JP2009238663A - Nonaqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous secondary battery which can restrain expansion/contraction of a negative electrode during charge and discharge and is excellent in cycle characteristics at high capacity. <P>SOLUTION: In the nonaqueous secondary battery having a negative electrode in which at least one alloy material selected from elements capable of alloying with lithium by charging and its compound and a carbon material, a positive electrode, and a nonaqueous electrolyte, a ratio of the alloy material on the total amount of the alloy material and the carbon material is 1-30 wt.%, and an average particle diameter of the alloy material is 2/5 or less that of the carbon material. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous secondary battery using, as a negative electrode, at least one alloying material selected from an element that can be alloyed with lithium by charging and a compound thereof.

  In order to achieve high energy density of the non-aqueous secondary battery, it is effective to use a material having a larger energy density as the active material. In order to increase the energy density, in place of a carbon material such as graphite that has recently been put to practical use as a negative electrode active material, Al, Sn, which can occlude lithium by an alloying reaction with lithium during charging, It has been proposed to use materials such as elements such as Si, and compounds such as alloys and oxides containing these elements, and various studies have been conducted.

When such a material containing an element that can be alloyed with lithium by charging (hereinafter referred to as an alloying material) is used as a negative electrode active material, a high capacity can be expected. At this time, the material expands and contracts due to insertion and extraction of lithium, and a large volume change occurs. For example, in the case of a negative electrode using a thin film of Sn or Si, when Li is electrochemically inserted up to x = 4.4 in the composition formula of Li _x M (M: Sn or Si), the volume of the thin film is quadrupled. It will expand to. For this reason, pulverization of the negative electrode active material and peeling from the negative electrode current collector occur, resulting in a problem that the conductivity in the negative electrode is lowered and excellent charge / discharge cycle characteristics cannot be obtained.

In order to solve the above problem, it has been proposed to use an alloy thin film formed by a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, or a plating method as a negative electrode (see, for example, Patent Documents 1 to 5). . When a thin film electrode is formed by these methods, the current collector foil (current collector) and the active material layer are more firmly integrated, and even if the active material is pulverized due to charge / discharge, the active material is activated from the current collector foil. It is difficult for the substance to fall off, and a certain effect can be obtained in improving the cycle characteristics.
JP 2001-68094 A JP 2001-256968 A JP 2005-108523 A JP 2003-7295 A International Publication No. WO01 / 31720

  However, if the adhesion between the current collector foil and the active material is increased, the electrode itself is more susceptible to the volume change of the active material thin film due to the insertion / desorption of Li. The occurrence of cracks and cracks in the active material thin film appears remarkably. As a result, the conductivity in the electrode is lowered, and the cycle characteristics of the battery are deteriorated. In addition, since such an alloyed material is very hard and easily broken when stress is applied, the active material thin film is easily cracked when the electrode is wound, and the end face is cut when the electrode sheet is cut. As a result, burrs are likely to be formed, and an internal short circuit is likely to occur in a wound body made of a negative electrode using the same. In particular, in the case of a prismatic battery, since it is common to press the wound body at the stage of inserting the wound body into a metal can, an internal short circuit is more likely to occur.

On the other hand, it has also been proposed to attach an alloying material to a negative electrode current collector with a binder, as in the case of conventional coated electrodes (see, for example, Patent Documents 6 and 7). In Patent Document 6, due to alloying of Si with Co, Mg, and the like, and in Patent Document 7, the ratio of graphite to the total weight of the alloyed material and graphite, using the alloyed material and graphite as a negative electrode active material. The ratio of the average particle size of the alloying material to the average particle size of graphite (average particle size of alloying material / average particle size of graphite) is in the range of 0.15 to 0.90. By doing so, a non-aqueous secondary battery having a high capacity and excellent cycle characteristics is formed.
JP 2000-243389 A JP 2006-164952A

  However, even the configurations in Patent Documents 6 and 7 cannot sufficiently suppress the expansion of the negative electrode during charging and discharging, and there is still room for study in improving the cycle characteristics.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a non-aqueous secondary battery having a high capacity and excellent cycle characteristics.

