Secondary battery

Abstract

Provided is a secondary battery having improved charge / discharge characteristics. The secondary battery includes a positive electrode, a negative electrode, and an electrolyte solution. The electrolyte solution contains at least one of first compounds represented by formula (1), formula (2), or formula (3). (n is an integer from 0 to 5 (inclusive). ).

Description

Technical Field

[0001] This technology relates to a secondary battery. Background Technology

[0002] Non-Patent Document 1 discloses a secondary battery using an electrolyte containing anisole.

[0003] Existing technical documents

[0004] Non-patent literature

[0005] Non-patent literature 1: Moon, J., Kim, DO, Bekaert, L. et al. Non-fluorinated non-solvating cosolvent enabling superior performance of lithium metal negative electrode battery. Nature Communications 13, 4538 (2022).

[0006] However, in the secondary battery described in Non-Patent Document 1, the charge-discharge characteristics may be reduced due to the coordination of anisole with lithium ions.

[0007] The present invention was made in view of the above-mentioned technical problems, and its object is to provide a secondary battery with improved charge and discharge characteristics. Summary of the Invention

[0008] One aspect of the present invention relates to a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte comprises at least one of a first compound represented by formula (1), formula (2), or formula (3).

[0009]

[0010] (n is an integer greater than 0 and less than 5.)

[0011] According to the present invention, a secondary battery with improved charge and discharge characteristics can be provided. Attached Figure Description

[0012] Figure 1 This is a perspective view showing the structure of a secondary battery according to one embodiment.

[0013] Figure 2 It is shown in magnification Figure 1 The diagram shows a cross-sectional view of the structure of the battery element.

[0014] Explanation of reference numerals in the attached figures

[0015] 1: Secondary battery; 21: Positive electrode; 22: Negative electrode. Detailed Implementation

[0016] Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to this embodiment.

[0017] <1. Secondary Battery>

[0018] The secondary battery described in this embodiment will be explained. The secondary battery described in this embodiment is a secondary battery in which battery capacity is obtained by intercalation and deintercalation of electrode reactive materials, and it includes a positive electrode, a negative electrode, and an electrolyte.

[0019] There are no particular limitations on the types of substances used in the electrode reactions; specifically, they are light metals such as alkali metals and alkaline earth metals. Specific examples of alkali metals include lithium, sodium, and potassium. Specific examples of alkaline earth metals include beryllium, magnesium, and calcium.

[0020] In the following explanation, lithium is used as the electrode reactant as an example. A secondary battery that utilizes the insertion and extraction of lithium to obtain battery capacity is, for example, a lithium-ion secondary battery. In a lithium-ion secondary battery, lithium is inserted and extracted in an ionic state.

[0021] <1-1. Structure>

[0022] Figure 1 This is a perspective view showing the structure of a secondary battery according to one embodiment. Figure 2 It is shown in magnification Figure 1 A cross-sectional view of the structure of the battery element shown. Figure 1 The image shows the outer packaging film 10 and the battery element 20 separated from each other, with the cross-section of the battery element 20 shown by dashed lines. Figure 2 Only a cross-section of a portion of the battery element 20 is shown in the image.

[0023] like Figure 1 as well as Figure 2 As shown, the secondary battery 1 includes an outer packaging film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and 42.

[0024] As mentioned above, Figure 1 The secondary battery 1 in question uses an outer packaging film 10 as the outer packaging component for housing the battery element 20. Therefore, Figure 1 The secondary battery 1 shown is a so-called laminated film type secondary battery.

[0025] [Outer packaging film]

[0026] like Figure 1As shown, the outer packaging film 10 is a flexible or soft outer packaging component with a bag-like structure that is sealed when the battery element 20 is housed. Thus, the outer packaging film 10 houses the positive electrode 21, the negative electrode 22, the separator 23, and the electrolyte (not shown), which will be described later.

[0027] exist Figure 1 In this example, the outer packaging film 10 is a film-like component folded in the folding direction F. The outer packaging film 10 has a recess 10U for accommodating the battery element 20. The recess 10U is a so-called deep-stretched portion.

[0028] Specifically, the outer packaging film 10 is a three-layer laminate consisting of a welding layer, a metal layer, and a surface protective layer, stacked sequentially from the inside. When the outer packaging film 10 is folded, the outer peripheries of the opposing welding layers are welded together. The welding layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protective layer contains a polymer compound such as nylon. Furthermore, the structure (number of layers) of the outer packaging film 10 is not particularly limited; it can be one layer, two layers, or four or more layers.

[0029] [Battery Components]

[0030] The battery element 20 is housed within the recess 10U of the outer packaging film 10. The battery element 20 is a so-called power generation element. Figure 1 as well as Figure 2 As shown, the battery element 20 includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown).

