JP2012064544A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery capable of maintaining a high capacity retention ratio and hardly causing lithium deposition.SOLUTION: A lithium ion secondary battery provided by the present invention includes a negative electrode comprising a negative electrode collector and a negative electrode mixture layer formed on the negative electrode collector. The negative electrode mixture layer has a negative electrode active material comprising a carbon material. When a volume per unit mass of pores with a diameter of 0.1 to 0.4 μm in the negative electrode mixture layer is defined as A [cc/g], and a specific surface area of the surface area of the negative electrode measured by the BET method in terms of a surface area per unit mass of the negative electrode mixture layer is defined as B [m/g], A/B falls in 0.0079 to 0.0091 [cc/m].

Description

  The present invention relates to a lithium ion secondary battery, and particularly to a lithium ion secondary battery including a negative electrode active material made of a carbon material.

  In recent years, lithium secondary batteries have become increasingly important as on-vehicle power supplies or personal computers and portable terminals. In particular, a lithium ion secondary battery that is lightweight and obtains a high energy density is expected to be preferably used as a high-output power source mounted on a vehicle. In one typical configuration of a lithium ion secondary battery, charging and discharging are performed by lithium ions traveling between a positive electrode mixture layer containing a positive electrode active material and a negative electrode mixture layer containing a negative electrode active material. Is called. A carbon material is preferably used as the negative electrode active material.

  Patent Document 1 discloses that in a lithium ion secondary battery including a negative electrode mixture layer containing graphite powder, the pore volume in the negative electrode mixture layer is less likely to cause a decrease in capacity during high rate charge / discharge. And that the average pore diameter is defined within a predetermined range. Specifically, the volume occupied by pores in the range of 0.1 to 10 μm in diameter measured by the mercury intrusion method is 80% or more of the total pore volume, and the average pore diameter is 0.00. It is described that the thickness is 5 μm or more and 1.5 μm or less.

JP 9-129232 A

  The present inventor has found that even if the conditions described in Patent Document 1 are followed, lithium may be deposited on the negative electrode. That is, even if the capacity retention rate can be kept high by satisfying the above conditions, lithium may be deposited on the negative electrode, and a suitable lithium ion secondary battery cannot always be obtained. I found.

  In view of the above, an object of the present invention is to provide a lithium ion secondary battery that can maintain a high capacity retention rate and hardly deposit lithium.

According to the present invention, a negative electrode having a positive electrode, a negative electrode current collector, and a negative electrode mixture layer formed on the negative electrode current collector, a separator interposed between the positive electrode and the negative electrode, and the positive electrode Provided is a lithium ion secondary battery comprising a nonaqueous electrolyte solution impregnated in the negative electrode and the separator and containing lithium ions. The negative electrode mixture layer has a negative electrode active material made of a carbon material. The volume per unit mass of pores having a diameter of 0.1 μm to 0.4 μm in the negative electrode mixture layer was defined as A [cc / g] (cc / g is synonymous with cm ³ / g), and measured by the BET method. When the specific surface area obtained by converting the surface area of the negative electrode into the surface area per unit mass of the negative electrode mixture layer is B [m ² / g], A / B = 0.0079 to 0.0091 [cc / m ² ]. It is.

  When the pore volume having a diameter of 0.1 μm to 0.4 μm in the negative electrode mixture layer is small, SEI (Solid Electrolyte Interface) is hardly generated, and the capacity retention rate is improved. However, if the pore volume is too small, lithium precipitation tends to occur. On the other hand, although the influence is small compared with the amount of pores having a diameter of 0.1 μm to 0.4 μm, an SEI film tends to be formed when the specific surface area of the negative electrode mixture layer is large, and lithium precipitation tends to occur when the specific surface area is small. It is in. The A / B is a parameter that decreases as the pore volume with a diameter of 0.1 μm to 0.4 μm in the negative electrode mixture layer decreases and increases as the specific surface area decreases. According to the lithium ion secondary battery, since the value of A / B is set in the above range, it is possible to effectively realize both the improvement of the capacity retention rate and the suppression of lithium deposition.