  The non-aqueous secondary battery of the present invention includes at least one alloying material selected from an element that can be alloyed with lithium by charging and a compound thereof, a negative electrode having a carbon material as an active material, a positive electrode, A non-aqueous secondary battery comprising a non-aqueous electrolyte, wherein the ratio of the alloying material in the total amount of the alloying material and the carbon material is 1 to 30% by weight, and the average of the alloying material The particle size is 2/5 or less of the average particle size of the carbon material.

  ADVANTAGE OF THE INVENTION According to this invention, the non-aqueous secondary battery which suppressed the expansion | swelling / contraction of the negative electrode in charging / discharging, was high, and was excellent in cycling characteristics can be obtained.

  In the present invention, the negative electrode active material is a mixture of at least one alloying material selected from an element that can be alloyed with lithium by charging and a compound thereof, and a carbon material, and the alloying material and the carbon material The ratio of the alloying material in the total amount is 1 to 30% by weight, and the average particle size of the alloying material is 2/5 or less of the average particle size of the carbon material. In the case of secondary particles, the average particle size represents the average particle size of secondary particles. The average particle diameter may be a number average particle diameter, and is determined by measuring a sample dispersed in water using a laser diffraction particle size distribution measuring device or the like. However, when the measurement is difficult, such as when the particle size is very small, an average value may be obtained from the particle size observed with an electron microscope.

  By using a part of the active material as a carbon material, the conductivity inside the negative electrode can be increased. It can also function as a buffer material when the alloying material expands.

  In order to enhance the effect of the carbon material, the proportion of the alloying material in the total amount of the alloying material and the carbon material needs to be 30% by weight or less, and more preferably 20% by weight or less. . On the other hand, in order to obtain the effect of increasing the capacity by the alloying material, the proportion of the alloying material needs to be 1% by weight or more, preferably 5% by weight or more, and preferably 10% by weight or more. Is more preferable.

  Further, in the present invention, in order to keep the alloying material particles in the gaps between the carbon material particles as much as possible and to enhance the function of the carbon material as a buffer material, the average particle size of the alloying material is changed to the average particle size of the carbon material. 2/5 or less. By making the particle size of the alloying material sufficiently smaller than the particle size of the carbon material, the periphery of the alloying material is surrounded by the carbon material, and the volume change of the alloying material can be accommodated. In addition, the conductivity of the negative electrode can be maintained well. The average particle size of the alloying material is more preferably 1/3 or less of the average particle size of the carbon material.

  Examples of the element that can be alloyed with lithium by the charging include elements such as Si, Sn, Ga, Ge, In, and Al. Examples of the alloying material to be a negative electrode active material include the element alone or an alloy of the elements, an alloy of the element with Co, Ni, Fe, Mn, Ti, Zr, and the like, an oxide of the element, and nitriding. Compounds such as products and carbides. Among these, as an element that can be alloyed with lithium by charging, Si or Sn is preferable, and simple substances of these elements, alloys containing these elements, and oxides of these elements are preferably used as active materials.

  The average particle diameter of the alloying material is preferably 3 μm or less, more preferably 2 μm or less in order to prevent pulverization due to volume change and formation of lithium dendrite during charging, while preventing an increase in contact resistance. Therefore, the thickness is preferably 0.1 μm or more.

In addition, the carbon material used as the negative electrode active material includes graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads capable of inserting and extracting lithium by charging and discharging. Carbon fiber, activated carbon, etc. are used. Among these, graphite having a (002) plane spacing of d ₀₀₂ of 0.340 nm or less, particularly graphite having d ₀₀₂ of 0.337 nm or less is preferably used. This is because the use of such an active material makes it possible to increase the capacity of the battery. _The lower limit of _{d 002} is not particularly limited, it is theoretically substantially 0.335 nm.

  The c-axis direction crystallite size Lc in the graphite crystal structure is preferably 3 nm or more, more preferably 8 nm or more, and further preferably 25 nm or more. This is because, within this range, insertion and extraction of lithium becomes easier. The upper limit of Lc is not particularly limited, but is usually about 200 nm.

  The average particle diameter of the carbon material depends on the coating thickness of the active material layer and the capacity per unit area, but is preferably 40 μm or less, and preferably 25 μm or less in order to form a good conductive network in the negative electrode. It is more preferable that the thickness is 15 μm or less. On the other hand, in order to reduce the irreversible capacity, it is preferably 1 μm or more, and more preferably 3 μm or more. The average particle size of the carbon material needs to be smaller than at least the coating thickness of the negative electrode active material layer.

