JP6132158B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a secondary battery (non-aqueous electrolyte secondary battery) provided with a non-aqueous electrolyte.

  Lithium ion secondary batteries and other non-aqueous electrolyte secondary batteries are becoming increasingly important as on-vehicle power supplies or personal computers and portable terminals. In particular, a lithium ion secondary battery that is lightweight and has a high energy density is preferably used as a high-output power source for mounting on a vehicle (for example, Patent Document 1). When such a nonaqueous electrolyte secondary battery is overcharged, excessive charge carriers are released from the positive electrode, and excessive charge carriers are inserted into the negative electrode. For this reason, both the positive electrode and the negative electrode are thermally unstable. When both the positive electrode and the negative electrode become thermally unstable, the organic solvent of the electrolytic solution is eventually decomposed, and an exothermic reaction occurs, thereby impairing the stability of the battery.

  To solve this problem, for example, the battery case is equipped with a current interruption mechanism that interrupts charging when the gas pressure inside the battery exceeds a predetermined pressure, and gas is generated when a predetermined overcharge state is reached in the electrolyte. A non-aqueous electrolyte secondary battery to which a gas generating agent is added is disclosed. As such a gas generating agent, for example, cyclohexylbenzene (CHB) or biphenyl (BP) is used (for example, Patent Document 1). CHB and BP activate the polymerization reaction during overcharge and generate hydrogen gas. Thereby, the pressure in a battery case becomes high, an electric current interruption mechanism act | operates, and an overcharge electric current is interrupted | blocked.

JP 2003-077478 A

  By the way, in order to operate the current interruption mechanism efficiently, it is effective to increase the addition amount of the gas generating agent. However, since the gas generating agent works as a resistance component of the battery, simply increasing the amount of the gas generating agent may cause a decrease in battery performance. On the other hand, if the addition amount of the gas generating agent is decreased in order to keep the battery performance high, the generation of gas may be slowed during overcharging, and the operation of the current interruption mechanism may be delayed. As described above, in the conventional configuration, it is difficult to achieve both ensuring of the amount of gas generation and maintenance of battery performance. It would be very useful if a large amount of gas could be generated with a small amount of gas generating agent.

  The present inventor uses a cyclohexylbenzene (CHB) and another gas generating agent (for example, BP) as a gas generating agent in a non-aqueous electrolyte secondary battery equipped with a gas generating agent and a current interruption mechanism. In addition, by fixing another gas generating agent to the conductive material in the positive electrode active material layer, it is possible to increase the gas generating agent without increasing the amount of the gas generating agent (and without degrading the battery performance). The inventors have found that the amount generated can be effectively increased, and have completed the present invention.

  That is, the non-aqueous electrolyte secondary battery provided by the present invention includes a positive electrode in which a positive electrode active material layer is held on a positive electrode current collector, a negative electrode in which a negative electrode active material layer is held on a negative electrode current collector, An electrode body comprising a separator interposed between a positive electrode and the negative electrode; a battery case containing the electrode body together with a non-aqueous electrolyte; an external terminal provided in the battery case and connected to the electrode body; And a current interrupt mechanism that interrupts electrical connection between the electrode body and the external terminal when the internal pressure of the battery case becomes higher than a predetermined pressure. Cyclohexylbenzene (CHB) is added to the non-aqueous electrolyte. The positive electrode active material layer includes at least a positive electrode active material and a conductive material. A gas generating agent that generates gas when a voltage equal to or higher than a predetermined voltage is fixed to the conductive material.

  According to such a configuration, it is possible to effectively increase the amount of gas generated during overcharge without increasing the amount of gas generating agent added. Therefore, when the overcharge state is reached, the internal pressure quickly increases, and the current interrupt mechanism can be appropriately operated. Therefore, according to the present invention, a nonaqueous electrolyte secondary battery having high performance (low resistance) and excellent stability can be provided.

FIG. 1 is a diagram illustrating an example of the structure of a non-aqueous electrolyte secondary battery. FIG. 2 is a schematic view showing a part of a cross section of the wound electrode body cut in the radial direction. FIG. 3 is a diagram schematically illustrating an example of the conductive material to which the BP is fixed. FIG. 4 is a graph showing the gas generation amounts of Examples and Comparative Examples 1 and 2. FIG. 5 is a graph showing the relationship between voltage and time when the batteries of Examples and Comparative Examples 1 and 2 were discharged. FIG. 6 is a diagram schematically illustrating an example of a conductive material to which CHB is fixed.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. In the present specification, the “secondary battery” refers to a general power storage device that can be repeatedly charged and discharged, and is a term including a so-called storage battery such as a lithium ion secondary battery and a power storage element such as an electric double layer capacitor. The “non-aqueous electrolyte secondary battery” refers to a battery provided with a non-aqueous electrolyte (typically, an electrolyte containing a supporting salt (supporting electrolyte) in a non-aqueous solvent). The “lithium ion secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by the movement of lithium ions between positive and negative electrodes. The electrode active material refers to a material capable of reversibly occluding and releasing chemical species (lithium ions in a lithium ion secondary battery) serving as a charge carrier.

