JP2001243943A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve cycle characteristics of a non-aqueous electrolyte battery using a positive electrode active material made of compounds including lithium nickel oxide compound and spinel type lithium manganese oxide when it is used as an assembled battery. SOLUTION: With a non-aqueous electrolyte secondary battery using a positive electrode active material made of compounds including lithium nickel oxide compound and spinel type lithium manganese oxide, rate of change of discharge- ending voltage can be made small, by using a negative electrode active material 4 of a compound of graphite containing non-graphite. Since variation of discharge-ending voltage of batteries gets small when these non-aqueous electrolyte secondary batteries are connected in series, enough discharge is realized without letting any specific battery over-discharge.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery in which a mixture of two kinds of positive electrode active materials is connected in series to form a battery pack. The present invention relates to a nonaqueous electrolyte secondary battery suitable for use.

[0002]

2. Description of the Related Art In recent years, portable personal computers and cordless devices have rapidly spread, and public interest in electric vehicles has been increasing in order to reduce air pollution.
Inexpensive and high performance rechargeable batteries are required for such power sources, and assembled batteries formed by connecting a plurality of unit cells to cope with required voltages and currents are used.

As such a secondary battery, a non-aqueous electrolyte secondary battery using a material capable of occluding and releasing lithium ions as a material for a positive electrode and a negative electrode has been developed, and has already been put into practical use as a power source for small electronic devices. A lithium-cobalt composite oxide is widely used as a positive electrode material of this non-aqueous electrolyte secondary battery, and graphite is widely used as a negative electrode material. However, in response to increasing demand for non-aqueous electrolyte secondary batteries, lithium-cobalt composite oxide as a positive electrode material is expensive because cobalt is used as its raw material, and the amount of resources is not sufficient. For this reason, spinel-type lithium manganese oxide, lithium nickel composite oxide, and the like have been proposed as alternative materials, and research is being actively conducted.

It is known that spinel-type lithium manganese oxide is particularly inexpensive and has high safety.
Since the discharge capacity per unit weight is small and the bulk density is small as compared with lithium cobalt oxide, there is a disadvantage that the energy density as a battery is reduced.

On the other hand, the lithium-nickel composite oxide has a larger discharge capacity per unit weight than the lithium-cobalt composite oxide, and can realize a battery with a high energy density. If the battery temperature is small and the battery temperature rises due to internal short circuit or the like, the thermal decomposition reaction of the positive electrode may proceed rapidly.

In order to solve the above problems, a positive electrode using a mixture of two types of composite oxides such as a lithium nickel composite oxide and a spinel type lithium manganese oxide as a positive electrode active material has been proposed. (Japanese Patent Laid-Open No. 9-
180718). This proposal is expected as a method for obtaining a relatively safe positive electrode that is inexpensive and has a large discharge capacity as compared with a positive electrode using a lithium-cobalt composite oxide.

[0007] However, the discharge end voltage usually set in the lithium nickel composite oxide is 2.7 V to 3.0 V.
In contrast to V, the spinel-type lithium manganese oxide is in an overdischarged state in a voltage region of 3.0 V or less. Therefore, in the positive electrode in which both are combined, the discharge capacity of 100
%, And in order not to fall into an overdischarge state, it is necessary to adopt a charging / discharging method that is accurately controlled so that the discharge end voltage is optimally set according to the composition of the positive electrode. .

However, in a positive electrode using a mixture of two types of positive electrode active materials having different discharge end voltages, even if a mixture is prepared with the same composition ratio, the dispersion of the capacity of each battery depends on the dispersion state of the two types of active materials. As a result, a particular unit cell is likely to be in an overdischarged state, particularly when individual cells are used as an assembled battery connected in series, which causes a problem that the cycle characteristics of the assembled battery are further deteriorated. .

As a method for avoiding this, there is a method of selecting a combination having no capacity variation as each of the cells constituting the assembled battery. However, it is impossible to select cells having no variation in capacity, and even if there is no variation in capacity at the time of manufacturing, the cycle deterioration rates of the batteries are not necessarily the same, and this method is not used. It is not realistic to completely suppress overdischarge.

