WO2007083583A1 - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

Disclosed is a nonaqueous electrolyte secondary battery which is characterized in that a material containing silicon is used as a negative electrode active material and a mixed solvent of propylene carbonate and a dialkyl carbonate is used as a solvent for the electrolyte solution. It is preferable that the volume ratio between propylene carbonate and the dialkyl carbonate in the mixed solvent (the former:the latter) is from 5:95 to 95:5. It is also preferable to use a sulfur-containing lithium salt, which is capable of forming a coating film containing sulfur on the active material surface made of the silicon-containing material, as a supporting electrolyte for the electrolyte solution.

Description

 Specification

 Non-aqueous electrolyte secondary battery

 Technical field

 The present invention relates to a nonaqueous electrolyte secondary battery such as a lithium secondary battery.

 Background art

 [0002] As an electrolyte for current lithium secondary batteries, a supporting electrolyte such as LiPF is used in an organic solvent.

 6

 It is common to use what was dissolved in a mixed solvent of a certain ethylene carbonate (hereinafter also referred to as EC) and jetyl carbonate (hereinafter also referred to as DEC). However, this mixed solvent could not be said to have sufficient temperature characteristics (high temperature and low temperature).

 [0003] In addition to EC and DEC, propylene carbonate (hereinafter also referred to as PC) is known as a typical organic solvent. PC is cheaper than EC and DEC, and is a solvent with excellent temperature and cycle characteristics. However, when PC is used together with Graphite, which is the negative electrode material of the current lithium secondary battery, even if Graphite attempts to occlude lithium, it will not be occluded, and instead PC will be decomposed. Is known (see Non-Patent Document 1). Therefore, in a lithium secondary battery using graphite as the negative electrode material, PC cannot be used as a solvent for the electrolyte.

 [0004] Non-Patent Document 1: Mio Nishi, “The Story of Lithium Ion Secondary Batteries”, 3rd edition, Houhuabo, 1999

 April 10th, p. 43

 Disclosure of the invention

 [0005] An object of the present invention is to provide a non-aqueous electrolyte secondary battery that can eliminate the above-mentioned drawbacks of the prior art.

[0006] The present invention provides a non-aqueous electrolyte secondary battery characterized in that a material containing silicon is used as a negative electrode active material, and a mixed solvent of propylene carbonate and dialkyl carbonate is used as a solvent for the electrolyte. This achieves the object. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described based on preferred embodiments thereof. The non-aqueous electrolyte secondary battery of the present invention typically includes a negative electrode, a positive electrode, and a separator interposed between both electrodes. It has. A non-aqueous electrolyte exists between the negative electrode and the positive electrode. The negative electrode has an active material layer formed on at least one surface of a current collector. The active material layer includes a negative electrode active material. In the present invention, a material containing silicon (Si) is used as the negative electrode active material. Silicon has the advantage of high capacity compared to the graphite currently used as the negative electrode material for non-aqueous electrolyte secondary batteries.

 As the negative electrode active material used in the present invention, not only silicon alone but also an alloy of silicon and another metal, an intermetallic compound of silicon and another metal, silicon oxide, or the like can be used. . Examples of other metals include metals with low ability to form lithium compounds such as Co, Ni, Cu, Fe, V, Ti, Mn, Cr, W, Mg, and Nd. “Lithium compound formation ability is low” means that an intermetallic compound or solid solution with lithium is not formed, or even if lithium is formed, the amount of lithium is very small or very unstable. means. Li can also be used as another metal. Furthermore, the negative electrode active material described in US2006 / 051675A1 of the previous application of the present applicant can be used. Specific examples include mixed particles of silicon particles and carbon particles; mixed particles of silicon particles and metal particles; mixed particles of silicon and metal compound particles and metal particles. .

 [0009] The negative electrode active material made of a material containing silicon can be, for example, in the form of a thin film. In this case, an active material layer having a thin film force is formed on at least one surface of the current collector by various thin film forming means such as chemical vapor deposition, physical vapor deposition, and sputtering. The thin film may be etched to form a number of voids extending in the thickness direction. For etching, a wet etching method using a sodium hydroxide aqueous solution or the like, or a dry etching method using a dry gas or plasma can be employed.

