JP5288448B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a high-capacity nonaqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics obtained by adding an additive to a negative electrode.

  Currently, with the spread of mobile devices such as mobile phones and notebook personal computers, the role of secondary batteries as the power source is regarded as important. These secondary batteries are small, lightweight, and have a high capacity, and are required to have a performance that does not easily deteriorate even after repeated charging and discharging. Currently, lithium ion secondary batteries are most frequently used.

  Carbon such as graphite and hard carbon is mainly used for the negative electrode of the lithium ion secondary battery. Although carbon can repeat charge / discharge cycles satisfactorily, the capacity has already been used up to near the theoretical capacity, and therefore no significant improvement in capacity can be expected in the future. On the other hand, there is a strong demand for increasing the capacity of lithium ion secondary batteries, and negative electrode materials having a higher capacity than carbon, that is, a higher energy density are being studied.

  An example of a material that can realize a high energy density is silicon. In fact, Non-Patent Document 1 describes use as a negative electrode active material.

  Although the negative electrode using silicon has a large amount of occlusion and release of lithium ions per unit volume and a high capacity, pulverization progresses due to large expansion and contraction of the electrode active material itself when lithium ions are occluded and released. However, the irreversible capacity in the first charge / discharge is large and the charge / discharge cycle life is short.

  As a measure for reducing the initial irreversible capacity using silicon and improving the charge / discharge cycle life, Patent Document 1 proposes a method using silicon oxide as a negative electrode active material. In Patent Document 1, since the volume expansion / shrinkage per unit weight of the active material can be reduced by using silicon oxide as the active material, improvement in cycle characteristics has been confirmed. On the other hand, since the conductivity of the oxide is low, the current collecting property is lowered, and the irreversible capacity in charge / discharge is large. Furthermore, as a measure for improving capacity and charge / discharge cycle life, Patent Document 2 proposes a method using particles obtained by combining a carbon material with silicon or silicon oxide as an active material. Although the improvement of the cycle characteristics was confirmed by this, it is still insufficient, and the improvement of the first charge / discharge efficiency is insufficient.

  As countermeasures for this initial irreversible capacity, an electrode formation method in which the irreversible capacity is electrochemically charged in advance or a method of making up for the irreversible capacity by attaching metallic lithium to the negative electrode has been attempted.

  The electrode formation method is excellent in that the amount of electricity according to the purpose can be controlled by controlling the amount of electricity supplied. However, since the electrode is once charged and then reassembled as a battery, it is cumbersome and the productivity is extremely poor. The lithium metal sticking method automatically moves Li between an oxide in a short-circuited state and lithium metal by injecting an electrolytic solution. However, in the case of this method, depending on the electrode plate form, there is a problem in quality such that Li migration is insufficient and metallic lithium remains, causing variations in characteristics and problems in safety.

In addition, as a countermeasure for this first irreversible capacity, the negative electrode contains silicon oxide and lithium represented by the general formula Li _3-X M _X N (M is a transition metal, 0.2 <X ≦ 0.8) A non-aqueous electrolyte secondary battery using a mixed active material with a composite nitride is proposed in Patent Document 3. Li _3-X M _X N (M is a transition metal, 0.2 <X ≦ 0.8) is excellent in supplementing the irreversible capacity of the oxide of silicon, but the capacity per weight of the lithium-containing composite nitride Is about 800 mAh / g, which is smaller than the oxide of silicon, and the capacity of the battery is smaller than the capacity of the battery when only the silicon oxide is used as the negative electrode active material.

Japanese Patent No. 2999741 JP 2004-139886 A JP 2000-164207 A Li et al., “A High Capacity Nano-Si Composite Anode Material for Lithium Rechargeable Batteries, Electrochemical and Solid State Letters” -State Letters), Vol. 2, No. 11, p547-549 (1999)

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery having high charge / discharge efficiency in the first charge / discharge and high energy density.

The non-aqueous electrolyte secondary battery of the present invention for solving the above problems is the non-aqueous electrolyte secondary battery using lithium in the positive electrode and capable of absorbing and releasing a lithium-containing material as a cathode active material, Si negative electrode, SiO ₂ and C This composite is used as a negative electrode active material, and Li ₃ N is used as an additive.

