JP3309719B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of battery characteristics after high-temperature storage by improving an electrolyte.

[0002]

2. Description of the Related Art As electronic devices have become smaller and lighter, there has been an increasing demand for smaller and lighter batteries as power sources. Among them, non-aqueous electrolyte secondary batteries using lithium metal for the negative electrode have been greatly expected because of their large theoretical energy density. However,
When lithium metal is used for the negative electrode, dendritic lithium (dendrites) are generated during charging, and the dendrites grow and penetrate the separator during repeated charge / discharge of the battery, causing an internal short circuit in the battery. Further, in such extreme cases, there are problems such as a rapid rise in the temperature of the battery, and the problems have not been completely solved until now.

As a means for solving this problem, various attempts have been made to use not only lithium metal alone but also an alloy of lithium and a low-melting metal such as aluminum, lead, indium, bismuth, and cadmium as a negative electrode. During charging and discharging of the battery, the alloy becomes finer as the occlusion and release of lithium are repeated, and this fine alloy penetrates through the separator and, like the lithium metal anode, the battery is short-circuited internally and the temperature rises sharply. It is hard to say that it was done.

On the other hand, as a solution to the above problem,
A battery using carbon for the negative electrode has been proposed. The battery using carbon as the negative electrode of the non-aqueous electrolyte secondary battery was 1986.
27th Battery Symposium Abstracts P. 97 or 198
7th 28th Battery Symposium Abstracts P. 201, it is said that lithium ion as an active material is supported on carbon of the negative electrode by an electrochemical method outside the battery system, and the positive electrode active material is vanadium pentoxide, manganese dioxide, Alternatively, chromium oxide is used. Above all, batteries using vanadium pentoxide for the positive electrode and carbon for the negative electrode have been put to practical use as coin-type batteries mainly used for memory backup applications and the like. In this battery, as a method of supporting lithium on the negative electrode, a method of electrically contacting lithium metal and carbon in the battery is adopted.

Recently, the summary of the 33rd Battery Symposium in 1992, P.S. At 83, the positive electrode is Li
A cylindrical battery using CoO ₂ and carbon as the negative electrode has been proposed, and it is reported that the capacity of 70% or more of the initial capacity was maintained even after 1200 cycles in deep charge / discharge. At present, this battery system has been put to practical use as a 4V class lithium ion secondary battery. The characteristics of this battery system are
The charge / discharge reaction of the negative electrode is a reaction of occluding and releasing lithium ions into the carbon of the negative electrode. Lithium does not deposit on the negative electrode upon charging, and therefore, no dendrites are generated, so that good cycle characteristics are obtained. Another point is that carbon does not undergo miniaturization like a lithium alloy even when lithium occlusion and release reactions are repeated, and a rapid temperature rise of the battery does not occur. Another feature of this battery system is that a lithium-containing composite oxide called LiCoO ₂ is used for the positive electrode, and lithium ions, which are the negative electrode active material, are supplied from the positive electrode. There is no need to carry lithium ions.

As the positive electrode active material of the 4V class lithium ion secondary battery, not only LiCoO ₂ but also LiNi
O ₂ , LiMn ₂ O ₄ , LiFeO ₂ , or these C
O, Ni, Mn, and Fe in which other metal elements are partially substituted have been studied. In addition, in addition to the fact that so-called amorphous carbon, such as coke, pyrolytic carbon, or low-temperature calcined products of various organic substances, was initially studied as carbon as a negative electrode material, lithium ion as an active material was also considered. In recent years, attention has been paid to highly crystalline carbon, so-called graphite-based carbon, from the viewpoint of the ability to occlude and release carbon.

