JP2013140734A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery that offers a high energy density, and enables the estimation of remaining battery power based on battery voltage over a wide range.SOLUTION: A nonaqueous electrolyte secondary battery 1 includes: a positive electrode that contains an olivine-type positive electrode active material showing an open circuit potential curve with a flat portion and an inclined portion, and represented by LiMPO(where M represents at least one kind of metal); and a negative electrode that shows an open circuit potential curve with a flat portion and an inclined portion, and contains graphite containing boron, and graphite containing no boron. The mixing ratio of the graphite containing boron to the graphite containing no boron is such that the flat portion of the positive electrode and the inclined portion of the negative electrode coexist over an SOC (State of Charge) range, and the inclined portion of the positive electrode and the flat portion of the negative electrode coexist over an SOC range.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  Various proposals have been made for electrodes of non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries. (For example, refer to Patent Documents 1 to 5).

  Patent Document 1 discloses a lithium ion battery using boron-added graphite containing 0.1 to 10% by mass of boron or boron-added hard carbon as a negative electrode. Patent Document 1 describes that by using boron-added carbon for the negative electrode, the negative electrode potential is increased as compared with general graphite, and precipitation of metallic lithium on the negative electrode surface can be suppressed.

  Patent Document 2 discloses a lithium ion battery using an olivine-based positive electrode made of two or more metal elements of Fe, Ni, Mn, Co, and Mg and a negative electrode made of soft carbon. In Patent Document 2, an olivine-based positive electrode is formed of two or more kinds of metal elements to provide a two-step flat region (plateau region) in the positive electrode potential curve.

  In Patent Document 3, a negative electrode having a two-layer structure of a layer made of mesocarbon microbeads (MCMB) on the current collector side and a layer made of graphite containing 0.05% by mass of boron on the surface side was used. A lithium ion battery is disclosed. Patent Document 3 discloses that the open circuit potential curve of MCMB has a flat region and an inclined region.

  In Patent Document 4, a lithium ion battery using a negative electrode formed by mixing 50% by mass of coated graphite coated with low crystalline carbon and 50% by mass of boron-containing graphite containing 0.014% by mass of boron. Is disclosed. Patent Document 4 describes that the open-circuit potential of the negative electrode at the time of charging can be lowered relatively by boron-containing graphite by using coated graphite.

  In Patent Document 5, a carbon material such as coke, graphite, or hard carbon is used for the negative electrode, and the slope of the open circuit potential with respect to the change in the amount of charge per unit weight of the carbon material in the charged state, and the trueness of the carbon material. A lithium ion battery having a density set within a predetermined range is disclosed.

JP 2004-335167 A JP 2009-104983 A JP 2001-307717 A JP 2007-122975 A JP 2007-265831 A

  In a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery, a depth of charge (SOC) is calculated based on the battery voltage, and charge / discharge is controlled based on the depth of charge. However, both the olivine-based positive electrode disclosed in Patent Document 2 and the negative electrode using MCMB and general graphite disclosed in Patent Document 3 have a wide flat region in the potential curve. In a battery using a positive electrode and a negative electrode, the battery voltage is constant over a wide range of charge depth. In general, the charging depth of the battery is often calculated from the battery voltage, and when the battery voltage has a slope, the charging depth is easily calculated. However, it is difficult to calculate the charge depth based on the difference in the battery voltage in the case where the battery voltage is constant over a wide range of the charge depth, that is, the battery voltage is flat. There was a problem that control became difficult.

  On the other hand, since the negative electrode made of boron-added carbon disclosed in Patent Document 1 has a potential curve with a gentle slope, the battery using this carbon as the negative electrode calculates the charging depth based on the battery voltage. Becomes easy. However, since the negative electrode made of boron-added carbon has a higher potential than carbon not added with boron, there arises a problem that the energy density of the battery is lowered. In addition, even in batteries using non-graphitizable carbon or graphitizable carbon having a slope in the potential curve as the negative electrode, it is easy to calculate the charging depth based on the battery voltage. Since the capacity is small and the negative electrode potential is high, the energy density of the battery is reduced.