  The shape of the carbon material is not particularly limited, but it is desirable that the carbon material is spherical in order to easily hold the alloying material in the voids between the particles.

  The alloying material and the carbon material may be combined with a conductive auxiliary agent or a binder as necessary, applied onto a current collector such as a copper foil, and formed into a strip-shaped molded body to form a negative electrode. Is done. Since the conductive assistant contributes to the improvement of the conductivity of the alloying material, it is desirable to add several weight percent when an oxide having a low electronic conductivity is used as an active material. As the conductive assistant, a material having a large specific surface area is desirable. For example, carbon black is preferable, and among them, acetylene black and ketylene black are preferable. In addition, carbon fiber, for example, vapor-grown carbon fiber contributes to maintaining the current collecting property between the particles of the alloying material or between the alloying material and the active carbon material. Can be improved.

  The thickness of the negative electrode mixture layer can be increased as compared with the case where the alloy thin film is used as the negative electrode, and can be, for example, 30 to 70 μm.

  In order to improve the electroconductivity of the negative electrode, the alloying material and the conductive auxiliary agent may be integrated and compounded by a method such as a granulation method, or the surface of the alloying material. In addition, a carbon coating layer, which is a conductive aid, may be formed.

  Further, as the binder, a material that is electrochemically inactive with respect to Li in the working potential range of the negative electrode and does not affect other substances as much as possible is selected. For example, styrene-butadiene rubber, polyvinylidene fluoride, carboxymethylcellulose, methylcellulose, polyimide, polyamideimide and the like are suitable as the binder. These may be used alone or in combination.

The positive electrode is produced by the same method as the negative electrode. The positive electrode active material is mixed with a conductive additive, a binder, and the like as necessary, and is applied onto a current collector such as an aluminum foil and formed into a strip-shaped molded body to be a positive electrode. The positive electrode active material is not particularly limited, and a generally usable active material such as a Li-containing transition metal oxide may be used. Specific examples of the Li transition metal oxide include, for example, Li _x CoO ₂ , Li _x NiO ₂ , Li _x MnO ₂ , Li _x Co _y Ni _1-y O ₂ , Li _x Co _y M _1-y O ₂ , _{Li x Ni 1-y M y} O 2, Li x Mn y Ni z Co 1-y-z O 2, Li x Mn 2 O 4, Li x Mn 2-y M y O 4 ( in the structural formula of the , M is at least one metal element selected from the group consisting of Mg, Mn, Fe, Co, Ni, Cu, Zn, Al, Ti, Ge and Cr, and 0 ≦ x ≦ 1.1, 0 < y <1.0, 2.0 <z <1.0).

  The electrolytic solution is not particularly limited, and examples thereof include those prepared by dissolving the following inorganic ion salt in the following solvent.

  Examples of the solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1, 3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, diethyl Use one or more aprotic organic solvents such as ether and 1,3-propane sultone It can be.

As the inorganic ion salt, Li salt, for _{example, LiClO 4, LiBF 4, LiPF} 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiB 10 Cl 10, lower aliphatic carboxylic acids Li, LiAlCl ₄ , LiCl, LiBr, LiI, chloroborane Li, Li tetraphenylborate, or the like can be used alone or in combination.

Among the electrolytic solutions in which the inorganic ion salt is dissolved in the solvent, at least one selected from the group consisting of 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate, ethylene carbonate or propylene carbonate, An electrolyte solution in which at least one inorganic ion salt selected from the group consisting of LiClO ₄ , LiBF ₄ , LiPF _6, and LiCF ₃ SO ₃ is dissolved in a solvent containing s is preferable. An appropriate concentration of the inorganic ion salt in the electrolytic solution is, for example, 0.2 to 3.0 mol / dm ³ .

  As for the configuration other than the above of the nonaqueous secondary battery of the present invention, those conventionally used can be used.

