 is positioned in the battery case 300 by the electrode terminals 420 and 440 fixed to the lid 340 (battery case 300).

≪Electrolytic solution≫
Thereafter, an electrolytic solution is injected into the battery case 300 from a liquid injection hole 350 provided in the lid 340. As the electrolytic solution, a so-called non-aqueous electrolytic solution that does not use water as a solvent is used. In this example, an electrolytic solution in which LiPF ₆ is contained at a concentration of about 1 mol / liter in a mixed solvent of ethylene carbonate and diethyl carbonate (for example, a mixed solvent having a volume ratio of about 1: 1) is used. Yes. Thereafter, a metal sealing cap 352 is attached (for example, welded) to the liquid injection hole 350 to seal the battery case 300. The electrolytic solution is not limited to the electrolytic solution exemplified here. For example, non-aqueous electrolytes conventionally used for lithium ion secondary batteries can be used as appropriate.

≪Hole≫
Here, the positive electrode active material layer 223 has minute gaps 225 that should also be referred to as cavities, for example, between the positive electrode active material particles 610 and the conductive material 620 (see FIG. 4). An electrolytic solution (not shown) can penetrate into the minute gaps of the positive electrode active material layer 223. In addition, the negative electrode active material layer 243 has minute gaps 245 that should also be referred to as cavities, for example, between the particles of the negative electrode active material 710 (see FIG. 5). Here, the gaps 225 and 245 (cavities) are appropriately referred to as “holes”. As shown in FIG. 2, the wound electrode body 200 has uncoated portions 222 and 242 spirally wound on both sides along the winding axis WL. On both sides 252 and 254 along the winding axis WL, the electrolytic solution can permeate from the gaps between the uncoated portions 222 and 242. For this reason, in the lithium ion secondary battery 100, the electrolytic solution is immersed in the positive electrode active material layer 223 and the negative electrode active material layer 243.

≪Gas escape route≫
In this example, the flat internal space of the battery case 300 is slightly wider than the wound electrode body 200 deformed flat. On both sides of the wound electrode body 200, gaps 310 and 312 are provided between the wound electrode body 200 and the battery case 300. The gaps 310 and 312 serve as a gas escape path. For example, when the temperature of the lithium ion secondary battery 100 becomes abnormally high, for example, when overcharge occurs, the electrolyte may be decomposed and gas may be generated abnormally. In this embodiment, the abnormally generated gas moves toward the safety valve 360 through the gaps 310 and 312 between the wound electrode body 200 and the battery case 300 on both sides of the wound electrode body 200, and from the safety valve 360 to the battery case 300. Exhausted outside.

  In the lithium ion secondary battery 100, the positive electrode current collector 221 and the negative electrode current collector 241 are electrically connected to an external device through electrode terminals 420 and 440 that penetrate the battery case 300. Hereinafter, the operation of the lithium ion secondary battery 100 during charging and discharging will be described.

≪Operation when charging≫
FIG. 7 schematically shows the state of the lithium ion secondary battery 100 during charging. At the time of charging, as shown in FIG. 7, the electrode terminals 420 and 440 (see FIG. 1) of the lithium ion secondary battery 100 are connected to the charger 290. Due to the action of the charger 290, lithium ions (Li) are released from the positive electrode active material in the positive electrode active material layer 223 to the electrolytic solution 280 during charging. In addition, charges are released from the positive electrode active material layer 223. The discharged electric charge is sent to the positive electrode current collector 221 through a conductive material (not shown), and further sent to the negative electrode sheet 240 through the charger 290. In the negative electrode sheet 240, electric charges are stored, and lithium ions (Li) in the electrolytic solution 280 are absorbed and stored in the negative electrode active material in the negative electrode active material layer 243.

