Non-aqueous secondary battery

Abstract

To provide a non-aqueous secondary battery that enables repeated Li dissolution deposition in a stable manner, improves discharge capacity maintenance, and improves cycle characteristics.SOLUTION: A non-aqueous secondary battery includes a negative electrode containing metallic Li, a positive electrode, a separator, and an electrolyte, the electrolyte contains fluorinated ethylene carbonate as a solvent at least 85 vol% based on the total amount of solvent, and contains 1.0 to 2.0 mol / L of LiPF6 as an electrolyte.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a non-aqueous secondary battery.

Background Art

[0002] Patent Document 1 describes an electrode assembly of an anode-free primary battery including a separator having a positive electrode side and a negative electrode side facing each other, a positive electrode located on the positive electrode side of the separator and including a positive electrode current collector and a positive electrode material, and a negative electrode current collector disposed on the negative electrode side of the separator. And it is described that the electrolyte may include, as lithium salts, LiPF6, LiBF4, LiClO4, LiAsF6, LiSbF6, LiAlCl4, LiGaCl4, LiNO3, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF2CF3, LiC6F5SO3, LiO2CCF3, LiSO3F, LiB(C6H5)4, LiCF3SO3, and LiDFOB, and may include fluoroethylene carbonate (FEC) as an organic solvent.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, a primary battery using an electrolyte containing a high concentration of FEC for an anode-free Li negative electrode has been proposed. However, when used in a secondary battery, further improvement in battery performance is required, and there is room for improvement.

[0005] An object of the present disclosure is to provide a non-aqueous secondary battery in which repeated Li dissolution and precipitation can be stably performed, the maintainability of the discharge capacity is improved, and the cycle characteristics can be improved.

Means for Solving the Problem

[0006] The means for solving the above problems include the following aspects. <1> A non-aqueous secondary battery having a negative electrode containing metallic Li, a positive electrode, a separator, and an electrolytic solution, wherein the electrolytic solution contains fluorinated ethylene carbonate as a solvent in an amount of 85% by volume or more based on the total amount of the solvent, and LiPF6 as an electrolyte in an amount of 1.0 to 2.0 mol / L. <2> The non-aqueous secondary battery according to <1>, wherein the electrolytic solution further contains a non-fluorinated cyclic carbonate as the solvent. <3> The non-aqueous secondary battery according to <2>, wherein the non-fluorinated cyclic carbonate is at least one of ethylene carbonate and propylene carbonate. <4> The ratio of the content of the fluorinated ethylene carbonate to the total content of the ethylene carbonate and the propylene carbonate in the solvent is 85:15 to 95:5, The non-aqueous secondary battery according to <3>, wherein the content of the LiPF6 is 1.5 to 2.0 mol / L.

Advantages of the Invention

[0007] According to the present disclosure, a non-aqueous secondary battery is provided in which repeated Li dissolution and precipitation can be stably achieved, the maintainability of the discharge capacity is improved, and the cycle characteristics can be improved.

Embodiments for Carrying Out the Invention

[0008] Hereinafter, an embodiment of the non-aqueous secondary battery of the present disclosure will be described in detail.

[0009] ≪Non-aqueous Secondary Battery≫ The non-aqueous secondary battery according to this embodiment has a negative electrode containing metallic Li, a positive electrode, a separator, and an electrolytic solution. The electrolytic solution contains, as a solvent, fluorinated ethylene carbonate (hereinafter, may be simply referred to as "FEC") in an amount of 85% by volume or more based on the total amount of the solvent. The electrolytic solution contains LiPF6 as an electrolyte in an amount of 1.0 to 2.0 mol / L.

[0010] According to the non-aqueous secondary battery according to this embodiment, repeated Li dissolution and deposition can be stably performed, and the maintainability of the discharge capacity is improved. That is, the cycle characteristics can be improved.

[0011] Although the mechanism by which this effect is achieved is not necessarily clear, it is presumed as follows. As factors for the decrease in the maintainability of the discharge capacity, for example, (1) charge consumption due to electrolytic solution decomposition, (2) charge consumption due to micro short circuit, (3) overvoltage due to an increase in resistance, etc. can be mentioned.

