JP2006120564A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve load characteristics at discharging of a nonaqueous electrolyte secondary battery which uses phosphoric acid iron lithium as the positive electrode active material. <P>SOLUTION: The nonaqueous electrolyte secondary battery comprises a positive electrode, which contains phosphoric acid iron lithium as positive electrode active material, negative electrode, nonaqueous electrolyte, and battery can 1 for housing the positive electrode, negative electrode, and nonaqueous electrolyte. Since the heating at battery discharging is accumulated in the battery can 1, the external surface of the battery can 1 is covered with a heat insulating material 2. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  Currently, non-aqueous electrolyte secondary batteries that use a non-aqueous electrolyte and charge and discharge by moving lithium ions between a positive electrode and a negative electrode are used as high-energy density energy batteries.

In such a non-aqueous electrolyte secondary battery, LiCoO ₂ is generally used for the positive electrode, and lithium metal, a lithium alloy, or a carbon material capable of occluding and releasing lithium is used for the negative electrode. A solution in which an electrolyte made of a lithium salt such as LiBF ₄ or LiPF ₆ is dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate is used.

However, Co has limited reserves and is a scarce resource, which increases production costs. In addition, in the case of a battery using LiCoO ₂ , there is a problem that the thermal stability is lowered when the battery in a charged state becomes a high temperature that cannot be considered in a normal use state.

For this reason, utilization of LiMn ₂ O ₄ , LiNiO ₂ or the like as a positive electrode material replacing LiCoO ₂ has been studied. However, LiMn ₂ O ₄ cannot be expected to have a sufficient discharge capacity, and has problems such as dissolution of manganese when the battery temperature increases. Further, LiNiO ₂ has problems such as a low discharge voltage.

Therefore, in recent years, olivine type lithium phosphate such as LiFePO ₄ has attracted attention as a positive electrode material replacing LiCoO ₂ .

The olivine-type lithium phosphate is a lithium composite compound represented by a general formula LiMPO ₄ or the like (M is at least one element selected from Co, Ni, Mn, and Fe), and the metal element M that is a nucleus. The operating voltage varies depending on the type. Therefore, the battery voltage can be arbitrarily set by selecting M, and the theoretical capacity is relatively high at about 140 mAh / g to 170 mAh / g, so that there is an advantage that the battery capacity per unit mass can be increased. . Furthermore, iron can be selected as M in the general formula. Since iron is produced in a large amount and is inexpensive, there is an advantage that the production cost can be greatly reduced by using iron.

  However, there are still problems to be solved in order to use olivine type lithium phosphate as a positive electrode active material of a non-aqueous electrolyte secondary battery. In particular, the following matters are serious problems. That is, olivine-type lithium phosphate has a problem that the lithium desorption / insertion reaction during battery charging / discharging is slow, and the electronic conductivity is very low compared with lithium cobaltate, lithium nickelate, lithium manganate, and the like. . For this reason, the battery using olivine type lithium phosphate as the positive electrode active material has a problem that the polarization is increased particularly during high-rate discharge, and the battery characteristics are remarkably deteriorated.

In order to solve the above problem, Patent Document 1 proposes to use LiFePO ₄ as a positive electrode active material, in which the primary particle size is as small as 3.1 μm or less and the specific surface area is sufficiently large. By reducing the particle size, the reaction area can be increased, and the diffusion distance of Li is shortened. In addition, the contact area with the conductive agent is increased, and the electronic conductivity of the positive electrode active material is improved. Although these load effects are improved by these effects, since a positive electrode active material having a small particle diameter is used, there arises a new problem that the packing density of the positive electrode is lowered and the energy density of the battery is also lowered.
JP 2002-110162 A

  An object of the present invention is to provide a non-aqueous electrolyte secondary battery using a lithium iron phosphate as a positive electrode active material and having excellent load characteristics during discharge.

  The present invention is a non-aqueous electrolyte secondary battery comprising a positive electrode containing lithium iron phosphate as a positive electrode active material, a negative electrode, a non-aqueous electrolyte, and a battery outer body containing the positive electrode, the negative electrode, and the non-aqueous electrolyte, In order to accumulate heat generated during battery discharge in the battery exterior body, the outer surface of the battery exterior body is covered with a heat insulating material.