  Hereinafter, a non-aqueous secondary battery according to an embodiment of the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected suitably to the member and site | part which show | play the same effect | action. Each drawing is schematically drawn and does not necessarily reflect the real thing. Each drawing shows an example only and does not limit the present invention unless otherwise specified. Hereinafter, embodiments of the present invention will be described by taking as an example the case where the present invention is applied to a lithium ion secondary battery, but the present invention is not intended to be limited.

  FIG. 1 shows a lithium ion secondary battery 100 according to an embodiment of the present invention. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 20 and a battery case 30. As shown in FIG. 1, a lithium ion secondary battery 100 according to an embodiment of the present invention includes a flat rectangular battery case 20 having a flat wound electrode body 20 together with a liquid electrolyte (electrolytic solution) (not shown). That is, it is accommodated in the exterior container 30.

  The battery case 30 has a box-shaped (that is, bottomed rectangular parallelepiped) case body 32 having an opening at one end (corresponding to the upper end in a normal use state of the battery), and the opening attached to the opening. It is comprised from the cover body 34 which consists of a rectangular-shaped plate member which plugs up a part. The material of the battery case 30 is exemplified by aluminum, for example. As shown in FIG. 1, a positive electrode terminal 42 and a negative electrode terminal 44 for external connection are formed on the lid 34. A safety valve 36 is formed between both terminals 42 and 44 of the lid 34.

  The wound electrode body 20 includes a long sheet-like positive electrode (positive electrode sheet 50) and a long sheet-like negative electrode (negative electrode sheet 60) similar to the positive electrode sheet 50, in total, two long sheet-like separators (separators). 70).

  The positive electrode sheet 50 includes a strip-shaped positive electrode current collector 52 and a positive electrode active material layer 54. For the positive electrode current collector 52, for example, a strip-shaped aluminum foil having a thickness of about 15 μm is used. A positive electrode active material layer non-forming portion 52 a is set along an edge portion on one side in the width direction of the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is held on both surfaces of the positive electrode current collector 52 except for the positive electrode active material layer non-forming portion 52 a set in the positive electrode current collector 52. The positive electrode active material layer 54 includes a positive electrode active material 56 (see FIG. 2), a conductive material 58 (see FIG. 2), and a binder.

As the positive electrode active material, a material used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of cathode active materials include lithium transition metal oxides such as LiNiCoMnO ₂ (lithium-nickel-cobalt-manganese composite oxide). For example, a conductive material can be mixed with the positive electrode active material. As the conductive material, carbon black such as acetylene black (AB) and ketjen black and other powdery carbon materials (graphite and the like) can be mixed. The conductive material will be further described later. In addition to the positive electrode active material and the conductive material, a binder such as polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR), or carboxymethyl cellulose (CMC) can be added. A positive electrode active material layer forming composition (paste) can be prepared by dispersing these in a suitable dispersion medium and kneading. The positive electrode active material layer 54 is formed by applying the positive electrode active material layer forming composition to the positive electrode current collector 52, drying it, and pressing it to a predetermined thickness.

  The negative electrode sheet 60 includes a strip-shaped negative electrode current collector 62 and a negative electrode active material layer 64. For the negative electrode current collector 62, for example, a strip-shaped copper foil having a thickness of about 10 μm is used. On one side in the width direction of the negative electrode current collector 62, a negative electrode active material layer non-formation part 62a is set along the edge. The negative electrode active material layer 64 is held on both surfaces of the negative electrode current collector 62 except for the negative electrode active material layer non-forming portion 62 a set in the negative electrode current collector 62. The negative electrode active material layer 64 includes a negative electrode active material, a thickener, a binder, and the like.

  As the negative electrode active material, one type or two or more types of materials conventionally used in lithium ion secondary batteries can be used without any particular limitation. Preferable examples include carbon-based materials such as graphite carbon. And like a positive electrode, this negative electrode active material is disperse | distributed to a suitable dispersion medium with binders, such as PVDF, SBR, and CMC (it can function also as a thickener), and is kneaded, The composition for negative electrode active material layer formation A product (paste) can be prepared. The negative electrode active material layer 64 is formed by applying this negative electrode active material layer forming composition to the negative electrode current collector 62, drying it, and pressing it to a predetermined thickness.

