JP4701595B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery provided with an electrolyte together with a positive electrode and a negative electrode, and in particular, a simple substance, an alloy, and a compound of a metal element or a metalloid element in which the negative electrode can occlude and release lithium (Li). The present invention relates to a lithium ion secondary battery including at least one member selected from the group consisting of :

  With the miniaturization of electronic devices, development of batteries having high energy density is required. As a battery that meets this requirement, there is a lithium secondary battery. However, the lithium secondary battery has a problem that the cycle life is short because lithium dendrites on the negative electrode during charging and becomes inactive.

  As a battery with improved cycle life, a lithium ion secondary battery has been commercialized. The negative electrode of a lithium ion secondary battery has a negative electrode active material such as a graphite material using lithium intercalation reaction between graphite layers, or a carbonaceous material applying lithium occlusion / release action into pores. It is used. Therefore, in a lithium ion secondary battery, lithium does not deposit dendrites and the cycle life is long. In addition, since the graphite material or the carbonaceous material is stable in the air, there is a great merit in industrial production.

However, the negative electrode capacity due to intercalation has an upper limit as defined by the composition C ₆ Li of the first stage graphite intercalation compound. In addition, it is industrially difficult to control the fine pore structure of the carbonaceous material and causes a decrease in the specific gravity of the carbonaceous material, which is effective in improving the negative electrode capacity per unit volume and thus the battery capacity per unit volume. It cannot be a means. Certain low-temperature calcined carbonaceous materials are known to exhibit negative electrode discharge capacities exceeding 1000 mAh / g. However, since they have a large capacity at a noble potential of 0.8 V or more with respect to lithium metal, metal oxides or the like are used. When a battery is configured for use as the positive electrode, there are problems such as a decrease in discharge voltage.

  For these reasons, it is considered difficult for the current carbonaceous materials to cope with longer use time of electronic devices and higher energy density of the power source in the future. A negative electrode active material having a large capacity is desired.

  On the other hand, as negative electrode active materials capable of realizing higher capacities, materials that apply the electrochemical and reversible generation and decomposition of certain lithium alloys have been widely studied. For example, a Li—Al alloy has been widely studied, and Patent Document 1 reports a Si alloy. However, when these alloys are used for the negative electrode of a battery, there is a problem that the cycle characteristics are deteriorated. One of the causes is that these alloys expand and contract with charging / discharging and are pulverized every time charging / discharging is repeated.

Therefore, in order to suppress the pulverization of such an alloy, for example, it has been studied to partially substitute with an element that does not participate in expansion and contraction associated with insertion and extraction of lithium. For example, LiSi _a O _b (0 ≦ a, 0 <b <2) (see Patent Document 2), Li _c Si _1-d M _d O _e (M is a metal element excluding alkali metal or semi-metal element excluding silicon) And 0 ≦ c, 0 <d <1, 0 <e <2) (see Patent Document 3), Li—Ag—Te alloy (see Patent Document 4), and the like have been proposed. In addition, a compound including at least one non-metallic element and a group 14 metal element or metalloid element in the long-period periodic table has been proposed (see Patent Document 5).
U.S. Pat. No. 4,950,566 JP-A-6-325765 JP-A-7-230800 JP 7-288130 A JP-A-11-102705

  However, even if these negative electrode active materials are used, the deterioration of cycle characteristics due to expansion and contraction is large, and the fact that the characteristics of high capacity cannot be fully utilized.

This invention is made | formed in view of this problem, The objective is to provide the lithium ion secondary battery which can improve cycling characteristics.

Lithium-ion secondary battery according to the present invention, e Bei a cathode, an anode, negative electrode, a single metal element or a metalloid element capable of lithium (Li) occluding and releasing the group consisting of alloys and compounds The electrolyte includes a first lithium salt composed of at least one of lithium salts represented by chemical formulas (1) to (5) , and LiPF ₆ , LiBF ₄ , LiN. An electrolytic solution containing (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiClO ₄ and a second lithium salt made of at least one of LiAsF ₆ , The concentration of 1 lithium salt is 0.01 mol / dm ³ or more and less than 0.5 mol / dm ³ .