  The olivine-based positive electrode active material disclosed in Patent Document 2 has higher thermal stability than other positive electrode active materials, and can increase the safety of the battery. On the other hand, batteries using this olivine-based positive electrode are required to have high energy density and ease of charge / discharge control in a wide range of charge depth. No specific solution is presented in Patent Documents 1 to 5 for such problems.

  The present invention has been made to solve the above-described problems, and an object of the present invention is a non-aqueous electrolyte secondary battery using an olivine-based positive electrode, which has a high energy density, and It is an object of the present invention to provide a nonaqueous electrolyte secondary battery in which the charge depth based on the battery voltage can be easily calculated.

In order to achieve the above object, the nonaqueous electrolyte secondary battery according to the first aspect of the present invention is represented by LiMPO ₄ (M is at least one kind of metal) whose open circuit potential curve has a flat region and an inclined region. A positive electrode containing an olivine-based positive electrode active material, a graphite whose open circuit potential curve is flat and inclined, and containing boron (hereinafter referred to as “boron-containing graphite”) and non-boron-containing graphite (hereinafter referred to as “boron-containing graphite”) The mixture ratio of boron-containing graphite and boron-free graphite has a negative electrode slope region at a charging depth where a flat region of the positive electrode exists. The mixing ratio is such that there is a flat region of the negative electrode at the charging depth where the positive region of the positive electrode exists.

  Here, the mixing ratio of boron-containing graphite and non-boron-containing graphite is a mass ratio. Here, the flat region is a region where the change in the open circuit potential with respect to the capacity per mass of the active material is less than 1 mV / mAh · g−1, and the inclined region is the open circuit with respect to the capacity per mass of the active material. This is a region where the change in potential is 1 mV / mAh · g−1 or more.

  In the non-aqueous electrolyte secondary battery according to the first aspect of the present invention, with the above configuration, the potential curve can be inclined in the low potential range of the negative electrode potential. Therefore, a battery using the negative electrode has a high battery voltage. A slope can be provided in the battery voltage in the range. As a result, it is possible to achieve both high energy density of the battery due to the high battery voltage and easy calculation of the charge depth due to the provision of the inclined region in the battery voltage. it can. Since the calculation of the charging depth is facilitated, charging / discharging control and estimation of the remaining battery level are facilitated.

  In the nonaqueous electrolyte secondary battery according to the first aspect described above, preferably, the slope of the negative electrode is flat with the depth of charge between the change point A between the flat region and the slope of the positive electrode and the charge termination point of the positive electrode. There is a change point B with the region. If comprised in this way, an inclination can be provided in a battery voltage in the whole range of the charge depth to a charge termination point.

  In the nonaqueous electrolyte secondary battery according to the first aspect, preferably, the mass ratio of graphite containing boron and graphite not containing boron is 30/70 or more and 90/10 or less. If comprised in this way, the inclination area | region of a negative electrode can be expanded.

A nonaqueous electrolyte secondary battery according to a second aspect of the present invention includes an olivine-based positive electrode active material represented by LiMPO ₄ (M is at least one metal) whose open circuit potential curve has a flat region and an inclined region. A positive electrode, an open circuit potential curve having a flat region and a slope region, and a negative electrode including boron-containing graphite and boron-free graphite, the mixing ratio of boron-containing graphite and boron-free graphite is The mixing ratio is such that the negative electrode inclined region exists at the charging depth of the inclined region, and the charging depth at the positive electrode charging end point exists in the negative electrode inclined region.

  In the non-aqueous electrolyte secondary battery according to the second aspect of the present invention, as described above, the gradient ratio of the negative electrode exists with the mixing ratio of boron-containing graphite and boron-free graphite at the charging depth of the gradient area of the positive electrode. In addition, by setting the mixing ratio such that the charging depth at the charging end point of the positive electrode exists in the inclined region of the negative electrode, it is possible to reliably provide the inclined region in the battery voltage over the entire range up to the charging end point of the positive electrode. .

