JP2005108476A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery capable of suppressing a voltage drop to minimize voltage dispersion. <P>SOLUTION: The lithium ion secondary battery is completed by producing a positive electrode using lithium manganate as a positive electrode active material and a negative electrode using amorphous carbon power as a negative electrode active material, winding the positive electrode and the negative electrode through a separator followed by impregnating in a nonaqueous electrolyte. The nonaqueous electrolyte is prepared by dissolving 1 mol/l of LiPF<SB>6</SB>as a solute to a 1:1 mixed solvent of EC and DMC by volume, further adding VC within the range of 0.3 to 1.5 wt% to the nonaqueous electrolyte. The VC is substantially entirely reduced on the negative electrode surface by initial charging after assembling the battery to generate a proper solid electrolyte membrane on the negative electrode surface. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a lithium ion secondary battery, and more particularly, to a lithium ion secondary battery in which a negative electrode using a carbon material mainly composed of amorphous carbon as a negative electrode active material and a positive electrode are infiltrated with a non-aqueous electrolyte. .

  Lithium secondary batteries represented by lithium ion secondary batteries are mainly used in portable devices such as VTR cameras, notebook computers, and mobile phones, taking advantage of the high energy density. Particularly in recent years, lithium ion secondary batteries using a carbon material capable of occluding and releasing lithium ions such as amorphous carbon and graphite-based carbon as a negative electrode active material have become widespread.

When amorphous carbon is used as the negative electrode active material, the voltage characteristics during charging and discharging have a slope, so the battery state can be estimated easily and accurately by voltage measurement, but the irreversible capacity is large and the capacity of the battery is increased. Is difficult. On the other hand, when a graphite-based material is used, the irreversible capacity is small and the capacity can be increased. However, since the volume change of the crystal accompanying charge / discharge is large, the life is shortened. These carbon materials are in a state in which lithium ions are completely released, that is, in a discharged state when the battery is assembled. Accordingly, an active material in a discharged state, for example, a transition metal double oxide such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), or lithium manganate (LiMnO ₂ ) is usually used for the positive electrode. Since such a positive electrode active material does not have sufficient electronic conductivity, the positive electrode of a lithium ion secondary battery generally has a low cost such as graphite or carbon black as a conductive agent in addition to the positive electrode active material, and In the battery, a stable conductive powder is contained, and a binder (binder) is further added and mixed.

  Moreover, the lithium ion secondary battery has the advantages of high capacity and high output. For this reason, recently, it has also been used as a power source for electric vehicles, hybrid electric vehicles using both an internal combustion engine and an electric motor (hereinafter, both are referred to as electric vehicles). When a lithium ion secondary battery is used as a power source for an electric vehicle, a plurality of lithium ion secondary batteries (cells) are electrically connected in series as a battery module in order to secure a high voltage. The voltage is controlled by a control circuit in the battery module.

  If one of the cells constituting the battery module has different battery characteristics such as voltage and capacity from other cells, or if the battery characteristics deteriorate due to changes over time, etc., the abnormal characteristics of the cells It becomes a cell load and deteriorates the characteristics of the entire battery module. In particular, when the self-discharge is different, the voltage drop of each cell varies, the characteristics of the entire battery module are lowered, and the life is extremely shortened. If the variation in the voltage drop of each cell is too large, the control circuit described above cannot adjust and control the cell voltage and the like, and the CPU in the control circuit runs out of control, leading to a decrease in the reliability of the battery module. For this reason, in general, by measuring the voltage of a cell for a certain period after the battery is assembled, cells that show an abnormality in voltage drop are removed.

  The cause of the voltage drop is the presence of impurities, particularly metallic impurities, mixed in the battery material such as the positive electrode active material. The mixed metal impurities are eluted into the non-aqueous electrolyte as metal ions due to charging / discharging of the battery, etc., and the eluted metal ions are deposited on the negative electrode and grow in a dendrite shape. As it discharges, a voltage drop occurs. In order to solve this problem, the applicant generally discloses a technique for limiting the positive electrode active material layer and the non-aqueous electrolyte so as not to substantially contain metallic impurities (see, for example, Patent Document 1). ).

