JP2015035359A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery arranged to be able to maintain both a battery performance when working normally and the reliability when subjected to overcharging.SOLUTION: The present invention provides a nonaqueous electrolyte secondary battery which comprises: a positive electrode; a negative electrode; a nonaqueous electrolyte; and a battery case in which the positive and negative electrodes and the nonaqueous electrolyte are enclosed. The positive electrode includes a positive electrode current collector, and a positive electrode active material layer formed on the positive electrode current collector; in the positive electrode active material layer, a positive electrode active material and dicyclohexylbenzene are almost uniformly mixed together on the whole. In addition, the nonaqueous electrolyte contains cyclohexylbenzene. The battery case includes a current interruption mechanism which is activated when the internal pressure of the battery case is increased by gas produced as a result of the decomposition of the cyclohexylbenzene and/or the dicyclohexylbenzene.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery. Specifically, the present invention relates to the battery provided with a current interruption mechanism that operates by increasing the internal pressure.

  In recent years, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries and nickel metal hydride batteries have been widely used as power sources for portable electronic devices and transportation devices. 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 driving a vehicle.

  Such a non-aqueous electrolyte secondary battery is generally used in a state in which the voltage is controlled so as to be within a predetermined region (for example, 3.0 V to 4.2 V). May be overcharged exceeding a predetermined voltage. Therefore, for example, in a large-sized and / or high-capacity battery used for driving a vehicle, a current interruption device (current interrupt device) that cuts off the charging current and stops the progress of overcharging when the pressure in the case exceeds a predetermined value : CID) is used. As techniques related to this, Patent Documents 1 to 3 are cited. For example, Patent Document 1 describes that by providing a polymer having a cyclohexyl group on the surface of the positive electrode, the CID can be activated in a short time during overcharge.

JP 2012-109219 A JP 2013-004305 A JP 2004-063114 A

In the technique described in Patent Document 1, a polymer is generated on the surface of the positive electrode by immersing the electrode body in a nonaqueous electrolytic solution containing the polymer and performing charging. For this reason, it is necessary to add a large amount of polymer in the non-aqueous electrolyte, and battery characteristics (for example, charge / discharge characteristics and output characteristics) during normal operation may be deteriorated.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a secondary battery capable of achieving both high-level battery characteristics during normal operation and reliability during overcharge.

  As a result of intensive studies by the present inventors, a means capable of solving the above problems has been found and the present invention has been completed. ADVANTAGE OF THE INVENTION According to this invention, the nonaqueous electrolyte secondary battery with which the positive electrode, the negative electrode, and the nonaqueous electrolyte solution were accommodated in the battery case is provided. The positive electrode is a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material and dicyclohexylbenzene (hereinafter also referred to as “DCHB”). And a positive electrode active material layer mixed almost uniformly. The non-aqueous electrolyte contains cyclohexylbenzene (hereinafter also referred to as “CHB”). The battery case includes a current interrupt mechanism that operates when gas is generated by decomposition of the cyclohexylbenzene and / or the dicyclohexylbenzene and the internal pressure of the battery case increases.

By mixing dicyclohexylbenzene (DCHB) homogeneously in the positive electrode active material layer and including cyclohexylbenzene (CHB) in the non-aqueous electrolyte, a large amount of gas can be quickly discharged when overcharged. Can be generated. As a result, the pressure in the battery case can be quickly increased, and the CID can be operated accurately.
Although this effect is not clear, it is considered that DCHB in the positive electrode active material layer serves as a reactive species and activates the reaction of CHB in the nonaqueous electrolytic solution. More specifically, when the battery is overcharged, DCCHB in the positive electrode active material layer is oxidatively decomposed to become a highly reactive cation radical, and the radical attacks the CHB in the non-aqueous electrolyte. It is considered that the polymerization proceeds or a cation radical of CHB is generated to attack another monomer. Thus, it is thought that the speed and efficiency of the gas generation at the time of overcharge are raised because DCHB used as the starting point of reaction is contained in the positive electrode as a reaction field.

In addition, DCB mixed in the positive electrode active material layer can perform anion insertion / extraction at a cyclohexyl group and / or a 6-membered ring portion. For this reason, it is thought that the resistance at the time of charging / discharging can be kept low, and the resistance at the time of normal operation can be reduced as compared with the conventional case.
Furthermore, compounds capable of decomposing during overcharge to generate gas (DCCH and CHB here; these are collectively referred to as “gas generating agent” hereinafter) are converted into a positive electrode and a non-aqueous electrolyte. By allocating, the CHB concentration in the non-aqueous electrolyte can be kept relatively low. For this reason, the resistance at the time of normal operation can be further reduced as compared with the conventional case, and excellent battery characteristics can be exhibited.
Therefore, according to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery that ensures reliability that allows the CID to be accurately operated at the time of overcharge and that has good battery performance during normal operation. it can.

