JP2014154220A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery capable of suppressing a short circuit due to dendrite-like metal lithium even when the metal lithium is formed, without complicating the manufacturing process thereof.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode including a positive electrode active material layer capable of absorbing/desorbing lithium ions; a negative electrode including a negative electrode active material layer capable of absorbing/desorbing lithium ions; a separator; a nonaqueous electrolyte; and an insulating coating layer provided on the surface of the positive electrode active material layer and including inorganic fine particles capable of absorbing/desorbing lithium.

Description

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

  In recent years, non-aqueous electrolyte secondary batteries typified by lithium ion batteries, which have high energy density, low self-discharge, and good cycle performance, have attracted attention as power sources for portable devices such as mobile phones and laptop computers, and electric vehicles. ing.

  The current mainstream of lithium ion batteries is a small lithium ion battery mainly for mobile phones having a positive electrode potential of up to about 4.2 V. However, the lithium ion battery is currently used in the automobile field such as electric cars, hybrid cars, and plug-in hybrid cars. The application of large or large lithium ion batteries is being studied, and some have been put into practical use. In particular, in an electric vehicle, all devices are driven by electric power supplied from a lithium ion battery installed. For this reason, excellent safety and reliability are strongly demanded for lithium ion batteries together with high energy density.

  When the lithium ion battery is overcharged, dendritic metallic lithium is deposited and grows on the negative electrode. Here, when the tip of the metallic lithium contacts the positive electrode, a micro short circuit occurs, leading to a thermal escape that causes the battery temperature to rise at a stretch.

  On the other hand, Patent Document 1 (Japanese Patent Application Laid-Open No. 2006-12788) discloses a technique that aims to suppress an internal short circuit of a lithium ion battery. That is, Patent Document 1 discloses a normal separator that includes a positive electrode made of composite lithium oxide, a negative electrode made of a material capable of reversibly occluding and releasing lithium, and a non-aqueous electrolyte, and separating the positive electrode and the negative electrode. A lithium secondary battery comprising: at least one of a positive electrode and a negative electrode has a porous film containing an inorganic oxide filler and a binder on a surface facing the other electrode, and the electrode surface is partially A lithium ion battery characterized by having a convex portion has been proposed.

  In the lithium ion battery, a gap is generated between the convex portion of the porous film or electrode and the opposing electrode. Since this gap can hold more electrolytic solution than other parts, the exchange of ions is activated. As a result, an overcharge reaction proceeds intensively at the site during overcharge, and conductive chemical species are deposited before the overcharge as a whole battery progresses so much. For this reason, it is possible to suppress the progress of overcharging and avoid the problem of overheating.

JP 2006-12788 A

  However, in the above-mentioned Patent Document 1, not only the process for forming the porous film or the convex portion of the electrode is complicated, but a short circuit occurs when the conductive chemical species contacts the electrode. Therefore, an object of the present invention is to reduce the short circuit due to the metallic lithium without complicating the production process and even when the dendritic metallic lithium as the conductive chemical species is generated. The object is to provide a water electrolyte secondary battery.

  As a result of intensive studies by the present inventors to solve the above-described problems, the surface of the positive electrode active material layer is coated with an oxide that is insulative and capable of occluding lithium, so that a negative electrode can be obtained during overcharge. Even when the metal lithium deposited in (1) reaches the positive electrode, the metal lithium is occluded in the coating layer, so that the thermal escape due to the short circuit is suppressed, and the present invention has been completed.

That is, the present invention includes a positive electrode including a positive electrode active material layer capable of occluding and releasing lithium ions, a negative electrode including a negative electrode active material layer capable of occluding and releasing lithium ions, a separator, a non-aqueous electrolyte, There is provided a non-aqueous electrolyte secondary battery comprising an insulating coating layer containing inorganic fine particles capable of occluding lithium provided on a surface of a positive electrode active material layer.

  In the nonaqueous electrolyte secondary battery of the present invention, the inorganic fine particles are preferably lithium-containing titanium oxide, and more preferably anatase-type titanium dioxide or lithium titanate.