In the lithium ion secondary battery according to the present invention, since the electrolyte contains the first lithium salt, a stable film is generated on the negative electrode, and the irreversible reaction generated between the negative electrode and the electrolyte is suppressed. In addition, the first lithium salt is difficult to dissolve in a non-aqueous solvent and may reduce the conductivity of the electrolyte. However, since the second lithium salt having a high conductivity is included, the conductivity of the first lithium salt is low. Reduction is suppressed.

According to the lithium ion secondary battery of the present invention, the electrolyte contains an electrolyte containing a predetermined amount of the first lithium salt and the second lithium salt, so that high conductivity is maintained. A stable film can be formed on the negative electrode, and cycle characteristics can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a cross-sectional structure of a secondary battery according to an embodiment of the present invention. This secondary battery is a so-called coin-type battery. A disc-shaped positive electrode 12 accommodated in the outer can 11 and a disc-shaped negative electrode 14 accommodated in the outer cup 13 are interposed via a separator 15. It is a laminated one. The peripheral portions of the outer can 11 and the outer cup 13 are sealed by caulking through an insulating gasket 16. The outer can 11 and the outer cup 13 are made of, for example, a metal such as stainless steel or aluminum (Al).

The positive electrode 12 includes, for example, a positive electrode current collector 12A having a pair of opposed surfaces and a positive electrode active material layer 12B provided on the positive electrode current collector 12A. The positive electrode current collector 12A is made of, for example, a metal foil such as an aluminum foil, a nickel (Ni) foil, or a stainless steel foil. The positive electrode active material layer 12B includes, for example, any one or more of positive electrode materials capable of inserting and extracting lithium as a positive electrode active material, and a conductive agent and a binder as necessary. May be included. Examples of the positive electrode material capable of inserting and extracting lithium include lithium such as titanium sulfide (TiS ₂ ), molybdenum sulfide (MoS ₂ ), niobium selenide (NbSe ₂ ), and vanadium oxide (V ₂ O ₅ ). Examples thereof include metal sulfides or metal oxides containing no lithium, or lithium-containing compounds containing lithium.

Among these, lithium-containing compounds are preferable because some compounds can obtain a high voltage and a high energy density. Examples of such lithium-containing compounds include those represented by the chemical formula Li _x MIO ₂ or Li _y MIIPO ₄ . In the formula, MI and MII represent one or more transition metals, and preferably contain at least one of cobalt (Co), nickel (Ni) and manganese (Mn). This is because a higher voltage can be obtained. Among these, it is more preferable that nickel is included. This is because the lithium-containing compound containing nickel has a high diffusion rate of lithium ions, and thus it is possible to suppress an increase in internal resistance even when an electrolytic solution having a low conductivity is used. In the formula, the values of x and y vary depending on the charge / discharge state of the battery, and are generally 0.05 ≦ x ≦ 1.10 and 0.05 ≦ y ≦ 1.10. Specific examples of the lithium-containing compound represented by the chemical formula Li _x MIO ₂ include lithium cobalt composite oxide (LiCoO ₂ ), lithium nickel composite oxide (LiNiO ₂ ), and lithium nickel cobalt composite oxide (Li _z Ni _v Co). _1-v O ₂ (wherein z and v vary depending on the charge / discharge state of the battery, and usually 0 <z <1, 0.7 <v <1.02)) or lithium manganese having a spinel structure Examples include composite oxide (LiMn ₂ O ₄ ).

  The negative electrode 14 includes, for example, a negative electrode current collector 14A having a pair of opposing surfaces, and a negative electrode active material layer 14B provided on the negative electrode current collector 14A. The negative electrode current collector 14A is made of, for example, a metal foil such as a copper (Cu) foil, a nickel foil, or a stainless steel foil. The negative electrode active material layer 14B includes, for example, one or more of a simple substance, an alloy, and a compound of a metal element or a metalloid element capable of inserting and extracting lithium as a negative electrode active material. Thereby, a high capacity can be obtained in this secondary battery. Note that in this specification, alloys include those composed of one or more metal elements and one or more metalloid elements in addition to those composed of two or more metal elements. There are structures in which a solid solution, a eutectic (eutectic mixture), an intermetallic compound, or two or more of them coexist.

