JP2019160683A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery which suppresses deterioration of the secondary battery to a higher degree and has improved high temperature durability without impairing output characteristics. The present invention provides a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte. Here, the non-aqueous electrolyte contains at least one selected from the group consisting of croconic acid, rhodisonic acid, and heptagonic acid alkali metal salts and derivatives thereof in a proportion of 0.05% by mass or more and 5% by mass or less. .. [Selection diagram] None

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 applications. For example, for non-aqueous electrolyte secondary batteries, both initial characteristics such as initial resistance are good and high durability means that the characteristics do not change significantly even after long-term use of the secondary battery. There is a problem that it is difficult. Therefore, for example, in Patent Documents 1 and 2, by adding specific squaric acid or the like as an additive to the non-aqueous electrolyte, it is possible to improve the capacity maintenance rate in the normal temperature region of the non-aqueous electrolyte secondary battery. It is disclosed.

JP 2008-262900 A JP 2012-109089 A

  On the other hand, for secondary batteries that are exposed to high-temperature environments, for example, in addition to the battery characteristics at room temperature, various characteristics are also required for battery characteristics after being exposed to high-temperature environments where deterioration is accelerated. Is done. However, in the secondary battery using the non-aqueous electrolyte to which the above-described squaric acid or the like is added, although the capacity retention rate in the normal temperature region is improved, there is a problem that other characteristics such as initial resistance are deteriorated. Further, no particular improvement is seen in the resistance characteristics after exposure to a high temperature environment. Therefore, the non-aqueous electrolyte secondary battery has room for improvement of characteristics at a higher level.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a non-aqueous electrolyte solution in which the deterioration of the secondary battery is more highly suppressed and the high-temperature durability is improved without impairing the output characteristics. The next battery is to provide.

  The technology 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 electrolyte contains 0.05% by mass or more and 5% by mass or less of at least one additive selected from the group consisting of alkali metal salts of croconic acid, rosic acid and heptagonic acid and derivatives thereof. Include in percentage. According to such a configuration, even if an additive is added for the purpose of improving the capacity retention ratio in the normal temperature region as is well known, the initial resistance can be reduced without deteriorating the initial resistance. In addition to this, even after being exposed to a high temperature environment for a long time, it is possible to satisfactorily suppress a decrease in capacity retention rate. In other words, the capacity retention rate and resistance characteristics at high temperatures in addition to room temperature are improved, so that the deterioration of the secondary battery is suppressed to a higher degree, and the initial resistance is reduced, so the output characteristics are also reduced. An improved secondary battery is provided.

  Hereinafter, preferred embodiments of the present invention will be described. It should be noted that matters other than the matters specifically mentioned in this specification and necessary for the implementation of the present invention (for example, secondary battery components such as a positive electrode and a negative electrode that do not characterize the present invention, and general battery Can be understood as a design matter 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 this specification and common technical knowledge in the field. In addition, description with "A-B" which shows a numerical range shall mean "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, an aluminum foil is suitable. The positive electrode active material layer has a porous structure. The positive electrode active material layer includes 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 a suitable solvent to the surface of the positive electrode current collector and drying it.

  The positive electrode active material may be any material that can reversibly occlude and release charge carriers. Preferred examples of the positive electrode active material include 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. Metal composite oxide is mentioned. 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 whose working potential during normal use is 4.2 V or less based on metallic lithium is preferable. Further, from the viewpoint of realizing a high output at a time, a so-called 5V-class positive electrode such as a spinel-type lithium nickel manganese oxide having an operating potential of over 4.2V and not more than about 5.2V with respect to metallic lithium. An active material is preferred. As the binder, for example, a halogenated vinyl resin such as polyvinylidene fluoride (PVdF), or a polyalkylene oxide such as polyethylene oxide (PEO) can be preferably used. The positive electrode active material layer may include an optional component (for example, a conductive material) other than the positive electrode active material. Examples of the conductive material include carbon blacks such as acetylene black and ketjen black, and carbon materials such as activated carbon, graphite, and carbon fibers.

  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 includes at least a negative electrode active material and a binder (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 a suitable solvent to the surface of the negative electrode current collector and drying.