As another method, there is a method in which the voltages of all the cells constituting the assembled battery are measured, and the discharge of the entire assembled battery is stopped so that each of the cells does not fall into an overdischarged state. However, this method is difficult to realize when the number of batteries in the assembled battery increases, resulting in a large increase in the cost of the entire assembled battery and a decrease in energy density, and the lithium nickel oxide and lithium manganese oxide There is a problem in that the advantages of high capacity and low cost due to the use of a mixed positive electrode are offset.

[0011]

As described above, a non-aqueous electrolyte secondary battery using a mixture of two kinds of positive electrode active materials for a positive electrode is excellent in battery capacity and safety, but is not In the case where a plurality of cells are connected in series to form a battery pack, a specific battery cell is over-discharged and the cycle characteristics of the battery pack deteriorate.

The present invention has been made in view of the above problems, and is a battery using a mixture of a positive electrode active material, which can have a high capacity and high safety, for a positive electrode, and is used as an assembled battery. It is another object of the present invention to provide a non-aqueous electrolyte secondary battery having good cycle characteristics.

[0013]

A nonaqueous electrolyte secondary battery according to the present invention comprises a positive electrode having a positive electrode active material comprising a mixture containing a lithium nickel composite oxide and a spinel type lithium manganese oxide; A negative electrode including a negative electrode active material composed of a graphitic carbon material and a carbon material mixture containing graphite; and a nonaqueous electrolyte interposed between the positive electrode and the negative electrode, wherein the negative electrode active material operates the negative electrode. It is a carbon material mixture having a composition in which the ratio of the non-graphite carbon material and graphite is controlled such that the discharge potential change rate at the potential is 20 mV / (mAh / g) or less.

Another non-aqueous electrolyte secondary battery of the present invention comprises a positive electrode having a positive electrode active material comprising a mixture containing a lithium nickel composite oxide and a spinel-type lithium manganese oxide, and a non-graphitic carbon material. A negative electrode including a negative electrode active material comprising a carbon material mixture containing graphite and a non-aqueous electrolyte sandwiched between the positive electrode and the negative electrode,
The negative electrode active material is a carbon material mixture having a composition in which a ratio of a non-graphitic carbon material and graphite is controlled such that a discharge potential change rate at an operating potential of the negative electrode is 20 mV / (mAh / g) or less. Features.

In the nonaqueous electrolyte secondary battery of the present invention, the battery capacity is increased by using a positive electrode active material comprising a mixture containing a lithium nickel composite oxide and a spinel type lithium manganese oxide for the positive electrode, And it is possible to improve safety.

Further, since the negative electrode active material used for the negative electrode contains graphite, the battery capacity can be increased.

On the other hand, in a cell using a positive electrode active material composed of a mixture as a positive electrode, the battery capacity of each cell tends to vary, and when these cells are connected in series to form an assembled battery, The battery is likely to be in an overdischarged state, and as a result, the cycle characteristics of the assembled battery are significantly deteriorated.

In the present invention, the battery capacity is increased by including graphite in the negative electrode active material, and the non-graphite active material is included to reduce the discharge change rate of the negative electrode.
The power charged in the assembled battery can be used without waste, and the overdischarge state of each cell can be reduced.

This will be described with reference to the drawings.

FIG. 1 is a diagram showing a discharge change of each battery when two unit cells A1 and A2 having variations in discharge characteristics are connected in series and B1 and B2 are connected in series. A1 and B1 are batteries that can be created as designed, A2 and
B2 is a battery with variations).

In a battery pack in which a plurality of cells are connected in series, the discharge end voltage of the cells when the quantity of electricity Q is discharged at the time of battery design is V _SF , and the difference between the obtained discharge capacity of each cell ( ΔQ, ΔQ
Assuming that the voltage change rate at the end of discharge of the unit cell is dV / dQ, the difference (variation in the end-of-discharge voltage of each cell) ΔV _SF between V _SF and the actual end-of-discharge voltage of each unit cell is expressed by the formula (1) ).