  According to the present invention, as described above, it is possible to increase the energy density of the battery due to the high battery voltage, and to easily calculate the charging depth due to the provision of the inclined region in the battery voltage. It is possible to provide a non-aqueous electrolyte secondary battery capable of achieving both of the above.

It is sectional drawing which showed the whole structure of the battery by one Embodiment of this invention. It is the figure which showed the open circuit potential curve of the negative electrode at the time of changing the mixing ratio of the graphite containing boron and the graphite which does not contain boron. It is the figure which showed the open circuit potential curve of the positive electrode of a battery by Examples 1-3 of this invention, and a negative electrode.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the drawings.

  First, with reference to FIGS. 1-3, the structure of the battery 1 by one Embodiment of this invention is demonstrated. The battery 1 is an example of the “nonaqueous electrolyte secondary battery” in the present invention.

  A battery 1 according to an embodiment of the present invention includes a power generation element 2 as shown in FIG. The power generation element 2 is formed by winding a positive electrode 3, a negative electrode 4, and a separator 5. In the positive electrode 3, a positive electrode mixture containing a positive electrode active material is applied to both surfaces of a positive electrode current collector made of an aluminum foil or an aluminum alloy foil. In the negative electrode 4, a negative electrode mixture containing a negative electrode active material is applied to both surfaces of a negative electrode current collector made of copper foil.

  In addition, the power generation element 2 is housed inside the battery case 6, and the battery cover 7 is connected to the battery so as to close the opening of the battery case 6 in a state where the power generation element 2 is housed inside the battery case 6. It is attached to the case 6. The positive electrode 3 of the power generation element 2 is connected to the battery lid 7, and the negative electrode 4 of the power generation element 2 is connected to the negative electrode terminal 8 of the battery lid 7. The battery case 6 has a liquid injection port (not shown). The battery 1 is formed by sealing the liquid injection port after injecting a non-aqueous electrolyte into the battery case 6 through the liquid injection port.

In the present embodiment, the positive electrode active material includes an olivine-based positive electrode active material represented by LiMPO ₄ (M is at least one metal) whose open circuit potential curve has a flat region and a gradient region, The active material includes a mixed material of boron-containing graphite and boron-free graphite.

  As the non-boron-containing graphite contained in the negative electrode active material, natural graphite, artificial graphite, or a graphite material (coated graphite) coated with a carbon material other than graphite may be used. Further, the boron content of the boron-containing graphite is not particularly limited as long as the effects of the present invention can be achieved. Moreover, in the mixed material of boron-containing graphite and boron-free graphite, boron-containing graphite and boron-free graphite may be mixed uniformly, the first layer made of boron-containing graphite, and boron-free It may be divided into a second layer made of graphite. In this case, it is preferable to dispose the first layer on the surface side (positive electrode side) and the second layer on the current collector side.

  In this embodiment, the mixing ratio of boron-containing graphite and non-boron-containing graphite of the negative electrode active material is such that the negative electrode slope region exists at the charge depth where the positive electrode flat region exists, and the positive electrode slope region exists. The charging depth is set to a mixing ratio (mass ratio) such that a flat area of the negative electrode exists or a charging end point of the positive electrode exists in the inclined area of the negative electrode. Specifically, as shown in FIG. 2, it is possible to adjust the range of the flat region and the inclined region in the open circuit potential curve of the negative electrode by the mixing ratio of boron-containing graphite and boron-free graphite. For example, “with / without B = 30/70” in FIG. 2 means that the mixing ratio (mass ratio) of boron-containing graphite and boron-free graphite in the negative electrode active material is 30:70. To do.