  According to the present invention, the load characteristics during discharge can be improved by covering the outer surface of the battery outer package with a heat insulating material. As described above, in a battery using lithium iron phosphate as a positive electrode active material, there is a problem in that the lithium desorption / insertion reaction is slow at room temperature (about 25 ° C.). However, lithium iron phosphate increases the diffusion rate of lithium in the active material at a high temperature of about 40 ° C. to 60 ° C., and increases the rate of lithium deinsertion reaction during charge and discharge. That is, in a battery using lithium iron phosphate as a positive electrode active material, load characteristics can be improved by charging and discharging at a high temperature.

  In the present invention, since the outer surface of the battery outer body is covered with a heat insulating material, the heat generated when the battery is discharged can be accumulated in the battery outer body, thereby increasing the temperature inside the battery. For this reason, high-rate discharge characteristics can be improved, and load characteristics during discharge can be improved.

Moreover, since lithium iron phosphate is excellent in thermal stability as compared with LiCoO ₂ and LiMn ₂ O ₄ , even if it is used at a high temperature, there is no particular problem.

  The heat insulating material used in the present invention is not particularly limited as long as it can accumulate heat generated during battery discharge in the battery outer body by covering the outer surface of the battery outer body. Cotton-like heat insulating materials such as ceramic wool and glass wool are preferably used. As the ceramic wool, wool made of alumina / silica fibers is preferably used. Other heat insulating materials include heat insulating materials using pearlite, diatomaceous earth, alumina powder, etc., heat insulating materials using asbestos, etc., heat insulating materials using foam materials such as foam glass, foam concrete, foam styrene, and urethane foam, Examples thereof include a heat insulating material using laminated glass. Moreover, the heat insulating material which formed the vacuum structure in the outer surface of the battery exterior body, and gave the heat insulation effect by this vacuum structure may be sufficient. The thermal conductivity of the heat insulating material is preferably 0.08 W / mK or less.

  According to the present invention, when the outer surface of the battery outer package is covered with a heat insulating material, the unit cell may be covered with the heat insulating material, or the assembled battery or the entire battery pack may be covered with the heat insulating material. Even when the assembled battery or the battery pack is entirely covered with a heat insulating material, the same effect as in the case of the unit cell can be obtained. Moreover, you may use it as an assembled battery or a battery pack combining the cell covered with the heat insulating material.

Lithium iron phosphate used as the positive electrode active material in the present invention include, for example, the general formula _{Li x Fe 1- (d + t} + q + r) D d T t Q q R r (PO 4) ( wherein, D is A divalent metal, selected from Mg, Ni, Co, Zn, and Cu; T is a trivalent metal, selected from Al, Ti, Cr, Fe, Mn, Ga, Zn, and V; Q is a tetravalent metal, selected from Ti, Ge, Sn, and V, R is a pentavalent metal, selected from V, Nb, and Ta, and x, d, t, q, and r Satisfying 0 ≦ x ≦ 1, 0 ≦ d, t, q, r ≦ 1) can be used. Examples of such lithium iron phosphate include Li _0.90 Ti _0.05 Nb _0.05 Fe _0.30 Co _0.30 Mn _0.30 PO ₄ , and typical examples include LiFePO ₄ .

  Lithium iron phosphate has a low electronic conductivity. Therefore, in order to increase the electronic conductivity, the surface of the lithium iron phosphate particles may be coated with carbon or treated with carbon attached. Good. Moreover, you may use what substituted some lithium of lithium iron phosphate with the transition metal.

The lithium iron phosphate used in the present invention may have a controlled particle size. As the particle diameter, both the median diameter (R _median ) and the mode diameter (R _mode ) measured with a laser diffraction particle size distribution measuring apparatus are preferably 10 μm or less, and more preferably 5 μm or less. By controlling the particle diameter in this way, the diffusion distance of lithium accompanying charging / discharging in the particle can be controlled, the resistance accompanying lithium insertion / extraction can be reduced, and the electrode characteristics can be improved. In addition, the contact area between the lithium iron phosphate particles and the current collector can be increased.

  In the present invention, lithium iron phosphate is included as the positive electrode active material, but other materials may be mixed and used as the positive electrode active material.