[0010] The negative electrode active material may also be in the form of particles. In this case, the particles are applied to at least one surface of the current collector in a slurry state mixed with a binder, a solvent, and the like. As a result, an active material layer composed of the slurry coating is formed. You can fire this coating to sinter the particles. As a sintering method, for example, the method described in US2004Z043294A1 can be used. Alternatively, it is also preferable that a metal having a low lithium compound forming ability is infiltrated between the particles. The permeation of the metal between the particles can effectively prevent the active material that has been pulverized due to volume change due to charge / discharge. Say here Penetration refers to a state in which a metal material having a low lithium compound forming ability exists in the space between the particles so as to cover the surface of the particle, and the space between the particles is filled with the metal material. I don't need it. Rather, the metallic material preferably covers the surface of the particles so that there is a space between the particles. The presence of the metal material in such a state has an advantage that the electrolytic solution surely reaches the deep part of the active material layer. In addition, there is an advantage that the volume increase caused by the expansion of the lithium occluded particles is alleviated.

[0011] In order to infiltrate the metal between the particles, the slurry coating film may be electrolyzed to deposit a metal having a low lithium compound forming ability between the particles. In order to deposit a metal between particles by electrolytic plating, for example, a method described in US2006 / 11 5735A1 according to the previous application of the present applicant can be used. Specifically, a coating film is formed by applying a slurry containing active material particles on a current collector. The slurry contains active material particles, conductive carbon material particles, a binder, a diluent solvent, and the like. As the binder, polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene diene monomer (EPDM) or the like is used. N-methylpyrrolidone, cyclohexane, etc. are used as the dilution solvent. After the slurry coating is formed, it is immersed in a plating bath containing a metal material having a low ability to form a lithium compound and electroplating is performed. For example, when using copper, or when using a copper sulfate solution, the concentration of copper is 30 to 100 gZl, the concentration of sulfuric acid is 50 to 200 gZl, the concentration of chlorine is 30 ppm or less, and the liquid temperature is 30. to 80 ° C, the current density may be set l~100AZdm ^2. When using a copper pyrophosphate solution, set the copper concentration to 2 to 50 g / l, the potassium pyrosilicate concentration to 100 to 700 g / l, f the night temperature to 30 to 60 ° C, pH to 8 to 12, current density:! may be set to ~ 10A / dm ^2. By appropriately adjusting these electrolysis conditions, a metal material having a low lithium compound-forming ability penetrates into the coating film, and the target active material layer is formed.

On the other hand, the positive electrode has an active material layer formed on at least one surface of a current collector. The active material layer contains a positive electrode active material. A Li-containing compound is used as the positive electrode active material. As the Li-containing compound, a substance capable of electrochemically inserting and extracting lithium is used. For example, LiCoO or LiNiO which are layered compounds containing lithium can be used. Or LiMo 2 O, which is a spinel structure compound containing lithium, can be used. The active material layer is formed by applying the positive electrode active material particles to at least one surface of the current collector in a slurry state in which the particles are mixed with a binder, a solvent, and the like.

 In the negative electrode, the current collector that supports the active material layer is generally composed of a metal material having a low lithium compound forming ability. Examples of such a metal material include copper, nickel, iron, cobalt, and alloys thereof. On the other hand, an aluminum foil is generally used as a current collector for supporting the active material layer in the positive electrode.

 [0014] The kind of separator separating the positive electrode and the negative electrode is not particularly limited, and the same separator as conventionally used as this type of material can be used. For example, a synthetic resin nonwoven fabric, a polyethylene or polypropylene porous film, or the like is preferably used.

