JP6909408B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

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

As non-aqueous electrolyte secondary batteries such as lithium-ion batteries are put into practical use, there is an increasing demand for improvement of various characteristics according to the application. For example, for a non-aqueous electrolyte secondary battery, both good initial characteristics such as initial resistance and high durability, which means that the characteristics do not change significantly even after long-term use of the secondary battery, are compatible. There is a problem that it is difficult to do. Therefore, for example, in Patent Documents 1 and 2, it is possible to improve the capacity retention rate of the non-aqueous electrolyte secondary battery in the normal temperature range by adding a specific squaric acid or the like as an additive to the non-aqueous electrolyte solution. It is disclosed.

Japanese Unexamined Patent Publication No. 2008-262900 Japanese Unexamined Patent Publication No. 2012-109089

On the other hand, for example, for secondary batteries for applications exposed to high temperature environments, it is required to improve various characteristics not only in the battery characteristics in the normal temperature environment but also in the battery characteristics after being exposed to the high temperature environment where deterioration is promoted. Will be done. However, in the secondary battery using the non-aqueous electrolytic solution to which the above-mentioned squaric acid or the like is added, although the capacity retention rate in the normal temperature range is improved, there is a problem that other characteristics such as initial resistance are deteriorated. In addition, no particular improvement is seen in the resistance characteristics after exposure to a high temperature environment. Therefore, there is room for improving the characteristics of the non-aqueous electrolyte secondary battery at a higher level.

The present invention has been made in view of this point, and an object thereof is a non-aqueous electrolyte solution in which deterioration of a secondary battery is suppressed to a higher degree and high temperature durability is improved without impairing output characteristics. The next battery is to be provided.

The technique disclosed herein provides a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte. Here, the non-aqueous electrolytic solution contains at least one additive selected from the group consisting of alkali metal salts of croconic acid, logisonic acid and heptagonic acid and derivatives thereof, in an amount of 0.05% by mass or more and 5% by mass or less. Include in proportion. According to such a configuration, as is known, even if an additive is added for the purpose of improving the capacity retention rate in the room temperature range, the initial resistance can be lowered without deteriorating the initial resistance. In addition to this, it is possible to satisfactorily suppress a decrease in the capacity retention rate even after being exposed to a high temperature environment for a long period of time. In other words, the capacity retention rate and resistance characteristics at high temperatures in addition to room temperature are improved, which suppresses the deterioration of the secondary battery to a higher degree, and the initial resistance is reduced, so the output characteristics are also improved. An improved secondary battery is provided.

Hereinafter, preferred embodiments of the present invention will be described. It should be noted that matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention (for example, secondary battery components such as positive electrodes and negative electrodes that do not characterize the present invention, and general batteries). Manufacturing process, etc.) can be grasped as a design item of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art. The description of "A to B" indicating the numerical range means "A or more and B or less".

The non-aqueous electrolyte secondary battery disclosed herein (hereinafter, simply referred to as “battery”, “secondary battery”, etc.) includes a positive electrode, a negative electrode, and an electrolytic solution.
Hereinafter, each component will be described in order.

The positive electrode includes a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. As the positive electrode current collector, a metal sheet having good conductivity, for example, aluminum foil is suitable. The positive electrode active material layer has a porous structure. The positive electrode active material layer contains at least a positive electrode active material and a binder. Such a positive electrode is produced, for example, by applying a positive electrode paste obtained by kneading a positive electrode active material and a binder in an appropriate solvent to the surface of a positive electrode current collector and drying the positive electrode.

The positive electrode active material may be any material that can reversibly store and release charge carriers. Preferable examples of the positive electrode active material are lithium transitions such as lithium nickel-containing composite oxide, lithium cobalt-containing composite oxide, lithium nickel cobalt-containing composite oxide, lithium manganese-containing composite oxide, and lithium nickel cobalt manganese-containing composite oxide. Examples include metal composite oxides. From the viewpoint of high durability, a so-called 4V class positive electrode active material such as a layered lithium nickel cobalt manganese-containing composite oxide having an operating potential of 4.2V or less based on metallic lithium is preferable. From the viewpoint of achieving high output at one time, a so-called 5V class cathode such as lithium nickel manganese oxide having a spinel structure having an operating potential exceeding 4.2V and about 5.2V or less based on metallic lithium is used. Active material is preferred. Further, as the binder, for example, a vinyl halide resin such as polyvinylidene fluoride (PVdF) and a polyalkylene oxide such as polyethylene oxide (PEO) can be preferably used. The positive electrode active material layer may contain an arbitrary component (for example, a conductive material) other than the positive electrode active material. Examples of the conductive material include carbon black such as acetylene black and Ketjen black, and carbon materials such as activated carbon, graphite and carbon fiber.

