JP2013125661A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve storage characteristics of a battery by reducing dissolution of an electrolyte and an effect of cobalt and manganese eluted from a positive electrode with an increase in capacity of a nonaqueous electrolyte secondary battery.SOLUTION: A nonaqueous electrolyte secondary battery includes: a positive electrode including a positive electrode mixture layer which contains a positive electrode active material and is formed on a surface of a positive electrode collector; a negative electrode including a negative electrode mixture layer which contains a negative electrode active material and is formed on a surface of a negative electrode collector; a separator provided between the positive electrode and the negative electrode; and a nonaqueous electrolyte containing a solvent and a solute. An inorganic particle layer containing an inorganic particle and lithium fluoride is formed at least one of between the positive electrode and the separator and between the negative electrode and the separator.

Description

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

  In recent years, non-aqueous electrolyte secondary batteries used as driving power sources for mobile information terminals such as mobile phones, notebook computers, and PDAs have been required to have higher capacities. To meet this demand, non-aqueous electrolyte secondary batteries High capacity is being promoted.

  In addition, as the capacity of nonaqueous electrolyte secondary batteries is increased, elemental technologies that ensure the safety and reliability of nonaqueous electrolyte secondary batteries have been developed. For example, it has been proposed to ensure the safety of a nonaqueous electrolyte secondary battery by forming an inorganic particle layer on a specific electrode surface (see Patent Documents 1 and 2).

JP 2007-280171 A JP 2007-280918 A

  It is known that the capacity of the battery can be increased by charging the end-of-charge voltage of the nonaqueous electrolyte secondary battery to a voltage higher than the current 4.2 V (see Patent Documents 1 and 2).

  When the end-of-charge voltage of the nonaqueous electrolyte secondary battery is increased, the oxidizability of the charged positive electrode active material is increased. For this reason, not only the decomposition of the electrolytic solution is promoted, but also the stability of the crystal structure of the positive electrode active material delithiated by charging is lost. For example, a non-aqueous electrolyte secondary battery using a positive electrode active material such as lithium cobaltate, lithium manganate, or nickel-cobalt-manganese lithium composite oxide is charged to a voltage higher than the current 4.2V. When stored in a high temperature environment, cobalt and manganese are ionized and eluted from the positive electrode. This is because, for example, in the case of a layered positive electrode active material such as lithium cobaltate, the valence of cobalt increases by extracting lithium ions, but since tetravalent cobalt is unstable, the crystal structure is not stable. Cobalt ions elute from the crystals in an attempt to change to a stable structure. For example, when spinel type lithium manganate is used as the positive electrode active material, trivalent manganese ions are disproportionated and eluted as divalent manganese ions.

  By reducing these eluted elements at the negative electrode, cobalt and manganese are deposited on the negative electrode and the separator. Due to this precipitation, the internal resistance of the battery is increased, and as a result, there is a problem that the storage characteristics of the battery are deteriorated.

  The nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode in which a positive electrode mixture layer containing a positive electrode active material is formed on the surface of a positive electrode current collector, and a negative electrode containing a negative electrode active material on the surface of the negative electrode current collector A negative electrode on which a mixture layer is formed, a separator provided between the positive electrode and the negative electrode, and a water electrolyte containing a solvent and a solute, and between the positive electrode and the separator and between the negative electrode and the separator. At least one of these is formed with an inorganic particle layer including inorganic particles and lithium fluoride.

  According to the present invention, the storage characteristics of the nonaqueous electrolyte secondary battery can be improved.

  Hereinafter, the present invention will be described in detail based on specific examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications without departing from the scope of the present invention. It is a thing.

Example 1
The production of the positive electrode will be described.

  Limix Cobalt made by Primes using lithium cobaltate as a positive electrode active material, acetylene black as a carbon conductive agent, PVdF (polyvinylidene fluoride) as a binder, NMP (N-methyl-2-pyrrolidone) as a diluent solvent The mixture was stirred to prepare a positive electrode mixture slurry. The mass ratio of the positive electrode active material, the carbon conductive agent, and the binder was positive electrode active material: carbon conductive agent: binder = 95: 2.5: 2.5. Then, a positive electrode mixture slurry is applied to both surfaces of an aluminum foil as a positive electrode current collector, dried and rolled to produce a positive electrode mixture layer on the positive electrode current collector. did. The thickness of the positive electrode mixture layer was 127 μm. The packing density of the positive electrode mixture layer was 3.60 g / cc.