ΔV _SF = dV / dQ × ΔQ (1) As shown, batteries A1 and A2 having a small voltage change rate
In the discharge end voltage variation [Delta] V _SF A is small,
In large batteries B1, B2 of the voltage change rate, it is seen that a large discharge end voltage variation [Delta] V _SF B.

[0023] For example, if the battery is designed to be over-discharged state V _SF to the boundary, B2 of the battery large [Delta] V _SF becomes larger over-discharge state than the batteries of small [Delta] V _SF A2.

Therefore, it can be seen that it is effective to reduce the rate of voltage change dV / dQ as shown by the batteries A1 and A2 in order to reduce the overdischarge state of a plurality of batteries connected in series.

The voltage change rate dV / dQ in a single battery is given by the difference between the positive electrode potential and the negative electrode potential. In a battery using a composite positive electrode of lithium nickel oxide and lithium manganese oxide, the voltage change rate dV of the negative electrode is dV / dQ. By reducing / dQ, the voltage change rate of the unit cell can be reduced.

The inventors of the present invention have conducted detailed studies on the relationship between the negative electrode potential change rate at the end of battery discharge and the overdischarge and cycle deterioration of the battery pack. It was confirmed that when a negative electrode having a discharge potential change rate of 20 mV / (mAh / g) or less was used when discharging was performed, in particular, overdischarge and cycle deterioration of the assembled battery could be effectively suppressed. .

That is, the discharge potential change rate is always 20 mV / (mAh / g) in a potential region of 0.5 (V vs Li / Li ⁺ ) or less, which is generally the operating potential of the negative electrode active material.
The following negative electrode may be used.

However, in order to realize a battery having as large an energy density as possible, it is essential to properly balance the positive and negative electrode capacities of the battery. Therefore, the negative electrode active material is particularly required to have a small potential change rate at the end of discharge of the negative electrode itself. For example, graphite and the like have a flat discharge curve, but the potential rises sharply at the end of discharge, and the negative electrode discharge potential 0.5, which is normally set to the negative electrode discharge termination potential.
(Vvs Li / Li ⁺ ), the discharge potential change rate is 2
It exceeds 0 mV / (mAh / g) and does not satisfy the above conditions.

Examples of the negative electrode active material satisfying such conditions include non-graphitizable materials such as non-graphitizable carbon materials and graphitizable carbon materials. However, non-graphite materials generally have disadvantages such as high irreversible capacity and low density. Therefore, by mixing the non-graphitic carbon material and graphite, the density of the irreversible capacity is suppressed without significantly lowering the density compared to graphite alone, and the potential change rate at the end of discharge can be arbitrarily designed according to the mixing ratio. It is possible to obtain a negative electrode that has been used.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A non-aqueous electrolyte secondary battery according to the present invention will be described below with reference to FIG.

FIG. 2 is a sectional view showing the right half of the cylindrical non-aqueous electrolyte secondary battery.

The electrode group 5 has a structure in which a band formed by laminating the positive electrode 2, the separator 3, and the negative electrode 4 is spirally wound. The electrode group 5 is housed in a bottomed cylindrical container 1 made of, for example, stainless steel, and is fixed by a hollow cylindrical electrode group holder 12 made of, for example, polypropylene. The separator 3 is formed of, for example, a nonwoven fabric, a microporous polypropylene film, a microporous polyethylene film, or a microporous polyethylene-polypropylene laminated film.

The container 1 is hermetically sealed by welding a sealing plate 9 on an upper part, and contains an electrolyte therein. A safety valve 10 is welded to the opening of the sealing plate 9, and the positive electrode terminal 8 is fixed to the sealing plate 9 by, for example, a hermetic seal 11. One end of the positive electrode current collecting lead 6 is connected to the positive electrode 2.
The other end is connected to the positive terminal 8.
The negative electrode 4 is connected to the container 1 serving as a negative electrode terminal via a negative electrode current collecting lead 7.

Next, the positive electrode 2, the negative electrode 4, the non-aqueous electrolyte and the like will be described.

1) Positive Electrode 2 The positive electrode 2 is not particularly limited as long as it has a positive electrode active material composed of a mixture containing a lithium nickel composite oxide and a spinel-type lithium manganese oxide. Can be used.