The negative electrode includes a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. As the negative electrode current collector, a metal sheet having good conductivity, for example, a metal foil such as copper or nickel is suitable. The negative electrode active material layer has a porous structure. The negative electrode active material layer contains at least a negative electrode active material and a binder. Such a negative electrode is produced, for example, by applying a negative electrode paste obtained by kneading a negative electrode active material and a binder in an appropriate solvent to the surface of a negative electrode current collector and drying the negative electrode.

The negative electrode active material may be any material that can reversibly occlude and release the charge carrier. Preferable examples of the negative electrode active material include carbon materials such as graphite (graphite), non-graphitized carbon (hard carbon), easily graphitized carbon (soft carbon), and carbon nanotubes, or a material in which these are combined. Among them, graphite-based materials such as natural graphite (stone ink) and artificial graphite can be preferably used from the viewpoint of energy density. As the graphite-based material, those in which amorphous carbon is arranged on at least a part of the surface can be preferably used. More preferably, almost the entire surface of the granular carbon is coated with an amorphous carbon film. As the binder, the same binder as that used for the negative electrode of this type of battery can be appropriately adopted. For example, the same binder as in the positive electrode can be used, but in a preferred form, an aqueous solvent is used, and rubbers such as styrene-butadiene rubber (SBR), polyethylene oxide (PEO), and water-soluble polymer such as vinyl acetate copolymer are used. A sex-based polymer material or a water-dispersible polymer material can be preferably adopted. The negative electrode active material layer may contain an optional component (for example, a thickener or the like) other than the negative electrode active material. Examples of the thickener include celluloses such as carboxymethyl cellulose (CMC).

The positive electrode and the negative electrode are arranged so as to face each other in a state of being insulated via a separator. The separator may be composed of a sheet-like material that has electrical insulation and allows the transfer of electrolytes between the positive and negative electrodes. Such separators are made of various materials that are stable against electrochemical reactions in the battery. As such a separator material, for example, a polyolefin resin such as polyethylene and polypropylene can be mentioned as a preferable example. Further, such a separator can be suitably configured by, for example, a woven fabric, a non-woven fabric, a microporous sheet or the like having an air permeability of 100 seconds or more and 1000 seconds or less. Instead of this separator, a gel-like electrolyte made of a gel-like polymer material may be used.

The non-aqueous electrolyte solution contains a non-aqueous solvent, an electrolyte supporting salt, and additives characteristic of the techniques disclosed herein.
The electrolytic solution typically exhibits a liquid state at a temperature below room temperature (25 ° C.). The electrolyte supporting salt is, for example, a lithium salt. The non-aqueous solvent and the lithium salt are not particularly limited, and may be the same as those used in the electrolytic solution of the conventional secondary battery.
Preferable examples of non-aqueous solvents include aprotic solvents such as carbonates, esters and ethers. Among them, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), chain carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), and these carbonates are fluorine. It is preferable to contain one or more fluorinated chain or cyclic carbonates. Preferable examples of the lithium salt include, for example, LiPF ₆ , LiBF _{4, and the} like. The concentration of the lithium salt in the electrolytic solution can be, for example, 0.8 to 1.3 mol / L.

The non-aqueous electrolyte solution contains at least one of the alkali metal salts of croconic acid, logisonic acid and heptagonic acid and derivatives thereof as additives. Croconic acid, logisonic acid and heptagonic acid are a type of compound called oxocarbonic acid. The alkali metal salts of these oxocarbonic acids are represented by the following general formulas (I) to (III). In addition, M in the formula is at least one kind selected from alkali metal elements. The M is not particularly limited and may be any of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr). As M, for example, lithium, sodium, or potassium is preferable, and lithium is particularly preferable. Further, as M in the formula, a functional group such as hydrogen (H) or a hydrocarbon group is not included.