[0031] exist Figure 1 In this example, battery element 20 is a so-called wound electrode. Therefore, the positive electrode 21 and the negative electrode 22 are positioned opposite each other across a separator 23 and wound around a winding axis P. In the following description, the direction along the winding axis P will sometimes be defined as the Y direction, the direction of the long side of battery element 20 perpendicular to the winding axis P as the X direction, and the direction of the short side of battery element 20 perpendicular to the winding axis P as the Z direction.

[0032] exist Figure 1 In the example, the battery element 20 has a flat, three-dimensional shape. That is, the shape of the cross-section (along the XZ plane) of the battery element 20 intersecting the winding axis P is a flat shape defined by the major axis J1 and the minor axis J2. The major axis J1 is an imaginary axis extending in the X-axis direction and has a length greater than that of the minor axis J2. The minor axis J2 is an imaginary axis extending in the Z-axis direction and has a length less than that of the major axis J1. Thus, the cross-sectional shape of the battery element 20 is a flat, approximately elliptical shape. However, the three-dimensional shape of the battery element 20 is just one example and is not limited to the shape described above.

[0033] (positive electrode)

[0034] like Figure 2 As shown, the positive electrode 21 includes a positive current collector 21A and a positive active material layer 21B. However, the positive current collector 21A can also be omitted.

[0035] The positive current collector 21A has one side with a positive active material layer 21B disposed thereon. The positive current collector 21A contains conductive materials such as aluminum or other metallic materials.

[0036] The positive electrode active material layer 21B contains at least one of the positive electrode active materials that are capable of lithium insertion / extraction. However, the positive electrode active material layer 21B may also contain one or more positive electrode binders and other materials such as positive electrode conductive agents. The method for forming the positive electrode active material layer 21B is not particularly limited; specifically, it can be a coating method, etc.

[0037] exist Figure 2 In the example, the positive electrode active material layer 21B is disposed on both sides of the positive electrode current collector 21A. However, the positive electrode active material layer 21B may also be disposed on only one side of the positive electrode current collector 21A, which is opposite to the negative electrode 22.

[0038] There are no particular limitations on the type of positive electrode active material; specifically, it can be lithium-containing compounds. Lithium-containing compounds are compounds that contain lithium and one or more transition metal elements as constituent elements. Lithium-containing compounds may also contain one or more other elements as constituent elements. The types of other elements are not particularly limited as long as they are elements other than lithium and transition metal elements; specifically, they are elements belonging to groups 2 to 15 of the long-period periodic table. There are no particular limitations on the type of lithium-containing compound; specifically, oxides, phosphoric acid compounds, silicate compounds, and borate compounds can be used.

[0039] Specific examples of oxides are LiNiO2, LiCoO2, and LiCo. 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni0.22 Co 0.13 O2 and LiMn2O4, etc. Specific examples of phosphoric acid compounds are LiFePO4, LiMnPO4, and LiFe 0.5 Mn 0.5 PO4 and LiFe 0.3 Mn 0.7 PO4, etc.

[0040] The positive electrode binder includes at least one of the following materials: synthetic rubber and polymeric compounds. Specific examples of synthetic rubbers include styrene-butadiene rubber, fluorinated rubber, and ethylene propylene diene monomer (EPDM) rubber. Specific examples of polymeric compounds include polyvinylidene fluoride (PVDF), polyimide, and carboxymethyl cellulose.

[0041] The positive electrode conductive agent includes at least one of conductive materials such as carbon materials, metallic materials, and conductive polymer compounds. Specific examples of carbon materials include graphite, carbon black, acetylene black, and Ketjen black.

[0042] (negative electrode)

[0043] like Figure 2 As shown, the negative electrode 22 includes a negative electrode current collector 22A and a negative electrode active material layer 22B. However, the negative electrode current collector 22A can also be omitted.

[0044] The negative current collector 22A has one side with a negative active material layer 22B disposed thereon. The negative current collector 22A contains conductive materials such as copper or other metallic materials.

[0045] The negative electrode active material layer 22B comprises at least one of the negative electrode active materials that are capable of lithium insertion / extraction. However, the negative electrode active material layer 22B may also comprise at least one of other materials such as a negative electrode binder and a negative electrode conductive agent. The method for forming the negative electrode active material layer 22B is not particularly limited, but specifically, it is at least one of the following: coating method, vapor phase method, liquid phase method, spraying method, and sintering method.

[0046] exist Figure 2 In the example, the negative electrode active material layer 22B is disposed on both sides of the negative electrode current collector 22A. However, the negative electrode active material layer 22B may also be disposed on only one side of the negative electrode current collector 22A, which is opposite to the positive electrode 21.

