JP4296580B2 - Nonaqueous electrolyte lithium secondary battery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte lithium secondary battery, and more particularly to an active material used for a lithium secondary battery.
[0002]
[Prior art]
Lithium secondary batteries using non-aqueous electrolytes are small, lightweight, and excellent batteries with high energy density. To date, negative electrode active materials include metallic lithium, lithium alloys, or carbon that can store and release lithium. Material has been used. However, metallic lithium has a problem in terms of cycle life due to dendritic precipitation of lithium (dendrite) produced during charging, and this dendrite penetrates the separator and may cause internal short circuit or cause ignition. there were. In addition, the lithium alloy used for the purpose of preventing the dendrite generated during charging as described above has problems such as fine powdering of the negative electrode and falling off of the negative electrode active material when the charge amount is large, and a sufficient cycle life. Was not obtained.
[0003]
On the other hand, since the carbon material can occlude and release lithium, the above-described problems are remarkably improved, and it is effective for extending the life and safety of the lithium secondary battery. However, it is known that a lithium secondary battery using a carbon material as a negative electrode active material has a large self-discharge and poor storage characteristics compared to a lithium secondary battery using metallic lithium or a lithium alloy. Although details about the self-discharge mechanism are not necessarily clear, it is thought to be due to a side reaction between the carbon material and the electrolyte, and is considered to be a phenomenon caused by the carbon material.
[0004]
[Problems to be solved by the invention]
Carbon materials as negative electrode active materials used in conventional lithium ion batteries are occluded by the carbon material in the charged state of the battery because the potential for occlusion / release of lithium is close to that of metallic lithium. Lithium is highly active and is expected to cause a reaction that reduces electrolytes and the like. In addition, since the carbon material itself is composed of a skeleton consisting of only carbon elements, it reacts with a solvent or supporting salt, which is a compound containing oxygen, to form an oxide film as an intermediate layer at the carbon-electrolyte interface. It is expected to be easy to do.
[0005]
Therefore, in order to obtain a safe non-aqueous electrolyte lithium secondary battery with high energy density and low self-discharge storage characteristics, it is possible to occlude and release lithium and to prevent negative reactions with electrolytes and the like. Development of material materials is desired.
[0006]
The present invention is intended to solve the above-described problems, and uses a negative electrode active material that can occlude and release lithium and hardly causes a side reaction with an electrolyte, and has a high potential and high energy density. By combining with a substance, an object is to provide a safe lithium secondary battery having high energy density and excellent self-discharge and storage characteristics.
[0007]
[Means for Solving the Problems]
The present invention was made in view of the above problems, the _{Li x Ti 5/3-y} L y O 4 (L having a spinel structure as a main component of the anode active material including B, and Co or Zn Li _m MPO ₄ (M is 1) having an olivine structure as a main component of the positive electrode active material using an oxide fired body represented by the elements 4/3 ≦ x ≦ 7/3, 0 <y ≦ 5/3) A non-aqueous electrolyte lithium secondary battery using an oxide fired body expressed by 0 ≦ m ≦ 2.1) with at least one kind of transition metal.
[0008]
That is, the present inventors have found that the use of a negative electrode active material made of an oxide capable of occluding and releasing lithium has a great effect on improving the storage characteristics, and have led to the present invention.
[0009]
The negative electrode active material used in the battery of the present invention has a potential of about 1.5 V with respect to the potential of metallic lithium. On the other hand, the positive electrode active material used in the battery of the present invention has a potential of about 4.5 to 5.0 V with respect to the potential of metallic lithium. Therefore, by combining the negative electrode active material and the positive electrode active material, a lithium secondary battery that operates at a high voltage of about 3.0 to 3.5 V despite the high negative electrode potential is formed. Therefore, it is possible to provide a safe lithium secondary battery having high storage characteristics and excellent storage characteristics.
[0010]
The non-aqueous electrolyte used in the present invention is preferably an organic electrolyte in order to achieve a high energy density, but in order to obtain a lithium secondary battery with higher safety, a gel electrolyte, an organic solid electrolyte, Alternatively, an inorganic solid electrolyte may be used.