Examples of metal elements or metalloid elements capable of inserting and extracting lithium include magnesium (Mg), boron (B), aluminum (Al), gallium (Ga), indium (In), and silicon (Si). , Germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), cadmium (Cd), silver (Ag), zinc (Zn), hafnium (Hf), zirconium (Zr) Or yttrium (Y) is mentioned. These alloys or compounds, such as those represented by the chemical formula Ma _s Mb _t. In this chemical formula, Ma represents at least one of a metal element and a metalloid element capable of inserting and extracting lithium, and Mb represents at least one of elements other than Ma. The values of s and t are s> 0 and t ≧ 0, respectively.

  Among these, a single element, alloy or compound of a group 14 metal element or metalloid element in the long-period periodic table is preferable, and silicon or tin, or an alloy or compound thereof is particularly preferable. This is because a higher capacity can be obtained. These may be crystalline or amorphous.

Specific examples of such alloys or compounds include SiB ₄ . _{SiB 6, Mg 2 Si, Mg} 2 Sn, Ni 2 Si, TiSi 2, MoSi 2, CoSi 2, NiSi 2, CaSi 2, CrSi 2, Cu 5 Si, FeSi 2, MnSi 2, NbSi 2, TaSi 2, VSi ₂ , an alloy or compound represented by McMd _u such as WSi ₂ or ZnSi ₂ (Mc represents silicon or tin, Md represents one or more metal elements, and u ≧ 0), or SiC, Si ₃ N ₄ , Si ₂ N ₂ O, Ge ₂ N ₂ O, SiO _v (0 <V ≦ 2), SnO _w (0 <W ≦ 2), LiSiO, or LiSnO.

  In addition, as other alloys or compounds, for example, LiAl alloy, LiAlMe alloy (Me represents at least one of the group consisting of group 2-11 elements and group 14 elements in the long-period periodic table), An AlSb alloy or a CuMgSb alloy can be used.

  Such an alloy or compound can be obtained, for example, by a mechanical alloying method or a method in which raw materials are mixed and heat-treated in an inert atmosphere or a reducing atmosphere.

  The negative electrode active material layer 14B may also include other negative electrode active materials, binders, conductive agents, or other materials that do not contribute to charging. Examples of other negative electrode active materials include carbon materials capable of inserting and extracting lithium. This carbon material is excellent in cycle characteristics, and can be used together with one or more of simple substances, alloys and compounds of metal elements or metalloid elements capable of occluding and releasing lithium. It is preferable because a high capacity can be obtained and cycle characteristics can be improved. Moreover, since it can also play the role of a electrically conductive agent, it is preferable. Examples of the carbon material include non-graphitizable carbon, graphitizable carbon, graphite, carbon black, fibrous carbon, and pyrolytic carbon.

  The separator 15 separates the positive electrode 12 and the negative electrode 14 and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. The separator 15 is composed of, for example, a porous film made of a synthetic resin made of polytetrafluoroethylene, polypropylene, polyethylene, or the like, or a porous film made of an inorganic material such as a ceramic nonwoven fabric. A structure in which the above porous films are laminated may be employed.

  The separator 15 is impregnated with an electrolytic solution that is a liquid electrolyte. The electrolytic solution contains a solvent such as an organic solvent and an electrolyte salt. Any organic solvent may be used as long as it dissolves and dissociates the electrolyte salt, and one kind may be used alone, or two or more kinds may be mixed and used. For example, propylene carbonate, ethylene carbonate, vinylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, γ-valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1, Examples include 3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, anisole, acetate ester, butyrate ester, propionate ester, or fluorobenzene.

Lithium electrolyte salt represented by the chemical formula _{LiB (C x H 2 (x} -2) O 4) (C y H 2 (y-2) O 4) (x and y is an integer of 2 or more, respectively) A first lithium salt comprising at least one of the salts, and at least one of LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiClO _4, and LiAsF _6. And a second lithium salt made of seeds. This is because the cycle characteristics can be improved. The cause is not clear, but it is presumed as follows. By including the first lithium salt, a stable film is generated on the negative electrode 14 and an irreversible reaction occurring between the negative electrode 14 and the electrolytic solution is suppressed. However, the electrical conductivity of the electrolytic solution is lowered only by the first lithium salt. Therefore, it is considered that the irreversible reaction can be suppressed while keeping the conductivity high by including the second lithium salt capable of increasing the conductivity.

