Sodium ion secondary battery and non-aqueous electrolyte for sodium ion secondary batteries

Abstract

The present invention provides a sodium ion secondary battery (NIB) that has excellent cycle characteristics, which are important for secondary batteries mounted on a vehicle such as an electric vehicle, and an electrolyte that makes it possible to produce such a sodium ion secondary battery. A sodium ion secondary battery according to the present invention is characterized by comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte in which a supporting salt is dissolved in a non-aqueous solvent, wherein: the positive electrode comprises a material that contains a sodium composite oxide; the negative electrode comprises a material that contains hard carbon; and the non-aqueous electrolyte contains more than 0.01 wt% but not more than 0.5 wt% of at least one substance selected from among lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate.

Description

Sodium ion secondary battery and non-aqueous electrolyte for sodium ion secondary battery

[0001] The present invention relates to a sodium ion secondary battery having excellent battery characteristics such as battery cycle life, and a nonaqueous electrolyte for the sodium ion secondary battery.

[0002] In recent years, sodium-ion batteries (NIBs) have attracted attention due to the limited availability of lithium in lithium-ion batteries (LIBs). The lithium used in these batteries is produced in a limited number of countries, and due to issues with raw material costs and resource availability, NIBs have attracted attention. The operating principle of NIBs is the same as that of LIBs, except that the supporting electrolyte is changed from lithium ions to sodium ions. While it is often assumed that the same cathode and anode materials used in LIBs can be directly adapted for NIBs, the reality is that sodium ions are larger than lithium ions, making the same cathode and anode materials used in LIBs in NIBs in practice unsuitable for NIBs. For example, graphite is used as the anode material for LIBs. However, the use of graphite anodes in NIBs prevents interphase intercalation and deintercalation, resulting in insufficient charge and discharge. Therefore, research into the use of hard carbon (HC) as anode materials for NIBs is actively underway.

[0003] It is known that when hard carbon is used for the anode of an NIB, the cycle characteristics are worse than those of a battery using graphite for the anode of an LIB. Therefore, when additives such as vinylene carbonate (VC) and 1,3-propane sultone (PS), which are used in the electrolyte of an LIB, were used in an NIB, it was found that they actually worsened the cycle characteristics of the battery. As mentioned above, although the operating principle of an NIB is the same as that of an LIB, the components used in an LIB cannot be directly reused, and research into new electrolytes for an NIB using hard carbon as the anode material is actively underway.

[0004] In Patent Document 1, lithium methyl sulfate (CH ) is used as a non-aqueous electrolyte for LIB. ３ OSO ３ Li), lithium ethyl sulfate (C ２ H５ OSO ３ Li), sodium methyl sulfate (CH ３ OSO ３ Na, DC1), sodium ethyl sulfate (C ２ H ５ OSO ３ Na, DC3), potassium methyl sulfate (CH ３ OSO ３ K, DC2), potassium ethyl sulfate (C ２ H ５ OSO ３ In Example I-5 of the same document, LiCoO is used as the positive electrode material. ２ The negative electrode uses artificial graphite, and the electrolyte is a non-aqueous solvent (EC, VC, DMC, MEC, etc.) with a supporting electrolyte of LiPF ６ 0.3 wt % of CH ３ OSO ３ The electrolyte solution of Example V-23 contains a non-aqueous solvent (EC, VC, DMC, MEC, etc.) and a supporting electrolyte of LiPF ６ 0.12 wt % of CH ３ OSO ３ A battery using Na has been proposed. All of the LIBs show excellent discharge capacity retention rates.

[0005] However, the CH described in the examples of Patent Document 1 ３ OSO ３ Li and CH ３ OSO ３ In LIB, non-aqueous electrolytes using Na as an additive are used with graphite anodes and supporting electrolytes such as LiPF ６ The effectiveness of this method has been found in the hard carbon anode of NIB and the supporting electrolyte NaPF4 in a non-aqueous solvent. ６ There is no mention whatsoever of the effectiveness of the method for non-aqueous electrolytes containing the above.

[0006] International Publication No. 2014 / 163055

[0007] The present invention aims to solve the above-mentioned problems and provide a sodium ion secondary battery (NIB) with excellent cycle characteristics, which are important for secondary batteries for vehicles such as electric vehicles, and also aims to provide an electrolyte capable of producing such an NIB.

[0008] As a result of extensive research by the present inventors, it has been found that by using a specific positive electrode, negative electrode, and non-aqueous electrolyte in a sodium ion secondary battery (NIB), an NIB can be obtained that can improve the cycle characteristics of the battery.