Moreover, it is preferable that all the Li in the additive Li ₃ N of the negative electrode is released during the first discharge and the additive Li ₃ N is decomposed, and the initial irreversible capacity is replenished.

Further, when the amount of Li corresponding to the first irreversible capacity of the Si, SiO ₂ , and C composite of the negative electrode is α and the amount of Li released by the Li ₃ N during the initial discharge is γ, the Si, SiO of the negative electrode _2. It is preferable that the weight ratio between the C composite material and the Li ₃ N satisfies α = γ.

In addition, when the initial irreversible capacity of the Si, SiO ₂ , and C composite of the negative electrode is X, the charge capacity of the negative electrode is Y, and the charge capacity of the positive electrode is Z, these relationships should be Y ≧ X + Z. Characteristically, all the active material in the positive electrode is used for charging and discharging, and a high capacity non-aqueous electrolyte secondary battery is obtained.

According to the present invention, the initial irreversible capacity inherent to the negative active material Si, SiO ₂ , and C composite is supplemented during initial discharge with Li in Li ₃ N present as an additive in the negative electrode, and the initial charge / discharge efficiency is improved. By improving, a high-capacity nonaqueous electrolyte secondary battery can be obtained.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery of the present invention. As shown in FIG. 1, the non-aqueous electrolyte secondary battery of the present invention has a negative electrode 3 composed of a negative electrode active material layer 1 formed on a negative electrode current collector 2 such as a copper foil and a positive electrode current collector 5 such as an aluminum foil. The positive electrode 6 formed of the positive electrode active material layer 4 is disposed so as to face the separator 7. As the separator 7, a polyolefin such as polypropylene or polyethylene, or a porous film such as a fluororesin can be used. A negative electrode lead tab 9 and a positive electrode lead tab 10 for taking out electrode terminals are drawn out from the negative electrode 3 and the positive electrode 6, respectively, and are covered with an outer film 8 such as a laminate film except for the respective ends.

The material structure of the negative electrode is composed of Si, SiO ₂ , C composite powder, which is an active material of the negative electrode, Li ₃ N powder as an additive, and a binder resin. Is formed. These mixtures are processed into a known form by a manufacturing method such as applying a paste kneaded with a solvent onto a metal foil such as copper foil and rolling it, or directly pressing into a pressure-formed electrode plate. Specifically, Si powder, SiO ₂ powder, composite particles in which C powder and lithium nitride powder are mixed, and polyimide, polyamide, polyamideimide, polyacrylic acid resin, polymethacrylic acid resin as binder And a binder having thermosetting properties such as N-methyl-2-pyrrolidone (NMP) dispersed and kneaded, and applied onto the negative electrode current collector 2 made of a metal foil, and a high temperature atmosphere It is formed by drying with. In the active material layer 1 of the negative electrode, carbon black, acetylene black, or the like may be mixed in order to impart conductivity as necessary. The electrode density of the generated negative electrode active material layer 1 is preferably 0.5 g / cm ³ or more and 2.0 g / cm ³ or less. When the electrode density is low, the absolute value of the discharge capacity is small, and a merit over the conventional carbon material cannot be obtained. On the other hand, when it is high, it is difficult to impregnate the electrode with the electrolytic solution, and the discharge capacity is also lowered. The thickness of the negative electrode current collector 2 is preferably 4 to 100 μm because it is preferable to maintain a thickness that can maintain strength, and more preferably 5 to 30 μm in order to increase the energy density.

Moreover, as electrolyte solution used for a battery, cyclic carbonates, such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC) Chain carbonates such as ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate, and γ-lactones such as γ-butyrolactone, , 2-Ethoxyethane (DEE), chain ethers such as ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethyl ether Tylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2 -Using an aprotic organic solvent such as oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, or a mixture of two or more thereof. A lithium salt that dissolves in an organic solvent is dissolved. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ). ₂ , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, lithium chloroborane, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, imides and the like. Further, a polymer electrolyte may be used instead of the electrolytic solution.

