Lithium-ion secondary battery

Abstract

In the present embodiment, a lithium ion secondary battery is provided, which includes a positive electrode having Li a Ni x Co y M z O2 (0.9 ≤ a ≤ 1.2, 0.3 ≤ x ≤ 0.8, 0.2 ≤ y + z ≤ 0.7, M is a metal element other than Li, Ni, and Co) as a positive electrode active material, and a negative electrode having non-graphitic carbon as a negative electrode active material. Moreover, in a portion where the positive electrode and the negative electrode face each other, the weight per unit area (P) of the positive electrode active material and the weight per unit area (N) of the negative electrode active material satisfy the relational expression of 0.65 ≤ P / N ≤ 1.05.

Description

Technical Field

[0001] The present invention relates to a lithium ion secondary battery. Background Art

[0002] In the past, lithium secondary batteries with the following internal electrode bodies are known, namely, the internal electrode bodies are formed by winding or stacking a positive electrode plate in which a positive electrode active material composed of a lithium transition metal composite oxide is arranged in a specified area on the surface of a positive electrode metal foil and a negative electrode plate in which a negative electrode active material composed of a carbonaceous material is arranged in a specified area on the surface of a negative electrode metal foil, with an insulating member interposed therebetween (for example, Patent Document 1).

[0003] In the lithium secondary battery described in Patent Document 1, the mass of the negative electrode active material per unit area of ​​the surface of the negative electrode metal foil (C (g / cm 2 )) and the mass of the positive electrode active material per unit area of ​​the surface of the positive electrode metal foil (A (g / cm 2 ))'s ratio (C / A) satisfies the relationship 0.9≤(C / A)≤2.5, and its output is above 200W.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2004-288405 Summary of the invention

[0007] Lithium secondary batteries can produce obvious performance degradation or unusable problems due to overcharging. In the past, abuse was prevented by using an overcharge protection circuit. However, when the protection circuit cannot work fully for some reason, abuse sometimes occurs. In contrast, by changing the positive electrode material of the lithium secondary battery to a material with high thermal stability such as lithium manganate and lithium iron phosphate, a prescribed durability can be obtained for overcharging. However, at the same time, the placement durability is reduced, and it is difficult to have both high overcharge resistance and high placement durability.

[0008] An object of the present embodiment is to provide a lithium ion secondary battery that is resistant to overcharge and has high storage durability.

[0009] The lithium ion secondary battery of this embodiment has a a Ni x Co y M z O 2(0.9≤a≤1.2, 0.3≤x≤0.8, 0.2≤y+z≤0.7, M is a metal element other than Li, Ni, and Co) as a positive electrode with a positive electrode active material and a negative electrode with a non-graphite carbon as a negative electrode active material, in the portion where the positive electrode and the negative electrode are opposite to each other, and the unit area weight (P) of the positive electrode active material and the unit area weight (N) of the negative electrode active material satisfy the relationship of 0.65≤P / N≤1.05.

[0010] According to the present embodiment, it is possible to provide a lithium ion secondary battery that is resistant to overcharge and has high storage durability. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a perspective view of the lithium ion secondary battery according to the present embodiment.

[0012] Figure 2 yes Figure 1 Cross-sectional view of the II-II line. DETAILED DESCRIPTION

[0013] Below, refer to Figure 1 and Figure 2 An embodiment of the lithium ion secondary battery of the present invention will be described. It should be noted that the names of the components (components) of this embodiment are names used in this embodiment and may be different from the names of the components (components) in the background art.

[0014] The lithium-ion secondary battery 1 of the present embodiment is a battery that utilizes electron transfer that occurs with the movement of lithium ions. The lithium-ion secondary battery 1 provides electrical energy. The lithium-ion secondary battery 1 is used in a single or multiple form. Specifically, when the required output and the required voltage are small, the lithium-ion secondary battery 1 is used in a single form. On the other hand, when at least one of the required output and the required voltage is large, the lithium-ion secondary battery 1 is combined with other lithium-ion secondary batteries 1 and used in a power storage device (battery module). In the above-mentioned power storage device, the lithium-ion secondary battery 1 used in the power storage device provides electrical energy.