The above derivatives of alkali metal salts of croconic acid, logisonic acid and heptagonic acid are not particularly limited. For example, it may be a compound in which the oxygen atom (O) in the above general formulas (I) to (III) is bonded or substituted with an arbitrary functional group. Examples of such a functional group include a hydrocarbon group, an alkylsilyl group, and a halogen-containing group in which a part or all of hydrogen of these functional groups is replaced with a halogen. Specifically, examples of the functional group include a saturated aliphatic hydrocarbon group, an unsaturated aliphatic hydrocarbon group, an aromatic hydrocarbon group, an alicyclic hydrocarbon group and the like, and more specifically, for example. , Methyl group, ethyl group, propyl group, isopropyl group, butyl group, allyl group, phenyl group, trimethylalkylsilyl group, triethylalkylsilyl group and the like. However, it can be said that an embodiment in which such a functional group is not contained is more preferable.

In a secondary battery, maintaining a high discharge capacity for a long period of time does not always correspond to maintaining a high input / output characteristic for a long period of time. This means that the decrease in the discharge capacity of a non-aqueous secondary battery used at a low load generally changes the balance between the charge levels of the positive electrode and the negative electrode due to the decomposition reaction of the non-aqueous electrolyte solution on the electrode surface. Is one of the main causes, whereas the decrease in input / output characteristics, that is, the increase in DC resistance (IV resistance) is due to the current collection resistance, the liquid resistance of the non-aqueous electrolyte solution, and the charge between the positive and negative electrodes and the electrolyte. This is because the increase in movement resistance and the like is the main cause. Therefore, it is important to suppress an increase in DC resistance in a secondary battery for applications that require high input / output. Although the details are not clear, by adding the alkali metal salt of oxocarbonic acid having a specific ring structure as described above to the non-aqueous electrolyte solution, the initial resistance is not increased in the normal temperature range (for example, the initial resistance is reduced). However, it is considered that a good film having excellent resistance to the battery reaction in a high temperature environment can be formed on the surface of the electrode. This makes it possible to improve the capacity retention rate without increasing the initial resistance. At the same time, it is possible to obtain the effect of suppressing an increase in battery resistance and a decrease in capacity even after being exposed to a high temperature environment of, for example, 40 ° C. or higher (60 ° C. for example) for a long period of time.

The alkali metal salt of oxocarbonic acid and its derivative include compounds having a 5-membered ring, a 6-membered ring or a 7-membered ring in the molecular structure represented by the above general formulas (I) to (III). It is known that there is a squaric acid having a 4-membered ring structure. However, according to the study by the present inventors, when this squaric acid is used as an additive, although it is effective in improving the capacity retention rate of the non-aqueous electrolyte secondary battery in the normal temperature range, the initial resistance is increased. In addition to this, it has been clarified that the battery characteristics (for example, capacity retention rate and resistance increase rate) after storage at a high temperature of 40 ° C. or higher are deteriorated. Therefore, in the technique disclosed herein, it is preferable not to use such an alkali metal salt of squaric acid as an additive and a derivative thereof. Although the details are not clear, it is known that in squaric acid having a 4-membered ring structure, the carbon atoms are arranged in a quadrangular shape, so that the reactivity with various nucleophiles is high. In a high temperature environment, it is considered that such high reactivity causes an undesired reaction with a non-aqueous solvent or the like.

Oxocarbonic acid having such a specific ring structure and its derivative (hereinafter, may be simply collectively referred to as "oxocarbonic acid") are 0 when the total amount of the non-aqueous electrolytic solution is 100% by mass. It is preferably contained in a proportion of 0.05% by mass or more and 5% by mass or less. As a preferable example, the ratio of the oxocarbonic acid is preferably 0.1% by mass or more, more preferably 0.3% by mass or more. As a result, the above effect can be expressed more clearly. However, the addition of an excessive amount of oxocarbonic acid is not preferable because not only the above effect cannot be obtained in proportion to the amount of addition, but also the ratio of the above effect may be reduced. From this point of view, the proportion of oxocarbonic acid is preferably 3% by mass or less, more preferably 2% by mass or less, and particularly preferably 1.5% by mass or less, for example, 1% by mass or less.