Examples of the lithium-nickel composite oxide include a composite oxide represented by the composition formula LiNiO ₂ , a composite oxide in which a part of lithium, nickel or oxygen of LiNiO ₂ is replaced by an appropriate element, Those in which the ratio of nickel and oxygen is changed to an appropriate value can be used.

[0037] Particularly, the composition formula _{Li 1-x Ni 1-xy} M y (O
_2-z F _z) (where the M represents at least one element selected boron, niobium, and aluminum, the x, y
Is (z + 0.05) / 2 ≦ x <(z + 1) / 3 and 0
<X + y ≦ 0.5 and 0 ≦ z <0.66) or a lithium-nickel composite metal oxide represented by the formula:
_{iNi 1-xy Co x M y} O2 ( provided that at least one element M may be selected from aluminum, boron, and niobium, wherein x, y are 0 <x ≦ 0.5,0 <y < 0.5, And 0 <x + y ≦ 0.5) are preferable because they have high safety and can increase the capacity of the battery.

More specifically, Li_1.075Ni_0.755Co
_0.17O_1.9F_0.1, Li_1.10Ni_0.74Co_0.16O_1.85F
_0.15, Li_1.075Ni_0.705Co_0.17Al_0.05O
_1.9F_0.1, Li _1.10Ni_0.72Co_0.16Nb_0.02O_1.85F
_0.15And LiNi_0.795Co_0.175B_0.03O_Two, LiNi
_0.795Co_0.175Nb_0.03O_Two, LiNi_0.725Co_0.17N
b_0.02O_{Two And the like.}The spinel type
As the lithium manganese oxide, specifically, Li_{1 + a}
Mn_{Two -a}O_Four, Li_{1 + a}Mn_2-abCo_bO_Four, Li_{1 + a}Mn
_2-abAl_bO_Four, Li_{1 + a}Mn_{2- ab}Fe_bO_Four, Li_{1 + a}M
n_2-abMg_bO_Four, Li_{1 + a}Mn_2-abTi_bO_Four, Li_{1 + a}
Mn_2-abNb_bO_Four, Li_{1 + a}Mn_2-abGe_bO_FourEtc.
(Where a is 0 <a and 2> a + b
Shown).

In the positive electrode active material, the proportion of the lithium-nickel composite metal oxide is at least 60% by weight and at least 80% by weight.
It is preferably less than.

The particle diameter of the lithium composite oxide is preferably such that the value of the cumulative average diameter _DN50 measured by a laser diffraction particle size distribution analyzer (Microtrac HRA particle size distribution analyzer: manufactured by Leeds & Northup) is in the range of 5 μm to 50 μm. .

The value of the cumulative average diameter D _M50 of the spinel-type lithium manganese oxide is preferably in the range of 1 μm to 30 μm.

With such a configuration, a non-aqueous electrolyte secondary battery having an excellent balance between safety and capacity characteristics can be formed.

The positive electrode 2 can be produced, for example, by applying a positive electrode material paste obtained by dispersing the positive electrode active material, a conductive agent and a binder in an appropriate solvent to one side or both sides of a current collector.

The coating amount of the positive electrode active material per one side of the positive electrode layer is preferably in the range of 80 g / m ² to 200 g / m ² . With such a configuration, a non-aqueous electrolyte secondary battery having an excellent balance between safety and capacity characteristics and high output characteristics can be obtained. The weight per one side of the positive electrode layer is more preferably 100 g /
m ² to 150 g / m ² .

Examples of the conductive agent include acetylene black, graphite, carbon black and the like.

As the binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVd)
F), a terpolymer of polyvinylidene fluoride-tetrafluoroethylene-6-propylene fluoride, an ethylene-propylene-diene copolymer (EPDM) and the like can be used. Among them, polyvinylidene fluoride (PVdF)
Is preferred because of its excellent adhesion to the substrate and good binding between the active materials.

As the organic solvent for dispersing the binder, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) and the like are used.

The amount of the binder is determined by the amount of the positive electrode active material 1
It is preferable that the amount is in the range of 2 parts by weight to 20 parts by weight with respect to 00 parts by weight (100 parts by weight in total when the conductive agent is included).

