JP2007250299A - Nonaqueous electrolyte solution secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte solution secondary battery capable of easily detecting a charging state without the use of a complicated electronic circuit. <P>SOLUTION: A laminated-film-sheathed battery 1 has two or more kinds of cathode active materials mixed in a cathode. A charge and discharge curve with a battery residual capacity as a horizontal axis and a battery voltage as a vertical axis at the charging and discharging of the sheathed battery 1 has both a quasi-flat part in the vicinity of 3.7 V of lithium-containing transition metal complex oxide with a lamellar crystal structure and a spinel crystal structure and a quasi-flat part in the vicinity of 3.4 V of lithium-containing transition metal complex oxide with an olivine crystal structure. Between the quasi-flat parts, there is a variable part in which the battery voltage varies against the change of the battery residual capacity by a portion equivalent to a difference of battery voltages of the two quasi-flat parts. By detecting changes of battery voltages at the variable part, a battery residual voltage equivalent to the variable part can be detected. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery including a positive electrode and a negative electrode capable of inserting and removing lithium ions.

  Conventionally, non-aqueous electrolyte secondary batteries have a high energy density and are therefore widely used for small consumer applications as power sources for home appliances and the like. Large non-aqueous electrolyte secondary batteries have also been put into practical use as power sources for electric vehicles. In these applications, a single non-aqueous electrolyte secondary battery may be used, or a battery system incorporating a plurality of non-aqueous electrolyte secondary batteries may be used.

  In such a non-aqueous electrolyte secondary battery or battery system, the state of charge (the ratio of the remaining battery capacity to the full charge capacity of the non-aqueous electrolyte secondary battery) is detected in order to avoid overcharge or overdischarge. This is very important. In general, a complicated electronic circuit is used to detect the state of charge of a nonaqueous electrolyte secondary battery. For example, a technique for detecting a charge state from a battery voltage actually detected using a table representing a relationship between a charge state and a battery voltage stored in advance in a storage element is disclosed (see Patent Document 1). It also has a circuit and memory element for detecting internal impedance, and compares the impedance value stored in advance in the memory element with the impedance value calculated from the actually detected battery current and voltage to determine the state of charge. A technique for detection is disclosed (for example, see Patent Document 2).

JP 2000-78769 A JP 2004-138586 A

  However, the above-described techniques of Patent Documents 1 and 2 both require a complicated electronic circuit, and in order to detect the battery voltage and internal resistance for each battery and detect the state of charge, the battery system and the like are individually designed. Since it is necessary to do this, the cost increases and the versatility also decreases. Further, at the time of charging / discharging, since the battery voltage changes gradually with respect to the change of the charging state, that is, the change of the remaining battery capacity, detection accuracy may be lowered when the charging state is detected from the battery voltage. In this regard, since the battery voltage greatly decreases with respect to the change in the remaining battery capacity at the end of discharge, the detection accuracy of the charged state from the detected battery voltage can be improved, but in this state the charged state decreases. Therefore, it is unavoidable to immediately stop discharging and shift to a charging operation.

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery that can easily detect the state of charge without using a complicated electronic circuit.

  In order to solve the above problems, the present invention provides a non-aqueous electrolyte secondary battery including a positive electrode and a negative electrode capable of inserting and releasing lithium ions, the positive electrode including two or more positive electrode active materials, and The charge / discharge curve representing the relationship between the battery remaining capacity and the battery voltage when charging / discharging the non-aqueous electrolyte secondary battery has the battery remaining capacity on the horizontal axis and the battery voltage on the vertical axis. The battery voltage is substantially flat, and has two or more flat portions in which the change in the battery voltage is small with respect to the change in the remaining battery capacity, and the battery with respect to the change in the remaining battery capacity from the flat portion is between the flat portions. A slope portion with a large voltage change is interposed.

  In the present invention, since the positive electrode contains two or more positive electrode active materials, the charge / discharge curve representing the relationship between the remaining battery capacity and the battery voltage when the nonaqueous electrolyte secondary battery is charged / discharged is the remaining battery capacity. When the horizontal axis is taken and the battery voltage is taken on the vertical axis, the battery voltage is substantially flat and has two or more flat portions with small changes in the battery voltage with respect to changes in the remaining battery capacity. In the slope part, when changing from one flat part to the other flat part, it corresponds to the difference in battery voltage between the two flat parts, and the change in the battery voltage is larger than the flat part. Since the remaining battery capacity corresponding to the slope is detected only by the circuit that detects the change in battery voltage, the state of charge of the non-aqueous electrolyte secondary battery can be easily detected without using a complicated electronic circuit. It can be.

