JP3705728B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

＜組電池のサイクル特性評価＞
作製した実施例１〜３及び比較例１〜３で得られた各組電池について、サイクル試験を実施した。
【００８８】
充電は、単電池の容量確認試験によって得られている放電容量を１時間で放電する際の電流値（１C）で１６．８Vまで行った後、１６．８Vの定電圧で保持し、計３時間行った。放電は、１C定電流で行い、放電終止電圧は１２Vとした。充電、放電の後の休止時間はそれぞれ３０分間とした。このような充放電を繰り返し行い、各サイクル毎に放電容量を測定した。また、放電終止時には組電池を構成する各４本の単電池の電圧をそれぞれ測定し、ばらつきを調べた。
【００８９】
そして、サイクル数が３００サイクルに達した時の放電容量と１サイクル目の放電容量の比、すなわち、（１００サイクル目の放電容量）／（１サイクル目の放電容量）を容量維持率として求めた。単電池の容量確認試験結果及び組電池のサイクル特性評価結果を表２に示す。
【表２】

表１及び表２から明らかなように、負極の放電電圧が１Ｖ時の負極放電電圧変化率が２０ｍＶ／（ｍＡｈ／ｇ）以上の値を示す比較例１及び２では、組電池の放電終止電圧を１２．０Ｖとした時の各単電池電圧のばらつきが１．７Ｖ及び１．５Ｖと著しく大きくなり、一部の単電池が過放電状態となっていることが確認され、３００サイクル後の容量維持率は７０％以下という低い値となった。
【００９０】
一方、充電状態から放電電位が０．５（ＶvsＬｉ/Ｌｉ⁺）になるまで、負極放電電位変化率が２０ｍＶ／（ｍＡｈ／ｇ）以下の値を示す実施例１から４においては、組電池の放電終止電圧を１２．０Ｖとした時の各単電池電圧のばらつきは１Ｖ以下であり、過放電が抑制された。これら実施例における３００サイクル後の容量維持率は最低でも７９％であり、優れた値を示した。
【００９１】
また、負極活物質として難黒鉛化性炭素（HC）１００％を使用した比較例３などと比較し、難黒鉛化性炭素（HC）とメソフェーズピッチ炭素繊維を黒鉛化した繊維状黒鉛（MCF）の混合比を３０：７０とした実施例２、１０：９０とした実施例３では初期容量、初期効率などの電池の初期特性でも優れた値を示すことが確認された。
【００９２】
なお、前述した実施例においては、円筒形非水電解液二次電池に適用した例を説明したが、ラミネートフィルムからなる外装材の内部に正極、負極、セパレータ及び非水電解液が収納された構造の薄型非水電解質二次電池にも同様に適用することができる。
【００９３】
【発明の効果】
本発明の非水電解液二次電池によれば、高容量、安全性の高い正極活物質を使用し、組電池として使用してもサイクル劣化を低減させることが可能になる。
【図面の簡単な説明】
【図１】 直列接続時の各単電池の放電特性を示す図。
【図２】 本発明に係わる円筒形非水電解質二次電池を示す図。
【図３】 本発明の非水電解液二次電池の組電池を示す図。
【符号の説明】
１・・・容器
２・・・ 正極
３・・・セパレータ
４・・・負極
５・・・電極群
６・・・正極集電リード
７・・・負極集電リード
８・・・正極端子
９・・・封口板
１０・・・安全弁
１１・・・ハーメチックシール
１２・・・電極群押さえ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery, and in particular, a non-aqueous electrolyte suitable for use as an assembled battery by connecting in series a non-aqueous electrolyte secondary battery using a mixture of two positive electrode active materials as a positive electrode. The present invention relates to a liquid secondary battery.
[0002]
[Prior art]
In recent years, portable personal computers and cordless devices have spread rapidly, and social interest in electric vehicles has been increasing in order to reduce air pollution. As those power sources, inexpensive and high-performance secondary batteries are required, and assembled batteries formed by connecting a plurality of single cells are used in order to cope with necessary voltages and currents.