  In the present invention, the positive electrode can be generally produced by forming a mixture layer on the positive electrode current collector. The mixture layer can be formed by applying the mixture slurry onto the positive electrode current collector and then drying. Moreover, electroconductive powder can be mixed with a mixture layer. By mixing the conductive powder, a conductive network made of the conductive powder can be formed around the active material particles, so that the current collecting property in the electrode can be further improved. As the conductive powder, conductive carbon powder is preferably used, but a conductive metal oxide or the like can also be used. The amount of the conductive powder added is preferably 10% by weight or less of the total weight with the active material. When there is too much addition amount of electroconductive powder, since the quantity of an active material material will decrease relatively, the charging / discharging capacity | capacitance of an electrode will become small.

  The mixture layer is preferably placed on the current collector and dried, and then rolled. Such rolling can be performed using a rolling roller or a press. By performing rolling, the active material density in the electrode is improved, and the volume energy density can be improved accordingly. Moreover, since the contact area of a positive electrode active material and electroconductive powder increases by rolling, electroconductivity improves and load characteristics can further be improved.

  The negative electrode in the present invention is not particularly limited as long as it can be used for a nonaqueous electrolyte secondary battery. Examples of the negative electrode active material include carbon materials that can store and release lithium, metals and alloys such as Si and Sn that can store lithium by alloying with lithium, and lithium metal.

  Examples of the nonaqueous electrolyte solvent in the present invention include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, and the like. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, etc., and those in which some or all of these hydroxyl groups are fluorinated can be used, such as trifluoropropylene carbonate and fluoroethyl carbonate. Is mentioned. Examples of chain carbonic acid esters include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, etc., and those in which part or all of these hydrogens are fluorinated are also used. It is possible. Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, and γ-butyrolactone. As cyclic ethers, 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5 -Trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether and the like. As chain ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl Ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1 -Dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethy Such as glycol dimethyl and the like. Nitriles include acetonitrile and the like, and amides include dimethylformamide and the like. In particular, from the viewpoint of voltage stability, it is preferable to use cyclic carbonates such as ethylene carbonate and propylene carbonate, and chain carbonates such as dimethyl carbonate, diethyl carbonate and dipropyl carbonate.

The lithium salt dissolved in the non-aqueous electrolyte in the present invention is not particularly limited as long as it can be generally used for a non-aqueous electrolyte secondary battery. For example, LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (C _l F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (l and m are integers of 1 or more), LiC (C _p F _{2p + 1} SO ₂ ) ( C _q F _{2q + 1} SO ₂ ) (C _r F _{2r + 1} SO ₂ ) (p, q, and r are integers of 1 or more). Moreover, the difluoro (oxalato) lithium borate etc. which have the structure shown below are mentioned.

  The lithium salt may be used alone or in combination of two or more. The amount of lithium salt dissolved is preferably about 0.1 to 1.5 mol / liter, more preferably 0.5 to 1.5 mol / liter.

  According to the present invention, the load characteristics during discharge can be improved by covering the outer surface of the battery outer package with a heat insulating material.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to a following example, In the range which does not deviate from the summary, it can change suitably and can implement.

Example 1
[Preparation of positive electrode and negative electrode]
LiFePO ₄ as an active material, acetylene black as a conductive agent, and a binder are mixed at a weight ratio of 86: 5: 9, and this is added to N-methylpyrrolidone (NMP). A slurry was prepared. This slurry was applied onto an aluminum foil as a current collector and dried to produce a positive electrode.

  Graphite and a binder are mixed at a weight ratio of 98: 2, and water is added thereto to produce a slurry. The slurry is applied onto a copper foil as a current collector and dried. A negative electrode was produced.

  In addition, the capacity | capacitance of the negative electrode per unit area was designed so that it might be 1.1 times the capacity | capacitance of the positive electrode per unit area, and the positive electrode and the negative electrode were produced.

  The obtained positive electrode and negative electrode were each rolled with a rolling roller. After rolling, the positive electrode was cut to a width of 50 mm and a length of 750 mm, the negative electrode was cut to a width of 54 mm and a length of 800 mm, and a positive electrode lead was attached to the positive electrode and a negative electrode lead was attached to the negative electrode.

