Electrolyte for secondary batteries, and secondary batteries

Abstract

To provide electrolyte 14 of a secondary battery 10, the electrolyte 14 being suppressed in decomposition.SOLUTION: Electrolyte 14 of a secondary battery 10 includes a nonaqueous solvent and a random copolymer of A segments and B segments, the A segment includes an ionic site comprised of an anionic alkali metal salt, and the B segment is comprised of a nonionic site.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] Embodiments of the present invention relate to an electrolyte for a secondary battery and a secondary battery. [Background technology]

[0002] With the proliferation of mobile phones and electric vehicles, there is a growing demand for high-capacity rechargeable batteries. Lithium-ion rechargeable batteries are widely used as such. However, lithium reserves are limited compared to the increasing demand, and lithium prices are rising.

[0003] Compared to lithium, sodium is abundant on Earth and inexpensive. Therefore, research into sodium batteries, which use sodium ions (Na ions) as charge carriers, is progressing. In particular, metallic sodium batteries, which use metallic sodium as the negative electrode active material, offer high capacity. However, a major challenge with metallic sodium batteries is the decomposition of the solvent and electrolyte at the negative electrode, even more so than with metallic lithium batteries, which use metallic lithium as the negative electrode active material.

[0004] International Publication No. 2017 / 149204 discloses that the electrolyte (ether-based solvent) of an electrochemical cell having a metallic sodium anode contains an electrolyte such as NaTFSA (sodium (trifluoromethylsulfonyl)amide), and forms a solid electrolyte interphase (SEI) on the anode to suppress the decomposition of the electrolyte. Ether-based solvents have good reduction stability but poor oxidation stability, making it difficult to use them with high-voltage cathodes.

[0005] International Publication No. 2013 / 034848 discloses a lithium battery having a solid electrolyte containing a copolymer. The copolymer is a block copolymer comprising a PEO (polyethylene oxide) polymer block and an anionic polymer block based on LiTFSA (lithium (trifluoromethylsulfonyl)amide). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2017 / 149204 [Patent Document 2] International Publication No. 2013 / 034848 [Overview of the project] [Problems that the invention aims to solve]

[0007] Embodiments of the present invention aim to provide an electrolyte for a secondary battery in which decomposition is suppressed, and a secondary battery comprising an electrolyte in which decomposition is suppressed. [Means for solving the problem]

[0008] The electrolyte of the secondary battery according to an embodiment of the present invention comprises a non-aqueous solvent and a random copolymer of A segments and B segments, wherein the A segments include ionic moieties made of an anionic alkali metal salt, and the B segments include nonionic moieties Therefore, the aforementioned metal salt is a sodium salt.

[0009] Another embodiment of the secondary battery comprises a metallic sodium negative electrode, a positive electrode that intercepts and deintercepts sodium, a non-aqueous solvent, and an electrolyte containing a polyanionic alkali metal salt, wherein the polyanionic alkali metal salt is a random copolymer of A segments and B segments, the A segments containing ionic moieties made of anionic alkali metal salts, and the B segments containing nonionic moieties. [Effects of the Invention]

[0010] According to embodiments of the present invention, it is possible to provide an electrolyte for a secondary battery in which decomposition is suppressed, and a secondary battery comprising such an electrolyte. [Brief explanation of the drawing]

[0011] [Figure 1] This is a diagram showing the configuration of the battery in the embodiment. [Figure 2] It is a diagram showing the charge-discharge characteristics of the battery of the embodiment. [Figure 3] It is a diagram showing the charge-discharge characteristics of the battery of the embodiment. [Figure 4] It is a diagram showing the rate characteristics of the battery of the embodiment. [Figure 5] It is a diagram showing the rate characteristics of the battery of the embodiment. [Figure 6] It is a diagram showing the charge-discharge characteristics of the battery of the embodiment. [Figure 7] It is a diagram showing the stability evaluation of the electrolytic solution (comparative example and embodiment) by charge and discharge. [Figure 8A] It is a diagram showing the analysis result of the electrolytic solution of the comparative example by XPS. [Figure 8B] It is a diagram showing the analysis result of the electrolytic solution of the embodiment by XPS. [Figure 9A] It is a diagram showing an example of voltage sweep in SEIRAS of the electrolytic solution. [Figure 9B] It is a diagram showing the analysis result of the electrolytic solution of the comparative example by SEIRAS. [Figure 9C] It is a diagram showing the analysis result of the electrolytic solution of the comparative example by SEIRAS. [Figure 10A] It is a diagram showing the analysis result of the electrolytic solution of the embodiment by SEIRAS. [Figure 10B] It is a diagram showing the analysis result of the electrolytic solution of the embodiment by SEIRAS.