  Moreover, it is desirable that the discharge end voltage value of the nonaqueous electrolyte secondary battery manufactured as described above is 1.5 V or more and 2.7 V or less. There is a problem that the deterioration of the discharge capacity due to repeated charge / discharge increases as the discharge end voltage value decreases. Setting the voltage to 1.5 V or less has a high degree of difficulty in circuit design. In the case of 2.7 V or more, the absolute value of the discharge capacity is small and no merit over the conventional carbon material can be obtained.

In the battery of the present invention, Si of Fukyokuchu, Si from Li ₃ N to SiO _2, C complexes, recruitment of Li corresponding to the irreversible capacity of SiO _2, C complex by a discharge operation after the cell structure Made. In this negative electrode, Si, SiO ₂ , C composite and Li ₃ N constitute a local battery, but the Li occlusion potential of Si, SiO ₂ , C composite is around 1 V where Li ₃ N is the Li release potential. Basically, there is no automatic movement of Li due to a potential difference because it is in a more base area. In the case of this battery, Li is supplied from the lithium-containing composite oxide positive electrode only to the Si, SiO ₂ , and C composite of the negative electrode at the first charge, and the charge is completed. The amount of Li that returns from the Si, SiO ₂ , and C composite to the positive electrode in the next discharge is insufficient (irreversible capacity). Therefore, Li is first supplied from Li ₃ N to the positive electrode at this time. The negative electrode contains Li ₃ N containing the same amount of Li as the irreversible capacity Li, and all Li in the Li ₃ N is released by completely discharging the battery during the first discharge, Li ₃ N decomposes. Although it has been stated that the replenishment of Li is performed by the discharging operation after the battery construction, this is also a very effective advantage in the manufacture of the battery. This is also the reason for choosing the combination of Si, SiO ₂ , C composite and lithium nitride of the present invention. In the case of Li ₃ N, the initial Li emission potential is in the vicinity of 1 V, but most of the oxides promising as negative electrodes have a Li storage potential nobler than 1 V. That is, when a mixed active material similar to that of the present invention is formed of such an oxide, automatic Li movement occurs rapidly due to a potential difference simultaneously with the injection of the electrolytic solution. Therefore, the combination of Si, SiO ₂ , C complex and Li ₃ N is preferable in that the release of Li can be controlled.

Furthermore, Li ₃ N has Li and N as its raw materials, and there is no particular problem in terms of cost.

Since Li ₃ N, which is an additive for the negative electrode, releases and decomposes Li only at the time of the first discharge, the initial irreversible capacity of Si, SiO ₂ , C complex is set to X, the negative electrode charge capacity is set to Y, and the positive electrode is charged If the relationship is Y ≧ X + Z when the capacity is Z, the effect of increasing the capacity peculiar to Si, SiO ₂ and C composites can be obtained. Further, if it is Y <X + Z Si, be supplemented with Li ₃ N irreversible capacity of SiO _2, C complex during initial discharge accept Li at the next charging Si, there is SiO _2, C complex Therefore, the capacity of the active material in the positive electrode cannot be fully utilized, and the battery capacity is reduced.

When the amount of Li corresponding to the initial irreversible capacity of the Si, SiO ₂ , C composite in the negative electrode is α, and the amount of Li released by decomposition and release of Li ₃ N in the negative electrode during the first discharge is γ, When the weight ratio of the Si, SiO ₂ , C composite and Li ₃ N in the negative electrode satisfies the relationship of α = γ, replenishment of the irreversible capacity at the first charge / discharge can be utilized to the maximum. Further, even when compared with replenishment of the initial irreversible capacity with lithium-containing composite nitride, a merit is obtained in charge / discharge capacity per weight.

As described above, by replenishing the initial irreversible capacity inherent to Si, SiO ₂ , and C complex with Li in Li ₃ N present as an additive in the negative electrode at the time of initial discharge, and improving the initial charge and discharge efficiency, A high-capacity nonaqueous electrolyte secondary battery can be produced.

In the present invention, Si, SiO ₂ , and C are used as the negative electrode active material. In this example, the ratio of the respective molecular weights is 1: 1: 0.8 as a representative example.