[0011]
The reason why the lithium secondary battery of the present invention is excellent in storage characteristics with less self-discharge is not necessarily clear, but is considered as follows. That is, Li _x Ti _{5 / 3-y} L _y O ₄ , which is the main component of the negative electrode active material, absorbs and releases lithium at a relatively high potential of about 1.5 V with respect to the potential of metallic lithium. In the charged state of the battery, the activity of lithium occluded in the molecular structure of the negative electrode active material is low, and the action of reducing the electrolyte and the like is considered to be very small. In addition, since the negative electrode active material is not a carbon material but an oxide, even if the solvent or the supporting salt constituting the electrolyte is a compound containing oxygen, it reacts with the intermediate and enters the interface with the electrolyte. The effect of forming an oxide film as a layer is also expected to be very small. Therefore, a side reaction on the electrode surface hardly occurs even in a charged state of the battery, so that a lithium secondary battery excellent in storage characteristics with little self-discharge can be obtained.
[0012]
Li _x Ti _{5 / 3-y} L _y O ₄ , which is the main component of the negative electrode active material used in the battery of the present invention, has a spinel structure, and L in the chemical formula is one or more elements other than Ti. Here, x is preferably 4/3 or more and 7/3 or less, and y is preferably 0 or more and less than 5/3.
[0013]
By substituting a part of Ti with the different element L, the electrode performance is further improved. The reason for this is not necessarily clear, but is considered as follows. Usually, titanium oxide is bulky, and when applied as an electrode, it tends to be an electrode having a large gap. In the present invention, this bulkiness can be suppressed by substituting a part of titanium with a different element, and it is considered that there is a function of smoothly transferring ions and electrons between particles. Substitution elements other than Ti are conceivable, but preferably, Be, B, C, Mg, Al, Si, P, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Examples include Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb, Te, Ba, La, Ta, W, Au, Hg, and Pb.
[0014]
Li _m MPO ₄ which is a main component of the positive electrode active material used in the battery of the present invention has an olivine structure, and M in the chemical formula is one or more transition metal elements. Here, m is 0 or more and 2.1 or less, and the transition metal M is Co, Ni, Fe, Mn, Cu, Zn, Cd, etc., and varies slightly depending on the main transition metal species. Japanese Patent Laid-Open No. 9-134724 describes that lithium is absorbed and released at a high potential of 4.5 to 5.0 V with respect to the dissolution / precipitation potential. For example, when the transition metal M is mainly Co, it is around 5.0V. In addition, in the case of a single phase containing only Fe, it may be around 3.5 V. Padhi, AK; Nanjundaswamy, KS; Goodenough, JB Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 144, 4, 1997, 1188-1194.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Examples of the non-aqueous electrolyte used in the battery of the present invention include a non-aqueous electrolyte and a solid electrolyte. Examples of the solid electrolyte for improving the safety of the lithium secondary battery include an inorganic solid electrolyte, an organic solid electrolyte, and an inorganic electrolyte. Organic solid electrolytes, molten salts, and the like can be used. Non-aqueous electrolytes include, as organic solvents, esters such as propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and γ-butyrolactone, substituted tetrahydrofuran such as tetrahydrofuran and 2-methyltetrahydrofuran, and dioxolane. , Ethers such as diethyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, dimethyl sulfoxide, sulfolane, methyl sulfolane, acetonitrile, methyl formate, methyl acetate, N-methylpyrrolidone, dimethylformamide, etc. can be used alone or as a mixed solvent, a supporting electrolyte salt _{thereto, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, Li It obtained by dissolving a _{F 3 SO 3, LiN (CF} 3 SO 2) 2 and the like. As the inorganic solid electrolyte, lithium nitride, halide, oxyacid salt, phosphorus sulfide compound, and the like are well known, and one or more of these can be used in combination. Among _{them, Li 3 N, LiI, Li} 5 NI 2, Li 3 N-LiI-LiOH, Li 4 SiO 4, Li 4 SiO 4 -LiI-LiOH, xLi 3 PO 4- (1-x) Li 4 SiO 4 , Li ₂ SiS ₃ , LiLaTiO ₃ , LiTi ₂ (PO ₄ ) _{3 and the} like, and similar compounds are effective. On the other hand, in the organic solid electrolyte, a polyethylene oxide derivative or a polymer containing at least the above derivative, a polypropylene oxide derivative or a polymer containing at least the above derivative, a polyphosphazene or a polyphosphazene derivative, a polymer containing an ion dissociation group, a phosphate ester polymer derivative, Furthermore, a polymer matrix material (gel electrolyte) in which the above non-aqueous electrolyte is contained in a polyvinylpyridine derivative, a bisphenol A derivative, polyacrylonitrile, polyvinylidene fluoride, fluorine rubber or the like is effective. Further, a method of using these inorganic solid electrolyte and organic solid electrolyte in combination is also effective.