JP 2002-75460 A

  However, in a lithium ion secondary battery using amorphous carbon as the negative electrode active material, the discharge curve indicating a voltage drop is gentle, and a relatively large voltage drop occurs in the initial stage after battery assembly. Therefore, there is a problem that it takes a considerable time to select a defective battery having a micro short circuit and a non-defective battery by a voltage drop. In addition, since there is a variation in the rate of voltage drop, voltage variations occur between batteries even in a non-defective battery. In the battery module described above, the voltage is adjusted by the control circuit. However, if the voltage variation is too large, the control circuit cannot adjust the voltage, so that the reliability of the battery module is lowered.

  An object of the present invention is to provide a lithium ion secondary battery that can suppress voltage drop and has little voltage variation.

  In order to solve the above problems, the present invention provides a negative electrode using a carbon material mainly composed of amorphous carbon as a negative electrode active material, and a lithium ion secondary battery in which a positive electrode is infiltrated with a non-aqueous electrolyte. The vinylene carbonate is added in the range of 0.3% to 1.5% by weight with respect to the non-aqueous electrolyte before completion of the initial charge, and added to the non-aqueous electrolyte by the initial charge. Is substantially all reduced to form a film on the surface of the negative electrode.

  In the present invention, one of the causes of the voltage drop is that metal ions of metallic impurities mixed in the lithium ion secondary battery concentrate and deposit on the positive and negative electrodes during charge and discharge to cause a micro short circuit (self-discharge), Paying attention to voltage variation between lithium ion secondary batteries, vinylene carbonate is added in a range of 0.3 wt% to 1.5 wt% with respect to the non-aqueous electrolyte as a countermeasure. The added vinylene carbonate is substantially entirely reduced by the initial charge, and a film that suppresses the reduction reaction after the completion of the initial charge is generated on the surface of the negative electrode. If vinylene carbonate is less than 0.3% by weight, the formation of the coating is insufficient, and metal ions concentrate and precipitate, resulting in micro short-circuiting, leading to a voltage drop, and conversely, if exceeding 1.5% by weight. Since the film thickness of the coating is increased to inhibit the movement of lithium ions, the output is reduced. According to the present invention, since vinylene carbonate is reduced and a film is formed on the surface of the negative electrode, since concentrated precipitation of metal ions is suppressed, voltage drop due to a micro short circuit can be suppressed, and the negative electrode can be formed by the film. Since the surface is protected and charged and discharged almost uniformly, voltage variation can be reduced.

  In this case, the amount of vinylene carbonate added to the non-aqueous electrolyte may be 1.2% by weight or more. Moreover, you may use a lithium transition metal double oxide for the active material of a positive electrode.

  According to the present invention, since vinylene carbonate is reduced and a film is formed on the surface of the negative electrode, since concentrated precipitation of metal ions is suppressed, voltage drop due to a micro short circuit can be suppressed, and the negative electrode can be formed by the film. Since the surface is protected and charged and discharged almost uniformly, an effect that voltage variation can be reduced can be obtained.

  Hereinafter, the best mode in which the present invention is applied to an 18650 type lithium ion secondary battery having a diameter of 18 mm and a height of 65 mm, which is widely spread as a small consumer lithium ion secondary battery, will be described.