  The separator 70 is a member that separates the positive electrode sheet 50 and the negative electrode sheet 60. In this example, the separator 70 is composed of a strip-shaped base material having a predetermined width and having a plurality of minute holes. Examples of the base material include a sheet material having a single layer structure (for example, a single layer structure of polyethylene) made of a porous polyolefin-based resin, or a sheet material having a laminated structure (for example, a three-layer structure of polypropylene, polyethylene, and polypropylene). Can be used.

  The wound electrode body 20 is attached to electrode terminals 42 and 44 attached to the battery case 30 (in this example, the lid body 34). The wound electrode body 20 is housed in the battery case 30 in a state where the wound electrode body 20 is flatly pushed and bent in one direction orthogonal to the winding axis. In the wound electrode body 20, the positive electrode active material layer non-formed portion 52 a of the positive electrode sheet 50 and the negative electrode active material layer non-formed portion 62 a of the negative electrode sheet 60 protrude on the opposite sides in the winding axis direction. Among these, one electrode terminal 42 is fixed to the positive electrode active material layer non-forming portion 52 a of the positive electrode current collector 52 via the positive electrode current collector plate 42 a, and the other electrode terminal 44 is the negative electrode current collector 62. The negative electrode active material layer non-forming portion 62a is fixed via a negative electrode current collector plate 44a. The wound electrode body 20 is accommodated in the flat internal space of the case body 32. The case body 32 is closed by the lid 34 after the wound electrode body 20 is accommodated.

As the electrolytic solution (non-aqueous electrolytic solution), the same non-aqueous electrolytic solution conventionally used for lithium ion secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. As the non-aqueous solvent, for example, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like can be used. Moreover, as said support salt, lithium salts, such as LiPF ₆ , can be used, for example.

  Cyclohexylbenzene (CHB) is added to the non-aqueous electrolyte. CHB activates the polymerization reaction and generates hydrogen gas at the time of overcharge of about 4.35V to 4.6V. The amount of CHB added to the nonaqueous electrolytic solution may be, for example, about 0.1% by mass to 10% by mass (preferably 3% by mass to 8% by mass, for example, 6 ± 1% by mass).

  Further, the lithium ion secondary battery 100 includes a current interruption mechanism 90. The current interruption mechanism 90 is a mechanism that interrupts the current path when the pressure in the battery case becomes abnormally high. In this embodiment, as shown in FIG. 1, the current interruption mechanism 90 is constructed inside the positive electrode terminal 42 so that the conduction path of the battery current in the positive electrode is interrupted.

  Hereinafter, the lithium ion secondary battery 100 will be described in more detail. FIG. 2 schematically shows a cross section of the positive electrode sheet 50 and the separator 70 overlapped in the wound electrode body 20 taken along the winding axis direction (for example, the width direction of the positive electrode sheet 50).

  As shown in FIG. 2, the positive electrode sheet 50 includes a positive electrode current collector 52 and a positive electrode active material layer 54 held by the positive electrode current collector 52. The positive electrode active material layer 54 includes a positive electrode active material 56 and a conductive material 58. As described above, the conductive material 58 is made of a powdered carbon material (for example, AB). A gas generating agent that generates gas when a voltage equal to or higher than a predetermined voltage is fixed to the conductive material 58.

  The gas generating agent fixed to the conductive material 58 is preferably a gas generating agent that can easily introduce a functional group derived from the gas generating agent to the surface of the conductive material 58. It is preferable to use a gas generating agent that can exist stably on the surface of the conductive material 58. Further, a gas generating agent that activates a polymerization reaction and generates hydrogen gas at the time of overcharge of about 4.35V to 4.6V is preferable. A gas generating agent that satisfies such conditions can be used without particular limitation. Examples of the gas generating agent include phenyl compounds, cycloalkylbenzene compounds, alkylbenzene compounds, organic phosphorus compounds, fluorine atom-substituted aromatic compounds, carbonate compounds, cyclic carbamate compounds, and alicyclic hydrocarbons. For example, it is preferable to use a phenyl compound having a phenyl group. Specific examples include biphenyl (BP), alkyl biphenyl, terphenyl, 2-fluorobiphenyl, 3-fluorobiphenyl, 4-fluorobiphenyl, 4,4'-difluorobiphenyl, and the like. Of these, the use of biphenyl (BP) is preferred (this embodiment). FIG. 3 schematically shows the conductive material 58 to which biphenyl is fixed. In the example shown in FIG. 3, the surface of the conductive material 58 is modified with a functional group derived from biphenyl (for example, a biphenyl group).

