Secondary battery

Abstract

Provided is a secondary battery capable of achieving excellent battery characteristics. This secondary battery is provided with a positive electrode, a negative electrode containing a lithium metal, and an electrolyte solution containing a solvent. The solvent contains an ortho-carbonate ester compound represented by formula (1), and the content of the ortho-carbonate ester compound in the solvent is 40 wt% or more.

Description

secondary battery

[0001] The present technology relates to a secondary battery.

[0002] Due to the widespread use of various electronic devices such as mobile phones, secondary batteries have been developed as small, lightweight power sources that can provide high energy density. These secondary batteries contain a positive electrode, a negative electrode, and an electrolyte solution, and various studies have been conducted on the configuration of these secondary batteries.

[0003] Specifically, the positive electrode active material contains manganese oxide, the negative electrode active material contains lithium metal, and the solvent of the electrolyte contains a chain tetraether, with the content of the chain tetraether in the solvent being 1% by volume to 20% by volume (see, for example, Patent Document 1). Also, the nonaqueous electrolyte contains an orthocarbonate ester, with the content of the orthocarbonate ester in the nonaqueous electrolyte being 0.001 mmol / g to 0.18 mmol / g (see, for example, Patent Document 2).

[0004] Japanese Patent Laid-Open No. 09-045339 Japanese Patent Laid-Open No. 2002-270222

[0005] Although various studies have been conducted on the configuration of secondary batteries, the battery characteristics of the secondary batteries are still insufficient and there is room for improvement.

[0006] There is a demand for a secondary battery that can provide excellent battery characteristics.

[0007] A secondary battery according to an embodiment of the present technology includes a positive electrode, a negative electrode containing lithium metal, and an electrolyte solution containing a solvent, wherein the solvent contains an orthocarbonate ester compound represented by formula (1), and the content of the orthocarbonate ester compound in the solvent is 40 wt % or more.

[0008] (Each of R1, R2, R3 and R4 is a hydrocarbon group.)

[0009] According to a secondary battery of one embodiment of the present technology, the negative electrode contains lithium metal, the solvent of the electrolyte contains the orthocarbonate ester compound represented by formula (1), and the content of the orthocarbonate ester compound in the solvent is 40 wt % or more, so that excellent battery characteristics can be obtained.

[0010] Note that the effects of the present technology are not necessarily limited to the effects described here, but may be any of a series of effects related to the present technology described below.

[0011] Fig. 1 is a perspective view illustrating a configuration of a secondary battery according to an embodiment of the present technology. Fig. 2 is an enlarged cross-sectional view illustrating the configuration of a battery element illustrated in Fig. 1. Fig. 3 is a block diagram illustrating the configuration of an application example of the secondary battery.

[0012] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order: 1. Secondary battery 1-1. Overall configuration 1-2. Detailed configuration of electrolyte solution 1-3. Operation 1-4. Manufacturing method 1-5. Actions and effects 2. Modifications 3. Uses of secondary battery

[0013] 1. Secondary Battery First, a secondary battery according to an embodiment of the present technology will be described.

[0014] The secondary battery described here is a so-called lithium metal secondary battery, since it is a secondary battery that obtains battery capacity by utilizing the deposition and dissolution of lithium.

[0015] <1-1. Overall Configuration> Fig. 1 shows a perspective view of a secondary battery, and Fig. 2 shows an enlarged cross-sectional view of a battery element 20 shown in Fig. 1.

[0016] However, Fig. 1 shows a state in which the exterior film 10 and the battery element 20 are separated from each other. Also, Fig. 1 shows a cross section of the battery element 20 along the XZ plane with a dashed line. Fig. 2 shows only a part of the battery element 20.

[0017] As shown in FIGS. 1 and 2, this secondary battery includes an exterior film 10, a battery element 20, a positive electrode lead 31, a negative electrode lead 32, and sealing films 41 and .

[0018] As described above, the secondary battery described here uses flexible or pliable exterior film 10 as an exterior member for housing battery element 20. Therefore, the secondary battery shown in Figures 1 and 2 is a so-called laminate film type secondary battery.

[0019] 1, the exterior film 10 has a bag-like structure that is sealed when the battery element 20 is housed therein. As a result, the exterior film 10 houses a positive electrode 21, a negative electrode 22, and a separator 23, which will be described later.

[0020] Here, the exterior film 10 is a single film-like member that is folded in a folding direction F. The exterior film 10 is provided with a recessed portion 10U (a so-called deep drawn portion) for accommodating the battery element 20.

[0021] Specifically, the exterior film 10 is a three-layer laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order from the inside out, and when the exterior film 10 is folded, the outer peripheral edges of the opposing fusion layers are fused to each other. The fusion layer contains a polymer compound such as polypropylene. The metal layer contains a metal material such as aluminum. The surface protection layer contains a polymer compound such as nylon.

[0022] However, the number of layers of the exterior film 10 is not particularly limited, and may be one or two layers, or four or more layers.

[0023] [Battery Element] The battery element 20 is housed in an exterior film 10. This battery element 20 is a so-called power generation element, and as shown in Figures 1 and 2, includes a positive electrode 21, a negative electrode 22, a separator 23, and an electrolyte (not shown).

