JP2007194107A - Non-aqueous electrolytic solution secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous electrolytic solution secondary battery superior in temperature characteristics and cycle characteristics at low temperatures. <P>SOLUTION: A material including silicon is used as a negative electrode active material and a mixture solvent of propylene carbonate and dialkyl carbonate is used as a solvent of electrolytic solution. It is desirable that the volumetric ratio (the former:the latter) of propylene carbonate and dialkyl carbonate is made 5:95-95:5. It is also desirable that a sulfur-contained lithium salt that can form a coating containing sulfur on the surface of the active material consisting of the material containing silicon is used as a supporting electrolyte of the electrolytic solution. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

As an electrolyte for current lithium secondary batteries, a supporting electrolyte such as LiPF ₆ was dissolved in a mixed solvent of an organic solvent, ethylene carbonate (hereinafter also referred to as EC) and diethyl carbonate (hereinafter also referred to as DEC). It is common to use one. However, this mixed solvent cannot be said to have sufficient temperature characteristics (high temperature and low temperature).

  As the organic solvent, in addition to EC and DEC, propylene carbonate (hereinafter also referred to as PC) is known as a representative one. PC is cheaper than EC and DEC, and is a solvent excellent in temperature characteristics and cycle characteristics. However, it is known that when PC is used in combination with graphite, which is the negative electrode material of the present lithium secondary battery, even if lithium is occluded in graphite, occlusion is not performed, but instead PC is decomposed. (Refer nonpatent literature 1). Therefore, in a lithium secondary battery using graphite as the negative electrode material, PC cannot be used as a solvent for the electrolytic solution.

Nishio, “The Story of Lithium Ion Secondary Batteries”, 3rd edition, Hankabo, April 10, 1999, p.43

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

  The present invention provides a non-aqueous electrolyte secondary battery using a material containing silicon as a negative electrode active material and using a mixed solvent of propylene carbonate and dialkyl carbonate as a solvent of an electrolytic solution. The object has been achieved.

  The non-aqueous electrolyte secondary battery of the present invention is excellent in temperature characteristics at low temperatures and cycle characteristics.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The nonaqueous electrolyte secondary battery of the present invention typically includes a negative electrode, a positive electrode, and a separator interposed between both electrodes. 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 an advantage that it has a higher capacity than graphite, which is currently used as a 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 other metal, an intermetallic compound of silicon and other metal, silicon oxide, or the like can be used. Examples of the other metal include metals having low ability to form lithium compounds such as Co, Ni, Cu, Fe, V, Ti, Mn, Cr, W, Mg, and Nd. “Low lithium compound forming ability” means that lithium does not form an intermetallic compound or solid solution, or even if formed, lithium is in a very small amount or very unstable. Li can also be used as another metal. Furthermore, what is described in Unexamined-Japanese-Patent No. 2005-63767 based on the previous application of this applicant can also be used as a negative electrode active material. 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.

  The negative electrode active material made of a material containing silicon can be in the form of a thin film, for example. In this case, an active material layer made of a thin film 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 large number of voids extending in the thickness direction. In addition to the wet etching method using a sodium hydroxide aqueous solution or the like, a dry etching method using a dry gas or plasma can be employed for the etching.

  The negative electrode active material can 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 and a solvent. As a result, an active material layer made of the slurry coating is formed. The coating film may be fired to sinter the particles. As a sintering method, for example, a method described in JP-A-2002-260637 can be used. Alternatively, it is also preferable that a metal having a low lithium compound forming ability permeates between the particles. By allowing the metal to permeate between the particles, it is possible to effectively prevent the active material that has been pulverized due to volume change due to charge and discharge. The term “penetration” as used herein refers to a state in which a metal material having a low lithium compound forming ability exists in the space between particles so as to cover the surface of the particle, and the space between particles is completely filled with the metal material You don't need to be. Rather, it is preferable that the metal material 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 reliably reaches the deep part of the active material layer. In addition, there is an advantage that the increase in volume caused by the expansion of the lithium-occluded particles is alleviated.