[0012] (1): Improving the film characteristics and suppressing the decomposition of the solvent can be effective countermeasures. The film components change depending on the type and concentration of the electrolyte. It is considered that the harder the film, the more the growth of Li dendrites is suppressed, and the smaller the surface area, the more the consumption of the electrolytic solution is suppressed. Also, the higher the flexibility of the film, the more the film breakage when Li dissolves and deposits is suppressed, and thus the decomposition of the electrolytic solution on the fresh surface of the Li metal is suppressed. By appropriately controlling the hardness and flexibility of the film, the maintainability of the discharge capacity can be increased. Also, generally, the higher the concentration, the fewer the solvents coordinated with Li and the fewer the solvents that are easily decomposed.

[0013] (2): This can occur when the needle-like grown Li penetrates the separator and reaches the positive electrode. Also, it is considered that needle-like growth can be promoted even when the diffusion of Li ions is slow. (3): This can be caused by the deterioration of the surface and bulk of the electrode material and the deterioration of the electrolytic solution.

[0014] On the other hand, fluorinated ethylene carbonate (FEC) in the solvent and LiPF6 in the electrolyte are considered to form an inorganic film such as LiF during decomposition, and the hardness of the film can be controlled by the contents of FEC and LiPF6. On the other hand, cyclic carbonates containing FEC are considered to easily form an organic film with a long carbon chain during decomposition, and the flexibility of the film can be controlled by its content. Also, depending on the salt concentration, the coordination state of Li ions with the solvent and anion (PF6 - ) changes, and the decomposition of the anion is promoted as the concentration increases. If the concentration is too high, the viscosity of the electrolyte increases, the diffusion of Li slows down, and dendrite growth can be promoted.

[0015] In the non-aqueous secondary battery according to this embodiment, the electrolyte contains FEC in an amount of 85% by volume or more based on the total amount of the solvent, and LiPF6 in an amount of 1.0 to 2.0 mol / L in the electrolyte. As a result, the hardness and flexibility of the film are controlled, and an electrolyte composition is obtained in which a good film is formed while ensuring the diffusion of Li. As a result, it is presumed that repeated Li dissolution and precipitation can be stably performed, the discharge capacity retention is improved, and the cycle characteristics are improved.

[0016] Next, each member constituting the non-aqueous secondary battery will be described.

[0017] The non-aqueous secondary battery includes, for example, an electrode body having a negative electrode containing metallic Li, a positive electrode, and a separator disposed between the positive electrode and the negative electrode, and this electrode body is disposed in a battery case together with an electrolyte (non-aqueous electrolyte).

[0018] <Electrolyte> · Solvent The electrolyte (non-aqueous electrolyte) contains a solvent (non-aqueous solvent) and an electrolyte. The solvent contains 85% by volume or more of fluorinated ethylene carbonate (FEC, fluoroethylene carbonate) based on the total amount of the solvent. When the content of fluorinated ethylene carbonate (FEC) in the solvent is less than 85% by volume, repeated Li dissolution precipitation cannot be stably performed, the discharge capacity retention decreases, and the cycle characteristics deteriorate. The upper limit of the content of fluorinated ethylene carbonate (FEC) based on the total amount of the solvent may be 100% by volume. The content of fluorinated ethylene carbonate (FEC) based on the total amount of the solvent is 85 to 100% by volume, preferably 85 to 92% by volume, more preferably 88 to 92% by volume, and even more preferably 88 to 90% by volume.

[0019] When the electrolytic solution contains a solvent other than fluorinated ethylene carbonate (FEC), it preferably contains a non-fluorinated cyclic carbonate as the solvent. Examples of the non-fluorinated cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). The non-fluorinated cyclic carbonate is preferably at least one of ethylene carbonate (EC) and propylene carbonate (PC).

[0020] Examples of other solvents include chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC); lactones such as γ-butyrolactone (GBL) and δ-valerolactone; cyclic ethers such as tetrahydrofuran (THF), 1,3-dioxolane, and 1,4-dioxane; chain ethers such as 1,2-dimethoxyethane (DME); carboxylic acid esters such as methyl formate (MF), methyl acetate (MA), and methyl propionate (MP), and the like.

[0021] However, the solvent preferably consists only of fluorinated ethylene carbonate (FEC) or consists only of fluorinated ethylene carbonate (FEC) and non-fluorinated cyclic carbonate. The content of solvents other than fluorinated ethylene carbonate (FEC) is 15% by volume or less based on the total amount of solvents.

[0022] ·Electrolyte The electrolyte solution contains 1.0 to 2.0 mol / L of LiPF6 (lithium hexafluorophosphate) as the electrolyte. When the amount of LiPF6 as the electrolyte is less than 1.0 mol / L or more than 2.0 mol / L, in either case, repeated Li metal precipitation cannot be stably performed, the discharge capacity maintenance property deteriorates, and the cycle characteristics deteriorate. The amount of LiPF6 as the electrolyte is 1.0 to 2.0 mol / L, preferably 1.0 to 1.5 mol / L.

