JPH11204145A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery with high capacity high energy density, superior in charging/discharging cycle characteristics with less irreversible capacity even in quick charge/discharge. SOLUTION: At least one compound selected from among the compounds of barium, bismuth, magnesium, titanium and calcium is added to the inside of a lithium secondary battery comprising a positive electrode 1, a negative electrode 2, a nonaqueous electrolyte and a separator 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having a large discharge capacity and a high output density and excellent cycle characteristics.

[0002]

2. Description of the Related Art As a negative electrode of a lithium secondary battery, lithium metal and lithium alloy have been conventionally used.
These batteries have a short cycle life due to a short circuit between the positive and negative electrodes due to the precipitation of resinous lithium (dendrites).
Therefore, in order to compensate for the deterioration, lithium equivalent of three times the battery capacity is required, and there is a disadvantage that the energy density is low. In recent years, studies to use carbon particles for the negative electrode have been active in order to solve these problems. When using this type of negative electrode, especially graphitized graphite, for example, using lithium cobalt oxide for the positive electrode, the battery voltage becomes flat, and it is superior in terms of capacity when used in portable equipment using single cells. There is. However, when high-rate charging is performed using this graphite, the doping voltage at the time of charging becomes close to 0 V, which is a competitive reaction with the precipitation of lithium. Therefore, for example, Japanese Patent Application Laid-Open
The structure in No. 299073 is such that the surface of the highly crystalline carbon particles forming the core is coated with a film containing a Group VIII metal element, and a carbon composite formed by coating carbon on the film is used as an electrode material. It is stated that the carbon particles having a turbostratic structure on the surface assist lithium intercalation, and that the charge / discharge capacity and charge / discharge rate are significantly improved due to the large surface area of the electrode. But,
The irreversible capacity of carbon in the negative electrode carbon particles increased, and as a result, the energy density was not yet sufficient.

[0003]

As described above, when carbon particles and a composite material are used as a negative electrode, there is a problem that the irreversible capacity of carbon is increased and that it is difficult to manufacture an electrode. The present invention solves this problem by adding at least one or more of a barium compound, a bismuth compound, a magnesium compound, a titanium compound, and a calcium compound to the inside of a battery, thereby achieving high capacity and high capacity even during rapid charge and discharge. It is an object of the present invention to provide a lithium secondary battery having an energy density and an excellent charge / discharge cycle characteristic with a small irreversible capacity.

[0004]

Means for Solving the Problems When carbon is considered as the negative electrode active material, the reaction mainly occurs in the occlusion and release (intercalation, deintercalation) of lithium into carbon particles, but the reaction governs the reaction. As one of the factors,
It was found that the state of the film formed between the electrolyte and the carbon surface was involved. For example, as typified by lithium metal as the negative electrode active material, a dense and highly ion-conductive film has excellent battery characteristics, while a thick, low-ion conductive film has a rate characteristic and cycle characteristics. It is known that the characteristics are bad. In that case, it is reported that the former is a coating such as lithium carbonate or lithium oxide, and the latter is a coating such as lithium fluoride. The same is conceivable for a coating formed on the carbon surface. That is, one of the factors that hinder the rate characteristics of the carbon particles is formation of a film having low ionic conductivity such as lithium fluoride on the surface of the carbon particles. The present inventors have conducted various studies to solve the problems with respect to this coating film. As a result, by adding at least one of a barium compound, a bismuth compound, a magnesium compound, a titanium compound, and a calcium compound to the inside of the battery, It has been found that it traps the fluorine anions present therein and suppresses the fluorine anions from coming to the interface between the electrolyte and the carbon particles.

[0005] The compound to be added to the inside of the battery may be any compound as long as it can be combined with barium, bismuth, magnesium, titanium, and calcium, such as halides, oxides, anhydrous oxides, sulfates, nitrates, and phosphates. Examples thereof include, but are not limited to, salts.
Preferred are anhydrides such as phosphates and oxides.

[0006] Addition locations of each compound include, for example, addition to the inside of the electrode, a separator other than the electrode,
Insulating plate, winding core, winding slip prevention pin, tape to stop outer circumference,
Any location inside the battery, such as the filler, the inside of the outer can, the current collector, the current collecting terminal, the safety valve, etc. is possible. Examples of the method of addition include a method of physically mixing or adhering and holding each compound by mechanofusion, a vapor deposition method, a sputtering method, a wet reduction method, an electrochemical reduction method, a gas phase reduction gas treatment method, a laser ablation, ion A method of chemically and electrochemically treating after adhering and holding on the surface by a vapor deposition method or the like may be used, but is not limited thereto.

