JP2010192459A - Nonaqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous secondary battery enhancing reliability in overcharge by preventing the occurrence of cracks or cutoff of a negative current collector even when the volume change attendant on charge discharge of a negative mix layer is large, and enhancing cycle characteristics by preventing deterioration of cycle characteristics caused by cracks or cutoff of the negative current collector. <P>SOLUTION: The nonaqueous secondary battery has a spiral electrode laminate formed by winding a positive electrode, a negative electrode, and a separator, and an electrolyte, and the negative electrode is formed by forming a negative mix layer on at least one surface of a current collector, the maximum volume change rate after charge and after discharge of the negative mix layer is 8% or more, degree of breaking extension of the negative current collector is 5% or more. In addition, preferably, the negative current collector has a contact angle of less than 40° as wettability. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous secondary battery, and more particularly to a non-aqueous secondary battery having high reliability particularly during overcharge and excellent cycle characteristics.

  Non-aqueous secondary batteries represented by lithium-ion batteries have a large capacity, high voltage, high energy density, and high output, and therefore demand is increasing.

  In the non-aqueous secondary battery, the negative electrode is formed by forming a layer of a negative electrode mixture containing a negative electrode active material or a binder on at least one surface of a metal current collector. In the course of further study of water secondary batteries, this non-aqueous secondary battery has a negative electrode mixture layer with a large volume change due to charge / discharge. It was found that the current collector was cracked, cut, etc., and the subsequent deterioration of the cycle characteristics increased.

  In view of the circumstances as described above, the present invention prevents the negative electrode current collector from cracking, cutting, etc. during overcharge even when the volume change accompanying charging / discharging of the negative electrode mixture layer is large. An object of the present invention is to provide a non-aqueous secondary battery having excellent cycle characteristics by improving reliability during charging and preventing deterioration of cycle characteristics due to occurrence of cracks, cuts, etc. of the negative electrode current collector.

  The present invention is a non-aqueous secondary battery having a spiral electrode laminate in which a positive electrode, a negative electrode and a separator are wound, and an electrolyte, wherein the negative electrode has a negative electrode mixture layer formed on at least one surface of a current collector When the maximum volume change after charging and discharging of the negative electrode mixture layer is 8% or more, the current collector of the negative electrode has a breaking elongation of 5% or more. In this case, cracks and cuts are prevented from occurring in the current collector of the negative electrode, and the above problems are solved. Further, in addition to the above characteristics, the use of a negative electrode current collector having a wettability of less than 40 degrees in contact angle can more effectively prevent deterioration of cycle characteristics.

  In the present invention, even when using a negative electrode mixture having a large volume change with a maximum volume change rate of 8% or more accompanying charge / discharge of the negative electrode mixture layer, preventing the occurrence of cracking, cutting, etc. of the negative electrode current collector, A non-aqueous secondary battery having excellent cycle characteristics can be provided by improving reliability during overcharge and preventing deterioration of cycle characteristics due to cracking or cutting of the negative electrode current collector.

It is a longitudinal cross-sectional view which shows an example of the non-aqueous secondary battery which concerns on this invention.

  In the present invention, the current collector of the negative electrode is made of, for example, copper, nickel, stainless steel, and, for example, a foil or a net-like material is used. It is necessary to be 5% or more. This is because if the elongation at break of the current collector of the negative electrode is not more than 5%, the effect of preventing the occurrence of cracking, cutting, etc. of the current collector due to charge / discharge cannot be sufficiently exhibited. The elongation at break is particularly preferably 7% or more. In order to obtain such elongation at break, it is suitable to use a current collector made of copper.

  In the present invention, the elongation at break of the current collector of the negative electrode means that the battery is discharged at a rate of 1C up to 2.75 V at 20 ° C., then decomposed and subjected to a tensile test together with the current collector or the negative electrode mixture layer. The elongation until the electric material breaks. The reason why the current collector is less cut when the current collector is larger is as follows.