In this case, the positive electrode active material is at least one of a lithium-containing transition metal composite oxide having an olivine crystal structure, a lithium-containing transition metal composite oxide having a layered crystal structure, and a lithium-containing transition metal composite oxide having a spinel crystal structure. One of them may be included. At this time, the lithium-containing transition metal composite oxide having an olivine crystal structure is represented by the chemical formula Li _{1 + y} M _1-y PO ₄ (M is one or more selected from Mn, Co, Ni, Cr, Al, Mg, Fe). A lithium-containing transition metal composite having a layered crystal structure, which is at least one of a compound represented by formula ( ₁ ) and a compound represented by the chemical formula Li _{1 + y} M _1-y PO _4. The oxide may be a compound represented by the chemical formula Li _{1 + x} M _1-x O ₂ , and the lithium-containing transition metal composite oxide having a spinel crystal structure may be the compound represented by the chemical formula Li _{1 + x} M _2-x O ₄ .

  According to the present invention, since the positive electrode includes two or more positive electrode active materials, the charge / discharge curve when the non-aqueous electrolyte secondary battery is charged / discharged has the remaining battery capacity on the horizontal axis and the battery voltage on the vertical axis. Since the battery voltage becomes substantially flat when it is taken, and there are two or more flat portions in which the change in the battery voltage is small with respect to the change in the remaining battery capacity, the slope voltage between the flat portions causes a change in the battery voltage larger than the flat portion. Therefore, since the remaining battery capacity corresponding to the slope portion is detected only by the circuit that detects the change in the battery voltage, the state of charge of the nonaqueous electrolyte secondary battery can be easily achieved without using a complicated electronic circuit. The effect that it can detect can be acquired.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of a lithium ion secondary battery (hereinafter referred to as a laminate film exterior battery) using a laminate film as a battery exterior to which the present invention can be applied will be described with reference to the drawings.

(Laminated film battery)
As shown in FIG. 1, the laminated film exterior battery 1 has two rectangular laminate films 2 used for the battery exterior. A laminated electrode group (not shown) is enclosed between the two laminated films 2. The upper laminate film 2 is formed in a convex shape along a laminated electrode group not shown, and the lower laminate film 2 is formed substantially flat. The outer edges of the two laminated films 2 are heat-welded to each other, and the laminated film exterior battery 1 has a sealed structure. On one side of the outer edge portion of the laminated film-clad battery 1, a positive electrode terminal 3 is sandwiched between two laminate films 2 with a positive electrode terminal 3 projecting on one side and a negative electrode terminal 4 projecting on the other side. .

  The laminate film 2 uses an aluminum foil as a base material. The laminate film 2 includes a nylon film for insulation protection, an aluminum foil as a base material, and a polypropylene (hereinafter abbreviated as PP) film as a heat-welded resin layer in this order through an adhesive. It is pressed. For this reason, the laminate film 2 has a three-layer structure.

  An aluminum plate is used for the positive electrode terminal 3, and a tape made of PP is attached to the outer periphery of the aluminum plate as a seal tape. A nickel plate is used for the negative electrode terminal 4, and a PP tape is attached to the outer periphery of the nickel plate as a seal tape. Around the positive electrode terminal 3 and the negative electrode terminal 4, the PP resin of the PP film softened at the time of heat welding of the laminate film 2 is in close contact with the gap.

  A mixed positive electrode active material in which two or more positive electrode active materials are mixed is used for the positive electrode constituting the laminated electrode group. The positive electrode active material to be mixed is capable of inserting and removing lithium ions from the same negative electrode described later, the electrolyte constituting the nonaqueous electrolyte and its solvent (operable), and the insertion of lithium ions. The battery voltage range that accompanies the separation (hereinafter referred to as the operating voltage range) is different. The mixed positive electrode active material includes at least lithium-containing transition metal composite oxide having an olivine crystal structure, lithium-containing transition metal composite oxide having a layered crystal structure, and lithium-containing transition metal composite oxide having a spinel crystal structure. One and the other are included.

