JP7096979B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Lithium-ion secondary batteries are preferably used as portable power sources, high-output power sources for vehicle mounting, and the like because they are lightweight and have a high energy density. This lithium ion secondary battery has a laminated structure in which a plurality of storage elements having a positive electrode and a negative electrode insulated by a separator or the like are laminated in one battery case. Here, the positive electrode and the negative electrode are designed so that the widthwise dimension of the negative electrode is wider than the widthwise dimension of the positive electrode in order to prevent the precipitation of lithium ions on the negative electrode. As a conventional technique for preventing a short circuit between the positive electrode and the negative electrode of such a secondary battery, it is disclosed that the surface of the positive electrode current collector is provided with an insulating layer containing an inorganic filler along the end of the positive electrode active material layer. (For example, see Patent Document 1).

JP-A-2017-143004

By the way, in a secondary battery for high output and high rate charging / discharging, there is a problem that the battery tends to generate heat even when a normal battery is used (charging / discharging). In particular, the current collector portion of the positive electrode current collector has a high current density and a high potential, and therefore tends to be in a highly oxidized state. According to the diligent research by the present inventors, the secondary battery provided with the insulating layer in the positive electrode current collector disclosed in Patent Document 1 can prevent a short circuit between the positive electrode and the negative electrode, but is at the end of the positive electrode active material layer during overcharging. It was found that the resistance becomes high in the part, the end part generates heat, and the battery tends to be in an unstable state.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a lithium ion secondary battery having both high rate characteristics and overcharge resistance.

As a solution to the above problems, the technique disclosed herein provides a lithium ion secondary battery including a positive electrode and a negative electrode. In this lithium ion secondary battery, the positive electrode is a positive electrode current collector, a positive electrode active material layer provided on a part of the surface of the positive electrode current collector and containing a positive electrode active material, and a surface of the positive electrode current collector. The other part is provided adjacent to the positive electrode active material layer, and includes an inorganic filler, an insulating layer containing trilithium phosphate, and a binder. Further, the negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on a part of the surface of the negative electrode current collector and containing a negative electrode active material. Here, the negative electrode active material layer includes a first region facing the positive electrode active material layer and a second region facing the insulating layer. The specific surface area S1 of the negative electrode active material contained in the first region and the specific surface area S2 of the negative electrode active material contained in the second region are as follows: 0.33 <S2 / S1 <3.33; Satisfy the relationship.

Trilithium phosphate (Li ₃ PO ₄ ; hereinafter, may be simply referred to as “LPO”) contained in the secondary battery reacts with an acid generated by oxidative decomposition of the electrolytic solution, and is a phosphate ion. It becomes (PO ₄ ^3- ) and elutes. The phosphate ions reach the negative electrode and can form a film that suppresses the exothermic reaction of the negative electrode to suitably improve the overcharge resistance of the secondary battery. According to the study by the present inventors, by providing an insulating layer in the vicinity of the positive electrode current collector, which tends to be in a high potential state due to current concentration, and arranging the LPO in this insulating layer, the film derived from this LPO generates heat of the negative electrode. It was revealed that the reaction can be suppressed particularly effectively. At this time, it was also found that, for example, the formation mode of the LPO-derived film can be suitably adjusted by controlling the specific surface area of the negative electrode active material. Furthermore, it was found that the overcharge resistance of the battery and the high rate characteristics can be effectively compatible with each other. This technique has been completed based on such knowledge. Although a configuration in which an insulating layer is provided at the positive electrode coating end is known (see, for example, Patent Document 1 and the like), it has been found that arranging an LPO on this insulating layer is particularly effective in overcharging resistance. It is a new technical matter that is unknown.

The above-mentioned lithium ion secondary battery can be provided as having high safety by suppressing heat generation at the time of overcharging. Taking advantage of these characteristics, for example, the temperature of a battery having a laminated structure (including a laminated electrode body and a wound electrode body) in which a plurality of power storage elements are laminated and repeatedly charging and discharging a large current at a high rate. The rise can be suitably suppressed. Therefore, the lithium ion secondary battery disclosed herein can be particularly preferably used as a main power source (driving power source) of, for example, a hybrid vehicle or a plug-in hybrid vehicle among vehicles.

It is a notch perspective view schematically showing the structure of the lithium ion secondary battery which concerns on one Embodiment. It is a partially developed view explaining the structure of the winding type electrode body. It is sectional drawing of the main part of the lithium ion secondary battery which concerns on one Embodiment. It is sectional drawing of the main part explaining the structure of the positive electrode end part which concerns on one Embodiment.

Hereinafter, an embodiment of the lithium ion secondary battery disclosed herein will be described. It should be noted that matters other than those specifically mentioned in the present specification (for example, the configuration of the insulating layer and the characteristics of the negative electrode active material) and necessary for carrying out the present invention (for example, the present invention is not characterized). The structure of the secondary battery, the manufacturing process, etc.) can be grasped as a design item of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in the present specification and the common general technical knowledge in the art. Further, the dimensional relations (length, width, thickness, etc.) in the drawings shown below do not necessarily reflect the actual dimensional relations. In the present specification, the notation "A to B" indicating a numerical range means "A or more and B or less".

Further, in the present specification, the "lithium ion secondary battery" refers to a battery in which lithium ion is used as an electrolyte and can be repeatedly charged and discharged by the transfer of electric charge between the positive and negative electrodes accompanying the lithium ion. The electrolyte in the lithium ion secondary battery may be, for example, a gel-like electrolyte, a solid electrolyte, or an electrolytic solution. Such a lithium ion secondary battery includes a battery generally called a lithium ion battery, a lithium secondary battery, or the like, a lithium polymer battery, a lithium ion capacitor, or the like. Above all, this technique is preferable because it can exert its effect in any way by applying it to a non-aqueous electrolyte secondary battery in which heat generation is likely to occur due to decomposition of the electrolyte during overcharging. Hereinafter, the techniques disclosed herein will be described by taking as an example the case where the lithium ion secondary battery is a non-aqueous electrolytic solution secondary battery.