(Positive electrode)
Lithium transition metal double oxide lithium manganate (LiMn ₂ O ₄ ) was used as the positive electrode active material. A dispersion solvent containing 80% by weight of LiMn ₂ O ₄ powder (hereinafter referred to as wt%), 15% by weight of carbon powder as a conductive agent, and 5% by weight of polyvinylidene fluoride (hereinafter referred to as PVDF) as a binder. In N-methyl-2-pyrrolidone (hereinafter referred to as NMP) and kneaded to obtain a slurry. The obtained slurry was applied to both surfaces of an aluminum foil serving as a positive electrode current collector using a comma roll and dried to form a positive electrode active material layer. This positive electrode active material layer was pressed at a press pressure of 0.2 to 0.7 kg / cm with a roll press having a roll heated to 80 ° C. to 120 ° C., and the bulk density of the positive electrode active material layer was 2.8 g. After compression to / m ³ , it was cut into a strip of 50 × 450 mm to obtain a positive electrode.

(Negative electrode)
Using an amorphous carbon powder capable of inserting and removing lithium ions into the negative electrode active material, NMP was added to a mixture of 90 wt% amorphous carbon powder and 10 wt% PVDF and kneaded to obtain a slurry. The obtained slurry was applied to both sides of a copper foil serving as a negative electrode current collector and dried to form a negative electrode active material layer. This negative electrode active material layer was pressed at a press pressure of 0.2 to 0.7 kg / cm with a roll press having a roll heated to 80 ° C. to 120 ° C., and the bulk density of the negative electrode active material layer was 1.04 g. After being compressed to / m ³ , it was cut into a strip of 50 × 480 mm to obtain a negative electrode.

(Nonaqueous electrolyte)
The non-aqueous electrolyte was prepared as follows. As a solvent for the non-aqueous electrolyte, a mixed solvent in which ethylene carbonate (hereinafter referred to as EC) and dimethyl carbonate (hereinafter referred to as DMC) are mixed at a volume ratio of 1: 1 is used. (Hereinafter referred to as VC) was added and mixed so that the amount added to the non-aqueous electrolyte was in the range of 0.3 to 1.5 wt%. As the solute of the nonaqueous electrolytic solution, lithium hexafluorophosphate (LiPF ₆ ) was used, and dissolved in a mixed solvent at a concentration of 1.0 mol / liter. The content of metallic impurities in the non-aqueous electrolyte solution was set to 1 ppm or less.

(Battery assembly)
The produced belt-like positive electrode and negative electrode were wound with a belt-like separator overlapped to produce a wound electrode body. This wound electrode body was put into a cylindrical battery can, 5 ml of non-aqueous electrolyte was injected, an upper lid was attached, and the battery was sealed to complete a 18650 type lithium ion secondary battery.

  Hereinafter, the battery of the Example produced according to this embodiment is demonstrated. In addition, it describes together about the battery of the comparative example produced for the comparison.

(Example 1 to Example 4)
As shown in Table 1 below, in Examples 1 to 4, lithium ion secondary batteries were manufactured by changing the amount of VC added to the non-aqueous electrolyte. Example 1 was 0.3 wt%, Example 2 was 0.8 wt%, Example 3 was 1.2 wt%, and Example 4 was 1.5 wt%. The number of fabricated batteries was 100 for each example (the same applies to the following comparative examples).

(Comparative Examples 1 to 2)
As shown in Table 1, in Comparative Example 1, a lithium ion secondary battery was prepared without adding VC to the non-aqueous electrolyte, and in Comparative Example 2, the amount of VC added to the non-aqueous electrolyte was 1.8 wt. As a percentage, lithium ion secondary batteries were produced.

(test)
Each battery of the manufactured example and comparative example was left after the initial charge under the following conditions, the voltage dropped from the first week to the eighth week was measured, and the voltage drop per day (mV) for each week. / Day) was calculated. In order to prevent the voltage drop amount of a defective battery from being included when comparing the variations in the voltage drop amount, the batteries of each of the examples and comparative examples (100 batteries each) were each 90 batteries in order of increasing voltage drop amount. Were selected as non-defective products, and the minimum value and the maximum value of the voltage drop amount were determined among 90 non-defective batteries. Moreover, the resistance value was calculated | required by measuring the voltage of the 5th second at the time of discharging 1800mA from a full charge state at 25 degreeC. Table 2 below shows the test results of the minimum value, maximum value, and resistance value of the voltage drop amount. In Table 2, the upper numerical value of each column indicates the minimum value, and the lower numerical value indicates the maximum value.
(First charge / discharge conditions)
(1) Charging: constant voltage charging 4.1V, limiting current 600mA, 4h, 25 ° C
(2) Discharge: constant current discharge 600 mA, final voltage 2.7 V, 25 ° C.
(3) Charging: constant voltage charging 3.7V, limiting current 900mA, 3h, 25 ° C