Mode for Carrying Out the Invention

[0012] As shown in FIG. 1, the secondary battery 10 (hereinafter also referred to as "battery 10") of the embodiment of the present invention includes a positive electrode (cathode) 11, a negative electrode (anode) 12, a spacer 13, and an electrolytic solution 14.

[0013] In the coin-type battery 10, the positive electrode 11 and the negative electrode 12 are stacked with a spacer 13 in between, and sealed in the coin cell case 15. A spring 16 is placed on top of the negative electrode 12, and the coin cell case 15 is sealed with a lid 17. A gasket 18 is interposed on the side wall of the coin cell case 15.

[0014] The positive electrode 11 is a positive electrode active material, sodium manganese composite oxide (NMO:Na 0.44 It contains MnO2. The negative electrode 12 is a copper plate with a 200 μm thick metallic sodium sheet attached to it. The spacer 13 is a 200 μm thick glass fiber filter that absorbs and holds the electrolyte 14.

[0015] Electrolyte 14 is a non-aqueous solvent, propylene carbonate (PC), in which the copolymer shown below (hereinafter referred to as "first copolymer" or "NaSTFSA-BA") is dissolved as an electrolyte at a concentration of 1.3 M.

[0016] [ka]

[0017] m:n=50:50, number average molecular weight (Mn)=200kDa, weight average molecular weight (Mw)=580kDa

[0018] The first copolymer is a random copolymer of segment A and segment B. Segment A contains an ionic moiety consisting of an anionic alkali metal salt and has an amide-type structure with a fluoro group at its terminal. Segment B consists of a nonionic moiety.

[0019] The first copolymer is a polyanionic alkali metal salt produced by radical polymerization of an ionic alkali metal salt monomer, which serves as the ionic site, and a neutral monomer, which serves as the nonionic site.

[0020] Specifically, the first copolymer is prepared by reacting 50 mol% sodium styrene (trifluoromethylsulfonyl)amide (NaSTFSA), 50 mol% butyl acrylate (BA), and 1 mol% azobisisobutyronitrile (AIBN), which is a polymerization initiator, with 60 mL of dimethyl sulfoxide (DMSO) as the solvent under an argon atmosphere at 60°C for 24 hours.

[0021] [ka]

[0022] <Battery Characteristics> Figures 2 and 3 show the charge-discharge cycle characteristics (60°C) of battery 10. The cycle voltage is 2.0V-3.8V. Below, the voltage is (Na / Na + The voltage is 0.1 mA / cm² per unit area of ​​the electrode. 2 That is the case.

[0023] Battery 10 was able to charge and discharge for 200 cycles while maintaining a Coulomb efficiency of over 99%.

[0024] Figures 4 and 5 show the rate characteristics of battery 10. Low current density (0.5 mA / cm²) 2 The current density at which 50% of the capacitance is maintained is 5.0 mA / cm² at a cutoff voltage of 2.0 V. 2 Therefore, at a cutoff voltage of 1.0V, the current is 10.0mA / cm². 2 That was the case.

[0025] Figure 6 shows the charge-discharge cycle characteristics of battery 10A, which has the same configuration as battery 10 except that hard carbon is used as the positive electrode 11. Battery 10A was able to charge and discharge even after 200 cycles, and although not shown, the Coulomb efficiency was maintained at 98% or higher.