[0024] Here, battery element 20 is a so-called wound electrode body, and therefore positive electrode 21 and negative electrode 22 are wound around winding axis P while facing each other with separator 23 interposed therebetween. This winding axis P is a virtual axis extending in the Y-axis direction, as shown in FIG.

[0025] There are no particular limitations on the three-dimensional shape of battery element 20. Here, battery element 20 has a flat three-dimensional shape, and therefore the shape of a cross section (cross section along the XZ plane) of battery element 20 intersecting winding axis P is a flat shape defined by major axis J1 and minor axis J2.

[0026] 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 intersecting the X-axis direction and has a length less than that of the major axis J1. Here, the three-dimensional shape of the battery element 20 is a flattened cylinder, and therefore the cross-sectional shape of the battery element 20 is a flattened, approximately elliptical shape.

[0027] (Positive Electrode) As shown in Fig. 2, the positive electrode 21 includes a positive electrode current collector 21A and a positive electrode active material layer 21B. However, the positive electrode current collector 21A may be omitted.

[0028] The positive electrode current collector 21A has a pair of surfaces on which the positive electrode active material layer 21B is provided. The positive electrode current collector 21A contains a conductive material such as a metal material, and a specific example of the conductive material is aluminum.

[0029] The positive electrode active material layer 21B contains one or more types of positive electrode active materials that absorb and release lithium. However, the positive electrode active material layer 21B may further contain one or more types of other materials such as a positive electrode binder and a positive electrode conductive agent. The method for forming the positive electrode active material layer 21B is not particularly limited, but specifically includes a coating method.

[0030] Here, the positive electrode active material layer 21B is provided on both sides of the positive electrode current collector 21A. However, the positive electrode active material layer 21B may be provided on only one side of the positive electrode current collector 21A, on the side where the positive electrode 21 faces the negative electrode 22.

[0031] The type of positive electrode active material is not particularly limited, but specifically includes a lithium-containing compound. This lithium-containing compound is a compound containing lithium and one or more transition metal elements as constituent elements, and may further include one or more other elements as constituent elements. The type of other element is not particularly limited as long as it is an element other than lithium and transition metal elements, but specifically includes elements belonging to Groups 2 to 15 of the long period periodic table. The type of lithium-containing compound is not particularly limited, but specifically includes oxides, phosphate compounds, silicate compounds, borate compounds, and the like.

[0032] A specific example of the oxide is LiNiO 2　 , LiCoO 2　 , LiCo 0.98 Al 0.01 Mg 0.01 O 2　 , LiNi 0.5　 Co 0.2　 Mn 0.3　 O 2　 , LiNi 0.8　 Co 0.15 Al 0.05 O 2　 , LiNi 0.33 Co 0.33 Mn 0.33 O 2　 , Li 1.2　 Mn 0.52 Co 0.175　 Ni 0.1　 O 2　 , Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 ) O 2　 and LiMn 2　 O 4　 Specific examples of phosphate compounds include LiFePO 4　 , LiMnPO 4　 , LiFe 0.5　 Mn 0.5　 P.O. 4　 and LiFe 0.3　 Mn 0.7　 P.O. 4　 And so on.

[0033] The positive electrode binder contains one or more of materials such as synthetic rubber and polymer compounds. Specific examples of synthetic rubber include styrene-butadiene rubber, fluorine-containing rubber, and ethylene-propylene-diene. Specific examples of polymer compounds include polyvinylidene fluoride, polyimide, and carboxymethyl cellulose.

[0034] The positive electrode conductive agent contains one or more conductive materials such as a carbon material, a metal material, and a conductive polymer compound, and specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black.

[0035] (Negative Electrode) As shown in FIG. 2, the negative electrode 22 faces the positive electrode 21 via the separator 23, and contains lithium metal.

[0036] This lithium metal is the so-called elemental lithium. However, the purity of the lithium metal is not necessarily limited to 100%. Therefore, the lithium metal may unintentionally contain any amount of impurities or may intentionally contain any amount of additives.

[0037] (Separator) As shown in Fig. 2, the separator 23 is an insulating porous film interposed between the positive electrode 21 and the negative electrode 22. The separator 23 allows lithium to pass through in an ionic state while preventing the occurrence of a short circuit due to contact between the positive electrode 21 and the negative electrode 22. The separator 23 contains one or more insulating polymer compounds, and a specific example of the insulating polymer compound is polyethylene.

[0038] (Electrolyte) The electrolyte is a liquid electrolyte, and is impregnated into each of the positive electrode 21 and the separator 23. The electrolyte contains a solvent and an electrolyte salt, and the solvent contains an orthocarbonate ester compound. The detailed composition of the electrolyte will be described later.

[0039] 1 and 2 , the positive electrode lead 31 is a positive electrode wiring connected to the positive electrode current collector 21A of the positive electrode 21, and is led out of the exterior film 10. The positive electrode lead 31 contains one or more types of conductive materials such as metal materials, and a specific example of the conductive material is aluminum. The positive electrode lead 31 may have a thin plate shape, a mesh shape, or the like.

