JP2017027653A - Nonaqueous electrolyte secondary battery and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the cycle characteristic of a nonaqueous electrolyte secondary battery.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode; a negative electrode; and an electrolytic solution. The negative electrode includes boron (B). In the electrolytic solution, the content of lithium bis(oxalate)borate (LiBOB) per electrode area is 0.1 mg/cmor less. The electrolytic solution is prepared so that the LiBOB content in the electrolytic solution becomes 0.1 mg/cmor less per electrode area, thereby suppressing the excess formation of a coating. Thus, the cycle characteristic of the nonaqueous electrolyte secondary battery can be enhanced.SELECTED DRAWING: None

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery and a method for producing the same.

  Vehicles such as EV (Electric Vehicle) and PHV (Plug in Hybrid Vehicle) are equipped with a lithium ion secondary battery as a power storage device having power supplied to a traveling motor. A lithium ion secondary battery mainly includes a positive electrode, a negative electrode, and an electrolytic solution. Each electrode has an active material and a current collector coated with the active material.

  A film is formed on the surface of the negative electrode active material during charge and discharge. The coating prevents the electrolytic solution from coming into direct contact with the negative electrode active material and suppresses the deterioration of the electrolytic solution. However, in this film, cracks may occur due to a volume change of the negative electrode active material. When a crack occurs in the coating, the electrolytic solution directly contacts the negative electrode active material, the electrolytic solution deteriorates, and charge / discharge cycle characteristics (hereinafter simply referred to as “cycle characteristics”) may be deteriorated.

  In order to improve the cycle characteristics, Patent Document 1 shows that the film formed on the surface of the negative electrode active material is stabilized by adding lithium bis (oxalate) borate (LiBOB) to the electrolytic solution. Yes. Patent Document 2 discloses an electrolytic solution containing 0.1 to 5 wt% of a lithium bisborate-based additive with respect to the total amount of the electrolytic solution.

JP 2010-010095 A JP 2010-192430 A

As a standard non-aqueous solvent, a mixed solvent prepared by mixing ethylene carbonate (EC), which is a cyclic carbonate, and ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), which are chain carbonates, and LiPF ₆ as an electrolyte. There is a standard electrolyte dissolved.

By adding lithium bis (oxalate) borate (LiBOB) to a standard electrolyte solution in which LiPF ₆ is dissolved in the above standard non-aqueous solvent, the cycle characteristics of a lithium ion secondary battery using the lithium bis (oxalate) borate are improved. It is predicted that However, according to the study by the present inventors, it has been found that even when LiBOB is added to the standard electrolyte, the cycle characteristics of the lithium ion secondary battery may not be greatly improved depending on the addition amount. In particular, if the amount added is too large, an excessive coating is formed, leading to an increase in electrode resistance. That is, it was found that even if LiBOB was used, the cycle characteristics could not always be improved depending on the amount used and the charge / discharge conditions.

In view of the above-described problems, an object of the present invention is to provide a non-aqueous electrolyte secondary battery and a method for manufacturing a non-aqueous electrolyte secondary battery that can reliably exhibit the effect of film formation.

A non-aqueous electrolyte secondary battery that solves the above problems is a non-aqueous electrolyte secondary battery that includes a positive electrode, a negative electrode, and an electrolytic solution, wherein the negative electrode has boron (B), and the electrolytic solution The lithium bis (oxalate) borate (LiBOB) content is 0.1 mg / cm ² or less with respect to the electrode area.

In the non-aqueous electrolyte secondary battery according to the present invention, the LiBOB content of the non-aqueous electrolyte may be 0.05 mg / cm ² or less with respect to the electrode area.

  In the non-aqueous electrolyte secondary battery according to the present invention, the electrolyte is added with LiBOB.

  In the non-aqueous electrolyte secondary battery according to the present invention, the electrolyte contains LiBOB.

A method for producing a non-aqueous electrolyte secondary battery according to the present invention is a method for producing a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and an electrolytic solution, wherein LiBOB is added to the electrolytic solution, A conditioning treatment and an aging treatment are performed on the non-aqueous electrolyte secondary battery so that the LiBOB content in the non-aqueous electrolyte is 0.1 mg / cm ² or less with respect to the electrode area.

According to the above, the secondary battery including the electrolytic solution to which LiBOB is added easily forms a stable coating derived from LiBOB on the surface of the negative electrode active material in the initial stage of use. In the present invention, a non-aqueous electrolyte secondary battery to which LiBOB is added is subjected to conditioning treatment and aging treatment to form a LiBOB-derived film on the surface of the negative electrode active material. And it suppresses that a coating film is formed excessively by preparing so that content of LiBOB in electrolyte solution may be 0.1 mg / cm < ² > or less with respect to an electrode area. As a result, the cycle characteristics of the nonaqueous electrolyte secondary battery can be improved.