[0026] The capacities of batteries 10 and 10A are smaller than the theoretical capacity of 1165 mAh / g of a cell with metallic sodium as the negative electrode. This is presumed to be a problem with the positive electrode 11. That is, in battery 10, the mass of the positive electrode active material is 4.7 mg / cm 2 whereas in battery 10A, the mass of the positive electrode active material was 5.7 mg / cm 2 was.

[0027] <Electrolyte characteristics> Figure 7 shows the cycle test results of a simple cell with metallic sodium at both electrodes. The separator and electrolyte are the same as those of battery 10. Simulating the charge and discharge of a secondary battery, at 0.2 mA / cm 2 , (deposition of metallic sodium for 2 hours / dissolution of metallic sodium for 2 hours) was repeated. As the electrolyte for the comparative example, a PC solution of 1M-Na[TFSA] was used.

[0028] In the electrolyte of the comparative example, the voltage increased and measurement became impossible in less than 10 hours. In contrast, for electrolyte 14, stable charge and discharge were performed even at 200 hours. In electrolyte 14, the increase in voltage with an increase in the number of cycles is presumed to be due to an increase in interfacial resistance due to the growth of the SEI (polyanion coating layer) described later.

[0029] Next, after washing and drying the negative electrode after the test, surface observation was performed. In the cell with electrolyte 14, no change was observed in the copper plate. However, in the cell with the comparative electrolyte, the surface of the copper plate changed to black.

[0030] Figures 8A and 8B show the carbon peaks by X-ray photoelectron spectroscopy (XPS) of the negative electrode surface. In the cell with the electrolyte of the comparative example shown in Figure 8A, a plurality of peaks considered to be decomposition products of the electrolyte were observed. In contrast, in the cell with electrolyte 14, only peaks considered to be copolymers were observed. Separately, the measured film thickness of the SEI was 120 nm.

[0031] Next, as shown in Figure 9A, surface-enhanced infrared spectroscopy (SEIRAS) was performed on the negative electrode while sweeping the potential. In this analysis, a negative electrode with a platinum thin film plated on a silicon prism was used. As shown in Figure 9B, when the potential was swept from 1.8V (natural potential, OCV) to 0.1V, peaks (PC, propylene) appeared on the surface of the negative electrode using the comparative electrolyte, due to the decomposition products of the electrolyte. Subsequently, as shown in Figure 9C, when the potential was swept from 0.1V to 1.8V, the peaks (PC, propylene) disappeared. This is because the decomposition products diffused into the liquid.

[0032] As shown in Figure 10A, when the potential was swept from 1.8V (OCV) to 0.1V on the negative electrode surface using the electrolyte 14 of the embodiment, a peak attributable to polyanions appeared. Furthermore, as shown in Figure 10B, the peak attributable to polyanions remained even when the potential was swept from 0.1V to 1.8V (spontaneous potential, OCV). This peak attributable to polyanions suggests the formation of SEI.

[0033] Furthermore, in a separate analysis, the presence of NaF was confirmed near the negative electrode of the SEI formed on the negative electrode surface using electrolyte 14. Therefore, it is thought that the copolymer partially decomposed, and the fluorine at the terminal end of the NaSTFSA group bonded with sodium.

[0034] From the above results, it can be seen that the electrolyte 14 of the battery 10 in the embodiment is decomposed because it forms SEI on the surface of the metallic sodium anode. In other words, since SEI is formed by copolymers, it is less likely to be destroyed than SEI formed by monomolecules, and therefore less likely to come into contact with the electrolyte and metallic sodium.

[0035] As already explained, the first copolymer is a random copolymer of A segments and B segments. Segment A contains ionic moieties consisting of anionic alkali metal salts. Segment B consists of nonionic moieties.

[0036] Polymers consisting solely of A segments, and copolymers containing blocks consisting solely of A segments, exhibit low dissociation of alkali metal ions, specifically Na ions. In contrast, the first copolymer, in which A segments are randomly copolymerized with nonionic B segments, shows increased ionic dissociation of Na ions in the A segments, resulting in increased ionic conductivity and improved solubility in solvents.