The amount of the compound to be added is 30% by weight or less, preferably 10% by weight or less of the total weight of the active material. Further, the particle size of the compound held and adhered is desirably 1 μm or less.

As the negative electrode active material used in the present invention, carbon particles capable of occluding and releasing lithium are preferable. In particular, the plane spacing (d002) by X-ray diffraction method is 3.354 to 3.36.
Carbon particles having a crystal size (Lc) of 200 ° or more in the C-axis direction at 9 ° are preferable because a high capacity can be obtained.

It is desirable that the carbon particles used in the present invention have an average particle size of 100 μm or less. In obtaining a predetermined shape, a pulverizer or a classifier is used to obtain a powder. For example, a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air jet mill, a sieve, and the like are used. At the time of pulverization, wet pulverization in which an organic solvent such as water or hexane coexists can be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like is used as needed in both dry and wet methods.

Examples of the negative electrode material that can be used in the present invention include lithium metals, lithium alloys, and the like, chalcogen compounds, and organic compounds containing lithium such as methyllithium. In addition, it is possible to insert lithium in advance into the carbon particles used in the present invention by using lithium metal, a lithium alloy, and an organic compound containing lithium in combination.

[0011] In the battery to which the compound of the present invention is added,
When the positive electrode and the negative electrode are manufactured, a conductive agent, a binder, a filler, and the like can be added as the electrode mixture. Any conductive material may be used as long as it does not adversely affect battery performance. Usually, natural graphite (scale graphite,
Flake graphite, earthy graphite, etc.), artificial graphite, carbon black, acetylene black, Ketjen black, carbon whiskers, carbon fiber and metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, conductive ceramics A conductive material such as a material can be included as one type or a mixture thereof. Among them, acetylene black and Ketjen black are preferably used in combination. The addition amount is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight.

In a battery to which the compound of the present invention is added,
When producing the positive electrode and the negative electrode, at least the surface layer portion of the positive and negative electrode active material powders can be treated with a substance other than the compound of the present invention. For example, gold, silver, carbon,
It is possible to coat a material with good electronic conductivity such as nickel and copper, or a material with good ion conductivity such as lithium carbonate, boron glass and solid electrolyte by applying techniques such as plating, sintering, mechanofusion, and vapor deposition. No.

As the binder, there are usually tetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, ethylene-propylene diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoro rubber, carboxymethyl cellulose and the like. Such as a thermoplastic resin, a polymer having rubber elasticity, a polysaccharide and the like can be used alone or as a mixture of two or more. Further, it is desirable that a binder having a functional group that reacts with lithium, such as a polysaccharide, has its functional group deactivated by, for example, methylation. The addition amount is preferably 1 to 50% by weight, particularly 2 to 50% by weight.
30% by weight is preferred.

As the filler, any material may be used as long as it does not adversely affect battery performance. Usually, olefin polymers such as polypropylene and polyethylene, aerosil, zeolite, glass, carbon and the like are used. The addition amount of the filler is preferably 0 to 30% by weight.

The current collector of the electrode active material may be any current collector that does not adversely affect the battery. For example, as the current collector for the positive electrode, in addition to aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, and the like, for the purpose of improving adhesiveness, conductivity, and oxidation resistance, aluminum And the surface of copper or the like treated with carbon, nickel, titanium, silver or the like can be used. As the current collector for the negative electrode, besides copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., the adhesiveness, conductivity, and oxidation resistance are improved. The surface of copper or the like may be treated with carbon, nickel, titanium, silver or the like for the purpose of the above. These materials can be oxidized on the surface. As these shapes, in addition to the foil shape, a film shape, a sheet shape, a net shape, a punched or expanded material, a lath body, a porous body, a foamed body, a formed body of a fiber group, and the like are used. The thickness is not particularly limited, but a thickness of 1 to 500 μm is used.