[0003]
As such a secondary battery, a non-aqueous electrolyte secondary battery using a substance capable of inserting and extracting lithium ions as a positive electrode and a negative electrode material has been developed and has already been put into practical use as a power source for small electronic devices. Lithium cobalt composite oxide is widely used as the positive electrode material of this non-aqueous electrolyte secondary battery, and graphite is widely used as the negative electrode material. However, in response to the increasing demand for non-aqueous electrolyte secondary batteries, lithium cobalt composite oxide as a positive electrode material is expensive because it uses cobalt as a raw material, and the amount of resources is not sufficient. For this reason, spinel type lithium manganese oxide, lithium nickel composite oxide, etc. are proposed as an alternative material, and research is actively performed.
[0004]
It is known that spinel type lithium manganese oxide is particularly inexpensive and has high safety, but since the discharge capacity per unit weight is small and the bulk density is small compared to lithium cobalt oxide, There exists a fault that the energy density as a battery will become small.
[0005]
On the other hand, the lithium nickel composite oxide has a large discharge capacity per unit weight compared to the lithium cobalt composite oxide, and a high energy density battery can be realized. On the other hand, the thermal stability at high temperature is small in the charged state, When the battery temperature rises due to an internal short circuit or the like, the positive electrode thermal decomposition reaction may proceed rapidly.
[0006]
In order to solve the above problems, a positive electrode using a mixture of two kinds 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 ( JP-A-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 compared to a positive electrode using a lithium cobalt composite oxide.
[0007]
However, the discharge end voltage normally set in the lithium nickel composite oxide is 2.7 V to 3.0 V, whereas the spinel type lithium manganese oxide is overdischarged in a voltage region of 3.0 V or less. Therefore, in the positive electrode that combines both, the discharge end voltage is set optimally according to the composition of the positive electrode in order to use nearly 100% of the discharge capacity and not to fall into an overdischarge state. Therefore, it is necessary to take a charge / discharge method controlled accurately.
[0008]
However, in a positive electrode using a mixture of two types of positive electrode active materials having different discharge end voltages, the capacity variation of each battery occurs depending on the dispersion state of the two types of active materials even if the mixture is prepared with the same composition ratio. As a result, when used as an assembled battery in which individual batteries are connected in series, a specific unit cell tends to be in an overdischarged state, which causes a problem that the cycle characteristics of the assembled battery are further deteriorated.
[0009]
As a method for avoiding this, there is a method of selecting a combination having no capacity variation as each unit cell constituting the assembled battery. However, it is impossible to select cells that have no capacity variation. Even if there is no capacity variation at the time of manufacture, the cycle deterioration rate of each battery is not always the same. It is not realistic to completely suppress overdischarge.
[0010]
As another method, there is a method in which the voltage of all the unit cells constituting the assembled battery is measured, and the discharge of the entire assembled battery is stopped so that each unit cell does not fall into an overdischarged state. However, this method is difficult to realize when the number of batteries of the assembled battery increases, leading to a significant increase in the cost of the entire assembled battery and a decrease in energy density, and lithium nickel oxide and lithium manganese oxide. There is a problem that the advantages of high capacity and low cost are offset by using the mixed positive electrode.
[0011]
[Problems to be solved by the invention]
As described above, the non-aqueous electrolyte secondary battery using a mixture of two types of positive electrode active materials for the positive electrode is excellent in terms of battery capacity and safety, but has variations in individual battery capacities, When a plurality of unit cells are connected in series to form an assembled battery, a specific unit cell is overdischarged, resulting in a problem that the cycle characteristics of the assembled battery are degraded.
[0012]
The present invention has been made in view of such problems, and is a battery that uses a positive electrode active material mixture that can be increased in capacity and is highly safe when used as an assembled battery. Another object of the present invention is to provide a non-aqueous electrolyte secondary battery having good cycle characteristics.