  In a preferred aspect of the lithium ion secondary battery disclosed herein, the negative electrode mixture layer has a first peak at a predetermined diameter in the range of 0.1 μm to 0.4 μm, and is 0.4 μm. It has a pore distribution with a second peak with a larger diameter. According to the knowledge of the present inventor, the pores of the second peak correspond to pores (in other words, gaps) between the negative electrode active materials, and the pores of the first peak exist on the surface of the negative electrode active material itself. Presumed to be a hole. With respect to the volume A per unit mass of the pores existing on the surface of the negative electrode active material, the above-described relationship is satisfied, so that both improvement of the capacity retention rate and suppression of lithium deposition can be achieved.

  In another preferable aspect of the lithium ion secondary battery, the negative electrode active material includes a carbon material having a graphite structure at least partially. In another preferred embodiment, the negative electrode active material is made of carbon particles having an average particle diameter of 5 μm to 50 μm. In another preferred embodiment, the non-aqueous electrolyte solution includes an aprotic non-aqueous solvent and a lithium compound that can be dissolved in the solvent to supply lithium ions. In another preferred embodiment, the negative electrode current collector is made of copper or an alloy containing copper as a main component. According to these aspects, it is possible to obtain a lithium ion secondary battery in which improvement in capacity retention rate and suppression of lithium deposition are highly compatible.

  The technology disclosed herein can be preferably applied to, for example, a lithium ion secondary battery for vehicle drive power supply. As another aspect of the present invention, a vehicle provided with such a lithium ion secondary battery (typically provided as a vehicle driving power source) is provided.

It is a characteristic view which shows the pore distribution of the negative electrode sheet which concerns on one Embodiment. It is a fragmentary sectional view which illustrates the structure of the lithium ion secondary battery concerning one embodiment. It is a side view of the motor vehicle provided with the lithium ion secondary battery which concerns on one Embodiment. It is a graph which shows the relationship between A / B, a capacity | capacitance maintenance factor, and the limiting current per unit area. It is a graph which shows typically the waveform of 1 cycle of a charge and discharge cycle.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The technology disclosed herein can be applied to various lithium ion secondary batteries using a carbon material as a negative electrode active material. The shape (outer shape) of the battery is not particularly limited, and may be, for example, a cylindrical shape, a square shape, a coin shape, or the like.

  As the negative electrode active material, for example, a particulate carbon material (carbon particles) having a graphite structure at least partially is preferably used. Any carbon material of a so-called graphitic material (graphite), non-graphitizable carbon material (hard carbon), graphitizable carbon material (soft carbon), or a combination of these can be used. . Carbon particles in which amorphous (amorphous) carbon is added to the surface of graphite may be used. As the properties of the negative electrode active material, for example, particles having an average particle size of about 5 μm to 50 μm are preferable. Among them, it is preferable to use carbon particles having an average particle diameter of about 5 μm to 25 μm (typically 5 μm to 15 μm, for example, about 8 μm to 12 μm). Since carbon particles having a relatively small particle size have a large specific surface area (which can be grasped as a reaction area per unit mass), they become a negative electrode active material suitable for more rapid charge / discharge (for example, high input charge). obtain. Therefore, a lithium ion secondary battery having such a negative electrode active material can be suitably used as, for example, a lithium ion secondary battery for vehicle mounting (typically, a lithium ion secondary battery for vehicle driving power source).

  The negative electrode in the technology disclosed herein typically has a configuration in which a negative electrode mixture layer containing the negative electrode active material as a main component is held by a negative electrode current collector. As the negative electrode current collector, a sheet-like member mainly composed of a highly conductive metal can be preferably used. In particular, it is preferable to use a negative electrode current collector made of copper (Cu) or an alloy containing copper as a main component (copper alloy). The size of the negative electrode current collector is not particularly limited, and can be appropriately selected according to the shape of the target lithium ion secondary battery. For example, a metal foil having a thickness of about 5 μm to 30 μm can be preferably used as the negative electrode current collector.

  The negative electrode mixture layer, for example, applies a liquid composition (typically a paste or slurry composition) in which a negative electrode active material is dispersed in a suitable solvent to a negative electrode current collector, and the composition (negative electrode) The mixture layer-forming composition) can be preferably prepared by drying. As the solvent (dispersion medium of the negative electrode active material), any of water, an organic solvent, and a mixed solvent thereof can be used. For example, an aqueous solvent (referred to as water or a mixed solvent containing water as a main component) can be preferably employed.