  In a non-aqueous secondary battery having a maximum volume change rate of 8% or more after charging and discharging of the negative electrode mixture layer, the expansion and contraction associated with charging and discharging of the negative electrode mixture layer is large. The current collector is stretched by being pulled. At this time, if the elongation at break of the current collector is small, the current collector is cut and some of the negative electrode mixture cannot be used, and the deterioration of the cycle characteristics increases.

  In addition, the influence is even greater in a non-aqueous secondary battery in which the maximum volume change after charging and discharging of the negative electrode mixture layer is 11% or more. The maximum volume change rate after charging and discharging of the negative electrode mixture layer is determined by measuring the thickness of the negative electrode mixture layer when it is decomposed by charging at a 1C rate for 2 hours and a half to the standard charging voltage of the battery. The thickness of the negative electrode mixture layer was measured when another battery produced in the same manner was discharged at a 1C rate to 2.75 V and decomposed, and the value of the portion with the largest volume change rate was obtained.

  The surface roughness of the current collector also affects the cutting of the current collector. If the surface of the current collector is smooth, slipping occurs between the current collector and the negative electrode mixture layer when charged and expands, making it difficult to cut. The surface roughness of the negative electrode current collector is preferably 0.3 μm or less, more preferably 0.25 μm or less, in terms of Ra (IPC-MF-150F).

In addition, when the elongation at break of the current collector of the negative electrode is large, the wettability is usually poor, and the cycle characteristics tend to be greatly deteriorated when the battery is charged and discharged. In such a case, if the contact angle is less than 50 degrees, the deterioration of the cycle characteristics is reduced. Further, if the contact angle is less than 40 degrees, the effect is further increased, which is more desirable. As a method for improving the wettability, for example, when chromating the current collector, the chromate treatment is performed by alkali chromate treatment, or reducing the chromate amount when chromating the current collector. Is also effective. Then, the chromate amount 0.15 mg / ^{m 2} or less is preferable, 0.1 mg / ^{m 2} or less is more preferable.

  Note that the wettability in the present invention is evaluated by a contact angle. The contact angle is determined by fixing a sample having a length of 4 cm and a width of 3 cm on a slide glass with a tape, and dropping 1 μl of water in a droplet amount onto the sample. This image is taken into a computer and means the average of three values measured by image analysis. The analysis method is "a new contact angle measurement method using computer image analysis system" [The 45th Colloid and Interface Chemistry Conference Summary of Lecture, p99 (1992)].

In addition, the effect of the present invention is remarkably exhibited when the capacity in the normal charge per unit volume of the electrode laminate inside the battery is 130 mAh / cm ³ or more, and more remarkably when the capacity is 140 mAh / cm ³ or more. Therefore, the present invention is suitable for application to such a high capacity battery. The electrode laminate unit volume is the bulk volume of the positive electrode, negative electrode and separator laminated or wound in the battery, and does not include the volume of the winding shaft. The total volume of

  When a liquid electrolyte (electrolytic solution) is used as the electrolyte, an ester is often used as the solvent component. A chain ester particularly often used is a chain ester having a chain COO- bond such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, or methyl propionate.

  Further, it is more desirable to use the above-mentioned chain ester with the following ester having a high dielectric constant (dielectric constant of 30 or more). Examples of such an ester having a high dielectric constant include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), gamma-butyrolactone (γ-BL), and ethylene glycol sulfite (EGS). In particular, those having a cyclic structure are desirable, cyclic carbonates are particularly desirable, and ethylene carbonate (EC) is most desirable.

  The high dielectric constant ester is desirably less than 40% by volume in the total solvent of the liquid electrolyte, more desirably 30% by volume or less, and even more desirably 25% by volume or less. And the improvement of safety by these esters having a high dielectric constant is such that the battery characteristics are improved when the ester is 10% by volume or more in the total solvent of the liquid electrolyte, and further improvement is seen when the ester reaches 20% by volume. become.