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

  Below, the form for implementing the non-aqueous-electrolyte secondary battery of this invention is demonstrated. Unless otherwise specified, the numerical range “ab” described herein includes the lower limit “a” and the upper limit “b”. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples.

  As the nonaqueous electrolyte secondary battery of the present invention, a lithium ion secondary battery is preferable. A lithium ion secondary battery includes a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode having a negative electrode active material capable of occluding and releasing lithium ions, and an electrolytic solution.

<Positive electrode>
A positive electrode used for a lithium ion secondary battery has a positive electrode active material capable of occluding and releasing lithium ions. The positive electrode is preferably composed of a current collector and a positive electrode active material layer that has a positive electrode active material and covers the surface of the current collector. The positive electrode active material may constitute a positive electrode material together with a binder and / or a conductive aid. The conductive auxiliary agent and the binder are not particularly limited as long as they can be used in the lithium ion secondary battery.

As the positive electrode active material, LiCoO ₂ , LiNi _p Co _q Mn _r O ₂ (0 <p <1, 0 + p <q <1-p, 0+ (p + q) <r <1- (p + q)), Li ₂ MnO ₂ , Li ₂ MnO ₃ , LiNi _s Mn _t O ₂ (0 <s <1, 0 + s <t <1-s), LiFePO ₄ , Li ₂ FeSO ₄ , or a lithium-containing metal oxide having a basic composition or 1 each. Examples thereof include solid solution materials containing two or more species. Any metal oxide may have the above basic composition, and a compound in which a metal element contained in the basic composition is substituted with another metal element can also be used.

  The positive electrode current collector can employ a metal mesh, a porous body, a metal foil, or the like, but is not particularly limited as long as it has a shape according to the purpose. The current collector is not particularly limited as long as it is generally used for a positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel.

In addition to the positive electrode active material, the positive electrode active material layer preferably contains a binder, a conductive additive, and the like.
The binder is not particularly limited, and a known one may be used. For example, a resin that does not decompose even at a high potential, such as a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, can be used.

  As the conductive auxiliary agent, a material generally used for an electrode of a lithium ion secondary battery may be used. For example, it is preferable to use conductive carbon materials such as carbon black (carbonaceous fine particles) such as acetylene black and ketjen black, and carbon fibers. Besides conductive carbon materials, known conductive materials such as conductive organic compounds are also used. An auxiliary agent may be used. One of these may be used alone or in combination of two or more.

<Negative electrode>
A negative electrode used for a lithium ion secondary battery has a negative electrode active material capable of occluding and releasing lithium ions. The negative electrode may include a current collector and a negative electrode active material layer that has a negative electrode active material and covers the surface of the current collector. The negative electrode active material may constitute a negative electrode material together with a binder and / or a conductive aid. The conductive auxiliary agent and the binder are not particularly limited as long as they can be used in the lithium ion secondary battery.

As the negative electrode active material, a material capable of inserting and extracting lithium ions can be used. Therefore, there is no particular limitation as long as it is a simple substance, alloy, or compound that can occlude and release lithium ions. For example, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, B
Examples include a negative electrode active material containing at least one of a, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Ge, Sn, Pb, Sb, and Bi. Specifically, Cu-S
Tin-based materials such as n-alloy and Co-Sn alloy, carbon-based materials such as various graphites, elemental silicon, SiO _x
Examples thereof include silicon-based materials such as (0.5 ≦ x ≦ 1.5), and one or more of these materials can be used.

  The negative electrode active material constitutes a negative electrode active material layer that covers at least the surface of the current collector. Generally, a negative electrode is formed by covering a current collector with a negative electrode active material layer. As the current collector, for example, a metal mesh such as copper or copper alloy, a porous body, or a metal foil may be used.

  As the binder resin and the conductive auxiliary agent, the same ones as described for the positive electrode can be used.

  Usually, the active material and binder resin are mixed with a conductive aid and an appropriate amount of an organic solvent as necessary, and the resulting slurry is rolled, dip-coated, doctor blade, spray-coated, curtained. An electrode can be produced by coating on a current collector by a method such as a coating method and curing the binder resin.