[0037] Therefore, it is important that the first copolymer is not a block copolymer in which the same type of segment (A segment, B segment) is arranged in long, continuous lengths, but rather a random copolymer (r polymer) in which the arrangement of the two types of segments (A segment, B segment) is not orderly. The random copolymer is prepared from a mixed solution of monomers that will become the A segment, monomers that will become the B segment, and a polymerization initiator. The first copolymer may also be an alternating copolymer in which the A segment and B segment are arranged alternately. However, since alternating copolymers are not easy to manufacture, it is particularly preferable that the first copolymer be a random copolymer.

[0038] The monomer of segment A contains an ionic moiety consisting of an anionic alkali metal salt. In particular, it is preferable that it has a fluorosulfonylamide group.

[0039] Monomers of segment A are synthesized from at least one monomer containing, for example, an alkali metal salt of at least one of the following: styrene-sulfonyl(trifluoromethylsulfonyl)amide, sulfonyl(trifluoromethylsulfonyl)amide acrylate, sulfonyl(trifluoromethylsulfonyl)amide methacrylate, sulfonyl(trifluoromethylsulfonyl)amide acrylamide, sulfonyl(trifluoromethylsulfonyl)amide methacrylamide, ethylene-sulfonyl(trifluoromethylsulfonyl)amide, propylene-sulfonyl(trifluoromethylsulfonyl)amide, sulfonyl(trifluoromethylsulfonyl)amide diene, and maleamide-sulfonyl(trifluoromethylsulfonyl)amide.

[0040] As monomers for segment A, sodium styrene(trifluoromethylsulfonyl)amide (NaSTFSA) and the following, sodium methacrylate(trifluoromethylsulfonyl)amide (NaMTFSA), sodium styrene(fluoromethylsulfonyl)amide (NaSFSA), and sodium methacrylate(fluoromethylsulfonyl)amide (NaMFSA), are particularly preferred as monomers for synthesizing the first copolymer because they are easy to synthesize.

[0041] [ka]

[0042] [ka]

[0043] [ka]

[0044] In order to increase the ionic dissociability of the first copolymer, it is preferable that the B segment has weak coordination properties for capturing cations (Na ions). For example, segments having polyethylene oxide (PEO) groups, which are widely used in solid electrolytes to ensure conductivity, are undesirable as the B segment of the first copolymer because they have strong coordination properties for capturing cations.

[0045] B segments with weak cation-binding coordination properties are preferably synthesized from at least one monomer having an acrylic group or an allyl group. B segments are particularly preferably synthesized from at least one alkyl acrylate or alkyl methacrylate, for example.

[0046] As alkyl methacrylates, methyl methacrylate, ethyl methacrylate, etc., can be used. As acrylic monomers, butyl acrylate (BA), methyl acrylate (MA), ethyl acrylate (EA), etc., as shown below, can be used.

[0047] [ka]

[0048] [ka]

[0049] The ratio of segment A to segment B (m:n, m+n=100) is preferably between (30:70) and (90:10), and the ratio (m:n) is most preferably (50:50). If the ratio of segment A (m) is less than the lower limit (30), the number of Na ions that ensure conductivity is small, resulting in decreased ionic conductivity. If the ratio of segment A (m) exceeds the upper limit (90), the dissociability of Na ions decreases, resulting in decreased ionic conductivity and decreased solubility in the solvent.

[0050] The molar concentration of the copolymer in the electrolyte 14 is preferably greater than 0.1 M and less than 2.0 M. If the molar concentration of the first copolymer is greater than the lower limit, the electrolyte contains sufficient Na ions, and thus ionic conductivity can be ensured. If the molar concentration of the first copolymer is less than the upper limit, the ionic conductivity of the electrolyte will not decrease due to an increase in viscosity.

[0051] Copolymers have a large molecular weight and therefore do not move easily in the electrolyte. As a result, in electrolyte 14, only Na ions move, while the anions (polymers) do not, resulting in a high Na ion transport rate. On the other hand, if the molecular weight of the copolymer is too large, its solubility in the solvent decreases, and the viscosity of the electrolyte increases, slowing down the movement rate of Na ions. For this reason, the number-average molecular weight of the copolymer is preferably between 50 kDa and 500 kDa.