  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 carrying out the present invention (for example, general manufacturing processes of components and batteries not characterizing the present invention) It can be grasped as a design matter of those skilled in the art based on the prior art. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The non-aqueous electrolyte secondary battery disclosed herein includes an electrode body including a positive electrode and a negative electrode, a non-aqueous electrolyte, and a battery case. The electrode body and the non-aqueous electrolyte are accommodated in the battery case. Hereinafter, each component will be described in order.

The positive electrode of the nonaqueous electrolyte secondary battery disclosed herein includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Such a positive electrode active material layer is characterized in that the positive electrode active material and dicyclohexylbenzene (DCHB) are mixed almost uniformly throughout. More specifically, for example, when the positive electrode active material layer is divided into two in the thickness direction, the ratio of DCB in the upper layer portion and the lower layer portion is substantially equal. Alternatively, for example, the ratio of DCHB in the central portion and the end portion in the width direction of the positive electrode active material layer is substantially equal. When DCHB is present uniformly throughout the positive electrode active material layer, high electron conductivity can be imparted to the entire positive electrode active material layer, and excellent charge / discharge characteristics can be exhibited during normal operation. In addition, when overcharge occurs, DCHB can be quickly oxidized and decomposed in a short time.
Such a positive electrode is, for example, applied to the surface of the positive electrode current collector with a slurry or paste composition in which the positive electrode active material is dispersed in an appropriate solvent (for example, N-methylpyrrolidone), and then dried. It can be made by removing the solvent.

As the positive electrode current collector, a conductive member made of a metal having good conductivity (for example, aluminum, nickel, titanium, stainless steel, etc.) can be employed. Examples of the positive electrode active material include lithium composite metal oxides such as layered and spinel (for example, LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , LiFePO _4, etc.) can be suitably employed. Examples of structural isomers of dicyclohexylbenzene include 1,4-dicyclohexylbenzene, 1,3-dicyclohexylbenzene, and 1,2-dicyclohexylbenzene, and among these, 1,4-dicyclohexylbenzene can be preferably used. Further, the positive electrode active material layer may contain components other than those described above as long as the effects of the present invention are not significantly impaired. Examples of such optional components include a binder and a conductive material. As the binder, polymer materials such as polyvinylidene fluoride (PVdF) and polyethylene oxide (PEO) can be preferably used. As the conductive material, a carbon material such as carbon black (for example, acetylene black or ketjen black) can be preferably used.

  The proportion of the positive electrode active material in the entire positive electrode active material layer is suitably about 60% by mass or more (typically 60% by mass to 99% by mass), and usually about 70% by mass to 95% by mass. It is preferable that The proportion of dicyclohexylbenzene (DCHB) in the entire positive electrode active material layer can be, for example, approximately 0.3% by mass to 3% by mass, and is generally preferably approximately 1% by mass to 2% by mass. . Thereby, the discharge resistance during normal operation can be improved by about 10% (preferably about 20%). Moreover, when using a binder, the ratio of the binder which occupies for the whole positive electrode active material layer can be about 0.5 mass%-10 mass%, for example, Usually, you may be about 1 mass%-5 mass%. Is preferred. When the conductive material is used, the ratio of the conductive material in the entire positive electrode active material layer can be, for example, approximately 1% by mass to 20% by mass, and is preferably approximately 2% by mass to 10% by mass. .

The negative electrode of the nonaqueous electrolyte secondary battery disclosed herein typically includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer contains at least a negative electrode active material.
As the negative electrode current collector, a conductive member made of a metal having good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be employed. As the negative electrode active material, carbon materials such as graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon) and the like can be suitably used, and among them, graphite can be preferably used. The negative electrode active material layer can also contain components other than the active material as long as the effects of the present invention are not significantly impaired. Examples of such optional components include a binder, a thickener, a dispersant, and a conductive material. As the binder, for example, a polymer material such as styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE) or the like can be preferably used. As the thickener, for example, carboxymethylcellulose (CMC), methylcellulose (MC) and the like can be preferably used.

  In addition to the positive electrode and the negative electrode, the electrode body of the nonaqueous electrolyte secondary battery disclosed herein typically includes a separator as an insulating layer that insulates both. As the separator, a porous resin sheet made of a resin such as polyethylene (PE) or polypropylene (PP) can be suitably used. Especially, what equips the single side | surface or both surfaces of a porous resin sheet with the heat resistant layer containing an inorganic compound particle (inorganic filler) is preferable. As the inorganic filler, alumina, boehmite, magnesia or the like can be adopted.