Example 1
[Production of positive electrode]
92 parts by mass of LiCoO ₂ and 5 parts by mass of flaky graphite were mixed. To this mixture, a solution prepared by dissolving 3 parts by mass of polyvinylidene fluoride in N-methyl-2-pyrrolidone was added and mixed. A mixture paste was obtained. The positive electrode mixture paste was passed through a stainless steel net to remove aggregates, and then the positive electrode mixture weight after drying was 24.0 mg per unit area on one side of a positive electrode current collector made of aluminum foil having a thickness of 15 μm. / Cm ² , uniformly applied and dried to form a positive electrode mixture layer, and then compression molded with a roller press at a pressure of 1.4 × 10 ⁷ N / m ² , and then 40 mm × 25 mm And the lead body was welded to produce a positive electrode. The thickness of the positive electrode mixture layer was 69 μm, and the capacity when charged to 4.3 V with respect to the Li potential was 34 mAh.

(Production of negative electrode)
A mixture of a composite material constituted by coating Si on the surface of Si powder having an average particle diameter of 1.5 μm and carbon, and mesocarbon microbeads (MCMB) (weight ratio = 20: 80) ) Was used as the negative electrode active material. The carbon coating was performed by CVD, and a carbon coating layer was formed by treating methane gas as a raw material at 1100 ° C. for 1 to 5 hours in N ₂ gas. The coating amount of carbon was 40 wt% of the entire composite material, and the formed carbon had conductivity but had little charge / discharge capacity, and did not act as a negative electrode active material. . MCMB has a d ₀₀₂ of 0.337 nm, an Lc of 95.0 nm, an average particle size of 8 μm, and a weight ratio of Si powder to MCMB of 13:87. The ratio to the average particle size was 0.19.

The mixture is mixed with an N-methyl-2-pyrrolidone (NMP) solution in which polyvinylidene fluoride (PVDF) is dissolved, and carbon black (weight ratio of the mixture to PVDF and carbon black is 90: 9: 1), Further, an NMP solution was added and mixed to obtain a negative electrode mixture paste. This negative electrode mixture paste was uniformly applied to one side of an electrolytic copper foil having a thickness of 10 μm so that the weight of the negative electrode mixture after drying was 5.6 mg / cm ² per unit area, and vacuum was applied in a heated dryer. After removing NMP by heat treatment, it is compression-molded twice with a roller press at a pressure of 1.4 × 10 ⁷ N / m ² , then cut to 42 mm × 27 mm, and the lead body is welded to produce a negative electrode did. The thickness of the negative electrode mixture layer was 41 μm, and the capacity of the entire active material was 821 mAh / g, which was 1.1 times the capacity of the positive electrode.

[Production of battery]
The coated surfaces of the positive electrode and the negative electrode face each other, overlap each other through a porous polyethylene separator (50 mm × 35 mm) having a thickness of 20 μm, put into an exterior body made of an aluminum laminate film having a thickness of 110 μm, and 0.7 ml of an electrolyte solution. After putting in and degassing in vacuum, it was sealed to produce a non-aqueous secondary battery. The above operations were performed in a dry atmosphere. Further, the electrolyte solvent mixture of ethylene carbonate and diethyl carbonate (volume ratio 3: 7) were used those obtained by dissolving LiPF ₆ in 1.0 mol / l to. In addition, the capacity | capacitance when charging the produced cell to 4.2V was 32.4 mAh.

[Battery evaluation]
The above cell was charged at 20 ° C. with a constant current of 5 mA until it reached 4.2 V, and further charged with a constant voltage of 4.2 V until the current value of charging decreased to 0.5 mA. The charging capacity was measured as charging. Thereafter, the discharge current was set to 5 mA and discharged to 3 V, and the discharge capacity at that time was measured. Further, the change in thickness of the battery due to charging was determined by the following equation, and the degree of expansion of the negative electrode was evaluated assuming that this was mainly due to expansion of the negative electrode.
[Battery thickness change] = [Difference in battery thickness before and after charging] / [Battery thickness before charging] × 100%

  In addition, for the battery after the discharge capacity measurement, charge and discharge were repeated 50 cycles as the constant current-constant voltage charge conditions as described above except that the current value during constant current charge was 17 mA. The ratio of the discharge capacity at the 50th cycle with respect to was evaluated as the capacity retention rate (%). Tables 1 and 2 show the configuration of the produced negative electrode and the measured battery characteristics, respectively.