As described above, the non-aqueous electrolytic solution secondary battery disclosed herein has improved characteristics not only in the normal temperature range but also in the high temperature range. Specifically, since the capacity retention rate and resistance characteristics at high temperatures in addition to room temperature are improved, deterioration of the secondary battery is suppressed to a higher degree. Further, since such improvement of high temperature characteristics can be realized while reducing the initial resistance in the normal temperature region without increasing, a secondary battery having improved output characteristics is provided. That is, the technology disclosed here realizes a secondary battery having both excellent output characteristics and high temperature durability. It should be noted that such output characteristics and high temperature durability are easily obtained in a secondary battery used in a mode of charging / discharging at a high output density or a high rate. In addition, it is likely to be required for secondary batteries for applications that may be exposed to a high temperature environment, such as those stored outdoors or installed together with a high temperature heating device such as an engine. Therefore, the non-aqueous electrolyte secondary battery disclosed herein is used for driving a motor mounted on a vehicle such as a plug-in hybrid vehicle, a hybrid vehicle, or an electric vehicle, which is required to have high output characteristics and high temperature durability. It can be particularly preferably used as a secondary battery (main battery) or the like.

Hereinafter, some examples of the present invention will be described, but the present invention is not intended to be limited to such examples.

[Manufacturing of evaluation battery]
(Example 1)
Natural graphite powder (C) having an average particle size of 20 μm as a negative electrode active material, styrene-butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener, C: SBR. A negative electrode paste was prepared by mixing at a mass ratio of CMC = 98: 1: 1 and kneading with ion-exchanged water as a dispersion medium. Then, the obtained negative electrode paste was applied to the surface of a copper foil as a negative electrode current collector, dried and rolled to prepare a negative electrode.

Lithium nickel cobalt manganese-containing composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ : LNCMO) as a positive electrode active material, polyvinylidene fluoride (PVdF) as a binder, and a conductive material. Acetylene black (AB) and N-methyl-2-pyrrolidone (NMP) as an organic solvent are mixed at a mass ratio of LNCMO: PVdF: AB = 87: 10: 3 and kneaded to prepare a positive electrode paste. did. Then, the obtained positive electrode paste was applied to the surface of an aluminum foil as a positive electrode current collector, dried and rolled to prepare a positive electrode.

The prepared positive electrode and negative electrode were laminated via a separator and housed in a laminated type battery case together with a non-aqueous electrolyte solution to construct a lithium ion secondary battery for evaluation in Example 1. As the separator, a microporous sheet having a three-layer structure made of polypropylene (PP) / polyethylene (PE) / polypropylene (PP) having an air permeability resistance of 300 seconds obtained by the Garley test method was used. As the non-aqueous electrolyte solution, LiPF ₆ as an electrolyte and ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of EC: DMC: EMC = 30: 40: 30 A mixture dissolved in a mixed solvent mixed at a ratio so as to have a concentration of 1 mol / L was used.

(Examples 2 to 12)
To the non-aqueous electrolytic solution of Example 1, an alkaline salt of oxocarbonic acid as an additive was added in the types and amounts shown in Table 1 below, and the others were the same as in Example 1 above, in Examples 2 to 10. A non-aqueous electrolyte secondary battery was obtained. The amount of the additive added is the ratio of the additive when the total amount of the non-aqueous electrolytic solution is 100% by mass, which is shown as a percentage based on the mass.

[Initial characteristics]
The lithium-ion secondary batteries of each example are charged with a constant current at a rate of 0.3 C until the battery voltage reaches 4.10 V in a constant temperature bath at 25 ° C., and then the battery voltage rises at a rate of 0.3 C. Initial charging was performed by discharging a constant current until the voltage reached 3.00 V.
Next, the battery was charged with a constant current at a rate of 0.2 C until the battery voltage became 4.10 V, and then charged with a constant voltage until the current value reached a rate of 1/50 C to make a full charge. Then, the capacity when discharged until the battery voltage became 3.00 V with a constant current of 0.2 C was measured and used as the initial capacity. This initial capacity is shown in the column of "Initial resistance ratio" in Table 1 as a relative value when the capacity of the battery of Example 1 is 100%.

In a constant temperature bath at 25 ° C., when the initial capacity is set to 100% of the charging depth (State Of Charge: SOC), constant current charging is performed at 0.3C until the SOC reaches 30%, and then 5C, 15C, 30C, And the battery voltage was measured 10 seconds after discharging at a rate of 45C. Each current value and voltage value at this time were plotted to obtain the IV characteristic at the time of discharge, and the resistance (Ω) at the time of discharge was obtained from the slope of the obtained straight line, which was used as the initial resistance.