  The composition for forming a negative electrode mixture layer is a kind that can be blended in a liquid composition used for forming a negative electrode mixture layer in the production of a general negative electrode for a lithium ion secondary battery, in addition to the negative electrode active material and the above solvent. Or 2 or more types of materials can be contained as needed. Examples of such materials include polymers that can function as binders and / or fluidity modifiers. For example, as appropriate from polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), etc. One or two or more selected ones can be suitably used as the binder and / or fluidity modifier (typically a viscosity modifier such as a thickener).

  Although not particularly limited, the solid content of the composition for forming a negative electrode mixture layer (nonvolatile content, that is, the ratio of the negative electrode mixture layer forming component in the entire composition; hereinafter, sometimes referred to as NV). Can be about 40 mass% to about 60 mass%, for example. The proportion of the negative electrode active material in the solid content (negative electrode mixture layer forming component) is typically 50% by mass or more, and usually 85% by mass or more (typically 85 to 99.9% by mass). %), Preferably 90 to 99.5% by mass (for example, 95 to 99% by mass).

In applying such a composition to the negative electrode current collector, a technique similar to a conventionally known method can be appropriately employed. For example, a predetermined amount of the composition for forming a negative electrode mixture layer may be applied to the surface of the current collector by using an appropriate coating apparatus (such as a gravure coater, slit coater, die coater, or comma coater). The coating amount per unit area of the current collector is not particularly limited, and can be appropriately varied depending on the negative electrode sheet and battery shape, target performance, and the like. For example, the above composition is applied to both sides of a foil-like current collector (for example, a metal foil (copper foil, etc.) having a thickness of about 5 μm to 30 μm can be preferably used). Is preferably applied so that the total amount of both surfaces is about 5 to 20 mg / cm ² .

  After the coating, the coated material is dried by an appropriate drying means, and pressed as necessary to form a negative electrode mixture layer on the surface of the negative electrode current collector. As the pressing method, various conventionally known pressing methods such as a roll pressing method and a flat plate pressing method can be appropriately employed.

  As the positive electrode in the technique disclosed herein, a positive electrode mixture layer having a positive electrode active material as a main component and held in a sheet-like positive electrode current collector can be preferably used. As the positive electrode active material, one or more of various materials known to be usable as an electrode active material of a lithium ion secondary battery (for example, an oxide having a layered structure or an oxide having a spinel structure) It can be used without particular limitation. Examples thereof include lithium-containing composite oxides such as lithium nickel composite oxides, lithium cobalt composite oxides, and lithium manganese composite oxides. Another example of the positive electrode active material is a polyanionic material such as olivine type lithium phosphate.

  Here, the lithium nickel-based composite oxide is an oxide having lithium (Li) and nickel (Ni) as constituent metal elements, and at least one other metal element (that is, Li and nickel) in addition to lithium and nickel. An oxide containing a transition metal element other than Ni and / or a typical metal element) as a constituent metal element at a rate equivalent to or less than nickel in terms of the number of atoms (typically less than nickel) It means to include. Examples of the metal element other than Li and Ni include, for example, Co, Al, Mn, Cr, Fe, V, Mg, Ti, Zr, Nb, Mo, W, Cu, Zn, Ga, In, Sn, La, and Ce. It may be one or more metal elements selected from the group consisting of: The same meaning is applied to the lithium cobalt complex oxide and the lithium manganese complex oxide. In a preferred embodiment of the technology disclosed herein, a lithium transition metal composite oxide containing at least Ni, Co, and Mn as constituent metal elements is used as the positive electrode active material. For example, a lithium transition metal composite oxide containing approximately the same amount of three elements of Ni, Co, and Mn in terms of the number of atoms can be preferably used.

  In the positive electrode, such a positive electrode active material is attached to a positive electrode current collector as a layered positive electrode mixture (positive electrode mixture layer) together with a conductive material, a binder, etc. used as necessary. It may be a form. As the conductive material, a carbon material such as carbon black (acetylene black or the like), a conductive metal powder such as nickel powder, or the like can be used. As the binder, the same as the negative electrode mixture layer can be used. As the positive electrode current collector, a sheet-like member (typically a metal foil having a thickness of about 5 μm to 30 μm, for example, an aluminum foil) mainly composed of a conductive metal such as aluminum, nickel, titanium, and stainless steel is preferably used. can do.