[0040] [Negative Electrode Lead] As shown in FIGS. 1 and 2 , the negative electrode lead 32 is a negative electrode wiring connected to the negative electrode 22, and is led out of the exterior film 10. Here, the lead-out direction of the negative electrode lead 32 is the same as the lead-out direction of the positive electrode lead 31. The negative electrode lead 32 contains one or more types of conductive materials such as metal materials, and a specific example of the conductive material is copper. The details regarding the shape of the negative electrode lead 32 are the same as the details regarding the shape of the positive electrode lead 31.

[0041] [Sealing Film] As shown in Fig. 1, the sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31. As shown in Fig. 1, the sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32. However, one or both of the sealing films 41, 42 may be omitted.

[0042] The sealing film 41 is a sealing member that prevents outside air and the like from entering the inside of the exterior film 10. This sealing film 41 contains a polymer compound such as polyolefin that has adhesiveness to the positive electrode lead 31, and a specific example of the polymer compound is polypropylene.

[0043] The configuration of the sealing film 42 is the same as the configuration of the sealing film 41, except that the sealing film 42 is a sealing member that has adhesiveness to the negative electrode lead 32. That is, the sealing film 42 contains a polymer compound such as polyolefin that has adhesiveness to the negative electrode lead 32.

[0044] <1-2. Detailed Configuration of Electrolyte Solution> Details regarding the configuration of the electrolyte solution are as described below.

[0045] [Solvent] As described above, the electrolytic solution contains a solvent, which is a medium for dissolving and ionizing the electrolyte salt.

[0046] (Orthocarbonate Ester Compound) The solvent contains one or more of the orthocarbonate ester compounds represented by formula (1). Therefore, an electrolyte containing an orthocarbonate ester compound as a non-aqueous solvent is a so-called non-aqueous electrolyte.

[0047] (Each of R1, R2, R3 and R4 is a hydrocarbon group.)

[0048] As is clear from formula (1), this orthocarbonate compound is a compound in which four oxygen-containing groups (-OR1, -OR2, -OR3, and -OR4) are bonded to a carbon atom. As described above, each of R1 to R4 is a hydrocarbon group, and the types of R1 to R4 may be the same or different. Of course, any two types of R1 to R4 may be the same, or any three types of R1 to R4 may be the same.

[0049] A hydrocarbon group is a general term for a group containing carbon and hydrogen as constituent elements, and the number of carbon atoms in the hydrocarbon group is not particularly limited. This hydrocarbon group may be a chain group, a cyclic group, or a group in which the chain group and the cyclic group are bonded to each other. The chain group may be linear or branched with one or more side chains.

[0050] Specifically, the hydrocarbon group includes an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a cycloalkyl group, a linking group, etc. The linking group will be described in detail later.

[0051] Specific examples of alkyl groups include methyl, ethyl, propyl, and butyl groups. Specific examples of alkenyl groups include vinyl and allyl groups. Specific examples of alkynyl groups include ethynyl groups. Specific examples of aryl groups include phenyl and naphthyl groups. Specific examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl groups.

[0052] The linking group is a general term for a monovalent group in which two or more of alkyl, alkenyl, alkynyl, aryl, and cycloalkyl groups are linked together. A specific example of the linking group is a benzyl group, which is a monovalent group in which an aryl group (phenyl group) and an alkyl group (methyl group) are linked together.

[0053] In particular, it is preferable that the hydrocarbon group contains an alkyl group having 3 or less carbon atoms, because this improves the solubility and compatibility of the orthocarbonate ester compound and makes it easier to synthesize the orthocarbonate ester compound.

[0054] Specific examples of the orthocarbonate ester compound include tetramethyl orthocarbonate (R1=R2=R3=R4=methyl groups), tetraethyl orthocarbonate (R1=R2=R3=R4=ethyl groups), tetrapropyl orthocarbonate (R1=R2=R3=R4=propyl groups), and tetraisopropyl orthocarbonate (R1=R2=R3=R4=isopropyl groups).

[0055] However, the content of the orthocarbonate ester compound in the solvent is set to a predetermined amount, specifically, the content of the orthocarbonate ester compound in the solvent is set to 40% by weight or more.

[0056] The solvent contains an orthocarbonate ester compound, and the content of the orthocarbonate ester compound in the solvent is 40% by weight or more because this suppresses the decomposition reaction of the electrolyte on the surface of the negative electrode 22 during charging and discharging, even if the negative electrode 22 contains lithium metal.

[0057] Specifically, orthocarbonate ester compounds have the property of being able to sufficiently dissolve and ionize electrolyte salts even when used alone, making them excellent solvents for use in electrolyte solutions, and therefore the electrolyte solution functions effectively even when using only the orthocarbonate ester compound as the solvent.

[0058] Furthermore, orthocarbonate ester compounds have a high lowest unoccupied molecular orbital (LUMO) and therefore have excellent reduction resistance. Therefore, the orthocarbonate ester compound contained in the solvent is less likely to decompose during charge and discharge, and the electrolyte salt and other compounds described below are more likely to decompose preferentially than the orthocarbonate ester compound. As a result, a coating derived from the electrolyte salt and other solvents is more likely to form on the surface of the negative electrode 22, and the surface of the negative electrode 22 is electrochemically protected by the coating.