  The method of fixing the gas generating agent to the conductive material (typically, a method of introducing a functional group derived from the gas generating agent into the conductive material) is not particularly limited. For example, a method in which a conductive material and a gas generating material are mixed and superheated can be preferably employed. For example, when the gas generating agent fixed to the conductive material 58 is BP, first, the raw material conductive material is introduced into the reactor, deoxygenated and purged with argon, and then 3-aminobiphenyl, tetrahydrofuran (THF), O— Add dichlorobenzene and isoamyl nitrite and stir with heating. After allowing to cool, the mixture is filtered, and the filtrate is washed with acetone, whereby a conductive material to which BP is fixed can be obtained.

  In the non-aqueous electrolyte secondary battery having such a configuration, as described above, CHB is added to the non-aqueous electrolyte, and BP is fixed to the conductive material 58 in the positive electrode active material layer 54. By adding CHB to the non-aqueous electrolyte and fixing BP to the conductive material 58 in this manner, the amount of gas generated during overcharging is effectively increased without increasing the amount of BP and CHB added. Can be able to. In other words, a combination of the fact that BP is fixed to the conductive material 58 that controls the potential and the addition of CHB in the non-aqueous electrolyte (due to a synergistic effect), generates a small amount of gas. Even the agent can generate a large amount of gas. Therefore, when the overcharge state occurs, the internal pressure quickly increases, and the current interrupt mechanism 90 (FIG. 1) can be appropriately operated. Therefore, according to this configuration, it is possible to provide a non-aqueous electrolyte secondary battery having high performance (low resistance) and excellent stability.

  An evaluation cell (lithium ion secondary battery) is constructed using a positive electrode in which CHB is added to a non-aqueous electrolyte and a gas generating agent is fixed to a conductive material, and gas is generated during an overcharge test of the battery. Quantity and IV resistance were evaluated. A specific method will be described below.

<Example>
In this example, an evaluation cell was constructed using a conductive material (conductive material with BP) to which BP was fixed as a gas generating agent.

  The conductive material with BP of the evaluation cell was produced as follows. First, acetylene black (AB) as a conductive material was put into a four-necked reactor equipped with a stirring blade, a thermometer, and a Liebig condenser, and deoxygenated and purged with argon. Subsequently, 3-aminobiphenyl, tetrahydrofuran (THF), O-dichlorobenzene and isoamyl nitrite were added to the reactor and stirred with heating. The mixture was allowed to cool and then filtered, and the filtrate was washed with acetone and dried to obtain a colorless solid sample (conductive material with BP). The obtained sample and the raw material conductive material were each subjected to thermogravimetric analysis (TG), and the change in mass was measured. From the difference in mass change between the two, it was confirmed that a substituent derived from BP was introduced into the conductive material in the sample.

The positive electrode of the evaluation cell was produced as follows. First, LiNiCoMnO ₂ powder as the positive electrode active material, the conductive material with BP (AB) and PVdF as the binder are mixed in NMP so that the mass ratio of these materials is 93: 4: 3, A composition for a positive electrode active material layer was prepared. The positive electrode active material layer was provided on both sides of the positive electrode current collector by applying the composition for positive electrode active material layer in a strip shape on both sides of a long sheet-like aluminum foil (positive electrode current collector) and drying it. A positive electrode sheet was produced.

  The negative electrode of the evaluation cell was produced as follows. First, amorphous coated graphite as a negative electrode active material, SBR as a binder and CMC as a thickener are dispersed in water so that the mass ratio of these materials is 98: 1: 1, and the negative electrode active material layer A composition was prepared. This composition for negative electrode active material layers was applied to both sides of a long sheet-like copper foil (negative electrode current collector) to prepare a negative electrode sheet having a negative electrode active material layer provided on both sides of the negative electrode current collector.

  A porous polyethylene (PE) sheet was used as a separator for the evaluation cell.

As the non-aqueous electrolyte of the evaluation cell, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 3: 4, and LiPF as a supporting salt. ₆ was used at a concentration of about 1 mol / liter. In addition, 6% by mass of CHB was added to the nonaqueous electrolytic solution.

<Comparative Example 1>
An evaluation cell was fabricated using a conductive material (AB) to which BP was not fixed. The other conditions were the same as in the example.