[Lithium-ion secondary battery]
FIG. 1 is a notched perspective view showing the configuration of the lithium ion secondary battery 1 disclosed herein. The lithium ion secondary battery 1 is configured such that a wound electrode body 20 including a positive electrode 30 and a negative electrode 40 is housed in a battery case 10 together with a non-aqueous electrolytic solution (not shown). W in the figure indicates the width direction of the battery case 10 and the winding type electrode body 20, and coincides with the winding axis WL of the winding type electrode body 20 shown in FIG. As shown in FIG. 2, the wound electrode body 20 includes a positive electrode 30, a negative electrode 40, and two separators 50.

The positive electrode 30 includes a positive electrode current collector 32, a positive electrode active material layer 34, and an insulating layer 36.
The positive electrode active material layer 34 is a porous body containing the positive electrode active material and may be impregnated with the electrolytic solution. The positive electrode active material releases lithium ions, which are charge carriers, into or occludes the electrolytic solution. The positive electrode active material layer 34 may additionally contain a conductive material or LPO. The positive electrode active material layer 34 is provided on a part of the surface (one side or both sides) of the positive electrode current collector 32. The positive electrode current collector 32 is a member that holds the positive electrode active material layer 34 and recovers electric charges from the positive electrode active material layer 34 as the positive electrode active material layer 34 releases lithium ions. The positive electrode current collector 32 is more suitable for a conductive member which is electrochemically stable in the positive electrode environment in the battery and is made of a metal having good conductivity (for example, aluminum, aluminum alloy, nickel, titanium, stainless steel, etc.). It is composed.

The positive electrode active material layer 34 is typically bonded to the positive electrode current collector 32 while the granular positive electrode active material is bonded to each other together with the conductive material by a binder (binding agent). As the positive electrode active material, various materials conventionally used as the positive electrode active material of the lithium ion secondary battery can be used without particular limitation. Suitable examples include lithium nickel oxide (eg LiNiO ₂ ), lithium cobalt oxide (eg LiCoO ₂ ), lithium manganese oxide (eg LiMn ₂ O ₄ ) and composites thereof (eg LiNi _0.5 Mn ₁ ). _.5 O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) and other oxide particles containing lithium and transition metal elements as constituent metal elements (lithium transition metal oxide particles) and phosphoric acid Examples thereof include phosphate particles containing lithium and a transition metal element as constituent metal elements, such as lithium manganese (LiMnPO ₄ ) and lithium iron phosphate (LiFePO ₄ ). Such a positive electrode active material layer 34 may be, for example, a positive electrode active material, a conductive material, a binder (for example, an acrylic resin such as a methacrylic acid ester polymer, a vinylidene halide resin such as polyvinylidene fluoride (PVdF), or a polyethylene oxide (for example). A positive electrode paste obtained by dispersing a polyalkylene oxide such as PEO) in an appropriate dispersion medium (for example, N-methyl-2-pyrrolidone) is supplied to the surface of the positive electrode current collector 32, and then dried and dispersed. It can be produced by removing the medium. In the configuration including the conductive material, for example, a carbon material such as carbon black (typically acetylene black or Ketjen black), activated carbon, graphite, carbon fiber or the like can be preferably used as the conductive material. Any one of these may be used alone or in combination of two or more.

The average particle size (D ₅₀ ) of the positive electrode active material particles is not particularly limited, and is typically 0.5 μm or more, preferably 1 μm or more, for example, 3 μm or more, typically 15 μm or less, preferably 10 μm or less. For example, it is 8 μm or less. The ratio of the positive electrode active material to the entire positive electrode active material layer 34 may be about 50% by mass or more, typically 60% by mass or more, for example, 70% by mass or more, and typically 95% by mass or less, for example. It can be 90% by mass or less. The ratio of the conductive material in the positive electrode active material layer 34 is typically 0.1 part by mass or more, preferably 1 part by mass or more, for example, 3 parts by mass or more with respect to 100 parts by mass of the positive electrode active material. Is 15 parts by mass or less, preferably 12 parts by mass or less, for example, 10 parts by mass or less. The ratio of the binder in the positive electrode active material layer 34 is typically 0.5 parts by mass or more, preferably 1 part by mass or more, for example, 2 parts by mass or more, and typically, with respect to 100 parts by mass of the positive electrode active material. Can be 10 parts by mass or less, preferably 8 parts by mass or less, for example, 5 parts by mass or less. The thickness of the positive electrode active material layer 34 after pressing (average thickness; the same applies hereinafter) is typically 10 μm or more, for example, 15 μm or more, and typically 50 μm or less, for example, 30 μm or less. be able to. The density of the positive electrode active material layer 34 is not particularly limited, but is typically 1.5 g / cm ³ or more, for example 2 g / cm ³ or more, and 3 g / cm ³ or less, for example 2.5 g / cm ³ . It can be as follows.
In the present specification, the "average particle size" is a cumulative 50% particle size (D ₅₀ ) in the volume-based particle size distribution obtained by the laser diffraction / scattering method, unless otherwise specified. Further, the particle diameter corresponding to the cumulative 10% from the small particle size side in the particle size distribution is referred to as D ₁₀ , the particle diameter corresponding to the cumulative 90% is referred to as D ₉₀ , and the maximum frequency diameter is referred to as D _max .

According to the studies by the present inventors so far, in the configuration where the positive electrode active material layer 34 contains LPO, the ratio of LPO is the effect of improving the overcharge resistance by LPO and the increase in the viscosity of the positive electrode paste at the time of producing the positive electrode. From the viewpoint of achieving both productivity improvement and productivity improvement, it is preferable that the LPO is 0.88 to 8.8 parts by mass with respect to 100 parts by mass of the positive electrode active material. Further, the specific surface area of the LPO is preferably 0.9 to 20.3 m ² / g from the viewpoint of achieving both improvement of overcharge resistance and reduction of reaction resistance. The average particle size of the LPO is preferably 1 μm or more, more preferably 2 μm or more, for example, 2.5 μm or more, preferably 30 μm or less, more preferably 8 μm or less, and for example, 5 μm or less. The D ₉₀ of LPO is preferably 60 μm or less, more preferably 40 μm or less, and may be 20 μm or less. The D ₁₀ of LPO is preferably 0.3 μm or more, more preferably 0.6 μm or more, and may be 0.8 μm or more. The D _max is preferably 80 μm or less, more preferably 60 μm or less, and preferably 50 μm or less.