  Examples of solvents that can be used in addition to the above ester include 1,2-dimethoxyethane (DME), 1,3-dioxolane (DO), tetrahydrofuran (THF), 2-methyl-tetrahydrofuran (2Me-THF), diethyl ether ( DEE). In addition, amine imide organic solvents, sulfur-containing or fluorine-containing organic solvents, and the like can also be used.

The solute of a liquid electrolyte, for example _{LiClO 4, LiPF 6, LiBF 4} , LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group], etc. However, LiPF ₆ or LiC ₄ F ₉ SO ₃ is particularly desirable. The concentration of the solute in the liquid electrolyte is not particularly limited, but it is desirable that the concentration be 1 mol / l or more because safety is further improved, and 1.2 mol / l or more is more desirable. Further, if it is less than 1.7 mol / l, it is desirable because electrical characteristics are improved, and if it is less than 1.5 mol / l, it is more desirable.

Examples of the positive electrode active material include metal oxides such as lithium cobalt oxide such as LiCoO ₂ , lithium manganese oxide such as LiMn ₂ O ₄ , lithium nickel oxide such as LiNiO ₂ , manganese dioxide, vanadium pentoxide, and chromium oxide. Or metal sulfides such as titanium disulfide and molybdenum disulfide are used. For the positive electrode, for example, a conductive additive or a binder such as polyvinylidene fluoride is appropriately added to the positive electrode active material as necessary. The finished positive electrode mixture is finished into a molded body using a current collector such as an aluminum foil as a core material.

In particular, when a lithium composite oxide, such as LiNiO ₂ , LiCoO ₂ , or LiMn ₂ O _4, whose open circuit voltage during charging is 4 V or more on the basis of Li is used as the positive electrode active material, it is desirable because a high energy density can be obtained.

  The active material used for the negative electrode may be any material that can be doped and dedoped with lithium ions. Examples of such a negative electrode active material include graphite, pyrolytic carbons, cokes, glassy carbons, and organic polymer compounds. Carbon compounds such as calcined bodies, mesocarbon microbeads, carbon fibers, activated carbon and the like can be used. Particularly, carbon compounds calcined at 2000 ° C. or more have a large volume change due to charge / discharge, and the present invention has such volume changes. When applied to a case where an active material having a large size is used, the effect is particularly prominent. In the present invention, an alloy such as Si, Sn, In, or a compound such as an oxide such as Si, Sn, In that can be charged and discharged at a low potential close to Li can be used as the negative electrode active material. When used as a material, the maximum volume change rate of the negative electrode mixture layer may exceed 10% with charge and discharge, and the present invention is particularly effective when applied to the case of using such a negative electrode active material. To express.

When a carbon material is used as the negative electrode active material, the carbon material preferably has the following characteristics. That is, the interlayer distance d ₀₀₂ of the (002) plane is preferably 3.4 mm or less. The crystallite size Lc in the c-axis direction is preferably 30 mm or more, more preferably 80 mm or more, and further preferably 250 mm or more. The average particle size is preferably 8 to 15 μm, particularly preferably 10 to 13 μm, and the purity is preferably 99.9% or more.

  The negative electrode is prepared by, for example, dissolving or dispersing, in a solvent, a negative electrode mixture obtained by adding a binder such as polyvinylidene fluoride or latex rubber or a conductive assistant such as artificial graphite to the negative electrode active material as described above. The negative electrode mixture slurry prepared in this manner is applied to the current collector, dried to volatilize and remove the solvent in the slurry, and a negative electrode mixture layer is formed on at least one surface of the current collector. The

  Next, the present invention will be described more specifically with reference to examples. However, this invention is not limited only to those Examples.

Example 1
Methyl ethyl carbonate and ethylene carbonate are mixed at a volume ratio of 75:25, LiPF ₆ is dissolved in 1.4 mol / l in this mixed solvent, and the composition becomes 1.4 mol / l Li.
A liquid electrolyte represented by PF ₆ / EC: MEC (25:75 volume ratio) was prepared.