[0013]
[Means for Solving the Problems]
  The nonaqueous electrolyte secondary battery of the present invention comprises 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 non-aqueous electrolyte of 10 wt% to 60 wt%. From graphite carbon material and carbon material mixture consisting of graphiteAnd the operating potential is 0.5 (VvsLi / Li ⁺ ) The discharge potential change rate is 20 mV / (mAh / g) or less in the following potential regions.A plurality of single cells each including the positive electrode active material and the negative electrode active material having the same composition ratio, each including a negative electrode having a negative electrode active material and the positive electrode and a non-aqueous electrolyte sandwiched between the negative electrode in series. It is characterized by being connected to.
[0014]
  Another non-aqueous electrolyte secondary battery of the present invention is the non-aqueous electrolyte secondary battery, wherein the negative electrode active material is 10% by weight or more and 50% by weight or less non-graphitizable carbon, and a graphitized mesophase pitch. It is a carbon material mixture made of carbon fiber.
[0015]
The non-aqueous electrolyte secondary battery of the present invention uses a positive electrode active material made of a mixture containing lithium nickel composite oxide and spinel type lithium manganese oxide for the positive electrode, thereby increasing battery capacity and safety. It is possible to increase.
[0016]
In addition, since the negative electrode active material used for the negative electrode contains graphite, the battery capacity can be increased.
[0017]
On the other hand, a unit cell using a positive electrode active material made of a mixture as a positive electrode tends to vary in the battery capacity of each cell, and when this unit cell is connected in series to form an assembled battery, a specific battery is excessive. It becomes easy to be in a discharged state, and as a result, the cycle characteristics of the assembled battery are significantly deteriorated.
[0018]
In the present invention, by adding graphite to the negative electrode active material, the battery capacity is increased, and by adding a non-graphitic active material to reduce the discharge change rate of the negative electrode, power charged in the assembled battery is wasted. Can be used and the overdischarge state of each unit cell can be reduced.
[0019]
This will be described with reference to the drawings.
[0020]
FIG. 1 is a diagram showing changes in discharge of batteries when two cells A1 and A2 having variations in discharge characteristics are connected in series and B1 and B2 are connected in series (A1 and B1). Is a battery created as designed, and A2 and B2 are batteries with variations.
[0021]
In an assembled battery in which a plurality of cells are connected in series, the cell discharge end voltage when the amount of electricity Q is discharged at the time of battery design is expressed as V_{science fiction}When the difference from the actual discharge capacity of each obtained unit cell (variation in discharge capacity of each unit cell) is ΔQ, and the rate of change in voltage at the end of discharge of the ΔQ unit cell is dV / dQ, V_{science fiction}And the actual discharge end voltage of each unit cell (variation in discharge end voltage of each battery) ΔV_{science fiction}Is represented by equation (1).
[0022]
ΔV_{science fiction}= DV / dQ × ΔQ (1)
As illustrated, in the batteries A1 and A2 having a small voltage change rate, the discharge end voltage variation ΔV_{science fiction}In the batteries B1 and B2 having a small A and a large voltage change rate, the discharge end voltage variation ΔV_{science fiction}You can see that B is large.
[0023]
For example, V_{science fiction}In the case of a battery designed to be overdischarged at_{science fiction}B2 battery with a large_{science fiction}Compared to the A2 battery with a small A2, the battery is overdischarged.
[0024]
Therefore, it can be seen that it is effective to reduce the voltage change rate dV / dQ as shown in the batteries A1 and A2 in order to reduce the overdischarge state of the plurality of batteries connected in series.
[0025]
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 / dQ of the negative electrode is By making it small, the voltage change rate in a single cell can be suppressed small.
[0026]
Therefore, as a result of repeated detailed studies on the relationship between the rate of change in potential of the negative electrode at the end of battery discharge and the overdischarge and cycle deterioration of the assembled battery, the present inventors performed charging and discharging as a negative electrode active material by a predetermined method. It was confirmed that overdischarge and cycle deterioration of the assembled battery can be effectively suppressed especially when the negative electrode has a discharge potential change rate of 20 mV / (mAh / g) or less.
[0027]
That is, it is generally 0.5 (VvsLi / Li, which is the operating potential of the negative electrode active material.⁺) A negative electrode whose discharge potential change rate is always 20 mV / (mAh / g) or less may be used in the following potential region.