The lithium-containing transition metal composite oxide having an olivine crystal structure includes a chemical formula Li _{1 + y} M _1-y PO ₄ (M is one or more transitions selected from Mn, Co, Ni, Cr, Al, Mg, Fe) And a compound represented by the chemical formula Li _{1 + y} M _1-y PO ₄ and at least one compound in which carbon is supported can be used. When the lithium-containing transition metal composite oxide having this olivine crystal structure is used as the positive electrode active material, the operating voltage range is around 3.4V. In addition, when the charge / discharge curve representing the relationship between the remaining battery capacity and the battery voltage during charging / discharging takes the remaining battery capacity on the horizontal axis and the battery voltage on the vertical axis, the battery voltage with respect to the change in the remaining battery capacity Has a quasi-flat portion (flat portion) that is substantially constant (substantially flat) around 3.4V.

On the other hand, a compound represented by the chemical formula Li _{1 + x} M _1-x O ₂ can be used for the lithium-containing transition metal composite oxide having a layered crystal structure, and the lithium-containing transition metal composite oxide having a spinel crystal structure can be used. May be a compound represented by the chemical formula Li _{1 + x} M _2-x O ₄ . When a lithium transition metal composite oxide having a layered crystal structure or a spinel crystal structure is used as the positive electrode active material, the operating voltage range is around 4V. Further, the charge / discharge curve has a quasi-flat portion where the battery voltage is substantially constant near the lower limit of the operating voltage range, that is, near 3.7 V with respect to the remaining battery capacity.

  By using such a mixed positive electrode active material, the mixed positive electrode active material inserts and desorbs lithium ions in each operating voltage range during charging and discharging. In the laminated film exterior battery 1, the charge / discharge curve has a quasi-flat portion near 3.7 V of the lithium-containing transition metal composite oxide having a layered crystal structure or a spinel crystal structure, and a lithium-containing transition metal composite oxide having an olivine crystal structure. It has two quasi-flat parts with a quasi-flat part near 3.4V of the object. Further, in the charge / discharge curve of the laminated film-clad battery 1, a change in which the battery voltage changes between two quasi-flat portions corresponding to the difference in battery voltage between the two quasi-flat portions with respect to the change in the remaining battery capacity. Part (slope part). The change in the battery voltage at the changing portion is larger than that in the quasi-flat portion where the battery voltage is substantially constant.

  The mixed positive electrode active material powder, the carbon material of the conductive agent, and the binder polyvinylidene fluoride (hereinafter abbreviated as PVDF) previously dissolved in the solvent normal methylpyrrolidone (hereinafter abbreviated as NMP). A positive electrode mixture slurry was prepared by stirring and mixing together. The obtained positive electrode mixture slurry was applied almost uniformly on both sides of an aluminum foil (positive electrode current collector) and dried. This was press-molded so as to have a thickness of 90 μm, for example, and cut into a rectangular shape to obtain a positive electrode. In addition, a plain portion where no positive electrode mixture slurry is applied for current collection is left on one side of the positive electrode, and a positive electrode lead piece is formed in this plain portion.

  On the other hand, amorphous carbon powder is used for the negative electrode as a negative electrode active material. The amorphous carbon powder was thoroughly stirred and mixed with PVDF as a binder and NMP as a solvent to prepare a negative electrode mixture slurry. The obtained negative electrode mixture slurry was applied almost uniformly on both sides of a copper foil (negative electrode current collector) and dried. This was press-molded so as to have a thickness of 70 μm, for example, and cut into a rectangular shape to obtain a negative electrode. Note that a plain portion where no negative electrode mixture slurry is applied for current collection is left on one side of the negative electrode, and a negative electrode lead piece is formed on this plain portion.

  Four sheets of the prepared positive electrode and five sheets of the negative electrode were alternately laminated one by one through, for example, a polyethylene microporous membrane separator having a thickness of 40 μm to produce a laminated electrode group. At this time, both ends of the laminated electrode group were made negative, and one separator was laminated between the positive electrode and the negative electrode. Moreover, it laminated | stacked so that the positive electrode lead piece may be located in the one side of the laminated electrode group, and the negative electrode lead piece may be located in the other side. Four positive electrode lead pieces and five negative electrode lead pieces were assembled and attached to the positive electrode terminal 3 and the negative electrode terminal 4 by ultrasonic welding, respectively.