EC in the liquid electrolyte is an abbreviation for ethylene carbonate, and MEC is an abbreviation for methyl ethyl carbonate. Therefore, 1.4 mol / l Li indicating the above liquid electrolyte
PF ₆ / EC: MEC (25:75 volume ratio) shows that 1.4 mol / l of LiPF ₆ is dissolved in a mixed solvent of 75% by volume of methyl ethyl carbonate and 25% by volume of ethylene carbonate. ing.

In addition to the above, scale-like graphite as a conductive additive is added to LiCoO ₂ at a weight ratio of 100: 7 and mixed, and this mixture is mixed with a solution in which polyvinylidene fluoride is dissolved in N-methylpyrrolidone to obtain a slurry. I made it. This positive electrode mixture slurry is passed through a 70 mesh net to remove large particles, and then uniformly applied to both surfaces of a positive electrode current collector made of aluminum foil having a thickness of 20 μm and dried to volatilize the solvent in the slurry. Then, a positive electrode mixture layer was formed on both surfaces of the positive electrode current collector, and then compression-molded by a roller press, cut, and the lead body was welded to produce a belt-like positive electrode.

Further, the graphite-based carbon material [where (002) plane interlayer distance d ₀₀₂ = 3.37 Å, c-axis direction crystallite size Lc = 950 Å, average particle size 10 μm, purity 99.9%. The carbon material] was mixed with a solution of polyvinylidene fluoride dissolved in N-methylpyrrolidone to form a slurry. This negative electrode mixture slurry was passed through a 70 mesh net to remove large particles, and then uniformly applied to both sides of a negative electrode current collector made of a strip-shaped copper foil having a thickness of 10 μm and dried to obtain a solvent in the slurry. Was removed by volatilization, a negative electrode mixture layer was formed on both surfaces of the negative electrode current collector, and then compression molded by a roller press machine and cut, and then the lead body was welded to produce a strip-shaped negative electrode. Here, the elongation at break of the negative electrode current collector used was 8%, and the surface roughness Ra was 0.2 μm. The contact angle representing the wettability of the surface of the negative electrode current collector used was 35 degrees, and the amount of chromate on the surface was 0.01 mg / m ² .

  The belt-like positive electrode is stacked on the belt-like negative electrode through a microporous polyethylene film having a thickness of 25 μm and wound into a spiral electrode laminate, and then a cylindrical battery case having a bottomed cylindrical shape with an outer diameter of 18 mm. The lead body of the positive electrode and the negative electrode was welded.

  Next, a liquid electrolyte is injected into the battery case, and after the liquid electrolyte has sufficiently penetrated into the separator, etc., it is sealed, precharged, and aged to produce a cylindrical non-aqueous secondary battery having the structure shown in FIG. did.

  Referring to the battery shown in FIG. 1, 1 is the positive electrode and 2 is the negative electrode. However, in FIG. 1, in order to avoid complication, the current collectors used for manufacturing the positive electrode 1 and the negative electrode 2 are not shown. The positive electrode 1 and the negative electrode 2 are spirally wound through a separator 3 and are housed in a battery case 5 together with the liquid electrolyte 4 as a spiral electrode laminate.

  The battery case 5 is made of stainless steel as described above, and an insulator 6 made of polypropylene is disposed at the bottom of the battery case 5 prior to the insertion of the spiral electrode laminate. The sealing plate 7 is made of aluminum and has a disk shape. A thin portion 7a is provided in the center portion on the inner side from both end faces in the thickness direction, and the internal pressure of the battery acts on the explosion-proof valve 9 around the thin portion 7a. A hole is provided as a pressure introduction port 7b. And the protrusion part 9a of the explosion-proof valve 9 is welded to the upper surface of this thin part 7a, and the welding part 11 is comprised. Note that the thin-walled portion 7a provided on the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are shown only on the cut surface for easy understanding on the drawing, and the contour line behind the cut surface is shown. Is not shown. In addition, the welded portion 11 between the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 is also shown in an exaggerated state so as to facilitate understanding on the drawing.