  The negative electrode active material may be any material that can reversibly occlude and release charge carriers. Preferable examples of the negative electrode active material include carbon materials such as graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), and carbon nanotubes, or materials combining these materials. Among these, from the viewpoint of energy density, graphite-based materials such as natural graphite (graphite) and artificial graphite can be preferably used. As such a graphite-based material, a material in which amorphous carbon is disposed on at least a part of the surface can be preferably used. More preferably, almost all of the surface of the granular carbon is covered 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 employed. For example, a binder similar to that in the positive electrode can be used, but as a preferred form, an aqueous solvent is used, rubbers such as styrene butadiene rubber (SBR), water such as polyethylene oxide (PEO), vinyl acetate copolymer, etc. A water-soluble polymer material or a water-dispersible polymer material can be preferably used. The negative electrode active material layer may contain an optional component other than the negative electrode active material (for example, a thickener). Examples of the thickener include celluloses such as carboxymethyl cellulose (CMC).

  The positive electrode and the negative electrode are arranged to face each other while being insulated via a separator. The separator can be made of a sheet-like material that has electric insulation and enables the electrolyte to move between the positive and negative electrodes. Such a separator is made of various materials that are stable against an electrochemical reaction in the battery. As such a separator material, for example, a polyolefin resin such as polyethylene and polypropylene can be cited as a suitable example. Moreover, such a separator can be suitably composed of, for example, a woven fabric, a nonwoven fabric, a microporous sheet or the like having a gas permeability of about 100 seconds or more and about 1000 seconds or less. Instead of this separator, a gel electrolyte made of a gel polymer material may be used.

The nonaqueous electrolytic solution includes a nonaqueous solvent, an electrolyte supporting salt, and an additive characteristic in the technology disclosed herein.
The electrolytic solution typically exhibits a liquid state at a temperature of room temperature (25 ° C.) or lower. The electrolyte supporting salt is, for example, a lithium salt. The nonaqueous solvent and the lithium salt are not particularly limited, and may be the same as those used in the electrolyte solution of the conventional secondary battery.
Preferable examples of the non-aqueous solvent 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 that 1 type, or 2 or more types of the fluorinated chain | strand-shaped or cyclic carbonate which was made into are included. Preferable examples of the lithium salt include LiPF ₆ and LiBF ₄ . The density | concentration of the lithium salt in electrolyte solution can be 0.8-1.3 mol / L, for example.

  The nonaqueous electrolytic solution contains at least one of alkali metal salts of croconic acid, rosic acid and heptagonic acid and derivatives thereof as additives. Croconic acid, rosic acid and heptagonic acid are a kind of compounds called oxocarbonic acids. These oxocarbonic acid alkali metal salts are represented by the following general formulas (I) to (III). M in the formula is at least one selected from alkali metal elements. 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, M in the formula does not include a functional group such as hydrogen (H) or a hydrocarbon group.

  The above-mentioned derivatives of alkali metal salts of croconic acid, rosic acid, and heptagonic acid are not particularly limited. For example, it may be a compound in which the oxygen atom (O) in the general formulas (I) to (III) is bonded or substituted with an arbitrary functional group. Examples of such functional groups include hydrocarbon groups, alkylsilyl groups, and halogen-containing groups obtained by substituting some or all of these functional groups with halogen. Specifically, examples of the functional group include a saturated aliphatic hydrocarbon group, an unsaturated aliphatic hydrocarbon group, an aromatic hydrocarbon group, and an alicyclic hydrocarbon group. 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 necessarily match maintaining a high input / output characteristic for a long period of time. This means that a decrease in the discharge capacity of a non-aqueous secondary battery used at low load generally changes the balance between the charge level of the positive electrode and the negative electrode due to the decomposition reaction of the non-aqueous electrolyte on the electrode surface. One of the main causes is that the decrease in input / output characteristics, that is, the increase in DC resistance (IV resistance) is due to current collection resistance, liquid resistance of non-aqueous electrolyte, and charge between positive and negative electrodes and electrolyte. This is because an increase in movement resistance or the like is the main cause. Therefore, it is important to suppress an increase in DC resistance in secondary batteries for applications requiring high input / output. Although details are not clear, by adding an alkali metal salt of an oxocarbonic acid having a specific ring structure as described above to the non-aqueous electrolyte, 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 against a battery reaction in a high temperature environment can be formed on the surface of the electrode. This can improve the capacity maintenance ratio without increasing the initial resistance. At the same time, even after being exposed to a high temperature environment of, for example, 40 ° C. or higher (60 ° C. as an example) for a long time, an effect of suppressing an increase in battery resistance and a decrease in capacity can be obtained.