Si powder, SiO ₂ powder, C powder, and Li ₃ N powder are commercially available as reagents, and this powder was obtained and used.

Oxide Si to be used in advance, was confirmed Discharge Performance of SiO _2, C composite anode electrode (confirmation capacitance characteristic metallic lithium by the model cell having a counter electrode), Si in the first charge, SiO _2, C composite The body occluded about 2500 mAh / g of Li, but only discharged about 1750 mAh / g in the next discharge and had an irreversible capacity of about 750 mAh / g.

In addition, when the charge / discharge performance of the lithium nitride Li ₃ N used in advance was confirmed (capacity characteristics using a model cell with metal lithium as the counter electrode), approximately 2300 mAh / g of Li was released in the first discharge. However, it was disassembled and hardly charged in the next charging.

The active material layer of the negative electrode is a mixture of Si, SiO ₂ , C composite material particles and Li ₃ N powder corresponding to an initial irreversible capacity of about 750 mAh / g of Si, SiO ₂ , C composite, and polyimide as a binder the electrode material obtained by mixing NMP as a solvent was coated onto a copper foil of 10 [mu] m, 125 ° C., dried for 5 minutes, subjected to compression molding by a roll press, 350 ° C. again at drying oven, 10 minutes N ₂ A drying process was performed in an atmosphere. The active material layer formed on this copper foil was punched out to 30 × 28 mm to form a negative electrode, and a negative electrode lead tab made of nickel for extracting electric charge was fused by ultrasonic waves. For the positive electrode active material layer, an active material particle made of lithium nickelate, an electrode material mixed with polyvinylidene fluoride as a binder, and NMP as a solvent are applied on a 20 μm aluminum foil, and dried at 125 ° C. for 5 minutes. Made. The active material layer formed on the aluminum foil was punched out to 30 × 28 mm to form a positive electrode, and a positive electrode lead tab made of aluminum for taking out electric charges was fused by ultrasonic waves. After laminating in order of the negative electrode, the separator, and the positive electrode so that the active material layer faces the separator, the laminate film was sandwiched, the electrolyte solution was poured, and the laminate type battery was sealed under vacuum. Note that the electrolytic solution, the EC, and DEC, the volume ratio of EMC 3: 5: was used LiPF ₆ was dissolved in 1 mol / L to 2 mixture of.

In making laminated battery, the charge capacity of the charge capacity and the negative electrode of the positive electrode ratio, Si, the initial irreversible capacity of SiO _2, C composite anode electrode X Distant, Si, the SiO _2, C composite anode electrode X: Y: Z = 0.3: 1.3: 1.0 (Example 1) so that the relationship satisfies Y ≧ X + Z when the charge capacity is Y and the charge capacity of the positive electrode is Z. X: Y: Z = 0.3: 1.5: 1.0 (Example 2), X: Y: Z = 0.3: 1.6: 1.0 (Example 3), X: Y: Z = 0.3: 1.7: 1.0 (Example 4).

The charge / discharge test of the battery produced as described above was performed at a constant current of 3 mA, a charge end voltage of 4.2 V, and a discharge end voltage of 2.5 V. Table 1 shows the initial charge capacity, initial discharge capacity, and initial charge / discharge efficiency of the battery in this test, and Table 2 shows the charge capacity in the second charge. Although this battery starts from charging, there is almost no difference between the initial charge capacity and the initial discharge capacity, and the irreversible capacity of the Si, SiO ₂ , C complex is effectively supplemented from Li ₃ N. Recognize. In the second charge, there is almost no difference from the initial charge capacity.

 Next, the capacity per weight of the active material of each positive and negative electrode in this battery was calculated. As a result, the lithium nickelate of the positive electrode worked about 200 mAh / g as an active material.

On the other hand, although the active material is Si, SiO ₂ , and C composite of the negative electrode, the capacity per weight and the capacity per total weight of the negative electrode are as shown in Table 3.

  Considering that the carbon material of the negative electrode of a lithium ion battery currently in practical use has a capacity density of about 300 to 370 mAh / g, the battery of the present invention has realized an extremely high capacity battery.