In a battery to which the compound of the present invention is added,
As the positive electrode active material, MnO ₂ , MoO ₃ , V ₂ O
₅ , Lix CoO ₂ , Lix NiO ₂ , Lix Mn ₂ O
₄ or the like, TiS ₂ , MoS ₂ , NbSe ₃
Metallic chalcogenides, such as, graphite intercalation compounds such as polyacene, polyparaphenylene, polypyrrole, and polyaniline, and various substances capable of absorbing and releasing anions and alkali metal ions such as conductive polymers can be used.

In particular, when used as a battery to which the compound of the present invention is added, from the viewpoint of high energy density, V
₂ O ₅ , MnO ₂ , Li _x CoO ₂ , Li _x NiO ₂ ,
Those having an electrode potential of 3 to ₄ V, such as Li _x Mn ₂ O _4, are desirable. In particular, Li _x CoO ₂ , Li _x NiO ₂ , L
i _x Mn ₂ O lithium-containing transition metal oxides such as ₄ are preferred, for the lithium-containing transition metal oxide elution of the metal is greater manganese content under the influence of HF in the electrolyte solution (hydrofluoric acid), the effect is remarkable is there.

As the electrolyte, for example, an organic electrolyte, a polymer solid electrolyte, an inorganic solid electrolyte, a molten salt, or the like can be used, and among them, the organic electrolyte is preferable. As the organic solvent of the organic electrolyte, propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, esters such as γ-butyrolactone, tetrahydrofuran, substituted tetrahydrofuran such as 2-methyltetrahydrofuran, dioxolane, Ethers such as diethyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, dimethylsulfoxide, sulfolane, methylsulfolane,
Acetonitrile, methyl formate, methyl acetate, N-methylpyrrolidone, dimethylformamide and the like can be mentioned, and these can be used alone or as a mixed solvent. Further, as supporting electrolyte salts, LiClO ₄ , LiP
F ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ ,
LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ )
₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) and the like. On the other hand, as the polymer solid electrolyte, those obtained by dissolving the above-described supporting electrolyte salt in a polymer such as polyethylene oxide or a crosslinked product thereof, or polyphosphazene or a crosslinked product thereof can be used. Further, inorganic solid electrolytes such as Li ₃ N and LiI can be used. That is, any non-aqueous electrolyte having lithium ion conductivity may be used.

As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. Olefin polymers such as polypropylene and polyethylene, glass fiber, and organic solvent resistant and hydrophobic
Polyvinylidene fluoride, polytetrafluoroethylene,
Sheets made of cellulose, etc., microporous membranes, nonwoven fabrics,
Cloth is used. The pore diameter of the separator is in a range generally used for batteries, for example, 0.01 to 10 μm
It is. The same applies to the thickness, which is in the range generally used for batteries, for example, 5 to 300 μm.
It is.

It is also possible to include a filler in the separator. As the filler in this case, any material may be used as long as it does not adversely affect battery characteristics, but a barium compound, a bismuth compound, a magnesium compound, a titanium compound, and a calcium compound are preferable.

In the battery of the present invention, when the electrode is spirally wound, it is possible to insert a winding core and a winding prevention pin at the center. The core may be any material such as stainless steel (SUS) that does not adversely affect battery characteristics, but a sintered body of a barium compound, a bismuth compound, a magnesium compound, a titanium compound, and a calcium compound is preferable.

The reason why the charge / discharge characteristics, particularly the rate characteristics, are improved is not necessarily clear, but is considered as follows. Generally, the battery often contains various impurities that do not contribute to the charge and discharge of the battery. For example, when LiPF ₆ is used for the electrolyte, it is conceivable that the salt itself introduces impurities or reacts with a trace amount of water contained in the battery or in the solvent to generate hydrofluoric acid.
On the surface of the carbon particles during lithium occlusion, a film having high ion conductivity such as lithium carbonate is formed between the electrolytic solution and the carbon particles, but when an acid such as hydrofluoric acid is present during or after the formation of the film. , Resulting in lithium halides with low ionic conductivity. Lithium halide generated at the interface between the carbon particles and the electrolytic solution is considered to be one of the causes of preventing the absorption and release of lithium and, as a result, reducing the rate characteristics of the negative electrode. We thought that this problem could be solved by keeping hydrofluoric acid away from the interface between the carbon particles and the electrolyte, and tried to add a compound that did not dissolve in the electrolyte as fluoride due to hydrofluoric acid reaction. Was. Ac, Am, Ba, B, assuming that the fluoride hardly dissolves in the electrolytic solution
i, Ca, Ce, Cm, Dy, Er, Ga, Ir, M
g, Nd, Np, Ru, Sc, Sm, Tb, Th, T
i, Tm, V, Y, and Yb fluorides. Anhydrous compounds such as these oxides are added to the inside of the battery to occlude the halogen anions, especially the fluorine anions, or to prevent the fluorine compounds from attracting hydrofluoric acid to the interface between the carbon particles and the electrolyte due to its ionic effect. In particular, it was confirmed that the compound of barium, bismuth, magnesium, titanium, and calcium improved the rate characteristics of the negative electrode, and the present invention was achieved.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments.