[0009] That is, the sodium ion secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution in which a supporting salt is dissolved in a non-aqueous solvent, wherein the positive electrode is made of a material containing a sodium composite oxide, the negative electrode is made of a material containing hard carbon, and the non-aqueous electrolyte solution contains at least one selected from lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate in an amount of more than 0.01 wt % and not more than 0.5 wt %.

[0010] The nonaqueous electrolyte for a sodium ion secondary battery of the present invention is used in a sodium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte in which a supporting salt is dissolved in a nonaqueous solvent, the positive electrode being made of a material containing a sodium composite oxide, and the negative electrode being made of a material containing hard carbon, and is characterized in that it contains at least one selected from lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate in an amount greater than 0.01% by weight and not greater than 0.5% by weight.

[0011] In the non-aqueous electrolyte for a sodium ion secondary battery of the present invention, the non-aqueous solvent is preferably at least two solvents selected from a cyclic carbonate and a chain carbonate.

[0012] In the nonaqueous electrolyte solution for a sodium ion secondary battery of the present invention, the nonaqueous solvent is a solvent in which a cyclic carbonate and a chain carbonate are combined, and the volume ratio of the cyclic carbonate to the chain carbonate is preferably in the range of 5:95 to 50:50.

[0013] The non-aqueous electrolyte for a sodium ion secondary battery of the present invention preferably contains dinitriles having a carbon chain length of 2 to 5 in an amount within the range of 0.1% by weight to 5% by weight.

[0014] The NIB of the present invention is a sodium-ion secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte solution in which a supporting salt is dissolved in a nonaqueous solvent, in which the positive electrode is made of a material containing a sodium composite oxide, the negative electrode is made of a material containing hard carbon, and the nonaqueous electrolyte solution contains more than 0.01 wt % but not more than 0.5 wt % of one or more selected from lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate, thereby improving the cycle characteristics of the NIB. This makes it possible to produce an NIB with excellent cycle characteristics, which are important for automotive secondary batteries such as those used in electric vehicles.

[0015] The following describes examples of embodiments and configurations of the present invention, but the present invention is not limited to these. Anything that conforms to the intent of the claims, problem-solving means, effects of the invention, etc. is included in the present invention.

[0016] The non-aqueous electrolyte solution is composed of a supporting salt and a non-aqueous solvent. The supporting salt in the present invention is not particularly limited as long as it is a sodium salt. For example, SO ２ NaN(SO ２ F) ２ (hereinafter also referred to as NaFSI), NaPF containing phosphorus (P) ６ , NaPO ２ F ２ , NaBF with boron (B) ４ Sodium salts such as the following can be used as supporting salts.

[0017] The sodium salt of the supporting salt may be used alone or in combination of two or more. A preferred combination of these sodium salts is a sodium salt having phosphorus (P) and SO ２ A combination of a sodium salt having a group, a combination of a sodium salt having phosphorus (P) and a sodium salt having boron (B) is preferred. Specifically, NaPF ６ Also NaFSI, NaPF ６ and NaBF ４ is preferred. ６ and other Na salts, NaPF ６ The weight ratio of NaPF to other Na salts is ６ The ratio of the supporting salt to the other Na salt is preferably in the range of 100:0 to 1:99, more preferably 100:0 to 50:50, and most preferably 100:0 to 70:30. The total concentration of the supporting salt is preferably in the range of 0.5 to 3 mol, more preferably 1 to 2 mol, per 1 L of the total volume of the NIB electrolyte solution of the present invention.

[0018] On the other hand, it was found that in the NIB using the hard carbon of the present invention as the negative electrode material, when a lithium salt is added as a supporting salt in an amount exceeding 0.5 wt%, the cycle characteristics deteriorate. Specific examples of lithium salts that cause such a result include SO ２ LiN(SO ２ F) ２ etc., SO ４ C with group ２ H ５ OSO ３ LiPF with phosphorus (P) such as Li ６ , LiPO ２ F ２ , lithium difluorobis(oxalato)phosphate (LiDFOP), etc., which has boron (B), LiBF ４ Examples of supporting salts include lithium salts such as lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB).

[0019] The non-aqueous solvent in the present invention is not particularly limited, and examples thereof include cyclic carbonates, chain carbonates, etc. Suitable examples of cyclic carbonates include ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), etc. However, vinylene carbonate (VC), a cyclic carbonate, is excluded because it reduces cycle characteristics. Suitable examples of chain carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc.