The laminated electrode group produced between the two laminate films 2 was sandwiched, and the outer edge portions of the laminate film 2 were overlapped. At this time, the leading end portions of the positive electrode terminal 3 and the negative electrode terminal 4 protrude outward from the outer edge portion of the laminate film 2. The outer edge of the laminated film 2 was heat-pressed with a metal plate heated to the melting temperature of the PP film, and the outer edges of the two laminated films 2 were heat-welded. After injecting a non-aqueous electrolyte from an injection port (not shown) arranged in advance between the positive electrode terminal 3 and the negative electrode terminal 4, the injection port was sealed to complete the assembly of the laminated film-clad battery 1. For the non-aqueous electrolyte, lithium hexafluorophosphate (LiPF ₆ ) as a lithium salt (electrolyte) in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC), for example, 1 mol / liter (1M) It was dissolved in and used.

(Action etc.)
Next, the operation and the like of the laminated film-clad battery 1 of this embodiment will be described.

  In the laminated film-clad battery 1 of this embodiment, a mixed positive electrode active material containing two or more positive electrode active materials having different operating voltage ranges is used as the positive electrode active material. For this reason, at the time of charging / discharging, the battery voltage changes in each operating voltage range, and the battery voltage becomes substantially constant near the lower limit of the operating voltage range. In the charge / discharge curve, the battery voltage on the vertical axis becomes substantially constant with respect to the change in the remaining battery capacity on the horizontal axis, so that it becomes substantially flat and forms a quasi-flat portion. The remaining battery capacity corresponding to the changing portion interposed between the quasi-flat portions is determined by the mixing ratio of the respective positive electrode active materials. Therefore, by using a mixed positive electrode active material, a quasi-flat portion due to two or more mixed positive electrode active materials can be reflected in the charge / discharge curve.

  Moreover, in the laminate film exterior battery 1 of this embodiment, the charge / discharge curve has two quasi-flat portions for both charging and discharging. Except for these quasi-flat portions and the changing portions interposed between the quasi-flat portions, the battery voltage gradually changes with respect to the change in the remaining battery capacity. For this reason, at the time of charging / discharging, a relatively large voltage change corresponding to the difference in battery voltage between the quasi-flat portions occurs in a short time at the changing portion when moving from one quasi-flat portion to another quasi-flat portion. By detecting the change in battery voltage at this time, the remaining battery capacity corresponding to the boundary (change part) of the quasi-flat part can be detected. Therefore, since the change of the battery voltage between the quasi-flat portions can be easily detected without using a high-accuracy voltage detection device, the state of charge of the battery can be easily detected. In other words, an approximate charge state can be detected only by a circuit that detects a change in battery voltage between the quasi-flat portions without using a complicated electronic circuit or the like to detect the charge state.

  Furthermore, in the laminated film-clad battery 1 of this embodiment, the positive electrode active material has a lithium-containing transition metal composite oxide having an olivine crystal structure, a lithium-containing transition metal composite oxide having a layered crystal structure, and a spinel crystal structure. At least one of the lithium-containing transition metal composite oxides is mixed. In the lithium-containing transition metal composite oxide having this olivine crystal structure, the operating voltage range is around a battery voltage of 3.4 V, and a quasi-flat portion is present near this 3.4 V. On the other hand, in the lithium-containing transition metal composite oxide having a layered crystal structure or a spinel crystal structure, the operating voltage range is around the battery voltage of 4 V, and has a quasi-flat portion around 3.7 V that is the lower limit of the operating voltage range. ing. For this reason, two quasi-flat portions can be formed in the charging / discharging curve of the laminated film-covered battery 1 in the vicinity of the battery voltage of 3.4 V and in the vicinity of 3.7 V both during charging and discharging.

  Furthermore, in the laminated film-clad battery 1 of this embodiment, a mixed positive electrode active material is used, and the range of the remaining battery capacity corresponding to the quasi-flat portion is determined by the mixing ratio of the mixed positive electrode active material. By adjusting the ratio, it is possible to set the boundary of the quasi-flat portion near the desired remaining battery capacity, that is, in the vicinity of the charged state to be detected.

  In the detection of the state of charge of a non-aqueous electrolyte secondary battery typified by a conventional lithium ion secondary battery, a complicated electronic circuit is used to detect the state of charge by detecting the battery voltage and internal resistance for each battery. ing. For this reason, since it is necessary to design a battery system etc. separately, cost increase is caused and versatility is also lowered. Further, at the time of charging / discharging of the nonaqueous electrolyte secondary battery, the battery voltage gradually changes with respect to the state of charge, that is, the remaining battery capacity, except at the end of discharge. For this reason, when the state of charge is detected from the battery voltage, the battery remaining capacity has a width with respect to the battery voltage, and thus the detection accuracy may be reduced. At the end of discharge, the battery voltage greatly decreases with respect to changes in the remaining battery capacity, so the detection accuracy can be increased when detecting the charged state from the detected battery voltage, but in this state the charged state has decreased. Therefore, it is necessary to immediately stop discharging and shift to a charging operation. The present embodiment is a non-aqueous electrolyte secondary battery that can solve these problems.