Examples of the lithium salt represented by the chemical formula _{LiB (C x H 2 (x} -2) O 4) (C y H 2 (y-2) O 4), those x and y is 7 or less each integer Preferably, an integer of 6 or less is more preferable. This is because if a molecular weight higher than this is used, the viscosity of the electrolytic solution increases and the electrical conductivity may decrease. Among these, LiB (C ₂ O ₄ ) ₂ is more preferable.

Concentration of the first lithium salt in the electrolytic solution is preferably less than 0.01 mol / dm ³ or more 0.5 mol / dm ^3, more if 0.05 mol / dm ³ or more 0.45 mol / dm ³ or less preferable. If the lithium salt is too small, a film having a desired thickness may not be formed. Conversely, if the lithium salt is too large, the film formed on the surface of the negative electrode 14 becomes too thick, and the negative electrode 14 is charged and discharged. This is because the lithium occlusion and release may be hindered.

In addition to the first lithium salt and the second lithium salt, the electrolyte salt may contain any one or more of other lithium salts other than these. Examples of other lithium salts include LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiCl, or LiBr.

A gel electrolyte may be used in place of the electrolytic solution. The gel electrolyte is obtained, for example, by holding an electrolytic solution in a polymer compound. The electrolytic solution (that is, the solvent, the electrolyte salt, etc.) is as described above. As the polymer compound, for example, any polymer that absorbs an electrolytic solution and gelates can be used. Examples of such a polymer compound include polyvinylidene fluoride or a copolymer of vinylidene fluoride and hexafluoropropylene. And a fluorine-based polymer compound such as polyethylene oxide or an ether-based polymer compound such as a crosslinked product containing polyethylene oxide, or polyacrylonitrile. In particular, a fluorine-based polymer compound is desirable from the viewpoint of redox stability. The concentration of the first lithium salt in this gel electrolyte is the same as that of the electrolytic solution. That is, the concentration of the first lithium salt in the electrolytic solution in the gel electrolyte is preferably less than 0.01 mol / dm ³ or more ^{0.5mol / dm 3, 0.05mol / dm} 3 or more 0.45 mol / dm It is more preferable if it is ³ or less.

  In the secondary battery, when charged, for example, lithium ions are released from the positive electrode active material layer 12B and inserted in the negative electrode active material layer 14B through the electrolyte. When the discharge is performed, for example, lithium ions are released from the negative electrode active material layer 14B and inserted into the positive electrode active material layer 12B through the electrolyte. At that time, a stable film based on the first lithium salt is generated on the negative electrode 14, and an irreversible reaction between the negative electrode 14 and the electrolyte is suppressed.

  For example, the secondary battery can be manufactured as follows.

  First, for example, a positive electrode active material, and optionally a conductive agent and a binder are mixed to prepare a positive electrode mixture, and this positive electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone. A positive electrode mixture slurry is obtained. Next, the positive electrode mixture slurry is applied to the positive electrode current collector 12 </ b> A and dried, followed by compression molding to form the positive electrode active material layer 12 </ b> B, thereby producing the positive electrode 12.

  In addition, for example, any one or more of a single element, alloy, or compound of a metal element or a metalloid element capable of inserting and extracting lithium, and a binder as necessary may be mixed to form a negative electrode A mixture is prepared, and this negative electrode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidone to obtain a negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to the negative electrode current collector 14A and dried, and then compression-molded to form the negative electrode active material layer 14B, whereby the negative electrode 14 is produced.

  After that, for example, the negative electrode 14, the separator 15 impregnated with the electrolyte, and the positive electrode 12 are laminated and placed in the outer cup 13 and the outer can 11, and they are caulked. Thereby, the secondary battery shown in FIG. 1 is completed.

  As described above, in the present embodiment, since the electrolyte contains the first lithium salt and the second lithium salt, a stable coating film is generated on the negative electrode 14 while maintaining high conductivity. And cycle characteristics can be improved.

In particular, if the concentration of the first lithium salt in the electrolytic solution is 0.01 mol / dm ³ or more and less than 0.5 mol / dm ³ , a higher effect can be obtained.

  Further, specific embodiments of the present invention will be described in detail.