[0059] For these reasons, even if the negative electrode 22 contains highly reactive lithium metal, the decomposition reaction of the electrolyte on the surface of the negative electrode 22 during charging and discharging is suppressed.

[0060] In this case, in particular, the content of the orthocarbonate ester compound in the solvent is optimized, which satisfies the protective function of the orthocarbonate ester compound to protect the surface of the negative electrode 22. As a result, the surface of the negative electrode 22 is sufficiently and stably protected by utilizing a coating derived from the electrolyte salt and other solvents, and the decomposition reaction of the electrolyte solution is also sufficiently and stably suppressed.

[0061] In particular, the content of the orthocarbonate ester compound in the solvent is preferably 60% by weight or more, and more preferably 80% by weight or more, because the protective function of the orthocarbonate ester compound is more effectively exerted, thereby further suppressing the decomposition reaction of the electrolyte solution.

[0062] Furthermore, the orthocarbonate ester compound preferably contains tetramethyl orthocarbonate, because the protective function of the orthocarbonate ester compound is sufficiently exhibited, and the decomposition reaction of the electrolyte solution is also sufficiently suppressed.

[0063] In order to confirm that the solvent contains an orthocarbonate ester compound and to measure the content of the orthocarbonate ester compound in the solvent, the electrolyte is analyzed by any method, including, but not limited to, one or more of inductively coupled plasma (ICP) atomic emission spectroscopy, nuclear magnetic resonance spectroscopy (NMR), and gas chromatography-mass spectrometry (GC-MS).

[0064] When analyzing the electrolyte, the secondary battery is disassembled to recover the electrolyte, and the recovered electrolyte is then analyzed, thereby identifying the type of component (orthocarbonate ester compound) contained in the electrolyte and the amount of the component.

[0065] (Other Compounds) The solvent may further contain one or more of the other compounds. As is clear from the range of the content of the orthocarbonate ester compound in the solvent described above, the solvent may contain other compounds in addition to the orthocarbonate ester compound.

[0066] The other compounds are non-aqueous solvents (organic solvents), except that the above-mentioned orthocarbonate ester compounds are excluded from the other compounds described here.

[0067] The non-aqueous solvent is an ester, an ether, or the like, more specifically, a carbonate ester compound, a carboxylic acid ester compound, a lactone compound, or the like, because it improves the dissociation of the electrolyte salt and also improves the mobility of ions.

[0068] The carbonate ester compounds include cyclic carbonate esters and chain carbonate esters, etc. Specific examples of cyclic carbonate esters include ethylene carbonate and propylene carbonate, and specific examples of chain carbonate esters include dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0069] The carboxylic acid ester compound is a chain carboxylic acid ester, etc. Specific examples of the chain carboxylic acid ester include ethyl acetate, ethyl propionate, propyl propionate, and ethyl trimethylacetate.

[0070] The lactone compound is lactone, etc. Specific examples of lactone include γ-butyrolactone and γ-valerolactone.

[0071] The ethers may be 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, or the like.

[0072] The non-aqueous solvent may be an unsaturated cyclic carbonate, a fluorinated cyclic carbonate, a sulfonate, a phosphate, an acid anhydride, a nitrile compound, an isocyanate compound, etc. The use of these compounds can improve the electrochemical stability of the electrolyte solution.

[0073] Specific examples of unsaturated cyclic carbonates include vinylene carbonate, vinylethylene carbonate, and methyleneethylene carbonate. Specific examples of fluorinated cyclic carbonates include monofluoroethylene carbonate and difluoroethylene carbonate. Specific examples of sulfonic acid esters include propane sultone and propene sultone. 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 succinonitrile. Specific examples of isocyanate compounds include hexamethylene diisocyanate.

[0074] (Electrolyte Salt) The electrolyte salt contains one or more types of light metal salts such as lithium salts.

[0075] A specific example of the lithium salt is lithium hexafluorophosphate (LiPF 6　 ), lithium tetrafluoroborate (LiBF 4　 ), lithium trifluoromethanesulfonate (LiCF 3　 SO 3　 ), lithium bis(fluorosulfonyl)imide (LiN(FSO 2　 ) 2　 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3　 SO 2　 ) 2　), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF 3　 SO 2　 ) 3　 ), lithium bis(oxalato)borate (LiB(C 2　 O 4　 ) 2　 ), lithium monofluorophosphate (Li 2　 PFO 3　 ) and lithium difluorophosphate (LiPF 2　 O 2　 ) With this content, high ionic conductivity can be obtained.

[0076] The content of the electrolyte salt is not particularly limited, but specifically, it is 0.3 mol / kg to 5.0 mol / kg relative to the solvent, because high ionic conductivity can be obtained.

[0077] <1-3. Operation> The secondary battery operates in the battery element 20 as follows.

[0078] During charging, lithium is released in an ionic state from the positive electrode 21. As a result, the lithium moves to the negative electrode 22 through the electrolyte, and lithium metal is deposited on the surface of the negative electrode 22.