  The ratio of the positive electrode active material to the entire positive electrode mixture is typically 50% by mass or more (for example, 50 to 95% by mass), and usually about 70 to 95% by mass (for example, 75 to 90% by mass). It is preferable. Moreover, the ratio of the electrically conductive material to the whole positive electrode mixture can be made into 2-20 mass% (preferably 2-15 mass%), for example. In the composition using a binder, the ratio of the binder to the whole positive electrode mixture can be, for example, 1 to 10% by mass (preferably 2 to 5% by mass).

  An example of the lithium ion secondary battery disclosed herein includes an electrode body having a configuration in which the positive electrode sheet and the negative electrode sheet are overlapped via a separator. As a typical example of such an electrode body, two long electrode sheets are wound together with two separator sheets or via a separator layer provided on the electrode surface or the like and wound in the long direction. An electrode body is mentioned. As said separator, the thing similar to the separator used for a general lithium secondary battery can be used, and it does not specifically limit. For example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide, a nonwoven fabric, or the like can be used.

  Such an electrode body (typically a wound electrode body) is accommodated in a suitable container (a metal or resin casing, a bag body made of a laminate film, etc.) together with a non-aqueous electrolyte containing lithium ions. Thus, a lithium ion secondary battery is constructed.

  The non-aqueous electrolyte includes a non-aqueous solvent and a lithium compound (supporting electrolyte) that can be dissolved in the solvent and supply lithium ions. As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used. For example, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile, propionitrile, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide, sulfolane, γ-butyrolactone, etc. One type or two or more types selected from non-aqueous solvents that are known to be usable for electrolytes of secondary batteries can be used.

Examples of the supporting electrolyte include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ ; LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , organic lithium salts such as LiC (SO ₂ CF ₃ ) ₃ ; lithium compounds such as; An electrolytic solution containing one of these lithium compounds alone may be used, or an electrolytic solution containing two or more types in appropriate combination may be used. In a preferred embodiment of the technology disclosed herein, a nonaqueous electrolytic solution containing LiPF ₆ alone is used as a supporting electrolyte.

  The concentration of the supporting electrolyte (supporting salt) in the non-aqueous electrolytic solution can be, for example, 0.3 mol / L to 5 mol / L (preferably 0.3 mol / L to 1.5 mol / L). In general, it is appropriate to adjust to about 0.7 mol / L to 2 mol / L (preferably about 0.7 mol / L to 1.5 mol, for example, 0.7 mol / L to 1.2 mol / L). It is.

The lithium ion secondary battery in the technology disclosed herein was measured by the BET method with the volume per unit mass of pores having a diameter of 0.1 μm to 0.4 μm in the negative electrode mixture layer being A [cc / g]. When the specific surface area obtained by converting the surface area of the negative electrode into the surface area per unit mass of the negative electrode mixture layer is B [m ² / g], it is characterized by A / B being within a predetermined numerical range. Note that a general nitrogen adsorption method is preferably used as the BET method.

  In the lithium ion secondary battery, a part of the nonaqueous electrolytic solution is decomposed during charging, and a coating made of the decomposition product, that is, a SEI (Solid Electrolyte Interface) film can be formed on the negative electrode surface. The SEI film plays a role of protecting the negative electrode, but if the amount is large, it causes a decrease in capacity retention rate. Lithium ion batteries have a large capacity deterioration particularly under high temperature conditions. It is considered that this is mainly due to the generation of the SEI film on the negative electrode and the trapping of lithium ions on the negative electrode. Therefore, in order to reduce the capacity degradation under high temperature conditions, it is considered important not to generate a lot of SEI film on the negative electrode.