The insulating layer 36 is a porous layer having electrical insulating properties and allowing the passage of charge carriers. As shown in FIGS. 2 and 3, the insulating layer 36 is a part of the surface (one side or both sides) of the positive electrode current collector 32 and is provided in a region adjacent to the positive electrode active material layer 34. As will be described later, the insulating layer 36 is provided in a region adjacent to the positive electrode active material layer 34 (a region in which the positive electrode active material layer 34 is not formed) and at least in a region facing the negative electrode active material layer 44. .. In FIG. 3, the insulating layer 36 may protrude outward from the negative electrode active material layer 44 by the dimension α in the width direction. The dimension α is such that even if the negative electrode active material layer 44 is displaced, the negative electrode active material layer 44 and the positive electrode current collector 32 face each other only through the separator 50. The dimensions are designed so that the insulating layer 36 can sufficiently cover the end portion of the material layer 44. On the side of the insulating layer 36 that is not adjacent to the positive electrode active material layer 34, a non-coated portion 32A in which the positive electrode current collector 32 is exposed for current collection is provided. Further, the insulating layer 36 is configured to prevent a short circuit between the positive electrode current collector 32 and the negative electrode active material layer 44 even if the separator 50 described later is unintentionally damaged. Such an insulating layer 36 is typically formed by binding an inorganic filler with a binder, but the insulating layer 36 disclosed herein is characterized by further containing LPO. By including the inorganic filler in the insulating layer 36, electrochemical stability and thermal stability can be provided. Further, since the insulating layer 36 contains the LPO, the LPO is eluted from the insulating layer 36 at the time of initial charging or overcharging, effectively forming an inert high-quality film on the surface contributing to heat generation of the negative electrode, and the negative electrode. Further exothermic reaction on the surface can be suitably suppressed. In other words, for example, the overcharge resistance of the battery can be suitably increased.

Although the thickness of the insulating layer 36 (average thickness; the same applies hereinafter) is not strictly limited, for example, when a metallic foreign substance is mixed between the positive electrode and the negative electrode, the positive electrode current collector 32 and the negative electrode active material layer due to the metallic foreign substance are mixed. It is preferable that the thickness is such that a short circuit with 44 can be sufficiently suppressed. From this point of view, the thickness of the insulating layer 36 may be 1 μm or more, preferably 3 μm or more, and more preferably 4 μm or more, for example. However, it is desirable that the volume of the insulating layer 36 is as small as possible because it may cause a decrease in workability of foil collection and welding. From this point of view, the insulating layer 36 may be 10 μm or less, preferably 8 μm or less, and more preferably 6 μm or less, for example. Further, as shown in FIG. 4, when the thickness of the insulating layer 36 is T1 and the thickness of the positive electrode active material layer is T2, for example, the ratio of the thicknesses T1 and T2 (T1 / T2) is 1 or less. Typically, it is 1/2 or less, preferably 2/5 or less, more preferably 1/3 or less, more preferably 1/4 or less, 1/5 or less, and the like. Further, from the viewpoint that the insulating layer 36 fully exerts its function, the ratio (T1 / T2) is preferably 1/10 or more, and may be, for example, 1/8 or more or 1/6 or more.

The inorganic filler constituting such an insulating layer 36 does not soften or melt at a temperature of 600 ° C. or higher, typically 700 ° C. or higher, for example, 900 ° C. or higher, and can maintain insulation between positive and negative electrodes. A material having heat resistance and electrochemical stability can be used. Typically, it can be composed of the above-mentioned heat-resistant and insulating inorganic materials, glass materials, and composite materials thereof. Specific examples of such an inorganic filler include inorganic oxides such as alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ), and titania (TIO ₂ ), aluminum hydroxide, and silicon nitride. Examples thereof include metal hydroxides such as nitrides, calcium hydroxide, magnesium hydroxide and aluminum hydroxide, clay minerals such as mica, talc, boehmite, zeolite, apatite and kaolin, and glass materials. Among them, as the inorganic filler, boehmite (Al ₂ O ₃・ H ₂ O), alumina (Al ₂ O ₃ ), silica (SiO ₂ ), etc., which have stable quality and are inexpensive and easily available, are used. Is preferable, and boehmite having an appropriate hardness is more preferable. These may contain any one kind alone, or may contain two or more kinds in combination. The shape of the inorganic filler is not particularly limited, and may be, for example, various shapes such as granular, plate-like, or fibrous.

The shape of the inorganic filler is not particularly limited. From the viewpoint of preferably forming the insulating layer 36 having the above thickness, the inorganic filler typically has an average particle diameter of 3 μm or less, preferably 2 μm or less, and for example, 1 μm or less. However, an inorganic filler that is too fine is not preferable because it is inferior in handleability and uniform dispersibility with LPO. Therefore, the average particle size of the inorganic filler is typically 0.05 μm or more, preferably 0.1 μm or more, for example 0.2 μm or more.

The shape of LPO is not particularly limited. As the LPO, for example, an LPO having the same properties as that contained in the positive electrode active material layer 34 can be used. However, since LPO is relatively expensive, for example, the insulating layer 36 having the above thickness can be suitably formed, and when the battery reaches an overcharged state, it can be rapidly eluted into the electrolytic solution. preferable. From this point of view, the average particle size of LPO is typically 10 μm or less, preferably 8 μm or less, for example 5 μm or less, typically 1 μm or more, preferably 2 μm or more, for example 2.5 μm or more. Is.