[0028]
However, in order to realize a battery having as high an energy density as possible, it is essential to have a proper balance between the positive and negative electrode capacities of the battery. Accordingly, 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 suddenly rises at the end of discharge, and the negative electrode discharge potential of 0.5 (VvsLi / Li), which is normally set to the discharge end potential of the negative electrode.⁺), The discharge potential change rate exceeds 20 mV / (mAh / g), which does not satisfy the above conditions.
[0029]
Examples of the negative electrode active material satisfying such conditions include non-graphite materials such as non-graphitizable carbon materials and graphitizable carbon materials. However, non-graphite materials generally have drawbacks such as large irreversible capacity and low density. Therefore, by mixing non-graphitic carbon material and graphite, the increase in irreversible capacity is suppressed without significantly reducing the density compared to graphite alone, and the potential change rate at the end of discharge is arbitrarily designed by the mixing ratio It becomes possible to obtain a negative electrode.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the nonaqueous electrolyte secondary battery according to the present invention will be described with reference to FIG.
[0031]
FIG. 2 is a cross-sectional view of the right half of the cylindrical nonaqueous electrolyte secondary battery.
[0032]
The electrode group 5 has a structure in which a strip-like material in which the positive electrode 2, the separator 3, and the negative electrode 4 are laminated is wound in a spiral shape. The electrode group 5 is housed in a bottomed cylindrical container 1 made of stainless steel, for example, and is fixed by a hollow cylindrical electrode group presser 12 made of polypropylene, for example. The separator 3 is formed of, for example, a nonwoven fabric, a polypropylene microporous film, a polyethylene microporous film, or a polyethylene-polypropylene microporous laminated film.
[0033]
The container 1 is hermetically sealed with a sealing plate 9 welded to the top, and an electrolyte is accommodated therein. A safety valve 10 is welded to the opening of the sealing plate 9, and the positive terminal 8 is fixed to the sealing plate 9 with a hermetic seal 11, for example. One end of the positive electrode current collecting lead 6 is connected to the positive electrode 2, and the other end is connected to the positive electrode terminal 8. The negative electrode 4 is connected to the container 1 which is a negative electrode terminal via a negative electrode current collecting lead 7.
[0034]
Next, the positive electrode 2, the negative electrode 4, the non-aqueous electrolyte and the like will be described.
[0035]
1) Positive electrode 2
The positive electrode 2 is not particularly limited as long as it includes a positive electrode active material made of a mixture containing a lithium nickel composite oxide and a spinel type lithium manganese oxide, and a known structure can be used.
[0036]
The lithium nickel composite oxide includes a composition formula LiNiO.₂Or a composite oxide represented by LiNiO₂A composite oxide obtained by substituting a part of lithium, nickel, or oxygen with an appropriate element, or a composition in which the ratio of lithium, nickel, and oxygen is changed to an appropriate value can be used.
[0037]
In particular, the composition formula Li_{1 - x}Ni_1-xyM_y(O_2-zF_z(Wherein M is at least one element selected from boron, niobium, and aluminum, x and y are (z + 0.05) / 2 ≦ x <(z + 1) / 3, and 0 <x + y ≦ 0. 5 and 0 ≦ z <0.66) or LiNi_1-xyCo_xM_yO2 (wherein M is at least one element selected from aluminum, boron and niobium, x and y are 0 <x ≦ 0.5, 0 <y <0.5, and 0 <x + y ≦ 0.5. Lithium nickel composite oxide represented by (2) is preferable in that it has high safety and can increase the capacity of the battery.
[0038]
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_0.15LiNi_0.795Co_0.175B_0.03O₂, LiNi_0.795Co_0.175Nb_0.03O₂, LiNi_0.725Co_0.17Nb_0.02O_{2 Etc.}
As the spinel type lithium manganese oxide, specifically, Li_{1 + a}Mn_2-aO_Four, Li_{1 + a}Mn_2-abCo_bO_Four, Li_{1 + a}Mn_2-abAl_bO_Four, Li_{1 + a}Mn_2-abFe_bO_Four, Li_{1 + a}Mn_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_Four(Wherein a represents 0 <a and 2> a + b).