  The production of the negative electrode will be described.

  Artificial graphite as a negative electrode active material, CMC (carboxymethylcellulose) as a dispersant, and SBR (styrene butadiene rubber) as a binder were mixed in water as a solvent to prepare a negative electrode mixture slurry. The mass ratio of the negative electrode active material, lithium fluoride, CMC, and SBR was negative electrode active material: lithium fluoride: CMC: SBR = 97.3: 0.2: 1: 1.5. When preparing the negative electrode mixture slurry, CMC having 1% by mass of CMC dissolved in pure water was added to the negative electrode active material and the dispersant. The prepared negative electrode mixture slurry is applied to both sides of a copper foil as a negative electrode current collector, dried and rolled to produce a negative electrode mixture layer on the negative electrode current collector. A negative electrode was obtained. The thickness of the negative electrode mixture layer was 142 μm. The packing density of the negative electrode mixture layer was 1.60 g / cc.

  The production of the inorganic particle layer will be described.

Inorganic particles, lithium fluoride (LiF), a binder, and a dispersant were mixed in water as a solvent to prepare an aqueous slurry a1. As the inorganic particles, alumina (Al ₂ O ₃ , average particle diameter 0.6 μm) was used. The solid content concentration of the inorganic particles in the aqueous slurry a1 was 25% by mass. LiF used was passed through a 100 μm sieve. The addition amount of LiF was 0.5 mass% with respect to 100 mass% of inorganic particles. As the binder, a copolymer (rubbery polymer) containing an acrylonitrile structure (unit) was used. The addition amount of the binder was 3.75% by mass with respect to 100% by mass of the inorganic particles. CMC (carboxymethylcellulose) was used as the dispersant. The addition amount of the dispersant was 0.5% by mass with respect to 100% by mass of the inorganic particles.

  The aqueous slurry a1 was applied to both sides of the positive electrode by a dip coating method, and the solvent water was removed by drying. This formed the inorganic particle layer containing an inorganic particle on both surfaces of a positive electrode. In addition, the thickness of the inorganic particle layer was 4 μm on both sides (2 μm on one side).

  The preparation of the nonaqueous electrolytic solution will be described.

The electrolyte solution is non-dissolved in a mixed solvent in which EC (ethylene carbonate) and MEC (methyl ethyl carbonate) are mixed so that LiPF ₆ (lithium hexafluorophosphate) has a ratio of 1.0 mol / L. Used as a water electrolyte. The volume ratio of EC to MEC was EC: MEC = 30: 70.

  The assembly of the battery will be described.

  First, the positive electrode tab was disposed so as to be positioned on the outermost peripheral portion in the positive electrode, and the negative electrode tab was disposed so as to be positioned on the outermost peripheral portion in the negative electrode. And the positive electrode and the negative electrode were arrange | positioned so that it might oppose through the sheet-like separator made from polyethylene (film thickness of 16 micrometers, porosity 47%). This was wound in a spiral shape and then crushed to produce a flat electrode body. A flat electrode body was placed in a battery exterior body formed of an aluminum laminate, and a nonaqueous electrolytic solution was injected and sealed to prepare a battery A1. The design capacity of the battery A1 is 800 mAh on the basis of the end-of-charge voltage up to 4.35V.

(Example 2)
In Example 2, the aqueous slurry a2 was prepared such that the amount of LiF added was 1% by mass with respect to 100% by mass of the inorganic particles. The aqueous slurry a2 was adjusted by the same method as the aqueous slurry a1 except that the amount of LiF added was changed. In Example 2, a battery A2 was produced in the same manner as the battery A1, except that the aqueous slurry a2 was used.

(Example 3)
In Example 3, the aqueous slurry a3 was prepared such that the amount of LiF added was 5% by mass with respect to 100% by mass of the inorganic particles. Except having changed the addition amount of LiF, the aqueous slurry a3 was adjusted with the method similar to the aqueous slurry a1. In Example 3, a battery A3 was produced in the same manner as the battery A1, except that the aqueous slurry a3 was used.

Example 4
In Example 4, the aqueous slurry a4 was prepared such that the amount of LiF added was 10% by mass with respect to 100% by mass of the inorganic particles. The aqueous slurry a4 was adjusted by the same method as the aqueous slurry a1 except that the amount of LiF added was changed. In Example 4, a battery A4 was produced in the same manner as the battery A1, except that the aqueous slurry a4 was used.