<< Operation during discharge >>
FIG. 8 schematically shows a state of the lithium ion secondary battery 100 during discharging. At the time of discharging, as shown in FIG. 8, charges are sent from the negative electrode sheet 240 to the positive electrode sheet 220, and lithium ions stored in the negative electrode active material layer 243 are released to the electrolyte solution 280. In the positive electrode, lithium ions in the electrolytic solution 280 are taken into the positive electrode active material in the positive electrode active material layer 223.

  Thus, in charging / discharging of the lithium ion secondary battery 100, lithium ions travel between the positive electrode active material layer 223 and the negative electrode active material layer 243 through the electrolytic solution 280. At the time of charging, electric charge is sent from the positive electrode active material to the positive electrode current collector 221 through the conductive material. On the other hand, at the time of discharging, the charge is returned from the positive electrode current collector 221 to the positive electrode active material through the conductive material.

  During charging, the smoother the movement of lithium ions and the movement of electrons, the more efficient and rapid charging is considered possible. At the time of discharging, it is considered that the smoother the movement of lithium ions and the movement of electrons, the lower the resistance of the battery, the amount of discharge, and the output of the battery.

≪Other battery types≫
The above shows an example of a lithium ion secondary battery. The lithium ion secondary battery is not limited to the above form. Similarly, an electrode sheet in which an electrode mixture is applied to a metal foil is used in various other battery forms. For example, as another battery type, a cylindrical battery or a laminate battery is known. A cylindrical battery is a battery in which a wound electrode body is accommodated in a cylindrical battery case. A laminate type battery is a battery in which a positive electrode sheet and a negative electrode sheet are stacked with a separator interposed therebetween.

  Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described.

≪Lithium ion secondary battery 100A≫
FIG. 9 shows a lithium ion secondary battery 100A according to an embodiment of the present invention, and FIG. 10 shows a cross section of the separators 262A, 264A.

  As shown in FIG. 9, in the separators 262 </ b> A and 264 </ b> A of the lithium ion secondary battery according to the embodiment of the present invention, a heat resistance layer 24 (HRL: Heat Resistance Layer) is held on the base material 22.

  Here, as the resin base material 22, for example, a porous sheet-like base material having a single layer structure or a laminated structure made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP) is used. Can do.

<< Production of heat-resistant layer 24 >>
The heat-resistant layer 24 contains a filler and a binder. Here, alumina is used as the filler. The heat-resistant layer 24 is prepared by preparing a paste-like mixture in which an alumina filler and a binder are mixed in a solvent, and applying the paste-like mixture to a resin substrate 22, drying and rolling. . Accordingly, separators 262A and 264A in which the porous heat-resistant layer 24 in which the alumina filler is bonded by the binder can be produced.

≪Alumina≫
Here, the alumina includes boehmite and α-alumina. Alumina is refined from bauxite, an ore containing aluminum. Bauxite contains only about 40% to 60% alumina, and includes silica (silicon dioxide), various iron oxides, titanium dioxide, and the like. The step of purifying alumina from such bauxite includes, for example, a step of heating bauxite in a sodium hydroxide solution. In this step, alumina is dissolved by, for example, a reaction represented by the following chemical formula.

Al ₂ O ₃ + 2OH ⁻ + 3H ₂ O → 2 [Al (OH) ₄ ] ⁻

  At this time, the alumina content in the bauxite dissolves into the alkaline solution as a sodium aluminate solution. Other undissolved components in the bauxite are separated by filtration. Aluminum hydroxide is obtained by adding seeds to a sodium aluminate solution from which the alumina content in bauxite is extracted to crystallize aluminum hydroxide. By pulverizing the obtained aluminum hydroxide, fine aluminum hydroxide is obtained.

The aluminum hydroxide thus obtained, by subjecting one molecule dehydrated in hot aqueous solution (heat treatment using an approximately 200 ° C. water) boehmite (AlO (OH) or _Al 2 _O 3 · _H 2 _O ) Is obtained. Also, α-alumina is obtained by heating aluminum hydroxide at about 1050 ° C. In addition, what was prepared by the method of adding the neutralization equivalent aluminum sulfate solution to the sodium aluminate solution as a seed added at the time of crystallization of aluminum hydroxide is known (for example, Unexamined-Japanese-Patent No. 1-19244).