[0079] During discharge, lithium metal is eluted from the negative electrode 22. As a result, lithium ions move to the positive electrode 21 through the electrolyte, and the lithium is absorbed in the positive electrode 21.

[0080] <1-4. Manufacturing Method> When manufacturing a secondary battery, the positive electrode 21 and the negative electrode 22 are each produced and an electrolyte solution is prepared according to the procedure described below as an example, and then the secondary battery is assembled and subjected to a stabilization process after assembly.

[0081] [Fabrication of Positive Electrode] First, a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed together to form a positive electrode mixture. Next, the positive electrode mixture is poured into a solvent to prepare a paste-like positive electrode mixture slurry. This solvent may be an aqueous solvent or an organic solvent. Finally, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 21A to form the positive electrode active material layer 21B. The positive electrode active material layer 21B may then be compression-molded using a compression device such as a roll press. In this case, the positive electrode active material layer 21B may be heated, or the compression molding may be repeated multiple times. This results in the positive electrode active material layer 21B being formed on both surfaces of the positive electrode current collector 21A, thereby fabricating the positive electrode 21.

[0082] [Fabrication of Negative Electrode] The negative electrode 22 containing lithium metal as the negative electrode active material is prepared. In this case, the negative electrode 22 may be fabricated by attaching a lithium metal foil or the like to a negative electrode current collector.

[0083] [Preparation of Electrolyte Solution] An electrolyte salt is added to a solvent containing an orthocarbonate ester compound, and the solvent is then stirred. In this case, the amount of the orthocarbonate ester compound added is adjusted so that the content of the orthocarbonate ester compound in the solvent falls within the above-mentioned range. This dissolves the electrolyte salt in the solvent, thereby preparing an electrolyte solution.

[0084] [Assembly of Secondary Battery] First, the positive electrode lead 31 is connected to the positive electrode current collector 21A of the positive electrode 21 using a joining method such as welding. Also, the negative electrode lead 32 is connected to the negative electrode 22 using a joining method such as welding.

[0085] Next, the positive electrode 21 and the negative electrode 22 are stacked together with the separator 23 interposed therebetween to form a laminate (not shown). Next, the laminate is wound to form a wound body (not shown), and then the wound body is pressed using a compression device such as a press to form a flat shape. The wound body after this formation has a configuration similar to that of the battery element 20, except that the positive electrode 21 and the separator 23 are not impregnated with an electrolyte solution.

[0086] Next, after the roll is accommodated in the recess 10U, the exterior film 10 (adhesive layer / metal layer / surface protection layer) is folded to face each other. Next, the outer peripheral edges of two sides of the opposing adhesive layers are joined together using an adhesive method such as heat fusion, thereby accommodating the roll in the bag-shaped exterior film 10.

[0087] Finally, after injecting the electrolyte solution into the bag-shaped exterior film 10, the outer peripheral edges of the remaining sides of the opposing fusion layers are joined together using an adhesive method such as heat fusion. In this case, a sealing film 41 is inserted between the exterior film 10 and the positive electrode lead 31, and a sealing film 42 is inserted between the exterior film 10 and the negative electrode lead 32.

[0088] As a result, the wound body is impregnated with the electrolyte and is enclosed in the bag-shaped exterior film 10, thereby assembling a secondary battery.

[0089] [Stabilization Treatment of Assembled Secondary Battery] The assembled secondary battery is charged and discharged. Stabilization conditions such as the ambient temperature, the number of charge / discharge cycles (number of cycles), and charge / discharge conditions can be set arbitrarily.

[0090] As a result, a coating is formed on each surface of the positive electrode 21 and the negative electrode 22. In this case, as described above, a coating derived from the electrolyte salt and other compounds is formed on the surface of the negative electrode 22.

[0091] As a result, the state of the battery element 20 becomes electrochemically stable, and the secondary battery is completed.

[0092] <1-5. Actions and Effects> In this secondary battery, the negative electrode 22 contains lithium metal, the solvent of the electrolyte contains an orthocarbonate ester compound, and the content of the orthocarbonate ester compound in the solvent is 40 wt % or more.

[0093] In this case, as described above, the properties of the orthocarbonate ester compound are utilized to improve the solubility of the electrolyte salt in the solvent, and a good coating derived from the electrolyte salt and other compounds is likely to be formed on the surface of the negative electrode 22 during charge and discharge. As a result, the surface of the negative electrode 22 is electrochemically protected by the coating, and even if the negative electrode 22 contains highly reactive lithium metal, the decomposition reaction of the electrolyte on the surface of the negative electrode 22 during charge and discharge is suppressed. Therefore, excellent battery characteristics can be obtained.

[0094] In particular, when the content of the orthocarbonate ester compound in the solvent is 60 wt % or more, the protective function of the orthocarbonate ester compound is utilized to further suppress the decomposition reaction of the electrolyte solution, thereby achieving a higher effect. In this case, when the content of the orthocarbonate ester compound in the solvent is 80 wt % or more, the decomposition reaction of the electrolyte solution is further suppressed, thereby achieving an even higher effect.