(Example 5)
In Example 5, the aqueous slurry a5 was prepared such that the amount of LiF added was 50% by mass with respect to 100% by mass of the inorganic particles. The aqueous slurry a5 was adjusted by the same method as the aqueous slurry a1 except that the amount of LiF added was changed. In Example 5, a battery A4 was produced in the same manner as the battery A1, except that the aqueous slurry a5 was used.

(Example 6)
Production of the inorganic particle layer will be described. A slurry a6 was prepared by mixing NMP (N-methyl-2-pyrrolidone) as a solvent with inorganic particles, lithium fluoride (LiF), and a binder. As the inorganic particles, alumina (Al ₂ O ₃ , average particle diameter 0.6 μm) was used. The solid content concentration of the inorganic particles in the slurry a6 was 25% by mass. LiF used was passed through a 100 μm sieve. The addition amount of LiF was 1.0 mass% with respect to 100 mass% of inorganic particles. As the binder, a copolymer (rubbery polymer) containing an acrylonitrile structure (unit) was used. The addition amount of the binder was 3.75% by mass with respect to 100% by mass of the inorganic particles. Slurry a6 was applied to both sides of the negative electrode by dip coating, and the solvent was dried and removed. Thereby, the inorganic particle layer containing inorganic particles was formed on both sides of the surface of the negative electrode. The thickness of the inorganic particle layer was 4 μm on each side (2 μm on one side).

  In Example 6, no inorganic particle layer was formed on the surface of the positive electrode.

  In Example 6, a battery A6 was produced in the same manner as the battery A1 and the battery A2, except that a negative electrode on which an inorganic particle layer was formed and a positive electrode on which no inorganic particle layer was formed were used.

(Comparative example)
In the comparative example, LiF was not added in the inorganic particle layer. A battery B was made in the same manner as the battery A1 except for this.

(High temperature storage test)
The batteries A1 to A6 and B1 were subjected to a high-temperature storage test under the following conditions, and the evaluation results are shown in Table 1.

  First, constant current charging was performed to 4.4 V with a current of 1 It (800 mA), and further charging was performed to a current of 1/50 It (16 mA) with a constant voltage of 4.4 V. After charging, a constant current discharge was performed through a 10-minute rest period until a current of 1.0 It (800 mA) reached 2.75 V. During the discharge, the discharge capacity before storage was measured.

  After discharging, charging was performed under the same conditions as described above. The charged battery was stored in a constant temperature bath at 60 ° C. for 5 days. After storage, the battery was allowed to stand until it reached room temperature. When the battery temperature reached room temperature, constant current discharge was performed until the battery temperature reached 2.75 V at a current of 1.0 It (800 mA). The discharge capacity after storage was measured during this discharge.

From the discharge capacity before and after storage, the discharge capacity residual rate was determined from the following formula.
Discharge capacity remaining rate (%) = discharge capacity at room temperature after storage ÷ discharge capacity before storage × 100

As a result of the high temperature storage test, the batteries A1 to A6 in which LiF was added in the inorganic particle layer had a higher discharge capacity remaining rate than the battery B in which LiF was not added in the inorganic particle layer. Therefore, it can be said that when LiF is added to the inorganic particle layer, the discharge capacity remaining rate is improved. This is presumably because LiF in the inorganic particle layer captured the transition metal eluted from the positive electrode during high temperature storage. Since LiF has a large polarity, fluorine atoms having a high electronegativity tend to attract electrons and become negatively charged. Therefore, it is considered that positively charged transition metal ions (Co ²⁺ , Co ³⁺ , Mn ²⁺ , Mn ^3+, etc.) are trapped by negatively charged fluorine atoms.

  The batteries A1 to A5 (particularly the battery A2) in which the inorganic particle layer was provided on the positive electrode surface had a higher discharge capacity remaining rate than the battery A6 in which the inorganic particle layer was provided on the negative electrode surface. Therefore, it can be said that the discharge capacity remaining rate is improved when the inorganic particle layer to which LiF is added is provided on the surface of the positive electrode. This is presumably because the transition metal is eluted from the positive electrode, and therefore the effect of capturing the transition metal is higher when LiF is arranged near the positive electrode. In addition, when an inorganic particle layer is provided between the separator and the negative electrode, the transition metal passes through the separator, and therefore, the transition metal may be deposited on the separator surface and the separator may be clogged.