(Example 1) First, a method of adding to the negative electrode will be described. Artificial graphite (particle size: 6 μm) and calcium orthophosphate were mixed at a weight ratio of 97: 3, and using this powder A as a negative electrode active material, a cylindrical nonaqueous electrolyte battery shown in FIG. 1 was prototyped as follows. First, 10% by weight of polyvinylidene fluoride (PVDF) was added as a binder, and a kneading liquid was prepared in an organic solvent. This coating solution was applied to both surfaces of a copper foil current collector having a thickness of 10 μm to prepare a negative electrode sheet A. In a similar manner, instead of calcium orthophosphate, artificial graphite (particle diameter: 6 μm) and barium orthophosphate, bismuth orthophosphate, magnesium orthophosphate, and titanium dioxide are respectively mixed to obtain powders B to E, respectively.
The obtained sheets were designated as negative electrode sheets B to E.

The positive electrode is made of LiMn ₂ O ₄ as a positive electrode active material.
And flaky graphite and PVDF powder in a weight ratio of 85:10:
The mixture was mixed at 5, and then a solvent was added and kneaded well to prepare a coating solution. This coating solution was applied to both sides of a 20 μm-thick aluminum foil current collector to prepare a positive electrode sheet.

The positive electrode sheet was cut into a width of 54 mm, the negative sheets A to E were cut into a width of 56 mm, and aluminum and nickel lead plates were spot-welded to the ends of the sheets. Drying under reduced pressure was performed for hours. Thereafter, as shown in FIG. 1, the positive electrode sheet (1), a polyethylene microporous membrane (3) as a separator, a negative electrode sheet (2), and a separator are stacked in this order, and are spirally wound by a winding machine. Turned. An insulating plate (9) made of an insulating polyethylene film is provided on the upper and lower portions of the wound body, and the wound body is housed in a nickel-plated iron cylindrical battery can (4). Was spot welded. In this battery can, a volume ratio of ethylene carbonate and diethyl carbonate of 1:
An electrolytic solution in which 1 mol / l of LiPF ₆ was dissolved in the mixed solvent of No. 1 was injected, and a stainless steel anti-slip pin (3) having a diameter of 3 mm was inserted into the center of the wound body. A battery lid (5) having a safety valve (7) and an aluminum lead plate were spot-welded and caulked via a gasket to produce a cylindrical battery having a diameter of 18 mm and a height of 65 mm. Batteries using the negative electrode sheets A to E are referred to as batteries A to E, respectively.

Comparative Example A battery was produced in the same manner as in Example 1 except that only artificial graphite (particle size: 6 μm) was used instead of powder A as the negative electrode active material. The obtained battery is referred to as Comparative Battery R.

(Example 2) LiMn ₂ as a positive electrode active material
LiMn ₂ instead of O ₄ O ₄ and calcium phosphate 9
A battery was produced in the same manner as in Comparative Example except that a powder mixed in a weight ratio of 7: 3 was used. The obtained battery was replaced with Battery F
And

Example 3 A battery was fabricated in the same manner as in the comparative example except that a sintered rod having a diameter of 3 mm containing calcium orthophosphate as a main component was used instead of using the stainless steel winding prevention pin (8). did. The obtained battery is referred to as Battery G.

(Example 4) In the same manner as in the comparative example except that an insulating polyethylene-based film (9) was applied to the upper and lower portions of the wound body and an insulating sintered plate containing calcium orthophosphate as a main component was used. To produce a battery. The obtained battery is referred to as a battery H.

Example 5 A battery was produced in the same manner as in the comparative example, except that a microporous film made of polypropylene using calcium orthophosphate as a filler was used instead of the microporous film made of polyethylene (3) as a separator. The obtained battery is referred to as Battery I.