[High temperature storage test]
A high-temperature storage test was conducted in which the lithium ion secondary batteries of each example were charged to 100% SOC, and the 100% SOC battery was stored in a constant temperature bath at 60 ° C. for 1 month (30 days). In this constant temperature storage test, the deterioration of the battery is accelerated. Then, for the batteries of each example after the high temperature storage test, the capacity and the resistance at the time of discharge were measured in the same manner as described above, and used as the capacity after storage and the resistance after storage.
From these results, the capacity retention rate after the high-temperature storage test was calculated based on the following formula: capacity retention rate (%) = capacity after storage ÷ initial capacity × 100; Shown in the column. In addition, the rate of increase in resistance after the high-temperature storage test was calculated based on the following formula: resistance increase rate (%) = (post-preservation resistance-initial resistance) ÷ initial resistance x 100; Shown in the "Rate" column.

As shown in Table 1, as an additive consisting of an oxocarbonate in a non-aqueous electrolyte solution, aquaric acid salt (Example 2), a squaric acid salt (Examples 3 to 8), and a logizonate (Example 9 to 9) are used. 11) It was found that when heptagone salt (Example 12) was added, the battery characteristics changed variously depending on the type and amount of the additive added.
Specifically, regarding the capacity characteristics after high-temperature storage, the battery of Example 2 to which the 4-membered ring compound squaric acid was added had a lower capacity retention rate than the battery of Example 1. On the other hand, for the batteries other than Example 2, it was found that the capacity retention rate was almost the same or increased. From these facts, it is preferable that the oxocarbonic acid to be added to the non-aqueous electrolytic solution is any one of a compound having a 5-membered ring or more, such as kutoconate, logizonate and heptagone, and their relatives. I understood it. Further, from the comparison of Examples 6, 9 and 12, it was found that the volume retention rate tends to be the highest when croconate is added as an additive. Further, from the comparison of Examples 6 and 8 and Examples 9 to 11, the oxocarbonate has a good effect regardless of whether it is a lithium salt, a potassium salt or a sodium salt, but it has an initial resistance and after high temperature storage. It can be said that it is particularly preferable to use a lithium salt when comprehensively judging the capacity retention rate and the resistance characteristics of the above.

Furthermore, from the comparison of Examples 3 to 7, in Example 3 in which 0.03% by mass of oxocarbonate was added, the capacity retention rate after high temperature storage was almost the same as that in Example 1, whereas oxocarbon It was confirmed that the volume retention rate after high temperature storage can be clearly increased by adding 0.5% by mass or more of the acid salt. It can be said that the above oxocarbonate should be added in an amount of 0.5% by mass or more. However, there was a mountainous relationship between the amount of oxocarbonate added and the volume retention rate after high-temperature storage, and the volume retention rate tended to decrease when the amount added was excessive. Therefore, it is preferable that the amount of the oxocarbonic acid added is not excessive, and it is understood that the amount of the oxocarbonic acid is preferably suppressed to about 1% by mass or less, for example.

It was also found that when oxocarbonate was added to the non-aqueous electrolyte solution, the initial resistance was reduced in all the batteries except Example 2 to which Kuguchi Conate was added. As for the resistance characteristics after high-temperature storage, it was found that the rate of increase in resistance after high-temperature storage decreased in all batteries in which oxocarbonate was added to the non-aqueous electrolyte solution. That is, it was found that the increase in resistance after high temperature storage can be suppressed by adding oxocarbonate to the non-aqueous electrolyte solution. The battery of Example 2 to which squaric acid was added and the battery of Example 3 to which a small amount of croconate was added had a slight effect of suppressing an increase in resistance as compared with the battery of Example 1. It was found that the other batteries of Examples 4 to 12 had a remarkable effect of suppressing the increase in resistance as compared with the batteries of Example 1. From this, it can be said that the amount of oxocarbonate added should be 0.5% by mass or more in order to preferably suppress the rate of increase in resistance after high temperature storage.

Although the present invention has been described in detail above, the above-described embodiments and examples are merely examples, and the inventions disclosed herein include various modifications and modifications of the above-mentioned specific examples.

Claims (1)

It has a positive electrode, a negative electrode, and a non-aqueous electrolyte solution.
The non-aqueous electrolytic solution contains at least one of a lithium salt of croconic acid, a lithium salt of logizonic acid and a lithium salt of heptagonic acid in a proportion of 0.05% by mass or more and 5% by mass or less. Liquid secondary battery.