The amount of the conductive agent is determined by the amount of the positive electrode active material 1
It is preferable that the amount is in the range of 0 to 18 parts by weight based on 00 parts by weight.

The compounding amount of the organic solvent is in the range of 65 parts by weight to 150 parts by weight based on 100 parts by weight of the positive electrode active material (100 parts by weight in total when the conductive agent is included). Is preferred.

The current collector has a thickness of 15 μm to 3 μm.
Examples include 5 μm aluminum foil, aluminum mesh, aluminum punched metal, aluminum lath metal, stainless steel foil, titanium foil, and the like.

2) Negative Electrode 4 The negative electrode 4 is prepared by suspending and mixing a negative electrode mixture 5b composed of, for example, a negative electrode active material, a conductive agent and a binder in an appropriate solvent, and coating the solution with one surface of a current collector. Alternatively, it is formed by coating on both sides and drying.

The negative electrode active material according to the present invention contains a mixture of a non-graphite carbon material and graphite.

The non-graphite carbon material includes, for example, coke, carbon fiber, pyrolysis gas phase carbon material, and fired resin, and the (002) plane spacing determined by the X-ray diffraction method is 0.34 nm. The above carbon materials are preferable because of their large discharge capacity.

Examples of graphite include mesophase pitch-based carbon fiber or mesophase spherical carbon in addition to natural graphite, and a fibrous carbon material having a (002) plane spacing of 0.34 nm or less determined by X-ray diffraction. It is preferred that

The ratio between the non-graphite carbon material and the graphite varies depending on the ratio of the lithium nickel composite oxide and the spinel type lithium manganese oxide as the positive electrode active material, that is, the discharge end voltage of the battery. It is preferable to set the ratio such that the discharge potential change rate of the negative electrode during the heating is 20 mV / (mAh / g).

Since the discharge change rate of the negative electrode usually becomes steep as the discharge proceeds, the mixing ratio of the negative electrode active material is adjusted so that the discharge voltage change rate becomes 20 mV / (mAh / g) at the discharge end voltage. Adjust it.

For example, when the discharge termination potential of the negative electrode is 0.5
When (VvsLi / Li ⁺ ), the ratio of the non-graphitic carbon material in the negative electrode active material is preferably 10% by weight or more.

If the proportion of graphite is too small, the battery capacity is reduced. Therefore, the proportion of the non-graphitic carbon material is desirably 60% by weight or less.

It is preferable that the amount of the carbon material to be applied is 30 to 100 g / m ² in a state where the negative electrode 5 is prepared.

With this configuration, the negative electrode potential change rate at the end of discharge is suitable for suppressing overdischarge,
In addition, a non-aqueous electrolyte secondary battery having a reduced irreversible capacity can be configured.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). it can.

The mixing ratio of the negative electrode material and the binder is preferably in the range of 80 to 98% by weight of the negative electrode material and 2 to 20% by weight of the binder.

The current collector has a thickness of 10 μm to 3 μm.
5 μm copper foil, copper mesh, copper punched metal, copper lath metal, stainless steel foil, nickel foil and the like can be used.

As the separator 3, for example, nonwoven fabric, polypropylene microporous film, polyethylene microporous film, polyethylene-polypropylene microporous laminated film, porous paper and the like can be used.

3) Non-aqueous electrolyte The non-aqueous electrolyte has a composition in which an electrolyte is dissolved in a non-aqueous solvent.

Examples of the non-aqueous solvent include propylene carbonate (PC) and ethylene carbonate (EC).
And cyclic carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), and the like, chains such as 1,2-dimethoxyethane (DME), and diethoxyethane (DEE). Ether, tetrahydrofuran (THF) or 2-methyltetrahydrofuran (2-
Cyclic ethers and crown ethers such as MeTHF),
At least one selected from fatty acid esters such as γ-butyrolactone (γ-BL), nitrogen compounds such as acetonitrile (AN), and sulfur compounds such as sulfolane (SL) and dimethyl sulfoxide (DMSO) can be used.