[0016]
In the nonaqueous electrolyte lithium secondary battery of the present invention, a conductive agent, a binder, a filler, or the like can be added to the positive electrode or the negative electrode active material as an electrode mixture. As the conductive agent, any electronic conductive material that does not adversely affect battery performance may be used. Usually, natural graphite (flaky graphite, earthy graphite, etc.), artificial graphite, carbon black, acetylene black, ketjen black, carbon whisker, carbon fiber and metal (copper, nickel, iron, silver, gold, etc.) powder, metal Conductive materials such as fibers, metal vapor deposition, and conductive ceramic materials can be included as one type or a mixture thereof. The addition amount is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight.
[0017]
As the binder, heat such as tetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, ethylene-propylene diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoro rubber, carbomethoxy cellulose, etc. A plastic resin, a polymer having rubber elasticity, a polysaccharide, or the like can be used as one kind or a mixture of two or more kinds. In addition, it is desirable that a binder having a functional group that reacts with lithium, such as a polysaccharide, be deactivated by, for example, methylation. The addition amount is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight.
[0018]
As the filler, any material that does not adversely affect the battery performance may be used. Usually, olefin polymers such as polypropylene and polyethylene, aerosil, alumina, carbon and the like are used. The amount of filler added is preferably 0 to 30% by weight.
[0019]
A separator can be used in combination with a solid electrolyte. As the separator, an insulating thin film having excellent ion permeability and mechanical strength can be used. Sheets, microporous membranes, and nonwoven fabrics made from olefin polymers such as polypropylene and polyethylene, glass fibers, polyvinylidene fluoride, polytetrafluoroethylene, etc. are used because of their organic solvent resistance and hydrophobicity. The pore diameter of the separator is in a range generally used for batteries, and is, for example, 0.01 to 10 μm. The thickness is also the same, generally in the range used for batteries, for example, 5 to 300 μm.
[0020]
The positive / negative electrode active material used in the present invention is preferably a powder having an average particle size of 0.1 to 100 μm. In obtaining a predetermined shape, a pulverizer, a classifier, or a granulator is used to obtain a powder. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill, a sieve, or the like is used. At the time of pulverization, water or wet pulverization in the presence of an organic solvent such as hexane may be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like is used as necessary for both dry and wet methods.
[0021]
The current collector for the electrode active material may be any electronic conductor as long as it does not adversely affect the constructed battery. For example, in addition to aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymer, conductive glass, etc., the positive electrode is made of aluminum or copper for the purpose of improving adhesiveness, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. In the negative electrode, in addition to copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., the purpose of improving adhesiveness, conductivity, reduction resistance Thus, a material obtained by treating the surface of copper, aluminum or the like with carbon, nickel, titanium, silver or the like can be used. In particular, since the negative electrode active material has a potential of about 1.5 V with respect to lithium, aluminum can be used for the purpose of weight reduction. The surface of these materials can be oxidized. Regarding these shapes, films, sheets, nets, punched metals, expanded products, lath bodies, porous bodies, foams, formed bodies of fiber groups, and the like are used in addition to foils. The thickness is not particularly limited, but a thickness of 1 to 500 μm is used.
[0022]
Examples of the shape of the nonaqueous electrolyte lithium secondary battery in the present invention include a cylindrical shape, a square shape, a coin shape, a button shape, a flat shape, and a film shape. Especially, in order to achieve a high energy density, it is desirable to use a film-like battery shape using a gel electrolyte or a solid electrolyte.