[0047] The type of negative electrode active material is not particularly limited, but specifically includes carbon materials and metallic materials. This results in high energy density. Specific examples of carbon materials include easily graphitized carbon, difficult-to-graphitize carbon, and graphite. Graphite can be one or both natural and artificial graphite. Metallic materials are a general term for materials containing at least one of the following constituent elements: metallic elements and half-metallic elements capable of forming alloys with lithium. Specific examples of these metallic and half-metallic elements include silicon and tin. Metallic materials can be monomers, alloys, or compounds. Furthermore, metallic materials can be mixtures of two or more materials, or materials containing two or more phases. Monomers can also contain any amount of impurities. Specific examples of metallic materials are TiSi2 and SiO. x (0<x≤2) etc.

[0048] The negative electrode binder can be made of the same material as the positive electrode binder described above. The negative electrode conductive agent can be made of the same material as the positive electrode conductive agent described above.

[0049] (Septum)

[0050] like Figure 2 As shown, the separator 23 is an insulating porous membrane located between the positive electrode 21 and the negative electrode 22. The separator 23 prevents short circuits caused by the contact between the positive electrode 21 and the negative electrode 22 while allowing lithium to pass through in an ionized state. The separator 23 contains at least one insulating polymer compound. Specific examples of insulating polymer compounds include polyethylene.

[0051] (electrolyte)

[0052] The electrolyte is impregnated in each of the positive electrode 21, the negative electrode 22, and the separator 23. Details of the electrolyte will be described later.

[0053] [Positive lead]

[0054] like Figure 1 as well as Figure 2 As shown, the positive lead 31 is a positive wiring connected to the positive current collector 21A of the positive electrode 21, and extends to the outside of the outer packaging film 10. The positive lead 31 contains at least one conductive material such as a metal, and specific examples of conductive materials are aluminum, etc. In addition, the shape of the positive lead 31 is not particularly limited, and it can be, for example, a thin plate or a mesh.

[0055] [Negative lead]

[0056] like Figure 1 as well as Figure 2As shown, the negative lead 32 is a negative electrode wiring connected to the negative current collector 22A of the negative electrode 22, and extends to the outside of the outer packaging film 10. The negative lead 32 contains at least one conductive material, such as a metal. Specific examples of conductive materials include copper. Furthermore, the shape of the negative lead 32 is not particularly limited; for example, it can be a thin plate or a mesh.

[0057] [Sealing film]

[0058] like Figure 1 As shown, the sealing film 41 is inserted between the outer packaging film 10 and the positive lead 31. Furthermore, as... Figure 1 As shown, the sealing film 42 is inserted between the outer packaging film 10 and the negative lead 32. However, one or both of the sealing films 41 and 42 can be omitted.

[0059] The sealing film 41 is a sealing component that prevents external gases or other substances from entering the interior of the outer packaging film 10. The sealing film 41 contains a polymer compound such as a polyolefin that has a tight seal with respect to the positive electrode lead 31. Specific examples of the polymer compound are polypropylene and the like.

[0060] The sealing film 42 is a sealing component that prevents external gases from entering the interior of the outer packaging film 10. The sealing film 42 contains a polymer compound such as a polyolefin that has a tight seal with respect to the negative electrode lead 32. Specific examples of the polymer compound are polypropylene and the like.

[0061] <2. Electrolyte>

[0062] The electrolyte involved in this embodiment will be described in detail below.

[0063] <2-1. Composition>

[0064] The electrolyte contains a solvent and an electrolyte salt.

[0065] [solvent]

[0066] (First compound)

[0067] The solvent contains at least one of the first compounds represented by formula (1), formula (2), or formula (3). The number of carbon atoms n in formulas (1), (2), and (3) is 0 or more and 5 or less. That is, the first compound is a compound in which one of the hydrogen atoms of the benzene ring bonded to anisole is replaced by a thiol group (-SH) or an alkylthiol group (-C). n H 2nThe first compound is a substituted compound (where n is an integer between 1 and 5). This increases the boiling point of the electrolyte, reduces its volatility and flammability, and improves safety. Furthermore, the first compound is difficult to solubilize with alkali metal ions such as lithium ions. Therefore, it can suppress the decrease in the concentration of unsolvated alkali metal ions in the electrolyte. Consequently, the presence of alkali metal ions promotes the decomposition of electrolyte anions on the negative electrode surface, thus forming a good coating of anions from the electrolyte, such as SEI (Solid Electrolyte Interphase), on the negative electrode 22, which improves charge-discharge characteristics.

[0068]

[0069] Here, the alkylthiol group in formulas (1), (2), and (3) refers to an alkyl (-C) group. n H 2n+ A functional group in which one hydrogen atom (n is an integer greater than or equal to 1 and less than or equal to 5) is replaced by a thiol group (-SH). Specific examples of alkyl groups are methyl, ethyl, propyl, butyl, and pentyl. Furthermore, alkyl groups can be linear or branched. Therefore, for example, butyl can be n-butyl, sec-butyl, isobutyl, or tert-butyl.