( Experimental Examples 1-1 to 1-10)
First, 91 parts by weight of lithium cobalt composite oxide (LiCoO ₂ ), 6 parts by weight of graphite (KS-6 made by Lonza) and 3 parts by weight of polyvinylidene fluoride as a binder were mixed to form a positive electrode mixture. Then, the mixture was dispersed in N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode current collector 12A made of an aluminum foil and dried, followed by compression molding to form a positive electrode active material layer 12B. Thereafter, the positive electrode 12 was produced by punching into pellets having a diameter of 15.2 mm.

  Moreover, 20 g of copper powder and 80 g of tin powder were mixed, this mixture was put into a quartz boat, heated to 1000 ° C. in an argon gas atmosphere, and allowed to cool to room temperature. The lump thus obtained was pulverized by a ball mill in an argon gas atmosphere to obtain a copper-tin (Cu-Sn) alloy powder. Next, using this copper-tin alloy powder as a negative electrode active material, 80 parts by weight of copper-tin alloy powder, 10 parts by weight of a conductive agent and negative electrode active material graphite (KS-6 made by Lonza), and 2 parts by weight of acetylene black And 8 parts by weight of polyvinylidene fluoride as a binder were mixed to prepare a negative electrode mixture, and then dispersed in N-methyl-2-pyrrolidone as a solvent to prepare a negative electrode mixture slurry. Subsequently, it apply | coated to the negative electrode collector 14A which consists of copper foil, and after making it dry, it compression-molded and formed the negative electrode active material layer 14B. After that, it was punched into a pellet having a diameter of 15.5 mm to produce a negative electrode 14.

Further, the first lithium salt and the second lithium salt were dissolved in a mixed solvent of 20% by weight of ethylene carbonate and 80% by weight of dimethyl carbonate to prepare an electrolytic solution. LiB (C ₂ O ₄ ) ₂ represented by the chemical formula ₂ was used for the first lithium salt, and LiPF ₆ was used for the second lithium salt. The concentrations of LiB (C ₂ O ₄ ) ₂ and LiPF _{6 in} the electrolytic solution were changed as shown in Table 1 in Experimental Examples 1-1 to 1-10.

  Next, a negative electrode 14 and a separator 15 made of a microporous polypropylene film having a thickness of 30 μm are laminated in this order on the outer cup 13, and after injecting an electrolytic solution from above, covering the outer can 11 containing the positive electrode 12, A coin-type secondary battery having a diameter of 20 mm and a height of 2.5 mm was manufactured by caulking the peripheral portions of the outer cup 13 and the outer can 11 with a gasket 16.

Further, as Comparative Examples 1-1 and 1-2 for Experimental Examples 1-1 to 1-10, the concentrations of LiB (C ₂ O ₄ ) ₂ and LiPF _{6 in} the electrolytic solution were changed as shown in Table 1. Other than the above, secondary batteries were fabricated in the same manner as in Experimental Examples 1-1 to 1-10. Further, as Comparative Examples 1-3 and 1-4 with respect to Experimental Examples 1-1 to 1-10, graphite (KS-44 made by Lonza) is used instead of copper-tin alloy and graphite (KS-6 made by Lonza). In addition, a secondary battery was fabricated in the same manner as in Experimental Examples 1-1 to 1-10 except that the concentrations of LiB (C ₂ O ₄ ) ₂ and LiPF ₆ were changed as shown in Table 1. .

For the fabricated secondary batteries of Experimental Examples 1-1 to 1-10 and Comparative Examples 1-1 to 1-4, the initial capacity and cycle characteristics were evaluated as follows. The results are shown in Table 1.

<Initial capacity>
In a 25 ° C. environment, 1 mA constant current constant voltage charging was performed up to an upper limit voltage of 4.2 V, then 1 mA constant current discharging was performed up to a final voltage of 2.5 V, and the discharge capacity was determined. The discharge capacity was determined by the following formula: discharge capacity (mAh) = 1 mA × discharge time (h).

<Cycle characteristics>
In a 25 ° C environment, 2 mA constant current and constant voltage charge was performed up to the upper limit voltage of 4.2 V, then 2 mA constant current discharge was performed up to a final voltage of 2.5 V, and 100 cycles of charge and discharge were performed under the same charge and discharge conditions. The discharge capacity retention rate (%) at the 100th cycle when the discharge capacity at the first cycle was set to 100 was determined.