The inventor has a pore distribution having at least two peaks in the negative electrode mixture layer, and if the amount of relatively smaller pores (pores having a diameter of 0.1 μm to 0.4 μm) is small, it is useless. It has been found that a stable SEI film is hard to be formed, and therefore the capacity retention rate can be improved. However, when the pore volume of the negative electrode mixture layer is small, lithium deposition occurs during rapid charging (particularly during rapid charging at a low temperature), which may reduce battery performance. On the other hand, although the influence is small compared with the amount of pores having a diameter of 0.1 μm to 0.4 μm, when the specific surface area of the negative electrode mixture layer is small, an SEI film is likely to be formed, and when the specific surface area is small, lithium precipitation tends to occur. It is in. Therefore, the present inventor uses the parameter A / B in order to improve the capacity retention rate and the resistance to lithium deposition (property of generating lithium) so that the parameter A / B is within a predetermined range. A negative electrode was constructed. The A / B decreases as the pore volume of the negative electrode mixture layer decreases, and increases as the specific surface area decreases. When the A / B is small, the capacity retention rate is improved, but the lithium precipitation resistance is considered to be lowered. Conversely, if the A / B is large, the lithium deposition resistance is improved, but the capacity retention rate is considered to be reduced. The A / B can also be grasped by an index representing the degree of penetration of the electrolyte into the active material particle surface. By specifying the above A / B, the improvement of the capacity retention rate and the suppression of the lithium precipitation can be effectively realized because the degree of the penetration of the electrolytic solution depends on the generation of SEI and the lithium precipitation. It is also considered to have a big influence. In the lithium ion secondary battery in the technology disclosed herein, A / B = 0.0001 to 0.0091 [cc / m ² ].

  The volume A [cc / g] per unit mass of pores having a diameter of 0.1 μm to 0.4 μm in the negative electrode mixture layer can be obtained, for example, as follows. That is, for a negative electrode (having a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector) to be measured, a diameter of 0.1 μm to 0.4 μm using a commercially available mercury porosimeter or the like. It can be calculated by measuring the pore volume V [cc] and dividing the measured value by the weight W2 [g] of the negative electrode mixture layer. A = V / W2. The weight W2 [g] of the negative electrode mixture layer can be calculated by subtracting the weight W1 [g] of the negative electrode current collector from the weight W0 [g] of the whole negative electrode. That is, first, the weight W0 of the whole negative electrode is measured. Thereafter, the negative electrode mixture layer is removed from the negative electrode, and the weight W1 of the remaining negative electrode current collector is measured. And weight W2 = W0-W1 of a negative mix layer is obtained by reducing weight W1 from weight W0.

  The pore distribution of the negative electrode shows a distribution as shown in FIG. 1, for example. The pore distribution of the negative electrode has two peaks, and has a first pore distribution region P1 having a relatively small diameter and a second pore distribution region P2 having a relatively large diameter. The horizontal axis in FIG. 1 represents the pore diameter, but the pore diameter increases toward the left. The first pore distribution region P1 has a peak at a predetermined pore diameter within a range of 0.1 μm to 0.4 μm. The second pore distribution region P2 has a peak at a predetermined pore diameter larger than 0.4 μm. The pores belonging to the first pore distribution region P1 are pores existing on the surface of the active material particles, and the pores belonging to the second pore distribution region P2 are pores formed between the active material particles. It is guessed. The pores having a diameter of 0.1 μm to 0.4 μm are pores belonging to the first pore distribution region P1.

The specific surface area B [m ² / g] of the negative electrode mixture layer is obtained by converting the surface area B ′ [m ² ] of the whole negative electrode measured by the BET method into the surface area per unit mass of the negative electrode mixture layer. . That is, B = B ′ / W2. In general, since the surface area of the negative electrode current collector is extremely smaller than the surface area of the negative electrode mixture layer, the surface area of the negative electrode current collector can be ignored in the calculation of B.

  Hereinafter, a schematic configuration of an embodiment of a lithium ion secondary battery to which the technology disclosed herein is applied will be described with reference to the drawings. A lithium ion secondary battery 10 shown in FIG. 2 includes a flat rectangular container 11 (typically made of metal and may be made of resin). In this container 11, a wound electrode body 30 formed by stacking and winding a positive electrode sheet 32, a negative electrode sheet 34, and two separator sheets 35 is accommodated. The positive electrode sheet 32 has a configuration in which a positive electrode mixture layer is provided on both surfaces of a long positive electrode current collector (for example, an aluminum foil) while leaving one end portion along the longitudinal direction of the current collector in a band shape. Have. The negative electrode sheet 34 has a configuration in which a negative electrode mixture layer is provided on both sides of a long negative electrode current collector (for example, a copper foil) while leaving one end along the longitudinal direction of the current collector in a strip shape. Have. The wound electrode body 30 is configured such that portions of the electrode sheets 32 and 34 where the mixture layer is not provided (mixture layer non-forming portions 32A and 34A) are on one end and the other along the longitudinal direction of the separator sheet 35. Each of the wound bodies is rolled up so as to protrude from the end of the container, and the wound body is pressed from the side surface direction to be ablated, thereby forming a flat shape that matches the shape of the container 11.