In order to infiltrate the metal between the particles, electrolytic plating is performed on the coating film of the slurry 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 Japanese Patent Application Laid-Open No. 2005-63929 related to an earlier application of the present applicant can be used. Specifically, a slurry containing active material particles is applied onto a current collector to form a coating film. The slurry contains active material particles, conductive carbon material particles, a binder, a diluting solvent, and the like. As the binder, polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene diene monomer (EPDM) or the like is used. As a diluting solvent, N-methylpyrrolidone, cyclohexane or the like is used. After the formation of the slurry coating film, electrolytic plating is performed by dipping in a plating bath containing a metal material having a low lithium compound forming ability. As conditions for electrolytic plating, for example, when using copper, when using a copper sulfate solution, the concentration of copper is 30 to 100 g / l, the concentration of sulfuric acid is 50 to 200 g / l, the concentration of chlorine is 30 ppm or less, The temperature may be 30 to 80 ° C. and the current density may be 1 to 100 A / dm ² . When using a copper pyrophosphate solution, the concentration of copper is 2 to 50 g / l, the concentration of potassium pyrophosphate is 100 to 700 g / l, the liquid temperature is 30 to 60 ° C., the pH is 8 to 12, and the current density is 1. ~10A / dm ² and may be set. By appropriately adjusting these electrolytic conditions, a metal material having a low lithium compound-forming ability penetrates into the coating film, and a 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 ₂ or LiNiO ₂ which is a layered compound containing lithium can be used. Alternatively, LiMo ₂ O ₄ which is a compound having a spinel structure including 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 and a solvent.

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

  There is no restriction | limiting in particular in the kind of separator which separates a positive electrode and a negative electrode, The thing similar to what was conventionally used as this kind of material can be used. For example, a synthetic resin nonwoven fabric, a polyethylene or polypropylene porous film, or the like is preferably used.

Accordingly, 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 a conventional non-aqueous electrolyte secondary battery, a mixed solvent of EC and DEC is 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. In addition, as a result of examinations by the present inventors, it has been found that when this mixed solvent is applied to a secondary battery using a material containing silicon as a negative electrode active material, there is a problem that the cycle life is deteriorated. The reason is considered as follows. If a material containing silicon is used as the negative electrode active material, and LiCoO ₂ is used as the positive electrode active material, and the charge / discharge voltage is 4.2-2.7 V, the polarization of Si at the end of discharge increases, resulting in this. Therefore, decomposition of the solvent is likely to occur. As a result, potential shift and negative electrode expansion are caused. For this reason, the cycle life of the battery deteriorates.

  In contrast to the above-mentioned drawbacks of the mixed solvent of EC and DEC, in the present invention, it is possible to eliminate the drawbacks by using a mixed solvent of PC and DAC as the solvent of the non-aqueous electrolyte. In the present invention, there is no risk of decomposition even if PC is used as the solvent of the non-aqueous electrolyte. Moreover, 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 a low temperature) are improved by using a DAC.

  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 (the former: latter) of PC and DAC in the mixed solvent is preferably 5:95 to 95: 5, and more preferably 20:80 to 70:30. When the volume ratio of PC exceeds 95%, the wettability with a separator generally used in a nonaqueous electrolyte secondary battery tends to be low, and the electrolyte may not flow smoothly. On the other hand, when the volume ratio of the DAC exceeds 95%, the polarity of the mixed solvent as a whole decreases, and it may be difficult to dissolve the electrolyte.

  For example, diethyl carbonate or dimethyl carbonate can be used as the DAC. Or these two can also be used together. In particular, diethyl carbonate is preferable because it has a low freezing point and can be used under a freezing point.

  The present invention is characterized in 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 that only PC and DAC are used as the solvent of the non-aqueous electrolyte and no other non-aqueous solvent is used.

As the supporting electrolyte of the nonaqueous electrolytic solution in the secondary battery of the present invention, the same one as conventionally used as this kind of substance can be used without particular limitation. For example _{LiC1O 4, LiA1Cl 4, LiPF 6} , LiAsF 6, LiSbF 6, LiSCN, LiC1, LiBr, LiI, etc. _{LiCF 3 SO 3, LiC 4 F} 9 SO 3 and the like. These supporting electrolytes can be used alone or in combination of two or more.

  Further, the various supporting electrolytes described above are used in combination with a sulfur-containing lithium salt (hereinafter also referred to as a film-forming lithium salt) capable of forming a film containing sulfur on the surface of a negative electrode active material made of a material containing silicon. It is preferable. By using the 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 has been found that SEI formed by using a film-forming lithium salt in combination has lithium ion conductivity and 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, 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 PC / DAC.

As the film-forming lithium salt, for example, a lithium salt containing sulfur that can react with silicon contained in the negative electrode active material, or a lithium salt containing sulfur that can react by itself can be used. . The film-forming lithium salt forms SEI on the surface of the negative electrode active material by reacting with silicon contained in the negative electrode active material or by itself during charge / discharge of the battery. Examples of such a lithium salt include LiS ₂ and 2-thienyl lithium (C ₄ H ₃ SLi) represented by the following chemical formula.