As is clear from Table 1, according to Experimental Examples 1-1 to 1-10 in which the first lithium salt and the second lithium salt are mixed and used, the first lithium salt and the second lithium salt are used. As compared with Comparative Examples 1-1 and 1-2 using any one of the above, a high capacity retention rate was obtained. The cause of this is not clear, but a film derived from LiB (C ₂ O ₄ ) ₂ is formed on the copper-tin alloy, and the conductivity of the electrolyte based on LiB (C ₂ O ₄ ) ₂ is increased by LiPF ₆ . This is probably because the decrease was suppressed.

On the other hand, in Comparative Examples 1-3 and 1-4 using only graphite as the negative electrode active material, Comparative Example 1-3 using only the second lithium salt is different from the first lithium salt and the first lithium salt. Both the initial capacity and the capacity retention rate were higher than those of Comparative Example 1-4, which was used by mixing 2 lithium salts. This is presumably due to the fact that a film derived from LiB (C ₂ O ₄ ) ₂ is hardly generated on graphite, which has a lower reactivity with the electrolyte than the alloy.

In Experimental Examples 1-1 to 1-10 using a copper-tin alloy as the negative electrode active material, the initial capacity is 7.0 mAh or more, whereas Comparative Examples 1-3 and − using only graphite are used. In 1-4, it was less than 5.5 mAh. That is, when the negative electrode 14 includes a tin alloy, if the electrolyte includes the first lithium salt and the second lithium salt, a large capacity can be obtained and cycle characteristics can be improved. I understood that I could do it.

Further, as apparent from Experimental Examples 1-1 to 1-10, the capacity retention ratio was improved when the concentration of the first lithium salt in the electrolytic solution was increased, and a tendency to decrease after showing the maximum value was observed. . That is, the concentration of the first lithium salt in the electrolytic solution is preferably 0.01 mol / dm ³ or more and less than 0.5 mol / dm ^3, and is 0.05 mol / dm ³ or more and 0.45 mol / dm ³ or less. It turned out to be more preferable.

( Experimental examples 2-1 to 2-10)
With a lithium salt shown in the second lithium salt in Table 2, except that the concentration of the lithium salt in the electrolytic solution were changed as shown in Table 2, other experimental examples 1-1 to 1-10 A secondary battery was fabricated in the same manner as described above. Regarding the secondary batteries of Experimental Examples 2-1 to 2-10 were evaluated initial capacity and cycle characteristics in the same manner as in Experimental Example 1-1 to 1-10. The results are shown in Table 2 together with the results of Experimental Examples 1-7 and 1-5 and Comparative Examples 1-1 and 1-2.

As is apparent from Table 2, according to Experimental Examples 2-1 to 2-10, the capacity retention rate is higher than Comparative Examples 1-1 and 1-2, similarly to Experimental Examples 1-5 and 1-7. was gotten. That is, even if LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiClO ₄ or LiAsF ₆ is used for the second lithium salt, cycle characteristics can be improved. I understood.

( Experimental examples 3-1 to 3-5)
A secondary battery was fabricated in the same manner as in Experimental Example 1-5, except that the lithium salt shown in Table 3 was used instead of LiB (C ₂ O ₄ ) ₂ for the first lithium salt. That is, the experimental example 3-1, which was used LiB represented by the structural formula of 3 to the first lithium salt _{(C 2 O 4) (C} 4 H 4 O 4), Experiment 3 2 uses LiB (C ₃ H ₂ O ₄ ) (C ₄ H ₄ O ₄ ) whose structural formula is represented by chemical formula ₄ , and Experimental Example 3-3 has a structural formula represented by chemical formula 5. LiB (C ₄ H ₄ O ₄ ) ₂ is used, and Experimental Example 3-4 is an example in which LiB (C ₆ H ₈ O ₄ ) (C ₇ H ₁₀ O ₄ ) represented by the chemical formula 6 is Experimental Example 3-5 uses LiB (C ₇ H ₁₀ O ₄ ) (C ₈ H ₁₂ O ₄ ) whose structural formula is represented by Chemical Formula 7. For the secondary batteries of Experimental Examples 3-1 to 3-5, the initial capacity and the cycle characteristics were evaluated in the same manner as in Experimental Example 1-5. The results are shown in Table 3 together with the results of Experimental Example 1-5 and Comparative Examples 1-1 and 1-2.