[Production of battery]
A battery was produced using the positive electrode and negative electrode produced as described above and a separator made of microporous polypropylene. A negative electrode, a separator, a positive electrode, and a separator were sequentially laminated in this order, and this was wound up in a plurality of turns to produce a battery element. After the insulating plates were attached to the upper and lower surfaces of the battery element, they were housed in the battery can, and the positive electrode lead was welded to the lid and the negative electrode lead was welded to the battery can.

Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 7, and LiPF ₆ was dissolved in this mixed solvent to a concentration of 1 mol / liter to obtain an electrolyte. This electrolytic solution was poured into the battery can so that the positive electrode, the separator, and the negative electrode were sufficiently wetted. Thereafter, the lid was caulked with a battery can through a gasket and sealed to produce a cylindrical battery having a diameter of 18 mm and a height of 65 mm.

Alumina / silica wool having a thickness of 10 mm, a length of 100 mm, and a width of 65 mm was wound around the outer periphery of the battery can of the battery produced as described above as a heat insulating material. As alumina / silica wool, trade names “Isowool” (Al ₂ O ₃ (alumina) 47.1% by weight, SiO ₂ (silica) 52.3% by weight, Fe ₂ O ₃ (Oxidation) Diiron) 0.2% by weight). This alumina / silica wool is obtained by electromelting a refined raw material and blowing it with a high-speed air stream to form a fiber. The thermal conductivity is 0.05 W / mK.

  1 and 2 are a cross-sectional view and a perspective view showing the lithium secondary battery in which the heat insulating material is wound as described above. As shown in FIGS. 1 and 2, a heat insulating material 2 is wrapped around and covered with the outer periphery of the battery can 1. In the cross-sectional view shown in FIG. 1, the inside of the battery can 1 is not shown.

(Comparative Example 1)
In Example 1, a lithium secondary battery was produced in the same manner as in Example 1 except that the outer periphery of the battery can was not covered with alumina / silica wool.

(Charge / discharge test)
The batteries of Example 1 and Comparative Example 1 were subjected to a charge / discharge test. Each battery was charged at a constant current (250 mA) to 4.0 V at room temperature, then stored for 4 hours in a constant temperature bath at 0 ° C., and discharged at a constant current (2000 mA) to 2.8 V while keeping the constant temperature bath at 0 ° C. did. The discharge curve at this time is shown in FIG.

  As is apparent from FIG. 3, according to the present invention, in the battery of Example 1 in which the outer periphery of the battery can was covered with a heat insulating material, a large discharge capacity was obtained at 0 ° C. discharge. This is excellent because the outer periphery of the battery can is covered with a heat insulating material, so that heat generated during the battery reaction is not released to the outside of the battery and can be efficiently used to increase the temperature of the battery and discharged at a high temperature. It seems that the high rate characteristic was shown.

  4 and 5 are diagrams showing a non-aqueous electrolyte secondary battery of another embodiment according to the present invention. 4 is a cross-sectional view, and FIG. 5 is a perspective view. As shown in FIGS. 4 and 5, in this embodiment, the unit cells 3 are combined to form an assembled battery 4, and the outer periphery of the assembled battery 4 is covered with a heat insulating material 5. Thus, even if the outer peripheral portion of the assembled battery is covered with a heat insulating material, the high-rate discharge characteristics can be improved as in the first embodiment.

Sectional drawing which shows the nonaqueous electrolyte secondary battery of one Example according to this invention. The perspective view which shows the nonaqueous electrolyte secondary battery of one Example according to this invention. The figure which shows the discharge curve of the nonaqueous electrolyte secondary battery of the Example according to this invention. Sectional drawing which shows the nonaqueous electrolyte secondary battery of the other Example according to this invention. The perspective view which shows the nonaqueous electrolyte secondary battery of the other Example according to this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Battery can 2 ... Heat insulation material 3 ... Single cell 4 ... Battery assembly 5 ... Heat insulation material

Claims (2)

In a non-aqueous electrolyte secondary battery comprising a positive electrode containing lithium iron phosphate as a positive electrode active material, a negative electrode, a non-aqueous electrolyte, and a battery outer body that houses the positive electrode, the negative electrode, and the non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery characterized in that heat generated during battery discharge is accumulated in the battery casing, and an outer surface of the battery casing is covered with a heat insulating material. The non-aqueous electrolyte secondary battery according to claim 1, wherein the heat insulating material is made of ceramic wool.

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