For example, Li ₂ S forms SEI made of Li ₄ SiS _{4 on} the surface of the negative electrode active material according to the following reaction formula (1).
Si + 2S ^2- + 2Li ₂ S-4e ⁻ → Li ₄ SiS ₄ (1)

Of the above-described film-forming lithium salts, LiS ₂ is easy to absorb moisture, and therefore, in a non-aqueous electrolyte secondary battery that dislikes moisture, attention should be paid to its handling. Further, 2-thienyl lithium may be directly dissolved in a non-aqueous solvent, or may be once dissolved in tetrahydrofuran (THF) and then added to the non-aqueous solvent.

  It is used in combination with a normal supporting electrolyte (that is, a lithium salt that does not contain sulfur and a lithium salt that contains sulfur but cannot form a film containing sulfur on the surface of a negative electrode active material made of silicon). The concentration of the film-forming lithium salt in the electrolytic solution is preferably 0.01 to 0.5 mol / l, particularly 0.05 to 0.2 mol / l from the viewpoint of the balance between the low temperature characteristics and the cycle life. .

  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 a negative electrode and a positive electrode, these three members are wound to form a wound body, and the wound body is accommodated in a battery container. A roll type battery (cylindrical battery or square battery) can be used.

  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 such examples. Unless otherwise specified, “%” and “part” mean “% by weight” and “part by weight”, respectively.

[Example 1]
(1) Production of Negative Electrode A current collector made of an electrolytic copper foil having a thickness of 18 μm 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 diameter (D ₅₀ ) of the particles was 2 μm. The composition of the slurry was particle: styrene butadiene rubber (binder) = 100: 1.7 (weight ratio).

The current collector on which the coating film was formed was immersed in a copper pyrophosphate bath having the following bath composition, and copper was deposited between the particles in the coating film by electrolytic plating to form an active material layer. Copper was deposited over the entire thickness direction of the coating film by this electrolytic plating. 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: 105 g / l
-Potassium pyrophosphate: 450 g / l
・ Potassium nitrate: 30 g / l
・ Bath temperature: 50 ° C
・ Current density: 3 A / dm ²
-PH: Ammonia water and polyphosphoric acid were added to adjust to pH 8.2.

(2) Manufacture of positive electrode LiCoO ₂ powder having an average particle diameter of 20 μm was used as the positive electrode active material. 90 parts of this powder and 5 parts of acetylene black as a conductive agent were mixed with a 5% N-methylpyrrolidone solution containing 5 parts of polyvinylidene fluoride as a binder to obtain a slurry. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to produce a positive electrode.

(3) Manufacture of secondary battery 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 at a concentration shown in the same table in a mixed solvent in which PC and DEC were mixed at a volume ratio shown in Table 1 was used.

(4) Evaluation About the obtained secondary battery, the low temperature characteristic and the 100 cycle capacity maintenance rate were measured with the following method. The results are shown in Table 1.

(Low temperature characteristics)
After 5 cycles of initial activity, the following ratios were calculated and quantified as low temperature characteristics.
{(Discharge capacity at 0.5 C rate at −10 ° C.) / (Discharge capacity at 0.5 C rate at 25 ° C.)} × 100

[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. Evaluation similar to Example 1 was performed about the obtained secondary battery. The results are shown in Table 1.

  As is clear from the results shown in Table 1, the secondary battery of each example using a mixed solvent of PC and DEC had a lower temperature characteristic and a lower battery than the secondary battery of the comparative example using a mixed solvent of EC and DEC. It can be seen that the 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 negative electrode active material 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.

Claims (6)

  A non-aqueous electrolyte secondary battery using a material containing silicon as a negative electrode active material and using a mixed solvent of propylene carbonate and dialkyl carbonate as a solvent of an electrolytic solution.   The nonaqueous electrolyte secondary battery according to claim 1, wherein a volume ratio of propylene carbonate and dialkyl carbonate (the former: the latter) in the mixed solvent is 5:95 to 95: 5.   The non-aqueous electrolyte secondary solution according to claim 1 or 2, wherein a sulfur-containing lithium salt capable of forming a film containing sulfur is used as a supporting electrolyte of the electrolyte solution on the surface of the active material made of the material containing silicon. battery. The non-aqueous electrolyte secondary battery according to claim 3, wherein the sulfur-containing lithium salt is Li ₂ S or 2-thienyl lithium.   The nonaqueous electrolyte secondary battery according to claim 3 or 4, wherein the concentration of the sulfur-containing lithium salt in the electrolyte is 0.01 to 0.5 mol / l. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, 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.