  ADVANTAGE OF THE INVENTION According to this invention, the nonaqueous electrolyte secondary battery which has the outstanding safety | security and reliability which can suppress the short circuit by dendritic metal lithium can be provided.

It is a conceptual diagram which shows the partial structure and effect of the nonaqueous electrolyte secondary battery of one Embodiment of this invention.

  Hereinafter, preferred embodiments of the nonaqueous electrolyte secondary battery of the present invention will be described in detail with reference to the drawings, but the present invention is not limited to these. In the following description, the same or corresponding parts are denoted by the same reference numerals, and duplicate descriptions may be omitted. The drawings are for conceptual description of the present invention. The dimensions of each component and their ratio may differ from the actual ones.

  FIG. 1 is a conceptual diagram showing a partial structure and effects of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. The nonaqueous electrolyte secondary battery of the present invention includes a positive electrode 2 including a positive electrode active material layer capable of occluding and releasing lithium ions, a negative electrode 4 including a negative electrode active material layer capable of occluding and releasing lithium ions, and a separator 6. And a non-aqueous electrolyte and an insulating coating layer 8 provided on the surface of the positive electrode active material layer and containing inorganic fine particles capable of occluding lithium.

  As the positive electrode 2, various conventionally known positive electrodes can be used as long as the effects of the present invention are not impaired, but a structure in which a positive electrode active material layer is formed on both surfaces of a positive electrode current collector made of an aluminum foil is used. Is preferred. The positive electrode active material layer can be obtained by applying a paste-like positive electrode mixture on both sides of the current collector, drying, and pressing.

The positive electrode active material layer formed by being applied to both surfaces of the positive electrode current collector contains a positive electrode active material. As a positive electrode active material, a conventionally well-known thing can be used in the field | area of this invention, For example, lithium containing transition metal oxides, such as lithium cobaltate, lithium nickelate, and lithium manganate, are mentioned. A part of the transition metal of the lithium-containing transition metal oxide may be substituted with another element. More specifically, for example, layered rock salt type oxides such as LiNi _1/3 Mn _1/3 Co _1/3 O ₂ and LiMPO ₄ (M = Mn, Fe, Co and / or Ni) and the like can be mentioned. Moreover, the surface of the lithium-containing transition metal oxide particles may be coated with another element. One type of positive electrode active material may be used alone, or two or more types of positive electrode active materials may be used in combination.

  The positive electrode active material layer is formed by applying and supporting a positive electrode mixture containing a positive electrode active material and a small amount of a binder (for example, polyethylene terephthalate (PTFE), polyvinylidene fluoride (PVDF)) on a positive electrode current collector. can do. A small amount of a conductive material (for example, acetylene black, ketjen black, graphite) or a dispersion medium (for example, N-methyl-2-pyrrolidone (NMP)) may be added to the positive electrode mixture.

As the negative electrode 4, various conventionally known negative electrodes can be used as long as the effects of the present invention are not impaired, but a structure in which a negative electrode active material layer is formed on both surfaces of a negative electrode current collector made of copper foil is used. Is preferred. The negative electrode mixture for forming the negative electrode active material layer is, for example, a mixture of a graphite powder part having a specific surface area of 1 m ² / g as a negative electrode active material and a polyvinylidene fluoride part as a binder, and appropriately NMP Can be added to prepare a paste. And the negative electrode 4 can be obtained by apply | coating the said negative mix on both surfaces of a collector, drying, and pressing.

The negative electrode active material layer formed by being applied to both surfaces of the negative electrode current collector contains a negative electrode active material. As the negative electrode active material, those conventionally known in the field of the present invention can be used. For example, carbon materials (for example, graphite (graphite), natural graphite, artificial graphite, hard carbon), elements that can be alloyed with lithium (for example, Al, Si, Zn, Ge, Cd, Sn, Pb), silicon compound (eg, SiO _x (0 <x <2)), tin compound (eg, SnO), lithium metal, alloy (eg, Ni—Si alloy, Ti—) Si alloy), conversion materials such as Co ₃ O ₄ , and oxide materials such as Li ₄ Ti ₅ O ₁₂ and Li _1.1 V _0.9 O ₂ can be used. One type of negative electrode active material may be used alone, or two or more types of negative electrode active materials may be used in combination.