<Comparative example 2>
An evaluation cell was produced using a conductive material (AB) to which BP was not fixed and adding 0.1% by mass of BP to the non-aqueous electrolyte. The other conditions were the same as in the example.

  The amount of overcharge gas generated in each of the evaluation cells of Examples and Comparative Examples 1 and 2 was measured as follows. The same electrode body as that used for the evaluation cell was placed in a laminating bag together with an electrolyte solution to which a gas generating agent was added, and vacuum-sealed to produce a laminate cell. This laminate cell was subjected to an overcharge test. Before the overcharge test, weigh the laminate cell in a fluorinated liquid in advance, and then charge the laminate cell in a 25 ° C test tank to 4.1 V (SOC 100%) at 1 C using the constant current and constant voltage method. The battery was further charged with a constant current of 1 C to obtain an overcharged state of SOC 140%. Thereafter, the discharge treatment was performed up to 3 V, taken out from the test tank, the weight in the fluorinated liquid was measured again, and the volume change of the laminate cell was measured based on the Archimedes method to obtain the amount of overcharge gas generated. The results are shown in Table 1 and FIG.

  Further, the change in voltage when the cells for evaluation in Examples and Comparative Examples 1 and 2 were charged to 3.69 V and then discharged at 2280 mA was plotted against the time on the horizontal axis. The results are shown in FIG. Here, it is suggested that the longer the time until the voltage drops to 0.7 V, the smaller the IV resistance.

As shown in Table 1 and FIG. 4, the battery according to the example in which BP was fixed to the conductive material had a gas generation amount of 25 cm ³ / Ah, and the gas generation amount during overcharge was good. Further, as shown in FIG. 5, the IV resistance was low and the battery characteristics were excellent. On the other hand, in the battery according to Comparative Example 1 in which BP is not fixed to the conductive material, the amount of gas generated was only about 40% of the example, and the amount of generated gas was insufficient. Further, the battery according to Comparative Example 2 in which BP was added to the non-aqueous electrolyte without fixing the BP to the conductive material also had a gas generation amount of about 60% of the example, and the gas generation amount was insufficient. . Furthermore, as shown in FIG. 5, the batteries according to Comparative Examples 1 and 2 had a tendency to increase IV resistance as compared with the Examples. From the above, by adding CHB to the non-aqueous electrolyte and fixing BP to the conductive material, a non-aqueous electrolyte secondary battery having high performance (low resistance) and excellent stability can be obtained. It was confirmed.

  Although the lithium ion secondary battery according to one embodiment of the present invention has been described above, the secondary battery according to the present invention is not limited to any of the above-described embodiments, and various modifications can be made. For example, in the above-described embodiment, the case where the BP is fixed to the conductive material is illustrated, but the present invention is not limited to this. For example, a secondary battery in which CHB is fixed to a conductive material may be used. FIG. 6 schematically shows a conductive material 158 to which CHB is fixed. In the example shown in FIG. 6, the surface of the conductive material 158 is modified with a functional group derived from CHB (for example, a cyclohexylphenyl group). Even in this case, an effect equivalent to the effect described above can be obtained.

20 Winding electrode body 30 Battery case 50 Positive electrode sheet 52 Positive electrode current collector 54 Positive electrode active material layer 56 Positive electrode active material 58 Conductive material 60 Negative electrode sheet 70 Separator 90 Current interruption mechanism 100 Lithium ion secondary battery

Claims (1)

An electrode body comprising: a positive electrode in which a positive electrode active material layer is held on a positive electrode current collector; a negative electrode in which a negative electrode active material layer is held on a negative electrode current collector; and a separator interposed between the positive electrode and the negative electrode; ,
A battery case containing the electrode body together with a non-aqueous electrolyte;
An external terminal provided in the battery case and connected to the electrode body;
When the internal pressure of the battery case becomes higher than a predetermined pressure, a current interruption mechanism that interrupts electrical connection between the electrode body and the external terminal,
In the non-aqueous electrolyte, cyclohexylbenzene is added in an addition amount of 0.1% by mass or more and 10% by mass or less ,
The positive electrode active material layer includes at least a positive electrode active material and a conductive material,
The surface of the conductive material is a functional group derived from a gas generating agent that generates hydrogen gas when a voltage equal to or higher than a predetermined voltage, and includes a biphenyl group, an alkylbiphenyl group, a terphenyl group, and a 2-fluorobiphenyl group. A non-aqueous electrolyte secondary battery modified with at least one functional group selected from the group consisting of: 3-fluorobiphenyl group, 4-fluorobiphenyl group, 4,4′-difluorobiphenyl group, and cyclohexylphenyl group .

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