  Further, in the battery of the present invention, a combination of X: Y: Z not satisfying Y ≧ X + Z is satisfied. X: Y: Z = 0.3: 1.0: 1.0, X: Y: Z = 0 .3: 1.1: 1.0, X: Y: Z = 0.3: 1.2: 1.0 was used to produce a battery, and a charge / discharge test was conducted in the same manner as in Examples 1-4. Table 4 shows the initial charge capacity, initial discharge capacity, and second charge capacity of the battery in this test.

  Compared to the case where Y ≧ X + Z is satisfied, the initial charge capacity and the charge capacity are smaller in the second charge, but the initial charge / discharge efficiency is the same.

Furthermore, in the active material layer of the negative electrode, when Si 3 SiO ₂ , C composite material particles are mixed with Li ₃ N powder more than the initial irreversible capacity of Si, SiO ₂ , C composite, and less than the initial irreversible capacity. In the case where the Li ₃ N powder was mixed, a battery was prepared in the same manner, and a charge / discharge test was performed. Implementing the case of mixing a Li ₃ N powder fold volume fraction relative to irreversible capacity Examples 8 and 9, irreversible capacity implementing the place of a mixture of Li ₃ N powder half volume fraction for the example 10, 11, the initial charge / discharge efficiency, the initial charge capacity, and the charge capacity per negative electrode active material layer are summarized in Table 5.

Compared to Examples 1 and 2 is the case of a mixture of Li ₃ N powder of the same capacity of the irreversible capacity, the case of mixing the multiple of Li ₃ N powder, the capacity per negative electrode active material layer is small, There is no change in the initial charge / discharge efficiency. Conversely, when half the Li ₃ N powder is mixed, the initial charge / discharge efficiency decreases, but the capacity per negative electrode active material layer is large. That is, when the same capacity of Li ₃ N powder as the irreversible capacity is mixed, the highest capacity non-aqueous electrolyte secondary battery can be obtained.

(Comparative example)
For comparison, a battery in which only a Si, SiO ₂ , and C composite was used as a negative electrode active material and a lithium nickelate positive electrode was fabricated (Comparative Example 1-4), and a charge / discharge test was performed. Table 6 shows the initial charge capacity, initial discharge capacity, and initial charge / discharge efficiency of the battery in this test. A battery using only a Si, SiO ₂ , and C composite has a reversible charge / discharge capacity due to its extremely large irreversible capacity.

As described above, when the negative electrode is made of Si, SiO ₂ , and C composite as the negative electrode active material and the lithium nitride represented by the general formula Li ₃ N is used as the additive in the negative electrode, the nonaqueous electrolyte battery has high initial charge / discharge efficiency. Confirmed that it can be provided.

Sectional drawing of the nonaqueous electrolyte secondary battery of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 (Negative electrode) Active material layer 2 Negative electrode collector 3 Negative electrode 4 (Positive electrode) Active material layer 5 Positive electrode collector 6 Positive electrode 7 Separator 8 Exterior film 9 Negative electrode lead tab 10 Positive electrode lead tab

Claims (4)

In a non-aqueous electrolyte secondary battery using a lithium-containing material capable of occluding and releasing lithium as a positive electrode as a positive electrode active material, a composite of Si, SiO ₂ and C is used as a negative electrode active material and Li ₃ N is used as an additive as a negative electrode A non-aqueous electrolyte secondary battery. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein all Li in the additive Li ₃ N is released and Li ₃ N is decomposed during the first discharge. When the amount of Li corresponding to the initial irreversible capacity of the Si, SiO ₂ , C composite of the negative electrode is α and the amount of Li released by the Li ₃ N during the initial discharge is γ, Si, SiO ₂ , The non-aqueous electrolyte secondary battery according to claim 1, wherein a weight ratio of the C composite material and the Li ₃ N satisfies α = γ. When the initial irreversible capacity of the Si, SiO ₂ , and C composite of the negative electrode is X, the charge capacity of the negative electrode is Y, and the charge capacity of the positive electrode is Z, these relations satisfy Y ≧ X + Z. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 3.

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