When the average particle size of the inorganic filler is D1, the average particle size of the LPO in the insulating layer 36 is D2, and the average particle size of the positive electrode active material is D3, it is preferable that D1 and D2 <D3 are satisfied. It is preferable to satisfy <D2, and it is more preferable to satisfy D1 <D2 <D3. Since D1 and D2 <D3, for example, as shown in FIG. 4, when the insulating layer 36 is formed so as to overlap the end portion of the positive electrode active material layer 34, the insulation from the surface of the positive electrode current collector 32 is provided. It is possible to preferably suppress that the surface height position of the layer 36 is higher than the surface height position of the positive electrode active material layer 34 from the surface of the positive electrode current collector 32. Further, since D1 <D2, the strength of the insulating layer 36 is secured by the fine inorganic filler, and the insulating property is secured even when the LPO is eluted, so that the positive electrode current collector 32 and the negative electrode active material layer are secured. The short circuit with 44 can be suppressed more preferably. Further, the LPO can be easily arranged on the insulating layer 36 so as to elute.

The specific surface area of LPO in the insulating layer 36 is preferably 0.1 m ² / g or more, more preferably 0.5 m ² / g or more, further preferably 1 m ² / g or more, and particularly preferably 2 m ² / g or more. For example, it may be 3 m ² / g or more. However, if the specific surface area is excessively increased, the uniform miscibility with the inorganic filler and the productivity of the insulating layer 36 are deteriorated, which is not preferable. Therefore, the specific surface area of the LPO in the insulating layer 36 is preferably about 20 m ² / g or less, may be 15 m ² / g or less, and may be, for example, 10 m ² / g or less. When the specific surface area of LPO in the insulating layer 36 is S1 and the specific surface area of LPO in the positive electrode active material is S2, these ratios (S1 / S2) are preferably 0.2 or more, preferably 0.5 or more. Is more preferable, 0.8 or more is particularly preferable, and 1 or more is particularly preferable.

As the binder contained in the insulating layer 36, for example, various binders that can be used for the positive electrode active material layer can be preferably used. Among them, the binder is made of polyvinylidene from the viewpoint of preferably forming the insulating layer 36 having the above thickness while imparting the flexibility when collecting a plurality of positive current collectors 32 to the insulating layer 36. A vinyl halide resin such as vinylidene (PVdF) can be preferably used. The proportion of the binder contained in the insulating layer 36 is typically 5% by mass or more, may be 10% by mass or more, preferably 15% by mass or more, and may be 20% by mass or more. The binder contained in the insulating layer 36 is typically, for example, 40% by mass or less, may be 35% by mass or less, and is preferably 30% by mass or less.

The negative electrode 40 is configured by providing a negative electrode active material layer 44 on a negative electrode current collector 42. The negative electrode current collector 42 is provided with a non-coated portion 42A in which the negative electrode active material layer 44 is not formed for current collection and the negative electrode current collector 42 is exposed. The negative electrode active material layer 44 contains a negative electrode active material. Typically, the particulate negative electrode active materials may be bonded to each other by a binder (binder) and bonded to the negative electrode current collector 42. The negative electrode active material occludes lithium ions, which are charge carriers, from the electrolytic solution and releases them to the electrolytic solution during charging and discharging. As the negative electrode active material, various materials conventionally used as the negative electrode active material of the lithium ion secondary battery can be used without particular limitation. Preferable examples thereof include carbon materials typified by artificial graphite, natural graphite, amorphous carbon and composites thereof (for example, amorphous carbon-coated graphite), or materials that form an alloy with lithium such as silicon (Si). Lithium alloy (for example, Li _X M, M is C, Si, Sn, Sb, Al, Mg, Ti, Bi, Ge, Pb or P, etc., and X is a natural number), silicon compound (SiO, etc.), etc. Lithium storable compounds are mentioned. The negative electrode 40 is, for example, a powdery negative electrode active material and a binder (for example, styrene butadiene copolymer (SBR), rubbers such as acrylic acid-modified SBR resin (SBR-based latex), carboxymethyl cellulose (CMC), etc. A negative electrode paste obtained by dispersing (for example, water or N-methyl-2-pyrrolidone, preferably water) in a suitable dispersion medium (for example, water or N-methyl-2-pyrrolidone) is supplied to the surface of the negative electrode current collector 42 and then dried. It can be produced by removing the dispersion medium. As the negative electrode current collector, a conductive member made of a metal having good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be preferably used.

The average particle size (D ₅₀ ) of the negative electrode active material particles is not particularly limited, and may be, for example, 0.5 μm or more, preferably 1 μm or more, and more preferably 5 μm or more. Further, it may be 30 μm or less, preferably 20 μm or less, and more preferably 15 μm or less. However, as will be described later, in order to adjust the specific surface area of the negative electrode active material layers in the regions R1 and R2, the average particle size of the negative electrode active material particles can be appropriately adjusted. The ratio of the negative electrode active material to the entire negative electrode active material layer 44 is preferably about 50% by mass or more, preferably 90% by mass to 99% by mass, for example, 95% by mass to 99% by mass. When a binder is used, the ratio of the binder to the negative electrode active material layer 44 can be, for example, about 0.1 part by mass to 5 parts by mass with respect to 100 parts by mass of the negative electrode active material, and is usually about 0. .It is appropriate to use 5 parts by mass to 2 parts by mass. The thickness of the negative electrode active material layer 44 (which is the average thickness; the same applies hereinafter) can be, for example, 10 μm or more, typically 20 μm or more, and 80 μm or less, typically 50 μm or less. The density of the negative electrode active material layer 44 is not particularly limited, but is, for example, 0.8 g / cm ³ or more, typically 1.0 g / cm ³ or more, and 1.5 g / cm ³ or less, typically. Can be 1.4 g / cm ³ or less, for example 1.3 g / cm ³ or less.