[0052] For polymerization of copolymers, azo radical polymerization initiators such as 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2-cyclopropylpropionitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and dimethyl 2,2'-azobisisobutyrate, as well as peroxide radical polymerization initiators such as benzoyl peroxide, t-butyl hydroperoxide, and cumene hydroperoxide may be used.

[0053] The concentration of the polymerization initiator, polymerization temperature, and polymerization time for polymerizing the copolymer are appropriately selected to obtain a copolymer with the desired molecular weight.

[0054] The electrolyte 14 may also contain low molecular weight salts other than the first copolymer, such as sodium bisfluorosulfonylamide (NaFSA), sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6), etc. However, it is preferable that 90% by weight or more of the electrolyte in the electrolyte 14 is the first copolymer in order to prevent a decrease in the Na ion transport fraction.

[0055] A carbonate-based solvent can be used as the solvent for the electrolyte 14. Carbonate-based solvents have better oxidation resistance than ether-based solvents, are less likely to decompose even at high-potential positive electrodes, and have a wider potential window. For this reason, the battery 10 can be driven at a higher voltage than a battery using an ether-based solvent.

[0056] As the carbonate-based solvent, at least one of propylene carbonate (PC), diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, and ethylene carbonate may be used.

[0057] The positive electrode 11 only needs to contain one or more positive electrode active materials that absorb and release sodium. The positive electrode active material is not limited to sodium manganese oxide (NMO) or hard carbon. As the positive electrode active material, one or more conventionally known materials such as various sodium-containing transition metal oxides, sodium-containing transition metal sulfides, and sodium-containing transition metal fluorides can be used, as long as they absorb and release sodium.

[0058] Examples of positive electrode active materials include sodium iron composite oxide (NaFeO2), sodium cobalt composite oxide (NaCoO2), sodium chromium composite oxide (NaCrO2), sodium manganese composite oxide (NaMnO2), sodium nickel composite oxide (NaNiO2), and sodium nickel titanium composite oxide (NaNi 0.5 Ti 0.5 O2), sodium nickel manganese composite oxide (NaNi 0.5 Mn 0.5 O2), sodium iron manganese complex oxide (Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2), sodium nickel cobalt manganese composite oxide (NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), sodium manganese composite oxide (NaMn2O4), sodium nickel manganese composite oxide (NaNi 1 / 2 Mn 3 / 2 These include O2, sodium iron phosphate compounds (NaFePO4), sodium manganese phosphate compounds (NaMnPO4), and sodium cobalt phosphate compounds (NaCoPO4).

[0059] In particular, sodium cobalt composite oxides, sodium iron cobalt composite oxides, and sodium cobalt phosphate fluoride can be used as high-potential positive electrode active materials.

[0060] The battery of the present invention is not limited to a metallic sodium battery, but may also be a metallic lithium secondary battery having an electrolyte containing a metallic lithium anode, a non-aqueous solvent, a (LiSTFSA-BA) copolymer, etc.

[0061] Furthermore, in a battery of the present invention that includes the electrolyte of the present invention, a film similar to the SEI formed on the negative electrode may also be formed on the positive electrode.

[0062] The battery 10 is not limited to coin-type batteries, but may have various known structures, such as wound-type or laminate-type batteries. The battery 10 may also have multiple unit cells (positive electrode / electrolyte / negative electrode), or it may have multiple units consisting of multiple unit cells.

[0063] The present invention is not limited to the embodiments described above. The present invention can be modified, altered, or combined in various ways, as long as the spirit of the invention is not altered. [Explanation of symbols]

[0064] 10, 10A…Battery 11...Positive electrode 12...Negative electrode 13…Spacer 14...Electrolyte 15… Coin cell case 16... Spring 17…Lid 18…Gasket

Claims

1. Non-aqueous solvents and A random copolymer of segment A and segment B, The A segment includes an ionic moiety made of an anionic alkali metal salt, The aforementioned B segment consists of nonionic sites, An electrolyte for a secondary battery, characterized in that the aforementioned metal salt is a sodium salt.