[0023]
【Example】
FIG. 1 is a cross-sectional view of a lithium secondary battery according to the present invention. In the positive electrode, a positive electrode mixture 1 containing a positive electrode active material, ketjen black as a conductive agent, and polytetrafluoroethylene (PTFE) as a binder is pressure-bonded onto a positive electrode current collector 6 made of aluminum. In the negative electrode, a negative electrode mixture 2 containing a negative electrode active material, ketjen black as a conductive agent, and polytetrafluoroethylene (PTFE) as a binder is pressure-bonded onto a negative electrode current collector 7 made of copper. The separator 3 is made of a polyethylene microporous film and is interposed between the positive and negative electrodes. The electrolytic solution is a non-aqueous electrolytic solution in which 1 mol of lithium tetrafluoroborate (LiBF ₄ ) is dissolved in a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 2: 1. The positive electrode, the negative electrode, and the separator are impregnated. The peripheral edge of the aluminum positive electrode can 4 containing the positive electrode and the peripheral edge of the stainless steel negative electrode lid 5 containing the negative electrode are sealed with a gasket 8.
[0024]
Reference Example 1 Lithium titanate (Li _4/3 Ti ₅ ) obtained by mixing lithium hydroxide (LiOH · H ₂ O) and titanium oxide (TiO ₂ ) and heat-treating them at 900 ° C. in an oxidizing atmosphere. _{/ 3} O ₄ ) was used as the negative electrode active material. On the other hand, tribasic cobalt oxide (Co ₃ O ₄ ), diammonium phosphate ((NH ₄ ) ₂ HPO ₄ ), and lithium hydroxide (LiOH · H ₂ O) were mixed, and the mixture was mixed at 750 ° C. for 20 hours in a nitrogen stream. Cobalt-lithium phosphate (LiCoPO ₄ ) obtained by heat treatment was used as the positive electrode active material.
[0025]
The positive electrode was obtained as follows. After mixing in a ratio of 87 parts by weight of the positive electrode active material, 10 parts by weight of the conductive agent and 3 parts by weight of the binder to make the positive electrode mixture 1, it was punched into a disk shape having a diameter of 16 mm using a molding die, and 150 ° C. Were dried in a vacuum for 10 hours to produce a positive electrode having a thickness of 0.55 mm.
[0026]
The negative electrode was obtained as follows. The mixture was mixed at a ratio of 87 parts by weight of the negative electrode active material, 10 parts by weight of the conductive agent, and 3 parts by weight of the binder to form the negative electrode mixture 2, and then punched into a disk shape having a diameter of 16 mm using a molding die. Were dried in a vacuum for 10 hours to produce a negative electrode having a thickness of 0.35 mm.
[0027]
A coin-type lithium secondary battery having a diameter of 20 mm and a thickness of 16 mm was prepared using the separator and non-aqueous electrolyte described above. This is referred to as Reference Example Battery 1.
[0028]
(Example 2) Titanium obtained by mixing lithium hydroxide (LiOH.H ₂ O), titanium oxide (TiO ₂ ) and boric anhydride (B ₂ O ₃ ) and heat-treating them at 900 ° C. in an oxidizing atmosphere. A coin-type lithium secondary battery was produced in the same manner as in Reference Example 1 except that lithium acid (Li _4/3 Ti _4/3 B _1/3 O ₄ ) was used as the negative electrode active material. This is referred to as the battery 2 of the present invention.
[0029]
(Example 3) Lithium hydroxide (LiOH.H ₂ O), titanium oxide (TiO ₂ ), and tribasic cobalt oxide (Co ₃ O ₄ ) were mixed and heat-treated at 900 ° C. in an oxidizing atmosphere. was except that lithium titanate _{(Li 4/3 Ti 4/3 Co 1/3 O} 4) was used as a negative electrode active material to prepare a coin-type lithium secondary batteries in the same manner as in reference example 1. This is referred to as the present invention battery 3.
[0030]
(Example 4) Lithium hydroxide (LiOH · H ₂ O), titanium oxide (TiO ₂ ) and zinc nitrate (Zn (NO ₃ ) ₂ · 6H ₂ O) were mixed, and these were mixed at 900 ° C. in an oxidizing atmosphere. except for using lithium titanate obtained by heat treatment _{(Li 4/3 Ti 4/3 Zn 1/3 O} 4) as a negative electrode active material was prepared a coin-type lithium secondary batteries in the same manner as in reference example 1 . This is the battery 4 of the present invention.
[0031]
Comparative Example 1 Graphite is used as the negative electrode active material, and manganese carbonate (MnCO ₃ ) and lithium hydroxide (LiOH · H ₂ O) are mixed as the positive electrode active material, and this is mixed at 750 ° C. for 20 hours in a dry air atmosphere. except for using lithium manganate obtained by heat treatment (LiMn ₂ O ₄₎ were prepared in the same manner as in a coin-type lithium batteries as in reference example 1. This is referred to as comparative battery 1.