Here, as shown in FIG. 3, the negative electrode active material layer 44 has a first region R1 facing the positive electrode active material layer 34 via the separator 50 and a second region facing the insulating layer 36 via the separator 50. Includes R2 and. The first region R1 and the second region R2 may have exactly the same configuration or different configurations, for example. When the configurations of the first region R1 and the second region R2 are different, it is preferable to change the specific surface area of the negative electrode active material contained therein in order to achieve both the overcharge resistance and the high rate characteristics of the battery in a well-balanced manner. For example, the specific surface area S1 of the negative electrode active material contained in the first region R1 and the specific surface area S2 of the negative electrode active material contained in the second region R2 are independently, for example, 0.5 m ² / g or more. It may be 1 m ² / g or more, and may be, for example, 1.5 m ² / g or more or 2 m ² / g or more. The specific surface area S1 of the negative electrode active material contained in the first region R1 and the specific surface area S2 of the negative electrode active material contained in the second region may be independently, for example, 15 m ² / g or less, which is typical. It may be 10 m ² / g or less, for example, 9 m ² / g or less, 7 m ² / g or less, and 6 m ² / g or less.

It can be said that the first region R1 of the negative electrode active material layer 44 has a large supply amount of lithium ions, and lithium ions are relatively easy to insert. It can be said that the supply amount of lithium ions is small in the second region R2 of the negative electrode active material layer 44, and it is relatively difficult to insert lithium ions. Therefore, a concentration gradient in which the concentration of lithium ions increases during charging tends to occur on the center side in the width direction of the negative electrode active material layer. If charging and discharging are repeated in such a form, metallic Li may be deposited near the center in the width direction of the negative electrode active material layer, and the initial resistance may increase. The ratio (S2 / S1) of the specific surface area S2 of the negative electrode active material contained in the second region R2 to the specific surface area S1 of the negative electrode active material contained in the first region R1 determines the insertability of lithium ions in the second region R2. From the viewpoint of increasing, suppressing the precipitation of metallic lithium, and reducing the resistance characteristics at the time of high-rate discharge, 0.33 or more (more than 0.33) is appropriate, and 0.4 or more is preferable, for example, 0. It may be 5 or more. In one embodiment, S2 / S1 may be 0.6 or more, 1.2 or more, or 1.5 or more. As a result, the precipitation of metallic Li can be suppressed and the increase in resistance can be suppressed.

Further, in the second region R2 facing the insulating layer 36, the LPO-derived film is less likely to be formed than in the first region R1, and the amount thereof may be smaller. In this case, the effect of improving the overcharge characteristic is reduced, which is not preferable. Therefore, the ratio S2 / S1 is 3.33 or less (3) from the viewpoint of sufficiently forming an LPO-derived film in the second region R2 and enhancing the occlusion of lithium ions to suppress the precipitation of metallic lithium. Less than .33) is appropriate, preferably 3 or less, and may be, for example, 2.5 or less or 2 or less. In one embodiment, S2 / S1 may be 1.8 or less, 1.6 or less, 1.5 or less, and the like. By appropriately adjusting the specific surface area of the negative electrode active material contained in the negative electrode active material layer 44 depending on the location, the balance between the overcharge resistance and the high rate characteristics can be better adjusted.

The surface of the negative electrode active material layer 44 may be provided with a coating film (not shown) derived from LPO. This film may be formed by initial charging after battery assembly, or may be formed by overcharging. The film derived from LPO can be confirmed by detecting a phosphate ion ( ^{PO 43-} ₎ or phosphorus (P) component on the surface of the negative electrode active material layer. As an example, the negative electrode active material layer is punched out to a predetermined size, and the surface thereof is washed with an acidic solvent (for example, sulfuric acid) to ^{elute the phosphate ion (PO 43-} ₎ or phosphorus (P) component. Then, from this eluate, for example, by quantifying phosphorus atoms by inductively coupled plasma emission spectrometry (ICP-OES) or by quantifying phosphate ions by ion chromatography, the negative electrode activity It is possible to grasp the existence of the LPO-derived film formed on the surface of the material layer and the amount of the film formed. The method for qualitative and quantitative analysis of phosphate ion ( ^{PO 43-3} ₎ or phosphorus (P) component is, for example, the above example or a known analytical chemistry method in consideration of the influence of additives in the electrolytic solution. Therefore, any person in the art can make an appropriate selection.

The separator 50 is a component that insulates the positive electrode 30 and the negative electrode 40 and provides a transfer path for the charge carrier between the positive electrode active material layer 34 and the negative electrode active material layer 44. Such a separator 50 is typically arranged between the positive electrode active material layer 34 and the negative electrode active material layer 44. The separator 50 may have a function of holding the non-aqueous electrolytic solution and a function of shutting down the movement path of the charge carrier at a predetermined temperature. Such a separator 50 can be suitably configured by a microporous resin sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Among them, the microporous sheet made of a polyolefin resin such as PE or PP preferably has a shutdown temperature in the range of 80 ° C to 140 ° C (typically 110 ° C to 140 ° C, for example 120 ° C to 135 ° C). It is preferable because it can be done. The shutdown temperature is a temperature at which the electrochemical reaction of the battery is stopped when the battery generates heat, and shutdown is typically expressed by melting or softening the separator 50 at this temperature. The separator 50 may have a single-layer structure composed of a single material, and two or more types of microporous resin sheets having different materials and properties (for example, average thickness, porosity, etc.) are laminated. (For example, a three-layer structure in which PP layers are laminated on both sides of the PE layer) may be used.

The thickness of the separator 50 (which is an average thickness; the same applies hereinafter) is not particularly limited, but can be usually 10 μm or more, typically 15 μm or more, for example, 17 μm or more. The upper limit can be 40 μm or less, typically 30 μm or less, for example, 25 μm or less. When the average thickness of the base material is within the above range, the permeability of the charge carrier can be kept good, and a minute short circuit (leakage current) is less likely to occur. Therefore, both input / output density and safety can be achieved at a high level.