Among them, one consisting of at least one selected from EC, PC and γ-BL, EC, PC and γ-B
L, DMC, MEC, DE
It is desirable to use a mixed solvent comprising at least one selected from C, DME, DEE, THF, 2-MeTHF, and AN. Further, when using a negative electrode containing a carbonaceous material that occludes and releases lithium ions, from the viewpoint of improving the cycle life of a secondary battery including the negative electrode, EC, PC and γ-BL, EC and PC And MEC, E
C and PC and DEC, EC and PC and DEE, EC and AN,
EC and MEC, PC and DMC, PC and DEC, or E
It is desirable to use a mixed solvent consisting of C and DEC.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ) and lithium hexafluorophosphate (Li
PF ₄ ), lithium borofluoride (LiBF ₄ ), lithium arsenic hexafluoride (LiAsF ₆ ), lithium trifluorometasulfonate (LiCF ₃ SO ₃ ), lithium aluminum tetrachloride (LiAlCl ₄ ), lithium bistrifluoromethylsulfonylimide [LiN (CF ₃ S
O ₂ ) ₂ ]. Among them, it is preferable to use LiPF _{6 ,} LiBF ₄ , and LiN (CF ₃ SO ₂ ) ₂ because conductivity and safety are improved.

The amount of the electrolyte dissolved in the non-aqueous solvent is preferably in the range of 0.5 mol / L to 1.5 mol / L.

FIG. 3 is a schematic diagram of an assembled battery in which nonaqueous electrolyte secondary batteries are connected in series.

A plurality of cells of a non-aqueous electrolyte secondary battery as shown in FIG. 2 are prepared, and a positive electrode terminal 8 and a negative electrode terminal (container 1) of each cell are connected by a conductor 20 to form an assembled battery. Accordingly, it is possible to deal with electric equipment that requires a large voltage.

[0073]

EXAMPLES Examples of the present invention will be specifically described below.

<Production of Battery> Example 1 LiNi _0.711 Co _0.206 as lithium nickel oxide
Al _0.083 O ₂ (D _N50 = 13 μm)
_{With Li 1.06 Mn 1.94 as a spinel-type manganese oxide
O 4} and _{(D N5 0} = _15μm) was used.

First, the lithium nickel oxide and the spinel-type manganese oxide were mixed at a mixing ratio (weight ratio) of 70% lithium nickel oxide and 30% spinel-type manganese oxide using a ball mill. To 100 parts by weight of the obtained mixture, 5 parts by weight of acetylene black as a conductive agent and 5 parts by weight of flake graphite (artificial graphite) were added and mixed again to prepare a positive electrode mixture. This positive electrode mixture was used as a binder with 5 parts by weight of polyvinylidene fluoride as N-
The mixture was dispersed in a solution of methyl-2-pyrrolidone to prepare a positive electrode material paste. This was applied to both sides of an aluminum foil as a current collector, dried, and then rolled to produce a positive electrode.

On the other hand, a mixture of 50% fibrous graphite (MCF) obtained by graphitizing mesophase pitch carbon fibers and 50% (weight ratio) of non-graphitizable carbon (HC) obtained by sintering a resin at low temperature was mixed. An active material was used. For 100 parts by weight of the negative electrode active material, a mixture consisting of 5 parts by weight of polyvinylidene fluoride was dispersed in N-methylpyrrolidone to form a paste, applied to both sides of a copper foil as a current collector substrate, and dried. Roll pressing was performed to produce a negative electrode.

After laminating the positive electrode, the separator made of a porous film made of polyethylene, and the negative electrode, they were spirally wound to form an electrode group.

As the electrolytic solution, 1 mol / l of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (mixing volume ratio 1: 2). Use
The electrode group and the electrolytic solution were respectively housed in a stainless steel bottomed cylindrical container, and 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled.

Subsequently, each of the produced single cells was used for 4.
The battery was charged at a constant voltage and constant current of 2 V and 1.2 A for 8 hours, and then discharged at a constant current of 0.8 A until the voltage reached 3 V. The discharge capacity of each cell was determined. The 50 cells are ranked in order of discharge capacity, and 5 cells from the larger discharge capacity and 5 from the smaller discharge capacity.
The remaining 40 batteries were removed, and the remaining 40 batteries were assembled into four batteries in the order of capacity, thereby producing 10 batteries in a series of four batteries with the smallest possible variation in capacity.