[0015] like Figure 1 and Figure 2 As shown, the lithium-ion secondary battery 1 includes an electrode body 2 including a positive electrode and a negative electrode, a case 3 for housing the electrode body 2, and an external terminal 7 disposed outside the case 3, the external terminal 7 being electrically connected to the electrode body 2. In addition to the electrode body 2, the case 3, and the external terminal 7, the lithium-ion secondary battery 1 also includes a current collecting component 5 for electrically connecting the electrode body 2 and the external terminal 7.

[0016] The electrode body 2 is formed by winding a stacked body 22 in which positive electrodes and negative electrodes are stacked in a state of being insulated from each other by a separator.

[0017] The positive electrode has a metal foil (current collector) and a positive electrode active material layer that overlaps the surface of the metal foil and contains active material particles. In this embodiment, the positive electrode active material layer overlaps both sides of the metal foil. It should be noted that the thickness of the positive electrode can be 40 μm to 150 μm.

[0018] The metal foil is in a strip shape. The metal foil of the positive electrode of this embodiment is, for example, aluminum foil. The positive electrode has a positive electrode active material layer non-covered portion (a portion where the positive electrode active material layer is not formed) at one end edge portion in the short side direction of the strip shape, that is, in the width direction.

[0019] The positive electrode active material layer includes a particulate active material (active material particles), a particulate conductive aid and a binder. The positive electrode active material layer may contain 80% to 98% by mass of the active material. The thickness of the positive electrode active material layer (one layer portion) may be 12 μm to 70 μm. The unit area weight of the positive electrode active material layer (one layer portion) may be 4 mg / cm 2 ~17mg / cm 2 The density of the positive electrode active material layer can be 1.5 g / cm 3 ~3.0g / cm 3 The basis weight and density are the basis weight and density of one layer arranged so as to cover one surface of the metal foil.

[0020] The weight per unit area of ​​the positive electrode active material layer can be calculated as follows. When measuring the density of the manufactured and used battery, the battery is discharged to 2.0V at a current of 3A (equivalent to a current of 1C when the rated capacity of the battery can be determined), and then maintained at 2.0V for 5 hours. After maintaining, rest for 5 hours, and take out the electrode body from the inside of the shell in a dry room or a glove box set to an argon atmosphere. Wash the positive electrode taken out of the electrode body more than 3 times with dimethyl carbonate (DMC) with a purity of more than 99.9% and a moisture content of less than 20ppm. Thereafter, remove the DMC by vacuum drying. Then, cut out the set area S (cm 2 ) For example, 4cm 2 The sample of size (2cm×2cm) is measured with mass W1 (mg). The active material layer is separated from the metal foil by immersion in pure water or the like. After separation, the mass W2 (mg) of the metal foil is measured. The weight per unit area of ​​the active material layer is calculated by (W1-W2) / S.

[0021] The active material of the positive electrode is a compound that can absorb and release lithium ions. The active material of the positive electrode contains at least Li a Ni x Co y M z O 2(0.9≤a≤1.2, 0.3≤x≤0.8, 0.2≤y+z≤0.7, M is a metal element other than Li, Ni, and Co). M in the above composition formula may contain at least one metal element selected from the group consisting of Mn, Al, Mg, Zr, W, Ti, and B.

[0022] In this embodiment, it is preferred that the potential of the positive electrode be set to 4.25 V with respect to lithium (charging current per electrode unit area: 0.5 mA / cm 2 ) is 110 mAh / g to 230 mAh / g. In addition, the above-mentioned charging capacity is more preferably 150 mAh / g to 200 mAh / g, and further preferably 160 mAh / g to 180 mAh / g.