  The negative electrode active material may be directly deposited on the negative electrode current collector, but a small amount of a binder (for example, PVDF, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid) may be used alone or in combination. The negative electrode mixture can also be formed by being applied to and supported on the negative electrode current collector. Similarly to the positive electrode mixture of the positive electrode 2, a small amount of a conductive material or a dispersion medium (for example, NMP, water) may be added to the negative electrode mixture.

  As the separator 6, those conventionally known in the field of the present invention can be used, and examples thereof include a microporous film made of a polyolefin resin such as polyethylene and / or polypropylene. Among these, a single layer film composed of polyethylene, a multilayer film composed of a polyethylene layer and a polypropylene layer, and the like can also be used. Moreover, you may use what coated the inorganic substance etc. on the surface of the separator.

As the electrolyte salt contained in the non-aqueous electrolyte, those conventionally known in the technical field of the present invention can be used. For example, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiC (CF ₃ ) ₃ , LiC (C ₂ F ₅ ) ₃ , LiCF ₃ SO ₃ , LiCF ₃ CF ₂ SO ₃ , LiCF ₃ CF ₂ CF ₂ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF _{3) 2, LiN (COCF 3} ) 2, LiN (COCF 2 CF 3) 2, LiPF 3 (CF 2 CF 3) 3, LiB (C 2 O 4) 2, LiBF 2 (C 2 O 4), LiPF 2 (C ₂ O ₄ ) ₂ and LiPF ₄ (C ₂ O ₄ ) and the like can be mentioned, and these can be used alone or in combination of two or more. . From the viewpoint of conductivity, LiPF ₆ is suitable as the electrolyte salt, and other compounds such as LiBF ₄ can be mixed and used with LiPF ₆ as the main component.

  Further, as the non-aqueous solvent contained in the non-aqueous electrolyte, those conventionally known in the technical field of the present invention can be used, and examples thereof include cyclic carbonates, chain carbonates, and cyclic sulfones. For example, ethylene carbonate (EC), propylene carbonate (PC), cyclic carbonates such as butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC). ), Chain carbonates such as dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate and ethyl propionate, lactones such as γ-butyrolactone and γ-valerolactone, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), chain ethers such as ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, pro Pionitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, Examples include tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, dimethyl sulfoxide, and N-methyl-2-pyrrolidone. These may be used alone, but may be used in combination of two or more from the viewpoint of adjusting the conductivity and viscosity of the non-aqueous electrolyte. Of these, a mixed solvent of a cyclic carbonate and a chain carbonate or a mixed solvent of a cyclic carbonate, a chain carbonate, and an aliphatic carboxylic acid ester is preferable. For example, a 3: 7 mixed nonaqueous solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) is preferably used.

  The mixing ratio of the electrolyte salt with respect to the nonaqueous solvent in the nonaqueous electrolytic solution may be appropriately selected within a range where the effect of the nonaqueous electrolytic solution is obtained based on a conventionally known technique. For example, the amount of electrolyte salt dissolved in the non-aqueous solvent is not particularly limited, but is preferably 0.2 to 5 mol / liter, and more preferably 0.5 to 2.5 mol / liter, for example.

  Various additives can be added to the non-aqueous electrolyte for the purpose of improving the charge / discharge characteristics of the lithium ion battery. Examples of the additive include vinylene carbonate (VC) and vinyl ethylene carbonate (VEC). These additives form a good film on the positive electrode 2 and / or the negative electrode 4 and contribute to extending the life of the battery. In addition, cyclohexylbenzene (CHB), fluorobenzene, and the like can be added to improve the stability during overcharge.

  The insulating coating layer 8 containing inorganic fine particles capable of occluding lithium is provided on the surface of the positive electrode active material layer of the positive electrode 2. The insulating coating layer 8 is formed by mixing inorganic fine particles capable of occluding lithium with a binder to form a paste, and uniformly applying the paste to the surface of the positive electrode active material layer of the positive electrode 2 and drying it. be able to. The insulating coating layer 8 is preferably continuous, but may be partially discontinuous. Moreover, it is preferable to use lithium-containing titanium oxide for the inorganic fine particles capable of occluding lithium, and it is more preferable to use anatase type titanium dioxide or lithium titanate. Moreover, polyvinylidene fluoride etc. can be used for a binder.