[0039]
In the positive electrode active material, the proportion of the lithium nickel composite metal oxide is preferably 60% by weight or more and less than 80% by weight.
[0040]
The particle size of the lithium composite oxide is a cumulative average diameter D measured by a laser diffraction particle size distribution meter (Microtrac HRA particle size distribution meter: manufactured by Lees & Northup)._N50Is preferably in the range of 5 μm to 50 μm.
[0041]
The cumulative average diameter D of the spinel type lithium manganese oxide_M50The value of is preferably in the range of 1 μm to 30 μm.
[0042]
By setting it as such a structure, the nonaqueous electrolyte secondary battery excellent in the balance of safety | security and a capacity | capacitance characteristic can be comprised.
[0043]
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.
[0044]
The coating amount of the positive electrode active material per one side of the positive electrode layer is 80 g / m.²To 200g / m²It is preferable to be in the range. By adopting such a configuration, it is possible to obtain a non-aqueous electrolyte secondary battery having an excellent balance between safety and capacity characteristics and high output characteristics. The weight per one side of the positive electrode layer is more preferably 100 g / m.²To 150 g / m²It is.
[0045]
Examples of the conductive agent include acetylene black, graphite, and carbon black.
[0046]
Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-tetrafluoroethylene-6-propylene terpolymer, ethylene-propylene-diene copolymer ( EPDM) or the like can be used. Among these, polyvinylidene fluoride (PVdF) is preferable because of excellent adhesion to the substrate and binding property between the active materials.
[0047]
As an organic solvent for dispersing the binder, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) or the like is used.
[0048]
The blending amount of the binder should be in the range of 2 parts by weight to 20 parts by weight with respect to 100 parts by weight of the positive electrode active material (100 parts by weight including the conductive agent when the conductive agent is included). preferable.
[0049]
The blending amount of the conductive agent is preferably in the range of 0 to 18 parts by weight with respect to 100 parts by weight of the positive electrode active material.
[0050]
The amount of the organic solvent is preferably in the range of 65 parts by weight to 150 parts by weight with respect to 100 parts by weight of the positive electrode active material (100 parts by weight including the conductive agent when the conductive agent is included). .
[0051]
Examples of the current collector include an aluminum foil having a thickness of 15 μm to 35 μm, an aluminum mesh, an aluminum punched metal, an aluminum lath metal, a stainless steel foil, and a titanium foil.
[0052]
2) Negative electrode 4
The negative electrode 4 is prepared by suspending and mixing a negative electrode mixture 5b made of, for example, a negative electrode active material, a conductive agent, and a binder in an appropriate solvent, and applying the applied liquid on one or both sides of a current collector, followed by drying. It is formed by doing.
[0053]
The negative electrode active material according to the present invention contains a mixture of a non-graphite carbon material and graphite.
[0054]
Non-graphite carbon materials include, for example, coke, carbon fiber, pyrolytic vapor phase carbon, and resin fired body, and carbon having a (002) plane spacing of 0.34 nm or more determined by X-ray diffraction method. A material is preferable because of its large discharge capacity.
[0055]
Graphite includes mesophase pitch-based carbon fiber or mesophase spherical carbon in addition to natural graphite, and is a fibrous carbon material having a (002) plane spacing of 0.34 nm or less determined by X-ray diffraction method. Is preferred.
[0056]
The ratio of the non-graphite carbon material and graphite varies depending on the ratio of the lithium nickel composite oxide of the positive electrode active material and the spinel type lithium manganese oxide, that is, the discharge end voltage of the battery, but during the discharge of the battery The ratio is preferably such that the rate of change in the discharge potential of the negative electrode is 20 mV / (mAh / g).
[0057]
Since the discharge change rate of the negative electrode usually becomes steeper as the discharge progresses, if the mixing ratio of the negative electrode active material is adjusted so that the discharge voltage change rate is 20 mV / (mAh / g) at the discharge end voltage. Good.