[0023] The active material of the positive electrode is a lithium transition metal oxide of the above composition, and the above charging capacity is within the above numerical range. Therefore, in relation to the charge and discharge capacity of the carbon material (non-graphitizable carbon) used in the negative electrode, it is possible to achieve both resistance to overcharge and high storage durability.

[0024] In this embodiment, the active material of the positive electrode can be Li a Ni x Co y Mn z O 2 A lithium transition metal composite oxide represented by a chemical composition of (wherein 0.9≤a≤1.2, x+y+z=1, 0.3≤x≤0.8, 0.2≤y≤0.7, 0.2≤z≤0.7). Alternatively, x≤0.55, y≤0.34, 0.25≤z≤0.34 may be used.)

[0025] As mentioned above by Li a Ni x Co y Mn z O 2 The lithium transition metal composite oxide represented by the chemical composition is, for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 、LiNi 0.55 Co 0.20 Mn 0.25 O 2 wait.

[0026] The average particle size (D50) of the active material particles of the positive electrode may be 2.0 μm to 20 μm.

[0027] The binder used in the positive electrode active material layer is, for example, polyvinylidene fluoride (PVdF), a copolymer of ethylene and vinyl alcohol, polymethyl methacrylate, polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, styrene butadiene rubber (SBR), carboxymethyl cellulose salt (CMC), etc. The binder of this embodiment is polyvinylidene fluoride.

[0028] The conductive additive of the positive electrode active material layer is a carbonaceous material. Examples of the carbonaceous material include Ketjen Black (registered trademark), acetylene black, and graphite. The positive electrode active material layer of this embodiment includes acetylene black as a conductive additive. The positive electrode active material layer may contain 1% to 15% by mass of the conductive additive.

[0029] The negative electrode has a metal foil (current collector) and a negative electrode active material layer formed on the metal foil. In the present embodiment, the negative electrode active material layers overlap on both sides of the metal foil. The thickness of the negative electrode can be 40 μm to 150 μm. The metal foil is strip-shaped. The negative electrode has a non-covered portion of the negative electrode active material layer (a portion where the negative electrode active material layer is not formed) at one end edge portion in the short side direction of the strip, i.e., the width direction.

[0030] The negative electrode has a metal foil (current collector) and a negative electrode active material layer formed on the metal foil. In the present embodiment, the negative electrode active material layers overlap on both sides of the metal foil. The metal foil is in the shape of a strip. The material of the metal foil is preferably aluminum or an aluminum alloy. In the present embodiment, the metal foil is an aluminum foil or an aluminum alloy foil, preferably an aluminum foil. An aluminum alloy is an alloy containing more than 90% by mass of aluminum. Preferably, a conductive layer can be formed on the surface of the metal foil containing aluminum. Since the metal foil contains aluminum, even when the storage element is in an over-discharged state, the dissolution of the metal foil is suppressed, and resistance to over-discharge is exerted. The negative electrode has a non-covered portion of the negative electrode active material layer (a portion where the negative electrode active material layer is not formed) at one end edge portion in the short side direction of the strip, that is, the width direction. The thickness of the negative electrode can be 40μm to 150μm.

[0031] The negative electrode active material layer contains at least a particulate active material (active material particles) and may contain a binder. The negative electrode active material layer is arranged to face the positive electrode via a separator. The width of the negative electrode active material layer is greater than that of the positive electrode active material layer.

[0032] The negative electrode active material layer may contain 80% to 98% by mass of active material. The thickness of the negative electrode active material layer (one layer) may be 10 μm to 100 μm. The unit area weight of the negative electrode active material layer (one layer) may be 2 mg / cm 2 ~10mg / cm 2The weight per unit area of ​​the negative electrode active material layer is measured by the same method as described above. The density of the negative electrode active material layer (one layer) can be 0.5 g / cm 3 ~6.0g / cm 3 .

[0033] The active material of the negative electrode can contribute to the electrode reactions of the charge reaction and the discharge reaction in the negative electrode. The active material of the negative electrode of this embodiment is non-graphite carbon. More specifically, the active material of the negative electrode is non-graphitizable carbon.