Japanese Patent Application Laid-Open No. 4-115457 states that a graphitic material comprising graphitizable spherical particles as a negative electrode exhibits excellent characteristics. C ₆ Li, an intercalation compound of graphite and lithium ions, has been known for a long time. When lithium ions are occluded and released (intercalated, deintercalated) electrochemically, the theoretical capacity is 1 g of carbon. It shows a very large value of 372 mAh. Nevertheless, the Journal of Ele was not initially adopted as the negative electrode of lithium ion secondary batteries.
As reported in the ctrochemical Society 117, No2 (1970) p. 222, when propylene carbonate, which is widely used as one of the solvent components of the electrolyte in the nonaqueous electrolyte primary battery, is used, the solvent molecule becomes Decomposes on the surface of graphite,
The intercalation reaction of lithium ions into graphite was not performed smoothly. On the other hand, the abstracts of the 59th Electrochemical Conference of 1992, P.A. 2
38, it is reported that this problem can be solved by mainly using ethylene carbonate as a solvent component of the electrolytic solution. Since then, natural graphite and various artificial graphites have been studied as negative electrodes of lithium ion secondary batteries, and graphite-based negative electrodes have become the mainstream at present.

One of the characteristics of these negative electrode carbon materials is that irreversible capacity is generated only at the initial charge and discharge. The irreversible capacity is determined by the gas generation reaction, lithium ions remaining in the negative electrode crystal, and the negative electrode between the negative electrode surface and the electrolyte. It is said to occur for some reactions, such as reactions. Therefore, the negative electrode carbon material is stabilized by forming a dense film containing lithium ions on its surface.

[0009]

However, the dense film formed on the carbon surface of the negative electrode hinders the movement of lithium ions due to charge and discharge. In particular, when the battery is left at a high temperature, the reaction between the negative electrode carbon material and the electrolyte proceeds, and a dense film is formed on the surface of the negative electrode carbon. Would. In particular, since the movement of lithium ions is remarkably inhibited by a dense film on the surface of the negative electrode, there is a problem that the capacity deterioration during high-rate discharge is increased.

An object of the present invention is to provide a non-aqueous electrolyte secondary battery having a small capacity deterioration even when left at high temperatures by improving the electrolyte. is there.

[0011]

SUMMARY OF THE INVENTION In order to solve this problem, the present invention provides a non-aqueous electrolyte secondary battery using a lithium-containing oxide for the positive electrode, a carbon material for the negative electrode, and a separator therebetween. In addition, a mixture in which another alkali metal salt is dissolved in an electrolytic solution in which a lithium salt is dissolved in an organic solvent is used as an electrolytic solution.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the nonaqueous electrolyte secondary battery of the present invention, a carbon material capable of occluding and releasing lithium ions is used for the negative electrode. The carbon material that formed the film and occluded lithium and the electrolyte were stabilized. Furthermore, when the battery is left at a high temperature, a film on the surface of the negative electrode carbon material grows and hinders the movement of lithium ions during charging and discharging.
However, by dissolving alkali metal ions having a larger ion radius than lithium ions, for example, sodium ions and potassium ions in the electrolytic solution, a part of the dense film containing lithium ions formed on the surface of the negative electrode carbon material is formed. Replaces with sodium or potassium ions to form a porous film, relieves lithium ions from migrating during charge and discharge, and has a small capacity deterioration even when the battery is left at high temperatures. In this case, the capacity deterioration is small.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. In the examples, a cylindrical battery was constructed and evaluated.

Embodiment 1 FIG. 1 is a longitudinal sectional view of a cylindrical battery used in this embodiment. In the figure, reference numeral 1 denotes a case in which a stainless steel plate made of an organic electrolytic solution is processed, 2 denotes a sealing plate provided with a safety valve, and 3 denotes an insulating packing. Reference numeral 4 denotes an electrode group, in which a positive electrode and a negative electrode are spirally wound a plurality of times via a separator and housed in the battery case 1. Then, a positive electrode lead 5 is pulled out from the positive electrode to form a sealing plate 2.
It is connected to the. A negative electrode lead 6 is drawn out from the negative electrode and connected to the bottom of the battery case 1. Reference numeral 7 denotes an insulating ring provided on the upper and lower portions of the electrode plate group 4, respectively. Hereinafter, the positive and negative electrode plates will be described in detail.