As is clear from Table 3, according to Experimental Examples 3-1 to 3-5, a capacity retention rate higher than that of Comparative Examples 1-1 and 1-2 was obtained as in Experimental Example 1-5. . That is, the first lithium salt, represented by LiB (C ₂ O _{4) 2} other than formula _{LiB (C x H 2 (x} -2) O 4) (C y H 2 (y-2) O 4) It was found that the cycle characteristics can be improved even when other lithium salts are used.

As can be seen from Experimental Examples 1-5, 3-1 to 3-5, in the chemical formula LiB (C _x H _{2 (x-2)} O ₄ ) (C _y H _{2 (y-2)} O ₄ ), As the values of x and y increased, the initial capacity and capacity retention rate tended to decrease. That is, the formula _{LiB (C x H 2 (x} -2) O 4) in _{(C y H 2 (y-} 2) O 4), it is preferable that x and y are each 7 an integer, of 6 or less It turned out that it is more preferable if it is an integer.

( Experimental examples 4-1, 4-2)
A secondary battery was fabricated in the same manner as in Experimental Example 1-5, except that LiNi _0.8 Co _0.2 O ₂ or LiNi _0.4 Mn _0.3 Co _0.3 O ₂ was used as the positive electrode active material instead of LiCoO ₂ . Further, except that as a comparative example 4-1 and 4-2 to the experimental examples 4-1 and 4-2, was not used first lithium salt and the other in the same manner as in Experimental Example 4-1 and 4-2 A secondary battery was manufactured. For the secondary batteries of Experimental Examples 4-1 and 4-2 and Comparative Examples 4-1 and 4-2, the initial capacity and cycle characteristics were evaluated in the same manner as in Experimental Example 1-5. The results are shown in Table 4 together with the results of Experimental Example 1-5 and Comparative Example 1-1.

As is apparent from Table 4, according to Experimental Examples 4-1 and 4-2, a higher capacity retention rate was obtained than in Experimental Example 1-5. This is thought to be because the positive electrode active material is made of a lithium-containing compound containing nickel, which has a high diffusion rate of lithium, and is therefore less susceptible to an electrolyte solution having low conductivity using LiB (C ₂ O ₄ ) _2. It is done.

On the other hand, in Comparative Examples 1-1, 4-1 and 4-2 using only the second lithium salt, Comparative Example 1-1 using a lithium-containing compound not containing nickel and lithium containing nickel In Comparative Examples 4-1 and 4-2 using the compound, there was almost no difference in both initial capacity and capacity retention. This is presumably because the electrolyte using LiPF ₆ alone has high electrical conductivity, so that the battery characteristics greatly affect the properties of the electrolyte and are hardly affected by the properties of the lithium-containing compound.

  That is, if the first lithium salt and the second lithium salt are used for the electrolyte salt and the lithium-containing compound containing lithium, nickel, and oxygen is used for the positive electrode 12, the characteristics can be further improved. I understood.

( Experimental examples 5-1 to 5-3)
A copper-silicon (Cu-Si) alloy was used in place of the copper-tin alloy, and the lithium salt shown in Table 5 was used as the second lithium salt, and the lithium content in the electrolyte was shown in Table 5. A secondary battery was fabricated in the same manner as in Experimental Example 4-2 except for the above changes. The copper-silicon alloy was prepared in the same manner as the copper-tin alloy of Experimental Example 1-1 except that 20 wt% copper powder and 80 wt% silicon powder were used as raw materials. Further, as Comparative Examples 5-1 and 5-2 with respect to Experimental Examples 5-1 to 5-3, the concentrations of LiB (C ₂ O ₄ ) ₂ and LiPF _{6 in} the electrolytic solution were changed as shown in Table 5. Other than the above, a secondary battery was fabricated in the same manner as in Experimental Example 5-1. Regarding the secondary batteries of Experimental Examples 5-1 to 5-3 and Comparative Examples 5-1 and 5-2 were evaluated for initial capacity and cycle characteristics in the same manner as in Experimental Example 4-2. The results are shown in Table 5.