  In this embodiment, as the positive electrode active material, a lithium-containing transition metal composite oxide having an olivine crystal structure, a lithium-containing transition metal composite oxide having a layered crystal structure, and a lithium-containing transition metal composite oxide having a spinel crystal structure are used. Although an example in which at least one of the compounds is blended is shown, the present invention is not limited to these crystal structures, and any crystal structure can be used as long as the charge / discharge curve has two or more quasi-flat portions. The positive electrode active material may be used. In the present embodiment, an example in which the charge / discharge curve has two quasi-flat portions has been described. However, the present invention is not limited to this, and two or more quasi-flat portions, for example, three quasi-flat portions. You may have a part. If it does in this way, the change part which can detect a charge condition easily can be increased. Furthermore, in order for the charge / discharge curve to have two or more quasi-flat portions, the positive electrode active material used must be operable with the same negative electrode, the electrolyte constituting the non-aqueous electrolyte, and the solvent thereof. Is preferred.

  In the present embodiment, amorphous carbon is exemplified as the negative electrode active material, but the present invention is not limited to this, and for example, a carbon material such as graphite may be used. As the negative electrode active material, it is preferable to use a material whose charge / discharge curve has a relatively constant gradient in the voltage range to be used. This is because, if the slope of the charge / discharge curve of the negative electrode changes non-uniformly in the voltage range used, the transition between the quasi-flat portions of the positive electrode is obscured in the battery charge / discharge curve. In this regard, amorphous carbon that exhibits a flat voltage change in an intermediate charge state and a uniform voltage change with respect to the charge state, rather than a material such as graphite that exhibits a rapid voltage change at the end of charge and discharge. It is preferable to use a material such as

  Furthermore, in this embodiment, although the laminated film exterior battery was illustrated, this invention is not limited to this. For example, the battery shape may be cylindrical or square, and the battery capacity and size are not particularly limited. Further, in the present embodiment, the laminated electrode group in which the positive electrode and the negative electrode are laminated is illustrated, but the present invention can also be applied to a battery using a wound electrode group in which a belt-like positive electrode and a negative electrode are wound.

  Next, examples of the laminated film-clad battery 1 manufactured according to the present embodiment will be described. In addition, it describes together about the battery of the comparative example produced for the comparison.

(Example)
In the examples, a lithium-containing transition metal composite oxide having a layered crystal structure represented by the chemical formula LiCo _0.33 Mn _0.33 Ni _0.34 O ₂ and lithium having an olivine crystal structure represented by the chemical formula LiFePO ₄ are used. A laminated film-covered battery 1 was produced using a mixed positive electrode active material obtained by mixing a transition metal composite oxide with a compound supporting 10% carbon by weight in a weight ratio of 2: 1.

(Comparative example)
In the comparative example, lamination was performed in the same manner as in the example except that a lithium-containing transition metal composite oxide having a layered crystal structure represented by the chemical formula LiCo _0.33 Mn _0.33 Ni _0.34 O ₂ was used as the positive electrode active material. A film-clad battery was produced.

(Evaluation)
About the produced laminate film exterior battery of the Example and the comparative example, it charged / discharged, respectively, and the change of the battery voltage with respect to the change of battery remaining capacity was measured. The result of the charge / discharge curve showing the change in the battery voltage with respect to the change in the remaining battery capacity is shown in FIG. In FIG. 2, the remaining battery capacity on the horizontal axis indicates the discharge capacity in the discharge curve.

  As shown in FIG. 2, in the laminated film exterior battery of the comparative example, the change of the battery voltage with respect to the change of the remaining battery capacity is the end of discharge of about 780 mAh or more of the remaining battery capacity. Except for the above)), there was a gradual change during both discharging and charging. From this, when the battery voltage is measured and the remaining battery capacity, that is, the state of charge, is detected, the battery remaining capacity has a width with respect to the battery voltage, so the detection accuracy of the state of charge may be lowered. In order to improve this point, it is desirable to detect the state of charge when the battery voltage changes greatly. The sudden change in the battery voltage in the charge / discharge curve is almost immediately before the end of the discharge. Even if the state of charge is detected at this part, there is almost no remaining battery capacity that can be discharged after that, so the load must be stopped immediately when it is installed in a device, etc., and it is necessary to perform the charging operation promptly It becomes.