The insulating coating layer 8 has an insulating property, but the AC resistance when a pressure of 1 kgf / cm ² is applied may be 100Ω or more, for example. When the lithium ion battery is overcharged, dendritic metallic lithium 12 precipitates and grows on the negative electrode 4 due to the movement of the lithium ions 10. Here, when the metal lithium 12 comes into contact with the positive electrode 2, a micro short-circuit occurs, leading to a thermal runaway in which the battery temperature rapidly increases.

  However, in the nonaqueous electrolyte secondary battery of the present invention, since the positive electrode 2 includes the insulating coating layer 8 containing inorganic fine particles capable of occluding lithium, the insulating property between the positive electrode 2 and the negative electrode 4 is high. Internal short circuit is suppressed. In addition, since the metal lithium 12 is occluded by the inorganic fine particles contained in the insulating coating layer 8, the metal lithium 12 itself causing the short circuit disappears, and the thermal escape can be more effectively suppressed.

As the inorganic fine particles capable of occluding lithium, lithium-containing titanium oxide such as lithium titanate or anatase-type titanium dioxide can be used. Examples of the lithium-containing titanium oxide include a composition formula Li ₄ Ti _5-x Me _x O ₁₂ (x = 0 to 1, Me is Mg, Al, Ti, Zr, Mo, Nb, Sr, Ni, Co, Mn , At least one of W) can be used. When lithium titanate is used as the inorganic fine particles capable of occluding lithium, the insulating coating layer 8 does not distort because lithium titanate does not cause expansion and contraction of the crystal lattice even when metallic lithium is occluded. Moreover, since anatase type titanium dioxide and lithium titanate are inactive in the positive electrode potential region, no trouble or the like occurs during normal use of the lithium ion battery.

  Moreover, although the lithium ion battery of this embodiment is used with the control circuit which controls charge, the said control circuit may each be mounted | worn with each lithium ion battery, and is 1 with respect to several lithium ion battery. Individual control circuits may be mounted.

  As mentioned above, although the typical example of the nonaqueous electrolyte secondary battery of this invention was demonstrated, this invention is not necessarily limited only to these, A various design change is within the range of the technical idea of this invention. All such design changes are included in the present invention. Hereinafter, the non-aqueous electrolyte secondary battery of the present invention will be described more specifically with reference to examples, but it goes without saying that the present invention is not limited to such examples.

Example 1: Use of anatase type titanium dioxide
(1) Preparation of positive electrode A positive electrode active material is prepared by applying a positive electrode paste using N-methylpyrrolidone as a solvent to both sides of a strip-shaped aluminum current collector (thickness: 15 μm), drying, and pressing with a roll press. A layer was made. Here, in addition to the solvent, the positive electrode paste contains LiCo _0.33 Ni _0.33 Mn _0.33 O ₂ as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride as a binder. The mass ratio of the positive electrode active material, the conductive material, and the binder in terms of solid matter is 92: 4: 4.

  Next, on the surface of the positive electrode active material layer, anatase-type titanium dioxide that is inorganic fine particles and polyvinylidene fluoride that is a binder are uniformly coated with an inorganic filler paste having a mass ratio of 94: 6 in terms of solid matter, The paste was dried under reduced pressure at room temperature for 24 hours to obtain a positive electrode having an insulating coating layer.

(2) Production of negative electrode A negative electrode paste containing graphite as a negative electrode active material, carboxymethyl cellulose as a binder, and styrene butadiene rubber in a solid-converted mass ratio of 97: 1.5: 1.5. A negative electrode active material layer was formed on both sides of a 10 μm copper foil and dried at 80 ° C. to remove moisture. Next, the negative electrode active material layer after moisture removal was pressure-formed with a roller press at room temperature, and then dried under reduced pressure for 24 hours at room temperature to completely remove moisture in the negative electrode active material layer, and both sides of the copper foil A plate-like negative electrode having a negative electrode active material layer was obtained.