[0058]
For example, the discharge end potential of the negative electrode is set to 0.5 (VvsLi / Li⁺), The proportion of the non-graphitic carbon material is preferably 10% by weight or more in the negative electrode active material.
[0059]
Further, since the battery capacity is reduced when the ratio of graphite is too small, it is desirable that the ratio of non-graphitic carbon material is 60% by weight or less.
[0060]
The carbon material is in a state in which the negative electrode 5 is produced, and the coating amount per side is 30 to 100 g / m.²It is preferable to be in the range.
[0061]
By adopting such a configuration, it is possible to configure a nonaqueous electrolyte secondary battery in which the negative electrode potential change rate at the end of discharge is suitable for overdischarge suppression and the irreversible capacity is also reduced.
[0062]
Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR).
[0063]
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.
[0064]
As the current collector, a copper foil having a thickness of 10 μm to 35 μm, a copper mesh, a copper punched metal, a copper lath metal, a stainless steel foil, a nickel foil, or the like can be used.
[0065]
As the separator 3, for example, a nonwoven fabric, a polypropylene microporous film, a polyethylene microporous film, a polyethylene-polypropylene microporous laminated film, porous paper, or the like can be used.
[0066]
3) Non-aqueous electrolyte
The non-aqueous electrolyte has a composition in which an electrolyte is dissolved in a non-aqueous solvent.
[0067]
Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC), for example, chain carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC). Chain ethers such as 1,2-dimethoxyethane (DME) and diethoxyethane (DEE), cyclic ethers and crown ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2-MeTHF), γ-butyrolactone (γ- At least one selected from fatty acid esters such as BL), nitrogen compounds such as acetonitrile (AN), sulfur compounds such as sulfolane (SL) and dimethyl sulfoxide (DMSO), and the like can be used.
[0068]
Among these, at least one selected from EC, PC, and γ-BL, at least one selected from EC, PC, and γ-BL and DMC, MEC, DEC, DME, DEE, THF, 2-MeTHF, It is desirable to use a mixed solvent composed of at least one selected from AN. Moreover, when using what contains the carbonaceous material which occludes / releases the said lithium ion for a negative electrode, from a viewpoint of improving the cycle life of the secondary battery provided with the said negative electrode, EC and PC, (gamma) -BL, EC and PC It is desirable to use a mixed solvent composed of EC and MEC, EC and PC and DEC, EC and PC and DEE, EC and AN, EC and MEC, PC and DMC, PC and DEC, or EC and DEC.
[0069]
Examples of the electrolyte include lithium perchlorate (LiClO)._Four), Lithium hexafluorophosphate (LiPF)_Four), Lithium borofluoride (LiBF)_Four), Lithium hexafluoroarsenide (LiAsF)₆), Lithium trifluorometasulfonate (LiCF)_ThreeSO_Three), Lithium aluminum tetrachloride (LiAlCl)_Four), Bistrifluoromethylsulfonylimide lithium [LiN (CF_ThreeSO₂)₂And the like. Among them, LiPF_{6 ,}LiBF_Four, LiN (CF_ThreeSO₂)₂Is preferable because conductivity and safety are improved.
[0070]
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.
[0071]
FIG. 3 shows a schematic diagram of an assembled battery in which nonaqueous electrolyte secondary batteries are connected in series.
[0072]
By preparing a plurality of cells of a nonaqueous electrolyte secondary battery as shown in FIG. 2 and making a battery assembly in which the positive electrode terminal 8 and the negative electrode terminal (container 1) of each single cell are connected by a conductor 20, It is possible to handle electrical equipment that requires a large voltage.
[0073]
【Example】
Examples of the present invention will be specifically described below.
[0074]
<Battery fabrication>
Example 1
LiNi as the lithium nickel oxide_0.711Co_0.206Al_0.083O₂(D_N50= 13μm)_{Li as spinel-type manganese oxide 1.06 Mn 1.94 O4 (}D_N50= 15 μm) was used.