[0070] The number of carbon atoms n in formulas (1), (2), and (3) is more preferably 0 or 1. That is, the solvent preferably includes at least one of the compounds in which one of the hydrogen atoms of the benzene ring bonded to anisole is replaced by a thiol group (-SH) or a methyl thiol group (-CH2SH) as the first compound. This improves the solubility of the solute relative to the first compound and its compatibility with solvents such as the second compound.

[0071] Specific examples of the first compound are compounds represented by each of formulas (4) to (6).

[0072]

[0073] Whether the electrolyte contains the first compound can be determined by analyzing the electrolyte. Specifically, the secondary battery 1 is disassembled, and the electrolyte is recovered and analyzed using a centrifuge. This allows determination of the types of components present in the electrolyte, including the first compound. The analytical method for the electrolyte is not particularly limited; specifically, it can be at least one of the following: inductively coupled plasma atomic emission spectrometry (ICP), nuclear magnetic resonance spectroscopy (NMR), and gas chromatography-mass spectrometry (GC-MS).

[0074] (Second compound)

[0075] The solvent preferably further comprises at least one of the second compounds. The second compound is a straight-chain ether. In this disclosure, a straight-chain ether refers to a straight-chain compound having at least one ether bond. Here, the straight-chain ether comprises a straight-chain ether in which hydrogen is substituted by a substituent that does not contain carbon. Specific examples of straight-chain ethers are 1,2-dimethoxyethane (DME), diethyl ether (DEE), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), etc. The solvent more preferably comprises 1,2-dimethoxyethane (DME). As a result, the viscosity of the electrolyte is reduced, the electrolytic activity of the electrolyte salt is improved, the mobility of ions in the electrolyte is increased, and thus the charge-discharge characteristics are improved.

[0076] Whether the electrolyte contains the second compound can be determined by analyzing the electrolyte using the same steps as those used to determine the presence or absence of the first compound.

[0077] (mixing ratio)

[0078] In this embodiment, the molar ratio of the first compound to the second compound in the electrolyte is preferably 1.8 or more, more preferably 2.0 or more. The molar ratio of the first compound to the second compound in the electrolyte is calculated by dividing the total mass (mol) of the first compound contained in the electrolyte by the total mass (mol) of the second compound contained in the electrolyte. This improves the effect based on the first compound—that is, the effect of forming a coating of anions from the electrolyte on the negative electrode—while simultaneously improving the electrolyte's physical properties through the second compound, thereby further enhancing the charge-discharge characteristics.

[0079] The molar ratio of the first compound to the second compound can be determined by analyzing the electrolyte. Specifically, the secondary battery 1 is disassembled, and the electrolyte is recovered and analyzed using a centrifuge. This allows for the determination of the amounts of the first and second compounds contained in the electrolyte. The analytical method for the electrolyte is at least one of ICP emission spectroscopy, NMR, and GC-MS.

[0080] (Other solvents)

[0081] In addition, the solvent may contain other solvents besides the first compound or the second compound.

[0082] Specifically, other solvents include esters and cyclic ethers, and more specifically, carbonate compounds, carboxylic acid ester compounds, and lactone compounds. Carbonate compounds include cyclic carbonates and chain carbonates. Specific examples of cyclic carbonates are ethylene carbonate and propylene carbonate. Specific examples of chain carbonates are dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Carboxylic acid ester compounds are chain carboxylic acid esters. Specific examples of chain carboxylic acid esters are ethyl acetate, ethyl propionate, propyl propionate, and trimethylethyl acetate. Lactone compounds are lactones. Specific examples of lactones are γ-butyrolactone and γ-valerolactone. Cyclic ethers include 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane.

[0083] In addition, other solvents may include unsaturated cyclic carbonates, fluorinated cyclic carbonates, acid esters, phosphate esters, acid anhydrides, nitrile compounds, and isocyanate compounds. This improves the electrochemical stability of the electrolyte. Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinyl ethylene carbonate, and methylene ethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonates include propane sulpholactone and propene sulpholactone. Specific examples of phosphate esters include trimethyl phosphate and triethyl phosphate. Specific examples of acid anhydrides include succinic anhydride, 1,2-ethanedisulfonic anhydride, and 2-sulfobenzoic anhydride. Specific examples of nitrile compounds include succinic nitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0084] [Electrolyte salts]

[0085] Electrolyte salts contain both cations and anions.

[0086] Examples of cations in electrolyte salts include alkali metal ions such as potassium ions, lithium ions, and sodium ions.