[0034] The non-graphite carbon in this specification is obtained by wide-angle X-ray diffraction using CuKα rays as a radiation source in a discharge state and an average interplanar spacing d of the (002) plane. 002 The average interplanar spacing d of the non-graphitizable carbon is 0.340 nm to 0.390 nm. 002 It is 0.360nm~0.390nm.

[0035] The binder (organic binder) that can be contained in the negative electrode active material layer is the same as the binder used in the positive electrode active material layer. As the binder, for example, styrene butadiene rubber (SBR), carboxymethyl cellulose salt (CMC) and the like are used.

[0036] The negative electrode active material layer may further contain a conductive auxiliary agent such as Ketjen Black (registered trademark) or acetylene black.

[0037] In the present embodiment, the ratio (P / N) of the unit area weight (P) of the positive electrode active material to the unit area weight (N) of the negative electrode active material is 0.65 to 1.05. The unit area weight (P) of the positive electrode active material can be calculated by multiplying the unit area weight of the above-mentioned positive electrode active material layer by the mass ratio of the positive electrode active material in the positive electrode active material layer. The unit area weight of the negative electrode active material is also calculated by multiplying the unit area weight of the negative electrode active material layer by the mass ratio of the negative electrode active material in the negative electrode active material layer. By making the above-mentioned ratio (P / N) greater than 0.65, the potential of the positive electrode is suppressed to a relatively low level and the durability is exerted. In addition, by being less than 1.05, the charging depth of the negative electrode is relatively shallow, and the resistance to overcharge is exerted. Therefore, the lithium-ion secondary battery 1 can be resistant to overcharge and has a high storage durability. In addition, as mentioned above, using Li a Ni x Co y M z O 2When a lithium transition metal oxide (0.9≤a≤1.2, 0.3≤x≤0.8, 0.2≤y+z≤0.7, M is a metal element other than Li, Ni, and Co) is used as the active material of the positive electrode, it may not be able to maintain the shelf life at high temperatures. However, by making the above-mentioned ratio (P / N) greater than 0.65, the shelf life at high temperatures can be achieved.

[0038] On the other hand, if the ratio (P / N) is less than 0.65, durability may be insufficient, and if it exceeds 1.05, the acceptance of Li ions in the negative electrode may decrease or the thermal stability of the negative electrode may lead to insufficient resistance to overcharge.

[0039] When the charging voltage of the lithium-ion secondary battery of the present embodiment is 3.6V, the potential of the negative electrode can be 300mV to 500mV in terms of lithium potential. By having the above potential of 300mV or more, the battery can be more tolerant to overcharge. By having the above potential of 500mV or less, the battery can have better storage durability. The potential of the above negative electrode can be 350mV or more. It should be noted that the potential of the above negative electrode can be adjusted by changing the unit area weight of the positive electrode active material and the unit area weight of the negative electrode active material. Specifically, by reducing the unit area weight of the positive electrode active material relative to the unit area weight of the negative electrode active material, the potential of the above negative electrode can be made higher.

[0040] In the electrode body 2 of the present embodiment, the positive electrode and the negative electrode constructed as above are wound in a state of being insulated by the separator. That is, in the electrode body 2 of the present embodiment, a stack 22 of the positive electrode, the negative electrode and the separator is wound. The separator is a component with insulating properties. The separator is arranged between the positive electrode and the negative electrode. Thus, in the electrode body 2 (more specifically, the stack 22), the positive electrode and the negative electrode are insulated from each other. In addition, the separator retains the electrolyte in the shell 3. Thus, when the lithium-ion secondary battery 1 is charged and discharged, lithium ions move between the positive electrode and the negative electrode alternately stacked with the separator sandwiched therebetween.

[0041] The separator is in a strip shape. The separator has a porous separator substrate. The separator is arranged between the positive electrode and the negative electrode to prevent a short circuit between the positive electrode and the negative electrode. The separator of this embodiment has only a separator substrate.