Example 6 Instead of using a solution obtained by dissolving 1 mol / l of LiPF _{6 in} a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 as an electrolytic solution, the above electrolytic solution and calcium orthophosphate were mixed. A battery was produced in the same manner as in the comparative example, except that the liquid was injected while the calcium orthophosphate was floating. The obtained battery is referred to as Battery J.

Using these batteries A to J and comparative battery R, a charge / discharge test was performed. The charge and discharge rates were 0.2 A and 1.0 A, and the upper and lower limit voltages of the charge and discharge were 4.1 V and 2.7 V, respectively. Table 1 shows the results of the obtained discharge capacity at the fifth cycle. Further, the number of cycles when the discharge capacity was reduced to 60% of the initial value was measured as the cycle life.

[0034]

[Table 1]

When the batteries A to J of the present invention and the comparative battery R are compared, it is understood that the battery of the present invention is excellent in all respects in terms of discharge capacity, rate characteristics, and cycle life.
Although the reasons for these phenomena are not clear,
It is considered that the addition of a compound such as calcium to the inside of the battery affects the state of the electrolyte, particularly the interface between the electrolyte and the material surface. That is, in the case of a conventional additive-free battery, a coating on the carbon surface generated by the insertion and extraction of lithium reacts with a small amount of hydrogen halide present inside the battery to generate lithium halide, and the ion conductivity is reduced. It is considered that the discharge characteristics, particularly the rapid charge / discharge characteristics, were reduced due to the decrease in the degree. In addition, by charging the negatively accepted negative electrode material,
It is considered that lithium is deposited as a metal at or above the acceptance capacity, resulting in an increase in irreversible capacity and a decrease in cycle life.

On the other hand, in the case of the battery of the present invention to which a compound such as calcium is added, a small amount of hydrogen halide present inside the battery is trapped before arriving at the interface between the carbon particles and the electrolyte, and the ionic effect of the halide is prevented. Thus, it is considered that there is a function of protecting the coating of carbon particles from hydrogen halide. The effect is not only kneading to the electrode,
It can be seen that the same effect can be obtained by adding to the gap of the battery, the insulating plate, and the separator.

It was found that the battery of the present invention was superior in charge and discharge efficiency in each cycle as compared with the comparative battery. That is, the discharge capacity and the cycle characteristics could be improved without lowering the charge / discharge efficiency by adding a calcium compound or the like.

In the above embodiment, the addition of a calcium compound or the like to the negative electrode and the addition of the calcium compound to the positive electrode, the pin for preventing winding deviation, the insulating plate, the separator, and the electrolytic solution have been described. Other compounds and addition methods were also confirmed. Incidentally, the present invention is a starting material of the active material described in the above examples, a production method, a positive electrode, a negative electrode,
It is not limited to electrolyte, separator, battery shape, and the like.

[0039]

Since the present invention is configured as described above, the decrease in ionic conductivity at the negative electrode active material interface is small,
As a result, rapid charge / discharge characteristics are improved, and cycle characteristics are also improved. In addition, since the treatment is simple and inexpensive, it is an excellent method of reforming the negative electrode material, and the resulting battery has high capacity, high energy density, and low irreversible capacity even in rapid charge and discharge. FIG.

[Brief description of the drawings]

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode sheet 2 Negative electrode sheet 3 Separator 4 Battery can 5 Battery lid 6 Gasket 7 Safety valve 8 Winding prevention pin 9 Insulation plate

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01M 4/62 H01M 4/62 Z

Claims (5)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, wherein at least one of a barium compound, a bismuth compound, a magnesium compound, a titanium compound, and a calcium compound is added. Lithium secondary battery. 2. The lithium secondary battery according to claim 1, wherein the compound is a phosphate or an anhydride of an oxide. 3. The active material of the negative electrode is a carbon particle,
2. The carbon particle according to claim 1, wherein the plane distance (d002) by X-ray diffraction is 3.354 to 3.369 °, and the crystal size (Lc) in the C-axis direction is 200 ° or more. Lithium secondary battery. 4. The lithium secondary battery according to claim 1, wherein the active material of the positive electrode is a lithium-containing transition metal oxide. 5. The lithium secondary battery according to claim 4, wherein the lithium-containing transition metal oxide contains manganese.