 [0015] Therefore, in the present invention, a mixed solvent of PC and dialkyl carbonate (hereinafter also referred to as DAC) is used as a solvent for the nonaqueous electrolytic solution. As described in the background section above, in the conventional non-aqueous electrolyte secondary battery, a mixed solvent of EC and DEC was used as a solvent for the non-aqueous electrolyte. However, this mixed solvent has a drawback that the temperature characteristics at high and low temperatures and the rate characteristics are not good. Further, as a result of the examination by the present inventors, there is a problem that when this mixed solvent is applied to a secondary battery using a material containing silicon as a negative electrode active material, the cycle life is deteriorated. The reason for this is thought to be as follows. If a material containing silicon is used as the negative electrode active material, and LiCoO is used as the positive electrode active material, and the charge / discharge voltage is 4.2 to 2.7 V, then at the end of discharge

The polarization of Si increases, and the decomposition of the solvent is likely to occur. As a result, potential shift and negative electrode expansion occur. For this reason, the cycle life of the battery deteriorates.

[0016] In contrast to the above-mentioned drawbacks of the mixed solvent of EC and DEC, in the present invention, by using a mixed solvent of PC and DAC as the solvent of the non-aqueous electrolyte, it has become possible to eliminate the drawbacks. . In the present invention, even if PC is used as the solvent for the non-aqueous electrolyte, the decomposition is not likely. In addition, temperature characteristics and cycle characteristics can be improved by using a mixed solvent of PC and DAC. Specifically, the battery cycle characteristics are improved by using a PC, and the battery temperature characteristics (temperature characteristics at low temperatures) are improved by using a DAC. [0017] As a result of the study by the present inventors, it was found that PC and DAC can be mixed within a wide range of volume ratios. Specifically, the volume ratio of PC to DAC in the mixed solvent (the former: latter) is preferably 5:95 to 95: 5, and more preferably 20:80 to 70:30. When the volume ratio of the PC exceeds 95%, the wettability with the separator generally used in the nonaqueous electrolyte secondary battery tends to be low, and the distribution of the electrolyte may not be smooth. On the other hand, if the volume ratio of DAC exceeds 95%, the polarity of the mixed solvent as a whole may decrease, and it may be difficult to dissolve the electrolyte.

 [0018] As the DAC, for example, jetyl carbonate or dimethyl carbonate can be used. Or these two can also be used together. In particular, jetyl carbonate is preferable because the battery can be used under freezing point where the freezing point is low.

 [0019] One feature of the present invention is that a mixed solvent of PC and DAC is used as a solvent for the non-aqueous electrolyte, but this does not preclude the use of a solvent other than PC and DAC. ,. That is, as the solvent used in the present invention, a solvent other than PC and DAC can be used as necessary. However, in order to maximize the effects of the present invention, it is most preferable to use only PC and DAC, and no other nonaqueous solvent as the solvent of the nonaqueous electrolytic solution.

 [0020] As the supporting electrolyte of the nonaqueous electrolytic solution in the secondary battery of the present invention, the same ones conventionally used as this type of substance can be used without particular limitation. For example, LiCl ○, LiAlCl, LiPF, LiAsF, LiSbF, LiSCN, LiCl, LiBr, Lil, LiCF SO

 4 4 6 6 6 3 3

, LiC F SO and the like. These supporting electrolytes are used alone or in combination of two or more.

4 9 3

 They can be used together.

Furthermore, the various supporting electrolytes described above are sulfur-containing lithium salts capable of forming a film containing sulfur on the surface of the negative electrode active material made of a material containing silicon (hereinafter also referred to as film-forming lithium salt). ) In combination. By using a film-forming lithium salt in combination, a film called SEI (solid electrolyte interface) is formed on the surface of the negative electrode active material. As a result of the study by the present inventors, it was found that SEI formed by using a film-forming lithium salt has lithium ion conductivity and has a property of suppressing decomposition of the electrolytic solution. Preventing the decomposition of the electrolytic solution leads to improvement of the cycle characteristics of the battery. Therefore, in the present invention However, when a normal supporting electrolyte is used in combination with a film-forming lithium salt, the cycle characteristics of the battery can be further improved by a synergistic effect with the use of a mixed solvent of PCZDAC.