The non-aqueous electrolyte solution typically dissolves or disperses a supporting salt as an electrolyte (for example, a lithium salt, a sodium salt, a magnesium salt, etc., and a lithium salt in a lithium ion secondary battery) in a non-aqueous solvent. It can be used without any particular limitation. Alternatively, it may be a so-called polymer electrolyte, a solid electrolyte, or the like, in which a polymer is added to a liquid non-aqueous electrolyte to form a gel. As the non-aqueous solvent, various organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones used as an electrolytic solution in a general lithium ion secondary battery may be used without particular limitation. can. For example, specific examples thereof include chain carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), and cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC). .. In particular, it is preferable to partially contain a solvent (for example, cyclic carbonate) that is decomposed in the acidic atmosphere of the positive electrode to generate hydrogen ions. Such a non-aqueous solvent may be fluorinated. Further, as the non-aqueous solvent, one kind may be used alone, or two or more kinds may be used as a mixed solvent. As the supporting salt, various types used in a general lithium ion secondary battery can be appropriately selected and adopted. For example, the use of lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, and LiCF ₃ SO ₃ is exemplified. Since the technique disclosed here has the effect of suppressing heat generation during overcharging, for example, a lithium compound containing fluorine that is decomposed during overcharging to generate hydrogen fluoride (HF) is used as a supporting salt. When used, it is preferable because the effect of this technique is clearly exhibited. Such a supporting salt may be used alone or in combination of two or more. The supporting salt is preferably prepared so that the concentration in the non-aqueous electrolyte is in the range of 0.7 mol / L to 1.3 mol / L.

Further, the non-aqueous electrolyte may contain various additives and the like as long as the characteristics of the lithium ion secondary battery of the present invention are not impaired. As such an additive, it can be used as a gas generating agent, a film forming agent, or the like for one or more purposes, such as improvement of input / output characteristics of a battery, improvement of cycle characteristics, improvement of initial charge / discharge efficiency, and the like. Specific examples of such additives include fluorophosphate (preferably difluorophosphate, for example, lithium difluorophosphate represented by LiPO ₂ F ₂ ), lithium bis (oxalat) borate (LiBOB), and the like. Oxalato complex compounds can be mentioned. It is appropriate that the concentration of these additives with respect to the entire non-aqueous electrolyte is usually 0.1 mol / L or less (typically 0.005 mol / L to 0.1 mol / L).

The lithium ion secondary battery 1 shown in FIG. 1 uses a flat square battery case as the battery case 10. However, the battery case 10 may be a non-flat square battery case, a cylindrical battery case, a coin battery case, or the like. Alternatively, the lithium ion secondary battery 1 may be a laminated bag formed in a bag shape by laminating a metal battery case sheet (typically an aluminum sheet) and a resin sheet. Further, for example, the battery case may be made of aluminum, iron, an alloy of these metals, high-strength plastic, or the like. Further, in the lithium ion secondary battery 1 shown in FIG. 1, for example, a long positive electrode 30 and a negative electrode 40 are laminated in a state of being insulated from each other by two separators 50, and a cross section is centered on a winding shaft WL. The so-called winding type electrode body 20 in the form of being wound in an oval shape is provided. As shown in FIGS. 2 and 3, the width W1 of the positive electrode active material layer 34, the width W2 of the negative electrode active material layer 44, and the width W3 of the separator satisfy the relationship of W1 <W2 <W3. Further, the negative electrode active material layer 44 covers the positive electrode active material layer 34 at both ends in the width direction, and the separator 50 covers the negative electrode active material layer 44 at both ends in the width direction. Further, the insulating layer 36 covers the positive electrode current collector 32 in a region facing at least the end of the negative electrode active material layer 44 while being adjacent to the positive electrode active material layer 34. However, the electrode body 20 of the lithium ion secondary battery 1 disclosed herein is not limited to the wound type electrode body, and for example, a plurality of positive electrodes 30 and 40 are insulated and laminated by a separator 50, respectively. It may be a so-called flat plate laminated type (also referred to as a single-wafer type) electrode body 20 having a different form. Alternatively, it may be a single cell in which one positive electrode 30 and one negative electrode 40 are housed in a battery case.

The battery case 10 is typically composed of a case body 11 having an opening on one side and a lid member 12 for covering the opening. Similar to the battery case of a conventional lithium ion secondary battery, the lid member 12 is provided with a safety valve for discharging the gas generated inside the battery case to the outside, a liquid injection port for injecting an electrolytic solution, and the like. May be done. Further, the lid member 12 may typically be provided with a positive electrode terminal 38 for external connection and a negative electrode terminal 48 in a state of being insulated from the battery case 10. The positive electrode terminal 38 and the negative electrode terminal 48 are electrically connected to the positive electrode 30 and the negative electrode 40 via the positive electrode current collecting terminal 38a and the negative electrode current collecting terminal 48a, respectively, and are configured to be able to supply electric power to an external load.

Although the lithium ion secondary battery disclosed herein can be used for various purposes, it may have higher safety, for example, during repeated charging and discharging at a high rate, as compared with the conventional product. Further, these excellent battery performance and reliability (including safety such as thermal stability at the time of overcharging) can be compatible at a high level. Therefore, by taking advantage of these characteristics, it can be preferably used in applications that require high energy density and high input / output density, and applications that require high reliability. Examples of such applications include drive power supplies mounted on vehicles such as plug-in hybrid vehicles, hybrid vehicles, and electric vehicles. It should be noted that such a secondary battery can be typically used in the form of an assembled battery in which a plurality of such secondary batteries are connected in series and / or in parallel.

Hereinafter, as a specific example, the lithium ion secondary battery disclosed here has been manufactured. It should be noted that the present invention is not intended to be limited to those shown in such specific examples.