Here, when only boron-free graphite (with / without B = 0/100) is used for the negative electrode, it has a wide flat region with a low open circuit potential (about 0.08 V). On the other hand, when only boron-containing graphite (with / without B = 100/0) is used for the negative electrode, the open circuit potential shifts more preciously as compared with the case of only graphite, and the open circuit potential The curve gently slopes throughout (no flat area). When boron-containing graphite and non-boron-containing graphite are mixed, depending on the mixing ratio, the nature of boron-free graphite having a flat region with a low open circuit potential and the open circuit potential shifts preciously and tilts Both the properties of boron-containing graphite with increasing area appear. That is, as the proportion of the boron-containing graphite is increased, the range of the flat region of the open circuit potential curve is narrowed, and the open circuit potential is shifted preciously as a whole. Focusing on the change point B (B0 to B3) that changes from the inclined region to the flat region, the change point B0 in the case of (with / without B = 0/100) is about 185 mAh · g ⁻¹ , (with / B). None = 30/70 to about 245 mAh ^{· g} -1 at the change point B1 of the case), (B Yes / B None = 50/50) to about 270 mAh ^{· g} -1 at the change point B2 in the case of, (B Yes / B None = 70/30), the change point B3 is about 305 mAh · g ⁻¹ .

  As described above, as the proportion of the boron-containing graphite is increased, the change point B that changes from the inclined region to the flat region shifts in the direction of higher charging depth (right side in FIGS. 2 and 3). On the other hand, the higher the proportion of non-boron-containing graphite, the lower the open circuit potential of the negative electrode, so that the battery voltage can be increased and the battery energy density can be increased. By adjusting the mixing ratio of the negative electrode active material boron-containing graphite and the non-boron-containing graphite as described above according to the relationship between the inclined region and the flat region of the positive electrode, the battery voltage is high. It is possible to achieve both a higher energy density and easier calculation of the charging depth due to the provision of the inclined region in the battery voltage. Since the calculation of the charging depth is facilitated, charging / discharging control and estimation of the remaining battery level are facilitated.

The positive electrode active material may be any type of active material as long as it is an olivine-based positive electrode active material represented by LiMPO ₄ (M is at least one kind of metal) having an open circuit potential curve having a flat region and an inclined region. It can be used. As the metal M, transition metals such as Fe, Ni, Mn, Cu, Co and V are preferable.

  In the present invention, “the mixing ratio of boron-containing graphite and non-boron-containing graphite is such that the negative electrode slope area exists at the charge depth where the flat area of the positive electrode exists, and the negative electrode slope exists at the charge depth where the positive electrode slope area exists. It has a configuration in which the mixing ratio is such that a flat region exists. As a method of realizing this configuration, after setting the mass ratio of the positive electrode active material and the negative electrode active material in the battery, there is a method of adjusting the mixing ratio of the boron-containing graphite and the boron-free graphite in the negative electrode active material. is there. That is, in this method, after the change point between the flat region and the inclined region of the positive electrode is grasped, the change point between the flat region and the inclined region of the negative electrode is adjusted in accordance with the change point of the positive electrode.

  In the present invention, it is preferable that there is a change point B between the slope region of the negative electrode and the flat region at the charging depth between the change point A between the flat region and the slope region of the positive electrode and the charge end point of the positive electrode ( See FIG. 3 Example 1). In this embodiment, the negative electrode potential can be lowered as compared with the embodiment in which the change point B of the negative electrode exceeds the charge end point of the positive electrode (Example 3 in FIG. 3), and at least the positive electrode and the negative electrode in the entire range of the charging depth. One inclined region can be provided. Further, in the present embodiment, the change point A of the positive electrode and the change point B of the negative electrode may exist at the same charge depth, or the change point B of the negative electrode is lower than the change point A of the positive electrode (FIG. 2). And on the left side of FIG. 3) (FIG. 3 Example 2). Moreover, the change point B of a negative electrode may exist in the charge depth (right side of FIG. 2 and FIG. 3) higher than the charge termination point of a positive electrode (FIG. 3 Example 3). In this embodiment, although the negative electrode potential is slightly higher, an inclined region can be provided in the battery voltage in the entire charging depth range based on the negative electrode potential.

  In addition, you may make a positive electrode mixture and a negative electrode mixture contain a conductive support agent, a binder, a viscosity modifier, etc. as needed. As the conductive aid, acetylene black (AB), carbon black, or the like can be used. As the binder, polyvinylidene fluoride, styrene-butadiene copolymer rubber, polyacrylonitrile, or the like can be used alone or in combination. As the viscosity modifier, N-methylpyrrolidone (NMP) or the like can be used.