  Further, the remaining discharge capacity rates of the batteries A3, A4, and A5 were similar. That is, it is considered that the effect obtained by adding LiF does not improve as LiF is added. That is, it is considered that the amount of LiF added is sufficient if LiF is added to the inorganic particle layer to capture the transition metal eluted from the positive electrode.

  In addition, when the result of the high temperature storage test of batteries A3, A4, and A5 is specifically compared, the battery A3 to which 5% by mass of LiF is added to 100% by mass of the inorganic particles is more than 100% by mass of the inorganic particles. In the batteries A4 and A5 to which 10% by mass or more of LiF was added, the discharge capacity remaining rate decreased by 0.1%. Therefore, it is considered that the effect of LiF capturing the transition metal is saturated when the amount of LiF added to 100% by mass of the inorganic particles is in the range of 5% by mass to 10% by mass. Therefore, the addition amount of LiF is preferably 10% by mass or less, more preferably 7% by mass or less, with respect to 100% by mass of the inorganic particles.

  Moreover, when comparing the results of the high temperature storage tests of the batteries A1, A2, and A3, the discharge capacity remaining rate was improved as the amount of LiF added was increased. Therefore, if the amount of LiF added is small, it is considered that all of the eluted transition metals cannot be captured. Therefore, the addition amount of LiF is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and further preferably 1.0% by mass or more with respect to 100% by mass of the inorganic particles. is there.

  In addition, since LiF has low electronic conductivity, it is considered that the battery characteristics are deteriorated when LiF is added in the positive electrode or the negative electrode. In addition, when LiF is added to the positive electrode or the negative electrode, in order to obtain the same effect as adding LiF to the inorganic particle layer, LiF is added to the inorganic particle layer. It is considered necessary to add LiF in a larger amount than that.

(Other matters)
(1) Positive Electrode As a positive electrode active material, instead of lithium cobaltate, a Co—Ni—Mn lithium composite oxide, a Ni—Mn—Al lithium composite oxide, a Ni—Co—Al composite oxide, etc. Lithium composite oxide containing cobalt or manganese, olivine type lithium phosphate represented by lithium iron phosphate LiFePO4, or the like can be used as the positive electrode active material. Even when these are used as a positive electrode active material, transition metals can be eluted from the positive electrode, and the non-aqueous electrolyte secondary battery using such a positive electrode active material also has an inorganic particle layer to which LiF is added. By providing, transition metal can be prevented from being deposited on the separator or the negative electrode, and the battery characteristics can be improved.

  In addition, as a positive electrode active material, the above-mentioned thing can also be used independently, and it is also possible to mix and use multiple types.

(2) Electrolytic Solution The electrolytic solution is not particularly limited to this example. Examples of the lithium salt include LiBF ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _6-x (C _n F _{2n + 1} ) _x [where 1 <x <6, n = 1 or 2] can be used, and one or a mixture of two or more of these can be used. The concentration of the lithium salt is desirably 1.0 to 1.8 mol per liter of the electrolytic solution.

  Moreover, as a solvent seed | species, it is preferable to use carbonate type | system | group solvents, such as EC, FEC, PC, GBL, DEC, EMC, and DMC, More preferably, it is using combining a cyclic carbonate and a chain carbonate.

The present invention can be applied to a drive power source of a mobile information terminal such as a mobile phone, a notebook personal computer, and a PDA, for example, in applications that require a particularly high capacity. In addition, it can be expected to be used in high output applications that require continuous driving at high temperatures and applications where the battery operating environment is severe, such as HEVs and electric tools.

Claims (4)

A positive electrode in which a positive electrode mixture layer containing a positive electrode active material is formed on the surface of the positive electrode current collector;
A negative electrode in which a negative electrode mixture layer containing a negative electrode active material is formed on the surface of the negative electrode current collector;
A separator provided between the positive electrode and the negative electrode;
A non-aqueous electrolyte containing a solvent and a solute;
With
A non-aqueous electrolyte secondary battery in which an inorganic particle layer including inorganic particles and lithium fluoride is formed between at least one of the positive electrode and the separator and between the negative electrode and the separator. .   The nonaqueous electrolyte secondary battery according to claim 1, wherein the inorganic particle layer is provided between the positive electrode and the separator.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the inorganic particle layer is provided in contact with the positive electrode. The non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein an addition amount of the lithium fluoride is 0.1 mass% or more and 5 mass% or less with respect to 100 mass% of the inorganic particles.