  Thus, the alumina used as the filler of the heat-resistant layer 24 can contain Na derived from a sodium aluminate solution or S derived from an aluminum sulfate solution.

Among these, as shown in the following reaction formula, for example, Na can hold 1 mol of H ₂ O with respect to 1 mol of Na.

Na ₂ O + H ₂ O → 2NaOH
2NaOH + 2H ₂ O → 2NaOH · H ₂ O

In addition, as shown in the following reaction formula, for example, 0.5 mol of H ₂ O can be removed with respect to 1 mol of S. Moreover, it can react with 2 mol of Na with respect to 1 mol of S.

SO ₂ + H ₂ O → H ⁺ + HSO ₃ ⁻
2HSO ₃ ⁻ → O ₂ S · SO ₃ ²⁻ + H ₂ O
SO ₃ ²⁻ + 2NaOH → Na ₂ SO ₄ + H ₂ O

  In the lithium ion secondary battery 100A according to an embodiment of the present invention, the heat resistant layer 24 of the separators 262A and 264A contains four elements of Na, S, Ca, and Si at a ratio of 1000 ppm or less.

In this case, Ca in the heat-resistant layer 24 can remove 1 mol of H ₂ O with respect to 1 mol of Ca, for example, as shown in the following reaction formula.

CaO + H ₂ O → Ca (OH) ₂

Further, H ₂ O may remain in the heat resistant layer 24. In this case, H ₂ O in the heat-resistant layer 24 and LiPF ₆ in the electrolytic solution react to generate HF. On the other hand, in this embodiment, Si is added to the heat-resistant layer 24. Si in the heat-resistant layer 24 exists, for example, as SiO ₂ . HF generated by the reaction of H ₂ O in the heat-resistant layer 24 and LiPF ₆ in the electrolytic solution is removed by reacting with SiO ₂ by the following reaction formula, for example.

SiO ₂ + 4HF → SiF ₄ + 2H ₂ O
SiO ₂ + 6HF → H ₂ SiF ₆ + 2H ₂ O

  In this embodiment, the heat-resistant layer 24 may contain Na derived from a sodium aluminate solution or S derived from an aluminum sulfate solution in alumina used as a filler. Here, Na and S contained in the heat-resistant layer 24 are reduced so that Na and S contained in the heat-resistant layer 24 are each 1000 ppm or less. Moreover, two elements of Ca and Si contained in the heat-resistant layer 24 are added in appropriate amounts so as to be in a range of 1000 ppm or less.

≪Adjustment of Na and S≫
In order to reduce Na and S contained in the heat-resistant layer 24, it is preferable to reduce Na and S remaining in the alumina filler used in the heat-resistant layer 24. In order to reduce Na and S remaining in the alumina filler, for example, purified alumina (alumina filler) may be washed with water. When Na and S are insufficient in the alumina filler, the deficiency may be added directly by alkali treatment or acid treatment, or the deficiency may be added as an oxide. For example, high-purity alumina may be prepared, and appropriate amounts of Na oxide and S oxide may be added to high-purity alumina.

≪Ca and Si adjustment≫
Ca and Si may be mixed as simple substances or oxides, for example, in an alumina filler when the heat-resistant layer 24 is formed.

  In Table 1, lithium ion secondary batteries were manufactured using separators in which four elements of Na, S, Ca and Si contained in the heat-resistant layer 24 were adjusted to predetermined amounts, and IV resistance was measured.