(3) Preparation of non-aqueous electrolyte solution LiPF ₆ as an electrolyte salt in a mixed solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) were mixed at an equal volume ratio, at a concentration of 1 mol / liter. By mixing, a non-aqueous electrolyte was obtained.

(4) Production of a lithium ion battery A pole group is produced by winding a belt-like positive electrode and a belt-like negative electrode flatly through a belt-like polyethylene microporous membrane as a separator, and accommodated in a rectangular battery case which is an exterior container, A non-aqueous electrolyte was injected and impregnated, and the lithium ion battery 1 having a design capacity of 20 Ah was manufactured through an initial charge / discharge cycle process.

<< Example 2: Use of lithium titanate >>
A lithium ion battery 2 was produced in the same manner as in Example 1 except that lithium titanate was used as the inorganic fine particles contained in the insulating coating layer.

<< Comparative Example 1: Use of alumina >>
A comparative lithium ion battery 1 was produced in the same manner as in Example 1 except that alumina was used as the inorganic fine particles contained in the insulating coating layer.

<< Comparative Example 2: Use of rutile titanium dioxide >>
A comparative lithium ion battery 2 was produced in the same manner as in Example 1 except that rutile titanium dioxide was used as the inorganic fine particles contained in the insulating coating layer.

<< Comparative Example 3: Use of silicon oxide >>
A comparative lithium ion battery 3 was produced in the same manner as in Example 1 except that silicon oxide was used as the inorganic fine particles contained in the insulating coating layer.

<< Comparative Example 4: Use of zirconium oxide >>
A comparative lithium ion battery 4 was produced in the same manner as in Example 1 except that zirconium oxide was used as the inorganic fine particles contained in the insulating coating layer.

<< Comparative Example 5: Use of anatase type titanium dioxide (negative electrode coating) >>
A comparative lithium ion battery 5 was produced in the same manner as in Example 1 except that the insulating coating layer was formed on the negative electrode instead of the positive electrode.

<< Comparative Example 6: Use of lithium titanate (negative electrode coating) >>
A comparative lithium ion battery 6 was produced in the same manner as in Example 2 except that the insulating coating layer was formed on the negative electrode instead of the positive electrode.

<< Comparative Example 7: No insulation coating >>
A comparative lithium ion battery 7 was produced in the same manner as in Example 1 except that the insulating coating layer was not formed on the positive electrode and the negative electrode.

[Overcharge test]
An overcharge test was performed on the lithium ion batteries 1 and 2 and the comparative lithium ion batteries 1 to 7 obtained as described above. All the batteries were put into the end-of-charge state by constant current and constant voltage charging for 0.2 hours, 4.2 V, and 10 hours before the test. At a temperature of 25 ° C., a direct current of 8.4 V and 32 A was applied to each battery using a direct current power source capable of knowing the accumulated amount of electricity supplied. Table 1 shows the state of each lithium ion battery after the overcharge test.

  From the results shown in Table 1, it can be seen that thermal escape of the lithium ion battery is suppressed only when anatase-type titanium dioxide and lithium titanate are used as the inorganic fine particles and an insulating coating layer is formed on the positive electrode.

2 ... Positive electrode 4 ... Negative electrode 6 ... Separator 8 ... Insulating coating layer 10 ... Lithium ion 12 ... Metallic lithium

Claims (3)

A positive electrode including a positive electrode active material layer capable of inserting and extracting lithium ions;
A negative electrode including a negative electrode active material layer capable of inserting and extracting lithium ions;
A separator;
A non-aqueous electrolyte,
An insulating coating layer containing inorganic fine particles capable of occluding lithium, provided on the surface of the positive electrode active material layer,
A non-aqueous electrolyte secondary battery. The inorganic fine particle is a lithium-containing titanium oxide;
The nonaqueous electrolyte secondary battery according to claim 1. The inorganic fine particles are anatase-type titanium dioxide or lithium titanate;
The nonaqueous electrolyte secondary battery according to claim 1.