[0095] Furthermore, if the hydrocarbon group in formula (1) relating to the orthocarbonate ester compound contains an alkyl group and the alkyl group has 3 or less carbon atoms, the solubility and compatibility of the orthocarbonate ester compound are improved, thereby achieving a greater effect.

[0096] Furthermore, if the orthocarbonate ester compound contains tetramethyl orthocarbonate, the protective function of the orthocarbonate ester compound is fully exerted, and therefore the decomposition reaction of the electrolyte is also sufficiently suppressed, thereby achieving a higher effect.

[0097] 2. Modifications The configuration of the secondary battery can be modified as appropriate, as described below. However, the series of modifications described below may be combined with each other.

[0098] [Modification 1] A porous film separator 23 is used. However, although not specifically shown here, a laminated separator including a polymer compound layer may also be used.

[0099] Specifically, the laminated separator includes a porous membrane having a pair of surfaces and a polymer compound layer provided on one or both surfaces of the porous membrane. This is because the separator improves adhesion to the positive electrode 21 and the negative electrode 22, respectively, thereby suppressing misalignment of the battery element 20. This suppresses miswinding of the positive electrode 21, the negative electrode 22, and the separator 23, thereby suppressing swelling of the secondary battery even if a decomposition reaction of the electrolyte occurs. The polymer compound layer includes polyvinylidene fluoride or the like. Polyvinylidene fluoride has excellent physical strength and is electrochemically stable.

[0100] One or both of the porous film and the polymer compound layer may contain one or more types of insulating particles. This is because the insulating particles dissipate heat when the secondary battery generates heat, improving the safety (heat resistance) of the secondary battery. The insulating particles contain one or more types of insulating materials, such as inorganic materials and resin materials. Specific examples of inorganic materials include aluminum oxide, aluminum nitride, boehmite, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of resin materials include acrylic resin and styrene resin.

[0101] When a laminated separator is produced, a precursor solution containing a polymer compound and an organic solvent is prepared, and then the precursor solution is applied to one or both sides of a porous film. In this case, the precursor solution may contain a plurality of insulating particles.

[0102] Even when this laminated separator is used, the same effect can be obtained because lithium can move in an ionic state between the positive electrode 21 and the negative electrode 22. In this case, as described above, swelling of the secondary battery is further suppressed, and therefore a greater effect can be obtained.

[0103] [Modification 2] An electrolytic solution that is a liquid electrolyte is used. However, although not specifically shown here, an electrolyte layer that is a gel electrolyte may also be used.

[0104] In the battery element 20 using the electrolyte layer, the positive electrode 21 and the negative electrode 22 are wound facing each other with the separator 23 and the electrolyte layer interposed therebetween. The electrolyte layer is interposed between the positive electrode 21 and the separator 23, and also between the negative electrode 22 and the separator 23.

[0105] Specifically, the electrolyte layer contains a polymer compound together with an electrolytic solution, and the electrolytic solution is held by the polymer compound. This is because leakage of the electrolytic solution is prevented. The composition of the electrolytic solution is as described above. The polymer compound contains polyvinylidene fluoride, etc. When forming the electrolyte layer, a precursor solution containing the electrolytic solution, the polymer compound, a solvent, etc. is prepared, and then the precursor solution is applied to one or both surfaces of each of the positive electrode 21 and the negative electrode 22.

[0106] Even when this electrolyte layer is used, the same effect can be obtained because lithium ions can move between the positive electrode 21 and the negative electrode 22 via the electrolyte layer. In this case, particularly, as described above, leakage of the electrolyte solution is prevented, so that a greater effect can be obtained.

[0107] 3. Uses of Secondary Batteries Finally, uses (application examples) of secondary batteries will be described.

[0108] The use of the secondary battery is not particularly limited. The secondary battery used as a power source may be a main power source or an auxiliary power source in electronic devices, electric vehicles, etc. The main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. The auxiliary power source may be a power source used in place of the main power source or a power source that can be switched from the main power source.

[0109] Specific examples of uses for secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals; Storage devices such as backup power supplies and memory cards; Power tools such as power drills and power saws; Battery packs installed in electronic devices; Medical electronic devices such as pacemakers and hearing aids; Electric vehicles such as electric cars (including hybrid cars); Power storage systems such as home or industrial battery systems that store power in preparation for emergencies, etc. In these uses, one secondary battery may be used, or multiple secondary batteries may be used.

[0110] The battery pack may include a single cell or a battery pack. The electric vehicle is a vehicle that runs on a secondary battery as a driving power source, and may be a hybrid vehicle that also includes a driving source other than the secondary battery. In a home power storage system, power stored in a secondary battery, which is a power storage source, can be used to power home electrical appliances, etc.

[0111] Here, an example of the use of the secondary battery will be specifically described. The configuration described below is merely an example and can be modified as appropriate.

[0112] Figure 3 shows the block diagram of a battery pack, which is an example of an application of a secondary battery. The battery pack described here is a battery pack (a so-called soft pack) that uses a single secondary battery, and is installed in electronic devices such as smartphones.