(Example 2)
As the alloying material, a SiO powder having an average particle diameter of 2 μm is used, and a composite material in which carbon is coated on the surface (the coating amount of carbon is 23 wt% of the entire composite material), and the average particle diameter of MCMC is set. Example 1 except that the thickness of the negative electrode mixture layer was adjusted to 10 μm, the weight ratio of the composite material to MCMB was 1: 2, and the negative electrode capacity was 1.1 times the positive electrode capacity. A non-aqueous secondary battery was produced in the same manner. The weight ratio of SiO powder to MCMB was 27.8: 72.2, and the value of the ratio between the average particle diameter of SiO powder and the average particle diameter of MCMB was 0.2.

(Examples 3 to 6)
The average particle size of the Si powder was changed as shown in Table 1, and the thickness of the negative electrode mixture layer was adjusted so that the negative electrode capacity was 1.1 times the positive electrode capacity. A water secondary battery was produced.

(Examples 7 to 9)
The same as in Example 1 except that the mixing ratio (weight ratio) of the composite material and MCMB was changed and the thickness of the negative electrode mixture layer was adjusted so that the negative electrode capacity was 1.1 times the positive electrode capacity. A non-aqueous secondary battery was produced.

(Comparative Example 1)
A nonaqueous secondary battery was prepared in the same manner as in Example 1 except that only MCMB was used as the negative electrode active material and the thickness of the negative electrode mixture layer was adjusted so that the negative electrode capacity was 1.1 times the positive electrode capacity. Produced. The thickness of the negative electrode mixture layer at this time was 65 μm, and the volume of the negative electrode mixture layer was about 1.6 times that of Example 1.

(Comparative Example 2)
The mixing ratio (weight ratio) of the composite material and MCMB is 50:50, the weight ratio of Si powder and MCMB is 37.5: 62.5, and the capacity of the negative electrode is 1.1 times the capacity of the positive electrode. A nonaqueous secondary battery was produced in the same manner as in Example 1 except that the thickness of the negative electrode mixture layer was adjusted so as to be.

(Comparative Example 3)
A non-aqueous secondary battery was produced in the same manner as in Example 1 except that the negative electrode was produced by setting the average particle size of the Si powder to 4 μm.

  The batteries were evaluated in the same manner as in Example 1 for Examples 2 to 9 and Comparative Examples 1 to 3. Tables 1 and 2 show the configuration of the produced negative electrode and the measured battery characteristics, respectively.

  In the nonaqueous secondary battery of the present invention, at least one alloying material selected from an element that can be alloyed with lithium by charging and a compound thereof and a carbon material are used as the negative electrode active material. As shown in FIG. 5, the capacity per unit weight of the negative electrode active material can be increased, and the volume of the negative electrode mixture layer can be increased compared to the non-aqueous secondary battery of Comparative Example 1 using only the carbon material as the negative electrode active material. Since it can be reduced, the capacity of the battery can be increased.

  Further, the ratio of the alloying material in the total amount of the alloying material and the carbon material is 1 to 30% by weight, and the average particle size of the alloying material is 2/5 or less of the average particle size of the carbon material, While suppressing changes in the thickness of the battery due to the expansion of the negative electrode due to charging, a decrease in capacity due to cycle charge / discharge was suppressed, and a nonaqueous secondary battery excellent in cycle characteristics could be constructed.

Claims (6)

A non-aqueous solution comprising at least one alloying material selected from an element that can be alloyed with lithium by charging and a compound thereof, a negative electrode having a carbon material as an active material, a positive electrode, and a non-aqueous electrolyte A secondary battery,
The ratio of the alloying material in the total amount of the alloying material and the carbon material is 1 to 30% by weight,
The non-aqueous secondary battery, wherein an average particle diameter of the alloying material is 2/5 or less of an average particle diameter of the carbon material.   The non-aqueous secondary battery according to claim 1, wherein the element that can be alloyed with lithium by charging is Si or Sn.   The nonaqueous secondary battery according to claim 2, comprising an oxide of Si or Sn as the alloying material.   The non-aqueous secondary battery according to claim 1, wherein the alloying material has an average particle size of 3 μm or less.   The nonaqueous secondary battery according to any one of claims 1 to 4, wherein a ratio of the alloying material in a total amount of the alloying material and the carbon material is 5% by weight or more.   The non-aqueous secondary battery according to claim 1, wherein the alloying material is combined with a conductive additive.