  As the separator 5 of the battery 1, a nonwoven fabric, a synthetic resin microporous film, or the like can be used. It is more preferable to use a polyolefin microporous film made of polyethylene, polypropylene, or the like from the viewpoint of ease of processing and durability improvement.

Further, as the non-aqueous electrolyte of the battery 1, a solution in which an electrolyte salt is dissolved in a non-aqueous solvent is used. As the electrolyte salt of the nonaqueous electrolyte, LiClO ₄ , LiPF ₆ , LiBF ₄ or the like can be used. Further, the above electrolyte salts may be used alone or in combination of two or more. From the viewpoint of improving conductivity, it is preferable to use LiPF ₆ as the electrolyte salt.

  Examples of nonaqueous solvents for nonaqueous electrolytes include ethylene carbonate (EC), propylene carbonate, butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate, ethyl methyl carbonate (EMC), dipropyl carbonate, methylpropyl carbonate, and dibutyl carbonate. It is possible to use. The nonaqueous solvent is preferably used by mixing a plurality of nonaqueous solvents described above from the viewpoint of adjusting the conductivity and viscosity of the nonaqueous electrolyte.

[Example]
Next, the evaluation test of the nonaqueous electrolyte secondary battery performed for confirming the effect of the present invention will be described. Specifically, as an example corresponding to the above embodiment shown in FIG. 1, a battery 1 according to Examples 1 to 3 below was manufactured.

Example 1
(1) Production of positive electrode plate Using LiFePO ₄ as a positive electrode active material, acetylene black as a conductive additive and polyvinylidene fluoride as a binder, the ratio of the positive electrode active material, the conductive additive and the binder is 88% by mass, An appropriate amount of NMP (N-methylpyrrolidone) was added to a mixture of 6% by mass and 6% by mass to prepare a positive electrode mixture slurry in which the viscosity was adjusted. This positive electrode mixture slurry was applied to both sides of an aluminum foil having a thickness of 20 μm and dried to produce positive electrode 3. The positive electrode 3 was provided with a portion where the aluminum foil not coated with the positive electrode mixture was exposed, and the portion where the aluminum foil was exposed and the positive electrode lead were joined. In addition, the coating mass of the positive electrode mixture and the negative electrode mixture was determined so that the mass ratio of the positive electrode active material and the negative electrode active material in the battery was positive electrode active material: negative electrode active material = 1.8: 1.0. .

(2) Preparation of boron-containing carbon The synthesis method of boron-containing carbon is as follows. Pitch coke and boric acid were mixed at a predetermined ratio, and the mixture was heated to 1000 ° C. in an argon stream and held at 1000 ° C. for 10 hours, and further heated to 2400 ° C. and held at 2400 ° C. for 20 hours. Then, the boron-containing carbon whose boron content is 0.05 mass% was obtained by cooling to room temperature and pulverizing. The boron content in the boron-containing carbon is preferably 0.01% by mass to 1.0% by mass.

(3) Production of Negative Electrode Plate A mixed material containing 50% by mass of boron-containing graphite and 50% by mass of boron-free graphite as a negative electrode active material (with / without B = 50/50), and polyvinylidene fluoride as a binder. A negative electrode mixture slurry was prepared by adjusting the viscosity by adding an appropriate amount of NMP to a mixture containing 90% by mass and 10% by mass of the negative electrode active material and the binder, respectively. This negative electrode mixture slurry was applied to both sides of a 15 μm thick copper foil and dried to prepare negative electrode 4. The negative electrode 4 was provided with a portion where the copper foil not coated with the negative electrode mixture was exposed, and the portion where the copper foil was exposed and the negative electrode lead were joined.