≪Heat resistant layer 24 of each sample≫
In the paste-like mixture produced when the heat-resistant layer 24 is produced, for example, an acrylic binder (for example, BM810B manufactured by Zeon Corporation) is used as the binder. Further, NMP (N-methyl-2-pyrrolidone, for example, manufactured by Kanto Chemical Co., Inc.) is used as the solvent. For the kneading of the paste mixture, a kneader (for example, an ultrasonic disperser CLEARMIX manufactured by M Technique Co., Ltd.) may be used. In such a paste-like mixture, for example, a filler, a binder, and NMP are mixed with a kneader so that the solid content (NV) is a predetermined amount (40%). At this time, the amount of filler and the amount of binder in the solid content may be adjusted to 96: 4 (weight ratio) so that the total amount of mixing is 1 kg.

Here, high-purity α-alumina powder was prepared as a filler, and the four elements Na, S, Ca, and Si were adjusted to predetermined amounts. Here, the α-alumina powder was the same for each sample. The α-alumina powder had an average particle size (D50) of 0.2 to 1.2 μm and a BET specific surface area of 1.3 to 18 m ² / g.

Application conditions of the paste-like mixture for producing the heat-resistant layer 24 are applied with a gravure coating machine. Here, the gravure roll of the gravure coating machine uses, for example, art (number of lines # 100 / inch, cell volume 19.5 cc / m ² ). Here, the coating speed (line speed) is, for example, 3 m / min, the gravure roll speed is 3.8 m / min, and the speed ratio between the gravure speed and the line speed (gravure speed / line speed) is 1.27. did.

  In addition, a porous film having a three-layer structure of PP / PE / PP and a porous film having a single-layer structure of PE were prepared as the base material 22 serving as a separator base material. Here, “PP” means polypropylene, and “PE” means polyethylene. In Table 1, in the item of “type of separator”, “three layers” means that a sample using a porous film having a three-layer structure of PP / PE / PP is used as the base material 22 as a base material of the separator. Show. In Table 1, the item “single layer” in the item of “type of separator” indicates a sample in which a porous film having a single layer structure of PE is used as the base material 22 serving as the base material of the separator. In addition, in the sample using the porous film having a three-layer structure of PP / PE / PP as the base material 22 serving as the separator base material, the drying temperature after the mixture application when preparing the heat-resistant layer 24 is set to 80 ° C. did. In a sample in which a porous film having a single layer structure of PE was used as the base material 22 serving as a separator base material, the drying temperature after application of the mixture at the time of preparing the heat-resistant layer 24 was set to 70 ° C. In Table 1, a porous film having a single-layer structure of PE was used as the base material 22 in Samples 2, 3, 6, and 7.

≪Positive electrode sheet for each sample≫
Here, the positive electrode sheet 220 (refer FIG. 9) is produced by the same method about the lithium ion secondary battery of each sample. Here, the same active material (here, LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide)) was used as the positive electrode active material. Acetylene black was used for the conductive material of the positive electrode, and PVDF was used for the binder. For the positive electrode current collector 221 (see FIG. 9), an aluminum foil having a length of 3000 mm, a width of 115 mm, and a thickness of 15 μm was used. A positive electrode active material layer 223 was formed on each side of the positive electrode current collector 221 with a length of 3000 mm, a width of 100 mm, and a thickness of 26 μm. Here, the dimension of the positive electrode active material layer 223 is a dimension after rolling. The basis weight after drying of the positive electrode active material layer 223 was 9.8 to 15.2 mg / cm ² . Here, the basis weight is the basis weight of both the positive electrode active material layers 223 produced on both sides of the positive electrode current collector 221. The theoretical density of the positive electrode active material layer 223 was 0.8 to 2.4 g / cm ³ .