[0023] The electrolyte solution may contain electrolytes other than LiPF6. Examples of other electrolytes include Li salts such as lithium tetrafluoroborate (LiBF4), Li[N(FSO2)2], and Li[N(CF3SO2)2]. However, it is preferable that the electrolyte consists only of LiPF6.

[0024] In addition to the solvent and the electrolyte, the electrolyte solution may contain various additives such as a thickening agent, a film-forming agent, and a gas-generating agent. The electrolyte is typically a non-aqueous electrolyte liquid at room temperature (for example, 25 ± 10 °C). The electrolyte solution typically remains liquid under the battery's operating environment (for example, in a temperature environment of -20 to +60 °C).

[0025] In this embodiment, it is preferable that the electrolyte solution contains a non-fluorinated cyclic carbonate in addition to fluorinated ethylene carbonate as the solvent, and the non-fluorinated cyclic carbonate is at least one of ethylene carbonate and propylene carbonate. Further, it is more preferable that the ratio of the content of fluorinated ethylene carbonate to the total content of ethylene carbonate and propylene carbonate is 85:15 to 95:5, and the content of LiPF6 is 1.5 to 2.0 mol / L.

[0026] <Negative electrode containing metallic Li> The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. As the negative electrode current collector, a conductive member made of a metal with good conductivity (such as copper) is suitable. The negative electrode active material layer contains a negative electrode active material. Examples of the negative electrode active material include graphite-based carbons such as natural graphite, artificial graphite, and amorphous coated graphite. The proportion of graphite in the graphite-based carbon is generally 50% by mass or more, preferably 80% by mass or more. The negative electrode active material layer may be composed only of the negative electrode active material, or may contain components other than the negative electrode active material, such as a thickener and a binder, as necessary. Examples of the thickener include celluloses such as carboxymethyl cellulose (CMC). Examples of the binder include rubbers such as styrene-butadiene copolymer (SBR) and vinyl halide resins such as polyvinylidene fluoride (PVdF). Note that in the negative electrode, when Li is deposited during charging, a negative electrode containing metallic Li is formed.

[0027] <Positive electrode> The positive electrode includes, for example, a positive electrode current collector and a positive electrode active material layer fixed on the positive electrode current collector. As the positive electrode current collector, a conductive member made of a metal with good conductivity (such as aluminum) is suitable. The positive electrode active material layer contains at least a positive electrode active material and a conductive material. The positive electrode active material layer may be composed of the positive electrode active material and the conductive material, or may contain other components, such as a binder and various additives. Examples of the binder include vinyl halide resins such as polyvinylidene fluoride (PVdF).

[0028] Examples of the positive electrode active material include lithium nickel cobalt manganese composite oxide (hereinafter sometimes simply referred to as "LNCM"). The simplest LNCM has the following general formula: LiNi x Co y Mn zIt is represented by O2 (where x, y, and z satisfy 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z = 1). LNCM may contain, in addition to Li, Ni, Co, and Mn, other additive elements, such as transition metal elements other than Ni, Co, and Mn, and typical metal elements other than Li. LNCM has a layered crystal structure. LNCM may exceed 50% by mass of the entire cathode active material and, for example, may account for 80 - 100% by mass. The cathode active material may be composed of only LNCM. Examples of other cathode active materials include lithium nickel composite oxides, lithium cobalt composite oxides, lithium nickel manganese composite oxides, etc.

[0029] Examples of the conductive material include graphitizable carbon such as non-graphitizable carbon and carbon black, and graphite.

[0030] <Separator> The separator is an electrically insulating porous membrane. The separator electrically isolates the cathode and the anode. The separator may have a thickness of, for example, 5 - 30 μm. The separator may be composed of, for example, a porous polyethylene (PE) membrane, a porous polypropylene (PP) membrane, etc. The separator may have a multilayer structure. For example, the separator may be composed of a porous PP membrane, a porous PE membrane, and a porous PP membrane laminated in this order. The separator may have a heat-resistant layer on its surface. The heat-resistant layer contains a heat-resistant material. Examples of the heat-resistant material include metal oxide particles such as alumina, and high melting point resins such as polyimide.