<Electrolyte>
The electrolytic solution is a composition in which an electrolyte salt is contained in a non-aqueous solvent. As the non-aqueous solvent, one or more selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like. Materials can be used. The electrolyte salt is preferably an alkali metal fluoride that is soluble in a non-aqueous solvent. Particularly preferred is a salt containing lithium such as lithium fluoride. Specifically, LiPF ₆ , LiBF ₄ , LiAsF ₆ , NaPF ₆ , NaBF ₄ , NaAsF _{6 and the} like are preferable, and at least one selected from these may be used. The content of the electrolyte salt may be about 0.5 to 1.7 mol / L with respect to 1 L of the electrolyte. In addition, the electrolyte may contain an additive for overcharge such as cyclohexylbenzene (CHB) or biphenyl (BP).

In the lithium secondary battery according to the present embodiment, lithium bis (oxalate) borate (LiBOB) is added to the electrolytic solution. LiBOB is a compound that is easily reduced and decomposed, and the reduced decomposition product becomes a coating component formed on the entire surface of the negative electrode active material and the positive electrode active material. For this reason, when such LiBOB is excessively contained in the electrolytic solution, the coating becomes thicker, leading to an increase in the electric resistance of the active material. Therefore, in order to maintain cycle characteristics, the content of LiBOB in the electrolytic solution after the conditioning process and the aging process is preferably suppressed to 0.1 mg / cm ² or less, further 0.05 mg / cm ² or less with respect to the electrode area.

A separator is used as needed. The separator separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.

  A separator is sandwiched between the positive electrode and the negative electrode as necessary to form an electrode assembly. Lithium ion secondary battery in which a non-aqueous electrolyte is impregnated in the electrode body after connecting between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collecting lead or the like It is good to do.

  The shape of the lithium ion secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, a coin shape, and a laminated shape can be adopted.

Such a lithium ion secondary battery may be charged and discharged within a voltage range suitable for the type of active material contained in the electrode. However, LiBOB contained in the electrolytic solution of the present invention produces a film at around 1.6 to 1.8V. Therefore, when the oxidation-reduction potential of Li is used as a reference potential, the charge / discharge voltage range of the lithium ion secondary battery is preferably 2 to 4.5 V, more preferably 2.25 to 4.25 V. Whether the negative electrode active material forms a LiBOB-derived film is determined by detecting B (boron) from the surface of the negative electrode active material by energy dispersive X-ray spectroscopy (EDX). Moreover, when B is detected from the electrolytic solution, the content of LiBOB can be measured by gas chromatography mass spectrometry (GC-MS).

As mentioned above, although embodiment of the electrolyte solution and lithium ion secondary battery of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be specifically described with reference to examples of the lithium ion secondary battery of the present invention.

  In order to evaluate the characteristics of the lithium ion secondary battery, four types of lithium ion secondary batteries with different types of electrolytic solutions were prepared, and the cycle characteristics of the batteries were measured.

(Battery 1)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ : 94% by mass, acetylene black (conductive agent): 2% by mass, flake graphite: 1% by mass, polyvinylidene fluoride (binder): 3% by mass Was mixed with N-methyl-2-pyrrolidone to prepare a positive electrode mixture paste. This positive electrode mixture paste was applied onto an aluminum foil (current collector), dried and pressurized, punched out to a predetermined size, and a positive electrode sheet was produced. The density of the portion excluding the current collector of the positive electrode was 3.3 g / cm ³ , and the basis weight on the aluminum foil was 18.4 mg / cm ² .

Further, 98.6% by mass of artificial graphite (negative electrode active material) is mixed with 0.7% by mass of SBR (binder) and 0.7% by mass of CMC (thickener) to prepare a negative electrode mixture paste. did. This negative electrode mixture paste was applied onto a copper foil (current collector), dried and pressurized, and punched into a predetermined size to produce a negative electrode sheet. The density of the portion excluding the current collector of the negative electrode was 1.4 g / cm ³ , and the basis weight on the copper foil was 10.4 mg / cm ² .

As the non-aqueous solvent, ethylene carbonate (EC) which is a cyclic carbonate, ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) which are chain carbonates were prepared. Cyclohexylbenzene (CHB) and biphenyl (BP) were prepared as additives for overcharge. Lithium bis (oxalate) borate (LiBOB) was prepared as an additive. Moreover, LiPF ₆ was prepared as an electrolyte. LiPF ₆ , CHB, BP, and further LiBOB were added to a non-aqueous solvent in which EC, EMC, and DMC were mixed at a predetermined volume ratio to obtain an electrolytic solution 1. LiPF ₆ was prepared to be 1 mol / L with respect to 1 L of the electrolytic solution. CHB was added to 2.5 wt% with respect to the electrolytic solution, and BP was added to 2.5 wt% with respect to the electrolytic solution. LiBOB was added at 0.095 mg / cm ² with respect to the electrode area.