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

  Such an oxocarbonic acid having a specific ring structure and derivatives thereof (hereinafter sometimes simply referred to as “oxocarbonic acid”) are 0% when the total amount of the non-aqueous electrolyte is 100% by mass. It is preferably contained in a proportion of not less than 0.05% by mass and not more than 5% by mass. As a preferred example, the proportion of oxocarbonic acid is preferably 0.1% by mass or more, and more preferably 0.3% by mass or more. Thereby, the said effect can be expressed more clearly. However, addition of excess oxocarbonic acid is not preferable because the above effect cannot be obtained in accordance with the amount added, and the ratio of the above effect may be reduced. From this viewpoint, 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 electrolyte secondary battery disclosed herein has improved characteristics when exposed to a high temperature region in addition to properties at a normal temperature region. Specifically, since the capacity retention rate and resistance characteristics at high temperatures are improved in addition to normal temperature, the deterioration of the secondary battery is suppressed to a higher degree. Further, such improvement in high temperature characteristics can be realized while reducing without increasing the initial resistance in the normal temperature region, and thus a secondary battery with improved output characteristics is provided. That is, the secondary battery that achieves both excellent output characteristics and high temperature durability is realized by the technology disclosed herein. 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 high rate. Moreover, it is easy to be demanded for a secondary battery that is used outdoors and may be exposed to a high temperature environment such as installed with a high temperature heating device such as an engine. Therefore, the non-aqueous electrolyte secondary battery disclosed herein is for driving a motor mounted on a vehicle such as a plug-in hybrid vehicle, a hybrid vehicle, or an electric vehicle, which requires high output characteristics, high temperature durability, and the like. It can be particularly suitably used as a secondary battery (main battery).

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

[Production of evaluation battery]
(Example 1)
A natural graphite powder (C) having an average particle diameter of 20 μm as a negative electrode active material, a styrene butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are represented by C: SBR. A negative electrode paste was prepared by mixing at a mass ratio of: CMC = 98: 1: 1 and kneading with ion exchange water as a dispersion medium. And the negative electrode was produced by apply | coating the obtained negative electrode paste on the surface of the copper foil as a negative electrode electrical power collector, and drying and rolling.

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 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. And the positive electrode was produced by apply | coating the obtained positive electrode paste on the surface of the aluminum foil as a positive electrode electrical power collector, and drying and rolling.

The prepared positive electrode and negative electrode were laminated via a separator, and housed in a laminate-type battery case together with a non-aqueous electrolyte, whereby a lithium ion secondary battery for evaluation of Example 1 was constructed. As the separator, a microporous sheet having a three-layer structure made of polypropylene (PP) / polyethylene (PE) / polypropylene (PP) having an air resistance of 300 seconds obtained by the Gurley test method was used. In addition, as the non-aqueous electrolyte, LiPF ₆ as an electrolyte, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC: DMC: EMC = 30: 40: 30 What was melt | dissolved in the mixed solvent mixed by the ratio so that it might become a concentration of 1 mol / L was used.

(Examples 2 to 12)
To the non-aqueous electrolyte of Example 1, an oxocarbonic acid alkali salt as an additive was added in the types and addition amounts shown in Table 1 below, and the other examples were carried out in the same manner as in Example 1 above. A non-aqueous electrolyte secondary battery was obtained. In addition, the addition amount shows the ratio of the additive when the total amount of the non-aqueous electrolyte is 100% by mass, as a percentage based on mass.