 [0022] As the film-forming lithium salt, for example, a lithium salt containing sulfur capable of reacting with silicon contained in the negative electrode active material, or a sulfur containing lithium salt capable of reacting itself. You can use it. The film-forming lithium salt forms SEI on the surface of the negative electrode active material by reacting with the silicon contained in the negative electrode active material or by itself during charge / discharge of the battery. Examples of such lithium salts include LiS and

2-Chenyllithium (C H SLi) represented by the following chemical formula.

 [0023] [Chemical 1]

For example, Li S is composed of Li SiS on the surface of the negative electrode active material according to the following reaction formula (1).

Form EI.

Si + 2S ² "+ 2Li S -4e" → Li SiS (1)

 [0025] Among the film-forming lithium salts described above, LiS easily absorbs moisture, and therefore, in a non-aqueous electrolyte secondary battery that dislikes moisture, attention should be paid to its handling. 2 Chenyllithium can be dissolved directly in a non-aqueous solvent or once dissolved in tetrahydrofuran (THF) or the like and then added to the non-aqueous solvent.

[0026] A normal supporting electrolyte (that is, a lithium salt that does not contain sulfur and a sulfur-containing film on which the sulfur-containing film can be formed on the surface of the negative electrode active material made of a material containing silicon). The concentration of the film-forming lithium salt used in combination with the lithium salt in the electrolyte is 0.01 to 0.5 mol / l, particularly 0.05 to 0.2 mol / l. Preferred from the standpoint of balance.

[0027] The form of the secondary battery of the present invention may be a coin type, a cylindrical type, or a square type. For example, in the secondary battery of the present invention, a separator is interposed between the negative electrode and the positive electrode, these three members are wound to form a wound body, and the wound body is accommodated in a battery container. Roll type electric It can be made into a pond (cylindrical battery or square battery).

 Example

 Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to the embodiments. Unless otherwise specified, “%” and “part” mean “% by weight” and “part by weight”, respectively.

 [Example 1]

 (1) Manufacture of negative electrode

 A current collector made of an electrolytic copper foil with a thickness of 18 μη was acid washed at room temperature for 30 seconds. After the treatment, it was washed with pure water for 15 seconds. A slurry containing Si particles was applied on the current collector to a thickness of 15 μm to form a coating film. The average particle size (D) of the particles was 2 μm. The composition of the slurry is

 50

 Particle: Styrene butadiene rubber (binder) = 100: 1.7 (weight ratio).

[0030] The current collector with the coating film formed is immersed in a copper pyrophosphate bath having the following bath composition, and copper is deposited between particles in the coating film by electrolysis to form an active material layer. did. By this electrolytic plating, copper was deposited over the entire thickness direction of the coating film. In this way, a negative electrode was produced. The electrolysis conditions were as follows. DSE was used for the anode. A DC power source was used as the power source.

 'Copper pyrophosphate trihydrate: 105gZl

 -Potassium potassium phosphate: 450g / l

 • Potassium nitrate: 30g / l

 • Bath temperature: 50 ° C

• Current density: 3A / dm ²

 • pH: Ammonia water and polyphosphoric acid were added to adjust to ρΗ8.2.

[0031] (2) Production of positive electrode

 LiCoO powder having an average particle size of 20 μm was used as the positive electrode active material. 90 parts of this powder,

 2

 A slurry was obtained by mixing 5 parts of acetylene black as a conductive agent with a 5% N-methylpyrrolidone solution containing 5 parts of polyvinylidene fluoride as a binder. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to produce a positive electrode.

[0032] (3) Manufacture of secondary batteries The obtained negative electrode and positive electrode were opposed to each other through a separator made of a polyethylene porous film, and housed in a battery case. As the electrolytic solution, a solution obtained by dissolving the supporting electrolyte shown in Table 1 in a mixed solvent in which PC and DEC were mixed at a volume ratio shown in Table 1 at a concentration shown in the same table was used.