  Moreover, the battery which can be judged to be a battery whose voltage drop amount is obviously large after the voltage measurement at the 8th week was determined as a defective battery, and the number thereof was examined. Table 3 below shows the test results of the number of defective batteries and the minimum value of the voltage drop amount from the 1st week to the 8th week of leaving the defective batteries.

  Furthermore, the defective batteries were disassembled, and the number of shorts caused by different metals, shorts caused by protrusions on electrode edges, and shorts caused by manufacturing defects were investigated for causes of short circuits. Dissimilar metals mixed in the battery show black traces when they are deposited on the separator. Therefore, disassemble the batteries with the average voltage drop in non-defective batteries, observe the separator, and observe the metal ions in the battery. The amount of precipitation (the number of traces of black spots on the separator) was examined. Table 4 below shows the test results of the number of defective batteries for each cause of short circuit and the number of black spots on good batteries.

  As shown in Table 2, in the battery of Comparative Example 1 in which VC was not added to the non-aqueous electrolyte, the voltage drop amount for the first week of the non-defective battery was 4.30 mV / day at the maximum value and 4.10 mV / day at the minimum value. The difference between the maximum value and the minimum value for each week, that is, the variation in the voltage drop amount was 0.18 to 0.22 mV / day. On the other hand, in the batteries of Examples 1 to 4 in which VC in the range of 0.3 to 1.5 wt% was added to the non-aqueous electrolyte, the voltage drop amount in the first week was both the maximum value and the minimum value. It became smaller, and the variation of the voltage drop every week was also reduced. In the battery of Example 1 in which the amount of VC added to the non-aqueous electrolyte was 0.3 wt%, the voltage drop amount in the first week showed a slightly large value. Further, in the battery of Comparative Example 2 in which 1.8 wt% of VC was added to the non-aqueous electrolyte, the voltage drop amount in the first week was small, and the variation in the voltage drop amount for each week was reduced. The resistance value has increased. From this, when the amount of VC added to the non-aqueous electrolyte is in the range of 0.3 to 1.5 wt%, the initial voltage drop amount becomes small, and the variation in the voltage drop amount up to the eighth week becomes small. It has been found. If the addition amount is in the range of 1.2 to 1.5 wt%, the variation in the voltage drop amount is further reduced.

  As shown in Table 3, the number of defective batteries was 7 in the battery of Comparative Example 1, whereas the number of defective batteries was reduced to 4 in the batteries of Examples 1 to 2, and Examples 3 to Examples. In the battery of 4, the number of defective batteries was further reduced to 2-3. Further, when the voltage drop amount is compared with the minimum value of the defective battery in Table 3 and the maximum value of the non-defective battery shown in Table 2, the battery of Comparative Example 1 has a minimum value of the defective battery after the fourth week. It is larger than the maximum value of a good battery. On the other hand, the minimum value of the defective battery is larger than the maximum value of the non-defective battery from the second week onward in the batteries of Example 1 and Example 2, and from the first week in the batteries of Example 3 and Example 4. . From this, in each battery of Examples 1 to 4, it was found that the difference in voltage drop between the defective battery and the non-defective battery clearly appears from the initial stage.