[0020] These solvents may be used alone or in combination of two or more.Suitable combinations of these cyclic carbonates include a combination of two types of EC and FEC, a combination of PC and FEC, and a combination of three types of EC, PC and PC, EC, PC and FEC, etc.Among these, it is particularly preferred to contain EC, and a combination of EC and FEC, a combination of EC, PC and PC, or a combination of EC, PC and FEC is preferred.

[0021] In addition, suitable combinations of these chain carbonates include a combination of two types of DMC and EMC, a combination of EMC and DEC, a combination of DMC and DEC, etc. Among these, it is particularly preferable to contain EMC, and a combination of DMC and EMC, or a combination of EMC and DEC is preferred.

[0022] When the cyclic carbonate in the nonaqueous electrolyte solution according to the present invention contains a chain carbonate, the ratio of the cyclic carbonate to the chain carbonate (volume ratio) is preferably 5:95 to 50:50, more preferably 10:90 to 40:60, from the viewpoint of improving electrochemical properties over a wide range of temperatures from high to low. In particular, when a plurality of cyclic carbonates are used, it is preferable that the ratio of EC is 50% or more.

[0023] The contents of lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate in the NIB electrolyte of the present invention are not particularly limited. However, since many of these are poorly soluble with solubilities of 0.5 wt% or less, an appropriate content is greater than 0.01 wt% relative to the total weight of the NIB electrolyte of the present invention, more preferably 0.03 wt% or more, and most preferably 0.05 wt% or more. The upper limit is preferably 0.5 wt% or less, more preferably 0.3 wt% or less, and most preferably 0.1 wt% or less relative to the total weight of the NIB electrolyte. The NIB electrolyte of the present invention can improve the cycle characteristics of the NIB by containing an appropriate amount of lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, or potassium ethyl sulfate in the electrolyte.

[0024] Furthermore, in the present invention, it has been found that adding dinitriles with carbon chain lengths of 2 to 5, such as succinonitrile, glutaronitrile, adiponitrile, and pimelonitrile, to a nonaqueous electrolyte improves the cycle characteristics of NIBs. These compounds are preferably present in an amount of 0.1 wt % or more, more preferably 0.5 wt % or more, and most preferably 1 wt % or more, based on the total weight of the NIB electrolyte of the present invention. The upper limit is preferably 5 wt % or less, more preferably 3 wt % or less, and most preferably 2 wt % or less, based on the total weight of the NIB electrolyte.

[0025] The NIB of the present invention comprises a positive electrode, a negative electrode, a separator, and the NIB electrolyte of the present invention. The separator of the present invention is not particularly limited as long as it can be used in the NIB. It is most preferable to use a separator made of a microporous membrane formed from a polyolefin material such as polypropylene or polyethylene, but a nonwoven fabric separator can also be used. The porous sheet or nonwoven fabric may have a single-layer or multilayer structure, and the separator surface may be coated with an oxide such as alumina. The thickness of the separator must be as thin as possible to increase the volumetric energy density of the battery. Therefore, a thickness of 20 μm or less is preferable, and a thickness of 10 μm or less is particularly preferable.

[0026] The negative electrode in the present invention is made of a material containing hard carbon. The negative electrode active material used in the negative electrode is a carbon material having a lattice spacing (d ００２ A suitable example is a non-graphitizable material (hard carbon) having a lattice spacing (d ) of 0.37 nm or more. Carbon-based materials are often used as general battery materials, but in the present invention, a non-graphitizable material (hard carbon) having a lattice spacing (d ) of 0.37 nm or more is preferably used. ００２ Graphite materials such as natural graphite and artificial graphite having a particle size of 0.340 nm or less are not preferred.

[0027] Examples of binders used in the negative electrode composite include ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene and butadiene copolymer (SBR), acrylonitrile and butadiene copolymer (NBR), carboxymethyl cellulose (CMC), etc. The negative electrode is produced, for example, by kneading the negative electrode active material with these binders to form a slurry negative electrode composite, applying this negative electrode composite to a copper foil or aluminum foil current collector, drying, pressure molding, and then heat treating, for example, under vacuum at 80°C.