[0087] The anion of the electrolyte salt preferably contains hexafluorophosphate ions (PC6). - ), tetrafluoroborate ion (BF4) -), trifluoromethanesulfonate ions (CF3SO3) - ), bis(trifluoromethanesulfonyl)imide ion (N(CF3SO2)2) - ), tri(trifluoromethanesulfonyl)methyl ion (C(CF3SO2)3) - ), bis(oxalate)borate ion (B(C2O4)2) - ) and nitrate ions (NO3) - At least one of the following. Thus, by decomposing the anions of the electrolyte salt, a coating containing inorganic substances such as LiF and Li3N can be formed on the negative electrode surface.

[0088] The anion of the electrolyte salt more preferably includes hexafluorophosphate ions (PC6). - ), tetrafluoroborate ion (BF4) - ), trifluoromethanesulfonate ions (CF3SO3) - ), bis(trifluoromethanesulfonyl)imide ion (N(CF3SO2)2) - ) and tri(trifluoromethanesulfonyl)methyl ion (C(CF3SO2)3 - At least one of the following. Thus, a fluorine-containing SEI can be formed on the negative electrode surface.

[0089] The anion of the electrolyte salt is further preferably composed of at least one anion with an imide bond, such as lithium bis(fluorosulfonyl)imide (LiN(FSO2)2) or lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2). This forms a good ion pair between the electrolyte anion and the alkali metal ion in the electrolyte, promoting the formation of the SEI.

[0090] Specific examples of electrolyte salts include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium tri(trifluoromethanesulfonyl)methylide (LiC(CF3SO2)3), lithium bis(oxalate)borate (LiB(C2O4)2), lithium monofluorophosphate (Li2PFO3), and lithium difluorophosphate (LiPF2O2). These lead to high battery capacity.

[0091] The content of the electrolyte salt is not particularly limited, but it is preferably 0.3 mol / kg or more and 3.0 mol / kg or less relative to the solvent. This improves the ionic conductivity of the electrolyte.

[0092] <3. Actions>

[0093] The secondary battery 1 in this embodiment operates in the battery element 20 in the following manner.

[0094] During charging, lithium is deintercalated from the positive electrode 21 and intercalated into the negative electrode 22 via the electrolyte. On the other hand, during discharging, lithium is deintercalated from the negative electrode 22 and intercalated into the positive electrode 21 via the electrolyte. During discharging and charging, lithium is intercalated and deintercalated in an ionic state, respectively.

[0095] <4. Manufacturing Method>

[0096] In manufacturing the secondary battery 1 according to this embodiment, the steps described below are as follows: after fabricating each of the positive electrode 21, the negative electrode 22, and the electrolyte, the secondary battery 1 is assembled, and a stabilization treatment of the secondary battery 1 is performed. Furthermore, the manufacturing method of the secondary battery 1 shown below is merely an example and is not limited thereto.

[0097] [The production of the positive electrode]

[0098] First, a positive electrode mixture is prepared by mixing the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent. Then, a paste-like positive electrode mixture slurry is prepared by adding the positive electrode mixture to a solvent. The solvent can be an aqueous solvent or an organic solvent.

[0099] Finally, a positive electrode active material layer 21B is formed by coating both sides of the positive electrode current collector 21A with a positive electrode slurry. During the formation of the positive electrode active material layer 21B, a compression device such as a roller press can be used to compress and shape the positive electrode active material layer 21B. When compressing and shaping the positive electrode active material layer 21B, it can be heated, or the compression and shaping process can be repeated multiple times. Through the above steps, a positive electrode active material layer 21B is formed on both sides of the positive electrode current collector 21A, thereby manufacturing the positive electrode 21.

[0100] [Making the negative electrode]

[0101] The negative electrode 22 is formed using the same steps as those used to fabricate the positive electrode 21. Specifically, a negative electrode mixture is prepared by mixing the negative electrode active material, the negative electrode binder, and the negative electrode conductive agent, and then adding this mixture to a solvent to prepare a paste-like negative electrode mixture slurry. The solvent can be an aqueous solvent or an organic solvent. Then, the negative electrode mixture slurry is coated onto both sides of the negative electrode current collector 22A to form a negative electrode active material layer 22B. The negative electrode active material layer 22B can then be compressed and molded. The negative electrode active material layer 22B can be compressed and molded in the same way as the positive electrode active material layer 21B during its formation. Through these steps, the negative electrode active material layer 22B is formed on both sides of the negative electrode current collector 22A, thereby fabricating the negative electrode 22.

[0102] [Electrolyte preparation]

[0103] In the manufacture of an electrolyte, an electrolyte salt is added to a solvent containing a first compound and a second compound. In this case, the mixing ratio of the first compound to the second compound is adjusted so that the molar ratio of the first compound to the second compound is within the aforementioned range. As a result, the electrolyte salt is dispersed or dissolved in the solvent, thereby preparing the electrolyte.