  As shown in Table 4, the number of defective batteries due to the short circuit due to the electrode edge protrusion and the short circuit due to the manufacturing failure was almost the same in the batteries of the example and the comparative example. Further, in the battery of Comparative Example 1, most of the defective batteries were caused by short circuits due to different metals, and the number of black spot traces observed in the non-defective battery was 13. On the other hand, in each battery of Examples 1 to 4, the number of defective batteries due to short-circuiting with different metals was as small as 0 to 1. Further, the number of sunspot traces was as small as 6 to 4 in the batteries of Examples 1 to 2, and was further as 1 to 2 in the batteries of Examples 3 to 4. From this, it was found that in the batteries of Examples 1 to 4, there was little precipitation of dissimilar metals and the occurrence of minute short circuits was small.

  When VC is added to the non-aqueous electrolyte of the lithium ion secondary battery, VC has a vinylene group double bond, so that it is rapidly reduced on the negative electrode surface by charging to produce a solid electrolyte membrane (film). Since the generated solid electrolyte membrane prevents contact between the non-aqueous electrolyte and the carbon material of the negative electrode, the reduction reaction is suppressed on the surface of the solid electrolyte membrane, so that concentrated precipitation of metal ions and decomposition of the non-aqueous electrolyte are less likely to occur. Further, when the solid electrolyte membrane is thin, lithium ions can pass through. However, as the thickness increases, it becomes a resistance component and inhibits movement of lithium ions. Therefore, if the amount of VC added to the non-aqueous electrolyte is less than 0.3 wt%, the reduction reaction cannot be suppressed because the formation of the solid electrolyte membrane is insufficient. When the content exceeds 1.5 wt%, the thickness of the solid electrolyte membrane to be generated increases, so that the migration of lithium ions is inhibited and the output is reduced.

  In the lithium ion secondary battery of the present embodiment, VC is added in the range of 0.3 to 1.5 wt% with respect to the non-aqueous electrolyte at the time of battery assembly (before completion of the initial charge). For this reason, VC is substantially entirely reduced on the negative electrode surface by the initial charge after battery assembly, and an appropriate solid electrolyte membrane is generated on the negative electrode surface. After the initial charge is completed, this solid electrolyte membrane suppresses the occurrence of micro short-circuits due to the concentrated precipitation of metal ions on the negative electrode surface and the increase in resistance component due to the decomposition of the non-aqueous electrolyte, thereby suppressing the voltage drop. And the variation in voltage between the batteries can be reduced. Therefore, the yield of battery production can be improved, and the reliability of the battery can be ensured even when used as a battery module.

  Amorphous carbon of the negative electrode active material exhibits a relatively large voltage drop characteristic in the early stage of discharge, and there is a variation in the voltage drop rate (voltage drop amount per day). For this reason, even if the voltage drop rate of a non-defective battery is measured, it is difficult to select a defective battery causing a micro short circuit, and it takes a long time for the selection. In the lithium ion secondary battery of this embodiment, the surface of the negative electrode is protected by the generated solid electrolyte membrane, so that the initial deterioration of amorphous carbon can be mitigated. For this reason, the voltage drop rate associated with the initial deterioration becomes slow and the variation in the voltage drop rate becomes small.Therefore, a clear difference occurs in the voltage drop rate between the non-defective battery and the defective battery in which the micro short circuit occurs. The sorting accuracy of non-defective batteries can be improved. In addition, since the difference between the non-defective battery and the defective battery occurs in a short period, the selection period can be shortened.

In the present embodiment, LiPF ₆ is dissolved in a solution obtained by mixing EC and DMC in a nonaqueous electrolytic solution, and a predetermined amount of VC is added. However, the present invention is not limited to this. It is sufficient that a predetermined amount of VC is added. As a non-aqueous electrolyte other than the present embodiment, organic solvents include propylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl carbonate, γ-butyllactone, tetrahydrofuran, diethyl ether, sulfolane, A single solvent such as acetonitrile or a mixed solvent in which two or more of these are mixed can be used, and LiClO ₄ , LiBF ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, LiAsF _6, etc. can be used as the electrolyte. Can do.