[0042] The separator substrate is porous. The separator substrate is, for example, a fabric, a nonwoven fabric, or a porous film. As the material of the separator substrate, polymer compounds, glass, ceramics, etc. can be cited. As the polymer compound, for example, at least one selected from polyesters such as polyacrylonitrile (PAN), polyamide (PA), polyethylene terephthalate (PET), polyolefins (PO) such as polypropylene (PP), polyethylene (PE), and cellulose can be cited.

[0043] The separator has a width (strip-shaped short-side dimension) slightly larger than the width of the negative electrode active material layer. The separator is disposed between the positive electrode and the negative electrode, which are stacked in a staggered state in the width direction so that the positive electrode active material layer and the negative electrode active material layer overlap.

[0044] The electrolyte is a non-aqueous electrolyte. The electrolyte is obtained by dissolving an electrolyte salt in an organic solvent. Examples of the organic solvent are cyclic carbonates such as propylene carbonate and ethylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. The electrolyte salt is LiClO 4 , LiBF 4 and LiPF 6 The electrolyte solution of this embodiment is prepared by dissolving 0.5 mol / L to 1.5 mol / L of LiPF in a mixed solvent of propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate at a predetermined ratio. 6 Obtained.

[0045] Case 3 includes case body 31 having an opening and cover plate 32 that plugs (closes) the opening of case body 31. Case 3 contains electrolyte in an internal space together with electrode body 2, current collector 5, etc. Case 3 is made of metal resistant to electrolyte.

[0046] The housing 3 is formed by joining the opening periphery of the housing body 31 and the periphery of the rectangular cover plate 32 in an overlapping state. In addition, the housing 3 has an internal space divided by the housing body 31 and the cover plate 32. In the present embodiment, the opening periphery of the housing body 31 and the periphery of the cover plate 32 are joined by welding.

[0047] The cover plate 32 has a gas discharge valve 321 that can discharge the gas in the housing 3 to the outside. The gas discharge valve 321 discharges the gas from the housing 3 to the outside when the internal pressure of the housing 3 rises to a predetermined pressure. The gas discharge valve 321 is provided at the center of the cover plate 32.

[0048] The housing 3 is provided with an injection hole for injecting electrolyte. The injection hole connects the inside of the housing 3 with the outside. The injection hole is provided on the cover plate 32. The injection hole is closed (plugged) by an injection plug 326. The injection plug 326 is fixed to the housing 3 (the cover plate 32 in the example of the present embodiment) by welding.

[0049] The external terminal 7 is a portion electrically connected to the external terminal 7 of another lithium ion secondary battery 1 or an external device. The external terminal 7 is formed of a conductive member. The external terminal 7 has a surface 71 to which a bus bar or the like can be welded. The surface 71 is a flat surface.

[0050] The current collecting component 5 is arranged in the housing 3 and is directly or indirectly connected to the electrode body 2 so that it can be energized. The current collecting component 5 of this embodiment is formed of a conductive component. The current collecting component 5 is arranged along the inner surface of the housing 3. The current collecting component 5 is respectively connected to the positive electrode and the negative electrode of the lithium ion secondary battery 1.

[0051] In the lithium ion secondary battery 1 of the present embodiment, the electrode body 2 (specifically, the electrode body 2 and the current collecting member 5 ) is housed in the case 3 in a state of being housed in a bag-shaped insulating cover 6 that insulates the electrode body 2 and the case 3 .

[0052] Next, a method for manufacturing the lithium ion secondary battery 1 according to the above-described embodiment will be described.

[0053] For example, in a method for manufacturing a lithium-ion secondary battery 1, first, a composition containing an active material is applied to a metal foil to form an active material layer, and a positive electrode and a negative electrode are prepared respectively. In addition, a commercially available separator is prepared, or a separator is prepared. Next, the positive electrode, the separator, and the negative electrode are overlapped to form an electrode body 2. The electrode body 2 is then placed in a housing 3, and an electrolyte is added to the housing 3, thereby assembling the lithium-ion secondary battery 1.