[0075]
First, the lithium nickel oxide and the spinel manganese oxide were mixed at a blending ratio (weight ratio) of 70% lithium nickel oxide and 30% spinel 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 dispersed in a solution prepared by dissolving 5 parts by weight of polyvinylidene fluoride as a binder in N-methyl-2-pyrrolidone to prepare a positive electrode material paste. This was applied to both surfaces of an aluminum foil as a current collector, dried and rolled to produce a positive electrode.
[0076]
On the other hand, 50% fibrous graphite (MCF) obtained by graphitizing mesophase pitch carbon fiber and 50% (weight ratio) non-graphitizable carbon (HC) obtained by low-temperature firing of the resin were mixed with the negative electrode active material. did. After mixing a mixture of 5 parts by weight of polyvinylidene fluoride with N-methylpyrrolidone into a paste form with respect to 100 parts by weight of the negative electrode active material, it was applied to both sides of a copper foil as a current collector substrate, dried, Roll press was performed to produce a negative electrode.
[0077]
After laminating the positive electrode, a separator made of a polyethylene porous film, and the negative electrode, they were wound in a spiral shape to produce an electrode group.
[0078]
As the electrolytic solution, a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (mixing volume ratio 1: 2), lithium hexafluorophosphate (LiPF) is used.₆) 1 mol / l dissolved, and the electrode group and the electrolyte solution were respectively stored in a stainless steel bottomed cylindrical container, and 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled. It was.
[0079]
Subsequently, the produced single cells were charged at a constant voltage and constant current of 4.2 V and 1.2 A for 8 hours, respectively, and then discharged at a constant current of 0.8 A until reaching 3 V. Asked. 50 cells are ranked in the order of discharge capacity, and the remaining 40 batteries are assembled into 4 batteries in order of capacity, except for 5 from the largest discharge capacity and 5 from the smallest. Ten sets of four series batteries as small as possible were produced.
[0080]
Example 2
As a negative electrode active material, a mixture of 70% fibrous graphite (MCF) graphitized mesophase pitch carbon fiber and 30% (weight ratio) non-graphitizable carbon (HC) obtained by low-temperature firing of resin. Then, 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled in the same manner as in Example 1 to prepare 10 battery packs in series.
[0081]
Example 3
As a negative electrode active material, a mixture of 90% fibrous graphite (MCF) graphitized mesophase pitch carbon fiber and 10% (weight ratio) non-graphitizable carbon (HC) obtained by low-temperature firing of resin Then, 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled in the same manner as in Example 1 to prepare 10 battery packs in series.
[0082]
Comparative Example 1
As the negative electrode active material, fibrous graphite (MCF) obtained by graphitizing mesophase pitch carbon fibers was used, and 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled in the same manner as in Example 1 and four in series. 10 sets of assembled batteries were produced.
[0083]
Comparative Example 2
As a negative electrode active material, flake graphite (artificial graphite) is used, and 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) are assembled in the same manner as in Example 1 to prepare 10 battery packs in series. did.
[0084]
Comparative Example 3
As the negative electrode active material, non-graphitizable carbon (HC) obtained by baking the resin at low temperature was used, and 50 cylindrical lithium ion secondary batteries (φ35 mm × 65 mm) were assembled in the same manner as in Example 1 below. Ten sets of four assembled batteries in series were produced.
[0085]
<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.
[0086]
A negative electrode containing 50 mg of the negative electrode active material and metal Li as the counter electrode, and LiPF in a solvent in which ethylene carbonate and methyl ethyl carbonate were mixed at a ratio of 1: 2 as the electrolyte solution₆Was dissolved at a concentration of 1 mol / l, and a single electrode evaluation cell was produced using the same electrolyte solution used for the battery. This cell was charged with 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 reached 8 hours, and was initially charged. Next, after a rest time of 30 minutes, the battery was discharged at a constant current of 5 mA until the negative electrode potential reached 0.5V.
[0087]
The rate of change of the negative electrode discharge potential when the negative electrode potential reached 0.5 V was determined for the negative electrodes of the respective examples and comparative examples. The results are shown in Table 1. For Examples 1 to 3 and Comparative Example 3, the condition that the discharge potential change rate was 20 mV / (mAh / g) or less was satisfied. For Comparative Examples 1 and 2, the discharge potential change rate exceeded 20 mV / (mAh / g).