[0104] [Assembly of a secondary battery]

[0105] First, the positive lead 31 is connected to the positive current collector 21A of the positive electrode 21 using a welding method or similar bonding method. Then, the negative lead 32 is connected to the negative current collector 22A of the negative electrode 22 using a welding method or similar bonding method.

[0106] Next, the positive electrode 21 and the negative electrode 22 are stacked with the membrane 23 in between to form a laminate. Then, by using... Figure 2 The winding shaft P is used to create the winding body by winding a central laminate. Then, the winding body is pressed using a compression device such as a press to form a flat shape. As a result, the shaped winding body has the same shape as the battery element 20.

[0107] Next, after the wound body is housed in the recessed portion 10U, the outer packaging film 10 is folded, thereby positioning the outer packaging films 10 opposite each other in the Z direction. Then, using an adhesive method such as heat fusion bonding, the wound body is housed in the bag-shaped outer packaging film 10 by joining the outer peripheral portions of the two opposing weld layers together.

[0108] Finally, after injecting the prepared electrolyte into the pouch-shaped outer packaging film 10, the outer peripheral portions of the remaining sides of the opposing welded layers are joined together using an adhesive method such as heat fusion. While the outer peripheral portions of these remaining sides are joined together, a sealing film 41 is inserted between the outer packaging film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the outer packaging film 10 and the negative electrode lead 32. Thus, by impregnating the wound body with electrolyte, a battery element 20 is fabricated.

[0109] Through the above steps, the battery element 20 is sealed in the bag-shaped outer packaging film 10, thereby assembling the secondary battery 1 involved in this embodiment.

[0110] [Stabilization treatment of the assembled secondary battery]

[0111] The assembled secondary battery 1 undergoes stabilization treatment through charge and discharge. Here, stabilization conditions such as ambient temperature, number of charge and discharge cycles, and charge and discharge conditions can be arbitrarily set. Through stabilization treatment, the aforementioned coating is formed on the surface of the negative electrode 22, thus completing the electrochemical stabilization of the secondary battery 1 (battery element 20).

[0112] <5. Functions and Effects>

[0113] As described above, the secondary battery 1 according to this embodiment includes a positive electrode 21, a negative electrode 22, and an electrolyte. The electrolyte contains at least one of a first compound represented by formula (1), formula (2), or formula (3).

[0114]

[0115] (n is an integer greater than 0 and less than 5.)

[0116] Therefore, the first compound can increase the boiling point of the electrolyte, reduce its volatility and flammability, and improve safety. Furthermore, the first compound is difficult to coordinate with alkali metal ions such as lithium ions, thus reducing the ionic conductivity of the electrolyte and improving its charge-discharge characteristics.

[0117] As a preferred embodiment, n in formula (1), formula (2), or formula (3) is 0 or 1. This improves the solubility of the solute relative to the first compound and its compatibility with solvents such as the second compound.

[0118] As a preferred embodiment, it also includes at least one of the second compounds that are straight-chain ethers. This reduces the viscosity of the electrolyte, improves the electrolytic properties of the electrolyte salt, increases the mobility of ions in the electrolyte, and thus improves charge-discharge characteristics.

[0119] As a more preferred embodiment, the molar ratio of the first compound to the second compound in the electrolyte is 1.8 or higher. This allows the first compound to promote the formation of a coating on the negative electrode, while the second compound ensures good electrolyte properties, thereby further improving charge-discharge characteristics.

[0120] <6. Variations>

[0121] Next, modified examples will be described. The configuration of the secondary battery according to this embodiment, as described below, can be appropriately modified. In addition, the series of modified examples described below can also be combined with each other.

[0122] [First Variation]

[0123] The difference between the secondary battery involved in the first variation and the secondary battery 1 mentioned above is that it utilizes a secondary battery that dissolves lithium through precipitation, namely a lithium metal secondary battery.

[0124] The secondary battery involved in the first variation is identical in structure to the secondary battery 1 described above, except that the negative electrode 22 contains lithium monomers and so-called lithium metal. Specifically, the negative electrode 22 is lithium metal foil or the like. However, the lithium metal may contain any amount of impurities.

[0125] In the secondary battery according to the first modification, if lithium is deintercalated from the positive electrode 21 in an ionic state during charging, lithium metal is deposited on the surface of the negative electrode 22. Furthermore, in the secondary battery according to the first modification, if lithium metal dissolves from the negative electrode 22 during discharging, lithium is intercalated into the positive electrode 21 in an ionic state.

[0126] The manufacturing method of the secondary battery involved in the first modification is the same as that of the secondary battery 1 described above, except that lithium metal is used as the negative electrode 22.

[0127] In the secondary battery involved in the first modification, the battery capacity is also obtained by the precipitation and dissolution of lithium, so the same effect as the secondary battery 1 described above can be obtained.