≪Negative electrode negative electrode sheet≫
The negative electrode sheet 240 (see FIG. 9) of the lithium ion secondary battery of each sample is produced by the same method. Here, the same active material (here, amorphous-coated graphite) was used as the negative electrode active material. SBR was used as the binder. For the negative electrode current collector 241 (see FIG. 9), a copper foil having a length of 3100 mm, a width of 210 mm, and a thickness of 10 μm was used. A negative electrode active material layer 243 was formed on each side of the negative electrode current collector 241 with a length of 3100 mm, a width of 105 mm, and a thickness of 32 μm. Here, the dimension of the negative electrode active material layer 243 is a dimension after rolling. The basis weight after drying of the negative electrode active material layer 243 was set to 4.8 to 10.2 mg / cm ² (weight per unit of both surfaces). The theoretical density of the negative electrode active material layer 243 was 0.8 to 1.4 g / cm ³ .

≪Electrolyte for each sample≫
The electrolyte solution of the lithium ion secondary battery of each sample is a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) (volume ratio is EC: EMC: DMC = 3: 4: 3). An electrolytic solution containing LiPF ₆ at a concentration of about 1.1 mol / liter is used.

  Each sample in Table 1 changes the amount of the four elements Na, S, Ca and Si contained in the heat-resistant layer 24, and the base material 22 of the separator. Other conditions are generally the same. Here, the porosity of the substrate 22 was 47%, the thickness of the substrate 22 was 20 μm, the porosity of the heat-resistant layer 24 was 53%, and the thickness of the heat-resistant layer 24 was 4 μm.

  In Sample 1 of Table 1, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 5 ppm, 10 ppm, 5 ppm and 10 ppm, respectively, and the IV resistance was 75 mΩ.

  In Sample 2, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 120 ppm, 90 ppm, 50 ppm and 100 ppm, respectively, and the IV resistance was 67 mΩ.

  In Sample 3, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 200 ppm, 200 ppm, 100 ppm and 200 ppm, respectively, and the IV resistance was 59 mΩ.

  In Sample 4, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 350 ppm, 410 ppm, 170 ppm and 350 ppm, respectively, and the IV resistance was 58 mΩ.

  In Sample 5, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 490 ppm, 520 ppm, 230 ppm and 500 ppm, respectively, and the IV resistance was 57 mΩ.

  In Sample 6, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 620 ppm, 710 ppm, 310 ppm and 600 ppm, respectively, and the IV resistance was 58 mΩ.

  In Sample 7, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 800 ppm, 800 ppm, 350 ppm and 800 ppm, respectively, and the IV resistance was 59.5 mΩ.

  In Sample 8, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 910 ppm, 890 ppm, 450 ppm and 900 ppm, respectively, and the IV resistance was 63 mΩ.

  In Sample 9, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 1000 ppm, 1000 ppm, 500 ppm and 1000 ppm, respectively, and the IV resistance was 65 mΩ.

  In Sample 10, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 2 ppm, 0 ppm, 0 ppm and 5 ppm, respectively, and the IV resistance was 115 mΩ.

  In Sample 11, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 450 ppm, 0 ppm, 0 ppm and 0 ppm, respectively, and the IV resistance was 125 mΩ.

  In Sample 12, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 2000 ppm, 5 ppm, 2 ppm and 5 ppm, respectively, and the IV resistance was 140 mΩ.

  In Sample 13, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 2300 ppm, 5 ppm, 2 ppm and 200 ppm, respectively, and the IV resistance was 135 mΩ.

  In Sample 14, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 500 ppm, 0 ppm, 0 ppm and 200 ppm, respectively, and the IV resistance was 130 mΩ.

  In sample 15, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 500 ppm, 200 ppm, 0 ppm and 0 ppm, respectively, and the IV resistance was 118 mΩ.

  In Sample 16, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 520 ppm, 0 ppm, 200 ppm and 0 ppm, respectively, and the IV resistance was 138 mΩ.

  In Sample 17, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 2100 ppm, 600 ppm, 2 ppm and 300 ppm, respectively, and the IV resistance was 125 mΩ.

  In Sample 18, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 350 ppm, 130 ppm, 0 ppm and 250 ppm, respectively, and the IV resistance was 115 mΩ.