[0054] In the production of the positive electrode, for example, a composition containing active material particles, a binder, a conductive aid and a solvent is coated on both sides of a metal foil to form a positive electrode active material layer. As a coating method for forming the positive electrode active material layer, a common method is adopted. The unit area weight of the positive electrode active material layer is adjusted by changing the coating amount. The coated positive electrode active material layer is rolled under a specified pressure. The thickness and density of the positive electrode active material layer can be adjusted by adjusting the pressing pressure. It should be noted that the negative electrode can also be made in the same way.

[0055] When forming the electrode body 2, the electrode body 2 is formed by winding a stacked body 22 with a separator sandwiched between the positive electrode and the negative electrode. Specifically, the positive electrode active material layer and the negative electrode active material layer are opposite to each other with a separator sandwiched therebetween, and the positive electrode, the separator, and the negative electrode are stacked to form the stacked body 22. The stacked body 22 is wound to form the electrode body 2.

[0056] When assembling the lithium ion secondary battery 1, the electrode body 2 is placed in the case body 31 of the case 3, the opening of the case body 31 is plugged with the cover plate 32, and the electrolyte is injected into the case 3. When the opening of the case body 31 is plugged with the cover plate 32, the electrode body 2 is placed inside the case body 31, and the opening of the case body 31 is plugged with the cover plate 32 in a state where the positive electrode is connected to one external terminal 7 and the negative electrode is connected to the other external terminal 7. When injecting the electrolyte into the case 3, the electrolyte is injected into the case 3 from the injection hole of the cover plate 32 of the case 3.

[0057] It should be noted that the lithium ion secondary battery of the present invention is not limited to the above-mentioned embodiment, and various changes can be made within the scope of the gist of the present invention. For example, the structure of another embodiment can be added to the structure of a certain embodiment, and a part of the structure of a certain embodiment can be replaced by the structure of another embodiment. In addition, a part of the structure of a certain embodiment can be deleted.

[0058] In the above embodiment, the positive electrode in which the active material layer containing the active material is directly in contact with the metal foil is described in detail. However, in the present invention, the positive electrode may have a conductive layer containing a binder and a conductive auxiliary agent, and the conductive layer is arranged between the active material layer and the metal foil.

[0059] In the above embodiment, the electrodes in which the active material layers are disposed on both sides of the metal foil of each electrode are described in detail. However, in the lithium ion secondary battery of the present invention, the positive electrode or the negative electrode may include an active material layer only on one side of the metal foil.

[0060] In the above embodiment, the lithium ion secondary battery 1 having the electrode body 2 formed by winding the stacked body 22 is described in detail, and the lithium ion secondary battery of the present invention may have an unwound stacked body 22. In detail, the lithium ion secondary battery may have an electrode body formed by stacking a plurality of layers of a positive electrode, a separator, a negative electrode, and a separator each formed in a rectangular shape. It should be noted that the shape and size (capacity) of the lithium ion secondary battery 1 are arbitrary.

[0061] The upper limit of the normal use voltage of the lithium ion secondary battery 1 is designed to be 3.6V. When a storage device is designed by connecting a plurality of such lithium ion secondary batteries 1 in series, preferably connecting four in series, the storage device has voltage compatibility with a power supply for an automobile equipped with a conventional lead-acid battery and can be used in place of a lead-acid battery. By using such a storage device as a replacement for a lead-acid battery, deep discharge that cannot be achieved with a lead-acid battery can be performed, and light weight can be achieved.

[0062] Example

[0063] A lithium ion secondary battery was manufactured as follows.