[0128] [Second variation]

[0129] In the second variation, the difference from the above-described secondary battery 1 is that a laminated membrane containing a polymer compound layer is used instead of the porous membrane 23.

[0130] Specifically, the laminated separator comprises a porous membrane and a polymer compound layer. The porous membrane has one and two sides. The polymer compound layer is disposed on one or both sides of the porous membrane. This improves the seal of the separator relative to each of the positive electrode 21 and the negative electrode 22, thus suppressing positional shifts of the positive electrode 21, the negative electrode 22, and the separator 23 during winding. Therefore, even if a decomposition reaction of the positive electrolyte occurs, the expansion of the secondary battery is suppressed.

[0131] The polymer compound layer preferably includes polyvinylidene fluoride (PVDF). This improves the physical strength and electrochemical stability of the polymer compound layer.

[0132] Furthermore, one or both of the porous membrane and the polymer compound layer may also contain at least one of multiple insulating particles. Therefore, when the secondary battery heats up, the multiple insulating particles dissipate heat, thus improving the heat resistance and safety of the secondary battery. The multiple insulating particles include at least one of inorganic materials and resin materials. Specific examples of inorganic materials are alumina, aluminum nitride, boehmite, silicon dioxide, titanium dioxide, magnesium oxide, and zirconium oxide. Specific examples of resin materials are acrylic resin and styrene resin.

[0133] The membrane involved in the second modification is fabricated by coating one or both sides of a porous membrane with the precursor solution after preparing a precursor solution containing a polymer compound and an organic solvent. In the fabrication of the membrane involved in the second modification, the precursor solution may also contain multiple insulating particles.

[0134] Even when using the laminated separator described in the second modification, lithium can move in an ionic state between the positive electrode 21 and the negative electrode 22, thus achieving the same effect as the secondary battery 1 described above. In particular, in the second modification, the expansion of the secondary battery can be suppressed.

[0135] [Third variation]

[0136] In the secondary battery described in the third variation, a gel-like electrolyte layer can also be used instead of a liquid electrolyte.

[0137] In the battery element 20 using the electrolyte layer according to the third modification, the positive electrode 21 and the negative electrode 22 are wound together while facing each other across the separator 23 and the electrolyte layer. The electrolyte layer is located between the positive electrode 21 and the separator 23 and between the negative electrode 22 and the separator 23.

[0138] Specifically, the electrolyte layer comprises an electrolyte and a polymer compound. Within the electrolyte layer, the electrolyte is held in place by the polymer compound. This helps to suppress electrolyte leakage. The composition of the electrolyte is the same as that involved in the secondary battery 1 described above. The polymer compound includes polyvinylidene fluoride, etc.

[0139] The electrolyte layer involved in the third variation is formed by coating one or both sides of the positive electrode 21 and the negative electrode 22 with the precursor solution after preparing a precursor solution containing an electrolyte, a polymer compound and a solvent.

[0140] Even when using the electrolyte layer involved in the third modification, lithium can move between the positive electrode 21 and the negative electrode 22 in an ionic state via the electrolyte layer, thus achieving the same effect. In particular, in the third modification, electrolyte leakage can be suppressed.

[0141] Hereinafter, embodiments of the secondary battery according to this embodiment will be described. However, the secondary battery according to this embodiment is not limited to the embodiments described below.

[0142] <Examples 1-5 and Comparative Example 1>

[0143] In Examples 1 to 5 and Comparative Example 1, the secondary battery was manufactured by the following steps. The secondary batteries involved in Examples 1 to 5 and Comparative Example 1 are simple lithium metal secondary batteries.

[0144] [Making a Secondary Battery]

[0145] The electrolyte is prepared by adding an electrolyte salt to a solvent obtained by mixing a first solvent and a second solvent and stirring.

[0146] As shown in Table 1 below, the first solvent used in Examples 1 to 3 was the compound represented by formula (4) (2-methoxybenzenethiol), in Example 4 it was the compound represented by formula (6) (4-methoxybenzenethiol), in Example 5 it was the compound represented by formula (5) (4-Methoxy-alpha-toluenethiol), and in Comparative Example 1 it was the compound represented by formula (7) (Anisole). That is, in Examples 1 to 5, the first compound involved in this embodiment was used as the first solvent, while in Comparative Example 1, anisole was used instead of the first compound.

[0147]

[0148] The second solvent used is 1,2-dimethoxyethane (DME). That is, in Examples 1 to 1 and Comparative Example 1, the second compound involved in this embodiment was used as the second solvent. Here, as shown in Table 1 described later, the solvent was prepared such that the molar ratio of the first solvent to the second solvent was 1.8 in Example 1 and 2.0 in Examples 2 to 5 and Comparative Example 1.