[0033] (4) Evaluation

 About the obtained secondary battery, the low temperature characteristic and the 100 cycle capacity maintenance rate were measured by the following methods. The results are shown in Table 1.

[0034] [Low temperature characteristics]

 After 5 cycles of initial activity, the following ratios were calculated and quantified as low temperature characteristics. {(— Discharge capacity at 0.5C rate at 10 ° C) / (Discharge capacity at 0.5C rate at 25 ° C)} X100

 [100 cycle capacity maintenance rate]

 The discharge capacity after 100 cycles was measured, and the value was divided by the maximum negative electrode discharge capacity and multiplied by 100.

[Examples 2 to 8 and Comparative Example 1]

 A secondary battery was obtained in the same manner as in Example 1 except that the solvents shown in Table 1 were used as the solvent and the supporting electrolyte in the electrolytic solution. The obtained secondary battery was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0037] [Table 1] Low-temperature characteristics 1 00 cycle Support electrolyte (mol ¾ degree) Solvent (volume ratio) (at -10, at 0.5C

 Capacity maintenance rate (%) Capacity ratio, 25

Example 1 LiPF ₆ (1M) PC: DEC = 3: 7 60 85 Example 2 LiPF ₆ (1) PC: DEC = 7: 3 40 90 Example 3 LiPF ₆ (1M) PC: DEC = 5: 95 50 80 Example 4 LiPF ₆ (1M) PC: DEC = 95: 5 35 92 Example 5 LiPF ₆ (1M) + Li ₂ S (0.05M) PC: DEC = 3: 7 60 92 Example 6 LiPF ₆ ( 1M) + Li ₂ S (0.1M) PC: DEC = 3: 7 60 93 Example 7 LiPF ₆ (1M) + 2-thio iiA lithium (0.05M) PC: DEC = 3: 7 60 95 Example 8 LiPF ₆ (1M) + 2-fi; Mf5A (0.1M) PC: DEC = 3: 7 60 96 Comparative Example 1 LiPF6 (1M) EC: DEC = 3: 7 10 65 [0038] As is apparent from the results shown in Table 1, the secondary batteries of the examples using the mixed solvent of PC and DEC were compared with the secondary batteries of the comparative examples using the mixed solvent of EC and DEC. It can be seen that the low temperature characteristics and cycle characteristics are improved. Further, as is clear from the comparison between Example 1 and Examples 5 to 8, it is understood that the cycle characteristics are further improved by using a film-forming lithium salt as the supporting electrolyte.

 Although not shown in the table, as a result of analysis, it was confirmed that a film containing sulfur was formed on the surface of the active material of the negative electrode in Examples 5 to 8. Specifically, the battery after the above evaluation was disassembled, the negative electrode was taken out and washed, and then the presence of sulfur was confirmed by analyzing the surface of the active material using XPS.

 Industrial applicability

As described above in detail, the non-aqueous electrolyte secondary battery of the present invention is excellent in temperature characteristics at low temperatures and cycle characteristics.

Claims

The scope of the claims  [1] A non-aqueous electrolyte secondary battery using a material containing silicon as a negative electrode active material and a mixed solvent of propylene carbonate and dialkyl carbonate as a solvent of an electrolyte.  [2] The nonaqueous electrolyte secondary battery according to claim 1, wherein the volume ratio of propylene carbonate to dialkyl carbonate (the former: the latter) in the mixed solvent is 5:95 to 95: 5.  [3] The non-claim according to claim 1, wherein a sulfur-containing lithium salt capable of forming a film containing sulfur is formed on the surface of the active material made of the material containing silicon as a supporting electrolyte of the electrolytic solution. Water electrolyte secondary battery.  [4] The sulfur-containing lithium salt is Li S or 2_ lithium lithium.  2  The nonaqueous electrolyte secondary battery according to item. 5. The nonaqueous electrolyte secondary battery according to claim 3, wherein the concentration of the sulfur-containing lithium salt in the electrolyte is 0.01 to 0.5 molZl. 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the material containing silicon is made of particles, and a metal having a low ability to form a lithium compound penetrates between the particles.