[0028] The positive electrode active material used in the positive electrode of the present invention is, for example, sodium chromite (NaCrO ２ ), sodium nickel manganate (NaNi ０．５ Mn ０．５ O２ , Na ２／３ Ni １／３ Mn ２／３ O ２ , NaNi ０．５ Mn ０．２ Ti ０．３ O ２ , Na ２／３ Ni １／３ Mn １／２ Ti １／６ O ２ , NaNi １／３ Mn １／３ Ti １／３ O ２ , NaNi ０．３３ Ti ０．３３ Mn ０．１６ Mg ０．１７ O ２ etc.), sodium ferromanganate (NaFe ０．５ Mn ０．５ O ２ , Na ２／３ Fe １／３ Mn ２／３ O ２ , NaFe ０．４ Ni ０．３ Mn ０．３ O ２ , NaFe １／３ Ni １／３ Mn １／３ O ２ etc.), sodium iron cobaltate (NaFe ０．５ Co ０．５ O ２ In particular, a positive electrode active material containing a sodium composite oxide containing Fe is preferably used. ４ , NaVPO ４ F, Na ３ V ２ (P.O. ４ ) ３ , Na ２ Fe ２ (SO ４ ) ３ Polyanionic compounds such as the above are also suitably used.

[0029] Examples of conductive additives used in the positive electrode composite include known or commercially available conductive additives such as carbon black (e.g., acetylene black, Ketjen black), carbon nanotubes, carbon fiber, activated carbon, and graphite. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethyl cellulose (CMC). The positive electrode is prepared, for example, by kneading the conductive additive and binder with the positive electrode active material to form a slurry-like positive electrode composite, applying the resulting positive electrode composite to an aluminum foil current collector, drying, pressure molding, and then heat-treating the composite at, for example, 80°C under vacuum. If the battery can be assembled without using a binder, the binder need not be used.

[0030] Other materials in the NIB of the present invention are not particularly limited as long as they can be used for NIB. The current collector used in the present invention is not particularly limited, but aluminum foil is preferred, and the current collector may be made porous to improve the permeability of the electrolyte.

[0031] In the present invention, the solvent used for the binder is not particularly limited, and various solvents can be selected depending on the active material or binder used. Specifically, when PVDF is used as the binder, it is preferable to use N-methyl-2-pyrrolidone as the solvent, while when a rubber-based binder such as styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, or carboxymethyl cellulose (CMC) is used, water is a suitable solvent.

[0032] The structure of the sodium ion secondary battery of the present invention is not particularly limited, but examples of the shape of the secondary battery having a positive electrode, a negative electrode, and a separator include a coin-type battery, a cylindrical battery, a square battery, a pouch-type battery, etc. In addition, the present invention is also applicable to a clay-like pouch-type NIB in which two electrode layers, a clay-like positive electrode and a clay-like negative electrode, are separated by a separator, instead of a sheet-like positive electrode or negative electrode.

[0033] EXAMPLES Next, the present invention will be specifically explained with reference to examples and comparative examples, but these do not limit the present invention in any way.

[0034] [Example 1] [Preparation of Electrolyte Solution] A non-aqueous solvent was mixed with EC / DMC at a volume ratio of 30 / 70, and NaPF 6 was added as a supporting electrolyte. ６ was dissolved at 1 M (mol / L). 0.1 wt % of sodium methyl sulfate was added to the solution obtained in this manner to prepare the electrolyte solution of Example 1. The weight % of sodium methyl sulfate (additive) in Table 1 represents the respective proportion relative to the weight of the entire electrolyte solution prepared, and the M (mol / L) of the supporting salt represents the respective proportion relative to the volume of the entire electrolyte solution prepared.

[0035] [Preparation of sodium ion secondary battery (NIB) and measurement of battery characteristics] NaFe １／３ Ni １／３ Mn １／３ O ２ A mixture of 96% by weight of ethylene black (positive electrode active material), 2% by weight of acetylene black (conductive additive), and 2% by weight of polyvinylidene fluoride (binder) was mixed, and 1-methyl-2-pyrrolidone was added to form a slurry to prepare a positive electrode composite, which was then applied to aluminum foil. The mixture was then dried and pressure-molded to prepare a positive electrode. Similarly, 98% by weight of hard carbon (negative electrode active material), 1% by weight of a styrene-butadiene copolymer binder, and 1% by weight of carboxymethyl cellulose were added to water and mixed to prepare a negative electrode composite in a slurry form, which was then applied to aluminum foil. The mixture was then dried, pressure-molded, and heat-treated to prepare a negative electrode sheet. A three-layer 20-micron microporous film consisting of polyethylene sandwiched between polypropylene was used as the separator, and the electrolytes of Examples 1 to 5 and Comparative Examples 1 and 2 were injected to prepare coin batteries (coin-shaped NIB: diameter 20 mm, thickness 3.2 mm).