On the other hand, in the laminated film-covered battery 1 of the example, since two types of positive electrode active materials having different operating voltage ranges are mixed, the positive electrode active material having a low operating voltage range is charged at the time of charging. A positive electrode active material having a high voltage range is charged. On the contrary, at the time of discharging, the positive electrode active material having a high operating voltage range is discharged, and then the positive electrode active material having a low operating voltage range is discharged. That is, at the time of discharging, the battery remaining capacity is around 450 mAh after the lithium-containing transition metal composite oxide having a layered crystal structure represented by the chemical formula LiCo _0.33 Mn _0.33 Ni _0.34 O ₂ has been discharged. A change part where the battery voltage changes rapidly is observed. Thereafter, the lithium-containing transition metal composite oxide having an olivine crystal structure represented by the chemical formula LiFePO ₄ was transferred to the discharge region, and the discharge was terminated when the remaining battery capacity was about 650 mAh or more. For this reason, in the discharge curve, a quasi-flat portion in the range of about 350 to 450 mAh remaining battery capacity and a quasi-flat portion in the range of about 500 to 600 mAh remaining battery capacity are recognized. By detecting a change in battery voltage that occurs between the two quasi-flat portions, it is possible to detect a remaining battery capacity of 450 to 500 mAh corresponding to the boundary between the quasi-flat portions. In this example, since the remaining capacity of the battery (discharge capacity for discharging) is about 450 mAh with respect to the total capacity of the battery of about 700 mAh, the state of charge is approximately 36%. Since the remaining battery capacity in which the battery voltage changes between the quasi-flat portions is determined by the mixing ratio of the mixed positive electrode active material, it has been found that the state of charge can be detected with a certain remaining battery capacity secured. Therefore, if the battery voltage change point (change part) is detected, the state of charge of the battery can be detected at a stage where the remaining battery capacity is sufficient, so that it is not necessary to immediately stop the device. That is, it is easy to detect the state of charge even with low-accuracy voltage detection, and the operation of the device can be ensured even after detection.

  Since the present invention provides a non-aqueous electrolyte secondary battery that can easily detect the state of charge without using a complicated electronic circuit, it contributes to the manufacture and sale of non-aqueous electrolyte secondary batteries. Has industrial applicability.

It is a perspective view which shows the external appearance of the laminated film exterior battery of embodiment which can apply this invention. It is a graph which shows the change of the battery voltage with respect to the change of the battery remaining capacity when charging / discharging the laminated film exterior battery of embodiment.

Explanation of symbols

1 Laminated film exterior battery (non-aqueous electrolyte secondary battery)
2 Laminate film

Claims (3)

  In a non-aqueous electrolyte secondary battery including a positive electrode and a negative electrode capable of inserting and removing lithium ions, the positive electrode includes two or more positive electrode active materials, and the non-aqueous electrolyte secondary battery is charged and discharged. When the charge / discharge curve representing the relationship between the remaining battery capacity and the battery voltage when the remaining battery capacity is taken on the horizontal axis and the battery voltage is taken on the vertical axis, the battery voltage becomes substantially flat and the change in the remaining battery capacity There are two or more flat portions with a small change in the battery voltage with respect to the battery, and a slope portion with a large change in the battery voltage with respect to the change in the remaining battery capacity is interposed between the flat portions. A non-aqueous electrolyte secondary battery.   The positive electrode active material includes a lithium-containing transition metal composite oxide having an olivine crystal structure, and at least one of a lithium-containing transition metal composite oxide having a layered crystal structure and a lithium-containing transition metal composite oxide having a spinel crystal structure. The nonaqueous electrolyte secondary battery according to claim 1, comprising: The lithium-containing transition metal composite oxide having the olivine crystal structure has the chemical formula Li _{1 + y} M _1-y PO ₄ (M is one or more transition metals selected from Mn, Co, Ni, Cr, Al, Mg, Fe) A lithium-containing transition metal composite having the above-mentioned layered crystal structure, which is at least one of a compound represented by the following formula and a compound represented by the chemical formula Li _{1 + y} M _1-y PO _4. The oxide is a compound represented by the chemical formula Li _{1 + x} M _1-x O ₂ , and the lithium-containing transition metal composite oxide having the spinel crystal structure is a compound represented by the chemical formula Li _{1 + x} M _2-x O _4. The non-aqueous electrolyte secondary battery according to claim 2.

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