[0064] (Test Example 1)

[0065] (1) Preparation of positive electrode

[0066] N-methyl-2-pyrrolidone (NMP) as a solvent, a conductive additive (acetylene black), a binder (PVdF), and an active material (LiNi) with a median particle size of 4.0 μm were used. 1 / 3 Co 1 / 3 Mn 1 / 3 O 2) particles were mixed and kneaded to prepare a positive electrode composition. The amounts of the conductive aid, binder, and active material were 4.5 mass %, 2.5 mass %, and 93 mass %, respectively. The prepared positive electrode composition was dried to a positive electrode active material layer with a unit area weight of 5.3 mg / cm 2 The positive electrode active material was coated on both sides of an aluminum foil (thickness 12 μm) (the unit area weight (P) of the positive electrode active material was 4.93 mg / cm 2 ). After drying by heating, roll pressing was performed. After that, vacuum drying was performed to remove moisture, etc. The thickness of the active material layer (1 layer portion) after pressing was 19 μm. The density of the active material layer was 2.8 g / cm 3 .

[0067] (2) Preparation of negative electrode

[0068] As the active material, non-graphite carbon (difficult to graphitize carbon) in particulate form with a median particle size of 4 μm was used. In addition, PVdF was used as a binder. Water, a binder, and an active material as a solvent were mixed and kneaded to prepare a composition for a negative electrode. The binder was formulated at 4% by mass, and the active material was formulated at 96% by mass. The composition for the prepared negative electrode had a unit area weight of 5.64 mg / cm2 of the negative electrode active material layer after drying. 2 The negative electrode active material was coated on both sides of an aluminum foil (thickness 12 μm) (the unit area weight (N) of the negative electrode active material was 5.42 mg / cm 2 ). After drying by heating, roll pressing was performed. After that, vacuum drying was performed to remove moisture, etc. The thickness of the active material layer (1 layer portion) after pressing was 86 μm. The density of the active material layer was 1.1 g / cm 3 .

[0069] The weight per unit area of ​​the negative electrode active material layer was set so that the potential of the negative electrode when the upper limit charging voltage of the battery was 3.6 V was 400 mV relative to lithium.

[0070] (3) Spacer (spacer substrate)

[0071] A polyethylene microporous film having a thickness of 22 μm was used as the separator substrate. The polyethylene microporous film had an air permeability of 100 sec / 100 cc.

[0072] (4) Preparation of electrolyte

[0073] As the electrolyte, an electrolyte prepared as follows was used. As the non-aqueous solvent, a mixture of 1 volume part of each of propylene carbonate, dimethyl carbonate, and ethyl methyl carbonate was used, and LiPF was dissolved in the non-aqueous solvent in such a way that the salt concentration was 1 mol / L. 6, prepare the electrolyte.

[0074] (5) The electrode body is arranged in the shell

[0075] A battery is assembled by a common method using the above-mentioned positive electrode, the above-mentioned negative electrode, the above-mentioned electrolyte, the separator, and the case.

[0076] First, the sheet-like object stacked by placing an isolator between the above-mentioned positive electrode and negative electrode is wound. Next, the wound electrode body is placed in the shell body of an aluminum square battery box as a shell. Next, the positive electrode and the negative electrode are electrically connected to two external terminals respectively. In addition, a cover plate is installed on the shell body. The above-mentioned electrolyte is injected into the shell from the injection port formed on the cover plate of the shell. Finally, the injection port of the shell is sealed to close the shell.

[0077] ·Ratio of the weight per unit area of ​​the positive electrode active material to the negative electrode active material

[0078] The ratio (P / N) of the weight per unit area of ​​the positive electrode active material (P) to the weight per unit area of ​​the negative electrode active material (N) was 0.91.

[0079] (Test Examples 2 to 7)

[0080] Without changing the weight per unit area of ​​the positive electrode active material layer and the positive electrode active material, the P / N ratio shown in Table 1 was set at 3.11 to 10.07 mg / cm 2 A lithium ion secondary battery was produced in the same manner as in Example 1 except that the weight per unit area of ​​the negative electrode active material layer was changed within a range of .