  In Sample 19, the four elements Na, S, Ca, and Si contained in the heat-resistant layer 24 were 570 ppm, 150 ppm, 100 ppm, and 0 ppm, respectively, and the IV resistance was 119 mΩ.

  In Sample 20, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 350 ppm, 0 ppm, 80 ppm and 270 ppm, respectively, and the IV resistance was 121 mΩ.

  In sample 21, the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 were 3250 ppm, 1050 ppm, 550 ppm and 1100 ppm, respectively, and the IV resistance was 124 mΩ.

  Here, the amount of the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 being 0 ppm means that the element was below the detection limit. Here, the amounts of the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 are determined from the separators 262A and 264A before assembling the lithium ion secondary battery 100A (see FIG. 9) of each sample. A sample (see FIG. 10) is taken and measured by a predetermined method. Here, before assembling the lithium ion secondary battery 100A (see FIG. 9) of each sample, the sample of the heat resistant layer 24 (see FIG. 10) is taken from the separators 262A and 264A to be included in the heat resistant layer 24. The amounts of the four elements Na, S, Ca and Si were measured by a predetermined method. The amount of the four elements Na, S, Ca and Si contained in the heat-resistant layer is not limited to this, and may be measured from the heat-resistant layer obtained by disassembling the lithium ion secondary battery.

  Among Na, S, Ca and Si contained in the heat-resistant layer 24, the amounts of Na, Ca and Si are quantified by ICP emission spectroscopic analysis. Here, it measured using iCAP6300 by Thermo Fisher Scientific. In this case, the detection limit of Na was about 1 ppm, the detection limit of Ca was about 1 ppm, and the detection limit of Si was about 5 ppm.

  The amount of S contained in the heat-resistant layer 24 is quantified by ion chromatography by burning the heat-resistant layer 24. The equipment used was AQF-100 manufactured by Mitsubishi Chemical as an automatic combustion device. As an ion chromatograph, ICS-1500 made by Dionex was used. The detection limit of S was about 5 ppm.

≪IV resistance≫
Here, the IV resistance indicates the IV resistance at SOC 60%. Here, a predetermined conditioning process was performed on each sample, and an IV resistance in a SOC 60% state of charge (SOC) was measured under an environment of 25 ° C.

<< conditioning >>
Next, the test battery constructed as described above was poured for about 10 hours after injecting the electrolytic solution, and was initially charged after the battery voltage became 2.0 V or higher. The conditioning process is performed by the following procedures 1 and 2.
Procedure 1: After reaching 4.1 V with a constant current charge of 1 C, pause for 5 minutes.
Procedure 2: After Procedure 1, charge for 1.5 hours by constant voltage charging and rest for 5 minutes.

≪Measurement of rated capacity≫
Next, the rated capacity is measured by the following procedures 1 to 3 at a temperature of 25 ° C. and a voltage range of 3.0 V to 4.1 V for the test battery after the conditioning step.
Procedure 1: After reaching 3.0V by constant current discharge of 1C, discharge by constant voltage discharge for 2 hours, and then rest for 10 seconds.
Procedure 2: After reaching 4.1 V by constant current charging at 1 C, charge for 2.5 hours by constant voltage charging, and then rest for 10 seconds.
Procedure 3: After reaching 3.0 V by constant current discharge of 0.5 C, discharge at constant voltage discharge for 2 hours, and then stop for 10 seconds.
Rated capacity: The discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge to the constant voltage discharge in the procedure 3 is defined as the rated capacity. In this test battery, the rated capacity is about 4.6 Ah.

≪SOC adjustment≫
The SOC adjustment is performed by the following procedures 1 and 2. Here, the SOC adjustment may be performed after the conditioning process and the measurement of the rated capacity. Here, in order to make the influence of temperature constant, SOC adjustment is performed in a temperature environment of 25 ° C.
Procedure 1: Charging at a constant current of 3V to 1C to obtain a charged state (SOC 60%) of about 60% of the rated capacity.
Procedure 2: After procedure 1, charge at constant voltage for 2.5 hours.
Thereby, the test battery can be adjusted to a predetermined state of charge.