  Further, in the present embodiment, amorphous carbon is exemplified as the negative electrode active material, but the present invention is not limited to this, and can be applied as long as it has a discharge curve peculiar to amorphous carbon. . For example, the effects of the present invention can be obtained in the same way even in a mixed carbon material of amorphous carbon and graphite-based carbon, and composite graphite in which the surface of amorphous carbon is coated with graphite-based carbon. The particle shape is not particularly limited, such as a scale shape, a spherical shape, a fiber shape, or a lump shape.

  Furthermore, in the present embodiment, lithium transition metal double oxide lithium manganate is exemplified as the positive electrode active material. However, other usable lithium transition metal double oxides include Li, V, Cr, Fe, and Co. Alternatively, a manganese site of lithium manganate or a lithium manganese complex oxide obtained by substituting the lithium site with at least one metal element selected from Ni, Mn, W, Zn, B, and Mg may be used. Further, the present invention is applicable not only to lithium manganese complex oxide but also to lithium cobalt complex oxide or lithium nickel complex oxide as a positive electrode active material, and further, cobalt site, nickel site, lithium The present invention can also be applied to lithium transition metal double oxides in which sites are substituted with one or more metal elements described above. Further, the crystal structure of the lithium transition metal double oxide is not particularly limited, such as a spinel crystal structure or a layered crystal structure.

  Furthermore, in the present embodiment, the 18650 type lithium ion secondary battery has been exemplified, but the present invention is not limited to the shape and size of the battery, and can be applied to a battery having a rectangular shape or other polygons. Further, the structure to which the present invention can be applied may be other than a battery having a structure in which a battery lid is attached to the battery container and sealed. As an example of such a structure, a battery in which positive and negative external terminals penetrate the battery lid and the positive and negative external terminals are pressed against each other via an axis in the battery container can be cited. Further, the present invention can be applied to a lithium ion secondary battery in which the positive electrode and the negative electrode are not wound structures but have a stacked structure.

  Furthermore, as the binder, in addition to the PVDF exemplified in this embodiment, for example, acrylic acid such as isobutyl acrylate, octyl acrylate, nonyl acrylate, butyl methacrylate and 2-ethylhexyl methacrylate, and / or carbon number of methacrylic acid Copolymerization of C4-C12 alkyl ester with unsaturated monomer having a functional group of carboxyl group or amide group such as polyacrylic acid such as methacrylic acid, itaconic acid, maleic acid, fumaric acid, acrylamide and methacrylamide Examples thereof include polyester, polyamide, polyamideimide, polyamide bismaleimide, polybutylene terephthalate, polyethylene terephthalate, etc., and these can be used alone or in combination.

  Further, in the present embodiment, an example in which heating is performed using a roll when performing the heat treatment in the press step for producing the positive and negative electrodes is shown. However, if the binder can be melted and solidified, it is heated by another heat treatment. Also good.

  According to the lithium ion secondary battery according to the present invention, since the voltage drop can be suppressed and the voltage variation is small, the reliability of the battery can be secured, so that it contributes to manufacturing, sales, etc., and can be used industrially. it can.

Claims (3)

  In a lithium ion secondary battery in which a negative electrode using a carbon material mainly composed of amorphous carbon as a negative electrode active material and a positive electrode are infiltrated in a non-aqueous electrolyte, before the completion of initial charge, The vinylene carbonate is added in the range of 0.3 wt% to 1.5 wt%, and the vinylene carbonate added to the non-aqueous electrolyte by the initial charge is substantially all reduced, and the surface of the negative electrode A lithium ion secondary battery characterized in that a film is formed on the lithium ion secondary battery.   2. The lithium ion secondary battery according to claim 1, wherein the amount of the vinylene carbonate added to the non-aqueous electrolyte is 1.2 wt% or more.   The lithium ion secondary battery according to claim 1, wherein the active material of the positive electrode is a lithium transition metal double oxide.