  A positive electrode terminal 14 and a negative electrode terminal 16 for external connection are electrically connected to the electrode sheets 32 and 34. In this connection, portions of the mixture sheet non-forming portions 32A, 34A of both electrode sheets 32, 34 that are protruded from the separator sheet 35 are gathered together in the radial direction of the wound electrode body 30, and a positive electrode is formed on the gathered portions. It can be suitably performed by connecting (for example, welding) the terminal 14 and the negative electrode terminal 16 respectively. The electrode body 30 to which the terminals 14 and 16 are connected is accommodated in the container 11, and after supplying an appropriate nonaqueous electrolytic solution therein, the container 11 is sealed, whereby the lithium ion secondary according to the present embodiment. Battery 10 is constructed.

  The lithium ion secondary battery 10 according to the present embodiment can be used as a secondary battery for various applications. For example, as shown in FIG. 3, it can be suitably used as a power source for a vehicle drive motor (electric motor) mounted on a vehicle 1 such as an automobile. Although the kind of vehicle 1 is not specifically limited, Typically, they are a hybrid vehicle, an electric vehicle, a fuel cell vehicle, etc. Such lithium ion secondary battery 10 may be used alone, or may be used in the form of an assembled battery that is connected in series and / or in parallel.

  Several embodiments relating to the present invention will be described below. However, it goes without saying that the present invention is not limited to the following examples.

<Production of lithium ion secondary battery>
Eight types of graphite powders C1 to C8 having different specific surface areas were prepared, and negative electrodes using these graphite powders as negative electrode active materials were prepared. That is, each graphite powder, SBR, and CMC were mixed with ion-exchanged water so that the mass ratio of these materials was 98: 1: 1 and NV was 45% by mass to form a negative electrode mixture layer. A composition was prepared. This composition was applied to both sides of a long copper foil (negative electrode current collector) having a thickness of 10 μm and dried to form a negative electrode mixture layer. The basis weight of the composition, that is, the coating amount (based on solid content) was adjusted to 9.4 mg / cm ^{2 on} both sides. After drying under the condition of a drying temperature of 120 ° C., pressing was performed so that the total thickness of the negative electrode current collector and the negative electrode mixture layers on both sides thereof became 60 μm. In this way, a total of eight types of sheet-like negative electrodes (that is, negative electrode sheets N1 to N8) corresponding to the graphite powders C1 to C8 were produced. In addition, the average particle diameters of the graphite powders C1 to C8 are all in the range of 6.5 μm to 22 μm.

As the positive electrode active material, a lithium nickel cobalt manganese composite oxide powder having a composition represented by LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was used. The positive electrode active material powder, acetylene black as a conductive material, and PVDF as a binder are such that the mass ratio of these materials is 87: 10: 3 and the solid content concentration (NV) is about 40% by mass. A composition for forming a positive electrode mixture layer was prepared by mixing with NMP. The composition was applied on both sides of a 15 μm thick long aluminum foil (positive electrode current collector) and dried to form a positive electrode mixture layer. The coating amount (based on solid content) of the composition was adjusted to be about 12.8 mg / cm ^{2 on} both sides. After drying, the positive electrode current collector and the positive electrode mixture layers on both sides thereof were pressed so as to have a total thickness of 25 μm to produce a sheet-like positive electrode (hereinafter referred to as a positive electrode sheet).

  Each of the negative electrode sheets N1 to N8 was laminated with the positive electrode sheet and two long separator sheets, and the laminated sheet was wound in the long direction to produce a wound electrode body. As the separator sheet, a porous polyethylene sheet having a thickness of 20 μm was used. In this way, a total of eight types of wound electrode bodies corresponding to the respective negative electrode sheets N1 to N8 were produced.