(4) Production of non-injected secondary battery A power generation element is formed by winding the positive electrode 3 and the negative electrode 4 with a separator 5 made of a polyethylene microporous film interposed between the produced positive electrode 3 and the negative electrode 4. 2 was produced. The power generation element 2 is housed in the battery case 6 from the opening of the battery case 6, the positive electrode lead is joined to the battery lid 7, the negative electrode lead is joined to the negative electrode terminal 8, and then the battery lid 7 is opened to the battery case 6. The battery case 6 and the battery lid 7 were joined to each other and joined by laser welding to produce a non-injected battery in which the nonaqueous electrolyte was not injected into the battery case 6.

(5) Preparation and injection of nonaqueous electrolyte LiPF6 in a mixed solvent of ethylene carbonate (EC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 3: 2: 5 (volume ratio) at a concentration of 1 mol / L To prepare a non-aqueous electrolyte. After pouring this non-aqueous electrolyte into the battery case 6 from a liquid injection port provided on the side surface of the battery case 6, the liquid injection port was sealed with a stopper to produce the battery 1 of Example 1.

(Example 2)
A battery 1 similar to that of Example 1 was prepared except that a mixed material containing 30% by mass of boron-containing graphite and 70% by mass of boron-free graphite (B present / no B = 30/70) was used as the negative electrode active material. did.

(Example 3)
A battery 1 similar to that of Example 1 was prepared except that a mixed material containing 70% by mass of boron-containing graphite and 30% by mass of boron-free graphite (70% / 30% with B) was used as the negative electrode active material. did.

(6) Initial Capacity Confirmation Test An initial discharge capacity confirmation test was performed using the batteries under the following charge / discharge conditions. The battery was charged to 3.5 V at a constant current of 450 mA at 25 ° C., charged at a constant voltage of 3.5 V, and charged for a total of 3 hours including constant current charging and constant voltage charging. After charging, discharging was performed at a constant current of 450 mA to a discharge end voltage of 2.0 V, and this discharge capacity was defined as “initial capacity”.

(7) Open circuit potential measurement of positive electrode plate and negative electrode plate A hole is provided in the side surface of each battery case, a reference electrode made of lithium metal covered with a separator is inserted, and the reference electrode and the electrolyte are brought into contact with each other. The potential of the positive electrode and the negative electrode was measured. Specifically, 1% of the charging depth was charged with a constant current of 450 mA from the discharged state, and then an open circuit potential of the positive electrode and the negative electrode was measured by providing a rest for 1 hour. This charging, resting, and potential measurement cycle was carried out to a charging depth of 100%.

  The measurement results of the open circuit potentials of the positive electrode and the negative electrode according to Examples 1 to 3 are shown in FIG. As shown in FIG. 3, in Example 1 (with / without B = 50/50), the change point B of the negative electrode exists between the change point A of the positive electrode and the charge end point of the positive electrode. In the form of Example 1, due to the slope region of at least one of the positive electrode potential and the negative electrode potential, a slope region can be provided in the battery voltage over the entire range of the charging depth used. Here, “the depth of charge used” is the charge depth of the battery from the end-of-discharge voltage to the end-of-charge voltage. On the horizontal axis in FIG. 3 in this example, the potential difference between the positive electrode potential and the negative electrode potential is 2.0 V. And the charging depth between 3.5V. Further, as apparent from FIG. 2, in Example 1, the open circuit potential of the negative electrode as a whole is lower than that of boron-containing graphite (Comparative Example 2), so that the battery voltage can be increased. Thereby, in Example 1, the high energy density of a battery can be achieved.

  In Example 2 (with / without B = 30/70), the change point B of the negative electrode exists at a lower charging depth than the change point A of the positive electrode. Moreover, in the form of Example 2, it is preferable that the change point B of a negative electrode exists in the range of 1/10 or less of the flat area | region of a positive electrode from the change point A of a positive electrode to the direction where a charge depth is low. In the form of Example 2, since most of the flat region of the positive electrode is the negative electrode inclined region, the inclined region can be provided in the battery voltage in a wide range of the charging depth used.