[0149] The electrolyte salt used was lithium bis(fluorosulfonyl)imide (LiFSI). Here, as shown in Table 1 below, the electrolyte was prepared with an electrolyte salt concentration of 2 mol / L relative to the solvent in Examples 1, 2, 4, 5 and Comparative Example 1, and 3 mol / L in Example 3.

[0150] The test electrode was fabricated by pressing a 0.1 mm thick lithium metal foil onto a 0.01 mm thick copper foil using a press. The counter electrode was a 0.012 mm thick copper foil. The separator was a 10 μm thick microporous polyethylene membrane. The separator was impregnated with a prepared electrolyte solution, with 0.01 mL of electrolyte added dropwise. Subsequently, the test electrode, the electrolyte-impregnated separator, and the counter electrode were stacked in that order to fabricate a secondary battery for charge-discharge testing.

[0151] [Charge / Discharge Test]

[0152] For the secondary battery manufactured above, as an evaluation of battery characteristics, charge-discharge tests were conducted using the following methods to determine the coulombic efficiency.

[0153] In the charge-discharge test, multiple charge-discharge cycles were performed for measurement. Specifically, in each cycle, the secondary battery was charged and discharged at an environment of 23°C, and the charging capacity and discharging capacity were measured respectively. During charging, the current was 0.22 mA / cm². 2 The current density was used to charge the battery until the total charging time reached 3 hours, and during discharge, the battery was discharged until the voltage reached 0.1V. Then, based on the formula Coulomb efficiency (%) = (discharge capacity / charge capacity) × 100, the Coulomb efficiency was calculated for each cycle.

[0154] In the charge-discharge test, the secondary battery was repeatedly charged and discharged for 25 cycles, and the coulombic efficiency was measured. Then, the average coulombic efficiency (%) was calculated by averaging the coulombic efficiencies of each of the 16 cycles calculated for each of the 10th to 25th cycles. Here, only the coulombic efficiencies of the 10th to 25th cycles are used in the calculation of the average coulombic efficiency to improve the accuracy and reproducibility of the evaluation of charge-discharge characteristics. The coulombic efficiency values ​​in the 1st to 9th cycles are prone to deviation; therefore, the coulombic efficiencies of the 1st to 9th cycles are not used to calculate the average coulombic efficiency.

[0155] Table 1 shows the composition and charge / discharge characteristics of the electrolytes of the secondary batteries involved in Examples 1 to 5 and Comparative Example 1. In Table 1, "first solvent / second solvent" refers to the ratio of the mass of the first solvent contained in the electrolyte to the mass of the second solvent contained in the electrolyte.

[0156] Table 1

[0157]

[0158] As shown in Table 1, in Examples 1 to 5, which used the first compound represented by any one of formulas (4) to (6) as the first solvent, the average coulombic efficiency was improved compared to Comparative Example 1, which used anisole represented by formula (7). Therefore, it can be seen that by including the first compound in the electrolyte, the charge-discharge characteristics are improved.

[0159] As shown in Table 1, in Examples 1 to 5, where the ratio of the first solvent to the second solvent, i.e., the ratio of the first compound to the second compound, was 1.8 or higher, the average coulombic efficiency was 99% or higher. Therefore, it can be seen that by making the ratio of the first compound to the second compound 1.8 or higher, the charge-discharge characteristics are good.

[0160] As shown in Table 1, in Examples 2 to 4, where the ratio of the first solvent to the second solvent, i.e., the ratio of the first compound to the second compound, was 2.0 or higher, the average coulombic efficiency was improved compared to Example 1, where the ratio of the first compound to the second compound was less than 2.0. Therefore, it can be seen that by making the ratio of the first compound to the second compound 2.0 or higher, the charge-discharge characteristics are further improved.

[0161] The above description is provided for ease of understanding of the invention and is not intended to limit or interpret its scope. The invention can be modified or improved without departing from its spirit, and its equivalents are also included.

[0162] Specifically, the battery structure of a secondary battery can also be cylindrical, cuboid, coin-shaped, or button-shaped, etc.

[0163] In addition, the structure of battery elements can also be stacked or folded. In a stacked structure, the positive and negative electrodes are alternately stacked with a separator in between. In a folded structure, the positive and negative electrodes are opposite each other with a separator in between and are folded into a serrated shape.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte. The electrolyte contains at least one of a first compound represented by formula (1), formula (2) or formula (3). n is an integer greater than 0 and less than 5.

2. The secondary battery according to claim 1, wherein, The value of n is 0 or 1.

3. The secondary battery according to claim 1 or 2, wherein, The electrolyte also contains at least one of the second compounds that are straight-chain ethers.

4. The secondary battery according to claim 3, wherein, The molar ratio of the first compound to the second compound in the electrolyte is 1.8 or more.

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