2. The electrolyte for a secondary battery according to claim 1, characterized in that the A segment is synthesized from a monomer having a fluorosulfonylamide group at its terminal end.

3. The electrolyte for a secondary battery according to claim 1, characterized in that the A segment is synthesized from at least one monomer comprising an alkali metal salt of at least one of the following: styrene-sulfonyl(trifluoromethylsulfonyl)amide, sulfonyl(trifluoromethylsulfonyl)amide acrylate, sulfonyl(trifluoromethylsulfonyl)amide methacrylate, sulfonyl(trifluoromethylsulfonyl)amide acrylamide, sulfonyl(trifluoromethylsulfonyl)amide methacrylamide, ethylene-sulfonyl(trifluoromethylsulfonyl)amide, propylene-sulfonyl(trifluoromethylsulfonyl)amide, sulfonyl(trifluoromethylsulfonyl)amide diene, and maleamide-sulfonyl(trifluoromethylsulfonyl)amide.

4. The electrolyte for a secondary battery according to claim 1, characterized in that the A segment is synthesized from a monomer containing an alkali metal salt of at least one of styrene(trifluoromethylsulfonyl)amide, methacrylate(trifluoromethylsulfonyl)amide, styrene(fluoromethylsulfonyl)amide, and methacrylate(fluoromethylsulfonyl)amide.

5. The electrolyte for a secondary battery according to claim 1, characterized in that the A segment is synthesized from a monomer containing an alkali metal salt of styrene(trifluoromethylsulfonyl)amide.

6. The electrolyte for a secondary battery according to claim 1, characterized in that the B segment is synthesized from at least one of a monomer having an acrylic group and a monomer having an allyl group.

7. The electrolyte for a secondary battery according to claim 1, characterized in that the B segment is synthesized from at least one of alkylacrylic and alkyl methacrylate.

8. The electrolyte for a secondary battery according to claim 1, characterized in that the B segment is synthesized from at least one of butyl acrylate, ethyl acrylate, methyl acrylate, methyl methacrylate, and ethyl methacrylate.

9. The electrolyte for the secondary battery according to claim 5, characterized in that the B segment is synthesized from butyl acrylate.

10. The electrolyte for a secondary battery according to claim 1, characterized in that the ratio of segment A to segment B is (30:70) to (90:10).

11. The electrolyte for a secondary battery according to claim 1, characterized in that the number-average molecular weight of the copolymer is greater than 50 kDa and less than 500 kDa.

12. The electrolyte for a secondary battery according to claim 1, characterized in that the non-aqueous solvent is a carbonate-based solvent.

13. The electrolyte for a secondary battery according to claim 1, characterized in that the non-aqueous solvent is at least one of propylene carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, and ethylene carbonate.

14. The electrolyte for a secondary battery according to claim 9, characterized in that the non-aqueous solvent is propylene carbonate.

15. A non-aqueous solvent, A random copolymer of segment A and segment B, The A segment includes an ionic moiety made of an anionic alkali metal salt, The aforementioned B segment consists of nonionic sites, The electrolyte for a secondary battery is characterized in that the B segment is synthesized from at least one of butyl acrylate, ethyl acrylate, methyl acrylate, methyl methacrylate, and ethyl methacrylate.

16. A non-aqueous solvent, A random copolymer of segment A and segment B, The A segment includes an ionic moiety made of an anionic alkali metal salt, The aforementioned B segment consists of nonionic sites, The electrolyte for a secondary battery is characterized in that the number-average molecular weight of the copolymer is greater than 50 kDa and less than 500 kDa.

17. Metallic sodium anode and A positive electrode that absorbs and releases sodium It comprises a non-aqueous solvent and an electrolyte containing a polyanionic alkali metal salt, The aforementioned polyanionic alkali metal salt is a random copolymer of A segment and B segment. The A segment includes an ionic moiety consisting of an anionic sodium salt, The B segment is characterized by comprising a nonionic portion.

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