The non-aqueous electrolyte of the non-aqueous electrolyte secondary battery disclosed herein typically contains a supporting salt and cyclohexylbenzene (CHB) in a non-aqueous solvent.
As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be suitably used. Of these, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be preferably used. As the supporting salt, a lithium salt, a sodium salt, a magnesium salt, or the like can be suitably used, and among them, a lithium salt such as LiPF ₆ or LiBF ₄ can be preferably used. It is preferable to prepare so that the density | concentration of the supporting salt in a non-aqueous electrolyte may be in the range of 0.7 mol / L-1.3 mol / L. As cyclohexylbenzene, cyclohexylbenzene and its derivatives can be adopted. The proportion of cyclohexylbenzene in the entire non-aqueous electrolyte can be, for example, about 0.01% by mass to 4% by mass, and preferably about 0.1% by mass to 2% by mass. Furthermore, various additives such as lithium difluorophosphate, vinylene carbonate, and fluoroethylene carbonate can also be included in the nonaqueous electrolytic solution as long as the effects of the present invention are not significantly impaired.

The battery case is a container that accommodates the electrode body and the non-aqueous electrolyte. As a battery case, the thing made from lightweight metal materials, such as aluminum, can be employ | adopted preferably, for example.
Further, a current interruption mechanism is provided inside the battery case of the nonaqueous electrolyte secondary battery disclosed herein. Generally, when a non-aqueous electrolyte secondary battery is overcharged, a non-aqueous electrolyte (typically a non-aqueous solvent) is oxidized and decomposed at the positive electrode, and gas is generated at the negative electrode starting from this. The current interruption mechanism can prevent further overcharge by cutting a conductive path from at least one electrode terminal to the electrode body based on the generated gas. In the technology disclosed herein, a large amount of gas is rapidly generated at the negative electrode starting from the fact that DCB homogeneously mixed in the positive electrode active material layer and / or CHB in the non-aqueous electrolyte is oxidatively decomposed at the positive electrode. Can be generated. As a result, the pressure in the battery case can be quickly increased, and the CID can be operated accurately.

  The non-aqueous electrolyte secondary battery disclosed herein is a highly reliable battery that can accurately operate the CID when overcharged, and the battery performance (for example, output characteristics) during normal operation is the conventional product. It is characterized by improvement. Therefore, taking advantage of such characteristics, the battery can be suitably used for a large or large capacity battery. As a specific example, there is a high output power source for driving mounted on a vehicle such as a plug-in hybrid vehicle, a hybrid vehicle, or an electric vehicle.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

<Construction of non-aqueous electrolyte secondary battery>
First, dicyclohexylbenzene (DCHB) as a gas generating agent was pulverized in an agate mortar. Next, the DCHB was added to Li _1.14 Ni _0.34 Co _0.33 Mn _0.33 O ₄ as a positive electrode active material and lightly mixed. Here, acetylene black (AB) as a conductive material, polyvinylidene fluoride (PVdF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium are added and mixed in a rotating / revolving mixer to obtain a positive electrode slurry. Was prepared. Table 1 shows the types (structural isomers) of DCHB used and the mixing ratio (% by mass). Note that “-” in the column indicates that DCHB was not added. And this positive electrode sheet | seat (Example 1-Example 12) was produced by apply | coating this slurry to the surface of aluminum foil (positive electrode collector), and drying.

  Next, natural graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC sodium salt) as a thickener are kneaded while adjusting the viscosity with ion-exchanged water. A negative electrode slurry was prepared. The slurry was applied to the surface of a copper foil (negative electrode current collector) and dried to prepare a negative electrode sheet.

The positive and negative electrodes prepared above are laminated so that the active material layers face each other through a separator sheet (here, a three-layer structure in which polypropylene is laminated on both sides of polyethylene), and an electrode body is obtained. Produced. Such an electrode body was disposed in a battery case, and a non-aqueous electrolyte was injected therein. As a non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC: EMC: DMC = 30: 30: 40 is used as a supporting salt. LiPF ₆ was dissolved at a concentration of 1.1 mol / L, and in Examples 1 to 11, cyclohexylbenzene (DCHB) was included as a gas generating agent in the ratio shown in Table 1. In this way, 12 types of lithium ion secondary batteries (Examples 1 to 12) were constructed.