  The terminal board 8 is made of rolled steel, has a nickel plating on the surface, and has a hat shape with a peripheral edge portion, and the terminal board 8 is provided with a gas discharge hole 8a. The explosion-proof valve 9 is made of aluminum and has a disk shape, and a central portion is provided with a protruding portion 9a having a tip portion on the power generation element side (lower side in FIG. 1), and a thin-walled portion 9b. As described above, the lower surface of the protruding portion 9a is welded to the upper surface of the thin portion 7a of the sealing plate 7 to constitute the welded portion 11. The insulating packing 10 is made of polypropylene and has an annular shape. The insulating packing 10 is arranged at the upper part of the peripheral portion of the sealing plate 7, and the explosion-proof valve 9 is arranged at the upper portion thereof, so that the sealing plate 7 and the explosion-proof valve 9 are insulated. At the same time, the gap between the two is sealed so that the liquid electrolyte does not leak between the two. The annular gasket 12 is made of polypropylene, the lead body 13 is made of aluminum, the sealing plate 7 and the positive electrode 1 are connected, an insulator 14 is disposed on the upper part of the spiral electrode laminate, and the negative electrode 2 and the battery case 5 are connected. Are connected by a nickel lead body 15.

Example 2
As the negative electrode current collector, a material having an elongation at break of 7%, a surface roughness Ra of 0.3 μm, a contact angle representing surface wettability of 65 degrees, and a surface chromate amount of 0.02 mg / m ² is used. A cylindrical non-aqueous secondary battery was produced in the same manner as in Example 1 except that.

Comparative Example 1
As the negative electrode current collector, a cylindrical shape was used in the same manner as in Example 1 except that a material having a breaking elongation of 4%, a surface roughness Ra of 0.3 μm, and a contact angle representing surface wettability of 79 degrees was used. A non-aqueous secondary battery was prepared.

Comparative Example 2
Instead of the graphite-based carbon material of Example 1, the (002) plane interlayer distance d ₀₀₂ is 3.44 mm, the crystallite size Lc in the c-axis direction is 32 mm, the average particle size is 10 μm, and the purity is 99.9%. A negative electrode mixture layer was formed in the same manner as in Example 1 except that the coke-based carbon material was used. However, in the carbon material of Comparative Example 2, although the volume change due to charging / discharging is small, the utilization factor of the negative electrode is reduced by about 30% and the electrode density is also reduced, so that the positive electrode active material must be reduced by about 20%. Therefore, the capacity was reduced by about 20%. As the negative electrode current collector, one having a breaking elongation of 4%, a surface roughness Ra of 0.3 μm, and a contact angle representing surface wettability of 79 degrees was used. Except for these, a cylindrical nonaqueous secondary battery was produced in the same manner as in Example 1.

  The batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were discharged at 1550 mA (1C) to 2.75 V, charged at 1550 mA, and after reaching 4.3 V, the voltage was adjusted to 4.3 V. The battery was overcharged for 3 and a half hours under the condition of keeping. Thereafter, the batteries were discharged at 1550 mA to 2.75 V, and then a part of the batteries was disassembled, and it was examined whether abnormalities such as cracks, cracks, and cutting occurred in the copper foil of the negative electrode current collector. The results are shown in Table 1. Table 1 also shows the maximum volume change rate of the negative electrode mixture layer when charged to 4.2 V and discharged to 2.75 V.

  In addition, the remaining battery is charged at 1550 mA, and after reaching 4.2 V, charging is performed for 2.5 hours under the condition of maintaining a constant voltage of 4.2 V, and further, a charging / discharging cycle of discharging to 2.75 V at 1550 mA is performed. Repeatedly. Then, the capacity retention ratio [(discharge capacity at the 50th cycle) / (discharge capacity at the 1st cycle) × 100] with respect to the first cycle at the 50th cycle was measured. The results are shown in Table 2. In the initial cycle, the battery was charged to 4.2 V, then discharged to 2.75 V, the discharge capacity was measured, and the capacity per unit volume of the electrode laminate was determined based on the discharge capacity. The results are also shown in Table 2.