Example 2 As the negative electrode active material, 70% of fibrous graphite (MCF) obtained by graphitizing mesophase pitch carbon fibers and 30% by weight of non-graphitizable carbon (HC) obtained by calcining a resin at a low temperature (weight ratio) ), 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled in the same manner as in Example 1 to produce 10 sets of four series assembled batteries.

Example 3 As the negative electrode active material, 90% of fibrous graphite (MCF) obtained by graphitizing mesophase pitch carbon fibers and 10% of a non-graphitizable carbon (HC) obtained by firing a resin at a low temperature (weight ratio) ), 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled in the same manner as in Example 1 to produce 10 sets of four series assembled batteries.

Comparative Example 1 As a negative electrode active material, fibrous graphite (MCF) obtained by graphitizing mesophase pitch carbon fibers was used, and in the same manner as in Example 1, a cylindrical lithium ion secondary battery (φ35 mm × 6) was used.
5 mm) were assembled to produce 10 sets of four battery packs in series.

Comparative Example 2 Fifty cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled in the same manner as in Example 1 using flaky graphite (artificial graphite) as the negative electrode active material. Ten sets of assembled batteries were produced.

Comparative Example 3 Using a non-graphitizable carbon (HC) obtained by baking a resin at a low temperature as a negative electrode active material, a cylindrical lithium ion secondary battery (φ35 mm × 65 mm) was produced in the same manner as in Example 1. ) Were assembled, and 10 sets of four battery packs in series were produced.

<Evaluation of Discharge Characteristics of Negative Electrode Active Material> First, the discharge characteristics of the negative electrode active materials used in Examples 1 to 3 and Comparative Examples 1 to 3 were evaluated.

Using a negative electrode containing 50 mg of the negative electrode active material and metallic Li as a counter electrode, LiPF ₆ was dissolved at a concentration of 1 mol / l in a solvent in which ethylene carbonate and methyl ethyl carbonate were mixed at a ratio of 1: 2 as an electrolytic solution. A single-electrode evaluation cell was prepared using the same electrolytic solution used for the battery. This cell was charged at a constant current of 5 mA (100 mA / g) until the negative electrode potential reached 10 mV, and then maintained at 10 mV until the total charging time was 8 hours, and the battery was initially charged. Next, after 30 minutes of rest time, 5m
The battery was discharged at a constant current of A until the negative electrode potential reached 0.5 V.

The rate of change of the negative electrode discharge potential at the time when the negative electrode potential reached 0.5 V was determined for each of the negative electrodes of Examples and Comparative Examples. Table 1 shows the results. Example 1
3 and Comparative Example 3, the discharge potential change rate was 20
The condition of mV / (mAh / g) or less was satisfied. For Comparative Examples 1 and 2, the discharge potential change rate was 20 mV / (m
Ah / g).

[Table 1] <Evaluation of Cycle Characteristics of Assembled Battery> A cycle test was performed on each of the assembled batteries obtained in Examples 1 to 3 and Comparative Examples 1 to 3.

For charging, the current value (1) at the time of discharging the discharge capacity obtained by the capacity confirmation test of the cell in one hour.
After performing to 16.8V in C), it was maintained at a constant voltage of 16.8V and performed for a total of 3 hours. The discharge was performed at a constant current of 1 C, and the discharge end voltage was 12 V. The rest time after charging and discharging was 30 minutes each. Such charge / discharge was repeated, and the discharge capacity was measured for each cycle. Further, at the end of the discharge, the voltages of the four cells constituting the assembled battery were measured, respectively, and the variation was examined.

The ratio of the discharge capacity when the number of cycles reaches 300 cycles to the discharge capacity in the first cycle, that is, (discharge capacity in the 100th cycle) / (discharge capacity in the first cycle) is the capacity retention ratio. Asked. Table 2 shows the results of the capacity confirmation test of the cells and the results of evaluating the cycle characteristics of the assembled battery.