≪IV resistance measurement method≫
The IV resistance in Table 1 is a constant current discharge at a predetermined current value (I) for 10 seconds from a state adjusted to SOC 60% in an environmental atmosphere of 25 ° C., and the voltage (V) after the discharge is measured. Then, based on the predetermined current value (I) and the voltage (V) after the discharge, I plotted on the X-axis and I on the Y-axis, and plotted based on the plots obtained by each discharge. And the slope is taken as IV resistance. Here, IV resistance was obtained based on the voltage (V) after each discharge obtained by performing constant current discharge at current values of 0.3C, 1C, and 3C.

  As shown in Table 1, since the heat-resistant layer 24 contains four elements of Na, S, Ca, and Si, the IV resistance is generally suppressed to a low level. This is thought to be due to the interaction of the four elements Na, S, Ca, and Si that suppresses the retention of moisture caused by Na and side reactions with the electrolyte solution that generates HF, for example. In this case, for example, as shown in samples 1 to 9 in Table 1, when the four elements of Na, S, Ca and Si contained in the heat-resistant layer 24 are in a ratio of 1000 ppm or less, the IV resistance is generally low. It can be suppressed. The heat-resistant layer 24 preferably contains four elements of Na, S, Ca, and Si at a ratio of 5 ppm or more.

  That is, as shown in Samples 10 to 21, when any of the four elements of Na, S, Ca and Si contained in the heat-resistant layer 24 is more than 1000 ppm each, the four elements of Na, S, Ca and Si are present. In each case of less than 5 ppm, there is a tendency that the IV resistance is generally higher than when the four elements Na, S, Ca and Si contained in the heat-resistant layer 24 are 5 ppm or more and 1000 ppm or less.

Further, as described above, 2 mol of Na reacts with 1 mol of S, and 1 mol of H ₂ O that can be held by 2 mol of Na is removed from the heat-resistant layer 24. Further, 1 mol of H ₂ O is removed from the heat-resistant layer 24 with respect to 1 mol of Ca. Therefore, the relationship between the number of moles of Na (Mna), the number of moles of S (Ms), and the number of moles of Ca (Mca) in the heat-resistant layer is preferably Mna <(Ms + Mca) / 2.

<< Samples 22-25 >>
Further, the alumina contained as a filler in the heat-resistant layer 24 is preferably boehmite rather than α-alumina. For example, as shown in Table 2, Sample 22 and Sample 24 using α-alumina as alumina used as a filler of the heat-resistant layer 24, and Sample 23 and Sample 25 using boehmite were prepared. Sample 22 and sample 23, and sample 24 and sample 25 have substantially the same structure except for the type of alumina used as the filler of heat-resistant layer 24, respectively. In this case, the IV resistance of the sample 22 using α-alumina as the alumina used as the filler of the heat-resistant layer 24 was 56 mΩ, whereas the IV resistance of the sample 23 using boehmite was 50 mΩ. The IV resistance of Sample 24 using α-alumina as the filler for the heat-resistant layer 24 was 57 mΩ, whereas that of Sample 25 using boehmite was 49 mΩ. Thus, in terms of keeping the IV resistance of the lithium ion secondary battery 100A low, the alumina used as the filler of the heat-resistant layer 24 tends to be more preferable than α-alumina.

  In the above, the lithium ion secondary battery which concerns on one Embodiment of this invention was demonstrated. Note that the present invention is not limited to any of the above-described embodiments unless otherwise specified.