As is apparent from Table 5, according to Experimental Examples 5-1 to 5-3, compared with Comparative Examples 5-1 and 5-2 using either the first lithium salt or the second lithium salt. A high capacity retention rate was obtained. That is, even when the negative electrode 14 contains a silicon alloy, it was found that the cycle characteristics can be improved if the electrolyte contains the first lithium salt and the second lithium salt.

In the above embodiment, the chemical formula _{LiB (C x H 2 (x} -2) O 4) (C y H 2 (y-2) O 4) lithium salt represented by by way of specific examples described However, the above effect is considered to be due to the structure of the lithium salt. Therefore, the formula _{LiB (C x H 2 (x} -2) O 4) be used _{(C y H 2 (y-} 2) O 4) other lithium salt represented by, is possible to obtain the same effect it can.

  Although the present invention has been described with reference to the embodiments and examples, the present invention is not limited to the embodiments and examples, and various modifications can be made. For example, in the above-described embodiments and examples, the case of using a gel electrolyte which is one type of electrolytic solution or solid electrolyte has been described, but another electrolyte may be used. Other electrolytes include, for example, a polymer solid electrolyte in which an electrolyte salt is dispersed in a polymer compound having ion conductivity, an ion conductive inorganic compound made of ion conductive ceramics, ion conductive glass or ionic crystals, Or what mixed these ion conductive inorganic compounds and electrolyte solution, or what mixed these ion conductive inorganic compounds, gel-like electrolyte, or polymer solid electrolyte is mentioned. Examples of the polymer compound of the solid polymer electrolyte include, for example, an ether polymer compound such as polyethylene oxide or a crosslinked product containing polyethylene oxide, an ester polymer compound such as polymethacrylate, and an acrylate polymer compound. Or may be copolymerized. As the inorganic solid electrolyte, lithium nitride, lithium iodide, or the like can be used.

Further, in the above embodiments and examples, a coin-type secondary battery has been specifically described, but the present invention is not limited to the use of an exterior member such as a cylindrical type, a button type, a square type, or a laminate film. two have a shape battery or Ru can be similarly applied to secondary batteries having other structures, such as winding structure.

It is sectional drawing showing the structure of the secondary battery which concerns on one embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Outer can, 12 ... Positive electrode, 12A ... Positive electrode collector, 12B ... Positive electrode active material layer, 13 ... Outer cup, 14 ... Negative electrode, 14A ... Negative electrode collector, 14B ... Negative electrode active material layer, 15 ... Separator, 16 ... gasket.

Claims (5)

Bei example a cathode, an anode,
The negative electrode includes at least one member selected from the group consisting of simple elements, alloys, and compounds of metal elements or metalloid elements capable of inserting and extracting lithium (Li),
The electrolyte includes a first lithium salt made of at least one of lithium salts represented by chemical formulas (1) to (5) , LiPF ₆ , LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ , An electrolyte solution containing LiN (C ₂ F ₅ SO ₂ ) ₂ , LiClO ₄ and a second lithium salt made of at least one of LiAsF ₆ ;
The concentration of the first lithium salt in the electrolytic solution is 0.01 mol / dm ³ or more and less than 0.5 mol / dm ^3.
Lithium ion secondary battery.

2. The lithium ion secondary battery according to claim 1, wherein the first lithium salt is at least one of lithium salts represented by the chemical formulas (1) to (3). 2. The lithium ion secondary battery according to claim 1, wherein a concentration of the first lithium salt in the electrolytic solution is 0.05 mol / dm ³ or more and 0.45 mol / dm ³ or less . The negative electrode of silicon (Si) or a single tin (Sn), at least one kind selected from the group consisting of alloys and compounds, lithium ion secondary battery according to claim 1, wherein. The positive electrode has the chemical formula Li _x MIO ₂ or Li _y MIIPO ₄ (MI and MII represent one or more transition metals including nickel (Ni), and x and y are 0.05 ≦ x ≦ 1.10. .05 is a value in the range of ≦ y ≦ 1.10.) contains a lithium-containing compound represented by a lithium ion secondary battery according to claim 1, wherein.

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