(Test Example 1)
[Positive electrode]
First, a layered structure lithium nickel cobalt manganese-containing composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ : NCM) as a positive electrode active material and trilithium phosphate (Li ₃ PO ₄ : LPO). To prepare a positive electrode paste by mixing polyvinylidene fluoride (PVdF) as a binder, acetylene black (AB) as a conductive auxiliary agent, and N-methyl-2-pyrrolidone (NMP) as a solvent. did. Here, a positive electrode paste was prepared with the mass ratio of NCM: AB: PVdF = 90: 8: 2 constant and the ratio of LPO per NCM (100 parts by mass) set to 0.88 parts by mass. The NCM used had an average particle diameter of 4 μm, and the LPO used had an average particle diameter of 2.9 μm and a specific surface area of 2.0 m ² / g.

Boehmite as an inorganic filler (F), trilithium phosphate (Li ₃ PO ₄ : LPO), and PVdF (B) as a binder are dispersed in N-methyl-2-pyrrolidone (NMP) as a dispersion medium. The inorganic filler pastes of Examples 1 to 7 (common) were prepared by kneading and kneading. Here, F: B = 82: 18 was constant, and the ratio of LPO per F (100 parts by mass) was set to 0.88 parts by mass. Further, the inorganic filler paste of Example 8 was prepared in the same manner except that LPO was not added. As the boehmite, one having an average particle diameter of 0.9 μm was used, and as the LPO, one having an average particle diameter of 2.9 μm and a specific surface area of 2.0 m ² / g was used.

Then, the prepared positive electrode paste and the inorganic filler pastes of Examples 1 to 8 are simultaneously applied to a long aluminum foil having a thickness of 12 μm as a positive electrode current collector using a die coater, and dried to activate the positive electrode. A material layer and an insulating layer were formed. The positive electrode paste was supplied from the central region in the width direction of the discharge slit of the die coater, which is twice the width of the positive electrode active material layer, to the central region in the width direction of the positive electrode current collector. The inorganic filler paste is supplied from both sides of the positive electrode paste in the width direction of the discharge slit of the die coater to the region adjacent to the positive electrode active material layer in the width direction of the positive electrode current collector to first obtain a double-width positive electrode. Made. Then, a double-width positive electrode was slit at the center of the positive electrode active material layer in the width direction to obtain two positive electrodes (Examples 1 to 8). The average thickness of the positive electrode active material layer was in the above range, and the average thickness of the insulating layer was 1/5 (less than 5 μm) of the positive electrode active material layer. In the positive electrode produced in this test example, a non-coated portion, an insulating layer, and a positive electrode active material layer are provided in order along the width direction of the positive electrode current collector, and the ratio of the width of the insulating layer to the width of the positive electrode active material layer is approximately. The ratio of the width of the insulating layer to the width of the uncoated portion was 5%, and was approximately 50%.

[Negative electrode]
Graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener, C: SBR: CMC = 98: 0.7: 0.5. The negative electrode paste was prepared by kneading with ion-exchanged water at the mass ratio of. Here, as the graphite of the negative electrode active material, the surface of natural graphite coated with amorphous carbon was used. In addition, as shown in Table 1 below, seven types of negative electrode active materials having different specific surface areas of 1.0 to 10 m ² / g are prepared, and examples 1 to 1 are used by using these negative electrode active materials respectively. The negative electrode paste of 7 was prepared. The specific surface area of the negative electrode active material is determined by drying the negative electrode active material having an adjusted particle size at 200 ° C. for 10 hours or more, and then using a specific surface area measuring device manufactured by Microtrac Bell Co., Ltd. to use nitrogen gas as the adsorption gas. It is a value measured by the BET 1-point method used.

Then, the prepared negative electrode paste was applied to both sides of a long copper foil having a thickness of 10 μm as a negative electrode current collector and dried to obtain a negative electrode provided with a negative electrode active material layer. Here, as the negative electrode paste, the negative electrode paste of Example 4 is used as the paste for forming the first region of the negative electrode active material layer facing the positive electrode active material layer, and the second region facing the insulating layer is formed. As the paste for this purpose, the negative electrode pastes of Examples 1 to 7 were used respectively. That is, for the coating of the negative electrode paste, the same die coater as the one coated with the positive electrode active material layer is used, and the example is from the central region in the width direction of the discharge slit, which is the same region as the positive electrode paste is discharged. The negative electrode paste of No. 4 was supplied to the central region in the width direction of the negative electrode current collector. Further, from the discharge slits on both sides in the width direction of the die coater similar to the one supplied with the inorganic filler paste, the negative electrode pastes of Examples 1 to 7 are applied to the region adjacent to the negative electrode active material layer in the width direction of the negative electrode current collector. Supplied at the same time. As a result, a double-width negative electrode was first produced, and then the double-width negative electrode was slit at the center of the negative electrode active material layer in the width direction to obtain two negative electrodes (Examples 1 to 8). The negative electrode was provided with a non-coated portion in which a negative electrode active material layer was not formed along one end in the width direction for current collection.

[Construction of lithium-ion battery]
The positive electrode and the negative electrode of Examples 1 to 8 prepared above were superposed with each other via two separators and wound to construct a wound electrode body. At this time, the non-coated portion of the positive electrode and the non-coated portion of the negative electrode are located on opposite sides in the width direction, and the first region of the positive electrode active material layer and the negative electrode active material layer is the insulating layer and the negative electrode active. The positive electrode and the negative electrode were overlapped so that the second regions of the material layer faced each other and the non-coated portion of the positive electrode and the non-coated portion of the negative electrode were arranged on opposite sides in the width direction. As the separator, a microporous sheet having a three-layer structure of PP / PE / PP was used. The positive electrode non-coated part and the negative electrode non-coated part of the prepared winding type electrode body are connected to the positive electrode terminal and the negative electrode terminal of the battery case, respectively, housed in the case body together with the non-aqueous electrolytic solution, and then sealed. Lithium-ion batteries for evaluation of Examples 1 to 8 were obtained. As the non-aqueous electrolytic solution, a mixed solvent containing ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of EC: EMC: DMC = 3: 3: 4 as a supporting salt. LiPF ₆ was dissolved at a concentration of 1 mol / L.