[Table 2] As is clear from Tables 1 and 2, the discharge voltage of the negative electrode is 1
The negative electrode discharge voltage change rate at V is 20 mV / (mAh / g)
In Comparative Examples 1 and 2 showing the above values, when the discharge end voltage of the assembled battery was set to 12.0 V, the variation of each cell voltage became remarkably large at 1.7 V and 1.5 V. Was confirmed to be in an overdischarged state, and the capacity retention after 300 cycles was as low as 70% or less.

On the other hand, from the charged state, the discharge potential is 0.5 (V
vsLi / Li ⁺ ) until the negative electrode discharge potential change rate is 20
In Examples 1 to 4 showing a value of not more than mV / (mAh / g), when the discharge end voltage of the assembled battery was set to 12.0 V, the variation of each cell voltage was 1 V or less, and overdischarge was suppressed. Was done. The capacity retention rate after 300 cycles in these examples was at least 79%, showing an excellent value.

In addition, as compared with Comparative Example 3 using 100% non-graphitizable carbon (HC) as the negative electrode active material, fibrous graphite obtained by graphitizing non-graphitizable carbon (HC) and mesophase pitch carbon fiber was used. In Example 2 in which the mixing ratio of (MCF) was 30:70 and Example 3 in which 10:90 was used, it was confirmed that the battery also exhibited excellent initial characteristics such as initial capacity and initial efficiency.

In the above-described embodiment, an example in which the present invention is applied to a cylindrical non-aqueous electrolyte secondary battery has been described. However, a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte are provided inside a packaging material made of a laminate film. The present invention can be similarly applied to a thin nonaqueous electrolyte secondary battery having a stored structure.

[0093]

According to the non-aqueous electrolyte secondary battery of the present invention,
Even if a positive electrode active material having high capacity and high safety is used and used as an assembled battery, cycle deterioration can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing discharge characteristics of each cell when connected in series.

FIG. 2 is a diagram showing a cylindrical non-aqueous electrolyte secondary battery according to the present invention.

FIG. 3 is a view showing an assembled battery of the non-aqueous electrolyte secondary battery of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Container 2 ... Positive electrode 3 ... Separator 4 ... Negative electrode 5 ... Electrode group 6 ... Positive electrode current collecting lead 7 ... Negative electrode current collecting lead 8 ... Positive electrode terminal 9. ..Sealing plate 10 Safety valve 11 Hermetic seal 12 Electrode group holder

Continuing from the front page (72) Inventor Moto Kanda 1 Tokoba R & D Center, Komukai Toshiba-cho, Saitama-ku, Kawasaki-shi, Kanagawa F-term (Reference) 5H029 AJ03 AJ12 AK03 AL06 AL07 AM03 AM04 AM05 AM07 BJ02 BJ14 DJ12 DJ17 DJ18 HJ18 HJ19 5H050 AA08 AA15 BA17 CA08 CA09 CB07 CB08 FA12 FA19 FA20 HA18 HA19

Claims (2)

[Claims] A positive electrode comprising a positive electrode active material comprising a mixture containing a lithium nickel composite oxide and a spinel type lithium manganese oxide; and a negative electrode comprising a carbon material mixture containing a non-graphitic carbon material and graphite. A negative electrode comprising a substance; and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode. The negative electrode active material has a discharge potential change rate at an operating potential of the negative electrode of 20 mV / (mAh / g) or less. A non-aqueous electrolyte secondary battery comprising a carbon material mixture having a composition in which the ratio of the non-graphite carbon material and graphite is controlled such that 2. A positive electrode comprising a positive electrode active material comprising a mixture containing a lithium nickel composite oxide and a spinel type lithium manganese oxide, and a discharge potential change rate at an operating potential of 20 mV / (mAh /
g) a negative electrode comprising a negative electrode active material comprising a non-graphitic carbon material and a carbon material mixture comprising graphite and a non-graphitic carbon material in which the ratio of graphite is controlled, and a non-aqueous material sandwiched between the positive electrode and the negative electrode. A non-aqueous electrolyte secondary battery comprising an electrolyte and a plurality of cells using the positive electrode active material and the negative electrode active material, each having the same composition ratio, connected in series.