The cathode mixes Li ₂ CO ₃ and Co ₃ O ₄ ,
100 parts by weight of LiCoO ₂ powder synthesized by firing at 900 ° C. for 10 hours, 3 parts by weight of acetylene black and 7 parts by weight of a fluororesin binder were mixed, and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. did. This paste was applied on both sides of an aluminum foil having a thickness of 0.03 mm, dried and rolled to obtain an electrode plate having a thickness of 0.18 mm, a width of 38 mm and a length of 240 mm.

The negative electrode was made of artificial graphite powder (12.6 μm, d0
02 = 3.36 °, Lc = 1000 °, surface area by BET method = 9.4 ° / g), 100 parts by weight, 5 parts by weight of styrene / butadiene rubber were mixed and suspended in an aqueous solution of carboxymethyl cellulose to form a paste. . This paste is applied to both sides of a copper foil having a thickness of 0.02 mm, dried, and then rolled to obtain a thickness of 0.19 mm, a width of 40 mm, and a length of 280 m.
m electrode plate.

An aluminum lead is attached to the positive electrode plate, and a nickel lead is attached to the negative electrode plate. The leads are spirally wound through a polyethylene porous film having a thickness of 0.025 mm, a width of 45 mm, and a length of 730 mm. And a battery case having a diameter of 14.0 mm and a height of 50 mm.
1 mol / liter of a solvent obtained by mixing ethylene carbonate (hereinafter abbreviated as EC), diethyl carbonate (hereinafter abbreviated as DEC), and methyl propionate (hereinafter abbreviated as MP) in a volume ratio of 30:50:20 is used as an electrolytic solution. LiPF6
Was used, and after injecting the solution, sealing was performed to produce a battery, which was designated as battery A. Here, the battery was used at a nominal voltage of 3.6 V and a nominal capacity of 550 mAh.

Further, as shown in Table 1, N in the electrolyte
except for changing the concentration of the APF ₆ is A battery was produced in the same manner as described above, as shown below, these batteries B~F
And

[0019]

[Table 1]

Using these batteries A to F, a high rate discharge test (1 C: 1 hour rate) was performed. The charge and discharge conditions are as follows: charge current 110 mA at an environmental temperature of 20 ° C., charge end voltage 4.
The test was performed at 2 V, a discharge current of 550 mA, and a discharge end voltage of 3.0 V. Thereafter, the battery was charged under the above-mentioned charging conditions, left at an environmental temperature of 60 ° C. for 30 days, and then repeated three cycles under the above-mentioned charging and discharging conditions. FIG. 2 shows the test results.

FIG. 2 shows that NaPF ₆ was added to the electrolyte at a concentration of 0.1%.
By dissolving M, the discharge capacity after standing at a high temperature (60 ° C., 30 days) was increased, and the recovery rate was almost constant even when the NaPF ₆ concentration was increased. On the other hand, when the discharge capacity before leaving is increased when the NaPF ₆ concentration is increased,
The capacity tended to decrease, and the capacity decreased particularly at 0.7 M or more. This is considered to be because the viscosity of the electrolyte increased due to the increase in the salt concentration.

Therefore, the concentration of NaPF _{6 to be} added is 0.1.
It is preferably from 1M to 0.5M.

(Example 2) Next, LiPF ₆ was used as an electrolyte as a lithium salt, and NaBF ₄ , NaClO ₄ , KPF ₆ , KBF ₄ , and KClO were used as other alkali metal salts.
₄ and the concentration of each alkali metal salt was 0.3 M
Batteries were prepared as shown in (Table 2) in the same manner as in the above example except that the above-mentioned conditions were used.

Using these batteries G to K, a high-rate discharge test was performed before and after standing at high temperature (60 ° C., 30 days) in the same manner as in (Example 1). (Table 2) shows the results of these tests.

[0025]

[Table 2]

From Table 2, it can be seen that in the batteries G to K than in the battery A, the capacity retention ratio after the high temperature storage (60 ° C., 30 days) was larger in all of the batteries regardless of the difference in the anion type of the sodium salt. The same effect was obtained when a potassium salt was used instead of the sodium salt.

This is because lithium ions in the dense film formed on the surface of the negative electrode are replaced by sodium ions or potassium ions having a large ionic radius, and the film changes into a porous film. It is considered that the movement was performed smoothly.