[0032]
Comparative Example 2 Reference Example 1 except that lithium titanate (Li _4/3 Ti _5/3 O ₄ ) was used as the negative electrode active material and lithium manganate (LiMn ₂ O ₄ ) was used as the positive electrode active material. We have created a coin-type lithium batteries in the same way. This is referred to as comparative battery 2.
[0033]
Using these batteries, a storage test was conducted using the self-discharge rate as an index. The storage test was performed at room temperature. Charging / discharging was performed at a rate of 10 hours, and in the reference battery 1 and the present invention batteries 2 to 4, the charge end voltage was 3.7V and the discharge end voltage was 3.0V. On the other hand, in the comparative battery 1, the charge end voltage was 4.2V and the discharge end voltage was 3.2V, and in the comparative battery 2, the charge end voltage was 2.8V and the discharge end voltage was 2.0V. Under the above charging / discharging conditions, the battery in the final charge state in the third cycle was stored for 30 days, and the discharge capacity after storage was measured to calculate the self-discharge rate according to the following formula.
[0034]
【formula】
[0035]
The storage test results are shown in Table 1 together with the battery voltage.
[0036]
[Table 1]
[0037]
As shown in the results of (Table 1), the reference battery 1, the inventive batteries 2 to 4 and the comparative battery 2 have a lower self-discharge rate during storage than the comparative battery 1, and improved storage characteristics. It is clear that However, the comparative battery 2 has a low battery discharge voltage of 3 V or less, and the storage characteristics are improved, but the energy density of the battery is lowered.
[0038]
In addition, it can be seen from the results of the batteries 2 to 4 of the present invention that the storage characteristics are slightly improved as compared with the reference battery 1 by substituting a part of titanium with another element. The reason for this is not necessarily clear, but it is considered that self-discharge can be suppressed by affecting the particle morphology after firing and reliably transferring ions and electrons inside the particles.
[0039]
In the above embodiment, B, Co, and Zn were used as the substitution elements, but the same effect has been confirmed for elements other than Ti. In addition, although Co is used as the transition metal M for the positive electrode active material, the same effect can be obtained by using a single element or a mixture of other transition metal elements.
[0040]
In addition, this invention is not limited to the starting material of the active material, the manufacturing method, the positive electrode, the negative electrode, the electrolyte, the separator etc. which were described in the said Example. The coin type battery is only for explaining the present invention, and the shape of the battery is not limited to this. Further, since the main negative electrode active material has a potential with respect to lithium of about 1.5 V, it is natural that the weight energy density is improved when aluminum is used instead of copper as a current collector.
[0041]
In addition, the conventional lithium secondary battery using a negative electrode active material in which lithium occlusion / release occurs in the vicinity of the potential of metallic lithium, when multiple series-parallel junctions are used, the environmental temperature and impedance between the batteries. Some of the batteries may be in a deeply charged state due to variations such as the above, and there is a risk of being led to a battery performance degradation or an unsafe mode due to the deposition of metallic lithium in the negative electrode. However, since the negative electrode active material of the present invention causes lithium insertion / release at a potential of about 1.5 V with respect to the potential of metallic lithium, when a plurality of lithium secondary batteries using the lithium secondary battery are connected in series and parallel, However, the above problem can be largely avoided.
[0042]
【The invention's effect】
Since the present invention is as described above, it is possible to provide a safe non-aqueous electrolyte lithium secondary battery having high energy density and excellent self-discharge and storage characteristics.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a battery of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Positive electrode mixture 2 Negative electrode mixture 3 Separator 4 Positive electrode can 5 Negative electrode lid 6 Positive electrode collector 7 Negative electrode collector 8 Gasket

Claims (1)

Li _x Ti _{5 / 3-y} L _y O ₄ having a spinel structure as a main component of the negative electrode active material (L is an element containing B, Co or Zn, 4/3 ≦ x ≦ 7/3, 0 <y ≦ 5 / 3 using an oxide sintered body represented by), the Li _m MPO _{4 (M} having the olivine structure as a main component of the positive electrode active material in one or more transition metals, expressed by 0 ≦ m ≦ 2.1) A non-aqueous electrolyte lithium secondary battery characterized by using an oxide fired body.