<Discharge characteristics test>
Each battery thus constructed was subjected to conditioning treatment and aging treatment in the usual manner, and then the initial discharge resistance of the constructed battery was measured under a temperature environment of 25 ° C. Specifically, CC charging was performed at a constant current of 1 C until the battery voltage reached 3.7 V, and then CV charging was performed at the voltage until the charging current reached 1/10 C. Thereafter, high-rate discharge was performed at a constant current of 10 C for 10 seconds, and an initial discharge resistance (IV resistance) was calculated from the voltage drop at this time using the following formula (I). The results are shown in Table 1. In Table 1, relative values based on Example 9 are shown.
Discharge resistance = (voltage immediately before discharge−voltage after 10 seconds of discharge) / (discharge current) (I)

<Overcharge test>
The battery after the initial characteristic measurement was subjected to an overcharge test under a temperature environment of 25 ° C. Specifically, first, CC charging was performed at a constant current of 1 C until the battery voltage reached 4.1 V, and then CV charging was performed at the voltage until the charging current reached 1/10 C to adjust to a fully charged state. Next, the fully charged battery was continuously charged with a constant current of 1 C, and the behavior of the battery at this time was observed. The results are shown in Table 1. In Table 1, the case where the CID operates normally in a safe state is indicated as “◯”, and when the battery temperature is excessively increased before the CID is operated, “X” is indicated. Indicated.

  When Example 9 to Example 11 differing only in the amount of addition of the gas generating agent (CHB) were compared, in Example 9 and Example 10, an excessive increase in battery temperature was observed during overcharge. This is presumably because a sufficient amount of gas for CID operation was not generated quickly because the amount of CHB added was small. On the other hand, in Example 11 with a large amount of CHB added, it was found that CID operates normally, that is, it has excellent overcharge resistance. However, in Example 11, the discharge resistance was deteriorated by 20% or more compared to Example 9. This is presumably because the diffusion and migration of lithium ions and / or anions were inhibited by the increase in the concentration of CHB in the non-aqueous electrolyte.

On the other hand, Example 4 had excellent overcharge resistance, and the discharge resistance was improved by 20% or more compared to Example 9. As the reason why Example 4 showed excellent overcharge resistance, it is considered that DCHB in the positive electrode active material layer and CHB in the non-aqueous electrolyte caused the above interaction. Moreover, the following (1) and (2) can be considered as the reason why the discharge resistance is improved in Example 4.
(1) The CHB concentration in the nonaqueous electrolytic solution could be kept relatively low.
(2) Since it is known that ions can be inserted into and desorbed from a six-membered ring as represented by graphite, the insertion and desorption of anions was similarly performed within the six-membered ring of DCBB. .

  All of Examples 1 to 6 which differ only in the proportion of DCHB in the positive electrode active material layer had excellent overcharge resistance. Furthermore, in Examples 1 to 5, it was found that the discharge resistance was improved as compared with Example 9, and good charge / discharge characteristics (low resistance) were exhibited. Among them, the discharge resistance was most improved in Example 4 in which the amount of DCHB added was 1.2 mass%. This is presumably because DCCHB in the positive electrode active material layer contributes to charge / discharge, but if added excessively, the reaction site of the positive electrode active material is blocked and insertion / extraction of lithium ions is inhibited. Therefore, in the positive electrode active material layer, by setting the proportion of DCB to 0.3 mass% to 3 mass%, the discharge resistance can be improved by about 10%, in particular, 1 mass% to 2 mass%, It was found that the discharge resistance can be improved by about 20%.

  In addition, comparing Example 4, Example 7 and Example 8 in which the position of the cyclohexyl group is different, Example 7 using 1,2-DCCH and Example 8 using 1,3-DCCH use 1,4-DCCH. The discharge resistance was not improved as much as it was (Example 4). This is thought to be due to the difference in the electronic state of the six-membered ring. That is, since the cyclohexyl group is an electron-withdrawing group, in 1,4-DCCH, the electrons in the six-membered ring are attracted symmetrically, and it is easy for the electron to be biased. It is inferred that it worked favorably during insertion and removal.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here.

Claims (1)

A non-aqueous electrolyte secondary battery in which a positive electrode, a negative electrode, and a non-aqueous electrolyte are housed in a battery case,
The positive electrode is a positive electrode current collector, a positive electrode active material layer formed on the positive electrode current collector, wherein the positive electrode active material and dicyclohexylbenzene are mixed almost uniformly throughout, and With
The non-aqueous electrolyte contains cyclohexylbenzene, and
The battery case is a non-aqueous electrolyte secondary battery provided with a current interruption mechanism that operates when gas is generated by decomposition of the cyclohexylbenzene and / or the dicyclohexylbenzene and an internal pressure of the battery case increases.