[0081] (Test Example 8)

[0082] Changing the active material of the positive electrode to LiNi 0.55 Co 0.20 Mn 0.25 O 2 The chemical composition of the active material changes the unit area weight of the negative electrode active material layer to 6.34 mg / cm 2 , the unit area weight of the positive electrode active material layer was changed to 5.30 mg / cm 2 A lithium ion secondary battery was produced in the same manner as in Example 1 except for the above.

[0083] (Test Examples 9, 10)

[0084] The active material of the positive electrode was changed to an active material with the chemical composition shown in Table 1. In addition, the concentration of 2 The weight per unit area of ​​the negative electrode active material layer was changed within the range of 5.30 mg / cm2 A lithium ion secondary battery was produced in the same manner as in Example 1 except for the above.

[0085] [Table 1]

[0086]

[0087] <Evaluation of resistance to overcharge>

[0088] Each battery was charged to 3.6 V at 3 A constant current at 25°C, and then charged at this voltage for a total of 3 hours. After that, an overcharge test was performed at 25°C at a constant current of 60 A and a final voltage of 20 V. In this test, the highest temperature was measured.

[0089] <Evaluation of (high temperature) storage durability>

[0090] After each battery is discharged at a constant current of 3A and a termination voltage of 2.4V, it is charged to 3.6V at a constant current of 3A, and then charged at a constant voltage of 3.6V for a total of 3 hours, and then discharged at a constant current of 3A and a termination voltage of 2.4V to determine the initial discharge capacity. Afterwards, each battery is charged to 3.6V at a constant current of 3A at 25°C, and further low-voltage charged at this voltage for a total of 3 hours. Store in a thermostatic chamber at 65°C for 30 days (1 month). After keeping at 25°C for 4 hours, the discharge capacity is determined in the same manner as above. Place for a total of 90 days, and calculate the discharge capacity retention rate after 90 days.

[0091] As shown in Table 1, the lithium ion secondary battery having a P / N ratio of 0.65 to 1.05 is resistant to overcharge and has high storage durability. On the other hand, the lithium ion secondary battery having a P / N ratio of less than 0.65 or greater than 1.05 is not resistant to overcharge or does not have sufficient storage durability.

[0092] When the P / N ratio is less than 0.65, the potential of the positive electrode is relatively high, thereby reducing durability. On the other hand, when the P / N ratio exceeds 1.05, polarization occurs at the negative electrode during charging, and Li electrolysis is likely to occur, and the tolerance to overcharge becomes low. It should be noted that since the negative electrode active material is non-graphitizable carbon, the potential of the negative electrode is relatively high, ensuring thermal stability of overcharge.

[0093] Explanation of symbols

[0094] 1: Lithium ion secondary battery (non-aqueous electrolyte secondary battery),

[0095] 2: Electrode body,

[0096] 3: Shell, 31: Shell body, 32: Cover plate,

[0097] 5: Current collecting parts,

[0098] 6: Insulation cover,

[0099] 7: External terminal, 71: Surface.

Claims

1. A lithium ion secondary battery having a positive electrode, a negative electrode and an electrolyte containing cyclic carbonates or chain carbonates, The positive electrode contains Li a Ni x Co y M z O 2 As the positive electrode active material, in, 0.9≤a≤1.2, 0.3≤x≤0.8, 0.2≤y+z≤0.7, x+y+z=1, M is Mn, The negative electrode contains non-graphite carbon as a negative electrode active material, Furthermore, at a portion where the positive electrode and the negative electrode are opposed to each other, the unit area weight P of the positive electrode active material and the unit area weight N of the negative electrode active material satisfy the relationship of 0.65≤P / N≤0.78, When the charging voltage of the lithium ion secondary battery is set to 3.6 V, the potential of the negative electrode is 300 mV to 500 mV relative to lithium.

2. The lithium ion secondary battery according to claim 1, in, The negative electrode has a negative electrode current collector containing aluminum.

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