[0113] 3, the battery pack includes a power supply 51 and a circuit board 52. The circuit board 52 is connected to the power supply 51 and includes a positive terminal 53, a negative terminal 54, and a temperature detection terminal 55.

[0114] The power source 51 includes one secondary battery. The positive electrode lead of this secondary battery is connected to a positive electrode terminal 53, and the negative electrode lead is connected to a negative electrode terminal 54. The power source 51 is connected to the outside via the positive electrode terminal 53 and the negative electrode terminal 54, and is therefore capable of charging and discharging. The circuit board 52 includes a control unit 56, a switch 57, a PTC element 58 which is a thermosensitive resistor, and a temperature detection unit 59. However, the PTC element 58 may be omitted.

[0115] The control unit 56 includes a central processing unit (CPU) and memory, and controls the operation of the entire battery pack. The control unit 56 detects and controls the usage state of the power source 51.

[0116] When the voltage of power supply 51 (secondary battery) reaches the overcharge detection voltage or the overdischarge detection voltage, control unit 56 turns off switch 57 to prevent charging current from flowing through the current path of power supply 51. The overcharge detection voltage is not particularly limited, but specifically, it is 4.20 V±0.05 V, and the overdischarge detection voltage is not particularly limited, but specifically, it is 2.40 V±0.10 V.

[0117] Switch 57 includes a charge control switch, a discharge control switch, a charging diode, a discharging diode, etc., and switches between the connection and disconnection of power supply 51 and an external device in response to instructions from control unit 56. Switch 57 includes a metal oxide semiconductor field effect transistor (MOSFET), etc., and the charge current and the discharge current are detected based on the ON resistance of switch 57.

[0118] Temperature detection unit 59 includes a temperature detection element such as a thermistor. Temperature detection unit 59 measures the temperature of power supply 51 using temperature detection terminal 55 and outputs the temperature measurement result to control unit 56. The temperature measurement result measured by temperature detection unit 59 is used when control unit 56 controls charging and discharging in the event of abnormal heat generation, and when control unit 56 performs correction processing when calculating the remaining capacity.

[0119] An embodiment of the present technology will be described.

[0120] Examples 1 to 4 and Comparative Examples 1 and 2 As will be described below, secondary batteries were manufactured, and then the battery characteristics of the secondary batteries were evaluated.

[0121] [Fabrication of Secondary Battery] In order to simply evaluate the battery characteristics, a test secondary battery was fabricated according to the following procedure: This test secondary battery was a simplified lithium metal secondary battery.

[0122] First, an electrolyte salt (lithium bis(trifluoromethanesulfonyl)imide) was added to a solvent, and the solvent was then stirred to prepare an electrolyte solution.

[0123] The solvent used was tetramethyl orthocarbonate (OTTM), an orthocarbonate ester compound, and 1,2-dimethoxyethane (DME), another compound. In this case, the mixing ratio of the orthocarbonate ester compound and the other compound was adjusted. The content of the electrolyte salt was 1 mol / L (=1 mol / dm 3　 )

[0124] The content (wt %) of the orthocarbonate compound in the solvent and the content (wt %) of other compounds in the solvent are as shown in Table 1.

[0125] Subsequently, a lithium metal foil (thickness=0.1 mm) was pressed onto a copper foil (thickness=0.01 mm) using a press to prepare a test electrode.

[0126] Next, the electrolyte solution was dropped onto a separator (microporous polyethylene film, thickness = 10 μm) to impregnate the separator with the electrolyte solution. The amount of the electrolyte solution dropped was 0.01 ml (= 0.01 cm 3　 )

[0127] Next, a copper foil (thickness = 0.012 mm) was prepared as a counter electrode, and the test electrode and the counter electrode were laminated together with a separator impregnated with an electrolytic solution interposed therebetween, thereby completing a test secondary battery.

[0128] [Evaluation of Battery Characteristics] The charge / discharge characteristics as the battery characteristics were evaluated according to the procedure described below, and the results shown in Table 1 were obtained.

[0129] When evaluating the charge / discharge characteristics, first, the secondary battery was charged in a room temperature environment (temperature = 23°C) to measure the charge capacity, and then the secondary battery was discharged to measure the discharge capacity.

[0130] During charging, 0.22mA / cm 2　 The battery was charged at a current density of 0.1 V for a total charging time of 3 hours.

[0131] Subsequently, the coulombic efficiency was calculated based on the formula: coulombic efficiency (%)=(discharge capacity / charge capacity)×100.

[0132] Subsequently, in the same environment, the secondary battery was repeatedly charged and discharged until the total number of cycles reached 25, while calculating the coulombic efficiency for each cycle. The charge and discharge conditions were as described above.

[0133] Finally, the average Coulombic efficiency, which is an index for evaluating charge-discharge characteristics, was calculated by averaging the 16 Coulombic efficiencies calculated for each of the 10th to 25th cycles. This average Coulombic efficiency value was rounded to one decimal place.