  A microporous separator was sandwiched between the positive electrode and the negative electrode. A plurality of these positive electrodes, separators and negative electrodes were laminated to form an electrode assembly. The periphery of the two aluminum films was sealed by heat-welding except for a part to make a bag shape. The electrode assembly was placed in a bag-like aluminum film, and the electrolyte 1 was further placed. Then, the opening part of the aluminum film was completely airtightly sealed while evacuating. At this time, the tips of the current collector on the positive electrode side and the negative electrode side were protruded from the edge portion of the film to be connectable to an external terminal to obtain a lithium ion secondary battery (battery 1).

(Battery 2)
The battery 2 had the same configuration as the battery 1 except that the electrolytic solution 2 in which 0.122 mg / cm ^{2 of} LiBOB was added to the electrode area was used.

(Battery 3)
The battery 2 had the same configuration as the battery 1 except that the electrolytic solution 3 in which 0.244 mg / cm ^{2 of} LiBOB was added to the electrode area was used.

(Battery 4)
The battery 2 has the same configuration as the battery 1 except that the electrolytic solution 4 in which 0.488 mg / cm ^{2 of} LiBOB was added to the electrode area was used.
The electrolytic solutions 1 to 4 used for the batteries 1 to 4 are shown in Table 1.

  The batteries 1 to 4 were subjected to conditioning treatment for initial charge / discharge at 25 ° C. After the conditioning treatment, the battery was subjected to an aging treatment that was held at 60 ° C. for 20 hours. By the conditioning treatment and the aging treatment, a film derived from LiBOB in the electrolytic solution is formed on the surface of the negative electrode active material.

(Energy dispersive X-ray spectroscopy)
Using the batteries 1 to 4 subjected to conditioning treatment and aging treatment, the surface of the negative electrode active material was analyzed by energy dispersive X-ray spectroscopy (EDX). As a result, in batteries 1 to 4, B (boron) was detected, and it was determined that a film was formed.

(Gas chromatograph mass spectrometry)
Next, the content of LiBOB remaining in the electrolytic solution was measured by gas chromatograph mass spectrometry (GC-MS) using the batteries 1 to 4 subjected to conditioning treatment and aging treatment. The measurement results are shown in Table 2.

(Cycle test)
Next, a charge / discharge cycle test was performed using the batteries 1 to 4 subjected to the conditioning process and the aging process. The batteries 1 to 4 used for the cycle test are prepared separately from the batteries used for gas chromatograph mass spectrometry. The cycle test was performed at 60 ° C. The charge condition of the cycle test was CC (constant current) charge up to 3.987V at 0.5C, and the discharge condition was CC discharge up to 3.455V at 0.5C. The first charge / discharge after the conditioning treatment was regarded as the first cycle, and the same charge / discharge was repeated until the 300th cycle. Then, the discharge capacity at the 300th cycle relative to the discharge capacity at the first cycle was calculated as “discharge capacity retention rate (%)”.
Discharge capacity retention ratio (%) = (discharge capacity at the 300th cycle / discharge capacity at the first cycle) × 100. The results are shown in Table 2.

As can be seen from Table 2, when the content of the remaining LiBOB is 0.1 mg / cm ² or less with respect to the electrode area, the reduction in the discharge capacity maintenance rate of the nonaqueous electrolyte secondary battery can be suppressed. I understood.

Further, from Table 2, it was found that excellent cycle characteristics were exhibited particularly when the remaining LiBOB content was 0.05 mg / cm ² or less with respect to the electrode area. This can be determined that the decrease in the discharge capacity maintenance rate could be suppressed without excessive formation of the coating derived from LiBOB.

The cycle characteristics of the battery 4 were the most inferior among the four types although the content of LiBOB remaining in the electrolytic solution was large. This is presumed that since the content of the remaining LiBOB is large, the coating is formed excessively, leading to an increase in the resistance of the electrode.
That is, it has been found that the effect of film formation can be surely exhibited by adjusting the content of LiBOB remaining in the electrolytic solution to an appropriate value.

Claims (5)

A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and an electrolyte solution,
The negative electrode has boron (B),
A non-aqueous electrolyte secondary battery, wherein the content of lithium bis (oxalate) borate (LiBOB) in the electrolyte is 0.1 mg / cm ² or less with respect to the electrode area. ^2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of LiBOB in the electrolyte is 0.05 mg / cm ² or less with respect to the electrode area.   The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the electrolytic solution is added with LiBOB.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the electrolytic solution contains LiBOB. A method for producing a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and an electrolyte solution,
LiBOB is added to the electrolyte,
By performing a conditioning process and an aging process on the non-aqueous electrolyte secondary battery,
A method for producing a non-aqueous electrolyte secondary battery, wherein the content of LiBOB in the electrolyte is 0.1 mg / cm ² or less with respect to the electrode area.