A non-aqueous electrolyte was prepared by dissolving 1.0 mol / L LiPF ₆ (supporting electrolyte) in a mixed solvent containing EC, EMC, and DMC in a volume ratio of 3: 4: 3.

  And the 18650 type lithium ion secondary battery was constructed | assembled by combining the said electrode body and the said electrolyte solution, and accommodating in an exterior case. Hereinafter, the lithium ion secondary batteries including the negative electrode sheets N1 to N8 are referred to as batteries of Examples A1 to A8, respectively.

[Measurement of pore volume A]
With respect to the negative electrode sheets N1 to N8, the volume A [cc / g] per unit mass of pores having a diameter of 0.1 μm to 0.4 μm was measured. For the measurement, “Pore master GT60” (pressure range 0.7 psi to 33000 psi) manufactured by Quantachtome was used. The obtained values are shown in Table 3. In Table 3, it is expressed as pore volume A [cc / g].

[Measurement of specific surface area B]
The specific surface area B [m ² / g] was measured for the negative electrode sheets N1 to N8. For measurement, “Macsorb” manufactured by MOUNTECH was used. The measurement was performed after drying for 1 hour under a temperature condition of 120 ° C. (pre-drying) and pre-deaeration for 5 minutes under a temperature condition of 120 ° C. The obtained values are shown in Table 3.

[Initial charge / discharge (conditioning)]
The batteries of Examples A1 to A8 were subjected to initial charge / discharge (conditioning) under the conditions shown in Table 1 below. That is, under the temperature condition of 25 ° C., the following charging, resting, and discharging were performed. Specifically, first, the battery was charged at a constant current of 1/4 C until the voltage between the terminals reached 4.1 V, and then charged at a constant voltage until the total charging time reached 6 hours (CCCV charging). Then, after resting for 10 minutes, the battery was discharged at a constant current of 1/3 C until the voltage between the terminals reached 3.0 V (CC discharge). Next, after resting for 10 minutes, the battery was charged at a constant current of 1 C until the voltage between the terminals reached 4.1 V, and then charged at a constant voltage until the total charging time reached 2 hours (CCCV charging). Then, after resting for 10 minutes, the battery was discharged at a constant current of 1 C until the voltage between the terminals reached 3.0 V (CC discharge). Next, after resting for 10 minutes, the battery was charged at a constant current of 1 C until the voltage between the terminals reached 4.1 V, and then charged at a constant voltage until the total charging time reached 2.5 hours (CCCV charging). . Then, after resting for 10 minutes, the battery was discharged at a constant current of 1/3 C until the voltage across the terminals reached 3.0 V (CC discharge). After resting for 10 minutes, it was further fixed until the total discharge time reached 4 hours. After discharging with voltage (CV discharge), the operation was stopped for 10 minutes.

[Measurement of capacity maintenance ratio]
About the battery of Examples A1-A8, the test which investigates how much capacity | capacitance is maintained after preserve | saving under predetermined | prescribed high temperature conditions was done.

  First, for the batteries of Examples A1 to A8, the initial capacity was measured under the conditions shown in Table 2. That is, under a temperature condition of 25 ° C., the battery was charged at a constant current of 1 C until the voltage between the terminals reached 4.1 V, and then charged at a constant voltage until the total charging time reached 2.5 hours (CCCV charging ). After 10 minutes from the end of charging, the battery is discharged at 25 ° C. with a constant current of 0.33 C until the voltage between the terminals reaches 3.0 V (CC discharge). After 10 minutes of rest, the total discharge time is 4 The battery was discharged at a constant voltage until time reached (CCCV discharge). The discharge capacity at this time was defined as the initial capacity Q1 of each example.

  The batteries of Examples A1 to A8 were stored for 60 days in a temperature environment of 60 ° C., and then discharged under the same conditions as in the initial capacity measurement at 25 ° C., and the battery capacity Q2 at this time was determined. And the capacity maintenance rate defined by Q2 / Q1 * 100 [%] was computed. The results obtained are shown in Table 3 and FIG.