[Initial characteristics]
The lithium ion secondary battery of each example was charged at a constant current in a constant temperature bath at 25 ° C. until the battery voltage reached 4.10 V at a rate of 0.3 C, and then the battery voltage at a rate of 0.3 C. Initial charging was performed by discharging at a constant current until 3.00V.
Next, constant current charging was performed until the battery voltage reached 4.10 V at a rate of 0.2 C, and then full charging was performed by constant voltage charging until the current value reached a rate of 1/50 C. Then, the capacity | capacitance when it discharged until the battery voltage became 3.00V with the constant current of 0.2C was measured, and it was set as the initial stage capacity | capacitance. 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 of 25 ° C., when the initial capacity is 100% of charge depth (State Of Charge: SOC), after charging with constant current at 0.3 C until SOC reaches 30%, 5C, 15C, 30C, 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 determine the IV characteristics during discharge, and the resistance (Ω) during discharge was determined from the slope of the obtained straight line, which was used as the initial resistance.

[High temperature storage test]
The lithium ion secondary battery of each example was charged until the SOC reached 100%, and a high temperature storage test was performed in which the battery of SOC 100% was stored in a constant temperature bath at 60 ° C. for one month (30 days). In this constant temperature storage test, deterioration of the battery is promoted. And about the battery of each example after a high temperature storage test, the capacity | capacitance and the resistance at the time of discharge were measured similarly to the above, and it was set as the capacity | capacitance after storage and the resistance after storage.
From these results, the capacity retention ratio after the high temperature storage test was calculated based on the following formula: capacity retention ratio (%) = capacity after storage ÷ initial capacity × 100; Shown in the column. Further, the ratio of increase in resistance after the high temperature storage test was calculated based on the following formula: resistance increase rate (%) = (resistance after storage−initial resistance) ÷ initial resistance × 100; It is shown in the column of “Rate”.

As shown in Table 1, squalate (Example 2), succinate (Examples 3 to 8), rhodizonate (Examples 9 to 8) are added to the non-aqueous electrolyte as an additive composed of an oxocarbonate. 11) It was found that when heptagonate (Example 12) was added, the battery characteristics varied depending on the type and amount of the additive.
Specifically, regarding the capacity characteristics after high-temperature storage, the battery of Example 2 to which squalinate, which is a four-membered ring compound, 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 ratio was approximately the same or increased. Therefore, the oxocarbonic acid to be added to the non-aqueous electrolyte may be any one of quinaconic acid salts, rhodizonates, heptagonates and their analogs, which are compounds having 5 or more members. I understood it. Moreover, it turned out that the capacity | capacitance maintenance factor tends to become the highest when a croconate is added as an additive from the comparison of Examples 6, 9, and 12. In addition, from comparisons of Examples 6 and 8 and Examples 9 to 11, although the oxocarbonic acid salt has a good effect with any of lithium salt, potassium salt, and sodium salt, the initial resistance and after high temperature storage When the capacity retention ratio and resistance characteristics of the film are comprehensively determined, it is particularly preferable to use a lithium salt.

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

  It was also found that when oxocarbonate was added to the non-aqueous electrolyte, the initial resistance was lowered in all the batteries other than Example 2 in which octaconate was added. And about the resistance characteristic after high temperature storage, it turned out that the resistance increase rate after high temperature storage falls in all the batteries which added the oxocarbonic acid salt to the non-aqueous electrolyte. That is, it was found that the increase in resistance after high-temperature storage can be suppressed by adding oxocarbonic acid salt to the non-aqueous electrolyte. In addition, about the battery of Example 2 which added squalinate, and the battery of Example 3 which added a small amount of croconate, compared with the battery of Example 1, the suppression effect of resistance increase was slight, About the other battery of Examples 4-12, it turned out that there exists a remarkable inhibitory effect of resistance raise compared with the battery of Example 1. FIG. From this, it can be said that the addition amount of the oxocarbonic acid salt should be 0.5% by mass or more in order to suitably suppress the rate of increase in resistance after high-temperature storage.

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

Claims (1)

A positive electrode, a negative electrode, and a non-aqueous electrolyte,
The non-aqueous electrolyte contains at least one selected from the group consisting of alkali metal salts of croconic acid, rosic acid and heptagonic acid and derivatives thereof in a proportion of 0.05% by mass or more and 5% by mass or less. Non-aqueous electrolyte secondary battery.