[0134] Note that the nine Coulombic efficiencies calculated during the initial charge / discharge cycles (1st to 9th cycles) were not used to calculate the average Coulombic efficiency. This is because the Coulombic efficiency tends to vary during the initial charge / discharge cycles. To calculate the average Coulombic efficiency, the Coulombic efficiencies calculated during the later charge / discharge cycles (10th to 25th cycles) are used instead of the Coulombic efficiencies calculated during the initial charge / discharge cycles, which reduces the variation in the Coulombic efficiency. This ensures the accuracy and reproducibility of the calculation of the average Coulombic efficiency.

[0135]

[0136] [Discussion] As shown in Table 1, the average coulombic efficiency varied depending on the solvent composition.

[0137] Specifically, when the content of the orthocarbonate compound in the solvent was less than 40% by weight (Comparative Examples 1 and 2), the average coulombic efficiency decreased.

[0138] In contrast, when the content of the orthocarbonate ester compound in the solvent was 40% by weight or more (Examples 1 to 4), the average Coulombic efficiency increased. In this case, the average Coulombic efficiency increased even more when the content of the orthocarbonate ester compound in the solvent was 60% by weight or more (Examples 2 to 4). Furthermore, when the content of the orthocarbonate ester compound in the solvent was 80% by weight or more (Examples 3 and 4), the average Coulombic efficiency increased even more.

[0139] In addition, in the above-mentioned Patent Document 2 (Japanese Patent Laid-Open Publication No. 2002-270222), an orthocarbonate ester similar to the orthocarbonate ester compound is contained.

[0140] However, the secondary battery disclosed in Patent Document 2 is a lithium ion secondary battery, not a lithium metal secondary battery. In addition, in the secondary battery disclosed in Patent Document 2, the orthocarbonate ester compound in the nonaqueous electrolyte solution is 0.001 mmol / g to 0.18 mmol / g, so the content of the orthocarbonate ester in the nonaqueous electrolyte solution should be less than 40 wt % when converted into the content of the orthocarbonate ester compound in the solvent.

[0141] Therefore, Patent Document 2 does not disclose the appropriate range of the content of orthocarbonate ester required to increase the average coulomb increase rate in a lithium metal secondary battery.

[0142] [Summary] From the results shown in Table 1, when the test electrode contained lithium metal, the electrolyte solvent contained an orthocarbonate ester compound, and the content of the orthocarbonate ester compound in the solvent was 40 wt % or more, a high average coulombic efficiency was obtained. Therefore, the charge / discharge characteristics were improved, resulting in excellent battery characteristics and excellent safety.

[0143] The present technology has been described above with reference to an embodiment and examples. However, the configuration of the present technology is not limited to the configuration described in the embodiment and examples, and can be modified in various ways.

[0144] Specifically, the secondary battery has been described as having a laminate film structure, but the structure of the secondary battery is not particularly limited, and may be cylindrical, prismatic, coin-shaped, button-shaped, or the like.

[0145] The battery element has been described as having a wound structure. However, the structure of the battery element is not particularly limited, and may be a stacked structure or a zigzag structure. In the stacked structure, positive and negative electrodes are alternately stacked with a separator interposed therebetween, while in the zigzag structure, the positive and negative electrodes are folded in a zigzag pattern while facing each other with the separator interposed therebetween.

[0146] The effects described in this specification are merely examples, and the effects of the present technology are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present technology.

[0147] The present technology may also be configured as follows: <1> A secondary battery comprising: a positive electrode; a negative electrode containing lithium metal; and an electrolyte solution containing a solvent, wherein the solvent contains an orthocarbonate ester compound represented by formula (1), and the content of the orthocarbonate ester compound in the solvent is 40 wt % or more. (Each of R1, R2, R3, and R4 is a hydrocarbon group.) <2> The secondary battery according to <1>, wherein the content of the orthocarbonate ester compound in the solvent is 60% by weight or more. <3> The secondary battery according to <2>, wherein the content of the orthocarbonate ester compound in the solvent is 80% by weight or more. <4> The secondary battery according to any one of <1> to <3>, wherein the hydrocarbon group contains an alkyl group, and wherein the alkyl group has 3 or less carbon atoms. <5> The secondary battery according to any one of <1> to <4>, wherein the orthocarbonate ester compound contains tetramethyl orthocarbonate.

[0148] 21...positive electrode, 22...negative electrode

Claims

1. A secondary battery comprising: a positive electrode; a negative electrode containing lithium metal; and an electrolyte solution containing a solvent, wherein the solvent contains an orthocarbonate ester compound represented by formula (1), and the content of the orthocarbonate ester compound in the solvent is 40% by weight or more. (Each of R1, R2, R3 and R4 is a hydrocarbon group.) 2. The secondary battery according to claim 1, wherein the content of the orthocarbonate compound in the solvent is 60% by weight or more.

3. The secondary battery according to claim 2, wherein the content of the orthocarbonate compound in the solvent is 80% by weight or more.

4. The secondary battery according to claim 1, wherein the hydrocarbon group includes an alkyl group, and the number of carbon atoms in the alkyl group is 3 or less.

5. The secondary battery according to any one of claims 1 to 4, wherein the orthocarbonate ester compound includes tetramethyl orthocarbonate.

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