[Evaluation test for lithium deposition]
A laminate type battery cell using the negative electrode sheets N1 to N8 was produced, and a test for evaluating the difficulty of depositing lithium was performed. That is, each of the electrode bodies including the negative electrode sheets N1 to N8 is covered with a laminate film (7 cm × 7 cm (one side)) so that a part of both terminal portions is exposed, an electrolyte is injected, and the laminate film is sealed. 8 types of laminated battery cells B1 to B8 were produced. The positive electrode, the separator, and the electrolytic solution are the same as in the batteries of Examples A1 to A8.

  After the same conditioning as the batteries of Examples A1 to A8, each battery cell B1 to B8 adjusted to a state of SOC 60% is shown in FIG. The charge / discharge cycle was repeated 250 cycles. That is, as one cycle, the center voltage was set to 3.75 V at a temperature of 0 ° C., charging was performed for 10 seconds at a predetermined current, and then discharging was performed for 10 seconds at the same rate. The pause time between charging and discharging was 10 minutes.

  The present inventor obtained from the experience of performing many similar tests that the lithium can be regarded as substantially deposited if the capacity retention rate of the battery is less than 97%. This is based on the accumulation of data obtained by actually disassembling the battery after the above test and visually confirming the presence or absence of lithium deposition. Therefore, after the above 250 cycles of charge / discharge, the capacity of the battery cells of Examples B1 to B8 was measured. If the capacity retention rate was 97% or more, lithium deposition did not occur. It was assumed that precipitation occurred. Then, the limit current at which lithium deposition does not occur was measured, and the limit current per unit area was calculated using the current as the limit current. The results obtained are shown in Table 3 and FIG.

From FIG. 4, it can be seen that Examples A1 to A5 using the negative electrode sheets N1 to N5 have a higher capacity retention rate than Examples A6 to A8 using the negative electrode sheets N6 to N8. Moreover, it turns out that the limiting current is higher in Examples B2 to B8 using the negative electrode sheets N2 to N8 than in Example B1 using the negative electrode sheet N1. As mentioned above, according to the negative electrode sheet | seat N2-N5 which exists in the range of A / B = 0.0079-0.0091 [cc / m < ² >], improvement of a capacity | capacitance maintenance factor and suppression of lithium precipitation can be highly compatible. confirmed.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here.

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Lithium ion secondary battery 11 Container 14 Positive electrode terminal 16 Negative electrode terminal 30 Winding electrode body 32 Positive electrode sheet 34 Negative electrode sheet 35 Separator sheet

Claims (7)

A negative electrode having a positive electrode, a negative electrode current collector, and a negative electrode mixture layer formed on the negative electrode current collector, a separator interposed between the positive electrode and the negative electrode, the positive electrode, the negative electrode, and the separator And a non-aqueous electrolyte containing lithium ions impregnated with, a lithium ion secondary battery comprising:
The negative electrode mixture layer has a negative electrode active material made of a carbon material,
The volume per unit mass of pores having a diameter of 0.1 μm to 0.4 μm in the negative electrode mixture layer is A [cc / g], and the surface area of the negative electrode measured by the BET method is the unit mass of the negative electrode mixture layer. A lithium ion secondary battery in which A / B is 0.0079 to 0.0091 [cc / m ² ], where B [m ² / g] is a specific surface area converted to a per surface area.   The negative electrode mixture layer has a pore distribution having a first peak at a predetermined diameter in a range of 0.1 μm to 0.4 μm and a second peak at a diameter larger than 0.4 μm. The lithium ion secondary battery according to claim 1.   The lithium ion secondary battery according to claim 1, wherein the negative electrode active material includes a carbon material having a graphite structure at least partially.   The lithium ion secondary battery according to claim 1, wherein the negative electrode active material is made of carbon particles having an average particle diameter of 5 μm to 50 μm.   The lithium according to any one of claims 1 to 4, wherein the non-aqueous electrolyte includes an aprotic non-aqueous solvent and a lithium compound that can be dissolved in the solvent to supply lithium ions. Ion secondary battery.   The lithium ion secondary battery according to any one of claims 1 to 5, wherein the negative electrode current collector is made of copper or an alloy containing copper as a main component.   The lithium ion secondary battery as described in any one of Claims 1-6 used as a vehicle drive power supply.

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