[Evaluation of overcharge characteristics]
Using the lithium-ion batteries of each example, the battery temperature in the overcharged state was measured, and the temperature rise rate was calculated. That is, after performing a predetermined initial charge treatment on the lithium ion batteries of each example in a constant temperature bath at 25 ° C., a thermocouple was attached to the outer surface of the battery case to stabilize the batteries. Then, in a temperature environment of 25 ° C., charging was continued at a charging rate of 1C, and the battery reaction was shut down by the separator. At this time, the battery temperature Ts when the battery smoked due to overcharging and the maximum temperature Tm of the battery whose temperature rose after the smoke was smoked were measured, and the temperature rise rate: (Tm-Ts) ÷ Ts × 100; was calculated. .. Then, using the temperature rise rate of the lithium ion battery of Example 4 as a reference (100), the temperature rise rate of each example was standardized, and the results are shown in the column of "overcharge characteristics" in Table 1.

[Evaluation of room temperature resistance characteristics]
Using the lithium ion batteries of each example, the battery resistance at room temperature was measured. That is, in a constant temperature bath at 25 ° C., the lithium ion batteries of each example are subjected to a predetermined initial charge treatment, and then the full charge is adjusted to 27% when the SOC (State of Charge) is 100% and stabilized. rice field. Next, CC discharge was performed at a discharge rate of 10 C in a temperature environment of 25 ° C., and the voltage drop width (ΔV) for 5 seconds from the start of discharge was divided by the discharge current value to calculate the battery resistance. The results were standardized using the battery resistance of the lithium-ion battery of Example 4 as a reference (100), and are shown in the column of "high rate resistance characteristics" in Table 1.

As shown in Table 1, the battery of Example 8 having an insulating layer on the positive electrode but not containing LPO in the insulating layer has remarkably high overcharging characteristics among all the examples, and after smoking due to overcharging. It was found that the temperature rise rate of the battery is high and there is a problem in safety. On the other hand, the battery of Example 4 is a battery containing LPO in the insulating layer of the positive electrode. As is clear from the comparison between Example 4 and Example 8, by adding LPO to the insulating layer of the positive electrode, the battery temperature rises after smoke generation due to overcharging without causing inconvenience such as high battery resistance. It was confirmed that the electrochemical reaction could be stopped safely by suppressing it. This is because the acid generated by the decomposition of the electrolytic solution due to the high potential state of the positive electrode decomposes the LPO contained in the insulating layer, and this decomposition component generates heat derived from the LPO on the surface of the negative electrode active material. It is considered that this is because the difficult film was effectively formed. From this, it was confirmed that the safety (overcharge resistance) at the time of overcharging of the battery is improved by adding LPO to the insulating layer of the positive electrode.

Here, from the comparison of Examples 1 to 7, even if the amount of LPO added to the insulating layer of the positive electrode is the same, the specific surface area of the active material contained in the negative electrode active material layer is partially different, so that the battery characteristics Turned out to be very different. That is, the specific surface area S2 of the negative electrode active material in the second region of the negative electrode active material layer facing the insulating layer is compared with the specific surface area S1 of the negative electrode active material in the first region of the negative electrode active material layer facing the positive electrode active material layer. It was found that as the value increased, the value of the overcharge characteristic increased, and the rate of temperature rise after smoke generation due to overcharge increased. In particular, it was found that when S2 / S1 exceeds 3.33, the overcharge resistance sharply decreases.

This is because the specific surface area of the negative electrode active material in the second region becomes too large, so that the amount of the LPO-derived film formed on the surface of the negative electrode decreases, and the heat generation of the negative electrode due to overcharging is suitably suppressed. It is thought that it will not be possible. For example, if the specific surface area of the negative electrode active material becomes too large, it is expected that the film derived from LPO will be decomposed and its suitable formation will be hindered. From this, it can be said that S2 / S1 is preferably about 3.33 or less, for example, less than 3.33 and 2.67 or less.

On the other hand, as the specific surface area S2 of the negative electrode active material in the second region becomes smaller than the specific surface area S1 of the negative electrode active material in the first region, the value of the overcharge characteristic becomes smaller, but at the time of high-rate discharge. It was found that the initial resistance tends to increase. In particular, it was found that the resistance increased sharply when S2 / S1 reached 0.33. This is because the specific surface area of the negative electrode active material in the second region is too small, so that smooth charging and discharging cannot be performed in the second region, a concentration gradient of Li ions is generated between the second region and the negative electrode active material, and Li precipitation occurs. It is probable that he was invited. From this, it is expected that if the specific surface area of the negative electrode active material in the second region is made too small, the resistance at the time of high-rate discharge may be increased and the battery characteristics may be deteriorated. From this, it can be said that S2 / S1 is preferably 0.33 or more, preferably 0.33 or more, for example, 0.5 or more.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The techniques described in the claims include various modifications and modifications of the specific examples exemplified above.

1 Lithium-ion secondary battery 30 Positive electrode 32 Positive electrode current collector 34 Positive electrode active material layer 36 Insulation layer 40 Negative electrode 42 Negative electrode current collector 44 Negative electrode active material layer R1 First region R2 Second region 50 Separator

Claims (1)

With a positive electrode and a negative electrode,
The positive electrode is
Positive current collector and
A positive electrode active material layer provided on a part of the surface of the positive electrode current collector and containing a positive electrode active material,
An insulating layer that is another part of the surface of the positive electrode current collector and is provided adjacent to the positive electrode active material layer and contains an inorganic filler, trilithium phosphate, and a binder.
Equipped with
The negative electrode is
Negative electrode current collector and
A negative electrode active material layer provided on a part of the surface of the negative electrode current collector and containing a negative electrode active material,
Equipped with
The negative electrode active material layer includes a first region facing the positive electrode active material layer and a second region facing the insulating layer.
The specific surface area S1 of the negative electrode active material contained in the first region and the specific surface area S2 of the negative electrode active material contained in the second region are as follows:
0.33 <S2 / S1 <3.33;
Lithium-ion secondary battery that meets the relationship of.

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