In this embodiment, the lithium salt is Li
Although PF ₆ was used, a lithium salt with another anion such as LiB
The same effect can be obtained with F ₄ and LiClO ₄ .

In this embodiment, LiCoO is used as a positive electrode.
₂ was used, but Li, a compound containing lithium ions, was used.
NiO ₂ and LiMn ₂ O ₄ , and also Co, Ni,
Alternatively, part of Mn is replaced with another element, for example, Co, Mn, F
Similar effects can be obtained even when a composite compound substituted with e, Ni, or the like is used. The composite oxide is, for example, a carbonate or oxide of lithium or cobalt as a raw material,
It can be easily obtained by mixing and firing according to the desired composition, and of course, it can be synthesized in the same manner when other raw materials are used. Usually, the firing temperature is set between 650 ° C and 1200 ° C.

In this embodiment, artificial graphite powder was used as the negative electrode. However, natural graphite, artificial graphite, natural graphite having a surface coated with a carbon material having a lower crystallinity than those, or graphitizable carbon was used. Similar effects can be obtained even when the material is graphitized at a high temperature. On the other hand, such as coke, pyrolytic carbon, or baked products of various organic substances,
Similar effects can be obtained even when so-called amorphous carbon is used.

As the electrolytic solution, a conventionally known electrolytic solution can be used. However, when a graphite material is used for the negative electrode, propylene carbonate (hereinafter abbreviated as PC) tends to undergo a decomposition reaction at the time of charging and generate gas. For this reason, it is not preferable, and it can be said that ethylene carbonate (EC), which is a similar cyclic carbonate used in this example, is suitable because it hardly involves side reactions such as PC. However, EC has a very high melting point and is a solid at ordinary temperature, so that it is difficult to use it as a single solvent. Therefore, it is preferable to use a mixed solvent with an aliphatic carboxylic acid ester such as 1,2-dimethoxyethane or diethyl carbonate (DEC) which is a solvent having a low melting point and a low viscosity, and further, methyl propionate (MP). .

[0032]

As is apparent from the above description, the present invention provides a non-aqueous electrolyte secondary battery using a lithium-containing composite oxide for the positive electrode and a carbon material for the negative electrode by dissolving a lithium salt in an organic solvent. By using a mixture obtained by dissolving another alkali metal salt in an electrolyte solution as an electrolyte solution and adjusting the concentration of the alkali metal salt to be mixed to 0.1 M or more and 0.5 M or less, the anode carbon can be removed even when left at a high temperature. Provide a non-aqueous electrolyte secondary battery with excellent high-rate discharge characteristics after being left at high temperatures because the dense film formed on the surface can be made into a porous film and the movement of lithium ions due to discharge can be smooth. be able to.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

FIG. 2 is a diagram showing the discharge capacity before and after high-temperature storage and the capacity retention ratio after before and after high-temperature storage for mixed NaPF ₆ concentrations.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode group 5 Positive electrode lead 6 Negative electrode lead 7 Insulation ring

──────────────────────────────────────────────────の Continuing on the front page (72) Takashi Takeuchi 1006 Kazuma Kadoma, Kazuma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. In-company (56) References JP-A-6-267589 (JP, A) JP-A-8-171936 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 10/40

Claims (3)

(57) [Claims] 1. A non-aqueous electrolyte secondary battery comprising a lithium-containing composite oxide for a positive electrode and a carbon material for a negative electrode with a separator interposed therebetween, wherein a lithium salt is dissolved in an organic solvent. Natori
Salt or potassium salt at a concentration of 0.1M or more and 0.5M
A non-aqueous electrolyte secondary battery dissolved as follows. 2. The lithium salt is LiPF ₆ , LiBF ₄ , Li
The non-aqueous electrolyte secondary battery according to claim 1, selected from ClO ₄ . 3. The sodium salt is NaPF ₆ , NaBF ₄ ,
NaClO ₄ , the potassium salt is KP
F _6, KBF _4, the non-aqueous electrolyte secondary battery 請 Motomeko 1, wherein selected from the group consisting of KClO _4.