  For example, although an embodiment using alumina as a typical example of the filler of the heat-resistant layer has been illustrated, in the present invention, the filler may not necessarily be alumina. The heat-resistant layer contains four elements of Na, S, Ca and Si at a ratio of 1000 ppm or less. For this reason, it can contribute to improving the output characteristic of a lithium ion secondary battery irrespective of the kind of filler. Further, in the above-described embodiment, the lithium ion secondary battery 100A (see FIG. 9) in which the wound electrode body 200A that is pressed and bent flat is accommodated in the rectangular battery case 300 is illustrated. The present invention is not limited to such a form unless otherwise specified.

  Further, as described above, the present invention contributes to improving the output characteristics of the lithium ion secondary battery 100A when the separators 262A, 264A including the heat resistant layer 24 (HRL) are used. For this reason, in particular, a secondary battery for a vehicle drive power source such as a drive battery for a hybrid vehicle (for example, a plug-in hybrid vehicle) or an electric vehicle that requires a high level of safety and requires a high output performance. Is preferred.

  In this case, the lithium ion secondary battery according to an embodiment of the present invention is, for example, in the form of an assembled battery in which a plurality of secondary batteries are connected and combined as shown in FIG. It can be suitably used as a vehicle drive battery 1000 for driving the motor (electric motor). In particular, the lithium ion secondary battery according to the present invention is a lithium ion having a high capacity (for example, a rated capacity of 3.0 Ah or more) as a driving battery for a hybrid vehicle (particularly, a plug-in hybrid vehicle) or an electric vehicle. Suitable for secondary batteries.

DESCRIPTION OF SYMBOLS 1 Vehicle 22 Base material 24 Heat-resistant layer 100, 100A Lithium ion secondary battery 200, 200A Winding electrode body 220 Positive electrode sheet 221 Positive electrode collector 222 Uncoated part 223 Positive electrode active material layer 224 Middle part 225 Gap 240 Negative electrode sheet 241 Negative electrode current collector 242 Uncoated portion 243 Negative electrode active material layer 245 Gaps 252, 254 Both sides 262, 264, 262A, 264A along the winding axis of the wound electrode body Separator 280 Electrolytic solution 290 Charger 300 Battery case 310, 312 Gap 320 Container body 322 Lid and container body joint 340 Lid body 350 Injection hole 352 Sealing cap 360 Safety valve 420, 440 Electrode terminal 420a, 440a Electrode terminal tip 610 Cathode active material particle 620 Conductive material 630 Binder 710 Negative electrode active material 730 Binder 1000 For vehicle drive Pond (lithium ion secondary battery)

Claims (7)

A positive electrode current collector;
A positive electrode active material layer containing a positive electrode active material held by the positive electrode current collector;
A negative electrode current collector;
A negative electrode active material layer containing a negative electrode active material held by the negative electrode current collector;
A separator interposed between the positive electrode active material layer and the negative electrode active material layer,
The separator includes a resin base material, and a heat resistant layer that is held on the base material and includes a filler and a binder,
The lithium ion secondary battery, wherein the heat-resistant layer contains four elements of Na, S, Ca, and Si at a ratio of 5 ppm to 1000 ppm. Wherein the heat-resistant layer, Na, 3 elements S and Si are included in the following proportions each 200ppm or 800 ppm, and, Ca is contained at a rate of 100ppm or 350ppm or less, as described in claim 1 Lithium ion secondary battery. The relationship among the number of moles of Na (Mna), the number of moles of S (Ms), and the number of moles of Ca (Mca) in the heat-resistant layer is Mna <(Ms + Mca) / 2. The lithium ion secondary battery described in 1. The lithium ion secondary battery according to any one of claims 1 to 3 , wherein the filler is alumina. The lithium ion secondary battery according to claim 4 , wherein the alumina is boehmite. The lithium ion secondary battery according to any one of claims 1 to 5 , wherein the rated capacity is 3.0 Ah or more. A resin base material, and a heat-resistant layer containing a filler and a binder, which is held by the base material,
The separator, wherein the heat-resistant layer contains four elements of Na, S, Ca, and Si at a ratio of 5 ppm to 1000 ppm.

Images