  As is clear from FIG. 2, the negative electrode potential can be further lowered by increasing the mass ratio of the boron-free graphite in the mixed material of boron-containing graphite and boron-free graphite. As a result, the battery voltage can be increased to increase the energy density of the battery.

  In Example 3 (with / without B = 70/30), the change point B exists in the negative electrode at a charging depth higher than the charging end point of the positive electrode. In the form of Example 3, since the mass ratio of the boron-containing graphite is large, the negative electrode potential is higher than those of Example 1 and Example 2, and the energy density of the battery is slightly reduced, but the charging is higher than the positive electrode end point E. Since the gradient region of the negative electrode potential exists up to the depth, the gradient region can be provided in the battery voltage over the entire range of the charging depth used.

  Also, in automobile applications such as hybrid vehicles, when charging with a large current is performed by regenerative energy, the polarization becomes large and the negative electrode potential drops temporarily. When the negative electrode potential is lower than the deposition potential of lithium, metallic lithium may be deposited on the negative electrode surface. On the other hand, in Example 3, the negative electrode potential at the end of charging exists in an inclined region that is higher than the flat region. By increasing the negative electrode potential at the end of charging, the negative electrode potential is less likely to fall below the lithium deposition potential, and the deposition of metallic lithium caused by charging with a large current can be suppressed.

  The embodiments and examples disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments and examples but by the scope of claims for patent, and includes all modifications within the meaning and scope equivalent to the scope of claims for patent.

  For example, in the above example, the mass ratio of the positive electrode active material and the negative electrode active material in the battery was set to be positive electrode active material: negative electrode active material = 1.8: 1.0. In addition, by changing the mass ratio of the positive electrode active material and the negative electrode active material in the battery, it is possible to adjust the relationship between the charging depth positions of the change point A of the positive electrode and the change point B of the negative electrode. For example, in Example 1, the change point A of the positive electrode can be shifted to a higher charging depth (right side in FIG. 3) by increasing the mass ratio of the positive electrode active material. A charging depth higher than the change point B2 can be present. By changing the mass ratio of boron-containing graphite and non-boron-containing graphite, not only the form of adjusting the relationship of the charging depth position between the change point A of the positive electrode and the change point B of the negative electrode, but also as described above By changing the mass ratio of the positive electrode active material and the negative electrode active material, the relationship of the charge depth position between the change point A of the positive electrode and the change point B of the negative electrode can be adjusted.

1 battery (non-aqueous electrolyte secondary battery)
3 Positive electrode 4 Negative electrode

Claims (4)

A positive electrode containing an olivine-based positive electrode active material represented by LiMPO ₄ (M is at least one kind of metal) having an open circuit potential curve having a flat region and an inclined region;
The open circuit potential curve has a flat region and a slope region, and includes a negative electrode including graphite containing boron and graphite not containing boron,
The mixing ratio of the boron-containing graphite and the boron-free graphite is such that at the charging depth at which the flat region of the positive electrode exists, the inclined region of the negative electrode exists and at the charging depth at which the inclined region of the positive electrode exists. Then, a non-aqueous electrolyte secondary battery having a mixing ratio such that a flat region of the negative electrode exists.   The change point A between the slope region and the flat region of the negative electrode exists at the charging depth between the change point A between the flat region and the slope region of the positive electrode and the charge end point of the positive electrode. The nonaqueous electrolyte secondary battery as described.   The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein a mass ratio of the graphite containing boron and the graphite not containing boron is 30/70 or more and 90/10 or less. A positive electrode containing an olivine-based positive electrode active material represented by LiMPO ₄ (M is at least one kind of metal) having an open circuit potential curve having a flat region and an inclined region;
The open circuit potential curve has a flat region and a slope region, and includes a negative electrode including graphite containing boron and graphite not containing boron,
The mixing ratio of the boron-containing graphite and the boron-free graphite is such that the negative electrode slope region exists at the charge depth of the positive electrode slope region, and the charge depth of the positive electrode charge end point is the charge depth. A non-aqueous electrolyte secondary battery having a mixing ratio that exists in the inclined region of the negative electrode.

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