[Table 1]

<Evaluation of cycle characteristics of battery pack>
A cycle test was performed on each assembled battery obtained in Examples 1 to 3 and Comparative Examples 1 to 3.
[0088]
Charging is performed at a constant current of 16.8 V after the discharge capacity obtained by the capacity confirmation test of the single cell is discharged to 16.8 V at the current value (1 C) when discharging in 1 hour. Went for hours. Discharging was performed at a constant current of 1 C, and the final discharge voltage was 12V. 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. At the end of discharge, the voltage of each of the four unit cells constituting the assembled battery was measured to examine variation.
[0089]
Then, the ratio of the discharge capacity when the number of cycles reached 300 and the discharge capacity at the first cycle, that is, (discharge capacity at the 100th cycle) / (discharge capacity at the first cycle) was obtained as the capacity maintenance rate. . Table 2 shows the results of the capacity confirmation test of the single cell and the cycle characteristic evaluation result of the assembled battery.
[Table 2]

As is clear from Tables 1 and 2, in Comparative Examples 1 and 2 where the negative electrode discharge voltage change rate when the negative electrode discharge voltage is 1 V is 20 mV / (mAh / g) or more, the discharge end voltage of the assembled battery When the voltage is 12.0V, the variation of each cell voltage is significantly increased to 1.7V and 1.5V, confirming that some of the cells are in an overdischarged state, and the capacity after 300 cycles The maintenance rate was as low as 70% or less.
[0090]
On the other hand, the discharge potential is 0.5 (VvsLi / Li from the charged state.⁺In Examples 1 to 4 in which the rate of change in the negative electrode discharge potential is 20 mV / (mAh / g) or less until each of the battery cell voltage is 12.0V, The variation was 1 V or less, and overdischarge was suppressed. In these examples, the capacity retention rate after 300 cycles was at least 79%, indicating an excellent value.
[0091]
Further, compared with Comparative Example 3 using 100% non-graphitizable carbon (HC) as the negative electrode active material, fibrous graphite (MCF) graphitized non-graphitizable carbon (HC) and mesophase pitch carbon fiber. In Example 2 in which the mixing ratio was 30:70 and Example 3 in which 10:90 was used, it was confirmed that the initial characteristics of the battery such as initial capacity and initial efficiency were excellent.
[0092]
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 accommodated in an exterior material made of a laminate film. The present invention can be similarly applied to a thin nonaqueous electrolyte secondary battery having a structure.
[0093]
【The invention's effect】
According to the non-aqueous electrolyte secondary battery of the present invention, cycle deterioration can be reduced even when a positive electrode active material with high capacity and high safety is used and used as an assembled battery.
[Brief description of the drawings]
FIG. 1 is a diagram showing discharge characteristics of each unit cell in series connection.
FIG. 2 is a view 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]
1 ... Container
2 ... Positive electrode
3 ... Separator
4 ... Negative electrode
5 ... Electrode group
6 ... Positive current collector lead
7 ... Negative electrode current collector lead
8 ... Positive terminal
9 ... Sealing plate
10 ... Safety valve
11 ... Hermetic seal
12 ... Electrode group holder

Claims (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;
It comprises a non-graphite carbon material of 10 wt% or more and 60 wt% or less and a carbon material mixture made of graphite , and the discharge potential change rate is 20 mV / (in a potential region where the operating potential is 0.5 (VvsLi / Li ⁺ ) or less. negative electrode comprising a negative electrode active material that is less than or equal to mAh / g) ;
The non-aqueous electrolyte sandwiched between the positive electrode and the negative electrode,
A nonaqueous electrolyte secondary battery, wherein a plurality of unit cells each using the positive electrode active material and the negative electrode active material having the same composition ratio are connected in series. 2. The non-aqueous electrolyte according to claim 1, wherein the negative electrode active material is a carbon material mixture composed of 10% by weight or more and 50% by weight or less non- graphitizable carbon and graphitized mesophase pitch carbon fiber. Secondary battery.

Images