[0031] <Battery case> The battery case may be, for example, rectangular (flat cuboid) or cylindrical. For example, metals such as aluminum (Al) and Al alloys constitute the battery case. However, as long as the battery case has a predetermined sealing property, a composite material of metal and resin may constitute the battery case. Examples of the composite material of metal and resin include aluminum laminate films. The battery case may be provided with external terminals, a liquid injection hole, a gas discharge valve, a current interruption mechanism (CID), etc.

[0032] <Use> The non-aqueous secondary battery according to this embodiment is expected to enable stable repeated Li dissolution and precipitation even when the battery is repeatedly charged and discharged, improve the maintainability of the discharge capacity, and improve the cycle characteristics. Examples of the use of the non-aqueous secondary battery according to this embodiment include power sources for hybrid vehicles (HV), plug-in hybrid vehicles (PHV), electric vehicles (EV), and the like.

Examples

[0033] Examples are shown below to more specifically explain the present disclosure.

[0034] - Preparation of electrolyte solution - A predetermined amount of LiPF6 was measured as the electrolyte. That is, if the target was 1 mol / L, it was measured to be equivalent to 0.01 mol. This was added to the solvent described in Table 1 and dissolved. The electrolyte dissolved in the solvent was transferred to a 10 mL volumetric flask and made up to an electrolyte liquid volume of 10 mL to obtain an electrolyte solution. The amounts (mol / L) of the electrolyte (LiPF6) and the amounts (volume %) of the solvents (fluoroethylene carbonate (FEC) and ethylene carbonate (EC)) used in each example and comparative example are shown in Table 1.

[0035] - Cell fabrication - LiNi x Co y Mn (1-x-y) O2 (x = 0.5, y = 0.2), a conductive aid, and a binder were mixed and applied to an Al foil and pressed to fabricate a positive electrode. An electrolytic copper foil was used as the negative electrode, opposed to the positive electrode through a separator made of polyethylene (PE), an electrolyte solution was injected, and a coin cell was fabricated. Note that a negative electrode containing metallic Li is formed by the precipitation of Li on the electrolytic copper foil during charging.

[0036] - Cell evaluation - 0.4 mA / cm 2The cell was activated at a current density of and in a voltage range of 3.0 to 4.3 V. Thereafter, a cycle test of 30 cycles was conducted at a current density of 4 mA / cm 2 and in a voltage range of 3.0 to 4.3 V. The discharge capacity retention rate was calculated by dividing the discharge capacity at the 30th cycle of the cycle test by the discharge capacity at the 1st cycle of the cycle test. Furthermore, the "discharge capacity retention rate ratio" was calculated by dividing the discharge capacity retention rate of each example and comparative example by the discharge capacity retention rate of Comparative Example 1. The results are shown in Table 1

[0037]

Table 1

[0038] What is shown in Table 1 is as follows. LiPF6: Lithium hexafluorophosphate EC: Ethylene carbonate PC: Propylene carbonate FEC: Fluoroethylene carbonate

[0039] Fluoroethylene carbonate (FEC) and LiPF6 are considered to form an inorganic film such as LiF during decomposition, and the hardness of the film can be controlled by their content. On the other hand, cyclic carbonates containing FEC are considered to easily form an organic film with a long carbon chain during decomposition, and the flexibility of the film can be controlled by their content. In Examples 1, 2, 3, and 6, the ratio of FEC and other cyclic carbonates in the solvent was appropriately blended, and it is considered that good cycle characteristics were shown.

[0040] Also, depending on the salt concentration, the coordination state of Li ions with the solvent and anion (PF6 - ) changes, and the higher the concentration, the more the decomposition of the anion is promoted. If the concentration is too high, the viscosity of the electrolyte increases, the diffusion of Li becomes slow, and dendrite growth can be promoted. In Examples 2, 4, and 5, an electrolyte composition was obtained in which the diffusion of Li was ensured while a good film was formed, and it is considered that the discharge capacity retention was improved and good cycle characteristics were exhibited.

Claims

【Claim 1】 having a negative electrode containing metallic Li, a positive electrode, a separator, and an electrolytic solution, wherein the electrolytic solution contains fluorinated ethylene carbonate as a solvent in an amount of 85% by volume or more based on the total amount of the solvent, contains LiPF ６ in an amount of 1.5 to 2.0 mol / L, the electrolytic solution further contains a non-fluorinated cyclic carbonate as the solvent, the non-fluorinated cyclic carbonate is at least one of ethylene carbonate and propylene carbonate, a non-aqueous secondary battery, wherein the ratio of the content of the fluorinated ethylene carbonate to the total content of the ethylene carbonate and the propylene carbonate in the solvent is 85:15 to 95:5.

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