  As shown in Table 1, in Examples 1 and 2, although the maximum volume change rate of the negative electrode mixture layer was 11%, the negative electrode current collector had no abnormality such as cracks, cracks, and cuts. Explaining this in detail, in Example 1 and Example 2, the maximum volume change rate of the negative electrode mixture layer was extremely 11%, because the negative electrode current collector had a breaking elongation of 5% or more. Despite being large, the negative electrode current collector had no abnormalities such as cracks, cracks and cuts.

  On the other hand, in Comparative Example 1 using a material having a small elongation at break of 4% as the negative electrode current collector, cracks occurred in the current collector as a result of charging and discharging. Therefore, in this comparative example 1, it is predicted that the deterioration of the cycle characteristics will increase. In Comparative Example 2, since the maximum volume change rate of the negative electrode mixture layer was as small as 1% or less, there was no abnormality such as cracking, cracking or cutting of the negative electrode current collector due to charging / discharging. The density of the negative electrode was lowered, the battery capacity was about 80% compared to Examples 1-2, and the capacity was low.

  Further, as shown in Table 2, in Comparative Example 1, the capacity retention ratio decreased to 50% or less at 50 cycles, whereas in Examples 1-2, the capacity retention ratio was as high as 85-91%. It was. In particular, the battery of Example 1 using the negative electrode current collector having a high wettability with a contact angle of 35 degrees representing the wettability of the surface of the current collector had the most excellent capacity retention rate of 91%. As described above, the reason why the capacity retention rate at 50 cycles in Comparative Example 1 was as low as 50% or less is considered to be that the negative electrode current collector was cracked, cut, or the like accompanying charging / discharging. .

1 Positive electrode 2 Negative electrode 3 Separator 4 Liquid electrolyte

Claims (4)

A non-aqueous secondary battery having a spiral electrode laminate around which a positive electrode, a negative electrode and a separator are wound, and an electrolyte,
The electrode laminate unit volume when charged at a 1C rate, charged for 4.2 hours after reaching 4.2V and maintained at a constant voltage of 4.2V, and further discharged to 2.75V at a 1C rate. The capacity per unit is 130 mAh / cm ³ or more,
The negative electrode is formed by forming a negative electrode mixture layer on at least one surface of the current collector,
Another non-aqueous secondary battery having the same configuration as the non-aqueous secondary battery and the thickness of the negative electrode mixture layer when the non-aqueous secondary battery was charged to 4.3 V at a 1C rate for 2 and a half hours and then decomposed After being charged to 4.3 V at 1C rate for 2 and a half hours, after discharging to 2.75 V and then the thickness of the negative electrode mixture layer when disassembled, charging and discharging of the negative electrode mixture layer obtained by the following formula A nonaqueous secondary battery, wherein a maximum volume change rate, which is a maximum value of the subsequent volume change rates, is 8% or more, and a breaking elongation rate of the current collector of the negative electrode is 5% or more.
Volume change rate = (thickness of negative electrode mixture layer after charging−thickness of negative electrode mixture layer after discharge)
÷ (Thickness of negative electrode mixture layer after discharge)   The nonaqueous secondary battery according to claim 1, wherein the wettability of the negative electrode current collector is less than 40 degrees in terms of contact angle.   The chain electrolyte and a cyclic structure ester having a dielectric constant of 30 or more are contained in the liquid electrolyte, and the amount of the cyclic structure ester in the total solvent of the liquid electrolyte is 40% by volume. Non-aqueous secondary battery. The nonaqueous secondary battery according to claim 1, wherein a carbon material having an interlayer distance d ₀₀₂ of (002) plane of 3.4 mm or less is used as the negative electrode active material.

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