[0036] This coin battery was charged at 25°C using a charge / discharge device ACD-MO1A (manufactured by Asuka Electronics) in CCCV mode at a constant current and constant voltage of 1C rate up to an upper limit voltage of 4.0 V, and then discharged in CC mode at a 1C rate down to a lower limit voltage of 2.0 V. The discharge capacity at the first cycle was measured using a non-aqueous solvent mixture of EC / DMC = 30 / 70 (volume ratio) as the electrolyte, and NaPF as the supporting electrolyte. ６ The discharge capacity at the first cycle was calculated as a relative ratio compared to that at the first cycle when an electrolyte solution containing only 1 mol / L of ethylenediaminetetraacetic acid (Comparative Example 1, no additives) was used. The cycle performance (%) was calculated by multiplying the obtained capacity (mAh / g) by 200th cycle / 1st cycle x 100. The results are shown in Table 1.

[0037] Comparative Example 1 An electrolyte solution was prepared in the same manner as in Example 1, except that sodium methyl sulfate was not added, and a coin battery was fabricated in the same manner as in Example 1, and the battery characteristics were measured. The results are shown in Table 1.

[0038] [Examples 2 to 11, Comparative Examples 2 to 8] Coin batteries were fabricated in the same manner as in Example 1, except that the electrolyte composition and additives (amounts of additives) were changed as shown in Table 1, and the battery characteristics were measured. The results are shown in Table 1.

[0039] A coin battery was fabricated in the same manner as in Example 1, except that the same electrolyte solution as in Comparative Example 1 was used and artificial graphite was used as the negative electrode active material of the coin battery, and the battery characteristics were measured. The results are shown in Table 1.

[0040]

[0041] The results in Table 1 show that a sodium-ion secondary battery (NIB) with excellent cycle characteristics can be provided by using a positive electrode containing a sodium composite oxide and a negative electrode made of a material containing hard carbon, and further using a nonaqueous electrolyte containing a specific amount of an additive selected from lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate. Example 3 shows that adding adiponitrile, a dinitrile with a carbon chain length of 2 to 5, in addition to the additive (sodium methyl sulfate) results in an NIB with excellent cycle characteristics of 92.1%. Comparative Example 2 shows that when the amount of additive (sodium methyl sulfate) added is insufficient, the effect of adding the additive is not observed in either the first-cycle discharge capacity (initial capacity) or the cycle characteristics (cycle life). It can be seen from Comparative Example 5 that when the amount of additive (lithium ethyl sulfate) added is excessive, both the first cycle discharge capacity (initial capacity) and cycle characteristics (cycle life) deteriorate. Also, it can be seen from Comparative Example 8 that when an artificial graphite negative electrode (a negative electrode material not containing hard carbon) is used, both the first cycle discharge capacity (initial capacity) and cycle characteristics (cycle life) deteriorate significantly compared to Comparative Example 1 using a hard carbon negative electrode.

[0042] By using the nonaqueous electrolyte of the present invention, it is possible to provide a sodium ion secondary battery with excellent battery characteristics such as battery cycle characteristics. The present invention aims to solve the problems of raw material cost and resource quantity seen in LIB while maintaining the performance of NIB, and the contribution of the present invention is immeasurable.

Claims

1. A sodium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution in which a supporting salt is dissolved in a non-aqueous solvent, wherein the positive electrode is made of a material containing a sodium composite oxide, the negative electrode is made of a material containing hard carbon, and the non-aqueous electrolyte solution contains at least one selected from the group consisting of lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate in an amount greater than 0.01% by weight and not greater than 0.5% by weight.

2. A nonaqueous electrolyte solution for use in a sodium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution in which a supporting salt is dissolved in a nonaqueous solvent, the positive electrode being made of a material containing a sodium composite oxide, and the negative electrode being made of a material containing hard carbon, the nonaqueous electrolyte solution for use in a sodium ion secondary battery comprising more than 0.01 wt % and not more than 0.5 wt % of at least one selected from the group consisting of lithium methyl sulfate, lithium ethyl sulfate, sodium methyl sulfate, sodium ethyl sulfate, potassium methyl sulfate, and potassium ethyl sulfate.

3. The non-aqueous electrolyte for a sodium ion secondary battery according to claim 2, wherein the non-aqueous solvent is at least two solvents selected from the group consisting of cyclic carbonates and chain carbonates.

4. The nonaqueous electrolyte solution for a sodium ion secondary battery according to claim 2, wherein the nonaqueous solvent is a solvent comprising a combination of a cyclic carbonate and a chain carbonate, and the volume ratio of the cyclic carbonate to the chain carbonate is within the range of 5:95 to 50:

50.

5. The nonaqueous electrolyte for a sodium ion secondary battery according to any one of claims 2 to 4, which contains dinitriles having a carbon chain length of 2 to 5 in an amount ranging from 0.1% by weight to 5% by weight.

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